an electrochemical biosensor for determining the level of a target chemical in a biological fluid includes an electrochemical system including a enzyme substrate which reacts with the target chemical to yield a system signal related to the concentration in the biological fluid of said target chemical . the biosensor includes a first membrane for immobilizing the enzyme substrate . the first membrane has a porosity permitting passage therethrough of the target chemical to react with the enzyme substrate and a surface exposed to the biological fluid characterized by electron donor site susceptible to facilitating attachment thereon of proteins and fibrin which impair the system signal . a second membrane is bonded to the electron donor sites of the first membrane . the second membrane is formed of a phenyl based polymer having connecting hydrogen atom donors which bond to the hydrogen atom donors bonding to the electron donor sites at least sufficiently to form an outer surface on the first membrane exposed to the biological fluid consisting of phenyl rings without significantly changing the porosity provided by the first membrane .