Patterned immobilization of polyoxometalate-loaded mesoporous silica particles via amine-ene Michael additions on alkene functionalized surfaces.

Polyoxometalates (POM) are anionic oxoclusters of early transition metals that are of great interest for a variety of applications, including the development of sensors and catalysts. A crucial step in the use of POM in functional materials is the production of composites that can be further processed into complex materials, e.g. by printing on different substrates. In this work, we present an immobilization approach for POMs that involves two key processes: first, the stable encapsulation of POMs in the pores of mesoporous silica nanoparticles (MSPs) and, second, the formation of microstructured arrays with these POM-loaded nanoparticles. Specifically, we have developed a strategy that leads to water-stable, POM-loaded mesoporous silica that can be covalently linked to alkene-bearing surfaces by amine-Michael addition and patterned into microarrays by scanning probe lithography (SPL). The immobilization strategy presented facilitates the printing of hybrid POM-loaded nanomaterials onto different surfaces and provides a versatile method for the fabrication of POM-based composites. Importantly, POM-loaded MSPs are useful in applications such as microfluidic systems and sensors that require frequent washing. Overall, this method is a promising way to produce surface-printed POM arrays that can be used for a wide range of applications.

Tailored Monolayers of N-Heterocyclic Carbenes by Kinetic Control.

Despite the substantial success of N-heterocyclic carbenes (NHCs) as stable and versatile surface modification ligands, their use in nanoscale applications beyond chemistry is still hampered by the failure to control the carbene binding mode, which complicates the fabrication of monolayers with the desired physicochemical properties. Here, we applied vibrational sum-frequency generation spectroscopy to conduct a pseudokinetic surface analysis of NHC monolayers on Au thin films under ambient conditions. We observe for two frequently used carbene structures that their binding mode is highly dynamic and changes with the adsorption time. In addition, we demonstrate that this transition can be accelerated or decelerated to adjust the binding mode of NHCs, which allows fabrication of tailored monolayers of NHCs simply by kinetic control.

Selective Removal of Gold: N-Heterocyclic Carbenes as Positive Etch Resists on Planar Gold Surfaces.

N-Heterocyclic carbene (NHC)-modified planar gold surfaces (NHC@Au) were found to be more susceptible toward wet chemical etching than undecorated surface areas. Site-selective decoration of NHCs on Au was achieved by microcontact printing (µCP) of the NHC precursors 1,3-bis(diisopropylphenyl)imidazol-3-ium hydrogen carbonate (IPr(H)[HCO3]) or 1,3-dimethylbenzimidazol-3-ium hydrogen carbonate (BIMe(H)[HCO3]). Strikingly, BIMe@Au showed concentration-dependent etching behavior, tunable from a positive resist to a negative resist. Surface patterning was verified by time-of-flight secondary-ion mass spectrometry and Kelvin probe force microscopy. Moreover, orthogonal µCP enabled the patterned functionalization of planar Au with both IPr and 1-eicosanethiol and the subsequent formation of three-dimensional structures with a single etching step. The selective removal of Au by functionalization with a surface ligand is unprecedented and enables novel applications of NHCs in materials chemistry and nanofabrication.

3D-Printed Oxygen-Carrying Nanocomposite Hydrogels for Enhanced Cell Viability under Hypoxic and Normoxic Conditions.

Insufficient and heterogeneous oxygen (O2) distribution within engineered tissues results in hypoxic conditions. Hypoxia is one of the characteristics of solid tumors. To date, very few studies have used an O2-deliverable injectable hydrogel for cancer treatment under hypoxic conditions. In this field, we describe a new O2-carrying nanomaterial and an injectable nanocomposite hydrogel (PMOF and AlgL-PMOF, respectively) that can provide extended oxygen levels for cell survival under hypoxia. Particularly, PMOF and AlgL-PMOF enhance cell viability under hypoxic and normoxic cell culturing conditions. Moreover, sustained oxygen availability in the presence of an anticancer drug within the 3D network of AlgL-PMOF results in a decrease in the viability of malignant and immortal cells, while the viability of healthy cells increases.

Mechanistic Understanding of the Heterogeneous, Rhodium-Cyclic (Alkyl)(Amino)Carbene-Catalyzed (Fluoro-)Arene Hydrogenation.

Recently, chemoselective methods for the hydrogenation of fluorinated, silylated, and borylated arenes have been developed providing direct access to previously unattainable, valuable products. Herein, a comprehensive study on the employed rhodium-cyclic (alkyl)(amino)carbene (CAAC) catalyst precursor is disclosed. Mechanistic experiments, kinetic studies, and surface-spectroscopic methods revealed supported rhodium(0) nanoparticles (NP) as the active catalytic species. Further studies suggest that CAAC-derived modifiers play a key role in determining the chemoselectivity of the hydrogenation of fluorinated arenes, thus offering an avenue for further tuning of the catalytic properties.

