Reversible aggregation of gold nanoparticles driven by inclusion complexation

Abstract

In this work, gold nanoparticles with an appreciably narrow size distribution are prepared by reduction of HAuCl₄ in the presence of mono-6-SH-β-cyclodextrins (CDs). The β-CD modified gold nanoparticles (β-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 β-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 β-CD. Further disassociation of the gold aggregation can be realized by addition of excess competitive host α-CD which favors forming inclusion complexes with the guest more than β-CD does. The aggregation–dissociation of β-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.