Thursday, March 31, 2011

Ultrahigh-resolution optical trap with single-fluorophore sensitivity

Matthew J Comstock, Taekjip Ha & Yann R Chemla

We present a single-molecule instrument that combines a time-shared ultrahigh-resolution dual optical trap interlaced with a confocal fluorescence microscope. In a demonstration experiment, we observed individual single fluorophore–labeled DNA oligonucleotides to bind and unbind complementary DNA suspended between two trapped beads. Simultaneous with the single-fluorophore detection, we clearly observed coincident angstrom-scale changes in tether extension. Fluorescence readout allowed us to determine the duplex melting rate as a function of force. The new instrument will enable the simultaneous measurement of angstrom-scale mechanical motion of individual DNA-binding proteins (for example, single-base-pair stepping of DNA translocases) along with the detection of properties of fluorescently labeled protein (for example, internal configuration).


Use of optical tweezers technology for long-term, focal stimulation of specific subcellular neuronal compartments

Elisa D'Este, Gabriele Baj, Paolo Beuzer, Enrico Ferrari, Giulietta Pinato, Enrico Tongiorgi and Dan Cojoc

Spatial regulation of secretory molecule release is a sophisticated mechanism used by the nervous system to control network development and finely tune the activity of each synapse. Great efforts have been made to develop techniques that mimic secretory molecule release with the aim of stimulating neurons as close as possible to physiological conditions. However, current techniques have poor spatial resolution or low flexibility. Here, we propose a novel approach to achieve focal and prolonged stimulation of neurons using optical tweezers and single microbeads functionalized with a secretory molecule, the neurotrophin brain-derived neurotrophic factor (BDNF). BDNF is a key regulator of neuronal development and plasticity. We show that single BDNF-coated microbeads can be extracted with optical tweezers from small reservoirs and positioned with submicrometric precision to specific sites on the dendrites of cultured hippocampal neurons. Localized contact of microbeads functionalized with BDNF, but not with bovine serum albumin (BSA), induced focal increase of calcium signaling in the stimulated dendrite, specific activation of the TrkB receptor pathway and influenced the development of growth cones. Remarkably, a single BDNF-coated bead localized on a dendrite was found to be enough for TrkB phosphorylation, an efficient and long-lasting activation of calcium signaling in the soma, and c-Fos signaling in the nucleus, comparable to bath stimulation conditions. These findings support the use of optical tweezer technology for long-term, localized stimulation of specific subcellular neuronal compartments.


A first step towards practical single cell proteomics: a microfluidic antibody capture chip with TIRF detection

Ali Salehi-Reyhani, Joseph Kaplinsky, Edward Burgin, Miroslava Novakova, Andrew J. deMello, Richard H. Templer, Peter Parker, Mark A. A. Neil, Oscar Ces, Paul French, Keith R. Willison and David Klug
We have developed a generic platform to undertake the analysis of protein copy number from single cells. The approach described here is ‘all-optical’ whereby single cells are manipulated into separate analysis chambers using an optical trap; single cells are lysed by a shock wave caused by laser-induced microcavitation, and the protein released from a single cell is measured by total internal reflection microscopy as it is bound to micro-printed antibody spots within the device. The platform was tested using GFP transfected cells and the relative precision of the measurement method was determined to be 88%. Single cell measurements were also made on a breast cancer cell line to measure the relative levels of unlabelled human tumour suppressor protein p53 using a chip incorporating an antibody sandwich assay format. These results suggest that this is a viable method for measuring relative protein levels in single cells.


Analytical partial wave expansion of vector Bessel beam and its application to optical binding: erratum

Jun Chen, Jack Ng, Pei Wang, and Zhifang Lin

In a previous Letter [Opt. Lett. 35, 1674 (2010)], the mathematical notations for Cx and Sx were unclearly defined. That error is corrected here.


Optical detection of target molecule induced aggregation of nanoparticles by means of high-Q resonators

Jeremy Witzens and Michael Hochberg

We theoretically investigate a novel scheme to detect target molecule induced, or suppressed, aggregation of nanoparticles. High-Q optical resonators are used to both optically trap gold nanoparticle clusters and to detect their presence via a shift in the resonance wavelength. The well depth of the optical trap is chosen to be relatively low compared to the thermal energy of the nanoparticles, so that trapping of single nanoparticles is marginal and results in a comparatively small wavelength shift. Aggregation of functionalized gold nanoparticles is mediated or suppressed via binding to a target molecule. The well depth for the resulting nanoparticle clusters scales much more favorably relative to Brownian motion, resulting in large nanoparticle concentration enhancements in the evanescent field region of the resonator. We predict a target molecule sensitivity in the tens of fM range. In order to predict the resonator response, a complete theory of time resolved nanoparticle cluster trapping dynamics is derived. In particular, the formalism of Kramers’ escape time is adapted to 2D (silicon wire) and 3D (ring resonator) optical traps.


Measuring the complete force field of an optical trap

Marcus Jahnel, Martin Behrndt, Anita Jannasch, Erik Schäffer, and Stephan W. Grill

The use of optical traps to measure or apply forces on the molecular level requires a precise knowledge of the trapping force field. Close to the trap center, this field is typically approximated as linear in the displacement of the trapped microsphere. However, applications demanding high forces at low laser intensities can probe the light-microsphere interaction beyond the linear regime. Here, we measured the full nonlinear force and displacement response of an optical trap in two dimensions using a dual-beam optical trap setup with back-focal-plane photodetection. We observed a substantial stiffening of the trap beyond the linear regime that depends on microsphere size, in agreement with Mie theory calculations. Surprisingly, we found that the linear detection range for forces exceeds the one for displacement by far. Our approach allows for a complete calibration of an optical trap.


Long-distance axial trapping with Laguerre–Gaussian beams

Raktim Dasgupta, Ravi Shanker Verma, Sunita Ahlawat, Deepa Chaturvedi, and Pradeep Kumar Gupta
We show that the axial spread of the focal volume of a tightly focused beam propagating through a glass–water interface is much reduced for Laguerre–Gaussian (LG) modes as compared to the TEM00mode. Therefore, use of the LG beam helps in achieving a significant improvement of the axial trapping range in optical tweezers. We demonstrate the use of LG modes to manipulate biological cells from the bottom layer of the medium to the top surface layer. Exposure of the cells to a higher oxygen concentration at the surface layer is used for estimation of the intramembrane oxygen diffusion rate.


Monday, March 28, 2011

Adding functionalities to precomputed holograms with random mask multiplexing in holographic optical tweezers

Josep Mas, Michelle S. Roth, Estela Martín-Badosa, and Mario Montes-Usategui

In this study, we present a method designed to generate dynamic holograms in holographic optical tweezers. The approach combines our random mask encoding method with iterative high-efficiency algorithms. This hybrid method can be used to dynamically modify precalculated holograms, giving them new functionalities—temporarily or permanently—with a low computational cost. This allows the easy addition or removal of a single trap or the independent control of groups of traps for manipulating a variety of rigid structures in real time.


Friday, March 25, 2011

Optical properties of photopolymerizable nanocomposites containing nanosized molecular sieves

I Naydenova, E Leite, Tz Babeva, N Pandey, T Baron, T Yovcheva, S Sainov, S Martin, S Mintova and V Toal
Acrylamide-based photopolymerizable nanocomposites containing three types of nanosized crystals with controlled microporosity, Silicalite-1 (MFI-structure), AlPO-18 (AEI-structure) and Beta (BEA-structure) are studied. The influence of the porous nanoparticles on the average refractive index, optical scattering and holographic recording properties of the nanocomposite are characterized. The redistribution of nanoparticles as a result of the holographic recording in the layers is investigated by Raman spectroscopy. It is observed that in all three nanocomposites the nanoparticles are redistributed according to the illuminating light pattern. This redistribution improves the refractive index modulation only in the case of the MFI nanoparticles, while no improvement is observed in AEI and BEA doped layers. The results can be explained by the hydrophobic/hydrophilic nature of the nanoparticles and their interactions, or absence of interactions, with the host photopolymer.


Observation and simulation of an optically driven micromotor

N K Metzger, M Mazilu, L Kelemen, P Ormos and K Dholakia

In the realm of low Reynolds number flow there is a need to find methods to pump, move and mix minute amounts of analyte. Interestingly, micro-devices performing such actuation can be initiated by means of the light–matter interaction. Light induced forces and torques are exerted on such micro-objects, which are then driven by the optical gradient or scattering force. Here, different driving geometries can be realized to harness the light induced force. For example, the scattering force enables micro-gears to be operated in a tangential setup where the micromotor rotors are in line with an optical waveguide. The operational geometry we investigate has the advantage that it reduces the complexity of the driving of such a device in a microfluidic environment by delivering the actuating light by means of a waveguide or fiber optic. In this paper we explore the case of a micromotor being driven by a fiber optically delivered light beam. We experimentally investigate how the driving light interacts with and diffracts from the motor, utilizing two-photon imaging. The micromotor rotation rate dependence on the light field parameters is explored. Additionally, a theoretical model based on the paraxial approximation is used to simulate the torque and predict the rotation rate of such a device and compare it with experiment. The results presented show that our model can be used to optimize the micromotor performance and some example motor designs are evaluated.


