Why are gold nanoshells for cancer treatment so interesting?

Gold nanoshells are 90- 130 nm particles of silica coated with a thin layer of gold that have unusual optical feature and can potentially be used for cancer therapy and diagnosis. The thin gold coating on the glassy substrate results in a product that can be designed to absorb and scatter light at very specific frequencies. This "tunable" property means by changing the ratio of the silica core to the gold ,gold nanoshells can be manufactured to respond to near infrared light frequencies (>800nm) that are very desirable for therapy and disease detection because near infrared light can pass through human tissue relatively easily.

We have chromophores in our body, like hemoglobin, that like to absorb light in the visible region of the electromagnetic spectrum. So if we can design particles that absorb at the near infrared spectrum (slightly outside of the visible spectra) then these light waves can travel through our tissues without interference from our chemicals that are naturally present in our bodies.

How do nanoshells kill cancer cells?
Called photoablation therapy, this process works because when specific frequency of light is directed at nanoshells, they heat up and the tissue where the nanoshells are located is destroyed via heat. The nanoshells collect around tumors because the blood vessels that are formed to feed the fast growing cancer cells are very abnormal and have a leaky characteristic that let nanoparticles somewhat selectively stay in the the cancerous area.
Because of the surface phenomena that is inherent in metal nanoparticles, gold nanoshells respond to an incident light beam and heat up.

Why do nanoshells heat up?
The valence electrons on the surfaces of metals that are free to move around. Because nanoparticles have a lot more surface area exposed than larger particles, the phenomena that occurs at the surface plays a much larger role in determining the physics of the system than if you had a large, bulk chunk of metal. In nanogold, the electrons on the surface of the metal particle can respond to incident light. They basically can vibrate or resonate in frequency with the color of the the light. This is analogous to a child being pushed on a swing. Just like there is an inherent frequency to the push on the swing that will keep the child in motion, light waves can induce a frequency response in the electron on the surface of a metal. This cloud of electrons, often called a plasmon, can swish back and forth across the nanoparticle in response to the incident light. Some of the light energy is transferred in heat and so the particles heat up at specific frequencies. This heat is what kills the cancer cells.