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 stud y of ultrathin organic and inorganic films adsorbed onto gold surfaces util izing near-infrared (NIR) excitation from 800 to 1152 mn is described. SPR scanning angle measurements of film thickness are demonstrated at 814 and 1 152 nm using low-power diode and HeNe laser sources, respectively. Several advantages of SPR in the NIR are noted. The in situ reflectivity versus ang le 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 f ilm 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 alkanethio l monolayers and composite biopolymer/SiO2 nanoparticle electrostatic multi layer films are given. SPR imaging experiments are also performed at variou s NIR wavelengths using an incoherent white light source and narrow band-pa ss filters. The incoherent white light source eliminates laser fringes that have been observed in previous SPR imaging experiments, and the use of nar row band-pass filters allows for the easy selection and variation of excita tion wavelength. The narrowness of the reflectivity versus angle curves lea ds to greater contrast in NIR SPR images compared to the same features exam ined with excitation from visible light. The combination of these changes r esults in nearly 1 order of magnitude enhancement in the SPR differential r eflectivity image, indicating that SPR imaging is best conducted with incoh erent NIR excitation. One disadvantage of using NIR wavelengths for SPR ima ging is that the surface plasmon propagation length increases in the NIR so that the lateral image resolution is reduced; however, image features larg er 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.