Analyses on intracellular Fe³⁺ with a rhodamine probe: "turn-on" response, specific recognition and bioimaging

To overcome the existing difficulty in distinguishing ferric from ferrous ions, a rhodamine-containing probe was designed to combine with Fe³⁺ based on an opening-closing transformation of the spirolactam ring in the rhodamine moiety. Through a specific and stoichiometric fluorescence response towards Fe³⁺ by 1 : 1 binding accompanied by an obvious color change in the recognition process of Fe³⁺, a "naked-eye" detection method of Fe³⁺ in an aqueous environment is possible. Theoretical calculations gave a possible recognition mechanism of the probe-Fe³⁺ system. Further cytotoxicity and bioimaging in living L929 cells suggested the probe's future applications as a real-time analytical method for intracellular Fe³⁺ in clinical diagnosis. Besides, bioimaging applications enable the dynamic labelling and tracking of Fe³⁺ in biological systems.