Parametric study of optical forces acting upon nanoparticles in a single, or a standing, evanescent wave

Martin Šiler and Pavel Zemánek

We compare four different methods for calculating the optical forces acting upon nanoparticles illuminated either by a single evanescent wave or by an interference field of two counter-propagating coherent evanescent waves (standing evanescent wave). Two of the employed methods consider the effect of multiple reflections of the scattered field from the water–prism interface that generates the illuminating evanescent wave(s). Subsequently, we present a parametric study of the optical forces acting upon nanoparticles for different polarizations and propagation constants of the illuminating evanescent wave, particle sizes and refractive indices.


Sagnac interferometer-enhanced particle tracking in optical tweezers

M A Taylor, J Knittel, M T L Hsu, H-A Bachor and W P Bowen

A set-up is proposed to enhance tracking of very small particles, by using optical tweezers embedded within a Sagnac interferometer. The achievable signal-to-noise ratio is shown to be enhanced over that for standard back-focal-plane interferometry. The enhancement factor increases asymptotically as the interferometer visibility approaches 100%, but is capped at a maximum given by four times the ratio of the trapping field intensity to the detector saturation threshold. For an achievable visibility of 95%, the signal-to-noise ratio can be enhanced by a factor of up to 158 and the minimum trackable particle size is 2.3 times smaller than without the interferometer. This technique is particularly useful for optical tweezers which require counter-propagating trap beams.


Micro-rheology near fluid interfaces

G M Wang, R Prabhakar, Y X Gao and E M Sevick

Using optical trapping, we have measured anisotropic and distance-dependent mobility of a colloidal particle near high surface tension fluid–fluid interfaces. These experimental results show that the parallel mobility is enhanced near a high surface tension liquid–gas surface, which is consistent with hydrodynamic predictions for a surface that does not support a stress, and that the parallel mobility is suppressed near a high surface tension liquid–liquid surface, consistent with a no-slip solid boundary. We demonstrate the potential for this optical technique to be applied to soft interfaces by predicting the normalized PSD using recent hydrodynamic predictions on particle mobility near soft surfaces.


Influence of rotational force fields on the determination of the work done on a driven Brownian particle

Giuseppe Pesce, Giovanni Volpe, Alberto Imparato, Giulia Rusciano and Antonio Sasso

For a Brownian system the evolution of thermodynamic quantities is a stochastic process, in particular the work performed on a driven colloidal particle held in an optical trap, changes for each realization of the experimental manipulation, even though the manipulation protocol remains unchanged. Nevertheless, the work distribution is governed by established laws. Here, we show how the measurement of the work distribution is influenced by the presence of rotational, i.e. nonconservative, radiation forces. Experiments on particles of different materials show that the rotational radiation forces, and therefore their effect on the work distributions, increase with the particle's refractive index.


Optically written optofluidic ice channels

S Anand, A Engelbrecht and D McGloin

We demonstrate that a range of liquid channels can be created within ice blocks using light. We show that channels with dimensions as small as 40 µm can be made using a 1064 nm laser beam coupled through single-mode fibre. This is in contrast to larger 'tapered' fibres that can be made using multimode fibre and more irregular channels using free space beams. The channels can be stabilized over timescales of minutes using powers as low as 30 mW. Furthermore we demonstrate that liquid samples containing particles may be inserted into the channels and present evidence that particles can be trapped and manipulated using a combination of optical and thermal forces within the light-created microchannels. Furthermore we suggest that such techniques could be used to create templates for conventional microfluidic channels.


iTweezers: optical micromanipulation controlled by an Apple iPad

R W Bowman, G Gibson, D Carberry, L Picco, M Miles and M J Padgett

The 3D interactive manipulation of multiple particles with holographic optical tweezers is often hampered by the control system. We use a multi-touch interface implemented on an Apple iPad to overcome many of the limitations of mouse-based control, and demonstrate an elegant and intuitive interface to multi-particle manipulation. This interface connects to the tweezers system hardware over a wireless network, allowing it to function as a remote monitor and control device.


Structural and Mechanical Properties of Klebsiella pneumoniae Type 3 Fimbriae

Feng-Jung Chen, Chia-Han Chan, Ying-Jung Huang, Kuo-Liang Liu, Hwei-Ling Peng, Hwan-You Chang, Gunn-Guang Liou, Tri-Rung Yew, Cheng-Hsien Liu, Ken Y. Hsu, and Long Hsu
This study investigated the structural and mechanical properties of Klebsiella pneumoniae type 3 fimbriae, which constitute a known virulence factor for the bacterium. Transmission electron microscopy and optical tweezers were used to understand the ability of the bacterium to survive flushes. An individual K. pneumoniae type 3 fimbria exhibited a helix-like structure with a pitch of 4.1 nm and a three-phase force-extension curve. The fimbria was first nonlinearly stretched with increasing force. Then, it started to uncoil and extended several micrometers at a fixed force of 66 ± 4 pN (n = 22). Finally, the extension of the fimbria shifted to the third phase, with a characteristic force of 102 ± 9 pN (n = 14) at the inflection point. Compared with the P fimbriae and type 1 fimbriae of uropathogenic Escherichia coli, K. pneumoniae type 3 fimbriae have a larger pitch in the helix-like structure and stronger uncoiling and characteristic forces.


Optical manipulation of colloids and defect structures in anisotropic liquid crystal fluids

R P Trivedi, D Engström and I I Smalyukh

Optical trapping in anisotropic fluids such as liquid crystals shows inherently different behavior compared to that in isotropic media. Anisotropic optical and visco-elastic properties of these materials result in direction-sensitive and polarization-dependent interaction of the focused laser beam with colloidal inclusions, defects and structures of long-range molecular order, providing new means of non-contact optical control. Optical trapping properties are further enriched by laser-induced realignment of the optical axis that can be observed in these liquid crystalline materials at relatively low trapping laser powers. Optical manipulation of particles and defects in these anisotropic fluids is of immense importance for their fundamental study and from the standpoint of technological applications such as light-directed colloidal self-assembly and generation of tunable photonic architectures in liquid crystals. We review the basic physical mechanisms related to optical trapping in anisotropic liquid crystalline fluids and demonstrate how it can be employed in quantitative studies of colloidal interactions and both topological and mechanical properties of defects.


Thursday, March 24, 2011

The effective hydrodynamic radius of single DNA-grafted colloids as measured by fast Brownian motion analysis

Olaf Ueberschär, Carolin Wagner, Tim Stangner, Christof Gutsche and Friedrich Kremer

Optical tweezers accomplished with fast position detection enable one to carry out Brownian motion analysis of single DNA-grafted (grafting density: 1000 molecules per particle, molecular weight: 4000 bp) colloids in media of varying NaCl concentration. By that the effective hydrodynamic radius of the colloid under study is determined and found to be strongly dependent on the conformation of the grafted DNA chains. Our results compare well both with recent measurements of the pair interaction potential between DNA-grafted colloids (Kegler et al. Phys Rev Lett 2008; 100:118302) and with microfluidic studies (Gutsche et al. Microfluid Nanofluid 2006; 2:381-386). The observed scaling of the brush height with the ion concentration is in full accord with the theoretical predictions by Pincus, Zhulina, Birshtein and Borisov.


Nano-manipulation performance with enhanced evanescent field close to near-field optical probes

B.H. Liu, L.J. Yang, Y. Wang and J.L. Yuan

Recently, interest in nano-manipulation using the evanescent wave generated by nano-objects has been growing, but the analyses of manipulation flexibility and performance haven't been solved. In this paper the near-field optical trap utilizing a tapered metalized probe used in NSOM is described in detail. By employing a generalization of the conservation law for momentum using three-dimensional FDTD method, rigorous calculations of field distributions and trapping forces in near-field region are conducted. Calculations show that the particle with radius larger than the aperture is pushed away from the metal-coated fiber probe, while it tends to be trapped in larger effective region as its radius becoming smaller. The particle that is placed very near the aperture and around two field peaks intends to be dragged to the aperture edge, while the particle placed at other position tends to be attracted to the center surface of the probe tip. Furthermore, a preferable method using the combination of the near-field optical fiber probe and the AFM metallic probe is proposed, for more efficient non-contact manipulation and better observation of one single nano-particle. The analyses of trapping potential along the probe axis and the near-field distribution show the possibility of particle trapping.


Microrheology and the fluctuation theorem in dense colloids

L. G. Wilson, A. W. Harrison, W. C. K. Poon and A. M. Puertas

We present experiments and computer simulations of a "tracer" (or "probe") particle trapped with optical tweezers and dragged at constant speed through a bath of effectively hard colloids with approximately the same size as the probe. The results are analyzed taking the single-particle case and assuming effective parameters for the bath. The effective microscopic friction coefficient and effective temperature of the tracer are obtained. At high probe velocities, the experimental microviscosity compares well with the viscosity from bulk rheology, whereas a correction due to hydrodynamic interactions (absent in the simulations) is necessary to collapse the simulation data. Surprisingly, agreement is found without any need of hydrodynamic corrections at small probe velocities. The dynamics of the tracer inside the trap shows, both in the simulations and experiments, a fast relaxation due to solvent friction and a slow one caused by the collisions with other particles. The latter is less effective in dissipating the energy introduced by the moving trap and causes increasing fluctuations in the tracer motion, reflected as higher effective temperature.


