Wendelin J. Stark

ETH Zurich, Zürich, Zurich, Switzerland

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Publications (279)1138.29 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Work-up in organic synthesis can be very time consuming, particularly when using reagents of solubility similar to the desired products and a low tendency to crystallize. In this respect, reactions involving organic bases would strongly profit from a tremendously simplified separation. Therefore, we synthesized a derivative of the superbasic proton sponge 1,8-bis(dimethylamino)naphthalene (DMAN) and covalently linked it to the strongest currently available nanomagnets based on carbon-coated cobalt metal nanoparticles. The immobilized magnetic superbase reagent was tested in Knoevenagel and Claisen-Schmidt type condensations and showed conversions up to 99%. In a particular example, high yields up to 97 % of isolated product could be obtained by simple recrystallization without using column chromatography. Recycling of the catalyst was simple and fast with insignificant decrease in catalytic activity.
    The Journal of Organic Chemistry 10/2014; DOI:10.1021/jo501913z · 4.64 Impact Factor
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    ABSTRACT: The capability of tracing a food product along its production chain is important to ensure food safety and product authenticity. For this purpose and as an application example, recently developed Silica Particles with Encapsulated DNA (SPED) were added to milk at concentrations ranging from 0.1 to 100 ppb (µg per kg milk). Thereby the milk, as well as the milk derived products yoghurt and cheese, could be uniquely labeled with a DNA tag. Procedures for the extraction of the DNA tags from the food matrixes were elaborated and allowed identification and quantification of previously marked products by quantitative polymerase chain reaction (qPCR) with detection limits below 1 ppb of added particles. The applicability of both synthetic as well as naturally occurring DNA sequences was shown. The usage of approved food additives as DNA carrier (silica = E551) and the low cost of the technology (< 0.1 USD per ton of milk labeled with 10 ppb of SPED) display the technical applicability of this food labeling technology.
    Journal of Agricultural and Food Chemistry 10/2014; 62(43). DOI:10.1021/jf503413f · 3.11 Impact Factor
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    ABSTRACT: Gelatin is an exceptional and versatile biopolymer with applications in various industries. As the most abundant structural protein in vertebrates it is available in megaton quantities. On these grounds, it would be a plausible substitute for synthetic polymers. Gelatin processing into fibers seems promising as continuous protein filaments do not have the limitation of natural fibers, i.e., small staple fiber length. Instead of spinning an aqueous gelatin solution, a protein precipitate from a phase-separated system is used. Robust wet spinning with subsequent fiber drawing allows production of a gelatin filament with similar mechanical properties as sheep wool. Different degrees of fiber drawing and addition of plasticizers enable to tailor the mechanical and thermal fiber properties and demonstrate the versatility of the proposed spinning process.
    Macromolecular Materials and Engineering 10/2014; 300(2). DOI:10.1002/mame.201400240 · 2.78 Impact Factor
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    ABSTRACT: We present a design and parameter study on 3D-printed, lost-wax-casted and combustion-powered soft silicone pumps, which are internally cooled by the conveyed liquid. Important factors influencing the pumping performance such as gas mixtures, feed rates, and actuation frequencies were thoroughly studied. Furthermore, we reinforced some of the here presented pumps with aramid fabrics in order to achieve partial blocking of the elastomeric flexibility upon combustion expansion. This design measure dramatically increased the pumping capabilities and allowed continuous conveying of water to 13 m (corresponding to 42 ft) of height. We were able to stably operate these novel pumps for more than 30 000 combustion cycles. Therefore, they represent a further step toward long-term stable soft machines with dense power characteristics.
    Industrial & Engineering Chemistry Research 08/2014; 53(31):12519-12526. DOI:10.1021/ie501991d · 2.24 Impact Factor
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    ABSTRACT: There is a strong interest in studying the cellular uptake of silica nanoparticles, particularly at medically relevant concentrations (ppb-ppm range) to understand their toxicology. At present, uptake analysis at these exposure levels is impeded by the high silica background concentration. Here we describe the use of DNA encapsulated within silica particles as a tool to quantify silica nanoparticles in in vitro cell-uptake experiments at low concentrations (down to 10 fg cell(-1)).
