Looking at avidity between a ligand-receptor, you're looking at an enhancement of the $K_d$ compared to a lone ligand.
Is it more appropriate to compare the $K_d$ using the concentration of the "free" ligand or the concentration of the free complex?
Using the ligand is odd at the lower regimes since the ligand efficency is fairly low but using the nanoparticle changes how you have to calculate the effective concentration. I've seen papers that do both and I'm curious which is most appropriate/correct