Solid-Phase Fluorescence Filter Effect Based Test Kit for Field Testing of COD Mn in Surface Water

On-site rapid monitoring of the permanganate index (COD Mn ) is critical for assessing the distribution and transformation of dissolved organic matter (DOM) in surface water. Herein, we proposed a portable field test kit for rapid, visual, and cost-effective COD Mn detection, complemented by an unmanned vessel for automated sampling. The method exploits the solid-phase fluorescence filter effect (SPFFE) between carbon dots (CDs) and iodine, enabling the quantification of residual KMnO 4 (calculated as COD Mn ). A smartphone-based reader and a user-friendly fluorescence reference card were developed for quantitative analysis of COD Mn and water quality classification. Under optimal conditions, a limit of detection (LOD) of 0.04 mg L –1 was obtained for COD Mn with relative standard deviations (RSDs) better than 2.0%. In a field application, 33 water samples were collected and analyzed across a 133,300 m 2 lake within 2 h. The results demonstrated a strong agreement with the standard method, exhibiting enhanced sensitivity, reproducibility, and portability. Spatial mapping further revealed that COD Mn distribution patterns were influenced by local environmental features. This work establishes a mechanism-driven analytical strategy that couples a previously unexplored SPFFE modulation with field-deployable test kit design and autonomous sampling, advancing practical COD Mn measurement for decentralized water quality monitoring and providing a scalable solution for real-time environmental surveillance.