Takahiro Sawaguchi

Effects of Saccharin and Thiourea on Sulfur Inclusion and Coercivity of Electroplated Soft Magnetic CoNiFe Film

During the course of our recent work performed to develop an electroplated CoNiFe temary alloy with high saturation magnetic flux density and low coercivity for use in magnetic recording heads, it was observed that two common sulfur-containing additives, saccharin and thiourea, behave differently with respect to the dependence of sulfur inclusion and coercivity of the alloy film on the additive concentration in the plating bath. To understand the cause of this difference, scanning tunneling microscopy (STM) was performed, using Au(111) as the substrate, to examine the structure of the adsorbed layers of the additive molecules. The result revealed that the nature of adsorption is fundamentally different for the two different additives; i.e., the adsorption of saccharin is physical and reversible, whereas thiourea undergoes irreversible chemisorption. This finding is consistent with the known behaviors of the two additives in the electroplating of nickel. In this paper the different effects of saccharin and thiourea in the electrodeposition of CoNiFe alloy are interpreted based on the STM results and relevant information available in the literature on the electrodeposition of nickel.

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.

Glucose oxidase/polyion complex-bilayer membrane for elimination of electroactive interferents in amperometric glucose sensor

Abstract An amperometric glucose-sensing electrode was prepared by immobilizing glucose oxidase (GOx) on a polyion complex membrane. First, a monolayer of 3-mercaptopropionic acid (MPA) was made on the surface of a gold electrode by immersing it in an ethanol solution containing MPA. Aqueous solutions of poly- l -lysine and poly-4-styrenesulfonate were successively placed on the electrode surface and allowed to dry. A GOx layer was then formed on the poly- l -lysine/poly-4-styrenesulfonate-complex layer by crosslinking the enzyme by the addition of a glutaraldehyde solution. The polyion complex layer was effective for eliminating electrochemical interferents such as l -ascorbic acid, uric acid and acetaminophen, whereas the hydrogen peroxide produced through the GOx-catalyzed reaction permeated rapidly through the layer. This resulted in a rapid response (100% response in l -ascorbic acid to that for the same concentration of glucose was 0.07). The electrode was applied to the assay of glucose in beverages and sera, and could be used for more than two months.

In situ STM imaging of individual molecules in two-component self-assembled monolayers of 3-mercaptopropionic acid and 1-decanethiol on Au(111)

Two-component self-assembled monolayers composed of 3-mercaptopropionic acid (MPA) and 1-decanethiol (CH3(HC2)9SH:C10SH) on Au(111) were investigated with in situ scanning tunneling microscopy (STM) and cyclic voltammetry, where the monolayers I and II were prepared in 0.1 mM ethanol+water solutions with the ratios of MPA:C10SH=95:5 and 90:10, respectively. In situ STM images revealed that both monolayer I and II consisted of phase-separated domains with molecularly ordered structures, each of which was predominantly formed by one of the constituent molecules. Based on the STM images, the surface fractions of constituent molecules were evaluated as MPA:C10SH=0.61:0.28 for monolayer I, and MPA:C10SH=0.34:0.57 for monolayer II. Cyclic voltammograms of the reductive desorption of the thiols provided the surface fractions of MPA:C10SH=0.69:0.31 for monolayer I, and MPA:C10SH=0.41 and 0.59 for monolayer II, which were in good agreement with the results obtained from STM images. Molecular resolution imaging allowed us to visualize the individual MPA and C10SH molecules and to determine the interfacial structures in the molecularly ordered domains of the phase-separated monolayer. The C10SH domains in the monolayer exhibited ordered phases with densely packed (√3×√3)R30° and p(3×2√3R−30°) structures, which are well-characterized structures for alkanethiol monolayers. On the other hand, a completely different molecular arrangement of MPA defined as a (3×3) structure was consistently observed in the MPA domains, where the molecular arrangement is almost the same as that of (√3×√3)R30° but intermolecular hydrogen bonding is thought to exist in the three neighboring MPA molecules located around the corner of the (3×3) unit cell. It was demonstrated that individual molecules of the monolayer constituents were successfully imaged in the two-component, phase-separated monolayer in solution.

Amperometric determination of pyruvate, phosphate and urea using enzyme electrodes based on pyruvate oxidase-containing poly(vinyl alcohol)/polyion complex-bilayer membrane

Abstract An amperometric pyruvate-sensing electrode was prepared by immobilizing pyruvate oxidase (PyOx) on a polyion complex membrane. First, aqueous solutions of poly- l -lysine and poly(4-styrenesulfonate) were successively placed on a mercaptopropionic acid-modified gold surface and allowed to dry. A photo-crosslinked poly(vinyl alcohol) layer containing PyOx was then formed on the poly- l -lysine/poly(4-styrenesulfonate)-complex layer. The polyion complex layer was effective for eliminating electrochemical interferents such as l -ascorbic acid, uric acid, l -cysteine and acetaminophen, whereas the hydrogen peroxide produced through the PyOx-catalyzed reaction permeated easily through the layer. This resulted in a high sensitivity (detection limit, 50 nM) and a low interference level (e.g. the ratio of response for l -ascorbic acid to that for the same concentration of pyruvic acid, 0.18). The electrode could be used for determining phosphoric acid (detection limit, 0.2 μM), since PyOx consumes phospholic acid as the co-substrate during the course of pyruvate oxidation. Further, an amperometric urea-sensing electrode (detection limit, 0.5 μM) was prepared by coupling the phosphate-sensing system with urea amidolyase which catalyzes an ATP-consuming urea hydrolyzation.

