- Researchers develop a new method that can identify an ultrafine dust particle with 10 times more sensitivity than previous techniques.
- According to the numerical simulations, it can be further improved to detect particles as small as 15 nanometers in diameter.
Tiny dust particles (smaller than 100 nanometers in diameter) can easily penetrate into the lungs and cause severe respiratory diseases. Unfortunately, environmental monitoring methods cannot detect such ultrafine particles.
In recent decades, scientists have built a powerful technique, known as surface-enhanced infrared absorption (SEIRA) to intensity the vibrational signals of thin molecular layers. It involves a process of amplifying the absorption that is achieved by directing a beam into a very small structure, for example, a narrow slit in a metallic foil. Due to this compact space, the interaction between beam and particle surface becomes stronger.
An intense infrared beam is focused on a small region where the nanoparticle absorbs energy at specific wavelengths, yielding signs of its composition. Scientists have used this method (SEIRA) to identify nanoparticles as small as 240 nanometers in diameter.
Now, researchers at the University of Heidelberg in Germany have taken this technique one step further and made it 10 times more sensitive. They used a tiny bowtie-shaped structure to increase the beam intensity at the particle’s location.
What Exactly They Did?
Researchers performed experiments with 50-nanometer thick crystalline wafer coated with a layer of gold. They carved out a bowtie-shaped aperture on the layer.
As you can see in the figure, the shape looks like a rectangle made of 2 triangles intruding inwards from the longer sides of the rectangle. The layer of gold strongly interacts with the infrared beam, and this bowtie-shaped aperture elevates the beam intensity at the position where two triangles almost touch each other.
In order to test this method, they trapped nanoparticles of silica (of different sizes) in the middle of the aperture. They then directed an infrared beam into the aperture and measured the reflected beam.
The silica particle showed one prominent absorption band at a wavelength characteristic of SiO2. On the other hand, in the absence of particle, the spectrum was mostly flat, which made data evaluation straightforward.
The system was able to detect particles as small as 34 nanometers in diameter. However, with the help of numerical simulations, researchers discovered that the bowtie-shaped structure can be further improved to detect 15-nanometer wide particles.
SEM (scanning electron microscope) images of bowtie antennas after sphere positioning | Courtesy of researchers
Moreover, they performed numerous calculations for nanoparticles made of different materials, such as plastic, and found that infrared spectra of particles can be used to distinguish different types of materials.
How It’s Useful?
The new technique has the potential to change the way of monitoring and detecting tiny dust particles in the atmosphere. This the first time someone has gone down to such tiny scales, especially with a single particle.
Nanoparticles are the main component of airborne particulate matter, which lead to multiple health issues. The system makes it possible to use a massive library of reference spectra when detecting different substances, including a single virus, which cannot be detected by using traditional methods.