Dena Izadi, Yujie Chen, Miles L Whitmore, Joseph D Slivka, Kevin Ching, Lisa J. Lapidus, and Matthew J Comstock
Over the past two decades, one of the standard models of protein folding has been the “two-state” model, in which a protein only resides in the folded or fully unfolded states with a single pathway between them. Recent advances in spatial and temporal resolution of biophysical measurements have revealed ‘beyond two-state’ complexity in protein folding, even for small, single-domain proteins. In this work, we used high-resolution optical tweezers to investigate the folding/unfolding kinetics of the B1 domain of immunoglobulin-binding protein G (GB1), a well-studied model system. Experiments were performed for GB1 both in and out of equilibrium using force spectroscopy. When the force was gradually ramped, simple single-peak folding force distributions were observed, while multiple rupture peaks were seen in the unfolding force distributions, consistent with multiple force-dependent parallel unfolding pathways. Force-dependent folding and unfolding rate constants were directly determined by both force-jump and fixed-trap measurements. Monte Carlo modeling using these rate constants was in good agreement with the force ramp data. The unfolding rate constants exhibited two different behaviors at low vs high force. At high force the unfolding rate constant increased with increasing force, as previously reported by high force, high pulling speed force ramp measurements. But at low force the situation reversed and the unfolding rate constant decreased with increasing force. Taken together, these data indicate that this small protein has multiple distinct pathways to the native state on the free energy landscape.
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Jie Yang, Ruo-Nan Zhang, Dong-Jie Liu, Xu Zhou, Tatsuya Shoji, Yasuyuki Tsuboi and Hu Yan
We have immobilized poly(ethylene glycol) (PEG) on surface of poly(lactic-co-glycolic acid) (PLGA) nanoparticles by two different chemical methods, i.e., SOCl2 halogennate-alcoholysis and DCC dehydration. The immobilized PLGA nanoparticles were characterized by DLS, 1H NMR, FT-IR and laser trapping/confocal Raman spectroscopic technique. As a result, especially the Raman spectra which were measured after optically trapping less than ten individual nanoparticles in solution, indicated that the PLGA nanoparticles were successfully immobilized with the PEG by the chemical method.
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Faris Sinjab, Dennis Awuah, Graham Gibson, Miles Padgett, Amir M. Ghaemmaghami, and Ioan Notingher
We present a new approach for combining holographic optical tweezers with confocal Raman spectroscopy. Multiple laser foci, generated using a liquid-crystal spatial light modulator, are individually used for both optical trapping and excitation of spontaneous Raman spectroscopy from trapped objects. Raman scattering from each laser focus is spatially filtered using reflective apertures on a digital micro-mirror device, which can be reconfigured with flexible patterns at video rate. We discuss operation of the instrument, and performance and viability considerations for biological measurements. We then demonstrate the capability of the instrument for fast, flexible, and interactive manipulation with molecular measurement of interacting live cell systems.
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Qiushi Huang, Sifeng Mao, Mashooq Khan and Jin-Ming Lin
Advances in microfluidic techniques have prompted researchers to study the inherent heterogeneity of single cells in cell populations. This would be helpful in the identification of major diseases and the design of personalized medicine. Different microfluidic approaches provide a variety of functions in the process of single-cell analysis. In this review, we take a broad overview of various microfluidic-based approaches for single-cell isolation, single-cell lysis, and single-cell analysis. Up-to-date flagship techniques and the pros and cons of these methods are discussed in detail.
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Xiao Zhang, Cui Song, Guanghui Ma and Wei Wei
As new materials, micro/nanoscale particles have been verified to have many novel and significant physicochemical properties as a result of their specific scale effects, and hence have received increasing amounts of attention. The understanding of the physiological effects of these particles on cells is fundamental to the biomedical field. Currently, it has been proven that the mechanical properties associated with the interaction between particles and cells dominate the final physiological response. Hence, it is vital and valuable to determine the mechanical features of particle–cell interactions. Based on these mechanical properties, many novel applications have been reported in the biomedical field, such as drug delivery, immune response, and cell mechanics. Here, we systematically summarize the mechanical measurement methods used to study particle–cell interactions and their subsequent applications regarding these mechanical properties. This work will further the understanding of particle-induced mechanical properties and the behavior of living cells as well as how such properties relate to cell function.
