Dually emitting carbon dots as fluorescent probes for ratiometric fluorescent sensing of pH values, mercury(II), chloride and Cr(VI) via different mechanisms.

The authors describe the preparation of carbon dots (CDs) that display both blue (~ 410 nm) and yellow (~ 565 nm) emission peaks. The CDs was synthesized by solvothermal treatment of o-phenylenediamine in aqueous ethyl alcohol at pH ~7.0. The CDs are shown to be useful fluorescent probes for pH values in that the ratio of fluorescences at 565 and 410 nm strongly depends on the pH value in the range from 4.5-6.5 and 10.0-13.0, respectively. The blue fluorescence is quenched by 91% by 100 µM solutions of Hg(II) through an electron transfer process, and is restored by 97% an addition of chloride (0.5 mM). The yellow fluorescence, in contrast, is hardly affected. The ratio of fluorescences at 414 and 565 nm drops linearly in the 30 to 60 μM of Hg(II) concentration range, and the limit of detection is 60 nM. Fluorescence is linearly restored in the 70 to 180 μM chloride concentration range, and the LOD is 2.8 nM. Both the blue and the yellow emission are reduced by Cr(VI) (chromate) due to an inner filter effect at pH 3.0. The ratio of fluorescences (410/565 nm) drops linearly in the 20 to 250 μM Cr(VI) concentration range, and the LOD is 260 nM. The method was utilized to analysis of chloride in salt lake brine and of Cr(VI) in spiked tap water. Graphical abstract Schematic presentation of carbon dots with pH-dependent dual emission (at ~ 410 nm and ~ 565 nm). They are shown to be viable fluorescent probes for ratiometric sensing of pH values, mercury(II), chloride and Cr(VI) via different mechanisms.