Stephanie Hoeppener

Fabrication of patterned silane based self-assembled monolayers by photolithography and surface reactions on silicon-oxide substrates

Self-assembled monolayers (SAMs) have received increasing attention since their introduction 30 years ago. Soon it was discovered that they can be used as alternative resist materials and are compatible with different established lithographic techniques commonly used in silicon semiconductor technology. Besides these possibilities to structure SAMs, other attractive properties emerged from the use of SAMs. E.g., the introduction of addressability into the patterns by selective functionalization with reactive precursor molecules and/or by applying suitable surface reactions was established. In this feature we highlight developments of photolithographic techniques that have been used in combination with SAMs serving either as resists for the patterning process or as precursor molecules for surface reactions, which can be performed on non-structured and mainly photochemically structured surfaces to obtain multifunctional surfaces with tunable surface properties. The aim is to provide an overview about the versatile possibilities to use silane based SAM systems to structure silicon-oxide substrates by introducing topographical as well as chemically heterogeneous surface structures. In particular the chemical activation of SAMs includes a large number of functionalization concepts which are intended to be summarized in this review. They will be introduced here according to the class of chemical reaction that has been used. Therefore, an introduction into the plethora of possible structures, which have been created by the combination of photolithographic structuring approaches, and the integration of tailor made surface functionalities into these systems will be highlighted. Additionally effective strategies to implement a diversity of chemical functionalities onto one substrate are summarized.

Synthesis and Modification of Carbon Nanomaterials utilizing Microwave Heating

Microwave-assisted synthesis and processing represents a growing field in materials research and successfully entered the field of carbon nanomaterials during the last decade. Due to the strong interaction of carbon materials with microwave radiation, fast heating rates and localized heating can be achieved. These features enable the acceleration of reaction processes, as well as the formation of nanostructures with special morphologies. A comprehensive overview is provided here on the possibilities and achievements in the field of carbon-nanomaterial research when using microwave-based heating approaches. This includes the synthesis and processing of carbon nanotubes and fibers, graphene materials, carbon nanoparticles, and capsules, as well as porous carbon materials. Additionally, the principles of microwave-heating, in particular of carbon materials, are introduced and important issues, i.e., safety and reproducibility, are discussed.

Tuning solution polymer properties by binary water–ethanol solvent mixtures

The solubility of polymers can be significantly altered by the use of solvent mixtures. The solvent composition also effects the self-assembly properties of amphiphilic copolymers. In addition, water-ethanol mixtures are known to exhibit abnormal physicochemical properties due to the presence of hydration shells around the ethanol molecules, while at the same time both solvents have very low toxicity. However, the solution properties of amphiphilic copolymers in water-ethanol mixtures have been scarcely studied. Here we show that the solution polymer properties of amphiphilic copoly(2-oxazoline)s can be significantly altered in binary water-ethanol mixtures resulting in increased solubility, tuneable lower critical solution temperatures as well as polymer-solvent combinations with both a LCST followed by an UCST and improved dispersion stability. Surprisingly, it was found that polymers insoluble in both ethanol and water could be dissolved in water-ethanol mixtures, opening the way to novel formulations for drug delivery or personal care applications. Our results represent a straightforward method for tuning solution polymer properties without the synthetic efforts that are generally required to change the copolymer composition and properties.

Terpyridine‐Functionalized Surfaces: Redox‐Active, Switchable, and Electroactive Nanoarchitecturesgland

Terpyridines represent versatile functional supramolecular building blocks that are easily integrated in numerous devices and can readily modify surfaces. In particular, redox-active complexes with terpyridine ligands have been attached to surfaces, either by covalent or non-covalent interactions, and form highly ordered mono- or multilayer systems, since electronic and charge transport properties are major topics of interest. Their applications in nanoelectronics are a driving force for understanding and enabling the utilization of the supramolecular properties of terpyridines for surface modification. This area of research has received increasing attention during the last decade leading into the supramacromolecular regime. This Progress Report presents an overview of the state-of-the-art of surface modifications utilizing terpyridine systems and highlights main results, as well as modern trends, in this research area.

Self-healing metallopolymers based on cadmium bis(terpyridine) complex containing polymer networks

The utilization of metal–ligand interactions within polymers generates materials which are of interest for several applications, including self-healing materials. In this work we use methacrylate copolymers containing terpyridine moieties in the side chain for the formation of self-healing metallopolymer networks. The materials were synthesized using the reversible addition–fragmentation chain transfer (RAFT) polymerization technique and subsequent crosslinking by the addition of a metal salt, here cadmium(II) salts, with different counter-ions. The influence of the counter-ions on the self-healing process within these structures was analyzed. The research resulted in a new polymeric material featuring a high (intrinsic) healing efficiency at relatively low temperatures (<75 °C).

A schizophrenic gradient copolymer: switching and reversing poly(2-oxazoline) micelles based on UCST and subtle solvent changes

The self-assembly of hydrophobic gradient copolymers of 2-nonyl-2-oxazoline and 2-phenyl-2-oxazoline in ethanol-water solvent mixtures is demonstrated to result in structures that respond to changes in temperature as well as subtle changes in the solvent composition leading to both switching and reversing of the formed micelles.