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Jicheng Wang, Chunyu Lu, Zheng-Da Hu, Chen Chen, Liang Pan, and Weiqiang Ding
We study the plasmonic properties of face-to-face phosphorene pairs, including their optical constraints and optical gradient forces. The symmetric and anti-symmetric plasmonic modes occur due to the strong anisotropic dispersion of phosphorene. Compared with the anti-symmetric mode, the symmetric mode has a stronger optical constraint and much larger gradient force. Especially, the optical constraint of the symmetric mode can even reach as high as 96% when the two phosphorene layers are along the armchair and zigzag direction respectively. We also propose a scheme of an ultra-small phase shifter using phosphorene-based photonic devices.
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Keiji Tanaka
Chalcogenide glasses including As2S3 and Se are known to exhibit a variety of photoinduced deformations such as volume expansions, anisotropic shape changes, and surface ripples. These deformations are produced by photoinduced viscous material flows which are caused by some driving forces, while the origins are controversial. We propose a guiding idea that the driving force arises from atomic and optical mechanisms; the former from structural disordering and intermolecular alignment and the latter from radiation force and torque. Previously proposed models such as Coulombic and electric-gradient forces are criticized. We also compare these deformations with those in azobenzene-containing organic materials.
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Azeem Ahmad, Vishesh Dubey, Vijay Raj Singh, Jean-Claude Tinguely, Cristina Ionica Øie, Deanna L. Wolfson, Dalip Singh Mehta, Peter T. C. So and Balpreet Singh Ahluwalia
Red blood cells (RBCs) have the ability to undergo morphological deformations during microcirculation, such as changes in surface area, volume and sphericity. Optical waveguide trapping is suitable for trapping, propelling and deforming large cell populations along the length of the waveguide. Bright field microscopy employed with waveguide trapping does not provide quantitative information about structural changes. Here, we have combined quantitative phase microscopy and waveguide trapping techniques to study changes in RBC morphology during planar trapping and transportation. By using interference microscopy, time-lapsed interferometric images of trapped RBCs were recorded in real-time and subsequently utilized to reconstruct optical phase maps. Quantification of the phase differences before and after trapping enabled study of the mechanical effects during planar trapping. During planar trapping, a decrease in the maximum phase values, an increase in the surface area and a decrease in the volume and sphericity of RBCs were observed. QPM was used to analyze the phase values for two specific regions within RBCs: the annular rim and the central donut. The phase value of the annular rim decreases whereas it increases for the central donut during planar trapping. These changes correspond to a redistribution of cytosol inside the RBC during planar trapping and transportation.
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Sandra M. Rodrigues, Joana S. Paiva, Rita S. R. Ribeiro, Olivier Soppera, João P. S. Cunha and Pedro A. S. Jorge
Optical fiber tweezers have been gaining prominence in several applications in Biology and Medicine. Due to their outstanding focusing abilities, they are able to trap and manipulate microparticles, including cells, needing any physical contact and with a low degree of invasiveness to the trapped cell. Recently, we proposed a fiber tweezer configuration based on a polymeric micro-lens on the top of a single mode fiber, obtained by a self-guided photopolymerization process. This configuration is able to both trap and identify the target through the analysis of short-term portions of the back-scattered signal. In this paper, we propose a variant of this fabrication method, capable of producing more robust fiber tips, which produce stronger trapping effects on targets by as much as two to ten fold. These novel lenses maintain the capability of distinguish the different classes of trapped particles based on the back-scattered signal. This novel fabrication method consists in the introduction of a multi mode fiber section on the tip of a single mode (SM) fiber. A detailed description of how relevant fabrication parameters such as the length of the multi mode section and the photopolymerization laser power can be tuned for different purposes (e.g., microparticles trapping only, simultaneous trapping and sensing) is also provided, based on both experimental and theoretical evidences.
