A self-assembly (SA) technique has been very widely used to construct ordered molecular layers of various functionalities on a solid substrate and self-assembled monolayers (SAMs) of alkylthiols on gold have been the most well studied system. In this article, the SA process and typical functionalities of the SAMs of thiol derivatives are described mainly based on the results obtained in our group over the last 10 years. The structure and mass transport during the self-assembly process of the thiol derivatives on Au(111) were investigated by quartz crystal microbalance (QCM) measurement, polarization modulation Fourier transform infrared spectroscopy (PM-FTIR) and scanning tunneling microscopy (STM). The electrochemical characteristics and related mass transport and structural changes of the SAMs were carefully examined. The SAMs with novel functions such as uni-directional electron transfer, electrochemically generated chemiluminescence (ECL), visible light induced uphill electron transfer, and electrochemically controllable SHG activity were constructed. Finally, future prospects are presented.

As a practical guideline, it was indicated that aniline is electrooxidatively polymerized in an acidic electrolytic solution under irradiation of ultrasound with a power higher than the cavitation threshold to form a highly uniform and dense polyaniline film available for electronic devices such as an electrolytic capacitor.

Introduction of hydroxyl group on the surface of carbon fiber was attained by the indirect anodic oxidation of carbon fiber with NO₃^• generated by anodic oxidation of NO₃⁻. This hydroxyl group was found to have enough chemical reactivity with electrophiles and the modification of carbon fiber surface was attained by using the reactivity of the hydroxyl group.

Molecular-resolution images of benzotriazole (BTAH) adsorbed on Cu(110) are investigated by in situ AFM and electrochemical measurement in 100 mM HClO₄ solution with 1 mM BTAH. At potential between −250 mV and −150 mV, a bare and unreconstructed Cu(110) surface was observed. At potential from −500 to −250 mV and from −150 to 400 mV, we observed adsorbed BTAH structures, c(2 × 3) and c(2 × 4) structures.

Two novel techniques of piezoelectric detection and laser-beam deflection were employed to examine the influence of the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox reaction on the changes in surface energy of a gold electrode in perchlorate solutions with and without iodide ions. In the absence of iodide ions, both piezoelectric signal curve and electrocapillary curve suggested the specific adsorption of Fe(CN)₆³⁻, Fe(CN)₆⁴⁻ or its decomposed product such as Prussian blue on the gold electrode. On the other hand, in the presence of iodide ions, it was found that the strong specific adsorption of iodide ions on the gold electrode inhibits the adsorption of the redox couple or Prussian blue.

In situ electrochemical scanning tunneling microscopy (EC-STM) was used to examine the zinc-terminated ZnO(0001)-Zn surface in electrolyte solution. Atomically-flat terrace-step structures were consistently observed on the surface which was prepared by chemical etching in alkaline solutions. Steps observed on the surface were composed of bilayers with a step height of 0.26 nm. Atomic images obtained by in situ EC-STM revealed that the surface has a (1 × 1) structure with hexagonal lattice.

First-order and second-order decomposition kinetics have been determined electrochemically for the monocationic and dicationic forms of self-assembled monolayer (SAM) of Bfc-CO(CH₂)₇SH (1, Bfc = l’-biferrocenyl) on Au(111), respectively. In the mixed SAMs of 1 and octanethiol, the heterogeneous electron-transfer rate constant, k⁰, at 25 °C obtained by the dependency of the peak-to-peak separation (ΔE_p) on the scan rate for both Bfc⁺/Bfc⁰ and Bfc²⁺/Bfc⁺ couples increases with decreasing the surface coverage of 1. The k⁰ value for the Bfc²⁺/Bfc⁺ couple is slightly lower than that for the Bfc⁺/Bfc⁰ couple in the SAMs.

We have studied the photooxidation of model long-chain organic compounds, i.e., octadecanoic acid, octadecane and glycerol trioleate on a polycrystalline anatase TiO₂ thin film. All of these compounds were mineralized to CO₂ and H₂O. Further, no gaseous intermediates and/or byproducts were produced. This suggests that the initial reactants as well as intermediates were continuously attached to the TiO₂ surface during the reaction.

