Mapping hydrogen sulfide in rats with a novel azo-based fluorescent probe

• The azo group was found for the first time to be able to act as a biocompatible reaction trigger for H2S.
• One azo-based fluorescent switch-on probe for endogenous H2S was developed.
• The probe features high specificity, good sensitivity, low detection limit.
• The probe was successfully applied to the quantification of endogenous H2S in rat tissues.

We report herein a reaction-based fluorescent switch-on sulfide sensor, azo3, for the quantification of endogenous sulfides in rat tissues. The sensor was exploited based on the novel azo-sulfide chemistry and designed by locking the rhodol fluorophore into its nonfluorescent form with an azo group. However, the azo group would undergo a specific and biocompatible reaction with sulfides, triggering significant fluorescence increasements which were linear to the concentrations of sulfides. Azo3 distinguished by its high sensitivity (148-fold fluorescent switch-on response), good selectivity (22-fold more selective towards sulfides than other bio-thiol species) and low detection limit (500 nM). Moreover, the azo3-based assay for biological sulfides displayed the unique advantage of being insusceptible to ultraviolet (UV) irradiation. Azo3 has been successfully applied to the quantification of endogenous sulfides in rat plasma and tissues including heart, brain, liver, spleen, lung and kidney. In addition to providing azo3 as a valuable tool to analyze sulfides in biological samples, we also discussed the influences of the electron effect on the sensitivity of the probes, which would shed some light on the design of future reaction-based probes.