Bart Jan Ravoo

University of Münster, Muenster, North Rhine-Westphalia, Germany

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Publications (156)918.05 Total impact

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    ABSTRACT: Coupling of plasmon resonances in metallic gap antennas is of interest for a wide range of applications due to the highly localized strong electric fields supported by these structures, and their high sensitivity to alterations of their structure, geometry, and environment. Morphological alterations of asymmetric nanoparticle dimer antennas with (sub)-nanometer size gaps are assigned to changes of their optical response in correlative dark-field spectroscopy and high-resolution transmission electron microscopy (HR-TEM) investigations. This multimodal approach to investigate individual dimer structures clearly demonstrates that the coupling of the plasmon modes, in addition to well-known parameters such as the particle geometry and the gap size, is also affected by the relative alignment of both nanoparticles. The investigations corroborate that the alignment of the gap forming facets, and with that the gap area, is crucial for their scattering properties. The impact of a flat versus a rounded gap structure on the optical properties of equivalent dimers becomes stronger with decreasing gap size. These results hint at a higher confinement of the electric field in the gap and possibly a different onset of quantum transport effects for flat and rounded gap antennas in corresponding structures for very narrow gaps.
    No preview · Article · Feb 2016 · Small
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    ABSTRACT: Fluorescent β-cyclodextrin vesicles (β-CDV) that display host cavities available for host-guest interactions at the vesicle surface were prepared by incorporation of hydrophobic spirobifluorene-based dye 1 in the membrane of unilamellar vesicles. Fluorescence quenching of dye 1 was observed in the presence of different quenchers. Methyl viologen 2 does not quench dye 1 because it does not bind to β CDV. 4 Nitrophenol 3 and 4 nitrophenol covalently connected to adamantane 4 quench the fluorescence of dye 1 in neutral solution, but by different mechanisms according to lifetime measurements. The quenching efficiency of 3 is pH dependent due to the presence of the phenolate form. Competition experiments with excess host and guest showed that 3 is likely to diffuse in and out of the membrane, while 4 forms and inclusion complex with β-CDV leading to close contact and efficient quenching. Our findings confirm that this dynamic supramolecular system is a versatile model to investigate quenching and recognition processes at bilayer membranes.
    No preview · Article · Jan 2016 · Photochemical and Photobiological Sciences
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    ABSTRACT: Surface immobilised polymer brushes containing azobenzene units were prepared using a combination of microcontact chemistry and surface-initiated atom transfer radical polymerisation (SI-ATRP). These brushes were investigated using AFM, XPS and UV/vis spectroscopy. It was shown that two surfaces bearing azobenzene brushes can be glued together in the presence of a β-cyclodextrin polymer and hold as much as 700 ± 150 g cm(-2).
    No preview · Article · Dec 2015 · Chemical Communications
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    ABSTRACT: Micropatterns of hydrophilic polymer brushes were prepared by micromolding in capillaries (MIMIC). The polymers are covalently bound to the surfaces by a rapid hetero Diels-Alder reaction, constituting the first example of polymers grafted to surfaces in a defined pattern by MIMIC. The polymers (poly(acrylic acid), poly(hydroxyethyl acrylate) and poly(tetraethyleneglycole acrylate)) ranging in molecular weight from 1500 to 6000 g mol-1) were prepared with narrow dispersities via the reversible addition fragmentation chain transfer (RAFT) process using a highly electron deficient RAFT agent able to react with surface anchored dienes such as cyclopentadiene. We demonstrate that the anchoring method is facile to perform and highly suitable to prepare patterned surfaces which are passivated against biological impact in well-defined areas.
    No preview · Article · Nov 2015 · Langmuir
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    ABSTRACT: Triazolinedione (TAD) click reactions were combined with microcontact chemistry to print, erase, and reprint polymer brushes on surfaces. By patterning substrates with a TAD-tagged atom-transfer radical polymerization initiator (ATRP-TAD) and subsequent surface initiated ATRP, it was possible to graft micropatterned polymer brushes from both alkene- and indole-functionalized substrates. As a result of the dynamic nature of the Alder–ene adduct of TAD and indole at elevated temperatures, the polymer pattern could be erased while the regenerated indole substrate could be reused to print new patterns. To demonstrate the robustness of the methodology, the write–erase cycle was repeated four times.
