Development and validation of a Mn/MOF-808 based colorimetric platform for rapid and reliable on-site monitoring of iodide in marine environments

Background Iodide (I − ) serves as a key indicator for monitoring 131 I contamination in marine environments. However, rapid on-site detection remains challenging due to the short half-life of radionuclides and the complex nature of seawater. Traditional nanocatalysts exhibit limited catalytic activity under near-neutral conditions and poor selectivity in high-salinity environments. There is therefore an urgent need to develop a robust, instrument-free analytical platform that can meet on-site environmental monitoring requirements by combining high sensitivity with operational simplicity. Results In this study, Mn-modified MOF-808 (Mn/MOF-808) nanozymes were synthesised using a sustainable aqueous post-synthetic approach. The incorporation of Mn centers significantly altered the electronic structure of the MOF nodes, resulting in exceptional peroxidase-like activity across a wide pH range (3.5-8.0). We found that I − acts as a potent synergistic amplifier, greatly accelerating the catalytic oxidation of chromogenic substrates. The sensing system achieved an ultra-low limit of detection (LOD) of 0.63 nM for I − . To enable point-of-care testing (POCT), we developed a smartphone-integrated platform using a customised WeChat mini-program for digital image colorimetry. The test results showed no significant deviation from those obtained using the standard ICP-MS method. Significance This work presents a cost-effective ($0.104/test), rapid, and user-friendly approach for I − monitoring. The Mn/MOF-808 platform demonstrates superior anti-interference capability in various environmental water samples and simulated seawater (recoveries: 96.5%-103.2%). By bridging advanced nanozyme engineering with mobile digitization, this research offers a practical and scalable solution for real-time environmental monitoring and emergency diagnostics of radioactive contaminants, aligning with the principles of green analytical chemistry.