Synthesis of fluorescent iron-doped carbon dots with Fenton activity for noninvasive dual-mode urinary uric acid monitoring toward point-of-care renal diagnostics.

Rapid, selective and point-of-care urinary uric acid (UA) monitoring is important for managing renal and metabolic disorders. This study employed solvothermal synthesis of fluorescent iron-doped carbon dots (Fe-CDs) exhibiting a fluorescence quantum yield of 44.5%. Pronounced fluorescence quenching of Fe-CDs was observed upon the introduction of hydrogen peroxide (H₂O₂), attributable to a Fenton reaction-based mechanism. In contrast, non-doped CDs exhibited no fluorescence quenching in the presence of H₂O₂, confirming the essential role of iron doping in the quenching process. The uricase-catalyzed oxidation of UA generates H₂O₂, enabling a turn-off sensing approach for UA that can be quantified using both smartphone-based colorimetric and spectrofluorometric analysis. This strategy confers high selectivity, effectively mitigating matrix interference from biological fluids. The smartphone-based visual assay achieved a limit of detection (LOD) of 1.67 μM (linear range: 5-600 μM), whereas the instrument-based fluorometric analysis yielded a superior LOD of 0.78 μM (linear range: 3-1000 μM). The assay effectively encompassed clinically relevant urinary UA concentrations, with spiked urine sample recoveries ranging from 95.5-105.6% and 95.0-105.7% for the fluorimeteric and the smartphone-mode, respectively. Statistical analysis of real urine samples demonstrated error percentages within ±2%. Paired t-test values of 2.053 and 0.509 were obtained for the fluorometric and smartphone-based modes, respectively, indicating no significant differences compared with the reference colorimetric enzymatic assay at the 95% and 99% confidence levels. Additionally, non-significant F-test results at both confidence levels confirmed the high accuracy, precision, and reliability of the proposed methods. Collectively, these findings highlight Fe-CDs as a versatile and practical platform for point-of-care detection of uric acid in clinical diagnostics, further underscored by their intrinsic catalytic oxidation of H₂O₂ via the Fenton reaction mechanism.