The salicylaldehyde derivative, 9-formyl-8-hydroxyjulolidine was developed as a fluorescent probe to detect weak acid ions. With the salicylaldehyde moiety as the chromophore, the probe exhibits pH sensitive detection of hydrogen sulfide and silicate in aqueous solution. It displays an excellent selectivity for hydrogen sulfide and silicate over other weak acid salts. Time-dependent density functional theory calculations confirmed that the fluorescence turn-on mechanism involved blocking intramolecular charge transfer. The probe detects S 2− in live cells, providing a powerful method to study H 2 S chemistry in biological systems.