    No preview · Article · Sep 2015 · Angewandte Chemie International Edition
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    ABSTRACT: Triazolindion-Click-Chemie und Mikrokontaktchemie wurden kombiniert, um Polymerbürsten auf Oberflächen zu drucken, zu löschen und anschließend erneut anzubinden. Die mikrostrukturierten Polymerbürsten wurden auf Alken- und Indol-Monoschichten gebildet, indem zuerst ein mit Triazolindion (TAD) substituierter Atomtransferradikalpolymerisationinitiator (ATRP-TAD) auf die Monoschichten gedruckt und anschließend eine oberflächeninitiierte Polymerisation durchgeführt wurde. Der dynamische Charakter des Alder-En-Addukts von TAD und Indol erlaubte es, die Polymerbürsten bei erhöhten Temperaturen von der Oberfläche zu löschen, während die regenerierten Indol-Monoschichten neu strukturiert wurden. Der Zyklus aus Schreiben und Löschen konnte vier Mal durchlaufen werden, wodurch die Robustheit der Methode bewiesen ist.
    No preview · Article · Sep 2015 · Angewandte Chemie
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    ABSTRACT: Herein we describe the synthesis of negatively charged N-heterocyclic carbene (NHC)-functionalized palladium and gold nanoparticles (NPs), which are stable in water for over 3 months. The formation of these NHC–NPs proceeds via an efficient ligand exchange procedure. This method was successfully applied to different negatively charged NHCs bearing sulfonate and carboxylate groups. The obtained PdNPs were investigated as catalysts in hydrogenation reactions and showed high catalytic activity (TON up to 2500 and TOF up to 2000 h–1).
    No preview · Article · Jul 2015 · ACS Catalysis
  • Moritz Buhl · Benjamin Vonhören · Bart Jan Ravoo
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    ABSTRACT: This communication describes a bioconjugation method for the generation of enzyme microarrays on surfaces using photochemical thiol-ene chemistry in combination with microcontact printing. Glucose oxidase and lactase were readily immobilized (i.e. printing time 2 min) on alkene terminated self-assembled monolayers on glass as demonstrated by X-ray photoelectron spectroscopy and fluorescence microscopy. Furthermore the activity of both immobilized enzymes was confirmed in single enzyme as well as cascade transformations.
    No preview · Article · Jun 2015 · Bioconjugate Chemistry
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    ABSTRACT: Photodetectors supply an electric response when illuminated. The detectors in this study consist of an active layer and a polyvinylidene fluoride (PVDF) blocking layer, which are sandwiched by an aluminum and an indium tin oxide (ITO) elec-trode. The active layer was prepared of zinc porphyrins and assembled by covalent layer-by-layer (LbL) deposition. The layer growth was monitored by UV/vis absorbance, ellipsometry and x-ray photoelectron spectroscopy. Upon exposure to chopped light, the detectors show an alternating transient photocurrent, which is limited by the accumulation of space charges at the blocking layer/active layer interface. We could show that the number of photoactive layers has a significant impact on the device performance. The fastest response was achieved with few layers. The highest photocurrents were measured for detectors with an intermediate number of layers, beyond which, more layers did not lead to an increase in photocurrent despite containing more active material.
    No preview · Article · Mar 2015 · ACS Applied Materials & Interfaces
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    ABSTRACT: Layer-by-layer deposition is a widely used method for surface functionalization. It is shown here that up to 58 covalently linked molecular layers could be assembled in 20 min at room temperature on a silicon wafer by the layer-by-layer click reaction of a divalent triazolinedione and a trivalent diene. The layer growth was found to be linear. The multilayers were analyzed by ellipsometry, atomic force microscopy, and X-ray photoelectron spectroscopy.
    No preview · Article · Mar 2015 · ACS Macro Letters
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    ABSTRACT: Gold nanoparticles (Au NPs) with tailor-made structures and properties are highly desirable for applications in catalysis and sensing. In this context, surface modifications of Au NPs are of particular relevance. Herein, we present a sequential surface modification of Au NPs with Ag(I) coordination complexes, which can be converted into Ag(0) -doped Au NPs by simple ligand-exchange reaction. The key innovative element of this surface modification is a multifunctional bioxazoline-based ligand that brings coordinated Ag(I) into close proximity to the particle surface. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Feb 2015 · Chemistry
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    Avik Samanta · Matthias Tesch · Armido Studer · Bart Jan Ravoo
