Concentration-dependent and pH response carbon dots and application in latent fingerprints recognition

Herein, red fluorescence carbon dots (R-CDs) were synthesized via solvothermal method using citric acid and urea as raw materials in DMF solvent. What is more interesting is that the synthesized CDs displayed concentration-dependent photoluminescence behavior, which can be ascribe to the fact that the intermolecular forces pulled the CDs particles closer, and the functional groups on the CDs surface combined together, thereby narrowing the energy gap, which ultimately led to the fluorescence emission peak moving to longer wavelength with the increase of concentration accompanied by a significant reduction in the fluorescent intensity. In addition, the diluted CDs displayed pH response. With the continuous increase of pH, the color of the solution changes from green to pink, and the fluorescence intensity increases significantly, which has a linear relationship in the range of pH 8–14. These characteristics are conducive to the visual detection of pH value. Moreover, in order to overcome aggregation-caused quenching (ACQ), a solid phosphor with an emission peak at 600 nm was obtained by incorporating chitosan matrix, which showed excellent performance in latent fingerprints (LFPs) detection.