Near-Infrared Luminescent Single-Atom Gold Modified Cadmium Sulfide Quantum Dots for Consuming Trace l-Cysteine

Exciton-recombination probability is an important factor that affects the fluorescence efficiency of semiconductor nanocrystal quantum dots (QDs), especially those that exhibit near-infrared (NIR) luminescence, making their fluorescence performances modulated via the modification of some receptors. Herein, we present a novel and facile method for the consumption of trace l-cysteine (l-Cys) under physiological conditions by using atomically dispersed gold modified cadmium sulfide QDs (sAu/CdS QDs) with remarkably enhanced NIR fluorescence efficiency compared to free CdS QDs. The enhancement is ascribed to electron transfer from CdS QDs to single-atom Au, increasing the recombination probability of conduction band electrons with valence band holes of CdS QDs and decreasing the fluorescence lifetime. Nevertheless, the electronic state density of single-atom Au modified on CdS QDs is increased upon its coordination with l-Cys as an electron donor, which restricts the electron transfer to decrease the exciton-recombination probability and increase the fluorescence lifetime again, partially lowering the NIR fluorescence efficiency. The efficiency change makes sAu/CdS QDs promising candidates for effective consumption of trace l-Cys with good anti-interference against other various biologically relevant species and extremely high sensitivity, for the limit of detection (LOD) down to 8.10 nM, which makes sAu/CdS QDs successfully applied in practical applicability of consuming trace l-Cys from real water samples. Our results broaden the application potential of NIR luminescent QDs in theoretical guidance and technical support and provide a novel approach to consume trace l-Cys through the modulation of their NIR fluorescence efficiency, which would be useful for diagnosing and treating a series of l-Cys-induced diseases.