The examples of upconversion (UC) described in the literature until now such as simultaneous or sequential absorption of two or more
photons with lower energy, second and higher harmonic generation of the fundamental wavelength, parametric processes has been
commonly associated with the use of coherent light sources (lasers) and very high excitation intensities.
In contrast to the all previously described methods the fundamental advantage of the energetically conjoined triplet-triplet
annihilation (TTA) photon up-conversion process is its inherent independence on the coherence of the excitation light.
Another principal advantage of this up-conversion process is the very low intensity (as low as 100 mWcm-2) and extremely low
spectral power density (as low as 125μWnm-1) of the excitation source needed (it can be the Sun).
A direct increase of the spectral brightness of the short wavelength region of the solar spectrum by using photons
from the longer wavelength region remains until now a very considerable challenge. For solar cell applications,
from a great importance are photons, notably spectrally blue shifted to the wavelength of the excitation photons
with an energy shift between 10kT – 100kT.
Conventional heterophase polymerization systems, i.e. o/w or w/o and multiphase systems employing oil or water, display important
limitations for the synthesis of polymers.
The first obvious limitation is that polymerizations requiring water-sensitive initiators or monomers cannot be performed to yield a clean polymer because of the presence of water. The second limitation is that emulsions and dispersions are used at temperature usually below 100 °C due to the presence of water. Besides, coalescence and Ostwald ripening are increased with temperature increase, leading to unstable colloids for polymerization systems governed by mass transfer through the continuous phase, e.g. emulsion polymerization. Polymer or inorganic colloids synthesized in emulsions at higher temperatures are hence only seldom reported in the literature.
We have shown that the use of non-aqueous miniemulsions to perform polymerizations circumvents both drawbacks. Thus, anionic polymerization of ε-caprolactam initiated by the water-sensitive sodium hydride could be performed in inverse miniemulsion and the polyaddition of diisocyanates and diols was carried out to yield hydrophilic polyurethane particles free from urea bonds.
Polyvinylpyrrolidone nanoparticles (Fig. 1) could be synthesized at temperature higher than 150 °C in non-aqueous inverse miniemulsions in the presence of an additional stabilizer. The latter synthesis can also be combined with the polyol process for the reduction of silver nitrate to yield polyvinylpyrrolidone/silver hybrid nanoparticles.
The use of supercritical fluids as continuous phase for heterophase polymerizations is gaining much attention. Supercritical CO2 (sc-CO2) is interesting since the supercritical pressure and temperature are low, 7.38 MPa and 304.1 K respectively. The last but probably most important advantage towards classical solvents is that sc-CO2 is non-toxic, non-flammable and environmentally friendly. Sc-CO2 as environmentally responsible solvent will be investigated as continuous phase for miniemulsion polymerizations.
Fig.1 TEM micrograph of polyvinylpyrrolidone particles.
Polymer colloids synthesized via heterophase polymerizations are usually spherical, which is not always the most suitable morphology
for some applications. In fact, significant advantages can be gained using anisotropic colloid structures. Spherical morphologies
involve a lower surface area/volume ratio compared to prolate, oblate or rod anisotropic structures. The shape of the colloids has
also a significant influence on their scattering, rheological, and coagulation properties. Since the shape of the colloids has been
recognized to play also a role in their interaction with biological systems, there is also an increasing interest in the synthesis
of anisotropic colloids.
The anisotropic colloid structures are usually obtained by stretching colloids or by the assembly of spherical colloids with directional interactions. The first approach is particularly attractive when the colloids or emulsions to be stretched are synthesized in miniemulsion since a large library of colloids with different chemical nature or morphologies are now available from this method.
We focus on two main methods, which are the mechanical deformation of polymer nanoparticles (Figure 1-2) and miniemulsion-electrospinning (Figure 3-4). Colloids synthesized by miniemulsion have also the advantage to be eventually nanostructured before the electrospinning process.
Figure 1. SEM micrograph of polystyrene ellipsoids after stretching spherical nanoparticles synthesized in miniemulsion.
Figure 2. SEM micrograph of “nanocanoes” obtained after stretching spherical core-shell nanoparticles synthesized in miniemulsion.
Figure 3. SEM micrograph of core-sheath polymer nanofibers synthesized by the miniemulsion-electrospinning technique.
