The ion-step method applied to disposable membranes for the development of ion-responding immunosensors

Abstract

Recently, a new method was introduced to operate an immunological field-effect transistor (ImmunoFET). By means of a so-called ion step, a transient diffusion of ions through a protein-charged membrane layer occurs, resulting in a transient membrane potential. The fixed charge density of the membrane, which is influenced by pH and the type of protein, determines the maximum transient membrane potential. The change in charge density of the membrane gives a direct measure of an immunochemical reaction. In this paper the ion-step method, applied to disposable filter strip membranes, is described. If on one side of a filter strip membrane (a porous carrier in which proteins can be incorporated) a stepwise salt concentration change is applied (an ion step), ions are transported through the membrane from the high towards the low salt concentration. The difference between the mobilities of the anions and the cations results in a potential difference over the membrane which is measured with two calomel reference electrodes. The formation of the potential difference across the membrane as a result of the ion step agrees with the results obtained with the ImmunoFET. In comparison with the ImmunoFET, the measurements are simpler, faster, and less expensive. Theoretical investigations of the intrinsic properties of proteins, such as the isoelectric point, amphoteric character, and charge-density-dependent effects (for instance, the formation of immune complexes) are easy to perform with this new technique. However, it is expected that it will not be applied to detect immunochemical reactions in the near future.