(Lysozyme Type VI)-Stabilized Au8 Clusters: Synthesis Mechanism and Application for Sensing of Glutathione in a Single Drop of Blood

This paper presents a one-pot approach for preparing highly fluorescent Au8 clusters by reacting the Au3+ precursor solution with lysozyme type VI (Lys VI) at pH 3. The fluorescence band of (Lys VI)-stabilized Au8 clusters is centered at 455 nm on the excitation at 380 nm. Blue-emitting Au8 clusters have a high quantum yield (similar to 56%), two fluorescence lifetimes, and a rare amount of Au+ on the surface of the Au core. When the pH of a solution of Au8 clusters increases suddenly to 12, the Au8 clusters gradually convert to Au25 clusters over time. This conversion is also observed in the case of (Lys VI)-directed synthesis of Au25 clusters at pH 12. The pH-induced conversion of Au8 to Au25 clusters suggests that the size of (Lys VI)-stabilized gold nanoclusters (AuNCs) relies on the secondary structure of Lys VI, which is susceptible to pH change. Based on these results and previous literature, this paper proposes the possible mechanism for growing (Lys VI)-stabilized Au8 and Au25 clusters. Additionally, (Lys VI)-stabilized Au8 clusters could sense glutathione (GSH) through GSH-induced core-etching of Au8 clusters; the limit of detection at a signal-to-noise ratio of 3 for GSH is determined to be 20 nm. Except for cysteine, the selectivity of (Lys VI)-stabilized Au8 clusters for GSH over amino acids is remarkably high. The practicality of using Au8 clusters to determine the concentration of GSH in a single drop of blood is also validated.