Nitrogen-Doped Biochar Derived from Starch for Enzyme-Free Colorimetric Detection of Uric Acid in Human Body Fluids

Uric acid (UA), the end product of purine metabolism in humans, is a crucial biomarker closely associated with various diseases. This study developed a novel enzyme-free colorimetric sensing platform based on starch-derived nitrogen-doped biochar (NC) for the highly sensitive and selective detection of UA in human body fluids. The NC material with a high specific surface area and abundant nitrogen active sites was prepared via a two-step strategy involving hydrothermal synthesis followed by high-temperature pyrolysis, using starch and urea as raw materials. It efficiently catalyzed dissolved oxygen to generate reactive oxygen species (·O2- and 1O2) under mild conditions, which oxidized 3,3',5,5'-tetramethylbenzidine (TMB) to produce a blue-colored product (TMBox). UA reduced TMBox back to colorless TMB, resulting in a decrease in absorbance at 652 nm, enabling the quantitative detection of UA. Key reaction conditions were systematically optimized. Material characterization and mechanistic investigations confirmed the catalytic performance. The method demonstrated a wide linear range of 10-500 μmol·L-1 and a low detection limit of 4.87 μmol·L-1, along with excellent selectivity, stability, and reproducibility. Practical application in human serum and urine samples yielded results consistent with clinical reference ranges, and spike-recovery rates ranged from 95.5% to 103.6%, indicating great potential for real-sample analysis.