Electrochemical Behaviour of Some Chlorogenic Acids and Their Characterization in Coffee by Square-Wave Voltammetry

The electrochemical behaviour of six chlorogenic acids (CGAs) isomers: three caffeoylquinic acids, CQAs (5-CQA, 4-CQA, 3-CQA) and three dicaffeoylquinic acids, diCQAs (3,4-diCQA, 3,5-diCQA, 4,5-CQA) was studied by SWV and compared with the electrochemical behaviour of caffeic acid (CFA), precursor of investigated CGAs. The study reveals that electrochemical behaviour of investigated CGAs strongly dependent on their chemical structure and electronic properties, particularly on electron-donating effect of -OH and -CH=CH- groups, and a strong electronwithdrawing effect of ester (-COOR) group presented in their structure. SWV measurements show that electrochemical oxidation/reduction of CGAs at GCE is reversible, pH-dependent, two electron-two proton process. The oxidation/reduction processes occurred on the catechol moiety (ortho-dihydroxyl groups) in the structure of CGAs molecules. The oxidation product(s) relatively strongly adsorb on the GCE surface especially at higher concentration of CGAs. The electrochemical behaviour of 5-CQA was investigate more in detail. It was observed that anodic peak current of 5-CQA show maximum in solution of pH 7, due to the maximum concentration of 5-CQA- anion at this pH value. The electrode reaction proceeded in solution of 5-CQAs is diffusion controlled process. The anodic peak current of 5-CQA shows linear relationship with the concentration in the range of 5-50 μmol L-1, with low LOD (7.7 x 10-7 mol L-1). SWV of coffee samples were also carried out, and the results show that electrochemical behaviour of coffee samples is very similar to that of investigated CGAs. Therefore, SWV was used for characterization of CGAs in coffee. It was shown that SWV is a very sensitive and selective method for determination of total CGAs content in coffee.