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. 2025 Jul 21;15(32):25811-25822.
doi: 10.1039/d5ra03656a.

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

Ramesh D K  1   2 , Prasad Pralhad Pujar  1   2
Affiliations

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

Ramesh D K et al. RSC Adv. .

Abstract

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 (K 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.

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Conflict of interest statement

There are no conflicts of interest to declare.

Figures

Scheme 1
Scheme 1. Synthesis of (1E,6E)-1,7-di(thiophen-2-yl)hepta-1,6-diene-3,5-dione (TAA).
Fig. 1
Fig. 1. Visual change of TAA in comparison with other metal ions: visible light (upper image), UV-light (365 nm) (lower image).
Fig. 2
Fig. 2. (a) UV-vis absorption and (b) fluorescence spectra of TAA (25 μM) in different solvents.
Fig. 3
Fig. 3. (a) UV-vis and (b) fluorescence spectra of TAA (25 μM) with various metal ions (50 μM) in CH3CN/H2O (v/v: 1/1).
Fig. 4
Fig. 4. Competitive study of TAA (25 μM) with Hg2+ upon addition of other interfering metal ions (50 μM): (a) absorption studies, (b) emission studies.
Fig. 5
Fig. 5. (a) UV-vis and (b) fluorescence titration plot of TAA (25 μM) upon addition of Hg2+ (50 μM), detection limit of TAA by (c) UV-vis spectroscopy, (d) fluorescence spectroscopy.
Fig. 6
Fig. 6. (a) Job's plot of TAA (25 μM) for Hg2+ (50 μM), (b) Benesi–Hildebrand (B–H) plot of TAA (25 μM) for Hg2+ (50 μM).
Fig. 7
Fig. 7. Time evolution of TAA (25 μM) towards Hg2+ (50 μM), (a) UV-vis spectroscopy, (b) fluorescence spectroscopy.
Fig. 8
Fig. 8. Geometry optimized structures of: (a) TAA, (b) TAA-Hg2+ complex, (c) frontier molecular orbital with energy difference of TAA and TAA-Hg2+ complex.
Fig. 9
Fig. 9. Effect of pH on TAA (25 μM) and TAA-Hg2+ in fluorescence spectroscopy.
Scheme 2
Scheme 2. Probable binding mode in the solution phase.
Fig. 10
Fig. 10. Reversibility cycle of TAA (25 μM) with EDTA (50 μM).
Fig. 11
Fig. 11. TAA-coated (1 mM) test strips for Hg2+ (1 mM) in comparison with other metal ions: (a) visible light, (b) UV-light (365 nm).
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