Fluorescence switch of gold nanoclusters stabilized with bovine serum albumin for efficient and sensitive detection of cysteine and copper ion in mice with Alzheimer's disease.

The near-infrared fluorescence of gold nanoclusters stabilized with bovine serum albumin (BSA -AuNCs) centered at 675 nm could be enhanced by cysteine and then effectively quenched by copper ion (Cu2+), therefore, cysteine and copper ion could be detected in sequence. At "on" state, fluorescence enhancement of BSA-AuNCs is generated due to the reaction between cysteine and BSA-AuNCs, via filling the surface defect of gold nanoclusters, while Cu2+ can further oxidize the reductive sulfydryl of cysteine and interact with amino acids presented in the BSA chain, inducing gold nanoclusters to aggregate, thus causing "off" state with fluorescence quenching. Fluorescence switch of BSA-AuNCs can be used for cysteine and Cu2+ detection in mice brain with Alzheimer's disease (AD) in vitro, with fast response, high chemical stability and sensitivity. Besides, it was able to image the endogenous Cu2+ in liver and heart of AD mice in situ. The results are promising, especially in the framework of early diagnosis of Alzheimer's disease.