Kô Takehara

Electrochemical studies of the terminally substituted alkanethiol monolayers formed on a gold electrode: Effects of the terminal group on the redox responses of Fe(CN)3- 6, Ru(NH3)3+ 6 and ferrocenedimethanol

Abstract Self-assembled monolayers of the alkanethiols having OH, COOH and NH 2 terminal group were formed on a gold electrode via a sulfur attachment. The effects of their terminal groups on the redox responses of Fe(CN) 3− 6 , Ru(NH 3 ) 3+ 6 and 1,1′-ferrocenedimethanol (FcDM 0 ) aqueous solutions were investigated by cyclic voltammetry. The voltammetric response of Fe(CN) 3− 6 was decreased in the order of NH 2 > OH > COOH terminal group. In contrast, the response of Ru(NH 3 ) 3+ 6 was increased in the order of NH 2 0 , the response was in the order of COOH > OH > NH 2 . These results were discussed in relations to the effect of charged terminal group on (a) the shift of the potential drop across the diffuse layer and (b) the change of the concentration of redox species at the electrode surface which is caused by the electrostatic interaction between the terminal group and ionic redox species.

Flow analysis of micro amounts of chromium(III) and (VI) in natural water by solid phase spectrophotometry using diphenylcarbazide

Specific determination of chromium(III) and chromium(VI) in natural water samples was performed without mutual separation. The chromium in sample solution was concentrated on cation-exchange resin packed in a flow-through cell as a reaction product of chromium(VI) with diphenylcarbazide. The absorbance increase caused by the accumulation of the complex on cation-exchange resin was continuously measured. By using peroxodisulfate as an oxidizing agent, the chromium(III) in sample solution was completely oxidized to chromium(VI). The chromium(III) concentration was calculated by the difference between the values of the total chromium and chromium(VI). The detection limit for chromium(VI) and total chromium was 70 pg and 120 pg, respectively. This method was applied to the specific determination of chromium in several natural water samples. The results of the total chromium obtained by the proposed method show moderate agreement with that obtained with ICP-MS. Unlike the previous results reported in chromium speciation studies, the predominant chromium species in natural water was chromium(VI).

An ion-gate response of the glutathione monolayer assembly formed on a gold electrode: Part 1. The effect of pH, K+ and Ca2+

Abstract The electrochemical behavior of the self-assembled glutathione (GSH) monolayer formed on a gold electrode was studied in aqueous Fe(CN) 6 3− solution in order to characterize the ion permeation properties of this monolayer assembly. It was found that the GSH monolayer showed ion-gate behavior for the permeation of Fe(CN) 6 3− probe ion. At pH 5.3, the monolayer assembly strongly hindered the electron transfer between the gold surface and the Fe(CN) 6 3− ion in solution. In contrast, the electron transfer was drastically improved at pH 2.9. The electron transfer was also found to improve with increasing Ca 2+ concentration. The rate constant of Fe(CN) 6 3− reduction increased linearly with Ca 2+ concentration. The ion-gate behavior of the GSH monolayer is discussed in terms of the conformational change in GSH caused by the coordination with Ca 2+ and the electrostatic interaction between GSH and Fe(CN) 6 3− .

An ion-gate response of the glutathione monolayer assembly formed on a gold electrode: Part 2. The effect of alkaline earth ions

Glutathione (y-L-glutamyl-L-cysteinyl-glycine) (GSH) is a well-known biologically active tripeptide which participates in the various functions of biological processes [ll. For example, GSH has been reported to act as a coenzyrne for ion-channel proteins and to participate in the biological redox systems 121. Longchain molecules containing thiol terminals, like n-alkanethiols, are known to form a well-ordered, densely packed and electron-insulating self-assembled monolayer film on a gold surface [3,4]. Furthermore, the ion-selective and ion-gate responses of such a monolayer assembly have also been reported by several authors [5,6]. The novel electrochemical characteristics of a GSH monolayer assembly formed on a gold surface, i.e. the ion-gate response which is controlled by the pH and the concentration of Ca*+ ion, have been reported previously [7]. In this communication we report electrochemical investigations of the effect of alkaline earth ions M*+ on the ion-gate response of the GSH monolayer assembly formed on a gold surface. The redox signal of the Fe(CN)zion was used as the marker of the ion-gate response of the GSH monolayer assembly.

Electrochemical behavior of ubiquinone and vitamin K incorporated into n-alkanethiol molecular assemblies on a gold electrode

The aim of the present note is to provide information about the effect of the alkyl side chains of ubiquinone and vitamin K on the voltammetric responses of these quinones incorporated into the n-alkanethiol molecular assembles which are modified on the gold electrode

Electrochemical properties of a gold electrode modified with a mixed monolayer

Abstract The electrochemical properties of a gold electrode modified with a mixed thiol monolayer containing both a polar and a non-polar head group have been investigated in aqueous Fe(CN) 6 4− , flavin adenine dinucleotide (FAD) and ubiquinone-0 (2,3-dimethoxy-5-methyl-1,4-benzoquinone, UQ 0 ) solutions. The cyclic voltammetric current-potential ( i-E ) response of Fe(CN) 6 4− was found to be affected considerably by the polarity of the head group contained in the mixed monolayer assembly, as compared with those of FAD and UQ 0 . It was also found that in the cases of UQ 0 and FAD the i-E responses for the modified electrode were affected by their own molecular size rather than the polarity of the mixed monolayer head group. Furthermore, compared with Fe(CN) 6 4− ion, these biologically related molecules are able to permeate readily into the well-organized and hydrophobic alkyl chains of the monolayer assembly. The voltammetric profile of UQ 0 was improved by the modification of aminoethanethiol, as compared with those of bare gold and the electrode modified with other polar thiols. Further, two different permeation paths of the electrode species into the mixed monolayer are suggested from the variation of the i-E response with the cycle of the potential scan.

