Light sailboats: Laser driven autonomous microrobots

Anrdás Búzás, Lóránd Kelemen, Anna Mathesz, László Oroszi, Gaszton Vizsnyiczai, Tamás Vicsek, and Pál Ormos

We introduce a system of light driven microscopic autonomous moving particles that move on a flat surface. The design is simple, yet effective: Micrometer sized objects with wedge shape are produced by photopolymerization, and they are covered with a reflective surface. When the area of motion is illuminated perpendicularly from above, the light is deflected to the side by the wedge shaped objects, in the direction determined by the position and orientation of the particles. The momentum change during reflection provides the driving force for an effectively autonomous motion. The system is an efficient tool to study self propelled microscopic robots.

DOI

Polarization-sensitive photophoresis

Vladlen G. Shvedov, Cyril Hnatovsky, Niko Eckerskorn, Andrei V. Rode, and Wieslaw Krolikowski

We photophoretically trap spherical airborne particles using a single radially or azimuthally polarized laser beam and show that the trapping efficiency is significantly higher for the radial polarization. The demonstrated polarization sensitivity of the photophoretic force, which is caused by polarization-dependent reflection from the particles, adds additional flexibility to the optical micromanipulation of light absorbing particles in gaseous media.

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Optical trapping microrheology in cultured human cells

E. Bertseva, D. Grebenkov, P. Schmidhauser, S. Gribkova, S. Jeney and L. Forró

We present the microrheological study of the two close human epithelial cell lines: non-cancerous HCV29 and cancerous T24. The optical tweezers tracking was applied to extract the several seconds long trajectories of endogenous lipid granules at time step of 1μs. They were analyzed using a recently proposed equation for mean square displacement (MSD) in the case of subdiffusion influenced by an optical trap. This equation leads to an explicit form for viscoelastic moduli. The moduli of the two cell lines were found to be the same within the experimental accuracy for frequencies 102 – 105 Hz. For both cell lines subdiffusion was observed with the exponent close to 3/4, the value predicted by the theory of semiflexible polymers. For times longer than 0.1s the MSD of cancerous cells exceeds the MSD of non-cancerous cells for all values of the trapping force. Such behavior can be interpreted as a signature of the active processes and prevents the extraction of the low-frequency viscoelastic moduli for the living cells by passive microrheology.

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Structural dynamics of nucleosomes at single-molecule resolution

John S. Choy, Tae-Hee Lee

The detailed mechanisms of how DNA that is assembled around a histone core can be accessed by DNA-binding proteins for transcription, replication, or repair, remain elusive nearly 40 years after Kornberg's nucleosome model was proposed. Uncovering the structural dynamics of nucleosomes is a crucial step in elucidating the mechanisms regulating genome accessibility. This requires the deconvolution of multiple structural states within an ensemble. Recent advances in single-molecule methods enable unprecedented efficiency in examining subpopulation dynamics. In this review, we summarize studies of nucleosome structure and dynamics from single-molecule approaches and how they advance our understanding of the mechanisms that govern DNA transactions.

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Optical trapping of microparticles using silicon nitride waveguide junctions and tapered-waveguide junctions on an optofluidic chip

Hong CAI and Andrew W Poon

We study optical trapping of microparticles on an optofluidic chip using silicon nitride waveguide junctions and tapered-waveguide junctions. We demonstrate trapping of single 1um-sized polystyrene particles using the evanescent field of waveguide junctions connecting a submicrometer-sized input-waveguide and a micrometer-sized output-waveguide. Particle trapping is localized in the vicinity of the junction. We also demonstrate trapping of one and two 1um-sized polystyrene particles using tapered-waveguide junctions connecting a submicrometer-sized singlemode input-waveguide and a micrometer-sized multimode output-waveguide. Particle trapping occurs near the taper output end, the taper center and the taper input end, depending on the taper aspect ratio.

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Force mapping of an optical trap using an acousto-optical deflector in a time-sharing regime

Ignacio A. Martínez and Dmitri Petrov

We suggest and study experimentally a time-sharing protocol for acousto-optical deflectors (AODs) that permits one to map the radial optical trapping force of optical tweezers without using a controllable flux control or an additional beam. Variations of the trapping potential due to modifications of the optical system are easily detected in terms of the force map. The protocol can be used in optical tweezers that already include an AOD without adding new elements in the existing optical system.

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Effect of spatial coherence on propagation, tight focusing, and radiation forces of an azimuthally polarized beam

Yiming Dong, Fei Wang, Chengliang Zhao, and Yangjian Cai

We report experimental generation of an azimuthally polarized (AP) beam with variable spatial coherence. The effect of spatial coherence on the propagation properties of an AP beam is studied both numerically and experimentally, and our experimental results agree well with the theoretical predictions. The dependence of the intensity distribution of an AP beam focused by a high numerical aperture objective lens on its initial spatial coherence is illustrated numerically, and it is found that we can shape the beam profile of a tightly focused AP beam by varying its initial spatial coherence. Furthermore, the radiation forces on Rayleigh particles induced by a tightly focused AP beam are studied, and we find that the tightly focused AP beam can be used to trap a Rayleigh particle whose refractive index is larger or smaller than that of the ambient by varying its initial spatial coherence. Our results will be useful for particle trapping and material thermal processing.

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High-resolution microrheology in the pericellular matrix of prostate cancer cells

Nadja Nijenhuis, Daisuke Mizuno, Jos A. E. Spaan and Christoph F. Schmidt

Many cells express a membrane-coupled external mechanical layer, the pericellular matrix (PCM), which often contains long-chain polymers. Its role and properties are not entirely known, but its functions are believed to include physical protection, mechanosensing, chemical signalling or lubrication. The viscoelastic response of the PCM, with polysaccharides as the main structural components, is therefore crucial for the understanding of its function. We have here applied microrheology, based on optically trapped micrometre-sized colloids, to the PCM of cultured PC3 prostate cancer cells. This technology allowed us to measure the extremely soft response of the PCM, with approximately 1 µm height resolution. Exogenously added aggrecan, a hyaluronan-binding proteoglycan, caused a remarkable increase in thickness of the viscoelastic layer and also triggered filopodia-like protrusions. The viscoelastic response of the PCM, however, did not change significantly. 

