Cysteine (CSH) readily stabilizes cadmium sulfide quantum dots (CdS QDs) that grow in aqueous buffered solutions. The oxidation of CSH by hydrogen peroxide (H2O2) at room temperature yields cystine (CSSC), which is less efficient in stabilizing CdS QDs compared to CSH. Herein, we demonstrate that such oxidation causes a decrease in the formation rate of CSH-capped CdS QDs from Cd2+ and S2− ions. For the first time, we combined the oxidation of CSH with the glucose oxidase (GOx)-assisted biocatalytic oxidation of D-glucose, which leads to a buildup of H2O2 in the reaction mixture. The enzymatically modulated in situ growth of CdS QDs was monitored using two techniques: fluorescence spectroscopy and photoelectrochemical (PEC) analysis. This system enables quantification of GOx and glucose in human serum.