Facile Fabrication of Silicon(IV)Phthalocyanine-Embedded Poly(vinyl alcohol)-Based Antibacterial and Antifouling Interfaces.

Interest in the photodynamic inactivation of bacteria as an alternative method to antibiotic treatment continues to grow. Based on this approach, light-activated anti-infective interfaces could be fabricated via incorporation of photosensitizers into the polymer-based materials. In order to combine photobactericidal and antifouling functions, the choice of the carrier polymer is of particular significance: it should enable fast and effective conjugation of photosensitizer and reduce the formation of bioburden on the artificial material in a biological environment. This study reports one-pot fabrication and characterization of two silicon(IV)phthalocyanine /poly(vinyl alcohol)-based electrospun mats. The method relies on the thermal cross-linking of components by esterification using sebacic acid as a cross-linking agent. Fabricated flexible mats showed photosensitizer-dependent antibacterial photoactivity against different Gram-positive bacteria with low cytotoxic effects on human fibroblasts and were effective against bacterial attachment, as an early step toward future biofilm formation. This work provides practical guidelines in developing photoactive materials and interfaces that can be used in nonadhesive wound dressings, food packaging, water, and air filtration.

3D bioprinting of triphasic nanocomposite hydrogels and scaffolds for cell adhesion and migration.

In this study we describe the first example of 3D bioprinted triphasic chiral nanocomposite (NC) hydrogels/scaffolds to simulate the complex 3D architecture, nano/micro scale topography, and chiral nature of extracellular matrix. These multifunctional constructs are prepared using a 3D bioprinting technique and are composed of three connected hydrogels/scaffolds, two of which are loaded with nanomaterials functionalized with opposite enantiomers of a biomolecule. With these constructs, we direct the migration of cells toward the part of the triphasic chiral NC hydrogels/scaffolds containing the cells' preferred biomolecule enantiomer.

Versatile Micropatterns of N-Heterocyclic Carbenes on Gold Surfaces: Increased Thermal and Pattern Stability with Enhanced Conductivity.

Patterned monolayers of N-heterocyclic carbenes (NHCs) on gold surfaces were obtained by microcontact printing of NHC-CO2 adducts and NHC(H)[HCO3 ] salts. The NHC-modified areas showed an increased conductivity compared to unmodified gold surface areas. Furthermore, the remaining surface areas could be modified with a second, azide-functionalized carbene, facilitating further applications and post-printing modifications. Thorough elucidation by a variety of analytical methods offers comprehensive evidence for the viability of the methodology reported here. The protocol enables facile access to versatile, microstructured NHC-modified gold surfaces with highly stable patterns, enhanced conductivity, and the option for further modification.

N-Heterocyclic-Carbene-Treated Gold Surfaces in Pentacene Organic Field-Effect Transistors: Improved Stability and Contact at the Interface.

N-Heterocyclic carbenes (NHCs), which react with the surface of Au electrodes, have been successfully applied in pentacene transistors. With the application of NHCs, the charge-carrier mobility of pentacene transistors increased by five times, while the contact resistance at the pentacene-Au interface was reduced by 85 %. Even after annealing the NHC-Au electrodes at 200 °C for 2 h before pentacene deposition, the charge-carrier mobility of the pentacene transistors did not decrease. The distinguished performance makes NHCs as excellent alternatives to thiols as metal modifiers for the application in organic field-effect transistors (OFETs).

Palladium Nanoparticle Loaded Bifunctional Silica Hybrid Material: Preparation and Applications as Catalyst in Hydrogenation Reactions.

Bifunctional mesoporous silica was prepared by co-condensation of tetraethyl orthosilicate (TEOS) with functionalized organosilanes containing azides or alkoxyamines. Orthogonal functional groups at the particles were selectively addressed in subsequent chemical modifications through "click"-chemistry ("click to ligand" strategy) and radical nitroxide exchange. Palladation with PdCl2 delivered Pd nanoparticle-loaded silica material bearing sulfoxides and additional aminoamides as stabilizing ligands by means of in situ reduction of the PdII -salt. These functional particles were successfully applied to the hydrogenation of alkynes and alkenes. Aldehyde hydrodeoxygenation and benzyl ether cleavage were achieved with these hybrid catalysts under mild conditions. Particles were analyzed by IR, TEM/STEM, EDX, and solid-state NMR spectroscopy.

8-Aza-2'-deoxyisoguanosine Forms Fluorescent Hydrogels whereas 8-Aza-2'-deoxyguanosine Assembles into Nucleoside Nanotubes.

