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ANALYTICAL CURRENTS Light-absorbing nanoparticle labels Here’s a potentially hot—literally—new way to detect molecules. Brahim Lounis and colleagues at Université Bordeaux (France) and Michel Orrit at Universiteit Leiden (The Netherlands) present the proof of concept that photothermal imaging can identify nanometer-sized metal particles, which are used to label molecules. Using metal particles to label individual molecules is not new, but previous methods typically used Rayleigh scattering to detect the particles and their attached molecules. Such light scattering can theoretically be used only with particles down to ~40 nm in diameter, but even at that size it can be difficult to reliably distinguish the signal from the background, the researchers say. To identify metal particles with diameters as small as 2.5 nm, Lounis and colleagues use light absorption properties instead of light scattering. As light is absorbed by the nanoparticle, the particle heats up and, in turn, dissipates heat to its surroundings. Using a polarization interference method, the researchers detect slight phase changes caused by the heat dissipation.
(a) A photothermal image of 5-nm-diam gold particles in polyvinyl alcohol film on glass and (b) a histogram of the spot intensities. (Adapted with permission. Copyright 2002 American Association for the Advancement of Science.)
According to the researchers’ rough calculations, the temperature rise on the surface of a 5-nm particle is ~15 K; farther from the center, the rise is not as great: ~3 K at a distance of 13 nm. The temperature increase, though small, may be too high to use with biomolecules. However, Lounis believes that the S/N can be reduced by a factor of 10, allowing the temperatures to be reduced and a broader range of molecules to be studied. (Science 2002, 297, 1160–1163)
Thick substrate for SERS Richard Dluhy and co-workers at the University of Georgia have developed a new thick substrate for surface-enhanced Raman scattering (SERS) of self-assembled monolayers (SAMs). The substrate, which consists of a dual-layer, vapor-deposited silver film, yields SERS enhancement factors of ~104, which compare favorably with the enhancements obtained using thin silver island film substrates, the researchers say. Thin silver films have been used successfully as SERS substrates, but a thick film would reduce background in the spectrum because less radiation would reach the underlying substrate, the researchers explain. Also, they add, thick films have less specular reflectance from the reflecting laser beam, and the same film could be used for a dual experiment using SERS and IR reflection– absorption spectroscopy. To develop the films, the researchers vapor-deposited a 45-nmthick silver underlayer onto a treated glass slide. At ambient conditions, the underlayer chemisorbed oxygen, forming an Ag2O interface. A 25-nm-thick silver overlayer, which had a surface morphology that approximated a thin silver island film, was then vapor-deposited on top. The Ag/Ag2O underlayer created an active interface that decreased the diffusion of the silver atoms from the overlayer. This produced silver particles with shapes favorable for SERS enhancement. The Ag/Ag2O/Ag film was characterized using atomic force
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(a) Atomic force microscopy images with corresponding line scan plots of (a) a traditional thick (45-nm) silver island film substrate and (b) the new 70-nm-thick Ag/Ag2O/Ag substrate. microscopy and X-ray photoelectron spectroscopy. For studies of the SERS spectra of 1-dodecanethiol SAMs, the researchers report an intensity increase of ~400%. They say that the Raman intensity enhancements achieved on this new substrate are the result of ideal morphology of the silver particles on the overlayer surface and a combined electric field effect produced from the Ag/Ag2O underlayer and silver overlayer. (J. Phys. Chem. B 2002, 106, 8747–8755)
