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J. Phys. Chem. B 2001, 105, 6846-6852
Tailoring the Polyelectrolyte Coating of Metal Nanoparticles David I. Gittins and Frank Caruso* Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany ReceiVed: March 28, 2001; In Final Form: April 17, 2001
Controlling the surface properties of nanoparticulate materials is necessary if they are to be exploited in applications such as colloidal crystals or biolabeling. By tailoring the polymer flexibility and the electrostatic forces involved in polyelectrolyte adsorption onto highly curved gold surfaces, through variation of the total salt concentrations suspending the chains and spheres, it is shown that irreversible polyelectrolyte wrapping of gold nanoparticles can be effected. By consecutively exposing the nanoparticles to polyelectrolyte solutions of opposite charge, polyelectrolytes can be deposited in a layer-by-layer sequence, yielding gold nanoparticles coated with uniform polyelectrolyte multilayers. Self-supporting polyelectrolyte multilayered nanocapsules are formed after dissolution of the metallic core. It has also been found that the gold nanoparticle surface charge, created by the adsorption of anions, is insufficient to overcome the ionic and hydrophobic polyanion/ polycation interactions, resulting in polymer desorption from the highly curved nanoparticle surface. One successful method to immobilize the charge on the gold nanoparticle surface is to covalently attach an anionic thiol before polyelectrolyte modification.
Introduction Controlling the surface properties of colloidal materials is a major technological research area encompassing studies in the pharmaceutical, mining, semiconductor, biological, and medical fields. Polymer coating of colloid particles is one method to modify surface properties, with the polymer type determining the final surface characteristics of the particles. Early studies on polymer modification of nanoparticle surfaces focused on the coating of oxide nanoparticles usually used in diagnostic applications and as pigments. This involved chemisorption or adsorption of a preformed polymer, or created coatings by in situ polymerization (suspension, emulsion, or vapor phase).1 Recently, these techniques have been applied to encapsulate metal nanoparticles with varying success. Quaroni and Chumanov have used emulsion polymerization of styrene and methacrylic acid in the presence of oleic acid-stabilized silver nanoparticles.2 Murray and co-workers have exploited the affinity of thiols to gold surfaces to covalently attach a thiolated poly(ethylene glycol) to gold nanoparticles.3 This approach has also been applied by the Feldheim4 and Mirkin5 research groups to attach monomers or initiation sites to the nanoparticle surface, followed by cross-linking or block copolymerization as an encapsulation step. Feldheim and co-workers have also aligned gold nanoparticles within filtration membranes followed by vapor-phase polymerization of pyrroles.6,7 Rotello and coworkers have used hydrogen-bonding to coagulate derivatized gold nanoparticles and preformed polymers to form either spherical or ‘extended-network’ aggregates.8 However, the studies mentioned above do not use electrostatic interactions for the controlled adsorption of preformed polymers to form uniform coatings on metal nanoparticles. A major area of polymer coating uses polyelectrolytes (ionic polymers) to modify surfaces9,10 and colloids,11 exploiting electrostatic attraction for their deposition. Previous studies of polyelectrolyte monolayer adsorption onto metal nanoparticles * Corresponding author. E-mail:
[email protected].
only provide an insight into the coupling chemistry involved.12,13 Polyelectrolytes have an advantage over uncharged polymers, because they can be deposited on surfaces layer-by-layer (LbL), enabling the total polymer thickness to be determined by the number of layers deposited.9 The LbL approach, in its simplest form, uses two solutions of oppositely charged polymers into which the substrate can be dipped (surface),9,10 or particles mixed (colloids).14 Sequential dipping or mixing causes material to be deposited on the surface because of electrostatic and hydrophobic interactions between the charged surface and polyelectrolyte. Once deposited, the layer of polyelectrolyte inverts the surface charge of the material it is adsorbed to, enabling a subsequent layer of polymer to be deposited from the second solution. This process can be repeated indefinitely to form a uniform multilayered film of polymeric material. The LbL process has been applied to a multitude of substrates, including various solid supports and a range of submicron- to micron-sized particles.9-11 In addition, one of the polyelectrolyte solutions can be replaced with a similarly charged species such as proteins, dyes, clays, nanoparticles, etc., to form composite multilayers.9-11 Despite the widespread use of the LbL technique, its application to nanoparticles (diameter