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Jiunn-Woei Liaw, Chiao-Wei Chien, Kun-Chi Liu, Yun-Cheng Ku & Mao-Kuen Kuo
3D optical vortex trapping upon a polystyrene nanoparticle (NP) by a 1D gold dimer array is studied theoretically. The optical force field shows that the trapping mode can be contact or non-contact. For the former, the NP is attracted toward a corresponding dimer. For the latter, it is trapped toward a stagnation point of zero force with a 3D spiral trajectory, revealing optical vortex. Additionally the optical torque causes the NP to transversely spin, even though the system is irradiated by a linearly polarized light. The transverse spin-orbit interaction is manifested from the opposite helicities of the spin and spiral orbit. Along with the growth and decline of optical vortices the trapped NP performs a step-like motion, as the array continuously moves. Our results, in agreement with the previous experiment, identify the role of optical vortex in the near-field trapping of plasmonic nanostructure.
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Vladlen Shvedov, Konrad Cyprych, M. Yadira Salazar-Romero, Yana Izdebskaya, and Wieslaw Krolikowski
We study nonlinear propagation of light in colloidal suspension of metallic nanoparticles, in the regime of particles surface plasmon resonance. We show that the propagation exhibits features typical for purely defocusing media and the observed spatial confinement is not a real self-trapping, as for solitons, but rather than is caused by the phase modulation of the beam via nonlocal defocusing nonlinearity. We also show that the light-induced refractive index change in the suspension leads to stabilization of structured light beams. In particular, we demonstrate a stable nonlinear propagation of bright ring beams with complex states of polarization, including practically important radial and azimuthal states.
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N. Sepehri Javan, R. Naderali, M. Hosseinpour Azad, and M. N. Najafi
Self-focusing of laser beam propagating through a dissipative suspension of metallic nanoparticles is studied. The impact of the imaginary part of nanoparticle polarizability on the optical force and consequently on the particles' rearrangement in the presence of laser fields with an initial Gaussian profile is considered. It is shown that the absorption of laser leads to the creation of optical force along the wave propagation direction which can cause longitudinal distribution of nanoparticles. Considering fifth order nonlinearity of wave amplitude that comes from the simultaneous considering of normal Kerr effect produced by the inhomogeneity of the refractive index resulted from the ponderomotive force acting on conducting electrons and artificial Kerr nonlinearity caused by the polarization optical force acting on electrically polarized particles, set of differential equations describing nonlinear steady-state evolution of laser beam is derived by using a non-paraxial method. Dynamics of laser for different frequencies is investigated and optimum frequency range for improving focusing property is determined. It is shown that the artificial Kerr effect causes localization of particles near the propagation axis that can substantially reduce the threshold power for occurring self-focusing in comparison with plasma and other rigid plasmonic systems.
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Reuven Gordon
Nanoaperture optical tweezers extend the range of optical tweezers to dielectric particles below 50 nm in size. This allows for optical trapping of proteins, DNA fragments and other biomolecules, as well as small viruses. With this label-free, tether-free approach proteins have been trapped, sized and their conformational changes observed in real-time. The molecular weight of proteins in the nanoaperture trap was determined from their Brownian motion statistics. This is useful for analysis of heterogeneous solutions: since this is a single molecule technique, it can be operated in “dirty” solutions with minimal sample preparation. The acoustic modes of proteins, DNA fragments and other trapped nanoparticles can be measured using a nanoaperture optical tweezer with two lasers creating a GHz to THz beat frequency. Interactions between proteins and DNA, small molecules (i.e., binding) and other proteins have also been demonstrated. This single molecule technique allows for measuring the dissociation constants of small molecules binding to proteins, both at equilibrium and at the single molecule level. For DNA fragments in the trap, it has been shown that the protein p53 can suppress unzipping and mutant p53 is ineffective to do so. This is promising for the discovery of drugs that effectively restore the function to p53. Integration of nanoapertures on the ends of fibers allows for translocation of the trapped object and may function as an optical “nanopipette” for microwell single molecule protein sampling. There is also potential to combine nanoapertures with nanopore translocation studies as well as fluorescence correlation microscopy studies and several researchers are already pursuing these areas of research.
