Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Covalent Attachment of 1-Alkenes to Oxidized Platinum Surfaces
    Alonso Carnicero, J.M. ; Fabre, B. ; Trilling, A.K. ; Scheres, L.M.W. ; Franssen, M.C.R. ; Zuilhof, H. - \ 2015
    Langmuir 31 (2015)9. - ISSN 0743-7463 - p. 2714 - 2721.
    self-assembled monolayers - organic monolayers - gold - alkanethiols - functionalization - spectroscopy - activation - alkenes - layers - films
    We report the formation of covalently bound alkyl layers onto oxidized Pt (PtOx) substrates by reaction with 1-alkenes as a novel way to bind organic molecules to metal surfaces. The organic layers were characterized by static contact angle, infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The grafted alkyl layers display a hydrolytic stability that is comparable to that of alkyl thiols on Au. PtOx-alkene attachment is compatible with terminal ester moieties enabling further anchoring of functional groups, such as redox-active ferrocene, and thus has great potential to extend monolayer chemistry on noble metals.
    Surface characterization and antifouling properties of nanostructured gold chips for imaging surface plasmon resonance biosensing
    Joshi, S. ; Pellacani, P. ; Beek, T.A. van; Zuilhof, H. ; Nielen, M.W.F. - \ 2015
    Sensors and Actuators B: Chemical 209 (2015). - ISSN 0925-4005 - p. 505 - 514.
    mass-spectrometry - nonspecific adsorption - zwitterionic polymers - organic monolayers - films - spr - functionalization - immobilization - microscopy - proteins
    Surface Plasmon Resonance (SPR) optical sensing is a label-free technique for real-time monitoring of biomolecular interactions. Recently, a portable imaging SPR (iSPR) prototype instrument, featuring a nanostructured gold chip, has been developed. In the present work, we investigated the crucial first steps, prior to eventual use of the nanostructured iSPR chip, i.e., its surface modification, in-depth surface characterization and the antifouling performance. Results were compared with conventional flat (i)SPR gold chips having the same surface chemistries, viz. different types of polyethylene glycol and zwitterionic polymers. Characterization of the (i)SPR chips before and after surface modification was performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), water contact angle (WCA), X-ray photoelectron spectroscopy (XPS) and direct analysis in real time high resolution mass spectrometry (DART-HRMS). The antifouling properties were then studied using the nanostructured chip in the portable iSPR instrument and the flat gold chip in conventional SPR setup. The zwitterionic polymer surface chemistries showed the best antifouling properties. Comparison of the nanostructured iSPR chips with conventional flat (i)SPR gold chips showed that the latter perform slightly better in terms of surface modification as well as antifouling properties. The portable iSPR instrument is almost as sensitive as conventional iSPR (IBIS) and nine times less sensitive than conventional SPR (Biacore 3000). The nanostructured iSPR chip, along with the portable instrument, offers the advantage of about ten-fold reduction in instrument size, weight and costs compared to conventional (i)SPR instruments using flat gold, thus making it highly interesting for future biosensing applications.
    Electrokinetics as an alternative to neutron reflectivity for evaluation of segment density distribution in PEO brushes
    Zimmermann, R. ; Romeis, D. ; Bihannic, I. ; Cohen Stuart, M.A. ; Sommer, J.U. ; Werner, C. ; Duval, J.F.L. - \ 2014
    Soft Matter 10 (2014)39. - ISSN 1744-683X - p. 7804 - 7809.
    poly(ethylene oxide) brushes - diffuse soft interfaces - grafted polymer brush - streaming current - aqueous-solution - behavior - films - water - thin - surfaces
    Unravelling details of charge, structure and molecular interactions of functional polymer coatings defines an important analytical challenge that requires the extension of current methodologies. In this article we demonstrate how streaming current measurements interpreted with combined self consistent field (SCF) and soft surface electrokinetic theories allow the evaluation of the segment distribution within poly(ethylene oxide) (PEO) brushes beyond the resolution limits of neutron reflectivity technique.
