Pillalamarri Srikrishnarka
Dielectica traverses through the literature on this topic – and summarizes as they appear.
Key words: SARS-CoV-2, RT-PCR, COVID, Biosensor, Liquid-gated FET
Chennai, India: Presently, the world is dealing with the outbreak of a respiratory illness caused by the severe acute respiratory syndrome coronavirus – 2 also known as (SARS-CoV-2). Being a successor to SARS-CoV-1 which was responsible for the 2002-2004 SARS outbreak, is highly contagious as the transmission primarily occurs via aerosols. [1] The virion typically ranges between 50-200 nm in diameter having four structural proteins known as the spike, envelope, membrane and nucleocapsid. [2] Testing-tracking and treating is the current strategy followed globally for controlling the pandemic. Nucleic acid testing using quantitative reverse transcription-polymerase chain reaction (qRT-PCR) is the golden standard for diagnosing COVID-19. However, this methodology requires sample purification, amplification and trained professionals which reduces the overall efficiency of the testing system.
To mitigate some of these limitations, there have been strides made using colloidal gold particles-based lateral flow assay, enzyme-linked immunosorbent assay and luminescence-based biosensors. However, for most of these sensors, the detection limit reaches a maximum of 2.8 fM. (where 1fM is 10-15 M). This limitation could become a factor while managing COVID when the antibody concentration is below this value. Therefore, while studying the vaccine efficacy, a sensor that can detect even the smallest concentration of antibodies is needed. In this regard, Hang et al., from Fudan University recently developed a graphene-based field-effect transistor for detecting SARS-CoV2 antibodies. Monolayered graphene films were initially prepared using chemical vapor deposition on copper foils. Graphene from these copper foils was then transferred onto SiO2/Si substrate using the wetting method. In this method, graphene is first transferred to phenyl methyl ether solution containing polymethyl methacrylate. Graphene was later transferred onto a SiO2/Si substrate from PMMA. The graphene is then functionalized with 1-pyrenebutyric acid N-hydroxysuccinimide ester (PASE) molecules through the pi-pi coupling. The spike S1 protein of SARS-CoV2 is then anchored to PACE via the amine group present on the protein with the hydroxyl-free succinimide esters present on PASE. Finally, an open well was created on the SiO2/Si substrate using PDMS, this helps in holding the sample solution.
This sensor comprises a liquid-gated FET with Ag/AgCl reference electrode which is inserted in the electrolyte as gate electrodes. The electrolyte/graphene interface serves as a dielectric layer when a liquid-gate bias is applied externally. Current response to external voltage was measured. At a particular voltage, a drop in current was observed. As soon as the antibody was dropped on the sensor within 15 min there was a shift in the voltage. A calibration curve was obtained by varying the concentration of antibodies and the voltage shifts were noted. Using this sensor, a concentration of 2.6 aM was achieved. How is this possible? Is a simple question that everyone gets, graphene is wonder material having an atomic layer thickness due to this the surface is covered completely with the receptor which helps in higher sensitivity. The detection result is read out directly from the electric response without needing any complex further processing and data analysis. Such chips can be manufactured large-scale at a relatively low cost. It does hold a great promise for on-site point-of-care detection of SARS-CoV-2 thus mitigating cross-infection and accessibility for testing at home. These results have been published in the journal NANO Letters. [3]
Sources:
[1] https://www.nature.com/articles/d41586-020-02058-1
[2]Philip V’kovski et al. Nat Rev Microbiol. 2020, 1–16.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7592455/
[3] Hua Kang et al. Nano Lett.2021, 21, 19, 7897–7904.
https://doi.org/10.1021/acs.nanolett.1c00837