Smartphone-based flow-controlled μPAD for automated nitrite determination

We report a smartphone-based microfluidic platform for automated nitrite determination using a paper colourimetric assay. The system combines a V-shaped microfluidic paper-based analytical device (μPAD), a wireless electronic module for capacitive flow monitoring and thermal actuation, and a custom Android application that coordinates the measurement sequence and processes the captured image. Fluid arrival at the sensing zone is identified from changes in capacitance, which trigger the heating step required to collapse a NIPAM-based hydrogel reservoir and release sulfanilic acid from the hydrogel reservoir. Images are acquired under ambient lighting conditions and analysed by an image-processing algorithm that includes circle detection, saturation-value (SV)-based pixel selection, and colour correction using in-frame black and white references. Calibration with standard nitrite solutions showed that the platform achieved a limit of detection (LOD) of 7.63 × 10^-6 M (0.35 mg L^-1 as NO₂^-) after colour correction, improving by about 17% with respect to the uncorrected response. The platform also showed good repeatability (CV < 6% at 10^-4 M) and consistent behaviour when tested with two different smartphones and several lighting conditions. Analysis of tap water and marine samples gave results in agreement with the reference method at the 95% confidence level. The platform combines automated fluid handling, thermally triggered reagent release, and adaptive image-based quantification in a portable format for nitrite analysis outside the laboratory.