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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    Hydrodynamic model for drying emulsions
    Feng Huanhuan, Huanhuan ; Sprakel, J.H.B. ; Gucht, J. van der - \ 2015
    Physical Review. E, Statistical nonlinear, and soft matter physics 92 (2015). - ISSN 2470-0045 - 8 p.
    latex film formation - atomic-force microscopy - foam drainage - phase inversion - coalescence - dynamics - surfactant - stability - fracture
    We present a hydrodynamic model for film formation in a dense oil-in-water emulsion under a unidirectional drying stress. Water flow through the plateau borders towards the drying end leads to the buildup of a pressure gradient. When the local pressure exceeds the critical disjoining pressure, the water films between droplets break and the droplets coalesce. We show that, depending on the critical pressure and the evaporation rate, the coalescence can occur in two distinct modes. At low critical pressures and low evaporation rates, coalescence occurs throughout the sample, whereas at high critical pressures and high evaporation rate, coalescence occurs only at the front. In the latter case, an oil layer develops on top of the film, which acts as a diffusive barrier and slows down film formation. Our findings, which are summarized in a state diagram for film formation, are in agreement with recent experimental findings.
    Dynamics of substituted Alkyl monolayers covalently bonded to silicon: A broadband admittance spectroscopy study
    Godet, C. ; Fadjie-Djomkam, A.B. ; Ababou-Girard, S. ; Tricot, S. ; Turban, P. ; Li, Y. ; Pujari, S.P. ; Scheres, L.M.W. ; Zuilhof, H. ; Fabre, B. - \ 2014
    The Journal of Physical Chemistry Part C: Nanomaterials and Interfaces 118 (2014)13. - ISSN 1932-7447 - p. 6773 - 6787.
    self-assembled monolayers - hydrogen-terminated silicon - atomic-force microscopy - oxide-free silicon - molecular-dynamics - organic monolayers - si(111) surfaces - dielectric-relaxation - electrical-properties - electronic-properties
    The relaxation dynamics of surface-bound n-alkyl chains was studied by broadband admittance spectroscopy (10 mHz–10 MHz) measured at low temperature (130–300 K) in the reverse bias regime of rectifying Hg//organic monolayer (OML)–n-doped Si tunnel junctions. To obtain molecular-level information on the structure and dynamics of grafted monolayers, carboxyl or amide dipolar moieties were located either at the top free surface with variable acid concentration (0%, 5%, and 100%) or at the inner position in the alkyl backbone (100% amide units). Two classes of dipolar relaxation mechanisms are found with different thermally activated behavior. At low T, only peak A is observed (f ˜ 102–105 Hz) with very small activation energy (EA = 20–40 meV) and pre-exponential factor (f 0A ˜ 103–106 Hz). With increasing T, peak B also appears, with higher values of activation energy, EB = 0.25–0.40 eV, and pre-exponential factor (f 0B ˜ 108–1010 Hz). The bias-independent relaxation mechanism A, with very low activation energy typical of dipole–dipole interaction, is attributed to extrinsic relaxation of adventitious H2O molecules in hydrogen-bond clusters. Mechanism B is attributed to intrinsic relaxation of the alkyl chain assembly. In the acid series, the relative intensity of peak B is consistent with the acid group coverage given by XPS, in contrast with peak A, and its activation energy reveals increased motional constraints in the acid-substituted OML. The shape of dipolar relaxation peaks, discussed in the framework of Dissado-Hill/Jonscher theories for many-body interactions, is useful to discriminate near-substrate and molecular tail relaxations through order/disorder effects.
    Comparison of waxy and normal potato starch remaining granules after chemical surface gelatinization: Pasting behavior and surface morphology
    Huang, J. ; Chen Zenghong, ; Xu, Yalun ; Li, Hongliang ; Liu, Shuxing ; Yang, Daqing ; Schols, H.A. - \ 2014
    Carbohydrate Polymers 102 (2014). - ISSN 0144-8617 - p. 1001 - 1007.
    atomic-force microscopy - internal structure - physicochemical properties - noodle quality - yellow pea - amylose - maize - acetylation - sites - wheat
    o understand the contribution of granule inner portion to the pasting property of starch, waxy potato starch and two normal potato starches and their acetylated starch samples were subjected to chemical surface gelatinization by 3.8 mol/L CaCl2 to obtain remaining granules. Native and acetylated, original and remaining granules of waxy potato starch had similar rapid visco analyzer (RVA) pasting profiles, while those of two normal potato starches behaved obviously different from each other. All remaining granules had lower peak viscosity than the corresponding original granules. Contribution of waxy potato starch granule's inner portion to the peak viscosity was significant more than those of normal potato starches. The shell structure appearing on the remaining granule surface for waxy potato starch was smoother and thinner than that for normal potato starches as observed by scanning electron microscopy, indicating a more regular structure of shell and a more ordered packing of shell for waxy potato starch granules. The blocklet size of waxy potato starch was smaller and more uniform than those of normal potato starches as shown by atomic force microscopy images of original and remaining granules. In general, our results provided the evidence for the spatial structure diversity between waxy and normal potato starch granules: outer layer and inner portion of waxy potato starch granule had similar structure, while outer layer had notably different structure from inner portion for normal potato starch granule.