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Aleksandr Kovrov, Andrey Novitsky, Alina Karabchevsky, Alexander S. Shalin
The ability to manipulate small objects with focused laser beams has opened a venue for investigating dynamical phenomena relevant to both fundamental and applied sciences. However, manipulating nano‐sized objects requires subwavelength field localization, provided by auxiliary nano‐ and microstructures. Particularly, dielectric microparticles can be used to confine light to an intense beam with a subwavelength waist, called a photonic nanojet (PNJ), which can provide sufficient field gradients for trapping nano‐objects. Herein, the scheme for wavelength‐tunable and nanoscale‐precise optical trapping is elaborated, and the possibility of lateral nanoparticle movement using the PNJ's side lobes is shown for the first time. In addition, the possibility of subwavelength positioning using polarization switching is shown. The estimated stability with respect to Brownian motion is higher compared to conventional optical trapping schemes.
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Anmar A Hussein, Haitham L Saadon, Asaad A Khalaf, Muzahim M Abdulah
Sickle cell disease (SCD) is prevalent in Basrah city and affects the red blood cell (RBC) deformability and thereby causes disease symptoms. Hydroxyurea (HU) is effective in reducing morbidity and mortality in SCD patients by different mechanisms. The aim of the study was to investigate the effect of HU on RBC deformability among SCD patients by direct laser optical trapping (OT) technique. Blood samples from SCD patients and control groups were prepared in the medical laboratory of Basrah Center for Hereditary Blood Diseases and transferred into physics laboratory wherein the laser system was presented and built-in. RBCs from each sample were exposed to three different powers of laser 5, 15, and 20 mW for 15 s and then were released and followed for 2 min. The images for each trapped RBC were obtained and at relaxation sequential times. The percentage changes in the diameters of trapped RBCs were measured for control and patient groups. SCD patients were divided into two groups depending on whether they were receiving HU (39 patients) or not (43 patients). They were matched with 50 healthy individuals (control) regarding age and gender. We found that all the trapped RBCs were affected during the trapping time and then returned toward near normal with some differences between the groups and according to the power used. The deformability of HU group was better and closure to the control. The presented laser system and OT technique with optimal power are effective to study the RBC characteristics and deformability. HU is effective in improving RBC deformability among SCD patients.
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Hamim Mahmud Rivy, M. R. C. Mahdy, Ziaur Rahman Jony, Nabila Masud, Sakin S. Satter, Rafsan Jani
Controlling the near field optical binding force is an emerging new area of optical manipulation. So far, there is no generic way to reverse the near field optical binding force for plasmonic or dielectric nano-dimers of distinct shapes (cube, cylinder, ring, sphere); and specially for homodimers. In this article, for both plasmonic and dielectric objects, we have demonstrated a general way to control the reversal of near field binding force for different shaped dimer sets applicable for both homodimers and heterodimers. The force reversal is achieved by simple breaking of symmetry, considering the nanoparticles are half or less than half immersed in an inhomogeneous dielectric background, i.e. at air–water interface. However, if the dimer set is placed over a dielectric interface or fully inside a homogeneous medium, the sign of binding force does not reverse. Such force reversals for plasmonic dimers have been explained based on Fano resonance, interference fields, Lorentz force and image charge theory. The mechanism of binding force reversal is quite different in dielectric dimers, which has been explained mainly based on electric dipole moment, magnetic dipole moment and the interaction between electric and magnetic dipole moment. Our proposed idea can be verified by simple experiment.
