Newkome-Type Dendron-Stabilized Gold Nanoparticles: Synthesis, Reactivity, and Stability

We report the synthesis and evaluation of four Newkome-type dendrons, G1-COOH, G2-COOH, SH-G1-COOH, and TA-G1-COOH, and their respective gold dendron conjugates, where GX represents the generation number. Gl- and G2-COOH are two-directional symmetric dendrons that have cystamine cores containing a disulfide group. SH-G1-COOH was prepared by treatment of G1-COOH with dithioerythritol to yield a free thiol group to replace the disulfide linkage. TA-G1-COOH has a thioctic acid moiety, which is a five-membered ring containing a disulfide group that cleaves to produce two anchoring thiols to bond with the gold surface. All dendrons have peripheral carboxylate groups to afford hydrophilicity and functionality. Gold nanoparticle conjugates were prepared by reaction of each dendron solution with a suspension of gold colloid (nominally 10 nm diameter) and purified by stirred cell ultrafiltration. Chemical structures were confirmed by H-1 and C-13 nuclear magnetic resonance spectroscopy and matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Particle size and surface plasmon resonance of the conjugates were characterized by dynamic light scattering (DLS) and UV-vis spectroscopy, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to confirm covalent bonding between the thiols on the dendron and the gold surface. XPS also revealed changes in the S/Au intensity ratio as a function of the dendron chemical structure, suggesting steric effects play a role in the reaction and/or conformation of dendrons on the gold surface. The colloidal and chemical stability of the conjugates as a function of temperature, pH, and suspending medium, and with respect to chemical resistance toward KCN, was investigated using DLS and UV-vis absorption.