Identifying three routes of the sensing mechanism for casein-directed gold nanoclusters

Although there have existed kinds of gold nanoclusters (AuNCs) employed for various sensing purposes, scarce studies were proposed for systematically exploring their multiple routes of the sensing mechanism since that plays the critical role for broadening their analysis applications. Herein, casein-directed gold nanoclusters (AuNCs@casein) have been successfully raised for identifying their multiple routes towards the sensing mechanism. Initially, the current nanoclusters were synthesized in aqueous solution with a quantum yield of nearly 2.1%, which obviously showed the red fluorescence at 600 nm. Subsequently, we have explored three routes of quenching the fluorescence of AuNCs@casein by introducing thiourea, bromelain, and graphene oxide with amino groups (GO-NH₂) as the representatives. To be specific, the formed bonds of Au-S between thiourea and AuNCs@casein achieved to quench their fluorescence, and the templates of AuNCs@casein destroyed by bromelain also showed the dramatically decreased fluorescence, and the energy transferring from the current AuNCs to GO-NH₂ indeed lead to the fluorescent variations of AuNCs@casein. On the basis of these phenomena, AuNCs@casein was further applied for simply sensing thiourea, bromelain and GO-NH₂ through the standard formats. Significantly, this study elaborated three quenching routes of AuNCs@casein and may provide more sensing strategies for other types of nanoclusters.