Highly sensitive and selective ESIPT-based fluorescent probes for detection of Pd²⁺ with large Stocks shifts

Highly sensitive and selective fluorescent probes 1 and 2 for detection of palladium ions (Pd²⁺) were synthesized based on excited-state intramolecular proton transfer process (ESIPT) using 2-(2′-hydroxy-)benzothiazole moiety. Different metal ions were used to optimize their sensitivity and selectivity but best results were obtained with Pd²⁺. Probe 1 produced green fluorescence with Stokes shift of about 163 nm upon addition of Pd²⁺, whereas probe 2 displayed red fluorescence with a large Stokes shift of about 217 nm under similar conditions. The detection limits of 1 and 2 to Pd²⁺ were found to be 285 nM and 14.6 nM, respectively. Mechanistic aspects of probes 1 and 2 were discussed to account for the optical changes leading to the strong ESIPT fluorescence in very short duration. Furthermore, the portable test strips were also prepared by direct deposition of the probe molecules onto the filter paper to detect the lowest possible ppm levels of Pd²⁺.