Gold Nanoparticles

Gold is an element used in jewelry, coins, dentistry, and electronic devices. Gold is even used in some medicines. Bulk gold is considered an inert material in that it doesn’t corrode or tarnish (which is why you paid so much for that engagement ring). As with all metals, gold has good electrical and thermal conductivity. Gold’s capability to resist corrosion as well as its high electrical conductivity make it useful for forming contacts in electronic devices.

Gold has been used in various medical treatments over the centuries without harmful affects. It was therefore natural for researchers to look to gold nanoparticles for medical applications rather than using elements such as platinum, which can be toxic in certain circumstances. Forming gold into nanoparticles allows researchers to use gold in areas that are too small for bulk gold to reach and brings with it new capabilities.

For targeted drug delivery uses, it will be interesting to see whether gold nanoparticles show any benefit versus cheaper types of nanoparticles, such as iron nanoparticles. For other uses, gold nanoparticles have some clear advantages.

When gold nanoparticles get really small, with a diameter of 5 nm or less, they can be used as a catalyst to help reactions that, for example, transform air pollutants into harmless molecules.

Using gold to clean up the air is somewhat surprising given that bulk gold is considered to be an inert material in that it doesn’t corrode or tarnish. Normally, gold would be a silly material to use as a catalyst for chemical reactions because it doesn’t do much. However, if you break down gold to nanosize (approximately 5 nanometers), it can act as a catalyst that can do things such as oxidizing carbon monoxide.

Researchers attach molecules to gold nanoparticles that are attracted to diseased regions of the body, such as cancer tumors, and other molecules such as therapeutic drug molecules. This enables the functionalized gold nanoparticles to be used to in targeted drug delivery.

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.

Excerpted from Nanotechnology For Dummies (2nd edition), from Wiley Publishing