Effect of electrostatic interactions on gate effect in molecularly imprinted polymers

The mechanism of the change in diffusive permeability of nanometer-ordered thin layers of molecularly imprinted polymer (MIP) in the presence of its template is examined and discussed based on electrostatic interactions. In this work, the theophylline-imprinted copolymer of ethyleneglycol dimethacrylate and methacrylic acid (Theo-MIP) is grafted onto indium-tin oxide (ITO) as an electrode for cyclic voltammetry of ferrocyanide with the grafted ITO, and the permeability of the Theo-MIP is estimated from the faradic current. The permeability is found to decrease with increasing pH, and the change in permeability due to the presence of the template is found to decrease with increasing concentration of the supporting electrolyte. These results indicate that the layer of grafted copolymer swells due to electric repulsion between carboxyl groups, representing a major factor in the gate effect of the Theo-MIP. If the grafted layer is considered to be porous, the porosity of Theo-MIP should increase as the polymer shrinks, and decrease as it swells. The increase in the permeability of the MIP in the presence of the template is therefore due to the increase in porosity due to shrinking.