Optimization of EPS-Mediated Biosynthesis Conditions of CdS QDs for Sensitive Fluorescent Detection of Cu2+ in Water

Owing to their excellent fluorescence and other advantageous properties, quantum dots (QDs) are promising candidates for environmental monitoring. Although we synthesized previously CdS QDs using extracellular polymeric substances (EPS) produced by sulfate-reducing bacteria ( Desulfovibrio desulfuricans subsp. desulfuricans ) as biological template, systematic optimization of the synthesis conditions and in-depth investigation of their fluorescence properties remain limited. In the present study, based on an EPS-templated strategy, key synthesis parameters (such as pH, EPS concentration, and precursor ratios) were systematically optimized, enabling the successful extracellular preparation of bright cyan-green fluorescent CdS QDs with an emission maximum at 520 nm and a quantum yield of 11%. Comprehensive characterization confirmed their size (∼4.8 nm), good crystallinity (lattice spacing of 0.209 nm for the (220) plane), and successful formation of Cd-S bonds. FT-IR spectroscopy further indicated that the surface of the QDs was rich in functional groups such as carboxyl, hydroxyl, amino, and thiol groups, which significantly enhanced the colloidal stability of the CdS QDs in solution. The probe exhibited a response time of 5 min, high sensitivity (detection limit of 48 nM), a wide linear range (0–60 μM, R 2 = 0.992), and excellent selectivity for Cu 2+ . Time-resolved fluorescence studies suggested a dynamic quenching mechanism. Satisfactory recovery rates (99.52-110.11%) in real water samples (river and tap water) validated its practical applicability. This work presents a novel EPS-templated biosynthesis route for CdS QDs and establishes an efficient sensing platform, offering a promising strategy for rapid, low-cost heavy metal detection in environmental waters.