Formation and Stabilization of Silver Nanoparticles with Cucurbit[n]urils (n = 5−8) and Cucurbituril-Based Pseudorotaxanes in Aqueous Medium

A series of silver/cucurbituril nanoparticles and aggregates have been prepared upon reduction of silver nitrate with sodium borohydride in the presence of different cucurbit[n]uril (CB[n]) macrocycles; CB[7] and CB[8] allow the formation of stable solutions of monocrystalline, narrowly dispersed nanoparticles (5.3 and 3.7 nm, respectively), while CB[5] and CB[6] induce rapid aggregation and sedimentation. The rigidity of CB[5] and CB[6], and their possible lack of suitable arrangement at the silver surface, may explain the poor stabilization of these silver assemblies, while the more flexible CB[7] and CB[8] may undergo some minor distortions and better adapt to the requirements of the metallic surface; computer modeling supports the existence of interactions between the silver nanoparticles and the oxygen atoms of the CB[n] carbonylated rim. The optimal silver nitrate/CB[7] ratio for the formation of stable nanoparticles is 1:1−2:1, while large excesses of silver or CB[7] trigger aggregation. Masking the portals of CB[7] by adding a bulky, positively charged guest into its cavity has a surprisingly minor effect on the stability of the silver/CB[7] assemblies; in such a case, the CB[7] rim is still expected to interact with the NPs, albeit via a fraction of its carbonyl oxygen atoms.