Next, assuming that the footprint of a thiolate ligand on a gold NP 47 corresponds to 0.22 nm 2 and that all MUA ligands are deprotonated (the pH at the onset of crystallization ≈ 9.2) leads to the conclusion that each Au•MUA/Au•TMA pair has to be accompanied by ~33 CO3 2-ions to compensate for the excess of the positively charged TMA ligands. Similarly, RDs obtained from larger Au•MUAs and smaller Au•TMAs must trap NH4 + ions from the surrounding solution as they form in order for electroneutrality to be met. In other words, (NH4)2CO3 acts like a "buffer", guiding the formation of colloidal crystals by providing them with small anions or cations to electrically balance the charge-mismatched NPs. Trapping of NH4 + and/or CO3 2-by superionic NPs is not unexpected given that the ratio of small ions to NPs at the onset of self-assembly is on the order of 10 7 (see also Supplementary Fig. ).