Multidepth, multiparticle tracking for active microrheology using a smart camera

Scott A. Silburn, Christopher D. Saunter, John M. Girkin, and Gordon D. Love

The quantitative measurement of particle motion in optical tweezers is an important tool in the study of microrheology and can be used in a variety of scientific and industrial applications. Active microheology, in which the response of optically trapped particles to external driving forces is measured, is particularly useful in probing nonlinear viscoelastic behavior in complex fluids. Currently such experiments typically require independent measurements of the driving force and the trapped particle's response to be carefully synchronized, and therefore the experiments normally require analog equipment. In this paper we describe both a specialized camera and an imaging technique which make high-speed video microscopy a suitable tool for performing such measurements, without the need for separate measurement systems and synchronization. The use of a high-speed tracking camera based on a field programmable gate array to simultaneously track multiple particles is reported. By using this camera to simultaneously track one microsphere fixed to the wall of a driven sample chamber and another held in an optical trap, we demonstrate simultaneous optical measurement of the driving motion and the trapped probe particle response using a single instrument. Our technique is verified experimentally by active viscosity measurements on water–ethylene glycol mixtures using a phase-shift technique.


Wednesday, March 23, 2011

Combining optical trapping, fluorescence microscopy and micro-fluidics for single molecule studies of DNA–protein interactions

Andrea Candelli, Gijs J. L. Wuite and Erwin J. G. Peterman

Complexity and heterogeneity are common denominators of the many molecular events taking place inside the cell. Single-molecule techniques are important tools to quantify the actions of biomolecules. Heterogeneous interactions between multiple proteins, however, are difficult to study with these technologies. One solution is to integrate optical trapping with micro-fluidics and single-molecule fluorescence microscopy. This combination opens the possibility to study heterogeneous/complex protein interactions with unprecedented levels of precision and control. It is particularly powerful for the study of DNA–protein interactions as it allows manipulating the DNA while at the same time, individual proteins binding to it can be visualized. In this work, we aim to illustrate several published and unpublished key results employing the combination of fluorescence microscopy and optical tweezers. Examples are recent studies of the structural properties of DNA and DNA–protein complexes, the molecular mechanisms of nucleo-protein filament assembly on DNA and the motion of DNA-bound proteins. In addition, we present new results demonstrating that single, fluorescently labeled proteins bound to individual, optically trapped DNA molecules can already be tracked with localization accuracy in the sub-10 nm range at tensions above 1 pN. These experiments by us and others demonstrate the enormous potential of this combination of single-molecule techniques for the investigation of complex DNA–protein interactions


Laser-guided cell micropatterning system

Russell K. Pirlo, Zhen Ma, Andrew Sweeney, Honghai Liu, Julie X. Yun, Xiang Peng, Xiaocong Yuan, George X. Guo, and Bruce Z. Gao
Employing optical force, our laser-guided cell micropatterning system, is capable of patterning different cell types onto and within standard cell research devices, including commercially available multielectrode arrays (MEAs) with glass culture rings, 35 mm Petri dishes, and microdevices microfabricated with polydimethylsiloxane on 22 mm × 22 mm cover glasses. We discuss the theory of optical forces for generating laser guidance and the calculation of optimal beam characteristics for cell guidance. We describe the hardware design and software program for the cell patterning system. Finally, we demonstrate the capabilities of the system by (1) patterning neurons to form an arbitrary pattern, (2) patterning neurons onto the electrodes of a standard MEA, and (3) patterning and aligning adult cardiomyocytes in a polystyrene Petri dish.


Spatially optimized gene transfection by laser-induced breakdown of optically trapped nanoparticles

Yoshihiko Arita, Maria Leilani Torres-Mapa, Woei Ming Lee, Tomáš Čižmár, Paul Campbell, Frank J. Gunn-Moore, and Kishan Dholakia

We demonstrate laser-induced breakdown of an optically trapped nanoparticle with a nanosecond laser pulse. Controllable cavitation within a microscope sample was achieved, generating shear stress to monolayers of live cells. This efficiently permeabilize their plasma membranes. We show that this technique is an excellent tool for plasmid-DNA transfection of cells with both reduced energy requirements and reduced cell lysis compared to previously reported approaches. Simultaneous multisite targeted nanosurgery of cells is also demonstrated using a spatial light modulator for parallelizing the technique.


Tuesday, March 22, 2011

Optical force of a TEM00 light beam on a sphere with refractive index less than its surrounding medium

Jiwei Huang and Yao-Xiong Huang

The force acting on a micro-sphere with refractive index less than its surrounding medium by a TEM00 mode focused light beam is investigated both theoretically and experimentally. It is demonstrated that although the force is mainly a repulsing one, optical trapping is available in the cases in which the sphere is closed to the beam axis of the light and located closely before its focus. By moving an aqueous micron sphere in an oil medium, then fusing it into a larger aqueous sphere encapsulating microbes and acting as a micron-bioreactor, we demonstrated the force and its potential applications.


Trapping Rayleigh particles using highly focused higher-order radially polarized beams

Youyi Zhuang, Yaoju Zhang , Biaofeng Ding and Taikei Suyama

The optical trapping characteristics of highly focused higher-order radially polarized beams (R-TEMp1*) acting on a Rayleigh particle are studied theoretically. Numerical results show that as the order p of beam increases and the numerical aperture NAo of the objective decreases, the axial trap distance increases but the trap depth and maximum restoring force decreases. In a limit of NAo = 1, three higher-order R-TEMp1* beams of p = 1, 2, 3, like the fundamental lowest-order radially polarized beam of p = 0, can three-dimensionally trap a particle to the focus but the axial trap stiffness decreases with the increase of p. When NAo = 0.95, the focus is still a stable trap point for the two beams of p = 0 and 1 but it becomes an unstable trap point for the two beams of p = 2 and 3. The trap stability is also discussed for higher-order radially polarized beam illumination.


Monopole antenna arrays for optical trapping, spectroscopy, and sensing

A. E. Çetin, Ahmet Ali Yanik, Cihan Yilmaz, Sivasubramanian Somu, Ahmed Busnaina, and Hatice Altug

We introduce a nanoplasmonic platform merging multiple modalities for optical trapping, nanospectroscopy, and biosensing applications. Our platform is based on surface plasmon polariton driven monopole antenna arrays combining complementary strengths of localized and extended surface plasmons. Tailoring of spectrally narrow resonances lead to large index sensitivities (S ∼ 675 nm/RIU) with record high figure of merits (FOM ∼ 112.5). These monopole antennas supporting strong light localization with easily accessible near-field enhanced hotspots are suitable for vibrational nanospectroscopy and optical trapping. Strong optical forces (350 pN/W/μm2) are shown at these hotspots enabling directional control with incident light polarization.


Wednesday, March 16, 2011

Electron Beam Fabrication of Birefringent Microcylinders

Zhuangxiong Huang, Francesco Pedaci, Maarten van Oene, Matthew J. Wiggin, and Nynke H. Dekker
Numerous biological and biotechnological applications rely on the use of micrometer- and nanometer-scale particles, benefiting tremendously from quantitative control of their physical and chemical properties. Here, we describe the use of electron beam lithography for the design, fabrication, and functionalization of micrometer-scale birefringent quartz cylinders for use in sensing and detection. We demonstrate excellent control of the cylinders’ geometry, fabricating cylinders with heights of 0.5−2 μm and diameters of 200−500 nm with high precision while maintaining control of their side-wall angle. The flexible fabrication allows cylinders to be selectively shaped into conical structures or to include centered protrusions for the selective attachment of biomolecules. The latter is facilitated by straightforward functionalization targeted either to a cylinder’s face or to the centered protrusion alone. The fabricated quartz cylinders are characterized in an optical torque wrench, permitting correlation of their geometrical properties to measured torques. Lastly, we tether individual DNA molecules to the functionalized cylinders and demonstrate the translational and rotational control required for single-molecule studies.


Plasmon-Enhanced Optical Trapping of Gold Nanoaggregates with Selected Optical Properties

Elena Messina, Emanuele Cavallaro, Adriano Cacciola, Maria Antonia Iat, Pietro G. Gucciardi, Ferdinando Borghese, Paolo Denti, Rosalba Saija, Giuseppe Compagnini, Moreno Meneghetti, Vincenzo Amendola, and Onofrio M. Marag
We show how light forces can be used to trap gold nanoaggregates of selected structure and optical properties obtained by laser ablation in liquid. We measure the optical trapping forces on nanoaggregates with an average size range 20−750 nm, revealing how the plasmon-enhanced fields play a crucial role in the trapping of metal clusters featuring different extinction properties. Force constants of the order of 10 pN/nmW are detected, the highest measured on a metal nanostructure. Finally, by extending the transition matrix formalism of light scattering theory to the optical trapping of metal nanoaggregates, we show how the plasmon resonances and the fractal structure arising from aggregation are responsible for the increased forces and wider trapping size range with respect to individual metal nanoparticles.


