Cyclic voltammetry is an analytical technique within electrochemistry wherein the current passed through a working electrode is ramped over time and then ramped in the opposite direction to return to the original electrode potential. Cyclic voltammetry is a powerful tool for probing electron transfer in reacting species in the presence of the electrodes, including biological mechanisms such as enzyme activity or protein folding events.
This article will discuss some of the applications of cyclic voltammetry within the life sciences and reflect on the future potential of the technique in clinical diagnostics.
Demystifying Cyclic Voltammetry in the Life Sciences
Cyclic voltammetry can be performed in a variety of solutions, including ionic buffer solutions suitable for the stability of sensitive biomolecules such as proteins, nucleic acids, lipids, and others.
The working electrode is commonly constructed from materials such as glassy carbon, platinum, and gold contained within a protective tube. A reference electrode is included in the cyclic voltammetry probe apparatus, usually containing silver salt and thus Ag+ ions, which generates a known potential difference to act as a control.
The purpose of the counter electrode is to provide the opposite surface to the working electrode upon which oxidation or reduction can occur, completing the electrical circuit. To not inhibit activity at the working electrode, the counter electrode is typically constructed from platinum and possesses a larger surface area.
Electrochemical Probes into Biomolecules
Various proteins undergo redox changes associated with structural changes as a result of bonding, electron transfer, or other environmental changes and thus generate distinctive cyclic voltammetry voltammograms that are quantitatively indicative of the ongoing process.
Proteins and other biomolecules, such as DNA, can be immobilized at the electrode surface, which may be constructed from specially compatible materials, such as SnO2, either electrostatically or by more permanent covalent interactions. Cyclic voltammetry can, therefore, be used to investigate the function of proteins and probe their activity, such as the binding affinity of an antibody to a particular antigen.
Insights into Disease Markers and Diagnostic Biomarkers
The ability to functionalize the surface of cyclic voltammetry probes with biomolecules allows them to be used in the highly specific detection of diagnostic biomarkers. For example, surface coating with antibodies specific to the SARS-CoV-2 spike protein has allowed the quantitative detection of the virus in biological samples.
Similarly, many diseases, such as cancer and various neurodegenerative conditions, generate specific biomolecules in abundance that are complimentary to specific enzymes or other biomolecules that can be coated onto the electrode surface.
These conditions are also known to cause a rise in oxidative stress in the surrounding tissue, as does inflammation and infection, which may also be detectable using cyclic voltammetry. In this case, the electrode can be coated with enzymes such as lactate dehydrogenase, which then engages in electron transfer with hydroxyl and peroxyl radicals in solution, reactive oxygen species indicative of oxidative stress.
Pharmaceuticals and Drug Development
Most drugs exert biological activity by bonding with proteins or other biological targets in a "lock and key" mechanism, wherein the specific shape and orientation of the drug molecule and it's functional groups allow them to interact with one another, very often by the insertion of the drug into a hydrophobic pocket on the protein, causing conformational change.
If the target protein or other biomolecule, such as DNA, is known, then it can be coated onto the cyclic voltammetry electrode and exposed to lead drug compounds for testing. Those with the greatest affinity for the drug target will produce the most rapid and intense change in current as strong bonds are formed, and then less easily break as the direction of current is reversed.
This type of study can be expanded to a wide range of drug-biomolecule interactions, and even within in vitro cell cultures, allowing the metabolic products of a collection of cells to be interpreted quantitatively over time.
For example, this could allow the extended examination of the generation of reactive oxygen species by cancer cells and the cellular response to oxidative stress. Cyclic voltammetry can also be used to study the redox properties of the drugs themselves in various synthetic biological compartments in specific media and with other particular environmental conditions, providing insight into the metabolic processes of the compound and, thus, it's pharmacokinetic and pharmacodynamic properties.
Challenges and Future Perspectives
Improvements in the construction and design of cyclic voltammetry electrodes, such as nanostructured materials like graphene, will further enhance the sensitivity and selectivity of detectors. In part, this is due to the comparatively massive surface area of nanostructured materials, allowing many more probes to be bound to the electrode for interaction with the target analyte.
These materials also offer enhanced conductivity and other unique optical and electronic properties that may be exploited in functionalizing the surface with probes. Many sub-types of cyclic voltammetry apparatus operate by measuring differing aspects of the electric circuit, such as impedance or voltage, and in many cases, these may be more sensitive and reliable than ordinary cyclic voltammeters.
Compared to many other laboratory-based analytical techniques, cyclic voltammetry can be highly cost-effective and efficient once the necessary electrodes are in place, which are generally expensive, being constructed from materials such as platinum, and are highly liable to degradation if functionalized with specific enzymes for the purpose of disease diagnosis or drug activity characterization.
In conclusion, cyclic voltammetry is a powerful analytical tool in the interpretation of redox mechanisms in chemistry, extending into the realm of biochemistry, including biological interactions and drug-target affinity. As innovative new materials are generated that improve and expand the gamut of electrodes available with high specificity and selectivity, cyclic voltammetry may be exploited increasingly within diagnostic healthcare.
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