Sunday, December 28, 2014

A monolithic glass chip for active single-cell sorting based on mechanical phenotyping

Christoph Faigle, Franziska Lautenschläger, Graeme Whyte, Philip Homewood, Estela Martín-Badosa and Jochen Guck

The mechanical properties of biological cells have long been considered as inherent markers of biological function and disease. However, the screening and active sorting of heterogeneous populations based on serial single-cell mechanical measurements has not been demonstrated. Here we present a novel monolithic glass chip for combined fluorescence detection and mechanical phenotyping using an optical stretcher. A new design and manufacturing process, involving the bonding of two asymmetrically etched glass plates, combines exact optical fiber alignment, low laser damage threshold and high imaging quality with the possibility of several microfluidic inlet and outlet channels. We show the utility of such a custom-built optical stretcher glass chip by measuring and sorting single cells in a heterogeneous population based on their different mechanical properties and verify sorting accuracy by simultaneous fluorescence detection. This offers new possibilities of exact characterization and sorting of small populations based on rheological properties for biological and biomedical applications.


Inherent Force-Dependent Properties of β-Cardiac Myosin Contribute to the Force-Velocity Relationship of Cardiac Muscle

Michael J. Greenberg, Henry Shuman, E. Michael Ostap

The heart adjusts its power output to meet specific physiological needs through the coordination of several mechanisms, including force-induced changes in contractility of the molecular motor, the β-cardiac myosin (βCM). Despite its importance in driving and regulating cardiac power output, the effect of force on the contractility of a single βCM has not been measured. Using single molecule optical-trapping techniques, we found that βCM has a two-step working stroke. Forces that resist the power stroke slow the myosin-driven contraction by slowing the rate of ADP release, which is the kinetic step that limits fiber shortening. The kinetic properties of βCM are affected by load, suggesting that the properties of myosin contribute to the force-velocity relationship in intact muscle and play an important role in the regulation of cardiac power output.


A micro-particle launching apparatus based on mode-division-multiplexing technology

Zhihai Liu, Peibo Liang, Yu Zhang, Jiaojie Lei, Yaxun Zhang, Jun Yang, Libo Yuan

We propose and demonstrate a trapped yeast cell being launched away from the fiber tip with a certain speed to a certain position without moving the optical fiber in a single fiber optical trapping apparatus. We excite both LP01 and LP11 mode beams in a same normal communication fiber core to generate the optical launching force and trapping force by molding the fiber tip into a special tapered-tip shape. A yeast cell of 6 μm diameter is trapped and then being launched away. We construct the optical trapping and launching potential wells by controlling the power of two mode beams. Besides that, we also build a physical model to analyze the micro-particle dynamic behavior characteristics during the launching process. This micro-particle directional launching function expands new features of fiber optical tweezers based on the normal communication fiber, providing for the possibility of more practical applications in the biomedical research fields.


Diffusion coefficients and particle transport in synthetic membrane channels

S. Pagliara, S. L. Dettmer, K. Misiunas, L. Lea, Y. Tan, U. F. Keyser

Diffusion in constrained geometries is paramount to transport across biological membranes and in mesoporous materials. Although the transported species vary from system to system, the underlying physical mechanisms are universal. However, there is an imbalance between theory and quantitative experimental model systems. We have recently introduced a new synthetic approach to mimic molecular diffusion based on colloidal particles, digital video microscopy, particle tracking, microfluidics and holographic optical tweezers. In this paper we report useful guidelines for the fabrication, handling and characterisation of the microfluidic chips and a study of diffusion coefficients, particle attempt and translocation rates through microfluidic channels with cross sections of different dimensions.


