Nitrogen‐Doped Graphene Quantum Dot Fluorescent Nanosensor for Sensitive and Quantitative Detection of Tyrosine in Urine

ABSTRACT To develop a sensitive and quantitative method for tyrosine detection in urine and evaluate its value in tumor screening. Methods: Nitrogen‐doped graphene quantum dots (N‐GQDs) were synthesized via a hydrothermal route. A fluorometric assay was established based on the formation of quinoneimine dye from tyrosine and 4‐aminoantipyrine in the presence of potassium periodate (KIO 4 ), with N‐GQDs serving as the fluorescent nanosensor. The analytical performance was assessed in terms of linear range, limit of detection (LOD), selectivity, spike recovery, and precision. Receiver operating characteristic (ROC) analysis was performed to evaluate the screening performance in clinical samples. Results: The proposed N‐GQDs‐based fluorescent nanosensor enabled sensitive determination of tyrosine with good linearity over 10–1000 µg mL − 1 and an LOD of 4.475 µg mL − 1 . For urinary tyrosine analysis, spike recoveries ranged from 87.4% to 106.6%, and the assay was not affected by common amino acids or inorganic ions present in urine. In clinical testing, urinary tyrosine levels measured by the N‐GQDs‐based fluorescent nanosensor were significantly higher in patients with malignant tumors, yielding an area under the ROC curve (AUC) of 0.832. Conclusion: A fluorescence nanosensor‐based method using N‐GQDs was developed for quantitative determination of urinary tyrosine, demonstrating promising value for tumor screening.