Constrained growth of anisotropic magnetic δ-FeOOH nanoparticles in the presence of humic substances. Polyakov A.Yu., Goldt A.E., Sorkina T.A., Perminova I.V., Pankratov D.A., Goodilin E.A., Tretyakov Y.D. //CrystEngComm. 2012. V.14. №23. P.8097-8102.
Natural polyelectrolytes, humic substances, are suggested to control in situ growth of feroxyhyte nanoparticles of a highly reduced mean size and with enhanced colloidal stability in salt solutions. The feroxyhyte is formed as 2-5 nm thick and 20 × 20 nm wide nanoflakes due to the blocking of developing facets of feroxyhyte and constraints caused by diffusion limitations of ionic constituents across partially charged branches of humic substances.
Superparamagnetic iron oxide nanoparticles (SPIONs) are known as effective biocompatible agents for various biomedical applications like drug delivery, in vivo magnetic resonance imaging, cell and protein separation, hyperthermia and transfection. At present, synthesis and application of rods, disks, fibers, tubes, sheets, ellipsoids, dumbbell-shaped, acorn-shaped and other anisotropic nanoparticles attract growing attention because of their unique properties. Aggregation is a serious problem in the preparation and storage of such magnetic nanoparticles limiting considerably their practical applications. This problem can be solved by a surface modification of nanoparticles with organic macromolecules, their preparation in the presence of surfactants, application of hydrophilic, surface-active ligands improving biocompatibility and resulting in multifunctional nanoparticles for medical diagnostics.
Contrary to other ferrimagnetic iron oxides, feroxyhyte can be formed under mild conditions and therefore has a large amount of surface hydroxyl groups readily available for targeted modifications. Moreover, feroxyhyte possesses a layered structure, a high specific surface area and pronounced adsorption properties. This compound is used as a sorbent for removal of toxic ions from wastewater, a precursor for high coercivity materials, and as a photocatalyst for water splitting. Feroxyhyte also is a participant of complex processes of organic–inorganic interactions in biogeochemical systems. The striking example is crystallinity control that natural organic matter imposes over amorphous iron hydroxides in soils and deep sea sediments leading to formation of extremely disperse feroxyhyte. While soil scientists see a template role of organic matter in this process serving as a crystallization inhibitor, the marine researchers explain it by degradation of organic matter which yields reducing conditions necessary for releasing iron ions from sedimentary minerals or clays.
The former mentioned global processes underline a unique synthetic niche of humic substances (HS) in superparamagnetic iron oxide nanoparticle and other nanomaterial transformation processes. Indeed, humic substances are believed to be a new candidate for the size and morphology control of superparamagnetic iron oxide nanoparticles; they are essentially a complex mixture of natural macromolecular compounds with vast functional peripheries dominated by carboxyl and hydroxyl groups. They possess properties of anionic polyelectrolytes and reveal a distinct affinity for surface complexation of superparamagnetic iron oxide nanoparticles. Perhaps they can bind nanoparticles both by electrostatic surface interactions and iron chelation. At least, it is highly possible that humic substances interact with iron oxides and stabilize them in the form of multinuclear hydroxy complexes or nanoparticles in their soils. In the present work we investigate humic substances as a new cheap and effective ‘‘green’’ stabilizer for feroxyhyte magnetic nanoparticles. We report a direct experimental evidence of the template role of this natural organic matter in the synthesis of δ-FeOOH. The results deepen our understanding of regulatory drivers in the organic–inorganic systems that may be used for improving iron bioavailability as well as for development of new classes of superparamagnetic iron oxide nanoparticles for nanomedicine.
An application of humic substances is found to result in reduction of a medium particle size of δ-FeOOH for at least one order of magnitude. The feroxyhyte nanoparticles reveal their high crystallographic anisotropy by forming nanoflakes possessing 2–5 nm thickness and 20 × 20 nm in-plane size. The influence of humic acids in the control of δ-FeOOH particle size seems to relate to both blocking of developing facets and producing constraints for fast diffusion of ionic constituents needed for fast growth due to a high molecular weight nature and partial charges of humic acid branches. The results deepen our understanding of regulatory drivers in the organic–inorganic systems that may be used for improving iron bioavailability as well as for development of new classes of superparamagnetic iron oxide nanoparticles for nanomedicine.