Near-infrared surface plasmon resonance measurements of ultrathin films. 1. Angle shift and SPR imaging experiments

The application of surface plasmon resonance (SPR) measurements to the study of ultrathin organic and inorganic films adsorbed onto gold surfaces utilizing near-infrared (NIR) excitation from 800 to 1152 mn is described. SPR scanning angle measurements of film thickness are demonstrated at 814 and 1152 nm using low-power diode and HeNe laser sources, respectively. Several advantages of SPR in the NIR are noted. The in situ reflectivity versus angle of incidence curves sharpen greatly (as compared to 632.8 nm) at longer wavelengths so that there is no loss in sensitivity in the measurement of film thickness despite a doubling of the excitation wavelength. The sharper resonance and longer wavelengths also allow for the measurement of thicker films. Examples of SPR thickness measurements for self-assembled alkanethiol monolayers and composite biopolymer/SiO2 nanoparticle electrostatic multilayer films are given. SPR imaging experiments are also performed at various NIR wavelengths using an incoherent white light source and narrow band-pass filters. The incoherent white light source eliminates laser fringes that have been observed in previous SPR imaging experiments, and the use of narrow band-pass filters allows for the easy selection and variation of excitation wavelength. The narrowness of the reflectivity versus angle curves leads to greater contrast in NIR SPR images compared to the same features examined with excitation from visible light. The combination of these changes results in nearly 1 order of magnitude enhancement in the SPR differential reflectivity image, indicating that SPR imaging is best conducted with incoherent NIR excitation. One disadvantage of using NIR wavelengths for SPR imaging is that the surface plasmon propagation length increases in the NIR so that the lateral image resolution is reduced; however, image features larger than 50 mu m can easily be resolved. A NIR SPR image of a DNA array onto which single-stranded DNA binding protein has bound is shown as an example of how NIR SPR imaging experiments have sufficient sensitivity to monitor DNA-protein interactions.