Redox potential sensors

Fulco, M., Schiltz, R.L., lezzi, S., King, T.M., Zhao, P., Kashiwaya, Y., Hoffman, E., Veech, R.L. and Sartorelli, V. (2003) Sir2 regulates skeletal muscle differentiation as a potential sensor of the redox state. Molecular Cell. 

Redox potential pH Ionic activities Inert redox electrodes (Pt, Au, glassy carbon, etc.) pH-glass electrode pH-ISFET iridium oxide pH-sensor Electrodes of the first land and M" /M(Hg) electrodes) univalent cation-sensitive glass electrode (alkali metal ions, NHJ) solid membrane ion-selective electrodes (F, halide ions, heavy metal ions) polymer membrane electrodes (F, CN", alkali metal ions, alkaline earth metal ions)... 

Recently, Yamazoe et al. observed an extremely high hydrogen sensitivity for Ag-Sn02 sensors ([16). They attributed this to the Fermi level of SnOx being pinned at the redox potential of Ag+/Ag° when the sensor was in air, and at the work function of Ag°... 

N. Oba, T. Yoshinobu and H. Iwasaki, Redox potential imaging sensor, Jpn. J. Appl. Phys. Pt. 2 -Lett. 35 (4A) (1996) L460-L463. 

Preorganisation of redox anion sensors on electrode surfaces is a promising new technique for electrochemical anion sensing. Self-assembled monolayers or thin polymer films of metal-based receptors have the potential to generate an amplified response to anion binding akin to the dendritic effect. 

An alternative to light-related detection is an electrochemical response. If the sensor and analyte are in solution then cyclic voltammetry can be used to detect changes in redox potential between the free sensor and its complex with the analyte. Supramolecular applications of this approach were pioneered by Beer who linked crown ethers to electrochemically responsive ferrocenium [1] and cobalticinium [14] groups. In the former case a response was detected when cations complementary to the crown ether cavity were added to acetonitrile solutions of the sensors in the latter, anions were detected by an acyclic receptor. 

Redox potential is measured potentiometrically with electrodes made of noble metals (Pt, Au) (Fig. 12). The mechanical construction is similar to that of pH electrodes. Accordingly, the reference electrode must meet the same requirements. The use and control of redox potential has been reviewed by Kjaergaard [218]. Considerations of redox couples, e.g. in yeast metabolism [47], are often restricted to theoretical investigations because the measurement is too unspecific and experimental evidence for cause-effect chains cannot be given. Reports on the successful application of redox sensors, e.g. [26,191], are confined to a detailed description of observed phenomena rather than their interpretation. 

Protons are relatively simple targets for sensor molecules and do not require engineered receptors, however, achievement of selective interactions with other chemical species requires much more elaborate receptors. In the most cases cations are bound via electrostatic or coordinative interactions within the receptors alkali metal cations, which are rather poor central ions and form only very weak coordination bonds, are usually bound within crown ethers, azacrown macrocycles, cryptands, podands, and related types of receptor moieties with oxygen and nitrogen donor atoms [8], Most of the common cation sensors are based on the photoinduced electron transfer (PET) mechanism, so the receptor moiety must have its redox potential (HOMO energy) adjusted to quench luminescence of the fluorophore (Figure 16.3). 

Molybdenum and tungsten complexes with three crown ether benzenedithio-lene ligands (21) have been reported (105) and the effect of alkali ion binding has been probed by CV (106). Upon binding with Li+, Na+, or K+, positive shifts in the redox potential have been observed for all complexes. This observation suggests that the tris(crown ether benzodithiolene) complexes of Mo and W may potentially be useful as sensors for alkali metal cations (106). 

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