Transmembrane potential difference electrode measurement

Overall, measurements of transmembrane potential differences in living cells is fairly well established with the methods fairly reliable and robust. These mostly take on two strategies either utilising electrodes or optical tools such as fluorescence to record the potential differences (Vm or Ai/ ) based on electrochromism or probe accumulation according to the Nernst equation (5.3). 

The membrane potential, that is, the potential difference between the two phases separated by the membrane, is the sum of the two phase-boundary potentials and the transmembrane potential difference. The latter is negligible in all practical cases since only one dominating exchangeable ion prevails in the manhrane. The phase-boundary potential on the inner membrane side does not depend on the composition of the sample and is thus constant. The observed electromotive force is the sum of all contributions in a measuring cell. Besides the ISM, the only sample-dependent contribution comes from the reference electrode. In well-designed systems, its contribution is nearly constant and can be included together with the other contributions, such as the transmembrane potential and the phase-boundary potential on the inner membrane side, into one potential term of the cell, K . Thus, the observed electromotive force, anf, is... 

Channel activity is best studied electrochemically as charged species cross a cell membrane or artificial lipid bilayer. There is a difference in electrical potential between the interior and exterior of a cell leading to the membrane itself having a resting potential between -50 and -100 mV. This can be determined by placing a microelectrode inside the cell and measuring the potential difference between it and a reference electrode placed in the extracellular solution. Subsequent changes in electrical current or capacitance are indicative of a transmembrane flux of ions. 

In potentiometric biosensors the biological recognition reaction causes a modulation of a redox potential, a transmembrane potential, or the activity of an ion. So the potentiometric biosensors utilize the measurement of a potential at an electrode in reference to another electrode (Bard and Faulkner, 1980). Mostly, it is comprised of a permselective outer layer and membrane or sensitive surface to a desired species (a bioactive material), usually an enzyme. The enzyme-catalyzed reaction generates or consumes a species, which is detected by an ion-selective electrode. Usually a high-impedance voltmeter is used to measure the electrical potential difference or electromotive force (EMF) between two electrodes at near-zero current. The basis of this type of biosensor is the Nemst equation, which relates the electrode potential (E) to the concentration of the oxidized and reduced species. For the reaction aA + ne bB, the Nemst equation can be described as the following. 

Potentiometry is a method in which the electrochemical cell potential is measured at equilibrium at which the current is zero. The properties of the interface region differ from the bulk properties. A potential is established at the phase boundaries, e.g., between the solution and the electrode surface. The potential of electrochemical cells is the sum of all interface potentials including electrode/electrolyte interface and liq-uid/Uquid interface (i.e., the two electrolyte solutions of different compositions that are in contact with each other). Ideally the measured potential should depend only on the potential between the interfaces of interest for analytical purpose. This is typically accomplished by keeping all other interfaces constant through a suitable electrode construction. Potentiometric sensors (e.g., ion selective electrodes) usually consist of a manbrane that contains ion exchangers, lipophilic salts, and plasticizers, and the transmembrane potential gives the activity of the analyte ion in solution. 

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