A hyperbolic metamaterial developed at the University of California, San Diego enables ordinary light-sheet microscopes to observe subcellular structures — in other words, enabling an ordinary light microscope to see in superresolution. The material, made up of nanometers-thin alternating layers of silver and silica glass, shortens the wavelength of light as the sample is illuminated, which allows the microscope to see beyond the diffraction limit.
In the application, the light-shrinking material converts low-resolution light to high-resolution light, said Zhaowei Liu, a professor of electrical and computer engineering at UC San Diego. “It’s very simple and easy to use. Just place a sample on the material, then put the whole thing under a normal microscope — no fancy modification needed," Liu said.
The work overcomes a major limitation of conventional light microscopes, as well as a limitation in superresolution imaging methods more generally. Light microscopes are useful for the observation of live cells, and other methods, such as electron microscopy, allow the observer to see subcellular structures, though the sample must be placed in a vacuum chamber. Other methods involve altering the cells, such as expansion microscopy. Light microscopy allows researchers to view living cells in their natural state, though conventional light microscopes, with a resolution of 200 nm, are unable to observe objects closer than that distance as separate objects.
“The major challenge is finding one technology that has very high resolution and is also safe for live cells,” Liu said.
The technology Liu's team developed combines both features. With the hyperbolic metamaterial in place, coated on a slide, an ordinary light microscope is able to see subcellular structures with a resolution of up to 40 nm. As light passes through, its wavelengths shorten and scatter to generate a series of random high-resolution speckled patterns. When a sample is mounted on the slide, it gets illuminated in different ways by this series of speckled light patterns, creating a series of low-resolution images. The images are captured and then pieced together by a reconstruction algorithm to produce a high-resolution image.
The researchers tested the technology with a commercial inverted microscope. They were able to image fine features, such as actin filaments, in fluorescently labeled Cos-7 cells, which are not discernable when using the microscope alone. The technology also enabled the researchers to clearly distinguish tiny fluorescent beads and quantum dots that were spaced 40 to 80 nm apart.
This light-shrinking material turns a conventional light microscope into a superresolution microscope. Courtesy of Junxiang Zhao.
According to the researchers, the technology has great potential for high-speed operation. Their goal is to incorporate high speed, superresolution, and low phototoxicity in one system for live cell imaging.
The team is working to expand the technology for high-resolution imaging in 3D space; the current paper shows that the technology can produce high-resolution images in a 2D plane. The team previously published a paper showing that the technology is also capable of imaging with ultrahigh axial resolution of about 2 nm. They are now working to combine the two.
The work was supported by the Gordon and Betty Moore Foundation and the National Institutes of Health.
The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-21835-8).