Single microfluidic paper-based analytical device for nitrite, nitrate, chloride, and pH as markers for food safety and quality control of local fermented fish

Abstract

A microfluidic paper-based analytical device (μPAD) was developed for the simultaneous measurement of nitrite, nitrate, chloride, and pH in local sour fermented fish. The chloride detection zone connects to the nitrite and nitrate reservoir, enabling analysis with a single sample drop. Chloride detection uses a distance-based, indicator-free argentometric reaction, with AgCl precipitate length measured 1 min after UV irradiation. Nitrite and nitrate are detected via the colorimetric Griess reaction, while pH is determined by bromophenol blue and bromothymol blue indicators, which enabled visual detection within the pH range of 3–7. Calibration curves for chloride, nitrite, and nitrate were established using mixed standards at concentrations of 105-1500 mg L⁻¹ Cl⁻, 0.3–3.0 mg L⁻¹ N–NO<inf>2</inf>^-, and 3–68 mg L⁻¹ N–NO<inf>3</inf>^-, respectively. The limits of detection (LOD) were 105 mg L⁻¹ Cl⁻, 0.3 mg L⁻¹ N–NO<inf>2</inf>^-, and 3 mg L⁻¹ N–NO<inf>3</inf>^-. Percent recoveries ranged from 90 to 112 % for chloride, 95–112 % for nitrite, and 91–118 % for nitrate. The relative standard deviations (RSDs) were 2.9 % for chloride (1000 mg L⁻¹ Cl⁻, n = 10), 3.3 % for nitrite (1.52 mg L⁻¹ N, n = 10), and 6.6 % for nitrate (22.6 mg L⁻¹ N, n = 10). The total analysis time is 16 min per sample. The μPAD was successfully applied to local fermented fish samples for food quality and safety control, with results showing good agreement with reference methods.