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August 24, 2010

SERS Hot Spots Probed

ACS Meeting News: Imaging shows larger-than-expected SERS-analyzable spots on nanoparticles

Celia Arnaud

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Superresolution imaging by single-molecule surface-enhanced Raman spectroscopy (SERS) shows that SERS-active sites called "hot spots" on metal nanoparticles may be larger than previously thought. Katherine A. Willets, an assistant chemistry professor at the University of Texas, Austin, presented her group's results at the American Chemical Society meeting in Boston and in a recent paper (Nano Lett., DOI: 10.1021/nl102559d). Their findings could help scientists reach a deeper level of understanding about the way single-molecule SERS works.

Only certain regions of nanoparticle SERS substrates are able to generate single-molecule SERS spectra. Characterizing these hot spots is difficult because both the molecule being analyzed and the hot spot in which it is found are smaller than the diffraction limit of light. To overcome this challenge, Willets and grad student Sarah M. Stranahan pinpointed the location of individual rhodamine 6G molecules by fitting their SERS emission to a two-dimensional Gaussian function, a method they borrowed from superresolution fluorescence microscopy.

"We've known for a number of years that the intensity of the molecules sitting in the hot spots fluctuates," Willets says. "What we see now is that these intensity fluctuations are coupled to motion of the molecule on some level."

They find that these intensity fluctuations correlate with motion of the molecule over distances of up to 100 nm, whereas the hot spots have been assumed to be closer to the size of an individual molecule, about 10 nm.

"One of the main difficulties in SERS is lack of ability to locate precisely the source of the effect as a function of geometry," says John R. Lombardi, a Raman expert at City College of New York. "Willets' work presents an ideal opportunity to elucidate the true nature of SERS."

The findings suggest that "our models of single-molecule SERS are not capturing the hot-spot properties properly," says George C. Schatz, a chemistry professor at Northwestern University. The study makes it "clear that our notion of what is a hot spot needs refinement."

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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