An electrokinetic-combined electrochemical study of the glucose electro-oxidation reaction: effect of gold surface energy

Abstract

The glucose electro-oxidation reaction typically involves several steps and it is strongly influenced by the crystalline structure. In this paper, gold with typical {111} defects (namely Au_{111}) and gold with defects enclosed in the (200) plane (Au_{200}) were used to determine the effect of the surface energy in the adsorption and electro-oxidation of D-(+)-glucose. To this end, an electrokinetic analysis of surface species was made by means of zeta potential (ζ) measurements and was correlated with an electrochemical study. At low glucose concentration (0.1 mM), the system Au_{200} showed a positive and large ζ value of 261.26 mV related to protons from the glucose dehydrogenation. Au_{111} presented a negative ζ value of −98.11 mV associated to the glucose chemisorption plus OH⁻ adsorption from the electrolyte. At a higher concentration (>20 mM) both systems exhibited positive ζ values (from 40 to 60 mV) related to the glucose dehydrogenation because of saturation of the electrical double layer by glucose molecules. Through cyclic voltammetry, it was observed that at low glucose concentration (<20 mM), both materials had preference for oxidation of glucose by-products. However, at higher concentrations, Au_{111} favors glucono-lactone oxidation (0.4 V vs. NHE); meanwhile Au_{200} favors glucose oxidation (−0.43 V vs. NHE). Through the electrokinetic analysis, the behavior of Au_{111} can be related to its affinity toward the chemisorption of glucose molecules, and that of Au_{200} to weak glucose chemisorption, which allows the desorption of glucose by-products renewing the gold surface for the further oxidation of glucose molecules.