Development of dual-mode nitrite detection and multi-platform sensors based on magnetic COF-MOF hybrid nanocatalysts.

Nitrite (NO2-), a highly toxic contaminant, poses a severe threat to human health even at trace levels, necessitating rapid and precise detection methods. This study employed a dual-mode sensing strategy integrating covalent organic frameworks with Fe3O4/Ce-metal organic framework (MOF) nanozymes to construct peroxidase-mimetic hybrids. A Ce-MOF/TbBd catalyst (where Tb and Bd stand for the monomers) was used to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2, generating a blue product (oxTMB, λmax = 651 nm). Subsequent diazotization with nitrite under acidic conditions yielded a yellow chromophore, enabling dual-wavelength colorimetric analysis via absorbance shifts at 445 nm (increase) and 651 nm (decrease). Three portable sensors-hydrogel, cotton fabric, and cotton swab platforms-were developed for on-site nitrite monitoring; they achieved detection limits of 0.21 μM (colorimetric) and 23.61 μM (RGB). The hydrogel sensor leverages ionic crosslinking for controlled chromogenic reactions, whereas cotton-based substrates utilize capillary action to enhance sensitivity and reduce matrix interference. This work not only advances multifunctional nanozyme design but also provides a practical toolkit for environmental and food safety applications, addressing critical challenges in real-time pollutant tracking with cost-effectiveness and operational simplicity.