Friday, December 19, 2014

Trapping of nanoparticles in a liquid by laser-induced microbubbles

V I Yusupov, S I Tsypina and V N Bagratashvili

Nanoparticles a few nanometers in diameter contained in a colloidal solution can be captured by the surface of a microbubble produced by a well-focused beam of a continuous wave visible laser. The concentration of the nanoparticles on the surface of the microbubble is found to gradually increase during the course of irradiation. The mechanism of such a trapping is associated with the laser-induced Marangoni convection developing in the vicinity of the microbubble as a result of the temperature gradient obtained at its surface.


Organic Component Vapour Pressures and Hygroscopicities of Aqueous Aerosol Measured by Optical Tweezers

Chen Cai, David Stewart, Jonathan Philip Reid, Yun-Hong Zhang, Peter Ohm, Cari S. Dutcher, and Simon Leslie Clegg

Measurements of the hygroscopic response of aerosol and the particle-to-gas partitioning of semi-volatile organic compounds are crucial for providing more accurate descriptions of the compositional and size distributions of atmospheric aerosol. Concurrent measurements of particle size and composition (inferred from refractive index) are reported here using optical tweezers to isolate and probe individual aerosol droplets over extended timeframes. The measurements are shown to allow accurate retrievals of component vapour pressures and hygroscopic response through examining correlated variations in size and composition for binary droplets containing water and a single organic component. Measurements are reported for an homologous series of dicarboxylic acids, maleic acid, citric acid, glycerol or 1,2,6-hexanetriol. An assessment of the inherent uncertainties in such measurements when measuring only particle size is provided to confirm the value of such a correlational approach. We also show that the method of molar refraction provides an accurate characterisation of the compositional dependence of the refractive index of the solutions. In this method, the density of the pure liquid solute is the largest uncertainty and must be either known or inferred from subsaturated measurements with an error of <± 2.5 % to discriminate between different thermodynamic treatments.


The electromagnetic force in the terahertz band generated by a cross-shaped absorber

Jiahui Fu, Wan Chen, Bo Lv, Lei Zhu, Qun Wu

A 2D periodic force generator in terahertz band is presented. A multi-layered structure is designed, which consists of two adjacent dielectric layers and a cross-shaped metal patch. Once the structure is resonant, strong coupling effect takes place between the two dielectric layers. Due to the coupling effect, the electromagnetic field is enhanced, which leads to a large electromagnetic force. The force is calculated by the Maxwell Stress Tensor. The magnitude of the force is three orders higher than the force generated in the optical band presented in other papers. The power density of the incident light is studied for the demonstration of the usefulness of the force in terahertz band generated by the structure. Dielectric loss is also taken into consideration. The result shows that the magnitude of the force is still enough to offset the gravity of the unit cell even with a high dielectric loss.


Heterogeneous oxidation of nitrite anion by gas-phase ozone in an aqueous droplet levitated by laser tweezers (optical trap): is there any evidence for enhanced surface reaction?

Oliver R. Hunt, Andrew D. Ward and Martin D. King

The oxidation of nitrite anion within an aqueous atmospheric droplet may be a sink for HONO in the lower atmosphere. An optical trap with Raman spectroscopy is used to demonstrate that the oxidation of aqueous nitrite anion in levitated, micron sized, aqueous droplets by gas-phase ozone is consistent with bulk aqueous-phase kinetics and diffusion. There is no evidence of an enhanced or retarded reaction at the droplet surface at the concentrations used in the experiment or likely to be found in the atmosphere. The oxidation of nitrite in an aqueous droplet by gas-phase ozone does not cause the droplet to hydrodynamically change in size and demonstrates use of an optical trap as a wall-less reactor to measuring aqueous-phase rate coefficients.