    Chemical Communications 07/2014; 50(73). DOI:10.1039/c4cc04480k · 6.72 Impact Factor
  • R. A. Raso, P. R. Stoessel, W. J. Stark
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    ABSTRACT: Removal of volatile organic compounds (VOC) and indoor air quality regulation through adsorbers required exchange or maintenance of active materials. In this work, we combine well known VOC adsorbers with oxidation catalysts as intimate particulate mixtures. We demonstrate how typical VOC can subsequently adsorb on such mixed material fixed beds (usually days to weeks; the common state of the system, adsorption phase) using small organic compounds (diethyl ether, triethylamine), monoterpenes such as linalool and limonene, and hexanoic acid. Occasional regeneration runs through heat up of the fixed bed results in simultaneous desorption and oxidation of the accumulated VOC, thus regenerating full adsorption capacity for a next adsorption phase. We investigated both small pore zeolites (H-ZSM-5) and larger pore zeolites (13X) and found a distinct interplay between the pore size and the type of VOC. Thermogravimetry coupled with mass spectroscopy was used to quantitatively study the effects of mixing composition and temperature on adsorber performance and regeneration. The here investigated bi-functional systems combine very low maintenance costs and materials requirement with low air flow and exchange costs, thus suggesting mixed (two-functional) bed adsorbers with catalytic function as sustainable alternatives to currently used single use systems based on granulated zeolites or activated carbon. In this work we show the ability of zeolite/cerium oxide physical mixtures to adsorb and capture different classes of VOC at room temperature and release them for oxidation at higher temperatures in a regenerative and sustainable process.
    07/2014; 2(34). DOI:10.1039/C4TA02317J
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    ABSTRACT: Elastic silicone composites with mechanically switchable transparency are generated by incorporating micron‐sized aluminum platelets into a highly flexible silicone. Physisorbing Fe3O4 nanoparticles onto the platelets surface allow magnetic pre‐alignment during the polymer curing. One‐dimensional or two‐dimensional stretching of the resulting silicone composites permits orientation of the incorporated flakes and alters light transmittance of the polymer coating.
    Advanced Engineering Materials 07/2014; 16(7). DOI:10.1002/adem.201300478 · 1.51 Impact Factor
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    ABSTRACT: To investigate chemo-mechanical effects of incorporating alkaline bioactive glass nanoparticles into a light-curable dental resin matrix.
    Dental materials: official publication of the Academy of Dental Materials 06/2014; DOI:10.1016/j.dental.2014.05.029 · 4.16 Impact Factor
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    ABSTRACT: A novel technique for transforming cubic α-NaYF4 phosphors into the hexagonal modification is presented for the removal of O impurities that hinder upconversion luminescence.
    ChemInform 06/2014; 45(22). DOI:10.1002/chin.201422215
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    ABSTRACT: Background Fractures with a critical size bone defect are associated with high rates of delayed- and non-union. The treatment of such complications remains a serious issue in orthopedic surgery. Adipose derived stem cells (ASCs) combined with biomimetic materials can potentially be used to increase fracture healing. Nevertheless, a number of requirements have to be fulfilled; in particular the insufficient vascularization of the bone constructs. Here, the objectives were to study the impact of ASC-derived osteoblasts on ASC-derived endothelial cells in a 3D co-culture and the effect of 40 wt % of amorphous calcium phosphate nanoparticles on the proliferation and differentiation of ASC-derived endothelial cells when present in PLGA. Materials and Methods Five primary ASC lines were differentiated towards osteoblasts (OB) and endothelial cells (EC) and two of them were chosen based on quantitative PCR results. Either a mono-culture of ASC-derived EC or a co-culture of ASC-derived EC with ASC-derived OB (1:1) was seeded on an electrospun nanocomposite of poly-(lactic-co-glycolic acid) and amorphous calcium phosphate nanoparticles (PLGA/a-CaP; reference: PLGA). The proliferation behavior was determined histomorphometrically in different zones and the expression of von Willebrand Factor (vWF) was quantified. Results Independently of the fat source (biologic variability), ASC-derived osteoblasts decelerated the proliferation behavior of ASC-derived endothelial cells in the co-culture compared to the mono-culture. However, expression of vWF was clearly stronger in the co-culture, indicating further differentiation of the ASC-derived EC into the EC lineage. Moreover, the presence of a-CaP nanoparticles in the scaffold slowed the proliferation behavior of the co-culture cells, too, going along with a further differentiation of the ASC-derived OB, when compared to pure PLGA scaffolds. Conclusions This study revealed significant findings for bone tissue-engineering. Co-cultures of ASC-derived EC and ASC-derived OB stimulate each other's further differentiation. A nanocomposite with a-CaP nanoparticles offers higher mechanical stability, bioactivity and osteoconductivity compared to mere PLGA and can easily be seeded with pre-differentiated EC and OB.