A novel scanning electrochemical/chemiluminescence microscopy (SECM/SCLM) was used for imaging of a spot of immobilized horse radish peroxidase (HRP) on a substrate. When the SECM tip scanned over the spot in an aqueous luminol solution, the immobilized HRP catalyzed the oxidation of luminol by the electrogenerated H₂O₂ at the tip to emit chemiluminescence which was detected by a photon-counter. The photon-counting intensity was plotted against the position of the tip to give the image indicating the activity of immobilized HRP. The topography image was also obtained by mapping the oxygen reduction current, since the current was sensitive to the topography of substrate.

An amperometric glucose-sensing electrode based on the cathodic detection of oxygen consumption during the glucose oxidase (GOx) reaction was prepared as follows. A platinum electrode was modified with polydimethyl-siloxane (PDMS), and a GOx-layer was placed on the PDMS layer. The permselectivity of PMDS made it possible to monitor the oxygen consumption rate without interference from the hydrogen peroxide. The concentration of glucose (0.01-1.2 mM) could be determined from the decrease in the cathodic current at −0.1 V vs. Ag/AgCl, without any interference from uric acid, acetaminophen and L-cysteine.

The effect of light irradiation with a 442 nm He-Cd laser on the electrodeposition was studied for polycrystalline CdS films. Controlled rectangular voltages were applied on indium tin oxide/glass substrates for the electrodeposition from low concentration of CdSO₄ and Na₂S₂O₃ in aqueous solution of pH = 3.1 at room temperature. The crystallinity of the CdS films obtained under the irradiation was better than that made in dark.

Structural effects on the rate of CO₂ reduction were studied on the low index planes of Pt (Pt(111), Pt(100) and Pt(110)) using macro-electrolysis in acetonitrile containing 0.1 M (M = mol dm⁻³) tetraethyl anmmonium perchrolate. CO₂ was reduced at −3.1 V (vs. Ag/Ag⁺) with the concentration of water controlled at 25 ± 2 mM. Oxalic acid was the main reduction product on each electrode. Formic acid and trace of CO were also produced. The order of the partial current density of oxalic acid formation is Pt(110) < Pt(111) < Pt(100): the partial current on Pt(100) is twice as high as that on Pt(110).

Organic thin films composed of cationic and anionic polymers were prepared on the surface of glucose biosensor by an alternate deposition technique, to improve the response characteristics of the biosensor. The polymer thin film significantly suppressed the permeation of both glucose and ascorbic acid, which resulted in a suppression of ascorbate interference or an improvement of the dynamic range, depending on the arrangement of the enzyme and polymer layers on the electrode surface.

Formation of Ni microstructure with high aspect ratio without thicker photoresist was tried by an electrodeposition method. The Ni microprobes thicker than photoresist thickness were electrodeposited on a patterned substrate using thin photoresist patterned with UV radiation. It was important for preparing Ni microprobe with high aspect ratio to keep the overpotential of patterned cathode higher, and moreover the agitation using a paddle plating cell system was so effective to form uniform microprobes.

Surface morphology change of highly oriented pyrolytic graphite was investigated in 1 M (M = mol dm⁻³) LiClO₄ dissolved in propylene carbonate (PC) and PC + 0.5 M 12-crown-4 using electrochemical scanning tunneling microscopy to elucidate the mechanism of passive film formation on graphite electrode in rechargeable lithium batteries. In PC, vigorous exfoliation of graphite layers was observed after potential cycling down to 0 V vs. Li/Li⁺. Such exfoliation was greatly suppressed by 12-crown-4 addition. The original step structure was nearly maintained, but swellings of graphite surface appeared after potential cycling. From these results, the role of 12-crown-4 in passive film formation was discussed.

A thick and Ti-rich oxide layer was obtained on Fe-22 at % Ti alloy by polarization in a 0.1 M (M = mol dm⁻³) H₂SO₄ solution with the applied potential being modulated as square wave. The surface cation concentration of up to 75 at% Ti was attained at favorable electrochemical conditions. However, the high Ti content was observed in the most surface. The photo electrochemical response reveals this oxide layer shows electronic properties similar to that for TiO₂ layer.

Polypyrrole/ion-conducting polymer electrolyte composites are prepared by in situ electro-polymerization of pyrrole in polymer electrolyte matrix. The existence of the concentration gradient of PPy is characterized at the PPy/polymer electrolyte interface and this concentration gradient leads to the facile electron transfer due to the fast counterion-transport across the interface.

