Digestive-ripening agents for gold nanoparticles: Alternatives to thiols

Several ligands, such as alkylthiols, -amines, -silanes, -phosphines, -halides, and simple alkanes, were employed for digestive ripening, a process in which a colloidal suspension in a solvent is refluxed at the solvent boiling temperature in the presence of a surface-active ligand to convert a highly polydisperse colloid into a nearly monodisperse one. Apart from thiols, which are the only established digestive-ripening ligands, amines, silanes, and phosphines were found to be similarly efficient for this purpose. The important steps involved in the digestive ripening were identified to be (1) breaking the polydisperse colloid into smaller size particles upon addition of the ligand, (2) isolating this colloid from the reaction side products, and finally (3) heating this isolated colloid in the presence of the ligand to form a nearly monodisperse colloid. The successful ligands could be differentiated from the others based on their effectiveness to perform the different tasks in each step. Namely, they broke the bigger nanoparticles into smaller ones in the first step, formed a stable redispersable colloid in toluene after the second step, and at the end of the third step lead to a nearly monodisperse colloid. The ability of the different ligands to break the bigger, prismatic as-prepared particles in the first step varied as RSH approximate to RNH2 approximate to R3P approximate to RSiH3 > RI > ROH approximate to RBr and simple alkanes completely failed to induce any changes in the size and shape of the as-prepared colloid. Ligands such as RI, RBr, and ROH failed in the second step, possibly because of the poor ligand-gold interaction. The ligand-gold interaction trends observed here could be rationalized semiqualitatively by invoking the hard and soft acid and base theory, which suggests that a soft acid-like gold likes to interact with softer bases such as RSH and R3P rather than hard bases such as ROH. After the third step, the sizes of the nearly monodisperse particles depended on the ligand used for digestive ripening and correlated well with the ligand-gold interaction trends.