A highly effective curcumin analogue as “naked eye” colorimetric and fluorescent sensor for sensitive and selective detection of Hg²⁺ ions and its application on test strips and real sample analysis

A thiophene appended curcumin-based colorimetric and fluorescent receptor (TAA) for selective recognition of Hg²⁺ ions was synthesized and characterized using ¹H NMR, ¹³C NMR and LC-MS spectroscopic techniques. TAA facilitates detection of Hg²⁺ by a "naked-eye" color change from yellow to colorless in visible light, and fluorescence 'turn-off' in UV light (365 nm). The observed fluorescence quenching is due to the chelation-enhanced fluorescence quenching (CHEQ). TAA exhibited excellent selectivity and sensitivity toward Hg²⁺ ions, even in the presence of competing cations. The binding constant (<i>K</i> _a) for Hg²⁺ ions was found to be 3.4 × 10⁵ M^-1, indicating a strong binding affinity. The binding mechanism was elucidated using DFT calculations and supported by LC-MS and FT-IR studies. TAA forms a 1 : 1 complex with Hg²⁺ ions, as confirmed by Job's plot analysis. Additionally, the colorimetric limit of detection was found to be 0.67 μM, while the fluorometric limit of detection was found to be 0.24 μM, which demonstrates the high sensitivity of TAA towards Hg²⁺. Furthermore, TAA probe exhibited successful detection of Hg²⁺ ions in real water samples. Also, it can serve as an effective on-site detection tool for mercury ions by a simple test strip method that requires no additional instrumentation.