A Portable Detection System Based on a Functionalized Carbon Nanotube Electrode for Rapid Determination of Nitrate in Agricultural Runoff

Highlights An integrated portable electrochemical system integrating a CdS/CNT-SPE sensor is developed for on-site nitrate analysis in agricultural runoff. Exhibits a 0.05 to 10 mmol/L linear detection range and strong correlation with standard lab methods (R2 = 0.987). Enables rapid, high-density spatial mapping of nitrate concentrations, identifying agricultural pollution hotspots. Abstract. Excessive nitrogen fertilization in agriculture leads to significant nitrate (NO₃⁻) pollution in runoff, posing a major threat to water quality and requiring effective on-site monitoring solutions. Traditional laboratory methods are time-consuming and ill-suited for rapid field assessment. This study presents a portable electrochemical system designed for rapid nitrate detection in agricultural runoff, where on-site analysis is performed by manually dropping a water sample onto a disposable sensor surface. The core sensing element is a commercial carbon nanotube-enhanced screen-printed electrode (CNT-SPE) functionalized with cadmium sulfide nanorods (CdS NRs) to enhance electrocatalytic activity towards nitrate reduction. The system integrates the modified sensor with a miniaturized potentiostat, an embedded microcontroller for data processing, and a wireless communication module into a compact, portable device. Laboratory characterization of the system demonstrated excellent performance, including a wide linear detection range (0.05-10 mmol/L), high repeatability (<5% RSD), sufficient storage stability, and strong anti-interference capabilities. Field validation conducted at 21 agricultural runoff sites showed high agreement between the portable system‘s measurements and standard UV spectrophotometry results (R²=0.987, RMSE=0.110 mmol/L). The system enables efficient, high-density spatial monitoring of nitrate concentration, effectively identifying pollution hotspots linked to agricultural management practices and hydrological pathways. This portable system provides a reliable, rapid, and practical tool for on-site nitrate assessment, significantly supporting precision fertilization and agricultural non-point source pollution control efforts.