Wednesday, December 30, 2015

Methods for Determining the Cellular Functions of Vimentin Intermediate Filaments

Karen M. Ridge, Dale Shumaker, Amélie Robert, Caroline Hookway, Vladimir I. Gelfand, Paul A. Janmey, Jason Lowery, Ming Guo, David A. Weitz, Edward Kuczmarski, Robert D. Goldman

The type III intermediate filament protein vimentin was once thought to function mainly as a static structural protein in the cytoskeleton of cells of mesenchymal origin. Now, however, vimentin is known to form a dynamic, flexible network that plays an important role in a number of signaling pathways. Here, we describe various methods that have been developed to investigate the cellular functions of the vimentin protein and intermediate filament network, including chemical disruption, photoactivation and photoconversion, biolayer interferometry, soluble bead binding assay, three-dimensional substrate experiments, collagen gel contraction, optical-tweezer active microrheology, and force spectrum microscopy. Using these techniques, the contributions of vimentin to essential cellular processes can be probed in ever further detail.

Optical trapping performance of dielectric-metallic patchy particles

Joseph L. Lawson, Nathan J. Jenness, and Robert L. Clark

We demonstrate a series of simulation experiments examining the optical trapping behavior of composite micro-particles consisting of a small metallic patch on a spherical dielectric bead. A full parameter space of patch shapes, based on current state of the art manufacturing techniques, and optical properties of the metallic film stack is examined. Stable trapping locations and optical trap stiffness of these particles are determined based on the particle design and potential particle design optimizations are discussed. A final test is performed examining the ability to incorporate these composite particles with standard optical trap metrology technologies.


Tuesday, December 29, 2015

Charged hydrophobic colloids at an oil–aqueous phase interface

Colm P. Kelleher, Anna Wang, Guillermo Iván Guerrero-García, Andrew D. Hollingsworth, Rodrigo E. Guerra, Bhaskar Jyoti Krishnatreya, David G. Grier, Vinothan N. Manoharan, and Paul M. Chaikin

Hydrophobic poly(methyl methacrylate) (PMMA) colloidal particles, when dispersed in oil with a relatively high dielectric constant, can become highly charged. In the presence of an interface with a conducting aqueous phase, image-charge effects lead to strong binding of colloidal particles to the interface, even though the particles are wetted very little by the aqueous phase. We study both the behavior of individual colloidal particles as they approach the interface and the interactions between particles that are already interfacially bound. We demonstrate that using particles which are minimally wetted by the aqueous phase allows us to isolate and study those interactions which are due solely to charging of the particle surface in oil. Finally, we show that these interactions can be understood by a simple image-charge model in which the particle charge q is the sole fitting parameter.


Use of Raman optical tweezers for cell cycle analysis

Sunita Ahlawat, Aniket Chowdhury, Abha Uppal, Nitin Kumar and Pradeep Kumar Gupta

We report the results of our investigations on the use of Raman optical tweezers for label free analysis of cells in different phases of their cell cycle. The studies performed on human colon adenocarcinoma (Colo-205) cells synchronized in G0/G1 and G2/M phases showed that the DNA Raman band at 783 cm-1 in the Raman spectra of optically trapped cells can provide information about the DNA content in the nucleus of the cell without the need for isolation of nucleus. The histograms of intensity of this band among the cell populations were found to corroborate the results obtained from fluorescence image cytometry performed on DAPI stained cells.


Spinning of a submicron sphere by Airy beams

Kyoung-Youm Kim and Saehwa Kim

We show that by employing two incoherent counter-propagating Airy beams, we can manipulate a submicron sphere to spin around a transverse axis. We can control not only the spinning speed, but also the direction of the spinning axis by changing the polarization directions of Airy beams.


Contact efflorescence as a pathway for crystallization of atmospherically relevant particles

Ryan D. Davis, Sara Lance, Joshua A. Gordon, Shuichi B. Ushijima, and Margaret A. Tolbert

Inadequate knowledge of the phase state of atmospheric particles represents a source of uncertainty in global climate and air quality models. Hygroscopic aqueous inorganic particles are often assumed to remain liquid throughout their atmospheric lifetime or only (re)crystallize at low relative humidity (RH) due to the kinetic limitations of efflorescence (salt crystal nucleation and growth from an aqueous solution). Here we present experimental observations of a previously unexplored heterogeneous nucleation pathway that we have termed “contact efflorescence,” which describes efflorescence initiated by an externally located solid particle coming into contact with the surface of a metastable aqueous microdroplet. This study demonstrates that upon a single collision, contact efflorescence is a pathway for crystallization of atmospherically relevant aqueous particles at high ambient RH (≤80%). Soluble inorganic crystalline particles were used as contact nuclei to induce efflorescence of aqueous ammonium sulfate [(NH4)2SO4], sodium chloride (NaCl), and ammonium nitrate (NH4NO3), with efflorescence being observed in several cases close to their deliquescence RH values (80%, 75%, and 62%, respectively). To our knowledge, these observations represent the highest reported efflorescence RH values for microdroplets of these salts. These results are particularly important for considering the phase state of NH4NO3, where the contact efflorescence RH (∼20–60%) is in stark contrast to the observation that NH4NO3 microdroplets do not homogeneously effloresce, even when exposed to extremely arid conditions (<1% RH). Considering the occurrence of particle collisions in the atmosphere (i.e., coagulation), these observations of contact efflorescence challenge many assumptions made about the phase state of inorganic aerosol.


Monday, December 28, 2015

Development and characterization of a eukaryotic expression system for human type II procollagen

Andrew Wieczorek, Naghmeh Rezaei, Clara K. Chan, Chuan Xu, Preety Panwar, Dieter Brömme, Erika F. Merschrod S. and Nancy R. Forde

Triple helical collagens are the most abundant structural protein in vertebrates and are widely used as biomaterials for a variety of applications including drug delivery and cellular and tissue engineering. In these applications, the mechanics of this hierarchically structured protein play a key role, as does its chemical composition. To facilitate investigation into how gene mutations of collagen lead to disease as well as the rational development of tunable mechanical and chemical properties of this full-length protein, production of recombinant expressed protein is required.
Here, we present a human type II procollagen expression system that produces full-length procollagen utilizing a previously characterized human fibrosarcoma cell line for production. The system exploits a non-covalently linked fluorescence readout for gene expression to facilitate screening of cell lines. Biochemical and biophysical characterization of the secreted, purified protein are used to demonstrate the proper formation and function of the protein. Assays to demonstrate fidelity include proteolytic digestion, mass spectrometric sequence and posttranslational composition analysis, circular dichroism spectroscopy, single-molecule stretching with optical tweezers, atomic-force microscopy imaging of fibril assembly, and transmission electron microscopy imaging of self-assembled fibrils.
Using a mammalian expression system, we produced full-length recombinant human type II procollagen. The integrity of the collagen preparation was verified by various structural and degradation assays. This system provides a platform from which to explore new directions in collagen manipulation.


Single optical tweezers based on elliptical core fiber

Yu Zhang, Li Zhao, Yunhao Chen, Zhihai Liu, Yaxun Zhang, Enming Zhao, Jun Yang, Libo Yuan

We propose and demonstrate a new single optical tweezers based on an elliptical core fiber, which can realize the trapped yeast cell rotation with a precise and simple control. Due to the elliptical shape of the fiber core, the LP11 mode beam can propagate stably. When we rotate the fiber tip, the LP11 mode beam will also rotate along with the fiber tip, which helps to realize the trapped micro-particle rotation. By using this method, we can easily realize the rotation of the trapped yeast cells, the rotating angle of the yeast cell is same as the elliptical core fiber tip.


Thursday, December 17, 2015

Optically Induced Forces Imposed in an Optical Funnel on a Stream of Particles in Air or Vacuum

Niko Eckerskorn, Richard Bowman, Richard A. Kirian, Salah Awel, Max Wiedorn, Jochen Küpper, Miles J. Padgett, Henry N. Chapman, and Andrei V. Rode

Optical trapping of light-absorbing particles in a gaseous environment is governed by a laser-induced photophoretic force, which can be orders of magnitude stronger than the force of radiation pressure induced by the same light intensity. In spite of many experimental studies, the exact theoretical background underlying the photophoretic force and the prediction of its influence on the particle motion is still in its infancy. Here, we report the results of a quantitative analysis of the photophoretic force and the stiffness of trapping achieved by levitating graphite and graphite-coated glass shells of calibrated sizes in an upright diverging hollow-core vortex beam, which we refer to as an “optical funnel”. The measurements of forces are conducted in air at various gas pressures in the range from 5 mbar to 2 bar. The results of these measurements lay the foundation for mapping the optically induced force to the intensity distribution in the trap. The mapping, in turn, provides the necessary information to model flight trajectories of particles of various sizes entering the beam at given initial speed and position relative to the beam axis. Finally, we determine the limits of the parameter space for the particle speed, size, and radial offset to the beam axis, all linked to the laser power and the particular laser-beam structure. These results establish the grounds for developing a touch-free optical system for precisely positioning submicrometer bioparticles at the focal spot of an x-ray free-electron laser, which will significantly enhance the efficiency of studying nanoscale morphology of proteins and biomolecules in femtosecond coherent diffractive imaging experiments.


Optical Trapping of Nanoparticles on a Silicon Subwavelength Grating and Their Detection by an Ellipsometric Technique

Naoya Taki, Yasuhiro Mizutan, Tetsuo Iwata, Takao Kojima, Hiroki Yamamoto & Takahiro Kozawa

A method and setup are proposed for trapping and detecting nanoparticles dispersed in a nanocomposite solution using periodically localized light generated by a subwavelength transmission grating. By numerical simulations, it is shown that there is an optimum duty ratio of the grating to produce the periodically localized light. Experimental results are presented for Au/γ-Fe2O3 composite nanoparticles having a diameter of 21.0 nm trapped on a silicon subwavelength rectangular grating and detected ellipsometrically. The technique should prove useful for evaluating optical and mechanical properties of nanocomposite materials.


