Soichiro Yoshimoto

Voltammetric and in situ STM studies on self-assembled monolayers of 4-mercaptopyridine, 2-mercaptopyridine and thiophenol on Au(111) electrodes

Voltammetric and in situ STM studies were carried out for self-assembled monolayers of 4-mercaptopyridine (4-PySH), 2-mercaptopyridine (2-PySH) and thiophenol (PhSH) on well-defined single-crystal Au(111) electrodes in aqueous solutions. A reversible voltammetric response for cytochrome c was clearly observed only at the 4-PyS/Au(111) electrode, showing that only the 4-pyridinethiolate monolayer promotes facial electron transfer reaction between the Au(111) and cytochrome c. On the basis of reductive desorption, the surface coverages of the three aromatic thiolate monolayers were found to be similar to each other; 4.6×10−10 mol/cm2 for 4-PyS/Au(111), 4.7×10−10 mol/cm2 for 2-PyS/Au(111), and 4.4×10−10 mol/cm2 for PhS/Au(111). High-resolution STM images in perchloric acid solutions revealed p(5×√3R-30°) and p(4×√7R-40.9°) structures for the 4- and 2-pyridinethiolate monolayers on Au(111), respectively. No structure order was observed for the PhSH monolayers. While the pyridine units of both 4- and 2-pyridinethiolate monolayers were found to be oriented normal to the surface, 2-pyridinethiolates adsorbed through not only sulfur but also nitrogen atom of the pyridine ring. From these STM images, the orientation of the N atom of the pyridine moiety must face to the bulk solution, as in the case of 4-PyS/Au(111), in order to obtain a facile electrochemical reaction for cytochrome c.

Advances in supramolecularly assembled nanostructures of fullerenes and porphyrins at surfaces

The ‘bottom-up’ strategy is an attractive and promising approach for the construction of nanoarchitectures. Supramolecular assemblies based on non-covalent interactions have been explored in an attempt to control surface properties. In this minireview, we focus on advances made in the past three years in the field of scanning tunneling microscopy (STM) on supramolecular assembly and the function of porphyrins, phthalocyanines, and fullerenes, non-covalently bound on metal single crystal surfaces. Well-defined adlayers, consisting of porphyrin and phthalocyanine for the design of supramolecular nanoarchitectures, supramolecular traps of C60 on hydrogen bond networks, a unique approach for controlling molecular orientation by a 1:1 supramolecularly assembled film consisting of C60 and the related derivatives and metallooctaethylporphyrins, and nanoapplications of fullerenes, either induced by tip manipulation or driven by thermal fluctuations at surfaces, were clearly visualized by STM.

Supramolecular Nanostructures of Phthalocyanines and Porphyrins at Surfaces Based on the “Bottom-Up Assembly”

The “bottom-up” strategy is an attractive and promising approach for the construction of nanoarchitectures. Supramolecular assemblies based on noncovalent interactions have been explored in an attempt to control the surface properties. In this chapter, we focus on advances made in the past 5 years in the field of scanning tunneling microscopy (STM) on supramolecularly nanostructured phthalocyanines and porphyrins on single-crystal surfaces. The design of supramolecular nanoarchitectures consisting of phthalocyanines and porphyrins, supramolecular traps of C60 and coannulene, direct metallation on phthalocyanines and porphyrins adlayers, direct synthesis of porphyrin oligomers at surfaces, axial coordination of phthalocyanines and porphyrins, and nanoapplications induced by tip manipulation at surfaces were clearly visualized by STM.

Simple methods for preparation of a well-defined 4-pyridinethiol modified surface on Au(111) electrodes for cytochrome c electrochemistry

Abstract A very small amount of sulfide impurity in 4-pyridinethiol (4-PySH) modifier solution was found to interfere with the proper formation of the 4-PySH modified surface for cytochrome c electrochemistry on an Au(111) electrode. When the modification was conducted in an alkaline (e.g. 0.1 M KOH) solution, in aqueous solutions under applying a potential more positive than 0.3 V vs. Ag/AgCl, or at a low modifier concentration (e.g. 20 μM), the proper 4-PySH modified surface was obtained even using 4-PySH as received, which contained a small amount of sulfide. The selective adsorption of 4-PySH in the presence of a small amount of sulfide under these conditions was due to the rapid formation of proper 4-PySH modified surface, which prevented the sulfide from reacting with the electrode surface.

