Clemens K. Weiss

High surface area crystalline titanium dioxide: potential and limits in electrochemical energy storage and catalysis

Titanium dioxide is one of the most intensely studied oxides due to its interesting electrochemical and photocatalytic properties and it is widely applied, for example in photocatalysis, electrochemical energy storage, in white pigments, as support in catalysis, etc. Common synthesis methods of titanium dioxide typically require a high temperature step to crystallize the amorphous material into one of the polymorphs of titania, e.g. anatase, brookite and rutile, thus resulting in larger particles and mostly non-porous materials. Only recently, low temperature solution-based protocols gave access to crystalline titania with higher degree of control over the formed polymorph and its intra- or interparticle porosity. The present work critically reviews the formation of crystalline nanoscale titania particles via solution-based approaches without thermal treatment, with special focus on the resulting polymorphs, crystal morphology, surface area, and particle dimensions. Special emphasis is given to sol-gel processes via glycolated precursor molecules as well as the miniemulsion technique. The functional properties of these materials and the differences to chemically identical, non-porous materials are illustrated using heterogeneous catalysis and electrochemical energy storage (battery materials) as example.

From soft to hard: the generation of functional and complex colloidal monolayers for nanolithography

Take polymer colloids—simple to make and ubiquitously available—let them self-assemble into a monolayer and you have, readily engineered, a sophisticated mask to create highly symmetric surface patterns with nanometre precision. Thirty years after its invention, this process, commonly referred to as colloidal lithography, is far from being old news and an ever-increasing range of scientists continues to use their creativity to come up with increasingly more complex surface patterns. As intriguing this technique is, the devil is in the details and it is far from trivial to achieve high order in a colloidal monolayer. This article reviews available crystallization techniques and discusses their scope and limitations in order to provide just the right method for your next experiment.

Miniemulsion Polymerization as a Means to Encapsulate Organic and Inorganic Materials

The miniemulsion technique greatly enhances the possibilities for the preparation of hybrid nanomaterials by encapsulating molecular compounds, liquids, or solid material. Using this technique, a wide variety of novel functional nanocomposites can be generated that are not accessible with other techniques. After briefly introducing miniemulsions and the miniemulsion polymerization techniques for the preparation of polymeric nanoparticles, this review focuses on the preparation of functional nanostructures by encapsulation of organic or inorganic material in polymeric matrices. The examples presented highlight the possibility to either protect the encapsulated material (e.g., dyes, drugs, magnetite, or DNA) and create completely new properties that emerge in a synergistic manner from the components of the nanocomposites, or to perform reactions in polymer-enclosed vessels of submicrometer size.

TiO2 Anatase Nanoparticle Networks: Synthesis, Structure, and Electrochemical Performance

Nanocrystalline anatase TiO(2) materials with different specific surface areas and pore size distributions are prepared via sol-gel and miniemulsion routes in the presence of surfactants. The samples are characterized by X-ray diffraction, nitrogen sorption, transmission electron microscopy, and electrochemical measurements. The materials show a pure anatase phase with average crystallite size of about 10 nm. The nitrogen sorption analysis reveals specific surface areas ranging from 25 to 150 m(2) g(-1) . It is demonstrated that the electrochemical performance of this material strongly depends on morphology. The mesoporous TiO(2) samples exhibit excellent high rate capabilities and good cycling stability.

Encapsulation by Miniemulsion Polymerization

The miniemulsion technique offers the possibility for the encapsulation of different materials, ranging from liquid to solid, from organic to inorganic, and from molecularly dissolved to aggregated species into polymeric nanoparticles or nanocapsules. Using this technique, a wide variety of novel functional nanomaterials can be generated. This review focuses on the preparation of functional nanostructures by encapsulating organic or inorganic material in polymeric nanoparticles. The examples demonstrate the possibilities to protect the encapsulated material as dyes, pigments, fragrances, photo-initiators, drugs, magnetite, or even DNA, use them as marker systems (dyes, magnetite), or create nanoparticles with completely new properties.

Ordered Arrays of Gold Nanostructures from Interfacially Assembled Au@PNIPAM Hybrid Nanoparticles

In this Article, we report on the assembly of hybrid Au@PNIPAM core-shell particles at the air/water interface, their transfer onto solid substrates, and the controlled combustion of the organic material to produce arrays of gold nanoparticles. A detailed investigation on the assembly behavior of such soft hybrid colloids at the air/water interface was performed by correlating the surface pressure-area isotherms with SEM and AFM images from samples transferred at different surface pressures. The hybrid particles display a complex behavior at the interface, and we could distinguish three distinct phases with varying interparticle spacings at different compression. The transfer process presented enables the decoration of topologically structured substrates with gold nanoparticle arrays, and the order of the initial monolayers is retained in the arrays of inorganic gold nanoparticles. The change in monolayer morphology upon compression can therefore be used to tailor the interparticle distance between approximately 650 and 300 nm without exchanging the colloids. More sophisticated gold nanostructures can be patterned into symmetric arrays using a similar protocol, which we demonstrate for nanostars and nanorods.