    Dans met mij : een vierkante kilometer in Dreischor
    Alterra - Centrum Landschap, ; Loo, S. van der; Zaken, D. van der - \ 2014
    films - wijnbouw - cultuur - landbouwbedrijven - plattelandsbevolking - landgebruik - mensen - menselijk gedrag - platteland - boeren - zeeuwse eilanden - levensgeschiedenis - films - viticulture - culture - farms - rural population - land use - people - human behaviour - rural areas - farmers - zeeuwse eilanden - life history
    De film gaat over één willekeurige vierkante kilometer in Dreischor, waar de makers ruim een jaar hebben gefilmd en de verhalen van een aantal mensen hebben gevolgd. De documentaire is onderdeel van een groter project onder de titel 'Venster op de wereld' waarbij ook een boek verschijnt, waarin onder andere een aantal wetenschappers zich over de kilometer hebben gebogen. Het project is een initiatief van Joop Schaminée en Anton Stortelder, Alterra Wageningen UR
    Hydrolytic and Thermal Stability of Organic Monolayers on Various Inorganic Substrates
    Bhairamadgi, N.S. ; Pujari, S.P. ; Trovela, F.G. ; Debrassi, A. ; Khamis, A.A.M. ; Alonso Carnicero, J.M. ; Zahrani, A.A. Al; Wennekes, T. ; Al-Turaif, H.A. ; Rijn, C.J.M. van; Alhamed, Y.A. ; Zuilhof, H. - \ 2014
    Langmuir 30 (2014)20. - ISSN 0743-7463 - p. 5829 - 5839.
    self-assembled monolayers - hydrogen-terminated silicon - oxidized si(100) surface - alkyl monolayers - nitride surfaces - aluminum-oxide - gold - phosphonate - films - attachment
    A comparative study is presented of the hydrolytic and thermal stability of 24 different kinds of monolayers on Si(111), Si(100), SiC, SiN, SiO2, CrN, ITO, PAO, Au, and stainless steel surfaces. These surfaces were modified utilizing appropriate organic compounds having a constant alkyl chain length (C18), but with different surface-reactive groups, such as 1-octadecene, 1-octadecyne, 1-octadecyltrichlorosilane, 1-octadecanethiol, 1-octadecylamine and 1-octadecylphosphonic acid. The hydrolytic stability of obtained monolayers was systematically investigated in triplicate in constantly flowing aqueous media at room temperature in acidic (pH 3), basic (pH 11), phosphate buffer saline (PBS) and deionized water (neutral conditions), for a period of 1 day, 7 days, and 30 days, yielding 1152 data points for the hydrolytic stability. The hydrolytic stability was monitored by static contact angle measurements and X-ray photoelectron spectroscopy (XPS). The covalently bound alkyne monolayers on Si(111), Si(100), and SiC were shown to be among the most stable monolayers under acidic and neutral conditions. Additionally, the thermal stability of 14 different monolayers was studied in vacuum using XPS at elevated temperatures (25–600 °C). Similar to the hydrolytic stability, the covalently bound both alkyne and alkene monolayers on Si(111), Si(100) and SiC started to degrade from temperatures above 260 °C, whereas on oxide surfaces (e.g., PAO) phosphonate monolayers even displayed thermal stability up to ~500 °C.
    Adhesion and friction properties of fluoropolymer brushes: On the tribological inertness of fluorine
    Bhairamadgi, N.S. ; Pujari, S.P. ; Rijn, C.J.M. van; Zuilhof, H. - \ 2014
    Langmuir 30 (2014)42. - ISSN 0743-7463 - p. 12532 - 12540.
