Little Adjustments Significantly Simplify the Gram-Scale Synthesis of High-Quality Iron Oxide Nanocubes.

This work presents a facile one-step protocol for the gram-scale synthesis of iron oxide nanocubes with adjustable sizes ranging from 13 to 20 nm and with size distributions between 7 and 12%. As X-ray diffraction indicated the initial formation of the wüstite phase, a formation mechanism of the nanocubes based on the wüstite crystal structure is proposed. When exposed to ambient conditions, the nanoparticles rapidly oxidize to magnetite/maghemite with a remaining wüstite core. The cubic morphology is attributed to the thermodynamic stability of the exposed {100} facets and the control over the growth rate via the use of a sodium oleate/oleic acid mixed ligand system. In contrast to previously reported procedures, the described synthetic approach does not require the initial preparation and isolation of iron oleate. Therefore, the amount of work and the consumption of hazardous solvents are significantly reduced. Thus, the method presented is much more efficient and environmentally more friendly while maintaining excellent control over the particles' shape, size, and size distribution.

Cysteine: an overlooked energy and carbon source.

Biohybrids composed of microorganisms and nanoparticles have emerged as potential systems for bioenergy and high-value compound production from CO2 and light energy, yet the cellular and metabolic processes within the biological component of this system are still elusive. Here we dissect the biohybrid composed of the anaerobic acetogenic bacterium Moorella thermoacetica and cadmium sulphide nanoparticles (CdS) in terms of physiology, metabolism, enzymatics and transcriptomic profiling. Our analyses show that while the organism does not grow on L-cysteine, it is metabolized to acetate in the biohybrid system and this metabolism is independent of CdS or light. CdS cells have higher metabolic activity, despite an inhibitory effect of Cd2+ on key enzymes, because of an intracellular storage compound linked to arginine metabolism. We identify different routes how cysteine and its oxidized form can be innately metabolized by the model acetogen and what intracellular mechanisms are triggered by cysteine, cadmium or blue light.

Cross-Linked Polystyrene Shells Grown on Iron Oxide Nanoparticles via Surface-Grafted AGET-ATRP in Microemulsion.

Most applications of nanoparticles require robust stabilization, for example, by surface-bound ligands or the encapsulation within polymer shells. Furthermore, for biomedical applications, the particles must be dispersible in a complex biological environment. Thus, high-quality nanoparticles synthesized in organic solvents must be transferred into aqueous media. Here, we present a novel scalable method enabling the robust hydrophilic encapsulation of non-agglomerated nanoparticles by growing polystyrene shells via AGET-ATRP in microemulsion. To demonstrate this approach, we encapsulate iron oxide nanoparticles (diameter: 13.7 ± 0.6 nm). Because the ATRP initiator is grafted onto the nanoparticles' surface, the shells are covalently attached to the iron oxide cores. By varying the amount of monomers, the shell thickness can be adjusted precisely, as indicated by the increasing hydrodynamic size from ∼22 to 26 nm (DLS, number mean) with an increasing amount of added monomers. Moreover, the degree of cross-linking can be controlled by the amount of added divinylbenzene (DVB). To evaluate the robustness of the polymer shells against ion infusion, we introduce a novel colorimetric method, which is based on the formation of the red iron thiocyanate complex. After addition of HCl, the increase in absorbance at 468 nm indicates leaching of iron ions from the polymer-encapsulated core particles. These measurements confirm that with increasing shell thickness, significantly improved shielding is achieved. Furthermore, high concentrations of added DVB [33-50% (v/v) in a monomer mixture] improve the shielding effect. However, when smaller amounts of DVB were added [10-25% (v/v)], the shielding effect was diminished, even in comparison to non-cross-linked polymer shells. This finding suggests a higher porosity of shells with a low degree of cross-linking.

Chemistry of Shape-Controlled Iron Oxide Nanocrystal Formation.

