Self-Folded Three-Dimensional Graphene with a Tunable Shape and Conductivity.

Three-dimensional (3D) graphene architectures are of great interest as applications in flexible electronics and biointerfaces. In this study, we demonstrate the facile formation of predetermined 3D polymeric microstructures simply by transferring monolayer graphene. The graphene adheres to the surface of polymeric films via noncovalent π-π stacking bonding and induces a sloped internal strain, leading to the self-rolling of 3D microscale architectures. Micropatterns and varied thicknesses of the 2D films prior to the self-rolling allows for control over the resulting 3D geometries. The strain then present on the hexagonal unit cell of the graphene produces a nonlinear electrical conductivity across the device. The driving force behind the self-folding process arises from the reconfiguration of the molecules within the crystalline materials. We believe that this effective and versatile way of realizing a 3D graphene structure is potentially applicable to alternative 2D layered materials as well as other flexible polymeric templates.

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.

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.

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.

Novel "lipid-flow chip" configuration to determine donor-to-acceptor ratio-dependent fluorescence resonance energy transfer efficiency.

We report on the determination of fluorescence resonance energy transfer (FRET) efficiency, which is dependent on the donor-to-acceptor (D-A) ratio, by using a new type of microchannel device called a "lipid-flow chip". The chip comprises two supported lipid bilayers (SLBs) that self-spread from either side of 10 microm wide straight lines and carry molecules embedded in them. We first show that the diffusion process that occurs when the two SLBs collide with each other in the channel and form a unified SLB can be expressed by a one-dimensional diffusion equation. Next we describe a method for determining the FRET efficiency between NBD (donor) and Texas Red (acceptor) from observations using the lipid-flow chip by employing a one-dimensional diffusion model. The advantages of our method are that all the D-A ratios are achieved in one chip, and a large number of data are recorded in one chip. The FRET efficiency varies depending on the D-A ratio under conditions whereby the concentration of the sum of the donors and acceptors is constant. The Förster radius is also estimated from our results using a known model describing two-dimensional FRET systems, which yields a radius consistent with the previously reported value for NBD and Texas Red.

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.

Supported lipid bilayer self-spreading on a nanostructured silicon surface.

We report on the self-spreading behavior of a supported lipid bilayer (SLB) on a silicon surface with various 100 nm nanostructures. SLBs have been successfully grown from a small spot of a lipid molecule source both on a flat surface and uneven surfaces with 100 nm up-and-down nanostructures. After an hour, the self-spreading SLB forms a large circle or an ellipse depending on the nanostructure pattern. The results are explained by a model that shows that a single-layer SLB grows along the nanostructured surfaces. The model is further supported by a quantitative analyses of our data. We also discuss the stability of the SLB on nanostructured surfaces in terms of the balance between its bending and adhesion energies.

A new morphology of copper 7,7,8,8-tetracyano-p-quinodimethane.

Morphology control is a long-standing problem that needs to be solved for making the switching mechanism of copper 7,7,8,8-tetracyano-p-quinodimethane (CuTCNQ) understood all the time, but up till now how many morphologies CuTCNQ possesses and which morphology should be responsible for the on/off switching phenomenon are still unclear. A new morphology of CuTCNQ, namely the tubular structure, has been obtained and characterized in our experiment, whose formation mechanism has also been investigated. Through characterizing, we can conclude that the tubular structure belongs to the phase I, which can be further confirmed by the electrical measurements. From the I-V plots, the carrier mobility of the tubular structure is estimated to be approximately 0.1 cm2 V-1 s-1, which suggests the potential application of CuTCNQ in 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.

Electron transport in self-assembled polymer molecular junctions.

A molecular junction of a poly(p-phenyleneethynylene)s derivative with thioacetate end groups (TA-PPE) was fabricated by self-assembling. Nanogap electrodes made by electroplating technique was used to couple thiol end groups of TA-PPE molecules. Room temperature current-voltage characteristics of the molecular junction exhibited highly periodic, repeatable, and identical stepwise features. First-principles calculations suggest that one possibility for the equidistant step is due to the opening of different conducting channels that corresponds to the unoccupied molecular orbitals of the polymer in the junction. It is interesting to see that an 18 nm long polymer is of quantized electronic structures and behaves like a quantum transport device.

A self-assembled nano optical switch and transistor based on a rigid conjugated polymer, thioacetyl-end-functionalized poly(para-phenylene ethynylene).

A nanometer-scale optical switch and transistor were fabricated with thioacetyl-end-functionalized poly(para-phenylene ethynylene)s and Au nanogap electrodes by self-assembly. With photoirradiation, the switch can be switched on/off quickly with a switching ratio as high as 1000. Moreover, the device works well as a p-type transistor. With an increase in gate bias, strong conductance oscillation was observed in this self-assembled transistor (under low temperature 147 K), which is very likely due to single-electron charging oscillations arising from electron tunneling through the nanometer-scale transistor.

Selective chemisorption of end-functionalized conjugated polymer on macro- and nanoscale surfaces.

Linear and conjugated poly(p-phenylene ethynylene)s (PPEs) with three different types of functionalized end groups (thiolacetate, isocyanide, and carboxylic acid groups) were synthesized, and their selective chemisorption behavior on various substrate surfaces were investigated using UV/vis transmission absorption spectroscopy. The UV/vis spectra of the PPEs were clearly dependent on the chemical affinity between the PPE end group and the solid surfaces. Furthermore, regarding the chemisorption of thiolacetate modified polymer on a nanoscopic gold particle surface, we visualized novel polymer-colloid nanoarchitectures such as a barbell-type nanohybrid and interconnected polymer nanowire structures that are successively linked through gold nanoparticles.