RAPID MEASUREMENT OF TRANSAMINASE ACTIVITIES USING AN AMPEROMETRIC L-GLUTAMATE-SENSING ELECTRODE BASED ON A GLUTAMATE OXIDASE-POLYION COMPLEX-BILAYER MEMBRANE

Abstract An amperometric l -glutamate-sensing electrode was prepared by immobilizing glutamate oxidase (GlOx) on a polyion complex layer-modified electrode. First, a monolayer of 3-mercaptopropionic acid was made on the surface of a gold electrode by immersing it in an ethanol solution containing the modifier. Next, aqueous solutions of poly- l - lysine and poly(4-styrenesulfonate) were successively placed on the electrode surface and allowed to dry. Finally, a GlOx layer was formed on the poly- l- lysine/poly(4-styrenesulfonate)-complex layer by crosslinking the enzyme by the addition of a glutaraldehyde solution. The use of thin bilayer system with the inner, polyion complex membrane, which showed permselectivity based on the solute size with the molecular cut-off of ≈100, brought high performance characteristics to the l- glutamate-sensing electrode; it showed high sensitivity (detection limit, 20 nM), rapid response (100% response time, 3 s), low interferential level (the ratio of response for l -ascorbic acid to that for the same concentration of l -glutamic acid, 8×10−2), and high stability (usable for more than a month). The bilayer-based electrode was useful for the rapid measurement of glutamate–oxaloacetate transaminase (GOT) and glutamate–pyruvate transaminase (GPT) in serum sample: each transaminase (0.2–1000 U l−1) could be determined within 10 s.

Surface modification of GC and HOPG with diazonium, amine, azide, and olefin derivatives.

Surface modification of glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) was carried out with diazonium, amine, azide, and olefin derivatives bearing ferrocene as an electroactive moiety. Features of the modified surfaces were evaluated by surface concentrations of immobilized molecule, blocking effect of the modified surface against redox reaction, and surface observation using cyclic voltammetry and electrochemical scanning tunneling microscope (EC-STM). The measurement of surface concentrations of immobilized molecule revealed the following three aspects: (i) Diazonium and olefin derivatives could modify substrates with the dense-monolayer concentration. (ii) The surface concentration of immobilized amine derivative did not reach to the dense-monolayer concentration reflecting their low reactivity. (iii) The surface modification with the dense-monolayer concentration was also possible with azide derivative, but the modified surface contained some oligomers produced by the photoreaction of azides. Besides, the blocking effect against redox reaction was observed for GC modified with diazonium derivative and for HOPG modified with diazonium and azide derivatives, suggesting fabrication of a densely modified surface. Finally, the surface observation for HOPG modified with diazonium derivative by EC-STM showed a typical monolayer structure, in which the ferrocene moieties were packed densely at random. On the basis of those results, it was demonstrated that surface modification of carbon substrates with diazonium could afford a dense monolayer similar to the self-assembled monolayer (SAM) formation.

In situ characterization of copolymers of pyrrole and N-methylpyrrole at microarray electrodes

Abstract Electrosynthesized copolymers of pyrrole and N -methylpyrrole were characterized by in situ conductivity measurements at microarray electrodes and cyclic voltametry in aqueous solution. The redox properties were easily controlled by changing the monomer ratio in the polymerization solution. The microarray electrode coated with polypyrrole or the copolymer served as redox switching devices showing the rapid ‘on’ and ‘off’ responses (typically, within a few seconds) to redox reagents in solutions with high reproducibility. The copolymerization was effective to give selectivity and sensitivity to this device.

Electrochemical catalytic reduction of molecular oxygen by iron porphyrin ion-complex modified electrode

Abstract The iron porphyrin ion-complex, which is electrostatic aggregate of iron meso-tetrakis- (N-methyl-4-pyridyl)porphyrin (FeTMPyP, cationic) and iron meso-tetrakis(p-sulfophenyl)porphyrin (FeTPPS, anionic), has a catalytic activity for the reduction of molecular oxygen in aqueous solutions. Rotating ring—disk voltammetric measurements show the two different reaction pathways for the oxygen reduction; one is in a lower and other in a higher overpotential region. The main product in lower overpotentials is hydrogen peroxide but 12% of molecular oxygen is reduced to water directly via four-electron process. In higher overpotentials, the iron porphyrin ion-complex also acts as a catalyst for the reduction of hydrogen peroxide. The four-electron process and the two step pathway through intermediate hydrogen peroxide occur. From these results, it can be presumed that some of iron porphyrin aggregates form a “face-to-face”-like structure by the electrostatic interaction and provide the effective sites for the reduction of molecular oxygen.

Enzyme electrodes based on self-assembled monolayers of thiol compounds on gold

Thiol monolayers prepared on gold electrodes have been proved to be useful as the anchor layers to immobilize enzymes molecules. An ion complex layer containing enzyme molecules can be prepared by the co-adsorption of the enzyme and poly-l-lysine onto a mercaptopropionic acid-modified gold surface. Enzymes, such as glutamate oxidase, glucose oxidase and lactate oxidase, were immobilized with high surface densities, and the enzyme-immobilized electrodes could be used for detecting the corresponding enzyme substrate. The UV irradiation onto the surface of gold electrode modified with an N3-functionalized thiol in an enzyme solution produced an enzyme layer, which was formed through the photoreaction between the N3-group and enzyme molecule. The activity of the immobilized enzyme (glucose oxidase) was rather low. However, the photochemical method is still interesting because it can provide the photolithographic patterning of enzyme layers.