The Effect of Aniline Concentration in the Ligand Exchange Reaction with Citrate-Stabilized Gold Nanoparticles

Sodium citrate reduction of hydrogen tetrachloroaurate is one of the most efficient routes in the synthesis of gold nanoparticles (AuNPs). However, a major limitation of this method is that it results only in citrate-stabilized AuNPs, and exchange reactions of the citrate for another stabilizer must be undertaken in cases where other surface functionalities are desired. These exchange reactions can be studied using a variety of techniques. Two techniques we have used for analysis are ultraviolet-visible spectroscopy, which takes advantage of the plasmon resonance (PR) of AuNPs, and liquid chromatography (LC) to examine residual exchanger in the supernatant at equilibrium. The exchange of the citrate on the surface of AuNPs with aniline was studied where different concentrations of aniline are added. While the equilibrium PR band spectra were similar for high and low concentrations of added aniline, the spectra as a function of time differed markedly. Surprisingly, increasing the concentration of added aniline slowed the evolution of a second red-shifted band. Transmission electron microscopy (TEM) imaging showed that addition of neither high nor low concentrations of aniline caused a significant difference in particle size, and the appearance of the large second band was most likely due to the presence of an oligomeric aniline species surrounding the particles. As aniline is known to undergo polymerization under a variety of conditions, it is postulated that aniline polymerization is a competitive event to the ligand exchange process, and increasing the aniline concentration increases the likelihood of polymerization kinetically competing with association of the aniline species interacting with the AuNP surface. This was further supported by LC data, which demonstrated that the aniline exchanged at approximately 4% of the initial amount of citrate present in the synthetic mixture.