Individual and simultaneous voltammetric determination of ascorbic acid, uric acid and folic acid by using a glassy carbon electrode modified with gold nanoparticles linked to bentonite via cysteine groups

A voltammetric sensor for both the individual and the simultaneous determination of ascorbic acid (AA), uric acid (UA) and folic acid (FA) is described. It is based on a glassy carbon electrode (GCE) that was modified with bentonite (Bnt) that was first functionalized with cysteine (Cys) to which gold nanoparticles were linked. The resulting material (referred to as Au-Cys-Bnt) and the other materials were characterized by UV-vis spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray analysis and electrochemical methods. The XRD peak positions of bentonite and Cys-functionalized bentonite prove the incorporation of Cys into bentonite. The XPS spectrum of Au-Cys-Bnt confirms the interaction of gold nanoparticles with the thiol group of Cys. The modified GCE displays high electrocatalytic activity for the oxidation of AA, UA and FA, typically at 0.19, 0.41, and 0.73 V (vs. SCE), respectively. Differential pulse voltammetric data show a linear response that covers the 1 mu M to 25 mM concentration range for AA, the 1 to 200 mu M concentration range for UA, and two linear ranges for FA, one from 5 to 100 mu M and one from 100 mu M to 1.5 mM. The sensor was applied to the determination of AA, UA and FA in (spiked) multi-vitamin syrup, bird serum and milk samples.