Patrick Theato

Stimuli responsive materials

Dramatic developments in the burgeoning field of polymer science are enabling new materials designs, synthetic methods, functional architectures, and applications. Today’s polymers are finding utility in broad areas, ranging from everyday commodity plastics to emerging, specialized, and high-tech materials. Moreover, it is apparent that the continued development of polymeric systems will be facilitated by ever-increasing understanding of advanced polymer synthesis and characterization techniques. This enhanced toolbox and knowledge-base will foster the facile design of next-generation precision materials with predictable and changeable properties. The present themed issue focuses on recent developments in the design of polymers that change properties in response to a single stimulus or multiple stimuli. These so-called ‘smart’ or stimuliresponsive polymers represent a growing cadre of materials that support various applications (e.g., controlled release agents, responsive coatings, and adaptive shape memory materials). Stimuli-responsive materials have benefited from significant advances in polymer science in recent years, and this themed issue highlights several of the fascinating developments that could have a major impact on the implementation of new smart materials. To fully address the field of stimuli responsive polymers, first we must understand the breadth of available stimuli that can induce a desired response, then we must design the polymer functionalities and systems that enable such a response; finally, we must develop methods to characterize the macromolecular changes as a result of that response. As demonstrated in this issue, many of the interesting properties of responsive materials arise from a transition in solubility or conformation of a macromolecule in the presence of a solvent. In this manner, transitions at the molecular level can be amplified to result in a change in nanoscale structure and/or materials properties. Gibson and O’Reilly (DOI: 10.1039/C3CS60035A) overview these transitions in the specific area of thermoresponsive polymers with particular attention to the effect of nanoscale geometry on the resulting change in chain conformation following a temperature change. Sumerlin and co-workers (DOI: 10.1039/ C3CS35499G) also highlight temperatureresponsive polymers with particular emphasis on design parameters that facilitate tuning of the specific transition temperatures. Light-responsive materials have received significant attention, as discussed by Gohy and Zhao (DOI: 10.1039/ C3CS35469E) in a review focused on reversible and irreversible transitions of photoresponsive copolymer micelles. Further, many systems can be designed a Institute for Technical and Macromolecular Chemistry, University of Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany. E-mail: theato@chemie.uni-hamburg.de b George & Josephine Butler Polymer Research Laboratory, Department of Chemistry, University of Florida, Gainesville, FL 32611, USA. E-mail: sumerlin@chem.ufl.edu c Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. E-mail: Rachel.OReilly@warwick.ac.uk d Department of Chemical & Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716, USA. E-mail: thepps@udel.edu

Au@MnO Nanoflowers: Hybrid Nanocomposites for Selective Dual Functionalization and Imaging

Recently, the development of hybrid nanostructures consisting of various materials has attracted considerable interest. The assembly of different nanomaterials with specific optical, magnetic, or electronic properties to multicomponent composites can change and even enhance the properties of the individual constituents. Specifically tuning the structure and interface interactions within the nanocomposites has resulted in novel platforms of materials that may lead the way to various future technologies, such as synchronous biolabeling, protein separation and detection, heterogeneous catalysis, and multimodal imaging in biomedicine. Of the various kinds of nanomaterials, gold nanorods show an unusually high polarizability at optical frequencies arising from the excitation of localized surface-plasmon resonances (LSPRs). Furthermore, gold nanorods have promising therapeutic properties as hyperthermal agents because the local temperature around the gold nanorods can be increased by laser illumination through the tunable surface plasmon bands in the near infrared (NIR) region. Using NIR radiation for hyperthermal therapy is beneficial because of the low absorption and low scattering by blood and tissue in this spectral range. Magnetic nanoparticles constitute another major class of nanomaterials that have attracted much research effort over the past decades. In particular, exchange-coupled magnetic nanocomposites, such as antiferromagnetic/ferromagnetic core–shell nanoparticles, such as MnO/Mn3O4, have magnetic properties that are quite different from those of the individual components. Concerning biomedical applications, superparamagnetic nanoparticles are attractive as contrast agents for magnetic resonance imaging (MRI). The majority of nanoparticles that have been investigated in this field comprise iron oxides (Fe3O4, g-Fe2O3), which are known to shorten the transverse (or spin–spin) relaxation time T2. [11] Recently, manganese oxide nanoparticles (MnO NPs) have been shown to be interesting candidates as contrast agents for shortening of the longitudinal (or spin-lattice) relaxation time T1. [12] Consequently, a nanoparticulate system containing both an optically active plasmonic gold unit and a magnetically active MnO component would be advantageous for simultaneous optical and MRI detection. Although considerable research efforts have been put into the chemical design of suitable surface ligands, one of the major obstacles for biocompatible applications remains the lack of surface addressability. Therefore, a nanocomposite made up of individually addressable Au and MnO domains offers two functional surfaces for the attachment of different kinds of molecules, thus increasing both diagnostic and therapeutic potential. Furthermore, the size of either of the two components can be varied to optimize the magnetic and optical properties. Herein we present the successful synthesis of Au@MnO nanocomposites consisting of both paramagnetic MnO NPs and Au crystallites followed by separate surface functionalization of both domains with fluorescent ligands. Scheme 1 depicts a functionalized Au@MnO nanoflower with selective attachment of catechol anchors to the metal oxide petals and thiol anchors to the gold core. The nanoflowers were synthesized by decomposition of manganese acetylacetonate [Mn(acac)2] in diphenyl ether in the presence of preformed Au NPs (“seeds”), with oleic acid and oleylamine as surfactants, following a similar procedure for the preparation of Au@Fe3O4 heteroparticles by Sun et al. [15] The [*] T. D. Schladt, Dr. M. I. Shukoor, K. Schneider, Dr. M. N. Tahir, O. K hler, Prof. Dr. W. Tremel Institut f r Anorganische Chemie und Analytische Chemie Johannes-Gutenberg-Universit t Duesbergweg 10–14, 55099 Mainz (Germany) Fax: (+49)6131-39-25605 E-mail: tremel@uni-mainz.de

