Johanna Forsting, Julia Kraxner, Hannes Witt, Andreas Janshoff, Sarah Köster
Intermediate filaments (IFs) are part of the cytoskeleton of eukaryotic cells and, therefore, are largely responsible for the cell’s mechanical properties. IFs are characterized by a pronounced extensibility and remarkable resilience that enable them to support cells in extreme situations. Previous experiments showed that, under strain, α-helices in vimentin IFs might unfold to β-sheets. Upon repeated stretching, the filaments soften; however, the remaining plastic strain is negligible. Here, we observe that vimentin IFs do not recover their original stiffness on reasonable time scales, and we explain these seemingly contradicting results by introducing a third, less well-defined conformational state. Reversibility on the nanoscale can be fully rescued by introducing cross-linkers that prevent transition to the β-sheet. Our results classify IFs as a nanomaterial with intriguing mechanical properties, which is likely to play a major role for the cell’s local adaption to external stimuli.
DOI
Thursday, October 31, 2019
Direct measurement of electrostatic interactions between poly(methyl methacrylate) microspheres with optical laser tweezers
Kyu Hwan Choi, Dong Woo Kang, Kyung Hak Kim, Jiwon Kim, Youngbok Lee, Sang Hyuk Im and Bum Jun Park
In this study, we measured the force of electrostatic interactions between poly(methyl methacrylate) (PMMA) particles dispersed in organic solvent mixtures of cyclohexyl bromide (CHB) and n-decane. Optical laser tweezers were employed to directly measure interactive forces between paired PMMA particles in a CHB medium that contained n-decane in various volume ratios. CHB, having a moderate dielectric constant, provided an environment with a high charge storage capacity. The addition of n-decane lowered the effective refractive index of the medium, which increased the optical trapping efficiency. We also fabricated microscope flow cells with a commonly used UV-curable adhesive and quantified the effects of dissolved adhesive compounds through interactive force measurements and nuclear magnetic resonance analysis. In addition, we studied the impact of CHB dissociation into H+ and Br− ions, which could screen electrostatic interactions.
DOI
In this study, we measured the force of electrostatic interactions between poly(methyl methacrylate) (PMMA) particles dispersed in organic solvent mixtures of cyclohexyl bromide (CHB) and n-decane. Optical laser tweezers were employed to directly measure interactive forces between paired PMMA particles in a CHB medium that contained n-decane in various volume ratios. CHB, having a moderate dielectric constant, provided an environment with a high charge storage capacity. The addition of n-decane lowered the effective refractive index of the medium, which increased the optical trapping efficiency. We also fabricated microscope flow cells with a commonly used UV-curable adhesive and quantified the effects of dissolved adhesive compounds through interactive force measurements and nuclear magnetic resonance analysis. In addition, we studied the impact of CHB dissociation into H+ and Br− ions, which could screen electrostatic interactions.
DOI
Two-Stage Collapse of PNIPAM Brushes: Viscoelastic Changes Revealed by an Interferometric Laser Technique
David van Duinen, Hans-Jürgen Butt, Rüdiger Berger
Many temperature-responsive polymers exhibit a single-phase transition at the lower critical solution temperature (LCST). One exception is poly(N-isopropylacryamide) (PNIPAM). PNIPAM brush layers (51 ± 3 nm thick) that are end-grafted onto glass beads collapse in two stages. The viscoelastic changes of a PNIPAM brush layers were investigated with an interferometric laser method at different temperatures. This method is able to measure the two-stage collapse of beads coated with a polymer brush layer. When these beads are situated close to a hydrophilic glass surface, they exhibit Brownian motion. As this Brownian motion changes with temperature, it reveals the collapse of the polymer layer. The characteristic spectrum of the Brownian motion of beads are modelled by a damped harmonic oscillator, where the polymer layer acts as both spring and damping. The change of the Brownian motion spectrum with temperature indicates two transitions of the PNIPAM brush layer, one at 36 °C and one at 46 °C. We attribute the first transition to the LCST volume collapse of PNIPAM. Here, changes of the density and viscosity of the brush dominate. The second transition is dominated by a stiffening of the brush layer.
DOI
Many temperature-responsive polymers exhibit a single-phase transition at the lower critical solution temperature (LCST). One exception is poly(N-isopropylacryamide) (PNIPAM). PNIPAM brush layers (51 ± 3 nm thick) that are end-grafted onto glass beads collapse in two stages. The viscoelastic changes of a PNIPAM brush layers were investigated with an interferometric laser method at different temperatures. This method is able to measure the two-stage collapse of beads coated with a polymer brush layer. When these beads are situated close to a hydrophilic glass surface, they exhibit Brownian motion. As this Brownian motion changes with temperature, it reveals the collapse of the polymer layer. The characteristic spectrum of the Brownian motion of beads are modelled by a damped harmonic oscillator, where the polymer layer acts as both spring and damping. The change of the Brownian motion spectrum with temperature indicates two transitions of the PNIPAM brush layer, one at 36 °C and one at 46 °C. We attribute the first transition to the LCST volume collapse of PNIPAM. Here, changes of the density and viscosity of the brush dominate. The second transition is dominated by a stiffening of the brush layer.
DOI
Cardiomyopathy-associated mutations in tropomyosin differently affect actin–myosin interaction at single-molecule and ensemble levels
Galina V. Kopylova, Daniil V. Shchepkin, Salavat R. Nabiev, Alexander M. Matyushenko, Natalia A. Koubassova, Dmitrii I. Levitsky, Sergey Y. Bershitsky
In the heart, mutations in the TPM1 gene encoding the α-isoform of tropomyosin lead, in particular, to the development of hypertrophic and dilated cardiomyopathies. We compared the effects of hypertrophic, D175N and E180G, and dilated, E40K and E54K, cardiomyopathy mutations in TPM1 gene on the properties of single actin–myosin interactions and the characteristics of the calcium regulation in an ensemble of myosin molecules immobilised on a glass surface and interacting with regulated thin filaments. Previously, we showed that at saturating Ca2+ concentration the presence of Tpm on the actin filament increases the duration of the interaction. Here, we found that the studied Tpm mutations differently affected the duration: the D175N mutation reduced it compared to WT Tpm, while the E180G mutation increased it. Both dilated mutations made the duration of the interaction even shorter than with F-actin. The duration of the attached state of myosin to the thin filament in the optical trap did not correlate to the sliding velocity of thin filaments and its calcium sensitivity in the in vitro motility assay. We suppose that at the level of the molecular ensemble, the cooperative mechanisms prevail in the manifestation of the effects of cardiomyopathy-associated mutations in Tpm.
DOI
In the heart, mutations in the TPM1 gene encoding the α-isoform of tropomyosin lead, in particular, to the development of hypertrophic and dilated cardiomyopathies. We compared the effects of hypertrophic, D175N and E180G, and dilated, E40K and E54K, cardiomyopathy mutations in TPM1 gene on the properties of single actin–myosin interactions and the characteristics of the calcium regulation in an ensemble of myosin molecules immobilised on a glass surface and interacting with regulated thin filaments. Previously, we showed that at saturating Ca2+ concentration the presence of Tpm on the actin filament increases the duration of the interaction. Here, we found that the studied Tpm mutations differently affected the duration: the D175N mutation reduced it compared to WT Tpm, while the E180G mutation increased it. Both dilated mutations made the duration of the interaction even shorter than with F-actin. The duration of the attached state of myosin to the thin filament in the optical trap did not correlate to the sliding velocity of thin filaments and its calcium sensitivity in the in vitro motility assay. We suppose that at the level of the molecular ensemble, the cooperative mechanisms prevail in the manifestation of the effects of cardiomyopathy-associated mutations in Tpm.
DOI
Monday, October 28, 2019
Memory effects in single-molecule force spectroscopy measurements of biomolecular folding
Andrew G.T. Pyo and Michael T Woodside
Folding is generally assumed to be a Markov process, without memory. When the molecular motion is coupled to that of a probe as in single-molecule force spectroscopy (SMFS) experiments, however, theory predicts that the coupling to a second Markov process should induce memory when monitoring a projection of the full multi-dimensional motion onto a reduced coordinate. We developed a method to evaluate the time constant of the induced memory from its effects on the autocorrelation function, which can be readily determined from experimental data. Applying this method to both simulated SMFS measurements and experimental trajectories of DNA hairpin folding measured by optical tweezers as a model system, we validated the prediction that the linker induces memory. For these measurements, the timescale of the induced memory was found to be similar to the time required for the force probe to respond to changes in the molecule, and in the regime where the experimentally observed dynamics were not significantly perturbed by probe-molecule coupling artifacts. Memory effects are thus a general feature of SMFS measurements induced by the mechanical connection between the molecule and force probe that should be considered when interpreting experimental data.
DOI
Folding is generally assumed to be a Markov process, without memory. When the molecular motion is coupled to that of a probe as in single-molecule force spectroscopy (SMFS) experiments, however, theory predicts that the coupling to a second Markov process should induce memory when monitoring a projection of the full multi-dimensional motion onto a reduced coordinate. We developed a method to evaluate the time constant of the induced memory from its effects on the autocorrelation function, which can be readily determined from experimental data. Applying this method to both simulated SMFS measurements and experimental trajectories of DNA hairpin folding measured by optical tweezers as a model system, we validated the prediction that the linker induces memory. For these measurements, the timescale of the induced memory was found to be similar to the time required for the force probe to respond to changes in the molecule, and in the regime where the experimentally observed dynamics were not significantly perturbed by probe-molecule coupling artifacts. Memory effects are thus a general feature of SMFS measurements induced by the mechanical connection between the molecule and force probe that should be considered when interpreting experimental data.
DOI
Formation Mechanism and Fluorescence Characterization of a Transient Assembly of Nanoparticles Generated by Femtosecond Laser Trapping
Wei-Yi Chiang, Jim Jui-Kai Chen, Anwar Usman, Tetsuhiro Kudo, kangwei Xia, Jia Su, Teruki Sugiyama, Johan Hofkens, Hiroshi Masuhara
Femtosecond (fs) laser trapping of dielectric nanoparticles displays a unique trapping and directional ejection phenomenon which is never observed in continuous-wave (cw) laser trapping. We studied the fs laser trapping and considered its dynamics and mechanism in terms of gathering, association and ejection of nanoparticles. By tuning solution viscosity through adding ethylene glycol (EG), the trapping behavior by fs and cw lasers are examined and compared with each other. Viscous solution slows down the diffusional and rotational movement of nanoparticles and increases the chance of nanoparticle assembly in the trapping site, which eventually leads to the novel ejection. We took advantage of fluorescence measurement to confirm the formation of the nanoparticle assembly during fs laser trapping. A red emission irregularly appeared in the fluorescence spectra, which strongly supports the transient assembly formation in molecular level.
DOI
Femtosecond (fs) laser trapping of dielectric nanoparticles displays a unique trapping and directional ejection phenomenon which is never observed in continuous-wave (cw) laser trapping. We studied the fs laser trapping and considered its dynamics and mechanism in terms of gathering, association and ejection of nanoparticles. By tuning solution viscosity through adding ethylene glycol (EG), the trapping behavior by fs and cw lasers are examined and compared with each other. Viscous solution slows down the diffusional and rotational movement of nanoparticles and increases the chance of nanoparticle assembly in the trapping site, which eventually leads to the novel ejection. We took advantage of fluorescence measurement to confirm the formation of the nanoparticle assembly during fs laser trapping. A red emission irregularly appeared in the fluorescence spectra, which strongly supports the transient assembly formation in molecular level.
DOI
TGFβ1 single-nucleotide polymorphism C-509T alters mucosal cell function in pediatric eosinophilic esophagitis
L. D. Duong, R. Rawson, A. Bezryadina, M. C. Manresa, R. O. Newbury, R. Dohil, Z. Liu, K. Barrett, R. Kurten & S. S. Aceves
Eosinophilic esophagitis (EoE) is a chronic Th2 antigen-driven disorder associated with tissue remodeling. Inflammation and remodeling lead to esophageal rigidity, strictures, and dysphagia. TGFβ1 drives esophageal remodeling including epithelial barrier dysfunction and subepithelial fibrosis. A functional SNP in the TGFβ1 gene that increases its transcription (C-509T) is associated with elevated numbers of esophageal TGFβ1-expressing cells. We utilized esophageal biopsies and fibroblasts from TT-genotype EoE children to understand if TGFβ1 influenced fibroblast and epithelial cell function in vivo. Genotype TT EoE esophageal fibroblasts had higher baseline TGFβ1, collagen1α1, periostin, and MMP2 (p < 0.05) gene expression and distinct contractile properties compared with CC genotype (n = 6 subjects per genotype). In vitro TGFβ1 exposure caused greater induction of target gene expression in genotype CC fibroblasts (p < 0.05). Esophageal biopsies from TT-genotype subjects had significantly less epithelial membrane-bound E-cadherin (p < 0.01) and wider cluster distribution at nanometer resolution. TGFβ1 treatment of stratified primary human esophageal epithelial cells and spheroids disrupted transepithelial resistance (p < 0.001) and E-cadherin localization (p < 0.0001). A TGFβ1-receptor-I inhibitor improved TGFβ1-mediated E-cadherin mislocalization. These data suggest that EoE severity can depend on genotypic differences that increase in vivo exposure to TGFβ1. TGFβ1 inhibition may be a useful therapy in subsets of EoE patients.
DOI
Eosinophilic esophagitis (EoE) is a chronic Th2 antigen-driven disorder associated with tissue remodeling. Inflammation and remodeling lead to esophageal rigidity, strictures, and dysphagia. TGFβ1 drives esophageal remodeling including epithelial barrier dysfunction and subepithelial fibrosis. A functional SNP in the TGFβ1 gene that increases its transcription (C-509T) is associated with elevated numbers of esophageal TGFβ1-expressing cells. We utilized esophageal biopsies and fibroblasts from TT-genotype EoE children to understand if TGFβ1 influenced fibroblast and epithelial cell function in vivo. Genotype TT EoE esophageal fibroblasts had higher baseline TGFβ1, collagen1α1, periostin, and MMP2 (p < 0.05) gene expression and distinct contractile properties compared with CC genotype (n = 6 subjects per genotype). In vitro TGFβ1 exposure caused greater induction of target gene expression in genotype CC fibroblasts (p < 0.05). Esophageal biopsies from TT-genotype subjects had significantly less epithelial membrane-bound E-cadherin (p < 0.01) and wider cluster distribution at nanometer resolution. TGFβ1 treatment of stratified primary human esophageal epithelial cells and spheroids disrupted transepithelial resistance (p < 0.001) and E-cadherin localization (p < 0.0001). A TGFβ1-receptor-I inhibitor improved TGFβ1-mediated E-cadherin mislocalization. These data suggest that EoE severity can depend on genotypic differences that increase in vivo exposure to TGFβ1. TGFβ1 inhibition may be a useful therapy in subsets of EoE patients.
DOI
Probing the Functional Role of Physical Motion in Development
Moritz Kreysing
Spatiotemporal organization during development has frequently been proposed to be explainable by reaction-transport models, where biochemical reactions couple to physical motion. However, whereas genetic tools allow causality of molecular players to be dissected via perturbation experiments, the functional role of physical transport processes, such as diffusion and cytoplasmic streaming, frequently remains untestable. This Perspective explores the challenges of validating reaction-transport hypotheses and highlights new opportunities provided by perturbation approaches that specifically target physical transport mechanisms. Using these methods, experimental physics may begin to catch up with molecular biology and find ways to test roles of diffusion and flows in development.
DOI
Spatiotemporal organization during development has frequently been proposed to be explainable by reaction-transport models, where biochemical reactions couple to physical motion. However, whereas genetic tools allow causality of molecular players to be dissected via perturbation experiments, the functional role of physical transport processes, such as diffusion and cytoplasmic streaming, frequently remains untestable. This Perspective explores the challenges of validating reaction-transport hypotheses and highlights new opportunities provided by perturbation approaches that specifically target physical transport mechanisms. Using these methods, experimental physics may begin to catch up with molecular biology and find ways to test roles of diffusion and flows in development.
DOI
Friday, October 25, 2019
Emulsions Stabilized by Fine Dust Particles
Yong Woo Kima, Donghyun Lim, Hyerin Han, Seunghyun Lee, Kyu Hwan Choi, Bum Jun Park
We investigated the capability of fine dust particles (FDPs) to be used as stabilizers for preparing emulsions. With the aid of cationic surfactants, it was found that the FDPs formed oil in water emulsions that were extremely stable for a long time period. The stabilization mechanism was quantitatively analyzed via ζ-potential measurements of surface modified FDPs and interface adsorption experiments of FDP-coated microbeads by using optical laser tweezers.
DOI
We investigated the capability of fine dust particles (FDPs) to be used as stabilizers for preparing emulsions. With the aid of cationic surfactants, it was found that the FDPs formed oil in water emulsions that were extremely stable for a long time period. The stabilization mechanism was quantitatively analyzed via ζ-potential measurements of surface modified FDPs and interface adsorption experiments of FDP-coated microbeads by using optical laser tweezers.
