A pencil graphite biosensor that detects SARS-CoV-2

Scientists around the world generally agreed that early detection of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is the virus responsible for the coronavirus disease 2019 (COVID-19), followed by isolation of the infected person is an effective method to prevent further spread of the disease. At present, owing to the high accuracy and specificity of the reverse-transcriptase polymerase chain reaction (RT-PCR) assay, this analytical technique is considered the gold standard for detecting the presence of SARS-CoV-2.

Nasopharyngeal (NP) or oropharyngeal (OP) samples are the two primary types that are used to detect SARS-CoV-2 by RT-PCR. However, this test is expensive, time-consuming, and requires a skilled technician to be performed. As a result, the RT-PCR assay is unsuitable for mass testing efforts, particularly those conducted in low-income countries with limited resources.

COVID-19 test kits

Several rapid test kits are currently available for the diagnosis of COVID-19. Despite their widespread use, these antigen-based diagnostic kits are associated with reduced accuracy and sensitivity as compared to the RT-PCR assay. These kits are therefore considered unreliable for accurately detecting the presence of COVID-19, particularly in asymptomatic individuals.

Study: Minute-scale detection of SARS-CoV-2 using a low-cost biosensor composed of pencil graphite electrodes. Image Credit: danielmarin / Shutterstock.com

As a result, there remains an urgent need for a rapid, low-cost, and highly sensitive diagnostic kits for the early detection of SARS-CoV-2. In this context, several portable devices have been designed and based on electrochemical, colorimetric, serological, and molecular-based technologies to offer a rapid diagnosis of the virus. Although these diagnostic kits are accurate, they tend to be slower and more expensive than current test kits, while also requiring materials and equipment that are not easily accessible.

In a new study published in the Proceedings of the National Academy of Sciences, scientists discuss the development of a rapid COVID-19 diagnostic tool known as the Low-cost Electrochemical Advanced Diagnostic (LEAD). The LEAD technology is based on highly accessible and inexpensive materials, of which include graphite pencil lead and a plastic vial.

LEAD, a rapid and low-cost electrochemical biosensor. (A) Schematic representation of graphite electrodes used in LEAD. (B) AuNPs-cys functionalization on graphite electrodes after modification with glutaraldehyde. (C) Modification of AuNPs-cys with ACE2 using EDC and NHS to enable amide bond formation and BSA for surface blockage. The analytical response of LEAD in the presence of SARS-CoV-2 was based on current suppression due to selective binding of viral SP with the ACE2-functionalized electrode, which partially blocked the electrodic area, leading to decreased peak current of the redox probe ([Fe(CN)6]3−/4− solution). (D) Cost and detection time comparison between LEAD and existing FDA-approved antigen, serological, and molecular tests (47).

Design of the new graphite biosensor-based SARS-CoV-2 diagnostic kit

This do-it-yourself (DIY) diagnostic kit consists of a transducer made of graphite leads modified with human angiotensin-converting enzyme 2 (ACE2) receptor and a plastic vial. This electrochemical device exploits the binding affinity of the SARS-CoV-2 spike (S) protein and human ACE2 receptors. The electrochemical reaction takes place in the graphite electrode, which ultimately gets converted to a detectable analytical signal that is functionalized through the drop-casting method.

In addition to the transducer and vial, a set of modifiers is also present within this kit, which is crucial to maintaining the high sensitivity of the sensor. These modifiers include glutaraldehyde (GA), gold nanoparticles (AuNPs), cysteamine (cys), ACE2, and bovine serum albumin (BSA).

In this diagnostic tool, the graphite working electrode (WE) has been modified with AuNPs that were stabilized using cys, which helps to anchor ACE2. The remaining active sites present on the surface of the electrode are blocked by BSA, which prevents nonspecific interactions from occurring between the clinical sample and the biosensor.

The graphite working electrode of this sensor can be activated in less than 3 hours. Once activated, the electrode remains stable for over 5 days when kept in phosphate-buffered saline (PBS) solution at 4°C. Additionally, the modified graphite leads were found to be highly sensitive for detecting the S protein of SARS-CoV-2, while simultaneously eliminating any cross-reactivity from occurring between the sensor and other relevant viruses. The accuracy of this device was studied by evaluating 113 OP, NP, and saliva patient samples.

Advantages of the new SARS-CoV-2 diagnostic kit

This testing device offers on-site SARS-CoV-2 detection within 6.5 minutes, which is considerably faster than the diagnostic kits that have been approved by the United States Food and Drug Administration (FDA).

The LEAD unit can be manufactured using materials that are not expensive, thus supporting the use of this technology for low-cost SARS-CoV-2 diagnostic kits. In fact, the researchers estimate that the cost of a single LEAD unit is around $1.50. Therefore, owing to its cost-effectiveness and DIY design, the new rapid COVID-19 diagnostic test could provide an opportunity for population-wide high-frequency testing in lower-income countries.

Applicability of LEAD

Further studies will be needed to evaluate the efficacy of the LEAD technology in detecting newer SARS-CoV-2 variants. If successful, these kits could enable the early detection of the new variants, which would assist in reducing the spread and transmission of new variants before they become dominant.

The main advantages of this device include its low cost, high sensitivity, and accuracy. This device would therefore be an ideal candidate for population surveillance, which could help prevent future outbreaks from occurring. Mass production of this diagnostic kit is possible because the modification of many electrodes can be performed simultaneously, thereby ensuring that automated manufacturing of this device would be possible.  

The researchers of the current study are optimistic that their novel sensor can be applied beyond the COVID-19 virus. This technology can be used for the detection of a wide range of pathogens such as other viruses, bacteria, and fungi, as long as the infectious agent and its receptor are available.

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.