Ultrafine dust particles less than 100 nanometres in size can penetrate deep into our lungs and cause respiratory diseases. Currently, however, the best environmental monitoring techniques fail for particles smaller than about 240 nm. Now, researchers have demonstrated a way to detect single particles as small as 34 nm, almost 10 times smaller than the previous limit. This was achieved by refining a technique called surface-enhanced infrared absorption spectroscopy, which involves focussing an intense infrared laser beam into a region where a particle may be present. By observing the energy absorbed by the particle at specific wavelengths, its composition is inferred. The absorption achieved gets amplified if the light is focussed into a nanoscale structure, such as a narrow slit in a metal foil, hence the use of the term “surface-enhanced”. Using this method, detection of particles as small as 240 nm in diameter was achieved.
Now Christian Huck and others of the University of Heidelberg, Germany, have refined this method and achieved a nearly tenfold improvement in sensitivity. The scientists used a crystalline wafer coated with a gold layer approximately 50 nm thick on which a bow-tie-shaped nanoscale aperture was etched. Gold interacts strongly with infrared light, and the bow-tie geometry increases the intensity of an incident beam at the meeting point of the two triangles.
The system was tested by trapping single silica nanoparticles of varying sizes in the centre of the bow tie. A broadband infrared beam was focussed into the structure, and the scattered light showed a strong absorption peak at a wavelength characteristic of silicon dioxide in the presence of the nanoparticle. The detection limit turned out to be 34 nm, but numerical simulations suggest that detection of a 15 nm nanoparticle should be possible by fine-tuning the shape of the nanostructure. Simulations for particles made from several other materials showed that the infrared absorption spectra could distinguish different material particles.
The method would make a difference for practical monitoring and chemically identifying fine dust particles in the environment, said the scientists. “Ultrafine particles are the main constituents of airborne particulate matter,” said Huck. “With this technique,” he said, “it’s possible to make use of an incredibly large library of reference spectra when identifying materials, which is not possible with many other methods.” The method could have many uses in diverse areas of monitoring for health and safety, including identification of a single virus, the scientists believe.