DOI
Interaction of a Polyarginine Peptide with Membranes of Different Mechanical Properties
Matías A. Crosio, Matías A. Via, Candelaria I. Cámara, Agustin Mangiarotti, Mario G. Del Pópolo and Natalia Wilke
The membrane translocation efficiency of cell penetrating peptides (CPPs) has been largely studied, and poly-arginines have been highlighted as particularly active CPPs, especially upon negatively charged membranes. Here we inquire about the influence of membrane mechanical properties in poly-arginine adsorption, penetration and translocation, as well as the subsequent effect on the host membrane. For this, we selected anionic membranes exhibiting different rigidity and fluidity, and exposed them to the nona-arginine KR9C. Three different membrane compositions were investigated, all of them having 50% of the anionic lipid 1,2-dioleoyl-sn-glycero-3-phospho-(1’-rac-glycerol) (DOPG), thus, ensuring a high affinity of the peptide for membrane surfaces. The remaining 50% was a saturated PC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC), an unsaturated PC (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) or a mixture of DOPC with cholesterol. Peptide-membrane interactions were studied using four complementary models for membranes: Langmuir monolayers, Large Unilamellar Vesicles, Black Lipid Membranes and Giant Unilamellar Vesicles. The patterns of interaction of KR9C varied within the different membrane compositions. The peptide strongly adsorbed on membranes with cholesterol, but did not incorporate or translocate them. KR9C stabilized phase segregation in DPPC/DOPG films and promoted vesicle rupture. DOPC/DOPG appeared like the better host for peptide translocation: KR9C adsorbed, inserted and translocated these membranes without breaking them, despite softening was observed.
DOI
The membrane translocation efficiency of cell penetrating peptides (CPPs) has been largely studied, and poly-arginines have been highlighted as particularly active CPPs, especially upon negatively charged membranes. Here we inquire about the influence of membrane mechanical properties in poly-arginine adsorption, penetration and translocation, as well as the subsequent effect on the host membrane. For this, we selected anionic membranes exhibiting different rigidity and fluidity, and exposed them to the nona-arginine KR9C. Three different membrane compositions were investigated, all of them having 50% of the anionic lipid 1,2-dioleoyl-sn-glycero-3-phospho-(1’-rac-glycerol) (DOPG), thus, ensuring a high affinity of the peptide for membrane surfaces. The remaining 50% was a saturated PC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC), an unsaturated PC (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) or a mixture of DOPC with cholesterol. Peptide-membrane interactions were studied using four complementary models for membranes: Langmuir monolayers, Large Unilamellar Vesicles, Black Lipid Membranes and Giant Unilamellar Vesicles. The patterns of interaction of KR9C varied within the different membrane compositions. The peptide strongly adsorbed on membranes with cholesterol, but did not incorporate or translocate them. KR9C stabilized phase segregation in DPPC/DOPG films and promoted vesicle rupture. DOPC/DOPG appeared like the better host for peptide translocation: KR9C adsorbed, inserted and translocated these membranes without breaking them, despite softening was observed.
DOI
Bidirectional Transport of Nanoparticles and Cells with a Bio‐Conveyor Belt
Xiaoshuai Liu, You Wu, Xiaohao Xu, Yuchao Li, Yao Zhang, Baojun Li
The bidirectional transport of nanoparticles and biological cells is of great significance in efficient biological assays and precision cell screening, and can be achieved with optical conveyor belts in a noncontact and noninvasive manner. However, implantation of these belts into biological systems can present significant challenges owing to the incompatibility of the artificial materials. In this work, an optical conveyor belt assembled from natural biological cells is proposed. The diameter of the belt (500 nm) is smaller than the laser wavelength (980 nm) and, therefore, the evanescent wave stably traps the nanoparticles and cells on the belt surface. By adjusting the relative power of the lasers injected into the belt, the particles or cells can be bidirectionally transported along the bio‐conveyor belt. The experimental results are numerically interpreted and the transport velocities are investigated based on simulations. Further experiments show that the bio‐conveyor belt can also be assembled with mammalian cells and then applied to dynamic cell transport in vivo. The bio‐conveyor belt might provide a noninvasive and biocompatible tool for biomedical assays, drug delivery, and biological nanoarchitectonics.
DOI
The bidirectional transport of nanoparticles and biological cells is of great significance in efficient biological assays and precision cell screening, and can be achieved with optical conveyor belts in a noncontact and noninvasive manner. However, implantation of these belts into biological systems can present significant challenges owing to the incompatibility of the artificial materials. In this work, an optical conveyor belt assembled from natural biological cells is proposed. The diameter of the belt (500 nm) is smaller than the laser wavelength (980 nm) and, therefore, the evanescent wave stably traps the nanoparticles and cells on the belt surface. By adjusting the relative power of the lasers injected into the belt, the particles or cells can be bidirectionally transported along the bio‐conveyor belt. The experimental results are numerically interpreted and the transport velocities are investigated based on simulations. Further experiments show that the bio‐conveyor belt can also be assembled with mammalian cells and then applied to dynamic cell transport in vivo. The bio‐conveyor belt might provide a noninvasive and biocompatible tool for biomedical assays, drug delivery, and biological nanoarchitectonics.
DOI
Plasmonic optical trapping of nanoparticles with precise angular selectivity
Ruo-Heng Chai, Wen-Jun Zou, Jun Qian, Jing Chen, Qian Sun, and Jing-Jun Xu
In this paper, a plasmonic trapping scheme including a polystyrene nanoparticle with gold cap and a metal tip tweezers was proposed. We numerically investigated the optical trapping behavior of the metal tip to this asymmetric particle. The results show that the metal tip can capture the particle at the position of the gold cap due to the strong plasmonic interaction, while other positions of the particle cannot be captured by metal tip. Furthermore, the trapping angle of the nanoparticle can be adjusted by changing the incident wavelength. Precisely controlling the trapping angle of the nanoparticles in our study has important potential applications of optical tweezers, such as in single molecule manipulation.
DOI
In this paper, a plasmonic trapping scheme including a polystyrene nanoparticle with gold cap and a metal tip tweezers was proposed. We numerically investigated the optical trapping behavior of the metal tip to this asymmetric particle. The results show that the metal tip can capture the particle at the position of the gold cap due to the strong plasmonic interaction, while other positions of the particle cannot be captured by metal tip. Furthermore, the trapping angle of the nanoparticle can be adjusted by changing the incident wavelength. Precisely controlling the trapping angle of the nanoparticles in our study has important potential applications of optical tweezers, such as in single molecule manipulation.
DOI
Schools of skyrmions with electrically tunable elastic interactions
Hayley R. O. Sohn, Changda D. Liu & Ivan I. Smalyukh
Coexistence of order and fluidity in soft matter often mimics that in biology, allowing for complex dynamics and applications-like displays. In active soft matter, emergent order can arise because of such dynamics. Powered by local energy conversion, this behavior resembles motions in living systems, like schooling of fish. Similar dynamics at cellular levels drive biological processes and generate macroscopic work. Inanimate particles capable of such emergent behavior could power nanomachines, but most active systems have biological origins. Here we show that thousands-to-millions of topological solitons, dubbed “skyrmions”, while each converting macroscopically-supplied electric energy, exhibit collective motions along spontaneously-chosen directions uncorrelated with the direction of electric field. Within these “schools” of skyrmions, we uncover polar ordering, reconfigurable multi-skyrmion clustering and large-scale cohesion mediated by out-of-equilibrium elastic interactions. Remarkably, this behavior arises under conditions similar to those in liquid crystal displays and may enable dynamic materials with strong emergent electro-optic responses.
DOI
Coexistence of order and fluidity in soft matter often mimics that in biology, allowing for complex dynamics and applications-like displays. In active soft matter, emergent order can arise because of such dynamics. Powered by local energy conversion, this behavior resembles motions in living systems, like schooling of fish. Similar dynamics at cellular levels drive biological processes and generate macroscopic work. Inanimate particles capable of such emergent behavior could power nanomachines, but most active systems have biological origins. Here we show that thousands-to-millions of topological solitons, dubbed “skyrmions”, while each converting macroscopically-supplied electric energy, exhibit collective motions along spontaneously-chosen directions uncorrelated with the direction of electric field. Within these “schools” of skyrmions, we uncover polar ordering, reconfigurable multi-skyrmion clustering and large-scale cohesion mediated by out-of-equilibrium elastic interactions. Remarkably, this behavior arises under conditions similar to those in liquid crystal displays and may enable dynamic materials with strong emergent electro-optic responses.
DOI
Research on the special bottle beam generated by asymmetric elliptical Gaussian beams through an axicon-lens system
Zhikun Yang, Xinglei Lin, He Zhang, Yingtian Xu, Liang Jin, Yonggang Zou, Xiaohui Ma
A special bottle beam was obtained by shaping the elliptical Gaussian beam through an axicon optical system. When elliptical Gaussian beams have a different eccentricity (σ = 0, 0.3, 0.6, 0.9), the optical field distribution after axicon and the focusing lens was investigated. By comparison with a circular Gaussian beam, this asymmetry leads to the formation of a non-diffracting beam from a quasi-Bessel beam characteristic to a quasi-Mathieu beam characteristic after the incident axicon. At the same time, the bottle beams produced by the asymmetric light source incident on the axicon-lens system showed changes in the front and rear sides. In addition, the variation of the diffraction spot in the optical space region on the bottle beam axis was also analyzed. On this basis, a hollow light spot tweezer with “quasi-Pearcey beam handle” was obtained, which is expected to expand the application of particle trapping operation.
DOI
A special bottle beam was obtained by shaping the elliptical Gaussian beam through an axicon optical system. When elliptical Gaussian beams have a different eccentricity (σ = 0, 0.3, 0.6, 0.9), the optical field distribution after axicon and the focusing lens was investigated. By comparison with a circular Gaussian beam, this asymmetry leads to the formation of a non-diffracting beam from a quasi-Bessel beam characteristic to a quasi-Mathieu beam characteristic after the incident axicon. At the same time, the bottle beams produced by the asymmetric light source incident on the axicon-lens system showed changes in the front and rear sides. In addition, the variation of the diffraction spot in the optical space region on the bottle beam axis was also analyzed. On this basis, a hollow light spot tweezer with “quasi-Pearcey beam handle” was obtained, which is expected to expand the application of particle trapping operation.
DOI
Wednesday, October 23, 2019
Combined High-Resolution Optical Tweezers and Multicolor Single-Molecule Fluorescence With Automated Single Molecule Assembly Line
Cho-Ying Chuang, Matthew Zammit, Miles L. Whitmore, Matthew J Comstock
We present an instrument that combines high-resolution optical tweezers and multi-color confocal fluorescence spectroscopy along with automated single molecule assembly. Multi-color allows the simultaneous observation of multiple molecules or multiple degrees of freedom which allows e.g., the observation of multiple proteins simultaneously within a complex. The instrument incorporates three fluorescence excitation lasers, with a reliable alignment scheme, that will allow three independent fluorescent probe or fluorescence resonance energy transfer (FRET) measurements and also increases flexibility in the choice of fluorescent molecules. We demonstrate the ability to simultaneously measure angstrom-scale changes in tether extension and fluorescence signals. Simultaneous tweezers and fluorescence measurement are particularly challenging due to fluorophore photobleaching, even more so if multiple fluorophores are to be measured. Therefore, 1) fluorescence excitation and detection is interlaced with time-shared dual optical traps. 2) We investigated the photostability of common fluorophores. The mean number of photons emitted before bleaching was unaffected by the trap laser and decreased only slightly with increasing excitation laser intensity. Surprisingly, we found that Cy5 outperforms other commonly used fluorophores by more than 5-fold. 3) We devised computer-controlled automation, which conserves fluorophore lifetime by quickly detecting fluorophore-labeled molecule binding, turning off lasers, and moving to add the next fluorophore-labeled component. The single-molecule assembly line enables the precise assembly of multi-molecule complexes while preserving fluorophores.
DOI
We present an instrument that combines high-resolution optical tweezers and multi-color confocal fluorescence spectroscopy along with automated single molecule assembly. Multi-color allows the simultaneous observation of multiple molecules or multiple degrees of freedom which allows e.g., the observation of multiple proteins simultaneously within a complex. The instrument incorporates three fluorescence excitation lasers, with a reliable alignment scheme, that will allow three independent fluorescent probe or fluorescence resonance energy transfer (FRET) measurements and also increases flexibility in the choice of fluorescent molecules. We demonstrate the ability to simultaneously measure angstrom-scale changes in tether extension and fluorescence signals. Simultaneous tweezers and fluorescence measurement are particularly challenging due to fluorophore photobleaching, even more so if multiple fluorophores are to be measured. Therefore, 1) fluorescence excitation and detection is interlaced with time-shared dual optical traps. 2) We investigated the photostability of common fluorophores. The mean number of photons emitted before bleaching was unaffected by the trap laser and decreased only slightly with increasing excitation laser intensity. Surprisingly, we found that Cy5 outperforms other commonly used fluorophores by more than 5-fold. 3) We devised computer-controlled automation, which conserves fluorophore lifetime by quickly detecting fluorophore-labeled molecule binding, turning off lasers, and moving to add the next fluorophore-labeled component. The single-molecule assembly line enables the precise assembly of multi-molecule complexes while preserving fluorophores.
DOI
Highly efficient dual-fibre optical trapping with 3D printed diffractive Fresnel lenses
Asa Asadollahbaik, Simon Thiele, Ksenia Weber, Aashutosh Kumar, Johannes Drozella, Florian Sterl, Alois Herkommer, Harald Giessen, Jochen Fick
Highly efficient counter-propagating fibre-based optical traps are presented which utilize converging beams from fibres with 3D printed diffractive Fresnel lenses on their facet. The use of converging beam instead of diverging beam in dual fiber traps creates a strong trapping efficiency in both axial and transverse direction. Converging beams with a numerical aperture of up to 0.7 are produced by diffractive Fresnel lenses. These lenses also provide a large focal distance of up to 200um in a moderately high refractive index medium. Fabrication of such diffractive lenses with micro-sized features at the tip of a fibre is possible by femtosecond two photon lithography. In comparison to chemically etched fiber tips, the normalized trap stiffness of dual fiber tweezers is increased by an substantial factor of 35-50 when using converging beam produced by diffractive Fresnel lenses. The large focal length provided by these diffractive structures allows to work at large fibre-to-fibre distance which leads to larger space and the freedom to combine other spectroscopy and analytical methods in combination with trapping.
DOI
Highly efficient counter-propagating fibre-based optical traps are presented which utilize converging beams from fibres with 3D printed diffractive Fresnel lenses on their facet. The use of converging beam instead of diverging beam in dual fiber traps creates a strong trapping efficiency in both axial and transverse direction. Converging beams with a numerical aperture of up to 0.7 are produced by diffractive Fresnel lenses. These lenses also provide a large focal distance of up to 200um in a moderately high refractive index medium. Fabrication of such diffractive lenses with micro-sized features at the tip of a fibre is possible by femtosecond two photon lithography. In comparison to chemically etched fiber tips, the normalized trap stiffness of dual fiber tweezers is increased by an substantial factor of 35-50 when using converging beam produced by diffractive Fresnel lenses. The large focal length provided by these diffractive structures allows to work at large fibre-to-fibre distance which leads to larger space and the freedom to combine other spectroscopy and analytical methods in combination with trapping.
DOI
Algorithmic approach for designingplasmonic nanotweezers
Neuton Li, Jasper Cadusch, and Kenneth Crozier
We use machine learning (simulated annealing) to design plasmonic nanoapertures that function as optical nanotweezers. The nanoapertures have irregular shapes that are chosen by our algorithm. We present electromagnetic simulations that show that these produce stronger field enhancements and extraction energies than nanoapertures comprising double nanoholes with the same gap geometry. We show that performance is further improved by etching one or more rings into the gold surrounding the nanoaperture. Lastly, we provide a direct comparison between our design and work that is representative of the state of the art in plasmonic nanotweezers at the time of writing.
DOI
We use machine learning (simulated annealing) to design plasmonic nanoapertures that function as optical nanotweezers. The nanoapertures have irregular shapes that are chosen by our algorithm. We present electromagnetic simulations that show that these produce stronger field enhancements and extraction energies than nanoapertures comprising double nanoholes with the same gap geometry. We show that performance is further improved by etching one or more rings into the gold surrounding the nanoaperture. Lastly, we provide a direct comparison between our design and work that is representative of the state of the art in plasmonic nanotweezers at the time of writing.
DOI
Cell swelling, softening and invasion in a three-dimensional breast cancer model
Yu Long Han, Adrian F. Pegoraro, Hui Li, Kaifu Li, Yuan Yuan, Guoqiang Xu, Zichen Gu, Jiawei Sun, Yukun Hao, Satish Kumar Gupta, Yiwei Li, Wenhui Tang, Hua Kang, Lianghong Teng, Jeffrey J. Fredberg & Ming Guo
Control of the structure and function of three-dimensional multicellular tissues depends critically on the spatial and temporal coordination of cellular physical properties, yet the organizational principles that govern these events and their disruption in disease remain poorly understood. Using a multicellular mammary cancer organoid model, we map here the spatial and temporal evolution of positions, motions and physical characteristics of individual cells in three dimensions. Compared with cells in the organoid core, cells at the organoid periphery and the invasive front are found to be systematically softer, larger and more dynamic. These mechanical changes are shown to arise from supracellular fluid flow through gap junctions, the suppression of which delays the transition to an invasive phenotype. These findings highlight the role of spatiotemporal coordination of cellular physical properties in tissue organization and disease progression.
DOI
Control of the structure and function of three-dimensional multicellular tissues depends critically on the spatial and temporal coordination of cellular physical properties, yet the organizational principles that govern these events and their disruption in disease remain poorly understood. Using a multicellular mammary cancer organoid model, we map here the spatial and temporal evolution of positions, motions and physical characteristics of individual cells in three dimensions. Compared with cells in the organoid core, cells at the organoid periphery and the invasive front are found to be systematically softer, larger and more dynamic. These mechanical changes are shown to arise from supracellular fluid flow through gap junctions, the suppression of which delays the transition to an invasive phenotype. These findings highlight the role of spatiotemporal coordination of cellular physical properties in tissue organization and disease progression.
