Self-Assembling Microspheres from Charged Functional Polyelectrolytes and Small-Molecule Counterions
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Self-Assembling Microspheres from Charged Functional Polyelectrolytes and Small-Molecule Counterions Brandon McKenna1, Henrik Birkedal1, Michael H. Bartl1,3, Timothy J. Deming1,2, and Galen D. Stucky1,2 1
Department of Chemistry and Biochemistry, 2Materials Department, and 3California NanoSystems Institute University of California, Santa Barbara, CA-93106-9510, USA
ABSTRACT Micrometer-sized spheres have been found to assemble from homopolymer electrolytes and small, multivalent counterions in water. In contrast to previous efforts, these vesicles do not use preformed templates, do not require block copolymers, and do not necessarily employ nanoparticles. We have investigated the requirements for vesicle formation with regards to both components of the assembly. Self-assembly occurs with a variety of poly-amino acids and counterions, all of which require a minimum number of charged groups to promote non-covalent crosslinking. We show how the assembly process is controlled by pH and how, in consequence, the pKa's of the reactants can be used to reliably predict sphere formation. By varying the nature of the small counterions, we have determined the requirements for assemblies. The assemblies have been further investigated using confocal microscopy and fluorescent labeling of the different components.
INTRODUCTION The directed formation of hierarchically arranged silica seen in diatoms and sponges provides a promising framework for designing synthetic patterned nanoscale materials. Using a biomimetic approach to silica condensation has many benefits, including: room-temperature synthesis, neutral or moderate pH’s, the opportunity for hierarchical ordering, and the ability to vary the resulting structure by tailoring the active components. Stucky and Morse have shown how silicateins from marine sponges act as catalysts for silica condensation and as scaffolds for the directed growth of polysiloxanes [1]. Kröger and Sumper have proven how diatom silaffins template organized silica condensation [3, 4]. The construction of silica spheres is important in this context, and they have been made using both natural peptides [4] and synthetic polyamines [5]. Self-assembled microspheres are important for their potential to contain and protect one material, while displaying the properties of a different one on the exterior. Such systems could find applications in chemical storage and transport. In particular, biocompatible microspheres are desirable for applications in drug delivery. For this reason several kinds of systems have been discovered that allow encapsulation, including micelles, liposomes, and hollow capsules made using layer-by-layer deposition onto a sacrificial core [6]. We have previously reported the discovery of organic-inorganic hybrid microspheres that are made from citrate-stabilized nanoparticles and poly-L-lysine (PLL) [7]. Here we report that these types of assemblies can be made using a variety of small multivalent anions in place of
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nanoparticles, and several different polycations. As
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