Stable dispersions of ferromagnetic carbon-coated metal nanoparticles: preparation via surface initiated atom transfer radical polymerization

Abstract

Magnetic nanoparticle dispersions are traditionally made from superparamagnetic materials since the absence of magnetic particle–particle attraction under normal conditions (no external field) easily allows preparation of stable dispersions. For inductive heating in medicinal chemistry or material science, however, the much higher magnetization of ferromagnetic metals over the currently used oxides is attractive. Traditional attempts to prepare stable dispersions of ferromagnetic particles, however, failed since the strong magnetic particle–particle attraction usually overcomes repulsive effects from surfactants or steric stabilizers (typically polymers). In the present work, we demonstrate how the direct, covalent attachment of highly charged polymers can circumvent stabilizer detachment and loss, and permits preparation of stable dispersions of ferromagnetic particles. More specifically, carbon-coated metal nanoparticles were covalently functionalized with positively charged polymer brushes via surface initiated atom transfer radical polymerization (SI-ATRP). Particle size distributions with an average diameter of 24 nm provided a ferromagnetic liquid with unprecedented stability in water over several months. The stability was discussed by comparison of the potentials of non-functionalized and modified nanomagnets within a modified Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. Measurements for inductive heating at different frequencies and various field strengths showed an average specific absorption rate of 360 W g⁻¹. Altogether, this suggests that efficiently stabilized dispersions of ferromagnetic nanoparticles could be an alternative to superparamagnetic iron oxide particles in a number of applications.