    poly(methyl methacrylate) brushes - self-lubricating composites - organic mono layers - polymer brushes - silicon(111) surfaces - assembled monolayers - coatings - density - films - wear
    The effects of fluorination on the adhesion and friction properties of covalently bound poly(fluoroalkyl methacrylate) polymer brushes (thickness ~80 nm) were systematically investigated. Si(111) surfaces were functionalized with a covalently bound initiator via a thiol–yne click reaction to have a high surface coverage for initiator immobilization. Surface-initiated atom-transfer radical polymerization (SI-ATRP) was employed for the synthesis of four different fluoropolymer brushes (SPFx, where x = 0, 3, 7, or 17 F atoms per monomer), based on fluoroalkyl methacrylates. All polymer brushes were characterized with static contact angle measurements, X-ray photoelectron spectroscopy (XPS), and infrared absorption reflection spectroscopy (IRRAS). The polymer brushes exhibited an excellent hydrophobicity, with static water contact angles of up to 121° depending on the number of fluorine atoms per side chain in fluoroalkyl methacrylate. The degree of swelling was precisely studied by using ellipsometry in different solvents such as acetone, hexadecane, hexafluoroisopropanol, nonafluorobutyl methyl ether, and Fluorinert FC-40. The polymer brushes have shown nanoscale swelling behavior in all solvents except hexadecane. The grafting density decreased upon increasing fluorine content in polymer brushes from 0.65 chains/nm2 (SPF0) to 0.10 chains/nm2 (SPF17) as observed in Fluorinert FC-40 as a good solvent. Adhesion and friction force measurements were conducted with silica colloidal probe atomic force microscopy (CP-AFM) under ambient, dry (argon), and lubricating fluid conditions. SPF17 showed the lowest coefficient of friction 0.005 under ambient condition (RH = 44 ± 2%) and a further decrease with 50% under fluidic conditions. These polymer brushes also showed adhesion forces as low as 6.9 nN under ambient conditions, which further went down to 0.003 nN under fluidic conditions (Fluorinert FC-40 and hexadecane) at 10 nN force.
    Barrier properties and storage stability of edible coatings prepared with electrospraying
    Khan, M.K.I. ; Schutyser, M.A.I. ; Schroen, C.G.P.H. ; Boom, R.M. - \ 2014
    Innovative Food Science and Emerging Technologies 23 (2014). - ISSN 1466-8564 - p. 182 - 187.
    fatty-acids - films - powder
    Electrospraying is a novel technique for the application of coating to foods. In this study, thin lipid-based coatings were prepared by electrospraying on model surface and evaluated for their moisture barrier functionality. Sunflower oil and chocolate based coating materials were electrosprayed at elevated temperature (60 °C) using a multiple nozzle system. Sunflower oil coated the sides and top surface of the target surfaces, while chocolate based material deposited primarily on the top surface. In chocolate based coatings, larger droplet size and related lower charge to mass ratio explained the limited “wrap-around effect”. Sunflower oil based coating penetrated into the target surfaces, which could be reduced by the addition of stearic acid (up to 0.15 g/g). However, this addition resulted in crystallization and crack formation during storage, and ultimately reduced barrier functionality. Conversely, chocolate-based material produced thicker coatings (up to 0.3 mm), which were more stable during storage and exhibited enhanced barrier properties.
    Intrinsic and Ionic Conduction in Humidity-Sensitive Sulfonated Polyaniline
    Doan, D.C.T. ; Ramaneti, R. ; Baggerman, J. ; Tong, H.D. ; Marcelis, A.T.M. ; Rijn, C.J.M. van - \ 2014
    Electrochimica Acta 127 (2014). - ISSN 0013-4686 - p. 106 - 114.
