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A team of researchers from Empa, ETH Zurich and the University Hospital of Zurich have successfully developed a new sensor for the detection of the new coronavirus. In the future, it could be used to determine the concentration of viruses in the environment, for example, in places where there are many people or in ventilation systems in hospitals.
Jing Wang and his team at Empa and ETH Zurich typically conduct research to measure, analyze, and reduce air pollutants, such as aerosols and artificially produced nanoparticles. But the current challenge facing the entire world is also changing goals and strategies in research labs. The new approach: a sensor that can quickly and reliably detect SARS-CoV-2, the new corona virus.
However, the idea is not so far from the group’s previous research work: Even before the COVID-19 virus began to spread, first in China and then around the world, Wang and his employees investigated sensors, bacteria, and viruses. . in the air The idea to use these basics matured in January, and he further developed the sensor to reliably identify a specific virus. 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.
Rapid and reliable tests for COVID-19 are urgently needed to control the pandemic as soon as possible. Most laboratories use a molecular method called “Reverse Transcription Polymerase Chain Reaction”, RT-PCR for short, to detect viruses in respiratory infections. This is established and can already detect small amounts of the virus, but at the same time testing is often time consuming.
An optical sensor for RNA samples
Jing Wang and his team have developed an alternative test method in the form of an optical biosensor. The sensor combines two different effects to detect the virus safely and reliably: an optical and a thermal one.
The sensor is based on small structures made of gold, called nano gold islands, on a glass substrate. Artificially produced DNA sequences that match certain RNA sequences of the SARS-CoV-2 virus apply to the nano islands. The new corona virus is the so-called RNA virus: its genome, unlike humans, animals and plants, does not consist of double strands of DNA, but a single strand of RNA. The artificial DNA receptors on the sensor are, therefore, the complementary sequences of the unique sequences of the virus RNA genome, which can clearly identify it.
The technology that researchers use for virus detection is called LSPR (“localized surface plasmon resonance”). This is an optical phenomenon that occurs with metallic nanostructures: when excited, they modulate the incident light in a certain wavelength range and generate the so-called plasmonic near field around the nanostructure. When molecules dock at the surface, the optical refractive index in this plasmonic near field changes precisely at this point. This can be measured with an optical sensor, which is located on the back of the sensor, and therefore it can be determined if the searched RNA strands are in the sample.
Heat increases reliability
Of course, it is of central importance that only those RNA strands be captured by the DNA receptor on the sensor that exactly matches them. This is where a second effect comes into play: the plasmonic photothermic effect (PPT). If the nanostructure itself is excited at the sensor with a laser of a certain wavelength, this produces heat.
And how does that help reliability? As already mentioned, the genetic makeup of the virus consists of a single strand of RNA. If this chain finds its complementary counterpart, the two combine to form a double chain, a process called hybridization. The opposite, when a double strand is divided into single strands, is called fusion or denaturation. This happens at a certain temperature, the melting temperature. However, if the ambient temperature is now much lower than the melting temperature, strands that are not 100% complementary to each other can also be attached. This can lead to incorrect test results. However, if the ambient temperature is only slightly lower than the melting temperature, only the complementary threads can be joined. And this is exactly the result of the increase in ambient temperature caused by the PPT effect.
Test with SARS-CoV already 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-CoV2, differ only slightly in their RNA, making a clear distinction extremely 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,” explains Jing Wang.
Some development steps are still missing
At the moment, the sensor is not yet ready, for example, to measure the concentration of the corona virus in the air at the Zurich main station. This requires a few more steps, such as a system that sucks in air, concentrates aerosols, and isolates RNA from viruses. “It still needs development work,” says Wang. But once the sensor is finished, the principle could also apply to other viruses, and help ensure that future epidemics can be caught early and even stopped. <<
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