Synthesis of carbon quantum dots with iron and nitrogen from Passiflora edulis and their peroxidase-mimicking activity for colorimetric determination of uric acid.

Carbon quantum dots co-doped with iron and nitrogen (Fe@NCDs) were synthesized by using Passiflora edulis Sims (P. edulis) as a precursor. The Fe@NCDs exhibit outstanding peroxidase-mimetic activity owing to successful doping with iron resulting in a behavior similar to that of iron porphyrins. In the presence of H2O2, the Fe@NCDs catalyze the oxidation of the peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) with a color change from colorless to blue. The blue oxidation product has a characteristic absorption peaking at 652 nm. A colorimetric assay was worked out for uric acid (UA) that measures the hydrogen peroxide produced during oxidation of UA by uricase. Response is linear in the 2-150 µM UA concentration range, and the limit of detection is 0.64 µM. The method was applied to the determination of UA in (spiked) urine, and recoveries ranged from 92.0 to 103.4%. Graphical abstract Schematic representation of the fabrication of iron and nitrogen co-doped carbon dots (Fe@NCDs) using Passiflora edulis Sims as carbon-based materials. First, uric acid (UA) was oxidized to generate H2O2 by uricase. Then, the Fe@NCDs catalyzed the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to form blue-colored oxidized TMB (oxTMB) in the presence of H2O2. UA can be quantified based on the theory.