DOI
Probing force in living cells with optical tweezers: from single-molecule mechanics to cell mechanotransduction
Claudia Arbore, Laura Perego, Marios Sergides, Marco Capitanio
The invention of optical tweezers more than three decades ago has opened new avenues in the study of the mechanical properties of biological molecules and cells. Quantitative force measurements still represent a challenging task in living cells due to the complexity of the cellular environment. Here, we review different methodologies to quantitatively measure the mechanical properties of living cells, the strength of adhesion/receptor bonds, and the active force produced during intracellular transport, cell adhesion, and migration. We discuss experimental strategies to attain proper calibration of optical tweezers and molecular resolution in living cells. Finally, we show recent studies on the transduction of mechanical stimuli into biomolecular and genetic signals that play a critical role in cell health and disease.
DOI
The invention of optical tweezers more than three decades ago has opened new avenues in the study of the mechanical properties of biological molecules and cells. Quantitative force measurements still represent a challenging task in living cells due to the complexity of the cellular environment. Here, we review different methodologies to quantitatively measure the mechanical properties of living cells, the strength of adhesion/receptor bonds, and the active force produced during intracellular transport, cell adhesion, and migration. We discuss experimental strategies to attain proper calibration of optical tweezers and molecular resolution in living cells. Finally, we show recent studies on the transduction of mechanical stimuli into biomolecular and genetic signals that play a critical role in cell health and disease.
DOI
Internal and near-surface fields for a charged sphere irradiated by a vector Bessel beam
Yiming Yang, Zizhuo Nie, Yinan Feng, Renxian Li
The interaction of an axicon-generated vector Bessel beam (AGVBB) with a charged sphere is investigated in the framework of generalized Lorenz–Mie theory (GLMT). The incident, internal, and scattered fields are expanded using vector spherical wave functions (VSWFs), beam shape coefficients (BSCs), and internal and scattered coefficients. An analytical expressions of beam shape coefficients (BSCs), which are derived using angular spectrum decomposition method (ASDM), are given. The internal and scattered coefficients are derived by considering the boundary conditions. The internal and near-surface electric fields of a charged sphere illuminated by AVGBBs are numerical calculated, and the effects of polarization, order of beam, half-cone angle are mainly discussed. The results are compared with that for neutral particles. The effect of the surface charge are discussed by the comparison of the results for charged spheres with that for neutral particles. Numerical results show that the internal and near-surface fields are sensitive to the surface charge. The internal fields and the near-surface fields can be locally enhanced. Internal and near-surface fields, especially its local enhancement, are very sensitive to the beam parameters, including polarization, order, half-cone angle, etc.
DOI
The interaction of an axicon-generated vector Bessel beam (AGVBB) with a charged sphere is investigated in the framework of generalized Lorenz–Mie theory (GLMT). The incident, internal, and scattered fields are expanded using vector spherical wave functions (VSWFs), beam shape coefficients (BSCs), and internal and scattered coefficients. An analytical expressions of beam shape coefficients (BSCs), which are derived using angular spectrum decomposition method (ASDM), are given. The internal and scattered coefficients are derived by considering the boundary conditions. The internal and near-surface electric fields of a charged sphere illuminated by AVGBBs are numerical calculated, and the effects of polarization, order of beam, half-cone angle are mainly discussed. The results are compared with that for neutral particles. The effect of the surface charge are discussed by the comparison of the results for charged spheres with that for neutral particles. Numerical results show that the internal and near-surface fields are sensitive to the surface charge. The internal fields and the near-surface fields can be locally enhanced. Internal and near-surface fields, especially its local enhancement, are very sensitive to the beam parameters, including polarization, order, half-cone angle, etc.
DOI
Monday, October 21, 2019
Using the Belinfante momentum to retrieve the polarization state of light inside waveguides
Vincent Ginis, Lulu Liu, Alan She & Federico Capasso
Current day high speed optical communication systems employ photonic circuits using platforms such as silicon photonics. In these systems, the polarization state of light drifts due to effects such as polarization mode dispersion and nonlinear phenomena generated by photonic circuit building blocks. As the complexity, the number, and the variety of these building blocks grows, the demand increases for an in-situ polarization determination strategy. Here, we show that the transfer of the Belinfante momentum to particles in the evanescent field of waveguides depends in a non-trivial way on the polarization state of light within that waveguide. Surprisingly, we find that the maxima and minima of the lateral force are not produced with circularly polarized light, corresponding to the north and south poles of the Poincaré sphere. Instead, the maxima are shifted along the great circle of the sphere due to the phase differences between the scattered TE and TM components of light. This effect allows for an unambiguous reconstruction of the local polarization state of light inside a waveguide. Importantly, this technique depends on interaction with only the evanescent tails of the fields, allowing for a minimally invasive method to probe the polarization within a photonic chip.
DOI
Current day high speed optical communication systems employ photonic circuits using platforms such as silicon photonics. In these systems, the polarization state of light drifts due to effects such as polarization mode dispersion and nonlinear phenomena generated by photonic circuit building blocks. As the complexity, the number, and the variety of these building blocks grows, the demand increases for an in-situ polarization determination strategy. Here, we show that the transfer of the Belinfante momentum to particles in the evanescent field of waveguides depends in a non-trivial way on the polarization state of light within that waveguide. Surprisingly, we find that the maxima and minima of the lateral force are not produced with circularly polarized light, corresponding to the north and south poles of the Poincaré sphere. Instead, the maxima are shifted along the great circle of the sphere due to the phase differences between the scattered TE and TM components of light. This effect allows for an unambiguous reconstruction of the local polarization state of light inside a waveguide. Importantly, this technique depends on interaction with only the evanescent tails of the fields, allowing for a minimally invasive method to probe the polarization within a photonic chip.
DOI
Flow with nanoparticle clustering controlled by optical forces in quartz glass nanoslits
Tetsuro Tsuji, Yuki Matsumoto, Satoyuki Kawano
In this paper, we demonstrate nanoparticle flow control using an optical force in a confined nanospace. Using nanofabrication technologies, all-quartz-glass nanoslit channels with a sudden contraction are developed. Because the nanoslit height is comparable to the nanoparticle diameter, the motion of particles is restricted in the channel height direction, resulting in almost two-dimensional particle motion. The laser irradiates at the entrance of the sudden contraction channel, leading the trapped nanoparticles to form a cluster. As a result, the translocation of nanoparticles into the contraction channel is suppressed. Because the particle translocation restarts when the laser irradiation is stopped, we can control the nanoparticle flow into the contraction channel by switching the trapping and release of particles, realizing an intermittent flow of nanoparticles. Such a particle flow control technique in a confined nanospace is expected to improve the functions of nanofluidic devices by transporting a target material selectively to a desired location in the device.
DOI
In this paper, we demonstrate nanoparticle flow control using an optical force in a confined nanospace. Using nanofabrication technologies, all-quartz-glass nanoslit channels with a sudden contraction are developed. Because the nanoslit height is comparable to the nanoparticle diameter, the motion of particles is restricted in the channel height direction, resulting in almost two-dimensional particle motion. The laser irradiates at the entrance of the sudden contraction channel, leading the trapped nanoparticles to form a cluster. As a result, the translocation of nanoparticles into the contraction channel is suppressed. Because the particle translocation restarts when the laser irradiation is stopped, we can control the nanoparticle flow into the contraction channel by switching the trapping and release of particles, realizing an intermittent flow of nanoparticles. Such a particle flow control technique in a confined nanospace is expected to improve the functions of nanofluidic devices by transporting a target material selectively to a desired location in the device.
DOI
Optical hoovering on plasmonic rinks
John Canning
Excitation of surface waves on conducting materials provides a near resistance-free interface capable of a material glissade either by plasmon forces or by optical beam tractors. Analogous to an ice hockey rink, as proof-of-principle plasmon-assisted optical traction, or hoovering, of water drops on a gold surface is demonstrated. Changes in the contact angle provide a novel, low-cost nanoscale method of quantifying observable and potentially tunable changes. Variability in thresholds and movement, including jumps, is observed and can be explained by the presence of significant roughness, measured by scanning electron microscopy, with water tension. The demonstration opens a path to directly integrate various optical and plasmonic traction technologies. Implications of the phenomena and ways of improving transport and potential applications spanning configurable microfluidics, antennas, tunable lenses, diagnostics, sensing, and active Kerr and other devices are discussed.
DOI
Excitation of surface waves on conducting materials provides a near resistance-free interface capable of a material glissade either by plasmon forces or by optical beam tractors. Analogous to an ice hockey rink, as proof-of-principle plasmon-assisted optical traction, or hoovering, of water drops on a gold surface is demonstrated. Changes in the contact angle provide a novel, low-cost nanoscale method of quantifying observable and potentially tunable changes. Variability in thresholds and movement, including jumps, is observed and can be explained by the presence of significant roughness, measured by scanning electron microscopy, with water tension. The demonstration opens a path to directly integrate various optical and plasmonic traction technologies. Implications of the phenomena and ways of improving transport and potential applications spanning configurable microfluidics, antennas, tunable lenses, diagnostics, sensing, and active Kerr and other devices are discussed.
DOI
Plasmonic Tweezers towards Biomolecular and Biomedical Applications
Xue Han and Changsen Sun
With the capability of confining light into subwavelength scale, plasmonic tweezers have been used to trap and manipulate nanoscale particles. It has huge potential to be utilized in biomolecular research and practical biomedical applications. In this short review, plasmonic tweezers based on nano-aperture designs are discussed. A few challenges should be overcome for these plasmonic tweezers to reach a similar level of significance as the conventional optical tweezers.
DOI
With the capability of confining light into subwavelength scale, plasmonic tweezers have been used to trap and manipulate nanoscale particles. It has huge potential to be utilized in biomolecular research and practical biomedical applications. In this short review, plasmonic tweezers based on nano-aperture designs are discussed. A few challenges should be overcome for these plasmonic tweezers to reach a similar level of significance as the conventional optical tweezers.
DOI
Light Induced Inverse-Square Law Interactions between Nanoparticles: “Mock Gravity” at the Nanoscale
J. Luis-Hita, M. I. Marqués, R. Delgado-Buscalioni, N. de Sousa, L. S. Froufe-Pérez, F. Scheffold, and J. J. Sáenz
The interaction forces between identical resonant molecules or nanoparticles, optically induced by a quasimonochromatic isotropic random light field, are theoretically analyzed. In general, the interaction force exhibits a far-field oscillatory behavior at separation distances larger than the light wavelength. However, we show that the oscillations disappear when the frequency of the random field is tuned to an absorption Fröhlich resonance, at which the real part of the particle’s electric polarizability is zero. At the resonant condition, the interaction forces follow a long-range gravitylike inverse square distance law which holds for both near- and far-field separation distances.
DOI
The interaction forces between identical resonant molecules or nanoparticles, optically induced by a quasimonochromatic isotropic random light field, are theoretically analyzed. In general, the interaction force exhibits a far-field oscillatory behavior at separation distances larger than the light wavelength. However, we show that the oscillations disappear when the frequency of the random field is tuned to an absorption Fröhlich resonance, at which the real part of the particle’s electric polarizability is zero. At the resonant condition, the interaction forces follow a long-range gravitylike inverse square distance law which holds for both near- and far-field separation distances.
DOI
Bulk phase water diffusion is significantly inhibited by inhomogeneity of single non-crystal particle at low relative humidity
Chen Cai, Stephen Ingram, Chunsheng Zhao
It has been suggested by recent studies that atmospheric particles adopt non-crystalline states which significantly impact aerosol-cloud interactions and atmospheric chemistry. In this study, the effect of non-crystalline states on water diffusion is detailed investigated from single multi-component particles levitated in aerosol optical tweezers. We infer the time-dependent particle size from Raman spectra using Mie fitting, thus derive the water diffusion coefficient (Dwater) from particle radius changes during evaporation or condensation processes. In both glassy states (in saccharide particles) and gel states (in MgSO4 particles), the bulk phase water diffusion is shown to be severely restricted, thus limiting the gas-particle water partitioning on the particle surface. The Dwater of glassy particles generally gradually decreases as the RH decreases, while the relative humidity (RH) - Dwater relationship of particle in gel state is complicated and brings huge deviation of Dwater determination. We therefore present the time dependent water content at different location (radial coordinate) of the particle. The time scale required for particle to get equilibrium to environmental RH is vastly extended by the kinetic inhibition of bulk phase water transfer. This can give direct and quantitative indication of water diffusion within single non-crystalline particle and its effect on gas-particle partitioning and equilibrium.
DOI
It has been suggested by recent studies that atmospheric particles adopt non-crystalline states which significantly impact aerosol-cloud interactions and atmospheric chemistry. In this study, the effect of non-crystalline states on water diffusion is detailed investigated from single multi-component particles levitated in aerosol optical tweezers. We infer the time-dependent particle size from Raman spectra using Mie fitting, thus derive the water diffusion coefficient (Dwater) from particle radius changes during evaporation or condensation processes. In both glassy states (in saccharide particles) and gel states (in MgSO4 particles), the bulk phase water diffusion is shown to be severely restricted, thus limiting the gas-particle water partitioning on the particle surface. The Dwater of glassy particles generally gradually decreases as the RH decreases, while the relative humidity (RH) - Dwater relationship of particle in gel state is complicated and brings huge deviation of Dwater determination. We therefore present the time dependent water content at different location (radial coordinate) of the particle. The time scale required for particle to get equilibrium to environmental RH is vastly extended by the kinetic inhibition of bulk phase water transfer. This can give direct and quantitative indication of water diffusion within single non-crystalline particle and its effect on gas-particle partitioning and equilibrium.
DOI
Thursday, October 17, 2019
A review of experimental techniques for aerosol hygroscopicity studies
Mingjin Tang, Chak K. Chan, Yong Jie Li, Hang Su, Qingxin Ma, Zhijun Wu, Guohua Zhang, Zhe Wang, Maofa Ge, Min Hu, Hong He, and Xinming Wang
Hygroscopicity is one of the most important physicochemical properties of aerosol particles and also plays indispensable roles in many other scientific and technical fields. A myriad of experimental techniques, which differ in principles, configurations and cost, are available for investigating aerosol hygroscopicity under subsaturated conditions (i.e., relative humidity below 100 %). A comprehensive review of these techniques is provided in this paper, in which experimental techniques are broadly classified into four categories, according to the way samples under investigation are prepared. For each technique, we describe its operation principle and typical configuration, use representative examples reported in previous work to illustrate how this technique can help better understand aerosol hygroscopicity, and discuss its advantages and disadvantages. In addition, future directions are outlined and discussed for further technical improvement and instrumental development.
DOI
Hygroscopicity is one of the most important physicochemical properties of aerosol particles and also plays indispensable roles in many other scientific and technical fields. A myriad of experimental techniques, which differ in principles, configurations and cost, are available for investigating aerosol hygroscopicity under subsaturated conditions (i.e., relative humidity below 100 %). A comprehensive review of these techniques is provided in this paper, in which experimental techniques are broadly classified into four categories, according to the way samples under investigation are prepared. For each technique, we describe its operation principle and typical configuration, use representative examples reported in previous work to illustrate how this technique can help better understand aerosol hygroscopicity, and discuss its advantages and disadvantages. In addition, future directions are outlined and discussed for further technical improvement and instrumental development.
DOI
Impact of the crystal orientation of LiNbO3:Fe on the dynamic behaviors of the particles trapped through the photovoltaic tweezer
Zhitao Zan, Donghui Wang, Feifei Li, Lihong Shi, Wenbo Yan
A real-time investigation of photovoltaic tweezer is performed on both c-cut and y-cut LiNbO3:Fe crystals. We study the impact of the crystal orientation on the dynamic behaviors of the particles trapped through the photovoltaic tweezer. By analyzing both amount and the distribution of the trapped particles and also by calculating the particle number of all trapped particles, it is found that the trapping efficiency highly depends on the crystal orientation. As compared with the case of the y-cut LiNbO3:Fe, the photovoltaic tweezer on the c-cut LiNbO3:Fe is capable to trap more particles in a unit time. The impact of the crystal orientation on the photovoltaic tweezer is explained by the simulations of the dielectrophoretic interaction for different crystal orientations.
DOI
A real-time investigation of photovoltaic tweezer is performed on both c-cut and y-cut LiNbO3:Fe crystals. We study the impact of the crystal orientation on the dynamic behaviors of the particles trapped through the photovoltaic tweezer. By analyzing both amount and the distribution of the trapped particles and also by calculating the particle number of all trapped particles, it is found that the trapping efficiency highly depends on the crystal orientation. As compared with the case of the y-cut LiNbO3:Fe, the photovoltaic tweezer on the c-cut LiNbO3:Fe is capable to trap more particles in a unit time. The impact of the crystal orientation on the photovoltaic tweezer is explained by the simulations of the dielectrophoretic interaction for different crystal orientations.
