Influence of Organic Ligands on the Surface Oxidation State and Magnetic Properties of Iron Oxide Particles

For the application of iron oxide nanoparticles from thermal decomposition approaches as contrast agents in magnetic resonance imaging (MRI), their initial hydrophobic ligands have to be replaced by hydrophilic ones. This exchange can influence the surface oxidation state and the magnetic properties of the particles. Here, the effect of the anchor group of three organic ligands, citric acid and two catechols, dihydrocaffeic acid and its nitrated derivative nitro dihydrocaffeic acid on iron oxide nanoparticles is evaluated. The oleate ligands of Fe3O4/γ-Fe2O3 nanoparticles prepared by the thermal decomposition of iron oleate were exchanged against the hydrophilic ligands. X-ray absorption spectroscopy, especially X-ray magnetic circular dichroism (XMCD) measurements in the total electron yield (TEY) mode was used to investigate local magnetic and electronic properties of the particles’ surface region before and after the ligand exchange. XMCD was combined with charge transfer multiplet calculations which provide information on the contributions of Fe2+ and Fe3+ at different lattice sites, i.e. either in tetrahedral or octahedral environment. The obtained data demonstrate that nitro hydrocaffeic acid leads to least reduction of the magnetizability of the surface region of the iron oxide nanoparticles compared to the two other ligands. For all hydrophilic samples, the proportion of Fe3+ ions in octahedral sites increases at the expense of the Fe2+ in octahedral sites whereas the percentage of Fe3+ in tetrahedral sites hardly changes. These observations suggest that an oxidation process took place, but a selective decrease of the Fe2+ ions in octahedral sites ions due to surface dissolution processes is unlikely. The citrate ligand has the least oxidative effect, whereas the degree of oxidation was similar for both catechol ligands regardless of the nitro group. Twenty-four hours of incubation in isotonic saline has nearly no influences on the magnetic properties of the nanoparticles, the least on those with the nitrated hydrocaffeic acid ligand.

Individually Stabilized, Superparamagnetic Nanoparticles with Controlled Shell and Size Leading to Exceptional Stealth Properties and High Relaxivities

Superparamagnetic iron oxide nanoparticles (SPION) have received immense interest for biomedical applications, with the first clinical application as negative contrast agent in magnetic resonance imaging (MRI). However, the first generation MRI contrast agents with dextran-enwrapped, polydisperse iron oxide nanoparticle clusters are limited to imaging of the liver and spleen; this is related to their poor colloidal stability in biological media and inability to evade clearance by the reticuloendothelial system. We investigate the qualitatively different performance of a new generation of individually PEG-grafted core-shell SPION in terms of relaxivity and cell uptake and compare them to benchmark iron oxide contrast agents. These PEG-grafted SPION uniquely enable relaxivity measurements in aqueous suspension without aggregation even at 9.4 T magnetic fields due to their extraordinary colloidal stability. This allows for determination of the size-dependent scaling of relaxivity, which is shown to follow a d2 dependence for identical core-shell structures. The here introduced core-shell SPION with ∼15 nm core diameter yield a higher R2 relaxivity than previous clinically used contrast agents as well as previous generations of individually stabilized SPION. The colloidal stability extends to control over evasion of macrophage clearance and stimulated uptake by SPION functionalized with protein ligands, which is a key requirement for targeted MRI.

Modular Fabrication of Polymer Brush Coated Magnetic Nanoparticles: Engineering the Interface for Targeted Cellular Imaging

Development of efficient and rapid protocols for diversification of functional magnetic nanoparticles (MNPs) would enable identification of promising candidates using high-throughput protocols for applications such as diagnostics and cure through early detection and localized delivery. Polymer brush coated magnetic nanoparticles find use in many such applications. A protocol that allows modular diversification of a pool of parent polymer coated nanoparticles will lead to a library of functional materials with improved uniformity. In the present study, polymer brush coated parent magnetic nanoparticles obtained using reversible addition-fragmentation chain transfer (RAFT) polymerization are modified to obtain nanoparticles with different "clickable" groups. In this design, trithiocarbonate group terminated polymer brushes are "grafted from" MNPs using a catechol group bearing initiator. A postpolymerization radical exchange reaction allows installation of "clickable" functional groups like azides and maleimides on the chain ends of the polymers. Thus, modified MNPs can be functionalized using alkyne-containing and thiol-containing moieties like peptides and dyes using the alkyne-azide cycloaddition and the thiol-ene conjugation, respectively. Using the approach outlined here, a cell surface receptor targeting cyclic peptide and a fluorescent dye are attached onto nanoparticle surface. This multifunctional construct allows selective recognition of cancer cells that overexpress integrin receptors. Furthermore, the approach outlined here is not limited to the installation of azide and maleimide functional groups but can be expanded to a variety of "clickable" groups to allow nanoparticle modification using a broad range of chemical conjugations.

Evaluation of High-Yield Purification Methods on Monodisperse PEG-Grafted Iron Oxide Nanoparticles

Fundamental research on nanoparticle (NP) interactions and development of next-generation biomedical NP applications relies on synthesis of monodisperse, functional, core-shell nanoparticles free of residual dispersants with truly homogeneous and controlled physical properties. Still, synthesis and purification of e.g. such superparamagnetic iron oxide NPs remain a challenge. Comparing the success of different methods is marred by the sensitivity of analysis methods to the purity of the product. We synthesize monodisperse, oleic acid (OA)-capped, Fe3O4 NPs in the superparamagnetic size range (3-10 nm). Ligand exchange of OA for poly(ethylene glycol) (PEG) was performed with the PEG irreversibly grafted to the NP surface by a nitrodopamine (NDA) anchor. Four different methods were investigated to remove excess ligands and residual OA: membrane centrifugation, dialysis, size exclusion chromatography, and precipitation combined with magnetic decantation. Infrared spectroscopy and thermogravimetric analysis were used to determine the purity of samples after each purification step. Importantly, only magnetic decantation yielded pure NPs at high yields with sufficient grafting density for biomedical applications (∼1 NDA-PEG(5 kDa)/nm(2), irrespective of size). The purified NPs withstand challenging tests such as temperature cycling in serum and long-term storage in biological buffers. Dynamic light scattering, transmission electron microscopy, and small-angle X-ray scattering show stability over at least 4 months also in serum. The successful synthesis and purification route is compatible with any conceivable functionalization for biomedical or biomaterial applications of PEGylated Fe3O4 NPs.

Synthesis and application of glycoconjugate-functionalized magnetic nanoparticles as potent anti-adhesion agents for reducing enterotoxigenic Escherichia coli infections

Polyethylene oxide stabilized magnetic nanoparticles (PEO-MNPs) bio-functionalized with glycoconjugate (Neu5Ac(α2-3)Gal(β1-4)Glcβ-sp) (GM3-MNPs) are synthesized using click chemistry. Interaction of GM3-MNPs with Enterotoxigenic Escherichia coli (ETEC) strain K99 (EC K99) is investigated using different microscopic techniques. Our results suggest that GM3-MNPs can effectively act as non-antibiotic anti-adhesion agents for treating ETEC infections.