    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.
    Two modes of phase inversion in a drying emulsion
    Feng, H. ; Sprakel, J.H.B. ; Ershov, D.S. ; Krebs, T. ; Cohen Stuart, M.A. ; Gucht, J. van der - \ 2013
    Soft Matter 9 (2013)10. - ISSN 1744-683X - p. 2810 - 2815.
    latex film formation - atomic-force microscopy - foam drainage - coalescence - stability - kinetics - droplet - system
    We report two different modes of phase inversion in surfactant-stabilized oil-in-water emulsions subjected to a unidirectional drying stress. Coalescence occurs either through a nucleation-and-growth mechanism, where coalesced pockets form and grow randomly throughout the sample, or through a coalescence front that propagates into the sample from the drying end. This drying-induced coalescence results from the development of a pressure gradient from the drying front into the bulk of the sample, established by limited water transport through the Plateau borders. Depending on the steepness of this pressure profile, coalescence occurs throughout the sample or only at the drying front. Moreover, we find that surfactant concentration plays a significant role through its effect on the critical disjoining pressure at which coalescence occurs. Very stable emulsions, characterized by a high critical pressure, tend to develop steep pressure profiles, which favours front-dominated coalescence, while unstable emulsions with low critical pressures develop shallow pressure gradients, favouring nucleation-and-growth dominated coalescence. These results offer new insights into the microscopic mechanisms governing the complex and poorly understood macroscopic phenomena during phase inversion of drying emulsions
    Covalently Attached Organic Monolayers onto Silicon Carbide from 1-Alkynes: Molecular Structure and Tribological Properties
    Pujari, S.P. ; Scheres, L.M.W. ; Weidner, T. ; Baio, J.E. ; Cohen Stuart, M.A. ; Rijn, C.J.M. van; Zuilhof, H. - \ 2013
    Langmuir 29 (2013)12. - ISSN 0743-7463 - p. 4019 - 4031.
    self-assembled monolayers - wide-bandgap semiconductor - atomic-force microscopy - diamond-like carbon - nanotribological characterization - adhesion measurements - terminated si(111) - sic films - surfaces - friction
    In order to achieve improved tribological and wear properties at semiconductor interfaces, we have investigated the thermal grafting of both alkylated and fluorine-containing ((CxF2x+1)–(CH2)n-) 1-alkynes and 1-alkenes onto silicon carbide (SiC). The resulting monolayers display static water contact angles up to 120°. The chemical composition of the covalently bound monolayers was studied by X-ray photoelectron spectroscopy (XPS), infrared reflection–absorption spectroscopy (IRRAS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. These techniques indicate the presence of acetal groups at the organic–inorganic interface of alkyne-modified SiC surfaces. The tribological properties of the resulting organic monolayers with fluorinated or nonfluorinated end groups were explored using atomic force microscopy (AFM). It was found that the fluorinated monolayers exhibit a significant reduction of adhesion forces, friction forces, and wear resistance compared with non-fluorinated molecular coatings and especially bare SiC substrates. The successful combination of hydrophobicity and excellent tribological properties makes these strongly bound, fluorinated monolayers promising candidates for application as a thin film coating in high-performance microelectronic devices.
    Colloids in Flatland: a perspective on 2D phase-separated systems, characterisation methods, and lineactant.
    Bernardini, C. ; Stoyanov, S.D. ; Arnaudov, L.N. ; Cohen Stuart, M.A. - \ 2013
    Chemical Society Reviews 42 (2013)5. - ISSN 0306-0012 - p. 2100 - 2129.