Extended and knotted optical traps in three dimensions

Elisabeth R. Shanblatt and David G. Grier

We describe a method for projecting holographic optical traps that are extended along arbitrary curves in three dimensions, and whose amplitude and phase profiles are specified independently. This approach can be used to create bright optical traps with knotted optical force fields.


Forcing a Connection: Impacts of Single-Molecule Force Spectroscopy on In Vivo Tension Sensing

Michael D. Brenner, Ruobo Zhou, Taekjip Ha

Mechanical tension plays a large role in cell development ranging from morphology to gene expression. On the molecular level, the effects of tension can be seen in the dynamic arrangement of membrane proteins as well as the recruitment and activation of intracellular proteins. Forces applied to biopolymers during in vitro force measurements offer greater understanding of the effects of tension on molecules in live cells, and experimental techniques involving test tubes and live cells can often overlap. Indeed, when forces exerted on cellular components can be calibrated ex vivo with force spectroscopy, a powerful tool is available for researchers in probing cellular mechanotransduction on the molecular scale. This review will discuss the techniques used in measuring both cellular traction forces and single-molecule force spectroscopy. Emphasis will be placed on the use of fluorescence reporter systems for the development of in vivo tension sensors that can be used for calibration with single molecule force methods.


Brownian diffusion of gold nanoparticles in an optical trap studied by fluorescence correlation spectroscopy

Wang, J.; Li, Z.; Yao, C.; Xue, F.; Zhang, Z.; Hüttmann, G.

The effect of thermal-induced Brownian motion on gold nanoparticles (Au NPs) in optical traps is studied by fluorescence correlation spectroscopy (FCS) method. The Brownian motion and optical trapping potential are investigated by the decay time of the FCS curve and the laser power. It is shown that that the probability of finding a gold nanoparticle in the trap depends on the ratio of the optical energy of the particle to its thermal energy. A power threshold is observed by the decay time as a function of laser power. The experimental studies show that the temperature rise does not seriously affect the average number of particles in the focal spot, but the average residence time is more sensitively affected by the temperature.


Optical assembly of microparticles into highly ordered structures using Ince–Gaussian beams

Mike Woerdemann, Christina Alpmann, and Cornelia Denz

Ince–Gaussian (IG) beams are a third complete family of solutions of the paraxial Helmholtz equation. While many applications of Hermite–Gaussian and Laguerre–Gaussian beams have been demonstrated for manipulation of microparticles, the potential of the more general class of IG beams has not yet been exploited at all. We describe the unique properties of IG beams with respect to optical trapping applications, demonstrate a flexible experimental realization of arbitrary IG beams and prove the concept by creating two- and three-dimensional, highly ordered assemblies of typical microparticles. The concept is universal and can easily be integrated into existing holographic optical tweezers setups.


Excitable particles in an optical torque wrench

Francesco Pedaci, Zhuangxiong Huang, Maarten van Oene, Stephane Barland & Nynke H. Dekker

The optical torque wrench is a laser trapping technique capable of applying and directly measuring torque on microscopic birefringent particles using spin momentum transfer, and has found application in the measurement of static torsional properties of biological molecules such as single DNAs. Motivated by the potential of the optical torque wrench to access the fast rotational dynamics of biological systems, a result of its all-optical manipulation and detection, we focus on the angular dynamics of the trapped birefringent particle, demonstrating its excitability in the vicinity of a critical point. This links the optical torque wrench to nonlinear dynamical systems such as neuronal and cardiovascular tissues, nonlinear optics and chemical reactions, all of which display an excitable binary (‘all-or-none’) response to input perturbations. On the basis of this dynamical feature, we devise and implement a conceptually new sensing technique capable of detecting single perturbation events with high signal-to-noise ratio and continuously adjustable sensitivity.


Tuesday, March 15, 2011

Optically based manufacturing with polymer particles

Reza Ghadiri, Mario Surbek, Cemal Esen and Andreas Ostendorf

We present a new single-laser optical trapping technique for the exact manipulation and durable assembly of transparent polymer microparticles. This technique comprises the trapping of microparticles and the assembly by using a laser-driven thermal process for the joining of the particles. The thermal energy necessary for the systematic joining is applied partly by global heating of the processing chamber and by absorption of the electromagnetic radiation of the laser tweezer. The main advantage of this contact free joining technology is to use the same laser for the optical trapping, positioning and the durable assembly. The generated joints are stable and cannot be broken up with optical forces. In summary, a new micromanufacturing process based on an optical machining process is reported with promising applications in the MEMS and photonics area.


The Pel Polysaccharide Can Serve a Structural and Protective Role in the Biofilm Matrix of Pseudomonas aeruginosa

Kelly M. Colvin, Vernita D. Gordon, Keiji Murakami, Bradley R. Borlee, Daniel J. Wozniak, Gerard C. L. Wong, Matthew R. Parsek
Bacterial extracellular polysaccharides are a key constituent of the extracellular matrix material of biofilms. Pseudomonas aeruginosa is a model organism for biofilm studies and produces three extracellular polysaccharides that have been implicated in biofilm development, alginate, Psl and Pel. Significant work has been conducted on the roles of alginate and Psl in biofilm development, however we know little regarding Pel. In this study, we demonstrate that Pel can serve two functions in biofilms. Using a novel assay involving optical tweezers, we demonstrate that Pel is crucial for maintaining cell-to-cell interactions in a PA14 biofilm, serving as a primary structural scaffold for the community. Deletion of pelB resulted in a severe biofilm deficiency. Interestingly, this effect is strain-specific. Loss of Pel production in the laboratory strain PAO1 resulted in no difference in attachment or biofilm development; instead Psl proved to be the primary structural polysaccharide for biofilm maturity. Furthermore, we demonstrate that Pel plays a second role by enhancing resistance to aminoglycoside antibiotics. This protection occurs only in biofilm populations. We show that expression of the pel gene cluster and PelF protein levels are enhanced during biofilm growth compared to liquid cultures. Thus, we propose that Pel is capable of playing both a structural and a protective role in P. aeruginosa biofilms.

Nanomanipulation using near field photonics

David Erickson, Xavier Serey, Yih-Fan Chen and Sudeep Mandal

In this article we review the use of near-field photonics for trapping, transport and handling of nanomaterials. While the advantages of traditional optical tweezing are well known at the microscale, direct application of these techniques to the handling of nanoscale materials has proven difficult due to unfavourable scaling of the fundamental physics. Recently a number of research groups have demonstrated how the evanescent fields surrounding photonic structures like photonic waveguides, optical resonators, and plasmonic nanoparticles can be used to greatly enhance optical forces. Here, we introduce some of the most common implementations of these techniques, focusing on those which have relevance to microfluidic or optofluidic applications. Since the field is still relatively nascent, we spend much of the article laying out the fundamental and practical advantages that near field optical manipulation offers over both traditional optical tweezing and other particle handling techniques. In addition we highlight three application areas where these techniques namely could be of interest to the lab-on-a-chip community, namely: single molecule analysis, nanoassembly, and optical chromatography.


Exact analytical expansion of an off-axis Gaussian laser beam using the translation theorems for the vector spherical harmonics

Lars Boyde, Kevin J. Chalut, and Jochen Guck

The interaction of a Gaussian laser beam with a particle that is located off axis is a fundamental problem encountered across many scientific fields, including biological physics, chemistry, and medicine. For spherical geometries, generalized Lorenz–Mie theory affords a solution of Maxwell’s equations for the scattering from such a particle. The solution can be obtained by expanding the laser fields in terms of vector spherical harmonics (VSHs). However, the computation of the VSH expansion coefficients for off-axis beams has proven challenging. In the present study, we provide a very viable, theoretical framework to efficiently compute the sought-after expansion coefficients with high numerical accuracy. We use the existing theory for the expansion of an on-axis laser beam and employ Cruzan’s translation theorems [Q. Appl. Math.20, 33 (1962)QAMAAY0033-569X] for the VSHs to obtain a description for more general off-axis beams. The expansion coefficients for the off-axis laser beam are presented in an analytical form in terms of an infinite series over the underlying translation coefficients. A direct comparison of the electromagnetic fields of such a beam expansion with the original laser fields and with results obtained using numerical quadratures shows excellent agreement (relative errors are on the order of ≲10−3). In practice, the analytical approach presented in this study has numerous applications, reaching from multiparticle scattering problems in atmospheric physics and climatology to optical trapping, sorting, and sizing techniques.


