Sunday, April 13, 2014

Plasmonic Optical Trapping in Biologically Relevant Media

Brian J. Roxworthy, Michael T. Johnston, Felipe T. Lee-Montiel, Randy H. Ewoldt, Princess I. Imoukhuede, Kimani C. Toussaint Jr

We present plasmonic optical trapping of micron-sized particles in biologically relevant buffer media with varying ionic strength. The media consist of 3 cell-growth solutions and 2 buffers and are specifically chosen due to their widespread use and applicability to breast-cancer and angiogenesis studies. High-precision rheological measurements on the buffer media reveal that, in all cases excluding the 8.0 pH Stain medium, the fluids exhibit Newtonian behavior, thereby enabling straightforward measurements of optical trap stiffness from power-spectral particle displacement data. Using stiffness as a trapping performance metric, we find that for all media under consideration the plasmonic nanotweezers generate optical forces 3–4x a conventional optical trap. Further, plasmonic trap stiffness values are comparable to those of an identical water-only system, indicating that the performance of a plasmonic nanotweezer is not degraded by the biological media. These results pave the way for future biological applications utilizing plasmonic optical traps.


Magnetic dipole super-resonances and their impact on mechanical forces at optical frequencies

Iñigo Liberal, Iñigo Ederra, Ramón Gonzalo, and Richard W. Ziolkowski

Artificial magnetism enables various transformative optical phenomena, including negative refraction, Fano resonances, and unconventional nanoantennas, beamshapers, polarization transformers and perfect absorbers, and enriches the collection of electromagnetic field control mechanisms at optical frequencies. We demonstrate that it is possible to excite a magnetic dipole super-resonance at optical frequencies by coating a silicon nanoparticle with a shell impregnated with active material. The resulting response is several orders of magnitude stronger than that generated by bare silicon nanoparticles and is comparable to electric dipole super-resonances excited in spaser-based nanolasers. Furthermore, this configuration enables an exceptional control over the optical forces exerted on the nanoparticle. It expedites huge pushing or pulling actions, as well as a total suppression of the force in both far-field and near-field scenarios. These effects empower advanced paradigms in electromagnetic manipulation and microscopy.


Direct evidence for three-dimensional off-axis trapping with single Laguerre-Gaussian beam

T. Otsu, T. Ando, Y. Takiguchi, Y. Ohtake, H. Toyoda, and H. Itoh

Optical tweezers are often applied to control the dynamics of objects by scanning light. However, there is a limitation that objects fail to track the scan when the drag exceeds the trapping force. In contrast, Laguerre-Gaussian (LG) beams can directly control the torque on objects and provide a typical model for nonequilibrium systems such as Brownian motion under external fields. Although stable “mid-water” trapping is essential for removing extrinsic hydrodynamic effects in such studies, three-dimensional trapping by LG beams has not yet been clearly established. Here we report the three-dimensional off-axis trapping of dielectric spheres using high-quality LG beams generated by a special holographic method. The trapping position was estimated as ~ half the wavelength behind the beam waist. These results establish the scientific groundwork of LG trapping and the technical basis of calibrating optical torque to provide powerful tools for studying energy-conversion mechanisms and the nonequilibrium nature of biological molecules under torque.


Optical tweezers: Dressed for success

Patrick C. Chaumet & Adel Rahmani

Controlled optical manipulation of a single dielectric nanoparticle is achieved with a bowtie nanoantenna placed at the end of the probe of a near-field scanning microscope.


Force-dependent isomerization kinetics of a highly conserved proline switch modulates the mechanosensing region of filamin

Lorenz Rognoni, Tobias Möst, Gabriel Žoldák, and Matthias Rief

Biological processes in the cell are highly dynamic and complex, and their correct interplay is ensured by a multitude of regulatory mechanisms. Among these, proline isomerization acts as a molecular switch that toggles two protein conformations, and thus functions, over time. In mechanosensing, mechanical stress is transduced into chemical signals. The molecular mechanisms underlying this regulation are crucial for understanding cell behavior and development. However, only a few experimental techniques are capable of studying force at the molecular level. In this paper, we use single-molecule mechanical experiments to investigate how the force-sensing region of the cytoskeletal cross-linker filamin is modulated by a proline switch.


Wednesday, April 9, 2014

Label-Free Biosensing over a Wide Concentration Range with Photonic Force Microscopy

Seungjin Heo, Prof. Kipom Kim and Prof. Yong-Hoon Cho

We present a label-free biosensor that measures molecular interactions between biomolecules on the surface of a probe bead and substrate over a wide concentration range. This system is capable of detecting target biomolecules with concentrations varying from 10 nm to 0.1 pm, with high selectivity and sensitivity.


Single-cell control of initial spatial structure in biofilm development using laser trapping

Jaime B. Hutchison , Christopher A Rodesney , Karishma Kaushik , Henry Le , Daniel Hurwitz , Yasuhiko Irie , and Vernita Gordon

Biofilms are sessile communities of microbes that are spatially structured by embedding matrix. Biofilm infections are notoriously intractable. This arises, in part, from changes in bacterial phenotype that result from spatial structure. Understanding these interactions requires methods to control the spatial structure of biofilms. We present a method for growing biofilms from initiating cells whose positions are controlled with single-cell precision using laser trapping. The native growth, motility, and surface adhesion of positioned microbes are preserved, as we show for model organisms Pseudomonas aeruginosa and Staphylococcus aureus. We demonstrate that laser-trapping and placing bacteria on surfaces can reveal the effects of spatial structure on bacterial growth in early biofilm development.


Theory for optical assembling of anisotropic nanoparticles by tailored light fields under thermal fluctuations

Mamoru Tamura, Syoji Ito, Shiho Tokonami, Takuya Iida
In order to evaluate the assembling processes of arbitrary-shaped nanoparticles (NPs) by the irradiation of a tailored laser beam under thermal fluctuations, we have developed a “Light-induced-force Nano Metropolis Method (LNMM)” as a new theoretical method based on the stochastic algorithm in the energy region and the general formula of light-induced force. By using LNMM, we have investigated the change of configurations of silver NPs with anisotropic shapes under the irradiation of laser beams with various polarizations and intensity distributions (Gaussian beam and axially-symmetric vector beams) in an aqueous solution at room temperature. As a result, it has been clarified that silver NPs can be selectively arranged into a characteristic spatial configuration reflecting the properties of an irradiated laser beam (wavelength, intensity distribution, and polarization distribution), and that the assembled structures possess broadband spectra and exhibit a strong optical response to the irradiated laser beam through the optimization with the help of fluctuations.