The first osmium(II), ruthenium(II) and iron(II) complexes of [3n]cyclophanes (n = 2-4): Synthesis and electrochemical study

Abstract ( η 6 -C 6 H 6 )( η 6 -[3 n ]Cyclophane)Ru(II) [BF 4 ] 2 and corresponding Os(II) [PF 6 ] 2 , as well as bis( η 5 -C 5 H 5 )( η 6 , η 6 -[3 n ]cyclophane)Fe(II)Fe(II) [PF 6 ] 2 ([3 n ]cyclophane=[3 2 ](1,4)cyclophane 2 , [3 3 ](1,3,5)cyclophane 3 , [3 4 ](1,2,3,5)cyclophane 4 , [3 4 ](1,2,4,5)cyclophane 5 ) have been synthesized and characterized. The complexation shifts of the 1 H-NMR signals of the metal-bound aromatic protons (H b ) are ca. 0.5–0.7 and 0.1–0.4 ppm for Fe(II) and Ru(II) complexes, respectively, whereas those of Os(II) complexes are ca. −0.2–0.1 ppm. The complexation shifts of the 13 C-NMR signals of the tertiary aromatic carbons of the metal-bound benzene ring are ca. 39–42 and 45–50 ppm for Ru(II) and Os(II) complexes, respectively. Thus the 1 H- and 13 C-NMR chemical shifts of the metal-bound aromatic hydrogens and carbons are strongly influenced by the anisotropy effect of the metal. The Ru(II) complexes showed electrochemically reversible responses. In the case of Os(II) complexes, a well-defined cathodic peak was also observed, but the rising portion of the corresponding anodic peak was somewhat deviated from the ordinary CV profile. In both cases, the redox process was attributed to the two-electron one-step mechanism, M(II)⇌M(0) (M=Ru and Os). An analysis of the redox properties of the Ru(II) and Os(II) complexes suggested that the Os(II)[3 4 ](1,2,4,5)cyclophane complex would be the most suitable subunit of an anticipated one-dimensional organometallic polymer.

Synthesis and characterisation of the six-membered cyclopalladated complexes of 2-benzylbenzothiazole

Abstract The six-membered cyclopalladated complex di- μ -acetato-bis[2-(2′-thiazolylmethyl)phenyl- C 1 , N ]dipalladium(II), [{Pd( μ -O 2 CMe) ( η 2 -bbt)} 2 ] ( η 2 -bbt = 2-(2′-thiazolylmethyl) phenyl- C 1 , N ), has been prepared by the reaction of palladium(II) acetate and 2-benzylbenzothiazole. Its di-μ-halogeno analogues [{Pd( μ -K) ( η 2 -bbt)} 2 ], synthesised by the metathetical reaction with LiCL or NaX, undergo bridge-splitting reactions with some pyridine derivatives and thallium(I) acetylacetonate to yield corresponding mononuclear cyclopalladated complexes. The cyclopalladated feature and the six-membered structure have been confirmed by the 1 H NMR spectra of the mononuclear pyridine complexes. Temperature-dependent 1 H NMR spectra have been observed for [{Pd( μ -O 2 CMe) ( η 2 -bbt)} 2 ] and [Pd(acac) ( η 2 -bbt)] and are attributed to inversions of acetato bridges and the boat-formed six-membered ring, respectively.

ADSORPTION OF LANTHANIDE(III) IONS FROM AQUEOUS SOLUTIONS BY SELF-ASSEMBLED MONOLAYERS OF GLUTATHIONE ON GOLD

ABSTRACT The adsorption of lanthanide(III) ions by the glutathione-(α-l-glutamyl-l-cysteinyl-glycine, GSH) monolayer on gold substrate (GSH/Au) is described and demonstrated for the recovery of lanthanide(III). This monolayer has been applied to the adsorption of lanthanide(III) ions in aqueous solutions as an adsorbent. The pH dependencies of the adsorption for lanthanide(III) ions have been examined to clarify the cation-exchange behavior of the adsorbent. Adsorption of lanthanide(III) ions was found to depend on the pH of the solution. The adsorption recovery for the 14 lanthanide(III) ions was investigated.

Electrochemical behaviour of the ubiquinone-Q10 film coated onto a glassy carbon electrode by the spinner method

Abstract The charge transfer mechanism of ubiquinone-Q10 (UQ10, coenzyme-Q10) coated onto a glassy carbon electrode (UQ-GCE) by the spin coating method was investigated in aqueous electrolyte solution. Two different charge transfer processes controlling the redox reaction of UQ10 at different sites of the UQ10 film are proposed. One process is assigned to the inter- and/or intra-molecular proton transfer process occurring at monolayer sites of the UQ10 film. The other process is assigned to the electron transfer process between adjacent UQ10 molecules taking place at the inner layer of multilayer sites of the film. The apparent diffusion coefficients of these charge transport species in monolayer and multilayer film are calculated to be 9.7 × 10−14 and 2.7 × 10−11 cm2/s, respectively.