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New method for crown detection in progress The optical sensor must detect viruses in the air.
Smear-free crown detection? Swiss researchers want to use an optical biosensor to detect the new Cornona virus using air samples. For example, the virus concentration could be measured in highly visited places or in ventilation systems in hospitals. Initial tests have already shown that the technology distinguishes even very similar viruses. However, development work is still required for the finished system.
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Zurich, Dübendorf / Switzerland – Tests for the new SARS-CoV-2 coronavirus are in high demand and are not yet available in sufficient quantities to meet current needs. That’s why test manufacturers are increasing their production as much as possible, and new laboratory test methods are being investigated, including Dr. Prof. Jing Wang and his team from Empa and ETH Zurich.
Researchers often develop methods to measure and avoid air pollutants, such as aerosols and artificially produced nanoparticles. But the current situation has brought new objectives and strategies into focus. They are now developing a sensor that should quickly and reliably detect SARS-CoV-2.
From pollutant analysis to virus sensors
However, the new idea is not so far from the group’s previous research work: Wang and his team were already researching sensors that could detect bacteria and viruses in the air before the corona pandemic. Back in January, the plan matured to further develop a sensor to reliably identify a specific virus like SARS-CoV-2.
The sensor should not necessarily replace established laboratory tests, but could be used as an alternative method for clinical diagnosis. And in particular to measure the concentration of viruses in the air in real time, for example, in high traffic places such as train stations or hospitals.
Most laboratories use “Reverse Transcription Polymerase Chain Reaction”, RT-PCR for short, to detect viruses in respiratory infections. This molecular testing method is well established and can already detect small amounts of the virus, but it is not the only way to detect the new type of coronavirus.
Detect molecules on nano islands of gold.
Wang and his team have developed an alternative test method that is based on an optical biosensor and that could detect viruses from the breath or ambient air in the future. The sensor combines two effects to detect the virus: an optical and a thermal one.
The optical effect is localized surface plasmon resonance (LSPR). It allows the sensor to detect molecules at all. The surface is covered with small gold structures. If light of a certain wavelength falls on these golden nano islands, a nearby field called plasmonic is created around the nanostructure. This phenomenon is LSPR.
However, when the molecules dock on a gold nano island, the optical refractive index in the plasmonic near field changes there, which can be measured with an optical sensor.
The right temperature is crucial for virus detection.
In order for the sensor to not simply detect any molecules, but to specifically recognize the new types of corona viruses, the researchers loaded the gold nano islands with artificially produced DNA sequences. These fit exactly to certain RNA sequences of the SARS-CoV-2 virus, so that only the RNA of the targeted virus can dock on the nano islands. However, this only works at a certain temperature, so the researchers take advantage of another effect: the plasmonic photothermic effect (PPT), which ensures that a nanostructure becomes warmer due to certain light.
In this way, the temperature of the nano-gold islands can be increased until the melting temperature of the artificial DNA molecules in them is reached. The double strands are then divided into two individual strands. If the temperature drops below the melting temperature again, double chains can form again, even with the complementary virus RNA, which is naturally a single chain.
It is important that the temperature be minimally below the DNA melting temperature, since at significantly lower temperatures, other RNA molecules than SARS-CoV-2 can bind to artificial DNA. That would lead to false test results. However, if the ambient temperature is only slightly lower than the DNA fusion temperature, only the complementary strands can bind. And this is exactly the result of the increase in ambient temperature, which is caused by the PPT effect in the Wang team optical biosensor.
Differentiation between similar coronaviruses was successful
To show how reliably the new sensor detects the current COVID-19 virus, the researchers tested it with a closely related virus: SARS-CoV. It is the virus that triggered the SARS pandemic in 2003. The two viruses, SARS-CoV and SARS-CoV-2, have very similar RNA; Therefore, a clear distinction is difficult. But the experiment was successful: “Our tests showed that the sensor can clearly differentiate between the very similar RNA sequences of the two viruses,” says study leader Wang.
At the moment, the sensor is not yet ready to measure the concentration of corona virus in the air. This requires a few more steps, such as a system that sucks in air, concentrates aerosols, and isolates RNA from viruses. “That still needs development work,” emphasizes Wang. But once the sensor is finished, the principle could apply to other viruses as well, and help ensure that future epidemics can be caught early and even stopped.
Original publication: G Qiu, Z Gai, Y Tao, J Schmitt, G A Kullak-Ublick, J Wang: Dual-function plasmonic photothermal biosensors for the detection of highly accurate severe acute respiratory syndrome coronavirus 2, ACS Nano 2020; DOI: 10.1021 / acsnano.0c02439
* K. Weinmann, EMPA Swiss Institute for Materials Testing and Research, 8600 Dübendorf / Switzerland
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