Abstract: The electrochemical quartz crystal microbalance (EQCM) technique was used to investigate the electrodeposition of the charge-transfer complex (CTC) generated during electrooxidation of o-tolidine (o-TD) in Britton-Robinson (B-R) buffers and effects of coexisting chondroitin sulfate (CS). A V-shaped frequency response to the cyclic voltammetric switching of o-TD indicated the precipitation and dissolution of the poorly soluble CTC, an oxidation intermediate, formed at the Au electrode during the redox switching of o-TD in a neutral or a weakly acidic medium (pH=4.07-6.50). The effects of potential scan rate, solution pH, and several supporting electrolytes were examined. The depth of the V-shaped frequency curves (-⊿f0V) was related to the supporting electrolyte used, with a decreasing sequence for -⊿f0V as 0.20 mol·L-1 NaNO3>0.20 mol·L-1 NaClO4>0.10 mol·L-1 Na2SO4. The -⊿f0V response to the redox switching of the CTC/o-TD“couple”was enhanced by the introduction of CS because of the formation of the CTC-CS adduct, as also characterized and supported by UV-Vis and FTIR spectrophotometry. The molar ratio (x) of the CTCto CSin the adduct and the electrode-collection efficiency of the CTC (η) were estimated using EQCM. The values of -⊿f0V increased with the increase in CS concentration, with a linear range from 0.75 to 15.2 μmol·L-1, and a detection limit down to 50 nmol·L-1. The new method proposed for CS assay was characterized by a dynamically renewed surface of the detection electrode.

Key words: Electrochemical quartz crystal microbalance, Charge-transfer complex, o-Tolidine, Assay of chondroitin sulfate, Dynamically renewed surface of the electrode