On the adsorption and kinetics of phase transients of adenosine at the different carbon electrodes modified with a mercury layer

Abstrakt

Optical roughness of a pyrolytic graphite electrode with basal (PGEb) and edge orientation (PGEe), a glassy carbon electrode (GCE), and the same electrodes modified with mercury (MFE) was studied by an optical diffractive element (DOE) based sensor. Electrochemical characterisation of these electrodes used was performed by cyclic voltammetry (CV) and capacitance measurements (C-E curves and electrochemical impedance spectroscopy, EIS). The kinetics of phase transients of adenosine adsorbed on PGEb, and GCE modified with mercury layer of different thicknesses (thickness was changed from 0.02 to 2 mum) was studied by chronoamperometry (j-t curves) and capacitance measurements (C-E curves). In acidic (pH 5) solution adenosine forms two different two-dimensional (2D) physisorbed condensed layers on the MFE. The first of these (region 1) is located at more positive potential; the centre of this adlayer is situated around -0.4 V. The second 2D physisorbed film (region 111) is formed at more negative potentials; the centre of the region III is around -1.3 V. The 2D condensed films (adlayer I and III) of adenosine still exist on the PGEb substrate modified by 0.02 mum thick mercury layer. Adlayers III and I of adenosine exist on the GCE modified with mercury layer down to 0.02 and 0.2 mum, respectively. The kinetics of phase transients of the adenosine films taking place by a potential jump from dilute adsorption region (state II and IV) to the 2D physisorbed film (region I and III) at the PGEb and GCE substrates modified with mercury (Hg-PGEb and Hg-GCE) were studied, respectively. During both of the phase transients of IV -->III and II -->III of adenosine a polynucleation and growth process on both the Hg-PGEb and Hg-GCE was detected. The phase transients of II -->I are characterised by an exponential decay of the current without the current maximum (the adsorption process took place only). It was observed that the phase transients of IV -->III are the fastest at the HMDE and gradually slow down on 2 mum Hg-GCE and 2 mum Hg-PGEb, respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.