Jens Ulstrup

Technical University of Denmark, Lyngby, Capital Region, Denmark

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Publications (285)1115.19 Total impact

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    ABSTRACT: In situ scanning tunneling microscopy combined with density functional theory molecular dynamics simulations reveal a complex structure for the self-assembled monolayer (SAM) of racemic 2-butanethiol on Au(111) in aqueous solution. Six adsorbate molecules occupy a (10×√3)R30° cell organized as two RSAuSR adatom-bound motifs plus two RS species bound directly to face-centered-cubic and hexagonally close-packed sites. This is the first time that these competing head-group arrangements have been observed in the same ordered SAM. Such unusual packing is favored as it facilitates SAMs with anomalously high coverage (30 %), much larger than that for enantiomerically resolved 2-butanethiol or secondary-branched butanethiol (25 %) and near that for linear-chain 1-butanethiol (33 %).
    ChemPhysChem 02/2015; 16(5). DOI:10.1002/cphc.201402904 · 3.36 Impact Factor
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    ABSTRACT: Among the low-index single-crystal gold surfaces, the Au(110) surface is the most active towards molecular adsorption, and the one with fewest electrochemical adsorption data reported. Cyclic voltammetry (CV), electrochemically controlled scanning tunnelling microscopy (EC-STM), and density functional theory (DFT) calculations have been employed in the present study to address the adsorption of the four nucleobases adenine (A), cytosine (C), guanine (G), and thymine (T), on the Au(110)-electrode surface. Au(110) undergoes reconstruction to the (1×3) surface in electrochemical environment, accompanied by a pair of strong voltammetry peaks in the double layer region in acid solutions. Adsorption of the DNA bases gives featureless voltammograms with lower double layer capacitance, suggesting that all the bases are chemisorbed on the Au(110) surface. Further investigation of the surface structures of the adlayers of the four DNA bases by EC-STM disclosed lifting of the Au(110) reconstruction, specific molecular packing in dense monolayers, and pH dependence of the A and G adsorption. DFT computations based on a cluster model for the Au(110) surface were carried out to investigate the adsorption energy and geometry of the DNA bases in different adsorbate orientations. The optimized geometry is further used to compute models for STM images which are compared with the recorded STM images. This has provided insight into the physical nature of the adsorption. The specific orientations of A, C, G, and T on Au(110) and the nature of the physical adsorbate/surface interaction based on the combination of the experimental and theoretical studies are proposed, and differences from nucleobase adsorption on Au(111)- and Au(100)-electrode surfaces discussed.
    The Journal of Physical Chemistry B 01/2015; 119(7). DOI:10.1021/jp511909f · 3.38 Impact Factor
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    ABSTRACT: The triazatriangulene (TATA) ring system was investigated as binding group for tunnel junctions of molecular wires on gold surfaces. Self-assembled monolayers (SAMs) of TATA platforms with three different lengths of phenylene wires were fabricated and their electrical conductance recorded by both conducting probe-atomic force microscopy (CP-AFM) and scanning tunneling microscopy (STM). Similar measurements were performed for phenylene SAMs with thiol anchoring groups as references. It was found that despite the presence of a sp3 hybridized carbon atom in the conduction path, the TATA platform, displays a contact resistance only slightly larger than the thiols. This surprising finding has not been reported before and was analyzed by theoretical computations of the transmission functions of the TATA anchored molecular wires. The relatively low contact resistance of the TATA platform along with its high stability and directionality makes this binding group very attractive for molecular electronic measurements and devices.
    Langmuir 11/2014; DOI:10.1021/la504056v · 4.38 Impact Factor
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    ABSTRACT: The rich stereochemistry of the self-assembled monolayers (SAMs) of the four butanethiols on Au(111) is described, SAMs containing up to 12 individual C, S, or Au chiral centers per surface unit cell. This is facilitated by synthesis of enantiomerically pure 2-butanethiol (the smallest unsubstituted chiral alkanethiol), followed by in situ scanning tunneling microscopy (STM) imaging combined with density-functional theory (DFT) molecular dynamics STM-image simulations. Even though butanethiol SAMs manifest strong head-group interactions, steric interactions are shown to dominate SAM structure and chirality. Indeed, steric interactions are shown to dictate the nature of the head-group itself: whether it takes on the adatom-bound motif RS•Au(0)S•R or else involves direct binding of RS• to face-centered cubic (FCC) or hexagonal close-packed (HCP) sites. Binding as RS• produces large organizationally chiral domains even when R is achiral, while adatom binding leads to rectangular plane groups that suppress long-range expression of chirality. Binding as RS• also inhibits the pitting intrinsically associated with adatom binding, desirably producing more regularly structured SAMs.
