One of the acute problems we face is the lack of a rapid test to detect SARS CoV-2, the
virus that causes COVID-19. The nations that have instituted widespread testing have fared
better in the current pandemic. They have been able to identify those individuals who
have been infected with the virus and track down their close contacts for quarantine.
We require an inexpensive mass-produced test for rapid screening.
The conventional test for the virus is reverse-transcription polymerase chain reaction (or RT-PCR) test.
This involves prep work in the lab to isolate the viral RNA, make a DNA copy from the RNA
template, and then make multiple copies of DNA using polymerase chain reaction. From
sample collection to result, the test usually takes a few hours to a couple of days.
This new discovery is very exciting. A group of scientists in the Republic of Korea have
created a bio-sensor for SARS-CoV-2. They designed a device that would detect the spike
protein which is displayed on the outside of the virus particle. The graphene field
effect transistor (or GFET) was derivatized with antibodies for the spike protein. This
discovery is a marriage of several disciplines including electronics, nanofabrication, chemistry
and virology to address a pressing medical need.
Let’s take a closer look at how they did this.
The team took advantage of the remarkable electronic properties of graphene. Graphene
is a sheet of carbon a single atom thick. Its structure is like an extended network
of benzene rings. The freely mobile electrons in the conjugated pi bonds create a highly
conductive material.
To construct a graphene-based transistor, the team used established nanofabrication
techniques. They used a wet-transfer method to deposit graphene onto a silicon dioxide
substrate. Channels were formed in the graphene layer using photolithography and reactive
ion etching forming orderly patterns of graphene on the substrate.
Then the metal electrodes were added using a thin-film deposition and lift-off method.
In this way, Graphene bridges the source and drain electrodes. Each transistor measured
100 by 100 microns. At this point the graphene was ready to be derivatized to make a COVID-19
sensor.
Let’s zoom in to see what’s going on at the molecular level. The graphene was soaked
with a PBASE solution. PBASE is a linker that contains a pyrene group that non-covalently
attaches to graphene through ? ? ? stacking. At the other end is an activated ester which
reacts with amines. Then they added the SARS-CoV-2 spike protein antibody which reacted with
the linker to form a chemical bond.
After the graphene was derivatized with antibody, they could test their device’s sensitivity.
Their design used a buffered water droplet with an electrode as the transistor’s gate
(a so-called aqueous-solution gated FET). Now, if a voltage is applied across the source
and drain electrodes, a current flow through the graphene layer can be measured as a function
of the gate voltage. Next, they tested whether their transistor would respond to the spike
protein. The spike protein bound to the antibodies
with high affinity. This event changes the charge distribution in the immediate vicinity
of the graphene layer thus changing its electrical conductivity. The change in conductivity alters
the amount of current that can flow between the source and drain electrodes. They established
that their device could detect the spike protein down to 1 femtogram per milliliter, a very
high sensitivity. Next, they tested the response to a related
but distinct spike protein from the MERS virus. They found that their sensor had no response
to this spike protein, establishing that their GFET sensor was speci?c for COVID-19.
They then established that the sensor could detect the whole virus. Their GFET sensor
detected concentrations as low as 16 PFU per milliliter. A PFU is a unit defining the number
of virus particles. They also showed that addition of higher concentrations of virus
gave a linear increase in response. To top it all off, they took an actual swab
sample from a COVID-19 patient and demonstrated that their device could detect the virus,
even if diluted down 100,000-fold. [pause] The higher the concentration of virus in the
sample, the greater signal they saw in their biosensor.
They carried out the negative control experiment also: a swab sample from a healthy, uninfected
person showed no response. This critical experiment demonstrated proof of concept that the device
could be used to distinguish between healthy and infected people.
This research team has designed and constructed the first COVID-19 biosensor based on GFET
technology. They have demonstrated that their device is fast, selective and highly sensitive.
What’s more, they can detect virus directly from patient swab samples without lengthy
sample prep in the lab. The device has not been approved by regulatory agencies for diagnostic
use in people, yet. A number of validation studies establishing the stability of the
test will need to be carried out first. Congratulations to the research team for making
a timely and impactful contribution. And thank you for watching!