A Straightforward, One-Step Synthesis of Alkyl Thiol Capped Silver Nanoparticles

Classical nucleation theory and Derjaguin, Landau, Verwey, Overbeek (DLVO) theory for colloidal stability were applied to gain insight into the synthesis of dodecanethiol (DDT)-functionalized silver nanoparticles (NPs) by reduction of silver nitrate with sodium borohydride in ethanol. This analysis indicated the importance of quickly establishing a dense DDT 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 DDT was available to form a densely packed ligand layer on the NPs. These insights were applied to design a new straightforward, one-step, one-phase synthesis for the production of alkyl thiol-functionalized silver NPs. To test the insights from DLVO theory, a battery of 16 samples were synthesized in the parameter space R=3-12, S=1-12 where R=[NaBH4]/[AgNO3], S=[DDT]/[AgNO3], and [AgNO3]=10 mM. 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. Ultimately, the insights gained from DLVO theory successfully guided the development of a one-step, one-phase technique for the synthesis of uniform, spherical alkyl thiol-functionalized silver NPs. In future work, this technique will be extended to facilitate the grafting of polymers from phase-separated ligand domains on NP surfaces.