MAISOE

MAISOE - ANESIS
Microlaboratoires d’Analyses In situ pour des Observatoires Environnementaux
Autonomous Nutrient Electrochemical Sensors In Situ

Scientific and societal issues

Development of in situ oceanic autonomous and multidisciplinary observatories is the XXIst century challenge in oceanography. This climatic monitoring requires instrumentation able to achieve excellent figures of merit: long lifetime, high precision and reproducibility, low detection limit, low energy requirements and resistance to biofouling and high pressure. Nutrients such as silicates, (Si(OH)₄), phosphates (PO₄^3-) and nitrates (NO₃^-) are ocean tracers. Their knowledge is used to evaluate the mixing of oceanic water masses and determine their origin. Long term monitoring enables to determine if water masses have evolved. It is possible, after deconvolution, to obtain information on biological activity of the ecosystem. Numerical models need in situ data. Changes in ecosystems and biogeochemical cycles in the context of global change have to be monitored over long periods of time. Sensors implemented onto these biogeochemical observatories do provide this sea truth. Networks of autonomous platforms implemented with chemical and/or physical sensors are the solution to solve the critical issue of under sampling in the coastal and open ocean. So far, more than 3,600 Argo floats equipped with physical sensors CTD (conductivity, temperature, depth) are dispersed into the oceans while only fourty are implemented with biogeochemical sensors (nitrate). There is no autonomous sensor to detect silicates and phosphates adaptable on underwater vehicles.

To address this problem, electrochemistry has been chosen as a detection method to develop autonomous, miniaturized, low power consumption sensors, to detect nutrients in situ without liquid reagents addition or calibration step.

Electrochemical cell for silicates detection without reagent addition (molybdenum oxidation)	1st prototype of ANESIS-Silicate with all the electrodes in the same cell

General organisation

 Research and development during the MAISOE-ANESIS project consisted of:

• Feasibility study of nitrates detection by electrochemistry

• Development of electrochemical detection protocol of silicates without any addition of liquid reagent and any calibration step

• Development and fabrication of silicates sensor prototype (mechanical study)

• Electrochemical characterization of microdevices in silicon technology integrating electrodes needed for silicomolybdic complex detection

• Low consumption electronics development

Results

The MAISOE-ANESIS project resulted in four publications (fifth one is in progress) and the deposit of a "SOLEAU envelope" in May 2013 which will be followed by a patent in 2015.

This project has enabled increasing the technological readiness level (TRL) from 3 to 6 for silicates sensor and 1 to 3 for nitrates detection. The validation of prototypes incorporating molybdenum oxidation step, silicomolybidic complex formation and electrochemical detection is in progress in terms of mechanics. Characterisation of silicon technology microdevices showed better stability of platinum than gold layers. Work in electronics has been taken over by the French company NKE currently developing complete sensor electronics and its integration. A full version of ANESIS-Silicate sensor will be manufactured in 2014 during the European project SenseOCEAN (FP7).

Research on nitrate detection showed that a gold electrode covered with silver nanoparticles allows to decrease the limit of detection thanks to the catalytic reaction in oxygenated medium. This work is under progress with new funding from RTRA-STAE (MIACTIS project) as part of the ICE initiative (Instrumentation and Environmental Sensors).

2nd prototype of ANESIS silicate sensor showing the pump, Mo oxidation cell and detection cell.

Mechanism of nitrate electrocatalytic reduction on gold electrode covered by silver nanoparticles