Thursday, December 18, 2014

Toward optical-tweezers-based force microscopy for airborne microparticles

Rory M. Power, Daniel R. Burnham, and Jonathan P. Reid

Optical tweezers have found widespread application in biological and colloidal physics for the measurement of pN forces over nanometer to micrometer length scales. Similar aerosol-phase measurements of interparticle force have not been reported in spite of the potential to better resolve particle coagulation kinetics. Various refractive index mismatches in the beam path as well as the need to explicitly account for gravity and inertial particle motion provide a number of challenges that must be overcome to make such measurements tractable. In this regard, we demonstrate schemes by which the particle position and trap stiffness may be unambiguously measured using bright-field microscopy with resolution comparable with analogous condensed-phase measurements. Moreover, some of the challenges of working with highly dynamic aqueous particles are introduced and exploited to observe size-dependent phenomena in aerosol optical tweezers. Notably, when combined with cavity-enhanced Raman spectroscopy, this provides a unique opportunity to explore trapping forces over a continuum of particle size and refractive index. It is expected that the methods developed will provide a basis for the measurement of pairwise interaction forces in aerosol optical tweezers while providing a probe of fundamental airborne particle trapping dynamics.


Stretching of red blood cells using an electro-optics trap

Md. Mozzammel Haque, Mihaela G. Moisescu, Sándor Valkai, András Dér, and Tudor Savopol

The stretching stiffness of Red Blood Cells (RBCs) was investigated using a combination of an AC dielectrophoretic apparatus and a single-beam optical tweezer. The experiments were performed at 10 MHz, a frequency high enough to avoid conductivity losses, but below the second turnover point between positive and negative dielectrophoresis. By measuring the geometrical parameters of single healthy human RBCs as a function of the applied voltage, the elastic modulus of RBCs was determined (µ = 1.80 ± 0.5 µN/m) and compared with similar values of the literature got by other techniques. The method is expected to be an easy-to-use, alternative tool to determine the mechano-elastic properties of living cells, and, on this basis, to distinguish healthy and diseased cells.


Evaluating the toxic effect of an antimicrobial agent on single bacterial cells with optical tweezers

Akbar Samadi, Chensong Zhang, Joseph Chen, S. N. S. Reihani, and Zhigang Chen

We implement an optical tweezers technique to assess the effects of chemical agents on single bacterial cells. As a proof of principle, the viability of a trapped Escherichia coli bacterium is determined by monitoring its flagellar motility in the presence of varying concentrations of ethyl alcohol. We show that the “killing time” of the bacterium can be effectively identified from the correlation statistics of the positional time series recorded from the trap, while direct quantification from the time series or associated power spectra is intractable. Our results, which minimize the lethal effects of bacterial photodamage, are consistent with previous reports of ethanol toxicity that used conventional culture-based methods. This approach can be adapted to study other pairwise combinations of drugs and motile bacteria, especially to measure the response times of single cells with better precision.


Wednesday, December 17, 2014

Measurements of liposome biomechanical properties by combining line optical tweezers and dielectrophoresis

Ellas Spyratou, Efrosini Cunaj, George Tsigaridas, Elena A. Mourelatou, Costas Demetzos, Alexander A. Serafetinides, and Mersini Makropoulou

Liposomes are well-known cell simulators and are currently studied as drug delivery systems, for a targeted delivery of higher drug concentrations, in specific cells. Novel biophotonic techniques for manipulation and characterization of liposomes have been developed; among which are optical tweezers. In our work, we demonstrate a novel use of line optical tweezers to manipulate and cause liposome deformations. Optical forces induce tension on liposomes, which are stretched along the line optical trap. The method of dielectrophoresis, combined with optical tweezers, was used to measure the exerted optical forces. As a consequence, in the case of reversible liposome deformations, the value of the shear and bending moduli of liposomes was calculated. We anticipate that the selective manipulation of liposomes will help us toward a better understanding of the cellular–liposome interactions. Studying the biomechanical properties of liposomes will provide an insight into the mechanical behavior of individual living cells, which have recently been implicated in many aspects of human physiology and patho-physiology. The biomechanical properties of cells (i.e. deformability, stiffness and elasticity) can be useful biomarkers for various disease processes and changes of the cell state.


