Heterogeneous membrane electrodes

An electrode for use in an electrochemical sensing device for the analysis of an aqueous sample, comprising;

• an insulating substrate layer having first and second sides, the insulating substrate layer having an opening defining an electrode region;
• an electric conductor located on the first side of the insulating substrate layer and closing the aperture on the first side;
• and
• a dry heterogeneous membrane for direct contact with the sample, the heterogeneous membrane being located on the second side of the insulating substrate layer and closing the aperture on the second side, the heterogeneous membrane being in electric contact with the conductor in the electrode region, the heterogeneous membrane including a hydrophobic compartment, a hydrophilic compartment and a dry internal reagent reservoir, the hydrophobic compartment being gas permeable and having a water vapor diffusion coefficient, the hydrophilic compartment being electrolyte salt permeable and having an aqueous electrolyte diffusion coefficient, wherein the hydrophobic compartment is in excess of the hydrophilic compartment for the water vapor diffusion coefficient of the hydrophobic compartment to be higher than the aqueous electrolyte diffusion coefficient of the ion conducting electrolyte compartment to maintain the electrode in a substantially dry, inactive condition at ambient conditions and to render the electrode convertible into a wet-up, active condition by water vapor penetration through the hydrophobic compartment into the internal reagent reservoir upon exposure of the heterogeneous membrane to an aqueous liquid.