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Aromatic residue mutations reveal direct correlation between HIV-1 nucleocapsid protein's nucleic acid chaperone activity and retroviral replication

Hao Wu, Mithun Mitra, Micah J. McCauley, James A. Thomas, Ioulia Rouzina, Karin Musier-Forsyth, Mark C. Williams, Robert J. Gorelick

The human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) protein plays an essential role in several stages of HIV-1 replication. One important function of HIV-1 NC is to act as a nucleic acid chaperone, in which the protein facilitates nucleic acid rearrangements important for reverse transcription and recombination. NC contains only 55 amino acids, with 15 basic residues and two zinc fingers, each having a single aromatic residue (Phe16 and Trp37). Despite its simple structure, HIV-1 NC appears to have optimal chaperone activity, including the ability to strongly aggregate nucleic acids, destabilize nucleic acid secondary structure, and facilitate rapid nucleic acid annealing. Here we combine single molecule DNA stretching experiments with ensemble solution studies of protein-nucleic acid binding affinity, oligonucleotide annealing, and nucleic acid aggregation to measure the characteristics of wild-type (WT) and aromatic residue mutants of HIV-1 NC that are important for nucleic acid chaperone activity. These in vitro results are compared to in vivo HIV-1 replication for viruses containing the same mutations. This work allows us to directly relate HIV-1 NC structure with its function as a nucleic acid chaperone in vitro and in vivo. We show that replacement of either aromatic residue with another aromatic residue results in a protein that strongly resembles WT NC. In contrast, single amino acid substitutions of either Phe16Ala or Trp37Ala significantly slow down NC's DNA interaction kinetics, while retaining some helix-destabilization capability. A double Phe16Ala/Trp37Ala substitution further reduces the latter activity. Surprisingly, the ensemble nucleic acid binding, annealing, and aggregation properties are not significantly altered for any mutant except the double aromatic substitution with Ala. Thus, elimination of a single aromatic residue from either zinc finger strongly reduces NC's chaperone activity as determined by single molecule DNA stretching experiments without significantly altering its ensemble-averaged biochemical properties. Importantly, the substitution of aromatic residues with Ala progressively decreases NC's nucleic acid chaperone activity while also progressively inhibiting viral replication. Taken together, these data support the critical role of HIV-1 NC's aromatic residues, and establish a direct and statistically significant correlation between nucleic acid chaperone activity and viral replication.

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Optically-actuated translational and rotational motion at the microscale for microfluidic manipulation and characterization

Samarendra Mohanty

The single beam optical trap (Optical tweezers), a highly focused beam, is on its way to revolutionizing not only the fields of colloidal physics and biology, but also material science and engineering. Recently, spatially-extended three-dimensional light patterns are gaining considerable usage for exerting force to alter, manipulate, organize and characterize materials. To advance the degree of manipulation, such as rotation of materials in microfluidic environments along with spatial structuring, other beam parameters such as phase and polarization have to be configured. These advancements in optical tweezers technology have enabled complex microfluidic actuation and sorting. In addition to remotely (in a non-contact way) applying force and torques in three-dimensions, which can be continuously varied unlike mechanical manipulators, optical tweezers-based methods can be used for sensing the force of interaction between microscopic objects in a microfluidic environment and for the characterization of micro-rheological properties. In this review, we place emphasis on applications of optical actuation based on novel beams in performing special functions such as rotation, transportation, sorting and characterization of the microscopic objects. Further, we have an extended discussion on optical actuation (transport and rotation) with fiber optic microbeams and spectroscopic characterization in microfluidic environment. All these advancements in optical manipulation would further facilitate the growing use of optical tools for complex microfluidic manipulations.

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Switching of myosin-V motion between the lever-arm swing and Brownian search-and-catch

Keisuke Fujita, Mitsuhiro Iwaki, Atsuko H. Iwane, Lorenzo Marcucci & Toshio Yanagida

Motor proteins are force-generating nanomachines that are highly adaptable to their ever-changing biological environments and have a high energy conversion efficiency. Here we constructed an imaging system that uses optical tweezers and a DNA handle to visualize elementary mechanical processes of a nanomachine under load. We apply our system to myosin-V, a well-known motor protein that takes 72 nm 'hand-over-hand' steps composed of a 'lever-arm swing' and a 'Brownian search-and-catch'. We find that the lever-arm swing generates a large proportion of the force at low load (<0.5 pN), resulting in 3 kBT of work. At high load (1.9 pN), however, the contribution of the Brownian search-and-catch increases to dominate, reaching 13 kBT of work. We believe the ability to switch between these two force-generation modes facilitates myosin-V function at high efficiency while operating in a dynamic intracellular environment.

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Click-Chemistry Assisted Single-Molecule Fingerprinting Reveals a 3D Biomolecular Folding Funnel

Zhongbo Yu, Deepak Koirala, Yunxi Cui, Leah F.Easterling, Yuan Zhao, and Hanbin Mao

A 3D folding funnel was proposed in the nineteen nineties to explain the fast kinetics exhibited by a biomacromolecule in presence of seemingly unlimited folding pathways. Over the years, numerous simulations have been performed with this concept, however, experimental verification is yet to attain even for simplest proteins. Here, we have used a click-chemistry based strategy to introduce six pairs of handles in a human telomeric DNA sequence. A laser-tweezers based single-molecule structural fingerprinting on the six inter-handle distances reveals the formation of a hybrid-1 G-quadruplex in the sequence. Kinetic and thermodynamic fingerprinting on the six trajectories defined by each handle-pairs depict a 3D folding funnel and a kinetic topology in which the kinetics pertaining to each handle residue is annotated for this G-quadruplex. We anticipate the methods and the concepts developed here are well applicable to other biomacromolecules, including RNA and proteins.

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Trapping colloidal dielectric microparticles with overlapping evanescent optical waves

R. Khakimov, A. Shevchenko, A. Havukainen, K. Lindfors, M. Kaivola

We experimentally demonstrate the creation of a stable surface trap for colloidal microparticles in a high-intensity evanescent optical field that is produced by total internal reflection of two counter-propagating and mutually incoherent laser beams. While the particles confined in the trap undergo fast Brownian motion, they never “stick” to the surface – not even at high optical powers – but rather levitate above the surface. If many particles are stored in the trap, they tend to form a well ordered self-organized array. We apply a numerical model based on the general energy-momentum tensor formalism to evaluate the overall optical force acting on a trapped particle. The optical-field parameters are calculated using the finite element method. The simulations show that for small particles a sharp repulsive potential at the surface – required for the levitation – can have neither optical nor light-induced thermal origin. Among the possible non-optical forces, electrostatic double-layer repulsion is often considered to be the origin of the levitation. We find, however, that the experimentally observed levitation of small particles in a high-intensity evanescent-wave trap cannot be explained by this effect.

