Nanolenses Help Researchers Pick Out Tiny Objects

With the growing popularity of nanotechnology in electronics and biology, scientists want simple, portable ways to image objects as small as a few nanometers in size. One method to do so involves making nanoscale lenses so that researchers can use optical imaging techniques to detect synthetic nanoparticles or even viruses. Now, researchers at the University of California, Los Angeles, have devised a new type of nanolens, formed right around the object being imaged, that can find targets only 20 nm wide (ACS Nano 2014, DOI: 10.1021/nn502453h).

Existing techniques use photolithography or thin-film deposition to make lenses on substrates containing nanoparticles to be imaged, instead of on the particles themselves. For the new method, UCLA engineer Aydogan Ozcan and his colleagues start by placing the nanoparticles they want to look at onto a glass substrate using one of several common methods, such as evaporating a solvent containing the particles. Then they suspend the sample slide upside down over a shallow pool of liquid polyethylene glycol (PEG) that has been preheated to 105 °C. PEG vapors rising off the pool condense on the sample, forming a thin film. The surface tension of the PEG causes the film to rise to form a nanolens over each particle. The nanolenses are convex on one side and concave on the other.

To image the particles, the researchers place the glass slide over a detector and shine 480-nm (blue) light from a fiber optic onto the slide. They are able to detect polymer and metal spheroids smaller than 40 nm, compared to 100 nm for previous techniques. They also detected rod-shaped particles with diameters less than 20 nm.

Because the nanolenses formed on each particle, the researchers could detect objects over an area of more than 20 mm², whereas a traditional light microscope would cover an area less than 0.2 mm². Ozcan hopes to improve the techniques still further—for instance, by using different liquids—to be able to see particles less than 20 nm in diameter. By selecting a liquid with the right material properties and controlling the timing of the evaporation, the researchers can control the shape of the lens they end up with, which in turn controls the quality of the image. Ozcan says the method could work with many liquids so long as they are transparent and can be deposited through evaporation.

Having lenses this small could be useful for monitoring the synthesis of carbon nanotubes. They might also be used in mobile sensing or diagnostic applications—for example, imaging particles captured from a sample of air or water, or viruses isolated from blood samples. Ozcan says the process is relatively easy to perform and cost effective enough that it could be used in developing countries.

“It is a really neat and innovative technique with a twist,” since the lenses form around the particles rather than on top of the glass slide, says Ashutosh Sharma, a chemical engineer at the Indian Institute of Technology Kanpur. Tony Jun Huang, an engineer at Pennsylvania State University, says the technique should make a nice complement to a method Ozcan developed (ACS Nano 2013, DOI: 10.1021/nn4037706) to image viruses with cell phone cameras. “It is an excellent and convenient way to image small nano-objects,” Huang says.