Optically-assisted thermophoretic reversible assembly of colloidal particles and E. coli using graphene oxide microstructures.

Optically-assisted large-scale assembly of nanoparticles have been of recent interest owing to their potential in applications to assemble and manipulate colloidal particles and biological entities. In the recent years, plasmonic heating has been the most popular mechanism to achieve temperature hotspots needed for extended assembly and aggregation. In this work, we present an alternative route to achieving strong thermal gradients that can lead to non-equilibrium transport and assembly of matter. We utilize the excellent photothermal properties of graphene oxide to form a large-scale assembly of silica beads. The formation of the assembly using this scheme is rapid and reversible. Our experiments show that it is possible to aggregate silica beads (average size 385 nm) by illuminating thin graphene oxide microplatelet by a 785 nm laser at low intensities of the order of 50-100 µW/µm2. We further extend the study to trapping and photoablation of E. coli bacteria using graphene oxide. We attribute this aggregation process to optically driven thermophoretic forces. This scheme of large-scale assembly is promising for the study of assembly of matter under non-equilibrium processes, rapid concentration tool for spectroscopic studies such as surface-enhanced Raman scattering and for biological applications.

Quantifying the Role of the Surfactant and the Thermophoretic Force in Plasmonic Nano-optical Trapping.

Plasmonic nanotweezers use intense electric field gradients to generate optical forces able to trap nano-objects in liquids. However, part of the incident light is absorbed into the metal, and a supplementary thermophoretic force acting on the nano-object arises from the resulting temperature gradient. Plasmonic nanotweezers thus face the challenge of disentangling the intricate contributions of the optical and thermophoretic forces. Here, we show that commonly added surfactants can unexpectedly impact the trap performance by acting on the thermophilic or thermophobic response of the nano-object. Using different surfactants in double nanohole plasmonic trapping experiments, we measure and compare the contributions of the thermophoretic and the optical forces, evidencing a trap stiffness 20× higher using sodium dodecyl sulfate (SDS) as compared to Triton X-100. This work uncovers an important mechanism in plasmonic nanotweezers and provides guidelines to control and optimize the trap performance for different plasmonic designs.

Adhesion layer influence on controlling the local temperature in plasmonic gold nanoholes.

Gold films do not adhere well on glass substrates, so plasmonics experiments typically use a thin adhesion layer of titanium or chromium to ensure a proper adhesion between the gold film and the glass substrate. While the absorption of light into gold structures is largely used to generate heat and control the temperature at the nanoscale, the influence of the adhesion layer on this process is largely overlooked. Here, we quantify the role of the adhesion layer in determining the local temperature increase around a single nanohole illuminated by a focused infrared laser. Despite their nanometer thickness, adhesion layers can absorb a greater fraction of the incoming infrared light than the 100 nm thick gold layer leading to a significant increase of the local temperature. Different experimental designs are explored, offering new ways to promote or avoid the temperature increase inside nanoapertures. This knowledge further expands the plasmonic toolbox for temperature-controlled experiments including single molecule sensing, nanopore translocation, polymerization, or nano-optical trapping.

Reconstitution reveals how myosin-VI self-organises to generate a dynamic mechanism of membrane sculpting.

One enigma in biology is the generation, sensing and maintenance of membrane curvature. Curvature-mediating proteins have been shown to induce specific membrane shapes by direct insertion and nanoscopic scaffolding, while the cytoskeletal motors exert forces indirectly through microtubule and actin networks. It remains unclear, whether the manifold direct motorprotein-lipid interactions themselves constitute another fundamental route to remodel the membrane shape. Here we show, combining super-resolution-fluorescence microscopy and membrane-reshaping nanoparticles, that curvature-dependent lipid interactions of myosin-VI on its own, remarkably remodel the membrane geometry into dynamic spatial patterns on the nano- to micrometer scale. We propose a quantitative theoretical model that explains this dynamic membrane sculpting mechanism. The emerging route of motorprotein-lipid interactions reshaping membrane morphology by a mechanism of feedback and instability opens up hitherto unexplored avenues of membrane remodelling and links cytoskeletal motors to early events in the sequence of membrane sculpting in eukaryotic cell biology.

Self-organization of actin networks by a monomeric myosin.

The organization of actomyosin networks lies at the center of many types of cellular motility, including cell polarization and collective cell migration during development and morphogenesis. Myosin-IXa is critically involved in these processes. Using total internal reflection fluorescence microscopy, we resolved actin bundles assembled by myosin-IXa. Electron microscopic data revealed that the bundles consisted of highly ordered lattices with parallel actin polarity. The myosin-IXa motor domains aligned across the network, forming cross-links at a repeat distance of precisely 36 nm, matching the helical repeat of actin. Single-particle image processing resolved three distinct conformations of myosin-IXa in the absence of nucleotide. Using cross-correlation of a modeled actomyosin crystal structure, we identified sites of additional mass, which can only be accounted for by the large insert in loop 2 exclusively found in the motor domain of class IX myosins. We show that the large insert in loop 2 binds calmodulin and creates two coordinated actin-binding sites that constrain the actomyosin interactions generating the actin lattices. The actin lattices introduce orientated tracks at specific sites in the cell, which might install platforms allowing Rho-GTPase-activating protein (RhoGAP) activity to be focused at a definite locus. In addition, the lattices might introduce a myosin-related, force-sensing mechanism into the cytoskeleton in cell polarization and collective cell migration.

An electrically excited nanoscale light source with active angular control of the emitted light.

We report on the angular distribution, polarization, and spectrum of the light emitted from an electrically controlled nanoscale light source. This nanosource of light arises from the local, low-energy, electrical excitation of localized surface plasmons (LSP) on individual gold nanoparticles using a scanning tunneling microscope (STM). The gold nanoparticles (NP) are chemically synthesized truncated bitetrahedrons. The emitted light is collected through the transparent substrate and the emission characteristics (angular distribution, polarization, and spectrum) are analyzed. These three observables are found to strongly depend on the lateral position of the STM tip with respect to the triangular upper face of the gold NP. In particular, the resulting light emission changes orientation when the electrical excitation via the STM tip is moved from the base to the vertex of the triangular face. On the basis of the comparison of the experimental observations with an analytical dipole model and finite-difference time-domain (FDTD) calculations, we show that this behavior is linked to the selective excitation of the out-of-plane and in-plane dipolar LSP modes of the NP. This selective excitation is achieved through the lateral position of the tip with respect to the symmetry center of the NP.

Edge scattering of surface plasmons excited by scanning tunneling microscopy.

The scattering of electrically excited surface plasmon polaritons (SPPs) into photons at the edges of gold metal stripes is investigated. The SPPs are locally generated by the inelastic tunneling current of a scanning tunneling microscope (STM). The majority of the collected light arising from the scattering of SPPs at the stripe edges is emitted in the forward direction and is collected at large angle (close to the air-glass critical angle, θ(c)). A much weaker isotropic component of the scattered light gives rise to an interference pattern in the Fourier plane images, demonstrating that plasmons may be scattered coherently. An analysis of the interference pattern as a function of excitation position on the stripe is used to determine a value of 1.42 ± 0.18 for the relative plasmon wave vector (kSPP/k0) of the corresponding SPPs. From these results, we interpret the directional, large angle (θ~θ(c)) scattering to be mainly from plasmons on the air-gold interface, and the diffuse scattering forming interference fringes to be dominantly from plasmons on the gold-substrate interface.