Silicon-based electrochemical microdevices for silicate detection in seawater

This paper describes the electrochemical characterisation of gold and platinum microdevices mass fabricated using silicon technology. Specific attention was paid to allow in situ electrochemical detection of silicate in seawater. Thus, using a silicon nitride (Si3N4) inorganic passivation layer patterned using Inductively Coupled Plasma Chemical Vapor Deposition (ICP-CVD), coupled with a non-aggressive liftoff based process, different electrodes were isolated electrically: one gold or platinum working electrode named "macroelectrode" (2 mm of diameter), four gold or platinum working ultramicroelectrodes (UME) (15 mu m of diameter), one platinum counter electrode and one silver electrode which can be used as a reference electrode after its chlorination. Their small size and mass fabrication make them very promising for oceanographic applications. As some components of microdevices release silicate and contaminate the solution, after being immersed in seawater, these microdevices were inserted in a specific cell that only puts the electrodes in contact with the seawater solution. Gold has been tested as a possible material for working electrodes but its lack of adherence to the passivation layer in seawater solutions led to nonaccurate measurements. On the contrary, pas sivation layer on platinum electrodes resists to the seawater corrosive medium. The analytical performances of the platinum microdevices has been tested through different silicate calibrations and shows an outstanding accuracy and reproducibility when measurements are performed, especially with the macroelectrodes which showed only 2.8% signal variation after four months of use and a limit of quantification of 0.50 timol L-1 suitable for oceanographic applications. (C) 2015 Elsevier B.V. All rights reserved.