Synthesis and Self-Assembly Behavior of Charged Au Nanocrystals in Aqueous Solution

A series of water-soluble Au nanocrystals with different core sizes coated by either negatively or positively charged ligands are synthesized. We find a ligand interexchange process takes place when positively and negatively charged nanocrystals are mixed together and heated, resulting in mixed charged zwitterionic nanocrystals. The ligand exchange process between nanocrystals is studied in detail by electrophoresis. Self-assembly properties of the monocharged and zwitterionic nanocrystals are studied subsequently. By using the solvent evaporation process only the zwitterionic and positively charged nanocrystals can pack into well-ordered fcc lattice films. Under the nonsolvent diffusion condition, only the zwitterionic nanocrystals can aggregate and form shaped supracrystals. Structural analysis shows that the interparticle distance of the shaped supracrystal made of zwitterionic nanocrystals is 1 nm larger than that of the film one. The different interparticle distance is ascribed to the different fabrication process. We consider that nanocrystals adopt the closest packing in the film supracrystal due to the destroyed electrical double layer during the drying process, while the electrostatic repulsion plays an important role in determining the interparticle distance in the shaped supracrystal