Regulating Host–Guest Interactions between Cucurbit[7]uril and Guests on Gold Surfaces for Rational Engineering of Gold Nanoparticles

Although macrocyclic host engineered nanoparticles have drawn a lot of attention in the past decade, little attention was paid to the binding affinity between the host and guest assembled on nanoparticles. In this work, the interactions between cucurbit[7]uril (CB[7]) and three guests with phenyl (1), adamantyl (2), and ferrocene (3) groups on a gold surface were investigated. Their interactions were classified into four groups. The binding constants between CB[7] and three guests in the solution (group I) were determined by proton nuclear magnetic resonance (1H NMR) and isothermal titration calorimetry (ITC) experiments. The binding constants between CB[7]-stabilized gold nanosphere and guests 1 and 3 (group II) and those between 1-stabilized gold nanosphere and CB[7] (group III) were determined by ITC experiments. The binding constants determined in groups II and III decrease to approximately 20% of those in group I. The interactions between CB[7] and three guests on gold (group IV) were measured by single-molecule force spectroscopy (SMFS). Much lower binding constants than those in other groups were deduced. The hydrophobic and ion–dipole interactions play important roles in the pulling-out process in a thorough SMFS analysis. Furthermore, the CB[7]-capped gold nanospheres with different sizes were prepared, and their binding constant with the free guest (group II) depends on the nanoparticle size. All of these results are of great benefit to the rational design of macrocyclic host engineered nanoparticles for drug delivery and many other applications.