In this particular novel, an assassin is using a tranquilizer gun to take out his target. The dart is loaded with a solution that has gold dust suspended in liquid; the gold dust is what's important, since it's going to kill the (non-human) target.
My question is, WHAT should the gold dust be in? It doesn't have to be a lot of gold dust; if the thing's too heavy, it obviously won't work. But I don't know what it should be mixed with; water is probably not dense enough, but I can't say that for sure.
Anyone have any thoughts?