DOI
Evaporation of mixed citric acid/(NH4)2SO4/H2O particles: Volatility of organic aerosol by using optical tweezers
Xi-Juan Lv, Zhe Chen, Jia-Bi Ma, Yun-Hong Zhang
The condensation and evaporation processes of semi-volatile organic compounds (SVOCs) in atmospheric aerosols can induce significant evolutions of their chemical and physical properties. Hence, for interpreting and predicting composition changes of atmospheric aerosols, it is indispensable to provide insight into the partitioning behaviors of SVOCs between condensed and gas phases. In this research, optical tweezers coupled with cavity-enhanced Raman spectroscopy were employed to observe the volatility of internally mixed citric acid (CA)/(NH4)2SO4 (AS) particles, and the effect of AS on the gas/particle partitioning behaviors of atmospheric organic acids was investigated. The radii and refractive indexes of the levitated droplets were determined in real time from the wavelength positions of simulated Raman spectra and the effective vapor pressures of CA at different relative humidities (RHs) were obtained according to Maxwell equation. For the CA/AS particle with organic to inorganic mole ratio (OIR) of 1:1, the effective vapor pressure of CA decreased with the decreasing of RH. When the RH decreased from 67% to 8.2%, the effective vapor pressure of CA decreased from (1.35 to (3.0. Meanwhile, the CA/AS particles with OIR of 3:1, 1:3 were also studied, and the results show the same phenomenon compared to the particles with OIR of 1:1. When under constant RHs, the effective vapor pressures of CA decreased with the increasing of AS contents, suggesting that the presence of AS suppressed the partitioning of CA to aqueous particles. In addition, the mass transfer processes of water in CA and CA/AS/H2O systems were further studied. The characteristic time ratio between the droplet radius and RH was used to describe the water mass transfer difference dependent on RH. Compared to the characteristic time ratio of pure CA, the characteristic time ratio of CA/AS particles apparently increased. For CA/AS particles under the same RH steps, the characteristic time ratio increased with the AS content increase. According to the differential isotherm, the diffusion coefficients of citric acid and citric acid/ammonium sulfate at low RHs (RH 7%–1%, RH≈1%–7%) were calculated respectively. Generally, the key aspect of the current work was to deeply explore the relationship between the evaporation rates of SVOCs and water transport process.
DOI
The condensation and evaporation processes of semi-volatile organic compounds (SVOCs) in atmospheric aerosols can induce significant evolutions of their chemical and physical properties. Hence, for interpreting and predicting composition changes of atmospheric aerosols, it is indispensable to provide insight into the partitioning behaviors of SVOCs between condensed and gas phases. In this research, optical tweezers coupled with cavity-enhanced Raman spectroscopy were employed to observe the volatility of internally mixed citric acid (CA)/(NH4)2SO4 (AS) particles, and the effect of AS on the gas/particle partitioning behaviors of atmospheric organic acids was investigated. The radii and refractive indexes of the levitated droplets were determined in real time from the wavelength positions of simulated Raman spectra and the effective vapor pressures of CA at different relative humidities (RHs) were obtained according to Maxwell equation. For the CA/AS particle with organic to inorganic mole ratio (OIR) of 1:1, the effective vapor pressure of CA decreased with the decreasing of RH. When the RH decreased from 67% to 8.2%, the effective vapor pressure of CA decreased from (1.35 to (3.0. Meanwhile, the CA/AS particles with OIR of 3:1, 1:3 were also studied, and the results show the same phenomenon compared to the particles with OIR of 1:1. When under constant RHs, the effective vapor pressures of CA decreased with the increasing of AS contents, suggesting that the presence of AS suppressed the partitioning of CA to aqueous particles. In addition, the mass transfer processes of water in CA and CA/AS/H2O systems were further studied. The characteristic time ratio between the droplet radius and RH was used to describe the water mass transfer difference dependent on RH. Compared to the characteristic time ratio of pure CA, the characteristic time ratio of CA/AS particles apparently increased. For CA/AS particles under the same RH steps, the characteristic time ratio increased with the AS content increase. According to the differential isotherm, the diffusion coefficients of citric acid and citric acid/ammonium sulfate at low RHs (RH 7%–1%, RH≈1%–7%) were calculated respectively. Generally, the key aspect of the current work was to deeply explore the relationship between the evaporation rates of SVOCs and water transport process.
DOI
Tunable plasmonic force switch based on graphene nano-ring resonator for nanomanipulation
Mohammad Mahdi Abbasi, Sara Darbari, and Mohammad Kazem Moravvej-Farshi
Using a plasmonic graphene ring resonator of resonant frequency 10.38 THz coupled to a plasmonic graphene waveguide, we design a lab-on-a-chip optophoresis system that can function as an efficient plasmonic force switch. Finite difference time domain numerical simulations reveal that an appropriate choice of chemical potentials of the waveguide and ring resonator keeps the proposed structure in on-resonance condition, enabling the system to selectively trap a nanoparticle. Moreover, a change of 250 meV in the ring chemical potential (i.e., equivalent to 2.029 V change in the corresponding applied bias) switches the structure to a nearly perfect off-resonance condition, releasing the trapped particle. The equivalent plasmonic switch ON/OFF ratio at the waveguide output is −15.519 dB. The designed system has the capability of trapping, sorting, controlling, and separating PS nanoparticles of diameters ≥30 nm with a THz source intensity of 14.78 mW/µm2 and ≥22 nm with 29.33 mW/µm2.
DOI
Using a plasmonic graphene ring resonator of resonant frequency 10.38 THz coupled to a plasmonic graphene waveguide, we design a lab-on-a-chip optophoresis system that can function as an efficient plasmonic force switch. Finite difference time domain numerical simulations reveal that an appropriate choice of chemical potentials of the waveguide and ring resonator keeps the proposed structure in on-resonance condition, enabling the system to selectively trap a nanoparticle. Moreover, a change of 250 meV in the ring chemical potential (i.e., equivalent to 2.029 V change in the corresponding applied bias) switches the structure to a nearly perfect off-resonance condition, releasing the trapped particle. The equivalent plasmonic switch ON/OFF ratio at the waveguide output is −15.519 dB. The designed system has the capability of trapping, sorting, controlling, and separating PS nanoparticles of diameters ≥30 nm with a THz source intensity of 14.78 mW/µm2 and ≥22 nm with 29.33 mW/µm2.
DOI
Super-low-power optical trapping of a single nanoparticle
Xiaoyun Tang, Yu Zhang, Wenjie Su, Yaxun Zhang, Zhihai Liu, Xinghua Yang, Jianzhong Zhang, Jun Yang, and Libo Yuan
We propose and demonstrate a simple approach for noncontact, three-dimensional, and stable trapping of a single nanoparticle with a super-low incident laser power (0.7 mW) via the single-fiber optical tweezers. We splice a section of single-mode fiber and a section of multimode fiber to construct a Bessel-like beam, which produces narrow output laser beams. We integrate a high-refractive-index glass microsphere on the tip of the multimode fiber to focus the narrow output laser beams. The focused beams provide a nanoscale optical trap for a single nanoparticle (polystyrene sphere, diameter of 200 nm). This optical fiber probe has the advantages of high laser transmission efficiency, high spatial resolution, and minimum joule heating. The proposed approach extends the application potential of fiber-based optical manipulations, such as nanoparticle sorting, single-cell organelle analysis, and bio-sensing.
DOI
We propose and demonstrate a simple approach for noncontact, three-dimensional, and stable trapping of a single nanoparticle with a super-low incident laser power (0.7 mW) via the single-fiber optical tweezers. We splice a section of single-mode fiber and a section of multimode fiber to construct a Bessel-like beam, which produces narrow output laser beams. We integrate a high-refractive-index glass microsphere on the tip of the multimode fiber to focus the narrow output laser beams. The focused beams provide a nanoscale optical trap for a single nanoparticle (polystyrene sphere, diameter of 200 nm). This optical fiber probe has the advantages of high laser transmission efficiency, high spatial resolution, and minimum joule heating. The proposed approach extends the application potential of fiber-based optical manipulations, such as nanoparticle sorting, single-cell organelle analysis, and bio-sensing.
DOI
Tele–Robotic Platform for Dexterous Optical Single-Cell Manipulation
Edison Gerena, Florent Legendre, Akshay Molawade, Youen Vitry, Stéphane Régnier and Sinan Haliyo
Single-cell manipulation is considered a key technology in biomedical research. However, the lack of intuitive and effective systems makes this technology less accessible. We propose a new tele–robotic solution for dexterous cell manipulation through optical tweezers. A slave-device consists of a combination of robot-assisted stages and a high-speed multi-trap technique. It allows for the manipulation of more than 15 optical traps in a large workspace with nanometric resolution. A master-device (6+1 degree of freedom (DoF)) is employed to control the 3D position of optical traps in different arrangements for specific purposes. Precision and efficiency studies are carried out with trajectory control tasks. Three state-of-the-art experiments were performed to verify the efficiency of the proposed platform. First, the reliable 3D rotation of a cell is demonstrated. Secondly, a six-DoF teleoperated optical-robot is used to transport a cluster of cells. Finally, a single-cell is dexterously manipulated through an optical-robot with a fork end-effector. Results illustrate the capability to perform complex tasks in efficient and intuitive ways, opening possibilities for new biomedical applications.
DOI
Single-cell manipulation is considered a key technology in biomedical research. However, the lack of intuitive and effective systems makes this technology less accessible. We propose a new tele–robotic solution for dexterous cell manipulation through optical tweezers. A slave-device consists of a combination of robot-assisted stages and a high-speed multi-trap technique. It allows for the manipulation of more than 15 optical traps in a large workspace with nanometric resolution. A master-device (6+1 degree of freedom (DoF)) is employed to control the 3D position of optical traps in different arrangements for specific purposes. Precision and efficiency studies are carried out with trajectory control tasks. Three state-of-the-art experiments were performed to verify the efficiency of the proposed platform. First, the reliable 3D rotation of a cell is demonstrated. Secondly, a six-DoF teleoperated optical-robot is used to transport a cluster of cells. Finally, a single-cell is dexterously manipulated through an optical-robot with a fork end-effector. Results illustrate the capability to perform complex tasks in efficient and intuitive ways, opening possibilities for new biomedical applications.
DOI
Miniaturized Metalens Based Optical Tweezers on Liquid Crystal Droplets for Lab-on-a-Chip Optical Motors
Satayu Suwannasopon, Fabian Meyer, Christian Schlickriede, Papichaya Chaisakul, Jiraroj T-Thienprasert, Jumras Limtrakul, Thomas Zentgraf and Nattaporn Chattham
Surfaces covered with layers of ultrathin nanoantenna structures—so called metasurfaces have recently been proven capable of completely controlling phase of light. Metalenses have emerged from the advance in the development of metasurfaces providing a new basis for recasting traditional lenses into thin, planar optical components capable of focusing light. The lens made of arrays of plasmonic gold nanorods were fabricated on a glass substrate by using electron beam lithography. A 1064 nm laser was used to create a high intensity circularly polarized light focal spot through metalens of focal length 800 µm, N.A. = 0.6 fabricated based on Pancharatnam-Berry phase principle. We demonstrated that optical rotation of birefringent nematic liquid crystal droplets trapped in the laser beam was possible through this metalens. The rotation of birefringent droplets convinced that the optical trap possesses strong enough angular momentum of light from radiation of each nanostructure acting like a local half waveplate and introducing an orientation-dependent phase to light. Here, we show the success in creating a miniaturized and robust metalens based optical tweezers system capable of rotating liquid crystals droplets to imitate an optical motor for future lab-on-a-chip applications.
DOI
Surfaces covered with layers of ultrathin nanoantenna structures—so called metasurfaces have recently been proven capable of completely controlling phase of light. Metalenses have emerged from the advance in the development of metasurfaces providing a new basis for recasting traditional lenses into thin, planar optical components capable of focusing light. The lens made of arrays of plasmonic gold nanorods were fabricated on a glass substrate by using electron beam lithography. A 1064 nm laser was used to create a high intensity circularly polarized light focal spot through metalens of focal length 800 µm, N.A. = 0.6 fabricated based on Pancharatnam-Berry phase principle. We demonstrated that optical rotation of birefringent nematic liquid crystal droplets trapped in the laser beam was possible through this metalens. The rotation of birefringent droplets convinced that the optical trap possesses strong enough angular momentum of light from radiation of each nanostructure acting like a local half waveplate and introducing an orientation-dependent phase to light. Here, we show the success in creating a miniaturized and robust metalens based optical tweezers system capable of rotating liquid crystals droplets to imitate an optical motor for future lab-on-a-chip applications.
DOI
Wednesday, October 16, 2019
Optimal condition for optical trapping of large particles: tuning the laser power and numerical aperture of the objective
Hossein Gorjizadeh Alinezhad and S. Nader S. Reihani
A Gaussian laser beam tightly focused through a high-numerical-aperture objective lens, so-called optical tweezers, is widely used for piconewton-range force spectroscopy. Utilizing a proper value for parameters such as bead size, numerical aperture of the objective, and power of the laser is always a challenge. Here, we show which set of values for the parameters can maximize lateral trapping efficiency. Our results show that for a high-numerical-aperture force spectroscopy, a bead with a diameter of 4–5 µm is suitable, and that for manipulation using large beads, utilizing a proper value for laser power and numerical aperture of the objective is crucial. We present a practical method for choosing the power of the laser that maximizes lateral trapping efficiency.
DOI
A Gaussian laser beam tightly focused through a high-numerical-aperture objective lens, so-called optical tweezers, is widely used for piconewton-range force spectroscopy. Utilizing a proper value for parameters such as bead size, numerical aperture of the objective, and power of the laser is always a challenge. Here, we show which set of values for the parameters can maximize lateral trapping efficiency. Our results show that for a high-numerical-aperture force spectroscopy, a bead with a diameter of 4–5 µm is suitable, and that for manipulation using large beads, utilizing a proper value for laser power and numerical aperture of the objective is crucial. We present a practical method for choosing the power of the laser that maximizes lateral trapping efficiency.
DOI
Probing the Basis of α-Synuclein Aggregation by Comparing Simulations to Single-Molecule Experiments
Cassandra D.M.Churchill, Mark A.Healey, JordanePreto, Jack A.Tuszynski, Michael T.Woodside
Intrinsically disordered proteins often play an important role in protein aggregation. However, it is challenging to determine the structures and interactions that drive the early stages of aggregation because they are transient and obscured in a heterogeneous mixture of disordered states. Even computational methods are limited because the lack of ordered structure makes it difficult to ensure that the relevant conformations are sampled. We address these challenges by integrating atomistic simulations with high-resolution single-molecule measurements reported previously, using the measurements to help discern which parts of the disordered ensemble of structures in the simulations are most probable while using the simulations to identify residues and interactions that are important for oligomer stability. This approach was applied to α-synuclein, an intrinsically disordered protein that aggregates in the context of Parkinson’s disease. We simulated single-molecule pulling experiments on dimers, the minimal oligomer, and compared them to force spectroscopy measurements. Force-extension curves were simulated starting from a set of 66 structures with substantial structured content selected from the ensemble of dimer structures generated at zero force via Monte Carlo simulations. The pattern of contour length changes as the structures unfolded through intermediate states was compared to the results from optical trapping measurements on the same dimer to discern likely structures occurring in the measurements. Simulated pulling curves were generally consistent with experimental data but with a larger number of transient intermediates. We identified an ensemble of β-rich dimer structures consistent with the experimental data from which dimer interfaces could be deduced. These results suggest specific druggable targets in the structural motifs of α-synuclein that may help prevent the earliest steps of oligomerization.
DOI
Intrinsically disordered proteins often play an important role in protein aggregation. However, it is challenging to determine the structures and interactions that drive the early stages of aggregation because they are transient and obscured in a heterogeneous mixture of disordered states. Even computational methods are limited because the lack of ordered structure makes it difficult to ensure that the relevant conformations are sampled. We address these challenges by integrating atomistic simulations with high-resolution single-molecule measurements reported previously, using the measurements to help discern which parts of the disordered ensemble of structures in the simulations are most probable while using the simulations to identify residues and interactions that are important for oligomer stability. This approach was applied to α-synuclein, an intrinsically disordered protein that aggregates in the context of Parkinson’s disease. We simulated single-molecule pulling experiments on dimers, the minimal oligomer, and compared them to force spectroscopy measurements. Force-extension curves were simulated starting from a set of 66 structures with substantial structured content selected from the ensemble of dimer structures generated at zero force via Monte Carlo simulations. The pattern of contour length changes as the structures unfolded through intermediate states was compared to the results from optical trapping measurements on the same dimer to discern likely structures occurring in the measurements. Simulated pulling curves were generally consistent with experimental data but with a larger number of transient intermediates. We identified an ensemble of β-rich dimer structures consistent with the experimental data from which dimer interfaces could be deduced. These results suggest specific druggable targets in the structural motifs of α-synuclein that may help prevent the earliest steps of oligomerization.
DOI
Quantitative generation of microfluidic flow by using optically driven microspheres
Songyu Hu, Ruifeng Hu, Liping Tang, Weiwei Jiang, Banglin Deng
Microfluidic flow generation plays a fundamental role in microfluidic systems and shows potential for applications in basic biology and clinical medicine. In this study, an enabling technology is proposed to quantitatively generate microfluid flow through the automatic movement of a microsphere in liquid by using optical tweezers. A closed-loop control strategy with visual servoing feedback is introduced to achieve high precision and robustness. The theoretical solution of the generated microfluid is obtained on the basis of Stokes equations. An experimental method is proposed, and experiments are performed to verify the effectiveness of our approach. This method does not impose any dedicated fabrication of microtool, and the microfluidic flow can be dexterously adjusted by controlling the direction, speed, and distance of the microsphere from a target location. To the best of our knowledge, this is the first demonstration of optically actuating liquids through the translational movement of microspheres with closed-loop control. The proposed method will be useful in various biomedical applications needing quantitative, precise and controllable localized microfluid.