    Journal of the American Chemical Society 11/2014; 136(49). DOI:10.1021/ja508100c · 11.44 Impact Factor
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    ABSTRACT: We report a new method for controlling H- and J-stacking in supramolecular self-assembly. Graphene nanosheets act as structure inducers to direct the self-assembly of a versatile organic dye, perylene into two distinct types of functional nanostructures, i.e. one-dimensional nanotubes via J-stacking and two-dimensional branched nanobuds through H-stacking. Graphene integrated supramolecular nanocomposites are highly stable and show significant enhancement of photocurrent generation in these two configurations of photosensing devices, i.e. solid-state optoelectronic constructs and liquid-junction solar cells.
    Nanoscale 08/2014; DOI:10.1039/c4nr02308k · 6.74 Impact Factor
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    ABSTRACT: Nanoporous gold (NPG) is composed of three-dimensional (3D) bicontinuous nanostructures with large surface area. Nano-channels inside NPG provide an ideal local environment for immobilization of enzyme molecules with expected stabilization of the protein molecules. In this work, glucose oxidase (GOx) has been brought to assemble on NPG via surface chemical reactions to form enzyme modified NPG nanomaterial with promising sensitivity for glucose detection. Cyclic voltammetry and single-potential step chronoamperometry (SPSC) are employed to study the electrochemical behavior of both bare and enzyme-modified NPG. Two redox mediators, p-benzoquinone (BQ) and ferrocenecarboxylic acid (FCA) are used to shuttle electrons between the enzyme redox center inside of GOx and the NPG electrode. Diffusion patterns at the functionalized NPG electrode are found significantly different from those on planar gold electrodes. This is mainly caused by internal 3D single crystal-like structures of NPG. Electrostatically neutral BQ mediator gives much higher voltammetric sensitivity than negatively charged FCA for GOx modified NPG electrodes. This study provides insight into the understanding of the intrinsic properties of NPG materials aiming at evolving enzymatic biosensors with high performance.
    Electrochimica Acta 06/2014; 130:559-567. DOI:10.1016/j.electacta.2014.02.146 · 4.09 Impact Factor
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    ABSTRACT: Galactose oxidase (GAOX) is a special metalloenzyme in terms of its active site structure and catalytic mechanisms. This work reports a study where the enzyme confined in a nanoscale chemical environment provided by mesoporous silicas (MPS) is probed. Two types of MPS, i.e. SBA-15 and MCF, were synthesized and used to accommodate GAOX. SBA-15-ROD is rod-shaped particles with periodically ordered nanopores (9.5 nm), while MCF has a mesocellular foam-like structure with randomly distributed pores (23 nm) interconnected by smaller windows (8.8 nm). GAOX is non-covalently confined in SBA-15-ROD, while it is covalently immobilized in MCF. Relatively high loadings in the range of 50–60 mg g À1 are achieved. Electron spin resonance (ESR) spectroscopy is used to probe the active site structures of the enzyme. The similar ESR spectra observed for GAOX in the free and immobilized states support that the electronic structure, particularly the copper catalytic centre of confined GAOX is well retained. The catalytic activity of confined enzyme is high, although the catalytic kinetics is slowed down, mainly attributed to the diffusion limitation of substrate and product in the nanoscale channels. The apparent Michaelis constant (K M) of the enzyme is largely unchanged upon immobilization, while the turnover number (k cat) is slightly reduced. The overall catalytic efficiency, represented by the ratio of k cat /K M , is retained around 70% and 60% for SBA-15 and MCF immobilization, respectively. The thermal resistance is enhanced up to 60 C, but with no further enhancement above 60 C.