Wednesday, December 16, 2015

Constructing 3D microtubule networks using holographic optical trapping

J. Bergman, O. Osunbayo & M. Vershinin

Developing abilities to assemble nanoscale structures is a major scientific and engineering challenge. We report a technique which allows precise positioning and manipulation of individual rigid filaments, enabling construction of custom-designed 3D filament networks. This approach uses holographic optical trapping (HOT) for nano-positioning and microtubules (MTs) as network building blocks. MTs are desirable engineering components due to their high aspect ratio, rigidity, and their ability to serve as substrate for directed nano-transport, reflecting their roles in the eukaryotic cytoskeleton. The 3D architecture of MT cytoskeleton is a significant component of its function, however experimental tools to study the roles of this geometric complexity in a controlled environment have been lacking. We demonstrate the broad capabilities of our system by building a self-supporting 3D MT-based nanostructure and by conducting a MT-based transport experiment on a dynamically adjustable 3D MT intersection. Our methodology not only will advance studies of cytoskeletal networks (and associated processes such as MT-based transport) but will also likely find use in engineering nanostructures and devices.


Plasmonic absorption activated trapping and assembling of colloidal crystals with non-resonant continuous gold films

Zhiwen Kang, Jiajie Chen, Shu-Yuen Wu and Ho-Pui Ho

Here we report the realization of trapping and assembly of colloidal crystals on continuous gold thin films based on the combined effect of thermophoresis and thermal convection associated with plasmonic optical heating. In the system, the stabilized trapping phenomenon is driven by thermophoretic forces caused by a temperature gradient which pushes the target particles from cold to hot regions and always in an opposite direction to the axial convective drag forces. Furthermore, the lateral convective flow of an aqueous medium accelerates the formation of the trap considerably by dragging target particles into the hot region from a long distance. The influence of salt concentration on the trapping behavior has also been investigated. Typically the threshold optical power density is in the order of microwatts per square micrometer (∼μW μm−2). We anticipate that the reported optical trapping approach may find many potential applications in biophysics, life sciences, and lab-on-a-chip devices.


Zebrafish as a model system for characterization of nanoparticles against cancer

Lasse Evensen, Patrick L. Johansen, Gerbrand Koster, Kaizheng Zhu, Lars Herfindal, Martin Speth, Federico Fenaroli, Jon Hildahl, Shahla Bagherifam, Claudia Tulotta, Lina Prasmickaite, Gunhild M. Mælandsmo, Ewa Snaar-Jagalska and Gareth Griffiths

Therapeutic nanoparticles (NPs) have great potential to deliver drugs against human diseases. Encapsulation of drugs in NPs protects them from being metabolized, while they are delivered specifically to a target site, thereby reducing toxicity and other side-effects. However, non-specific tissue accumulation of NPs, for example in macrophages, especially in the spleen and liver is a general problem with many NPs being developed for cancer therapy. To address the problem of non-specific tissue accumulation of NPs we describe the development of the zebrafish embryo as a transparent vertebrate system for characterization of NPs against cancer. We show that injection of human cancer cells results in tumor-like structures, and that subsequently injected fluorescent NPs, either made of polystyrene or liposomes can be imaged in real-time. NP biodistribution and general in vivo properties can be easily monitored in embryos having selective fluorescent labeling of specific tissues. We demonstrate in vitro, by using optical tweezer micromanipulation, microscopy and flow cytometry that polyethylene glycol (PEG) coating of NPs decreases the level of adhesion of NPs to macrophages, and also to cancer cells. In vivo in zebrafish embryos, PEG coating resulted in longer NP circulation times, decreased macrophage uptake, and reduced adhesion to the endothelium. Importantly, liposomes were observed to accumulate passively and selectively in tumor-like structures comprised of human cancer cells. These results show that zebrafish embryo is a powerful system for microscopy-based screening of NPs on the route to preclinical testing.


Tuesday, December 15, 2015

Simple method to measure and analyze the fluctuations of a small particle in biopolymer solutions

Masafumi Kuroda and Yoshihiro Murayama
We developed a simple method to investigate the motion of a small particle in biopolymer solutions. Using optical tweezers with low stiffness, a trapped probe particle fluctuates widely for a long time along the light axis, which reflects the rheological properties of the surrounding environment. We present a convenient technique for three-dimensional position tracking and the analysis focused on the distribution of particle positions and its variance in a given time interval. It allows us to obtain useful information about the dynamics of a small particle in a wide range from a free diffusive motion to a constrained motion with statistical significance. We applied this method to investigate the dynamics in collagen and DNA solutions; it was found that a collagen solution behaves as a simple viscous liquid and a DNA solution has apparent elasticity due to the slow relaxation of the configuration of molecules.


Effect of elastic colored noise in the hopping dynamics of single molecules in stretching experiments

M. Hidalgo-Soria, A. Pérez-Madrid, and I. Santamaría-Holek

The influence of colored noise induced by elastic fluctuations in single-molecule stretching experiments is theoretically and numerically studied. Unlike in the thermal white noise case currently considered in the literature, elastically induced hopping dynamics between folded and unfolded states is manifested through critical oscillations showing smaller end-to-end distance fluctuations (δx∼1.25nm) within the free energy wells corresponding to both states. Our results are derived by analyzing the elastic coupling between the Handle-Molecule-Handle system and the laser optical tweezers (LOT) array. It is shown that an Ornstein-Uhlenbeck process related to this elastic coupling may trigger the hopping transitions via a colored noise with an intensity proportional to the elastic constant of the LOT array. Evolution equations of the variables of the system were derived by using the irreversible thermodynamics of small systems recently proposed. Theoretical expressions for the corresponding stationary probability densities are provided and the viability of inferring the shape of the free energy from direct measurements is discussed.


Cellobiohydrolase 1 from Trichoderma reesei degrades cellulose in single cellobiose steps

Sonia K. Brady, Sarangapani Sreelatha, Yinnian Feng, Shishir P. S. Chundawat & Matthew J Lang

Cellobiohydrolase 1 from Trichoderma reesei (TrCel7A) processively hydrolyses cellulose into cellobiose. Although enzymatic techniques have been established as promising tools in biofuel production, a clear understanding of the motor’s mechanistic action has yet to be revealed. Here, we develop an optical tweezers-based single-molecule (SM) motility assay for precision tracking of TrCel7A. Direct observation of motility during degradation reveals processive runs and distinct steps on the scale of 1 nm. Our studies suggest TrCel7A is not mechanically limited, can work against 20 pN loads and speeds up when assisted. Temperature-dependent kinetic studies establish the energy requirements for the fundamental stepping cycle, which likely includes energy from glycosidic bonds and other sources. Through SM measurements of isolated TrCel7A domains, we determine that the catalytic domain alone is sufficient for processive motion, providing insight into TrCel7A’s molecular motility mechanism.


Fiber-Based, Injection-Molded Optofluidic Systems: Improvements in Assembly and Applications

Marco Matteucci, Marco Triches, Giovanni Nava, Anders Kristensen, Mark R. Pollard, Kirstine Berg-Sørensen and Rafael J. Taboryski

We present a method to fabricate polymer optofluidic systems by means of injection molding that allow the insertion of standard optical fibers. The chip fabrication and assembly methods produce large numbers of robust optofluidic systems that can be easily assembled and disposed of, yet allow precise optical alignment and improve delivery of optical power. Using a multi-level chip fabrication process, complex channel designs with extremely vertical sidewalls, and dimensions that range from few tens of nanometers to hundreds of microns can be obtained. The technology has been used to align optical fibers in a quick and precise manner, with a lateral alignment accuracy of 2.7 ± 1.8 μm. We report the production, assembly methods, and the characterization of the resulting injection-molded chips for Lab-on-Chip (LoC) applications. We demonstrate the versatility of this technology by carrying out two types of experiments that benefit from the improved optical system: optical stretching of red blood cells (RBCs) and Raman spectroscopy of a solution loaded into a hollow core fiber. The advantages offered by the presented technology are intended to encourage the use of LoC technology for commercialization and educational purposes.


Monday, December 14, 2015

Single exosome study reveals subpopulations distributed among cell lines with variability related to membrane content

Zachary J. Smith, Changwon Lee, Tatu Rojalin, Randy P. Carney, Sidhartha Hazari, Alisha Knudson, Kit Lam, Heikki Saari, Elisa Lazaro Ibañez, Tapani Viitala, Timo Laaksonen, Marjo Yliperttula and Sebastian Wachsmann-Hogiu

Current analysis of exosomes focuses primarily on bulk analysis, where exosome-to-exosome variability cannot be assessed. In this study, we used Raman spectroscopy to study the chemical composition of single exosomes. We measured spectra of individual exosomes from 8 cell lines. Cell-line-averaged spectra varied considerably, reflecting the variation in total exosomal protein, lipid, genetic, and cytosolic content. Unexpectedly, single exosomes isolated from the same cell type also exhibited high spectral variability. Subsequent spectral analysis revealed clustering of single exosomes into 4 distinct groups that were not cell-line specific. Each group contained exosomes from multiple cell lines, and most cell lines had exosomes in multiple groups. The differences between these groups are related to chemical differences primarily due to differing membrane composition. Through a principal components analysis, we identified that the major sources of spectral variation among the exosomes were in cholesterol content, relative expression of phospholipids to cholesterol, and surface protein expression. For example, exosomes derived from cancerous versus non-cancerous cell lines can be largely separated based on their relative expression of cholesterol and phospholipids. We are the first to indicate that exosome subpopulations are shared among cell types, suggesting distributed exosome functionality. The origins of these differences are likely related to the specific role of extracellular vesicle subpopulations in both normal cell function and carcinogenesis, and they may provide diagnostic potential at the single exosome level.


