Cyclic phosphazene-based fluorescent probes with schiff base units for ultralow-level Hg2+ detection in real samples and paper-based devices

The design and synthesis of new fluorescence sensors based on Schiff base derivatives are essential for environmental and biological applications. In this study, novel anthracene-appended cyclic phosphazenes (compounds 4-6) were developed as selective and effective sensors for Hg2+ ions. Standard spectroscopic techniques confirmed their structures. Photophysical and fluorescence behaviors were investigated using UV-vis absorption, steady-state and time-resolved fluorescence, and excitation-emission matrix (EEM) analyses. Compounds 4 and 5 exhibited excellent sensitivity and selectivity toward Hg2+, with detection limits (LOD) and quantification limits (LOQ) of 0.15-0.28 mu M and 0.45-0.84 mu M, respectively, along with broad linear ranges (0.90-10.0 mu M for 4 and 0.50-6.00 mu M for 5). Density functional theory (DFT) calculations supported the proposed sensing mechanisms. Real sample analyses using spike/recovery methods confirmed their practical applicability. Importantly, both compounds were successfully applied as paper-based test kits, enabling rapid and visible detection of Hg2+ ions in environmental diagnostics due to their clear colorimetric responses.