Figure 4. TEM micrograph of core-sheath polymer nanofibers synthesized by the miniemulsion-electrospinning technique.
Lipid and polymer aggregates are versatile and very useful model systems and are intensely studied with respect to biomedical applications. Compared to their low molecular counterpart, i. e. the liposomes, polymeric vesicles show higher stability and robustness as well as a highly designable permeability. Therefore, they have received the attention of many research groups all over the world. Furthermore, compared with the extensive research on encapsulation of biomolecules into liposomes, the studies on incorporation of large biological units into block copolymer aggregates are very limited, in part because large hydrophilic biomolecules are sensitive to organic solvents and the pH of the solution. The paucity of studies dealing with the encapsulation of biological species into block copolymer aggregates underlines the inherent difficulties of the process and suggests the need for further research. On this account a simplified synthetic model system for systematic investigations is introduced, with the aim of improving the understanding of the encapsulation and supramolecular structure formation mechanisms and controlling the parameters which influence these processes. We study the encapsulation process of globular amine-terminated, poly(amido amine) dendrimer (PAMAM) as well as rodlike PAMAM-dye complexes into block copolymer assemblies, including micelles and vesicles. A polystyrene block poly(acrylic acid) copolymer (PS-b-PAA) is chosen because it is a well-known and well-understood system and the self-organization has been studied extensively. On the other hand, dendrimers are very useful model compounds for complex structures, such as proteins, which have similar sizes and/or similar terminal groups. The presence of carboxylic acid moieties on the block copolymer and of amine groups on the dendrimer, in a medium of relatively low polarity, such as dioxane, undoubtedly leads to strong interactions, which manifest themselves immediately on mixing of the two components.
The complex structure of the formed aggregates as well as the distribution and the localization of the dendrimer units inside these aggregates are determined by comparative studies of dynamic light scattering and turbidity measurements and transmission electron microscopy. The strong, complex and specific interactions, which influence the dynamic behavior of the system, have a strong influence on the structure formation, which proceeds at different hierarchical levels. The PAMAM concentration as well as the character of the terminal groups of the dendrimer influence the strength of these interactions and consequently affect the structure formation process. As shown by fluorescence quenching experiments, on all supramolecular hierarchical structure levels, and specifically in vesicles, the dendrimer is coated by the PAA chains of the block copolymer due to the strong interactions; since the PAA blocks are connected to the PS blocks, which form the corona, the dendrimer is surrounded by PS chains and is thus encapsulated into the hydrophobic regions of the block copolymer aggregates.
Attributed to their high energy storage capacity, lithium-ion batteries are one of the most popular types of batteries for portable consumer electronic devices. Basically, a lithium-ion battery is composed of four primary functional components; the anode and the cathode serve as storage for lithium ions, the electrolyte provides the necessary Li-conductivity and a separator membrane prevents electrical contact between the electrodes. On discharging, the Li-ions migrate from the cathode to the anode and vice versa during the charging process. In commercial batteries graphite is the common functional anode material, which has a significantly lower specific capacity compared to that of LiCoO2, being the usual anode material. Hence the specific capacity of the anode material limits the performance of the battery. Metal oxides are potential candidates to replace the graphite as anode material. In order to optimize the overall performance of the battery, one has to understand the mechanism how Li is reversibly stored in the respective material. For metal oxides one potential process is the reduction of the host oxide under simultaneous formation of lithium oxide (conversion mechanism MeO + 2 Li -> Me + Li2O). However, exploring the storage mechanism requires, amongst others, the identification of light elements (e.g. Li) on the nanoscale.
Fig. 1: TEM micrograph and NAED
pattern of lithiated NiO.
Transmission electron microscopy (TEM) in combination with electron energy loss spectroscopy (EELS) provides the needed lateral resolution and yields information on the electronic structure. During the first discharge cycle Li-ions are inserted into the formerly micron sized metal oxide usually leading to its disintegration and formation of nano sized crystallites. Fig. 1 shows a NiO anode material from a battery which has been discharged completely. The corresponding EEL spectrum (Fig. 2) reveals the presence of Li through the significant peak at 60 eV. Moreover, from the observed peak position one can deduce information on the valence, showing, that in this case the lithium does not exist in form of Li2O, as expected from the conversion mechanism. Consistent with this, nano-area electron diffraction (NAED) identifies the individual nano-particles as NiO. Moreover, EELS can provide very local information as shown in Fig. 3. Here the individual spectra were taken at different positions on a partly lithiated LiCoO2 particle. Only an area of approx 20 nm diameter contributes to the respective spectra. With increasing distance from the edge of the particle one can observe that the Co core loss peak shifts to higher energies, indicative of an increasing valence of the Co, corresponding to a decrease of Li content.
