Voltammetric studies of the redox mediator, cobalt phthalocyanine, with regard to its claimed electrocatalytic properties

It is widely reported in the literature that cobalt phthalocyanine (CoPC) facilitates a wide variety of redox processes. This conclusion is often made without any comparison of analytical parameters (such as sensitivity and detection limit) for the reaction at both CoPC modified and a bare unmodified electrodes. Another important issue is a lack of full and proper study of the electrochemical properties of dissolved and adsorbed CoPC before adding an analyte to the solution. It is also generally reported, without presenting convincing argument, that the nature of active form of CoPC at the surface of carbon electrodes is always a monolayer. We report a thorough electrochemical study on the solution phase and solid phase cobalt phthalocyanine with regard to its claimed electrocatalytic properties towards oxidation of nitrite. The cyclic voltammetry of cobalt phthalocyanine has been examined in dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) at a glassy carbon (GC) electrode. In both solvents three redox couples are present: one ring based and two cobalt based processes. The aqueous voltammetry of CoPC adsorbed on basal plane pyrolytic graphite (BPPG) and edge plane pyrolytic graphite (EPPG) was also studied in phosphate buffer solution. Two couples are present: Co(II/I) and Co(III/II). Cyclic voltammetry experiments suggest that the number of electroactive adsorbed CoPC molecules on EPPG is approximately 3.5 times higher than on BPPG. The surface properties of both electrodes modified by immersing in 0.1 mM CoPC in dimethylformamide (DMF) solution were characterized by SEM and energy dispersive X-ray spectroscopy (EDS). It was observed that CoPC adsorbed on EPPG and BPPG electrodes exists as microcrystals, not as a monolayer reported in the literature for other carbon electrodes. The use of EPPG-CoPC modified electrode for sensing nitrite (NO2-) was also investigated. It was found that CoPC on EPPG has no influence on the oxidation of nitrite. © 2010 Elsevier B.V. All rights reserved

Electrochemical determination of nitrite at a bare glassy carbon electrode; why chemically modify electrodes?

The oxidation of nitrite was studied at a bare glassy carbon (GC) electrode in aqueous solution using cyclic voltammetry, square wave voltammetry and chronoamperometry. A mechanism for the electrode reaction is proposed. A limit of detection (LOD) of 4 × 1 0- 7 M was obtained for amperometry and this is evaluated with reference to literature reports for NO2- detection; in particular, the possible merits of using chemically modified electrodes as compared to 'bare' unmodified electrodes are critically assessed. © 2009 Elsevier B.V. All rights reserved

Application of Marcus-Hush-Chidsey Theory to Two Electron Systems. The Influence of Electrode Material on the Electro-Reduction Kinetics of Covalently Attached 2-Anthraquinonyl Groups: Gold Vs. Carbon

A gold electrode was derivatized by electrochemical reduction of anthraquinone-2-diazonium tetrafluoroborate (AQ2-N 2+BF 4-) giving an Au-AQ2 modified electrode of a surface coverage below a monolayer. Simulations of cyclic voltammograms using Marcus-Hush-Chidsey theory for 2e - process and assuming a uniform surface were not able to achieve a good fit for the overall shape of the baseline subtracted experimental voltammograms. Subsequently two models of surface inhomogeneity based on Marcus-Hush-Chidsey theory were investigated: a distribution of formal potentials, E o′, and a distribution of electron tunneling distances, r 0. The simulation of cyclic voltammograms involving the distribution of formal potentials allowed a good agreement between theory and experiment. The parameters of the Au-AQ2/Au-AQ2 2- process compared to the parameters of the EPPG-AQ2/EPPG-AQ2 2- process reported previously by Kozub et al. (ChemPhysChem, 12 (2011) 2806) show much faster kinetics, attributed to the higher density of electronic states for a gold electrode. © 2011 by ESG

Voltammetric Responses of Surface-Bound and Solution-Phase Anthraquinone Moieties in the Presence of Unbuffered Aqueous Media

The voltammetry of solution-phase redox species involving proton transfer are known to be qualitatively altered under conditions of nonbuffered media (Quan, M. et al. J. Am. Chem. Soc. 2007, 129, 12847-12856). We now report first the voltammetric response of solution-phase anthraquinone monosulphonate on a gold macroelectrode in the presence of a limited concentrations of protons; further we provide quantitative analysis of the voltammetry under unbuffered conditions whereby it is possible to demonstrate through simulation that in some conditions the pH at the electrode may alter by up to 5-6 pH units as compared to that of the bulk solution. This change in local environment adjacent to the electrode is caused by the consumption of protons during the electrochemical process. As a result, in conditions of low buffering the electrochemical reduction is limited by the availability of protons, leading to a measured voltammetric signal with two voltammetric waves. Second the work is developed through the study of an anthraquinone modified pyrolytic graphite electrode under conditions of finite proton concentration. We demonstrate experimentally how analogous split wave results occur for surface confined species. These results provide physical insight into the consumption of protons during the electrochemical process and highlight how methods of pH measurement based upon the use of redox modified electrode surfaces are nonpassive. © 2010 American Chemical Society

Edge plane pyrolytic graphite electrode covalently modified with 2-anthraquinonyl groups: theory and experiment.

An edge plane pyrolitic graphite (EPPG) electrode was modified by electrochemical reduction of anthraquinone-2-diazonium tetrafluoroborate (AQ2-N(2)(+)BF(4)(-)), giving an EPPG-AQ2-modified electrode of a surface coverage below a monolayer. Cyclic voltammograms simulated using Marcus-Hush theory for 2e(-) process assuming a uniform surface gave unrealistically low values of reorganisation energies, λ, for both electron transfer steps. Subsequently, two models of surface inhomogeneity based on Marcus-Hush theory were investigated: a distribution of formal potentials, E', and a distribution of electron tunneling distances, r(0). The simulation of cyclic voltammograms involving the distribution of formal potentials showed a better fit than the simulation with the distribution of tunneling distances. Importantly the reorganization energies used for the simulation of E' distribution were similar to the literature values for adsorbed species