Nitrogen and sulfur co-doped graphene quantum dot-modified electrode for monitoring of multivitamins in energy drinks

A glassy carbon electrode was modified with nitrogen and sulfur co-doped graphene quantum dots immobilized in chitosan for the monitoring of multivitamins. The graphene quantum dots were synthesized using a simple citric acid/l-cysteine pyrolysis procedure. The co-doping with nitrogen and sulfur in the graphene matrix was confirmed by spectroscopic techniques. Electron microscopy results showed that the synthesized quantum dots had a diameter of 3.4 ± 1.4 nm. Electrochemical techniques showed excellent current responses to vitamin oxidation provided by the modified electrode compared to the bare electrode. The parameters of square wave voltammetry were optimized in order to obtain the best current responses and to study the electrochemical oxidation of vitamins. The calibration plots for vitamins B₂, B₆ and B₁₂ were constructed in 0.1 mol L^-1 sodium acetate buffer (pH 5.0) with limits of detection of 0.30, 30.1 and 0.32 nmol L^-1, respectively. Lastly, the modified electrode was effectively implemented in the quantification of vitamins in classic and fruit-based energy drink samples.