pH-Responsive Layered Hydrogel Microcapsules as Gold Nanoreactors Veronika Kozlovskaya, † Eugenia Kharlampieva, † Sehoon Chang, ‡ Rachel Muhlbauer, † and Vladimir V. Tsukruk* ,†,‡ School of Materials Science and Engineering, and School of Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed February 3, 2009. ReVised Manuscript ReceiVed April 2, 2009 We demonstrate that layered hydrogel poly(methacrylic acid) capsules (PMAA), produced from hydrogen-bonded (PMAA/poly-N-vinylpyrrolidone) (PMAA/PVPON) multilayer precursors through cross- linking with ethylenediamine (EDA), can facilitate in situ synthesis of gold nanoparticles within hydrogel walls under ambient conditions. The necessary amine groups are available within the (PMAA) shells due to one-end attached cross-linker molecules. We also show that the nanoparticle size can be controlled through changing the pH-dependent balance of amine/ammonium groups in the ionic cross-links within the shells. Importantly, the pH-responsive properties of the ultrathin hydrogel shells are preserved after gold nanoparticle synthesis within the capsule walls. The reported in situ synthesis of gold nanoparticles within the ultrathin and pH-responsive shells can find a potential use for straightforward and facile fabrication of the hybrid organic-gold nanomaterials for biochemical sensing and delivery applications. Introduction In past years, the research on metal nanoparticles as re- enforcing, functional, and responsive blocks for hybrid organic/nanocomposite materials has gained significant at- tention due to the unique optical, electrical, catalytic, and antibacterial properties of inorganic nanoparticles. 1-4 Dif- ferent inorganic materials such as silica nanoparticles, magnetite nanocrystals, metal nanoparticles, and semicon- ducting nanowires have been deposited onto or within polymer matrices to enhance their mechanical and optical properties. 5-7 Inclusion of metal nanoparticles within polymer materials was shown to be an effective way for a combination of unique optical properties of gold nanostructures with responsive behavior of ultrathin membranes enhancing their function as easily handable adaptive nanomaterials. 8 Such a combination allowed a robust control over various important properties of the responsive polymer films 9 such as microroughness, 10,11 wettability, 12 biocompatibility, 13 and optical response, 14 which are important for a variety of fields in separation, 15 sensing, 16,17 and delivery of functional molecules. 18 Various methods have been used for embedding gold nanoparticles within the polymer matrix including Lang- muir-Blodgett (LB) deposition, 19,20 electrostatic-driven adsorption, 21-24 and hydrogen-bonded assembly. 25 The layer- by-layer assembly (LbL) of polyelectrolyte structures on sacrificial substrates has been widely used to fabricate free- standing planar polymer films with inorganic particles for the improvement of their mechanical, delivery, and optical properties. 26-29 Polyelectrolyte capsules with nanocomposite organic/inorganic shells with embedded nanoparticles have demonstrated significant mechanical rigidity and maintain * To whom correspondence should be addressed. 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