Fluorescent turn on sensing of Caffeine in food sample based on sulfur-doped carbon quantum dots and optimization of process parameters through response surface methodology

We have demonstrated a fluorescence approach for rapid, simple and selective detection of caffeine in real media using sulfur-doped carbon quantum dots (S-CQDs) as probe. S-CQDs were synthesized by microwave-assisted treatment using citric acid as carbon source and sodium thiosulfate as passivator, respectively. The obtained S-CQDs emitted strong blue fluorescence under 365 nm UV light excitation with a high quantum yield of 21.4%. The fluorescence of S-CQDs is strongly quenched by Cu2+ mainly due to dynamic quenching. A detailed investigation of the quenching mechanisms was carried out. Upon addition of caffeine, the fluorescence of S-CQDs was recovered due to the formation of copper-caffeine complexes. Therefore, we designed an off-on fluorescent probe for detection of caffeine based on this phenomenon. The effective parameters in sensing process, including time, temperature, pH and concentration of S-CQDs, were optimized using response surface methodology (RSM) based on central composite design. Under optimal conditions, the fluorescence intensity of the system increased linearly with increasing concentrations of caffeine from 0.2 μM to 70 μM. The limit of detection of caffeine was 0.05 μM. The established method showed a high selectivity for caffeine activity compared with other interference components. Finally, the carbon quantum dots −based fluorescent probe was successfully employed for the caffeine detection in real sample, and satisfactory results were achieved.