A new air quality monitor can detect any variant of the SARS-CoV-2 virus in near real time. The first-of-its-kind device, which comprises a high-flow air sampler and a nanobody-based biosensor, could also be adapted to detect other respiratory pathogens such as influenza, rhinovirus and respiratory syncytial virus (RSV), according to its developers at Washington University in St. Louis, US.

While we are no longer in the emergency phase of the COVID-19 pandemic, it is still important to prevent people from becoming infected, especially if they are clinically vulnerable to the coronavirus or its long-term effects. One way to do that would be to survey indoor environments for the coronavirus – ideally in real time, so that people can evaluate the risks and take appropriate actions. “There is nothing at the moment that tells us how safe a room is,” explains John Cirrito, a WashU neurologist and member of the research team. “If you are in a room with 100 people, you don’t want to find out five days later whether you could be sick or not. The idea with this device is that you can know essentially in real time, or every five minutes, if there is a live virus.”

Micro-immunoelectrode biosensor

The new device is an adaptation of a micro-immunoelectrode (MIE) biosensor that Cirrito and his psychiatrist colleague Carla Yuede previously developed to detect amyloid beta, the plaque-forming amino acids that are thought to be implicated in Alzheimer’s disease. To make this biosensor sensitive to SARS-CoV-2, Cirrito and Yuede exchanged the antibody that bonds to amyloid beta for a nanobody obtained from llamas that binds to the spike protein from the coronavirus.

Their next task was to combine this modified sensor with an air sampler. For this, they turned to Joseph Puthussery, an engineer in Rajan Chakrabarty’s Complex Aerosol Systems Research Laboratory at WashU. Because levels of viruses in indoor air are typically very low, the team chose a sampler called a wet cyclone that takes in large volumes of air in a short period of time. Aerosols enter this sampler at high speeds and impact its wetted inner walls, creating a downward vortex flow that traps any airborne virus particles present.

Once the sample is collected, the device sends the virus-liquid mixture to the MIE biosensor using an automated liquid transfer pump. Yuede explains that the SARS-CoV-2 virus then binds to nanobodies on the sensor, and a technique called square wave voltammetry is used to oxidise amino acids called tyrosines that sit on the virus’ surface.

The strength of the resulting oxidation current is related to the amount of virus in the sample, and Chakrabarty says the device is sensitive enough to detect as few as 7-35 copies of viral RNA in a cubic metre of air. “It is like finding a needle in a haystack,” he observes. “The high virus recovery by the wet cyclone can be attributed to its extremely high flow rate of around 1000 litres per minute, which allows it to sample a larger volume of air over a 5-minute sample collection compared with commercially available samplers.”

Real-time benefits

A further advantage compared to commercial samplers is the device’s speed. “The nanobody-based electrochemical approach is faster at detecting the virus because it doesn’t need a reagent or a lot of processing steps,” Yuede explains.

Puthussery adds that conventional aerosol sampling involves two main steps. First, samples are collected from the air using either filter-based sampling or a particle-into-liquid sampler. This collection process can take anywhere from several tens of minutes to 24 hours or more. Once the aerosol samples are collected, they must be carefully stored in a medical-grade storage container for transport to a testing facility. There, they are tested for the virus, typically using the reverse transcription-quantitative polymerase chain reaction (Rt-qPCR) technique.

This approach is time-consuming, expensive, and has poor temporal resolution. In contrast, the WashU team’s device could be programmed to light up, beep, or simply display the raw biosensor oxidation current signal whenever it detects that SARS-CoV-2 is present, enabling users to take practical steps such as opening windows or increasing airflow in other ways. “The choice of notification would be location-specific so as not to create panic among the building occupants,” he says. “We have not finalized what would be the ideal.”

The researchers now plan to diversify their biosensor by adding different target-specific nanobodies so that it can detect other common respiratory pathogens. They will then start working on commercializing their system. “In a hospital setting, the monitor could be used to measure for staph or strep, which cause all kinds of complications for patients,” says Cirrito. “This could really have a major impact on people’s health.”

The new device is detailed in Nature Communications.

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