No abstract

No abstract

Impedance measurements for the reduction of Tl+ on gramicidin-modified dioleoylphosphatidylcholine-coated mercury electrodes have been performed. The frequency dependence of the admittance data fits well to a Randles circuit, and the Warburg coefficient, σ, and the irreversibility coefficient, p‘, can be obtained at every dc potential from the frequency analysis conforming to this circuit. However, the potential dependence of the Warburg coefficient is different from the one expected for a simple electron transfer. Instead, the σ−E data can be analyzed conforming to a mechanism including preceding and following homogeneous chemical steps to the electron transfer (CEC mechanism). In addition, from the irreversibility coefficient, p‘, a value of the standard rate constant for the electron transfer of 0.035 cm s-1 and a potential independent value for the transfer coefficient, α, of close to 0.5 are obtained. The possibility that the CEC mechanism originates partly from nonlinear diffusion is considered, and the results are discussed in comparison with those given in the literature for Tl+ reduction on pure mercury.

The effect of the electrode potential on the orientation and conformation of the 1,2-dipalmitoyl-sn-glycero-3cytidine monolayer deposited on a gold (111) electrode surface was described. The potential of zero free charge (E pzc ) for the monolayer-covered electrode was determined to be −0.2 V vs SCE. The differential capacitance and charge density data indicated that the monolayer is stable at the electrode surface when (E − E pzc ) > 0.0 V. At negative rational potentials, a progressive detachment (electrodewetting) of the monolayer occurs. The monolayer is fully detached from the electrode surface at (E − E pzc ) < −0.6 V. The conformation and orientation of the acyl chains and the orientation of the cytosine moiety were determined with the help of photon polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The IR measurements demonstrate that the acyl chains are predominantly in the gel phase in the adsorbed state and tilted at an angle of ∼30°with respect to the electrode surface normal. The tilt angle of the acyl chains increases when the film is detached from the gold surface, indicating that the monolayer becomes more disordered. At (E − E pzc ) > 0.0 V, the plane of the cytosine moiety assumes a small angle of ∼20°with respect to the surface. At negative potentials, the tilt angle of the cytosine fragment increases and rotates. With the help of DFT calculations, these changes were explained by the repulsion of the positive pole of the cytosine permanent dipole moment by the positively charged gold surface and its attraction to the metal surface at negative electrode potentials. This work provides unique information for the future development of sensors based on the molecular recognition of nucleoside targets.

Analytical theory is developed for heterogeneous ECE reactions occurring at channel electrodes. Simple expressions are presented that allow the ready mechanistic interpretation of experimental data and the deduction of corresponding rate constants. These are shown to be in excellent agreement with numerical simulations and consistent with experiments conducted on the reduction of nitromethane in buffered aqueous solution at Hg/Cu electrodes deposited on a platinum substrate thought to proceed via a heterogeneous ECEEE mechanism. The experimental resolution between heterogeneous and homogeneous ECE processes in general is discussed.