Extrinsic Antifouling Sulfur Quantum Dots: Mechanistic Insights and Their Application in Tyrosinase Detection

Sulfur quantum dots (SQDs), as an emerging member of the fluorescent quantum dot family, hold great promise for application in biological analysis and environmental health. However, their intrinsic resistance to fouling in complex matrix pollution is rarely reported. Herein, we report the remarkable antifouling properties of polyethylene glycol (PEG)-modified SQDs, which exhibit 100% fluorescence retention under harsh conditions, including high salinity (2 M), high temperature (90 °C), and high protein concentration (150 mg/mL). Through fluorescence quenching experiments and density functional theory (DFT) calculations, we elucidate the mechanistic origin of the differential antifouling behavior between PEG-SQDs and carboxymethyl cellulose (CMC)-modified SQDs (CMC-SQDs). DFT calculations reveal that CMC strongly polarizes H₂O (d_O-H = 1.026 Å), making it more prone to protein adsorption. In contrast, PEG induces substantially weaker polarization of H₂O (d_O-H = 0.980 Å), effectively inhibiting the adsorption of various hydrophilic biomacromolecules. Leveraging the excellent antifouling performance of PEG-SQDs, we developed a strategy for detecting tyrosinase (TYR) activity. This strategy is based on TYR-catalyzed oxidation of l-tyrosine to melanin-like products, which efficiently quench the fluorescence of PEG-SQDs. Ultimately, a highly sensitive, selective, simple, and environmentally friendly method for TYR detection has been constructed. This method exhibits a linear response range for TYR from 0.5 to 20 U/mL, with a detection limit of 0.08 U/mL (S/N = 3). Furthermore, it enables accurate quantification of TYR activity in human serum samples, underscoring the excellent antifouling capability of PEG-SQDs in complex biological matrices. Overall, this work not only provides a novel tool for the sensitive detection of TYR but also offers new insights into the antifouling mechanism of SQDs.