Bioélectrochimie, biointerfaces et biotechnologiesBioelectrochemistry, biointerfaces and biotechnologies
Cyclic voltammetry (CV) is a common electrochemical method used to study redox systems diffusing in the solution or adsorbed on the electrode surface. A typical setup consists of three electrodes: working electrode (WE), reference electrode (RE) and counter electrode (CE). The potential of WE (versus RE) is changed linearly with time between two border values E1 and E2 at a certain scan rate. The current passing between WE and CE is recorded, which may be positive (oxidation) or negative (reduction). The resulting current vs potential graph (voltammogram) depends on the nature of redox system. For the simplest fast one-electron redox system in the solution it represents two symmetric waves with peak-to-peak separation close to 57 mV at 25 °C. The midpoint potential (E1/2) between two peak is characteristic of the redox couple.
Upon careful experiment design CV can give a multitude of information about the thermodynamics (potential) and the kinetics (current) of the redox system. CV of adsorbed redox proteins has some peculiarities and it is often called protein film voltammetry. In the absence of enzyme substrate and when the electron exchange rate between the biomolecule and electrode is fast enough, an enzyme monolayer should give a pair of symmetric peaks whose charge is proportional to the enzyme coverage. These peaks are virtually undetectable for most of the large enzymes though, due to the consequent low coverage. The signals can be significantly amplified in the presence of enzyme substrate(s) giving rise to so-called catalytic current.
Besides a faradaic current from the redox system of interest, CV always has a contribution of non-faradaic current caused by the formation of the electric double-layer on the electrode boundery, so-called capacitive current. Depending on the electrode material and surface area (applicable to nanostructured electrodes in particular), capacitive current may reach high values and completely mask the faradaic response on CV.
Different formes of potential rampe can be used in such case to increase the contribution of the faradaic current compared with the capacitive one. All such methods make use of different kinds of potential pulses for this purpose, e.g. differential pulse voltammetry (DPV) and squarewave voltammetry (SWV). Thanks to the particular current sampling, the resulting voltammogram has minimal capacitive current contribution while the faradaic current waveform has broad diagnostic use.