    acid-doped polyaniline - poly(vinyl alcohol) - charge-transfer - electrical-conductivity - impedance spectroscopy - polymer electrolytes - gaseous substances - water-absorption - sensors - films
    The influence of humidity on the conductivity of sulfonated polyaniline (SPANI) and polyaniline (PANI) is investigated with electrochemical impedance spectroscopy (EIS). Separation of intrinsic (q) and ionic charge (i) mobility was observed using combination of ac and dc impedance measurements at relative humidity concentrations (RH) ranging from 5% to 90%. Nyquist plots for both acid-doped PANI and SPANI show single semicircles at all humidity levels, indicating pure intrinsic charge conduction. These semicircles are well-described with a fairly constant capacitance (C1 = 10-10 F) of the interdigitated electrodes and with a large varying resistance (10 kO - 10 MO) of the polymer bulk with RC times in the order of 10-3-10-6 sec. In all samples the intrinsic charge conductivity increases for increasing humidity, with emeraldine base PANI showing the strongest (2 orders of magnitude) increase upon raising the RH from 5 - 90%. A partial exchange of protons with sodium ions in SPANI (SPAN-Na) induces a second semicircle in the Nyquist plot at humidity levels above 40% with large RC (t) values (10-1 - 10-3 sec), which is attributed to a contribution of ionic mobility at the polymer/electrode interface. Blends of SPAN-Na with poly(vinyl alcohol) (PVA) and polyacrylamide (PAA) also show a second semicircle at slightly higher humidity levels from 50% RH onwards. In all cases this second semicircle is best described by a Constant Phase Element (CPE) with a value (order typical 10-9 - 10-7 F) increasing at higher humidity, and a varying exponential coefficient (0.5
    Shape recognition of microbial cells by colloidal cell imprints
    Borovicka, J. ; Stoyanov, S.D. ; Paunov, V.N. - \ 2013
    Nanoscale 5 (2013)18. - ISSN 2040-3364 - p. 8560 - 8568.
    nanoparticles - silver - shells - films
    We have engineered a class of colloids which can recognize the shape and size of targeted microbial cells and selectively bind to their surfaces. These imprinted colloid particles, which we called "colloid antibodies", were fabricated by partial fragmentation of silica shells obtained by templating the targeted microbial cells. We successfully demonstrated the shape and size recognition between such colloidal imprints and matching microbial cells. High percentage of binding events of colloidal imprints with the size matching target particles was achieved. We demonstrated selective binding of colloidal imprints to target microbial cells in a binary mixture of cells of different shapes and sizes, which also resulted in high binding selectivity. We explored the role of the electrostatic interactions between the target cells and their colloid imprints by pre-coating both of them with polyelectrolytes. Selective binding occurred predominantly in the case of opposite surface charges of the colloid cell imprint and the targeted cells. The mechanism of the recognition is based on the amplification of the surface adhesion in the case of shape and size match due to the increased contact area between the target cell and the colloidal imprint. We also tested the selective binding for colloid imprints of particles of fixed shape and varying sizes. The concept of cell recognition by colloid imprints could be used for development of colloid antibodies for shape-selective binding of microbes. Such colloid antibodies could be additionally functionalized with surface groups to enhance their binding efficiency to cells of specific shape and deliver a drug payload directly to their surface or allow them to be manipulated using external fields. They could benefit the pharmaceutical industry in developing selective antimicrobial therapies and formulations.
    Surface Pressure and Elasticity of Hydrophobin HFBII Layers on the Air-Water Interface: Rheology Versus Structure Detected by AFM Imaging
    Stanimirova, R.D. ; Gurkov, T.D. ; Kralchevsky, P.A. ; Balashev, K.T. ; Stoyanov, S.D. ; Pelan, E.G. - \ 2013
    Langmuir 29 (2013)20. - ISSN 0743-7463 - p. 6053 - 6067.
