New applications of organic/inorganic hybrid nanoparticles

Hans-Juergen P. Adler and Andrij Pich, Institute of Macromolecular Chemistry and Textile Chemistry, Technische Universitaet Dresden, Dresden, Germany (hans-juergen.adler@chemie.tu-dresden.de )

The potential use of polymeric colloidal particles (microgels and latex particles) as templates or microreactors for synthesis, storage and transport of different nano-structured materials has been recently demonstrated. [1,2] General scheme in Fig. 1 shows the pathways for the preparation of composite or hybrid particles based on polymeric colloids.

Different functional materials can be incorporated into the microgel or latex particle by following approaches. The first method requires nanoparticle (NPs) preparation on the surface of the latex particle or inside of the microgel (both acting as template) by means of polymerization processes, reduction/oxidation or precipitation reactions. In this case one can expect that the structure and functionality of a latex and microgel will influence the properties of formed nanoparticles as well as their fixation and distribution in the polymeric template. The use of the latex particles as templates will result in formation of core-shell structures with variable thickness of the nano-structured shell. When microgels applied as templates nanoparticles will be deposited mainly in the microgel interior and well separated by swollen polymeric network.

The second approach allows preparation of composite latex particles or microgels by mixing polymeric colloids with nanoparticles. In this case NPs will diffuse in the microgel interior or to the latex particle surface and can be fixed by electrostatic or hydrophobic interactions with functional groups of the polymer colloid. However, compared to the first method this approach leads to the limited amount of the NPs which can be incorporated into polymeric beads mainly due to the limited diffusion and some sterical effects. In the case of the microgels this drawback can be partly eliminated by using so called “breathing in” method e.g. soaking pre-designed nanoparticles into the microgel network during swelling process. However, the risk of the NPs leakage from such composite colloids is high because electrostatic interactions can be strongly influenced by pH and ionic strength of the continuous medium.

The third approach utilizes nanoparticle incorporation into polymeric colloids by encapsulation during polymerization process. In the case of latex particles this approach can be realized during emulsion or miniemulsion polymerization and in the case of microgel synthesis by inverse heterophase polymerization or precipitation polymerization process. In this case filler material with pre-selected properties (size, morphology etc.) is mechanically “locked” within the latex particle or microgel network. It seems to be the best approach to “isolate” nanoparticles from environment. In this case the risk of NPs leakage as well as probability of their undesired interactions with cells, tissues etc. is extremely low.

Based on the typical microgel one can predict following advantages when using them as microreactors comparing to other template systems: -easy preparation; -variable size and flexible functionalization by reactive groups; -highly porous structure with adjustable crosslinking degree; -enhanced colloidal stability; -stimuli-responsive change of the microgel dimension (T-, pH-sensitivity). With regard to these features the preparation of different materials in form of nanoparticles inside of the microgels can offer: -controlled NPs synthesis in microgels (localization of reactive sites and controlled growth, homogeneous distribution within microgel); -adjustable NPs properties (particle size and morphology control; separation and stabilization in polymer network); -NPs accessibility (high surface area, no diffusion limitation); -control of the distance between NPs by swelling or collapse of microgel as its response to the environmental conditions; -high colloidal stability provided by microgel particles. However, one should be aware of some undesired effects which can be expected at high microgel loading by another material. The specific interactions between NPs and polymer chains within microgel can lead to the reduction of the chain mobility and shift of the phase transition temperature as well as colloidal destabilization.

By using latex particles as templates one can expect some advantages originating from the formation of well-defined core-shell particles with controlled thickness of the shell being inherently nanostructured. The removal of the polymeric core provides a simple route for the preparation of hollow spheres. Composite latex particles can be designed for the preparation of the thin films or colloidal crystals due to the easy variation of their size and polymer composition.

As it will be demonstrated, all approaches described above can be used to integrate different nanomaterials into polymeric spheres serving as: a) templates for their growth, providing better control over the NPs size, loaded amount and distribution inside of the polymeric particles; b) carrier to maintain NPs stability and avoid aggregation; c) shuttle to transport NPs through the phase boundaries from one solvent to another etc. Some selected results on the preparation of the sub-micrometer latex particles (based on copolymers of styrene and different acrylates) and microgels (based on copolymers of vinylcaprolactam and different acrylates) and their functionalization with noble metals (Au, Ag, Pt), semiconductors (ZnS, CdS), conjugated polymers (polypyrrole or PEDOT), metal oxides (Fe2O3, ZnO), biominerals (CaCO3, hydroxyapatite) etc will be discussed. [3-7] Some new synthetic routes for the preparation of hybrid colloids and possible application fields, e.g. conducting films, particles for biotechnical applications, dried particles as fillers and particles for sensors and actuators will be presented.

References:

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