One of the most pressing problems in the current corona pandemic is the bottleneck in testing. So far, the reliable virus tests take too long and require a lot of laboratory capacity. A biosensor developed by Swiss researchers could now provide a remedy. Because it can detect RNA pieces of Sars-CoV-2 with the help of a physical effect. According to tests, this detection is specific for this coronavirus and sensitive enough to detect the pathogen in patient samples. However, the biosensor could even be expanded to detect the virus in the ambient air, for example, as the researchers report.
So far, the gold standard for the detection of acute Covid 19 disease has been the so-called RT-PCR test. In this method, which is carried out in the laboratory, the DNA of the coronavirus, which is present as RNA, is first translated into DNA using an enzyme. The resulting pieces of DNA are then amplified by the polymerase chain reaction (PCR). The detection is then carried out by the attachment of these DNA pieces to specific markers that match this sequence. These tests are considered the most reliable method to detect an acute Sars-CoV-2 infection. However, because the test takes several hours and requires special laboratory equipment and reagents, not all suspected corona cases can be tested in Germany and many other countries – the capacity of the laboratories is simply not sufficient.
Evidence of optical effects
That is why a team of researchers headed by Jing Wang from ETH Zurich has now developed an alternative test method that could at least complement the common RT-PCR tests. The scientists use a sensor concept that they have developed in recent years for the detection of aerosols, but also bacteria and viruses in the ambient air. Wang and his team have now developed this biosensor so that it can detect the current corona virus. “Our biosensor can thus provide a reliable and easy-to-use diagnostic platform that can improve diagnosis in clinical tests and reduce the pressure on PCR-based tests,” say the researchers. As you emphasize, your sensor should not necessarily replace the established laboratory tests, but rather supplement it by being used above all where a quick diagnosis is necessary.
The basis of the new test is a technology called “Localized Surface Plasmon Resonance” (LSPR). This bulky term hides an optical phenomenon that occurs with metallic nanostructures. If, for example, gold nanoparticles are excited with laser light on a glass surface, they generate an energy field that oscillates the electrons of the surface atoms. This so-called plasmonic near field creates a certain optical pattern that can be measured with a photosensor, as the scientists explain. However, if foreign molecules dock onto the nanoparticles, the optical pattern of the plasmonic field changes at this point. It is exactly this effect that Wang, first author Guangyu Qiu from ETH and her colleagues have now used to detect specific RNA fragments of Sars-Cov-2.
Specific for Sars-CoV-2 RNA
For the coronavirus biosensor, the researchers added two DNA sections to the gold nanoparticles on the glass support, which are quasi the negative for the corresponding sections of the viral RNA from Sars-CoV-2. The sequence of these genetic fragments is so specific that only RNA strands of this virus can attach to the DNA complementary sequences. In this specific case, Wang and his team chose the same viral gene signatures that are used in the common PCR tests. In order to make the biosensor specific for only this one corona virus, the researchers supplemented the LSPR technology with a second, thermal component, the so-called plasmonic photothermal effect (PPT). For this purpose, a laser heats the test surface with the nanoparticles to such an extent that the temperature is only slightly below the value at which double strands of RNA or DNA normally split into single strands. This increases the specificity of the attachment and prevents almost matching RNA strands from attaching to the marker DNA on the biosensor, as the researchers explain.
To test how reliable and specific the new test actually works, Wang and his team conducted test runs with solutions that either contained the RNA sections of Sars-CoV-2 or the very similar sections of the closely related coronavirus Sars-CoV , the causative agent of the Sars pandemic in 2003. The result: “The LSPR sensor without the thermal component displayed a false positive signal when it detected the Sars sequences,” the researchers report. The sensor that combined LSPR with the photothermal effect, however, did not have such false positive results. “Our tests showed that the sensor can clearly differentiate between the very similar RNA sequences of the two viruses,” explains Wang. Closer analyzes showed that the lower detection limit for the biosensor is around 22,600 copies of the viral RNA strand from Sars-Cov-2. As the researchers explain, this is a hundred times lower than the virus densities that are typically detected in smears from Covid 19 patients shortly after the onset of symptoms.
“This suggests that our system has the potential to detect SARs-CoV-2 in samples from the airway area,” say the scientists. You will see the area of application of your biosensor primarily where a quick initial diagnosis of Covid-19 is necessary – for example when suspected cases are admitted to the hospital. In the longer term, the system could even be expanded so that it can also test air samples from the environment for the virus. “It still needs development work,” says Wang. To do this, a system has to be developed that sucks in the air, concentrates the aerosols in it and then isolates the RNA from the viruses. But once the sensor is finished, you could use it to measure the corona virus concentration in the air, for example in train stations, in the vicinity of hospitals or in airports.
Source: Guangyu Qiu (ETH Zurich) et al., ACS Nano, doi: 10.1021 / acsnano.0c02439