Author: Sun ShinPublisher:
ISBN:
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Languages : en
Pages :
Book Description
A wide range of molecules, polymers, and particles which possess both hydrophilic and hydrophobic domains are desirable as surface active agents. Unlike their molecular analogs, inorganic nanoparticle (NP) amphiphiles have the ability both to adapt their surface chemistry in response to environmental changes and to respond to external stimuli acting on the particle cores. These unique capabilities enable new strategies for a variety of applications in drug delivery, emulsification, assembly of hierarchical structures, surface coatings, and optoelectronic devices. This dissertation describes experimental methods for preparing and characterizing NP amphiphiles and also discusses their applications in assembly and biomedical engineering.The synthesis of gold NPs functionalized asymmetrically with domains of hydrophobic and hydrophilic ligands on their respective hemispheres is described in Chapter 2. These particle amphiphiles form spontaneously by a thermodynamically controlled process, in which the particle cores and two competing ligands assemble at the interface between two immiscible liquids to reduce the interfacial energy. The asymmetric surface chemistry resulting from this process is confirmed using contact angle measurements of water droplets on NP monolayers deposited onto hydrophobic and hydrophilic substrates. The spontaneous assembly process is rationalized by a thermodynamic model, which accounts both for the energetic contributions driving the assembly and for the entropic penalties that must be overcome. In addition to gold NPs and thiolate ligands, generality of this approach is demonstrated by extending it to the preparation of amphiphilic iron oxide NPs using two types of diol-terminated ligands. The synthetic strategy described here provides a comprehensive solution for designing NP amphiphiles with controlled surface composition and ligand distribution.The self-assembly of gold NP amphiphiles in water is discussed in Chapter 3. Unlike typical amphiphilic particles with "fixed" surface chemistries, the ligands used here are not bound irreversibly but can rearrange dynamically on the particles' surface during their assembly from solution. Depending on the assembly conditions, these adaptive amphiphiles form compact micellar clusters or extended chain-like assemblies in aqueous solution. By controlling the amount of hydrophobic ligands on the particles' surface, the average number of nearest neighbors -- that is, the preferred coordination number -- can be varied systematically from ~1 (dimers) to ~2 (linear chains) to ~3 (extended clusters). To explain these experimental findings, an assembly mechanism is presented in which hydrophobic ligands organize dynamically to form discrete patches between proximal NPs to minimize contact with their aqueous surroundings. Monte Carlo simulations incorporating these adaptive hydrophobic interactions reproduce the three-dimensional assemblies observed in experiment.The spontaneous incorporation of amphiphilic gold NPs into the walls of surfactant vesicles is reported in Chapter 4. When the NP amphiphiles are mixed with preformed surfactant vesicles, the hydrophobic ligands on the NP surface interact favorably with the hydrophobic core of the bilayer structure and guide the incorporation of NPs into the vesicle walls. Unlike previous strategies based on small hydrophobic NPs, the approach described here allows for the incorporation of water soluble particles even when the size of the particles greatly exceeds the bilayer thickness.Finally, a strategy to stabilize open bilayer structures using NP amphiphiles is described in Chapter 5. Molecular amphiphiles self-assemble in polar media to form ordered structures such as micelles and vesicles essential to a broad range of industrial and biological processes. Some of these architectures such as bilayer sheets, helical ribbons, and hollow tubules are potentially useful but inherently unstable owing to the presence of open edges that expose the hydrophobic bilayer core. The particle amphiphiles described here bind selectively to the open edge of bilayer membranes to stabilize otherwise transient amphiphile assemblies. It is demonstrated how such particles can precisely control the size of lipid tubules, how they can inhibit the formation of undesirable assemblies such as gallstone precursors, and how they can stabilize free-floating lipid microdiscs.