Gold does not dissolve in N-hexane or most solvents for that matter.

[Aqua regia (nitric acid:HCl::3:1) will do the trick but I doubt that is what you have in mind... or may be you do., if you really want gold to form solution.]

'Nanopowder' is an oxymoron.  At least,no powder has free, independent particles in the < 100 nm size range.  A powder with small primary size is a collection of fused aggregates sub- and post-micron.  See:

Nov 11th, 2008. Dispersion and nanotechnology

http://www.brainshark.com/brainshark/vu/view.asp?pi=106756756&text=M021507

http://tinyurl.com/hpywsge

As Tausif states, gold (or gold powder) is not soluble (or dispersible) in hexane.  You need to build your 15 nm nanoparticles bottom-up with a Au-containing precursor that's soluble in hexane.  Any resultant gold sol will need to be kept in the liquid state and not dried.

Thanks all for your helpful comments especially these two useful links.

the expression "nanopowder" was written by the company we bought it. this gold nanopowder is a gift from my master that bought 1 year ago. the gold is not miscible in toluene too and became precipitated immediately after adding to toluene.

thus my clear question is that this spherical 15 nm gold nanopowder would be dissolved in which solvents? in its MSDS just wrote that it does not dissolve in water, nothing else.

thanks a lot

 "thus my clear question is that this spherical 15 nm gold nanopowder would be dissolved in which solvents?"

It would seem that either you do not understand the nature of very poorly reactive metals (gold and platinum) or the meaning of word dissolve. In my previous comment, I have already mentioned exactly what it takes to dissolve gold.

Perhaps you should elaborate on what is it that you actually wish do - that way may be someone can actually help you.

Yes you are right Tausif, i mean disperse not dissolve. i wrote disperse in my question title.

Once you have the material in powder form you'll not get back ('disperse') to individual, independent, stable, nano particles.  Period.  You need to make such a (nano) system from a bottom up process.

Forget the powder - or sell it - it's useless for your needs in the form it is in.  You can dissolve it in aqua regia in order to obtain a possible precursor for nanoparticle production.

"All that glisters is not gold" The Merchant of Venice (Shakespeare)