Urea detection in milk by urease-assisted pH-sensitive carbon dots

Excessive urea in milk will lead to serious health problems. To detect whether the urea concentration in milk exceeds the standard and ensure the quality of milk, it is necessary to develop detection technology for urea in milk. But it is difficult to detect urea in milk conveniently and accurately by traditional methods. To measure the concentration of urea in milk, stable green light carbon dots (CDs) were synthesized by a one-step method as a fluorescent probe. Then, 3, 5-diaminobenzoic acid was used as the precursor for CD synthesis. Experimental results showed that CDs can generate strong fluorescence when excited by light (350-450 nm). The fluorescence peak wavelength is 490 nm, and the optimum excitation wavelength is 390 nm. The fluorescence intensity of CDs has a significant change with variations of pH (pH of 6-9), and the higher the pH, the lower the fluorescence intensity. Additionally, urea can be hydrolyzed by urease to produce ammonia and carbon dioxide. Ammonia is ionized in water to produce OH^-, which increases the pH of the solution. After adding standard urea to milk, urease and CDs are added. The fluorescence intensity of CDs in the mixed solution decreases as the concentration of standard added urea increases. Thus the concentration of urea in milk can be calculated. The experimental results show that the CD method for detecting urea in milk has advantages of high sensitivity and wide measurement range. The linear interval is 25-500 mg/L, R² is 0.998, and the limit of detection is 6.27 mg/L. The concentration of urea in the milk used in the experiment is 265.46 mg/L. CDs are easy to fabricate, and the advantages of the method are simple operation, no pretreatment, safety, and low cost. A new method for the detection of urea in milk was established, to the best of our knowledge, and this method can aid in food quality control.