Fig. 2: EEL spectrum of lithiated NiO (red)
and Li2O (blue).
Fig. 3: EEL spectra of Co core loss (left) from different positions on a partly discharged LiCoO2 particle (TEM micrograph on the right part).
Nanoparticles in biomedical applications have attracted a lot of interest in the last years. First applications of nanoparticles in
the biomedical field involved submicron particles attached to cells by antibodies in order to sort a specific cell from a cell mixture
(e.g. hematopoetic stem cells from bone marrow or blood). Superparamagnetic iron oxide nanoparticles have also been developed as
contrast agents for magnetic resonance imaging (MRI).
Recently, it became clear that nanoparticles are taken up by cells so that homing and migration of cells can be followed. Therefore, we started investigating nanoparticles (commercially available and specially designed) as to how nanoparticle uptake into cells could be enhanced. First, we used nanoparticles with specifically designed surface properties. A series of fluorescent nanoparticles from a co-polymer (e.g. styrene/acrylic acid, styrene/2-aminoethyl methacrylate hydrochloride [AEMH]) was synthesized by the miniemulsion technique. In this process a fluorescent dye was also incorporated (perylenemonoimide, PMI). Consequently cellular uptake was observed by flow cytometry (FACS), confocal fluorescent microscopy (see Fig. 1) and also by TEM and SEM. These surface modifications resulted in an approximately 15x higher uptake of nanoparticles compared with uncharged nanoparticles, when the surface was decorated by carboxy groups (styrene/acrylic acid). A positive charge by amino groups (styrene/AEMH) resulted in ca. 70x higher cellular uptake rate. Using the amino acid lysine coupled to the carboxy group resulted in an even higher uptake, hereby mimicking the effect of transfection agents.
By using polyethyleneglycol on the nanoparticle surface, uptake by cells was hindered (“stealth nanoparticles”). Furthermore, we investigated other types of non-biodegradable (polyisoprene, polymethylmethacrylate) and biodegradable (poly-L-lactide, poly-ε-caprolactone) nanoparticles and their uptake behavior.
Nanoparticles from butylcyanoacrylate were used for in-vitro and in-vivo studies in a blood brain barrier model. While nanoparticles in the in-vitro model (human brain endothelial microvasculature cells (HBMEC) in a transwell system) were not transcytosed, the particles crossed the blood brain barrier in the animal model.
The endocytosis of the nanoparticles is a key factor for understanding the interaction of nanoparticles with cell. By using pharmacological inihibitors we dissected the different ways of nanoparticle uptake. These studies are ongoing and are extended to specific surface modifications and polymeric materials.
Besides fluorescent labels also superparamagnetic iron oxide particles and gadolinium complexes as reporters, were also incorporated into these polymeric nanoparticles. The later two can be used in a clinical MRI scanner, making these double labelled nanoparticles very versatile and flexible tools for fluorescence and MRI imaging (“dual reporter nanoparticles”).
We have started to investigate nanoparticles as drug carriers and will work on making them responsive to the outer environment (e.g. degradation upon decrease in pH as in lysosomal compartments inside the cells).
Fig. 1: On the left confocal laser scanning microscopy images are shown demonstrating intracellular uptake of carboxy functionalized nanoparticles (styrene/acrylic acid NPs). On the right uptake of side amino functionalized nanoparticles is quantified by FACS.
Driven by the ability of polymers to confine geometries in which crystallization takes place, the technique of inverse (water-in-oil)
miniemulsion in the presence of amphiphilic copolymers is an easy and versatile approach to produce a variety of inorganic
nanoparticles (e.g., transition metal oxides) with tunable size. The inorganic aqueous precursor is dispersed in a continuous oil
phase in the form of nearly monodisperse droplets stabilized by the copolymers. Ideally, each precursor droplet acts an an independent
crystallization “nanoreactor”. Nanoparticles with morphologies ranging from spheres to cylinders, different degrees of crystallinity
and different crystal phase compositions can be obtained depending on reaction parameters, as well as on chemical structures and
concentration of copolymer.
