Enzyme electrode with enhanced specificity using outer polymeric membrane doped with substrate selective ditopic carrier

A new method for enhancing the specificity of enzyme electrodes in which th e immobilized enzymatic layer is a relatively nonselective biocatalyst is r eported. The approach is based on the use of an outer polymeric membrane do ped with a substrate selective carrier that transports the target analyte s electively into the underlying enzyme layer. As an initial model for this c oncept, an amperometric enzyme electrode with enhanced selectivity for tryp tophan is described. The sensor is prepared by coating a layer of L-amino a cid oxidase (AAO) on the surface of a platinum working electrode and then c overing this layer with an outer hydrophobic plasticized polyurethane (PU) film containing the ditopic carrier, manganese(III)-4,5-di(3,5-di-t-butylsa licylideneimine)benzo-18-crown-6 tetraphenylborate (Mn(III)tBuSalph-B 18C6( BPh4). Tryptophan is transported selectively from sample solution by the ca rrier into the L-amino acid oxidase layer, yielding production of hydrogen peroxide that can be detected amperometrically at the platinum electrode at 650 mV (vs. Ag/AgCl reference electrode). The resulting enzyme electrode i s shown to exhibit a dramatic increase in selectivity for tryptophan when c ompared to the amino acid response of an analogous enzyme electrode prepare d with a nonselective outer dialysis membrane. Independent measurements of the PU-carrier membrane's substrate permselectivity using a diffusion cell correlate with the amperometric enzyme electrode results. The practical lim itations of this approach for preparing enzyme electrodes (i.e., slower res ponse times and reduced substrate fluxes) are discussed and potential solut ions to these problems are proposed.