Fluorescence Enhancement of Terminal Amine Assembled on Gold Nanoclusters and Its Application to Ratiometric Lysine Detection.

Ratiometric fluorescent sensors have emerged as an attractive tool for analytical sensing and optical imaging due to their providing a built-in self-calibration for environmental effects. However, cumbersome processes of nanoparticles modified with fluorophores for constructing traditional ratiometric sensors limit their further application. Herein, we report a facile and label-free strategy for constructing a ratiometric sensor based on an aggregation-induced-emission (AIE)-active amine-terminated small molecule on the surface of gold nanoclusters (AuNCs). Intrinsic fluorescence of the terminal primary amine of the small molecule lysine resulting from AIE was first observed in the presence of glutathione-stabilized gold nanoclusters (GSH-AuNCs). Using lysine as both the fluorophore and the analyte, the synthesized GSH-AuNCs showed a good lysine-responsive ratiometric property. The AIE-active dual-emitting fluorescence property of the GSH-AuNCs/lysine complex made it feasible to achieve ratiometrically detection of the analyte without conjugated fluorogen. This AIE-active GSH-AuNC-based biosensor possesses high selectivity, rapid response, and excellent photostability. Moreover, the strategy opens a new pathway for the construction of a label-free ratiometric fluorescent sensor with various applications.