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Renxian Li, Ping Li, Jiaming Zhang, Chunying Ding, Zhiwei Cui
Optical tractor beams can reel in an object towards the source, and are becoming a topic of significant worldwide research. Previous works considered the axial and transverse radiation pressure cross-sections (RPCSs) of optical tractor Bessel polarized beams on a dielectric sphere. However, most particles are charged, and it is important to investigate the tractor beam effect on charged particles. The aim of this work is therefore directed toward this goal, where the axial and transverse RPCSs for a charged sphere illuminated by a vector Bessel beam are computed in the framework of generalized Lorenz-Mie theory (GLMT). Numerical computations of the RPCSs are performed, with emphasis on the emergence of a negative pulling force and its dependence on the half-cone angle α0, the order l, and the polarization. A higher-order (l ≠ 0) Bessel beam possesses a hollow core and is of vortex nature, while the fundamental mode () is of non-vortex type and has a bright maximum intensity at the center of the beam. In our calculation, both and are considered. The axial PRCSs versus ka and α0 are first calculated, and the negative axial forces can arise. Moreover, the axial and transverse RPCSs in the plane perpendicular to the beam axis are computed. However, numerical results show that the RPCSs are same to that for a neutral particle. To explain this, the ratios of axial RPCSs for charged and neutral spheres are investigated taking ka as a parameter. The results show that charges only affect the RPCSs for small particles. Finally, the RPCSs for a charged sphere of relatively small are considered. The charge can affect the magnitude of the RPCSs, however, it does not affect the direction of axial optical forces. These results are of great importance in the development of novel optical tweezers and tractor beams.
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Aniket Chowdhury, Deepak Waghmare, Raktim Dasgupta, Shovan K. Majumder
Rapid membrane damage of optically trapped red blood cells (RBCs) was observed at trapping powers ≥280 mW. An excellent agreement between the estimated laser‐induced thermal gradient across trapped cell's membrane and that typically required for membrane electropermeabilization suggests a mechanism involving temperature gradient‐induced electropermeabilization of membrane. Also the rapid collapse of the trapped cell due to membrane rupture was seen to cause shock waves in the surroundings permeabilizing nearby untrapped cells. When the experiments were carried out with RBCs collected from type II diabetic patients, a noticeable change in the damage rate compared to normal RBCs was seen suggesting a novel optical diagnosis method for the disease.
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Jeneffer P. England, Yuxin Hao, Lihui Bai, Virginia Glick, H. Courtney Hodges, Susan S. Taylor, and Rodrigo A. Maillard
Protein kinases are dynamic molecular switches that sample multiple conformational states. The regulatory subunit of PKA harbors two cAMP-binding domains [cyclic nucleotide-binding (CNB) domains] that oscillate between inactive and active conformations dependent on cAMP binding. The cooperative binding of cAMP to the CNB domains activates an allosteric interaction network that enables PKA to progress from the inactive to active conformation, unleashing the activity of the catalytic subunit. Despite its importance in the regulation of many biological processes, the molecular mechanism responsible for the observed cooperativity during the activation of PKA remains unclear. Here, we use optical tweezers to probe the folding cooperativity and energetics of domain communication between the cAMP-binding domains in the apo state and bound to the catalytic subunit. Our study provides direct evidence of a switch in the folding-energy landscape of the two CNB domains from energetically independent in the apo state to highly cooperative and energetically coupled in the presence of the catalytic subunit. Moreover, we show that destabilizing mutational effects in one CNB domain efficiently propagate to the other and decrease the folding cooperativity between them. Taken together, our results provide a thermodynamic foundation for the conformational plasticity that enables protein kinases to adapt and respond to signaling molecules.
DOI
R M Abraham Ekeroth
A recent study of the photonic coupling between metallic nanowires has revealed new degrees of freedom in the system. Unexpected spin torques were induced on dimers when illuminated with linearly polarized plane waves. As near-field observables, the spectra of torques showed more resolved resonances than the peaks in typical far-field spectra. Here, the study is extended to silicon dimers. The optical properties of high-dielectric systems are governed by volume resonances, not by surface resonances as is the case in plasmonic arrangements. Differently from plasmonic systems, which show strong mechanical inductions only for p-polarized light, high-dielectric systems experience the action of strong forces and torques for both polarizations s and p. The asymmetry in strong near-fields is responsible for the unusual mechanics of the system. Some consequences of this may include the breaking of the action−reaction principle or the appearance of pulling forces. This numerical study is based on an exact method. The work provides ideas for the design of nanorotators and nanodetectors. It suggests a new viewpoint about optical forces: the resultant dynamics of topological variations of electromagnetic fields.
