news of the week OCTOBE R 26, 2009 EDITED BY WILLIAM G. SCHULZ & ALICIA J. CHAMBERS
SEEING MOLECULES IN A NEW LIGHT
MARCUS HALEVI
croscopic samples often leads to absorption too weak to be detected by conventional microscopy. The technique can visualize drugs as well. Xie and coworkers used the method to image hemoglobin in blood cells and transdermal delivery of a nonfluorescent drug. MICROSCOPY: Technique relies “It’s an impressive outcome,” says microscopist Stesolely on light that nonfluorescent fan W. Hell of the Max Planck Institute for Biophysical compounds absorb Chemistry, in Göttingen, Germany, adding that he has “a lot of admiration” for Xie and coworkers. “They have opened up NEW MICROSCOPY technique makes it possia new way to detect molecules that ble to visualize nonfluorescent molecules based would otherwise be left in the dark.” only on their ability to absorb light. “What I love about this techThe light that objects absorb and reflect is responnique is its simplicity,” comments sible for the colorful world people see. But because spectroscopist and microscopist molecular-level absorption is often too weak to detect Martin T. Zanni of the University under a microscope, imaging of molecules typically reof Wisconsin, Madison. “It is just lies on fluorescence. The new approach overcomes this a standard stimulated emission sensitivity problem, allowing imaging of nonfluoresexperiment, albeit very carefully cent compounds that would be difficult or impossible done. It could have been done five to label with fluorescent tags. or 10 years ago, which is one of the The technique depends on stimulated emission things that makes it so beautiful.” (photon-induced electron de-excitation), which was inIn conventional absorption spectroduced in 1917 by physicist Albert Einstein and is the troscopy, a molecule raised to an basis for lasers. Postdoc Wei Min, grad student Sijia Lu, excited state relaxes back to its ground state principally spectroscopist X. Sunney Xie, and coworkers at Harvard by converting that energy to heat. In the new technique, University demonstrate for the first time that stimua stimulation beam de-excites a molecule and converts lated emission can be used as a contrast mechanism for its excitation energy into a photon in an amplified stimumolecular microscopy (Nature 2009, 461, 1105). lated emission beam. This signal is generated at the comThe method could make it possible to image biomol- mon foci of two laser beams, which are scanned across ecules like hemoglobin, cytochrome, and melanin in or through the sample to build 2- or 3-D images. Xie and living cells and organisms. Such molecules have undecoworkers use intensity modulation, timing adjusttectable fluorescence, and their low abundance in miments, and lock-in amplification to manipulate pulsed excitation and stimulation beams in such a way that the stimulated emission can be detected. “The technique potentially will allow in vivo imaging with micron-scale resolution deep into intact thick tissues,” says Chris B. Schaffer of Cornell University, whose group uses optical techniques to observe and manipulate biological systems. For example, it could make it possible to study blood oxygenation by monitoring hemoglobin in single brain capillaries or to study redox changes in cytochrome c during the respiratory cascade, he says. “The idea would be to do stimulated emission microscopy at two wavelengths, where the amount of emission is different for two different states, such as the oxygenation or redox states of a molecule.” The technique is less sensitive than fluorescence microscopy “but it’s four to five orders of CELL VISUALIZATION Stimulated emission image magnitude more sensitive than would normally of hemoglobin (red) in blood cells (inset) in mouse-ear be possible with absorption,” Schaffer adds. “It’s capillaries. an enormous gain in sensitivity.”—STU BORMAN
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Xie (from left), Min, and Lu adjust their stimulated emission microscopy apparatus.
