Effects of Sonication and Hydrothermal Treatments on the Optical and Chemical Properties of Carbon Dots.

In our study, we have tested the effects of sonication and hydrothermal treatments on the properties of carbon dots synthesized from a microwave-assisted method (C-dots). When the carbon dots are sonicated in an aerobic environment, the fluorescence quantum yield decreases drastically because the molecular fluorophores attached to the surface of the carbon dots are oxidized during the sonication process. Meanwhile, the sonicated C-dots also lose their Hg ion sensing and photoreduction activity due to the oxidization of surface functional groups. After the hydrothermal treatment, the fluorescence quantum yield of C-dots increases due to the formation of new fluorophores; however, the Hg ion sensitivity and photoreduction activity of C-dots decrease significantly due to the oxidization of surface functional groups. By autoclaving the C-dots at 100 °C, we have demonstrated that we can enhance the fluorescence quantum yield of C-dots without losing their Hg ion sensitivity. This finding can be used to improve the fluorescence quantum yield of the fluorescent ion sensor based on C-dots.