Surface-modified complex SU-8 microstructures for indirect optical manipulation of single cells

Badri L. Aekbote, Tamás Fekete, Jaroslaw Jacak, Gaszton Vizsnyiczai, Pál Ormos, and Lóránd Kelemen

We introduce a method that combines two-photon polymerization (TPP) and surface functionalization to enable the indirect optical manipulation of live cells. TPP-made 3D microstructures were coated specifically with a multilayer of the protein streptavidin and non-specifically with IgG antibody using polyethylene glycol diamine as a linker molecule. Protein density on their surfaces was quantified for various coating methods. The streptavidin-coated structures were shown to attach to biotinated cells reproducibly. We performed basic indirect optical micromanipulation tasks with attached structure-cell couples using complex structures and a multi-focus optical trap. The use of such extended manipulators for indirect optical trapping ensures to keep a safe distance between the trapping beams and the sensitive cell and enables their 6 degrees of freedom actuation.


Photoinduced magnetic force between nanostructures

Caner Guclu, Venkata Ananth Tamma, Hemantha Kumar Wickramasinghe, and Filippo Capolino

Photoinduced magnetic force between nanostructures, at optical frequencies, is investigated theoretically. Till now optical magnetic effects were not used in scanning probe microscopy because of the vanishing natural magnetism with increasing frequency. On the other hand, artificial magnetism in engineered nanostructures led to the development of measurable optical magnetism. Here two examples of nanoprobes that are able to generate strong magnetic dipolar fields at optical frequency are investigated: first, an ideal magnetically polarizable nanosphere and then a circular cluster of silver nanospheres that has a looplike collective plasmonic resonance equivalent to a magnetic dipole. Magnetic forces are evaluated based on nanostructure polarizabilities, i.e., induced magnetic dipoles, and magnetic-near field evaluations. As an initial assessment on the possibility of a magnetic nanoprobe to detect magnetic forces, we consider two identical magnetically polarizable nanoprobes and observe magnetic forces on the order of piconewtons, thereby bringing it within detection limits of conventional atomic force microscopes at ambient pressure and temperature. The detection of magnetic force is a promising method in studying optical magnetic transitions that can be the basis of innovative spectroscopy applications.


Friday, December 11, 2015

Cellular Temperature Measurement by Dielectrophoretic Impedance Measurement Method


Dielectrophoretic impedance measurement (DEPIM) method has recently attracted attention because the method is simple and immediately. We demonstrated that the impedance between the electrodes with trapped cell has thermal property and measured cellular temperature by the impedance change even if the number of trapped cells did not change. However, this result was not observed when cells were not trapped between the electrodes.
In summary, the cellular temperature could be measured by observing the change in shunt voltage. In this study, we developed cellular temperature measurement system and it was expected to be used in biosensor application. In the future, we want to measure single cellular temperature with isolation by optical tweezers.


Femtosecond Fiber Laser Applying for Cell Fusion

Trang Dang NGUYEN, Yoshihiro MIZUTA, Kozo TAGUCHI

We developed an actively mode-locked fiber laser that can generate 295 fs pulses at 9.188 MHz repetition rate. We built up a laser-induced cell fusion, in which the developed femtosecond laser was used as the laser source for both optical tweezers mode and laser scalpel mode, and thus improving cost-effectiveness. The cell fusion system also used a transparent dielectrophoresis chip as the specimen stage to create and manipulate the pearl chain of two or multiple cells for facilitating the cell fusion processes. We successfully developed the first optical tweezers using femtosecond fiber laser operating at 1530 nm, which can trap and transport cells effectively. With this developed system, we obtained the laser-induced fusion of red cabbage protoplasts. We also proposed a experimental cell fusion procedure which allows precisely selective cell fusion at the single-cell level. Therefore, the developed system would benefit basic research in biotechnology and biomedicine.


Metal Coated Chemically Etched Fiber Probe for Single Cell Manipulation and Isolation

Kozo TAGUCHI, Ryo KIDO, Yoshihiro MIZUTA

In this paper, dielectrophoresis tweezers using metal coated chemically etched fiber was proposed for cell manipulation and isolation. We proposed a simple and low cost dielectrophoretic device for picking out and relocating single target cells. The device consists of metal coated chemically etched fibers and an AC signal generator. It does not require microfabrication technologies or sophisticated electronics. From experimental results, it was found that our proposed dielectrophoretic manipulator could discriminate between live and dead cells. We also could see the cell reproduction of yeast cells trapped and isolated using our proposed dielectrophoresis tweezers.

Lab-on-Chip System Combined Optical Tweezers and Dielectrophoresis

Yoshihiro MIZUTA, Kozo TAGUCHI

Cell manipulation and operation have played important roles in modern biotechnology, hence a number of researchers have developed them during the past decades. In this paper we introduce two techniques based on dielectrophoresis (DEP) and optical tweezers: Distinction between viable cells and non-viable cells using DEP, Manipulating cells captured by positive DEP (pDEP) using optical tweezers. And then we suggest a system combined both DEP and optical tweezers, that would be a useful tool in biotechnology. This system will be useful to perform cell operation and other applications in biotechnology.


Thursday, December 10, 2015

Self-induced back-action optical trapping in nanophotonic systems

Lukas Neumeier, Romain Quidant and Darrick E Chang

Optical trapping is an indispensable tool in physics and the life sciences. However, there is a clear trade off between the size of a particle to be trapped, its spatial confinement, and the intensities required. This is due to the decrease in optical response of smaller particles and the diffraction limit that governs the spatial variation of optical fields. It is thus highly desirable to find techniques that surpass these bounds. Recently, a number of experiments using nanophotonic cavities have observed a qualitatively different trapping mechanism described as 'self-induced back-action trapping' (SIBA). In these systems, the particle motion couples to the resonance frequency of the cavity, which results in a strong interplay between the intra-cavity field intensity and the forces exerted. Here, we provide a theoretical description that for the first time captures the remarkable range of consequences. In particular, we show that SIBA can be exploited to yield dynamic reshaping of trap potentials, strongly sub-wavelength trap features, and significant reduction of intensities seen by the particle, which should have important implications for future trapping technologies.


Depletion interactions and modulation of DNA-intercalators binding: Opposite behavior of the “neutral” polymer poly(ethylene-glycol)

F. A. P. Crisafuli, L. H. M. da Silva, G. M. D. Ferreira, E. B. Ramos and M. S. Rocha

In this work we have investigated the role of high molecular weight poly(ethylene-glycol) 8000 (PEG 8000) in modulating the interactions of the DNA molecule with two hydrophobic compounds: Ethidium Bromide (EtBr) and GelRed (GR). Both compounds are DNA intercalators and are used here to mimic the behavior of more complex DNA ligands such as chemotherapeutic drugs and proteins whose domains intercalate DNA. By means of single-molecule stretching experiments, we have been able to show that PEG 8000 strongly shifts the binding equilibrium between the intercalators and the DNA even at very low concentrations (1% in mass). Additionally, microcalorimetry experiments were performed to estimate the strength of the interaction between PEG and the DNA ligands. Our results suggest that PEG, depending on the system under study, may act as an “inert polymer” with no enthalpic contribution in some processes but, on the other hand, it may as well be an active (non-neutral) osmolyte in the context of modulating the activity of the reactants and products involved in DNA-ligand interactions.


Wednesday, December 9, 2015

Transformation and patterning of supermicelles using dynamic holographic assembly

Oliver E.C. Gould, Huibin Qiu, David J. Lunn, John Rowden, Robert L. Harniman, Zachary M. Hudson, Mitchell A. Winnik, Mervyn J. Miles & Ian Manners

Although the solution self-assembly of block copolymers has enabled the fabrication of a broad range of complex, functional nanostructures, their precise manipulation and patterning remain a key challenge. Here we demonstrate that spherical and linear supermicelles, supramolecular structures held together by non-covalent solvophobic and coordination interactions and formed by the hierarchical self-assembly of block copolymer micelle and block comicelle precursors, can be manipulated, transformed and patterned with mediation by dynamic holographic assembly (optical tweezers). This allows the creation of new and stable soft-matter superstructures far from equilibrium. For example, individual spherical supermicelles can be optically held in close proximity and photocrosslinked through controlled coronal chemistry to generate linear oligomeric arrays. The use of optical tweezers also enables the directed deposition and immobilization of supermicelles on surfaces, allowing the precise creation of arrays of soft-matter nano-objects with potentially diverse functionality and a range of applications.


Probing the Evaporation Dynamics of Ethanol/Gasoline Biofuel Blends Using Single Droplet Manipulation Techniques

Stella Corsetti, Rachael E.H. Miles, Craig McDonald, Yuri Belotti, Jonathan Philip Reid, Johannes Kiefer, and David McGloin

Using blends of bio-ethanol and gasoline as automotive fuel leads to a net decrease in the production of harmful emission compared to the use of pure fossil fuel. However, fuel droplet evaporation dynamics change depending on the mixing ratio. Here we use single particle manipulation techniques to study the evaporation dynamics of ethanol/gasoline blend micro-droplets. The use of an electrodynamic balance (EDB) enables measurements of the evaporation of individual droplets in a controlled environment, while optical tweezers facilitate studies of the behavior of droplets inside a spray. Hence, the combination of both methods is perfectly suited to obtain a complete picture of the evaporation process. The influence of adding varied amounts of ethanol to gasoline is investigated, and we observe that droplets with a greater fraction of ethanol take longer to evaporate. Furthermore, we find that our methods are sensitive enough to observe the presence of trace amounts of water in the droplets. A theoretical model, predicting the evaporation of ethanol and gasoline droplets in dry nitrogen gas, is used to explain the experimental results. Also a theoretical estimation of the saturation of the environment, with other aerosols, in the tweezers is carried out.