Template-Assisted Fabrication of Free-Standing Nanorod Arrays of a Hole-Conducting Cross-Linked Triphenylamine Derivative: Toward Ordered Bulk-Heterojunction Solar Cells

Free-standing nanorod arrays of a thermally cross-linked semiconducting triphenylamine were fabricated on conductive ITO/glass substrates via an anodic aluminum oxide (AAO) template-assisted approach. By using a solution wetting method combined with a subsequent thermal imprinting step to fill the nanoporous structure of the template with a cross-linkable triphenylamine derivative, a polymeric replication of the AAO was obtained after thermal curing and selective removal of the template. To obtain well-aligned and free-standing nanorod arrays, aggregation and collapse of the nanorods were prevented by optimizing their aspect ratio and applying a freeze-drying technique to remove the aqueous medium after the etching step. Because of their electrochemical properties and their resistance against organic solvents after curing, these high density nanorod arrays have potential application in organic photovoltaics.

From Defined Reactive Diblock Copolymers to Functional HPMA-Based Self-Assembled Nanoaggregates

This paper describes the synthesis of functional amphiphilic poly( N-(2-hydroxypropyl) methacrylamide)-block-poly(lauryl methacrylate) copolymers by RAFT polymerization via the intermediate step of activated ester block copolymers (pentafluoro-phenyl methacrylate). Block copolymers with molecular weights from 12000-28000 g/mol and PDIs of about 1.2 have been obtained. The amphiphilic diblock copolymers form stable super structures (nanoaggregates) by self-organization in aqueous solution. The diameters of these particles are between 100 and 200 nm and depend directly on the molecular weight of the block copolymer. Furthermore, we investigated the impact of these nanoaggregates on cell viability and on the motility of adherent cells. Cytotoxicity was investigated by the MTS test and the fluctuation in cell shape was monitored employing ECIS (electrical cell-substrate impedance sensing). In these investigations, the formed particles are not cell toxic up to a concentration of 2 mg/mL. Thus, our polymeric particles offer potential as polymer therapeutics.

Synthesis of Hetero-Telechelic α,ω Bio-Functionalized Polymers

Reversible addition-fragmentation chain transfer (RAFT) polymerization was used to synthesize poly[diethylene glycol monomethylether methacrylate] (PDEGMA) (M(n) = 6250 g/mol, PDI = 1.14) with a pentafluorophenyl (PFP) activated ester and a dithioester end group. The hormone thyroxin (T4) was quantitatively attached to the PFP activated ester alpha end group via its amino group. The omega-terminal dithioester was not harmed by this reaction and was subsequently aminolyzed in the presence of N-biotinylaminoethyl methanethiosulfonate, yielding a polymer with a thyroxin and a biotin end group with very high heterotelechelic functionality. The polymer was characterized by (1)H, (13)C, and (19)F NMR, UV-vis, and IR spectroscopy and gel permeation chromatography. The thyroxin transport protein prealbumin with two thyroxin binding sites and streptavidin, which has four biotin binding sites, was conjugated using the biotarget labeled polymer, resulting in the formation of a protein-polymer network, confirming the heterotelechelic nature of the polymer. Polymer-protein microgel formation was observed with dynamic light scattering. To realize a directed protein assembly, prealbumin was immobilized onto a surface, exposing one of its two thyroxin binding groups and thus allowing the conjugation with the thyroxin alpha end group of the heterotelechelic polymer. The biotin omega end group of the attached polymer layer enabled the subsequent immobilization of streptavidin, yielding a defined multilayer system of two proteins connected with the synthetic polymer (efficiency of streptavidin immobilization 81% based on prealbumin). Without the polymer, no streptavidin immobilization occurred. The layer depositions were monitored by surface plasmon resonance. The synthetic approach of combining PFP activated esters with functional MTS reagents presents a powerful method for obtaining well-defined heterotelechelic (bio-) functionalized polymers.