In situ seriate droplet coalescence under an optical force

Jin Ho Jung, Kyung Heon Lee, Ghulam Destgeer, Kang Soo Lee, Hyunjun Cho, Byung Hang Ha, Hyung Jin Sung

We demonstrated the induced coalescence of droplets under a highly accurate optical force control. Optical scattering and gradient forces were used to push and trap the droplets prior to coalescence within a microfluidic channel. The behavior of the droplets under the influence of an optical force was predicted using an analytical model that agreed well with the experimental data. The optical gradient force accelerated and decelerated the droplet within the laser beam region, and the drag force acting on the droplet was thoroughly characterized. A description of the optical trap was presented in terms of the momentum transfer from the photons to the droplet, effectively restricting droplet motion inside the microfluidic channel prior to coalescence. A phase diagram was plotted to distinguish between the three regimes of droplet coalescence, including the absence of coalescence, coalescence, and multiple coalescence events. The phase diagram permitted the laser power input and the net flow rate in the microfluidic channel to be estimated. This technique was applied to the synthesis of biodegradable gel microparticles.


Dynamic orientation of azopyridine units within the shell of vesicles PNIPAM-b-PAzPyn copolymers

Yingjue Wang, Guangyong Shen, Jiangang Gao, Gang Zou andQijin Zhang

Vesicles with hydrophobic shells have been self-assembled through three kinds of amphiphilic block copolymers containing pendent azopyridine groups with different spacers, namely PNIPAM-b-PAzPyn (n = 0, 2, 6), respectively. By polarization laser-trapping Raman spectroscopy, the photoinduced orientation behaviors of azopyridine groups within the vesicle shells have been investigated and it is found that spacer lengths affect the orientation of the azopyridine groups and the morphologic structure of the vesicle shells. The exact experimental results show that the orientation is dynamic for the pendent azopyridine groups with connecting spacers of 2 or 6 methylene units rather than those without spacers, so the vesicles of PNIPAM-b-PAzPy6 can be changed to show a typical “soft” character compared with its solid films when irradiated with a relatively weak polarized UV light of 190 µW/cm2. However, the vesicles of PNIPAM-b-PAzPy0 without spacers do not change even though the azopyridine units can be oriented. By quantitative Raman spectral analysis, it is found that the isomerization degree of azopyridine units is 70% for PNIPAM-b-PAzPy6 yet it is 10% for PNIPAM-b-PAzPy0, which shows a close relationship between aggregation and isomerization of azopyridine units under a weak UV light.


Optical force on a large sphere illuminated by Bessel beams: comparisons between ray optics method and generalized Lorenz–Mie theory

Shukun Song, Neng Wang, Wanli Lu, and Zhifang Lin

Optical forces are calculated for a dielectric spherical particle illuminated by a zero-order Bessel beam based on both the generalized Lorenz–Mie theory (GLMT) and the ray optics method (ROM). Particles with positive and negative refractive indices are examined. The peculiar characteristics of the Bessel beam allow for analytical expressions for the beam shape coefficients required in the GLMT as well as a decomposition of optical force into the gradient and the scattering forces irrespective of the particle size, which enable respective comparisons for the gradient and scattering forces between the results obtained from the GLMT and the ROM. Our results demonstrate that the discrepancy between the results obtained from the GLMT and the ROM depends on the particle refractive index np, the particle size, and, also, the particle location in the beam field. As the particle size increases, the difference between the results from the GLMT and the ROM shows a general tendency of decreasing, as can be expected, but the change may exhibit oscillatory rather than monotonic behavior. A phase diagram is presented that displays the regime for particle size and refractive index where a specified accuracy can be achieved for optical force by the ROM.


