Fluorescence intensity changes were investigated theoretically and experimentally using self-assembled colloidal structures on silver semitransparent mirrors. Using a simplified quasi-static model and finite element method, we demonstrate that near-field interactions of metallic nanostructures with a continuous metallic surface create conditions that produce enormously enhanced surface plasmon resonances. The results were used to explain the observed enhancements and determine the optimal conditions for the experiment. The theoretical parts of the studies are supported with reports on detailed emission intensity changes which provided multiple fluorescence hot spots with 2-3 orders of enhancements. We study two kinds of the fluorophores: dye molecules and fluorescent nanospheres characterized with similar spectral emission regions. Using a lifetime-resolved fluorescence/reflection confocal microscopy technique, we find that the largest rate for enhancement (~1000-fold) comes from localized areas of silver nanostructures.

Analytical expressions for the plasmon resonance frequencies of prolate and oblate spheroids and their dependence on ellipticity have been derived, and approximate bounds on these frequencies established. These formulas may be useful in tuning the plasmon resonance within certain limits. With increasing aspect ratio, the prolate spheroid resonance is red shifted relative to a sphere with no lower limit under the assumptions of a Drude dispersion model. On the other hand, the oblate resonances are blue shifted as the spheroid becomes increasingly flat, but up to a limit.

Anisotropic metal-based nanomaterials have been proposed as potential contrast agents due to their strong surface plasmon resonance. We evaluated the contrast properties of gold, silver, and gold-silver hybrid nanorods for molecular imaging applications in three-dimensional biological samples. We used diffuse reflectance spectroscopy to predict the contrast properties of different types of nanorods embedded in biological model systems of increasing complexity. The predicted contrast properties were then validated using wide-field and high-resolution imaging. Results demonstrated that silver nanorods yield images with higher positive-contrast than gold nanorods; however, it is more difficult to synthesize silver nanorods which are homogeneous in shape and size. Gold-silver hybrid nanorods combine the homogeneous synthesis of gold nanorods with the higher scattering properties of silver nanorods. The spectroscopic and imaging results demonstrated that the image contrast properties that can be achieved with anisotropic nanomaterials depend strongly on the material composition, mode of imaging, presence of targeting molecules, and the biological environment. We also found that gold, silver, and gold-silver hybrid nanorods are stable and biocompatible sources of positive and absorptive contrast for use in reflectance molecular imaging and are promising for future clinical translation.