Fluctuations and response in a non-equilibrium micron-sized system

Juan Ruben Gomez-Solano, Artyom Petrosyan, Sergio Ciliberto and Christian Maes

The linear response of non-equilibrium systems with Markovian dynamics satisfies a generalized fluctuation-dissipation relation derived from time symmetry and antisymmetry properties of the fluctuations. The relation involves the sum of two correlation functions of the observable of interest: one with the entropy excess and the second with the excess of dynamical activity with respect to the unperturbed process, without recourse to anything but the dynamics of the system. We illustrate this approach in the experimental determination of the linear response of the potential energy of a Brownian particle in a toroidal optical trap. The overdamped particle motion is effectively confined to a circle, undergoing a periodic potential and driven out of equilibrium by a non-conservative force. Independent direct and indirect measurements of the linear response around a non-equilibrium steady state are performed in this simple experimental system. The same ideas are applicable to the measurement of the response of more general non-equilibrium micron-sized systems immersed in Newtonian fluids either in stationary or non-stationary states and possibly including inertial degrees of freedom.


Optical macro-tweezers: trapping of highly motile micro-organisms

G Thalhammer, R Steiger, S Bernet and M Ritsch-Marte

Optical micromanipulation stands for contact-free handling of microscopic particles by light. Optical forces can manipulate non-absorbing objects in a large range of sizes, e.g., from biological cells down to cold atoms. Recently much progress has been made going from the micro- down to the nanoscale. Less attention has been paid to going the other way, trapping increasingly large particles. Optical tweezers typically employ a single laser beam tightly focused by a microscope objective of high numerical aperture to stably trap a particle in three dimensions (3D). As the particle size increases, stable 3D trapping in a single-beam trap requires scaling up the optical power, which eventually induces adverse biological effects. Moreover, the restricted field of view of standard optical tweezers, dictated by the use of high NA objectives, is particularly unfavorable for catching actively moving specimens. Both problems can be overcome by traps with counter-propagating beams. Our 'macro-tweezers' are especially designed to trap highly motile organisms, as they enable three-dimensional all-optical trapping and guiding in a volume of 2 × 1 × 2 mm3. Here we report for the first time the optical trapping of large actively swimming organisms, such as for instance Euglena protists and dinoflagellates of up to 70 µm length. Adverse bio-effects are kept low since trapping occurs outside high intensity regions, e.g., focal spots. We expect our approach to open various possibilities in the contact-free handling of 50–100 µm sized objects that could hitherto not be envisaged, for instance all-optical holding of individual micro-organisms for taxonomic identification, selective collecting or tagging.

The holographic optical micro-manipulation system based on counter-propagating beams

T. Čižmár, O. Brzobohatý, K. Dholakia, P. Zemánek

We present a system employing a dynamic diffractive optical element to control properties of two counterpropagating beams overlapping within a sample chamber. This system allows us to eliminate optical aberrations along both beam pathways and arbitrarily switch between various numbers of laser beams and their spatial profiles (i.e. Gaussian, Laguerre-Gaussian, Bessel beams, etc.). We successfully tested various counter-propagating dual-beam configurations including optical manipulation of both high and low index particles in water or air, particle delivery in an optical conveyor belt and the formation of colloidal solitons by optical binding. Furthermore, we realized a novel optical mixer created by particles spiraling in counter-propagating interfering optical vortices and a new tool for optical tomography or localized spectroscopy enabling sterile contactless rotation and reorientation of a trapped living cell.

Monday, March 14, 2011

Brownian motion in a Maxwell fluid

Matthias Grimm, Sylvia Jeney and Thomas Franosch

The equilibrium dynamics of a spherical particle immersed in a complex Maxwell fluid is analyzed in terms of velocity autocorrelation function (VACF), mean-square displacement (MSD), and power spectral density (PSD). We elucidate the role of hydrodynamic memory and its interplay with medium viscoelasticity for a free and a harmonically confined particle. The elastic response at high frequencies introduces oscillations in the VACF, which are found to be strongly damped by the coupling to the fluid. We show that in all Maxwell fluids hydrodynamic memory eventually leads to a power-law decay in the VACF as is already known for Newtonian fluids. The MSD displays asymptotically an intermediate plateau reflecting the elastic restoring forces of the medium. In the frequency domain, the PSD exhibits at high frequencies a step due to the trapping, whereas the low-frequency decay reflects the viscoelastic relaxation. Our results suggest that high-frequency microrheology is well-suited to infer the elastic modulus, which is sensitive over a wide range of Maxwell times.


Red blood cell dynamics: from spontaneous fluctuations to non-linear response

Young Zoon Yoon, Jurij Kotar, Aidan T. Brown and Pietro Cicuta

We studied experimentally the mechanical properties of the red blood cell. By attaching beads biochemically on the cell membrane at diametrically opposite positions, the membrane movements can be detected very accurately, and a deformation of the cell can be imposed. A measurement of the mechanical properties at very small amplitudes is obtained by fluctuation analysis, and compared to the stiffness at larger deformations, obtained by stretching the cells via optical traps whilst monitoring the force. The cells are also probed at various conditions of pre-strain. These measurements show clearly a stiffening with strain and with pre-strain, which is strongest at low frequencies of deformation. The cell is measured to be slightly softer from fluctuation analysis, but consistent simply with the fact that the oscillation amplitude in fluctuations is very small. There is no evidence in these experiments of non-thermal sources of membrane motion, although non-thermal noise may be present within experimental error.


Downstream DNA Tension Regulates the Stability of the T7 RNA Polymerase Initiation Complex

Gary M. Skinner, Bennett S. Kalafut and Koen Visscher

Gene transcription by the enzyme RNA polymerase is tightly regulated. In many cases, such as in the lac operon in Escherichia coli, this regulation is achieved through the action of protein factors on DNA. Because DNA is an elastic polymer, its response to enzymatic processing can lead to mechanical perturbations (e.g., linear stretching and supercoiling) that can affect the operation of other DNA processing complexes acting elsewhere on the same substrate molecule. Using an optical-tweezers assay, we measured the binding kinetics between single molecules of bacteriophage T7 RNA polymerase and DNA, as a function of tension. We found that increasing DNA tension under conditions that favor formation of the open complex results in destabilization of the preinitiation complex. Furthermore, with zero ribonucleotides present, when the closed complex is favored, we find reduced tension sensitivity, implying that it is predominantly the open complex that is sensitive. This result strongly supports the “scrunching” model for T7 transcription initiation, as the applied tension acts against the movement of the DNA into the scrunched state, and introduces linear DNA tension as a potential regulatory quantity for transcription initiation.


Dissociation of Bimolecular αIIbβ3-Fibrinogen Complex under a Constant Tensile Force

Rustem I. Litvinov, Valeri Barsegov, Andrew J. Schissler, Andrew R. Fisher, Joel S. Bennett, John W. Weisel and Henry Shuman

The regulated ability of integrin αIIbβ3 to bind fibrinogen plays a crucial role in platelet aggregation, adhesion, and hemostasis. Employing an optical-trap-based electronic force clamp, we studied the thermodynamics and kinetics of αIIbβ3-fibrinogen bond formation and dissociation under constant unbinding forces, mimicking the forces of physiologic blood shear on a thrombus. The distribution of bond lifetimes was bimodal, indicating that the αIIbβ3-fibrinogen complex exists in two bound states with different mechanical stability. The αIIbβ3 antagonist, abciximab, inhibited binding without affecting the unbinding kinetics, whereas Mn2+ biased the αIIbβ3-fibrinogen complex to the strong bound state with reduced off-rate. The average bond lifetimes decreased exponentially with increasing pulling force from 5 pN to 50 pN, suggesting that in this force range the αIIbβ3-fibrinogen interactions are classical slip bonds. We found no evidence for catch bonds, which is consistent with the known lack of shear-enhanced platelet adhesion on fibrinogen-coated surfaces. Taken together, these data provide important quantitative and qualitative characteristics of αIIbβ3-fibrinogen binding and unbinding that underlie the dynamics of platelet adhesion and aggregation in blood flow.


Optically Directed Assembly of Continuous Mesoscale Filaments

J. T. Bahns, S. K. R. S. Sankaranarayanan, S. K. Gray, and L. Chen

We demonstrate irreversible continuous filament formation when a weak laser focus is positioned near the edge of an evaporating colloidal droplet containing carbon and gold nanoparticles. Optical trapping, hydrothermal, and chemical interactions lead to controlled colloidal synthesis of stable, irreversible mesoscale filaments of arbitrary shape and size. Mechanisms for this optically directed assembly are discussed with fluid dynamics, molecular dynamics, and lattice kinetic Monte Carlo calculations.


Sunday, March 13, 2011

Optical tweezers and paradoxes in electromagnetism

Robert N C Pfeifer, Timo A Nieminen, Norman R Heckenberg and Halina Rubinsztein-Dunlop

The widespread application of optical forces and torques has contributed to renewed interest in the fundamentals of the electromagnetic force and torque, including long-standing paradoxes such as the Abraham–Minkowski controversy and the angular momentum density of a circularly polarized plane wave. We discuss the relationship between these electromagnetic paradoxes and optical tweezers. In particular, consideration of possible optical tweezers experiments to attempt to resolve these paradoxes strongly suggests that they are beyond experimental resolution, yielding identical observable results in all cases.


