Substrates Tuned for Surface-Enhanced Raman Scattering
Daniel S. Burgess
Researchers at the University of Victoria in British Columbia, Canada, have demonstrated that a wet chemistry technique for depositing layers of gold nanoparticles and linker molecules can optimize the Raman scattering signal from an absorbed sample for the excitation wavelength employed. The method could potentially find application in preparing substrates tuned for use in drug discovery, forensics and biomolecular sensing.
A wet chemistry technique enables fabrication of substrates for surface-enhanced Raman scattering that can be tuned to work best with the desired excitation wavelength. Courtesy of Alexandre G. Brolo.
Raman spectroscopy, despite its advantages as a highly sensitive, versatile and nondestructive approach to chemical analysis, is limited by a relatively low signal strength that makes it difficult to resolve the components of complex mixtures. In normal circumstances, only approximately one in 10 million photons scattered from an object under investigation offers Raman information. One solution to this problem is to exploit surface-enhanced Raman scattering, in which plasmonic and/or chemical phenomena amplify the signal from samples in proximity to metallic nanostructures — particularly silver, gold and copper.
This method increases the Raman scattering signal by tens of thousands to trillions of times to enable the collection of chemically specific vibrational information from molecular-scale sample volumes. It relies on a variety of fabrication techniques to produce the necessary nanostructures, including chemical vapor deposition and electron- and ion-beam lithography.
Alexandre G. Brolo, an assistant professor in the department of chemistry at the university, and graduate student Christopher J. Addison have shown that a relatively simple wet chemistry technique can be used to produce the substrates by alternately immersing a glass slide in solutions that respectively deposit gold nanoparticles and dithiol linker molecules. Their work, which expands on the efforts of researchers at Pennsylvania State University in University Park a little more than a decade ago, further demonstrates that it is possible to tweak the process to yield substrates best suited for the wavelength of excitation to be used.
Observing the effect of substrates with one to 17 layers of gold nanoparticles on the signal from the Raman-active analyte oxazine 720, they found that increasing the number of deposited layers shifted the absorption spectra of the substrate. Substrates with nine layers of nanoparticles offered maximum enhancement for samples under 632-nm laser light, and those with 13 layers of nanoparticles optimized the signal under 785-nm radiation. They employed an inVia Raman microscope system from Renishaw plc of Wotton-under-Edge, UK, in the experiments.
Brolo said that the scientists will investigate whether the technique can be improved by using rod- or prism-shaped nanoparticles, rather than the spherical ones in the initial demonstration. They also may employ various bimetallic or core-shell nanoparticles. Other avenues of inquiry include determining whether the substrates can be made to enhance second-harmonic generation or coherent anti-Stokes Raman scattering for biological imaging applications.
Langmuir, Oct. 10, 2006, pp. 8696-8702.
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