Electrochemical study on competitive adsorption of pyridinethiol with sulfide onto Au(111) surfaces

Abstract Using electrochemical reductive desorption of surface modifiers the structure of the modified surface of a single crystal electrode was studied. A very small amount (e.g. 1 mol %) of sulfide impurity in a 4-pyridinethiol (4-PySH) modifier solution was found to adsorb on an electrode in competing with 4-PySH and eventually adsorbed 4-PySH (or bis(4-pyridyl)disulfide, 4,4′-PySSPy) molecules were replaced completely by sulfide. In an ethanolic solution the sulfide impurity affected the modified surface structure of an Au(111) electrode more significantly than in an aqueous solution. The purified 4-PySH and 4,4′-PySSPy samples reproducibly gave a preferable surface for cytochrome c electrochemistry. The reductive desorption peak potential of a thiol adsorbed on the electrode surface was suggested to be a measure for predicting the structure of the modified surface. When the desorption peak potential of thiol becomes more positive, faster replacement of thiol by sulfide occurs. On the same lines, it was explained why an Au(111) single crystal surface is more sensitive to the sulfide impurity than are the Au(100) and Au(110) surfaces.

Adsorption and assembly of ions and organic molecules at electrochemical interfaces: Nanoscale aspects

We describe the history of electrochemical scanning tunneling microscopy (STM) and advances made in this field during the past 20 years. In situ STM allows one to monitor various electrode processes, such as the underpotential deposition of copper and silver ions; the specific adsorption of iodine and sulfate/bisulfate ions; electrochemical dissolution processes of silicon and gold single-crystal surfaces in electrolyte solutions; and the molecular assembly of metalloporphyrins, metallophthalocyanines, and fullerenes, at atomic and/or molecular resolution. Furthermore, a laser confocal microscope, combined with a differential interference contrast microscope, enables investigation of the dynamics of electrochemical processes at atomic resolution.

Design of Biomolecular Interface for Detecting Carbohydrate and Lectin Weak Interactions

A hybrid functional biomolecular interface designed at a molecular size level is very effective at capturing an analyte with high sensitivity even if the interaction is very weak, as when detecting proteins with carbohydrate. We designed and processed a protein (lectin) recognition molecular interface taking the following points into consideration: (1) the height (molecular length) difference between the capturing and spacer molecules; (2) the ratio of capturing molecules in the recognition interface. When the height difference between the maltoside part (Concanavalin A (Con A) recognition group) and the OH group terminated spacer molecules exceeded (>(CH(2))(6)), the association rate constant (k(a)) became larger (k(a)(1/Ms): ∼2.6 times) and the dissociation constant (K(D)) became much smaller (K(D)(M): 1.0 × 10(-6): ∼0.17 times) compared with the similar heights (lengths) of both molecular interfaces. With regard to maltoside density, a 100% maltoside monolayer was unsuitable for detecting Con A. We constructed a nanostructured recognition site with a maltoside part of 10%, which was the most suitable ratio for Con A detection. The binding interaction between Con A and the maltoside group was changed from monovalent binding to bivalent binding when the maltoside part was diluted in the recognition interface. From electrochemical measurements, even though there was a small amount of maltoside component on the suitable recognition monolayer, quality similar to that of 100% maltoside was observed.

