Poly(allylamine) and cysteine co-capped silver indium sulfide quantum dots with excellent photostability for specific Cd2+ and Zn2+ detection based on fluorescence enhancement

Metal chalcogenide quantum dots (MCQDs) have received tremendous attention in biomedical field for their fascinating optical properties and the admirable biomolecular modifiability. However, challenged by low stability and notorious biocompatibility, MCQDs make them limited for direct biomedical use. Herein, a novel designed and synthesized poly(allylamine) and cysteine co-capped silver indium sulfide quantum dots (PAA-Cys-AIS QDs) with excellent photostability is reported for the specific rapid detection of Cd2+ and Zn2+ ions. The obtained water-soluble PAA-Cys-AIS QDs emitted a stable orange fluorescence at 625 nm with an average size of 3.639 ± 1.006 nm. The unusual anti-photobleaching property of PAA-Cys-AIS QDs is demonstrated to be owing to the introducing of poly(allylamine) and cysteine, which uniquely increased the crystallinity and hydrodynamic size of the structure system. Unexpectedly, unlike most reported approaches for Cd2+ and Zn2+ detection based on the quenching fluorescence mechanism, the PAA-Cys-AIS QDs showed remarkable and rapid fluorescence enhancement after being exposed to Cd2+ and Zn2+ ions aqueous solutions. Notably, the detection limits of the new probe for Cd2+ and Zn2+ are 175 and 50 nM respectively. Complexation and metal ions doping mechanisms are proposed to be responsible for the enhancement of photoluminescence intensity based on the metal ions capture capacity coming from PAA-Cys-AIS QDs’ intrinsic defects and abundant surface amino functional groups. Moreover, the intracellular photostability and biocompatibility of PAA-Cys-AIS QDs are strong enough to prompt us to investigate the metal ions imaging applications in SH-SY5Y cells to further demonstrate the potential for real-live biomedical applications.