Imidazopyridine Based Fluorescent Turn-Off Sensor for Selective Recognition of Fe Ions and its Application in Test Strips.

A series of 2-ethoxycarbonyl imidazopyridines have been synthesised and characterised by various spectroscopic techniques (H NMR, C NMR, FTIR and HRMS). Among these compounds, 3e was selected for metal sensing applications due to its favourable photophysical properties. 3e demonstrated high selectivity and sensitivity for detecting Fe ions, even in the presence of other competing ions in a mixture of acetonitrile/water (1:9, v/v). It showed a turn-off fluorescence response with a fluorescence detection limit of 3.9 µM. The decrease in the fluorescence intensity of 3e is due to paramagnetic quenching resulting from its interaction with Fe ions. Job's plot analysis indicated a 1:1 binding ratio between 3e and Fe. Density Functional Theory (DFT) calculations revealed a reduced HOMO-LUMO energy gap for the 3e-Fe complex, suggesting successful complexation. The binding constant, calculated using the Benesi-Hildebrand equation, was determined to be 2.845 × 10 M, further confirming the strong interaction between 3e and Fe. Time-based studies revealed excellent photostability of 3e and its rapid response time for detecting Fe. Real-time detection of Fe was successfully achieved using sensor-coated filter paper, demonstrating the practical utility of 3e for rapid assays in real samples.