    class-ii hydrophobins - air/water interface - langmuir monolayers - trichoderma-reesei - proteins - films - adsorption - stability - emulsions - mechanisms
    Here, we combine experiments with Langmuir trough and atomic force microscopy (AFM) to investigate the reasons for the special properties of layers from the protein HFBII hydrophobin spread on the airwater interface. The hydrophobin interfacial layers possess the highest surface dilatational and shear elastic moduli among all investigated proteins. The AFM images show that the spread HFBII layers are rather inhomogeneous, (i.e., they contain voids, monolayer and multilayer domains). A continuous compression of the layer leads to filling the voids and transformation of a part of the monolayer into a trilayer. The trilayer appears in the form of large surface domains, which can be formed by folding and subduction of parts from the initial monolayer. The trilayer appears also in the form of numerous submicrometer spots, which can be obtained by forcing protein molecules out of the monolayer and their self-assembly into adjacent pimples. Such structures are formed because not only the hydrophobic parts, but also the hydrophilic parts of the HFBII molecules can adhere to each other in the water medium. If a hydrophobin layer is subjected to oscillations, its elasticity considerably increases, up to 500 mN/m, which can be explained with compaction. The relaxation of the layers tension after expansion or compression follows the same relatively simple law, which refers to two-dimensional diffusion of protein aggregates within the layer. The characteristic diffusion time after compression is longer than after expansion, which can be explained with the impedence of diffusion in the more compact interfacial layer. The results shed light on the relation between the mesoscopic structure of hydrophobin interfacial layers and their unique mechanical properties that find applications for the production of foams and emulsions of extraordinary stability; for the immobilization of functional molecules at surfaces, and as coating agents for surface modification.
    On the link between surface rheology and foam disproportionation in mixed hydrophobin HFBII and whey protein systems
    Blijdenstein, T.B.J. ; Ganzevles, R.A. ; Groot, P.W.N. de; Stoyanov, S.D. - \ 2013
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 438 (2013). - ISSN 0927-7757 - p. 13 - 20.
    class-ii hydrophobins - air/water interface - trichoderma-reesei - adsorption layers - shear rheology - films - displacement - stability - casein - sc3
    Here we study the surface dilational properties of spread and adsorbed layers of whey protein isolate (WPI) and hydrophobin HFBII at air/water interface using Langmuir trough and relate them to foam ability and stability. In spread and adsorbed systems, a gradual increase in modulus with the fraction of HFBII on the surface or in the bulk is observed and we can identify distinct regimes of WPI-dominated and HFBII-dominated behaviour. The dominance of HFBII is further substantiated by visual observation of microscopic wrinkles appearing on the surface of the trough. When comparing spread to adsorbed systems, it was found that a higher HFBII fraction is needed to obtain HFBII dominant behaviour in spread layers than in case of adsorbing layers (f(HFBII) = 0.6 and 0.2 respectively). Furthermore, our results indicate that the HFBII-contribution to the interfacial behaviour becomes more pronounced upon sequential large scale compression/expansion cycles. In order to explain this non-trivial behaviour, we propose that there is multi-layer formation at the interface, having a top layer enriched in HFBII and bottom layer enriched in WPI. It is also concluded that coarsening stability in foams corresponds more closely to the surface dilational properties measured in adsorbing layers than those in spread layers. Finally, it was observed that in mixed systems of HFBII and WPI, the coarsening process levels off, which corresponds to the increased dominance of HFBII in mixed WPI:HFBII-layers upon large surface deformation that is occurring at the surface of shrinking bubbles. (C) 2013 Elsevier B.V. All rights reserved.
    Highly wear-resistant ultra-thin per-fluorinated organic monolayers on silicon(III) surfaces
    Pujari, S.P. ; Zuilhof, H. - \ 2013
    Applied Surface Science 287 (2013). - ISSN 0169-4332 - p. 159 - 164.
    self-assembled monolayers - atomic-force microscopy - nanotribological characterization - mono layers - friction - films - phosphonate - mechanisms - 1-alkynes - stability
    This study reports on fluorine-containing alkyne-derived monolayers onto Si(1 1 1) substrates to obtain densely packed, highly wear-resistant surfaces. The nano-wear properties were measured using atomic force microscopy (AFM). The presence of the fluorinated monolayers was found to enhance the wear properties of the silicon surfaces, with a decrease of the depth of wear scratches of up to 120 times as compared to the unmodified surface. Ultimately, the scratch depth was only 6 nm for a heptadecafluoro-alkyl based monolayer for scratching normal forces as high as 38 µN.