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Optical-Tweezers Assembly-Line for the Construction of Complex Functional Zeolite L Structures

M. Veiga-Gutiérrez, M. Woerdemann, E. Prasetyanto, C. Denz, L. De Cola

An optical-tweezers assembly-line is presented, which has high potential for the construction of complex structures of zeolite L crystals and other microscopic building blocks. Different examples of assembled 2D and 3D zeolite structures are discussed. These include well-oriented monolayers, microtowers, and angle-aligned dye-loaded zeolites, which suggests exciting applications, for example as microscopic polarization sensors.

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Brownian motion in a designer force field: dynamical effects of negative refraction on nanoparticles

Aurelien Cuche, Benedikt Stein, Antoine Canaguier-Durand, Eloïse Devaux, Cyriaque Genet, and Thomas W. Ebbesen 

Photonic crystals (PC) have demonstrated unique features that have renewed the fields of classical and quantum optics. Although holding great promises, associated mechanical effects have proven challenging to observe. We demonstrate for the first time that one of the most salient properties of PC, namely negative refraction, can induce specific forces on metal nanoparticles. By integrating a periodically patterned metal film in a fluidic cell, we show that near-field optical forces associated with negatively refracted surface plasmons are capable of controlling particle trajectories. Coupling particle motions to PC band structures draws radically new approaches and strategies for parallel and high resolution all-optical control of particle flows, with applications for micro and nanofluidic systems.

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Optical Tweezers Array System Based on 2D Photonic Crystals

Xuechang Ren, Canhui Wang, Yanshuang Li, Shaoxin Shen, Shou Liu

A simple optical interference method for creating multiple optical tweezers from a single laser beam, using two dimentional photonic crystals (PhCs) as a diffractive beam splitter, was described. To obtained clear periodic traps, all diffracted beams sould be used and the intensity of each splitted beam should be same. So the period and the surface features of PhCs was adjusted in the present study As a demonstration of this technique, using 2D PhCs with 700 nanometer period, hexagonal lattice patterns with one micrometer period have been implemented. The image of periodic intensity gradient of light fabricated by this method is presented.

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Tertiary DNA Structure in the Single-Stranded hTERT Promoter Fragment Unfolds and Refolds by Parallel Pathways via Cooperative or Sequential Events

Zhongbo Yu, Vanessa Gaerig, Yunxi Cui, HyunJin Kang, Vijay Gokhale, Yuan Zhao, Laurence H.Hurley, and Hanbin Mao

The discovery of G-quadruplexes and other DNA secondary elements has increased the structural diversity of DNA well beyond the ubiquitous double helix. However, it remains to be determined whether tertiary interactions can take place in a DNA complex that contains more than one secondary structure. Using a new data analysis strategy that exploits the hysteresis region between the mechanical unfolding and refolding traces obtained by a laser-tweezers instrument, we now provide the first convincing kinetic and thermodynamic evidence that a higher order interaction takes place between a hairpin and a G-quadruplex in a single-stranded DNA fragment that is found in the promoter region of human telomerase. During the hierarchical unfolding or refolding of the DNA complex, a 15-nucleotide hairpin serves as a common species among three intermediates. Moreover, either a mutant that prevents this hairpin formation or the addition of a DNA fragment complementary to the hairpin destroys the cooperative kinetic events by removing the tertiary interaction mediated by the hairpin. The coexistence of the sequential and the cooperative refolding events provides direct evidence for a unifying kinetic partition mechanism previously observed only in large proteins and complex RNA structures. Not only does this result rationalize the current controversial observations for the long-range interaction in complex single-stranded DNA structures, but also this unexpected complexity in a promoter element provides additional justification for the biological function of these structures in cells.

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Real-Time Path Planning for Coordinated Transport of Multiple Particles Using Optical Tweezers

Banerjee, A.G., Chowdhury, S., Losert, W., Gupta, S.K.

Automated transport of multiple particles using optical tweezers requires real-time path planning to move them in coordination by avoiding collisions among themselves and with randomly moving obstacles. This paper develops a decoupled and prioritized path planning approach by sequentially applying a partially observable Markov decision process algorithm on every particle that needs to be transported. We use an iterative version of a maximum bipartite graph matching algorithm to assign given goal locations to such particles. We then employ a three-step method consisting of clustering, classification, and branch and bound optimization to determine the final collision-free paths. We demonstrate the effectiveness of the developed approach via experiments using silica beads in a holographic tweezers setup. We also discuss the applicability of our approach and challenges in manipulating biological cells indirectly by using the transported particles as grippers.

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Elastic interactions between colloidal microspheres and elongated convex and concave nanoprisms in nematic liquid crystals

Bohdan Senyuk and Ivan I. Smalyukh

We study mutual alignment and interactions between colloidal particles of dissimilar shapes and dimensions when dispersed in a nematic host fluid. Convex pentagonal and concave starfruit-shaped nanoprisms and microspheres induce dipolar or quadrupolar director structures. The ensuing elastic pair interactions between microspheres and nanoprisms are highly dependent on the nanoparticle shape being omnidirectionally attractive for convex prisms but strongly anisotropic for concave prisms. Elastic deformations due to spherical particles cause well-defined alignment of complex-shaped nanoparticles at distances much larger than the microsphere size. We characterize distance and angular dependencies of elastic pair interaction forces, torques, and binding energies. The studied elasticity-mediated self-assembly of metal and dielectric nanoparticles with dissimilar shapes and sizes opens new possibilities for self-assembly based fabrication of structured mesoscopic composites with predesigned properties.

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Material-Independent and Size-Independent Tractor Beams for Dipole Objects

Andrey Novitsky, Cheng-Wei Qiu, and Andrei Lavrinenko

A Bessel beam without an axial gradient can exert a pulling force on an object [A. Novitsky, C. W. Qiu, and H. Wang, Phys. Rev. Lett. 107 203601 (2011)]. However, it cannot be called a “tractor beam” per se, as long as the light pulling effect is ultrasensitive to the object’s material and size, a perturbation of which will make the optical traction go away. In this Letter, we investigate and report on the universality for a Bessel beam to be either a material-independent or size-independent optical tractor beam within the dipolar regime. Moreover, a general condition for a nonparaxial laser to be simultaneously a material- and size-independent tractor beam is proposed. These universal pulling effects and conditions are discussed in association with insight on modified far-field scattering, scattering resonances, and induced polarizabilities. Interestingly, we find that the acoustic pulling force exhibits only size independence, owing to the acoustic scattering theory in contrast to the light scattering counterpart. The findings pave the way for the realistic engineering and application of universal tractor beams pulling a wide variety of objects.

