With mass COVID-19 vaccinations being rolled out by countries across the world, there may be an end in sight for the ongoing pandemic. One long-term question that remains is whether the virus will eventually be eradicated or will instead become endemic, akin to a seasonal flu. Clearly with such an infectious disease, more rapid methods of diagnosis are essential to both assist in the current detection of cases and to avoid future mass outbreaks. Recent developments in graphene-based sensors can provide such methods.
Limitations of PCR tests
Polymerase chain reaction (PCR) tests are currently one of the main COVID-19 testing techniques. PCR is a highly sensitive technique, demonstrating a limit of detection of 100 copies of viral RNA per 1 mL of sample. Nevertheless, PCR tests have their limitations. Results may take a relatively long time to produce, with patients generally needing to wait a minimum of a few hours. The results themselves can also sometimes come out as false negatives, especially at very low detection levels.
Graphene-based sensors
Previously, we discussed the launch of a graphene-based respirator mask which exploited a microporous graphene filtration system (see our blog Graphene masks: facing up to coronavirus (COVID-19)). Graphene may also be used in sensing applications for the detection of COVID-19. Researchers from the Friedrich Schiller University Jena, the IMMS Institute for Microelectronic and Mechatronic Systems non-profit GmbH (IMMS) and FZMB, Research Center for Medical Technology and Biotechnology are developing a graphene sensor as part of their “Virograph” project. This focuses on the multiplex detection system for the detection of viruses based on graphene field-effect transistors (gFETs).
FETs are charge carrying devices which have three terminals: a “source”, a “drain” and a “gate”. When an electric field is applied at the gate, this changes the conductivity of a channel or recognition surface which is between the source and the drain. This change can be monitored allowing the FET to act as a sensor.
While FETs have been employed in other applications, such as in measuring pH values, a lack of sensitivity limited its use in the field of immunological diagnostics. For the purposes of COVID-19 testing, the ideal properties of any recognition surface are high conductivity, low electron transfer resistance and high surface-to-volume ratio. A FET utilising graphene (gFET) qualifies with all these characteristics and diminishes previously known weaknesses.
In the “Virograph” project, a new type of electronic gFET sensor, with the channel being a one nanometre thick graphene membrane, is able to capture the SARS-CoV-2 spike protein. Owing to the sensitivity of the technique, the accumulation of virus particles on the surface causes an observable change in the conductivity across the graphene surface which can then be quantitatively measured by an electrical impedance signal – the more virus present, the greater the signal.
Similarly, Caltech researchers used a graphene surface and functionalised it with antibodies and proteins as “hooks” which are specifically designed to capture the virus and aid in its detection.
It is the well-known high conductivity of graphene which allows the sensor to be sensitive to the smallest change in electrical currents to be measured, which can be in the range of a few nanoampere. Graphene also provides a very short response time of gFET, which can consequently provide rapid detection of the virus. The graphene has a high surface-to-volume ratio, which further increases the sensitivity of diagnosis even under very low detection limits.
In addition to the speed of results and sensitivity of the technique, graphene-based sensors provide some other advantages. Being easily printable, the 2-D material may find facile incorporation into lateral-flow assays. Production of testing kits can be a concern with millions of tests test done daily, but the relatively low cost of graphene is ideal for the fabrication of high-performance biosensing platforms.
Beyond sensing viruses
While some electronic applications involving graphene focus on the detection of viruses, there are also other applications involving graphene beyond such detection.
Graphene-based materials have been extensively explored for their antimicrobial potentials, with several studies demonstrating a broad degree of inhibition activity of graphene oxide against bacteria and fungi. In the fight against COVID-19, GrapheneCA developed a coating which may help protect people who come into contact with potentially infected surfaces.
Another area of interest is in making surgical masks easier to sterilise and re-use. Researchers at the Hong Kong Polytechnic University developed graphene masks with superhydrophobic and photothermal properties. They employed a laser manufacturing process to deposit a coating of graphene onto commercially available masks. The coating is superhydrophobic which reduces the possibility of infectious aqueous droplets adhering to them. Under exposure to sunlight, the functionalised mask may also be sterilised, owing to graphene’s strong light-absorption properties.
These alternative examples provide just a glimpse of how research can exploit the unique properties of graphene to combat the virus.
Outlook
The ultimate goal of the research discussed above is to have small, mobile devices available for incredibly rapid COVID-19 testing. This is why the Virograph project also aims to develop suitable miniaturised measurement technology for implementing into a “handy point-of-care device”, according to Dominik Gary from FZMB. The significance of point-of-care testing is to ensure that results are achieved in a matter of minutes, not hours, in a highly sensitive manner to make it suitable for use in fast-paced checkpoints like airports and other venues where high traffic is inevitable.
While such sensor technology has yet to be widely employed as the dominant method of testing, we may also see this expand into the detection of viruses other than SARS-CoV-2, especially in the prevention of and the fight against future pandemics. However these technologies evolve, though, it seems that once again graphene has the potential to be a game-changer.