Thursday, December 11, 2014

Study of hepatocyte plasma membrane mechanical properties using optical trapping

A D Vedyaykin, N E Morozova, G E Pobegalov, A N Arseniev, M A Khodorkoskii and A V Sabantsev

In this paper we describe the use of membrane tether formation technique which is widely used to study mechanical properties of plasma membranes. This method was successfully used for the direct measurement of parameters characterizing membranes mechanical properties (static tether tension force and effective membrane viscosity) of human hepatocytes (HepG2 hepatocellular carcinoma line). These results allow using this method in future for diagnostics of the cell membrane, evaluating the influence on the mechanical parameters of various factors, including toxins and drugs.


Optical trapping with Bessel beams generated from semiconductor lasers

G S Sokolovskii, V V Dudelev, S N Losev, K K Soboleva, A G Deryagin, V I Kuchinskii, W Sibbett and E U Rafailov

In this paper, we study generation of Bessel beams from semiconductor lasers with high beam propagation parameter M2 and their utilization for optical trapping and manipulation of microscopic particles including living cells. The demonstrated optical tweezing with diodegenerated Bessel beams paves the way to replace their vibronic-generated counterparts for a range of applications towards novel lab-on-a-chip configurations.


Rotation dynamics of particles trapped in a rotating beam

Huachao Yu and Weilong She

The rotation dynamics of particles trapped in a rotating beam is theoretically investigated. We find that there is a critical angular speed for the rotating beam. If the angular speed of the rotating beam is smaller than the critical value, the angular velocity of the trapped particle is nearly the same as that of the rotating beam, which is in accord with existing experimental observation. Otherwise, the angular velocity of the trapped particles will become periodic or quasi-periodic with time, depending on the beam polarization, which, to the best of our knowledge, has not been previously reported. Moreover, we also propose some methods to determine the ratio between the beam power and the maximal angular speed of the trapped particle, which can be used to estimate the minimum power required to rotate the particle at a given angular speed.


Nanoscale phase behavior on flat and curved membranes

Thomas Andersen, Azra Bahadori, Dino Ott, Anders Kyrsting, S Nader S Reihani and Poul M Bendix

The diverse physical properties of membranes play a critical role in many membrane associated biological processes. Proteins responsible for membrane transport can be affected by the lateral membrane order and lateral segregation of proteins is often controlled by the preference of certain membrane anchors for membrane phases having a physically ordered state. The dynamic properties of coexisting membrane phases are often studied by investigating their thermal behavior. Optical trapping of gold nanoparticles is a useful tool to generate local phase transitions in membranes. The high local temperatures surrounding an irradiated gold nanoparticle can be used to melt a part of a giant unilamellar lipid vesicle (GUV) which is then imaged using phase sensitive fluorophores embedded within the bilayer. By local melting of GUVs we reveal how a protein-free, one component lipid bilayer can mediate passive transport of fluorescent molecules by localized and transient pore formation. Also, we show how tubular membrane curvatures can be generated by optical pulling from the melted region on the GUV. This will allow us to measure the effect of membrane curvature on the phase transition temperature.


Tuesday, December 9, 2014

Coupled nano-plasmons

M. Apostol, S. Ilie, A. Petrut, M. Savu, S. Toba

A simple model of coupled plasmons arising in two neighbouring nano-particles is presented. The coupled oscillations and the corresponding eigenfrequencies are computed. It is shown that the plasmons may be periodically transferred between the two particles. For larger separation distances between the two particles the retardation is included. The oscillation eigenmodes are the polaritons in this case. There are distances for which the particles do not couple to each other, i.e. the polaritonic coupling gets damped. The van der Waals-London-Casimir force is estimated for the two particles; it is shown that for large distances the force is repulsive. We compute also the polarizabilities of the two coupled nano-particles and their cross-section under the action of an external monochromatic plane wave, which exhibit resonances indicative of light trapping and field enhancement. A resonant force is also identified, acting upon the particles both on behalf of the external field and of each other.


