David J. Fermín

Voltammetry at a liquid-liquid interface supported on a metallic electrode

Abstract A liquid–liquid interface supported on a metallic electrode has been used to study ion transfer (IT) and electron transfer (ET) reactions by cyclic voltammetry. The system is composed of an aqueous droplet supported on a platinum disc electrode and immersed into an organic electrolyte solution. Depending on the nature of the dissolved species present in the aqueous solution, and in the organic electrolyte solution, different electrochemical coupled reactions can be observed. This method enables a fast and convenient method to measure standard transfer potentials for example of ionised drug molecules.

Spectroelectrochemical approaches to heterogeneous electron transfer reactions at the polarised water ∣ 1,2-dichloroethane interfaces

Abstract Heterogeneous electron transfer (ET) reactions at the polarised waterxa0∣xa01,2-dichloroethane (DCE) interface are studied by in situ UV-Visible spectroscopy in total internal reflection mode. The reduction of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the oxidation of 1,1′-dimethylferrocene (DMFc) by the hexacyanoferrate redox couple are considered. The generation of products in the organic phase is monitored spectroscopically and correlated to the simultaneous current response. Both systems exhibit a good correlation between optical and electrochemical responses, highlighting the ideal behaviour of these redox couples for electron transfer studies at liquidxa0∣xa0liquid interfaces. The kinetics of ET between hexacyanoferrate and TCNQ is analysed also by chronoabsorptometry and potential modulated reflectance spectroscopy. The potential dependence of the ET rate constant is discussed within the framework of the ET models for ITIES.

Electronic properties of Ag nanoparticle arrays. A Kelvin probe and high resolution XPS study

The electronic properties of citrate stabilised Ag nanoparticles with sizes ranging from 4 to 35 nm were investigated by the Kelvin probe method and high resolution XPS. Two and three dimensional assemblies of the particles were prepared by electrostatic adsorption from aqueous solution onto poly-l-lysine modified surfaces. The work function of the Ag particles increased from 5.29 +/- 0.05 to 5.53 +/- 0.05 eV as the particle size decreased. These values are approximately 0.8 eV higher than for clean polycrystalline Ag surfaces. The origin of these remarkable high work functions cannot be explained in terms of either citrate induced changes in the surface dipole or image forces in the confined metallic domains. High resolution XPS spectra of the Ag 3d(5/2) core level were characterised by broad bands and a 0.4 eV shift towards lower binding energies for the smallest particles. Comparisons with reported studies on extended Ag surfaces indicate that as-grown particles exhibit partially oxidised surfaces. The behaviour of the work function further suggests that the strength of the Ag-O bonding increases with decreasing particle sizes. These findings are highly relevant to the interpretation of the catalytic properties of Ag nanoparticles.

Spectroelectrochemical Study of the Copper(II) Transfer Assisted by 6,7-dimethyl- 2,3- di(2-pyridyl)quinoxaline at the Water|1,2-Dichloroethane Interface

Reference LEPA-ARTICLE-1998-012doi:10.1016/S0022-0728(98)00183-1View record in Web of Science Record created on 2005-11-07, modified on 2017-12-03

Mechanistic aspects associated with the oxidation of l-ascorbic acid at the 1,2-dichloroethane ∣ water interface

Abstract The biologically relevant oxidation of l -ascorbic acid (AH 2 ) was studied at an externally polarised 1,2-dichloroethanexa0∣xa0water interface by a combination of electrochemical and spectroscopic techniques. In the presence of an electron acceptor (chloranil) in 1,2-dichloroethane, the basic features of the voltammetric signal are dependent on the concentration ratio of the redox species. In the presence of an excess of chloranil, a reversible signal is readily observed. A similar voltammetric response is developed when the semiquinone radical anion of chloranil (Q − ) is generated by homogeneous reduction in the presence of decamethylferrocene (DMFc) in the organic phase. These results suggest that Q − is formed homogeneously rather than by heterogeneous electron transfer from AH 2 to Q under these conditions. However, in the presence of an excess of ascorbate, an irreversible voltammetric response is the dominant feature. Replacing chloroanil by tetracyanoquinodimethane also provides irreversible features indicating that heterogeneous electron transfer can also take place. The occurrence of both homogeneous and heterogeneous electron transfer pathways is also suggested by potential modulated reflectance (PMR) spectroscopy.

Potential Modulated Reflectance Spectroscopy of the Methyl Orange Transfer Across the Water/1,2-Dichloroethane Interface

The kinetics of methyl orange transfer across the water1,2-dichloroethane interface is studied by potential-modulated reflectance (PMR) and ac-impedance. The frequency dependence analysis of both techniques provide complementary information about charge transfer kinetics and the potential distribution across the interface. The PMR signal is obtained under total internal reflection from the liquidliquid interface. It is concluded that the PMR responses are originated from the ac flux of ion crossing the interface, and therefore provide a direct analysis of the faradaic responses. This technique is also less sensitive to uncompensated resistance effects in comparison to the admittance analysis. In the case of methyl orange transfer, a standard apparent rate constant of (4.590.7) 10 2 cm s 1 and a transfer coefficient of 0.409 0.03 were estimated from the frequency dependence of the PMR.

Marangoni flow in micro-channels

Abstract A novel electrochemical method for driving fluids in micro-channels is presented. The principle is based upon the onset of Marangoni flow along the interface between an aqueous solution (mobile phase) and an organic electrolyte polymer gel coated on the inner walls of the micro-channel. The gradient of surface tension responsible for the fluid motion arises from local changes in the surface charge. The excess charge is determined by the ionisation of surfactant species at the gel coating|aqueous electrolyte interface which is effectively dependent on the Galvani potential difference. Potential differences of less than a volt between two closely spaced silver band electrodes along the micro-channel can generate zones of high and low surface tension, promoting the motion of the aqueous electrolyte.

Transfer and adsorption of 1-pyrene sulfonate at the water 1,2-dichloroethane interface studied by potential modulated fluorescence spectroscopy

The transfer mechanism of 1-pyrene sulfonate anion (PSA−) across the polarized water 1,2-dichloroethane (DCE) interface was investigated by a combination of electrochemical techniques and potential modulated fluorescence (PMF) spectroscopy under total internal-reflection. The dependence of the cyclic voltammogram and the ac voltammogram on the PSA− concentration showed an apparent reversible ion transfer process. However, the PMF responses exhibited a complex dependence on the PSA− concentration, revealing adsorption and dimerization processes taking place at the interface. Analysis of the dynamic spectroelectrochemical responses suggests that PSA− is adsorbed at the interface prior to the transfer step. Upon transferring to the organic phase, PSA− appears to accumulate at the interface undergoing a dimerization reaction.

Photocurrent responses at dye sensitised ultrathin polyelectrolyte multilayers supported on gold electrodes.

The photoelectrochemical behaviour of ionic conducting ultrathin multilayers formed by sequential deposition of poly-L-lysine and poly-L-glutamic acid on modified gold electrodes is investigated upon sensitisation by zinc mesotetrakis(p-sulfonatophenyl)porphyrin.