Herein, we demonstrate that meticulous and in-depth analysis of the reaction mechanisms of nanoparticle formation is rewarded by full control of the size, shape, and crystal structure of superparamagnetic iron oxide nanocrystals during synthesis. Starting from two iron sources, iron(II) and iron(III) carbonate, a strict separation of oleate formation from the generation of reactive pyrolysis products and concomitant nucleation of iron oxide nanoparticles was achieved. This protocol enabled us to analyze each step of nanoparticle formation independently in depth. The progress of the entire reaction was monitored via matrix-assisted laser desorption ionization time-of-flight mass spectrometry and gas chromatography, thus providing insight into the formation of various iron oleate species prior to nucleation. Interestingly, due to the intrinsic strongly reductive pyrolysis conditions of the oleate intermediates and redox process in early stages of the synthesis, pristine iron oxide nuclei were composed exclusively from wüstite irrespective of the oxidation state of the iron source. Controlling the reaction conditions provided a very broad range of size- and shape-defined monodispersed iron oxide nanoparticles. Curiously, after nucleation, star-shaped nanocrystals were obtained that underwent metamorphism toward cubic-shaped particles. Electron energy loss spectroscopy tomography revealed ex post oxidation of the primary wustite nanocrystal, providing a full 3D image of Fe2+ and Fe3+ distribution within. Overall, we developed a highly flexible synthesis, yielding multi-gram amounts of well-defined iron oxide nanocrystals of different sizes and morphologies.

Nanocomposites of Highly Monodisperse Encapsulated Superparamagnetic Iron Oxide Nanocrystals Homogeneously Dispersed in a Poly(ethylene Oxide) Melt.

Nanocomposite materials based on highly stable encapsulated superparamagnetic iron oxide nanocrystals (SPIONs) were synthesized and characterized by scattering methods and transmission electron microscopy (TEM). The combination of advanced synthesis and encapsulation techniques using different diblock copolymers and the thiol-ene click reaction for cross-linking the polymeric shell results in uniform hybrid SPIONs homogeneously dispersed in a poly(ethylene oxide) matrix. Small-angle X-ray scattering and TEM investigations demonstrate the presence of mostly single particles and a negligible amount of dyads. Consequently, an efficient control over the encapsulation and synthetic conditions is of paramount importance to minimize the fraction of agglomerates and to obtain uniform hybrid nanomaterials.

Determination of the packing fraction in photonic glass using synchrotron radiation nanotomography.

Photonic glass is a material class that can be used as photonic broadband reflectors, for example in the infrared regime as thermal barrier coating films. Photonic properties such as the reflectivity depend on the ordering and material packing fraction over the complete film thickness of up to 100 µm. Nanotomography allows acquiring these key parameters throughout the sample volume at the required resolution in a non-destructive way. By performing a nanotomography measurement at the PETRA III beamline P05 on a photonic glass film, the packing fraction throughout the complete sample thickness was analyzed. The results showed a packing fraction significantly smaller than the expected random close packing giving important information for improving the fabrication and processing methods of photonic glass material in the future.

Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties.

It is commonly accepted that the combination of the anisotropic shape and nanoscale dimensions of the mineral constituents of natural biological composites underlies their superior mechanical properties when compared to those of their rather weak mineral and organic constituents. Here, we show that the self-assembly of nearly spherical iron oxide nanoparticles in supercrystals linked together by a thermally induced crosslinking reaction of oleic acid molecules leads to a nanocomposite with exceptional bending modulus of 114 GPa, hardness of up to 4 GPa and strength of up to 630 MPa. By using a nanomechanical model, we determined that these exceptional mechanical properties are dominated by the covalent backbone of the linked organic molecules. Because oleic acid has been broadly used as nanoparticle ligand, our crosslinking approach should be applicable to a large variety of nanoparticle systems.

Quantitative Activity Measurements of Brown Adipose Tissue at 7 T Magnetic Resonance Imaging After Application of Triglyceride-Rich Lipoprotein 59Fe-Superparamagnetic Iron Oxide Nanoparticle: Intravenous Versus Intraperitoneal Approach.

