Nitrogen and sulfur co-doped carbon dots for the fluorescence detection of lactose in milk

For lactose-intolerant patients, controlling lactose intake is crucial. To address this need, we developed carbon dot-based sensors for lactose detection in milk. Using citric acid and sulfamic acid as precursors, we synthesized nitrogen and sulfur co-doped carbon dots (N,S-CDs) via hydrothermal method. The resulting N,S-CDs exhibit excellent water solubility and fluorescence properties, making them ideal for analytical applications. Lactose can be converted to hydrogen peroxide by the cascade enzyme method of lactase and glucose oxidase. Given the activity of hydrogen peroxide in the Fenton reaction and that the fluorescence of CDs can be significantly quenched by the Fenton reaction, a simple and convenient method for the detection of hydrogen peroxide and lactose was established. The strong oxidizing property of hydroxyl radicals produced in the Fenton reaction destroys the C-O bonds on the carboxylic acid groups of the CDs, which results in the quenched emission peak at 418 nm. The fluorescence intensity of the CDs is linearly related to the concentration of hydrogen peroxide (0.1-5 μM), with a detection limit of 0.09 μM. For lactose in milk, a detection limit of 0.36 μM (0.5-22 μM) was obtained. The method for the detection of hydrogen peroxide and lactose based on the Fenton reaction and N, S co-doped CDs exhibits good stability, specificity, and selectivity. Moreover, the application potential of the method was verified in milk.