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Tomaž Požar, Jernej Laloš, Aleš Babnik, Rok Petkovšek, Max Bethune-Waddell, Kenneth J. Chau, Gustavo V. B. Lukasievicz & Nelson G. C. Astrath
Electromagnetic momentum carried by light is observable through the mechanical effects radiation pressure exerts on illuminated objects. Momentum conversion from electromagnetic fields to elastic waves within a solid object proceeds through a string of electrodynamic and elastodynamic phenomena, collectively bound by momentum and energy continuity. The details of this conversion predicted by theory have yet to be validated by experiments, as it is difficult to distinguish displacements driven by momentum from those driven by heating due to light absorption. Here, we have measured temporal variations of the surface displacements induced by laser pulses reflected from a solid dielectric mirror. Ab initio modelling of momentum flow describes the transfer of momentum from the electromagnetic field to the dielectric mirror, with subsequent creation/propagation of multicomponent elastic waves. Complete consistency between predictions and absolute measurements of surface displacements offers compelling evidence of elastic transients driven predominantly by the momentum of light.
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Yangxi Chu, Erin Evoy, Saeid Kamal, Young Chul Song, Jonathan P. Reid, Chak K. Chan, and Allan K. Bertram
A previous study reported an uncertainty of up to 3 orders of magnitude for the viscosity of erythritol (1,2,3,4-butanetetrol)–water particles. To help reduce the uncertainty in the viscosity of these particles, we measured the diffusion coefficient of a large organic dye (rhodamine B isothiocyanate–dextran, average molecular weight ∼ 70 000 g mol−1) in an erythritol–water matrix as a function of water activity using rectangular-area fluorescence recovery after photobleaching (rFRAP). The diffusion coefficients were then converted to viscosities of erythritol–water particles using the Stokes–Einstein equation. In addition, we carried out new viscosity measurements of erythritol–water particles using aerosol optical tweezers. Based on the new experimental results and viscosities reported in the literature, we conclude the following: (1) the viscosity of pure erythritol is 184−73+122Pas (2 standard deviations); (2) the addition of a hydroxyl (OH) functional group to a linear C4 carbon backbone increases the viscosity on average by a factor of 27−5+6 (2 standard deviations); and (3) the increase in viscosity from the addition of one OH functional group to a linear C4 carbon backbone is not a strong function of the number of OH functional groups already present in the molecule up to the addition of three OH functional groups, but the increase in viscosity may be larger when the linear C4 carbon backbone already contains three OH functional groups. These results should help improve the understanding of the viscosity of secondary organic aerosol particles in the atmosphere. In addition, these results show that at water activity > 0.4 the rFRAP technique, aerosol optical tweezers technique, and bead-mobility technique give results in reasonable agreement if the uncertainties in the measurements are considered. At water activity < 0.4, the mean viscosity values determined by the optical tweezers technique were higher than those determined by the bead-mobility and rFRAP techniques by 1–2 orders of magnitude. Nevertheless, the disagreement in viscosity measured using multiple techniques reported in this paper is smaller than reported previously.
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M. O. Akanbi, L. M. Hernandez, M. H. Mobarok, J. G. C. Veinot and N. Tufenkji
Upon release to the environment, engineered nanoparticles (ENPs) undergo several chemical, physical and biological transformations that may affect their fate, transport, and bioavailability. The impact of ENP transformations (e.g., coating with natural organic matter or heteroaggregation with natural colloids) on ENP fate and transport has been systematically examined; however, the influence of soil enzymes that are ubiquitous in soils has not been considered. In this study, we examined the effect of a model extracellular soil enzyme (cellulase) – either free in an artificial porewater or adsorbed on a model aquifer grain (silica) surface – on the deposition kinetics of polyethylene glycol-coated titanium dioxide nanoparticles (PEG-nTiO2). A quartz crystal microbalance with dissipation monitoring (QCM-D) was used to study the interaction of PEG-nTiO2 with bare and cellulase-coated silica surfaces as well as in the presence of free cellulase over a range of sodium chloride concentrations. Significant reduction in PEG-nTiO2 deposition rates was observed in the presence of cellulase indicative of strong repulsive interactions between the nanoparticles and the layer of cellulase adsorbed on the silica surface. QCM-D observations were supported by measurements of the PEG-nTiO2–surface interaction energies using an optical NanoTweezer apparatus revealing more repulsive particle–surface interaction energies for the cellulase-coated silica. QCM-D measurements also indicated formation of more viscoelastic films in the presence of cellulase compared to bare silica except at the lowest ionic strength (IS) studied (10 mM NaCl). Overall, this work shows the potential for increased mobility of ENPs in subsurface environments in the presence of extracellular soil enzymes, motivating the need for further studies on the fate and behaviour of ENPs in the presence of these ubiquitous biomolecules.