OBJECTIVES: The aim of this study was to determine metabolic activity of brown adipose tissue (BAT) with in vivo magnetic resonance imaging (MRI) after intravenous (IV) and intraperitoneal (IP) injection of radioactively labeled superparamagnetic iron oxide nanoparticles (SPIOs) embedded into a lipoprotein layer. MATERIALS AND METHODS: Fe-labeled SPIOs were either polymer-coated or embedded into the lipid core of triglyceride-rich lipoproteins (TRL-Fe-SPIOs). First biodistribution and blood half time analysis in thermoneutral mice after IP injection of either TRL-Fe-SPIOs or polymer-coated Fe-SPIOs (n = 3) were performed. In the next step, cold-exposed (24 hours), BAT-activated mice (n = 10), and control thermoneutral mice (n = 10) were starved for 4 hours before IP (n = 10) or IV (n = 10) injection of TRL-Fe-SPIOs. In vivo MRI was performed before and 24 hours after the application of the particles at a 7 T small animal MRI scanner using a T2*-weighted multiecho gradient echo sequence. R2* and ΔR2* were estimated in the liver, BAT, and muscle. The biodistribution of polymer-coated Fe-SPIOs and TRL-Fe-SPIOs was analyzed ex vivo using a sensitive, large-volume Hamburg whole-body radioactive counter. The amount of Fe-SPIOs in the liver, BAT, and muscle was correlated with the MRI measurements using the Pearson correlation coefficient. Tissue uptake of Fe-SPIOs was confirmed by histological and transmission electron microscopy analyses. RESULTS: Triglyceride-rich lipoprotein Fe-SPIOs exhibited a higher blood concentration after IP injection (10.1% ± 0.91% after 24 hours) and a greater [INCREMENT]R2* in the liver (103 ± 5.0 s), while polymer-coated SPIOs did not increase substantially in the blood stream (0.19% ± 0.01% after 24 hours; P < 0.001) and the liver (57 ± 4.08 s; P < 0.001). In BAT activity studies, significantly higher uptake of TRL-Fe-SPIOs was detected in the BAT of cold-exposed mice, with [INCREMENT]R2* of 107 ± 5.5 s after IV application (control mice: [INCREMENT]R2* of 22 ± 5.8 s; P < 0.001) and 45 ± 5.5 s after IP application (control mice: [INCREMENT]R2* of 11 ± 2.9 s; P < 0.01). Fe radioactivity measurements and [INCREMENT]R2* values correlated strongly in BAT (r > 0.85; P < 0.001) and liver tissue (r > 0.85; P < 0.001). Histological and transmission electron microscopy analyses confirmed the uptake of TRL-Fe-SPIOs within the liver and BAT for both application approaches. CONCLUSIONS: Triglyceride-rich lipoprotein-embedded SPIOs were able to escape the abdominal cavity barrier, whereas polymer-coated SPIOs did not increase substantially in the blood stream. Brown adipose tissue activity can be determined via MRI using TRL-Fe-SPIOs. The quantification of [INCREMENT]R2* using TRL-Fe-SPIOs is feasible and may serve as a noninvasive tool for the quantitative estimation of BAT activity.

Determination of liver-specific r2 * of a highly monodisperse USPIO by (59) Fe iron core-labeling in mice at 3 T MRI.

Accurate determination of tissue concentration of ultrasmall superparamagnetic iron oxide nanoparticles (USPIO) using T2 * MR relaxometry is still challenging. We present a reliable quantification method for local USPIO amount with the estimation of the liver specific relaxivity r2 * using monodisperse (59) Fe-core-labeled USPIO ((59) FeUSPIO). Dynamic and relaxometric in vivo characteristics of unlabeled monodisperse USPIO were determined in MRI at 3 T. The in vivo MR studies were performed for liver tissue with (59) FeUSPIO using iron dosages of 9 (n = 3), 18 (n = 2) and 27 (n = 3) µmol Fe kg(-1) body weight. The R2 * of the liver before and after USPIO injection (∆R2 *) was measured and correlated with (59) Fe activity measurements of excised organs by a whole body radioactivity counter (HAMCO) to define the dependency of ∆R2 * and (59) FeUSPIO liver concentration and calculate the r2 * of (59) FeUSPIO for the liver. Ultrastructural analysis of liver uptake was performed by histology and transmission electron microscopy. ∆R2 * of the liver revealed a dosage-dependent accumulation of (59) FeUSPIO with a percentage uptake of 70-88% of the injection dose. Hepatic ∆R2 * showed a dose-dependent linear correlation to (59) FeUSPIO activity measurements (r = 0.92) and an r2 * in the liver of 481 ± 74.9 mm(-1) s(-1) in comparison to an in vitro r2 * of 60.5 ± 3.3 mm(-1) s(-1) . Our results indicate that core-labeled (59) FeUSPIO can be used to quantify the local amount of USPIO and to estimate the liver-specific relaxivity r2 *.

