A Lab-on-Chip analyzer for in situ measurement of soluble reactive phosphate: improved phosphate blue assay and application to fluvial monitoring

Here, we present a new in situ microfluidic phosphate sensor that features an improved “phosphate blue” assay which includes polyvinylpyrrolidone in place of traditional surfactants—improving sensitivity and reducing temperature effects. The sensor features greater power economy and analytical performance relative to commercially available alternatives, with a mean power consumption of 1.8 W, a detection limit of 40 nM, a dynamic range of 0.14–10 μM, and an infield accuracy of 4 ± 4.5%. During field testing, the sensor was continuously deployed for 9 weeks in a chalk stream, revealing complex relations between flow rates and phosphate concentration that suggest changing dominance in phosphate sources. A distinct diel phosphorus signal was observed under low flow conditions, highlighting the ability of the sensor to decouple geochemical and biotic effects on phosphate dynamics in fluvial environments. This paper highlights the importance of high resolution in situ sensors in addressing the current gross under-sampling of aquatic environments.