Conjugate Size and Surface Oxidation Synergistically Trigger Red Fluorescence in Carbon Dots for Detecting Trace Water in Organic Solvents

In this paper, o-phenylenediamine and dopamine hydrochloride were used as precursors, and the pH values of the reaction system(pH=7, 3, 1) were adjusted by phosphoric acid to obtain carbon dots(CDs) with gradually red-shifted fluorescence, namely CDs-7(green), CDs-3(orange) and CDs-1(red), respectively. Trans- mission electron microscopy(TEM), Raman spectroscopy(Raman), Fourier transform infrared spectroscopy (FTIR), X-ray electron spectroscopy(XPS), UV-Vis absorption spectrum and fluorescence lifetime decay curves analysis demonstrated that the decrease of the reaction system pH promoted the precursor carbonization and cross -linking, which led to the increase of sp(2) conjugate domain size and graphitization, resulting in the fluorescence red-shift of CDs. In addition, the acidic environment favors the formation of carboxyl groups by oxidation on the surface of CDs, which further promotes the red-shift of CDs fluorescence while improving the quantum efficiency (QY). Finally, because of its high QY(14.8%) and solvent-dependent luminescence, CDs-1 was used as a fluores-cent probe to detect trace water in ethyl alcohol(EtOH), N,N-dimethylformamide(DMF) and 1,4-dioxane(DIO), with detection limits of 0.86%, 0.123% and 0.023% respectively. It is proved that CDs-1 has great potential in the detection of trace water.