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

Abstract 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 μM 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.