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Changing the composition of a precursors mixture is a powerful tool to tune the structure and properties of carbonaceous nanoparticles synthesized via the solvothermal route. We have addressed the influence of the ratio of urea or thiourea to citric acid during their solvothermal treatment in dimethylformamide on the optical and sensing properties of the obtained colloidal product. It has been found that the urea-derived products are more diverse in comparison with the thiourea-based ones. The excitation-dependent fluorescence of the products and their sensitivity to mercury(II) ions have been investigated; one to three types of fluorophores have been observed in the products depending on the composition. The nanoparticles prepared in excess of urea have been found more sensitive to the heavy metal, with the sensitivity of the long-wave emission band being superior.

We have demonstrated that variation of the precursor concentration during hydrothermal synthesis of carbon nanodots is an efficient mean to tune the yield of the products keeping their optical properties the same. Moreover, we found that the nature of the precursor determined the sensitivity and selectivity of detection of metal ions in the solution via quenching the fluorescence.