What I have found is basically this:
http://bookshop.blackwell.com/jsp/id/Av ... 0121820855
It is a book about avidin-biotin technology. Anyway the biggest clue fo rme was during the search is this:
http://www.biocompare.com/itemdetails.asp?itemid=434283
This is a site where you can buy avidin or streptavidin-gold. And in the description it says:
streptavidin absorbed to colloidal gold particles
now this is very important. definiton of aDsorption(not absorption) is this:
Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a molecular or atomic film (the adsorbate).
So I'm thinking that there are these big gold particles, and the proteins are bound to the surface, making a film. This is not a chemical bond really. I personally think, but I can be wrong that the aminoacids in the protein that have some kind of charge, negative or positive, electrostatically bind the protein to the huge electron clouds of the gold atoms.
colloidal gold particles mean a specific particle size(1-100 nm) which are big enough to ineract with light, and make the solution opaque, or whatever, therefore can be seen in light microscope, and the huge nucleus of gold atoms make it possible for electrons to bounce off, therefore making it great for electronmicroscopes.
Anyway but you probably know this, that avidin has a strong affinity to biotin which is the vitamin B7 or vitamin H, and it is a coenzyme. So where this B7 there will be this avidin.
I have found on wikipedia this sentence about avidin:
Avidin immobilized onto solid supports is also used as purification media to capture biotin-labelled protein or nucleic acid molecules.
So avidin is mainly used to mark biotin labeled things, but because avidin is not showing up on a transmission electron microscope picture, somehow gold or other material with huge nucleus must bound to avidin.