Reversible aggregation of gold nanoparticles driven by inclusion complexation

In this work, gold nanoparticles with an appreciably narrow size distribution are prepared by reduction of HAuCl4 in the presence of mono-6-SH-beta-cyclodextrins (CDs). The beta-CD modified gold nanoparticles (beta-CD-AuNPs) tend to aggregate together upon addition of diazo, water soluble ditopic guest molecules with a double-azobenzene structure, due to the host-guest inclusion complexation between beta-CD and diazo. The aggregation leads to significant shifts in the absorption spectrum with concomitant visible color changes from pinkish-red to purple. Furthermore, the average number of gold particles in each gold aggregate is controllable as it depends on the molar ratio of diazo to beta-CD. Further disassociation of the gold aggregation can be realized by addition of excess competitive host alpha-CD which favors forming inclusion complexes with the guest more than beta-CD does. The aggregation-dissociation of beta-CD-AuNPs is confirmed to be reversible. This simple, yet highly efficient way to control the self-assembly of gold nanoparticles could be helpful in repeated usage of the gold nanoparticles applied to diverse technologies.