The cell-type specific uptake of polymer-coated or micelle-embedded QDs and SPIOs does not provoke an acute pro-inflammatory response in the liver.

Semiconductor quantum dots (QD) and superparamagnetic iron oxide nanocrystals (SPIO) have exceptional physical properties that are well suited for biomedical applications in vitro and in vivo. For future applications, the direct injection of nanocrystals for imaging and therapy represents an important entry route into the human body. Therefore, it is crucial to investigate biological responses of the body to nanocrystals to avoid harmful side effects. In recent years, we established a system to embed nanocrystals with a hydrophobic oleic acid shell either by lipid micelles or by the amphiphilic polymer poly(maleic anhydride-alt-1-octadecene) (PMAOD). The goal of the current study is to investigate the uptake processes as well as pro-inflammatory responses in the liver after the injection of these encapsulated nanocrystals. By immunofluorescence and electron microscopy studies using wild type mice, we show that 30 min after injection polymer-coated nanocrystals are primarily taken up by liver sinusoidal endothelial cells. In contrast, by using wild type, Ldlr (-/-) as well as Apoe (-/-) mice we show that nanocrystals embedded within lipid micelles are internalized by Kupffer cells and, in a process that is dependent on the LDL receptor and apolipoprotein E, by hepatocytes. Gene expression analysis of pro-inflammatory markers such as tumor necrosis factor alpha (TNFα) or chemokine (C-X-C motif) ligand 10 (Cxcl10) indicated that 48 h after injection internalized nanocrystals did not provoke pro-inflammatory pathways. In conclusion, internalized nanocrystals at least in mouse liver cells, namely endothelial cells, Kupffer cells and hepatocytes are at least not acutely associated with potential adverse side effects, underlining their potential for biomedical applications.

Polymer-assisted self-assembly of superparamagnetic iron oxide nanoparticles into well-defined clusters: controlling the collective magnetic properties.

The combination of superstructure-forming amphiphilic block copolymers and superparamagnetic iron oxide nanoparticles produces new nano/microcomposites with unique size-dependent properties. Herein, we demonstrate the controlled clustering of superparamagnetic iron oxide nanoparticles (SPIOs) ranging from discretely encapsulated SPIOs to giant clusters, containing hundreds or even more particles, using an amphiphilic polyisoprene-block-poly(ethylene glycol) diblock copolymer. Within these clusters, the SPIOs interact with each other and show new collective properties, neither obtainable with singly encapsulated nor with the bulk material. We observed cluster-size-dependent magnetic properties, influencing the blocking temperature, the magnetoviscosity of the liquid suspension, and the r2 relaxivity for magnetic iron oxide nanoparticles. The clustering methodology can be expanded also to other nanoparticle materials [CdSe/CdS/ZnS core/shell/shell quantum dots (QDs), CdSe/CdS quantum dots/quantum rods (QDQRs), gold nanoparticles, and mixtures thereof].

Controlling the physical and biological properties of highly fluorescent aqueous quantum dots using block copolymers of different size and shape.

