The Comparative Study of Two New 6-Hydrazinylpurine Derivatives as "Turn-On" Fluorescence Chemosensors for the Detection of Zn2.

Two simple Schiff bases PHMP((E)-2-((2-(9H-purin-6-yl)hydrazono)methyl)phenol) and PHMEP((E)-2-((2-(9H-purin-6-yl)hydrazono)methyl)-6-ethoxyphenol) were designed and synthesized based on 6-hydrazine-purine with salicylaldehyde and 3-ethoxy-salicylaldehyde, respectively. Among them, both PHMP and PHMEP exhibit significant "turn-on" response characteristics when coordinating with Zn2+ ions in an EtOH/H2O buffer solution, demonstrating remarkable selectivity and sensitivity, accompanied by a noticeable color change. Initially, neither probe exhibited significant fluorescence. However, upon forming complexes with Zn2+, PHMP and PHMEP exhibit bright blue and green fluorescence responses, respectively. In contrast, the ethoxy modification in PHMEP alters its steric hindrance and electron cloud density, resulting in fluorescence signals with both superior selectivity and sensitivity compared with PHMP. Additionally, the newly formed complex PHMEP-Zn2+ can serve as a selective "turn-off" fluorescence probe for Cu2+ ions. Experimental data indicate that the detection limits of PHMP and PHMEP for Zn2+ ions are 1.08 × 10-8 M and 8.85 × 10-9 M, respectively; the detection limit of PHMEP-Zn2+ for Cu2+ ions is 1.79 × 10-8 M. Job's plot analysis, ESI-MS titration, and theoretical calculations confirmed that the stoichiometric ratio of the probes to the target ions is 1:1. Additionally, PHMP and PHMEP demonstrate broad potential applications in quantitative detection, being suitable for both tap water and convenient test strips.