Metal-based nanorods as molecule-specific contrast agents for reflectance imaging in 3D tissues.

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.