In a different but related approach, latex particles obtained by miniemulsion polymerization and functionalized with polyelectrolyte chains of controlled length can be used to template the crystallization of inorganic materials on spherical surfaces. The obtained materials may have a great potential for catalytic application and have been shown to be highly efficient in oxidation reactions.
Polymers play different roles in crystallization phenomena. They can not only be used as surfactants or surface modifiers facilitate the dispersion in a given solvent, but they can also affect the nucleation and enhance or inhibit the crystal growth by adsorbing specifically on certain growing faces. Furthermore, macromolecules can be used as structural templates, because they can self-assemble in supramolecular structures that act as compartments for the subsequent crystal growth, especially in materials obtained by sol-gel processes. In some cases, metastable phases can also be stabilized by the presence of polymers. We investigate how the chemical structure of the polymer affects the crystallization parameters, the morphology, and the final physical properties of the materials obtained.
Nature is able to control the formation of inorganic structures by biogenic macromolecules,
mostly proteins. Calcium carbonate—present in eggs or shells of crustaceans—,
calcium phosphates—present in bones and teeth—, and calcium oxalate—found in plats
or kidney stones—are some well-known and important examples. Although much research
has been devoted to the field of biomineralization over the last two decades, many
mechanistic features are still far from being understood and the synthesis in the
laboratory of large monocrystalline structures, as natures does, remains a challenge.
We attempt to systematically investigate the effect of oligopeptides of well defined compositions and length on the crystallization from aqueous media of minerals of biological importance, such as calcium carbaonate, calcium oxalate, magnesium ammonium phosphate (struvite), and iron oxides. A solid-phase peptide synthesize is used to prepare the desired peptides. In a further approach, the peptide chains are attached to latex particles to act as models for the understanding of mineralization processes on spherical surfaces
In an effort to create particles for each particular application, it is essential and of immense interest to have the ability to control the particle size, morphology, composition, physico-colloidal properties, density of functional groups on the particle surface, and so on. This can be achieved by miniemulsion technique, which is being used widely for the production of many varieties of polymeric dispersions, mainly due to its flexibility in product design and process control. Such flexibility is acquired by adopting different start-up procedures (e.g. type of monomer(s), initiator, surfactant, functional additives, etc.), as well as the possibility to combine various synthetic approaches in one system. For example, polystyrene and poly(methyl methacrylate) nanoparticles with surface carboxyl-, amino-, epoxy-, phosphate or phosphonate groups were produced by radical copolymerization of styrene or methyl methacrylate and corresponding functional monomer (see Fig. 1). The density of functional groups and their distance from the particle surface greatly depends on the properties and the amount of functional comonomer as well as on the type of surfactant (ionic or non-ionic) used during the synthesis. Generally, particles stabilized with non-ionic surfactant are bigger in size and contain higher amount of functional groups as compared with polymer particles obtained in the presence of ionic surfactants.
Fig. 1. SEM image of phosphonated polystyrene-based nanoparticles.
Introduction of a hydrophobic fluorescent dye into the reaction mixture results in the composite particles which have found the
application as carriers for contrasting or imaging agents. Furthermore, the receptor-specific antibodies could be attached to the
functional groups via covalent bonds. The obtained particles were utilized for targeting to the cells of interest. non-specific
particle-cell interaction was reduced by introduction of polyethylene glycol (PEG) chains onto the particle surface. This was
achieved either through chemical linkage between amino-modified PEG and functional group on the particle surface or by employing
polymerizable PEG as a second comonomer during the synthesis.
Fig. 2. TEM image of poly(L-lactide) particles with encapsulated Fe3O4 (10 nm).
Over the past two decades an increased attention has gained to the core-shell particles consisting of a liquid core owing to their
utilization as sub-micrometer containers for the encapsulation of biologically active substances. The main advantages of nanocapsules
for drug delivery are the efficient protection of therapeutic agents against enzymatic and hydrolytic degradation. The ideal
nanocarrier will be one that is size- and morphology-specific, has the ability to encapsulate a variety of compounds, can be
functionalized with certain surface targeting ligands, and has the possibility to deliver and to release the encapsulated material
in a controlled way.