Investigation of the structure of self-assembled monolayers of asymmetrical disulfides on Au(111) electrodes by electrochemical desorption☆

Abstract In this paper the reductive desorption for homogeneously mixed self-assembled monolayers (SAMs) on Au(111) electrodes formed from an asymmetrical disulfide, i.e. butyl hexadecyl disulfide [C 4 H 9 –S–S–C 16 H 33 ; H4H16] or decyl 2-(perfluorohexyl)ethyl disulfide [C 10 H 21 –S–S–(CH 2 ) 2 (CF 2 ) 5 CF 3 ; H10F8] was studied by cyclic voltammetry. Peak potentials of electrochemical desorption waves for the SAMs of these asymmetrical disulfides were different from those of SAMs containing only one of the constituent chains. The peak potential deviation from those of the single component SAMs suggests mixing of the two constituent chains in molecular dimensions. The ratio of the two constituent chains of the asymmetrical disulfide in each of the SAMs was considered to be exactly 1:1 when the homogeneously mixed SAMs were prepared under favorable conditions with respect to reaction concentration and temperature. We will discuss features of cyclic voltammograms in terms of the structure of the SAMs.

STM and voltammetric studies on the structure of a 4-pyridinethiolate monolayer chemisorbed on Au(100)-(1 × 1) surface

The structure for a 4-pyridinethiolate monolayer chemisorbed on the Au(100)-(1×1) single crystal surface was characterized by in situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV). In situ STM observation showed a well-ordered p(√2 R 45°×5 R 53.1°) structure (abbreviated as √2×5) for the surface modified with either 4-pyridinethiol (4-PySH) or bis(4-pyridyl)disulfide (4,4′-PySSPy) in a 0.05 M HClO4 solution. On the Au(100)-(1×1) surface, 4-PySH molecules formed a dimer structure with the S–S bond length of about 2.4 nm. The observed dimer structure is similar to that previously reported on the Au(111) surface, and the orientation of the pyridine ring is mostly perpendicular to the surface normal. However, the adsorbed molecules were more densely packed (√3/√2 times) on the Au(100) surface than on the Au(111) surface. The surface excess was estimated to be 5.8 (±0.2)×10−10 mol cm−2 based on the voltammetric charge for the reductive desorption. This value is in good agreement with that (5.7×10−10 mol cm−2) calculated from the parallelogrammic (√2×5) unit cell. The Au(100)-(1×1) surface modified with 4-PySH gave a well-defined electrochemical response of cytochrome c.

Electrostatically controlled nanostructure of cationic porphyrin diacid on sulfate/bisulfate adlayer at electrochemical interface.

Two different cationic tetraphenyl porphyrins, one with two carboxyphenyl groups in cis-position and the other in trans-position (cis- and trans-H(4)DCPP(2+)), have been examined to control the structure of their 2D supramolecular assemblies in 0.05 M H(2)SO(4) at electrochemical interfaces. Electrochemical scanning tunneling microscopy (EC-STM) images revealed the formation of supramolecularly organized nanostructures of cis-H(4)DCPP(2+) such as dimer, trimer, and tetramer on the (square root(3) x square root(7)) sulfate/bisulfate adlayer, suggesting the importance of both electrostatic interaction between cationic porphyrin core and sulfate/bisulfate adlayer and the hydrogen bond formation between carboxyl groups of the nearest neighbor cationic porphyrins. Trans-H(4)DCPP(4+) ions were also found to be aligned in the square root(3) direction of the sulfate/bisulfate adlayer. The structure of these cationic porphyrin adlayers was found to depend upon the electrode potential; i.e., when the potential was changed in the negative direction, the (square root(3) x square root(7)) sulfate/bisulfate adlayer disappeared, and no ordered arrays were formed. In contrast, when 0.1 M HClO(4) was used as an electrolyte solution, only a disordered array was observed. The results of the present study indicate that the (square root(3) x square root(7)) sulfate/bisulfate adlayer formed on Au(111) in 0.05 M H(2)SO(4) plays a significant role as a nanorail template in the control of electrostatically assembled diacid porphyrin dicarboxylic acid derivative. In addition, the high-resolution STM clearly distinguished between cis-H(4)DCPP(2+) ion and cis-H(2)DCPP molecule. The cis-H(2)DCPP molecules on Au(111) provided an adlayer structure and an electrochemical behavior which are different from those of cis-H(4)DCPP(2+) ions.