Novel self-assembled monolayers of a mixed-valent (μ₃-oxo)triruthenium(II,III,III) complex containing a terminal CO ligand were constructed on Au(111) electrode surface and were characterized by cyclic voltammetry and in situ Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). A distinctive infrared ν(CO) absorption band was observed at 1950 cm⁻¹ for the original {Ru^II–CO}Ru^IIIRu^III complex in an aqueous 0.1 M NaClO₄ solution and it shifted to 2060 cm⁻¹ upon one-electron oxidation of the Ru^II center.

Adsorption behavior of three isomeric pyridinecarboxylic acids (picolinic acid: 2-PCA, nicotinic acid: 3-PCA, and isonicotinic acid: 4-PCA) in 0.1 M (M = mol dm⁻³) HClO₄ solution on a smooth polycrystalline gold electrode surface was investigated by in situ infrared reflection absorption spectroscopy (IRAS). As the electrode potential became more positive, adsorbed 3- and 4-PCAs were found to reorient from a flat configuration to a vertical one that binds to the surface through lone pairs of electrons on two oxygen atoms of the carboxylate group. However, no significant adsorption of 2-PCA occurred on the electrode surface, and this finding indicates the ortho effect on adsorption.

Electrochemically induced chemical deposition (EICD) of nanocrystalline CdS thin films has been realized in an aqueous mixture of CdSO₄, thioacetamide (TAA) and 2-mercaptoethanol (ME). Increasing the ME concentration as 0, 2, 4 and 6 mM (M = mol dm⁻³) in the reaction bath containing 0.05 M CdSO₄ and 0.1 M TAA resulted in a decrease of the crystal size as 23, 6.4, 4.8 and 4.1 nm, respectively. The bandgap energies of direct transition, estimated from the photocurrent action spectra of the deposited thin-film electrodes, increased as 2.43, 2.57, 2.65 and 2.69 eV in the same order, clearly exhibiting the size quantization effects expected for such nano-particulate CdS thin films.

The surface pKa of mercaptoundecanoic acid (MUA) in its self-assembled monolayer (SAM) and in mixed monolayers with alkanethiol has been examined using the surface mass titration method based on a quartz crystal microbalance. The pKa value, determined from the pH at half the total mass change, shifted in a positive direction with the increase in the surface coverage of MUA in both monolayers, showing the repulsive interaction between carboxylates. The pKa at zero coverage depends on the monolayer structure; 4.8 for the SAM and 5.6 for the mixed monolayer.

The electrochemical oxidation of ascorbic acid (AA) and dopamine (DA) at a gold electrode modified with a self-assembled monolayer (SAM) of cystamine (CYSA) has been studied. A large decrease in the overpotential for the oxidation of AA was noticed at CYSA-Au electrode. Well-separated square wave voltammetric peaks for AA and DA were observed at this electrode, which can be used for the simultaneous detection of these species.

Two-component self-assembled monolayers composed of 1-decanethiol (CH₃(HC₂)₉SH : C₁₀SH) and 3-mercaptopropionic acid (MPA) on Au(111) were studied with in situ scanning tunneling microscopy (STM). STM images with molecular resolution revealed that the monolayer consisted of phase-separated domains with molecularly ordered structures, each of which was dominantly formed by one of the constituent molecules. The C₁₀SH domains in the monolayer exhibited ordered phases with a (p × √3) structure, well-established for alkanethiols. On the other hand, a molecular arrangement of MPA defined as a (3 × 3) structure was observed for the MPA domains. It was demonstrated that individual molecules of the monolayer constituents were successfully visualized in the two-component, phase-separated monolayer in solution.

The redox-electrode reaction of Co(II)(phen)₃/Co(III)(phen)₃ (phen = 1,10-phenanthroline) in potassium halide solutions was kinetically studied on a Pt(111) single crystal electrode. The formal rate constants were found to depend on the anions and to decrease in the order: F⁻ > Cl⁻ > Br⁻. It was found that modification of the electrode surface with a monolayer of thiocyanate ion accelerates the heterogeneous electron-transfer reaction, whereas the reaction was retarded by the modification with iodide. These findings are discussed based on the adsorption of anions.

