Water-compatible fluorescent [2]rotaxanes for Au³⁺ detection and bioimaging

In this study, the synthesis of [2]rotaxanes as fluorescent metal ion sensors has been demonstrated. [2]Rotaxanes, RA-H·PF₆, RRA and RRB, undergo photoelectron transfer resulting in fluorescence quenching. Before the diimine (dynamic covalent bond) reduction on the macrocyclic ring, the dynamic [2]rotaxane RA-H·PF₆ can be hydrolyzed and turned fluorescent by trivalent metal ions, giving fluorescence at λ_max 424 nm. After reduction of the imines, the reduced [2]rotaxanes RRA and RRB are kinetically stable and highly selective to Au³⁺ binding among 27 metal ions in a water-compatible (50 vol%) solution with working fluorescence in the range of pH 4-10. 50-Fold and 1.2-fold fluorescence turn-on after addition of Au³⁺ has been observed for RRA and RRB, respectively. Metal interference on Au³⁺ detection is insignificant, and thereby the fluorescence intensity is linearly proportional to the concentration of Au³⁺ until excess. The solid-state crystal structure of RRA shows the mechanically interlocked structure (mechanical bond). The bioimaging experiment of RRB with HeLa cells demonstrates the potential application of these mechanically interlocked molecules for metal ion detection in aqueous media and biological systems.