Engineering a Mn3O4/Eu MOF Nanocomposite for Ratiometric Dual-Mode Sensing: Mechanistic Insight and Ultrasensitive Detection of Nitrite in Complex Food Matrices

Rapid, sensitive, and reliable detection of nitrite remains a critical analytical challenge, given its importance for food safety and public health. Herein, we report a dual-mode, dual-ratiometric colorimetric–fluorescent sensor based on a Mn₃O₄/Eu-MOF hybrid for the ultrasensitive detection of nitrite. The Mn₃O₄ component exhibits prominent oxidase-mimicking activity and can rapidly catalyze the oxidation of TMB to its blue oxidized product, ox-TMB. Owing to the highly specific diazotization reaction between nitrite and ox-TMB under acidic conditions, the blue reaction solution gradually fades, accompanied by the formation of a yellow diazotization product. This process generates a characteristic ratiometric colorimetric response, manifested by a marked decrease in absorbance at 652 nm and a simultaneous increase at 450 nm. Meanwhile, ox-TMB quenches the red emission of Eu³⁺ at 615 nm via the inner filter effect. Upon nitrite addition, consumption of ox-TMB restores Eu³⁺ fluorescence, whereas the ligand emission at 360 nm remains essentially unchanged, enabling built-in ratiometric fluorescence self-calibration. Together, these two sensing modes provide an ultrawide linear range of 0.2–55 μg·mL⁻¹ and low detection limits of 0.095 μg·mL⁻¹ for colorimetry and 0.084 μg·mL⁻¹ for fluorometry, meeting regulatory requirements for nitrite in drinking water and meat products. Practical application in sausage, pickled vegetable, tap water, and lake water samples shows excellent agreement with the Griess method, with relative deviations below 5%, recoveries ranging from 93.27% to 101.09%, and RSDs below 3.5%. This Mn₃O₄/Eu-MOF platform therefore enables simple, sensitive, and reliable on-site nitrite detection and offers a general strategy for constructing multifunctional MOF-based dual-ratiometric sensors.