| Metal nanoparticles are an interesting class of materials because they often exhibit properties different from those of the corresponding bulk metals. For example, bulk Au is not catalytically active, but recent studies show that Au nanoparticles can serve as catalysts for oxidation and hydrogenation reactions. Without a suitable support, however, metal particles aggregate, reducing surface area and eventually affecting the particle properties. To overcome this problem, this research employs the layer-by-layer (LbL) assembly technique, which was introduced by Decher in 1991, as a convenient method to prevent the aggregation of nanoparticles and immobilize them on solid supports. While the multilayers help in stabilizing the nanoparticles, they also aid in retaining important properties of Pd (catalytic) and silver (antibacterial) nanoparticles.; Catalytic Pd nanoparticles in multilayer polyelectrolyte films can be easily prepared by alternating depositions of poly(acrylic acid) (PAA) and a polyethylenimine (PEI)-Pd(II) complex on alumina, and subsequent reduction of the Pd(II) by NaBH4. The polyelectrolytes limit aggregation of the particles and impart catalytic selectivity in the hydrogenation of alpha-substituted unsaturated alcohols by restricting access to catalytic sites. Hydrogenation of allyl alcohol by encapsulated Pd(0) nanoparticles can occur as much as 24-fold faster than hydrogenation of 3-methyl-l-penten-3-ol. In a related system, alternating adsorption of PdCl42- and polyethylenimine (PEI), followed by reduction of Pd(II), yields catalysts with a higher activity than found in [PAA/PEI-Pd(0)]nPAA films due to greater accessibility of the Pd nanocatalysts. In the [PAA/PEI-Pd(0)] nPAA system, turnover frequency decreases with the number of layers deposited, suggesting that the outer layer of the film is primarily responsible for catalysis. In contrast, turnover frequency increases with the number of deposited layers for reduced [PdCl42-/PEI] n films.; We also report work examining the antibacterial properties of Ag nanoparticle-containing multilayer polyelectrolyte films deposited on polyethersulfone ultrafiltration membranes. Rubner and others suggested that the mechanism of antibacterial action by Ag nanoparticles in polyelectrolyte films presumably involves oxidation of nanoparticles and slow release of Ag+. In principle, this should lead to sustained antibacterial efficacy of membranes containing Ag nanoparticles compared to membranes containing Ag+ ions. Studies of silver leaching confirm that the rate of leaching of silver in Ag+-containing films is nearly an order of magnitude greater than that in Ag0-nanoparticle containing systems, confirming that the use of Ag nanoparticles rather than ions could enhance the longevity of an antibacterial coating. Filtration of bacteria-containing suspensions through modified membranes indicates that the flux decline associated with bacterial fouling in silver-containing films is lower than that in membranes without any silver coating, but it is difficult to distinguish between Ag + and Ag0-containing films in short-term fouling studies. |