Topological Switching and Orbiting Dynamics of Colloidal Spheres Dressed with Chiral Nematic Solitons

T. Porenta, S. Čopar, P. J. Ackerman, M. B. Pandey, M. C. M. Varney, I. I. Smalyukh & S. Žumer

Metastable configurations formed by defects, inclusions, elastic deformations and topological solitons in liquid crystals are a promising choice for building photonic crystals and metamaterials with a potential for new optical applications. Local optical modification of the director or introduction of colloidal inclusions into a moderately chiral nematic liquid crystal confined to a homeotropic cell creates localized multistable chiral solitons. Here we induce solitons that “dress” the dispersed spherical particles treated for tangential degenerate boundary conditions, and perform controlled switching of their state using focused optical beams. Two optically switchable distinct metastable states, toron and hopfion, bound to colloidal spheres into structures with different topological charges are investigated. Their structures are examined using Q-tensor based numerical simulations and compared to the profiles reconstructed from the experiments. A topological explanation of observed multistability is constructed.


Reconstructing Folding Energy Landscape Profiles from Nonequilibrium Pulling Curves with an Inverse Weierstrass Integral Transform

Megan C. Engel, Dustin B. Ritchie, Daniel A. N. Foster, Kevin S. D. Beach, and Michael T. Woodside

The energy landscapes that drive structure formation in biopolymers are difficult to measure. Here we validate experimentally a novel method to reconstruct landscape profiles from single-molecule pulling curves using an inverse Weierstrass transform (IWT) of the Jarzysnki free-energy integral. The method was applied to unfolding measurements of a DNA hairpin, replicating the results found by the more-established weighted histogram (WHAM) and inverse Boltzmann methods. Applying both WHAM and IWT methods to reconstruct the folding landscape for a RNA pseudoknot having a stiff energy barrier, we found that landscape features with sharper curvature than the force probe stiffness could not be recovered with the IWT method. The IWT method is thus best for analyzing data from stiff force probes such as atomic force microscopes.


Periodic dynamics, localization metastability, and elastic interaction of colloidal particles with confining surfaces and helicoidal structure of cholesteric liquid crystals

Michael C. M. Varney, Qiaoxuan Zhang, Mykola Tasinkevych, Nuno M. Silvestre, Kris A. Bertness, and Ivan I. Smalyukh

Nematic and cholesteric liquid crystals are three-dimensional fluids that possess long-range orientational ordering and can support both topological defects and chiral superstructures. Implications of this ordering remain unexplored even for simple dynamic processes such as the ones found in so-called “fall experiments,” or motion of a spherical inclusion under the effects of gravity. Here we show that elastic and surface anchoring interactions prompt periodic dynamics of colloidal microparticles in confined cholesterics when gravity acts along the helical axis. We explore elastic interactions between colloidal microparticles and confining surfaces as well as with an aligned ground-state helical structure of cholesterics for different sizes of spheres relative to the cholesteric pitch, demonstrating unexpected departures from Stokes-like behavior at very low Reynolds numbers. We characterize metastable localization of microspheres under the effects of elastic and surface anchoring periodic potential landscapes seen by moving spheres, demonstrating the important roles played by anchoring memory, confinement, and topological defect transformation. These experimental findings are consistent with the results of numerical modeling performed through minimizing the total free energy due to colloidal inclusions at different locations along the helical axis and with respect to the confining substrates. A potential application emerging from this work is colloidal sorting based on particle shapes and sizes.


Forces in Aharonov–Bohm optical setting

Sergey Sukhov, Veerachart Kajorndejnukul, John Broky, and Aristide Dogariu

The Aharonov–Bohm effect is usually associated with a path-dependent phase accumulated by a charged matter wave and determined by an effective vector potential. In a more general geometrical framework, such phase alterations have been the hallmark of a host of related phenomena in many different fields. However, besides phase changes, it was suggested that for finite wave-packets there is also an additional deflection leading to observable changes in the wave’s canonical momentum. In this paper, we create an optical scattering situation that permits observing nonconservative reaction forces, which result from the conservation of canonical momentum. We demonstrate experimentally, for the first time, the presence of such mechanical forces with the magnitude and direction determined by the phase dislocation of the vortex state. Our experimental results offer insights into essentially untested phenomena, where forces act on vortex fields, and allow examining the role of conservation laws and symmetries in complex interacting systems.