    Injury 06/2014; 45(6). DOI:10.1016/j.injury.2014.02.035 · 2.46 Impact Factor
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    ABSTRACT: The bombardier beetle uses attack-triggered mixing of reactants (hydrochinone, hydrogen peroxide (H2O2) and enzymes as catalysts) to defend itself against predators. Using multi-layer polymer sheets with H2O2 and catalyst (MnO2) filled compartments we developed a 2D analogous bio-inspired chemical defence mechanism for anti-vandalism applications. The reactants were separated by a brittle layer that ruptures upon mechanical attack and converts the mechanical energy trigger (usually a few Joules) into a chemical self-defence reaction involving release of steam, and optionally persistent dyes and a DNA-based marker for forensics. These surfaces effectively translate a weak mechanical trigger into an energetic chemical reaction with energy amplification of several orders of magnitude. Since the responsive materials presented here do not depend on electricity, they may provide a cost effective alternative to currently used safety systems in the public domain, automatic teller machines and protection of money transport systems. Anti-feeding protection in forestry or agriculture may similarly profit from such mechanically triggered chemical self-defending polymer surfaces.
    05/2014; 2(22). DOI:10.1039/C3TA15326F
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    ABSTRACT: Purpose: A low direct current can be used to disinfect dental titanium implants in simulated physiologic environments. The aim of this study was to determine whether this treatment affects implant surface structure and cytocompatibility. Materials and Methods: Titanium test disks with a sandblasted, acid-etched, large-grit (SLA) surface were placed as anodes in an electrolytic bath with physiologic saline and treated with 15 mA of current for 15 minutes. Surfaces were analyzed by light and electron microscopy and contact angle measurement. Depth profile analyses of SLA disks were run at subsurface levels from 0 to 1,000 nm. The proliferation and viability of preosteoblastic cells and human foreskin fibroblasts on implant surfaces were assessed. Alkaline phosphatase (ALP) activity was determined with and without exposure to bone morphogenetic protein-2 (BMP-2). Mineralization was determined after 4 weeks. Results: A blue discoloration was observed after treating the SLA disks, but no damage was recognized microscopically. An oxidation layer formed on the surface and the wettability of the disks increased significantly. Cell proliferation and initial maturation were not affected by the treatment. Mineralization and ALP activity of BMP-exposed cells, however, were slightly but significantly reduced on test disks. Conclusions: The current study showed that the alterations in implant color after electrochemical treatment did not reflect significant surface changes, which would preclude cell adhesion and growth or have a major impact on osteoblastic differentiation or maturation.
    The International journal of oral & maxillofacial implants 05/2014; 29(3):735-42. DOI:10.11607/jomi.3342 · 1.49 Impact Factor
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    ABSTRACT: Intravascular application of magnetic nanocarriers is a critical step in the development of new therapeutic strategies, including magnetic drug targeting or hyperthermia. However, injection of particulate matter bears the intrinsic risk of contact activation of the blood coagulation cascade. In this work, we use point-of-care assays to study coagulation dynamics and clotting parameters in blood samples exposed to relevant concentrations of surface-functionalized carbon-coated iron carbide nanomagnets using unmodified nanomagnets and poly(ethylene)glycol-functionalized nanomagnets with different end-groups, including –OCH3, –NH2, –COOH, –IgG, and –ProteinA-protected-IgG (–IgG-ProtA). Silica nanoparticles with a comparable surface area are used as a reference material. For magnetic nanoparticles, we observe a decrease in clotting time by 25% compared to native blood at concentrations of 1 mg mL−1, independent of the surface functionalization, and only minor differences in receptor expression on platelets (GP-IIb-IIIa, CD62, and CD63) relative to control samples were observed. Interestingly, the inter-subject variance of the clotting time is similar to the nanoparticle-induced effect in a single subject with average clotting time. Whilst the present study is based on in vitro assays and a small group of healthy blood donors, the comparison to broadly used silica nanoparticles, and the fact that experimental intergroup variability is comparable to the observed effects from the carbon-coated nanomagnets suggests continuing investigations on their potential clinical use.
