Tailoring the electronic structure of iron by molybdenum doping to enhance peroxidase-like activity for colorimetric analysis of natural organic acids.

Accurate quantification of added organic acids with antioxidant activity in food is critical for food safety and human health. Herein, newly-designed Mo-doped Fe3C (denoted as Mo-Fe3C) as nanozyme is prepared via a stepwise self-assembly strategy. Notably, the introduction of Mo not only facilitates efficient charge transfer but also creates additional active sites. More importantly, the Mo doping can also modulate the electronic structure of Fe site in Fe3C, thereby downshifting its d-band center. This modification endows Mo-Fe3C with moderate adsorption capacity toward H2O2, which prevents the over-activation of H2O2. As a result, the Mo-Fe3C exhibits significantly enhanced peroxidase-like activity. Taking advantage of the robust peroxidase-like activity of Mo-Fe3C, a highly effective colorimetric sensing array is developed using 3,3',5,5'-tetramethyl benzidine (TMB) as colorimetric substrate. Owing to the distinctive "molecular fingerprint" characteristic of electrophilic reactions between •OH and natural organic acids, proposed sensing array is capable of achieving colorimetric detection of four organic acids, including tannic acid (TA), phytic acid (PA), malic acid (MA), and ascorbic acid (AA). The limit of detection (LOD) values of TA, PA, MA and AA are calculated to be 0.63, 0.82, 2.40 and 2.88 μM, respectively. Even in mixed-component systems or real samples, the colorimetric sensing array still maintains high accuracy in discriminating and quantifying various organic acids. In short, the present work can provide novel insights into future research on nanozyme-based colorimetric sensing analysis.