Changes in the Porosity and Permeability of a Molecularly Imprinted Membrane Induced by the Adsorption of a Trace Quantity of Template

Abstract

It is known that the diffusive permeability of solutes within a thin layer of molecularly imprinted polymer (MIP) may be affected by specific binding of the MIP with its template molecule. This phenomenon, termed the gate effect, shows promise for the development of novel biomimetic sensors. However, the mechanism underlying this effect is still unclear; although the relationship between the specific adsorption of a template and the corresponding porosity and permeability of the polymeric film or membrane is very important, this association has not yet been examined in detail. We therefore studied this relationship using a molecularly imprinted self-supporting membrane (MISSM) possessing chiral specificity, specially developed as a tool for investigating the gate effect. Both the diffusive permeability and volume porosity of the MISSM were sensitive to the presence of the template compounds (D and L-phenylalanine) at concentrations as low as 5 µM, while, at the same time, insensitive to the enantiomer of the template. The relationship between the amount of adsorbed template and the equilibrium template concentration followed the expected Langmuir isotherm pattern, which indicates the thermodynamic homogeneity of binding sites in the MISSM. This study also demonstrated that the relative concentration of the adsorbed template in the membrane was only 3 ppm and relative site occupation was only 1% following exposure to a 5 µM concentration of the template. These results show that the gate effect may be advantageously exploited during application of MIPs in amplifiers or sensors offering high sensitivity. Molecularly imprinted polymers (MIPs) are synthetic polymers that contain specific binding sites formed by imprinting of a target molecule (or template) during the polymerization process. An MIP layer can be prepared by a simple and economical procedure (1-3). Before such MIPs may be employed as molecular recognition elements in chemical-sensing devices, however, it is necessary to develop a means of translating specific binding events at the MIP into an electric signal. The so-called gate effect, which refers to changes in the diffusive permeability of solutes within the MIP layer resulting from specific binding at the template, can be used as a mechanism for signal transference based on conductometry, utilizing changes in the ionic permeability of the MIP membrane associated with a specific interaction with the template (4-6). In this gate analogy, the template corresponds to the key while the MIP site which allows specific rebinding with the template corresponds to the keyhole. An amperometric method is also applicable, using a thin MIP layer grafted onto an electrode, in which the template can be detected by following changes in the faradic current resulting from the change in permeability of a redox marker across the MIP layer (7-11). The gate effect is a selective process capable of discriminating between