Takashi Kakiuchi

Reductive desorption of carboxylic-acid-terminated alkanethiol monolayers from Au(111) surfaces

Abstract The peak potential E p of the reductive desorption for COOH-terminated alkanethiols at the gold(111)|0.1 M KOH solution interface has been studied at several different alkyl chain lengths and compared with those for n -alkanethiols. The desorption of COOH-terminated alkanethiols is also confirmed by the increase in the capacitance of gold|solution interface and the reflectivity of gold electrode surface. The E p for both CH 3 - and COOH-terminated alkanethiols shift by 15 mV to the negative direction per methylene group as the alkyl chain length n increases. The E p vs. n plots for the two series of alkanethiols are parallel with each other, suggesting that the chemical interaction of thiol head group with gold, the chain-chain attractive interaction, and the electrostatic repulsion between the carboxylate groups make an additive contribution to the determination of E p . The E p values for COOH-terminated alkanethiols are 110 mV more positive than those for n -alkanethiols having the same alkyl chains, owing in part to the repulsive interaction between the negatively-charged carboxylate groups in a COOH-terminated alkanethiol monolayer. The positive shift of E p for mercaptopropionic acid with decreasing concentration of KOH solution also suggests the primary role of the electrostatic repulsive interaction between carboxylate groups in the shift of E p .

Phase separation of binary self-assembled thiol monolayers composed of 1-hexadecanethiol and 3-mercaptopropionic acid on Au(111) studied by scanning tunneling microscopy and cyclic voltammetry

Abstract The mixing characteristics of binary self-assembled monolayers composed of 1-hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) on Au(111) have been studied by cyclic voltammetry and scanning tunneling microscopy. Two distinctive peaks, ∼0.45V apart, are observed over the entire range of surface composition on cyclic voltammograms for the reductive desorption of the adsorbed thiol molecules, which reflects the presence of two different types of phase-separated domains greater than several tens of nm 2 which can be imaged by scanning tunneling microscopy. The peak potential of 1-hexadecanethiol is nearly independent of the surface composition whereas a slight shift of the peak potential is observed in the case of MPA, suggesting that the HDT is slightly soluble in the MPA domains, while MPA is insoluble in the HDT domains. The minimum number of the adsorbed thiol molecules required for exhibiting two distinctive peaks (i.e. two-dimensional bulk properties) is estimated to be ca. 50 by comparing the cyclic voltammograms with the distribution of the domain size observed by scanning tunneling microscopy.

Long-range electron-transfer reaction rates to cytochromec across long- and short-chain alkanethiol self-assembledmonolayers: Electroreflectancestudies

The kinetics of electron transfer (ET) between cytochrome c and a gold (111) electrode through self-assembled monolayers of alkanethiols with terminal carboxylic acid groups, COOH(CH 2 ) n SH, have been studied for n=2–11 using an ac potential-modulated UV–VIS reflectance spectroscopic technique (electroreflectance spectroscopy, ER). For 9⩽n⩽11, the standard ET rate constant, k app , depends exponentially on the chain lengths and the exponential decay factor is 1.09 per methylene group; for n<9, however, k app deviates from the exponential plot. The ET reaction through short-chain alkanethiol monolayers is controlled by the preceding chemical reaction. The rate-controlling step is very likely to be the reorganization of cytochrome c to the favourable conformation for the ET reaction. The ET reaction rate constant from cytochrome c in the favourable conformation to the electrode surface obeys Marcus theory for long-range ET. The ET reaction through long-chain alkanethiol monolayers is controlled by the ET rate through alkanethiols.

Investigation of the electrode reaction of cytochrome c through mixed self-assembled monolayers of alkanethiols on gold(111) surfaces☆

Horse heart cytochrome c was immobilized on mixed self-assembled monolayers (SAMs) of carboxyl- and methyl-terminated alkanethiolates (HOOC(CH2)10S + CH3(CH2)9S) on Au(111) electrodes. The apparent standard rate constant kobs of electron transfer for the electrode system of cytochrome c⧹mixed SAM⧹gold electrode was measured by a.c. impedance and a.c. modulated UV-visible electro reflectance techniques. A strong dependence of kobs on the composition of the mixed alkanethiol solution, from which the monolayers had been assembled, was observed. A maximum of kobs ≈ 200s−1 was found for SAMs formed from solutions containing a mole fraction χ ≈ 0.8 of carboxyl-terminated alkanethiols. This rate constant is about six times faster than that of a system with a single-component monolayer of HOOC(CH2)10S (ξ = 1). Reductive desorption suggested that the two alkane thiolates mix homogeneously in the monolayer and there is no sizable phase separation on the Au(111) electrode. Double-layer capacitance measurements determined the degree of protonation of the mixed SAMs in the pH range 6–9. The enhancement of electron-transfer rate is attributed to a favorable distribution of protein binding sites in the mixed monolayers.

Electroreflectance spectroscopic study of the electron transfer rate of cytochrome c electrostatically immobilized on the ω-carboxyl alkanethiol monolayer modified gold electrode

Abstract Horse heart cytochrome c was immobilized electrostatically on self-assembled monolayers (SAMs) of carboxylic-acid-terminated alkanethiols, HS(CH 2 ) n COOH ( n = 2, 10) on gold electrodes, The rate constants of the electrode reaction of cytochrome c at the SAM-modified gold electrodes were determined by the frequency dependence of a.c. modulated UV-visible electroreflectance signals. The apparent standard electrode reaction rate constants of cytochrome c were 880 s −1 and 72 s −1 at the HS(CH 2 ) 2 COOH and HS(CH 2 ) 10 COOH SAM modified gold electrodes respectively. The rate constant at HS(CH 2 ) 2 COOH was much smaller than expected from Marcus theory. However, the rate constant at HS(CH 2 ) 10 COOH could be explained in terms of the through-bond tunnelling mechanism with tunnelling parameter β = 8.2 nm −1 .

