Tripeptide-assisted gold nanocluster formation for sensing Fe3+ and Cu2+

Over the past decade, various fluorescent gold nanoclusters (AuNCs) have been studied for their potential as metal ion sensors. Due to the importance of the surface ligand in stabilizing nanocluster and sensing target metal ions, there is a great need of further investigations into possible surface ligands to develop selective and sensitive sensors. In this study, we designed very simple tripeptides to form fluorescent AuNCs by taking advantage of the reduction capability of tyrosine under alkaline conditions. In addition to studying the role of tyrosine in forming AuNC, we also investigated the role of tyrosine in sensing metal ions. Two tripeptides, tyrosine-cysteine-tyrosine (YCY) and serine-cysteine-tyrosine (SCY), were prepared and used for the formation of AuNCs. We obtained AuNCs with blue and red fluorescence from YCY peptides and AuNCs with blue fluorescence from SCY peptides. We found that the fluorescence of blue fluorescence-emitting YCY- and SCY-AuNCs is selectively quenched with Fe3+ and Cu2+, while the fluorescence of the red fluorescence-emitting YCY-AuNC is stable with 13 different metal ions. We have observed that the number of tyrosine residues affects the response of sensors. In the presence of different metal ions, different aggregation propensities were observed from DLS measurement. These results suggest the chelation between the peptide on the AuNC surface and the target ions results in aggregation and causes fluorescence quenching. Our study has shown that very simple and short peptides can be designed rationally for the formation of fluorescent AuNCs and utilized as the surface ligand for the metal ion sensing.