Broadband boundary effects on Brownian motion

Jianyong Mo, Akarsh Simha, and Mark G. Raizen

Brownian motion of particles in confined fluids is important for many applications, yet the effects of the boundary over a wide range of time scales are still not well understood. We report high-bandwidth, comprehensive measurements of Brownian motion of an optically trapped micrometer-sized silica sphere in water near an approximately flat wall. At short distances we observe anisotropic Brownian motion with respect to the wall. We find that surface confinement not only occurs in the long time scale diffusive regime but also in the short time scale ballistic regime, and the velocity autocorrelation function of the Brownian particle decays faster than that of a particle in bulk fluid. Furthermore, at low frequencies the thermal force loses its color due to the reflected flow from the no-slip boundary. The power spectrum of the thermal force on the particle near a no-slip boundary becomes flat at low frequencies. This detailed understanding of boundary effects on Brownian motion opens a door to developing a 3D microscope using particles as remote sensors.


Tractor beam for fully immersed multiple objects: Long distance pulling, trapping, and rotation with a single optical set-up

Md. Masudur Rahman, Ayed Al Sayem, M. R. C. Mahdy, Md. Ehsanul Haque, Rakibul Islam, S. Tanvir-ur-Rahman Chowdhury and Md. Abdul Matin

In this article, we have theoretically demonstrated the mechanism of an active tractor beam for multiple fully immersed objects with additional abilities to yielding stable long distance levitation, a controlled rotation and a desired 3D trapping. This is demonstrated with a single optical set-up by using two coaxial, or even non-coaxial, superimposed higher order monochromatic Bessel beams of reverse helical nature and different frequencies. The superimposed beams can possess periodic intensity variations both along and around the beam-axis due to a difference in longitudinal wave-numbers and beam orders, respectively. The difference in frequencies of the two laser beams makes the intensity pattern to move along and around the beam-axis in a continuous way without manual ramping of phase, which allows for bidirectional movement of completely immersed multiple particles. The condition for increasing or decreasing the dimension of binding regions is also proposed here to manipulate multiple immersed objects of different sizes under dipole approximation.


Monday, December 7, 2015

Exploded view of higher order G-quadruplex structures through click-chemistry assisted single-molecule mechanical unfolding

Sangeetha Selvam, Zhongbo Yu and Hanbin Mao

Due to the long-range nature of high-order interactions between distal components in a biomolecule, transition dynamics of tertiary structures is often too complex to profile using conventional methods. Inspired by the exploded view in mechanical drawing, here, we used laser tweezers to mechanically dissect high-order DNA structures into two constituting G-quadruplexes in the promoter of the human telomerase reverse transcriptase (hTERT) gene. Assisted with click-chemistry coupling, we sandwiched one G-quadruplex with two dsDNA handles while leaving the other unit free. Mechanical unfolding through these handles revealed transition dynamics of the targeted quadruplex in a native environment, which is named as native mechanical segmentation (NMS). Comparison between unfolding of an NMS construct and that of truncated G-quadruplex constructs revealed a quadruplex–quadruplex interaction with 2 kcal/mol stabilization energy. After mechanically targeting the two G-quadruplexes together, the same interaction was observed during the first unfolding step. The unfolding then proceeded through disrupting the weaker G-quadruplex at the 5′-end, followed by the stronger G-quadruplex at the 3′-end via various intermediates. Such a pecking order in unfolding well reflects the hierarchical nature of nucleic acid structures. With surgery-like precisions, we anticipate this NMS approach offers unprecedented perspective to decipher dynamic transitions in complex biomacromolecules.


FACS-style detection for real-time cell viscoelastic cytometry

Aditya Kasukurti, Charles Eggleton, Sanjay A Desai and David W M Marr

Cell mechanical properties have been established as a label-free biophysical marker of cell viability and health; however, real-time methods with significant throughput for accurately and non-destructively measuring these properties remain widely unavailable. Without appropriate labels for use with fluorescence activated cell sorters (FACS), easily implemented real-time technology for tracking cell-level mechanical properties remains a current need. Employing modulated optical forces and enabled by a low-dimensional FACS-style detection method introduced here, we present a viscoelasticity cytometer (VC) capable of real-time and continuous measure. We demonstrate utility of this approach by tracking the high-frequency cell physical properties of populations of chemically-modified cells at rates of ~ 1 s-1 and explain observations within the context of a simple theoretical model.


Simulation and measurement of stiffness for dual beam laser trap using residual gravity method

Zhenggang Li, Yu Shen, Huizhu Hu, Bo Jia, Minqiang Tie
Dual-beam optical trap is constituted by two beams of unfocused counter-propagating laser. It has advantages such as several equilibrium points, small optical power density, large capture range and easier integration with other technologies, which optical tweezers may not achieve. Dual-beam optical trap stiffness is an important parameter to describe its mechanical properties. On the basis of quartz-substrate dual-beam optical fibre trap unit, we demonstrate a simple method for measuring stiffness. Residual gravity offset by buoyancy may provide a tiny force for the optical trap. By measuring the relations between the captured particle's axial equilibrium position and residual gravity, we may calibrate the optical trap stiffness. This method simply requires measuring the relative position of the particle in the steady state of the optical trap, and dynamic measurement of the particles location is not needed, so we can get a high position accuracy by long time measuring, thus the accuracy of the stiffness calibration is improved. In this paper, we simulated and carried out experiments to measure the stiffness of a dual-beam fibre optical trap unit based on a quartz-substrate. We simulate the stiffness of several Mie particles with different materials and sizes in dual-beam optical trap as well as the relation between the waist radius of fundamental mode Gaussian beam and stiffness. Stiffness measurement experiment is conducted using the residual gravity method. Stiffness values measured in experiments are consistent with the simulation results, which verify the validity and accuracy of the method.


From transverse angular momentum to photonic wheels

Andrea Aiello, Peter Banzer, Martin Neugebauer & Gerd Leuchs

Scientists have known for more than a century that light possesses both linear and angular momenta along the direction of propagation. However, only recent advances in optics have led to the notion of spinning electromagnetic fields capable of carrying angular momenta transverse to the direction of motion. Such fields enable numerous applications in nano-optics, biosensing and near-field microscopy, including three-dimensional control over atoms, molecules and nanostructures, and allowing for the realization of chiral nanophotonic interfaces and plasmonic devices. Here, we report on recent developments of optics with light carrying transverse spin. We present both the underlying principles and the latest achievements, and also highlight new capabilities and future applications emerging from this young yet already advanced field of research.


Friday, December 4, 2015

Detecting Single Gold Nanoparticles (1.8 nm) with Ultrahigh-Q Air-Mode Photonic Crystal Nanobeam Cavities

Feng Liang and Qimin Quan

The growing applications of nanoparticles in energy and healthcare demand new metrology techniques with improved sensitivity, lower sample concentration, and affordable instrument cost. Here we demonstrate the first air-mode photonic crystal nanobeam cavity with ultrahigh Q-factor (Q = 2.5 × 105) and ultrasmall mode volume (V = 0.01λ3) at telecom wavelength. The air-mode cavity has strong field localization outside of its high-index material, thus significantly improving the sensitivity to detect nanoparticles. The strong field gradient attracts the nanoparticles to its field maximum, improving the detection efficiency. Combining these advantages, we report detecting and sizing single gold nanoparticles down to 1.8 nm in diameter (equivalently single polystyrene nanoparticle of 3 nm in diameter) with significantly reduced sample concentration (∼fM) than traditional optical techniques. In addition, the air-mode ultrahigh Q, ultrasmall V photonic crystal nanobeam cavity will be a useful platform to study strong light–matter interactions, nonlinear processes, and cavity quantum electrodynamics.


Topological binding and elastic interactions of microspheres and fibres in a nematic liquid crystal

M. Nikkhou, M. Škarabot, I. Muševič

We present a detailed analysis of topological binding and elastic interactions between a long, and micrometer-diameter fiber, and a microsphere in a homogeneously aligned nematic liquid crystal. Both objects are surface treated to produce strong perpendicular anchoring of the nematic liquid crystal. We use the opto-thermal micro-quench of the laser tweezers to produce topological defects with prescribed topological charge, such as pairs of a Saturn ring and an anti-ring, hyperbolic and radial hedgehogs on a fiber, as well as zero-charge loops. We study the entanglement and topological charge interaction between the topological defects of the fiber and sphere and we observe a huge variety of different entanglement topologies and defect-mediated elastic bindings. We explain all observed phenomena with simple topological rule: like topological charges repel each other and opposite topological charges attract. These binding mechanisms not only demonstrate the fascinating topology of nematic colloids, but also open a novel route to the assembly of very complex topological networks of fibers, spheres and other objects for applications in liquid crystal photonics.


Cell mechanics in biomedical cavitation

Qianxi Wang, Kawa Manmi, Kuo-Kang Liu
Studies on the deformation behaviours of cellular entities, such as coated microbubbles and liposomes subject to a cavitation flow, become increasingly important for the advancement of ultrasonic imaging and drug delivery. Numerical simulations for bubble dynamics of ultrasound contrast agents based on the boundary integral method are presented in this work. The effects of the encapsulating shell are estimated by adapting Hoff's model used for thin-shell contrast agents. The viscosity effects are estimated by including the normal viscous stress in the boundary condition. In parallel, mechanical models of cell membranes and liposomes as well as state-of-the-art techniques for quantitative measurement of viscoelasticity for a single cell or coated microbubbles are reviewed. The future developments regarding modelling and measurement of the material properties of the cellular entities for cutting-edge biomedical applications are also discussed.


