Magnetic rod-shaped Mn-based MOF as a multi-functional and recyclable platform for dual-mode ratiometric-based nitrite detection.

The development is shown of rod-shaped manganese-based metal-organic frameworks (Mn-MOFs) as hot- and cold-adapted oxidase-like nanozymes, with strong magnetic properties. These Mn-MOFs enable highly sensitive detection of nitrite ions, utilizing both convenient colorimetric ratio analysis and a visual instrument-free-based approach compatible with smartphone-based detection. The Mn-MOF showed multi-functional activity, such as cold/hot-adapted and magnetic oxidase-like activity, catalyzing the oxidation of chromogenic substrates 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized TMB (oxTMB). Mn-MOF shows high oxidase activity with Vmax of 1.39 × 10 M/s and Km of 0.068 mM for TMB oxidation. Nitrite ions further react with oxTMB to form a yellow color via diazotization resulting in the ratiometric change in absorbance (A/A). The color ratio is also quantified through the naked eye and/or smartphone app by analyzing RGB values, providing a rapid, portable, and cost-effective method for on-site detection. When applying Mn-MOF for smartphone-based nitrite detection, it performs excellent detection, with a linear range of 5.0-55.0 µM and a limit of detection of 0.18 µM, superior to most of the oxidase nanozyme-based nitrite sensing platforms. The detection platforms develop sensing probes using a reusable nanozyme that enables highly sensitive and selective detection of nitrite, featuring a broad linear range and a low limit of detection.