Fluorescent probes for Pd²⁺ detection by allylidene-hydrazone ligands with excellent selectivity and large fluorescence enhancement

Because palladium is widely used in various catalysts and converters, which results in a high level of contamination of water systems and the soil by residual palladium, there is an urgent need for Pd²⁺-sensitive and -selective probes. Based on the special affinity of Pd²⁺ to conjugated double-bond ligands, two fluorescence probes (RPd2 and RPd3) that contain conjugated allylidene-hydrazone ligands that link to colorless rhodamine-spirolactam have been developed. The results show that conjugated allylidene-hydrazones have a much better affinity toward Pd²⁺, and consequently provide the probes with more acute color change and fluorescence enhancement (≈170-fold), and better selectivity over other metal ions (especially platinum-group elements, or PGEs) than the unconjugated allyl-hydrazine. With richer electron density and a more suitable stereo effect in the allylidene-hydrazone group, RPd2 displays the best specificity toward Pd²⁺ and affords convenient detection by the naked eye. Its potential application for Pd²⁺-contaminated water and soil-sample analysis is revealed by proof-of-concept experiments. Sensitive and selective: Two fluorescent probes for Pd²⁺ have been developed. The results show that conjugated allylidene-hydrazone has much better affinity toward Pd ²⁺, and consequently better selectivity over other metal ions (especially platinum-group elements) than the unconjugated allyl-hydrazine (see graphic).