pH- and Ion-responsive Membranes from Self-Assembly/Sintering of SiO 2 @Au Core-Shell Nanospheres
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pH- and Ion-responsive Membranes from Self-Assembly/Sintering of SiO2@Au Core-Shell Nanospheres Patricia Anne A. Ignacio-de Leon and Ilya Zharov Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA ABSTRACT We prepared colloidal crystals by self-assembly of gold-coated silica nanospheres, and formed free-standing nanoporous membranes by sintering these colloidal crystals. We modified the nanopore surface with ionizable functional groups, by forming a monolayer of L-cysteine or by surface-initiated polymerization of methacrylic acid. Diffusion experiments for neutral and cationic dyes showed that transport through these surface-modified Au-coated colloidal membranes can be controlled by pH and presence of metal cations. INTRODUCTION Colloidal crystals made of metallic nanoparticles or coated with a metallic layer have been extensively investigated in the last decade because such structures possess properties suitable for applications in catalysis, [1] photonics, [2] surface-enhanced Raman spectroscopy, [3] and chemical and biological sensors. [4] Two main strategies have been used to prepare these materials. [5] Metallic nanoparticles of various shapes have been assembled into the ordered structures directly, either by filtration [6] and evaporation, [7] or using an electric field-mediated assembly. [8] Polystyrene and silica colloidal crystals have been coated with metallic layers using layer-by-layer deposition of gold nanoparticles, [9] or using a variety of dipping methods that utilized immobilization of metallic nanoparticles on the colloidal crystal surface and electroless plating. [10] An obvious alternative method for the preparation of the latter materials is the self-assembly of metal-shell dielectric-core nanoparticles into the ordered structures. However, to the best of our knowledge, this approach has not been previously reported. In the present work, we demonstrate that SiO2@Au core-shell nanospheres can indeed self-assemble into the ordered colloidal crystals. Ordered colloidal structures are attractive as nanoporous membrane materials. [11] Recently, we have prepared free-standing silica colloidal membranes by sintering silica colloidal crystals[12] which possess high molecular flux and size-selectivity[13] and can be used to prepare pH and ion-responsive membranes by their surface modification with cationic polyelectrolyte brushes.[14] Using SiO2@Au core-shell nanospheres to prepare free-standing colloidal membranes provide two advantages. First, sintering as a method of preparing freestanding silica colloidal membranes results in the loss of the silanol surface functionality at high temperature, which requires rehydroxylation if further surface modifications are desired. Using gold on the surface of the nanospheres comprising the colloidal membranes might solve this problem. Secondly, it would introduce new types of nanopore surface functionalization based on thiol-gold chemistry. Our paper describes the preparation, characterization and surface modification of gold-coated free
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