Micro- and nanotechnology for cell biophysics

Péter Galajda, Lóránd Kelemen, Gergely A. Végh
Procedures and methodologies used in cell biophysics have been improved tremendously with the revolutionary advances witnessed in the micro- and nanotechnology in the last two decades. With the advent of microfluidics it became possible to reduce laboratory-sized equipment to the scale of a microscope slide allowing massive parallelization of measurements with extremely low sample volume at the cellular level. Optical micromanipulation has been used to measure forces or distances or to alter the behavior of biological systems from the level of DNA to organelles or entire organisms. Among the main advantages is its non-invasiveness, giving researchers an invisible micro-hand to “touch” or “feel” the system under study, its freely and very often quickly adjustable experimental parameters such as wavelength, optical power or intensity distribution. Atomic force microscopy (AFM) opened avenues for in vitro biological applications concerning with single molecule imaging, cellular mechanics or morphology. As it can operate in liquid environment and at human body temperature, it became the most reliable and accurate nanoforce-tool in the research of cell biophysics. In this paper we review how the above three techniques help increase our knowledge in biophysics at the cellular level.


Plasmonic Coupling Dynamics of Silver Nanoparticles in an Optical Trap

Marc Blattmann and Alexander Rohrbach

We investigate binding and plasmonic coupling between optically trapped 80 nm silver spheres using a combination of spectroscopic sensing and 3D interferometric laser particle tracking on a 1 μs time scale. We demonstrate that nanoparticle coupling can be either spontaneous or induced by another particle through confinement of diffusion. We reveal ultrafast entries and exits of nanoparticles inside the optical trap, fast particle rearrangements before binding, and dimer formation allowing new insights into nanoparticle self-assembly.


Wednesday, December 2, 2015

‘Lissajous-like’ trajectories in optical tweezers

R. F. Hay, G. M. Gibson, S. H. Simpson, M. J. Padgett, and D. B. Phillips

When a microscopic particle moves through a low Reynolds number fluid, it creates a flow-field which exerts hydrodynamic forces on surrounding particles. In this work we study the ‘Lissajous-like’ trajectories of an optically trapped ‘probe’ microsphere as it is subjected to time-varying oscillatory hydrodynamic flow-fields created by a nearby moving particle (the ‘actuator’). We show a breaking of time-reversal symmetry in the motion of the probe when the driving motion of the actuator is itself time-reversal symmetric. This symmetry breaking results in a fluid-pumping effect, which arises due to the action of both a time-dependent hydrodynamic flow and a position-dependent optical restoring force, which together determine the trajectory of the probe particle. We study this situation experimentally, and show that the form of the trajectories observed is in good agreement with Stokesian dynamics simulations. Our results are related to the techniques of active micro-rheology and flow measurement, and also highlight how the mere presence of an optical trap can perturb the environment it is in place to measure.


Spin–orbit photonics

Filippo Cardano & Lorenzo Marrucci

Spin–orbit optical phenomena involve the interaction of the photon spin with the light wave propagation and spatial distribution, mediated by suitable optical media. Here we present a short overview of the emerging photonic applications that rely on such effects.


Monday, November 30, 2015

Influence of arbitray mode in cluster formation in optical tweezers

R. Kumar

Three dimension multiparticle optical trapping and dynamic manipulation in a pre-defined fashion is a routine affair often realized by spatio-temporal modulation of fundamental Gaussian beam either using lens or diffractive optical elements (DOE). We present multi-particle trapping which is achieved due to cumulative effects of both the weak focusing of arbitrary beam and constructive interference arising from scrambling outwardly from trapped polystyrene spheres. The arbitrary mode is generated from diode pumped solid state source due to cavity imperfections per-se. The prime motivation of this article is to establish the usability of arbitrary mode in arranging as external optics in OTs for achieving colloidal clusters of microspheres from which forwardly scattered light can be utilized as feedback in shaping the wavefronts to be focused in highly turbid media. In general, it offers overall cost-cutting of experimental set-up and particularly suitable for fundamental demonstration of trapping to undergraduates only.


Transient dynamics of a colloidal particle driven through a viscoelastic fluid

Juan Ruben Gomez-Solano and Clemens Bechinger
We study the transient motion of a colloidal particle actively dragged by an optical trap through different viscoelastic fluids (wormlike micelles, polymer solutions, and entangled λ-phage DNA). We observe that, after sudden removal of the moving trap, the particle recoils due to the recovery of the deformed fluid microstructure. We find that the transient dynamics of the particle proceeds via a double-exponential relaxation, whose relaxation times remain independent of the initial particle velocity whereas their amplitudes strongly depend on it. While the fastest relaxation mirrors the viscous damping of the particle by the solvent, the slow relaxation results from the recovery of the strained viscoelastic matrix. We show that this transient information, which has no counterpart in Newtonian fluids, can be exploited to investigate linear and nonlinear rheological properties of the embedding fluid, thus providing a novel method to perform transient rheology at the micron-scale.


Femtosecond Nanostructuring of Glass with Optically Trapped Microspheres and Chemical Etching

Aleksander Shakhov, Artyom Astafiev, Alexander Gulin, and Victor A. Nadtochenko

Laser processing with optically trapped microspheres is a promising tool for nanopatterning at sub-diffraction limited resolution in a wide range of technological and biomedical applications. In this paper, we investigate sub-diffraction limited structuring of borosilicate glass with femtosecond pulses in the near-field of optically trapped microspheres combined with chemical post-processing. Glass surface was processed by single laser pulses at 780 nm focused by silica microspheres and then subjected to selective etching in KOH, which produced pits in the laser affected zones (LAZs). Chemical post-processing allowed obtaining structures with better resolution and reproducibility. We demonstrate production of reproducible pits with diameter as small as 70 nm (λ/11). Complex 2-Dimensional structures with 100 nm (λ/8) resolution were written on the glass surface point by point with microspheres manipulated by optical tweezers. Furthermore, the mechanism of laser modification underlying selective etching was investigated with mass-spectrum analysis. We propose that increased etching rate of laser-treated glass result from change in its chemical composition and oxygen deficiency.


Versatile microsphere attachment of GFP-labeled motors and other tagged proteins with preserved functionality

Michael Bugiel, Horatiu Fantana, Volker Bormuth, Anastasiya Trushko, Frederic Schiemann, Jonathon Howard, Erik Schäffer, Anita Jannasch

Microspheres are often used as handles for protein purification or force spectroscopy. For example, optical tweezers apply forces on trapped particles to which motor proteins are attached. However, even though many attachment strategies exist, procedures are often limited to a particular biomolecule and prone to non-specific protein or surface attachment. Such interactions may lead to loss of protein functionality or microsphere clustering. Here, we describe a versatile coupling procedure for GFP-tagged proteins via a polyethylene glycol linker preserving the functionality of the coupled proteins. The procedure combines well-established protocols, is highly reproducible, reliable, and can be used for a large variety of proteins. The coupling is efficient and can be tuned to the desired microsphere-to-protein ratio. Moreover, microspheres hardly cluster or adhere to surfaces. Furthermore, the procedure can be adapted to different tags providing flexibility and a promising attachment strategy for any tagged protein.


Thursday, November 26, 2015

Holographic Raman tweezers controlled by multi-modal natural user interface

Zoltán Tomori, Peter Keša, Matej Nikorovič, Jan Kaňka, Petr Jákl, Mojmír Šerý, Silvie Bernatová, Eva Valušová, Marián Antalík and Pavel Zemánek

Holographic optical tweezers provide a contactless way to trap and manipulate several microobjects independently in space using focused laser beams. Although the methods of fast and efficient generation of optical traps are well developed, their user friendly control still lags behind. Even though several attempts have appeared recently to exploit touch tablets, 2D cameras, or Kinect game consoles, they have not yet reached the level of natural human interface. Here we demonstrate a multi-modal 'natural user interface' approach that combines finger and gaze tracking with gesture and speech recognition. This allows us to select objects with an operator's gaze and voice, to trap the objects and control their positions via tracking of finger movement in space and to run semi-automatic procedures such as acquisition of Raman spectra from preselected objects. This approach takes advantage of the power of human processing of images together with smooth control of human fingertips and downscales these skills to control remotely the motion of microobjects at microscale in a natural way for the human operator.


Optical trapping and manipulation of nanoparticles using a meta plasmonic structure

Rehab Kotb, Mahmoud El Maklizi, Yehea Ismail and Mohamed A Swillam
In this paper, a novel structure of nano optical tweezers using triple-slit plasmonic grating structure is introduced and analyzed. The tweezers have deep potential wells that can trap sub-10 nm dielectric particle stably and efficiently. The resultant 50 KT potential well provides tight 2D trapping to the particle. The plasmonic device allows for steering the particle by simply changing the angle of the incident plane. This simple control allows efficient manipulation of the trapped particle over wide range of angles.


Probing the Red Blood Cells Aggregating Force With Optical Tweezers

Kisung Lee, Danilina, A.V. ; Kinnunen, M. ; Priezzhev, A.V. ; Meglinski, I.

The red blood cells (RBC) aggregation is of current basic science and clinical interest, as a determinant of blood microcirculation. Thus, the measurement and assessment of the RBC aggregation property (aggregability) and aggregation state at different physiologic conditions of a human individual or laboratory animal are an important issue. In this paper, in order to assess the dynamics of RBC interaction, optical tweezers were used to probe the forces during the RBC doublet formation or disruption. We show that in autologous plasma, RBC aggregating and disaggregating forces have different absolute values, ca 2-4 pN and dozens of piconewton, correspondingly. We speculate that in plasma, RBC aggregation and disaggregation processes have different driving forces.


Probing the Casimir force with optical tweezers

D. S. Ether jr., L. B. Pires, S. Umrath, D. Martinez, Y. Ayala, B. Pontes, G. R. de S. Araújo, S. Frases, G.-L. Ingold, F. S. S. Rosa

We propose to use optical tweezers to probe the Casimir interaction between microspheres inside a liquid medium for geometric aspect ratios far beyond the validity of the widely employed proximity force approximation. This setup has the potential for revealing unprecedented features associated to the non-trivial role of the spherical curvatures. For a proof of concept, we measure femtonewton double-layer forces between polystyrene microspheres at distances above 400 nm by employing very soft optical tweezers, with stiffness of the order of fractions of a fN/nm. As a future application, we propose to tune the Casimir interaction between a metallic and a polystyrene microsphere in saline solution from attraction to repulsion by varying the salt concentration. With those materials, the screened Casimir interaction may have a larger magnitude than the unscreened one. This line of investigation has the potential for bringing together different fields including classical and quantum optics, statistical physics and colloid science, while paving the way for novel quantitative applications of optical tweezers in cell and molecular biology.