Monday, December 8, 2014

Interaction of G-quadruplexes in the Full-length 3´ Human Telomeric Overhang

Jibin Abraham Punnoose , Yunxi Cui , Deepak Koirala , Philip M. Yangyuoru , Chiran Ghimire , Prakash Shrestha , and Hanbin Mao

The 3´ human telomeric overhang provides ample opportunities for the formation and interaction of G-quadruplexes, which have shown impacts on many biological functions including telomerase activities in the telomere region. However, in the few investigations on DNA constructs that approach to the full length of the human telomeric overhang, the presence of higher order quadruplex-quadruplex interactions is still a subject of debate. Herein, we employed dynamic splint ligation (DSL) to prepare a DNA construct, 5´-(TTAGGG)24 or 24G, which has the length comparable to the full stretch of 3´ human telomeric overhang. Using mechanical unfolding assays in laser tweezers, we observed a minor population (~4.5%) of higher order interactions between G-quadruplexes while the majority of the quadruplexes follow the bead-on-a-string model. Analyses on the non-interacting G-quadruplexes in the 24G construct showed features similar to those of the stand-alone G-quadruplexes in the 5´-(TTAGGG)4 (4G) construct. As each 24G construct contains as many as six G-quadruplexes, this method offers increased throughput for the time-consuming mechanical unfolding experiments of non-B DNA structures.


A Universal Law for Plasmon Resonance Shift in Biosensing

Weihua Zhang and Olivier J.F. Martin

We derive an explicit expression for the resonance frequency shift for a subwavelength plasmonic nanocavity upon the adsorption or trapping of a single nanoparticle using rigorous perturbation theory. It reveals a simple linear dependence of the resonance frequency shift on the product of the local field intensity of a resonance mode, the material dispersion factor dω/dε of the nanocavity, and the polarizability of the nanoparticle. To verify this linear relation, we numerically simulate the nanoparticle-induced resonance shifts for subwavelength ellipsoids, rods, rod pairs, and split rings with different sizes and materials, and a very good agreement is found between the theory and the numerical results. Moreover, we discuss this approach from the energy perspective and find that the linear relation can be understood in the context of optical trapping. This work not only reveals the underlining physics of near-field couplings in plasmonic nanocavities, but also provides theoretical guidelines for the design of ultrasensitive nanosensors.


Thursday, December 4, 2014

Enhancing Optical Forces on Fluorescent Up-Converting Nanoparticles by Surface Charge Tailoring

Héctor Rodríguez-Rodríguez, Paloma Rodríguez Sevilla, Emma Martín Rodríguez, Dirk H. Ortgies, Marco Pedroni, Adolfo Speghini, Marco Bettinelli, Daniel Jaque and Patricia Haro-González

3D remote control of multifunctional fluorescent up-converting nanoparticles (UCNPs) using optical forces is being required for a great variety of applications including single-particle spectroscopy, single-particle intracellular sensing, controlled and selective light-activated drug delivery and light control at the nanoscale. Most of these potential applications find a serious limitation in the reduced value of optical forces (tens of fN) acting on these nanoparticles, due to their reduced dimensions (typically around 10 nm). In this work, this limitation is faced and it is demonstrated that the magnitude of optical forces acting on UCNPs can be enhanced by more than one order of magnitude by a controlled modification of the particle/medium interface. In particular, substitution of cationic species at the surface by other species with higher mobility could lead to UCNPs trapping with constants comparable to those of spherical metallic nanoparticles.


