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New Optical Biosensor for Detecting Coronavirus

A group of scientists from Empa, Zurich University Hospital, and ETH Zurich has successfully created a new sensor for identifying the novel coronavirus.

The sensor uses an optical and a thermal effect to detect the COVID-19-Virus safely and reliably. Image Credit: Fusion Medical Animation.

In the coming days, this sensor could be used for quantifying the concentration of the coronavirus in the environment, for instance, in hospital ventilation systems or in places where there are several people.

Jing Wang and his research team at ETH Zurich and Empa often work on quantifying, examining, and minimizing airborne pollutants such as aerosols as well as artificially-produced nanoparticles.

But the difficulties being faced by the entire world are also altering the goals and plans of the research laboratories. The latest focus is to design an innovative sensor that can reliably and rapidly identify the novel coronavirus—SARS-CoV-2.

However, this concept is more or less close to the earlier research work performed by the group—much before the COVID-19 pandemic started to spread, initially in China and subsequently across the world, Wang and his collaborators were studying sensors that can possibly detect atmospheric viruses and bacteria.

Back in January 2020, the researchers developed the idea of applying this basis to further improve the sensor so that it can consistently detect a particular virus.

Though the new sensor will not substitute the proven laboratory tests, it could be utilized as an alternative approach for clinical diagnosis, and more significantly, to perform real-time measurement of the concentration of the virus in the air—for instance, in busy places such as hospitals or railway stations.

There is an urgent need for rapid and reliable tests to help control the COVID-19 pandemic as soon as possible. Many laboratories employ a molecular technique known as a reverse transcription-polymerase chain reaction (RT-PCR) to identify viruses that cause respiratory infections. While RT-PCR is a proven technique and can identify trace amounts of viruses, it is also prone to errors and takes a significant amount of time.

An Optical Sensor for RNA Samples

Along with his research team, Jing Wang has devised an alternative test approach in the form of a new optical biosensor. This sensor integrates two types of effects to identify the virus in a reliable and safe manner: a thermal effect and an optical effect.

The biosensor is based on the so-called gold nanoislands—that is, tiny gold structures—on a glass substrate. DNA receptors, that were synthesized artificially and match certain RNA sequences of the SARS-CoV-2 virus, are grafted onto the gold nanoislands.

The so-called RNA virus is the coronavirus: the genome of this virus contains only a single RNA strand and not the DNA double-strand as found in other living organisms. The sensor’s receptors act as the complementary sequences to the unique RNA sequences of the virus. These unique RNA sequences can consistently detect the virus.

Localized surface plasmon resonance (LSPR) is the technology being adopted by the scientists for detection. This optical phenomenon takes place in metallic nanostructures and when these metallic nanostructures are stimulated, they produce a plasmonic near-field around the nanostructure by modulating the incident light in a particular wavelength range.

When molecules attach to the surface, a change occurs in the local refractive index inside the activated plasmonic near-field. This change can be measured by using an optical sensor, which is situated on the back of the sensor, and whether the sample comprises the RNA strands in question can be determined ultimately.

Heat Increases Reliability

But only those RNA strands that precisely match the DNA receptor on the sensor need to be captured. At this juncture, a second effect comes into play on the sensor—the plasmonic photothermal, or PPT, effect. If a laser of a specific wavelength is used to excite the same nanostructure on the sensor, the nanostructure will generate localized heat.

But how does that phenomenon aid reliability? As previously stated, the virus’ genome contains just one RNA strand. If this RNA strand locates its complementary counterpart, both strands will merge to create a double-strand—a process known as hybridization.

The counterpart—when a double-strand breaks into two single strands—is referred to as denaturation. This occurs at a specific temperature, that is, the melting temperature. But if the ambient temperature happens to be relatively lower than the melting temperature, then strands that are not complementary to one another can also link together. This process may lead to false test results.

Only complementary strands will join together if the ambient temperature happens to be just slightly lower than the melting temperature. This is precisely the outcome of the increased ambient temperature, which is induced by the PPT effect.

To show how the novel sensor can reliably identify the present-day COVID-19 virus, the team tested the sensor with SARS-CoV—a very closely related virus of COVID-19. SARS-CoV was the virus that emerged in 2003 and started the SARS pandemic. The RNA of both viruses—SARS-CoV2 and SARS-CoV—varies only slightly. And validation proved to be effective.

Tests showed that the sensor can clearly distinguish between the very similar RNA sequences of the two viruses.

Dr Jing Wang, Professor, Empa

The results are ready in just a few minutes.

But right now, the novel sensor is not yet ready to quantify the concentration of the coronavirus in the atmosphere, for instance, in the main railway station of Zurich. Several developmental steps are still required to perform this—for instance, a system that traps the air, then concentrates the aerosols present in it, and finally discharges the RNA from the viruses.

This still needs development work.

Dr Jing Wang, Professor, Empa

But as soon as new sensor is ready, the same principle can also be used for other types of viruses that might help identify and prevent the epidemics at an early stage.


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