Significant benefits of the miniemulsion technique offers the formation of polymeric nanocapsules with different particle size (50-1000 nm), morphology (single or multi core), versatility in the polymeric materials, and surface functionality. The high stability of the system allows performing reactions inside the droplets and at their interface. Due to the lack of monomer diffusion processes throughout the polymerization, an efficient encapsulation can be obtained by phase separation inside the nanodroplets throughout the polymerization process, by nanoprecipitation of the polymer onto nanodroplets, or by an interfacial reaction at the nanodroplet’s interphase. Under carefully chosen conditions of miniemulsification, it is possible to entrap different kinds of substances, e.g. fluorescent dye molecules, magnetite for magnetic resonance imaging, or DNA molecules, avoiding their damaging. Moreover, after the synthesis the surface of the capsules can be functionalized in order to employ them further for specific delivery of the encapsulated material.
Polymeric nanocapsules (size range 250 – 600 nm) with encapsulated dsDNA (790 bp) were produced via anionic polymerization of n-butylcyanoacrylate (BCA) carried out at the interface of homogeneously distributed aqueous droplets in the inverse miniemulsion. From the performed investigations it was found that, the average capsule size and polydispersity are mainly determined by the type and concentration of the used surfactant(s) as well as by the viscosity of the continuous phase. The capsule morphology (single or multi core) can be controlled by varying the monomer amount (see Fig. 1). In the case of single core, the polymeric shell thickness from 15 up to 40 nm can be obtained. In order to form the polymeric shell with high molecular weight, the use of low surfactant concentrations and monomer amounts are of the best choice. After encapsulation, at least about 15% of the initially loaded dsDNA molecules, that do not change their structural integrity, could be recovered from the capsules.
Fig. 1. TEM images of PBCA capsules obtained with different amounts of the monomer: (from left to right) 70 μl; 100 μl; 200 μl.
(Clemens K. Weiss)
Enzymatic catalysis (biocatalysis) is a highly efficient strategy for the synthesis of various organic compounds. In their natural environment, water and ambient temperatures, enzymes usually exhibit their highest activity. Adapting this concept for organic synthesis, the application of enzymes as catalysts can access highly specific and selective synthetic pathways at mild conditions (ambient pressure and temperature) in water. Water does not necessarily have to be the solvent but can also act as continuous phase in a two-phase reaction system. Typical enzymes catalyzing reactions at an oil-water interface are lipases, which are capable of catalyzing hydrolysis, esterification (schematically shown in Figure 1), perhydrogenation and in some cases even amidation reactions.
Figure 1: Enzymatic esterification in miniemulsion. A: The hydrophobic substrates are located in droplets, dispersed in water. The enzyme catalyzes the esterification at the interface between water and the hydrophobic substrates. B: As the generated water is expelled from the hydrophobic droplet, it is not available for hydrolysis, favoring ester formation in high yields.
Thus, systems with large interfacial area are most suitable for efficient lipase catalysis. Here, miniemulsions are the system of choice. Miniemulsions can easily be prepared from a great variety of lipase suitable hydrophobic substrates, which form the droplet phase. Having usually a size below 300 nm, such heterophase systems provide nearly 1000 times the interfacial area compared to conventional emulsions with droplet sizes of several 10s of μm, significantly accelerating the reaction (Figure 2).
Figure 2: Comparison of lipase PS catalyzed esterifications of 3-phenyl propanol with nonanoic acid in miniemulsion and emulsion. [E. M. Aschenbrenner et. al., Chem. Eur. J., 15, 2434-2444 (2009)]
Additionally, if the substrates are hydrophobic enough, the droplets remain stable during the course of the reaction and far beyond this time, providing constant reaction (micro)environment for efficient enzymatic catalysis until maximum yield is obtained. Such systems were obtained with linear carboxylic acids with more than 7 carbon atoms and ω-phenyl alkanoles (C1-C5) or medium chain linear alcohols (e.g. hexanol). With more hydrophilic substrates, e.g. hexanoic acid or ethanol, the stability of the miniemulsions is lower. Nevertheless, using a suitable lipase, significant ester yields (>50%) were obtained. As a further benefit, enzymes show distinct selectivity towards specific substrates (Figure 3), and interestingly, the selectivity of enzymes can be altered by conducting reactions in miniemulsion. This was observed for e.g. cutinase, which usually preferably catalyzes reactions of short chain substrates (< C4). In miniemulsion this selectivity is altered towards the catalysis of long chain substrates (C10 and more).
