Preparation of pH-resistant multicolor thiol fluorescent carbon dot using cysteine and neutral red as precursors and its application in detection and adsorption of apigenin

Conventional carbon dots (CDs) typically exhibit substantial variations in fluorescence intensity across different pH. This limitation underscores the pressing need for advancements in their stability and performance under diverse environmental conditions. Herein, l-cysteine and neutral red are selected as precursors, and three kinds of CDs, which can emit red, orange, and green fluorescent light (assigned as r-CD, o-CD, g-CD, respectively), are synthesized by simply changing the reaction conditions. The pH resistance of r-CD and o-CD in acidic and alkaline environments was demonstrated. A fluorescence detection system for natural flavonoid apigenin was established with r-CD, as r-CD exhibited obviously selective quenching of apigenin, without obvious response to other analogs. The linear range of apigenin detection was divided into low concentration at 2.85-30 μmol L^-1 and high concentration at 90-225 μmol L^-1, demonstrating excellent anti-interference ability in complex environments. In addition, the molecularly imprinted polymer containing r-CD (r-CD@MIP) was also prepared by surface molecular imprinting technique. The maximum adsorption capacity for apigenin was 28.39 mg g^-1, and the adsorption equilibrium reached in 90 min. A complete adsorption-fluorescence detection method of apigenin in actual samples was successfully established by using r-CD and r-CD@MIP. The results demonstrated that the developed CDs and detection methods have promising applications in the field of fluorescence sensing and adsorption.