Preparation of sulfur doped carbon quantum dots from lignin as sensor to detect Sudan I in acidic environment

To achieve a rapid and facile quantitative evaluation of Sudan I illegally added in ketchup, fluorescent carbon quantum dots with excellent stability in acidic environments are required as the actually pH value of ketchup is close to 4.0. In this paper, we developed a green approach to prepare sulfur doped carbon quantum dots (SCQDs) via hydrothermal treatment of lignin, isolated from pre-hydrolysis liquor, in the sulfuric acid solution. The resultant SCQDs possessed amounts of sulfur-containing groups, which exhibited excellent fluorescence with a quantum yield up to 13.5% and good stability in acidic environments with a wide pH range of 0-5.0. Therefore, the SCQDs were successfully employed as sensor to detect Sudan I in acidic solution with excellent selectivity and sensitivity. The linear range for Sudan I was 0-40 µM, while the limit of detections was 0.12 µM. In addition, the fluorescent indicator paper functionalized with SCQDs also showed outstanding selectivity to Sudan I. The proposed SCQDs sensing system not only displayed a prospective application for quantitative evaluation of Sudan dye in practical samples, but also provided a way to convert the lignin-based wastes into high-valued nanoscale materials.