Yoshiaki Kashimura

Fabrication of nano-gap electrodes using electroplating technique

Abstract We report a simple and controllable method for fabricating a pair of electrodes with a sub-10-nm gap, namely nano-gap electrodes, by using a conventional electroplating technique. A pair of initial electrodes with a 100–200 nm gap is fabricated by electron beam lithography, and then gold is deposited electrochemically to reduce this gap. A sub-10-nm gap is produced by halting the electroplating just before the electrodes make contact with each other. We also use our technique to fabricate nano-gap electrodes with a gold and a platinum finger, and with three gold fingers.

Self-Assembly of Gold Nanorods Induced by Intermolecular Interactions of Surface-Anchored Lipids

Surface-modified gold nanorods (Au NRs) with 1,2-dipalmitoyl- sn-glycero-3-phosphothioethanol (DPPTE) were synthesized, and their self-assembled structures on a silicon substrate were observed using a scanning electron microscope (SEM). The Au NR-DPPTE complex formed characteristic one- and two-dimensional self-assemblies induced by intermolecular interactions of surface-anchored lipids via simple drying process. The interparticle distance between neighboring NRs was uniform at around 5.0 nm, which was consistent with the thickness of the lipid bilayer. Furthermore, we observed the anisotropic configurations of the NR complex, preferentially oriented in a lateral or perpendicular fashion, in a two-dimensional assembled structure dependent on the interfacial hydrophilicity or hydrophobicity of the silicon surface.

Self-assembled rigid conjugated polymer nanojunction and its nonlinear current-voltage characteristics at room temperature

A gold/polymer/gold nanojunction was fabricated by the self-assembly of a rigid polymer, namely poly(p-phenyleneethynylene)s with thioacetyl groups, between gold nanogap electrodes. The self-assembly depends on: (i) the ideal rigidity of the polymer molecules and (ii) the strong affinity of the thioacetyl/thiol end groups of the polymer for the Au surface. The current–voltage (I–V) characteristics of the conjugated polymer nanojunction exhibited stepwise features (some steps appeared as peaks) at room temperature. The I–V can be explained as electron tunneling through the nanojunction.

Ca2+ ion transport through channels formed by α-hemolysin analyzed using a microwell array on a Si substrate

For the functional analysis of ion channel activity, an artificial lipid bilayer suspended over microwells was formed that ruptured giant unilamellar vesicles on a Si substrate. Ca(2+) ion indicators (fluo-4) were confined in the microwells by sealing the microwells with a lipid bilayer. An overhang formed at the microwells prevented the lipid membrane from falling into them and allowed the stable confinement of the fluorescent probes. The transport of Ca(2+) ions through the channels formed by α-hemolysin inserted in a lipid membrane was analyzed by employing the fluorescence intensity change of fluo-4 in the microwells. The microwell volume was very small (1-100 fl), so a highly sensitive monitor could be realized. The detection limit is several tens of ions/s/μm(2), and this is much smaller than the ion current in a standard electrophysiological measurement. Smaller microwells will make it possible to mimic a local ion concentration change in the cells, although the signal to noise ratio must be further improved for the functional analysis of a single channel. We demonstrated that a microwell array with confined fluorescent probes sealed by a lipid bilayer could constitute a basic component of a highly sensitive biosensor array that works with functional membrane proteins. This array will allow us to realize high throughput and parallel testing devices.

Microchannel device using self-spreading lipid bilayer as molecule carrier

We propose a microchannel device that employs a surface-supported self-spreading lipid bilayer membrane as a molecule carrying medium. The device has a micropattern structure fabricated on a SiO2 surface by photolithography, into which a self-spreading lipid bilayer membrane is introduced as the carrier medium. This system corresponds to a microchannel with a single lipid bilayer membrane height of approximately 5 nm, compared with conventional micro-fluidic channels that have a section height and width of at least several microm. The device is beneficial for detecting intermolecular interactions when molecules carried by the self-spreading lipid bilayer collide with each other in the microchannel. The validity of the device was confirmed by observing the fluorescence resonance energy transfer (FRET) between two dye molecules, coumarin and fluorescein.

