Johannes C. Brendel

Solid-State Dye-Sensitized Solar Cells Using Red and Near-IR Absorbing Bodipy Sensitizers

Boron-dipyrrin dyes, through rational design, yield promising new materials. With strong electron-donor functionalities and anchoring groups for attachment to nanocrystalline TiO(2), these dyes proved useful as sensitizers in dye-sensitized solar cells. Their applicability in a solid-state electrolyte regime offers additional opportunities for practical applications.

A High Transconductance Accumulation Mode Electrochemical Transistor

An organic electrochemical transistor operates in accumulation mode with high transconductance. The channel comprises a thiophene-based conjugated polyelectrolyte, which is p-type doped by anions injected from a liquid electrolyte upon the application of a gate voltage. The use of ethylene glycol as a co-solvent dramatically improves the transconductance and the temporal response of the transistors.

Semiconductor amphiphilic block copolymers for hybrid donor–acceptor nanocomposites

Block copolymers feature unique properties for organizing in a well-defined pattern on length scales of several tenths of nanometers. This special attribute enables the formation of ideal donor and acceptor domains for photovoltaic devices in the size of the exciton diffusion length. Thus we designed an amphiphilic block copolymer, able to act as a hole conductor and to coordinate inorganic semiconductor nanoparticles as electron acceptors. Utilizing controlled radical polymerization techniques, defined polymers were synthesized consisting of triphenylamine pendant groups in the hole conductor block and a hydrophilic polystyrene sulfonate block. This particular combination creates narrowly distributed micelles in aqueous solution exhibiting domain sizes suitable for photovoltaic applications. The strong anionic sulfonate groups offer high loading capacities for modified cationic nanoparticles. To guarantee a broad absorption and good conductivity, we synthesized cationic CdSe nanorods and combined them with our hole conductor micelles. The advantage of high loading combined with the processability from aqueous dispersions promises a novel “green” alternative for preparation of hybrid solar cells with controlled domain sizes in the desired length scale.

Efficient click-addition sequence for polymer–polymer couplings

Controlled radical polymerization methods and click chemistry form a versatile toolbox for creating complex polymer architectures. However, the incompatibility between the functional groups required for click reactions and the reaction conditions of radical polymerization techniques often limits application. Here, we demonstrate how combining two complementary click reactions in a sequence circumvents compatibility issues. We employ isocyanate-amine addition on a polymer obtained by RAFT without purification, thus allowing us to work at exact equimolarity. The addition of commercially available amine-functional azido or strained alkyne compounds, yields orthogonally modified polymers, which can be coupled together in a subsequent strain promoted cycloaddition (SPAAC). The efficiency of this reaction sequence is demonstrated with different acrylate, methacrylate, and acrylamide polymers giving block copolymers in high yield. The resulting diblock copolymers remain active towards RAFT polymerization, thus allowing access to multiblock structures by simple chain extension. The orthogonality of the isocyanate-amine reaction, SPAAC and RAFT polymerization (both in terms of monomer and chain end groups) is a key advantage and offers access to functional and challenging polymer architectures without the need for stringent reaction conditions or laborious intermediate purifications.

Polymer templated nanocrystalline titania network for solid state dye sensitized solar cells

We report a novel preparation method for nanocrystalline TiO2 networks with controlled pore sizes using spherical polyelectrolyte brushes (SPB) as templates. The SPB consists of a solid polystyrene core from which anionic polyelectrolytes are densely grafted. The SPB templates are synthesized via conventional photoemulsion polymerization with efficient control of core size and brush length. Subsequently, the TiO2 precursor is hydrolyzed at room temperature within the anionic brush to obtain anatase nanocrystals of 12–20 nm size. These stable and form-persistent composite particles of SPB decorated with anatase nanocrystals are then assembled on a conductive substrate. The subsequent calcination of this composite layer leads to a robust nanocrystalline TiO2 network, in which the pores and the wall thickness are directly correlated to the polystyrene core size and the amount of TiO2 hydrolyzed within the brush respectively. In this study, we optimized different thin-film preparation methods and characterized the resulting nanocrystalline TiO2 networks using SEM and XRD. Moreover, the applicability of these nanocrystalline networks as electron transport layers are tested in solid-state dye-sensitized solar cells (SDSCs). The first test devices exhibited efficiencies up to 0.8%. The precise and individual control of parameters such as porosity, thickness and crystallinity makes this concept highly attractive for the realization of efficient solid-state hybrid devices.

Cyclic peptide–polymer nanotubes as efficient and highly potent drug delivery systems for organometallic anticancer complexes

Functional drug carrier systems have potential for increasing solubility and potency of drugs while reducing side effects. Complex polymeric materials, particularly anisotropic structures, are especially attractive due to their long circulation times. Here, we have conjugated cyclic peptides to the biocompatible polymer poly(2-hydroxypropyl methacrylamide) (pHPMA). The resulting conjugates were functionalized with organoiridium anticancer complexes. Small angle neutron scattering and static light scattering confirmed their self-assembly and elongated cylindrical shape. Drug-loaded nanotubes exhibited more potent antiproliferative activity toward human cancer cells than either free drug or the drug-loaded polymers, while the nanotubes themselves were nontoxic. Cellular accumulation studies revealed that the increased potency of the conjugate appears to be related to a more efficient mode of action rather than a higher cellular accumulation of iridium.