Stereoscopic particle tracking for 3D touch, vision and closed-loop control in optical tweezers

Richard Bowman, Daryl Preece, Graham Gibson and Miles Padgett

Force measurement in an interactive 3D micromanipulation system can allow the user to make delicate adjustments, and to explore surfaces with touch as well as vision. We present a system to achieve this on the micron scale using stereoscopic particle tracking combined with holographic optical tweezers, which can track particles with nanometre accuracy. 2D tracking of particles in each of the stereo images gives 3D positions for each particle. This takes less than 200 µs per image pair, using a 1D 'symmetry transform' applied to each row and column of a 2D image, which can maintain tracking of particles throughout the 10 µm axial range. The only parameters required are the geometry of the imaging system, and therefore there is no need to recalibrate for different particle sizes or refractive indices. Consequently, we can calculate the force exerted by the optical trap in real time at 1 kilohertz, allowing us to implement a force-feedback interface (with a loop rate of 400 Hz). In combination with our OpenGL hologram calculation engine, the system has a closed-loop bandwidth of 20 Hz. This allows us to stabilize trapped particles axially through active feedback, cancelling out some Brownian motion. For the weak traps we use here (spring constant k≈2 pN µm − 1), this results in a threefold increase in axial stiffness. We demonstrate the 3D interface by probing an oil droplet, mapping out its surface in the y–z plane.


Optimizing active and passive calibration of optical tweezers

M Andersson, F Czerwinski and L B Oddershede

To obtain quantitative information from optical trapping experiments it is essential to perform a precise force calibration. Therefore, sources of noise should be pinpointed and eliminated. Fourier analysis is routinely used to calibrate optical trapping assays because it is excellent for pinpointing high frequency noise. In addition, Allan variance analysis is particularly useful for quantifying low frequency noise and for predicting the optimal measurement time. We show how to use Allan variance in combination with Fourier analysis for optimal calibration and noise reduction in optical trapping assays. The methods are applied to passive assays, utilizing the thermal motion of a trapped particle, and to active assays where the bead is harmonically driven. The active method must be applied in assays where, for example, the viscoelastic properties of the medium or the size or shape of the trapped object are unknown. For measurement times shorter than the optimal calibration time the noise is larger in active than in the passive assays. For times equal to or longer than the optimal measurement time, though, the noise on passive and active assays is identical. As an example, we show how to quantify the influence on measurement noise of bead size and chamber geometry in active and passive assays.


Agglutination of Histoplasma capsulatum by IgG Monoclonal Antibodies against Hsp60 Impacts Macrophage Effector Functions

Allan Jefferson Guimarães, Susana Frases, Bruno Pontes, Mariana Duarte de Cerqueira, Marcio L. Rodrigues, Nathan Bessa Viana, Leonardo Nimrichter, and Joshua Daniel Nosanchuk
Histoplasma capsulatum can efficiently survive within macrophages, facilitating H. capsulatum translocation from the lung into the lymphatics and bloodstream. We have recently generated monoclonal antibodies (MAbs) to an H. capsulatum surface-expressed heat shock protein of 60 kDa (Hsp60) that modify disease in a murine histoplasmosis model. Interestingly, the MAbs induced different degrees of yeast cell agglutination in vitro. In the present study, we characterized the agglutination effects of the antibodies to Hsp60 on H. capsulatum yeast cells by light microscopy, flow cytometry, dynamic light scattering, measuring zeta potential, and using optical tweezers. We found that immunoglobulin Gs (IgGs) to Hsp60 cause H. capsulatum aggregation dependent on the (i) concentration of MAbs, (ii) MAb binding constant, and (iii) IgG subclass. Furthermore, infection of macrophages using agglutinates of various sizes after incubation with different Hsp60-binding MAbs induced association to macrophages through distinct cellular receptors and differentially affected macrophage antifungal functions. Hence, the capacity of IgG MAbs to agglutinateH. capsulatum significantly impacted pathogenic mechanisms of H. capsulatum during macrophage infection, and the effect was dependent on the antibody subclass and antigen epitope.


High-speed video-based tracking of optically trapped colloids

O Otto, J L Gornall, G Stober, F Czerwinski, R Seidel and U F Keyser

We have developed an optical tweezer setup, with high-speed and real-time position tracking, based on a CMOS camera technology. Our software encoded algorithm is cross-correlation based and implemented on a standard computer. By measuring the fluctuations of a confined colloid at 6000 frames s − 1, continuously for an hour, we show our technique is a viable alternative to quadrant photodiodes. The optical trap is calibrated by using power spectrum analysis and the Stokes method. The trap stiffness is independent of the camera frame rate and scales linearly with the applied laser power. The analysis of our data by Allan variance demonstrates single nanometer accuracy in position detection.


Saturday, March 12, 2011

Understanding Optical Trapping Phenomena: A Simulation for Undergraduates

Mas, J.; Farré, A.; Cuadros, J.; Juvells, I.; Carnicer, A.

Optical trapping is an attractive and multidisciplinary topic that has become the center of attention to a large number of researchers. Moreover, it is a suitable subject for advanced students that requires a knowledge of a wide range of topics. As a result, it has been incorporated into some syllabuses of both undergraduate and graduate programs. In this paper, basic concepts in laser trapping theory are reviewed. To provide a better understanding of the underlying concepts for students, a Java application for simulating the behavior of a dielectric particle trapped in a highly focused beam has been developed. The program illustrates a wide range of theoretical results and features, such as the calculation of the force exerted by a beam in the Mie and Rayleigh regimes or the calibration of the trap stiffness. Some examples that are ready to be used in the classroom or in the computer lab are also supplied.


Optical tweezers: wideband microrheology

Daryl Preece, Rebecca Warren, R M L Evans, Graham M Gibson, Miles J Padgett, Jonathan M Cooper and Manlio Tassieri

Microrheology is a branch of rheology having the same principles as conventional bulk rheology, but working on micron length scales and microlitre volumes.
Optical tweezers have been successfully used with Newtonian fluids for rheological purposes such as determining fluid viscosity. Conversely, when optical tweezers are used to measure the viscoelastic properties of complex fluids the results are either limited to the material's high-frequency response, discarding important information related to the low-frequency behaviour, or they are supplemented by low-frequency measurements performed with different techniques, often without presenting an overlapping region of clear agreement between the sets of results. We present a simple experimental procedure to perform microrheological measurements over the widest frequency range possible with optical tweezers. A generalized Langevin equation is used to relate the frequency-dependent moduli of the complex fluid to the time-dependent trajectory of a probe particle as it flips between two optical traps that alternately switch on and off.


Simultaneous transfer of linear and orbital angular momentum to multiple low-index particles

Vincent Ricardo Daria, Mary Ann Go and Hans-A Bachor

We demonstrate simultaneous transfer of linear and orbital angular momentum (OAM) to hollow glass microbeads using a dynamic array of optical vortices. Previous reports have shown that the transfer of OAM is due to light scattering which creates a tangential force on a particle and causes it to move on a circular orbit around a vortex. In this paper we describe a case with reduced frictional force, as the low-index particle is pinned to the wall of the sample cell. This results in a more efficient transfer of OAM, which sets a hollow microbead into orbital motion around the optical vortex. We show that the localized OAM carried by each vortex in the array can be independently transferred to one microbead trapped per vortex. Finally, we present novel demonstrations showing simultaneous transfer of both orbital angular and linear momentum to multiple microbeads.


Moving into the cell: single-molecule studies of molecular motors in complex environments

Claudia Veigel and Christoph F. Schmidt

Much has been learned in the past decades about molecular force generation. Single-molecule techniques, such as atomic force microscopy, single-molecule fluorescence microscopy and optical tweezers, have been key in resolving the mechanisms behind the power strokes, 'processive' steps and forces of cytoskeletal motors. However, it remains unclear how single force generators are integrated into composite mechanical machines in cells to generate complex functions such as mitosis, locomotion, intracellular transport or mechanical sensory transduction. Using dynamic single-molecule techniques to track, manipulate and probe cytoskeletal motor proteins will be crucial in providing new insights.

Thursday, March 10, 2011

Optimizing the optical trapping stiffness of holographically trapped miJourcrorods using high-speed video tracking

D B Phillips, D M Carberry, S H Simpson, H Schäfer, M Steinhart, R Bowman, G M Gibson, M J Padgett, S Hanna and M J Miles
Dielectric microrods can be trapped horizontally in pairs of holographically controlled optical traps. External forces acting on these microrods are registered via the rotational and translational displacement of the microrod relative to the traps. In the following paper we demonstrate accurate, real-time tracking of this displacement in two dimensions. The precision of the method is estimated and the translational and rotational stiffness coefficients of the trapped microrod are evaluated by analysing the thermal motion and the Stokes drag. The variation of these stiffness coefficients relative to the displacement of the traps from the ends of the microrods is measured, and optimal trapping conditions are located.