Wednesday, November 25, 2015

Dimensionality constraints of light-induced rotation

László Oroszi, András Búzás, Péter Galajda, Lóránd Kelemen, Anna Mathesz, Tamás Vicsek, Gaszton Vizsnyiczai and Pál Ormos

We have studied the conditions of rotation induced by collimated light carrying no angular momentum. Objects of different shapes and optical properties were examined in the nontrivial case where the rotation axis is perpendicular to the direction of light propagation. This geometry offers important advantages for application as it fundamentally broadens the possible practical arrangements to be realised. We found that collimated light cannot drive permanent rotation of 2D or prism-like 3D objects (i.e., fixed cross-sectional profile along the rotation axis) in the case of fully reflective or fully transparent materials. Based on both geometrical optics simulations and theoretical analysis, we derived a general condition for rotation induced by collimated light carrying no angular momentum valid for any arrangement: Permanent rotation is not possible if the scattering interaction is two-dimensional and lossless. In contrast, light induced rotation can be sustained if partial absorption is present or the object has specific true 3D geometry. We designed, simulated, fabricated, and experimentally tested a microscopic rotor capable of rotation around an axis perpendicular to the illuminating light.


Direct observation of processive exoribonuclease motion using optical tweezers

Furqan M. Fazal, Daniel J. Koslover, Ben F. Luisi, and Steven M. Block

Bacterial RNases catalyze the turnover of RNA and are essential for gene expression and quality surveillance of transcripts. In Escherichia coli, the exoribonucleases RNase R and polynucleotide phosphorylase (PNPase) play critical roles in degrading RNA. Here, we developed an optical-trapping assay to monitor the translocation of individual enzymes along RNA-based substrates. Single-molecule records of motion reveal RNase R to be highly processive: one molecule can unwind over 500 bp of a structured substrate. However, enzyme progress is interrupted by pausing and stalling events that can slow degradation in a sequence-dependent fashion. We found that the distance traveled by PNPase through structured RNA is dependent on the A+U content of the substrate and that removal of its KH and S1 RNA-binding domains can reduce enzyme processivity without affecting the velocity. By a periodogram analysis of single-molecule records, we establish that PNPase takes discrete steps of six or seven nucleotides. These findings, in combination with previous structural and biochemical data, support an asymmetric inchworm mechanism for PNPase motion. The assay developed here for RNase R and PNPase is well suited to studies of other exonucleases and helicases.


Laser refrigeration of hydrothermal nanocrystals in physiological media

Paden B. Roder, Bennett E. Smith, Xuezhe Zhou, Matthew J. Crane, and Peter J. Pauzauskie

Coherent laser radiation has enabled many scientific and technological breakthroughs including Bose–Einstein condensates, ultrafast spectroscopy, superresolution optical microscopy, photothermal therapy, and long-distance telecommunications. However, it has remained a challenge to refrigerate liquid media (including physiological buffers) during laser illumination due to significant background solvent absorption and the rapid (∼ps) nonradiative vibrational relaxation of molecular electronic excited states. Here we demonstrate that single-beam laser trapping can be used to induce and quantify the local refrigeration of physiological media by >10 °C following the emission of photoluminescence from upconverting yttrium lithium fluoride (YLF) nanocrystals. A simple, low-cost hydrothermal approach is used to synthesize polycrystalline particles with sizes ranging from <200 nm to >1 μm. A tunable, near-infrared continuous-wave laser is used to optically trap individual YLF crystals with an irradiance on the order of 1 MW/cm2. Heat is transported out of the crystal lattice (across the solid–liquid interface) by anti-Stokes (blue-shifted) photons following upconversion of Yb3+ electronic excited states mediated by the absorption of optical phonons. Temperatures are quantified through analysis of the cold Brownian dynamics of individual nanocrystals in an inhomogeneous temperature field via forward light scattering in the back focal plane. The cold Brownian motion (CBM) analysis of individual YLF crystals indicates local cooling by >21 °C below ambient conditions in D2O, suggesting a range of potential future applications including single-molecule biophysics and integrated photonic, electronic, and microfluidic devices.


Lateral forces on circularly polarizable particles near a surface

Francisco J. Rodríguez-Fortuño, Nader Engheta, Alejandro Martínez & Anatoly V. Zayats

Optical forces allow manipulation of small particles and control of nanophotonic structures with light beams. While some techniques rely on structured light to move particles using field intensity gradients, acting locally, other optical forces can ‘push’ particles on a wide area of illumination but only in the direction of light propagation. Here we show that spin–orbit coupling, when the spin of the incident circularly polarized light is converted into lateral electromagnetic momentum, leads to a lateral optical force acting on particles placed above a substrate, associated with a recoil mechanical force. This counterintuitive force acts in a direction in which the illumination has neither a field gradient nor propagation. The force direction is switchable with the polarization of uniform, plane wave illumination, and its magnitude is comparable to other optical forces.


Tuesday, November 24, 2015

Plasmon-mediated binding forces on gold or silver homodimer and heterodimer

Jiunn-Woei Liaw, Ting-Yu Kuo, Mao-Kuen Kuo

This study theoretically investigates plasmon-mediated optical binding forces, which are exerted on metal homo or heterodimers, induced by the normal illumination of a linearly polarized plane wave or Gaussian beam. Using the multiple multipole method, we analyzed the optical force in terms of Maxwell׳s stress tensor for various interparticle distance at some specific wavelengths. Numerical results show that for a given wavelength there are several stable equilibrium distances between NPs of a homodimer, which are slightly shorter than some integer multiples of the wavelength in medium, such that metal dimer acts as bonded together. At these specific interparticle distances, the optical force between dimer is null and serves a restoring force, which is repulsive and attractive, respectively, as the two NPs are moving closer to and away from each other. The spring constant of the restoring force at the first stable equilibrium is always the largest, indicating that the first stable equilibrium distance is the most stable one. Moreover, the central line (orientation) of a dimer tends to be perpendicular to the polarization of light. For the cases of heterodimers, the phenomenon of stable equilibrium interparticle distance still exists, except there is an extra net photophoretic force drifting the heterodimer as one. Moreover, gradient force provided by a Gaussian beam may reduce the stability of these equilibriums, so larger NPs are preferred to stabilize a dimer under illumination of Gaussian beam. The finding may pave the way for using optical manipulation on the gold or silver colloidal self-assembly.


Optical torque reversal and spin-orbit rotational Doppler shift experiments

Davit Hakobyan and Etienne Brasselet

We report on optical rotational Doppler frequency shift experiments in the context of a counter-intuitive optomechanical phenomenon that is the angular analog of so-called negative optical radiation forces, which involves spin-orbit scattering of light. In practice, spin-orbit opto-mechanical effects arising from the interaction between polarized light and azimuthally varying birefringent optical elements are retrieved from mechano-optical experiments that involve spatial of the medium. Two kinds of experiments (single-beam and two-beam geometries) are performed and both approaches are discussed in the framework of previous dynamical geometric phase and rotational Doppler shift experiments based on spin and/or orbital angular momentum of light.


Droplet Manipulations in Two Phase Flow Microfluidics

Arjen M. Pit, Michèl H. G. Duits and Frieder Mugele

Even though droplet microfluidics has been developed since the early 1980s, the number of applications that have resulted in commercial products is still relatively small. This is partly due to an ongoing maturation and integration of existing methods, but possibly also because of the emergence of new techniques, whose potential has not been fully realized. This review summarizes the currently existing techniques for manipulating droplets in two-phase flow microfluidics. Specifically, very recent developments like the use of acoustic waves, magnetic fields, surface energy wells, and electrostatic traps and rails are discussed. The physical principles are explained, and (potential) advantages and drawbacks of different methods in the sense of versatility, flexibility, tunability and durability are discussed, where possible, per technique and per droplet operation: generation, transport, sorting, coalescence and splitting.


MD Simulation of Brownian Motion of Buckminsterfullerene Trapping in Nano-Optical Tweezers

M. Y. Abdollahzadeh Jamalabadi

Optical tweezers are a relatively new technique for non-invasive manipulation tool in biology and physics for studying single molecules. Brownian motion of a trapped particle poses a challenge to develop the Optical tweezers. Standard methods to analyze the optical tweezers data rely on using power spectrum of the Brownian motion of a dielectric bead trapped in the tweezers for macro scales. In this study the well-known MD code, GROMACS, is modified to find the variation of position and velocity of all atoms in the system of buckminsterfullerene solved in water. By applying the statistical methods our molecular dynamics simulations reveals the diffusion coefficient of the motion and the standard deviation of the Brownian motion. The simulation of system performs for the variety of trap constant and a model for estimation of the diffusion coefficient and the standard deviation of the Brownian motion is presented. Finally, experimental results are discussed based on the proposed model.