Potential modulation reflectance (PMR) was measured as a function of potential and wavelength of light for self-assembled monolayers (SAMs) formed from dialkyl disulfide-linked naphthoquinones with different spacer alkyl chain on a gold electrode. On the cyclic voltammogram a redox reaction between naphthoquinone and naphtho-hydroquinone in the SAMs is clearly observed. The oxidation and reduction peak currents are strongly dependent on the spacer alkyl chain length. On the PMR spectra two peaks are observed: one at 470 nm wavelength originates from the redox change between the naphthoquinone and naphtho-hydroquinone and the other peak at 530 nm wavelength may be due to formation of a charge transfer complex between naphthoquinone and naphtho-hydroquinone. From the phase retardations of PMR and differential capacitance, it is supposed that relatively slow processes like a configuration change of the naphthoquinone derivative accompanied by proton transfer are included in the redox reaction.

The electrochemical properties of MoO₃ on glassy carbon surface were evaluated. MoO₃ was highly dispersed on glassy carbon surface when the loaded amount of MoO₃ was 0.5 nmol cm⁻². This electrode exhibited extremely high pseudocapacitance (118 kC mol⁻¹), which is comparable to that of RuO₂.

We developed a new anion-imprinted electropolymerization method to form complementary cavity to the size of the used electrolyte anion as template in the cross-linked polyviologen film prepared by reductive coupling reaction of trifunctional monomer. The film has both size-dependent anion exchange property due to the anion-imprinted cavity and photoreduction property of the viologen moieties strongly dependent on the electron donability of the exchanged anion in the film. The anion-imprinted polyviologen film plays a role in a new type of optical sensing to recognize various anions.

A dinuclear ruthenium complex [Ru₂(dhpta)(μ-O₂CCH₃)₂]⁻ (H₅dhpta = 1,3-diamino-2-hydroxypropane-N,N,N′,N′-tetraacetic acid) is accumulated on a gold surface by using the ligand substitution reaction of bridging acetate in the complex by terminal benzoate in a self-assembled monolayer with 4-(6-mercaptohexoxy)benzoic acid. The accumulated complex, which shows a surface redox couple corresponding to Ru^IIIRu^III/Ru^IIIRu^II, covers the gold surface completely. The ligand substitution reaction is applicable for the accumulation of other complexes and useful for the designing of functional electrodes.

The electrocatalytic activity for H₂ oxidation in the presence of 100 ppm CO has been investigated on 40 species of binary Pt alloy electrocatalysts with different combinations or compositions. Pt-Fe, Pt-Ni, Pt-Co and Pt-Mo alloys have been found to exhibit the excellent CO-tolerance. On such alloy electrodes, the equilibrium coverage of CO was suppressed less than ca. 0.6. The surface of the alloys was found to be composed of thin Pt layer, but its electronic structure differed from that of pure Pt.

Potential and pH dependencies of surface structures of Au(111) electrodes modified with 2- and 4-pyridinethiol (2-PySH and 4-PySH, respectively) and 2-pyrimidinethiol (2-PymSH) were examined by surface-enhanced IR absorption spectroscopy (SEIRAS) and STM. At positive potentials, the pKa values of the adsorbed pyridinethiol and pyrimidine species decreased, and even in an acid solution the pyridine (or pyrimidine) N atom became less protonated. Therefore, STM images of self-assembled monolayers of these thiols observed at properly positive potentials (e.g., > ca. 0.3 V vs. Ag/AgCl for 2-PySH and 2-PymSH in a 0.1 M HClO₄ solution, and >0.7 V vs. Ag/AgCl for 4-PySH in a 0.01 M (M = mol dm⁻³) HClO₄ solution) can be used for evaluating the structures in a neutral solution.

The addition of aluminum iodide (AlI₃) to binary electrolytes, propylene carbonate mixed with dimethyl carbonate (PC/DMC) or ethylene carbonate mixed with DMC (EC/DMC), containing lithium bis(perfluoroethylsulfonylimide) (LiBETI) improved the charge-discharge cycle efficiency of a Li metal anode; the enhanced efficiency in EC/DMC with LiBETI was about 95% even without the stacking pressure against the anode. In contrast, the AlI₃ addition reduced the efficiency in a PC/DMC electrolyte containing lithium hexafluorophosphate (LiPF₆). The AlI₃ additive in the LiBETI electrolytes provided a low-resistance Li interface without predominant anion-decomposition as well as dendritic or mossy Li deposition.

