Dopamine-Modified AuCu Bimetallic Nanoclusters as Charge Transfer-Based Biosensors for Highly Sensitive Glycine Detection

Glycine is the simplest amino acid in living organisms and plays important roles in biology and medicine. However, few biosensors for glycine sensing have been reported. Herein, we present a facile strategy to construct dopamine-modified AuCu bimetallic nanoclusters (denoted as AuCu NC-DA) as charge transfer-based biosensors for highly sensitive glycine sensing. The AuCu NCs stabilized by bovine serum albumin (BSA) exhibited a fluorescence maximum at 400 nm. Because of the high affinity of BSA for dopamine (DA), the surface of the AuCu NCs was modified with DA without any complicated chemical reactions, resulting in fluorescence quenching through a charge transfer process. Among 20 amino acids, AuCu NC-DA exhibited an off/on fluorescence switching response specifically toward glycine through the formation of hydrogen bonds with oxidized DA, which inhibited the charge transfer process, leading to the emergence of a new emission peak at 475 nm. Spectroscopic and thermodynamic results combined with molecular docking analyses provided comprehensive understanding of the sensing mechanism. Furthermore, we showed that AuCu NC-DA was able to sense glycine in cells by imaging. Finally, the practicability of AuCu NC-DA for glycine detection was validated in milk drink samples. This study presents a promising type of a charge transfer-based sensor.