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 μM and 0.45–0.84 μM, respectively, along with broad linear ranges (0.90–10.0 μM for 4 and 0.50–6.00 μM 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.