Web Release Date: August 18,
Near-Infrared Surface Plasmon Resonance Measurements of Ultrathin Films. 2. Fourier Transform SPR Spectroscopy
and*
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin, 53706-1396, and GWC Instruments, 822 Oneida Place, Madison, Wisconsin, 53711
Received for review May 13, 1999. Accepted July 2, 1999.
Abstract:
The application of surface plasmon resonance (SPR)
measurements to the study of ultrathin organic films
adsorbed onto gold surfaces utilizing near-infrared (NIR)
excitation from a Fourier transform (FT) spectrometer is
described. The FT-SPR experiment measures the NIR
reflectivity spectrum from a prism/gold film/water assembly at a fixed angle of incidence approximately 1-2
greater than the critical angle. A strong reflectivity minimum is observed in the FT-SPR spectrum; this minimum
can be shifted from 12 000 to 6000 cm-1 by tuning the
angle of incidence. Upon adsorption of a thin biopolymer
film from solution, a shift in the minimum is observed
that can be correlated to a film thickness using Fresnel
calculations. From experiments on the adsorption of
electrostatically bound poly(lysine)/poly(glutamic acid)
multilayers, an ~60-cm-1 shift per 10-Å change in film
thickness was measured. Frequency shifts of 2 cm-1
(corresponding to a thickness change of the polymer layer
of ~0.3 Å) can be easily measured from the FT-SPR
spectra, demonstrating that this technique has sensitivity
equivalent to or better than other visible SPR angle shift
or wavelength shift measurements. Furthermore, the
ability to perform FT-SPR measurements over a wide
range of NIR wavelengths allows one to avoid any absorption bands that might otherwise interfere with the analysis.
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