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

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 C-13 NMR studies indicated that the basicity of the 'C=S' can be varied from delta 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 sub-stituent. Chemodosimeters 1-3 exhibit a stable response to Hg2+ from 2.5 x 10(-7) to 2.0 x 10(-5) M witha 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. (C) 2015 Elsevier B.V. All rights reserved.