Development of a High-Sensitivity Lab-On-Chip Sensor for In Situ Determinations of Phosphate in Coastal and Open-Ocean Environments

The development of field-deployable nutrient sensors with a dynamic quantification range suitable for high-frequency sampling in estuaries, coastal waters and oligotrophic seas is essential to improve our understanding of the fluxes and biogeochemical implications of nutrient distributions in the marine environment. By virtue of their compactness, low reagent (< 320 μL/sample) and power (1.5 W) consumption, microfluidic wet-chemical analysers based on Lab-On-Chip technology provide an attractive means to address this challenge while opening the possibility of in situ monitoring onboard autonomous seagoing platforms such as oceanic gliders.

Here we report the development and complete characterization of a stand-alone, microfluidic Lab-On-Chip sensor for phosphate determinations using the classic Molybdenum Blue (MB) method. The sensor is the second iteration of a Lab-On-Chip sensor designed at the National Oceanographic Centre, which was successfully deployed in shallow water environments and gliders for nitrate determinations. The phosphate chemical assay consists of merging streams of an acidic molybdate solution, sample and reducing agent into a 41 mm absorption cell fitted with a LED and photodiode for absorbance quantification. The assay is performed in stopped flow mode, which allows monitoring the kinetics of the reaction as well as altering the dynamic quantification range by varying the length of the stopped flow period.

A complete optimization of the assay parameters of the MB method to achieve optimal performance on a microfluidic chip will be described along with results of a deployment in a temperature controlled chamber to address the effect of ambient temperature on the sensitivity of the assay. Data with regards to the long-term stability of the reagents will be presented along with a trial deployment in a shallow water environment. The use of long-path absorbance cells for nanomolar phosphate determinations will also be discussed.