DOI
Microfluidic flow generation plays a fundamental role in microfluidic systems and shows potential for applications in basic biology and clinical medicine. In this study, an enabling technology is proposed to quantitatively generate microfluid flow through the automatic movement of a microsphere in liquid by using optical tweezers. A closed-loop control strategy with visual servoing feedback is introduced to achieve high precision and robustness. The theoretical solution of the generated microfluid is obtained on the basis of Stokes equations. An experimental method is proposed, and experiments are performed to verify the effectiveness of our approach. This method does not impose any dedicated fabrication of microtool, and the microfluidic flow can be dexterously adjusted by controlling the direction, speed, and distance of the microsphere from a target location. To the best of our knowledge, this is the first demonstration of optically actuating liquids through the translational movement of microspheres with closed-loop control. The proposed method will be useful in various biomedical applications needing quantitative, precise and controllable localized microfluid.
DOI
Complex dynamics under tension in a high-efficiency frameshift stimulatory structure
Matthew T. J. Halma, Dustin B. Ritchie, Tonia R. Cappellano, Krishna Neupane, and Michael T. Woodside
Specific structures in mRNA can stimulate programmed ribosomal frameshifting (PRF). PRF efficiency can vary enormously between different stimulatory structures, but the features that lead to efficient PRF stimulation remain uncertain. To address this question, we studied the structural dynamics of the frameshift signal from West Nile virus (WNV), which stimulates −1 PRF at very high levels and has been proposed to form several different structures, including mutually incompatible pseudoknots and a double hairpin. Using optical tweezers to apply tension to single mRNA molecules, mimicking the tension applied by the ribosome during PRF, we found that the WNV frameshift signal formed an unusually large number of different metastable structures, including all of those previously proposed. From force-extension curve measurements, we mapped 2 mutually exclusive pathways for the folding, each encompassing multiple intermediates. We identified the intermediates in each pathway from length changes and the effects of antisense oligomers blocking formation of specific contacts. Intriguingly, the number of transitions between the different conformers of the WNV frameshift signal was maximal in the range of forces applied by the ribosome during −1 PRF. Furthermore, the occupancy of the pseudoknotted conformations was far too low for static pseudoknots to account for the high levels of −1 PRF. These results support the hypothesis that conformational heterogeneity plays a key role in frameshifting and suggest that transitions between different conformers under tension are linked to efficient PRF stimulation.
DOI
Specific structures in mRNA can stimulate programmed ribosomal frameshifting (PRF). PRF efficiency can vary enormously between different stimulatory structures, but the features that lead to efficient PRF stimulation remain uncertain. To address this question, we studied the structural dynamics of the frameshift signal from West Nile virus (WNV), which stimulates −1 PRF at very high levels and has been proposed to form several different structures, including mutually incompatible pseudoknots and a double hairpin. Using optical tweezers to apply tension to single mRNA molecules, mimicking the tension applied by the ribosome during PRF, we found that the WNV frameshift signal formed an unusually large number of different metastable structures, including all of those previously proposed. From force-extension curve measurements, we mapped 2 mutually exclusive pathways for the folding, each encompassing multiple intermediates. We identified the intermediates in each pathway from length changes and the effects of antisense oligomers blocking formation of specific contacts. Intriguingly, the number of transitions between the different conformers of the WNV frameshift signal was maximal in the range of forces applied by the ribosome during −1 PRF. Furthermore, the occupancy of the pseudoknotted conformations was far too low for static pseudoknots to account for the high levels of −1 PRF. These results support the hypothesis that conformational heterogeneity plays a key role in frameshifting and suggest that transitions between different conformers under tension are linked to efficient PRF stimulation.
DOI
Interference of axially-shifted Laguerre–Gaussian beams and their interaction with atoms
K Koksal, Vasileios E Lembessis, J Yuan and M Babiker
Counter-propagating co-axial Laguerre–Gaussian (LG) beams are considered, not in the familiar scenario where the focal planes coincide at z = 0, but when they are separated by a finite axial distance d. The simplest case is where both beams are doughnut beams which have the same linear polarisation. The total fields of this system are shown to display novel amplitude and phase distributions and are shown to give rise to a ring or a finite ring lattice composed of double rings and single central ring. When the beams have slightly different frequencies the ring lattice pattern becomes a finite set of rotating Ferris wheels and the whole pattern also moves axially between the focal planes. We show that the fields of such an axially shifted pair of counter-propagating LG beams generate trapping potentials due to the dipole force which can trap two-level atoms in the components of the ring lattice. We also highlight a unique feature of this system which involves the creation of a new longitudinal optical atom trapping potential due to the scattering force which arises solely when $d\ne 0$. The results are illustrated using realistic parameters which also confirm the importance of the Gouy and curvature effects in determining the ring separation both radially and axially and gives rise to the possibility of atom tunnelling between components of the double rings.
Counter-propagating co-axial Laguerre–Gaussian (LG) beams are considered, not in the familiar scenario where the focal planes coincide at z = 0, but when they are separated by a finite axial distance d. The simplest case is where both beams are doughnut beams which have the same linear polarisation. The total fields of this system are shown to display novel amplitude and phase distributions and are shown to give rise to a ring or a finite ring lattice composed of double rings and single central ring. When the beams have slightly different frequencies the ring lattice pattern becomes a finite set of rotating Ferris wheels and the whole pattern also moves axially between the focal planes. We show that the fields of such an axially shifted pair of counter-propagating LG beams generate trapping potentials due to the dipole force which can trap two-level atoms in the components of the ring lattice. We also highlight a unique feature of this system which involves the creation of a new longitudinal optical atom trapping potential due to the scattering force which arises solely when $d\ne 0$. The results are illustrated using realistic parameters which also confirm the importance of the Gouy and curvature effects in determining the ring separation both radially and axially and gives rise to the possibility of atom tunnelling between components of the double rings.
Pixantrone anticancer drug as a DNA ligand: Depicting the mechanism of action at single molecule level
C. H. M. Lima, J. M. Caquito Jr., R. M. de Oliveira, M. S. Rocha
In this work we use single molecule force spectroscopy performed with optical tweezers in order to characterize the complexes formed between the anticancer drug Pixantrone (PIX) and the DNA molecule, at two very different ionic strengths. Firstly, the changes of the mechanical properties of the DNA-PIX complexes were studied as a function of the drug concentration in the sample. Then, a quenched-disorder statistical model of ligand binding was used in order to determine the physicochemical (binding) parameters of the DNA-PIX interaction. In particular, we have found that the PIX molecular mechanism of action involves intercalation into the double helix, followed by a significant compaction of the DNA molecule due to partial neutralization of the phosphate backbone. Finally, this scenario of interaction was quantitatively compared to that found for the related drug Mitoxantrone (MTX), which binds to DNA with a considerably higher equilibrium binding constant and promotes a much stronger DNA compaction. The comparison performed between the two drugs can bring clues to the development of new (and more efficient) related compounds.
DOI
In this work we use single molecule force spectroscopy performed with optical tweezers in order to characterize the complexes formed between the anticancer drug Pixantrone (PIX) and the DNA molecule, at two very different ionic strengths. Firstly, the changes of the mechanical properties of the DNA-PIX complexes were studied as a function of the drug concentration in the sample. Then, a quenched-disorder statistical model of ligand binding was used in order to determine the physicochemical (binding) parameters of the DNA-PIX interaction. In particular, we have found that the PIX molecular mechanism of action involves intercalation into the double helix, followed by a significant compaction of the DNA molecule due to partial neutralization of the phosphate backbone. Finally, this scenario of interaction was quantitatively compared to that found for the related drug Mitoxantrone (MTX), which binds to DNA with a considerably higher equilibrium binding constant and promotes a much stronger DNA compaction. The comparison performed between the two drugs can bring clues to the development of new (and more efficient) related compounds.
DOI
In-situ reflection imaging and microspectroscopic study on three-dimensional crystal growth of L-phenylalanine under laser trapping
Jim Jui-Kai Chen, Ken-ichi Yuyama, Teruki Sugiyama and Hiroshi Masuhara
We investigated growth behavior of an L-phenylalanine crystal formed by laser trapping with the use of reflection imaging and microspectroscopy. Optical reflection micrographs show colored images of the crystal due to constructive interference of incident white light. The color distribution on the crystal is dynamically changed under laser trapping, which is in addition to enlargement of the crystal plane area. The temporal change in the crystal thickness is examined by measuring reflection spectra of the crystal. We discuss the three-dimensional crystal growth under laser trapping condition by comprehensively considering the changes in the crystal thickness and the crystal plane area.
DOI
We investigated growth behavior of an L-phenylalanine crystal formed by laser trapping with the use of reflection imaging and microspectroscopy. Optical reflection micrographs show colored images of the crystal due to constructive interference of incident white light. The color distribution on the crystal is dynamically changed under laser trapping, which is in addition to enlargement of the crystal plane area. The temporal change in the crystal thickness is examined by measuring reflection spectra of the crystal. We discuss the three-dimensional crystal growth under laser trapping condition by comprehensively considering the changes in the crystal thickness and the crystal plane area.
DOI
Kinetochore-associated Stu2 promotes chromosome biorientation in vivo
Matthew P. Miller, Rena K. Evans, Alex Zelter, Elisabeth A. Geyer, Michael J. MacCoss, Luke M. Rice, Trisha N. Davis, Charles L. Asbury, Sue Biggins
Accurate segregation of chromosomes to daughter cells is a critical aspect of cell division. It requires the kinetochores on duplicated chromosomes to biorient, attaching to microtubules from opposite poles of the cell. Bioriented attachments come under tension, while incorrect attachments lack tension and must be released to allow proper attachments to form. A well-studied error correction pathway is mediated by the Aurora B kinase, which destabilizes low tension-bearing attachments. We recently discovered that in vitro, kinetochores display an additional intrinsic tension-sensing pathway that utilizes Stu2. The contribution of kinetochore-associated Stu2 to error correction in cells, however, was unknown. Here, we identify a Stu2 mutant that abolishes its kinetochore function and show that it causes biorientation defects in vivo. We also show that this Stu2-mediated pathway functions together with the Aurora B-mediated pathway. Altogether, our work indicates that cells employ multiple pathways to ensure biorientation and the accuracy of chromosome segregation.
DOI
Accurate segregation of chromosomes to daughter cells is a critical aspect of cell division. It requires the kinetochores on duplicated chromosomes to biorient, attaching to microtubules from opposite poles of the cell. Bioriented attachments come under tension, while incorrect attachments lack tension and must be released to allow proper attachments to form. A well-studied error correction pathway is mediated by the Aurora B kinase, which destabilizes low tension-bearing attachments. We recently discovered that in vitro, kinetochores display an additional intrinsic tension-sensing pathway that utilizes Stu2. The contribution of kinetochore-associated Stu2 to error correction in cells, however, was unknown. Here, we identify a Stu2 mutant that abolishes its kinetochore function and show that it causes biorientation defects in vivo. We also show that this Stu2-mediated pathway functions together with the Aurora B-mediated pathway. Altogether, our work indicates that cells employ multiple pathways to ensure biorientation and the accuracy of chromosome segregation.
DOI
Tuesday, October 15, 2019
Single‐Cell Biodetection by Upconverting Microspinners
Elisa Ortiz‐Rivero, Katarzyna Prorok, Michal Skowickł, Dasheng Lu, Artur Bednarkiewicz, Daniel Jaque, Patricia Haro‐González
Near‐infrared‐light‐mediated optical tweezing of individual upconverting particles has enabled all‐optical single‐cell studies, such as intracellular thermal sensing and minimally invasive cytoplasm investigations. Furthermore, the intrinsic optical birefringence of upconverting particles renders them light‐driven luminescent spinners with a yet unexplored potential in biomedicine. In this work, the use of upconverting spinners is showcased for the accurate and specific detection of single‐cell and single‐bacteria attachment events, through real‐time monitoring of the spinners rotation velocity of the spinner. The physical mechanisms linking single‐attachment to the angular deceleration of upconverting spinners are discussed in detail. Concomitantly, the upconversion emission generated by the spinner is harnessed for simultaneous thermal sensing and thermal control during the attachment event. Results here included demonstrate the potential of upconverting particles for the development of fast, high‐sensitivity, and cost‐effective systems for single‐cell biodetection.
DOI
Near‐infrared‐light‐mediated optical tweezing of individual upconverting particles has enabled all‐optical single‐cell studies, such as intracellular thermal sensing and minimally invasive cytoplasm investigations. Furthermore, the intrinsic optical birefringence of upconverting particles renders them light‐driven luminescent spinners with a yet unexplored potential in biomedicine. In this work, the use of upconverting spinners is showcased for the accurate and specific detection of single‐cell and single‐bacteria attachment events, through real‐time monitoring of the spinners rotation velocity of the spinner. The physical mechanisms linking single‐attachment to the angular deceleration of upconverting spinners are discussed in detail. Concomitantly, the upconversion emission generated by the spinner is harnessed for simultaneous thermal sensing and thermal control during the attachment event. Results here included demonstrate the potential of upconverting particles for the development of fast, high‐sensitivity, and cost‐effective systems for single‐cell biodetection.
DOI
Red‐Blood‐Cell Waveguide as a Living Biosensor and Micromotor
Yuchao Li, Xiaoshuai Liu, Xiaohao Xu, Hongbao Xin, Yao Zhang, Baojun Li
With great potential in intelligent sensing and actuating systems, biosensors and micromotors are expected to be powerful instruments for early diagnosis and drug delivery in precision medicine. However, it is difficult to ensure the synthetic biosensors and micromotors are compatible with biosystems because of their exogenous building blocks. Biocompatible biosensors and micromotors assembled are reported from living red blood cells (RBCs) optically bound into a waveguide using fiber probes. By monitoring light propagation of the RBC waveguide, the pH of blood solution is detected in real time with an accuracy of 0.05. This can be used for the diagnosis of pH‐related disorders of the blood. After diagnosis, optical torque is exerted on the RBC waveguide, allowing it to rotate as a micromotor and transport microparticles to a target region. The RBC waveguide is then constructed inside zebrafish blood vessels to validate in vivo application. The living biosensors and micromotors are expected to provide a “smart” platform for precise biosensing, medical analysis, and drug delivery.
DOI
With great potential in intelligent sensing and actuating systems, biosensors and micromotors are expected to be powerful instruments for early diagnosis and drug delivery in precision medicine. However, it is difficult to ensure the synthetic biosensors and micromotors are compatible with biosystems because of their exogenous building blocks. Biocompatible biosensors and micromotors assembled are reported from living red blood cells (RBCs) optically bound into a waveguide using fiber probes. By monitoring light propagation of the RBC waveguide, the pH of blood solution is detected in real time with an accuracy of 0.05. This can be used for the diagnosis of pH‐related disorders of the blood. After diagnosis, optical torque is exerted on the RBC waveguide, allowing it to rotate as a micromotor and transport microparticles to a target region. The RBC waveguide is then constructed inside zebrafish blood vessels to validate in vivo application. The living biosensors and micromotors are expected to provide a “smart” platform for precise biosensing, medical analysis, and drug delivery.
DOI
Nanophotonic Array-Induced Dynamic Behavior for Label-Free Shape-Selective Bacteria Sieving
Yuzhi Shi, Haitao Zhao, Kim Truc Nguyen, Yi Zhang, Lip Ket Chin, Tongtong Zhu, Yefeng Yu, Hong Cai, Peng Huat Yap, Patricia Yang Liu, Sha Xiong, Jingbo Zhang, Cheng-Wei Qiu, Che Ting Chan, Ai Qun Liu
Current particle sorting methods such as microfluidics, acoustics, and optics focus on exploiting the differences in the mass, size, refractive index, or fluorescence staining. However, there exist formidable challenges for them to sort label-free submicron particles with similar volume and refractive index yet distinct shapes. In this work, we report an optofluidic nanophotonic sawtooth array (ONSA) that generates sawtooth-like light fields through light coupling, paving the physical foundation for shape-selective sieving. Submicron particles interact with the coupled hotspots which impose different optical torques on the particles according to their shapes. Unstained S. aureus and E. coli are used as a model system to demonstrate this shape-selective sorting mechanism based on the torque-induced body dynamics, which was previously unattainable by other particle sorting technologies. More than 95% of S. aureus is retained within ONSA, while more than 97% of E. coli is removed. This nanophotonic chip offers a paradigm shift in shape-selective sorting of submicron particles and expands the boundary of optofluidics-based particle manipulation.
DOI
DOI
In Situ Measurement of Depletion Caused by SDBS Micelles on the Surface of Silica Particles Using Optical Tweezers
Shuai Liu, Yue Hu, Jing Xia, Shenwen Fang, Ming Duan
Dual-trap optical tweezers have been used to directly measure the interaction forces between two silica particles upon controlling the concentration of the ionic surfactant sodium dodecylbenzenesulfonate (SDBS). By capturing two silica particles in one spot optical trap and one linear optical trap and controlling the linear trap to bring one particle to approach another sufficiently closer, the interaction forces between these two particles can be measured as the separation distance changes. Results showed that with increasing concentrations of SDBS, the interaction force between the two silica particles emerges at closer surface distance between two silica particles. Only repulsive force exists between silica particles below the critical micelle concentration (cmc) of SDBS and it could be well-fitted using the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. However, the depletion attraction force appears above the cmc of SDBS which is induced by the generation of SDBS micelles. By in situ measurement of the interaction force between two silica particles in the presence of different concentrations of SDBS, the depletion force can be quantitatively calculated.
