Abstract
Natural halloysite clay nanotubes are cheap and available in large quantities, thus seem superior to the other nanotube-based materials. Halloysite nanotubes are used to form structured substrates for surface-enhanced Raman scattering (SERS). Specifically, layered aluminum-halloysite- (noble) metal substrates were prepared by deposition of halloysite nanotubes on aluminum foil, followed by thin metal film coatings via vacuum evaporation. The composite substrates required no capping agent on the metal surface and thus avoided background peaks in the Raman spectra, making them well suited for SERS studies. Gold-, silver-, and copper-coated substrates exhibited SERS for p-mercaptobenzoic acid, whereas uncoated and platinum-coated substrates did not. The SERS enhancement factors for the gold-, silver-, and copper-coated substrates were, by a conservative estimate, 1.4 × 105, 3.5 × 104, and 3.5 × 103, respectively. Interestingly enough, the SERS enhancement factors of substrates decrease in a row: Au > Ag > Cu, which differ from those for the corresponding metal nanoparticles.
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