Microfluidic-electrochemical sensor utilizing statistical modeling for enhanced nitrate detection in surface water towards environmental monitoring

The presence of nitrate (NO₃^-) in surface water and groundwater used for potable supply needs to be closely monitored since in elevated amounts it can adversely affect aquatic life and human health by causing hypoxia and methemoglobinemia. Many of the existing EPA-certified sensors used for environmental monitoring are expensive, bulky, and labor-intensive. To address these concerns, we have successfully developed a low-cost microfluidic electrochemical impedimetric sensor, consisting of a nitrate-binding nickel complex within a polyaniline/carbon nanocomposite (Ni@Pani/C) enabling nitrate monitoring in field samples. Under optimized conditions, our sensor demonstrated a high sensitivity of 2.31 ± 0.09 Ω ppm^-1 cm^-2 across a wide nitrate concentration range (0.6-10 ppm). It also showed a desirable low detection limit of 0.015 ppm and a swift response time under 20 seconds. It maintained repeatability over a wide temperature range (5-65 °C) and exhibited consistent performance over an extended period (∼1 month). The sensor exhibited high specificity towards nitrate when tested against potential interferences (SO₄^2-, C₂H₃O₂^-, HCO₃^-, NH₄⁺, Cl^-) and showed good reproducibility for test water samples collected from various streams in Maryland, U.S.A. A statistical model was used to confirm the sensor's accuracy, which yielded a maximum standard deviation of ±0.6 ppm (absolute value). Our sensor was also benchmarked against a commercial SUNA device resulting in comparable performance.