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2005 Nuggets

Highly Sensitive, Highly Reproducible Nanoporous Gold Surfaces For Surface Enhanced Raman Based Chemical Sensors Control of Chemistry Through Confinement
D. Allara, in collaboration with the PSU Applied Research Laboratory and Chem Imaging, Inc., Pittsburgh, PA. IRG3

Methods were developed in an initial MRSEC collaboration with Moses Chan to prepare thin films of nanoporous gold (n-PAu) in the thickness range of 40-5000 nm with nanostructures in the size scale of ~20-50 nm. The preparation was carried out by chemically dealloying silver from silver-gold alloy thin films deposited by specialized ion beam methods developed in the MRSEC project. The n-PAu films were prepared to explore their use as chemical confinement media with designed inner pore wall chemistry imparted by thiol-Au functionalization chemistry. During the preparation of a series of different thickness films the ability of these nanostructures to exhibit effects in surface enhanced Raman spectroscopy (SERS) of adsorbed molecules was explored in parallel, in concert with a joint project with the PSU Applied Research Labs (ARL). Several orders of magnitude enhancements were observed, attributed to the nanoscale interior structure of the films, and a maximum effect was observed for thicknesses in the range of ~50 nm. Further work with the ARL team showed that additional etching procedures, particularly through the use of electrochemical (e-chem) methods, gave additional orders of magnitude enhancement, in some cases approaching single molecule detection levels. Most important, through judicious control of preparation and etching conditions the SERS signals were shown to be highly reproducible spot to spot across the sample and from sample to sample, thus enabling the n-PAu films to be controllably manufactureable and suitable for commercial sensing applications. Figure 1 shows SEM images of the n-PAu before and after e-chem etching, which reveal an additional, finer scale nanostructure imparted to the Au nanopore morphology. Also shown is a typical spectral intensity enhancement from the additional etching. The bottom of the figure shows the evolution of resulting SERS spectra upon exposure to a test vapor from an adjacent sample of a low vapor pressure (~10-5 Torr) test molecule. To move this discovery into the commercial realm, a patent disclosure was filed,1 the work moved completely to the ARL project and a partnership was formed with Chem Imaging, Inc. of Pittsburgh, PA, a manufacturer of imaging Raman instruments with an interest in chem./bio sensing instrumentation.