Optical Sorting of Nanoparticles by Size

Schematic diagram of how the sorting system works. Particles of many different sizes pass through an optical standing wave in a laser cavity at the same oblique angle (Source: J. Curry / PML)

Schematic diagram of how the sorting system works. Particles of many different sizes pass through an optical standing wave in a laser cavity at the same oblique angle (Source: J. Curry / PML)

NIST scientists have devised and modeled a unique optical method of sorting micro­scopic and nano­scopic particles by size, with a reso­lution as fine as 1 nano­meter for particles of similar compo­sition. A stream of particles of various sizes enters the system at a single point, but the particles exit the system at different places, depending on their size. The process does not alter the particles in any way, so that those with dimen­sions of interest can be readily separated and analyzed. The sorting operates on a conti­nuous input of particles, with the particles moving through the system in only a couple of seconds.

That kind of capa­bility is of significant interest to climate scientists studying atmo­spheric chemistry and airborne conta­minants, as well as to national security and health care. “For example, if you wanted to know whether someone had released spores or bacteria or even viruses into the air in a closed space, such as a subway system,” says John Curry of NIST’s Physical Measurement Labo­ratory PML, “you could drama­tically speed up the inves­tigation by sorting, because a cubic centimeter of air can contain thousands of nano particles.”

The idea arose when “I got to thinking that there all these great tools that have been developed for atomic and molecular physics, but they haven’t really been applied to the study of particles,” says Curry, who co-developed the idea with PML colleague Zachary Levine. Although indi­vidual particles in fluids have been mani­pulated by optical tweezers and optical patterns have been used to sort particles in micro­fluidic channels, compa­ratively little attention has been devoted to aerosols, where the scientists decided to focus their work. “One of the things that sets it apart,” Curry says, “is that it has a large volume compared to optical tweezers. We wanted some­thing useful for batch pro­cessing, that could process a very large number of particles at the same time.”

“It became clear that the ability to sort by size was a primary concern,” Curry says. “Zachary and I started tossing around some ideas, and pretty soon we realized that setting up an optical standing wave, and then running a stream of particle-bearing air across it at an angle, was going to do the trick.” They determined the forces at work from first principles, and then subjected the model to two different kinds of rigorous numerical simu­lations. NIST has filed a provisional patent appli­cation. A proto­type device has not yet been developed.

The NIST system envi­sions a cavity with mirrors at both ends. A laser beam enters the cavity and forms a resonant standing wave, with regu­larly spaced regions of high and low optical field strength. Each high- or low-intensity region is about one-quarter of a wavelength wide. To model particles, the scientists used the well-charac­terized properties of poly­styrene spheres, which are widely employed in nano­particle research, at various diameters on the order of 200 nm. They assumed a flow rate of 1 mm per second, about 10 times faster than a typical micro­fluidic channel.

As a particle approaches the optical standing wave, the degree to which it feels the optical force is directly dependent on its size. The largest are affected earliest and displaced the most. Smaller sizes are deflected later and less. And some particles of a specific size move straight across the standing wave field with hardly any deflection at all. “If the particle is around half a wavelength wide, closely approa­ching the same length scale as the spacing in the sinu­soidal standing wave,” Levine says, “it’s going to have about the same amount of its volume in both low-intensity areas and high-intensity areas, resulting in a zero-force condition.” That pheno­menon had been observed before, but largely over­looked in terms of its sorting potential.

Even small devia­tions from the “zero-force” propor­tions would be dis­cernible in the particle path. “You could set it up so that you know exactly what kind of a regular sphere would pass right through the system,” Levine says. “But if an identical sphere had, say, an antigen bonded to it, you might be able to tell them apart even though the mass difference is only parts per thousand.” Another potential benefit of the system, Curry says, is that with a variable-wavelength light source, it could be used as a reliable standard for measuring the size of nano­particles in a way directly traceable: “Because the pheno­menon directly links the size of the particle that makes it all the way through the optical field to the wavelength of the laser light this system offers what may be the first way to connect particle size to a funda­mental SI unit.”

Although this work focused on aerosols, the researchers have determined that the same system can be used to sort particles in liquid, in­cluding protein-like particles in water. Researchers in the life sciences need to be able to sort out the individual compo­nents of protein agglo­merations, and the con­ventional method of filtering is not ideal. “You can also use optical tweezers,” Curry says, “but the throughput is very low. Our system, by contrast, could process a million particles per second. In addition, a large number of these devices could be operated in parallel.” (Source: NIST)

Reference: J. J. Curry & Zachary H. Levine: Continuous-feed optical sorting of aerosol particles, Optics Exp. 24, 14100 (2016); DOI: 10.1364/OE.24.014100

Link: Sensor Science Devision, National Institute of Standards and Technology NIST, Gaithersburg, Maryland, USA

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