Microfluidic-based chemical sensors take laboratory analytical protocols and miniaturise them into field-deployable systems for in situ monitoring of water chemistry. Here we present a prototype nitrate/nitrite sensor based on droplet microfluidics that in contrast to standard (continuous phase) microfluidic sensors, treats water samples as discrete droplets contained within a flow of oil. The new sensor device can quantify the concentrations of nitrate and nitrite within each droplet and provides high measurement frequency and low fluid consumption. Reagent consumption is at a rate of 2.8 ml/day when measuring every ten seconds, orders of magnitude more efficient than the current state of the art. The sensor's capabilities were demonstrated during a three-week deployment in a tidal river. The accurate and high frequency data (6 % error relative to spot samples, measuring at 0.1 Hz) elucidated the influence of tidal variation, rain events, diurnal effects, and anthropogenic input on concentrations at the deployment site. This droplet microfluidic-based sensor is suitable for a wide range of applications such as monitoring of rivers, lakes, coastal waters, and industrial effluents.

Point of care monitoring of chemical biomarkers in real-time holds great potential in rapid disease diagnostics and precision medicine. However, monitoring is still rare in practice, as measurement of biomarkers...

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