Introduction
In the present scenario of nanotechnology, gold nanoparticles (Au NPs) are at the forefront due to their amazing chemical and physical properties, which can be exploited in various scientific disciplines. Au NPs have a wide range of applications in areas such as catalysis, medical diagnostics, and biological imaging.
[1]
Ease of chemical synthesis and less toxicity as compared to some other nanomaterials are advantages. Gold as well as silver nanoparticles have been known to mankind for several centuries. Interestingly, if we look at old cathedrals and churches, which were built few centuries ago, one can see color glass windows. The reason is that glassmakers of the medieval time added minute amounts of gold and silver salts to molten glass. Thus, they were able to incorporate gold and silver nanoparticles. The glassmakers were unknowingly the first nanotechnologists.
[2]
In a modern chemical aspect, Michael Faraday was the first scientist who made gold colloids, around 1857.
[3]
Later on, the synthetic method was simplified by the pioneering work of Turkevich
[4]
and Frens.
[5]
The properties of gold nanoparticles at the nano scale (1-100 nm) remarkably differ from those of the bulk counter part because of quantum size confinement imposed by the nano-size regime. As a result, gold nanoparticles show colors ranging from ruby red to blue, green, and orange, etc., depending on the size and shape.
[6]
This prominent absorption in the visible is called surface plasmon absorption. The Au NPs show remarkable catalytic activity, whereas bulk gold is known to be catalytically inert.
[7]
Gold nanoparticles can be synthesized by simple chemical reduction of a tetrachloroauric acid (HAuCl4) solution with sodium citrate or sodium borohydride in both aqueous and organic media. Usually, SH- or NH-functional groups of organic moieties are known to bind to the surface of Au NPs. Colloidal solutions of Au NPs prepared by citrate
[4]
and borohydride
[5]
reduction have a red-wine color. Solutions of colloidal gold nanorods have a blue color
[8]
with different optical properties.
