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Nanomaterials are widely used as pseudostationary and stationary phases in electrically driven capillary separations. The advantages of nanomaterial incorporation into capillary electrophoresis (CE) are numerous and include tunable sizes, multiple core compositions, flexible injection/introduction methods in separation techniques, and diverse surface chemistry options. Nanomaterials, however, exhibit inherently large surface energies which induce aggregation and as a result, yield unpredictable function in separations. Because nanomaterials can modify buffer conductivity, viscosity, and pH; separation optimization and nanoparticle stability must be considered. Successful incorporation of nanomaterials into reproducible separations requires (1) strict nanomaterial synthetic control and (2) detailed characterization of the nanoparticle in terms of both core material and surface chemistry.
For this reason, this dissertation investigates how the surface chemistry on and morphology of gold nanoparticles impact capillary electrophoresis separations. The gold nanoparticle core composition, shape, size, self assembled monolayer (SAM) formation kinetics, and SAM ligand packing density are all evaluated for thioctic acid, 6-mercaptohexanoic acid, or 11-mercaptoundecanoic acid monolayers. Transmission electron microscopy (TEM), 1H NMR, extinction spectroscopy, zeta potential, X-ray photoelectron spectroscopy (XPS), and flocculation studies are used to assess the morphology, surface chemistry, optical properties, surface charge, SAM packing density, and effective stability of carboxylated nanoparticles, respectively.
Using these well-characterized nanostructures, applications of gold nanoparticle pseudostationary phases in capillary electrophoresis is studied. Gold nanoparticles functionalized with mixed SAMs composed of thioctic acid and either 6-mercaptohexanoic acid or 6-aminohexanethiol impact the mobility of possible Parkinson's disease biomarkers in a concentration and surface chemistry dependent manner. From these data, a critical nanoparticle concentrations is developed to characterized nanoparticle stability during capillary electrophoresis separations.
To understand the function of these and other carboxylated gold nanoparticles, extended DLVO theory is used to model interparticle interactions during electrically driven flow. 11-Mercaptoundecanoic acid functionalized gold nanoparticles suppress current, while 6-mercaptohexanoic acid and thioctic acid functionalized nanoparticles enhance separation current. Nanoparticle aggregation leads to electron tunneling effects between nanoparticles thereby increasing currents in poorly ordered SAMs while highly packed monolayers induce reversible flocculation characteristics and reduce current. In all cases, these effects are dependent on nanoparticle concentrations.
Finally, surface chemistry optimized carboxylic acid functionalized gold nanoparticles effect the separation of hypothesized Parkinson's disease biomarkers. SAM composition and surface coverage impact separation efficiency, resolution, and selectivity. These effects are most systematic with well ordered SAMs. To understand the mechanism functionalized gold nanoparticles exhibit during a separation, their zeta potential with and without dopamine are evaluated. Nanoparticle to dopamine mole ratios (i.e. large dopamine concentrations), neutralize the three functionalized gold nanoparticles according to a dose response curve. The positively charged dopamine molecules saturate the negatively charged nanoparticle surfaces and aggregate thereby providing a plausible explanation to the biomarker concentration trends observed in capillary electrophoresis. These and future studies provide a rigorous experimental and theroretical evalauation of how nanoparticle structure impacts their function as pseudostationary phases in separations and other applications.