On-chip optical trapping and Raman spectroscopy using a TripleX dual-waveguide trap

Martijn Boerkamp, Thijs van Leest, Jeroen Heldens, Arne Leinse, Marcel Hoekman, Rene Heideman, and Jacob Caro

We present a new approach to the dual-beam geometry for on-chip optical trapping and Raman spectroscopy, using waveguides microfabricated in TripleX technology. Such waveguides are box shaped and consist of SiO2 and Si3N4, so as to provide a low index contrast with respect to the SiO2 claddings and low loss, while retaining the advantages of silicon Si3N4. The waveguides enable both the trapping and Raman functionality with the same dual beams. Polystyrene beads of 1 µm diameter can be easily trapped with the device. In the axial direction discrete trapping positions occur, owing to the intensity pattern of the interfering beams. Trapping events are interpreted on the basis of simulated optical fields and calculated optical forces. The average transverse trap stiffness is 0.8 pN/nm/W, indicating that a strong trap is formed by the beams emitted by the waveguides. Finally, we measure Raman spectra of trapped beads for short integration times (down to 0.25 s), which is very promising for Raman spectroscopy of microbiological cells.


Tuesday, December 2, 2014

Probing the Raman-active acoustic vibrations of nanoparticles with extraordinary spectral resolution

Skyler Wheaton, Ryan M. Gelfand & Reuven Gordon

Colloidal quantum dots, viruses, DNA and all other nanoparticles have acoustic vibrations that can act as ‘fingerprints’ to identify their shape, size and mechanical properties, yet high-resolution Raman spectroscopy in this low-energy range has been lacking. Here, we demonstrate extraordinary acoustic Raman (EAR) spectroscopy to measure the Raman-active vibrations of single isolated nanoparticles in the 0.1–10 cm−1 range with ∼0.05 cm−1 resolution, to resolve peak splitting from material anisotropy and to probe the low-frequency modes of biomolecules. EAR employs a nanoaperture laser tweezer that can select particles of interest and manipulate them once identified. We therefore believe that this nanotechnology will enable expanded capabilities for the study of nanoparticles in the materials and life sciences.


Cytoplasmic dynein transports cargos via load-sharing between the heads

Vladislav Belyy, Nathan L Hendel, Alexander Chien & Ahmet Yildiz

Cytoplasmic dynein is a motor protein that walks along microtubules (MTs) and performs mechanical work to power a variety of cellular processes. It remains unclear how a dynein dimer is able to transport cargos against load without coordinating the stepping cycles of its two heads. Here by using a DNA-tethered optical trapping geometry, we find that the force-generating step of a head occurs in the MT-bound state, while the ‘primed’ unbound state is highly diffusional and only weakly biased to step towards the MT-minus end. The stall forces of the individual heads are additive, with both heads contributing equally to the maximal force production of the dimer. On the basis of these results, we propose that the heads of dynein utilize a ‘load-sharing’ mechanism, unlike kinesin and myosin. This mechanism may allow dynein to work against hindering forces larger than the maximal force produced by a single head.


Generation of microfluidic flow using an optically assembled and magnetically driven microrotor

J Köhler, R Ghadiri, S I Ksouri, Q Guo, E L Gurevich and A Ostendorf

The key components in microfluidic systems are micropumps, valves and mixers. Depending on the chosen technology, the realization of these microsystems often requires rotational and translational control of subcomponents. The manufacturing of such active components as well as the driving principle are still challenging tasks. A promising all-optical approach could be the combination of laser direct writing and actuation based on optical forces. However, when higher actuation velocities are required, optical driving might be too slow. Hence, a novel approach based on optical assembling of microfluidic structures and subsequent magnetic actuation is proposed. By applying the optical assembly of microspherical building blocks as the manufacturing method and magnetic actuation, a microrotor was successfully fabricated and tested within a microfluidic channel. The resulting fluid flow was characterized by introducing an optically levitated measuring probe particle. Finally, a freely moving tracer particle visualizes the generated flow. The tracer particle analysis shows average velocities of 0.4–0.5 µm s−1 achieved with the presented technology.