The bottom-up fabrication of molecular devices with vertical or lateral structure is one of the most challenging research subjects. For the fabrication of vertical nanostructure, the layer-by-layer growth of redox-active complexes based on surface coordination chemistry is applied. In order to control the molecular orientation on the surface, a novel ligand with tetrapod phosphonate groups (XP) has been introduced. The combination of self-assembly and successive metal coordination leads to provide novel well-defined multilayer structures, which act as molecular rectifying devices or switches on surfaces. Layer-by-layer fabrication of mono- and dinuclear metal complexes with XP ligand on surface made it possible to control the multilayer structure on the surface. The combinatorial approach to multilayer structures by use of the redox-active molecular units can produce the various types of potential gradients on the surface, which lead to the rectifying or memory devices. For the construction of two-terminal lateral device structure, the interconnection of molecular dots on the terminal surface by DNA has been studied. The point-to-point capture of DNA by surface metal complex on mica or Au patterned SiO2/Si wafer surface was successfully achieved, and the interconnected DNA acts as a scaffold for two-terminal lateral nanowiring.
Amphiphilic bis-tetrathiafulvalene (bis-TTF) annulated macrocycles (1) were designed from the viewpoints of supramolecular chemistry and surface science in order to construct a variety of nanostructures based on molecular conductors. Electrically active low-dimensional nanostructures such as molecular-assembly nanodots, nanowires and nanorings were fabricated from the charge-transfer (CT) complexes of donor 1 by spin-coating and Langmuir-Blodgett techniques. CT complex of bis (methylthio)-substituted derivative (1a) formed oriented molecular-assembly nanowires on mica surface, whose typical dimensions were 2.5×50×>1,000 nm. The nanowire growth directions were consistent with the directions of hexagonal lattice of K+ sites on the mica surface. Ethylenedithio-substituted derivative (1b) had an ability to form a new redox-active organogel. Chemical oxidation of donor 1b yielded size-controllable nanodot structures by applying spin-coating technique. The diameter of nanodots (20∼800 nm) depended on the rotation speed of the spinner. Conductance of carrier dopped nanodots was three or four orders of magnitude higher than those of neutral molecular-assembly nanostructures.
Nano-materials and devices designed by programmed self-organization processes are presented in a realistic style. The concept of programmed self-organization is the accumulation of elemental self-assembling steps with the control of time, space, and geometry by sequential procedures, fusion of top-down and bottom up processes, and multi-dimensional crystal growth, respectively. Molecule-incorporated DNA was formed by specific molecular recognition through the use of triple hydrogen bonding. For the construction of network-based molecular-scale devices, we developed the DNA-templated arrangement of Au-nanoparticles and top-contact electrode using nano-transfer printing. We also found that self-organized positioning of functional lipid vesicles on nano-well electrodes is useful for electrochemical bio-sensing.
A new process was proposed which exploited protein-supramolecules as scaffolds for producing inorganic, functional nano-structures. This new process was named as “Bio Nano Process” (the BNP) and is applicable in a vast field of nanotechnology. For example, a protein supramolecule, ferritin can be used to form inorganic nanodot in its inner cavity. Using the self-assemble property of the ferritin protein, a two-dimensional array of iron-oxide loaded ferritin molecules can be formed on a Si substrate. Moreover, it is also demonstrated that control of protein-substrate interaction in the solution can realize ferritin placements on designed substrate area at will. Heat treatment removed protein moiety of ferritin molecules and can leave only the nanodots on the substrate. This newly proposed method is a powerful tool for the fabrication of nanometric functional structures.
In vitro evolution system established early in the 1990s has enabled us to create a specific peptide aptamer (=binder) against various targets. Recently, the technique has been applied to isolate peptide aptamers against metals and semiconductors. For the purpose of creating artificial proteins that functionalize the surfaces of titanium and carbon nanohorns, we have isolated peptide aptamers, TBP-1 and NHBP-1. TBP-1 binds to the surfaces of titanium and carbon nano-materials, and has been shown to have a target specificity against Ti, Ag, and Si. Mutational studies have revealed that the peptide binds to these materials by electrostatic interaction. TBP-1 also has biomineralization activities for Ag, silica and oxidized titanium formation. Thus, TBP-1 is a bifunctional peptide, i.e.,it is a binder as well as a catalyst. By using these peptide aptamers against inorganic materials, novel hybrid materials have been developing.
We have found that micro porous polymer films were prepared by simple casting of polymer solution under humid atmospheric condition when water-immiscible organic solvent was used as evaporating solvent. Mono-dispersed water droplets are formed on the surface of the casting polymer solution by water condensation due to evaporation cooling. A surface monolayer of the self-packed water droplets formed on the polymer solution surface acts as a temporary template of micropores. Honeycomb-like hexagonally packed regular-sized micro-pores were formed in the polymer film after evaporation of solvent and water. Biomedical application is an emerging requirement for the honeycomb-patterned polymer flms, especially of biodegradable polymers. Hepatocytes cultured on the honeycomb-patterned film formed spheroid expressing liver functionality, e.g. albumin secretion. The shapes and function of the liver cells can be regulated by the pore size of the honeycomb film. Spheroid aggregates were also formed from neural stem cells on the honeycomb-patterned film.
Cell-materials interactions are deeply related to physicochemical nature of the surfaces. Poly(N-isopropylacrylamide) (PIPAAm) exhibits a transition from hydrophobic to hydrophilic across its lower-critical solution temperature (LCST) of 32oC.The surfaces on which PIPAAm is covalently grafted with the thickness of 15-20 nm achieve temperaturedependent cell adhesion/detachment control. When the temperature is reduced below 32oC, the surface becomes hydrophilic and swells, allowing cells to spontaneously detach from the surface without the need for enzymatic treatments. With this method, confluently cultured cells are harvested as tissue-like cell sheets with simply reducing temperature. In this review, we describe the clinical applications in regenerative medicine with cell sheet engineering using these intelligent ther morespoinsive surfaces.
We have measured electrical resistance conductance across a single atomic step through a metallic monolayer on a crystal surface by using three independent methods that provide consistent values. The two were direct electrical conductivity measurements with monolithic microscopic four-point probes and four-tip scanning tunneling microscope probes. The other was scanning tunneling microscopy/spectroscopy observations on electron standing waves near step edges, combined with analyses based on the Landauer formula for 2D conductors. The present experimental results and the recent theoretical caluclations imply that the electron transport across an atomic step is fairly modeled as a tunneling process.