DOI
Dual-trap optical tweezers have been used to directly measure the interaction forces between two silica particles upon controlling the concentration of the ionic surfactant sodium dodecylbenzenesulfonate (SDBS). By capturing two silica particles in one spot optical trap and one linear optical trap and controlling the linear trap to bring one particle to approach another sufficiently closer, the interaction forces between these two particles can be measured as the separation distance changes. Results showed that with increasing concentrations of SDBS, the interaction force between the two silica particles emerges at closer surface distance between two silica particles. Only repulsive force exists between silica particles below the critical micelle concentration (cmc) of SDBS and it could be well-fitted using the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. However, the depletion attraction force appears above the cmc of SDBS which is induced by the generation of SDBS micelles. By in situ measurement of the interaction force between two silica particles in the presence of different concentrations of SDBS, the depletion force can be quantitatively calculated.
DOI
Ring-broken optical vortices with an adjustable opening
Shubo Cheng, Tian Xia, Mengsi Liu, Yiping Xu, Shan Xu, Shufang Gao, Geng Zhang, Shaohua Tao, Wenxing Yang
A modified helical phase is presented in this paper. The generated vortex beams with the modified helical phases are characterized with ring-broken intensity distributions. The transverse intensity patterns of the generated vortex beams at a defined axial position and the focal plane are analyzed, respectively. The results demonstrate that the generated beam has an opening, whose size can be customized with a variable . The proposed vortex beams realized optical guiding of microparticles in the transverse planes of the beams. This kind of ring-broken beams will be promising for optical manipulation.
DOI
A modified helical phase is presented in this paper. The generated vortex beams with the modified helical phases are characterized with ring-broken intensity distributions. The transverse intensity patterns of the generated vortex beams at a defined axial position and the focal plane are analyzed, respectively. The results demonstrate that the generated beam has an opening, whose size can be customized with a variable . The proposed vortex beams realized optical guiding of microparticles in the transverse planes of the beams. This kind of ring-broken beams will be promising for optical manipulation.
DOI
Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities
Yijie Shen, Xuejiao Wang, Zhenwei Xie, Changjun Min, Xing Fu, Qiang Liu, Mali Gong & Xiaocong Yuan
Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin–orbital interactions, Bose–Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.
DOI
Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin–orbital interactions, Bose–Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.
DOI
Hidden Symmetry and Invariance in Optical Forces
Yikun Jiang, Haoze Lin, Xiao Li, Jun Chen, Junjie Du, Jack Ng
Like any physical quantity whose symmetry properties mimic its source, the optical force acting on a neutral spherical particle has a symmetry that mimics the incident field. The optical force consists of the gradient force and the scattering force. Here, we explicitly show that in optical lattices, the in-plane gradient force and scattering force have additional even and odd symmetries upon 2N-fold rotation, respectively, which are not shared by the incident field that is N-fold discrete rotationally symmetric. Similar hidden symmetries, namely, even and odd symmetries upon reflection about the focal plane, are also found in particles illuminated by a Gaussian beam, suggesting that it is a general property of the optical force. These are verified numerically in multiple examples and analytically for three incident plane waves, by which we also discover that the profiles of the gradient force and the scattering force are invariant with respect to material composition and particle size for a spherical particle. As such, one can tune the polarization to almost completely “turn off” either gradient force or scattering force, leaving behind a purely irrotational or solenoidal force field, opening a new freedom to control the conservativeness of optical forces.
DOI
Like any physical quantity whose symmetry properties mimic its source, the optical force acting on a neutral spherical particle has a symmetry that mimics the incident field. The optical force consists of the gradient force and the scattering force. Here, we explicitly show that in optical lattices, the in-plane gradient force and scattering force have additional even and odd symmetries upon 2N-fold rotation, respectively, which are not shared by the incident field that is N-fold discrete rotationally symmetric. Similar hidden symmetries, namely, even and odd symmetries upon reflection about the focal plane, are also found in particles illuminated by a Gaussian beam, suggesting that it is a general property of the optical force. These are verified numerically in multiple examples and analytically for three incident plane waves, by which we also discover that the profiles of the gradient force and the scattering force are invariant with respect to material composition and particle size for a spherical particle. As such, one can tune the polarization to almost completely “turn off” either gradient force or scattering force, leaving behind a purely irrotational or solenoidal force field, opening a new freedom to control the conservativeness of optical forces.
DOI
Proper measurement of pure dielectrophoresis force acting on a RBC using optical tweezers
Mehrzad Sasanpour, Ali Azadbakht, Parisa Mollaei, and S. Nader S. Reihani
The force experienced by a neutral dielectric object in the presence of a spatially non-uniform electric field is referred to as dielectrophoresis (DEP). The proper quantification of DEP force in the single-cell level could be of great importance for the design of high-efficiency micro-fluidic systems for the separation of biological cells. In this report we show how optical tweezers can be properly utilized for proper quantification of DEP force experienced by a human RBC. By tuning the temporal frequency of the applied electric field and also performing control experiments and comparing our experimental results with that of theoretically calculated, we show that the measured force is a pure DEP force. Our results show that in the frequency range of 0.1-3 𝑀𝐻𝑧 the DEP force acting on RBC is frequency independent.
DOI
The force experienced by a neutral dielectric object in the presence of a spatially non-uniform electric field is referred to as dielectrophoresis (DEP). The proper quantification of DEP force in the single-cell level could be of great importance for the design of high-efficiency micro-fluidic systems for the separation of biological cells. In this report we show how optical tweezers can be properly utilized for proper quantification of DEP force experienced by a human RBC. By tuning the temporal frequency of the applied electric field and also performing control experiments and comparing our experimental results with that of theoretically calculated, we show that the measured force is a pure DEP force. Our results show that in the frequency range of 0.1-3 𝑀𝐻𝑧 the DEP force acting on RBC is frequency independent.
DOI
Monday, October 14, 2019
Optically Addressable Array of Optomechanically Compliant Glass Nanospikes on the Endface of a Soft-Glass Photonic Crystal Fiber
Zheqi Wang, Shangran Xie, Xin Jiang, Fehim Babic, Jiapeng Huang, Riccardo Pennetta, Johannes R. Koehler, Philip St.J. Russell
Arrays of elongated nanoscale structures with suitable optical and mechanical properties can act as probes of numerous physical processes at the nanoscale, with applications in, for example, high-resolution optical imaging and atomic force microscopy. They can also be used to investigate optomechanical phenomena such as synchronization among large assemblies of mechanical oscillators. Here we report a novel and versatile technique for fabricating two-dimensional light-guiding arrays of mechanically compliant glass nanospikes with lengths up to several hundred micrometers. The procedure starts with a multicore fiber made by stacking and drawing capillaries and rods of two different germanate glasses with markedly different acid etching rates. After a suitable etching step, a free-standing nanospike array is created at the fiber endface. The parameters are chosen so that there is evanescent coupling between adjacent nanospikes, which gives rise to strong optomechanical forces that can be exploited to drive and control the mechanical motion of the nanospikes and thus the optical properties.
DOI
Arrays of elongated nanoscale structures with suitable optical and mechanical properties can act as probes of numerous physical processes at the nanoscale, with applications in, for example, high-resolution optical imaging and atomic force microscopy. They can also be used to investigate optomechanical phenomena such as synchronization among large assemblies of mechanical oscillators. Here we report a novel and versatile technique for fabricating two-dimensional light-guiding arrays of mechanically compliant glass nanospikes with lengths up to several hundred micrometers. The procedure starts with a multicore fiber made by stacking and drawing capillaries and rods of two different germanate glasses with markedly different acid etching rates. After a suitable etching step, a free-standing nanospike array is created at the fiber endface. The parameters are chosen so that there is evanescent coupling between adjacent nanospikes, which gives rise to strong optomechanical forces that can be exploited to drive and control the mechanical motion of the nanospikes and thus the optical properties.
DOI
Fabricating fiber probes for optical tweezers by an improved tube etching method
Y. X. Liu, B. Zhang, N. Zhang, and Z. L. Liu
An improved tube etching method to fabricate high-quality fiber probes for optical tweezers by reserving a certain length of bare fiber to form a T-type composite structure was proposed and implemented. This method can overcome the impact of fiber types on the quality of probes in the conventional tube etching effectively. Based on the influence of gravity and diffusion on the motion of reactants, the analysis of formation mechanism was proposed for this method. This procedure retained the advantage of smooth surface in traditional tube etching but shortened the etching time. Our results also demonstrated that light transmittance of the probe fabricated by this method was improved by 6.8 times, resulting in a greater force in cells trapping. This work provided a way of designing and fabricating optical fiber tweezers with a high trapping efficiency.
DOI
An improved tube etching method to fabricate high-quality fiber probes for optical tweezers by reserving a certain length of bare fiber to form a T-type composite structure was proposed and implemented. This method can overcome the impact of fiber types on the quality of probes in the conventional tube etching effectively. Based on the influence of gravity and diffusion on the motion of reactants, the analysis of formation mechanism was proposed for this method. This procedure retained the advantage of smooth surface in traditional tube etching but shortened the etching time. Our results also demonstrated that light transmittance of the probe fabricated by this method was improved by 6.8 times, resulting in a greater force in cells trapping. This work provided a way of designing and fabricating optical fiber tweezers with a high trapping efficiency.
DOI
Optical force microscopy: combining light with atomic force microscopy for nanomaterial identification
Nusrat Jahan, Hanwei Wang, Shensheng Zhao, Arkajit Dutta, Hsuan-Kai Huang, Yang Zhao, Yun-Sheng Chen
Scanning probe techniques have evolved significantly in recent years to detect surface morphology of materials down to subnanometer resolution, but without revealing spectroscopic information. In this review, we discuss recent advances in scanning probe techniques that capitalize on light-induced forces for studying nanomaterials down to molecular specificities with nanometer spatial resolution.
DOI
DOI
Light-controlled skyrmions and torons as reconfigurable particles
Hayley R. O. Sohn, Changda D. Liu, Yuhan Wang, and Ivan I. Smalyukh
Topological solitons, such as skyrmions, arise in field theories of systems ranging from Bose-Einstein condensates to optics, particle physics, and cosmology, but they are rarely accessible experimentally. Chiral nematic liquid crystals provide a platform to study skyrmions because of their natural tendency to form twisted structures arising from the lack of mirror symmetry at the molecular level. However, large-scale dynamic control and technological utility of skyrmions remain limited. Combining experiments and numerical modeling of chiral liquid crystals with optically controlled helical pitch, we demonstrate that low-intensity, unstructured light can control stability, dimensions, interactions, spatial patterning, self-assembly, and dynamics of these topological solitons.
DOI
Topological solitons, such as skyrmions, arise in field theories of systems ranging from Bose-Einstein condensates to optics, particle physics, and cosmology, but they are rarely accessible experimentally. Chiral nematic liquid crystals provide a platform to study skyrmions because of their natural tendency to form twisted structures arising from the lack of mirror symmetry at the molecular level. However, large-scale dynamic control and technological utility of skyrmions remain limited. Combining experiments and numerical modeling of chiral liquid crystals with optically controlled helical pitch, we demonstrate that low-intensity, unstructured light can control stability, dimensions, interactions, spatial patterning, self-assembly, and dynamics of these topological solitons.
DOI
Single-Molecule Topochemical Analyses for Large-Scale Multiplexing Tasks
Shankar Mandal, Xiaoqing Zhang, Shankar Pandey, Hanbin Mao
Multitasking is the pivotal feature in next-generation chemo- or bioanalyses. However, simultaneous analyses rarely exceed over three different tasks, which is ascribed to the limited space to accommodate analyzing units and the compromised signal-to-noise (S/N) level as the number of tasks increases. Here, by leveraging superior S/N of single-molecule techniques, we analyzed five microRNA biomarkers by spatially encoding miRNA recognition units with nanometers resolution in a DNA template, while decoding the analyte binding temporally in seconds. The hairpin stem is interspersed by internal loops to encode recognition units for miRNA. By mechanical unfolding of the hairpin, individual internal loops are sequentially interrogated for the binding of each miRNA. Using this so-called topochemical spatiotemporal analysis, we were able to achieve subpicomolar detection limits of miRNAs. We anticipate that this new single-molecule topochemical analysis can massively analyze single-molecule targets.
DOI
Multitasking is the pivotal feature in next-generation chemo- or bioanalyses. However, simultaneous analyses rarely exceed over three different tasks, which is ascribed to the limited space to accommodate analyzing units and the compromised signal-to-noise (S/N) level as the number of tasks increases. Here, by leveraging superior S/N of single-molecule techniques, we analyzed five microRNA biomarkers by spatially encoding miRNA recognition units with nanometers resolution in a DNA template, while decoding the analyte binding temporally in seconds. The hairpin stem is interspersed by internal loops to encode recognition units for miRNA. By mechanical unfolding of the hairpin, individual internal loops are sequentially interrogated for the binding of each miRNA. Using this so-called topochemical spatiotemporal analysis, we were able to achieve subpicomolar detection limits of miRNAs. We anticipate that this new single-molecule topochemical analysis can massively analyze single-molecule targets.
DOI
Stochastic buckling of self-assembled colloidal structures
Simon Stuij, Jan Maarten van Doorn, Thomas Kodger, Joris Sprakel, Corentin Coulais, and Peter Schall
The vast majority of soft and biological materials, gels, and tissues are made from micrometer-size slender structures such as biofilaments and colloidal and molecular chains, which are believed to crucially control their mechanics. These constituents show intriguing extreme mechanics, mechanical instabilities, and plasticity, which, besides attracting significant theoretical attention, have not been studied experimentally and as such remain poorly understood. Here we investigate, by experiments, simulations, and theory, the mechanical instabilities of a slender self-assembled colloidal structure, observing a form of stochastic buckling where thermal fluctuations and associated entropic force effects are amplified in the vicinity of a buckling instability. We fully characterize how the persistence length and plasticity control the stochastic buckling transition, leading to intriguing higher-order buckling modes. These results elucidate the interplay of geometrical, thermal, and plastic interactions in the nonlinear mechanics of thermal self-assembled structures, crucial to the mechanical response and function of fiber-based soft and biological materials, as well as the rational design of micro- and nanoscale architectures.
DOI
The vast majority of soft and biological materials, gels, and tissues are made from micrometer-size slender structures such as biofilaments and colloidal and molecular chains, which are believed to crucially control their mechanics. These constituents show intriguing extreme mechanics, mechanical instabilities, and plasticity, which, besides attracting significant theoretical attention, have not been studied experimentally and as such remain poorly understood. Here we investigate, by experiments, simulations, and theory, the mechanical instabilities of a slender self-assembled colloidal structure, observing a form of stochastic buckling where thermal fluctuations and associated entropic force effects are amplified in the vicinity of a buckling instability. We fully characterize how the persistence length and plasticity control the stochastic buckling transition, leading to intriguing higher-order buckling modes. These results elucidate the interplay of geometrical, thermal, and plastic interactions in the nonlinear mechanics of thermal self-assembled structures, crucial to the mechanical response and function of fiber-based soft and biological materials, as well as the rational design of micro- and nanoscale architectures.
DOI
Velocity-dependent optical forces and Maxwell’s demon
J. D. Franson
An atom placed in a focused laser beam will experience a dipole force due to the gradient in the interaction energy, which is analogous to the well-known optical tweezers effect. This force will be dependent on the velocity of the atom due to the Doppler effect, which could potentially be used to implement a Maxwell’s demon. Photon scattering and other forms of dissipation can be negligibly small, which would seem to contradict quantum information proofs that a Maxwell’s demon must dissipate a minimum amount of energy. We show that the velocity dependence of the dipole force is cancelled out by another force that is related to the gradient in the phase of the laser beam. As a result, a Maxwell’s demon cannot be implemented in this way.
DOI
DOI
Conformation and mechanical property of rpoS mRNA inhibitory stem studied by optical tweezers and X-ray scattering
Xinyao Hu , Xuanling Li , Lingna Yang, Yilin Zhu, Yunyu Shi, Yinmei Li, Haowei Wang , Qingguo Gong
3′ downstream inhibitory stem plays a crucial role in locking rpoS mRNA 5' untranslated region in a self-inhibitory state. Here, we used optical tweezers to study the unfolding/refolding of rpoS inhibitory stem in the absence and presence of Mg2+. We found adding Mg2+ decreased the free energy of the RNA junction without re-arranging its secondary structure, through confirming that this RNA formed a canonical RNA three-way junction. We suspected increased free energy might change the relative orientation of different stems of rpoS and confirmed this by small angle X-ray scattering. Such changed conformation may improve Hfq-bridged annealing between sRNA and rpoS RNA inhibitory stem. We established a convenient route to analyze the changes of RNA conformation and folding dynamics by combining optical tweezers with X-ray scattering methods. This route can be easily applied in the studies of other RNA structure and ligand-RNA.
DOI
3′ downstream inhibitory stem plays a crucial role in locking rpoS mRNA 5' untranslated region in a self-inhibitory state. Here, we used optical tweezers to study the unfolding/refolding of rpoS inhibitory stem in the absence and presence of Mg2+. We found adding Mg2+ decreased the free energy of the RNA junction without re-arranging its secondary structure, through confirming that this RNA formed a canonical RNA three-way junction. We suspected increased free energy might change the relative orientation of different stems of rpoS and confirmed this by small angle X-ray scattering. Such changed conformation may improve Hfq-bridged annealing between sRNA and rpoS RNA inhibitory stem. We established a convenient route to analyze the changes of RNA conformation and folding dynamics by combining optical tweezers with X-ray scattering methods. This route can be easily applied in the studies of other RNA structure and ligand-RNA.
