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 S^2- 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.