Surface Chemistry Matters: Carboxylated Graphene Quantum Dots Enable Selective Aptasensing of Lead Ions

Surface chemistry plays a pivotal role in the selection and identification of materials used in biosensor development. While previous studies have highlighted the significant influence of oxygen-containing groups (OCGs) on graphene oxide surfaces in bioassay performance, this work investigates the effect of OCGs on graphene quantum dots (GQDs) for the optical detection of Pb2⁺ ions—an environmentally toxic pollutant with adverse effects on ecosystems and human health. Three GQD variants were evaluated as both biosensing platforms and nanoquenchers: graphene oxide quantum dots (GQD-Ox), carboxylated graphene quantum dots (GQD-COOH), and hydroxylated graphene quantum dots (GQD-OH). A fluorescently labeled aptamer specific for Pb2⁺ ions served as the biorecognition element. Fluorescence quenching of the aptamer's fluorescein amidites (FAM) label was achieved via Förster resonance energy transfer (FRET) between the GQDs and the aptamer, with fluorescence recovery occurring upon Pb2⁺-induced formation of a G-quadruplex aptamer complex. Comparative analysis of calibration sensitivity, selectivity, and reproducibility identified GQD-COOH as the superior material over GQD-Ox and GQD-OH. These findings provide critical insights for the rational design and optimization of GQD-based homogeneous biosensors. By identifying the optimal surface chemistry for biorecognition element immobilization, this work advances the development of sensitive, selective, and reproducible sensing platforms essential for environmental monitoring and public health protection.