(474c) Multifunctional Magnetic Nanoparticles By Surface Initiated Atom Transfer Radical Polymerization

Superparamagnetic and ferromagnetic nanoparticles are of huge interest in various areas of science (e.g. chemistry, biology, bio-chemistry, medicine, sensor technology)¹.The requirements for particles are as multifaceted as the areas, in which they are applied. Nanoparticles enjoy great popularity as particles at the nanoscale exhibit a favorable surface to volume ration in comparison to the same bulk material. However, this property is not that valuable, if other requirements do not fit.

For chemical applications, the particles most often have to be quickly separable, inert and stable against harsh environment like temperature, challenging pH-values and solvents.^2,3 In contrast to this, medical and biological applications usually afford dispersion stability in aqueous solutions and compatibility with biological relevant media (e.g. phosphate buffered saline (PBS) or Dulbeccoâ??s modified eagleâ??s medium (DMEM))⁴. For sensor application, the requirements to the magnetic nanoparticles contain huge dispersion stability, a high saturation magnetization and the possibility for further modification (e.g. attachment of a target substance etc.)^4,5,6.Requirements to the magnetic nanoparticles which are highly demanded, often contain multiple combinations of different properties. This afford smart materials which contains amphiphilic character. Such smart materials can for example change the properties by an external trigger (temperature, pH, gas)⁷.

Most of the different requirements can be achieved using carbon coated ferromagnetic nanoparticles as a platform. The graphene-like layers protect the magnetic metal core from oxidation and allows the covalent attachment of chemical functionalities via diazonium chemistry⁸. There are numerous functionalities which can be implemented by this approach. Amongst them, alcohol-, amine-, carboxy-, azide- or an initiator moiety for atom transfer radical polymerization (ATRP) can be listed, just to mention a few^7,9,10. Adsorption of a polymer can usually not satisfy the requirements, as adsorbed polymers tend to form only single layers and desorption frequently occurs.

In contrast to this, surface initiated ATRP (SI-ATRP: polymer-growth from the surface) is suitable with a plethora of different monomers (desired functionality) and the polymerization can be well controlled (layer thickness and block-co-polymerization)^4,11. Thus SI-ATRP gives the toolbox for various modification of the surface that allows achievement of several desired requirements.

Using charged monomers (e.g. 3-Sulfopropyl methacrylate potassium salt (SPM) or 3-(Methacryloylamino)propyl]trimethylammonium chloride (MAPTAC)) leads to highly stable dispersions in aqueous solutions. The stability can be tuned by the number of repeating units^4,6. Using poly(ethylene glycol) methacrylate (PEGMA) as monomer the highly lipophilic graphene layers can be turned into hydrophilic and antifouling surfaces¹². The use of glycidyl methacrylate (GMA) gives access to post-modification by epoxide ring opening.⁴ Most challenging modifications require amphiphilic properties (dispersion stability vs. fast separation or lipophilicity vs. hydrophilicity). Such requirements need a coating with smart polymers, which can change their properties upon external trigger (temperature, pH etc.). By using N-isopropylacrylamine (NIPAM) as building block, the particles can reversibly change between hydrophilic and hydrophobic upon temperature change⁷. Here the promising potential of SI-ATRP on magnetic nanoparticles in terms of designing a desired nanomaterial for chemical, biological and medical applications is shown¹³.

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