Metal nanoshells are spherical nanoparticles consisting of a dielectric core such as silica covered with a thin layer of metal (typically gold). Gold nanoshells have optical properties similar to colloidal gold nanoparticles (such as strong optical absorption due to the collective electronic response of metal to light). However, gold nanoshells show a strong dependence of optical response due to the relative size of the core and the shell's thickness. By varying the core size and the thickness of the gold layer, the color of these nanoparticles can be fine-tuned across a wide spectrum spanning the visible and near-infrared regions (NIRs). Therefore, nanoshells show a great promise in biomedical imaging and therapeutic applications in general and cancer imaging and treatment in particular (Loo et al. 2004, 2005; Hirsch et al. 2003; Chen and Scott 2001). In addition, as with other types of nanoparticles, especially gold nanoparticles, drugs can be conjugated to nanoshells for drug delivery to tumors as seen in Fig. 8.
The optical properties of nanoshells are attributed to the plasmon resonance at the dielectric-gold interface. Plasmon resonance is the phenomenon in which
Dielectric core J
Fig. 8 (Top) Schematic illustration of a nanoshell. Polymers (such as hydrogels) can be coated onto a nanoshell surface and drug molecules can be entrapped within the polymer matrix. (Bottom) Transmission electron microscope images of gold/silica nanoshells during shell growth (Reprinted with permission from reference (Loo et al. 2004)). From left to right are the nanoshells with a growing metal shell
Fig. 8 (Top) Schematic illustration of a nanoshell. Polymers (such as hydrogels) can be coated onto a nanoshell surface and drug molecules can be entrapped within the polymer matrix. (Bottom) Transmission electron microscope images of gold/silica nanoshells during shell growth (Reprinted with permission from reference (Loo et al. 2004)). From left to right are the nanoshells with a growing metal shell light induces collective oscillations of conductive metal electrons at the dielectric-gold interface (Hirsch et al. 2003). The absorbing and scattering properties of the particle will then depend on the particle's plasmon resonance. Although many bulk metals have plasmon resonance behavior, it has been observed over a very small region of the visible spectrum. Depending on the relative thickness of the core and shell layers of a nanoshell, its plasmon resonance and the resultant optical absorption can be tuned across a broad region of the spectrum from the near-UV to the midinfrared. This range spans the NIR (Oldenburg et al. 1999), a region where optical absorption in tissues is minimal and penetration is optimal (Weissleder 2001).
The sensitive dependence of optical properties on the core diameter-shell thickness ratio can be understood by the hybridization model of Prodan et al. (2003) in which plasmon resonance of a nanoshell is considered as the result of the interaction between the nanosphere plasmon and cavity plasmon which are electromagnetic excitations that induce surface charges at the outer and inner interfaces of the metal shell, respectively. The sphere plasmon itself depends on the diameter of the sphere and the cavity plasmon is a sensitive function of the inner and outer radius of the metallic shell (Aden and Kerker 1951). In addition, the interaction of the sphere and cavity plasmons depends on the thickness of the shell layer.
Was this article helpful?
Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...