DOI
Thursday, October 10, 2019
Polarization-Dependent Lateral Optical Force of Subwavelength-Diameter Optical Fibers
Xiangke Wang, Wanling Wu, Yipeng Lun, Huakang Yu, Qihua Xiong and Zhi-yuan Li
It is highly desirable to design optical devices with diverse optomechanical functions. Here, we investigate lateral optical force exerted on subwavelength-diameter (SD) optical fibers harnessed by input light modes with different polarizations. It is interesting to find that input light modes of circular or elliptical polarizations would bring about lateral optical force in new directions, which has not been observed in previous studies. By means of finite-difference time-domain (FDTD) simulations, detailed spatial distributions of the asymmetric transverse force density are revealed, meanwhile dependence of optical force on input light polarizations, fiber diameters, and inclination angles of fiber endfaces are all carefully discussed. It is believed that polarization-sensitive reflection, refraction, and diffraction of optical fields occur at the interface, i.e., fiber oblique endfaces, resulting in asymmetrically distributed optical fields and thereafter non-zero transverse optical force. We believe our new findings could be helpful for constructing future steerable optomechanical devices with more flexibility.
DOI
It is highly desirable to design optical devices with diverse optomechanical functions. Here, we investigate lateral optical force exerted on subwavelength-diameter (SD) optical fibers harnessed by input light modes with different polarizations. It is interesting to find that input light modes of circular or elliptical polarizations would bring about lateral optical force in new directions, which has not been observed in previous studies. By means of finite-difference time-domain (FDTD) simulations, detailed spatial distributions of the asymmetric transverse force density are revealed, meanwhile dependence of optical force on input light polarizations, fiber diameters, and inclination angles of fiber endfaces are all carefully discussed. It is believed that polarization-sensitive reflection, refraction, and diffraction of optical fields occur at the interface, i.e., fiber oblique endfaces, resulting in asymmetrically distributed optical fields and thereafter non-zero transverse optical force. We believe our new findings could be helpful for constructing future steerable optomechanical devices with more flexibility.
DOI
Optical tweezers assisted controllable formation and precise manipulation of microdroplet
Shuai Li, Chunguang Hu, Xiaoqing Gao, Guoteng Ma, Hongbin Li, Xiaodong Hu and Xiaotang Hu
We demonstrate an optical method to realize controllable formation and precise manipulation of microdroplet using optical tweezers (OT). With the irradiation of a highly focused laser into the mixture of inorganic phosphate buffered saline (PBS) and organic solvent isopropanol, a microdroplet was gradually formed at the center of the trap. The size and the growth rate of the microdroplet could be precisely controlled by regulating the laser power and the proportion of two solvents in the mixture. Furthermore, the microdroplet could be manipulated by OT to build microstructures on the slide. We also discuss the possible mechanism behind our observations and the potential usage of such discoveries.
DOI
We demonstrate an optical method to realize controllable formation and precise manipulation of microdroplet using optical tweezers (OT). With the irradiation of a highly focused laser into the mixture of inorganic phosphate buffered saline (PBS) and organic solvent isopropanol, a microdroplet was gradually formed at the center of the trap. The size and the growth rate of the microdroplet could be precisely controlled by regulating the laser power and the proportion of two solvents in the mixture. Furthermore, the microdroplet could be manipulated by OT to build microstructures on the slide. We also discuss the possible mechanism behind our observations and the potential usage of such discoveries.
DOI
Three-dimensional crystals of adaptive knots
Jung-Shen B. Tai, Ivan I. Smalyukh
Starting with Gauss and Kelvin, knots in fields were postulated to behave like particles, but experimentally they were found only as transient features or required complex boundary conditions to exist and could not self-assemble into three-dimensional crystals. We introduce energetically stable, micrometer-sized knots in helical fields of chiral liquid crystals. While spatially localized and freely diffusing in all directions, they resemble colloidal particles and atoms, self-assembling into crystalline lattices with open and closed structures. These knots are robust and topologically distinct from the host medium, though they can be morphed and reconfigured by weak stimuli under conditions such as those in displays. A combination of energy-minimizing numerical modeling and optical imaging uncovers the internal structure and topology of individual helical field knots and the various hierarchical crystalline organizations that they form.
DOI
Starting with Gauss and Kelvin, knots in fields were postulated to behave like particles, but experimentally they were found only as transient features or required complex boundary conditions to exist and could not self-assemble into three-dimensional crystals. We introduce energetically stable, micrometer-sized knots in helical fields of chiral liquid crystals. While spatially localized and freely diffusing in all directions, they resemble colloidal particles and atoms, self-assembling into crystalline lattices with open and closed structures. These knots are robust and topologically distinct from the host medium, though they can be morphed and reconfigured by weak stimuli under conditions such as those in displays. A combination of energy-minimizing numerical modeling and optical imaging uncovers the internal structure and topology of individual helical field knots and the various hierarchical crystalline organizations that they form.
DOI
Varying crosslinking motifs drive the mesoscale mechanics of actin-microtubule composites
Shea N. Ricketts, Madison L. Francis, Leila Farhadi, Michael J. Rust, Moumita Das, Jennifer L. Ross & Rae M. Robertson-Anderson
The cytoskeleton precisely tunes its mechanics by altering interactions between semiflexible actin filaments, rigid microtubules, and crosslinking proteins. We use optical tweezers microrheology and confocal microscopy to characterize how varying crosslinking motifs impact the mesoscale mechanics and mobility of actin-microtubule composites. We show that, upon subtle changes in crosslinking patterns, composites can exhibit two distinct classes of force response – primarily elastic versus more viscous. For example, a composite in which actin and microtubules are crosslinked to each other but not to themselves is markedly more elastic than one in which both filaments are independently crosslinked. Notably, this distinction only emerges at mesoscopic scales in response to nonlinear forcing, whereas varying crosslinking motifs have little impact on the microscale mechanics and mobility. Our unexpected scale-dependent results not only inform the physics underlying key cytoskeleton processes and structures, but, more generally, provide valuable perspective to materials engineering endeavors focused on polymer composites.
DOI
The cytoskeleton precisely tunes its mechanics by altering interactions between semiflexible actin filaments, rigid microtubules, and crosslinking proteins. We use optical tweezers microrheology and confocal microscopy to characterize how varying crosslinking motifs impact the mesoscale mechanics and mobility of actin-microtubule composites. We show that, upon subtle changes in crosslinking patterns, composites can exhibit two distinct classes of force response – primarily elastic versus more viscous. For example, a composite in which actin and microtubules are crosslinked to each other but not to themselves is markedly more elastic than one in which both filaments are independently crosslinked. Notably, this distinction only emerges at mesoscopic scales in response to nonlinear forcing, whereas varying crosslinking motifs have little impact on the microscale mechanics and mobility. Our unexpected scale-dependent results not only inform the physics underlying key cytoskeleton processes and structures, but, more generally, provide valuable perspective to materials engineering endeavors focused on polymer composites.
DOI
Mechanosensing through immunoreceptors
Cheng Zhu, Wei Chen, Jizhong Lou, William Rittase & Kaitao Li
The immune response is orchestrated by a variety of immune cells. The function of each cell is determined by the collective signals from various immunoreceptors, whose expression and activity depend on the developmental stages of the cell and its environmental context. Recent studies have highlighted the presence of mechanical force on several immunoreceptor–ligand pairs and the important role of force in regulating their interaction and function. In this Perspective, we use the T cell antigen receptor as an example with which to review the current understanding of the mechanosensing properties of immunoreceptors. We discuss the types of forces that immunoreceptors may encounter and the effects of force on ligand bonding, conformational change and the triggering of immunoreceptors, as well as the effects of force on the downstream signal transduction, cell-fate decisions and effector function of immune cells.
DOI
The immune response is orchestrated by a variety of immune cells. The function of each cell is determined by the collective signals from various immunoreceptors, whose expression and activity depend on the developmental stages of the cell and its environmental context. Recent studies have highlighted the presence of mechanical force on several immunoreceptor–ligand pairs and the important role of force in regulating their interaction and function. In this Perspective, we use the T cell antigen receptor as an example with which to review the current understanding of the mechanosensing properties of immunoreceptors. We discuss the types of forces that immunoreceptors may encounter and the effects of force on ligand bonding, conformational change and the triggering of immunoreceptors, as well as the effects of force on the downstream signal transduction, cell-fate decisions and effector function of immune cells.
DOI
Raman Spectroscopy Study of Curvature-Mediated Lipid Packing and Sorting in Single Lipid Vesicles
Liam Collard, Faris Sinjab, Ioan Notingher
Cellular plasma membrane deformability and stability is important in a range of biological processes. Changes in local curvature of the membrane affect the lateral movement of lipids, affecting the biophysical properties of the membrane. An integrated holographic optical tweezers and Raman microscope was used to investigate the effect of curvature gradients induced by optically stretching individual giant unilamellar vesicles (GUVs) on lipid packing and lateral segregation of cholesterol in the bilayer. The spatially resolved Raman analysis enabled detection of induced phase separation and changes in lipid ordering in individual GUVs. Using deuterated cholesterol, the changes in lipid ordering and phase separation were linked to lateral sorting of cholesterol in the stretched GUVs. Stretching the GUVs in the range of elongation factors 1–1.3 led to an overall decrease in cholesterol concentration at the edges compared to the center of stretched GUVs. The Raman spectroscopy results were consistent with a model of the bilayer accounting for cholesterol sorting in both bilayer leaflets, with a compositional asymmetry of 0.63 ± 0.04 in favor of the outer leaflet. The results demonstrate the potential of the integrated holographic optical tweezers-Raman technique to induce deformations to individual lipid vesicles and to simultaneously provide quantitative and spatially resolved molecular information. Future studies can extend to include more realistic models of cell membranes and potentially live cells.
DOI
Cellular plasma membrane deformability and stability is important in a range of biological processes. Changes in local curvature of the membrane affect the lateral movement of lipids, affecting the biophysical properties of the membrane. An integrated holographic optical tweezers and Raman microscope was used to investigate the effect of curvature gradients induced by optically stretching individual giant unilamellar vesicles (GUVs) on lipid packing and lateral segregation of cholesterol in the bilayer. The spatially resolved Raman analysis enabled detection of induced phase separation and changes in lipid ordering in individual GUVs. Using deuterated cholesterol, the changes in lipid ordering and phase separation were linked to lateral sorting of cholesterol in the stretched GUVs. Stretching the GUVs in the range of elongation factors 1–1.3 led to an overall decrease in cholesterol concentration at the edges compared to the center of stretched GUVs. The Raman spectroscopy results were consistent with a model of the bilayer accounting for cholesterol sorting in both bilayer leaflets, with a compositional asymmetry of 0.63 ± 0.04 in favor of the outer leaflet. The results demonstrate the potential of the integrated holographic optical tweezers-Raman technique to induce deformations to individual lipid vesicles and to simultaneously provide quantitative and spatially resolved molecular information. Future studies can extend to include more realistic models of cell membranes and potentially live cells.
DOI
Near‐Field Optical Tweezers for Chemistry and Biology
Dr. Sheng Hu, Ziwei Liao, Lu Cai, Xiaoxiao Jiang
Near‐field optical tweezer techniques have been essential for the development for manipulation and identification of micro‐ and nano‐particles in lab on a chip systems. Optical waveguide, which has been exploited as a fascinating manipulation approach, affords valuable insights into the biology of the specific type of cells and potential for enhanced Raman spectroscopy. Specific waveguides and its own structures in near‐field optical techniques can be used to trap, transport, sort, levitate, and deform a micro‐ and nano‐particle in colloid chemistry and pharmaceutical biology. Given the precise manipulation and trapping stiffness with low light power, a diverse range of micro/nanostructured optical fiber, silicon‐on‐insulator resonator, photonic crystal cavity, and plasmonic waveguide have been proposed in many theoretical models and experimental observations, including polystyrene bead, silica bead, metal particle, erythrocyte, virus, and DNA. Taken into account the optical pressure fundamental, fabrication process, and structure categorization, this review describes recent mainstream developments and future perspectives of near‐field optical tweezers for chemical and biological applications.
DOI
Near‐field optical tweezer techniques have been essential for the development for manipulation and identification of micro‐ and nano‐particles in lab on a chip systems. Optical waveguide, which has been exploited as a fascinating manipulation approach, affords valuable insights into the biology of the specific type of cells and potential for enhanced Raman spectroscopy. Specific waveguides and its own structures in near‐field optical techniques can be used to trap, transport, sort, levitate, and deform a micro‐ and nano‐particle in colloid chemistry and pharmaceutical biology. Given the precise manipulation and trapping stiffness with low light power, a diverse range of micro/nanostructured optical fiber, silicon‐on‐insulator resonator, photonic crystal cavity, and plasmonic waveguide have been proposed in many theoretical models and experimental observations, including polystyrene bead, silica bead, metal particle, erythrocyte, virus, and DNA. Taken into account the optical pressure fundamental, fabrication process, and structure categorization, this review describes recent mainstream developments and future perspectives of near‐field optical tweezers for chemical and biological applications.
DOI
Photonic spin Hall effect on an ellipsoidal Rayleigh particle in scattering far-field
Wenjia Li, Jianlong Liu, Yang Gao, Keya Zhou, and Shutian Liu
We present the photonic spin Hall effect on an ellipsoidal Rayleigh particle, which amounts to a polarization-dependent shift in scattering far-field. Based on the dipole model, we demonstrate that such shift is unavoidable when the light incidence is inclined with respect to the main axis of the ellipsoidal Rayleigh particle. The result has general validity and can be applied to metal and dielectric materials. In addition, the photonic spin Hall effect also manifests itself in the optical force and torque exerted on the particle, which is promising for precision metrology, spin-optics devices and optical driven micro-machines. Due to wide existence of the Rayleigh particles in nature, we believe that our findings might provide a useful toolset for investigating polarization-dependent scattering of particles.
DOI
We present the photonic spin Hall effect on an ellipsoidal Rayleigh particle, which amounts to a polarization-dependent shift in scattering far-field. Based on the dipole model, we demonstrate that such shift is unavoidable when the light incidence is inclined with respect to the main axis of the ellipsoidal Rayleigh particle. The result has general validity and can be applied to metal and dielectric materials. In addition, the photonic spin Hall effect also manifests itself in the optical force and torque exerted on the particle, which is promising for precision metrology, spin-optics devices and optical driven micro-machines. Due to wide existence of the Rayleigh particles in nature, we believe that our findings might provide a useful toolset for investigating polarization-dependent scattering of particles.
DOI
Tuesday, October 8, 2019
Raman Tweezers Microspectroscopy of Functionalized 4.2 nm Diameter CdSe Nanocrystals in Water Reveals Changed Ligand Vibrational Modes by a Metal Cation
Randa Mrad, Sergei G. Kruglik, Nassim Ben Brahim, Rafik Ben Chaâbane, Michel Negrerie
We demonstrated the possibility of acquiring Raman spectra of colloidal quantum dots (QDs) at low concentration in water with a size as small as 2.5 nm in diameter using Raman tweezers microspectroscopy. We measured the spectra of CdSe QDs capped with thioglycerol and with l-cysteine. This technique was applied to probe the interaction between Co2+ and Cys–CdSe QDs whose fluorescence emission is quenched in the presence of this metal cation. The quenching mechanism was so far hypothetical. The Raman spectra of Cys–CdSe QDs recorded in the absence and in the presence of Co2+ demonstrated the binding of Co2+ cations to the carboxylate groups of the l-cysteine ligand grafted on the surface of the 4.2 nm CdSe QDs. The frequency of modes for the grafted ligand is changed with respect to the free ligand in solution. Considering the vibrational coupling between the excitonic state and the ligand, we inferred that the binding of a metal cation to the grafted ligand modifies this coupling, so that exciton relaxation through crystal defects is favored. This result rationalizes the fluorescence quenching observed during the metal cation–QD interaction.
DOI
We demonstrated the possibility of acquiring Raman spectra of colloidal quantum dots (QDs) at low concentration in water with a size as small as 2.5 nm in diameter using Raman tweezers microspectroscopy. We measured the spectra of CdSe QDs capped with thioglycerol and with l-cysteine. This technique was applied to probe the interaction between Co2+ and Cys–CdSe QDs whose fluorescence emission is quenched in the presence of this metal cation. The quenching mechanism was so far hypothetical. The Raman spectra of Cys–CdSe QDs recorded in the absence and in the presence of Co2+ demonstrated the binding of Co2+ cations to the carboxylate groups of the l-cysteine ligand grafted on the surface of the 4.2 nm CdSe QDs. The frequency of modes for the grafted ligand is changed with respect to the free ligand in solution. Considering the vibrational coupling between the excitonic state and the ligand, we inferred that the binding of a metal cation to the grafted ligand modifies this coupling, so that exciton relaxation through crystal defects is favored. This result rationalizes the fluorescence quenching observed during the metal cation–QD interaction.
DOI
Single-molecule trapping and measurement in solution
Maria I Bespalova, Sushanta Mahanta, Madhavi Krishnan
Trapping of a single molecule in the fluid phase was realized decades following developments in the gas-phase, because in some ways the solution phase posed a greater challenge. The key issues have since been addressed by several different means; techniques to confine nanometer scale entities in solution now abound and are gaining traction in a variety of single molecule studies. Available methods range from pure physical entrapment of a molecule on the one hand to electrokinetic and optical techniques, and approaches that exploit thermodynamic principles on the other. Some trapping techniques have also opened up new avenues to highly precise, accurate measurements of molecular physical properties in solution.
DOI
Trapping of a single molecule in the fluid phase was realized decades following developments in the gas-phase, because in some ways the solution phase posed a greater challenge. The key issues have since been addressed by several different means; techniques to confine nanometer scale entities in solution now abound and are gaining traction in a variety of single molecule studies. Available methods range from pure physical entrapment of a molecule on the one hand to electrokinetic and optical techniques, and approaches that exploit thermodynamic principles on the other. Some trapping techniques have also opened up new avenues to highly precise, accurate measurements of molecular physical properties in solution.
