Hideki Nabika

Hyper-Raman scattering enhanced by anisotropic dimer plasmons on artificial nanostructures.

Silver nanodimers with a small gap of a few nanometers aligned on glass substrates were used to enhance hyper-Raman scattering of crystal violet dye molecules. When localized surface plasmon of the dimer array was resonantly excited along the interparticle axis, hyper-Raman intensity was significantly enhanced. Moreover, the spectral appearance was slightly different between the two excitation polarizations, suggesting a possibility of two resonance contributions at one-photon and two-photon energies. Since the plasmonic property of dimer arrays can be controlled by the dimer geometry, the dimer arrays are expected to be well-defined substrates for surface-enhanced hyper-Raman spectroscopy.

Segregation of molecules in lipid bilayer spreading through metal nanogates.

A new methodology for nanoscopic molecular filtering was developed using a substrate with a periodic array of metallic nanogates with various widths between 75 and 500 nm. A self-spreading lipid bilayer was employed as the molecular transport and filtering medium. Dye-labeled molecules doped in the self-spreading lipid bilayer were filtered after the spreading less than a few tens of micrometers on the nanogate array. Quantitative analysis of the spreading dynamics suggests that the filtering effect originates from the formation of the chemical potential barrier at the nanogate region, which is believed to be due to structural change such as compression imposed on the spreading lipid bilayer at the gate. A highly localized chemical potential barrier affects the ability of the doped dye-labeled molecules to penetrate the gate. The use of the self-spreading lipid bilayer allows molecular transportation without the use of any external field such as an electric field as is used in electrophoresis. The present system could be applied micro- and nanoscopic device technologies as it provides a completely nonbiased filtering methodology.

Enhanced Brownian Ratchet Molecular Separation Using a Self-Spreading Lipid Bilayer

A new approach is proposed for two-dimensional molecular separation based on the Brownian ratchet mechanism by use of a self-spreading lipid bilayer as both a molecular transport and separation medium. In addition to conventional diffusivity-dependence on the ratchet separation efficiency, the difference in the intermolecular interactions between the target molecules and the lipid bilayer is also incorporated as a new separation factor in the present self-spreading ratchet system. Spreading at the gap between two ratchet obstacles causes a local change in the lipid density at the gap. This effect produces an additional opportunity for a molecule to be deflected at the ratchet obstacle and thus causes an additional angle shift. This enables the separation of molecules with the same diffusivity but with different intermolecular interaction between the target molecule and surrounding lipid molecules. Here we demonstrate this aspect by using cholera toxin subunit B (CTB)-ganglioside GM1 (GM1) complexes with different configurations. The present results will unlock a new strategy for two-dimensional molecular manipulation with ultrasmall devices.

Liesegang patterns engineered by a chemical reaction assisted by complex formation.

Liesegang rings based on a chemical reaction, not a conventional precipitation reaction, have been developed by appropriate design of the nucleation dynamics in a system involving complex formation in a matrix. The periodic and concentric rings consisted of well-dispersed Ag nanoparticles with diameters of a few nanometers. The approach modeled here could be applied to form novel micropatterns out of inorganic salts, metal nanoparticles, organic nanocrystals, or polymeric fibers, and it could also offer a scaffold for novel models of a wide variety of reaction-diffusion phenomena in nature.

Molecular separation in the lipid bilayer medium: electrophoretic and self-spreading approaches.

Molecular separation in a microchannel is a key technology for future miniature devices. Because of growing advances in microfabrication techniques, we now have various choices of microscopic molecular separation systems. Recent progress in this field is reviewed in this manuscript. In particular, the use of the lipid bilayer as a separation medium is highlighted, because of its possible application to the manipulation of relatively small biomaterials such as membrane proteins and lipids. In this context, an approach based on electrophoresis is reviewed for molecular separation in the bilayer. Although the methods based on electrophoresis are effective, we will also focus on their limitation, i.e., only charged molecules can be manipulated. To solve this dilemma, we review new techniques based on the self-spreading nature of the lipid bilayer. In this method, there is no need to input an external field, such as an electric field, to achieve molecular separation. Phenomenological insights into the self-spreading nature and newly proposed molecular separation effects are shown in detail. This novel concept enables us to establish a completely unbiased molecular separation system in future microscopic and nanoscopic devices.