Monday, November 23, 2015

Microfluidic Sample Preparation for Single Cell Analysis

Sanjin Hosic, Shashi K. Murthy, and Abigail N Koppes

Single cell analysis is the measurement of transcription, translation, regulatory, and signaling events within individual cells at the molecular level. The goal is to analyze and synthesize information from single cells in order to holistically understand the cell population. This reductionist approach allows researchers to unravel how molecular events within a single cell link to the behavior of tissues, organs, and eventually whole organisms. Single cell analysis has gained significant traction over the past decade, as evidenced by the number of recent reviews.1-3 The field continues to expand exponentially and necessitates a review of developments that have occurred over the past three years. The transition from bulk to single cell analyses has been fueled in part by studies highlighting single cell heterogeneity and stochasticity relative to whole cell populations.4-5 The random variability in these cell populations is likely due to intrinsic noise. Intrinsic noise refers to cell-to-cell variation in transcription and translation products such as ions, mRNA, and proteins. These components are governed by phenomena such as reaction rates and molecular collisions. Given the flexible and dynamic nature of the cell membrane, reactions and molecular collisions will occur stochastically. Thus, it is unreasonable to assume that all cells within a population are equal at any given moment, and only a large number of single cell measurements will reveal this heterogeneity and provide the statistical power to model it. Modeling approaches are necessary for interpreting the massive amount of data generated with single cell analyses such as whole genome sequencing. Furthermore, these models may ultimately guide the optimum operation of a bioprocess such as the production of valuable biotherapeutics via cell culture or deterministic stem cell reprogramming for regenerative medicine.6 Such findings have driven the development of new analytical systems to probe biology at the resolution of a single cell. In order to study single cells accurately and efficiently, systems with high sensitivity and throughput are needed. The small dimensions of microfluidic systems enable single cell and reagent manipulation with minimal dilution,8 resulting in high sensitivity assays. Furthermore, microfluidic systems offer several key advantages toward the study of single cells including facile automation, parallelization, and reagent reduction.8 Early researchers found that sample preparation such as cell manipulation, compartmentalization, and lysis was significantly more difficult to implement at the single cell scale compared to in bulk. However, sample preparation preceding molecular analysis has also been miniaturized, allowing facile sample processing. As such, microfluidic systems have been developed and applied toward the study of single cells extensively.9-10 Given microfluidics’ instrumental role in single cell analysis up to this point, we can expect continued innovations in microfluidics to better enable single cell biology. In this review, novel microfluidic techniques currently used toward sample preparation and subsequent single cell analysis are highlighted. Techniques are discussed in terms of discrete sample preparation steps that may be necessary for characterizing single cells; tissue dissociation into cell suspensions, sorting heterogeneous cell populations into homogenous populations, isolating, and lysing single cells (Figure 1). With each discrete step, conventional approaches are discussed first and then microfluidic based strategies are reviewed. Finally, the future direction for developing microfluidic single cell analysis technology is discussed.


Photodynamic assembly of nanoparticles towards designable patterning

Huan Wang, Yong-lai Zhang, Hong Xia, qidai chen, Kwang-Sup Lee and Hongbo Sun

Recent advancements in nanotechnology continue to stimulate the development of functional devices based on nanomaterials. However, controllable assembly of these tiny nanomaterials into functional structures is still a big challenge for practical applications, nowhere is this more obvious than in the fields of nanodevices. Currently, despite the fact that self-assembly technologies have revealed great potential to reach this end, serious problems with respect to morphology control, designable assembly and even flexible patterning set huge obstacle to the fabrication of functional devices. Nowadays, in addition to self-assembly technologies that make use of relative weak interaction force, photodynamic assembly (PDA) technology has emerged as a promising route to architect functional materials in a controlled manner. In this review, we summarize the recent development in PDA technology for flexible patterning of NPs. Basic fundamentals of PDA that resort to optical trapping (OT) and typical examples regarding to far-field/near-field OT for PDA of various nanoparticles (NPs) have been reviewed. Particularly, femtosecond laser induced photodynamic assembly (FsL-PDA) that enable designable patterning of NPs through a direct writing manner has been introduced. Finally, current challenges and future prospects of this dynamic field are discussed based on our own opinion.


Determining the 3D Orientation of Optically Trapped Upconverting Nanorods by in situ Single-particle Polarized Spectroscopy

Paloma Rodriguez Sevilla, Lucia Labrador, Dominika Wawrzynczyk, Marcin Nyk, Marek Samoc, Ajoy K. Kar, Mark Mackenzie, Lynn Paterson, Daniel Jaque García and Patricia Haro

An approach to unequivocally determine the three dimensional orientation of optically manipulated NaYF4:Er3+,Yb3+ upconverting nanorods (UCNRs) is demonstrated. Long-term immobilization of individual UCNRs inside single and multiple resonant optical traps allow for stable single UCNR spectroscopy studies. Based on the strongly polarization dependent upconverted luminescence of UCNRs it is possible to unequivocally determine, in real time, their three dimensional orientation when optically trapped. In single-beam traps, polarized single particle spectroscopy has concluded that UCNRs orientate parallel to the propagation axis of the trapping beam. On the other hand, when multiple-beam optical tweezers are used, single particle polarization spectroscopy demonstrated how full spatial control over UCNR orientation can be achieved by changing the trap-to-trap distance as well as on the relative orientation between optical traps. All these results show the possibility of real time three dimensional manipulation and tracking of anisotropic nanoparticles with wide potential application in modern nanobiophotonics.


Mass-manufacturable polymer microfluidic device for dual fiber optical trapping

Diane De Coster, Heidi Ottevaere, Michael Vervaeke, Jürgen Van Erps, Manly Callewaert, Pieter Wuytens, Stephen H. Simpson, Simon Hanna, Wim De Malsche, and Hugo Thienpont

We present a microfluidic chip in Polymethyl methacrylate (PMMA) for optical trapping of particles in an 80µm wide microchannel using two counterpropagating single-mode beams. The trapping fibers are separated from the sample fluid by 70µm thick polymer walls. We calculate the optical forces that act on particles flowing in the microchannel using wave optics in combination with non-sequential ray-tracing and further mathematical processing. Our results are compared with a theoretical model and the Mie theory. We use a novel fabrication process that consists of a premilling step and ultraprecision diamond tooling for the manufacturing of the molds and double-sided hot embossing for replication, resulting in a robust microfluidic chip for optical trapping. In a proof-of-concept demonstration, we show the trapping capabilities of the hot embossed chip by trapping spherical beads with a diameter of 6µm, 8µm and 10µm and use the power spectrum analysis of the trapped particle displacements to characterize the trap strength.


Thursday, November 19, 2015

Trapping of classical particles by an electromagnetic potential well deepening over time

A. Ch. Izmailov
On the basis of fundamental relations of classical mechanics, we established a mechanism of trapping and localization of sufficiently slow particles by an electromagnetic potential well that becomes deeper over time (up to a certain limit). It is assumed that these particles are contained in high vacuum, and acting upon them forces are not dissipative. Such potential wells can be created by means of an electromagnetic field (nonresonance radiation, in particular) with fixed spatial distribution and nondecreasing over time electric field strength. Trapping and localization of particles in such electromagnetic traps, which takes place due to gradient forces, is analyzed for laser beams with typical intensity distribution. The obtained results can be used in high-resolution spectroscopy of different particles, including, in some cases, atoms and molecules.


Plasmon-Exciton Interactions Probed Using Spatial Co-Entrapment of Nanoparticles by Topological Singularities

Paul J. Ackerman, Haridas Mundoor, Ivan I. Smalyukh, and Jao van de Lagemaat

We study plasmon-exciton interaction by using topological singularities to spatially confine, selectively deliver, co-trap and optically probe colloidal semiconductor and plasmonic nanoparticles. The interaction is monitored in a single quantum system in the bulk of a liquid crystal medium where nanoparticles are manipulated and nanoconfined far from dielectric interfaces using laser tweezers and topological configurations containing singularities. When quantum dot-in-a-rod particles are spatially co-located with a plasmonic gold nanoburst particle in a topological singularity core, its fluorescence increases because blinking is significantly suppressed and the radiative decay rate increases by nearly an order of magnitude owing to the Purcell effect. We argue that the blinking suppression is the result of the radiative rate change that mitigates Auger recombination and quantum dot ionization, consequently reducing nonradiative recombination. Our work demonstrates that topological singularities are an effective platform for studying and controlling plasmon-exciton interactions.


Optical forces in nanorod metamaterial

Andrey A. Bogdanov, Alexander S. Shalin & Pavel Ginzburg

Optomechanical manipulation of micro and nano-scale objects with laser beams finds use in a large span of multidisciplinary applications. Auxiliary nanostructuring could substantially improve performances of classical optical tweezers by means of spatial localization of objects and intensity required for trapping. Here we investigate a three-dimensional nanorod metamaterial platform, serving as an auxiliary tool for the optical manipulation, able to support and control near-field interactions and generate both steep and flat optical potential profiles. It was shown that the ‘topological transition’ from the elliptic to hyperbolic dispersion regime of the metamaterial, usually having a significant impact on various light-matter interaction processes, does not strongly affect the distribution of optical forces in the metamaterial. This effect is explained by the predominant near-fields contributions of the nanostructure to optomechanical interactions. Semi-analytical model, approximating the finite size nanoparticle by a point dipole and neglecting the mutual re-scattering between the particle and nanorod array, was found to be in a good agreement with full-wave numerical simulation. In-plane (perpendicular to the rods) trapping regime, saddle equilibrium points and optical puling forces (directed along the rods towards the light source), acting on a particle situated inside or at the nearby the metamaterial, were found.


Wednesday, November 18, 2015

Gold Nanorod Rotary Motors Driven by Resonant Light Scattering

Lei Shao, Zhong-Jian Yang, Daniel Andrén, Peter Johansson, and Mikael Käll

Efficient and robust artificial nanomotors could provide a variety of exciting possibilities for applications in physics, biology and chemistry, including nanoelectromechanical systems, biochemical sensing, and drug delivery. However, the application of current man-made nanomotors is limited by their sophisticated fabrication techniques, low mechanical output power and severe environmental requirements, making their performance far below that of natural biomotors. Here we show that single-crystal gold nanorods can be rotated extremely fast in aqueous solutions through optical torques dominated by plasmonic resonant scattering of circularly polarized laser light with power as low as a few mW. The nanorods are trapped in 2D against a glass surface, and their rotational dynamics is highly dependent on their surface plasmon resonance properties. They can be kept continuously rotating for hours with limited photothermal side effects and they can be applied for detection of molecular binding with high sensitivity. Because of their biocompatibility, mechanical and thermal stability, and record rotation speeds reaching up to 42 kHz (2.5 million revolutions per minute), these rotary nanomotors could advance technologies to meet a wide range of future nanomechanical and biomedical needs in fields such as nanorobotics, nanosurgery, DNA manipulation and nano/microfluidic flow control.