    RSC Advances 05/2014; 4(42):21939. DOI:10.1039/c4ra00653d · 3.71 Impact Factor
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    ABSTRACT: We have briefly overviewed recent efforts in the electrochemistry of single transition metal complex, redox metalloprotein, and redox-marked oligonucleotide (ON) molecules. We have particularly studied self-assembled molecular monolayers (SAMs) of several 5'-C6-SH single- (ss) and double-strand (ds) ONs immobilized on Au(111) electrode surfaces via AuS bond formation, using a combination of nucleic acid chemistry, electrochemistry and electrochemically controlled scanning tunnelling microscopy (in situ STM). Ds ONs stabilized by multiply charged cations and locked nucleic acid (LNA) monomers have been primary targets, with a view on stabilizing the ds-ONs and improving voltammetric signals of intercalating electrochemical redox probes. Voltammetric signals of the intercalator anthraquinone monosulfonate (AQMS) at ds-DNA/Au(111) surfaces diluted by mercaptohexanol are significantly sharpened and more robust in the presence than in the absence of [Co(NH3 )6 ](3+) . AQMS also displays robust Faradaic voltammetric signals specific to the ds form on binding to similar LNA/Au(111) surfaces, but this signal only evolves after successive voltammetric scanning into negative potential ranges. Triply charged spermidine (Spd) invokes itself a strong voltammetric signal, which is specific to the ds form and fully matched sequences. This signal is of non-Faradaic, capacitive origin but appears in the same potential range as the Faradaic AQMS signal. In situ STM shows that molecular scale structures of the size of Spd-stabilized ds-ONs are densely packed over the Au(111) surface in potential ranges around the capacitive peak potential.
    ChemPhysChem 07/2013; 14(10). DOI:10.1002/cphc.201300299 · 3.36 Impact Factor
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    ABSTRACT: Coordination chemistry has been a consistently active branch of chemistry since Werner's seminal theory of coordination compounds inaugurated in 1893, with the central focus on transition metal complexes. However, control and measurement of metal–ligand interactions at the single-molecule level remain a daunting challenge. Here we demonstrate an interdisciplinary and systematic approach that enables measurement and modulation of the coordinative bonding forces in a transition metal complex. Terpyridine is derived with a thiol linker, facilitating covalent attachment of this ligand on both gold substrate surfaces and gold-coated atomic force microscopy tips. The coordination and bond breaking between terpyridine and osmium are followed in situ by electrochemically controlled atomic force microscopy at the single-molecule level. The redox state of the central metal atom is found to have a significant impact on the metal–ligand interactions. The present approach represents a major advancement in unravelling the nature of metal–ligand interactions and could have broad implications in coordination chemistry.
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    ABSTRACT: Coordination chemistry has been a consistently active branch of chemistry since Werner's seminal theory of coordination compounds inaugurated in 1893, with the central focus on transition metal complexes. However, control and measurement of metal-ligand interactions at the single-molecule level remain a daunting challenge. Here we demonstrate an interdisciplinary and systematic approach that enables measurement and modulation of the coordinative bonding forces in a transition metal complex. Terpyridine is derived with a thiol linker, facilitating covalent attachment of this ligand on both gold substrate surfaces and gold-coated atomic force microscopy tips. The coordination and bond breaking between terpyridine and osmium are followed in situ by electrochemically controlled atomic force microscopy at the single-molecule level. The redox state of the central metal atom is found to have a significant impact on the metal-ligand interactions. The present approach represents a major advancement in unravelling the nature of metal-ligand interactions and could have broad implications in coordination chemistry.