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Gene expression of single human mesenchymal stem cell in response to fluid shear

Hu Zhang, Alasdair Kay, Nicholas R Forsyth, Kuo-Kang Liu, Alicia J El Haj

Stem cell therapy may rely on delivery and homing through the vascular system to reach the target tissue. An optical tweezer model has been employed to exert different levels of shear stress on a single non-adherent human bone marrow–derived mesenchymal stem cell to simulate physiological flow conditions. A single-cell quantitative polymerase chain reaction analysis showed that collagen type 1, alpha 2 (COL1A2), heat shock 70-kDa protein 1A (HSPA1A) and osteopontin (OPN) are expressed to a detectable level in most of the cells. After exposure to varying levels of shear stress, there were significant variations in gene transcription levels across human mesenchymal stem cells derived from four individual donors. Significant trend towards upregulation of COL1A2 and OPN gene expression following shear was observed in some donors with corresponding variations in HSPA1A gene expression. The results indicate that shear stress associated with vascular flow may have the potential to significantly direct non-adherent stem cell expression towards osteogenic phenotypic expression. However, our results demonstrate that these results are influenced by the selection process and donor variability.

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Optofluidic integrated cell sorter fabricated by femtosecond lasers

Francesca Bragheri , Paolo Minzioni , Rebeca Martinez Vazquez , Nicola Bellini , Petra Paiè , Chiara Mondello , Roberta Ramponi, Ilaria Cristiani and Roberto Osellame

The main trend in optofluidics is currently towards full integration of the devices, thus improving automation, compactness and portability. In this respect femtosecond laser microfabrication is a very powerful technology given its capability of producing both optical waveguides and microfluidic channels. The current challenge in biology is the possibility to perform bioassays at the single cell level to unravel the hidden complexity in nominally homogeneous populations. Here we report on a new device implementing a fully integrated fluorescence-activated cell sorter. This non-invasive device is specifically designed to operate with a limited amount of cells but with a very high selectivity in the sorting process. Characterization of the device with beads and validation with human cells are presented.

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Hydrodynamic Synchronization of Light Driven Microrotors

R. Di Leonardo, A. Búzás, L. Kelemen, G. Vizsnyiczai, L. Oroszi, and P. Ormos

Hydrodynamic synchronization is a fundamental physical phenomenon by which self-sustained oscillators communicate through perturbations in the surrounding fluid and converge to a stable synchronized state. This is an important factor for the emergence of regular and coordinated patterns in the motions of cilia and flagella. When dealing with biological systems, however, it is always hard to disentangle internal signaling mechanisms from external purely physical couplings. We have used the combination of two-photon polymerization and holographic optical trapping to build a mesoscale model composed of chiral propellers rotated by radiation pressure. The two microrotors can be synchronized by hydrodynamic interactions alone although the relative torques have to be finely tuned. Dealing with a micron sized system we treat synchronization as a stochastic phenomenon and show that the phase lag between the two microrotors is distributed according to a stationary Fokker-Planck equation for an overdamped particle over a tilted periodic potential. Synchronized states correspond to minima in this potential whose locations are shown to depend critically on the detailed geometry of the propellers.

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Stochastic optical active rheology

Hyungsuk Lee, Yongdae Shin, Sun Taek Kim, Ellis L. Reinherz, and Matthew J. Lang

We demonstrate a stochastic based method for performing active rheology using optical tweezers. By monitoring the displacement of an embedded particle in response to stochastic optical forces, a rapid estimate of the frequency dependent shear moduli of a sample is achieved in the range of 10−1–103 Hz. We utilize the method to probe linear viscoelastic properties of hydrogels at varied cross-linker concentrations. Combined with fluorescence imaging, our method demonstrates non-linear changes of bond strength between T cell receptors and an antigenic peptide due to force-induced cell activation.

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Trapping and rotating microparticles and bacteria with moiré-based optical propelling beams

Peng Zhang, Daniel Hernandez, Drake Cannan, Yi Hu, Shima Fardad, Simon Huang, Joseph C. Chen, Demetrios N. Christodoulides, and Zhigang Chen

We propose and demonstrate trapping and rotation of microparticles and biological samples with a moiré-based rotating optical tweezers. We show that polystyrene beads, as well as Escherichia coli cells, can be rotated with ease, while the speed and direction of rotation are fully controllable by a computer, obviating mechanical movement or phase-sensitive interference. Furthermore, we demonstrate experimentally the generation of white-light propelling beams and arrays, and discuss the possibility of optical tweezing and particle micro-manipulation based on incoherent white-light rotating patterns.

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Optical trap assisted laser nanostructuring in the near-field of microparticles

Ulf Quentin, Karl-Heinz Leitz, Lutz Deichmann, Ilya Alexeev, and Michael Schmidt

Particle based near-field nanostructuring is an excellent possibility to overcome the optical diffraction limit in laser based material processing. In the near-field of microspheres which are irradiated with pulsed laser radiation, it is possible to generate nanoholes with diameters below 100 nm using a laser wavelength of 800 nm. To improve this approach, it is possible to position the microparticles with an optical trap to generate arbitrary structure geometries. In this paper, the authors describe the basic principle of optical trap assisted nanostructuring and present simulational and experimental results demonstrating the potential of this innovative nanoscale optical material processing technology.

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Optofluidic particle manipulation in a liquid-core/liquid-cladding waveguide

Kang Soo Lee, Sang Youl Yoon, Kyung Heon Lee, Sang Bok Kim, Hyung Jin Sung, and Sang Soo Kim

This paper describes a method for particle manipulation in a liquid-core/liquid-cladding optical waveguide system. Step-index and graded-index waveguides were modeled with consideration for, respectively, miscible and immiscible core and cladding fluids. The characteristic motions of four different particles with refractive indices of 1.59, 1.48, 1.37, and 1.22 were examined. The guided beam was assumed to be Gaussian in shape. Our results showed that high-refractive-index particles converged at the center of the core fluid due to a positive gradient force, whereas low-refractive-index particles converged at the flow periphery. The nonlinearity of the particle motion increased as the flow velocity and the guided beam waist decreased and the laser power and the particle size increased. The initial beam waist of the guided beam in the graded-index waveguide did not significantly affect the characteristics of the particle motion due to the effects of diffusion.

