Relaxivity Optimization of a PEGylated Iron-Oxide-Based Negative Magnetic Resonance Contrast Agent for <i>T</i>₂-Weighted Spin–Echo Imaging

Concerning the outer sphere relaxation theory, the sensitivity of a <i>T</i>₂ MRI contrast agent, expressed by the transverse relaxivity <i>r</i>₂, depends on the diffusion length of water molecules relative to the particle size. For <i>T</i>₂-weighted spin–echo imaging, theoretical concepts reveal three regimes regarding the <i>r</i>₂ relaxivity depending on the nanocrystal size: the motional averaging regime (MAR), the static dephasing regime (SDR), and the echo-limiting regime (ELR). The <i>r</i>₂ maximum corresponds to the SDR, which represents a small size regime. To verify the theoretical concepts and to adjust the SDR, tailor-made <i>T</i>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 <i>T</i>₂ contrast agents by tuning the cluster size of SPIO nanocrystals

Ultrasmall Biocompatible Nanocomposites: A New Approach Using Seeded Emulsion Polymerization for the Encapsulation of Nanocrystals

We report a novel approach of seeded emulsion polymerization in which nanocrystals are used as seeds. Ultrasmall biocompatible polymer-coated nanocrystal with sizes between 15 and 110 nm could be prepared in a process that avoids any treatment with high shear forces or ultrasonication. The number of nanocrystals per seed, the size of the seeds, and the shell thickness can be independently adjusted. Single encapsulated nanocrystals in ultrasmall nanobeads as well as clusters of nanocrystals can be obtained. Polysorbat-80 was used as surfactant. It consists of poly­(ethylene glycol) (PEG) chains, giving the particles outstanding biofunctional characteristics such as a minimization of unspecific interactions

Synthesis of Iron Oxide Nanorods Using a Template Mediated Approach

Synthesis of Iron Oxide Nanorods Using a Template Mediated Approac

Radical Initiated Reactions on Biocompatible CdSe-Based Quantum Dots: Ligand Cross-Linking, Crystal Annealing, and Fluorescence Enhancement

Cross-linking of biocompatible ligand shells significantly improves the stability of nanocrystals in the biological environment. We report a detailed spectroscopic study of radical initiated reactions on poly­(isoprene)-<i>b</i>-poly­(ethelene glycol) encapsulated CdSe/CdS/ZnS core–shell–shell quantum dots. It was found that the radicals not only initiate cross-linking of the polyisoprene moieties but also may anneal the nanocrystal surfaces and improve their crystallinity

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-<i>block</i>-ethylene oxide) (PI-<i>b</i>-PEO) as a versatile system of amphiphilic diblock copolymers for the micellular encapsulation of nanoparticles (NPs). We show the synthesis of a novel PI-<i>b</i>-(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-<i>b</i>-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

Poly(ethylene oxide) and Polystyrene Encapsulated Quantum Dots: Highly Fluorescent, Functionalizable, and Ultrastable in Aqueous Media

The polymer encapsulation of quantum dots via seeded emulsion polymerization is a powerful method for the preparation of extraordinarily stable fluorescent particles and furthermore allows simple and straightforward <i>in situ</i> functionalization of the polymeric shell. Both features are inevitable for the application of quantum dots as targetable fluorescent probes in advanced biomedical studies. In particular, polymer encapsulated quantum dots showed only marginal loss of quantum yields when exposed to Cu²⁺ ions, which under nonoptimized conditions completely quenched quantum dot fluorescence. This will allow the application of copper-catalyzed click chemistry. Furthermore, by simple addition of functional surfactants or functional monomers during the seeded emulsion polymerization process, a broad range of <i>in situ</i> functionalized polymer-coated quantum dots were obtained. This was demonstrated by purposeful modulation of the zeta potential encapsulated of quantum dots and conjugation of dyestuff. Successful functionalization was unequivocally proven by total reflection X-ray fluorescence

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-<i>block</i>-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 <i>r</i>₂ 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]

Metal–Semiconductor Nanoparticle Hybrids Formed by Self-Organization: A Platform to Address Exciton–Plasmon Coupling

Hybrid nanosystems composed of excitonic and plasmonic constituents can have different properties than the sum of of the two constituents, due to the exciton–plasmon interaction. Here, we report on a flexible model system based on colloidal nanoparticles that can form hybrid combinations by self-organization. The system allows us to tune the interparticle distance and to combine nanoparticles of different sizes and thus enables a systematic investigation of the exciton–plasmon coupling by a combination of optical spectroscopy and quantum-optical theory. We experimentally observe a strong influence of the energy difference between exciton and plasmon, as well as an interplay of nanoparticle size and distance on the coupling. We develop a full quantum theory for the luminescence dynamics and discuss the experimental results in terms of the Purcell effect. As the theory describes excitation as well as coherent and incoherent emission, we also consider possible quantum optical effects. We find a good agreement of the observed and the calculated luminescence dynamics induced by the Purcell effect. This also suggests that the self-organized hybrid system can be used as platform to address quantum optical effects

<i>In Situ</i> 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 <i>in situ</i> 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

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)-<i>b</i>-poly­(ethylene glycol) diblock copolymer (PI-<i>b</i>-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-<i>manno</i>-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