Kinetic evaluation of the standard rate constant from cyclic voltammetric data at SWCNT-modified tungsten microelectrodes

The electrochemical properties and the kinetic evaluation at Single-Walled Carbon Nanotubes (SWCNTs) -modified tungsten (W) microelectrodes have been studied and compared with those of Glassy Carbon (GC) bare electrodes. Field Emission Scanning Electron Microscopy (FEG-SEM) and Raman spectroscopy have been used to characterize the modified-electrode morphology and microstructure, respectively. Cyclic voltammetry (CV) has been used to study the electrochemical performances, with seven different redox systems serving as probes (Fe(CN) 63-/4-; Ru(NH3) 63+/2+, Ir(Cl)62-/3-, catechol, dopamine, ferrocene monocarboxylic acid and caffeic acid). The analytical response for different systems is, highly reproducible for each type of tungsten microelectrode modified with SWCNT coating by Chemical Vapor Deposition method (CVD). For all seven redox systems, the forward reaction peak current varies linearly with the square root of scan rate (v)1/2, indicative of electrode reaction kinetics controlled by mass transport (semi-infinite linear diffusion) of the reactant. Apparent heterogeneous electron-transfer rate constants, Kºapp for all seven redox systems have been determined from ∆Ep-v experimental data, according to the method described by Nicholson. Kºapp values of 1.02-1.17 cm/s have been observed for Ru(NH3) 63+/2+, Ir(Cl)62-/3-, and Fe(CN) 63-/4- without any extensive electrode pretreatment (e.g., polishing). Lower Kºapp values of 10-6-10-2 crn/s have been found for catechol, dopamine, ferrocene monocarboxylic acid, and caffeic acid. The voltammetric responses for Ru(NH3) 63+/2+, Ir(Cl)62-/3-, and Fe(CN) 63-/4- have also been examined at SWCNT modified W electrodes in different solutions pH (1.1 -10.0), and the corresponding ∆Ep, ipox, ipred and Kºapp values for the most part, have been unaffected by the solution pH. This is probably related to the absence of oxygen-functionalities at SWCNT-modified W microelectrodes, which is in contrast to the typical behaviour of the oxygenated, Sp2 carbon electrodes, such as glassy carbon or graphite.