Monitoring of water chemistry using novel droplet-based microfluidic sensors - in a high frequency and at a low cost

Traditionally, water chemistry has been measured by taking spot samples, either manually or using autosamplers, transporting to a laboratory where the samples are then analysed.

In situ analysis removes the logistical problem of transporting samples, meaning larger data sets can be obtained. The quality of data can be higher by removing the risk of sample deterioration or contamination. Furthermore, it raises the possibility of large scale networks of sensors deployed on moorings and autonomous vehicles such as profiling floats and gliders to build up a spatially detailed picture of water chemistry.

In situ microfluidic analysers use already-established laboratory assays, has the potential to analyse a wide range of chemical parameters such as macrontutrients (nitrate, nitrite, phosphate, ammonia) and micronutrients (Mn, Fe). Relative to competitor technologies such as voltammetry and in situ UV-vis sensors they offer high limits of detection and non-drifting measurements.

Here we present an in situ chemical sensor based on droplet microfluidics. Water sample is drawn into the sensor device automatically through a mini pump. There it meets an analyte-specific reagent and the resulting mixture is immediately compartmentalised into discrete nano litre droplets by a fluorinated oil. As the droplets are carried downstream along the fluidic path, the droplet contents mix and the reagent reacts with the analyte producing a measurable colour. This operates continuously producing a continuous train of droplets with each droplet forms an independent reactor, the colour of which represent the chemical composition of the incoming sample. This droplet fluidic sensor reduces cross-contamination, allowing higher temporal resolution monitoring and more efficient use of power and consumables.

Our first product line can monitor nitrate, nitrite, silicate and phosphate separately, with a frequency as high as 8600 times per day, and consumes a power less than 2 Watts. The device has a small footprint and requires low maintenance (once per month), supports both wired and wireless data transmission. It has been tested in River Thames and estuary at Southampton for measuring both fresh and sea water, with high quality data correlates to offline analysis of spot samples and tidal information.