DOI
Radiation forces study of a Laguerre Gaussian beam type TEM∗01 on a dielectric sphere in the Rayleigh scattering regime
Darby Paez Amaya, Martha Lucía Molina Prado, Néstor Alonso Arias Hernández
From the invention of the Optical Tweezer (OT) in 1986, these devices have been considered as high-level tools for research in the areas such as biology and microbiology. A theoretical study obtaining equations for gradient and scattering forces that exert an OT when the illumination beam is a doughnut-shaped mode TEM∗01 linearly polarized is realized. This work focuses on the behavior of radiation forces on a dielectric sphere in the Rayleigh regime. In order to facilitate the phenomenological analysis of the behavior of the radiation forces a graphical user interface is created.
DOI
From the invention of the Optical Tweezer (OT) in 1986, these devices have been considered as high-level tools for research in the areas such as biology and microbiology. A theoretical study obtaining equations for gradient and scattering forces that exert an OT when the illumination beam is a doughnut-shaped mode TEM∗01 linearly polarized is realized. This work focuses on the behavior of radiation forces on a dielectric sphere in the Rayleigh regime. In order to facilitate the phenomenological analysis of the behavior of the radiation forces a graphical user interface is created.
DOI
Enhancing Double-Beam Laser Tweezers Raman Spectroscopy (LTRS) for the Photochemical Study of Individual Airborne Microdroplets
Jovanny A. Gómez Castaño, Luc Boussekey, Jean P. Verwaerde, Myriam Moreau and Yeny A. Tobón
A new device and methodology for vertically coupling confocal Raman microscopy with optical tweezers for the in situ physico- and photochemical studies of individual microdroplets (Ø ≤ 10 µm) levitated in air is presented. The coupling expands the spectrum of studies performed with individual particles using laser tweezers Raman spectroscopy (LTRS) to photochemical processes and spatially resolved Raman microspectroscopy on airborne aerosols. This is the first study to demonstrate photochemical studies and Raman mapping on optically levitated droplets. By using this configuration, photochemical reactions in aerosols of atmospheric interest can be studied on a laboratory scale under realistic conditions of gas-phase composition and relative humidity. Likewise, the distribution of photoproducts within the drop can also be observed with this setup. The applicability of the coupling system was tested by studying the photochemical behavior of microdroplets (5 µm < Ø < 8 µm) containing an aqueous solution of sodium nitrate levitated in air and exposed to narrowed UV radiation (254 ± 25 nm). Photolysis of the levitated NaNO3 microdroplets presented photochemical kinetic differences in comparison with larger NaNO3 droplets (40 µm < Ø < 80 µm), previously photolyzed using acoustic traps, and heterogeneity in the distribution of the photoproducts within the drop.
DOI
A new device and methodology for vertically coupling confocal Raman microscopy with optical tweezers for the in situ physico- and photochemical studies of individual microdroplets (Ø ≤ 10 µm) levitated in air is presented. The coupling expands the spectrum of studies performed with individual particles using laser tweezers Raman spectroscopy (LTRS) to photochemical processes and spatially resolved Raman microspectroscopy on airborne aerosols. This is the first study to demonstrate photochemical studies and Raman mapping on optically levitated droplets. By using this configuration, photochemical reactions in aerosols of atmospheric interest can be studied on a laboratory scale under realistic conditions of gas-phase composition and relative humidity. Likewise, the distribution of photoproducts within the drop can also be observed with this setup. The applicability of the coupling system was tested by studying the photochemical behavior of microdroplets (5 µm < Ø < 8 µm) containing an aqueous solution of sodium nitrate levitated in air and exposed to narrowed UV radiation (254 ± 25 nm). Photolysis of the levitated NaNO3 microdroplets presented photochemical kinetic differences in comparison with larger NaNO3 droplets (40 µm < Ø < 80 µm), previously photolyzed using acoustic traps, and heterogeneity in the distribution of the photoproducts within the drop.
DOI
Optical deformation of single aerosol particles
Aidan Rafferty, Kyle Gorkowski, Andreas Zuend, and Thomas C. Preston
Advancements in designing complex models for atmospheric aerosol science and aerosol–cloud interactions rely vitally on accurately measuring the physicochemical properties of microscopic particles. Optical tweezers are a laboratory-based platform that can provide access to such measurements as they are able to isolate individual particles from an ensemble. The surprising ability of a focused beam of light to trap and hold a single particle can be conceptually understood in the ray optics regime using momentum transfer and Newton’s second law. The same radiation pressure that results in stable trapping will also exert a deforming optical stress on the surface of the particle. For micron-sized aqueous droplets held in the air, the deformation will be on the order of a few nanometers or less, clearly not observable through optical microscopy. In this study, we utilize cavity-enhanced Raman scattering and a phenomenon known as thermal locking to measure small deformations in optically trapped droplets. With the aid of light-scattering calculations and a model that balances the hydrostatic pressure, surface tension, and optical pressure across the air–droplet interface, we can accurately determine surface tension from our measurements. Our approach is applied to 2 systems of atmospheric interest: aqueous organic and inorganic aerosol.
DOI
Advancements in designing complex models for atmospheric aerosol science and aerosol–cloud interactions rely vitally on accurately measuring the physicochemical properties of microscopic particles. Optical tweezers are a laboratory-based platform that can provide access to such measurements as they are able to isolate individual particles from an ensemble. The surprising ability of a focused beam of light to trap and hold a single particle can be conceptually understood in the ray optics regime using momentum transfer and Newton’s second law. The same radiation pressure that results in stable trapping will also exert a deforming optical stress on the surface of the particle. For micron-sized aqueous droplets held in the air, the deformation will be on the order of a few nanometers or less, clearly not observable through optical microscopy. In this study, we utilize cavity-enhanced Raman scattering and a phenomenon known as thermal locking to measure small deformations in optically trapped droplets. With the aid of light-scattering calculations and a model that balances the hydrostatic pressure, surface tension, and optical pressure across the air–droplet interface, we can accurately determine surface tension from our measurements. Our approach is applied to 2 systems of atmospheric interest: aqueous organic and inorganic aerosol.
DOI
Non-Occlusive Retinal Vascular Inflammation and Role of Red Blood Cell Deformability in Birdshot Chorioretinopathy
Rupesh Agrawal, Bryan Ang, Praveen Kumar Balne, Christopher Richards, Thomas Smart, João Cardoso, David Shima, Philip H. Jones & Carlos Pavesio
To investigate differences in red blood cell (RBC) deformability between birdshot chorioretinopathy (BCR) subjects and matched controls, and to postulate its relationship with lack of vascular occlusion in BCR. In a single center, prospective, non-randomized mechanistic study, blood samples were collected from eight healthy controls and nine BCR patients, and subjected to biochemical and hematological tests, as well as RBC indices assessment using dual-beam optical tweezers. The mean age of the controls was 52.37 ± 10.70 years and BCR patients was 53.44 ± 12.39 years. Initial cell size (Io) for the controls was 8.48 ± 0.25 μm and 8.87 ± 0.31 μm for BCR RBCs (p = 0.014). The deformability index (DI) for the controls was 0.066 ± 0.02 and that for BCR RBCs was 0.063 ± 0.03 (p = 0.441). There was no statistically significant difference in DI between RBCs from BCR and healthy controls. This may explain the rare occurrence of retinal vascular occlusion despite the underlying vasculitic pathophysiology of BCR.
DOI
To investigate differences in red blood cell (RBC) deformability between birdshot chorioretinopathy (BCR) subjects and matched controls, and to postulate its relationship with lack of vascular occlusion in BCR. In a single center, prospective, non-randomized mechanistic study, blood samples were collected from eight healthy controls and nine BCR patients, and subjected to biochemical and hematological tests, as well as RBC indices assessment using dual-beam optical tweezers. The mean age of the controls was 52.37 ± 10.70 years and BCR patients was 53.44 ± 12.39 years. Initial cell size (Io) for the controls was 8.48 ± 0.25 μm and 8.87 ± 0.31 μm for BCR RBCs (p = 0.014). The deformability index (DI) for the controls was 0.066 ± 0.02 and that for BCR RBCs was 0.063 ± 0.03 (p = 0.441). There was no statistically significant difference in DI between RBCs from BCR and healthy controls. This may explain the rare occurrence of retinal vascular occlusion despite the underlying vasculitic pathophysiology of BCR.
DOI
Cryo-EM structure of the CFA/I pilus rod
Weili Zheng, Magnus Andersson, Narges Mortezaei, Esther Bullitt and Edward Egelman
Enterotoxigenic Escherichia coli (ETEC) are common agents of diarrhea for travelers and a major cause of mortality in children in developing countries. To attach to intestinal cells ETEC express colonization factors, among them CFA/I, which are the most prevalent factors and are the archetypical representative of class 5 pili. The helical quaternary structure of CFA/I can be unwound under tensile force and it has been shown that this mechanical property helps bacteria to withstand shear forces from fluid motion. We report in this work the CFA/I pilus structure at 4.3 Å resolution from electron cryomicroscopy (cryo-EM) data, and report details of the donor strand complementation. The CfaB pilins modeled into the cryo-EM map allow us to identify the buried surface area between subunits, and these regions are correlated to quaternary structural stability in class 5 and chaperone–usher pili. In addition, from the model built using the EM structure we also predicted that residue 13 (proline) of the N-terminal β-strand could have a major impact on the filament's structural stability. Therefore, we used optical tweezers to measure and compare the stability of the quaternary structure of wild type CFA/I and a point-mutated CFA/I with a propensity for unwinding. We found that pili with this mutated CFA/I require a lower force to unwind, supporting our hypothesis that Pro13 is important for structural stability. The high-resolution CFA/I pilus structure presented in this work and the analysis of structural stability will be useful for the development of novel antimicrobial drugs that target adhesion pili needed for initial attachment and sustained adhesion of ETEC.
DOI
Enterotoxigenic Escherichia coli (ETEC) are common agents of diarrhea for travelers and a major cause of mortality in children in developing countries. To attach to intestinal cells ETEC express colonization factors, among them CFA/I, which are the most prevalent factors and are the archetypical representative of class 5 pili. The helical quaternary structure of CFA/I can be unwound under tensile force and it has been shown that this mechanical property helps bacteria to withstand shear forces from fluid motion. We report in this work the CFA/I pilus structure at 4.3 Å resolution from electron cryomicroscopy (cryo-EM) data, and report details of the donor strand complementation. The CfaB pilins modeled into the cryo-EM map allow us to identify the buried surface area between subunits, and these regions are correlated to quaternary structural stability in class 5 and chaperone–usher pili. In addition, from the model built using the EM structure we also predicted that residue 13 (proline) of the N-terminal β-strand could have a major impact on the filament's structural stability. Therefore, we used optical tweezers to measure and compare the stability of the quaternary structure of wild type CFA/I and a point-mutated CFA/I with a propensity for unwinding. We found that pili with this mutated CFA/I require a lower force to unwind, supporting our hypothesis that Pro13 is important for structural stability. The high-resolution CFA/I pilus structure presented in this work and the analysis of structural stability will be useful for the development of novel antimicrobial drugs that target adhesion pili needed for initial attachment and sustained adhesion of ETEC.
DOI
Monday, October 7, 2019
Synthetic optical vortex beams from the analogous trajectory change of an artificial satellite
Haiping Wang, Liqin Tang, Jina Ma, Xiuyan Zheng, Daohong Song, Yi Hu, Yigang Li, and Zhigang Chen
We propose a method to generate specially shaped high-order singular beams of pre-designed intensity distributions. Such a method does not a priori assume a phase formula, but rather relies on the “cake-cutting and assembly” approach to achieve the azimuthal phase gradient for beam shaping, inspired by the orbital motion trajectory change of an artificial satellite. Based on our method, several typical vortex beams with desired intensity patterns are experimentally generated. As an example, we realize optical trapping and transportation of microorganisms with a triangle-shaped vortex beam, demonstrating the applicability of such unconventional vortex beams in optical trapping and manipulation.
DOI
We propose a method to generate specially shaped high-order singular beams of pre-designed intensity distributions. Such a method does not a priori assume a phase formula, but rather relies on the “cake-cutting and assembly” approach to achieve the azimuthal phase gradient for beam shaping, inspired by the orbital motion trajectory change of an artificial satellite. Based on our method, several typical vortex beams with desired intensity patterns are experimentally generated. As an example, we realize optical trapping and transportation of microorganisms with a triangle-shaped vortex beam, demonstrating the applicability of such unconventional vortex beams in optical trapping and manipulation.
DOI
Measurement of Van der Waals force using oscillating optical tweezers
Avijit Kundu, Shuvojit Paul, Soumitro Banerjee, and Ayan Banerjee
We employ oscillating optical tweezers as a probe to measure the surface forces between polystyrene and silica. Thus, we modulate a trapped polystyrene particle with an external sinusoidal force in close proximity (∼80 nm) of a silica surface. The particle motion is influenced by several factors which include an increased drag force according to Faxen's correction, a spurious force that comes into play due to the diffusion coefficient of the medium becoming position dependent, and finally, the London-Van der Waals (LVdW) force which becomes substantial when the particle approaches the surface. By accounting for the other forces from the analytically known results, we are able to directly quantify the LVdW force from the experimentally measured amplitude of the oscillating particle. Thereby, we determine the Hamaker constant H for the LVdW force between polystyrene and silica, and obtain a good agreement with the value reported in the literature. Our method is general in nature and can be extended toward probing other surface effects or other interaction forces using oscillating optical tweezers.
DOI
We employ oscillating optical tweezers as a probe to measure the surface forces between polystyrene and silica. Thus, we modulate a trapped polystyrene particle with an external sinusoidal force in close proximity (∼80 nm) of a silica surface. The particle motion is influenced by several factors which include an increased drag force according to Faxen's correction, a spurious force that comes into play due to the diffusion coefficient of the medium becoming position dependent, and finally, the London-Van der Waals (LVdW) force which becomes substantial when the particle approaches the surface. By accounting for the other forces from the analytically known results, we are able to directly quantify the LVdW force from the experimentally measured amplitude of the oscillating particle. Thereby, we determine the Hamaker constant H for the LVdW force between polystyrene and silica, and obtain a good agreement with the value reported in the literature. Our method is general in nature and can be extended toward probing other surface effects or other interaction forces using oscillating optical tweezers.
DOI
Application of STED imaging for chromatin studies
Georgij Kostiuk, Jonas Bucevičius, Rūta Gerasimaitė and Gražvydas Lukinavičius
Chromatin is the information center of a cell. It comprises proteins and nucleic acids that form a highly complex and dynamic structure within the nucleus. Its multiple organization levels span from micrometre to nanometre scale. For many years, the lower levels of chromatin organization have been beyond the resolution limit of fluorescent microscopy, thus impeding research on nucleus architecture, transcription, translation and DNA repair. Recent development in super-resolution fluorescence microscopy enables us to more easily observe objects at the nanometre scale and allows the study of complex cellular structures at unprecedented detail. This review focuses on the application of stimulated emission depletion microscopy for imaging two main components of the chromatin-DNA and the proteins interacting with it.
DOI
Chromatin is the information center of a cell. It comprises proteins and nucleic acids that form a highly complex and dynamic structure within the nucleus. Its multiple organization levels span from micrometre to nanometre scale. For many years, the lower levels of chromatin organization have been beyond the resolution limit of fluorescent microscopy, thus impeding research on nucleus architecture, transcription, translation and DNA repair. Recent development in super-resolution fluorescence microscopy enables us to more easily observe objects at the nanometre scale and allows the study of complex cellular structures at unprecedented detail. This review focuses on the application of stimulated emission depletion microscopy for imaging two main components of the chromatin-DNA and the proteins interacting with it.
DOI
Stiffness of Cargo–Motor Linkage Tunes Myosin VI Motility and Response to Load
Rachit Shrivastava, Ashim Rai, Murti Salapaka Sivaraj Sivaramakrishnan
We examine the effect of cargo–motor linkage stiffness on the mechanobiological properties of the molecular motor myosin VI. We use the programmability of DNA nanostructures to modulate cargo–motor linkage stiffness and combine it with high-precision optical trapping measurements to measure the effect of linkage stiffness on the motile properties of myosin VI. Our results reveal that a stiff cargo–motor linkage leads to shorter step sizes and load-induced anchoring of myosin VI, while a flexible linkage results in longer steps with frequent detachments from the actin filament under load. Our findings suggest a novel regulatory mechanism for tuning the dual cellular roles of the anchor and transporter ascribed to myosin VI.
DOI
We examine the effect of cargo–motor linkage stiffness on the mechanobiological properties of the molecular motor myosin VI. We use the programmability of DNA nanostructures to modulate cargo–motor linkage stiffness and combine it with high-precision optical trapping measurements to measure the effect of linkage stiffness on the motile properties of myosin VI. Our results reveal that a stiff cargo–motor linkage leads to shorter step sizes and load-induced anchoring of myosin VI, while a flexible linkage results in longer steps with frequent detachments from the actin filament under load. Our findings suggest a novel regulatory mechanism for tuning the dual cellular roles of the anchor and transporter ascribed to myosin VI.
DOI
Transcription: Overcoming chromatin barriers
Babette E de Jong, John van Noort
Single-molecule experiments reveal the dynamics of transcription through a nucleosome with single-base-pair accuracy.
DOI
Single-molecule experiments reveal the dynamics of transcription through a nucleosome with single-base-pair accuracy.
DOI
Subscribe to:
Posts (Atom)