doi:10.1016/j.cplett.2007.12.042 
Copyright © 2007 Elsevier B.V. All rights reserved.
Surface enhanced infrared absorption (SEIRA) spectroscopy on nanoshell aggregate substrates
Janardan Kundub, d, Fei Lec, d, Peter Nordlanderc, d and Naomi J. Halasa, b, d, 
, 
aDepartment of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, TX 77005, United States
bDepartment of Chemistry, Rice University, 6100 Main Street, Houston, TX 77005, United States
cDepartment of Physics and Astronomy, Rice University, 6100 Main Street, Houston, TX 77005, United States
dThe Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston, TX 77005, United States
Received 6 November 2007;
accepted 6 December 2007.
Available online 23 December 2007.
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Abstract
Aggregates of Au 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.
Graphical abstract
Aggregates of Au nanoshells with infrared resonances are used as high-quality substrates for surface enhanced Infrared absorption (SEIRA) spectroscopy. High SEIRA reproducibility and sensitivity across the chemical fingerprint region 700–3300 cm−1, with enhancement factors in the 
104 range, are demonstrated.
Fig. 1. (A) Experimental extinction spectrum of nanoshell aggregate film on (i) glass substrate and (ii) calculated extinction spectrum of constituent nanoshell, with parameters specified in text. (B) Experimental near-IR – mid-IR extinction spectrum of typical nanoshell aggregate film on silicon shown with the SEM image of the film with a single nanoshell shown in inset.
Fig. 2. (i) Transmission spectra of neat pMA dispersed in KBr and (ii) transmission SEIRA spectra of pMA on nanoshell aggregates with table of peak assignments for normal IR and SEIRA of pMA. (δ,ν, and π stands for bend, stretch, and wag mode, respectively).
Fig. 3. Transmission spectrum of (i) neat adenine and (ii) SEIRA spectrum of adenine on nanoshell aggregates. Spectra are offset for clarity. Inset shows the high wavenumber SEIRA spectra.
Fig. 4. SEIRA signal-to-noise (S/N) ratio of some of the observed vibrational modes of pMA obtained from various concentrations of pMA solution.
Fig. 5. (A) Histogram plot of a representative SEIRA substrate characterizing nanoshell aggregation. (B) FDTD simulated local field enhancements for a (a) dimer, (b) trimer, and (c) quadrumer nanoshell aggregate. Each nanoshell, embedded in vacuum, has inner and outer shell dimensions of r1, r2 = [190, 210] nm with an interparticle separation distance of 4 nm. The incident direction and polarization of light, scale bar of near field enhancements, and local field maximum are shown. The hot spot sizes in each system are based on an estimation of the surface area on all nanoshells with local electric field enhancement factors >10. (C) Enhancement factors obtained for different IR modes of pMA.
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