Pregnancy tests and rapid coronavirus tests work almost similarly. The test kit contains a metal surface equipped with chemical nanosensors that detect specific compounds in a urine, saliva, or blood sample that indicate the presence of a given protein or part of a virus.
The test shows positive results if these nanosensors react with the target compound. This biological mechanism is invisible, but the metal structure is sensitive to light. The interaction of metals with light can interfere with the movement of light.
Olivier Martin, head of EPFL’s laboratory for nanophotonics and metrology, within the Technical Faculty, said, “These disturbances are what tell us that the sensor on the metal surface came into contact with the target mixture. The process creates an optical wave, which propagates and appears, for example, in the form of a red line on a pregnancy test. “
To make the technology more sensitive and efficient, EPFL scientists have developed a method.
In their method, the scientists used aluminum for metal surfaces and placed nanosensors on them. They then added a layer of silicon just below the aluminum. The silicone layer does not conduct electricity.
Martin said, “Silicon acts like a sound box. Imagine a kettle – its surface vibrates when the drummer hits it, and the sound box below us allows us to hear vibrations. In our system, the silicon layer serves as a resonator and enhances the metal reaction, making the system more sensitive. This means that we can detect lower proteins or lower concentrations of the virus. “
This sandwich system works on a nanometer scale. But why did scientists develop such miniature technology?
Martin said, “We have to work on the same scale as the objects we want to detect – in this case, proteins and viruses. Also, the optical response varies depending on the scale we use. A silver plate may look gray to us, but on a nanometer scale, silver particles appear blue. “
“This is the first time we have developed a system for medical testing by joining metal to an electrical insulator.”
“We have formulas to design nanostructures for metals and dielectric materials, but we still have to find one that combines the two. Developing our sandwich technology was a real challenge. Next, we plan to experiment with other metals, which will create new challenges. We also need to optimize the structure of our device to make the optical resonance as strong as possible. “
- Debdatta Ray et al. Hybrid metal-dielectric metasurfaces for refractive index recognition.DOI: 10.1021 / acs.nanolett.0c03613