Solubility and Thermoresponsiveness of PMMA in Alcohol-Water Solvent Mixtures

To reduce the environmental burden of polymer processing, the use of non-toxic solvents is desirable. In this regard, the improved solubility of poly(methyl methacrylate) (PMMA) in ethanol/water solvent mixtures is very appealing. In this contribution, detailed investigations on the solubility of PMMA in alcohol/water solvent mixtures are reported based on turbidimetry measurements. PMMA revealed upper critical solution temperature transitions in pure ethanol and ethanol/water mixtures. However, around 80 wt-% ethanol content a solubility maximum was observed for PMMA as indicated by a decrease in the transition temperature. Moreover, the transition temperatures increased with increasing PMMA molar mass as well as increasing polymer concentration. Careful analysis of both heating and cooling turbidity curves revealed a peculiar hysteresis behaviour with a higher precipitation temperature compared with dissolution with less than 60 wt-% or more than 90 wt-% ethanol in water and a reverse hysteresis behaviour at intermediate ethanol fractions. Finally, the transfer of poly(styrene)-block-poly(methyl methacrylate) block copolymer micelles from the optimal solvent, i.e. aqueous 80 wt-% ethanol, to almost pure water and ethanol is demonstrated.

Reversible supramolecular functionalization of surfaces: terpyridine ligands as versatile building blocks for noncovalent architectures.

We report on the reversible and selective functionalization of surfaces by utilizing supramolecular building blocks. The reversible formation of terpyridine bis-complexes, based on a terpyridine ligand-functionalized monolayer, is used as a versatile supramolecular binding motif. Thereby, click chemistry was applied to covalently bind an acetylene functionalized Fe(II) bis-complex onto azide-terminated self-assembled monolayers. By decomplexation of the formed supramolecular complex, the ligand modified monolayer could be obtained. These monolayers were subsequently used for additional complexation reactions, resulting in the reversible functionalization of the substrates. The proper choice of the coordinating transition metal ions allows the tuning of the binding strength, as well as the physicochemical properties of the formed complexes and thus an engineering of the surface properties.

Self-Assembling Doxorubicin−Tocopherol Succinate Prodrug as a New Drug Delivery System: Synthesis, Characterization, and in Vitro and in Vivo Anticancer Activity

Self-assembled prodrugs forming nanoaggregates are a promising approach to enhance the antitumor efficacy and to reduce the toxicity of anticancer drugs. To achieve this goal, doxorubicin was chemically conjugated to d-α-tocopherol succinate through an amide bond to form N-doxorubicin-α-d-tocopherol succinate (N-DOX-TOS). The prodrug self-assembled in water into 250 nm nanostructures when stabilized with d-α-tocopherol poly(ethylene glycol) 2000 succinate. Cryo-TEM analysis revealed the formation of nanoparticles with a highly ordered lamellar inner structure. NMR spectra of the N-DOX-TOS nanoparticles indicated that N-DOX-TOS is located in the core of the nanoparticles while PEG chains and part of the tocopherol are in the corona. High drug loading (34% w/w) and low in vitro drug release were achieved. In vitro biological assessment showed significant anticancer activity and temperature-dependent cellular uptake of N-DOX-TOS nanoparticles. In vivo, these nanoparticles showed a greater antitumor efficacy than free DOX. N-DOX-TOS nanoparticles might have the potential to improve DOX-based chemotherapy.

Polyelectrolyte Complexes of DNA and Linear PEI: Formation, Composition and Properties

In the present study, the complexation between linear 13.4 kDa poly(ethylene imine) (LPEI) and plasmid DNA was investigated. Analytical ultracentrifugation (AUC) was used for size and molar mass determination. Additionally, the morphology was studied by scanning force microscopy. The polyplex formation was investigated in a wide range of PEI nitrogen to DNA phosphate ratios (N/P). At N/P ratios below 1, the PEI/DNA complex formation is characterized by an incomplete DNA condensation and the formation of the primary DNA/PEI complexes. The merging of the initially formed polyplexes occurs at N/P ~2, resulting in the formation of polyplexes with much larger size and high aggregation rate. Stable and uniform polyplexes were formed at N/P > 10, with average sizes of the polyplexes of about 170 ± 65 nm. The content of uncomplexed PEI chains in the polyplex dispersion was estimated at four different N/P ratios, 6.2, 11.6, 28.6, and 57.8, by combining preparative centrifugation with a copper complex assay and by sedimentation velocity analysis as an alternative method. It is demonstrated that virtually all added PEI binds to the DNA at N/P < 2.5; further addition of PEI results in the appearance of a large amount of free PEI in solution. Nevertheless, PEI is able to bind in the whole range of N/P ratios tested. According to the data collected by sedimentation velocity analysis and scanning force microscopy, the single PEI/DNA complexes are composed on average of 8 to 32 single condensed DNA plasmids and 70 ± 25 PEI molecules.