Templated Organic and Hybrid Materials for Optoelectronic Applications

The review highlights different approaches to template organic materials as well as hybrid materials that find or are expected to find application in optoelectronic devices. The first templating approach focuses on the use of preformed nanoporous membranes as templates for organic materials and polymeric materials. Such nanoporous templates can be track-etched membranes, anodic aluminum oxide membranes and other variants thereof, or block copolymer templates. Further, opals have been described as templates. In the second part, we have summarized developments that take advantage of self-assembly processes to pattern hybrid materials. Examples are sol-gel templating techniques using amphiphiles, evaporation-induced self-assembly, lyotropic templating as well as templating from block copolymers. Both routes are very promising templating approaches for optoelectronic materials and represent complementary rather than competing techniques.

Monitoring the formation of biosilica catalysed by histidine-tagged silicatein{

Surface bound silicatein retains its biocatalytic activity, which was demonstrated by monitoring the immobilisation of silicatein using a histidine-tag chelating anchor and the subsequent biosilicification of SiO(2) on surfaces by surface plasmon resonance spectroscopy, atomic force microscopy and scanning electron microscopy.

UCST-type behavior of poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) in aliphatic alcohols: solvent, co-solvent, molecular weight, and end group dependences

Poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) is shown to possess insoluble–soluble transitions (UCST-type phase behavior) in a large variety of aliphatic alcohols. Samples of different molecular weights ranging from 5 kg mol−1 to 23 kg mol−1 prepared by the RAFT process and featuring different end groups at each end were analyzed by cloud point measurements. Transitions occurred sharply and were fully reversible. The UCST was found to increase with an increasing molecular weight. Hydrophobic (alkyl chain) end groups were found to lower the critical temperature in isopropanol, while rigid aromatic end groups raised the transition temperature. In ternary mixtures of isopropanol/chloroform/POEGMA, the UCST decreased with an increasing chloroform concentration, with 10 vol% of chloroform accounting for a 30 °C drop. In mixtures of isopropanol/hexane/POEGMA, the cloud point increased significantly only with hexane concentrations above 30 vol%, at which level a 2 °C transition temperature increase was found. Addition of water to isopropanol solutions had a strong effect, with 1 vol% of water causing a decrease of the transition temperature of 12.5 °C. With an increasing chain length of the solvent, the cloud point increased, while a branching of the hydrocarbon chains lowered the cloud point. Samples of 23 kg mol−1POEGMA were for instance found to have cloud points of 22.0 °C in ethanol, 35.7 °C in isopentanol, and 75.4 °C in dodecanol.

Multi-responsive copolymers: using thermo-, light- and redox stimuli as three independent inputs towards polymeric information processing

We report on triple responsive polymers, exhibiting a distinct and reversible lower critical solution temperature in water that can be altered by light and redox stimuli, and we suggest their evaluation for molecular information processing.

Temperature controlled dispersion of carbon nanotubes in water with pyrene-functionalized poly(N-cyclopropylacrylamide).

Despite their immense potential, the ability to control the dispersion and microstructure of carbon nanotubes remains a hurdle for their widespread use. Poly(N-cyclopropylacrylamide), containing 5 mol % pyrene-bearing repeat units (p-PNCPA), is shown to vary the dispersion state of single-walled carbon nanotubes (SWNTs) in water. This is a thermo-responsive polymer whose conformation changes with temperature, which in turn leads to changes in the nanotube dispersion state. Cryo-TEM micrographs show that SWNTs stabilized using p-PNCPA transitions from a more exfoliated to a more bundled state as the aqueous suspension temperature is raised above the lower critical solution temperature (LCST) of the polymer (approximately 30 degrees C). Viscosity measurements on SWNT/p-PNCPA aqueous suspensions show shear thinning and near Newtonian behavior at 10 and 50 degrees C, respectively. Drying of these suspensions produces composites whose microstructure and electrical conductivity vary with drying temperature. This behavior has significant implications for the processing of carbon nanotubes and tailoring of composite properties. Such stimuli-controlled dispersion of carbon nanotubes could have a variety of applications in nanoelectronics, sensing, and drug and gene delivery systems.