A theory of voltammetry of ion transfer across a liquid membrane in the absence of supporting electrolytes using the Nernst-Planck equation and electroneutrality assumption

A theory of cyclic voltammetry of ion transfer across a liquid membrane has been presented based on the Nernst–Planck equation and the electroneutrality assumption. The initial conditions are given by the partition equilibrium of ions between the membrane and the two bathing solutions. Current–potential curves are calculated for the case of reversible transfer of Na+ across the membrane/solution boundary and the complete dissociation of electrolytes in the membrane, taking account of time-dependent solution resistance and the diffusion potential. The peaks appear only in the limited range of the scan rate at a given thickness of the membrane. The model explains wide peak separation which has been reported in the voltammetry of ion transfer in the presence of lipophilic ions. Upon imposing the voltage across the membrane, the phase-boundary potential at each side of the membrane varies with time and, hence, the ion partitioning at the membrane/bathing solution interface is a time-dependent process.

Effect of the viscosity of the aqueous phase on the rate of ion transfer across the nitrobenzenevbwater interface

Abstract The effect of the solution viscosity on the rate of tetraethylammonium ion (TEA + ) transfer across the nitrobenzenevbwater interface has been studied by adding sucrose in the aqueous phase or replacing the aqueous phase with a corresponding heavy-water solution. The addition of sucrose into the aqueous phase up to 40 wt.% does not lower the value of the apparent standard rate constant of TEA + ion transfer, which is measured with phase-selective a.c. polarography. The substitution of H 2 O with D 2 O also introduces no significant deceleration of the rate of TEA + ion transfer. The observed insensitiveness of the rate of ion transfer to the change in bulk viscosity suggests that the thickness of the interfacial layer determining the rate of ion transfer is much less than the order of microns. The interfacial viscosity is not likely to be influenced by the increase in the viscosity of the bulk phase. No sign of the specific adsorption of sucrose is discernible in the differential capacitance.

Determination of the electrode kinetic parameters of a species immobilized on electrodes using the electroreflectance (ER) voltammogram

Abstract Ac-modulated UV-vis reflectance spectroscopy was applied to elucidate the electrode reaction rate of the species immobilized on electrode surfaces. The electroreflectance (ER) with respect to the electrode potential at constant wavelength, denoted as ER voltammetry, was analyzed theoretically using numerical calculation. In the present calculation, the amplitude of the ac modulation, the IR drop due to the solution resistance, and the double layer capacitance were taken into account. The results of the calculation were compared to the electrode reaction of cytochrome c immobilized on an SH(CH 2 ) 9 COOH self-assembled monolayer modified gold electrode, which has previously been studied. The standard rate constant of the electrode reaction of cytochrome c 3 adsorbed directly on gold electrode was determined to be 200 s −1 using non-linear least square fitting to the experimental data. This approach is applicable to unstable redox species at the electrode surface, e.g. cytochrome c 3 , which desorbs gradually from the gold electrode surface at negative electrode potential. That is, one can determine the kinetic parameters of adsorbed redox species by a single potential scan.

Probability theory of desorption kinetics of self-assembled alkanethiols stabilized with pair interaction

Abstract An electrode kinetic theory is presented for the reductive desorption of self-assembled monolayers of alkanethiols adsorbed on a Au(111) surface by taking account of the probability of pair interaction between closest neighboring alkyl chains. The model of the desorption is a honeycomb-arrangement unit of adsorption sites, the center of which is occupied by the alkanethiol to be desorbed. The six surrounding sites are occupied or not by alkyl chains, depending on the pair interaction energy. The probability of de-stabilizing the alkyl chains on the six sites is calculated for all the possible arrangements. On the assumption that the desorption kinetics have an exponential dependence on the potential, a non-linear kinetic equation is derived, from which voltammograms are obtained in terms of the interaction energy and two kinetic parameters. The peak potential varies linearly with the logarithm of the potential sweep rate. With a negative increase in the interaction energy, the desorption wave shifts linearly in the negative direction and it becomes narrower. Voltammograms for alkanethiols with several chain lengths were analyzed on the basis of these variations. The transfer coefficient is determined from the variation of the peak potential with the sweep rate. The pair interaction energy is evaluated from the variation of the peak potential with the chain length.

Free energy coupling of electron transfer and ion transfer in two-immiscible fluid systems

Abstract Free energy coupling of electron and ion transfers across a liquid-liquid interface is treated theoretically assuming electroneutrality of each phase and complete dissociation of electrolytes. A general method is presented for calculating the equilibrium inner potential difference between the two phases and equilibrium concentrations of redox species, as well as other ionic components from given initial concentrations, values of standard electrode potential of redox couples, and of standard ion transfer potential of other ionic components. The ion-electron exchange across the interface is exemplified for several simpler cases. The volume ratio has a dramatic effect on the equilibrium. The partition of indifferent-electrolyte ions can drive the redox reactions before the onset of electrolysis and can significantly alter the initial conditions for electron transfer studies at the liquid-liquid interface.