Imidazopyridine Based Fluorescent Turn-Off Sensor for Selective Recognition of Fe3+ Ions and its Application in Test Strips

Abstract

A series of 2-ethoxycarbonyl imidazopyridines have been synthesised and characterised by various spectroscopic techniques (¹H NMR, ¹³C 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 Fe³⁺ 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 Fe³⁺ ions. Job's plot analysis indicated a 1:1 binding ratio between 3e and Fe³⁺. Density Functional Theory (DFT) calculations revealed a reduced HOMO-LUMO energy gap for the 3e-Fe³⁺ complex, suggesting successful complexation. The binding constant, calculated using the Benesi-Hildebrand equation, was determined to be 2.845 × 10³ M^-1, further confirming the strong interaction between 3e and Fe³⁺. Time-based studies revealed excellent photostability of 3e and its rapid response time for detecting Fe³⁺. Real-time detection of Fe³⁺ was successfully achieved using sensor-coated filter paper, demonstrating the practical utility of 3e for rapid assays in real samples.

Keywords: DFT; Fluorescence; Imidazopyridine; Iron; Job's plot; Stern–Volmer analysis.