The phase transfer of fluorescent CdSe based quantum dots (QDs) while retaining their properties and offering some advantages concerning the stability and functionalization characteristics is an important and intensively investigated field of research. Here we report how to tune and control the properties of CdSe/CdS/ZnS core-shell-shell QDs in water, using poly(isoprene-block-ethylene oxide) (PI-b-PEO) as a versatile system of amphiphilic diblock copolymers for the micellular encapsulation of nanoparticles (NPs). We show the synthesis of a novel PI-b-(PEO)2 miktoarm star polymer and how this different architecture besides the variation of the polymers' molecular weight gives us the opportunity to control the size of the built constructs in water between 24 and 53 nm. Because of this size control, an upper limit of the construct's diameter for the cellular uptake could be determined by a systemic study with human alveolar epithelial cells (A549) and murine macrophage leukemia cell (RAW-264.7). Furthermore, fluorescence quenching experiments with copper(II) and iron(III) ions show a strong influence of the used polymer on the shielding against these ions. This enables us to control the permeability of the polymer shell from very porous shells, which allow an almost complete cation exchange up to very dense shells. These even offer the possibility to perform copper(I) catalyzed click reactions while keeping the fluorescence of the QDs. All these results underline the huge variability and controllability of the PI-b-PEO diblock copolymer system for the encapsulation and functionalization of nanoparticles for biological applications. As a general trend, it can be stated that those coatings, which were most stable against quenchers, also showed the best resistivity with respect to unspecific cellular uptake.

Gold nanoparticles functionalized with a fragment of the neural cell adhesion molecule L1 stimulate L1-mediated functions.

The neural cell adhesion molecule L1 is involved in nervous system development and promotes regeneration in animal models of acute and chronic injury of the adult nervous system. To translate these conducive functions into therapeutic approaches, a 22-mer peptide that encompasses a minimal and functional L1 sequence of the third fibronectin type III domain of murine L1 was identified and conjugated to gold nanoparticles (AuNPs) to obtain constructs that interact homophilically with the extracellular domain of L1 and trigger the cognate beneficial L1-mediated functions. Covalent conjugation was achieved by reacting mixtures of two cysteine-terminated forms of this L1 peptide and thiolated poly(ethylene) glycol (PEG) ligands (~2.1 kDa) with citrate stabilized AuNPs of two different sizes (~14 and 40 nm in diameter). By varying the ratio of the L1 peptide-PEG mixtures, an optimized layer composition was achieved that resulted in the expected homophilic interaction of the AuNPs. These AuNPs were stable as tested over a time period of 30 days in artificial cerebrospinal fluid and interacted with the extracellular domain of L1 on neurons and Schwann cells, as could be shown by using cells from wild-type and L1-deficient mice. In vitro, the L1-derivatized particles promoted neurite outgrowth and survival of neurons from the central and peripheral nervous system and stimulated Schwann cell process formation and proliferation. These observations raise the hope that, in combination with other therapeutic approaches, L1 peptide-functionalized AuNPs may become a useful tool to ameliorate the deficits resulting from acute and chronic injuries of the mammalian nervous system.

Glycoconjugated amphiphilic polymers via click-chemistry for the encapsulation of quantum dots.

Herein, we present a strategy for the glycoconjugation of nanoparticles (NPs), with a special focus on fluorescent quantum dots (QDs), recently described by us as "preassembly" approach. Therein, prior to the encapsulation of diverse nanoparticles by an amphiphilic poly(isoprene)-b-poly(ethylene glycol) diblock copolymer (PI-b-PEG), the terminal PEG appendage was modified by covalently attaching a carbohydrate moiety using Huisgen-type click-chemistry. Successful functionalization was proven by NMR spectroscopy. The terminally glycoconjugated polymers were subsequently used for the encapsulation of QDs in a phase transfer process, which fully preserved fluorescence properties. Binding of these nanoconstructs to the lectin Concanavalin A (Con A) was studied via surface plasmon resonance (SPR). Depending on the carbohydrate moiety, namely, D-manno-heptulose, D-glucose, D-galactose, 2-deoxy-2-{[methylamino)carbonyl]amino}-D-glucopyranose ("des(nitroso)-streptozotocin"), or D-maltose, the glycoconjugated QDs showed enhanced affinity constants due to multivalent binding effects. None of the constructs showed toxicity from 0.001 to 1 µM (particle concentration) using standard WST and LDH assays on A549 cells.

Effect of the spacer structure on the stability of gold nanoparticles functionalized with monodentate thiolated poly(ethylene glycol) ligands.

