A label-free turn-on-off fluorescent sensor for the sensitive detection of cysteine via blocking the Ag + -enhancing glutathione-capped gold nanoclusters.

A new label-free fluorescent turn-on-off mode for the sensitive detection of cysteine(Cys) and the identification of Cys over glutathione (GSH) was firstly proposed, mainly on the basis of two different combination ways between Ag+ ions and GSH-capped gold nanoclusters, or to say, two different contribution ways to the fluorescence enhancement, Ag+ ions combining with Au core structure and coordinating with carboxylate groups on the surface of gold nanoclusters, respectively (turn-on mode). The Au core-binding Ag+ ions could merely be captured by Cys for its relatively smaller steric hindrance than that of GSH, resulting in the identification of Cys over GSH. The carboxylate-binding Ag+ ions could be removed both by Cys and GSH, which, hence, could be used for the qualification of Cys and GSH (turn-off mode). Good selectivity and sensitivity for Cys over other amino acids were achieved. This new turn-on-off sensing strategy for Cys also had obvious advantages of simplicity, rapidity, ease of operation and a low cost. Under the optimized conditions, the linear range was 0.00-2.71 mu M with the limit of detection of 16.54 nM for Cys and 15.59 nM for GSH. The probe also had a good application in Cys containing health products.