Figure 3: left: conversion of lipase PS catalyzed esterification of 3-phenyl propanol with carboxylic acids of various chain lengths, right: enzymatic activities for these reactions. The activities as well as the conversion curve indicate the clear preference for nonanoic acid. [E. M. Aschenbrenner et. al., Chem. Eur. J., 15, 2434-2444 (2009)]
Porous oxide nanoparticles
(Clemens K. Weiss)
Gas storage, sorption, separation, or catalysis are only few examples of research areas where large specific surfaces of metal oxides are crucial for high performance applications. External surfaces can be created by finely dividing macroscopic matter to nanosized entities. Even higher surface areas can be created by giving matter an inner, porous structure. Pore-size tailoring further enhances the specificity and performance of such porous nanoparticles. A well known way for the generation of pores in the nanometer range is the templating with lyotropic phases. The metal oxide is generated during a sol-gel process in the presence of structure inducing surfactant. Confining these processes to a droplet of < 1 μm leads to the formation of metal oxide nanoparticles with an inner, (ordered) porous structure. This can be realized by creating a miniemulsion of an aqueous solution of water soluble precursors for metal oxide formation and, if desired, a structure inducing surfactant (e.g. CTAB, Pluronic P123), finely dispersed in an organic solvent. A non-ionic block-copolymer is required for droplet stabilization. After particle formation, an optional calcination step can be performed in order to remove residual organic material. Final particle characteristics as their morphology, internal structure, crystallinity and specific surface are delicately dependent on the choice and amount of the precursor, the reaction parameters, as temperature, pH, amount of surfactant, etc. This gives a toolbox for fine tuning the particles for the desired application. Silica particles, for example, evolve from solid to porous particles (Figure 4, left) and finally to hollow, porous capsules or structured porous grids according to the chosen reaction parameters. The application of the water soluble precursor bis(2-hydroxyethyl)titanate leads to aggregates of nanocrystalline titania (Figure 4, middle). While the reaction temperature determines the titania phase (anatase or rutile), the specific surface can be controlled by the amount of surfactant added for droplet stabilization. These anatase nanoparticles have successfully been used as anode materials for Li-ion batteries and as support materials for gold nanoparticles applied for CO-oxidation reactions. The combination of different water soluble precursors of e.g. titania and zirconia in one miniemulsion droplet yields mixed oxide particles (Figure 4, right). Furthermore, the inverse miniemulsion technique allows the use of ionic precursors as e.g Ce(NO3)3 for the preparation of further oxide nanoparticles. These examples demonstrate the potential of this approach. Virtually any water soluble precursor, suitable for sol-gel condensation, ionic or molecular, can be utilized, giving opportunity for the synthesis of a great number of (ordered) porous nanoparticles.
Figure 4: Porous particles prepared with the inverse miniemulsion technique. Left: porous silica particles. Middle: nanocrystalline anatase forming porous aggregates. Right: amorphous Zr/ TiO2 mixed oxide nanoparticles.
(Katharina Landfester, Clemens K. Weiss)
Among the many phenomena revealed and provided by nanoscience, the functionalization of a surface by nanopatterning certainly holds promise for a large number of attractive applications. A prominent example is the deposition of self-assembled monolayers by e.g. microcontact printing in order to control wettability, adhesion, friction, and wear. While for the manipulation of the wettability of a surface, highly ordered arrays of nanopillars are apparently not necessary, in case of field emitters a high degree of translational order of the nanopattern is extremely helpful in identifying individual emitters when applying scanning tunneling microscopy or spectroscopy for their characterization. The micellar approach for creating highly structured surfaces exhibits some inherent drawbacks, the maximum interparticle distance as given by the diameter of the micelles is limited to approximately 150 nm since for larger block-lengths of the involved polymer chains a spherical shape of the resulting micelles is no longer guaranteed. Larger distances between individual nanostructures like pillars are, however, desirable for many applications. Another characteristic difficulty with the micellar approach is related to the preparation of alloy particles, which is possible only for a few systems due to the problem of selectively ligating different alloy components within the micellar core. To overcome these drawbacks, as alternative approach a novel technique was introduced based on aqueous miniemulsions loaded with a metal precursor complex. After polymerization, metal complex containing latexes with uniform size distributions are obtained. They, like in the micellar approach, will self-assemble into hexagonally ordered arrays after deposition onto a substrate. Several hydrophobic complexes of a wide variety of metals (Pt, Fe, Zn, Cr, Ni,…) can be incorporated in defined quantities in polymeric nanoparticles. For some Pt complexes adapted emulsion polymerization techniques can be applied for further improvement of the particle size distribution.
