Complete Exchange of the Hydrophobic Dispersant Shell on Monodisperse Superparamagnetic Iron Oxide Nanoparticles

High-temperature synthesized monodisperse superparamagnetic iron oxide nanoparticles are obtained with a strongly bound ligand shell of oleic acid and its decomposition products. Most applications require a stable presentation of a defined surface chemistry; therefore, the native shell has to be completely exchanged for dispersants with irreversible affinity to the nanoparticle surface. We evaluate by attenuated total reflectance−Fourier transform infrared spectroscopy (ATR−FTIR) and thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) the limitations of commonly used approaches. A mechanism and multiple exchange scheme that attains the goal of complete and irreversible ligand replacement on monodisperse nanoparticles of various sizes is presented. The obtained hydrophobic nanoparticles are ideally suited for magnetically controlled drug delivery and membrane applications and for the investigation of fundamental interfacial properties of ultrasmall core–shell architectures

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 <i>d</i>² dependence for identical core–shell structures. The here introduced core–shell SPION with ∼15 nm core diameter yield a higher <i>R</i>₂ 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

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)/nm², 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

Interaction of Size-Tailored PEGylated Iron Oxide Nanoparticles with Lipid Membranes and Cells

Targeted nanomedicine builds on the concept that nanoparticles can be directed to specific tissues while remaining inert to others organs. Many studies have been performed on the synthesis and cellular interactions of core–shell nanoparticles, in which a functional inorganic core is coated with a biocompatible polymer layer that should reduce nonspecific uptake and cytotoxicity. However, work is lacking that relates structural parameters of the core–shell structure and colloidal properties directly to interactions with cell membranes and further correlates these interactions to cell uptake. We have synthesized monodisperse (SD < 10%), single-crystalline, and superparamagnetic iron oxide nanoparticles (SPION) of different core size (3–8 nm) that are densely grafted with nitrodopamine-poly­(ethylene glycol) (NDA-PEG­(5 kDa)) brushes. We investigated the interactions of the PEGylated SPION with biomimetic membranes and cancer and kidney cells. It is shown that a dense homogeneous PEG shell suppresses membrane interactions and cell uptake but that nanoparticle curvature can influence membrane interactions for similarly grafted nanoparticles. Weak adsorption to anionic lipid membranes is shown to correlate with eukaryote cell uptake and is attributed to double-layer interactions without direct membrane penetration. This attraction is strongly suppressed during physiological conditions and leads to unprecedented low cell uptake and full cell viability when compared to those of traditional dextran-coated SPION. Less curved (larger core) PEGylated SPION show weaker membrane adsorption and lower cell uptake due to effectively denser shells. These results provide a better understanding of design criteria for core–shell nanoparticles in terms of avoiding nonspecific uptake by cells, reducing toxicity, and increasing circulation time

Doping Method Determines Para- or Superparamagnetic Properties of Photostable and Surface-Modifiable Quantum Dots for Multimodal Bioimaging

Semiconductor quantum dots (QDs) are widely used for optical applications and bioimaging. In comparison to organic dyes used for fluorescent labeling, QDs exhibit very high photostability and can be further surface modified. Equipping QDs with magnetic properties (mQDs) makes it possible to combine fluorescence and magnetic resonance imaging analyses. For this purpose, we have prepared water-dispersible and magnetic CdTe/ZnS mQDs, whereby ferrous ions are selectively incorporated in either their cores or their shells. This study aims at understanding the differences in optical, structural, and magnetic properties between these core- and shell-doped mQDs. Field-dependent isothermal magnetic susceptibility measurements show that shell-doped mQDs exhibit paramagnetic and their core-doped equivalents super­paramagnetic behavior near room temperature. Shell doping results in about 1.7 times higher photoluminescence quantum yields and 1.4 times higher doping efficiency than core doping. X-ray diffraction patterns reveal that core doping leads to defects in the lattice and hence to a severe decrease in crystallinity, whereas shell doping has no significant impact on the crystal structure and consequently fewer disadvantages regarding the mQD’s quantum yield. These selective doping approaches, particularly shell doping, allow for the tailored design of paramagnetic QDs having modifiable and biocompatible particle surfaces. The organic ligandsin this study <i>N</i>-acetyl-l-cysteinesufficiently prevent leakage of toxic metal ions, as shown by cytotoxicity assays with HepG2 cells. Confocal laser scanning microscopy shows that mQDs are internalized by these cells and accumulated near their nuclei. This study shows that biocompatible, fluorescent, and paramagnetic QDs are promising photostable labels for multimodal bioimaging