Current optical technologies utilize changes in optical properties of tissue to distinguish diseased from normal tissue. This poses an important challenge to enhance this subtle intrinsic contrast with the use of novel nanoparticle based contrast agents. Gold nanoshells are a novel type of spherical concentric nanoparticle that possesses high optical efficiencies well into the near infrared. Gold nanoshells are typically made of a dielectric silica core and a thin metallic gold outer layer and a wide range of sizes are easily fabricated using current chemistries. Gold nanoshells can scatter and/or absorb light with optical cross-sections often several times larger than the geometric cross-section. To elucidate the effectiveness of gold nanoshells as a contrast agent for reflectance, it is important to understand how different optical properties of nanoshells affect reflectance, and ultimately provide insight into how reflectance is affected by gold nanoshells embedded in biological tissue. A fiber-probe based spectrometer was used to measure diffuse reflectance of gold nanoshells suspensions from 500nm to 900nm. We further characterize diffuse reflectance of gold nanoshell suspensions using Monte Carlo based computational tools. Our results show that gold nanoshells are capable of producing large changes in diffuse reflectance, and computer modeling results agreed well with the experimental observations. From the study, we also show that it may be feasible to use Monte Carlo based modeling to simulate biological medium embedded with gold nanoshells.
Many optical diagnostic approaches rely on changes in scattering and absorption properties to generate optical contrast between normal and diseased tissue. Recently, there has been increasing interest in using exogenous agents to enhance this intrinsic contrast with particular emphasis on the development for targeting specific molecular features of disease. Gold nanoshells are a class of core-shell nanoparticles with an extremely tunable peak optical resonance ranging from the near-UV to the mid-IR wavelengths. Using current chemistries, nanoshells of a wide variety of core and shell sizes can easily be fabricated to scatter and/or absorb light with optical cross sections often several times larger than the geometric cross section. Using gold nanoshells of different size and optical parameters, we employ Monte Carlo models to predict the effect of varying concentrations of nanoshells on tissue reflectance. The models demonstrate the importance of absorption from the nanoshells on remitted signals even when the optical extinction is dominated by scattering. Furthermore, because of the strong optical response of nanoshells, a considerable change in reflectance is observed with only a very small concentration of nanoshells. Characterizing the optical behavior of gold nanoshells in tissue will aid in developing nanoshells as contrast agents for optical diagnostics.