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The Measurement of Displacement and Optical Force in Multi-Optical Tweezers

Ling Lin, Guo Hong-Lian, Huang Lu, Qu E, Li Zhao-Lin and Li Zhi-Yuan

We set up a system of multiple optical tweezers based on a spatial light modulator, and measured the displacement and optical force of the trapped particles simultaneously. All of the trapped particles can be clearly imaged in three dimensions by several CCDs. The displacement is obtained by calculating the gray weighted centroid in the trapped particle's image. The stiffness of the trapped particles in the optical traps is measured by oscillating the sample stage in a triangular wave based on Stokes fluid dynamics. The optical force of each trapped particle can be calculated by the measured displacement and stiffness.

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Planar optical tweezers using tapered-waveguide junctions

Hong Cai and Andrew W. Poon

We demonstrate planar optical tweezers using the evanescent field of a silicon nitride tapered-waveguide junction between a singlemode waveguide and a multimode waveguide. Our experiments show that the junction embedded in a fluidic channel holds up to one and two polystyrene particles of sizes of 2.2 μm and 1 μm, respectively. The trapped particles are successively substituted by the incoming particles. Our experiments and numerical modeling reveal that the junction particle trapping depends on particle size and number.

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Tuning the structural and optical properties of gold/silver nano-alloys prepared by laser ablation in liquids for optical limiting, ultra-sensitive spectroscopy, and optical trapping

E. Messina, L. D’Urso, E. Fazio, C. Satriano, M.G. Donato, C. D’Andrea, O.M. Maragò, P.G. Gucciardi, G. Compagnini, F. Neri

The plasmon resonance of metallic Au/Ag nano-alloys in the colloidal state was tuned from 400 nm to 500 nm using a laser irradiated technique performed directly in the liquid state. Interesting optical nonlinearities, trapping effects and spectroscopic enhancements were detected by modifying the gold molar fraction in the nano-alloys. In particular a reduction of the limiting threshold was observed by increasing the gold amount. The SERS activity of the Au/Ag alloys was tested in liquid and in solid state in presence of linear carbon chains as probe molecules. The increased Raman signals for nanoparticles with different Au/Ag atomic ratio are presented. Finally, studies and prospects for optical and Raman tweezers experiments are discussed.

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Photo-actuation of liquids for light-driven microfluidics: state of the art and perspectives

Damien Baigl

Using light to control liquid motion is a new paradigm for the actuation of microfluidic systems. We review here the different principles and strategies to induce or control liquid motion using light, which include the use of radiation pressure, optical tweezers, light-induced wettability gradient, thermocapillary effect, photosensitive surfactants, chromocapillary effect, optoelectrowetting, photocontrolled electroosmotic flows and optical dielectrophoresis. We analyze the performance of these approaches to control using light many kinds of microfluidic operations involving discrete pL- to µL-sized droplets (generation, driving, mixing, reaction, sorting) or fluid flows in microchannels (valve operation, injection, pumping, flow rate control). We show that a complete toolbox is now available to control microfluidic systems by light. We finally discuss the perspectives of digital optofluidics as well as microfluidics based on all optical fluidic chips and optically reconfigurable devices.

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Cardiogenic Regulation of Stem-Cell Electrical Properties in a Laser-Patterned Biochip

Zhen Ma, Qiuying Liu, Honghai Liu, Huaxiao Yang, Julie X. Yun, Meifeng Xu, Carol A. Eisenberg, Thomas K. Borg, Roger Markwald and Bruce Z. Gao

Normal cardiomyocytes are highly dependent on the functional expression of ion channels to form action potentials and electrical coupling with other cells. To fully determine the scientific and therapeutic potential of stem cells for cardiovascular-disease treatment, it is necessary to assess comprehensively the regulation of stem-cell electrical properties during stem cell-cardiomyocyte interaction. It has been reported in the literature that contact with native cardiomyocytes induced and regulated stem-cell cardiogenic differentiation. However, in conventional cell-culture models, the importance of cell–cell contact for stem-cell functional coupling with cardiomyocytes has not been elucidated due to insufficient control of the cell-contact mode of individual cells. Using microfabrication and laser-guided cell micropatterning techniques, we created two biochips with contact-promotive and -preventive microenvironments to systematically study the effect of contact on cardiogenic regulation of stem-cell electrical properties. In contact-promotive biochips, connexin 43 expression was upregulated and relocated to the junction area between one stem cell and one cardiomyocyte. Only stem cells in contact with cardiomyocytes were induced by adjacent cardiomyocytes to acquire electrophysiological properties for action-potential formation similar to that of a cardiomyocyte.

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Winding single-molecule double-stranded DNA on a nanometer-sized reel

Huijuan You, Ryota Iino, Rikiya Watanabe and Hiroyuki Noji

A molecular system of a nanometer-sized reel was developed from F1–ATPase, a rotary motor protein. By combination with magnetic tweezers and optical tweezers, single-molecule double-stranded DNA (dsDNA) was wound around the molecular reel. The bending stiffness of dsDNA was determined from the winding tension (0.9–6.0 pN) and the diameter of the wound loop (21.4–8.5 nm). Our results were in good agreement with the conventional worm-like chain model and a persistence length of 54 ± 9 nm was estimated. This molecular reel system offers a new platform for single-molecule study of micromechanics of sharply bent DNA molecules and is expected to be applicable to the elucidation of the molecular mechanism of DNA-associating proteins on sharply bent DNA strands.

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Reconfigurable Optothermal Microparticle Trap in Air-Filled Hollow-Core Photonic Crystal Fiber

O. A. Schmidt, M. K. Garbos, T. G. Euser, and P. St. J. Russell

We report a novel optothermal trapping mechanism that occurs in air-filled hollow-core photonic crystal fiber. In the confined environment of the core, the motion of a laser-guided particle is strongly influenced by the thermal-gradient-driven flow of air along the core surface. Known as “thermal creep flow,” this can be induced either statically by local heating, or dynamically by the absorption (at a black mark placed on the fiber surface) of light scattered by the moving particle. The optothermal force on the particle, which can be accurately measured in hollow-core fiber by balancing it against the radiation forces, turns out to exceed the conventional thermophoretic force by 2 orders of magnitude. The system makes it possible to measure pN-scale forces accurately and to explore thermally driven flow in micron-scale structures.

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Optically controlled grippers for manipulating micron-sized particles

Graham Gibson, Louise Barron, Fiona Beck, Graeme Whyte and Miles Padgett

We report the development of a joystick controlled gripper for the real-time manipulation of micron-sized objects, driven using holographic optical tweezers (HOTs). The gripper consists of an arrangement of four silica beads, located in optical traps, which can be positioned and scaled in order to trap an object indirectly. The joystick can be used to grasp, move (lateral or axial), and change the orientation of the target object. The ability to trap objects indirectly allows us to demonstrate the manipulation of a strongly scattering micron-sized metallic particle.

