New Graphene Chemeo-Phononic Test for SARS-CoV-2 May Challenge PCR Assays
Sheets of functionalized graphene quickly detected the SARS-CoV-2 virus in research conducted at the University of Illinois Chicago (UIC), paving the way for an alternative to PCR testing in the form of an accurate, real-time optical test for COVID-19.
The graphene chemeo-phononic system described by UIC researchers in ACS Nano detects the effect of the spike protein on the phononics of graphene functionalized with antibody. “It appears to distinguish SARS-CoV-2 spike protein from a complex mixture of other proteins and enzymes in artificial saliva as well as another member in the betacorona virus family: MERS-CoV spike protein,” according to Professor Vikas Berry, head of chemical engineering at the UIC College of Engineering and senior author of the paper, and colleagues.
Because graphene is only one atom thick, any molecule attaching to its surface causes a noticeable change in the electronic energy. Therefore, both sensitivity and specificity are high.
The limit of detection was approximately 3.75 fg/mL in artificial saliva and about 1 fg/mL in phosphate-buffered saline. Importantly, the detection was specific to the SARS-CoV-2 spike protein. The system did not react to proteins in artificial human saliva or to the MERS-CoV spike protein. Results were returned in about five minutes.
In this detection system, antibody to the SARS-CoV-2 virus is chemically attached to graphene, which functions as a Raman transducer. That enables this graphene chemeo-phononic system to directly measure changes in the excitation of molecules in sheets of graphene (a process called graphene phononics).
The detection system has the potential to become a platform diagnostic technology. “The surface chemistry can be modified to diagnose other viral variants and diseases,” the authors wrote. This includes detecting COVID-19 variants and whole viral particles.
“We have been developing graphene sensors for many years. In the past, we have built detectors for cancer cells and amyotrophic lateral sclerosis (ALS),” Berry said in an interview with The Engineer, a British publication.
For COVID-19 diagnostics, the researchers functionalized the graphene with a SARS-CoV-2 spike RBD antibody. The proximity of the spike protein to the antibody triggers a blue-shift in the 2D peak when analyzed by Raman spectroscopy and, therefore, detection.
Unlike some other diagnostic approaches, graphene chemeo-phononics measures the graphene’s reaction to the binding of the SARS-CoV-2 spike protein rather than measuring the phonemic signature of the analyte being measured. Therefore, it must be – and is – sensitive to even small changes caused by analytes attaching themselves to the surface of the graphene.
The graphene can be modified by a molecule in close proximity by two methods: a charge transfer or dipole moment gating. This modification alters the electronic band and causes a change in the 2D Raman shift and in its scattering phononic energies.
Importantly, the detection system is easy and relatively inexpensive to manufacture. As the paper points out, this graphene chemeo-phononic system offers direct analyte measurement with few reagents, and does not require electrical connections or electrochemical side-reactions.
For this approach to work, however, high grade graphene is required. Otherwise, material defects can lead to non-specific binding and issues with detection.
The most significant limitation, Berry and colleagues say, is “the relatively high cost of the Raman spectrometer” to detect the virus and to analyze the data. They suggest this may not be an issue for high volume labs, however.
Today, RT-PCR is considered the gold standard for COVID-19 testing. In fact, it represents approximately 90% of the COVID-19 tests that had received U.S. Food & Drug Administration (FDA) authorization by April 2021.
PCR offers high throughput, high sensitivity, and high specificity, and can detect active cases of infection. It does, though, have some limitations, the authors point out. In particular, PCR tests require some level of skill to perform. The PRC equipment needed for high volume testing tends to be based in large, high-complexity, CLIA-certified laboratories, which limits availability. The logistics of getting patient samples to these labs and results back also can be time-consuming, causing delays in diagnosis that allow the disease to spread.
With a three to five-day incubation period, “These (limitations) necessitate the development of a virus detection platform that is simple and capable of providing quick results, while still being reliable,” Berry and colleagues wrote. They suggest this graphene chemeo-phononic may fill that need.
The research was funded by Ramaco Carbon, the Office of Naval Research, and University of Illinois at Chicago. Patents have been filed on this work.