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International sensor development network
  Autonomous Nutrient Electrochemical Sensors In Situ




Development of electrochemical sensor for in situ detection of phosphates in sea water:

  • Development of electrochemical protocol to detect phosphates in situ without any addition of liquid reagents

  • Development of a method to overcome the interference of silicates

Improvement of STOX sensor in order to determine dissolved oxygen concentration in oxygen minimum zones (OMZ)


Scientific and societal issues


Oceans play a role in natural regulation of CO2 in the atmosphere and thus exert a powerful influence on the climate. Many factors can influence carbon cycle such as temperature, salinity, availability of light and nutrients. Phosphate is one of the main components of the biologic carbon pump. By monitoring phosphate concentration and other factors (nutrients, oxygen…), information on biological activity and on water mass mixing can be deduced. It is thus critical to monitor with long time series phosphate concentration in the ocean. To palliate the issue of under-sampling in the ocean, in situ, miniaturized, autonomous instrumentation is needed. Electrochemical methods to detect phosphate have been chosen to reach this goal.


Dissolved oxygen is another important chemical element involved in biogeochemistry and needs to be properly monitored, particularly in the oxygen minimum zones (as off Peru, Chile or Namibia) where ultra low concentrations are observed. In order to decrease the limit of detection, an improvement of the actual STOX electrochemical sensor was needed. The STOX sensor is an amperometric sensor integrating 2 Clark’s electrodes.

General organisation

Research and development during the SENSEnet project consisted of:

  • Development of electrochemical method to detect phosphates in situ without liquid reagent addition

  • Development of an in situ silicate interference free method

  • First design of miniaturized prototype

  • Improvement of STOX sensor geometry to detect ultra low oxygen concentration



Work performed on STOX sensor was carried out in Denmark at Aarhus University in collaboration with UNISENSE company. A modification in the STOX sensor fabrication, using wet chemical method to form gold layer instead of electrodeposition of gold, allows to decrease the distance between the two gold electrodes and then decrease the time response to 7 s instead of 16 s and increase the sensitivity of the sensor to 2 or 3 times.

Electrochemical detection of phosphates without addition of liquid reagent requires the addition of a molybdenum electrode that will produce molybdates and protons ions during its oxidation, useful to complex phosphates. In order to avoid silicates interference and form only the phosphomolybdic complex, a 3rd compartment is added into the electrochemical cell. This new compartment has a 2nd molybdenum electrode and is connected to detection compartment through a proton exchange membrane. Therefore, by oxidizing this 2nd Mo electrode, a ratio H+/MoO42- of 70 is obtained, allowing to detect only phosphates.

A laboratory cell has been produced and tested during the SENSEnet project. A miniaturized cell is now in construction and will be characterized during the SenseOCEAN European project (FP7).

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