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Effects of cholesterol on nano-mechanical properties of the living cell plasma membrane

Nima Khatibzadeh , Sharad Gupta , Brenda Farrell , William E. Brownell and Bahman Anvari

In this study, we investigated the effects of membrane cholesterol content on the mechanical properties of cell membranes by using optical tweezers. We pulled membrane tethers from human embryonic kidney cells using single and multi-speed protocols, and obtained time-resolved tether forces. We quantified various mechanical characteristics including the tether equilibrium force, bending modulus, effective membrane viscosity, and plasma membrane–cytoskeleton adhesion energy, and correlated them to the membrane cholesterol level. Decreases in cholesterol concentration were associated with increases in the tether equilibrium force, tether stiffness, and adhesion energy. Tether diameter and effective viscosity increased with increasing cholesterol levels. Disruption of cytoskeletal F-actin significantly changed the tether diameters in both non-cholesterol and cholesterol-manipulated cells, while the effective membrane viscosity was unaffected by F-actin disruption. The findings are relevant to inner ear function where cochlear amplification is altered by changes in the membrane cholesterol content.

DOI

Calcium-dependent folding of single calmodulin molecules

Johannes Stigler and Matthias Rief

Calmodulin is the primary calcium binding protein in living cells. Its function and structure depend strongly on calcium concentration. We used single molecule force spectroscopy by optical tweezers to study the folding of calmodulin in the physiologically relevant range. We find that full-length calmodulin switches from a rich and complex folding behavior at high calcium to a simple folding pathway at apo conditions. Using truncation mutants, we studied the individual domains separately. Folding and stability of the individual domains differ significantly at low calcium concentrations. With increasing calcium, the folding rate constants increase while unfolding rate constants decrease. The complete kinetic as well as energetic behavior of both domains could be modeled using a calcium-dependent three-pathway model. We find that the dominant folding pathway at high calcium concentrations proceeds via a transition state capable of binding one calcium ion. The folding of calmodulin seems to be designed to occur fast robustly over a large range of calcium concentrations and hence energetic stabilities. 

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Comparing the mechanism of water condensation and evaporation in glassy aerosol

David L. Bones, Jonathan P. Reid, Daniel M. Lienhard, and Ulrich K. Krieger

Atmospheric models generally assume that aerosol particles are in equilibrium with the surrounding gas phase. However, recent observations that secondary organic aerosols can exist in a glassy state have highlighted the need to more fully understand the kinetic limitations that may control water partitioning in ambient particles. Here, we explore the influence of slow water diffusion in the condensed aerosol phase on the rates of both condensation and evaporation, demonstrating that significant inhibition in mass transfer occurs for ultraviscous aerosol, not just for glassy aerosol. Using coarse mode (3–4 um radius) ternary sucrose/sodium chloride/aqueous droplets as a proxy for multicomponent ambient aerosol, we demonstrate that the timescale for particle equilibration correlates with bulk viscosity and can be ≫103 s. Extrapolation of these timescales to particle sizes in the accumulation mode (e.g., approximately 100 nm) by applying the Stokes-Einstein equation suggests that the kinetic limitations imposed on mass transfer of water by slow bulk phase diffusion must be more fully investigated for atmospheric aerosol. Measurements have been made on particles covering a range in dynamic viscosity from < 0.1 to > 1013 Pa s. We also retrieve the radial inhomogeneities apparent in particle composition during condensation and evaporation and contrast the dynamics of slow dissolution of a viscous core into a labile shell during condensation with the slow percolation of water during evaporation through a more homogeneous viscous particle bulk.

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Force-Dependent Detachment of Kinesin-2 Biases Track Switching at Cytoskeletal Filament Intersections

Harry W. Schroeder III, Adam G. Hendricks, Kazuho Ikeda, Henry Shuman, Vladimir Rodionov, Mitsuo Ikebe, Yale E. Goldman, Erika L.F. Holzbaur

Intracellular trafficking of organelles often involves cytoskeletal track switching. Organelles such as melanosomes are transported by multiple motors including kinesin-2, dynein, and myosin-V, which drive switching between microtubules and actin filaments during dispersion and aggregation. Here, we used optical trapping to determine the unitary and ensemble forces of kinesin-2, and to reconstitute cargo switching at cytoskeletal intersections in a minimal system with kinesin-2 and myosin-V motors bound to beads. Single kinesin-2 motors exerted forces up to ∼5 pN, similar to kinesin-1. However, kinesin-2 motors were more likely to detach at submaximal forces, and the duration of force maintenance was short as compared to kinesin-1. In multimotor assays, force increased with kinesin-2 density but was not affected by the presence of myosin-V. In crossed filament assays, switching frequencies of motor-bound beads were dependent on the starting track. At equal average forces, beads tended to switch from microtubules onto overlying actin filaments consistent with the relatively faster detachment of kinesin-2 at near-maximal forces. Thus, in addition to relative force, switching probability at filament intersections is determined by the dynamics of motor-filament interaction, such as the quick detachment of kinesin-2 under load. This may enable fine-tuning of filament switching in the cell.

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How Malaria Parasites Reduce the Deformability of Infected Red Blood Cells

S. Majid Hosseini, James J. Feng

The pathogenesis of malaria is largely due to stiffening of the infected red blood cells (RBCs). Contemporary understanding ascribes the loss of RBC deformability to a 10-fold increase in membrane stiffness caused by extra cross-linking in the spectrin network. Local measurements by micropipette aspiration, however, have reported only an increase of ∼3-fold in the shear modulus. We believe the discrepancy stems from the rigid parasite particles inside infected cells, and have carried out numerical simulations to demonstrate this mechanism. The cell membrane is represented by a set of discrete particles connected by linearly elastic springs. The cytosol is modeled as a homogeneous Newtonian fluid, and discretized by particles as in standard smoothed particle hydrodynamics. The malaria parasite is modeled as an aggregate of particles constrained to rigid-body motion. We simulate RBC stretching tests by optical tweezers in three dimensions. The results demonstrate that the presence of a sizeable parasite greatly reduces the ability of RBCs to deform under stretching. With the solid inclusion, the observed loss of deformability can be predicted quantitatively using the local membrane elasticity measured by micropipettes.

