One-step Fabrication of Nitrogen-Doped Carbon Dots-Based Fluorescent Sensors with Red Emission for Efficient Detection of pH

Carbon dots have been widely noticed due to their excellent optical properties, such as good photostability and a wide excitation wavelength range. However, the emission peaks of most known carbon dots (CDs) are in the blue region, which hinders their extensive applications in long-wavelength emission. In this study, red-emitting carbon dots (RCDs) that are pH-responsive are synthesized using citric acid and urea as precursors, and their fluorescence intensity increased gradually with increasing solution pH from 1 to 12. A new mechanism based on H aggregation and Fermi energy level transfer is proposed to explain the pH-responsive emission of RCDs. In contrast to the narrow response range of most pH-sensitive CDs reported so far, the pH-responsive range of RCDs is large, especially in the alkaline range, and their pronounced red light emission is well visualized. The prepared carbon dots have the advantages of non-toxicity, low cost, and a wide pH response range. In addition, “rose” test paper and carbon dot PVA composite film, RCDs-PVA film, are made to demonstrate the visualization of carbon dots in pH detection. The “rose” test paper and carbon dot PVA still have good fluorescence intensity. The prepared carbon dots have good application prospects for pH detection.