Digestive Ripening of Thiolated Gold Nanoparticles:  The Effect of Alkyl Chain Length

Digestive ripening, heating a colloidal suspension at or near the solvent boiling point in the presence of a surface-active ligand, was applied to polydisperse colloidal gold in toluene using a series of alkylthiols, viz., octyl-, decyl-, dodecyl-, and hexadecylthiols. In all the instances, digestive ripening significantly reduced the average particle size and polydispersity. All the colloids remain suspended in solution above 80 °C, but at room temperature the tendency to form 3D superlattices and precipitate increased with declining alkyl chain length. For example, using octanethiol as the ligand makes the colloids aggregate into big 3D superlattices and precipitate; decane- and dodecanethiol also produce precipitated 3D superlattices along with separate particles, while hexadecanethiol-coated particles remain well separated from each other. The optical spectra at room temperature reveal, apart from the gold plasmon band at 530 nm, a large tail above 700 nm for Au−octanethiol and Au−decanethiol cases and a shoulder at 630 nm for Au−dodecanethiol attributed to the superlattices. Au−hexadecanethiol, on the other hand, shows only the gold plasmon band as expected from separate particles. However, at higher temperatures only the gold plasmon band is observed for all the colloids indicating the dissolution of the superlattices. The aggregation of the particles into 3D superlattices or their stability as a colloidal suspension is qualitatively explained on the basis of decreasing van der Waals attraction between the gold nanoparticles as the separation between them is increased through the alkyl chain length of the capping ligand from octyl to hexadecyl.