Near-Infrared Surface Plasmon Resonance Measurements of Ultrathin Films. 2. Fourier Transform SPR Spectroscopy

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.