A deep sea lab on a chip chemical sensor

Abstract

The measurement of chemical concentrations within the oceans is crucial for our understanding of its biogeochemical processes. Such data is vital for; studies of climate and environment change; natural resource management; assessment of pollution, human impact and biodiversity. Current measurement methods (sampling and large in situ instrumentation) cannot provide the quantity and quality of biogeochemical data that is required. The factors limiting the widespread collection of quality data include, sample degradation/contamination, instrument size, cost and insufficient sensitivity.

New technologies allow the manufacture of Lab On a Chip (L.O.C.) devices that can be used as small, low-cost and low power sensors. There are numerous demonstrations of these devices in the laboratory where it is possible to use standard bench top equipment to operate them. Within the National Oceanography Centre Southampton and the Nanoscale System Integration Research Group at Southampton University it has been proposed that integrated L.O.C. devices could be used autonomously and remotely in the marine environment. These innovative micro-devices could provide in situ real time synoptic data on processes with temporal and spatial scales smaller than currently sampled. This report details the development, laboratory testing and field trial of the world's first deep sea in situ L.O.C. chemical sensor. Preparatory work for the design of the next generation of marine L.O.C. devices including low-cost fabrication in fluoropolymer materials (which are naturally robust and chemically resistant) is also presented.

The L.O.C. devices within this study use a reagent based colorimetric protocol to determine the concentration of a chemical within a sea-water sample. The optical absorption when the reagent is mixed with a sample is proportional to the chemical concentration and is measured using a double beam spectrophotometer. This method can be used in the metrology of a number of chemical species including nutrients and pollutants and therefore this technology is generic. The detection of nitrite and nitrate at a wavelength of 540nm is used as a proof of concept within this report. Nitrite samples are combined with α-napthylamine and sulphanilamide to form a coloured dye. The absorption of the dye is proportional to the nitrite concentration. Nitrate is reduced to nitrite using a cadmium column and then measured in same manner. The L.O.C. devices are fabricated using negative photolithography on photosensitive epoxy resin. Micro channels measuring 500 by 500 µm are used to create micromixers, optical detection paths and fluid delivery ports on a device with a footprint of 45 by 45 mm. The absorption is measured with low powered portable electronics, a modulated light emitting diode source and photodiode detector both coupled to polymer fibres. The mixer uses a three dimensional split and recombine technique to ensure effective mixing of the chemicals and sample.

On the laboratory bench the sensor was capable of continuously sampling nitrite concentration levels in sea-water at 60?l/min with a limit of detection of 47.6nM and a precision of 89.3nM at 15µM. Once reconfigured it was capable of detecting nitrate in seawater, at the same flow rates with a limit of detection of 1.75µM and a precision of 9.26µM at 100µM. An in situ version of the sensor, packaged within a pressure compensated housing measuring Ø120mm by 300mm, was deployed in the mid-Atlantic. It provided key functionality and construction methodologies for future generation devices. These trials also identified the developments necessary for the sensor to work as efficiently at depths of 1500m as on the laboratory bench.

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Identifiers

ORCID for Matthew Mowlem: orcid.org/0000-0001-7613-6121

ORCID for Hywel Morgan: orcid.org/0000-0003-4850-5676

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