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Microscope slide may enable molecular imaging in living cells

BioPhotonics
Jun 2007
David L. Shenkenberg

What if you could view viruses and proteins as they move through living cells or observe proteins interacting with DNA? A new microscope slide could enable you to do just that.

Its developers, Igor I. Smolyaninov and colleagues at the University of Maryland in College Park, used the substrate with an ordinary microscope to image nonliving objects down to about 70 nm; conventional microscopy — without the slide — can achieve a resolution on the order of only 200 nm. If the microscope slide can reach a 70-nm resolution in living cells, it will show all but the smallest molecules. Smolyaninov said that the slide can theoretically resolve objects down to 10 nm, so it may be able to reveal even smaller objects.

BNSuper_Fig-1.jpg
Scientists created a microscope slide that could enable observation of molecules in living cells. The slide contains a superlens that magnifies objects via surface plasmon resonance. Laser light activates the slide by exciting surface plasmons, and plasmon rays propagate outward in a radial direction. Image reprinted with permission of Science.

He said that the group developed the slide because it can be mass-produced cheaply and because it can be used with existing optical microscopes without the complexity and expense of integration that an objective lens might require.

The microscope slide consists of concentric rings and doublets of polymethyl methacrylate, or PMMA, on a gold film that is attached to a glass slide. “PMMA-like compounds are often used in biological sample preparation, which makes our approach very compatible with biological applications,” Smolyaninov said. The PMMA acts as a superlens, enabling the microscope slide to magnify objects via surface plasmon resonance. To excite the plasmons, the researchers focused an argon-ion laser (~500 nm) on the slide.

BNSuper_Fig-2.jpg
This image shows an array of superlenses on a glass slide covered with gold film.

To demonstrate the efficacy of the superlens, they visualized the doublets with a conventional optical microscope. Without activating the plasmons, the scientists could not resolve the doublets, but upon plasmon excitation, they observed two lines, which were divergent plasmon beams generated by the doublets. These lines corresponded to a resolution on the order of 70 nm, well below the approximately 200-nm diffraction barrier.

Next they used the optical microscope and microscope slide to take images of plasmon rays from the PMMA circles. Overlaying these images with atomic force microscopy images of the PMMA circles showed that both techniques can produce the same information.

Smolyaninov said that the researchers plan to make improvements to the slide. However, he said that nothing will prevent the slide from being used with living cells right now. In fact, his group has begun to use the slide with living cells.

Science, March 23, 2007, pp. 1699-1701.


GLOSSARY
microscope
An instrument consisting essentially of a tube 160 mm long, with an objective lens at the distant end and an eyepiece at the near end. The objective forms a real aerial image of the object in the focal plane of the eyepiece where it is observed by the eye. The overall magnifying power is equal to the linear magnification of the objective multiplied by the magnifying power of the eyepiece. The eyepiece can be replaced by a film to photograph the primary image, or a positive or negative relay...
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