    Pearl-necklace complexes of flexible polyanions with neutral-cationic diblock copolymers
    Golinska, M.D. ; Wolf, F.A. de; Cohen Stuart, M.A. ; Hernandez Garcia, A. ; Vries, R.J. de - \ 2013
    Soft Matter 9 (2013)28. - ISSN 1744-683X - p. 6406 - 6411.
    polyelectrolyte complexes - core micelles - poor solvent - adsorption - stability - brushes - films
    We study the complexation of very asymmetric diblock copolymers (consisting of a cationic block of 12 lysines connected to a 400 amino acid long hydrophilic polypeptide block with a net charge that is nearly zero) with oppositely charged sodium poly(acrylic acid) (NaPAA) with a range of molar masses between 2 and 1300 kg mol-1. For shorter Na-PAA chains, spherical complex coacervate micelles are formed, but for long Na-PAA chains, with molar masses in excess of 250 kg mol-1, atomic force microscopy indicates the presence of pearl-necklace structures. Complexes most likely consist of only a single NaPAA chain, complexed to multiple diblocks. Hence, the size of the complexes can be fully controlled via the NaPAA molar mass. The occurrence of pearl-necklace complexes at higher NaPAA molar masses is attributed to the packing frustration that arises due to the small size of the cationic block of the diblock copolymers
    Protein Concentration and Protein-Exposed Hydrophobicity as Dominant Parameters Determining the Flocculation of Protein-Stabilized Oil-in-Water Emulsions
    Delahaije, R.J.B.M. ; Wierenga, P.A. ; Nieuwenhuijzen, N.H. van; Giuseppin, M.L.F. ; Gruppen, H. - \ 2013
    Langmuir 29 (2013)37. - ISSN 0743-7463 - p. 11567 - 11574.
    diffusing wave spectroscopy - beta-lactoglobulin - coalescence stability - disjoining pressure - drop size - adsorption - films - ph - interfaces - ovalbumin
    DLVO theory is often considered to be applicable to the description of flocculation of protein-stabilized oil-in-water emulsions. To test this, emulsions made with different globular proteins (ß-lactoglobulin, ovalbumin, patatin, and two variants of ovalbumin) were compared under different conditions (pH and electrolyte concentration). As expected, flocculation was observed under conditions in which the zeta potential is decreased (around the isoelectric point and at high ionic strength). However, the extent of flocculation at higher ionic strength (>50 mM NaCl) decreased with increasing protein-exposed hydrophobicity. A higher exposed hydrophobicity resulted in a higher zeta potential of the emulsion droplets and consequently increased stability against flocculation. Furthermore, the addition of excess protein strongly increased the stability against salt-induced flocculation, which is not described by DLVO theory. In the protein-poor regime, emulsions showed flocculation at high ionic strength (>100 mM NaCl), whereas emulsions were stable against flocculation if excess protein was present. This research shows that the exposed hydrophobicity of the proteins and the presence of excess protein affect the flocculation behavior.
    Surface shear rheology of hydrophobin adsorption layers: laws of viscoelastic behaviour with applications to long-term foam stability
    Danov, K.D. ; Radulova, G.M. ; Kralchevsky, P.A. ; Golemanov, K. ; Stoyanov, S.D. - \ 2012
    Faraday Discussions 158 (2012). - ISSN 1359-6640 - p. 195 - 221.
    class-ii hydrophobins - air-water-interface - hfbii hydrophobin - liquid interfaces - protein layers - beta-casein - films - elasticity - monolayers - viscosity
    The long-term stabilization of foams by proteins for food applications is related to the ability of proteins to form dense and mechanically strong adsorption layers that cover the bubbles in the foams. The hydrophobins represent a class of proteins that form adsorption layers of extraordinary high shear elasticity and mechanical strength, much higher than that of the common milk and egg proteins. Our investigation of pure and mixed (with added beta-casein) hydrophobin layers revealed that their rheological behavior obeys a compound rheological model, which represents a combination of the Maxwell and Herschel-Bulkley laws. It is remarkable that the combined law is obeyed not only in the simplest regime of constant shear rate (angle ramp), but also in the regime of oscillatory shear strain. The surface shear elasticity and viscosity, E-sh and eta(sh), are determined as functions of the shear rate by processing the data for the storage and loss moduli, G' and G ''. At greater strain amplitudes, the spectrum of the stress contains not only the first Fourier mode, but also the third one. The method is extended to this non-linear regime, where the rheological parameters are determined by theoretical fit of the experimental Lissajous plot. The addition of beta-casein to the hydrophobin leads to softer adsorption layers, as indicated by their lower shear elasticity and viscosity. The developed approach to the rheological characterization of interfacial layers allows optimization and control of the performance of mixed protein adsorption layers with applications in food foams.
