Microwave-synthesized narrow emitting carbon dots and their tunable fluorescence for sensing applications

Fluorescent carbon dots (CDs) with narrow emissions have been mostly prepared at high temperatures and high pressures, with long reaction times and tedious purification processes, which greatly limits their large-scale production. In contrast, the microwave-assisted solvothermal method can produce CDs simply and rapidly, but often results in wide-emissive CDs with full widths at half maximum (FWHM) larger than 80 nm. In this study, the microwave synthesis of CDs was improved by using ethylene glycol as the solvent and sodium thiosulfate as the oxidant. Consequently, narrow emissive CDs with a FWHM of only 35 nm were obtained from phloroglucinol, and were purified by simple precipitation using a mixed solvent of ethanol and dichloromethane. The structural characterization studies of the obtained CDs reveal that the sulfur element could be doped in the carbon structure with the addition of thiosulfate. It was also demonstrated that the photoluminescence efficiency of the CDs is greatly determined by their carbon core structures and edge groups. In addition, the fluorescence of CDs could be tuned significantly from the blue region to the green region by changing the dispersing solvent. Accordingly, the prepared CDs were successfully applied for the sensing of water content in organic solvents.