Development of a novel hybrid pH sensor for deployment on autonomous profiling platforms V. Rérolle, D. Angelescu, A. Hausot, P. Ea, Nathalie Lefèvre, Christine Provost, Matthieu Labaste To cite this version: V. Rérolle, D. Angelescu, A. Hausot, P. Ea, Nathalie Lefèvre, et al.. Develop- ment of a novel hybrid pH sensor for deployment on autonomous profiling platforms. OCEANS2019 doi: 10.1109/OCEANSE.2019.8867572, 2019, Marseille, France. pp.1-8, 10.1109/OCEANSE.2019.8867572. hal-03027418 HAL Id: hal-03027418 https://hal.archives-ouvertes.fr/hal-03027418 Submitted on 27 Nov 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Development of a novel hybrid pH sensor for deployment on autonomous profiling platforms V. Rérolle1, D. Angelescu1, A. Hausot1, P. Ea1, N. Lefèvre2, C. Provost2, M. Labaste2 1.Fluidion SAS, 75001, Paris ,France 2. Sorbonne Universités (UPMC, Univ Paris 06)-CNRS-IRD-MNHN, LOCEAN Laboratory, 75005, Paris, France [email protected] Abstract— Ocean acidification is a direct consequence of the Installation of sensors on profiling and drifting floats requires a atmospheric CO2 increase and represents a threat for marine small, solid and lightweight sensor capable of performing ecosystems, particularly in the Arctic. High-quality seawater pH highly accurate measurements, with fast response time and measurements with good spatial and temporal coverage are without significant measurement drift for extended deployment required to apprehend the ocean acidification phenomena. We are durations. Cost is also an important consideration, since working to develop a high-accuracy, high-resolution pH sensor that has the potential to allow global ocean acidification mapping profilers stop emitting their location at the end of battery life, through deployment on fleets of ARGO floats and other making very unlikely the prospect of their recovery. autonomous platforms already in existence. The instrument Traditionally, seawater pH has been measured using one of two implements a novel hybrid approach, utilizing the two different standard methods: potentiometry (pH electrodes) and and complementary measurement techniques (potentiometric and colorimetry (using pH indicators), which remains the gold colorimetric) to generate temporally dense and highly accurate pH standard in the field [8]. A third method, using MOS field effect data. Here we present the concept and initial results obtained from transistors (MOS-FET), has recently been adapted to marine a hybrid pH sensor. Results show that the potentiometric part of waters, and was only recently released as a commercial solution the sensor is capable to operate in real ocean pressure and for surface and potential profiling applications [9]; this new temperature conditions, including near-freezing temperatures typical of Arctic environmental conditions. The colorimetric part sensor can be categorized under potentiometry. provides a stable reference to perform periodic recalibrations and remove drift. Every type of traditional ocean pH measurement presents a set of advantages and disadvantages. Colorimetry, for example, is Keywords—Hybrid pH sensor, microfluidic, colorimetric, well adapted to many biogeochemical parameters because it electrode, acidification. enables high-quality measurements with low detection thresholds and without drift or need for periodic recalibration. I. INTRODUCTION: STATE OF THE ART It allows the acquisition of quality data in a wide range of temperature and salinity environments, with negligible deterioration in measurement quality even after long periods of Ocean acidification, a direct consequence of CO2 increase in the atmosphere, represents a real threat for marine ecosystems use (> 1 year). The colorimetric method does however consume [1], the Arctic Ocean being the most vulnerable region with the liquid pH indicator (m-cresol purple being routinely used in strongest pH decrease [2,3]. High-quality seawater pH ocean monitoring), which limits the total number of measurements with good spatial and temporal coverage are measurements available as well as their frequency. required to understand ocean acidification phenomena. While Colorimetric sensors require metered reagent dosage and several ocean parameters can now be measured globally thank accurate optical measurements that are not always easy to to a network of autonomous profiling and drifting floats [4], pH achieve in actual field conditions or during profiling operations is a notable exception: there are currently very few direct pH [10]. measurement options available, despite efforts to develop a continuous accurate pH measurement system by several The potentiometric method, meanwhile, does not require use of academic and industrial groups [5,6,7]. As a consequence, long- reagent. Potentiometric sensors (electrodes) are more easily term trends in surface pH are often inferred only indirectly from miniaturized, consume little energy and enable continuous other measured ocean carbon system parameters. The measurements with fast response times. Therefore, this method oceanographic community is currently in need of autonomous allows vertical profiling. However, these sensors can be very pH sensor technology that will affordably, accurately and sensitive to electromagnetic perturbations or to changes in efficiently measure ocean chemistry from its shallowest to its temperature and salinity, with phenomena such as electrolyte deepest waters even in remote areas with extreme conditions. diffusion and biofilm formation further leading to frequent recalibration requirements that are impossible to accomplish on a drifting float. There is consequent reluctance within the ocean 978-1-7281-1450-7/19/$31.00 ©2019 IEEE acidification community to deploy stand-alone electrode-type II. INSTRUMENTS AND METHODS sensors for extended periods without human intervention. A. Instruments The recently developed MOSFET-based sensors show a 1) Potentiometric sensor relatively good stability but require intricate and lengthy The potentiometric sensor is a combined glass-electrode and a location-dependent calibration procedures and still suffer from reference electrode (Ag/AgCl) in KCl gel. The potentiometric measurement drift after several weeks of deployment, requiring sensor uses an instrumentation amplifier circuit – whenever data correction [11]. referred to in text, the electrode voltage refers to the amplified signal. The potentiometric sensor can be outfitted with either a The paper presents the concept and initial results obtained from plastic or a stainless-steel casing, depending on weight a novel hybrid pH sensor using a combination of potentiometric constraints and on possible presence of electrical interferences, and colorimetric measurements to compensate for drift and and can perform high-frequency measurements (1 Hz) from generate high-frequency and accurate ocean pH data. Our aim surface to 2000 m depth in seawater and brines solution with is to develop an accurate (≤0.010 pH units) and precise (0.001 temperature ranging from -2 to 60 °C. pH units) high-resolution pH sensor that has the potential to allow global ocean acidification mapping through deployment 2) Colorimetric analyzer on fleets of ARGO floats and other autonomous platforms The colorimetric analyzer is based on the widely used meta- already in existence. The instrument implements a hybrid cresol purple (mCP) indicator for seawater pH measurement approach, utilizing the two different and complementary [12]. The pH indicator is added to the seawater and optical measurement techniques (potentiometric and colorimetric) to spectra are recorded using a combination of LED light sources generate temporally dense and highly-accurate pH data. The and a spectrophotometric detector, absorbance measurements hybrid system is ultimately aimed to be deployed on an being performed at 434, 578 and 700 nm. A fluidic manifold autonomous platform and will ensure the precise re-calibration integrates a pressure control block for reducing the underway of the potentiometric electrode by the periodic colorimetric supply pressure to 6 PSI, electro-fluidic valves for controlling measurements performed throughout the duration of the the flow of sample and reagent streams, a micromixer platform deployment. manufactured using micro-electromechanical systems (MEMS) technology and implementing the flow-folding principle, as We will start by characterizing a standalone electrode before well as MEMS hydraulic resistors for controlling flow rates presenting an initial ferry-box version of the hybrid sensor for throughout the microfluidic geometry. The fluidic manifold continuous underway measurements, and then present a new enables precise control of the reagent and sample dosage and version of the sensor that is adapted to surface water monitoring mixing prior to injection into a custom MEMS silicon/glass on moorings (currently under test). Results show that the 25mm-long optical cell. Coupling of the optical cell to LEDs potentiometric part of the sensor is capable to operate