    Ultralow Adhesion and Friction of Fluoro-Hydro Alkyne-Derived Self-Assembled Monolayers on H-terminated Si(III)
    Pujari, S.P. ; Spruijt, E. ; Cohen Stuart, M.A. ; Rijn, C.J.M. van; Paulusse, J.M.J. ; Zuilhof, H. - \ 2012
    Langmuir 28 (2012)51. - ISSN 0743-7463 - p. 17690 - 17700.
    atomic-force microscopy - covalently attached monolayers - extremely mild attachment - silicon surfaces - organic monolayers - porous silicon - visible-light - x-ray - gold - films
    New fluorine-containing terminal alkynes were synthesized and self-assembled onto Si(111) substrates to obtain fluorine-containing organic monolayers. The monolayers were analyzed in detail by ellipsometry, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS), static water contact angle measurements (CA), and atomic force microscopy (AFM). The SAMs exhibit excellent hydrophobicity, with static water contact angles of up to 119° and low critical surface tensions of 5-20 mN/m depending on the number of F atoms per molecule. IRRAS confirmed the formation of highly ordered monolayers, as indicated by the antisymmetric and symmetric stretching vibrations of the CH(2) moieties at 2918-2920 and 2850-2851 cm(-1), respectively. Upon increasing the number of fluorine atoms in the alkyne chains from 0 to 17, the adhesion of bare silica probes to the SAMs in air decreases from 11.6 ± 0.20 mJ/m(2) for fluorine-free (F0) alkyne monolayers to as low as 3.2 ± 0.03 mJ/m(2) for a heptadecafluoro-hexadecyne (F17)-based monolayer. Likewise, the friction coefficient decreases from 5.7 × 10(-2) to 1.2 × 10(-2). The combination of high ordering, excellent hydrophobicity, low adhesion, and low friction makes these fluoro-hydro alkyne-derived monolayers highly promising candidates for use in high-performance microelectronic devices.
    Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level
    Yaffe, O. ; Qi, Y. ; Scheres, L.M.W. ; Puniredd, S.R. ; Segev, L. ; Ely, T. ; Haick, H. ; Zuilhof, H. ; Vilan, A. ; Kronik, L. ; Kahn, A. ; Cahen, D. - \ 2012
    Physical Review. B : Condensed Matter and Materials Physics 85 (2012)4. - ISSN 1098-0121
    self-assembled monolayers - electronic transport - molecular electronics - organic-molecules - chain monolayers - spectroscopy - polyethylene - conductance - films - photoemission
    We compare the charge transport characteristics of heavy-doped p(++)- and n(++)-Si-alkyl chain/Hg junctions. Based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction we suggest that for both p(++)- and n(++)-type junctions, the energy difference between the Fermi level and lowest unoccupied molecular orbital (LUMO), i.e., electron tunneling, controls charge transport. This conclusion is supported by results from photoelectron spectroscopy (ultraviolet photoemission spectroscopy, inverse photoelectron spectroscopy, and x-ray photoemission spectroscopy) for the molecule-Si band alignment at equilibrium, which clearly indicate that the energy difference between the Fermi level and the LUMO is much smaller than that between the Fermi level and the highest occupied molecular orbital (HOMO). Furthermore, the experimentally determined Fermi level - LUMO energy difference, agrees with the non-resonant tunneling barrier height, deduced from the exponential length attenuation of the current.
