Why is gold so popular in nanotechnology?
First, let's look at gold's properties.


Gold is the most malleable of all metals. This malleability alone is very useful in aiding scientists to create such small gold nanoparticles.
Gold also reflects infrared radiation very well and is an excellent conductor of heat and electricity.
Generally, gold is not easily affected by oxygen and is generally nonreactive with most acids and bases. This property serves well especially in the human body where there are an abundant of acidic and basic fluids and the circulatory system is enveloped with oxygen. Because gold is generally unaffected by most acids, bases, and oxygen, there is more assurance for scientists that their golden nanoparticles for medicinal uses would not react with other things before reaching their specific target.
Going back to gold's ability to reflect infrared radiation, an essential reason why physicians, scientists, and researchers utilize nanotechnology is to track down certain cells (cancer, etc), molecules, or processes. Thus, it is important to use such a metal that can reflect radiation to tell them where their targeted molecules are or where their targets are going. In essence, gold nanoparticles are suitable for "staining," or showing contrasts in cells more visible, such as in tissue samples. This can provide higher spatial resolutions for labeling applications.
Gold, as a nice heat conductor, can also absorb heat very well, as I mentioned earlier, and in these biotechnologies, strong absorption of light energy is also another crucial aspect for allowing gold to then dissipate this absorbed light energy in its surroundings so that these heated areas can also be tracked and help with drug delivery purposes. 


Here are some references: 


Instant insight: A golden future
Nanotechnology Research into the Uses of Gold Nanoparticles