Colloidal Stability of Ligand-Stabilized Gold Nanoparticles

Gold nanoparticles (NP), coated with organic ligands for stabilization and functionalization, are being increasingly used in nano- and bio-technology applications.  The ultimate fate of such NP in the environment is a matter of increasing concern.  As a first step, it would be useful to have a predictive model for the stability of the NP against self-aggregation in various aqueous media.  When NP are very small (less than 100 nm), traditional models like DLVO fail to predict the stability, leading to incorporation of so-called extra-DLVO forces that are less understood.  In this work we carry out a systematic study of the short-time aggregation of a certain class of ligand-stabilized gold NP using a combination of experiment and modeling.  The NP are prepared with various gold cores, ranging from 2-6 nm in size, and coated with alkanethiols that terminate in a short hydrophilic segment and a charged head group (which can be varied).  Dynamic light scattering measurements are used to calculate the stability ratio, which is directly related to the total interaction potential, for a given NP-solvent system.  Atomistic modeling gives an independent first-principles prediction that aids in understanding the experimental results.  Finally, classical models (in the extra-DLVO regime) are parameterized using the quantitative understanding gained from these techniques.