An electrode for an electrochemical sensing device for the analysis of an aqueous sample, comprising - an electric conductor; - an insulating layer on the conductor, the insulating layer having a throughgoing aperture defining an electrode region; and - a dry heterogeneous polymeric membrane for direct contact with the sample, the heterogeneous polymeric membrane being in electrical contact with the conductor in the electrode region and in physical contact with the insulating layer over the electrode region, the heterogeneous polymeric membrane comprises two components as physically separate compartments, - a hydrophobic compartment and - a hydrophilic compartment, wherein the hydrophilic compartment constitutes an internal reagent reservoir, the membrane material having interpenetrating networks of the two compartments, 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 present in large excess by volume over the hydrophilic compartment for heterogeneous membrane to support water vapour transport through the hydrophobic compartment and constrained electrolyte transport through the hydrophilic compartment for the membrane material layer to be in a dry, inactive condition, but being convertible to a wet-up, active condition by water vapor transport through the hydrophobic compartment and then by equilibrium partitioning from the hydrophobic compartment to the hydrophilic compartment and the internal reagent reservoir upon exposure to the aqueous sample, characterized in that the hydrophobic compartment contains photo-cross-linking entities which are cross-linked by photo-irradiation, or wherein both the hydrophilic and the hydrophobic compartment are cross-linked. - an electric conductor; - an insulating layer on the conductor, the insulating layer having a throughgoing aperture defining an electrode region; and - a dry heterogeneous polymeric membrane for direct contact with the sample, the heterogeneous polymeric membrane being in electrical contact with the conductor in the electrode region and in physical contact with the insulating layer over the electrode region, - a hydrophobic compartment and - a hydrophilic compartment, wherein the hydrophilic compartment constitutes an internal reagent reservoir, - gas permeable and having a water vapor diffusion coefficient, - electrolyte salt permeable and having an aqueous electrolyte diffusion coefficient, - the hydrophobic compartment is present in large excess by volume over the hydrophilic compartment for heterogeneous membrane to support water vapour transport through the hydrophobic compartment and constrained electrolyte transport through the hydrophilic compartment for the membrane material layer to be in a dry, inactive condition, but being convertible to a wet-up, active condition by water vapor transport through the hydrophobic compartment and then by equilibrium partitioning from the hydrophobic compartment to the hydrophilic compartment and the internal reagent reservoir upon exposure to the aqueous sample,