Efficient Fluorescence Resonance Energy Transfer-Based Ratiometric Fluorescent Cellular Imaging Probe for Zn2+ Using a Rhodamine Spirolactam as a Trigger

This letter described the design and synthesis of a novel fluorescein-appended rhodamine spirolactam derivative and its preliminary application as a ratiometric fluorescent cellular imaging probe for Zn2+. The ratiometric fluorescent signal change of the probe is based on an intramolecular fluorescence resonance energy transfer (FRET) mechanism modulated by a specific metal ion induced ring-opening process of the rhodamine spirolactam (acting as a trigger). In the new developed sensing system, the emission peaks of the two fluorophores are well-resolved, which can avoid the emission spectra overlap problem generally met by spectra-shift type probes and benefits for observation of fluorescence signal change at two different emission wavelengths with high resolution. It also benefits for a large range of emission ratios, thereby a high sensitivity for Zn2+ detection. Under optimized experimental conditions, the probe exhibits a stable response for Zn2+ over a concentration range from 2.0 x 10(-7) to 2.0 x 10(-5) M, with a detection limit of 4.0 x 10(-8) M. Most importantly, the novel probe has well solved the problem of serious interferences from other transition metal ions generally met by previously reported typical fluorescent probes for Zn2+ with the di(2-picolyl)amine moiety as the receptor (in this case, the fluorescence response induced by Cd2+ is even comparable to that of Zn2+) and shows a reversible and fast response toward Zn2+. All these unique features make it particularly favorable for ratiometric cellular imaging investigations. It has been preliminarily used for ratiometric imaging of Zn2+ in living cells with satisfying resolution.