Aggregates of An nanoshells on silicon substrates with plasmon resonances tuned to the infrared region of the spectrum provide a remarkably high-quality substrate for surface enhanced infrared absorption (SEIRA) spectroscopy. SEIRA spectra spanning the chemical fingerprint region of 700-3300 cm(-1) with strong enhancement across this entire spectral range are obtained. SEIRA enhancement factors obtained for para-mercaptoaniline on these substrates are evaluated to be in the 104 range, based on a statistical evaluation of aggregate geometries combined with finite difference time domain modeling of the infrared 'hot spots' characteristic of the constituent structures. (c) 2007 Elsevier B.V. All rights reserved.