A sensor for a selected analyte in a test sample has (a) a semipermeable membrane with pores for retaining the analyte, where the membrane has been chemically modified by attachment of membrane modifiers; (b) immunoassay labels which have label binding ligands where these label binding ligands will have a binding affinity for the membrane modifiers in the presence of the analyte, and a measurably different binding affinity for the membrane modifiers in the absence of the analyte; and (c) a label detecting system, for detecting the presence of the labels on the membrane.