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    ABSTRACT: Polymer-shelled vesicles are prepared by using cyclodextrin vesicles as supramolecular templates and an adamantane functionalized poly(acrylic acid) additive anchored via host-guest recognition followed by cross-linking of carboxylic acid groups on the polymer. The polymer-shelled nanocontainers are highly stable and effective for encapsulating small hydrophilic molecules. We also show that a hollow cross-linked polymer cage can be obtained after dissolution of the template vesicles. The size and the shell thickness of the polymer cage can be tuned by variation of the template size and polymer length.
    Full-text · Article · Jan 2015 · Journal of the American Chemical Society
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    ABSTRACT: The development of an effective and general delivery method that can be applied to a large variety of structurally diverse biomolecules remains a bottleneck in modern drug therapy. Herein, we present a supramolecular system for the dynamic trapping and light-stimulated release of both DNA and proteins. Self-assembled ternary complexes act as nanoscale carriers, comprising vesicles of amphiphilic cyclodextrin, the target biomolecules and linker molecules with an azobenzene unit and a charged functionality. The non-covalent linker binds to the cyclodextrin by host–guest complexation with the azobenzene. Proteins or DNA are then bound to the functionalized vesicles through multivalent electrostatic attraction. The photoresponse of the host–guest complex allows a light-induced switch from the multivalent state that can bind the biomolecules to the low-affinity state of the free linker, thereby providing external control over the cargo release. The major advantage of this delivery approach is the wide variety of targets that can be addressed by multivalent electrostatic interaction, which we demonstrate on four types of DNA and six different proteins.
    Full-text · Article · Jan 2015 · Chemistry - A European Journal
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    ABSTRACT: The development of an effective and general delivery method that can be applied to a large variety of structurally diverse biomolecules remains a bottleneck in modern drug therapy. Herein, we present a supramolecular system for the dynamic trapping and light-stimulated release of both DNA and proteins. Self-assembled ternary complexes act as nanoscale carriers, comprising vesicles of amphiphilic cyclodextrin, the target biomolecules and linker molecules with an azobenzene unit and a charged functionality. The non-covalent linker binds to the cyclodextrin by host–guest complexation with the azobenzene. Proteins or DNA are then bound to the functionalized vesicles through multivalent electrostatic attraction. The photoresponse of the host–guest complex allows a light-induced switch from the multivalent state that can bind the biomolecules to the low-affinity state of the free linker, thereby providing external control over the cargo release. The major advantage of this delivery approach is the wide variety of targets that can be addressed by multivalent electrostatic interaction, which we demonstrate on four types of DNA and six different proteins.
    Full-text · Article · Jan 2015 · Chemistry - A European Journal
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    ABSTRACT: Aluminium- and gallium-functionalised alkenylalkynylgermanes, R12Ge(CCR2)[C{E(CMe3)2}C(H)R2] (E=Al, Ga), exhibit a close contact between the coordinatively unsaturated Al or Ga atoms and the α-C atoms of the intact ethynyl groups. These interactions activate the GeC(alkynyl) bonds and favour the thermally induced insertion of these C atoms into the EC(vinyl) bonds by means of 1,1-carbalumination or 1,1-carbagallation reactions. For the first time the latter method was shown to be a powerful alternative to known metallation processes. Germacyclobutenes with an unsaturated GeC3 heterocycle and endo- and exocyclic CC bonds resulted from concomitant GeC bond formation to the β-C atoms of the alkynyl groups. These heterocyclic compounds show an interesting photoluminescence behaviour with Stokes shifts of >110 nm. The fascinating properties are based on extended π-delocalisation including σ*-orbitals localised at Ge and Al. High-level quantum chemical DFT and TD-DFT calculations for an Al compound were applied to elucidate their absorption and emission properties. They revealed a biradical excited state with the transfer of a π-electron into the empty p-orbital at Al and a pyramidalisation of the metal atom.
    No preview · Article · Dec 2014 · Chemistry - A European Journal
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    ABSTRACT: Friction and adhesion between two β-cyclodextrin functionalized surfaces can be switched reversibly by external light stimuli. The interaction between the cyclodextrin molecules attached to the tip of an atomic force microscope and a silicon wafer surface is mediated by complexation of ditopic azobenzene guest molecules. At the single molecule level, the rupture force of an individual complex is 61±10 pN.
