Ion-gate membranes

P Burgmayer, RW Murray. An ion gate membrane Electrochemical control of ion permeability through a membrane with an embedded electrode. J Am Chem Soc 104 6139-6140, 1982. 

In addition, temperature-responsive properties of PNIPAAm have also been used in fabricating molecular ion gating membranes.37 38 In these studies, the concept of an ion gating molecular recognition membrane using synthetic host substances and a thermosensitive polymer was proposed. Figure 15.1038 illustrates the concept. The membrane was prepared by plasma graft copolymerization, which filled the pores of a porous polyethylene him with a copolymer of A -isopropyl... 

Figure 15.10 Schematic representation of a molecular recognition ion gating membrane. The copolymer of thermosensitive polymer fV-isopropylacrylamide, and crown ether polymer, BCAm, is grafted on the pore surface. The membrane senses a specific ion Mn+, by the crown receptor, and closes its pores rapidly.38 (Reprinted with permission from T. Ito et al., J. Am. Chem. Soc. 2002,124,7840—7846. Copyright 2002 American Chemical Society.)...

A novel principle for increasing the selectivity is the use of an ion gate membrane. Here, the ion permeability is controlled by electrochemical regulation of the redox state of the membrane material. For example, a polypyrrole film has to be in the reduced state to become permeable for anions. If the film is oxidized, no anion can permeate (Burgmayer and Murray, 1982). 

K. Kontturi, L. Murtomaki, P. Pentti, and G. Sundholm, Preparation and properties of a pyrrole-based ion-gate membrane as studied by the EQCM, Synth. Met., M(2), 179-185 (1998). 

Ito, T. Yamaguchi, T. Osmotic pressure control in response to a specific ion signal at physiological temperature using a molecular recognition ion gating membrane. J. Am. Chem. Soc. 2004,126, 6202-6203. 

Yamaguchi et al. demonstrated the multi-sensitive behavior of the ternary copolymer 8 of [3-(methacryloylamino)propyl]trimethylammonium chloride, acrylic acid, and benzo[18]crown-6-acrylamide. This system controlling thermosensitivity by cation recognition of the crown ether was applied to an ion gating membrane. 

The ISFET is an electrochemical sensor based on a modification of the metal oxide semiconductor field effect transistor (MOSFET). The metal gate of the MOSFET is replaced by a reference electrode and the gate insulator is exposed to the analyte solution or is coated with an ion-selective membrane as illustrated in Fig. 

Burgmayer and Murray [40] reported electrically controlled resistance to the transport of ions across polypyrrole membrane. The membrane was formed around a folded minigrid sheet by the anodic polymerization of pyrrole. The ionic resistance, measured by impedance, in 1.0 M aqueous KC1 solution was much higher under the neutral (reduced) state of the polymers than under the positively charged (oxidized) state. The redox state of polypyrrole was electrochemically controlled this phenomenon was termed an ion gate, since the resistance was varied from low to high and vice versa by stepwise voltage application. 

In ion-selective FETs (ISFETs), an ion-selective membrane replaces the gate electrode. When an enzyme-loaded gel is combined with the membrane, the device can be used to measure substrates which enzymically generate charged species. 

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