Throughout the last decades, magnetic nanoparticles (MNP) have gained tremendous interest in different fields of applications like biomedicine (e.g., magnetic resonance imaging (MRI), drug delivery, hyperthermia), but also more technical applications (e.g., catalysis, waste water treatment) have been pursued. Different surfactants and polymers are extensively used for surface coating of MNP to passivate the surface and avoid or decrease agglomeration, decrease or modulate biomolecule absorption, and in most cases increase dispersion stability. For this purpose, electrostatic or steric repulsion can be exploited and, in that regard, surface charge is the most important (hybrid) particle property. Therefore, polyelectrolytes are of great interest for nanoparticle coating, as they are able to stabilize the particles in dispersion by electrostatic repulsion due to their high charge densities. In this review article, we focus on polyzwitterions as a subclass of polyelectrolytes and their use as coating materials for MNP. In the context of biomedical applications, polyzwitterions are widely used as they exhibit antifouling properties and thus can lead to minimized protein adsorption and also long circulation times.

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We herein present the reversible formation of hybrid nanoparticles featuring a magnetic core and two consecutive polyzwitterion/polyelectrolyte (or protein) layers. Starting from multicore iron oxide nanoparticles, a first coating with zwitterionic poly­(dehydroalanine) is realized, and the resulting PDha@MCNP [PDha = poly­(dehydroalanine) and MCNP = multicore nanoparticle] shows pH-dependent (invertible) surface charge and dispersion stability. In a second step, this can be used as a versatile platform to adsorb either polycations {[poly­(N,N-dimethylamino)­ethyl methacrylate] (PDMAEMA) or poly­(aminomethyl acrylate)}, a polyanion [poly­(styrenesulfonic acid) (PSS)], or a model protein in a quasi layer-by-layer approach. The size, surface charge, and aggregation behavior of the resulting double-layer-coated particles are investigated via dynamic light scattering, transmission electron microscopy, ζ-potential, and turbidity measurements. In contrast to typical layer-by-layer coatings, the use of polyzwitterionic PDha as the first layer allows the pH-dependent release of the second polyelectrolyte shell (PDMAEMA and PSS) upon charge inversion. This turns such reversible multilayer coatings into interesting candidates for applications where controlled swelling or release is in focus and where it is important to control which part of a segmented or nanostructured system responds to changes in the surrounding medium.

We report on the coating of superparamagnetic iron oxide nanoparticles using polyanionic or polyzwitterionic materials based on polydehydroalanine. The resulting core–shell hybrid nanoparticles exhibit shells of different charge and thickness.