Graphical abstractH-type isotherms HA, PAM, GA, and CA Highlights  Organic acids either stabilize or destabilize oxide nanoparticles in natural waters.  The stabilizing/destabilizing effect depends on pH, salinity and organic concentration.  Specific configuration of carboxylic groups is necessary to surface complexation.  Surface complexation leads to high affinity adsorption isotherms.  Higher molecular weight organic acids provide better stability than smaller ones. AbstractThe adsorption of different organic acids and their influence on the pH-dependent charging, salt tolerance and so the colloidal stability of magnetite nanoparticles are compared. Adsorption isotherms of citric acid -CA, gallic acid -GA, poly(acrylic acid) -PAA, poly(acrylic-co-maleic acid) -PAM and humic acid -HA were measured. The pH-dependent charge state of MNPs was characterized by electrophoretic mobility and their aggregation by dynamic light scattering. The salt tolerance was tested in coagulation kinetic experiments. Although the adsorption capacities, the type of bonding (either H-bonds or metal ioncarboxylate complexes) and so the bond strengths are significantly different, the following general trends have been found. Small amount of organic acids at pH < ~8 (the pH of PZC of magnetite) -relevant condition in natural waters -only neutralizes the positive charges, and so promotes the aggregation and sedimentation of nanoparticles. Greater amounts of organic acid, above the charge neutralization, cause the sign reversal of particle charge, and at high

Highlights• Multicore magnetite nanoparticles (MNPs) were superparamagnetic.• PEG-carboxylate polyelectrolytes coat spontaneously MNPs and stabilize them electrosterically.• Biofunction can be attached to MNPs via carboxylated coating layer.• Multifunctional shell prevents MNPs' internalization into cells.• Superparamagnetic property is sustained after MNP coating. AbstractBiocompatible magnetite nanoparticles (MNPs) were prepared by post-coating the magnetic nanocores with a synthetic polymer designed specifically to shield the particles from nonspecific interaction with cells. Poly(ethylene glycol) methyl ether methacrylate (PEGMA) macromonomers and acrylic acid (AA) small molecular monomers were chemically coupled by quasi-living atom transfer radical polymerization (ATRP) to a comb-like copolymer, P(PEGMA-co-AA) designated here as P(PEGMA-AA). The polymer contains pendant carboxylate moieties near the backbone and PEG side chains. It is able to bind spontaneously to MNPs; stabilize the particles electrostatically via the carboxylate moieties and sterically via the PEG moieties; provide high protein repellency via the structured PEG layer; and anchor bioactive proteins via peptide bond formation with the free carboxylate groups. The presence of the P(PEGMA-AA) coating was verified in XPS experiments. The electrosteric (i.e., combined electrostatic and steric) stabilization is efficient down to pH 4 (at 10 mM ionic strength). Static magnetization and AC susceptibility measurements showed that the P(PEGMA-AA)@MNPs are superparamagnetic with a saturation magnetization value of 55 emu/g and that both single core nanoparticles and multicore structures are present in the samples. The multicore components make our product well suited for magnetic hyperthermia applications (SAR values up to 17.44 W/g). In vitro biocompatibility, cell internalization, and magnetic hyperthermia studies demonstrate the excellent theranostic potential of our product. Graphical abstract

The building blocks for the fabrication of biocompatible magnetoresponsive carriers are the subdomain magnetite nanoparticles surface coated by biocompatible molecular layers which ensure their stabilization and dispersion in an appropriate carrier liquid, in order to obtain stable magnetic nanofluids that are the primary materials for the envisaged magnetic nanocomposites. The synthesis by chemical coprecipitation is the most simple and cost effective route to obtain hydrophobic and hydrophilic magnetite nanoparticles at industrial scale. Manifold physico-chemical characterization of the magnetic nanofluids is employed in order to certify their requested composition and structure.

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In this paper, we investigate the influence of a vertically aligned magnetic field of a circular current loop on a ferrofluid layer located on a liquid substrate. It has been found that an increase in field strength leads to a threshold formation of the layer rupture in the form of a regular circle. The critical value of the field strength depends linearly on the initial layer thickness. The rupture initiated by the magnetic field does not disappear after removal of the field provided that the thickness of the ferrofluid layer is lower than the critical value. In the case of deformation of rather thick layers the occurrence of rupture is accompanied by the formation of well-ordered drop structures.