    Nature Communications 07/2013; 4:2121. DOI:10.1038/ncomms3121 · 10.74 Impact Factor
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    ABSTRACT: Coordination chemistry has been a consistently active branch of chemistry since Werner's seminal theory of coordination compounds inaugurated in 1893, with the central focus on transition metal complexes. However, control and measurement of metal–ligand interactions at the single-molecule level remain a daunting challenge. Here we demonstrate an interdisciplinary and systematic approach that enables measurement and modulation of the coordinative bonding forces in a transition metal complex. Terpyridine is derived with a thiol linker, facilitating covalent attachment of this ligand on both gold substrate surfaces and gold-coated atomic force microscopy tips. The coordination and bond breaking between terpyridine and osmium are followed in situ by electrochemically controlled atomic force microscopy at the single-molecule level. The redox state of the central metal atom is found to have a significant impact on the metal–ligand interactions. The present approach represents a major advancement in unravelling the nature of metal–ligand interactions and could have broad implications in coordination chemistry.
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    ABSTRACT: Although chemically synthesized gold nanoparticles (AuNPs) from gold salt (HAuCl4) are among the most studied nanomaterials, understanding the formation mechanisms is a challenge mainly due to limited dynamics information. A range of in situ methods with down to millisecond (ms) time resolution have been employed in the present report to monitor time-dependent physical and chemical properties in aqueous solution during the chemical synthesis. Chemical synthesis of AuNPs is a reduction process accompanied by release of ions and protons, and formation of solid particles. Dynamic information from redox potential, pH, conductivity, and turbidity of the solution enables distinct observation of reduction and nucleation/growth of AuNPs phases. The dynamics of the electrochemical potential shows that reduction of gold salt (HAuCl4 and its hydrolyzed forms) occurs via intermediate [AuCl2]− to form Au atoms during the early stage of the synthesis process. pH- and conductivity-dynamics point further clearly to formation of coating layers on AuNPs and adsorbate exchange between MES and starch.
    The Journal of Physical Chemistry C 05/2013; 117(22):11818–11828. DOI:10.1021/jp401883h · 4.84 Impact Factor
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    ABSTRACT: Along with reduced graphene oxide (RGO), water soluble Prussian blue nanoparticles (PBNPs, around 6 nm) are synthesized and broadly characterized. These two types of highly stable, low-cost and chemically compatible nanomaterials are exploited as building ingredients to prepare electrically enhanced and functionally endorsed nanohybrid electrocatalysts, which are further transformed into free-standing graphene papers. PBNPs doped graphene papers show highly efficient electrocatalysis towards reduction of hydrogen peroxide and can be used alone as flexible chemical sensors for potential applications in detection of hydrogen peroxide or/and other organic peroxides. The as-prepared PBNPs–RGO papers are further capable of biocompatible accommodation of enzymes for development of free-standing enzyme based biosensors. In this regard, glucose oxidase is used as an example for electrocatalytic oxidation and detection of glucose. The present work demonstrates a facile and highly reproducible way to construct free-standing and flexible graphene paper doped with electroactive catalyst. Thanks to high stability, low-cost and efficient electrocatalytic characteristics, this kind of nanohybrid material has potential to be produced on a large scale, and offers a broad range of possible applications, particularly in the fabrication of flexible sensing devices and as a platform for electrocatalytic energy conversion.
    Advanced Functional Materials 05/2013; 23(43):5297-5306. DOI:10.1002/adfm.2013605 · 10.44 Impact Factor
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    ABSTRACT: Redox reactions of solvated molecular species at gold-electrode surfaces modified by electrochemically inactive self-assembled molecular monolayers (SAMs) are found to be activated by introducing Au nanoparticles (NPs) covalently bound to the SAM to form a reactive Au-alkanedithiol-NP-molecule hybrid entity. The NP appears to relay long-range electron transfer (ET) so that the rate of the redox reaction may be as efficient as directly on a bare Au electrode, even though the ET distance is increased by several nanometers. In this study, we have employed a fast redox reaction of surface-confined 6-(ferrocenyl) hexanethiol molecules and NPs of Au, Pt and Pd to address the dependence of the rate of ET through the hybrid on the particular NP metal. Cyclic voltammograms show an increasing difference in the peak-to-peak separation for NPs in the order Au<Pt<Pd, especially when the length of the alkanedithiol increases from octanedithiol to decanedithiol. The corresponding apparent rate constants, k(app) , for decanedithiol are 1170, 360 and 14 s(-1) for NPs of Au, Pt and Pd, respectively, indicating that the efficiency of NP mediation of the ET clearly depends on the nature of the NP. Based on a preliminary analysis rooted in interfacial electrochemical ET theory, combined with a simplified two-step view of the NP coupling to the electrode and the molecule, this observation is referred to the density of electronic states of the NPs, reflected in a broadening of the molecular electron/NP bridge group levels and energy-gap differences between the Fermi levels of the different metals.
