Gold Nanocluster-Grafted Cyclodextrin Suprastructures: Formation of Nanospheres to Nanocubes with Intriguing Photophysics

Abstract

Glutathione (GSH)-coated gold nanoclusters (Au NCs) were synthesized in aqueous acidic medium. On deprotonation of the carboxyl groups of the GSH molecules under alkaline condition, the anionic ends react with the added cationic surfactant molecules to convert the Au NCs hydrophobic, resulting in loss of fluorescence due to apparent insolubility in water. The fluorescence is revived by adding cyclodextrins (CDs) that encapsulate the protruding hydrophobic tails of the surfactant molecules surrounding the GSH-coated Au NCs. While addition of β-CD showed maximum revival of the Au NC fluorescence, that by adding α-CD was lesser. Interestingly, on adding γ-CD, there was no increase in fluorescence of Au NCs at all. The size of CDs varies as γ- > β- > α-. It appears that the cavity size of the CD-hosts controls the fluorescence from the Au NCs abruptly, and the reason behind that was found to be formation of suprastructures, the shapes of which varied from spherical to cubic. The work shows the production of Au NC-grafted CD suprastructures that develop fluorescence on-off composites on the basis of their overall shapes.