Tailored leaky plasmon waves from a subwavelength aperture for optical particle trapping on a chip

M. S. Muradoglu, Tuck Wah Ng, Adrian Neild, and Ian Gralinski

Optical forces available on a chip that possess features of strong trapping at the subwavelength scale, in a coplanar geometry, and at specific and selective locations portend many useful applications. We demonstrate here a two-pronged approach to accomplish this. First, the plasmon fields emanating from a subwavelength aperture are manipulated so that they leak maximally to the sides on a surface through the use of tailored corrugations. Second, the location of secondary corrugation at some distance permits reflection of these leaky waves, which, with the coherence property of light used, generate optical standing wave fields capable of strong optical trapping. The linear optical forces generated with this scheme are presented here.


Optical aberration compensation in a multiplexed optical trapping system

T Čižmár, H I C Dalgarno, P C Ashok, F J Gunn-Moore and K Dholakia

In this paper we discuss optical aberrations within a multiplexed optical trapping system. We analyze two of the most powerful methods for optical trap multiplexing: time-shared beam steering and holographic beam shaping in a tandem system with an acousto-optic deflector and spatial light modulator. We show how to isolate and correct for the aberrations introduced by these individual optical components using the spatial light modulator and demonstrate the enhancement this provides to optical trapping.


Alternative modes for optical trapping and manipulation using counter-propagating shaped beams

D Palima, T B Lindballe, M V Kristensen, S Tauro, H Stapelfeldt, S R Keiding and J Glückstad
Counter-propagating beams have enabled the first stable three-dimensional optical trapping of microparticles and this procedure has been enhanced and developed over the years to achieve independent and interactive manipulation of multiple particles. In this work, we analyse counter-propagating shaped-beam traps that depart from the conventional geometry based on symmetric, coaxial counter-propagating beams. We show that projecting shaped beams with separation distances previously considered axially unstable can, in fact, enhance the axial and transverse trapping stiffnesses. We also show that deviating from using perfectly counter-propagating beams to use oblique beams can improve the axial stability of the traps and improve the axial trapping stiffness. These alternative geometries can be particularly useful for handling larger particles. These results hint at a rich potential for light shaping for optical trapping and manipulation using patterned counter-propagating beams, which still remains to be fully tapped.


Near-field optical trapping with an actively locked cavity

N J van Leeuwen, L J Moore, W D Partridge, R Peverall, G A D Ritchie and M D Summers

This paper details the construction and performance of a resonant cavity evanescent wave trap for the trapping and assembling of microparticles. The technique employs a low finesse resonator which incorporates total internal reflection (TIR) at the silica/water interface to generate an evanescent field which is coherently scattered by silica microparticles present in suspension to form optically bound structures. Circulating powers of 14 W over an area of 150 µm × 75 µm were generated with a 400 mW Nd:YAG source. This approach allows some degree of control over the shape of the evanescent field by locking to higher-order cavity modes.


Tuesday, March 8, 2011

Mechanical and electrical properties of red blood cells using optical tweezers

A Fontes, M L Barjas Castro, M M Brandão, H P Fernandes, A A Thomaz, R R Huruta, L Y Pozzo, L C Barbosa, F F Costa, S T O Saad and C L Cesar
Optical tweezers are a very sensitive tool, based on photon momentum transfer, for individual, cell by cell, manipulation and measurements, which can be applied to obtain important properties of erythrocytes for clinical and research purposes. Mechanical and electrical properties of erythrocytes are critical parameters for stored cells in transfusion centers, immunohematological tests performed in transfusional routines and in blood diseases. In this work, we showed methods, based on optical tweezers, to study red blood cells and applied them to measure apparent overall elasticity, apparent membrane viscosity, zeta potential, thickness of the double layer of electrical charges and adhesion in red blood cells.

Control and Manipulation of Pathogens with an Optical Trap for Live Cell Imaging of Intercellular Interactions

Jenny M. Tam, Carlos E. Castro, Robert J. W. Heath, Michael L. Cardenas, Ramnik J. Xavier, Matthew J. Lang, Jatin M. Vyas

The application of live cell imaging allows direct visualization of the dynamic interactions between cells of the immune system. Some preliminary observations challenge long-held beliefs about immune responses to microorganisms; however, the lack of spatial and temporal control between the phagocytic cell and microbe has rendered focused observations into the initial interactions of host response to pathogens difficult. This paper outlines a method that advances live cell imaging by integrating a spinning disk confocal microscope with an optical trap, also known as an optical tweezer, in order to provide exquisite spatial and temporal control of pathogenic organisms and place them in proximity to host cells, as determined by the operator. Polymeric beads and live, pathogenic organisms (Candida albicans and Aspergillus fumigatus) were optically trapped using non-destructive forces and moved adjacent to living cells, which subsequently phagocytosed the trapped particle. High resolution, transmitted light and fluorescence-based movies established the ability to observe early events of phagocytosis in living cells. To demonstrate the broad applicability of this method to immunological studies, anti-CD3 polymeric beads were also trapped and manipulated to form synapses with T cells in vivo, and time-lapse imaging of synapse formation was also obtained. By providing a method to exert fine control of live pathogens with respect to immune cells, cellular interactions can be captured by fluorescence microscopy with minimal perturbation to cells and can yield powerful insight into early responses of innate and adaptive immunity.


Raman spectroscopy of individual monocytes reveals that single-beam optical trapping of mononuclear cells occurs by their nucleus

Samantha Fore, James Chan, Douglas Taylor and Thomas Huser

We show that laser tweezers Raman spectroscopy of eukaryotic cells with a significantly larger diameter than the tight focus of a single-beam laser trap leads to optical trapping of the cell by its optically densest part, i.e. typically the cell's nucleus. Raman spectra of individual optically trapped monocytes are compared with location-specific Raman spectra of monocytes adhered to a substrate. When the cell's nucleus is stained with a fluorescent live cell stain, the Raman spectrum of the DNA-specific stain is observed only in the nucleus of individual monocytes. Optically trapped monocytes display the same behavior. We also show that the Raman spectra of individual monocytes exhibit the characteristic Raman signature of cells that have not yet fully differentiated and that individual primary monocytes can be distinguished from transformed monocytes based on their Raman spectra. This work provides further evidence that laser tweezers Raman spectroscopy of individual cells provides meaningful biochemical information in an entirely non-destructive fashion that permits discerning differences between cell types and cellular activity.


Monday, March 7, 2011

Optically levitating dielectrics in the quantum regime: Theory and protocols

O. Romero-Isart, A. C. Pflanzer, M. L. Juan, R. Quidant, N. Kiesel, M. Aspelmeyer, and J. I. Cirac

We provide a general quantum theory to describe the coupling of light with the motion of a dielectric object inside a high-finesse optical cavity. In particular, we derive the total Hamiltonian of the system as well as a master equation describing the state of the center-of-mass mode of the dielectric and the cavity-field mode. In addition, a quantum theory of elasticity is used to study the coupling of the center-of-mass motion with internal vibrational excitations of the dielectric. This general theory is applied to the recent proposal of using an optically levitating nanodielectric as a cavity optomechanical system [see Romero-Isart et al., New J. Phys. 12, 033015 (2010); Chang et al., Proc. Natl. Acad. Sci. USA 107, 1005 (2010)]. On this basis, we also design a light-mechanics interface to prepare non-Gaussian states of the mechanical motion, such as quantum superpositions of Fock states. Finally, we introduce a direct mechanical tomography scheme to probe these genuine quantum states by time-of- flight experiments.


Optical tweezers and non-ratiometric fluorescent-dye-based studies of respiration in sperm mitochondria

Timothy Chen, Linda Z Shi, Qingyuan Zhu, Charlie Chandsawangbhuwana and Michael W Berns

The purpose of this study is to investigate how the mitochondrial membrane potential affects sperm motility using laser tweezers and a non-ratiometric fluorescent probe, DiOC6(3). A 1064 nm Nd:YVO4 continuous wave laser was used to trap motile sperm at a power of 450 mW in the trap spot. Using customized tracking software, the curvilinear velocity (VCL) and the escape force from the laser tweezers were measured. Human (Homo sapiens), dog (Canis lupis familiaris) and drill (Mandrillus leucophaeus) sperm were treated with DiOC6(3) to measure the membrane potential in the mitochondria-rich sperm midpieces. Sperm from all three species exhibited an increase in fluorescence when treated with the DiOC6(3). When a cyanide inhibitor (CCCP) of aerobic respiration was applied, sperm of all three species exhibited a reduction in fluorescence to pre-dye levels. With respect to VCL and escape force, the CCCP had no effect on dog or human sperm, suggesting a major reliance upon anaerobic respiration (glycolysis) for ATP in these two species. Based on the preliminary study on drill sperm, CCCP caused a drop in the VCL, suggesting potential reliance on both glycolysis and aerobic respiration for motility. The results demonstrate that optical trapping in combination with DiOC6(3) is an effective way to study sperm motility and energetics.


