Colloidal gold
![Suspensions of gold nanoparticles of various sizes. The size difference causes the difference in colors.](/Images/godic/202502/04/Gold2551445.jpg")
![](/Images/godic/202502/04/C60-rods1445.png")
![This cranberry glass bowl was made by adding a gold salt (probably gold chloride) to molten glass.](/Images/godic/202502/04/Vintage_cranberry_glass1445.jpg")
![Potential difference as a function of distance from particle surface.](/Images/godic/202502/04/Gold_nanoparticle_Zeta-potential1445.png")
Colloidal gold is a sol or colloidal suspension of submicrometre-size nanoparticles of gold in a fluid, usually water. The liquid is usually either an intense red colour (for particles less than 100 nm) or blue/purple (for larger particles). Due to the unique optical, electronic, and molecular-recognition properties of gold nanoparticles, they are the subject of substantial research, with applications in a wide variety of areas, including electron microscopy, electronics, nanotechnology, and materials science.