The first step into the brain: uptake of NIO-PBCA nanoparticles by endothelial cells in vitro and in vivo, and direct evidence for their blood-brain barrier permeation.

By using fluorescent polysorbate 80 coated poly(n‐butylcyanoacrylate) (PBCA) nanoparticles in an in vivo study, direct evidence was found for the presence of nanoparticles entering the brain and retina of rats. The nanoparticles, prepared with a miniemulsion process, were labeled in situ with a fluorescent dye and coated with polysorbate 80. After preparation the particle size, ζ potential, and the molecular weight distribution were determined. BMEC cells were used as an in vitro model for the BBB. The cells showed significant uptake of the particles, but no transcytosis could be observed in vitro. After applying the particles to the animals at two concentrations, cryosections of the brains and retinas were prepared. Regarding the sections of the rats that received the lower dose, co‐localization of the applied fluorescent particles and the stained endothelial cells could be detected in the brain and retina, indicating particle internalization in the endothelial cells. Applied at higher doses, the particles could be detected within the brain and retina with few co‐localized signals, suggesting passage through the blood–brain and blood–retina barriers.

The Softer and More Hydrophobic the Better: Influence of the Side Chain of Polymethacrylate Nanoparticles for Cellular Uptake

Intracellular uptake of nanoparticles is highly interesting for labeling of cells, drug delivery, or non-viral gene delivery. In this study we have synthesized a wide variety of poly(alkyl methacrylate) nanoparticles with the same size and investigated their uptake into cells. The nanoparticles were prepared from alkylmethacrylates with different linear and branched ester chains as well as from benzylmethacrylate using the miniemulsion polymerizaiton technique. By adding a fluorescent dye as a marker, the internalization of the nanoparticles could be investigated quantitatively with flow cytometry and qualitatively with confocal laser scanning microscopy. With increasing side chain of the ester and therefore increasing hydrophobicity and at glass transition temperature (T(g)), below the incubation temperature of 37 degrees C the uptake of the nanoparticles into cells is favored.

Enzymatically degradable nanogels by inverse miniemulsion copolymerization of acrylamide with dextran methacrylates as crosslinkers

Nanogels consisting of polyacrylamide (PAAm), crosslinked with dextran methacrylate (Dex-MA), were designed to be partially biodegradable by enzymatic cleavage of the methacryl-functionalized polysaccharide chains. Important properties of the described hydrogels such as a low initial degree of swelling and a high crosslinking efficiency—in order to ensure the morphological integrity during nanogel preparation, purification and storage—are highly favored. Primarily, those requirements strongly depend on the amount of crosslinking points, i.e.methacrylate groups in the network. This parameter was adjusted by either increasing the ratio of Dex-MA/AAm for a fixed degree of substitution (DS) of Dex-MA or increasing the DS of Dex-MA for a fixed ratio of Dex-MA/AAm. Secondly, the distribution of crosslinking points in the network is another crucial parameter which was optimized by lowering the molecular weight of the Dex-MA for analogous DS or by simultaneously increasing the Dex-MA/AAm ratio and decreasing the respective DS. This resulted in nanogels with a reduced initial degree of swelling and sol content, therefore indicating a more homogeneous distribution of the same amount of crosslinking points. Degradation of the hybrid gel nanoparticles was examined by turbidity and DLS measurements upon treatment with dextranase. It was found that the degradation behavior depends on the total amount of methacrylate groups in the network and the degree of substitution of the individual Dex-MA chains.

Phase stability and photocatalytic activity of Zr-doped anatase synthesized in miniemulsion

A series of mesoporous anatase-type TiO(2) doped with zirconium (0-50 mol% Zr) was synthesized by combining the sol-gel process with the inverse miniemulsion technique. Nanoparticles between 100 and 300 nm were directly prepared from acidic precursor solutions of titanium glycolate (EGMT) and zirconium isopropoxide. The miniemulsion technique is a simple and convenient method to synthesize nanoparticles of homogeneous size because the reactions (here hydrolysis and condensation) take place in the confined space of nanodroplets (several hundreds of nanometres) and therefore in a highly controlled manner. For low doping levels (0-7.1 mol% Zr), Zr(x)Ti(1 - x)O(2) solid solutions were formed where Zr was uniformly dispersed into the anatase framework. For higher amounts of zirconium (Zr >or= 7.1 mol%), the crystallization of zirconium titanate (ZrTiO(4)) occurred at a low temperature of 650 degrees C and it was obtained as a pure material for 47.4 mol% <or= Zr <or= 50 mol%. The influence of the amount of zirconium on the crystallinity, crystallite size, phase composition and stability, morphology and specific surface area was investigated. For the characterization transmission electron microscopy (TEM), x-ray diffraction (XRD), nitrogen sorption (BET) and inductively coupled plasma-optical emission spectrometry (ICP-OES) were used. The photocatalytic activity of the crystalline mixed oxides (0-9.4 mol% Zr) was examined for the degradation of methylene blue under UV irradiation.