The present application discloses a planar enzyme sensor for measuring the concentration of an analyte in a solution comprising a substrate of an electrically insulating material supporting an electrode layer of an electrically conductive material. The substrate and electrode layer have a plurality of layers disposed thereon which include an enzyme layer and a microporous outer layer covering the enzyme layer, wherein the outer layer comprises a continuous phase of a water-resistant polymer (e.g. a polyvinylacetate or an acrylate copolymer), a protein (e.g. an enzyme) embedded in the continuous phase, and possibly polytetrafluoroethylene particles. The enzyme and the polytetrafluoroethylene particles provide a controlled porosity to the outer membrane.