    ChemPhysChem 04/2013; 14(5). DOI:10.1002/cphc.201200901 · 3.36 Impact Factor
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    ABSTRACT: An electrochemical system composed of two polarizable interfaces (the metallic electrode|water and water|ionic liquid interfaces), namely two-polarized-interface (TPI) technique, has been proposed to explore the ion transfer processes between water and moderately hydrophobic ionic liquids (W|mIL), typically 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C8mimC1C1N) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C6mimC1C1N). Within the classic four-electrode system, it is not likely that the ion transfer information at the W|mIL interface can be obtained due to an extremely narrow polarized potential window (ppw) caused by these moderately hydrophobic ionic components. In this article, we show that TPI technique has virtually eliminated the ppw limitation based on a controlling step of concentration polarization at the electrode|water interface. With the aid of this technique, the formal ion transfer potential differences between C1C1N− and Cnmim+ (n=6, 8) were accurately determined for 356 mV and 420 mV at a corresponding interface (W|C6mimC1C1N and W|C8mimC1C1N). Besides, this technique is used to monitor electrochemical polarization at the two W|mIL systems, which exhibits an adaptable polarizability (i.e., a conversion from a nonpolarized interface to a polarized interface). Some of the typical anion transfers at the W|C8mimC1C1N interface have also been investigated, as they are particularly important for ion extraction. The experimental results indicate that this facile TPI technique offers a general avenue to explore ion transfer in multifarious biphasic systems.
    Electroanalysis 04/2013; 25(4). DOI:10.1002/elan.201200123 · 2.82 Impact Factor
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    ABSTRACT: Many key chemical and biochemical reactions, particularly in living cells, take place in confined space at the mesoscopic scale. Toward understanding of physicochemical nature of biomacromolecules confined in nanoscale space, in this work we have elucidated fluorescence effects of a light harvesting complex LH2 in nanoscale chemical environments. Mesoporous silicas (SBA-15 family) with different shapes and pore sizes were synthesized and used to create nanoscale biomimetic environments for molecular confinement of LH2. A combination of UV-vis absorption, wide-field fluorescence microscopy, and in situ ellipsometry supports that the LH2 complexes are located inside the silica nanopores. Systematic fluorescence effects were observed and depend on degree of space confinement. In particular, the temperature dependence of the steady-state fluorescence spectra was analyzed in detail using condensed matter band shape theories. Systematic electronic-vibrational coupling differences in the LH2 transitions between the free and confined states are found, most likely responsible for the fluorescence effects experimentally observed.
    The Journal of Physical Chemistry C 01/2013; 117(6):2868-2878. DOI:10.1021/jp311239y · 4.84 Impact Factor

Publication Stats

5k Citations
1,115.19 Total Impact Points

Institutions

  • 1189–2015
    • Technical University of Denmark
      • Department of Chemistry
      Lyngby, Capital Region, Denmark
  • 2007
    • University of Liverpool
      • Department of Chemistry
      Liverpool, ENG, United Kingdom
  • 2002
    • Aarhus University
      • Department of Chemistry
      Aarhus, Central Jutland, Denmark
  • 1995–2002
    • IT University of Copenhagen
      København, Capital Region, Denmark
  • 1997
    • Forschungszentrum Jülich
      Jülich, North Rhine-Westphalia, Germany
  • 1990–1992
    • Newcastle University
      • School of Chemistry
      Newcastle-on-Tyne, England, United Kingdom
    • University of Southern Denmark
      Odense, South Denmark, Denmark
  • 1991
    • Georgian National Academy of Sciences
      Tbilsi, T'bilisi, Georgia
  • 1979
    • Tel Aviv University
      • Department of Chemistry
      Tell Afif, Tel Aviv, Israel
  • 1977
    • Max Planck Society
      München, Bavaria, Germany