©  Copyright 2013 Another property that gold nanoparticles have is the capability to convert certain wavelengths  of light into heat. As with all metals, gold contains electrons that are not tied to a particular atom  but free to move throughout the metal. These electrons help to conduct a current when a voltage  is applied across the conductor. Depending on the size and shape of the nanoparticles, these free  electrons will absorb the energy from a particular wavelength of light, at the right wavelength to  make the cloud of free electrons on the surface of the gold nanoparticle resonate.  It turns out that two types of gold nanoparticle shapes are more efficient in converting  light into heat:  • Gold nanorods: These solid cylinders of gold have a diameter as small as 10 nm. By using nanorods with different combinations of diameter and length, researchers can change the wavelength of light that the nanorod absorbs. • Nanospheres consist of a gold coating over a silica core: By using nanospheres with  variations in the thickness of the gold coating and the diameter of the silica core,  researchers can change the wavelength of the light that the nanosphere absorbs.  Various researchers are using either nanorods or nanospheres to develop methods for  localized heat treatment of diseased regions of the body. This method is called  hyperthermia therapy.  . Chemical Engineering & Gas Processing