Automated Droplet-Based Microfluidic Analyzer for <i>In Situ</i> Monitoring of Ammonium Ions in River Water

Accurate <i>in situ</i> monitoring of ammonium ion (NH₄ ⁺) levels in water bodies is fundamental for assessing water quality and potential contamination. Here, we present a novel automated droplet-based microfluidic analyzer for <i>in situ</i> monitoring of NH₄ ⁺ concentrations in river water. The developed analyzer miniaturizes the modified Berthelot colorimetric method into submicroliter droplets contained within oil flow. The use of droplets enables measurements at high frequency (10 s per measurement) with low reagent consumption (0.3 μL per measurement). NH₄ ⁺ measurements showed linear responses up to 2 mg L^-1, and a limit of detection (LOD) of 0.11 mg L^-1. The system was tested with river water samples collected from the River Itchen (Southampton, UK), with excellent agreement (<i>p</i> < 0.05) between the droplet microfluidic analyzer and standard benchtop analysis. The analytical performance of this microfluidic analyzer showed that it could be applicable to sustainable <i>in situ</i> monitoring of NH₄ ⁺ levels in a wide range of water bodies and scenarios, including rivers, lakes, aquaculture, and/or industrial effluents.