Designing Fe8−N2 Catalytic Sites of Nitrogen-Doped Iron-Based Nanoparticles with Oxidase-Like Activity: Characterization, Calculation and Application

Graphical Nitrogen-doped iron-based nanoparticles (Fe NPs-N@C), characterized by the presence of Fe8−N2 active sites, are synthesized to exhibit oxidase-like activity, facilitating the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to yield the blue product, oxTMB. Designing metal nanoparticles with oxidase-mimicking capabilities has garnered significant attention due to their promising attributes. However, understanding the intricate catalytic mechanisms underlying these nanoparticles poses a formidable challenge. In this study, a straightforward pyrolysis procedure was employed to synthesize nitrogen-doped iron-based nanoparticles (Fe NPs-N@C) with Fe8−N2 serving as active sites. The confirmation of these sites was thoroughly confirmed through density functional theory (DFT) calculations complemented by experimental validation. The resulting Fe NPs-N@C nanoparticles, averaging 5.45 nm in size, exhibited excellent oxidase-mimicking activity, with v max =1.11×10 −7 M s −1 and k m =1.67 mM, employing 3,3′,5,5′-tetramethylbenzidine as a substrate. The oxidation pathway and catalytic mechanism of Fe NPs-N@C involved 1O 2 ⋅ radicals, validated through electron paramagnetic resonance analysis and DFT calculations. Furthermore, Fe NPs-N@C/TMB system was devised for ascorbic acid and nitrite quantitative detection. This method demonstrated the capability to detect ascorbic acid within concentrations ranging from 1 to 55 μM, with a limit of detection (LOD) of 0.81 μM, and nitrite within concentrations from 1 to 160 μM, with a LOD value of 0.45 μM. These findings offer a comprehensive understanding of the catalytic mechanisms of Fe NPs-N@C nanoparticles at the atomic level, along with its potential for colorimetric sensor in future.