Preparation of Polymer-Coated Functionalized Silver Nanoparticles

In recent years, the study and preparation of inorganic crystalline particles on the nanometer scale has attracted considerable attention from both fundamental and applied research. 1 Metal nanoparticles, particularly silver, gold, and copper, have been the focus of great interest because of their unique optical properties, determined by the collective oscillations of electron density termed plasmons. 2 The efficiency for the absorption and scattering of light by metal nanoparticles can surpass that of any molecular chromophores. 3 Such a high efficiency for interaction with light, together with an extreme resistance to photodegradation, is stimulating the development of new applications of nanoparticles in photochemistry, analytical procedures, and as components of electronic and photonic devices. Chemical resistance and the ability to modify the metal surface are important prerequisites for many potential applications. For example, oxygen-driven etching of silver in the presence of strong ligands for Ag +, such as chloride ions, constrains the use of the particles in chloriderich biological environments. Silver nanoparticles can also be etched in the presence of thiols, particularly when exposed to visible light. At the same time, the adsorption of functionalized thiols is an efficient method for the tailoring of metal surfaces, as was recently demonstrated for gold nanoparticles. 4 Aqueous suspensions of unprotected silver and gold particles are also susceptible to irreversible aggregation. 5 This aggregation is traditionally overcome through the spontaneous adsorption on the particle surface of polymeric stabilizers. 6 However, this method requires the presence in the suspension of a large concentration of stabilizers which can interfere with surface functionalization; at lower concentrations, the stability of the suspension is limited by desorption of the stabilizers from the surface. Emulsion polymerization has been previously applied to the coating of organic and inorganic particles 7 on the micrometer and submicrometer scale. The method has the potential for producing a stable and compact polymer layer, completely enclosing the particle and improving its chemical resistance. Herein, we report the encapsulation of silver nanoparticles into a polymer shell via emulsion polymerization to produce a chemically stable system. The coating was produced by the polymerization of styrene and/ or methacrylic acid in emulsions of oleic acid. In this system, silver particles are coated with a uniform, well-defined layer and are in a nonaggregated suspension, contrary to previously reported systems in which silver particles appeared to be imbedded into a polymer matrix. The thickness of the layer can be readily controlled by changing the concentration of monomers. The coated particles were tested in concentrated chloride solutions and exhibited a strong resistance toward etching. These can also be efficiently purified by filtration and/or precipitation, and redispersed in different media for further chemical modification. As an example of chemical tailoring of particle surface, we attached bovine serum albumin (BSA) to the carboxylate functionalities derived from methacrylic acid monomers. TEM pictures of 100 nm silver particles uncoated and coated with the polymer are shown in Figure 1. The polymer layer has insufficient electron density to be directly observed by TEM and visualization of the coating was achieved through negative staining with phosphotungstic acid. The polymer appears as a white layer between the dark silver core and the gray halo of the stain surrounding the particles. Layers of varying thickness, ranging from 2 nm to about 10 nm, were produced and examined by TEM (Table 1). Whereas thin coatings follow the shape of the metal core, thicker ones ( >10 nm) tend to assume a globular shape. The presence of oleic acid is required to obtain particle encapsulation; when the polymerization was carried out in the absence of oleic acid, only polymer particles without metal core were produced and no polymer shell was observed around the silver particles. Because carboxylic acids have high affinity for the oxide layer that is present on the surface of silver, 8 oleic acid is readily adsorbed on the particles. Since the amount of oleic acid used largely exceeds the amount required for monolayer coverage of the particles, a double layer is presumably formed, producing a vesicle structure that surrounds the silver core. This structure is (1) Elghanian, R. ; Storhoff, J. J.; Mucic, R. C. ; Letsinger, R. L. ; Mirkin, C. A. Science1997, 277, 1078-1081. Alivisatos, A. P.NATO Sci. Ser. , Ser. C 1999, 519, 405-416. 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TEM images of silver particles: (A) uncoated particle, (B) polystyrene/methacrylate coated particles, (C) polystyrene/methacrylate coated particles with a covalently bound BSA layer, and (D) the same as panel C after exposure to gold colloids. Negative staining by phosphotungstic acid used for all images. 10642 J. Am. Chem. Soc. 1999,121,10642-10643