  Motivated by the fine compositional control observed in membraneless droplet
organelles in cells, we investigate how a sharp binding-unbinding transition
can occur between multivalent client molecules and receptors embedded in a
porous three-dimensional structure. In contrast to similar superselective
binding previously observed at surfaces, we have identified that a key effect
in a three-dimensional environment is that the presence of inert crowding
agents can significantly enhance or even introduce superselectivity. In
essence, molecular crowding initially suppresses binding via an entropic
penalty, but the clients can then more easily form many bonds simultaneously.
We demonstrate the robustness of the superselective behavior with respect to
client valency, linker length and binding interactions in Monte Carlo
simulations of an archetypal lattice polymer model.
