Rear-infrared surface plasmon resonance measurements of ultrathin films. 2. Fourier transform SPR spectroscopy

The application of surface plasmon resonance (SPR) measurements to the stud y of ultrathin organic films adsorbed onto gold surfaces utilizing near-inf rared (NIR) excitation from a Fourier transform (FT) spectrometer is descri bed. The FT-SPR experiment measures the NIR reflectivity spectrum from a pr ism/gold film/water assembly at a fixed angle of incidence approximately 1- 2 degrees greater than the critical angle. A strong reflectivity minimum is observed in the FT-SPR spectrum; this minimum can be shifted from 12 000 t o 6000 cm(-1) by tuning the angle of incidence. Upon adsorption of a thin b iopolymer film from solution, a shift in the minimum is observed that can b e correlated to a film thickness using Fresnel calculations. From experimen ts on the adsorption of electrostatically bound poly(lysine)/poly(glutamic acid) multilayers, an similar to 60-cm(-1) shift per 10-Angstrom change in film thickness was measured. Frequency shifts of 2 cm(-1) (corresponding to a thickness change of the polymer layer of similar to 0.3 Angstrom) 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-S PR measurements over a wide range of NIR wavelengths allows one to avoid an y absorption bands that might otherwise interfere with the analysis.