Microfluidic Chips for Rapid On-Site Detection of both Total Nitrogen and Total Phosphorus in Water Quality Monitoring

Pollution of nitrogen and phosphorus in aquatic environments poses a growing threat to global water quality, driving an urgent demand for rapid, portable, and reliable on-site detection tools. This study develops a portable microfluidic chip for rapid on-site detection of both total nitrogen (TN) and total phosphorus (TP) in water. The device employs on-chip UV-activated persulfate at 50 °C to efficiently digest diverse nitrogen and phosphorus species into nitrate and orthophosphate, followed by colorimetric quantification via the Griess diazotization and phospho-molybdenum blue reactions, respectively. By pre-loading all reagents into integrated bubble caps within the on-chip design, the system eliminates manual reagent handling, minimizes reagent consumption and operational complexity, and enables reliable field deployment across diverse matrices, including surface waters from multiple river systems and coastal sites, as well as groundwater. Complete analysis is achieved within 40 min with detection limits of 0.061 mg/L (TN) and 0.040 mg/L (TP), both below EPA standards (TN: 1.5 mg/L; TP: 0.2 mg/L). This platform offers a cost-effective, user-friendly solution for real-time water quality management, demonstrating significant potential for adaptive environmental sensing networks. Notably, quantitative readout is performed exclusively by a smartphone-based recognition app built on the uni-app framework: the user marks a blank reference region and a sample region on the captured chip image, and an embedded, pre-trained model [random forest (RF) for TN, support-vector regression (SVR) for TP] converts the normalized colors to concentration entirely on-device. In the on-site prototype, the smartphone replaces both the laboratory absorbance reader and most discrete on-chip photodetectors.