[PROPERTIES OF NANOSHELLS]

The Tissue Optical Window
As mentioned in the introduction, gold nanoshells are a new breed of biocompatible nanostructures that are garnering a huge amount of interest in the field of laser medicine.  The main reason is that these particles can be tuned to a specific wavelength and act as photosensitizers in the body.  Specifically, the near infrared has been found be an intriguing region of interest for laser-mediated therapy due to low absorption by tissue.  This region is often referred to as the "tissue optical window".  As shown below, the main absorbing components of tissue have the lowest absorption in the near-infrared region.  Consequently, light is the most deeply penetrating in this range and would be an ideal wavelength range for the application of nanoshells as a contrast to tissue.^[1]

The "Tissue Optical Window"

 source: http://www.nanospectra.com/physics/physics.asp

Optical Tuning
Nanoshells can be tuned to this wavelength by altering the geometry of the shell.  Simply put, the ratio of the inner core to the outer core contribute to the particles absorption and scattering properties.  This can be partially explained by plasmon resonance of the constituent metal.  However, since the shells are not solid structures the properties depend upon the inner and outer dimensions of the shell.  The diagram below shows how optical absorption values can be tuned by altering the geometry of the shell.^[2]

Optical Tunability of Nanoshells

source: http://www.nanospectra.com/physics/physics.asp

Biocompatibility
Gold is a completely biocompatible material making it a suitable choice for biomedical applications.  Sometimes, stealthing polymers such as poly(ethylene) glycol (PEG) can be attached to the surface to enhance compatibility and blood circulation times.  In addition, these nanoparticles have the ability to be tethered to specific antibodies to increase selectivity.  Thus, nanoshells with the appropriate antibodies can be targeted towards tumor sites.^[2]