We routinely quantify the biotin-binding capacity of our beads using fluorescein-biotin. However, since it is very difficult to ascertain the effect of the bead on quantum yield and other properties of the fluor, we avoid this situation by fluorescently quantifying the amount of unbound vs. bound bioin-fluorophore left in solution. By simple subtraction, we obtain the amount bound. If you use this technique, it is always wise to run a control bead without bound streptavidin if possible as well as a no bead control.