New microscopy method reveals internal structure of opaque materials

February 5, 2009--A new microscopy method has proven successful in revealing the fundamental structure of nearly opaque materials. Invented by scientists at the University of Utah, its application to biological specimens has been demonstrated. And the method, based on silver nanostructures, is equally promising for materials science--for instance, for detecting fatigue in carbon-fiber plastics used to build aircraft fuselage, wings, and tails.

February 5, 2009--A new microscopy method has proven successful in revealing the internal structure of nearly opaque materials. Invented by scientists at the University of Utah, its application to biological structures has been demonstrated. And the method is equally promising for materials science--for instance, for detecting fatigue in carbon-fiber plastics used to build aircraft.

The approach is based on silver nanoparticles, which the scientists say could be embedded in carbon-fiber plastic. Then, the integrity of the fuselage or other aircraft components could be inspected regularly by exciting the embedded particles with a laser, and measuring how much light from the particles is transmitted through the fuselage material. Changes in transmission of the light would indicate changes in the fuselage structure, a warning that closer inspections of fuselage integrity are required.

John Lupton, an associate professor of physics and leader of a study describing the method (published today online and in the March 2009 issue of Nano Letters), said the new method is a variation of fluorescence microscopy, but involves using an infrared laser to excite clusters of the nanoparticles. In their study, Lupton and his colleagues--Michael Bartl, an assistant professor of chemistry; Debansu Chaudhuri, a postdoctoral researcher in physics; and graduate students Jeremy Galusha in chemistry, and Manfred Walter and Nicholas Borys in physics--placed a mirror made of silver nanoparticles beneath a biological sample. The particles form "plasmonic hotspots," which act as beacons, shooting intensely focused white light upward through the overlying sample.

The spectrum or colors of transmitted light reveal information about the composition and structure of the substance examined.

Development of the new method began after Bartl, Galusha and others published a study last May revealing that a beetle from Brazil--a weevil named Lamprocyphus augustus--has shimmering green scales with an "ideal" photonic crystal structure. They wanted to know more about these naturally occurring crystals.

"A normal light microscope generally won't do the trick," Lupton says, because visible light is easily scattered by the scales, thwarting efforts to view their internal structure. The researchers found that if they placed a mirror made of silver nanoparticles beneath the beetle, and illuminated the specimen with "very intense infrared light, the silver starts to emit white light, but only at very discrete positions on the mirror."

The beacons of intense light were transmitted upward through the beetle scale, allowing scientists to view the scale's internal structure, including tiny differences in the angles of crystal facets and the existence of vertical stacks of crystals invisible to other microscope methods. An image created this way exhibits colored blotches that reveal information about the scale's internal structure.

"There really does not appear to be any other useful technique to look at these natural photonic crystals microscopically," Lupton says. "The silver nanoparticle approach to microscopy potentially could be very versatile, allowing us to view other highly scattering samples such as tumor cells, bone samples or amorphous materials in general."

The researchers are seeking a patent on the new method.

For more information see the paper, Toward Subdiffraction Transmission Microscopy of Diffuse Materials with Silver Nanoparticle White-Light Beacons, published by Nano Letters.

Posted by Barbara G. Goode, barbarag@pennwell.com.

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