The crystallinity of p-type porous silicon has been investigated with X-ray diffraction (XRD), thermal analysis, and transmission electron microscopy (TEM). Each XRD peak is composed of a sharp Bragg reflection peak and a diffuse diffraction. The diffuse diffraction is not related to the presence of amorphous phase judging from the thermal analysis and the comparison with the XRD of amorphous silicon. The shape of the XRD pattern changes with heat-treatment at temperature as low as 400 °C. The TEM observation also reveals the enhancement of crystallinity by the heat-treatment. The results imply that no amorphous phase is formed in porous silicon with high resistivity.

Electrochemical oxidation of ascorbic acid was significantly suppressed on a 11-mercaptoundecanoic acid (MUA)-assembled gold electrode, whereas redox reaction of ferrocenylmethanol was not retarded on the same electrode. The selective oxidation reaction of the hydrophobic mediator was utilized for an amperometric glucose sensing by using a glucose oxidase-immobilized MUA electrode. It was shown that the surface-ordered electrode was useful for selective amperometric biosensing in which interference from ascorbic acid was excluded.

Modified electrodes with cadmium sulfide nanoparticles (Q-CdS) were fabricated by immobilizing them on a self-assembled monolayer of hexanedithiol prepared on the Au electrode. Photoirradiation of the Q-CdS-modified electrodes in the presence of triethanolamine afforded stable anodic photocurrents. Appreciable reduction (∼3%) of photocurrent was observed in the presence of (external) magnetic fields (0.5 T). It is suggested that this magnetic field effect is caused by the quantum size effect of Q-CdS.

Using a two-dimensional electrogenerated chemiluminescence (ECL) observation system with a CCD detector, the dynamic transformation of the ECL images have been previously observed on the surface of the conventional disk electrode on which the microcrystals of the perylene dimer cation radical perchlorate (Pe₂ClO₄) were deposited. In the present work, the nanoelectrode ensembles were used for the analyses of the deposition process of Pe₂ClO₄ and the ECL reaction. For the deposition process of Pe₂ClO₄ with the nanoelectrode ensemble, it is confirmed that the oxidation of Pe occurs on the surfaces of the deposited Pe₂ClO₄ and causes nucleation growth of the needle-like crystals of Pe₂ClO₄. However, the ECL emission could not be observed using the nanoelectrode ensemble. This means that the formation of Pe^•− in the diffusion layer is an essential factor for the ECL emission using the conventional disk electrode. In addition, the changes in the contrast of the needle-like Pe₂ClO₄ crystals at the reduction potential indicated that the reduction of Pe₂ClO₄ occurred. In this manner, it was found that the nanoelectrode ensembles are effective for the elucidation of the electrode reaction mechanism involving the deposition of the conducting microcrystals.

The electric field (EF) effects on the proton transfer reaction that take place on the surfaces of Pt(111) and Ag(111) electrodes, were studied using density functional theory (DFT) at the level of B3LYP/LanL2DZ. According to calculations based on a “one-dimensional” proton transfer model, the potential energy curve of proton transfer between two water molecules on the surfaces, depends substantially on the direction of the EF. An EF shifts the equilibrium of proton transfer in the direction of electrostatic attraction. The potential energy surface for a “two-dimensional” proton transfer model, in which a proton transfers between two water molecules and the surface, revealed that an EF effectively accelerates the proton transfer from H₃O⁺ to the surface. The rate-determining step of the proton transfer is discussed in the light of the results.

Phase-separated binary self-assembled thiol monolayers of 2-mercaptoethanesulfonic acid and 1-octadecanethiol have been studied by cyclic voltammetry and scanning probe microscopy. Cyclic voltammograms for the reductive desorption of the binary self-assembled monolayers show a high degree of phase separation of the monolayers. The presence of two different types of domains caused by phase separation is imaged by atomic force microscopy.

Three types of electroactive peptides were synthesized with 16 amino acids, which had a flavin pendant at three different sites of the α-helical peptide. Each flavin-pendant α-helical peptide was self-assembled on a gold electrode through Cys-S(H) at the C-terminus, and electrochemically characterized. All of these flavin-pendant peptides performed excellent characteristics in electron transfer at the electrode/solution interface. In the results, it was strongly suggested that the rate constant of the flavin redox reaction was determined by the molecular environment surrounding the flavin.