Nano-Particle Size Detection by Interferometry
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Particles are detected optically with a laser beam split by a beam splitter into an attenuated reference beam and a beam that is focused on a nanoscale channel in a micro-fluidic system. Scattered light from a passing nano particle is recombined with the reference beam to provide a background-free signal.
This method can detect a single nano particle, such as a single virus in water. The method is based on interferometrically measuring the electromagnetic field amplitude of the scattered light, and is sensitive down to 5 nanometers. The technique is useful in detecting and sizing, not only viruses and other biological entities such as anthrax spores, but also other nano-particles of various sorts: catalysts, semiconductor nanoparticles and water-borne pollutants. It could be used to detect small contaminants in ultrapure water.
Because it depends on the third power of the particle size (R3), as opposed to the sixth power (R6) of other light scattering methods, the method scales to lower particle size than the earlier optical methods, which are limited to particles > 200 nm. Most viruses are in the range of 20 to 200 nm, where this technique works. The interferometric technique is background-free, with no signal in the absence of a particle. The detection of single particles occurs in real time with a resolution of 1 millisecond.
Apparatus and Method for Sizing Nanoparticles Based on Optical Forces and Interferometric Field Detection
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