Cetyltrimethylammonium Bromide/Chloride on Gold Nanocrystals Can Be Directly Replaced with Tri-Citrate

This work demonstrates an effective method for directly exchanging the toxic cetyltrimethylammonium bromide/chloride (CTAB/C) on Au nanocrystals with tri-citrate. Our experimental and computational studies indicate that counterion plays a vital role in the exchange process. Specifically, when citrate species bind to Au surface, they all evolve into tri-citrate with different counterions. In the case of three H⁺ counterions, tri-citrate could readily replace the CTAB/C due to a strong binding of the carboxylate group with the Au surface. The substitution of H⁺ counterion by Na⁺ or K⁺ weakens the binding strength and thus compromises the exchange. Additionally, our quantitative measurements and theoretical calculations indicate that Au nanospheres encased by high-index facets are advantageous over their counterparts enclosed by {111} and/or {100} facets for the exchange owing to the difference in binding strength. The mechanistic insights and experimental control should be extendable to other combinations of surface ligands and metal nanocrystals.