Poly(ethylene glycol)- (PEG-) based ligands are well-established for the stabilization of nanoparticles in aqueous solution and are especially interesting for applications in medicine and biotechnology because they are known to improve the pharmacokinetic properties of nanomaterials. In this study, we prepared gold nanoparticles (AuNPs) with ligand shells of different monodentate poly(ethylene glycol)-thiol (PEG-SH) ligands. These ligands differed only in the segment connecting the thiol group with the PEG moiety (Mw ≈ 2000 g/mol) through an ester bond, the spacer. All ligands were synthesized by straightforward esterification. Specifically, we used PEG ligands with a long (C10, PEGMUA) or short (C2, PEGMPA) alkylene spacer or a phenylene (PEGMPAA) spacer. The influence of the spacer on the stability of gold nanoparticle-PEG conjugates (AuNP@PEG) was tested by cyanide etching experiments, electrolyte-induced aggregation, and competitive ligand displacement with dithiothreitol (DTT). In the presence of 100 mM cyanide, AuNPs stabilized with PEGMPA or PEGMPAA were completely dissolved by oxidative etching within a few minutes, whereas AuNPs stabilized with PEGMUA needed more than 20 h to be completely etched. By complementary experiments, we deduced a simplified description for the etching process that takes into account the role of excess ligand. In the presence of free ligand, significantly fewer AuNPs are etched, suggesting a competition of etching and ligand binding to AuNPs. We also compared the stabilizing effect of PEGMUA with that of a bidentate PEG-thiol ligand (PEGLIP) and found a reversed stability against cyanide etching and DTT displacement, in agreement with previously reported observations. Our results clearly demonstrate the strong impact of the spacer structure on conjugate stability and provide valuable information for the rational design of more complex AuNP@PEG conjugates, which are of much interest in the context of biotechnology and medical applications.

Amphiphilic, cross-linkable diblock copolymers for multifunctionalized nanoparticles as biological probes.

Nanoparticles (NPs) play an increasingly important role in biological labeling and imaging applications. However, preserving their useful properties in an aqueous biological environment remains challenging, even more as NPs therein have to be long-time stable, biocompatible and nontoxic. For in vivo applications, size control is crucial in order to route excretion pathways, e.g. renal clearance vs. hepato-biliary accumulation. Equally necessary, cellular and tissue specific targeting demands suitable linker chemistry for surface functionalization with affinity molecules, like peptides, proteins, carbohydrates and nucleotides. Herein, we report a three stage encapsulation process for NPs comprised of (1) a partial ligand exchange by a multidentate polyolefinic amine ligand, PI-N3, (2) micellar encapsulation with a precisely tuned amphiphilic diblock PI-b-PEG copolymer, in which the PI chains intercalate to the PI-N3 prepolymer and (3) radical cross-linking of the adjacent alkenyl bonds. As a result, water-soluble NPs were obtained, which virtually maintained their primal physical properties and were exceptionally stable in biological media. PEG-terminal functionalization of the diblock PI-b-PEG copolymer with numerous functional groups was mostly straightforward by chain termination of the living anionic polymerization (LAP) with the respective reagents. More complex affinity ligands, e.g. carbohydrates or biotin, were introduced in a two-step process, prior to micellar encapsulation. Advantageously, this pre-assembly approach opens up rapid access to precisely tuned multifunctional NPs, just by using mixtures of diverse functional PI-b-PEG polymers in a combinatorial manner. All constructs showed no toxicity from 0.001 to 1 µM (particle concentration) in standard WST and LDH assays on A549 cells, as well as only marginal unspecific cellular uptake, even in serum-free medium.

In situ functionalization and PEO coating of iron oxide nanocrystals using seeded emulsion polymerization.