Oxidation-responsive micelles by a one-pot polymerization-induced self-assembly approach

The increased levels of reactive oxygen species (ROS) such as hydrogen peroxide in inflamed or cancerous tissue represent a promising trigger for the local and selective release of drugs at the affected areas. Despite new developments in the field of oxidation-responsive drug carrier systems, the preparation of the required materials remains in most cases tedious. Here, we present a novel system, which combines the advantages of a one-pot sequential controlled radical polymerization with the direct polymerization-induced self-assembly (PISA) process. By utilizing highly reactive acrylamide monomers, full conversion can be reached while maintaining a high chain end fidelity in RAFT polymerization, which enables the precise preparation of block copolymers or micelles, respectively, without intermediate purification steps. We demonstrate that the cyclic thioether N-acryloyl thiomorpholine is a versatile monomer for PISA resulting in a hydrophobic block, which upon oxidation can be transformed into a highly water-soluble sulfoxide. The micellar structures are tunable in size by the variation of the block length and feature a good sensitivity towards hydrogen peroxide even at low concentrations of 10 mM resulting in their disintegration. In vitro studies prove the uptake of these micelles into cells without signs of toxicity up to 500 μg mL−1. The straightforward preparation, the excellent biocompatibility and the selective disintegration in the presence of biologically relevant levels of hydrogen peroxide are features that certainly make the presented system an attractive new material for oxidation-responsive drug carriers.

SuFEx – a selectively triggered chemistry for fast, efficient and equimolar polymer–polymer coupling reactions

The synergy between controlled radical polymerization methods and click chemistry enables the design of complex and well-defined materials. To date, a number of highly efficient reactions have been reported to be suitable to couple polymers in equimolar amounts within minutes, although such reactions typically require high reactivity, and the active groups are not fully compatible under the conditions applied in radical polymerization, necessitating additional modification steps. Here, we demonstrate how the Sulfur(VI) Fluoride Exchange (SuFEx) reaction proves to be an efficient coupling reaction whilst avoiding the traditional issues encountered in other ligation reactions. Two chain transfer agents (CTAs) for RAFT polymerizations were created, which bear the required orthogonal groups for the SuFEx reaction. Both CTAs yield well-defined polymers, with no observable side reactions, even when both CTAs are utilised in the same polymerization. The choice of the catalyst for this click reaction is an important consideration in order to allow efficient polymer coupling using equimolar amounts. Besides the previously reported strong bases, we discovered that the salt tetrabutylammonium fluoride (TBAF) is an excellent catalyst yielding almost quantitative conversions within minutes for different polymers and solvents. This combination of orthogonality towards radical processes and high reactivity is unprecedented in the literature.

Beyond Gene Transfection with Methacrylate-Based Polyplexes—The Influence of the Amino Substitution Pattern

Methacrylate-based polymers represent promising nonviral gene delivery vectors, since they offer a large variety of polymer architectures and functionalities, which are beneficial for specific demands in gene delivery. In combination with controlled radical polymerization techniques, such as the reversible addition-fragmentation chain transfer polymerization, the synthesis of well-defined polymers is possible. In this study we prepared a library of defined linear polymers based on (2-aminoethyl)-methacrylate (AEMA), N-methyl-(2-aminoethyl)-methacrylate (MAEMA), and N,N-dimethyl-(2-aminoethyl)-methacrylate (DMAEMA) monomers, bearing pendant primary, secondary, and tertiary amino groups, and investigated the influence of the substitution pattern on their gene delivery capability. The polymers and the corresponding plasmid DNA complexes were investigated regarding their physicochemical characteristics, cytocompatibility, and transfection performance. The nonviral transfection by methacrylate-based polyplexes differs significantly from poly(ethylene imine)-based polyplexes, as a successful transfection is not affected by the buffer capacity. We observed that polyplexes containing a high content of primary amino groups (AEMA) offered the highest transfection efficiency, whereas polyplexes bearing tertiary amino groups (DMAEMA) exhibited the lowest transfection efficiency. Further insights into the uptake and release mechanisms could be identified by fluorescence and transmission electron microscopy, emphasizing the theory of membrane-pore formation for the time-efficient endosomal release of methacrylate-based vectors.

How To Tune the Gene Delivery and Biocompatibility of Poly(2-(4-aminobutyl)-2-oxazoline) by Self- and Coassembly

Despite their promising potential in gene transfection, the toxicity and limited efficiency of cationic polymers as nonviral vectors are major obstacles for their broader application. The large amount of cationic charges, for example, in poly(ethylene imine) (PEI) is known to be advantageous in terms of their transfection efficiency but goes hand-in-hand with a high toxicity. Consequently, an efficient shielding of the charges is required to minimize toxic effects. In this study, we use a simple mixed-micelle approach to optimize the required charge density for efficient DNA complex formation and to minimize toxicity by using a biocompatible polymer. In detail, we coassembled mixed poly(2-oxazoline) nanostructures ( d ≈ 100 nm) consisting of a hydrophobic-cationic block copolymer (P(NonOx52- b-AmOx184)) and a hydrophobic-hydrophilic stealth block copolymer (P(EtOx155- b-NonOx76) in ratios of 0, 20, 40, 60, 80, and 100 wt % P(NonOx52- b-AmOx184). All micelles with cationic polymers exhibited a very good DNA binding efficiency and dissociation ability, while the bio- and hemocompatibility improved with increasing EtOx content. Analytics via confocal laser scanning microscopy and flow cytometry showed an enhanced cellular uptake, transfection ability, and biocompatibility of all prepared micelleplexes compared to AmOx homopolymers. Micelleplexes with 80 or 100 wt % revealed a similar transfection efficiency as PEI, while the cell viability was significantly higher (80 to 90% compared to 60% for PEI).