Date of Award

1-1-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Jon Zubieta

Keywords

electrostatic, gold, nanoparticles, polyoxometalate, POMs, self assembly

Subject Categories

Physical Sciences and Mathematics

Abstract

Nanoscience has shifted its focus from studying properties of individual nanoparticles toward using these nanoparticles as building blocks to assemble larger systems. One approach to this end is to exploit electrostatic self-assembly through the interaction of oppositely charged nanoparticles to form larger superstructures and even crystalline superlattices. Metal nanoparticles, such as gold, have been used to design such structures, but understanding the mechanics of the self-assembly can be elusive due to their random size and charge distributions. However, a certain class of polyoxomolybdates can provide insights into how electrostatic self-assembly occurs with their precise size dimensions and charges.

In the first investigation, the electrostatic assembly between a series of the differently charged polyoxomolybdate-type Keplerates of (NH4)42[{(MoVI)MoVI5O21(H2O)6}12{MoV2O4(CH3COO)}30]∙ ≈300 H2O ∙ ≈ 10 CH3COONH4 (Mo-132a), (NH4)72[{(MoVI)MoVI5O21(H2O)6}12{MoV2O4(SO4)}30] ∙ ≈200 H2O (Mo-132b) and Na10(NH4)62[{(MoVI)MoVI5O21(H2O)6}12{MoV2O4(HPO4)}30] ∙ ≈300 H2O ∙ ≈2Na+ - ≈2NH4+ ∙ ≈4 H2PO4 (Mo-132c) with cationic gold nanoparticles (AuNPs) was explored for the first time. The rapid electrostatic assembly from nanoscopic entities to micron scale aggregates was observed upon precipitation, which closely matched the point of aggregate electroneutrality. Successful assembly was demonstrated using UV-vis, DLS, TEM, and zeta-potential analysis. Results indicate that the point at which precipitation occurs is related to charge balance or electroneutrality, and that counter-ions at both the Mo-132 and AuNP play a significant role in assembly.

The subsequent investigation, focused on certain counterintuitive results of the previous study of Mo-132 and AuNP aggregates by manipulating the AuNP core diameter. We hypothesized that a AuNP core diameter of comparable size to Mo-132 would lower the rE0 (equivalence molar ratio, r = [Mo-132]:[Au]) between the nanomaterials, whereas a larger AuNP core would raise the rE0. The range of sizes allowed assessment of how counter-ions affect assembly based on comparable dimensions of the nanomaterials. It was found that a smaller AuNP core size of 1.6 nm produced the naively anticipated results based on charge yielding a rE0, molar ratio of ca. 1.0 at electroneutrality. However, even at this favorable rE0 unorganized aggregates were observed. These findings suggested that crystallization could not be based on charge considerations alone, but that the size relation between the nanomaterials must also be taken into account to induce crystallization.

In the last investigation, the electrostatic self-assembly of the polyoxomolybdate (NH4)28[Mo154(NO)14O420(OH)14(H2O)70]∙ 350 H2O (Mo-154) and AuNPs of different size was studied to explore the aspect of asymmetry in these colloidal systems. The aim of this work was to observe the dependence electrostatic interactions may have on asymmetric shaped nanomaterials. The ring-shaped Mo-154 structure gave this study a unique view on how the polarity of electron density can affect the molar ratio, r = [Mo-154]:[Au]. It was shown that the van der Waals attraction forces of Mo-154, in comparison to Mo-132, were stronger resulting in a lower rE0.

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