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n e w s of t h e w e e k menters in many disciplines, including chemistry, molecular biology, and materials science. In general, Raman scattering is a very ineffi"Observing molecules one at a time is a cient process. The tiny chemist's favorite dream," says Shuming scattering cross sections Nie, assistant professor of chemistry at measured in liquid-phase Indiana University, Bloomington. And his samples are too small by a group is doing just that, having devised a factor of more than a trilnovel method for probing single mole- lion to allow single-molecules and nanoparticles. cule detection. Twenty By exploiting the enormous signal am- years ago, however, plifications from surface-enhanced Ra- chemistry professor Richman scattering and the resonance Raman ard P. Van Duyne of effect, Nie and graduate student Steven Northwestern University, Evanston, HI., disR. Emory have been able to measure the covered that molecules adsorbed on roughcharacteristic Raman spectrum from a ened silver surfaces exhibit a surfacesingle dye molecule adsorbed on a enhanced Raman signal that is more than 1 nanometer-sized silver particle [Science, million times larger than the signalfromliquid-phase molecules. 275,1102(1997)]. Nie stresses, however, that the surface There are other ways of detecting single molecules, Nie explains. But unlike enhancement factor from that earlier work electrochemical orfluorescencetechniques, is a value averaged over an ensemble of Raman scattering provides a molecular fin- molecules. "Many, many molecules were gerprint that uniquely identifies the ad- probed simultaneously in those experisorbed species. Methods of single-molecule ments, but very few of them produced detection provide detailed information large signals." that benefits theoreticians and experiVan Duyne agrees that most research-
Raman method detects single molecules, nanoparticles
Imaging single molecules in motion Continuous imaging of single molecules of CCD detection is used to view single mola highlyfluorescentcompound and a fluo- ecules continuously in real time as they rescent-tagged DNA has been achieved by diffuse in free solution. "By opening the postdoctoral associate Xiao-Hong Xu and shutter," says Yeung, "we record the enchemistry professor Edward S. Yeung of tire sequence of events in the same imthe department of chemistry at Iowa State age—not a movie, but a single picture." University, Ames [Science, 275, 1106 He says the new technique makes it pos(1997)]. sible to follow molecular motions 100 to Imaging single biological molecules 1,000 times faster than has previously generally requires that they be immobi- been possible. lized in a polymer matrix or on a solid The method enables Xu and Yeung to surface, or that their motion be frozen track the diffusion of single molecules. cryogenically. But such techniques don't "If you open your shutter in a camera show biological molecules in their natu- for a long time and single molecules are ral environment—aqueous solution. moving, you will see a blur for each Last year, chemistry professor W. E. one," explains Yeung. "The size of each Moerner of the University of California, blur reflects the random-walk diffasionSan Diego, and coworkers devised an al motion of the molecule." approach in which fluorescence miThe researchers also use the technique croscopy and a charge-coupled device to measure reaction rates of individual (CCD) detector were used to obtain dis- molecules undergoing a photoinduced crete images of single fluorescent and unimolecular reaction. The measuretagged molecules in aqueous solution ments show that each single molecule in the pores of polyacrylamide gels has its own rate. "The macroscopic rate [Science, 274, 966 (1996)]. A series of constants and equilibrium concepts that these individual images could be com- we are familiar with no longer hold on a bined into a kind of "movie" of molec- single-molecule basis," notes Yeung. "If ular motion. you look at millions of molecules and avNow Xu and Yeung have taken this ap- erage their properties, you get back the proach one stepfartherby developing a normal rate constant" technique in which a special mode of Stu Borman 10 FEBRUARY 24, 1997 C&EN
Nie (left) and Emory probe single molecules with Raman scattering; individual optically "hot" silver nanoparticles are revealed by Raman scattering from adsorbed rhodamine 6G.
ers have studied only the ensembleaveraged enhancement factor. But Nie, he says, "has taken a very important step by pushing surface-enhanced Raman technology to provide more localized information about single molecules in special environments." In their current study, Nie and Emory examined single, immobilized silver particles. The molecule-to-particle ratios were controlled to ensure that not more than one rhodamine 6G dye molecule was adsorbed on each particle. Then by isolating the lightemitting or "hot" particles from the nonemitting particles, the Indiana researchers were able to measure the enhanced Raman signal from single molecules. The result, Nie says, is that the mtrinsic enhancement factor attained in surface-enhanced Raman scattering is actually about 1014, roughly seven orders of magnitude larger than the population-averaged results. Similar results have been obtained recently by Katrin Kneipp at the Technical University of Berlin and physics professor Michael S. Feld at Massachusetts Institute of Technology and coworkers. In their study, soon to be published in Physical Review Letters, the researchers measured the Raman spectrum from a single molecule of crystal violet dye. Mitch Jacoby
