Strategically modified highly selective mitochondria-targeted two-photon fluorescent probe for Au3+ employing Schiff-base: Inhibited C = N isomerization vs. hydrolysis mechanism

Strategic modifications from N-containing through O-containing to S-containing Schiff-base receptors lead to the development of a highly selective Au3+ two-photon fluorescent probe that responded via an Au3+-induced hydrolysis mechanism. Here, we systematically demonstrate the two-step process of Au3+-induced hydrolysis reaction by implementing a series of well-designed experiments, which is of instructive significance for the structural design of Schiff-base receptors. The results reveal that Schiff-base receptors are first rapidly coordinated with Au3+ (Le. inhibited C = N isomerization mechanism), then hydrolyzed to the corresponding products by the attack of water. The rigid S-containing Schiff-base receptor (PyCM-3) has a better selectivity for Au3+ than that of the flexible S-containing Schiff-base receptor (PyCM-2), which is supported by experimental and DFT results. Confocal fluorescence imaging experiments indicate that PyCM-3 can be applied to monitor Au3+ in mitochondria, fresh liver tissues and live zebrafishes under two-photon excitation with large two photon action cross-sections, little cytotoxicity and good biocompatibility.