As the COVID-19 pandemic continues to spread across the world, testing remains a key strategy for tracking and containing the virus. Researchers have developed a rapid, ultrasensitive test using a paper-based electrochemical sensor that can detect the presence of the virus in less than five minutes.
There are two broad categories of COVID-19 tests on the market. The first uses reverse transcriptase real-time poly-merase chain reaction (RT-PCR) and nucleic acid hybridization strategies to identify viral RNA. Some drawbacks include the amount of time it takes to complete the test, the need for specialized personnel, and the availability of equipment and reagents. The second category focuses on the detection of antibodies; however, there could be a delay of a few days to a few weeks after a person has been exposed to the virus for them to produce detectable antibodies.
In recent years, researchers have had some success with creating point-of-care biosensors using 2D nanomaterials such as graphene to detect diseases. The main advantages of these biosensors are their sensitivity, low cost of production, and rapid detection turnaround. Graphene exhibits unique mechanical and electrochemical properties that make it ideal for the development of sensitive electrochemical sensors. The team created a graphene-based electrochemical biosensor with an electrical readout setup to selectively detect the presence of SARS-CoV-2 genetic material.
There are two components to the biosensor: a platform to measure an electrical readout and probes to detect the presence of viral RNA. To create the platform, researchers first coated filter paper with a layer of graphene nanoplatelets to create a conductive film. Then, they placed a gold electrode with a predefined design on top of the graphene as a contact pad for electrical readout. Both gold and graphene have high sensitivity and conductivity, which makes this platform ultrasensitive to detect changes in electrical signals.
Current RNA-based COVID-19 tests screen for the presence of the N-gene (nucleocapsid phosphoprotein) on the SARS-CoV-2 virus. In this research, the team designed antisense oligonucleotide (ASOs) probes to target two regions of the N-gene. Targeting two regions ensures the reliability of the senor in case one region undergoes gene mutation. Gold nanoparticles (AuNP) are capped with these single-stranded nucleic acids (ssDNA), which represents an ultra-sensitive sensing probe for the SARS-CoV-2 RNA.
The researchers previously showed the sensitivity of the developed sensing probes in earlier work. The hybridization of the viral RNA with these probes causes a change in the sensor electrical response. The AuNP caps accelerate the electron transfer and when broadcasted over the sensing platform, results in an increase in the output signal and indicates the presence of the virus.
The team tested the performance of the sensor by using COVID-19 positive and negative samples. The sensor showed a significant increase in the voltage of positive samples compared to the negative ones and confirmed the presence of viral genetic material in less than five minutes. Furthermore, the sensor was able to differentiate viral RNA loads in these samples. Viral load is an important quantitative indicator of the progress of infection and a challenge to measure using existing diagnostic methods.
This platform has far-reaching applications due to its portability and low cost. The sensor, when integrated with microcontrollers and LED screens or with a smartphone via Bluetooth or WiFi, could be used at the point-of-care in a doctor's office or even at home. Beyond COVID-19, the team also foresees the system to be adaptable for the detection of many different diseases.