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A non-contact single optical fiber multi-optical tweezers probe: Design and fabrication

Yu Zhang, Zhihai Liu, Jun Yang, Libo Yuan

Optical tweezers have developed into a versatile and widely used tool; compared with the traditional optical tweezers, optical traps built with a single optical fiber provide a more flexible solution towards compact, integrated multiple traps. But the single optical tweezers have their own short comings—the manipulation working on the micro-particles is not non-contact, because of the fabrication method of the optical fiber probe. However we propose new single fiber multi-optical tweezers which are simple, low-cost and can manipulate micro-particles in a non-contact manner and non-invasively.

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Electromagnetic plane-wave forces on homogeneous material

Kevin J. Webb and Shivanand

The electromagnetic plane-wave force on homogeneous materials having the possible set of constitutive parameter signs is evaluated. The force remains positive in all cases with loss, but a negative force results with gain. A negative force is shown to occur for an evanescent field when there is no component of the electric field in that direction. Both forces have significant magnitude and hence should be observable in experiments.

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Calculation of radiation force and torque exerted on a uniaxial anisotropic sphere by an incident Gaussian beam with arbitrary propagation and polarization directions

Zheng-Jun Li, Zhen-Sen Wu, Qing-Chao Shang, Lu Bai, and Chun-Hui Cao

On the basis of spherical vector wave functions and coordinate rotation theory, the expansion of the fields of an incident Gaussian beam with arbitrary propagation and polarization directions in terms of spherical vector wave functions is investigated, and beam shape coefficients are derived. Using the results of electromagnetic scattering by a uniaxial anisotropic sphere, the analytical expressions of the radiation force and torque exerted on a homogeneous absorbing uniaxial anisotropic sphere by the arbitrary incident Gaussian beam. We numerically analyze and discuss the following: the effects of an anisotropic absorbing dielectric on the axial and transverse radiation forces exerted by an off-axis Gaussian beam on a uniaxial anisotropic sphere; the variations in the axial, transverse, and resultant radiation forces (with incident angle β and polarization angle α) imposed by an obliquely Gaussian beam on a uniaxial anisotropic sphere; and the results on the characteristics of the three components of the radiation forces versus the center-to-center distance between the sphere and beam. Selected numerically results on the radiation torque exerted on a stationary uniaxial anisotropic transparent or absorbing sphere by a linearly polarized Gaussian beam are shown, and the results are compared with those exerted an isotropic sphere. The accuracy of the theory and code is confirmed by comparing the axial radiation forces with the results obtained from references.

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Phase, Morphology, and Hygroscopicity of Mixed Oleic Acid/Sodium Chloride/Water Aerosol Particles before and after Ozonolysis

Benjamin J. Dennis-Smither, Kate L. Hanford, Nana-Owusua A. Kwamena, Rachael E. H. Miles, and Jonathan P. Reid

Aerosol optical tweezers are used to probe the phase, morphology, and hygroscopicity of single aerosol particles consisting of an inorganic component, sodium chloride, and a water insoluble organic component, oleic acid. Coagulation of oleic acid aerosol with an optically trapped aqueous sodium chloride droplet leads to formation of a phase-separated particle with two partially engulfed liquid phases. The dependence of the phase and morphology of the trapped particle with variation in relative humidity (RH) is investigated by cavity enhanced Raman spectroscopy over the RH range <5% to >95%. The efflorescence and deliquescence behavior of the inorganic component is shown to be unaffected by the presence of the organic phase. Whereas efflorescence occurs promptly (<1 s), the deliquescence process requires both dissolution of the inorganic component and the adoption of an equilibrium morphology for the resulting two phase particle, occurring on a time-scale of <20 s. Comparative measurements of the hygroscopicity of mixed aqueous sodium chloride/oleic acid droplets with undoped aqueous sodium chloride droplets show that the oleic acid does not impact on the equilibration partitioning of water between the inorganic component and the gas phase or the time response of evaporation/condensation. The oxidative aging of the particles through reaction with ozone is shown to increase the hygroscopicity of the organic component.

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Tomographic imaging of a suspending single live cell using optical tweezer-combined full-field optical coherence tomography

Woo June Choi, Kwan Seob Park, Tae Joong Eom, Myung-Kyu Oh, and Byeong Ha Lee

We propose a label-free depth-resolved tomographic scheme for imaging a single live cell in fluid. This approach utilizes a modified time-domain full-field optical coherence tomography (FF-OCT) system combined with an optical tweezer technique. The optical trap for holding a moving specimen is made by tightly focusing a 1064 nm Q-switching pulsed laser beam with a 1.0 NA microscope objective in the sample arm of the FF-OCT part. By cosharing the probe for both systems, the optical actions of trapping and cellular resolution tomographic imaging could be achieved simultaneously. Feasibility of the combined system is demonstrated by imaging micron-sized polystyrene beads and a living suspension cell in medium.

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Optical tweezers with enhanced efficiency based on laser-structured substrates

D. G. Kotsifaki, M. Kandyla, I. Zergioti, M. Makropoulou, E. Chatzitheodoridis, and A. A. Serafetinides

We present an optical nanotrapping setup that exhibits enhanced efficiency, based on localized plasmonic fields around sharp metallic features. The substrates consist of laser-structured silicon wafers with quasi-ordered microspikes on the surface, coated with a thin silver layer. The resulting optical traps show orders of magnitude enhancement of the trapping force and the effective quality factor.

DOI

Optical Tweezers-Assisted Cross-Correlation Analysis for a Non-intrusive Fluid Temperature Measurement in Microdomains

Chih-Ming Cheng, Ming-Chih Chang, Yu-Fen Chang, Wei-Ting Wang, Chien-Ting Hsu, Jing-Shin Tsai, Chia-Yeh Liu, Chien-Ming Wu, Keng-Liang Ou, and Tzu-Sen Yang
An image-based approach to predict the fluid temperature in microfluidic flow cell is presented. We apply Fourier-based cross-correlation processing to determine the lateral displacement of the optically trapped bead; therefore, both the mean square displacement (MSD) and the diffusion coefficient (D) can be obtained. On the other hand, applying the Stokes–Einstein equation, together with Faxen's law correction, the theoretical relation showed that D is proportional to (T/η), where Tand η are temperature and temperature-dependent fluid viscosity, respectively. Hence, the fluid temperature can be determined by MSD-based thermometry.

