Cyclodextrin-stabilized Gold nanoclusters for bioimaging and selective label-free intracellular sensing of Co2+ ions

Abstract

Gold nanoclusters are a new class of emerging fluorescent probes which have an immense potential for the selective detection of heavy metal ions. Herein, we propose the fabrication of glutathione protected ultra-small fluorescent gold nanoclusters in the presence of cyclodextrin for fluorescence imaging and its unique sensing capabilities. A panel of characterization was elucidated to understand the characteristics and an electronic structure of cyclodextrin capped gold nanoclusters (AuCDs). Biocompatibility and cell cycle analysis were performed for AuCDs and were found to be highly biocompatible. Furthermore, confocal laser scanning microscopy showed a cytoplasmic localization of AuCDs in L929 cells with bright red fluorescence and their complete cellular internalization was confirmed using flow cytometry. The inherent fluorescence was pH-dependent and found to be specifically quenched by the addition of Co2+ ions at pH 3.5. The AuCDs were further explored to develop a label free fluorescent probe for the selective determination of Co2+ ions in aqueous solution. The Co2+ ions were selectively detectedin aqueous solutions (0–1000 μM) and in mammalian cells (0–800 μM) using AuCDs as a “turn-off” sensing mechanism in which the Co2+ ions bind to the sulphur atoms, thus, resulting in the disruption of charge transfer process through Au-S bond. Thus, AuCDs could form a simple nano-sensor for a label-free detection of heavy metal ions such as Co2+ in the environment.