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Tianjun Yao, Shengli Pu, Jie Rao & Jianming Zhang
The optical force acting on the magnetic nanoparticles (MNPs) is investigated with the magnetic-fluid-filled fiber-optic Fabry-Perot interferometer. The shift of interference spectra is related with the local refractive index variation in the light path, which is assigned to the optical-force-induced outward movement of MNPs. The influence of magnetic fluid’s viscosity, ambient temperature, strength and orientation of the externally applied magnetic field on the optical-force-induced MNPs’ movement is studied in details. The results of this work provide a further understanding of interaction between light and MNPs and clarify the dynamic micro-processes of MNPs within magnetic fluid under external stimuli. It may have the potentials in the fields of light-controllable magnetic-fluid-based devices and vector magnetic field detection.
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Tominaga, Makoto; Higashi, Yuki; Kumamoto, Takuya; Nagashita, Takashi; Nakato, Teruyuki; Suzuki, Yasutaka; Kawamata, Jun
Because inorganic nanosheets, such as clay minerals, are anisotropic, the manipulation of nanosheet orientation is an important challenge in order to realize future functional materials. In the present study, a novel methodology for nanosheet manipulation using laser radiation pressure is proposed. When a linearly polarized laser beam was used to irradiate a niobate (Nb6O17 4-) nanosheet colloid, the nanosheet was trapped at the focal point so that the in-plane direction of the nanosheet was oriented parallel to the propagation direction of the incident laser beam so as to minimize the scattering force. In addition, the trapped nanosheet was aligned along the polarization direction of the linearly polarized laser beam. Thus, unidirectional alignment of a nanosheet can be achieved simply by irradiation using a laser beam.
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Anjan Bhat Kashekodi, Tobias Meinert, Rebecca Michiels, and Alexander Rohrbach
Living cells are highly dynamic systems responding to a large variety of biochemical and mechanical stimuli over minutes, which are well controlled by e.g. optical tweezers. However, live cell investigation through fluorescence microscopy is usually limited not only by the spatial and temporal imaging resolution but also by fluorophore bleaching. Therefore, we designed a miniature light-sheet illumination system that is implemented in a conventional inverted microscope equipped with optical tweezers and interferometric tracking to capture 3D images of living macrophages at reduced bleaching. The horizontal light-sheet is generated with a 0.12 mm small cantilevered mirror placed at 45° to the detection axis. The objective launched illumination beam is reflected by the micro-mirror and illuminates the sample perpendicular to the detection axis. Lateral and axial scanning of both Gaussian and Bessel beams, together with an electrically tunable lens for fast focusing, enables rapid 3D image capture without moving the sample or the objective lens. Using scanned Bessel beams and line-confocal detection, an average axial resolution of 0.8 µm together with a 10-15 fold improved image contrast is achieved.
