The potential difference between the reference electrode and the indicator electrode is measured at zero current flow. The ideally nonpolarizable reference electrode provides a constant potential, while the indicator electrode shows an erratic potential depending on the concentration of the analytes. The zero current potentials applied between those two electrodes are recorded as a function of the concentrations of target analytes in a logarithmic manner [48]. The potential difference at the electrode-electrolyte interface arising from unbalanced activities of species i in the electrolyte phase (s) and in the electrode phase (��) is related by the following Nernst equation:E=Eo+RTZiF ln aisai��(2)where E0 is the standard electrode potential, R the gas constant, T the absolute temperature, F the faraday constant, ai the activity of species i, and Zi the number of moles of electron involved.
Potentiometric sensors are divided into the metal-oxide sensitive field effect transistor (MOSFET), the light-addressable potentiometric sensor (LAPS), the ion-sensitive field effect transistors (ISFET) and the ion-selective electrodes (ISE). Ali et al. developed a commercial MOSFET using a GOx modified ZnO nanowire. GOx-functionalized ZnO nanowires were grown on the Ag wire and then directly connected to the MOSFET gate (Figure 1) [54]. They tested the response time and the stability of the MOSFET sensor by using three different GOx/ZnO modified Ag electrodes, i.e.
, vertically aligned, uniform nonaligned, and nonuniform nonaligned ZnO nanowires.
The results showed that well aligned ZnO-modified electrode displayed a good stability, short response time (<100 ms), and improved detection limit. They also further demonstrated that the GOx/ZnO modified MOSFET is able to be used for the immobilization of other biomolecules to make versatile electrodes for biosensing.Figure 1.Schematic Brefeldin_A illustration of the configuration of the MOSFET-based potentiometric glucose detection using an extended-gate functionalized-ZnO nanowire as a working electrode and the Ag/AgCl reference electrode (reproduced with permission from [54]. Copyright …
ISFETs and LAPS AV-951 have attracted much attention for biosensing application being especially convenient for construction. The principles of LAPS are based on the activation of semiconductor by a light emitting diode [55]. Seki et al. developed a LAPS based on SiO2/Al2O3 film grown on an n-type Si substrate. The GOx was immobilized on the film at various pH in the range of 3 to 11. Upon exposure to the light emitting diode, the equilibrium potential of the GOx-modified SiO2/Al2O3 film was increased linearly with the increase of glucose concentration up to 4 mM.