Research labs that measure and analyze airborne pollutants have a new focus — to develop a sensor that can quickly and reliably detect the SARS-CoV-2 virus. Most laboratories use a molecular method to detect viruses that can lead to respiratory infections. This method, called reverse transcription polymerase chain reaction (RT-PCR), can detect viruses in tiny amounts, but can be time-consuming and prone to error. The team of professor Jing Wang, who leads the Air Quality and Particle Technology group at ETH Zurich and a group at the Swiss Federal Laboratories for Materials Science and Technology (Empa), has developed an alternative test method — an optical biosensor that uses thermal effects to detect the virus safely and reliably. The sensor uses an optical and a thermal effect to detect the COVID-19 virus safely and reliably. Courtesy of American Chemical Society/doi:10.1021/acs.nano.0c02439. The dual-functional plasmonic biosensor combines plasmonic photothermal (PPT) and localized surface plasmon resonance (LSPR) on a tiny, 2D, gold nanoisland (AuNI chip), which is on a glass substrate. Artificially produced DNA receptors that match specific RNA sequences of the SARS-CoV-2 virus are grafted onto the AuNI chips. The AuNI chips that are now functionalized with the DNA receptors perform a sensitive detection of selected sequences from the SARS-CoV-2 virus through nucleic acid hybridization. LSPR is used to excite the metallic nanostructure, causing a plasmonic near-field to be created around it. When molecules bind to the surface of the structure, the local refractive index within the excited plasmonic near-field changes. An optical sensor located on the back of the device measures this change, and the measurement is used to determine whether the sample contains the RNA strands of SARS-CoV-2. PPT boosts the ambient temperature, so that the sensor can more reliably confirm that only the RNA strands that match the DNA receptor on the sensor are captured. By using two different angles of incidence, the plasmonic resonances of PPT and LSPR can be excited at two different wavelengths, which significantly enhances the sensing stability, sensitivity, and reliability of the device. For better sensing performance, the thermoplasmonic heat is generated on the AuNI chips when they are illuminated at their plasmonic resonance frequency. The localized PPT heat is able to elevate the in situ hybridization temperature and facilitate the accurate discrimination of two similar gene sequences. The researchers tested the sensor with SARS-CoV, the virus that triggered the SARS pandemic of 2003. The two viruses — SARS-CoV and SARS-CoV-2 — differ only slightly in their RNA. “Tests showed that the sensor can clearly distinguish between the very similar RNA sequences of the two viruses,” Wang said. Test results were available in a matter of minutes. The sensor is not yet ready to measure concentrations of the virus in highly frequented locations such as a railway station or airport terminal. A number of developmental steps are still needed — for example, a system is needed that will draw in the air, concentrate the aerosols in it, and release the RNA from the viruses. “This still needs development work,” Wang said. Once the sensor is ready, the principles that it is based on could be applied to other viruses to help detect epidemics at an early stage. The research was published in ACS Nano (www.doi.org/10.1021/acsnano.0c02439). For more information, see COVID-19 Test Detects Viral DNA in Minutes (Photonics.com, April 2020)