Activity of Keggin and Dawson polyoxometalates toward model cell membrane

We present direct information showing that polyoxometalates (POMs) show destructive activity toward the model cell membrane, via the rapid adsorption of POMs on the vesicle surface. The crucial step in causing the vesicle destruction is the structural change into stable POM–lipid conjugates, which are known as surfactant-encapsulated clusters.

Force applied to a single molecule at a single nanogate molecule filter

We have investigated the origin of molecule filtering system based on a chemical potential barrier produced by thermodynamically driven molecular flow in a nanoscopic space at nanogates. Single molecule tracking experiments prove that the highly localized potential barrier allows for selective manipulation of the target molecule. We propose the presence of a force, a few fN per molecule, to decelerate the molecules movement at the nanogate, which is comparable to or larger than the force applied by conventional electrophoretic operation. The present force can be tuned by changing the nanogate width at the nanometre level. These findings allow us to propose an accurate design of novel devices for molecular manipulation on an ultra small scale using a very small number of molecules without any external biases.

Control of near-infrared optical response of metal nano-structured film on glass substrate for intense Raman scattering

Near-infrared SERS activity of the Ag film under electric polarization was evaluated in aqueous solution containing 1 mM glutamic acid. Spectra were obtained in situ from the near infrared laser Raman microscope system with an excitation wavelength of 785 nm. Intensity of the SERS increased significantly upon application of an external electric field to the film. Empirical signal enhancement factor, which was determined from the peak integration ratio of the SERS vibration to the unenhanced signal from the solution of a defined sample concentration, was estimated to be in the range between 10(5) and 10(9). The evolution of the scattering signal was not observed in the absence of an applied external field. Under the present conditions, the SERS intensity was fully controlled by the applied field and the time. Relatively strong enhancement observed at the present system could be attributable to closed-packed particulate structure characterized by the diameters of approximately 20-90 nm on the Ag film. Raman images prove that the scattering signals are highly localized at the specific sites on the films showing possible achievement of relatively larger enhancement more than 10(12). Importance of the control of the size and inter-particle distance for intense Raman scattering was proved by the preparation of the well-ordered chained Ag dot array showing stronger SERS signals than those at the Ag films.

Effective Brownian Ratchet Separation by a Combination of Molecular Filtering and a Self-Spreading Lipid Bilayer System

A new molecular manipulation method in the self-spreading lipid bilayer membrane by combining Brownian ratchet and molecular filtering effects is reported. The newly designed ratchet obstacle was developed to effectively separate dye-lipid molecules. The self-spreading lipid bilayer acted as both a molecular transport system and a manipulation medium. By controlling the size and shape of ratchet obstacles, we achieved a significant increase in the separation angle for dye-lipid molecules compared to that with the previous ratchet obstacle. A clear difference was observed between the experimental results and the simple random walk simulation that takes into consideration only the geometrical effect of the ratchet obstacles. This difference was explained by considering an obstacle-dependent local decrease in molecular diffusivity near the obstacles, known as the molecular filtering effect at nanospace. Our experimental findings open up a novel controlling factor in the Brownian ratchet manipulation that allow the efficient separation of molecules in the lipid bilayer based on the combination of Brownian ratchet and molecular filtering effects.

Tracking Single Molecular Diffusion on Glass Substrate Modified with Periodic Ag Nano-architecture

Diffusion of individual dye-labeled molecules in self-spreading lipid bilayer on glass substrate in aqueous solution was observed directly using total internal reflection fluorescence microscopy (TIRFM). The molecular diffusion was evaluated quantitatively by the mean square displacement (MSD) analysis for the trajectories of individual molecules. The mode of normal Brownian diffusion in lipid bilayer was changed in the presence of the periodic triangular Ag nanoparticles on the surface of the substrate. Suppressed diffusivity observed at the system with the triangular Ag nanoparticles was attributed to the confinement of the molecular motion in the micro-compartment surrounded by the Ag triangles.