Point-of-Care-Testing NO₃-N Detection Technology with Selected Transition-Metal-Based Colorimetric Sensor Arrays

Nitrate-nitrogen (NO₃-N) is a major contaminant in groundwater and seawater. Significant amounts of ammonia are oxidized to nitrate through nitrification, leading to an imbalance in the nitrogen cycle and causing nitrate pollution in water bodies. Controlling NO₃-N levels is a significant challenge for both marine aquaculture and human health. Traditional measurement methods, such as ion chromatography and continuous flow analysis, require pretreatment steps to detect NO₃-N in complex matrices, which is time-consuming. However, in this study, we developed a transition-metal-based sensor capable of measuring NO₃-N concentrations on-site without the need for pretreatment. We analyzed the color change of transition-metal-based sensors over time and obtained color data by mixing transition metals (Mn, V, Fe, Co, Cr, Cu, and Ni) with solvents and additives at fixed ratios, and combining them with standard solutions of NO₃-N at concentrations of 1, 2, 3, 5, 10, 20, 30, 40, 50, 75, and 100 ppm. We selected sensors that exhibited linearly increasing color velocity with increasing NO₃-N concentrations and developed an array sensor using the selected sensors. The performance of the array was validated by comparing its results with those of hierarchical cluster analysis (HCA) based on color data and compositional analysis, confirming its ability to detect NO₃-N in complex matrices. Additionally, by creating a large data set of color change patterns of the array sensor, we can develop selective array sensors for detecting specific substances, surpassing the capability of merely measuring the NO₃-N concentration.