    Journal of Materials Chemistry 04/2014; DOI:10.1039/C4TB00208C · 6.63 Impact Factor
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    ABSTRACT: Due to their porous structure, angora rabbit fibers make for some of the highest quality wool. The application of these fibers on a technical scale is not feasible due to their limited availability and high price. Here, a robust fiber preparation method is reported based on an unusual spinning process, where a non-equilibrated, ternary system of protein, solvent, and non-solvent is continuously processed into strong fibers with minimal energy input and harmless solvents. Gelatin - the degradation product of collagen - is chosen as the protein component because of its immense availability from slaughterhouse waste. Due to the sponge-like structure of the ternary gelatin/water/2-propanol spinning mixture, fibers with internal cavities are produced. The porous nature of these fibers resembles the morphology of angora rabbit fibers. Despite their high porosity, the here-obtained gelatin fibers show clear re-orientation of the fibrous protein and attain a mechanical performance similar to other bio- (e.g., wool, tendon collagen) and synthetic polymers (e.g., polytetrafluoroethylene, polyamide 6). These promising results motivate for broader investigations on the spinning of non-equilibrium protein mixtures and suggest the use of porous gelatin fibers in textiles.
    Advanced Functional Materials 04/2014; 24(13). DOI:10.1002/adfm.201303321 · 10.44 Impact Factor
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    ABSTRACT: The luxury angora rabbit fiber stands out from other animal hair. Its porous structure combines exceptional insulation and comfortable wear. On page 1831, W. J. Stark and co‐workers present a method by which a structurally equivalent protein filament can be synthesized in a continuous process. Raw material slaughterhouse waste‐derived gelatin is utilized, adding value to this low‐cost biopolymer.
    Advanced Functional Materials 04/2014; 24(13). DOI:10.1002/adfm.201470080 · 10.44 Impact Factor
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    ABSTRACT: Indoor odor management currently relies on energy-intensive high air exchange rates, or, more sustainable, on single use volatile organic compounds (VOC) adsorbers or ozonisation. This study investigates a more sustainable, multi-cycle use of an odor adsorber system that combines concepts from catalytic oxidation and air cleaning. Both pure and sodium doped, nanostructured CeO2 were tested as adsorber material for high volume removal of odorous compounds from air. As a representative compound for unpleasant odors, hexanoic acid (HA) was used. After air cleaning on fixed beds of CeO2 or Na/CeO2, both hexanoic acid loaded adsorber materials were heated under air and displayed considerable oxidation activity at 191 °C and 263 °C, respectively. Mass spectroscopy was used to confirm that no hexanoic acid desorbed during combustion. Cerium oxide showed an adsorber efficiency of ≥96.5% over a period of 60 h (Cin = 0.044 mg/L, gas hourly space velocity, GHSV = 440 h−1) and sodium doped cerium oxide adsorbed ≥97% for over 90 h (Cin = 0.056 mg/L, GHSV = 1100 h−1). CeO2 was regenerated at 220 °C in air and could be successfully re-used as adsorber without noticeable loss in performance. The study demonstrates that CeO2 has most promising properties for application as re-usable air cleaner due to its very good ability for adsorption even at highly dilute conditions (ppm-level) using a representative acidic test compound with rancid and sweaty odor. Sodium as a basic dopant further improved the adsorption of hexanoic acid but requires a higher regeneration temperature.