Herein we demonstrate that seeded emulsion polymerization is a powerful tool to produce multiply functionalized PEO coated iron oxide nanocrystals. Advantageously, by simple addition of functional surfactants, functional monomers, or functional polymerizable linkers-solely or in combinations thereof-during the seeded emulsion polymerization process, a broad range of in situ functionalized polymer-coated iron oxide nanocrystals were obtained. This was demonstrated by purposeful modulation of the zeta potential of encapsulated iron oxide nanocrystals and conjugation of a dyestuff. Successful functionalization was unequivocally proven by TXRF. Furthermore, the spatial position of the functional groups can be controlled by choosing the appropriate spacers. In conclusion, this methodology is highly amenable for combinatorial strategies and will spur rapid expedited synthesis and purposeful optimization of a broad scope of nanocrystals.

A simple and widely applicable method to 59Fe-radiolabel monodisperse superparamagnetic iron oxide nanoparticles for in vivo quantification studies.

A simple, fast, efficient, and widely applicable method to radiolabel the cores of monodisperse superparamagnetic iron oxide nanoparticles (SPIOs) with (59)Fe was developed. These cores can be used as precursors for a variety of functionalized nanodevices. A quality control using filtration techniques, size-exclusion chromatography, chemical degradation methods, transmission electron microscopy, and magnetic resonance imaging showed that the nanoparticles were stably labeled with (59)Fe. Furthermore, the particle structure and the magnetic properties of the SPIOs were unchanged. In a second approach, monodisperse SPIOs stabilized with (14)C-oleic acid were synthesized, and the stability of this shell labeling was studied. In proof of principle experiments, the (59)Fe-SPIOs coated with different shells to make them water-soluble were used to evaluate and compare in vivo pharmacokinetic parameters such as blood half-life. It could also be shown that our radiolabeled SPIOs embedded in recombinant lipoproteins can be used to quantify physiological processes in closer detail than hitherto possible. In vitro and in vivo experiments showed that the (59)Fe label is stable enough to be applied in vivo, whereas the (14)C label is rapidly removed from the iron core and is not adequate for in vivo studies. To obtain meaningful results in in vivo experiments, only (59)Fe-labeled SPIOs should be used.

Tailor-made quantum dot and iron oxide based contrast agents for in vitro and in vivo tumor imaging.

The biofunctionalization of CdSe/CdS/ZnS quantum dots and Fe(3)O(4) nanocrystals using a novel ligand system based on polyisoprene-block-poly(ethylene oxide) ligands is described. The synthesis includes a partial ligand exchange of the hydrophobic nanocrystals with amino-functionalized polyisoprene ligands, followed by seeded micelle formation of the diblock-copolymers in water. The resulting water-soluble quantum dots showed fluorescence quantum efficiencies in the 40 to 50% range and extraordinary fluorescence stability in the biological environment after cross-linking of the polyisoprene moiety of the ligand shell. No toxicity was detected by water-soluble tetrazolium (WST8) and lactate dehydrogenase (LDH) assays, even at very high nanoparticle concentrations, and almost no nonspecific cell adhesion was detected. The ligand shell was further coupled to the antigen-related cell adhesion molecule (CEACAM) specific monoclonal antibody T84.1. The so-conjugated Fe(3)O(4) nanocrystals allowed in vitro and in vivo tumor targeting by magnetic resonance imaging.

Relaxivity optimization of a PEGylated iron-oxide-based negative magnetic resonance contrast agent for T2-weighted spin-echo imaging.

Concerning the outer sphere relaxation theory, the sensitivity of a T(2) MRI contrast agent, expressed by the transverse relaxivity r(2), depends on the diffusion length of water molecules relative to the particle size. For T(2)-weighted spin-echo imaging, theoretical concepts reveal three regimes regarding the r(2) relaxivity depending on the nanocrystal size: the motional averaging regime (MAR), the static dephasing regime (SDR), and the echo-limiting regime (ELR). The r(2) maximum corresponds to the SDR, which represents a small size regime. To verify the theoretical concepts and to adjust the SDR, tailor-made T(2) contrast agents were synthesized by controlled self-assembly of superparamagnetic iron oxide nanocrystals (SPIOs) into raspberry-like nanoclusters with diameters of 30-200 nm using a PEG-based ligand. The results highlight an opportunity to optimize the relaxivity of T(2) contrast agents by tuning the cluster size of SPIO nanocrystals.