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Sakin S. Satter, M. R. C. Mahdy, M. A. R. Ohi, Farhan Islam, and Hamim Mahmud Rivy
Near-field optical binding force is an emerging new topic in the field of optical manipulation and plasmonics. However, so far, all the studies of near-field binding force and its counterintuitive reversal are only restricted to dimer sets. In this work, we have studied extensively the idea of near-field optical binding force and associated Lorentz force dynamics for more than two objects, such as plasmonic tetramers over different substrates. We have demonstrated that if closely positioned plasmonic cube tetramers are placed only over the plasmonic substrate and the circularly polarized light is impinged over them, all-optical control between their mutual attraction and repulsion is possible because of strong Fano resonance. In addition, the polarization state of light controls the shifting of the extinction spectra and the binding force reversal wavelength, making such nanostructures ideal for the polarization-dependent optical switching device. The high magnitude of attractive and repulsive binding forces has been obtained at the dark and bright resonant modes, respectively, because of strong induced currents in the plasmonic substrate. Because of its simple arrangement, our proposed tetramer configuration may open a novel route for all-optical particle clustering, aggregation, and crystallization, which can be verified by the simple experimental setup.
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Mahdi Sahafi and Amir Habibzadeh-Sharif
Waveguide-based optical tweezers have significant advantages for particle trapping and transporting in optofluidic chips due to their simple fabrication process. However, for effective trapping of nanoparticles, a high-power input field should be applied, which limits their applications. Here, we numerically show that a silicon-on-insulator-based V-groove waveguide has a much higher capability to trap nanoparticles compared with the traditional waveguides. According to the calculations, the trapping force exerted by the V-groove waveguide to a 5 nm radius nanoparticle can be 14 times greater than that of the strip waveguide. The scattering force is five times larger in the same conditions. This structure would be useful to be integrated into a lab-on-a-chip system to form a particle delivery and analysis device.
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Fransisca A. S. van Esterik, Arianne V. Vega, Kristian A. T. Pajanonot, Daniel R. Cuizon, Michelle E. Velayo, Jahazel Dejito, Stephen L. Flores, Jenneke Klein-Nulend, Rommel G. Bacabac
Fibrin promotes wound healing by serving as provisional extracellular matrix for fibroblasts that realign and degrade fibrin fibers, and sense and respond to surrounding substrate in a mechanical-feedback loop. We aimed to study mechanical adaptation of fibrin networks due to cell-generated forces at the micron-scale. Fibroblasts were elongated-shaped in networks with ≤ 2 mg/ml fibrinogen, or cobblestone-shaped with 3 mg/ml fibrinogen at 24 h. At frequencies f < 102 Hz, G′ of fibroblast-seeded fibrin networks with ≥ 1 mg/ml fibrinogen increased compared to that of fibrin networks. At frequencies f > 103 Hz, G″ of fibrin networks decreased with increasing concentration following the power-law in frequency with exponents ranging from 0.75 ± 0.03 to 0.43 ± 0.03 at 3 h, and of fibroblast-seeded fibrin networks with exponents ranging from 0.56 ± 0.08 to 0.28 ± 0.06. In conclusion, fibroblasts actively contributed to a change in viscoelastic properties of fibrin networks at the micron-scale, suggesting that the cells and fibrin network mechanically interact. This provides better understanding of, e.g., cellular migration in wound healing.
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Lena Wullkopf, Ann-Katrine V. West, Natascha Leijnse, Thomas R. Cox, Chris D. Madsen, Lene B. Oddershede, and Janine T. Erler
Increased tissue stiffness is a classic characteristic of solid tumors. One of the major contributing factors is increased density of collagen fibers in the extracellular matrix (ECM). Here, we investigate how cancer cells biomechanically interact with and respond to the stiffness of the ECM. Probing the adaptability of cancer cells to altered ECM stiffness using optical tweezers based micro-rheology and deformability cytometry, we find that only malignant cancer cells have the ability to adjust to collagen matrices of different densities. Employing micro-rheology on the biologically relevant spheroid invasion assay, we can furthermore demonstrate that even within a cluster of cells of similar origin there are differences in the intracellular biomechanical properties dependent on the cells’ invasive behavior. We reveal a consistent increase of viscosity in cancer cells leading the invasion into the collagen matrices in comparison to cancer cells following in the stalk or remaining in the center of the spheroid. We hypothesize that this differential viscoelasticity might facilitate spheroid tip invasion through a dense matrix. These findings highlight the importance of the biomechanical interplay between cells and their microenvironment for tumor progression.
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