    Applied Catalysis B Environmental 04/2014; 147:965-972. DOI:10.1016/j.apcatb.2013.10.030 · 6.01 Impact Factor
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    ABSTRACT: Palladium nanoparticles are deposited on the surface of highly magnetic carbon-coated cobalt nanoparticles. In contrast to the established synthesis of Pd nanoparticles via reduction of Pd(II) precursors, the microwave decomposition of a Pd(0) source leads to a more efficient Pd deposition, resulting in a material with considerably higher activity in the hydrogenation of alkenes. Systematic variation of the Pd loading on the carbon-coated cobalt nanoparticle surface reveals a distinct trend to higher activities with decreased loading of Pd. The activity of the catalyst is further improved by the addition of 10 vol.% diethyl-ether to iso-propanol that is found to be the solvent of choice. With respect to activity (turnover frequencies up to 11095/h), handling, recyclability through magnetic decantation, and leaching of Pd (≤6 ppm/cycle), this novel magnetic hybrid material compares favorably to conventional Pd/C or Pd@CNT catalysts.
    Advanced Functional Materials 04/2014; DOI:10.1002/adfm.201303277 · 10.44 Impact Factor
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    ABSTRACT: In the pursuit of robust and reusable biocatalysts for industrial synthetic chemistry, nanobiotechnology is cur-rently taking a significant part. Recently, enzymes have been immobilized on different nanoscaffold supports. Carbon coated metallic nanoparticles were found to be a practically useful support for enzyme immobilization due to their large surface area, high magnetic saturation and manipulative surface chemistry. In this study carbon coated cobalt nanoparticles were chemically functionalized (diazonium chemistry), activated for bioconjugation (N,N-Disuccinimidyl carbonate) and subsequently used in enzyme immobilization. Three enzymes: β-glucosidase, α-chymotrypsin and lipase B were successfully covalently immobilized on the magnetic nonsupport. The enzyme-particle conjugates formed retained their activity and stability after immobilization and were effi-ciently recycled from milliliter to liter scales in short recycle times.
    Bioconjugate Chemistry 03/2014; 25(4). DOI:10.1021/bc400476y · 4.82 Impact Factor
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    ABSTRACT: We present a gas combustion powered soft pump made from highly durable and flexible polydimethylsiloxane (soft silicone). Our soft pump was able to run for 10 000 combustion cycles at a constant combustion power rating of 500 watts and thus discloses novel prospects for long-lasting soft-machines at high specific energy-densities.
    RSC Advances 03/2014; 4(31):16039-16042. DOI:10.1039/C4RA01497A · 3.71 Impact Factor
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    ABSTRACT: Applications of upconversion phosphors have grown extensively in number during the past decade. Hexagonal sodium yttrium fluoride (β-NaYF4) is known to be one of the best host lattices for upconversion materials. We developed a novel technique for transforming cubic sodium yttrium fluoride (α-NaYF4) phosphors into the hexagonal modification and remove oxygen impurities that hinder the upconversion luminescence. We transformed cubic α-NaYF4 nanoparticles from flame-spray synthesis with a particle size less than 50 nm into more efficient β-NaYF4 phosphors. The application of SnF2 and ZnF2 as oxygen scavengers allowed the formation of the pure hexagonal phase and improved the upconversion luminescence intensity. The developed process utilizes no free HF gas in the production and does not contaminate the upconversion phosphors with scavenger material. The treatment increases the particle size to between approximately 500 nm and 1 μm. Upconversion luminescence spectra revealed the characteristic blue Tm3+ and green Er3+ emissions of β-NaYF4: Yb,Tm and Yb,Er, respectively.
    Chemistry of Materials 03/2014; 26(6):2015–2020. DOI:10.1021/cm403459v · 8.54 Impact Factor

Publication Stats

7k Citations
1,138.29 Total Impact Points


  • 2003–2015
    • ETH Zurich
      • • Institute for Chemical and Bioengineering
      • • Department of Mechanical and Process Engineering
      Zürich, Zurich, Switzerland
  • 2010–2014
    • Universität Regensburg
      • Institute of Organic Chemistry
      Ratisbon, Bavaria, Germany
    • Imperial College London
      • Department of Materials
      London, ENG, United Kingdom
  • 2011
    • University of Zurich
      • Institut für Anästhesiologie
      Zürich, ZH, Switzerland
  • 2002
    • Eawag: Das Wasserforschungs-Institut des ETH-Bereichs
      Duebendorf, Zurich, Switzerland