Fluorescent hydrogels have attracted attention for applications in tissue engineering, drug delivery or as molecular machines. This study describes a nucleoside hydrogel formed by 8-aza-2'-deoxyisoguanosine (z8 isoGd ). The new hydrogel is the first guanosine gel that has intrinsic fluorescence. It has long-term stability and a higher thermal stability (Tgel =77 °C) than gels of 2'-deoxyisoguanosine (Tgel =67 °C) and 2'-deoxyguanosine (Tgel =50 °C). Furthermore, the minimum gel concentration is lower at 0.3 mg per 100 µL for z8 isoGd compared to 0.7 mg for 2'-deoxyisoguanosine. Scanning electron microscopy images show that the z8 isoGd gel forms dense bundles of fibers. The related nucleoside 8-aza-2'-deoxyguanosine does not form a hydrogel but forms nanotube-like structures in the solid state. The fluorescence of the z8 isoGd hydrogel responds to external stimuli such as the addition of alkali metal ions, pH change, heat, and UV irradiation. In binary mixtures with nucleoside shape mimics such as 7-deaza-2'-deoxyisoguanosine, the z8 isoGd hydrogel disintegrates.

Supramolecular Isoguanosine Assemblies Form Hydrogels with Excellent Long-Term Stability.

Supramolecular nucleoside-based hydrogels have the potential to be utilized in drug delivery, regenerative medicine and for the construction of nanoscopic devices. Isoguanosine, its 2'-deoxyribonucleoside and the 2'-deoxy-2'-fluororibonucleoside form hydrogels in alkali metal salt solutions. Gel stability depends on the metal ions used (Li+ , Na+ , K+ , Rb+ , Cs+ ). All isoguanosine gels show long-term stability of several months, whereas guanosine gels collapse within minutes or hours. Rheological data confirm a 15 times higher hydrogel stability for isoguanosine compared to guanosine. Isoguanosine gel stability covers a broad pH range (pH 3-10), and the stability at physiological Na+ ion concentration is striking. SEM images reveal that isoguanosine self-assembles into helical stacked fibers that are interconnected whereas guanosine forms discrete ribbons. Moreover, isoguanosine gels display excellent loading and release ability for small molecules.

Supramolecular Isoguanosine Assemblies Form Hydrogels with Excellent Long-Term Stability.

Invited for this month's cover is the collaboration between Prof. Dr. Frank Seela at Osnabrück University and Center for Nanotechnology in Münster, Germany, Dr. Hang Zhao, former postdoctoral fellow in Prof. Seela's group and now Associate Professor at Sichuan University, P. R. China, and Dr. Andreas Schäfer at Nanoanalytics GmbH, Münster, Germany. The cover picture shows an isoguanosine hydrogel formed by nucleoside self-assembly that consists of interconnected helical fibers. Read the full text of the article at 10.1002/cplu.201600586.

Cell Growth on ("Janus") Density Gradients of Bifunctional Zeolite L Crystals.

Nanoparticle density gradients on surfaces have attracted interest as two-dimensional material surfaces that can mimic the complex nano-/microstructure of the native extracellular matrix, including its chemical and physical gradients, and can therefore be used to systematically study cell-material interactions. In this respect, we report the preparation of density gradients made of bifunctional zeolite L crystals on glass surfaces and the effects of the density gradient and biopolymer functionalization of zeolite L crystals on cell adhesion. We also describe how we created "Janus" density gradient surfaces by gradually depositing two different types of zeolite L crystals that were functionalized and loaded with different chemical groups and guest molecules onto the two distinct sides of the same glass substrate. Our results show that more cells adhered on the density gradient of biopolymer-coated zeolites than on uncoated ones. The number of adhered cells increased up to a certain surface coverage of the glass by the zeolite L crystals, but then it decreased beyond the zeolite density at which a higher surface coverage decreased fibroblast cell adhesion and spreading. Additionally, cell experiments showed that cells gradually internalized the guest-molecule-loaded zeolite L crystals from the underlying density gradient containing bifunctional zeolite L crystals.

Chemical modification of polymer brushes via nitroxide photoclick trapping.

The preparation of polymer brushes (PBs) bearing α-hydroxyalkylphenylketone (2-hydroxy-2-methyl-1-phenylpropan-1-one) moieties as photoreactive polymer backbone substituents is presented. Photoreactive polymer brushes with defined thicknesses (up to 60 nm) and high grafting densities are readily prepared by surface initiated nitroxide mediated radical polymerization (SINMP). The photoactive moieties can be transformed via Norrish-type I photoreaction to surface-bound acyl radicals. Photolysis in the presence of a persistent nitroxide leads to chemically modified PBs bearing acylalkoxyamine moieties as side chains resulting from trapping of the photogenerated acyl radicals with nitroxides. Application of functionalized nitroxides to the photochemical PB postmodification provides functionalized PBs bearing cyano, polyethylene glycol (PEG), perfluoroalkyl, and biotin moieties. As shown for one case, photochemical postfunctionalization of the PB through a mask using a biotin-conjugated nitroxide as the trapping reagent leads to the corresponding site-selective chemically modified PB, which is successfully used for site-specific streptavidin immobilization. Surface analysis of PBs was performed by contact angle (CA) measurements, X-ray photoelectron spectroscopy (XPS), attenuated total reflection (ATR), fourier transform infrared (FTIR) spectroscopy, and fluorescence microscopy.