By choosing different amounts of surfactant, the particle size can be adjusted between 100 and 260 nm. To obtain larger particles up to 500 nm for a given metal complex, an additional seeded emulsion polymerization step can be added. In this way, the particles can be easily modified by changing the polymer or the copolymer composition. Using a pure polymer core and adding semicontinuously a complex/monomer mixture leads to particles increased in size which can contain additional metal complexes. The methodology was also applied for the preparation of alloyed Fe/Pt particles. The concentration relations were analyzed by inductively coupled plasma spectroscopy (ICP-OES) and, by energy dispersive X-ray spectroscopy (EDX). Deposited onto a substrate, the high homogeneity of the particle size results in hexagonally well ordered monolayers of the metal containing polymer particles (see Figure 5).
Figure 5: Metal-containing polymeric nanoparticles before (left) and after (right) plasma etching. [Schreiber et. al., Chem. Mater., 21, 1750-1760 (2009)]
Once these nanoparticles are obtained, they can be used as nanomasks for subsequent etching steps (see Figure 15) according to the recently developed recipes to prepare hexagonally ordered arrays of nanopillars or cylindrical nanoholes in Si. It is the aim of the present work to demonstrate the potential of the miniemulsions by preparing highly ordered arrays of nanopillars and nanoholes in Si with distances between these structures well above those accessible to the micellar technique. On the other hand, the previously developed recipe to fabricate masks made of amorphous Si with hexagonally arranged nanoholes can immediately be applied to the miniemulsion technique thereby opening the possibility to transfer the hole pattern into any underlying substrate.
(Clemens K. Weiss)
Lanthanide compounds as complexes, clusters or sold state materials have unique optical or magnetic properties. Some of them are
difficult to process or sensitive towards the environment, thus their high potential cannot be easily exploited for any application.
Incorporation of such compounds in polymeric nanoparticles can shield and protect them from environmental influence and, in the form
of aqueous dispersions the hybrid nanoparticles can be very easily processed and handled.
The miniemulsion technique is a very convenient method for the preparation of polymeric nanoparticles and simultaneous encapsulation of various compounds. Taking advantage of this method it was possible to prepare hybrid particles from several hydrophobic lanthanide complexes (e.g. Ln(tmhd)3), multinuclear clusters (e.g. [H5[Nd5O5(Ph2acac)10]]) and even micro- and nanocrystalline solid state materials (Ce:YAG). Basically, the sizes of the particles can be adjusted from 70 – 250 nm. The polymeric matrix can be prepared from a wide variety of monomers or monomer mixtures (e.g. styrene, acrylates or copolymers) suitable for the desired application.
The nature of the incorporated material and the polymer determine the internal structure of the particles and their properties. With several ß-diketonato-lanthanide complexes, internal onionlike or pillarlike layered structures of the inorganic component and the polymeric matrix could be observed. Without the possibility for further coordination, the complex will be dispersed throughout the polymeric matrix. Here, detailed studies were performed on the encapsulation of several hydrophobic multinuclear rare earth clusters. Up to 10 wt% of the complexes could be embedded in polystyrene or poly(butylacrylate-co-methylmethacrylate) (PBA-co-PMMA) (Figure 6). Aqueous dispersions of polystyrene based hybrid nanoparticles as well as dispersions and thin films prepared from PBA-co-PMMA based nanoparticles exhibited similar optical properties as the pure complexes.
Figure 6: Luminescence spectrum and TEM micrograph of H5[Nd5O5(Ph2acac)10]/polystyrene hybrid nanoparticles.