A Promiscuous DNA Packaging Machine from Bacteriophage T4

Zhihong Zhang, Vishal I. Kottadiel, Reza Vafabakhsh, Li Dai, Yann R. Chemla, Taekjip Ha, Venigalla B. Rao
Complex viruses are assembled from simple protein subunits by sequential and irreversible assembly. During genome packaging in bacteriophages, a powerful molecular motor assembles at the special portal vertex of an empty prohead to initiate packaging. The capsid expands after about 10%–25% of the genome is packaged. When the head is full, the motor cuts the concatemeric DNA and dissociates from the head. Conformational changes, particularly in the portal, are thought to drive these sequential transitions. We found that the phage T4 packaging machine is highly promiscuous, translocating DNA into finished phage heads as well as into proheads. Optical tweezers experiments show that single motors can force exogenous DNA into phage heads at the same rate as into proheads. Single molecule fluorescence measurements demonstrate that phage heads undergo repeated initiations, packaging multiple DNA molecules into the same head. These results suggest that the phage DNA packaging machine has unusual conformational plasticity, powering DNA into an apparently passive capsid receptacle, including the highly stable virus shell, until it is full. These features probably led to the evolution of viral genomes that fit capsid volume, a strikingly common phenomenon in double-stranded DNA viruses, and will potentially allow design of a novel class of nanocapsid delivery vehicles.


Selective optical trapping based on strong plasmonic coupling between gold nanorods and slab

Y. J. Zheng, H. Liu, S. M. Wang, T. Li, J. X. Cao, L. Li, C. Zhu, Y. Wang, S. N. Zhu, and X. Zhang

A resonance plasmon mode is formed between a gold nanorod and an infinite slab in infrared range, with local electric field enhancement factor over 40. A strong optical attractive force is exerted on the rod by the slab at resonance frequency. Based on Maxwell stress tensor method and numerical simulations, the optical force was calculated to be over 2.0 nN/(mW/μm2). For a fixed incident wavelength, the enhanced optical force is obtained only for the rods with particular length when the diameter is fixed. This strong optical force could be used as a possible selective optical trapping technique in the future.


Optical tweezers for studying taxis in parasites

A A de Thomaz, A Fontes, C V Stahl, L Y Pozzo, D C Ayres, D B Almeida, P M A Farias, B S Santos, J Santos-Mallet, S A O Gomes, S Giorgio, D Feder and C L Cesar
In this work we present a methodology to measure force strengths and directions of living parasites with an optical tweezers setup. These measurements were used to study the parasites chemotaxis in real time. We observed behavior and measured the force of: (i) Leishmania amazonensis in the presence of two glucose gradients; (ii) Trypanosoma cruzi in the vicinity of the digestive system walls, and (iii) Trypanosoma rangeli in the vicinity of salivary glands as a function of distance. Our results clearly show a chemotactic behavior in every case. This methodology can be used to study any type of taxis, such as chemotaxis, osmotaxis, thermotaxis, phototaxis, of any kind of living microorganisms. These studies can help us to understand the microorganism sensory systems and their response function to these gradients.


Microparticle movements in optical funnels and pods

José A. Rodrigo, Antonio M. Caravaca-Aguirre, Tatiana Alieva, Gabriel Cristóbal, and María L. Calvo

Three-dimensional microparticle movements induced by laser beams with a funnel- and tubular pod-like structure, in the neighbourhood of the focal plane of an optical trapping setup, are experimentally studied. The funnel and pod beams constructed as coherent superpositions of helical Laguerre-Gaussian modes are synthesized by a computer generated hologram using a phase-only spatial light modulator. Particle tracking is achieved by in-line holography method which allows an accurate position measurement. It is experimentally demonstrated that the trapped particle follows different trajectories depending on the orbital angular momentum density of the beam. In particular applying the proposed pod beam the particle rotates in opposite directions during its movement in the optical trap. Possible applications of these single-beam traps for volumetric optical particle manipulation are discussed.


Friday, March 4, 2011

Laser tweezers Raman spectrum of normal breast cell line and carcinoma breast cell line

Xie, Y.-A., Leng, Z.-H., Meng, L.-J. , Luo, X.-L. , Zhang, W.-M. , Kuang, Z.-P.

OBJECTIVE: To detect the interaction between Raman spectrum of normal breast cell line (HBL-100) and carcinoma breast cell line (MCF-7), as well as the change of biological tissue composition from normal cell to carcinogenesis. In the end, Characteristic Raman spectra of cancer cells were presented. METHODS: Raman spectrum of normal breast cell line and carcinoma breast cell line were recorded, The result of which was performed by principal component analysis(PCA). RESULTS: Significant differences of average Raman spectrums were founded between the normal cell and the carcinoma one; The spectra line of carcinoma cell became strong at the whole; The strength of 936, 1002, 1298 and 1445 cm-1 increased; the peak of 1102 cm-1 shifted to 1094 cm-1; the peak of 484 cm-1 disappeared; Additionally, the structure and the amount of protein, nucleic acid, lipids and other such molecules had also changed. Average Raman spectrum of monocell was analyzed with the principal component of PCA, the result of which suggested normal cell and carcinoma cell could be differentiated with PCA, The discrimination rate was 87% (13/15). CONCLUSIONS: Judging normal breast cell and carcinoma cell with laser tweezers Raman spectrums is an efficient method. Single cells laser tweezers raman spectra technique can become a new way to diagnose cancer, which has very wide prospect.


Optical Forces in Hybrid Plasmonic Waveguides

Xiaodong Yang, Yongmin Liu, Rupert F. Oulton, Xiaobo Yin, and Xiang Zhang

We demonstrate that in a hybrid plasmonic system the optical force exerted on a dielectric waveguide by a metallic substrate is enhanced by more than 1 order of magnitude compared to the force between a photonic waveguide and a dielectric substrate. A nanoscale gap between the dielectric waveguide and the metallic substrate leads to deep subwavelength optical energy confinement with ultralow mode propagation loss and hence results in the enhanced optical forces at low input optical power, as numerically demonstrated by both Maxwell’s stress tensor formalism and the coupled mode theory analysis. Moreover, the hybridization between the surface plasmon modes and waveguide modes allows efficient optical trapping of single dielectric nanoparticle with size of only several nanometers in the gap region, manifesting various optomechanical applications such as nanoscale optical tweezers.


Heat profiling of three-dimensionally optically trapped gold nanoparticles using vesicle cargo release

Anders Kyrsting, Poul M. Bendix, Dimitrios G. Stamou, and Lene B. Oddershede

Irradiated metallic nanoparticles hold great promise as heat transducers in photothermal applications such as drug delivery assays or photothermal therapy. We quantify the temperature increase of individual gold nanoparticles trapped in three dimensions near lipid vesicles exhibiting temperature sensitive permeability. The surface temperature can increase by hundreds of degrees Celsius even at moderate laser powers. Also, there are significant differences of the heat profiles in two-dimensional and three-dimensional trapping assays.


Mechanistic Basis of Otolith Formation during Teleost Inner Ear Development

David Wu, Jonathan B. Freund, Scott E. Fraser and Julien Vermot

Otoliths, which are connected to stereociliary bundles in the inner ear, serve as inertial sensors for balance. In teleostei, otolith development is critically dependent on flow forces generated by beating cilia; however, the mechanism by which flow controls otolith formation remains unclear. Here, we have developed a noninvasive flow probe using optical tweezers and a viscous flow model in order to demonstrate how the observed hydrodynamics influence otolith assembly. We show that rotational flow stirs and suppresses precursor agglomeration in the core of the cilia-driven vortex. The velocity field correlates with the shape of the otolith and we provide evidence that hydrodynamics is actively involved in controlling otolith morphogenesis. An implication of this hydrodynamic effect is that otolith self-assembly is mediated by the balance between Brownian motion and cilia-driven flow. More generally, this flow feature highlights an alternative biological strategy for controlling particle localization in solution.


Photothermal trapping of dielectric particles by optical fiber-ring

Hongbao Xin, Hongxiang Lei, Yao Zhang, Xingmin Li, and Baojun Li

The removal of dielectric particles and bacteria from water is an extremely important global issue, particularly, for drinking and sanitation. This work provides a demonstration of optical purification of water using an optical fiber-ring. The size of particles suspended in water for trapping is 2.08 μm in diameter and the wavelength of light used for inducing photothermal effect is 1.55 μm with a power of 97 mW. The fiber, 6 μm in diameter, was formed to a racket-shaped ring with a minimum diameter of 167 μm and a maximum one of 350 μm. Experiment indicates that the particles moved toward the ring with the highest velocity of 4.2 μm/s and are trapped/assembled in the center of the ring once the laser beam of 1.55-μm wavelength was launched into the fiber. With a moving of the fiber-ring, the trapped/assembled particles were moved and the water can be purified by removal of the particles.


Behavior of colloidal particles at a nematic liquid crystal interface

Mohamed Amine Gharbi, Maurizio Nobili, Martin In, Guillaume Prévot, Paolo Galatola, Jean-Baptiste Fournier and Christophe Blanc
We examine the behavior of spherical silica particles trapped at an air–nematic liquid crystal interface. When a strong normal anchoring is imposed, the beads spontaneously form various structures depending on their area density and the nematic thickness. Using optical tweezers, we determine the pair potential and explain the formation of these patterns. The energy profile is discussed in terms of capillary and elastic interactions. Finally, we detail the mechanisms that control the formation of a hexagonal lattice and analyze the role of gravity for curved interfaces.