Pb²⁺-modified graphene quantum dots as a fluorescent probe for biological aminothiols mediated by an inner filter effect

A simple, yet efficient fluorescent method for detecting biological aminothiols has been developed based on the inner filter effect principle that utilizes graphene quantum dots (GQDs) as the donor and aminothiol-Pb²⁺ complex as the absorber. Well-defined diethanol amine modified graphene quantum dots (GQD-DEA) were first synthesized by a "synthesis-modification integration" strategy. Then, the addition of aminothiols can bind with Pb²⁺ and displace it from the surface of preformed GQD-DEA-Pb²⁺, leading to the formation of aminothiol-Pb²⁺ complex. Due to the complementary overlap between the excitation band of GQD-DEA and the absorption band of aminothiol-Pb²⁺ complex, the fluorescence of GQDs was quenched, thereby a turn-off fluorescent assay for the determination of aminothiols via the inner filter effect was constructed. This strategy enabled cost-effective and selective detection of aminothiols with theoretical simplicity and low technical demands. Moreover, the fluorescent probe offered high selectivity for aminothiol due to the strong binding of aminothiol with Pb²⁺ in comparison with other amino acids and the inner filter effect provided by thiol-Pb²⁺ complex. Under the optimum conditions, the linear concentration ranges were 5 × 10^-5-6 × 10^-4 M for cysteine, 5 × 10^-5-1 × 10^-3 M for homocysteine, 1 × 10^-4-2 × 10^-3 M for glutathione, respectively.