    Copper-Free Click Biofunctionalization of Silicon Nitride Surfaces via Strain-Promoted Alkyne-Azide Cycloaddition Reactions
    Manova, R.K. ; Pujari, S.P. ; Weijers, C.A.G.M. ; Zuilhof, H. ; Beek, T.A. van - \ 2012
    Langmuir 28 (2012)23. - ISSN 0743-7463 - p. 8651 - 8663.
    self-assembled monolayers - terminated monolayers - organic monolayers - one-step - functionalization - chemistry - films - immobilization - microarrays - dna
    Cu-free "click" chemistry is explored on silicon nitride (Si3N4) surfaces as an effective way for oriented immobilization of biomolecules. An omega-unsaturated ester was grafted onto Si3N4 using UV irradiation. Hydrolysis followed by carbodiimide-mediated activation yielded surface-bound active succinimidyl and pentafluorophenyl ester groups. These reactive surfaces were employed for the attachment of bicyclononyne with an amine spacer, which subsequently enabled room temperature strain-promoted azide alkyne cycloaddition (SPAAC). This stepwise approach was characterized by means of static water contact angle, X-ray photoelectron spectroscopy, and fluorescence microscopy. The surface-bound SPAAC reaction was studied with both a fluorine-tagged azide and an azide-linked lactose, yielding hydrophobic and bioactive surfaces for which the presence of trace amounts of Cu ions would have been problematic. Additionally, patterning of the Si3N4 surface using this metal-free click reaction with a fluorescent azide is shown. These results demonstrate the ability of the SPAAC as a generic tool for anchoring complex molecules onto a surface under extremely mild, namely ambient and metal-free, conditions in a clean and relatively fast manner.
    Monodisperse conjugated polymer particles by Suzuki-Miyaura dispersion polymerization
    Kuehne, A.J.C. ; Gather, M.C. ; Sprakel, J.H.B. - \ 2012
    Nature Communications 3 (2012). - ISSN 2041-1723
    aqueous-media - nanoparticles - laser - microparticles - films - gain - dots
    The self-assembly of colloidal building blocks into complex and hierarchical structures offers a versatile and powerful toolbox for the creation of new photonic and optoelectronic materials. However, well-defined and monodisperse colloids of semiconducting polymers, which would form excellent building blocks for such self-assembled materials, are not readily available. Here we report the first demonstration of a Suzuki–Miyaura dispersion polymerization; this method produces highly monodisperse submicrometer particles of a variety of semiconducting polymers. Moreover, we show that these monodisperse particles readily self-assemble into photonic crystals that exhibit a pronounced photonic stopgap.
    Rates of ionic reactions with charged nanoparticles in aqueous media
    Duval, J.F.L. ; Leeuwen, H.P. van - \ 2012
    The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment, & General Theory 116 (2012)25. - ISSN 1089-5639 - p. 6443 - 6451.
    diffusion-controlled reactions - streaming current - particles - electrokinetics - coagulation - complexes - films
    A theory is developed to evaluate the electrostatic correction for the rate of reaction between a small ion and a charged ligand nanoparticle. The particle is assumed to generally consist of an impermeable core and a shell permeable to water and ions. A derivation is proposed for the ion diffusion flux that includes the impact of the equilibrium electrostatic field distribution within and around the shell of the particle. The contribution of the extra- and intraparticulate field is rationalized in terms of a conductive diffusion factor, fel, that includes the details of the particle geometry (core size and shell thickness), the volume charge density in the shell, and the parameters defining the electrostatic state of the particle core surface. The numerical evaluation of fel, based on the nonlinear Poisson–Boltzmann equation, is successfully complemented with semianalytical expressions valid under the Debye–Hückel condition in the limits of strong and weak electrostatic screening. The latter limit correctly includes the original result obtained by Debye in his 1942 seminal paper about the effect of electrostatics on the rate of collision between two ions. The significant acceleration and/or retardation possibly experienced by a metal ion diffusing across a soft reactive particle/solution interphase is highlighted by exploring the dependence of fel on electrolyte concentration, particle size, particle charge, and particle type (i.e., hard, core/shell, and entirely porous particles).
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