Spectroscopic insights into the dual-mode sensing of uric acid by iron-doped carbon dots: peroxidase-like activity and the inner filter effect.

Enzymatic assays have been widely used for uric acid (UA) detection owing to their high specificity and favorable catalytic efficiency, playing an indispensable role in clinical diagnosis and point-of-care testing. Nevertheless, the practical applications of natural enzymes in sensing and detection are severely limited by their high cost and poor structural stability. Herein, a potent peroxidase-mimetic nanoagent was developed from iron-doped carbon nanodots (Fe-CDs). This material enabled a dual-signal output (colorimetric/fluorometric) for UA analysis. Fe-CDs exhibited robust peroxidase-like activity and catalyzed the conversion of colorless 3,3',5,5'-tetramethylbenzidine (TMB) into a visible blue oxidized TMB (oxTMB) signal via a superoxide radical (O2·-)-dominated pathway, while simultaneously suppressing the native fluorescence of Fe-CDs through the inner filter effect (IFE). Introduction of UA reduced the oxidized TMB, triggering a coordinated color-fading and fluorescence-recovery response. Exceptional sensitivity to urate is demonstrated by the dual-channel LODs, which stand at 0.177 μM (colorimetric) and 0.067 μM (fluorometric). The platform exhibited outstanding selectivity against both single interferents and a mixture of common reducing species, enabling reliable UA quantification in complex biological matrices. Practical performance was validated in fetal bovine serum and healthy human urine, with recoveries of 99.06-100.47% (RSD < 6.03%) and 95.2-106.8% (RSD < 4.37%), respectively. Our findings provide spectroscopic insights into the catalytic and fluorescence modulation mechanisms of Fe-CDs, highlighting their potential as a precise and cost-effective probe for clinical UA analysis.