Finally, it is important to confirm that Au•TMA and Au•MUA retain their identity during selfassembly experiments -i.e., no ligand hopping between the two types of NPs takes place to a significant extent at room temperature. To this end, we performed a series of gel electrophoresis experiments on Au•TMA and Au•MUA with a typical size of 4.83 nm (Supplementary Section 7). In the key experiment, we titrated Au•TMA with Au•MUA, collected the resulting precipitate, redissolved it in a concentrated (NH4)2CO3 solution, and allowed the NPs to move in an electric field. We were pleased to find that the mixture of NPs was separated into two bands migrating in opposite directions (Supplementary Fig. ).