(molar mass od gold = 196 g/mol) x (mass single nanoparticles) x (number concentration = 7,2e10/ml),
with
(mass of nanoparticles) = (Volume of 40 nm sphere) x (density of gold).
Practically, gold colloids have an isosbestic point in the UV/Vis around 380 nm, so that you can use Extinction (optical density) at 380 nm to calibrate concentration/molarity during dilution.