Anisotropic assembly of gold nanorods assisted by selective ion recognition of surface-anchored crown ether derivatives

Gold nanorods (NRs) mixed with crown ether derivatives exhibited the efficient and selective recognition of Na+ and K+ ions, which were detected by localized surface plasmon absorption in response to dispersed and aggregated gold NRs. Furthermore, in the aggregates preferential end-to-end or side-to-side assembly of NRs was observed which was dependent on the additive concentration.

Self-Spreading Behavior of Supported Lipid Bilayer through Single Sub-100-nm Gap

We report on the self-spreading behavior of a supported lipid bilayer passing through a sub-100-nm gap (nanogap). For this purpose, a device with a nanogap in a microchannel on a silicon substrate was designed and fabricated by electron beam lithography and photolithography. Fluorescence images of the lipid bilayer labeled with dye-conjugated lipids were observed by using a confocal laser scanning microscope. The time evolution of the self-spreading lipid bilayer passing through the nanogap was investigated at first. In the device, the lipid bilayer successfully passed through the nanogap without any stagnation. An analysis of the velocity of an advancing lipid bilayer showed no significant effect before or after passage though the nanogap. The effects of dye-conjugated lipid molecules and the size of the nanogap on the self-spreading behavior were examined next. We observed an abrupt decrease in the fluorescence intensity in the vicinity of the nanogap with Texas Red-DHPE and fluorescein-DHPE. It was revealed that the decrease depends on nanogap size as well as bulkiness of the dye molecule. The results suggest that bulkier dye molecules experience interference when they pass through narrower nanogaps.

Advancing conjugated polymers into nanometer-scale devices

In this article, we review the possibility of combining conjugated polymers with nanometer-scale devices (nanodevices), in order to introduce the properties associated with conjugated polymers into such nanodevices. This approach envisages combining the highly topical disciplines of polymer electronics and nanoelectronics to engender a new subdirection of polymer nanoelectronics, which can serve as a tool to probe the behavior of polymer molecules at the nanometer/molecular level, and contribute to clarifying transport mechanisms in conjugated polymers. In this study, we exemplify this combination, using a family of linear and conjugated polymers, poly(p-phenylene-ethynylene)s (PPEs) with thiolacetate-functionalized end groups.

Electrostatic control of lipid bilayer self-spreading using a nanogap gate on a solid support.

We have demonstrated for the first time that the self-spreading of supported lipid bilayers can be controlled by the temporal switching of an electric field applied between nanogap electrodes. To account for this phenomenon, we propose an electrostatic trapping model in which an electric double layer plays an important role. The validity of this mechanism was verified by the dependence of self-spreading on the nanogap width and the ionic concentration of the electrolyte. Our results provide a promising tool for the temporal and spatial control of lipid bilayer formation for nanobio devices.

Pattern formation and molecular transport of histidine-tagged GFPs using supported lipid bilayers.

We fabricated a heterogeneous supported lipid bilayer (SLB) by employing binary lipid mixtures comprising a saturated acyl chain DSPC and an unsaturated acyl chain nickel-chelating lipid. By using the specific adsorption properties of histidine-tagged proteins (His-tagged GFPs) in relation to nickel-chelating lipids, we demonstrated protein pattern formation on the SLB corresponding to the phase separation pattern of the SLB. In addition, by using a lipid mixture consisting of an unsaturated acyl chain DOPC and a nickel-chelating lipid, and His-tagged GFPs, we succeeded in transporting the proteins along a hydrophilic micropattern on a SiO(2) substrate. The protein transport is induced by the self-spreading behavior of a fluid SLB with a kinetic spreading coefficient beta = 10.4 microm(2) s(-1). This method provides a guide for strategically carrying various biomolecules to specific positions by using a soft biointerface on a solid surface. In addition, the results demonstrate the importance of using techniques that allow the controlled manipulation of biomolecules based on the static or dynamic properties of the SLB platform.