SEM image of Cu-coated MF particles
Optical microscopy image of 17 µm Au-coated PS particles
Optical microscopy image of 10 µm Au-coated MF particles
Metal-coated Polymer Particles
Monodisperse metal-coated microspheres have found wide applications in the fields of catalysis, anisotropically conductive adhesives (ACAs), microelectronics, Surface Enhanced Raman Scattering (SERS), Particle Image Velocimetry (PIV), and bioassays. During recent years, scientists at microParticles GmbH have successfully developed strategies to metallize chemically different microspheres like melamine resins, polystyrene, polystyrene-divinylbenzene copolymers, poly(methyl methacrylate) copolymers, or silica. The composite microspheres consist of a monodisperse polymer core whereas the shell either consists of metal nanoparticles or a thin metal film of e.g. gold, silver, copper, or others.
The use of the monodisperse melamine resin particles (MF) has a number of advantages:
- the surface of the MF particles is well-defined, having a large number of reactive amino and imino groups for immobilization of different metal ions or metal nanoparticles
- under acidic environments, the unmodified surface of MF particles is positively charged and shows a high binding capacity to complex anions like AuCl4-, PdCl42-, PtCl62-
- in particular, the AuCl4- adsorbance on MF microparticles of about 500 nm diameter reaches a very high value of about 0,5 g/g.
Generally, for the preparation of our metal-coated microspheres three approaches have been used:
In the self-assembly approach, preformed colloidal nanoparticles are adsorbed on the specifically modified particle surface by electrostatic interactions, for instance. The extent of nanoparticle surface coverage can be controlled by varying the pH value of the solution, the immersion time, the metal colloid concentration in suspension, nanoparticle dimensions, and the chemical surface properties of the polymer particles. In this way, monodisperse microspheres with individually dispersed nanoparticles of gold, silver, platinum, or palladium can be prepared.
In-situ synthesis approach: In the in-situ approach AuCl4-, PdCl42-, PtCl62- precursor ions are bound on the surface of chemically modified monodisperse particles by complexation and/or electrostatic interactions, followed by their in situ reduction to nanosized metal particles. TEM images show the in situ synthesized metal nanoparticles being homogeneously distributed on the particle surface without aggregation. The average diameter is around 2-3 nm with a narrow size distribution.
Electroless deposition approach: In the electroless deposition approach, the specifically surface-modified microspheres are immersed in an electroless plating bath, which consists of the metal ions and a reducing agent. Gold, silver, palladium, or platinum nanoparticles immobilized on the particle surface can be used as seeds for the auto-catalytic deposition of the metal. The result is a metal nanoshell with variable thickness of gold, silver, or copper on the particle surface. High-quality metal coatings can be achieved with particles in the size region above 1 µm. Research is underway to develop particles with gold nanoshells in the submicron range, too