    Full-text · Article · Dec 2014 · Chemical Communications
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    ABSTRACT: A novel method to prepare microstructured polymer brushes using TiO2 nanoparticles and photocatalytic microcontact printing is described. It is shown that ethanol amine can be polymerized to linear polyethyleneimine (PEI) driven by the photocatalytic action of TiO2. Upon UV irradiation during microcontact printing of ethanol amine with a stamp coated with TiO2 nanoparticles, patterned polymer brushes with a length of around 50 nm are obtained.
    Preview · Article · Nov 2014 · Chemical Communications
  • Avik Samanta · Bart Jan Ravoo
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    ABSTRACT: Für die Proteomik ist die einfache und effiziente Isolierung von Proteinen aus einer Mischung essenziell. Wir beschreiben hier einen supramolekularen Ansatz, um selektiv Proteine aus einer Proteinmischung einzufangen und mithilfe eines Magnetfeldes auszufällen. Dies geschieht durch die Bildung eines multivalenten selbstorganisierten Komplexes aus einer verdünnten Lösung der folgenden drei Komponenten: Cyclodextrin-beschichtete magnetische Nanopartikel, Adamantan- und Kohlenhydrat-funktionalisierte nichtkovalente Vernetzer und Lectine. Der selbstorganisierte ternäre Komplex wird im Magnetfeld ausgefällt und kann durch die Zugabe eines konkurrierenden Bindungspartners leicht wieder in Lösung gebracht werden. Wir zeigen, dass dieser supramolekulare Ansatz zur Aufreinigung von Proteinen mithilfe magnetischer Extraktion hoch selektiv und effizient erfolgt.
    No preview · Article · Nov 2014 · Angewandte Chemie
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    ABSTRACT: In this article we report the preparation and characterization of a peptide-based hydrogel, which possesses characteristic rheological properties, is pH responsive and can be functionalized at its thiol function. The tripeptide N-(fluorenyl-9-methoxycarbonyl)-L-Cys(acetamidomethyl)-L-His-L-Cys-OH 1 forms stable supramolecular aggregates in water leading to hydrogels above 1.5 wt%. Rheological analysis of the hydrogel revealed visco-elastic and shear thinning properties of samples containing 1.5 wt% of peptide 1. The hydrogel reversibly responds to pH changes. Below and above pH 6, electrostatic repulsion of the peptide results in a weakening of the three- dimensional gel network. Based on atomic force microscopy, small angle X-ray scattering and molecular dynamics simulations, it is proposed that the peptide assembles in bilayers, which form tapes that tend to entangle at higher concentrations in water. The development of functional materials based on the peptide assemblies was possible via thiol-ene-click chemistry of the free thiol function at the C-terminal cysteine unit. As a proof of concept, the functionalization with adamantyl units to give 1-Ad was shown by molecular recognition of β cyclodextrin vesicles. These vesicles were used as supramolecular cross-linkers of the assemblies of peptide 1 mixed with peptide 1-Ad leading to gel networks at a reduced peptide concentration.
    Preview · Article · Oct 2014 · Organic & Biomolecular Chemistry
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    ABSTRACT: Controlled fusion events between natural membranes composed of phospholipids with synthetic unnatural membranes will yield valuable fundamental information on the mechanism of membrane fusion. Here, fusion between vastly different phospholipid liposomes and cyclodextrin amphiphile based vesicles (CDVs) controlled by a pair of coiled coil forming lipidated peptides was investigated. Fusion events were characterized using lipid and content mixing assays and the resulting hybrid assemblies were characterized with cryo-TEM imaging. The secondary/quaternary structure of the lipidated peptides at the membrane interface was studied using circular dichroism spectroscopy. This is the first example of targeted fusion between natural and non-natural bilayer membranes and the in situ formation of hybrid CDV/liposome structures is of interest as it yields fundamental information about the mechanism through which fusion proceeds.
    No preview · Article · Oct 2014 · Soft Matter

Publication Stats

4k Citations
918.05 Total Impact Points

Institutions

  • 2008-2016
    • University of Münster
      • Institute of Organic Chemistry
      Muenster, North Rhine-Westphalia, Germany
  • 2004-2008
    • University of Twente
      • Institute for Nanotechnology (MESA+)
      Enschede, Overijssel, Netherlands
  • 2007
    • Technische Universiteit Eindhoven
      Eindhoven, North Brabant, Netherlands
  • 2000-2005
    • University College Dublin
      • Centre for Synthesis and Chemical Biology
      Dublin, Leinster, Ireland
  • 1994-2001
    • University of Groningen
      • • Groningen Biomolecular Sciences and Biotechnology Institute (GBB)
      • • Molecular Inorganic Chemistry Group
      Groningen, Groningen, Netherlands