Researchers at Purdue University (West Lafayette, IN) have found a way to see synthetic nanostructures and molecules using a new type of super-resolution microscopy that does not require fluorescent dyes, representing a practical tool for biomedical and nanotechnology research.
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Conventional optical microscopes can resolve objects no smaller than about 300 nmâa restriction known as the "diffraction limit," which is defined as half the width of the wavelength of light being used to view the specimen. However, researchers want to view molecules such as proteins and lipids as well as synthetic nanostructures like nanotubes, which are a few nanometers in diameter.
Such a capability could bring advances in a diverse range of disciplines, from medicine to nanoelectronics, says Ji-Xin Cheng, an associate professor of biomedical engineering and chemistry at Purdue University.
"The diffraction limit represents the fundamental limit of optical imaging resolution," Cheng said. "Stefan Hell at the Max Planck Institute and others have developed super-resolution imaging methods that require fluorescent labels. Here, we demonstrate a new scheme for breaking the diffraction limit in optical imaging of non-fluorescent species. Because it is label-free, the signal is directly from the object so that we can learn more about the nanostructure."
The imaging system, called saturated transient absorption microscopy (STAM), uses a trio of laser beams, including a doughnut-shaped laser beam that selectively illuminates some molecules but not others. Electrons in the atoms of illuminated molecules are kicked temporarily into a higher energy level and are said to be excited, while the others remain in their "ground state." Images are generated using a laser called a probe to compare the contrast between the excited and ground-state molecules.
The researchers demonstrated the technique, taking images of graphite "nanoplatelets" about 100 nm wide. The technique has great potential for the study of nanomaterialsâboth natural and synthetic, Cheng says.
The doughnut-shaped laser excitation technique, invented by researcher Stefan Hell, makes it possible to focus on yet smaller objects. Researchers hope to improve the imaging system to see objects about 10 nm in diameter, or about 30 times smaller than possible using conventional optical microscopes.
Future research may include work to use lasers with shorter wavelengths of light. Because the wavelengths are shorter, the doughnut hole is smaller, possibly allowing researchers to focus on smaller objects.
The work will be discussed during the third annual Spectroscopic Imaging: A New Window into the Unseen World workshop to take place May 23-24, 2013, at Purdue University. The workshop is hosted by the university's Weldon School of Biomedical Engineering; to learn more, please visit www.conf.purdue.edu/cheng.
Full findings are detailed in Nature Photonics; for more information, please visit www.nature.com/doifinder/10.1038/nphoton.2013.97.
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