Dynamic chromatin regulation from a single molecule perspective

Beat Fierz
Chromatin regulatory processes, like all biological reactions, are dynamic and stochastic in nature, but can give rise to stable and inheritable changes in gene expression patterns. A molecular understanding of those processes is key for fundamental biological insight into gene regulation, epigenetic inheritance, lineage determination and therapeutic intervention in the case of disease. In recent years great progress has been made in identifying important molecular players involved in key chromatin regulatory pathways. Conversely, we are only beginning to understand the dynamic interplay between protein effectors, transcription factors and the chromatin substrate itself. Single-molecule approaches employing both highly defined chromatin substrates in vitro, as well as direct observation of complex regulatory processes in vivo open new avenues for a molecular view of chromatin regulation. This review highlights recent applications of single-molecule methods and related techniques to investigate fundamental chromatin regulatory processes.


Lateral optical force on paired chiral nanoparticles in linearly polarized plane waves

Huajin Chen, Yikun Jiang, Neng Wang, Wanli Lu, Shiyang Liu, and Zhifang Lin

We demonstrate that a lateral optical force (LOF) can be induced on paired chiral nanoparticles with opposite handedness under the illumination of a linearly polarized plane wave. The LOFs on both chiral particles are equal and thus can move the pair sideways, with the direction depending on the separation between two particles, as well as the handedness of particle chirality. Analytical theory reveals that the LOF comes largely from the optical potential gradient established by the multiple scattering of light between the paired particles with asymmetric chirality. In addition, it is weakly dependent on the material loss of a particle, a feature of gradient force, while heavily dependent on the magnitude and handedness of particle chirality. The effect is expected to find applications in sorting and separating chiral dimers of different handedness.


Tuesday, November 17, 2015

In situ calibrating optical tweezers with sinusoidal-wave drag force method

Li Di, Zhou Jin-Hua, Hu Xin-Yao, Zhong Min-Cheng, Gong Lei, Wang Zi-Qiang, Wang Hao-Wei and Li Yin-Mei

We introduce a corrected sinusoidal-wave drag force method (SDFM) into optical tweezers to calibrate the trapping stiffness of the optical trap and conversion factor (CF) of photodetectors. First, the theoretical analysis and experimental result demonstrate that the correction of SDFM is necessary, especially the error of no correction is up to 11.25% for a bead of 5 μm in diameter. Second, the simulation results demonstrate that the SDFM has a better performance in the calibration of optical tweezers than the triangular-wave drag force method (TDFM) and power spectrum density method (PSDM) at the same signal-to-noise ratio or trapping stiffness. Third, in experiments, the experimental standard deviations of calibration of trapping stiffness and CF with the SDFM are about less than 50% of TDFM and PSDM especially at low laser power. Finally, the experiments of stretching DNA verify that the in situ calibration with the SDFM improves the measurement stability and accuracy.


Spectrally reconfigurable integrated multi-spot particle trap

Kaelyn D. Leake, Michael A. B. Olson, Damla Ozcelik, Aaron R. Hawkins, and Holger Schmidt

Optical manipulation of small particles in the form of trapping, pushing, or sorting has developed into a vast field with applications in the life sciences, biophysics, and atomic physics. Recently, there has been increasing effort toward integration of particle manipulation techniques with integrated photonic structures on self-contained optofluidic chips. Here, we use the wavelength dependence of multi-spot pattern formation in multimode interference (MMI) waveguides to create a new type of reconfigurable, integrated optical particle trap. Interfering lateral MMI modes create multiple trapping spots in an intersecting fluidic channel. The number of trapping spots can be dynamically controlled by altering the trapping wavelength. This novel, spectral reconfigurability is utilized to deterministically move single and multiple particles between different trapping locations along the channel. This fully integrated multi-particle trap can form the basis of high throughput biophotonic assays on a chip.


High-throughput linear optical stretcher for mechanical characterization of blood cells

Kevin B. Roth, Keith B. Neeves, Jeff Squier and David W. M. Marr

This study describes a linear optical stretcher as a high-throughput mechanical property cytometer. Custom, inexpensive, and scalable optics image a linear diode bar source into a microfluidic channel, where cells are hydrodynamically focused into the optical stretcher. Upon entering the stretching region, antipodal optical forces generated by the refraction of tightly focused laser light at the cell membrane deform each cell in flow. Each cell relaxes as it flows out of the trap and is compared to the stretched state to determine deformation. The deformation response of untreated red blood cells and neutrophils were compared to chemically treated cells. Statistically significant differences were observed between normal, diamide-treated, and glutaraldehyde-treated red blood cells, as well as between normal and cytochalasin D-treated neutrophils. Based on the behavior of the pure, untreated populations of red cells and neutrophils, a mixed population of these cells was tested and the discrete populations were identified by deformability.


Monday, November 16, 2015

Diffusion and Reactivity in Ultraviscous Aerosol and the Correlation with Particle Viscosity

Frances H. Marshall, Rachael E.H. Miles, Young-Chul Song, Peter B. Ohm, Rory M. Power, Jonathan P Reid and Cari S. Dutcher

The slow transport of water, organic species and oxidants in viscous aerosol can lead to aerosol existing in transient states that are not solely governed by thermodynamic principles but by the kinetics of gas-particle partitioning. The relationship between molecular diffusion constants and particle viscosity (for example, as reflected in the Stokes-Einstein equation) is frequently considered to provide an approximate guide to relate the kinetics of aerosol transformation with a material property of the aerosol. We report direct studies of both molecular diffusion and viscosity in the aerosol phase for the ternary system water/maleic acid/sucrose, considering the relationship between the hygroscopic response associated with the change in water partitioning, the volatilisation of maleic acid, the ozonolysis kinetics of maleic acid and the particle viscosity. Although water clearly acts as a plasticiser, the addition of minor fractions of other organic moieties can similarly lead to significant changes in the viscosity from that expected for the dominant component forming the organic matrix (sucrose). Here we highlight that the Stokes-Einstein relationship between the diffusion constant of water and the viscosity of the particle may be more than an order of magnitude in error, even at viscosities as low as 1 Pa s. We show that the thermodynamic relationships of hygroscopic response that underpin such kinetic determinations must be accurately known to retrieve accurate values for diffusion constants; such data are often not available. Further, we show that scaling of the diffusion constants of organic molecules of similar size to those forming the matrix with particle viscosity may be well represented by the Stokes-Einstein equation, suppressing the apparent volatility of semi-volatile components. Finally, the variation in uptake coefficients and diffusion constants for oxidants and small weakly interacting molecules may be much less dependent on viscosity than the diffusion constants of more strongly interacting molecules such as water.


Spatial Filtering of a Diode Laser Beam for Confocal Raman Microscopy

Kitt, Jay P.; Bryce, David A.; Harris, Joel M.

With the development of single-longitudinal mode diode lasers, there has been an increase in using these sources for Raman spectroscopy. This is largely due to the cost-effectiveness of diode lasers, which offer savings not only in initial capital cost, but also electrical, cooling, and replacement costs over time, when compared with ion lasers. The use of diode-lasers in confocal Raman microscopy has remained a challenge, however, due to poor transverse beam quality. In this work, we present the design and implementation of a simple spatial filter capable of adapting a single-mode diode laser source to confocal Raman microscopy, yielding comparable spatial resolution as a gas-ion laser beam for profiling and optical-trapping applications. For profiling applications, spatial filtering improved x,y resolution of the beam by a factor 10, which in turn increased optical-trapping forces by ∼90 times and yielded sevenfold greater Raman scattering signal intensity from an optically trapped phospholipid vesicle.


Friday, November 13, 2015

Nanoscale volume confinement and fluorescence enhancement with double nanohole aperture

Raju Regmi, Ahmed A. Al Balushi, Hervé Rigneault, Reuven Gordon & Jérôme Wenger
Diffraction ultimately limits the fluorescence collected from a single molecule, and sets an upper limit to the maximum concentration to isolate a single molecule in the detection volume. To overcome these limitations, we introduce here the use of a double nanohole structure with 25 nm gap, and report enhanced detection of single fluorescent molecules in concentrated solutions exceeding 20 micromolar. The nanometer gap concentrates the light into an apex volume down to 70 zeptoliter (10−21 L), 7000-fold below the diffraction-limited confocal volume. Using fluorescence correlation spectroscopy and time-correlated photon counting, we measure fluorescence enhancement up to 100-fold, together with local density of optical states (LDOS) enhancement of 30-fold. The distinctive features of double nanoholes combining high local field enhancement, efficient background screening and relative nanofabrication simplicity offer new strategies for real time investigation of biochemical events with single molecule resolution at high concentrations.


Enhanced Optical Manipulation of Cells Using Antireflection Coated Microparticles

Derek Craig, Alison McDonald, Michael Mazilu, Helen Rendall, Frank Gunn-Moore, and Kishan Dholakia

We demonstrate the use of antireflection (AR) coated microparticles for the enhanced optical manipulation of cells. Specifically, we incubate CHO-K1, HL60, and NMuMG cell lines with AR-coated titania microparticles and subsequently performed drag force measurements using optical trapping. Direct comparisons were performed between native, polystyrene microparticle and AR microparticle tagged cells. The optical trapping efficiency was recorded by measuring the Q value in a drag force experiment. CHO-K1 cells incubated with AR microparticles show an increase in the Q value of nearly 220% versus native cells. With the inclusion of AR microparticles, cell velocities exceeding 50 μm/s were recorded for only 33 mW of laser trapping power. Cell viability was confirmed with fluorescent dyes and cells expressing a fluorescent ubiquitination-based cell cycle protein (FUCCI), which verified no disruption to the cell cycle in the presence of AR microparticles.