Direct synthesis of large water-soluble functionalized gold nanoparticles using Bunte salts as ligand precursors

The widespread use of functionalized gold nanoparticles (AuNPs) in a rapidly increasing number of sensing and biomedical applications has made the development of synthetic methods that combine precise surface chemistry control (functionality) with effective core size control over the range of 1-20 nm crucial. Although a variety of effective methods exist for controlling the core size and functionality during gold nanoparticle synthesis, there is a lack of synthetic methods that permit the direct synthesis of thiol-protected gold nanoparticles with core diameters greater than 5.0 nm. Inspired by previous reports on the use of alkyl thiosulfates (Bunte salts) as ligand precursors, we anticipated that the slow passivation kinetics of these masked thiols would provide a method to synthesize large functionalized AuNPs directly. We found that Bunte salts produce larger AuNPs under the same synthesis conditions than do thiols. We investigated the effect of the ligand/gold ratio, temperature, and reducing agent concentration on the particle diameter and dispersity to understand better how to control particle size. The AuNP core size can be systematically controlled by varying the ratio of ligand precursor/gold (L/Au) and the temperature of the reaction. The synthesis produces functionalized AuNPs ranging from 1.5 to 20.0 nm in diameter. The use of Bunte salts provides a convenient synthetic platform for the synthesis of AuNPs across this size range that possess a variety of surface functionalities, including positive, negative, and neutral functional groups.