(418cp) Synthesis of Homogeneous and Mixed Ligand Alkanethiol Functionalized Silver Nanoparticles Via DLVO Modeling of Nucleation and Growth

Classical nucleation theory and Derjaguin, Landau, Verwey, Overbeek (DLVO) theory for colloidal stability were applied to gain insight into the synthesis of alkanethiol functionalized silver nanoparticles (NPs) by reduction of silver nitrate with sodium borohydride in ethanol. This analysis indicated the importance of quickly establishing a dense alkanethiol ligand brush on inherently unstable primary particles to achieve colloidal stability. The DLVO calculations also indicated that the electrostatic potential was a minor contributor to repulsive interactions, signifying that it would be possible to control NP size and uniformity in solutions with high ionic strength, as long as sufficient alkanethiol ligand was available to form a densely packed self-assembled monolayer (SAM) on the NPs. These insights were applied to design a new straightforward, one-step, one-phase synthesis for the production of a wide variety of alkanethiol-functionalized silver NPs. To test the insights from DLVO theory, 16 docanethiol samples were synthesized in the parameter space R=3-12, S=1-12 where R=[NaBH₄]/[AgNO₃], S=[DDT]/[AgNO₃], and [AgNO₃]=10mM. In general, samples with R=3 or S=1 were polydisperse, however, samples in the R=6-12 and S=3-12 range had uniform particle sizes with average diameters between 3.5-4.7 nm. Additionally, samples with R=72-108 and S=12 were synthesized to test particle stability at high ionic strength; again uniform NPs with average diameters from 3.5-3.8 nm were produced. Finally, the synthetic methods developed with dodecanethiol were extended to silver NPs having SAMs composed of single ligands from butanethiol through undecanethiol, as well as mixed SAMs composed of a combination of dodecanethiol and a ligand from the series butanethiol-undecanethiol. All syntheses were conducted with R=12, S=12, [AgNO₃] = 0.5 mM and resulted in similar quality of NPs to pure dodecanethiol functional NPs. NP size did not change appreciably from dodecanethiol as ligand length was shortened, however, polydispersity was observed to increase as the average ligand length of the SAM decreases. This result follows directly from our understanding of the growth processes of this system through DLVO, as the steric barrier to aggregation was calculated to be smaller for shorter ligand lengths, resulting in more NP aggregation events. Ultimately, the insights gained from DLVO theory successfully guided the development of a one-step, one-phase technique for the synthesis of uniform, spherical alkanethiol-functionalized silver NPs with both single-ligand and mixed SAMs composed of ligands from butanethiol through dodecanethiol.