Thanks for your question! Applications for nano-particles in general are just starting to be put into use, but there are many potential applications for which they may be useful.

As for gold nano-particles, the really cool thing about them is that bulk gold is inert (nonreactive) -you can think about gold jewelry: it doesn't react on you when you wear it and it lasts a long time.

However when you make nanoparticles out of gold the very small pieces of gold are reactive. This property is good for creating sensors for chemicals and biological agents.

Another property of gold nanoparticles is that they can emit a large amount of heat. This has a potential use in biomedical applications such as heating up a tumor to kill the cells.

Other nanoparticles that are coated in a gold shell are showing more biocompatibility as being less toxic than some nanoparticles not coated in gold. These are just a few of the applications that I am aware of.