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 KMnO₄ (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 m² 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.