DOI

Single-Molecule Measurements of the Binding between Small Molecules and DNA Aptamers

Philip M. Yangyuoru, Soma Dhakal, Zhongbo Yu,Deepak Koirala, Simon M. Mwongela, and Hanbin Mao

Aptamers that bind small molecules can serve as basic biosensing platforms. Evaluation of the binding constant between an aptamer and a small molecule helps to determine the effectiveness of the aptamer-based sensors. Binding constants are often measured by a series of experiments with varying ligand or aptamer concentrations. Such experiments are time-consuming, material nonprudent, and prone to low reproducibility. Here, we use laser tweezers to determine the dissociation constant for aptamer–ligand interactions at the single-molecule level from only one ligand concentration. Using an adenosine 5′-triphosphate disodium salt (ATP) binding aptamer as an example, we have observed that the mechanical stabilities of aptamers bound with ATP are higher than those without a ligand. Comparison of the change in free energy of unfolding (ΔGunfold) between these two aptamers yields a ΔG of 33 ± 4 kJ/mol for the binding. By applying a Hess-like cycle at room temperature, we obtained a dissociation constant (Kd) of 2.0 ± 0.2 μM, a value consistent with the Kdobtained from our equilibrated capillary electrophoresis (CE) (2.4 ± 0.4 μM) and close to that determined by affinity chromatography in the literature (6 ± 3 μM). We anticipate that our laser tweezers and CE methodologies may be used to more conveniently evaluate the binding between receptors and ligands and also serve as analytical tools for force-based biosensing.

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Scattering forces and electromagnetic momentum density in crossed circularly polarized standing waves

Manuel I. Marqués and Juan José Saénz

We analyze the forces on a small dipolar particle and the electromagnetic momentum density in a configuration consisting in two perpendicular circularly polarized stationary waves. The field distribution shows regions in which the electric and magnetic fields are parallel corresponding to a null Poynting vector. Although the average value of the momentum density, proportional to the Poynting vector, is zero in these regions, there are scattering forces acting on small particles due to light’s spin force. The total scattering force suggests a new definition of the average value of the momentum density for free propagating electromagnetic fields.

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Observation of trapping and transporting air-borne absorbing particles with a single optical beam

Ze Zhang, Drake Cannan, Jingjiao Liu, Peng Zhang, Demetrios N. Christodoulides, and Zhigang Chen

We demonstrate optical trapping and manipulation of micron-sized absorbing air-borne particles with a single focused Gaussian beam. Transportation of trapped nonspherical particles from one beam to another is realized, and the underlying mechanism for the trapping is discussed by considering the combined action of several forces. By employing a specially-designed optical bottle beam, we observe stable trapping and optical transportation of light-absorbing particles from one container to another that is less susceptible to ambient perturbation.

DOI

Optical fiber nano-tip and 3D bottle beam as non-plasmonic optical tweezers

Samir K. Mondal, Sudipta Sarkar Pal, and Pawan Kapur

We report a simple fiber nano-tip as non-plasmonic optical tweezer, which can manipulate submicron particles in a non-contact manner. The efficiency of an optical tweezer can be enhanced by using non-diffracting type optical beams such as Bessel beam or self-imaged Bessel beam (3D bottle beam). The present work, for the first time, realizes a non-plasmonic optical tweezer based on a miniaturized axicon like single-mode optical fiber nano-tip. The tip generates non-diffracting type 3D bottle beam by virtue of its changing wedge angle. The nano-tip is prepared from a photosensitive single-mode optical fiber by employing a novel chemical etching technique. We experimentally demonstrate trapping of ~60 nm silver particle and ~160 nm silica particle using this nano-tip optical tweezer. The nano-tweezer also succeeds to pick up the particles from aqueous solution. The proposed nano-tweezer working at smaller laser powers opens new avenues for nanomanipulation and analysis of sub-microscale specimens in the biological and physical sciences.

DOI

Trapping and deformation of microbubbles in a dual-beam fibre-optic trap

Susan E Skelton, Marios Sergides, Gianluca Memoli, Onofrio M Maragó and Philip H Jones

We present results of numerical calculations to evaluate the performance of a dual-beam fibre-optic trap for low refractive index particles such as ultrasound contrast agent microbubbles. Using a geometrical optics approach, we determine the range of parameters of microbubble size and beam dimensions over which the optical trap is stable and evaluate the trapping forces and spring constants. Additionally, we calculate the optically induced stress profile over the surface of the microbubble and evaluate the resulting deformation of the microbubble using elastic membrane theory. Our results suggest that such an experiment could be a useful tool for quantifying the mechanical properties (elastic modulus) of the shell material of an ultrasound contrast agent microbubble.

DOI

Intramolecular Folding in Human ILPR Fragment with Three C-Rich Repeats

Soma Dhakal, Javonne L. Lafontaine, Zhongbo Yu, Deepak

Enrichment of four tandem repeats of guanine (G) rich and cytosine (C) rich sequences in functionally important regions of human genome forebodes the biological implications of four-stranded DNA structures, such as G-quadruplex and i-motif, that can form in these sequences. However, there have been few reports on the intramolecular formation of non-B DNA structures in less than four tandem repeats of G or C rich sequences. Here, using mechanical unfolding at the single-molecule level, electrophoretic mobility shift assay (EMSA), circular dichroism (CD), and ultraviolet (UV) spectroscopy, we report an intramolecularly folded non-B DNA structure in three tandem cytosine rich repeats, 5'-TGTC4ACAC4TGTC4ACA (ILPR-I3), in the human insulin linked polymorphic region (ILPR). The thermal denaturation analyses of the sequences with systematic C to T mutations have suggested that the structure is linchpinned by a stack of hemiprotonated cytosine pairs between two terminal C4 tracts. Mechanical unfolding and Br2 footprinting experiments on a mixture of the ILPR-I3 and a 5′-C4TGT fragment have further indicated that the structure serves as a building block for intermolecular i-motif formation. The existence of such a conformation under acidic or neutral pH complies with the strand-by-strand folding pathway of ILPR i-motif structures.

DOI

Exploring the complexity of aerosol particle properties and processes using single particle techniques

Ulrich K. Krieger , Claudia Marcolli and Jonathan P. Reid

The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 μm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.

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Published Paper Statistics (first half of 2012)

Here is the results for the first half of 2012 for published papers on optical tweezers, micromanipulation and trapping.


The top Journals (more than 2% hits) are:
13,9% Optics Express
5,9% Optics Letters
5,1% Proceedings of the National Academy of Sciences
3,4% Applied Physics Letters
3,0% Nano Letters
3,0% Plos One
2,5% Physical Review Letters
2,1% Atti della Accademia Peloritana dei Pericolanti



Below is a tag cloud for the corresponding words found in the title and abstracts for the period:

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