Biothiols have been reported to involve in intracellular redox-homeostasis against oxidative stress. In this study, a highly selective and sensitive fluorescent probe for sensing biothiols is explored by using an ultrasmall gold nanodot (AuND), the dendrimer-entrapped Au8-cluster. This strategy relies upon a thiol/disulfide exchange to trigger the fluorescence change through a photoinduced electron transfer (PET) process between the Au8-cluster (as an electron donor) and 2-pyridinethiol (2-PyT) (as an electron acceptor) for sensing biothiols. When 2-PyT is released via the cleavage of disulfide bonds by biothiols, the PET process from the Au8-cluster to 2-PyT is initiated, resulting in fluorescence quenching. The fluorescence intensity was found to decrease linearly with glutathione (GSH) concentration (0-1500 μM) at physiological relevant levels and the limit of detection for GSH was 15.4 μM. Compared to most nanoparticle-based fluorescent probes that are limited to detect low molecular weight thiols (LMWTs; i.e., GSH and cysteine), the ultrasmall Au8-cluster-based probe exhibited less steric hindrance and can be directly applied in selectively and sensitively detecting both LMWTs and high molecular weight thiols (HMWTs; i.e., protein thiols). Based on such sensing platform, the surface-functionalized Au8-cluster has significant promise for use as an efficient nanoprobe for intracellular fluorescence imaging of biothiols including protein thiols in living cells whereas other nanoparticle-based fluorescent probes cannot.
