Efficient rhodamine-thiosemicarbazide-based colorimetric/fluorescent ‘turn-on’ chemodosimeters for the detection of Hg2+ in aqueous samples

• 1–3 exhibits the high selectivity and sensitivity toward Hg2+ ion under physiological condition.
• The substituent effect toward Hg2+ was provided by binding studies and 13C NMR titration study.
• The use of the test strip of the chemodosimeter 1 to detect Hg2+ in water was also demonstrated.

This paper describes the optimized design, synthesis and application of three novel thiosemicarbazide-appended rhodamine spirolactam derivatives as ‘turn-on’ colorimetric and fluorescent chemodosimeters (1–3) for the detection of Hg2+ in aqueous samples with a broad pH range (5.0–9.4). The ‘turn-on’ change in the color and fluorescence signals of 1–3 is based on a Hg2+-triggered reaction that converts the thiosemicarbazide form to the 1,3,4-oxadiazole form. The results of 13C NMR studies indicated that the basicity of the ‘CS’ can be varied from δ 183.05 (for nitro) to 181.3 ppm (for methyl) by changing the R group in the chemodosimeter. Electronic and spectrofluorimetric studies indicated that the order of the binding constants of these chemodosimeters with Hg2+ ions is 1 > 2 > 3, which may be dependent on the substituent. Chemodosimeters 1–3 exhibit a stable response to Hg2+ from 2.5 × 10−7 to 2.0 × 10−5 M with a detection limit of 10−8 M. The response of these chemodosimeters to Hg2+ is highly selective and pH insensitive, and the response time is rapid. All of these unique features make these sensors particularly favorable for Hg2+ test strip applications. These sensors have been preliminarily used for the highly sensitive monitoring of Hg2+ levels in aqueous media with satisfactory resolution.

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