    langmuir-blodgett-films - air-water-interface - perfluorinated carboxylic-acids - field optical microscopy - hydrogenated hybrid amphiphiles - fluorinated comb copolymers - brewster-angle microscopy - organized molecular films - atomic-force microscopy - mixed lan
    In 1861 Thomas Graham gave birth to a new field of science, today known as colloid science. Nowadays, the notion “colloid” is often used referring to systems consisting of two immiscible phases, one of which is finely dispersed into the other. Research on colloids deals mostly with sols (solids dispersed in a liquid), emulsions (liquids dispersed in liquid), and foams (gas dispersed in a liquid). Because the dispersed particles are small, there is a lot of interface per unit mass. Not surprisingly, therefore, the properties of the interface have often a decisive effect on the behaviour of colloids. Water–air interfaces have a special relevance in this field: many water-insoluble molecules can be spread on water and, given the right spreading conditions and enough available surface area, their spreading proceeds until a monolayer (a one-molecule thick layer) eventually remains. Several 2D phases have been identified for such monolayers, like “gas”, “liquid expanded”, “liquid condensed”, and “solid”. The central question of this review is whether these 2D phases can also exist as colloidal systems, and what stabilizes the dispersed state in such systems. We shall present several systems capable of yielding 2D phase separation, from those based on either natural or fluorinated amphiphiles, to polymer-based ones. We shall seek for analogies in 3D and we shall try to clarify if the lines between these 2D objects play a similar role as the interfaces between 3D colloidal systems. In particular, we shall consider the special role of molecules that tend to accumulate at the phase boundaries, that is, at the contact lines, which will therefore be denoted “line-actants” (molecules that adsorb at a 1D interface, separating two 2D colloidal entities), by analogy to the term “surfactant” (which indicates a molecule that adsorbs at a 2D interface separating two 3D colloidal entities).
    Bacterial cell surface deformation under external loading
    Chen, Y. ; Norde, W. ; Mei, H.C. van der; Busscher, H.J. - \ 2012
    mBio 3 (2012)6. - ISSN 2150-7511 - 8 p.
    atomic-force microscopy - staphylococcus-aureus - mechanical-properties - adhesion - biofilms - model - viscoelasticity - peptidoglycan - contact - removal
    Viscoelastic deformation of the contact volume between adhering bacteria and substratum surfaces plays a role in their adhesion and detachment. Currently, there are no deformation models that account for the heterogeneous structure and composition of bacteria, consisting of a relatively soft outer layer and a more rigid, hard core enveloped by a cross-linked peptidoglycan layer. The aim of this paper is to present a new, simple model to derive the reduced Young’s modulus of the contact volume between adhering bacteria and substratum surfaces based on the relationship between deformation and applied external loading force, measured using atomic force microscopy. The model assumes that contact is established through a cylinder with constant volume and does not require assumptions on the properties and dimensions of the contact cylinder. The reduced Young’s moduli obtained (8 to 47 kPa) and dimensions of the contact cylinders could be interpreted on the basis of the cell surface features and cell wall characteristics, i.e., surfaces that are more rigid (because of either less fibrillation, less extracellular polymeric substance production, or a higher degree of cross-linking of the peptidoglycan layer) had shorter contact cylinders and higher reduced Young’s moduli. Application of an existing Hertz model to our experimental data yielded reduced Young’s moduli that were up to 100 times higher for all strains investigated, likely because the Hertz model pertains to a major extent to the more rigid peptidoglycan layer and not only to the soft outer bacterial cell surface, involved in the bond between a bacterium and a substratum surface.
    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.
    Fibrillar structures in food
    Kroes-Nijboer, A. ; Venema, P. ; Linden, E. van der - \ 2012
    Food & Function 3 (2012)3. - ISSN 2042-6496 - p. 221 - 227.
    heat-induced denaturation - beta-lactoglobulin gels - globular protein gelation - atomic-force microscopy - amyloid fibrils - whey-protein - ionic-strength - spherulite formation - bovine insulin - hen lysozyme
    Assembly of proteins or peptides into fibrils is an important subject of study in various research fields. In the field of food research, the protein fibrils are interesting candidates as functional ingredients. It is essential to understand the formation and properties of the fibrils for successful application of the fibrils in food products. This paper describes the impact of recent research on the general view of the process of fibril formation from [small beta]-lg and the properties of the fibrils that are formed, leading to better control of applications for the fibrils. There is a need for a better understanding of the behavior of fibrils in more complex food systems.
    The effect of high pressure microfluidization on the structure and length distribution of whey protein fibrils
    Oboroceanu, D. ; Wang, L. ; Kroes-Nijboer, A. ; Brodkorb, A. ; Venema, P. ; Magner, E. ; Auty, M.A. - \ 2011
    International Dairy Journal 21 (2011)10. - ISSN 0958-6946 - p. 823 - 830.
    transform infrared-spectroscopy - atomic-force microscopy - beta-lactoglobulin gels - amyloid fibrils - milk-proteins - heat - aggregation - rheology - flow
    The effect of high pressure microfluidization on native ß-lactoglobulin (ß-lg) or whey protein isolate (WPI), both before and after heat-induced protein fibril formation at pH 2.0, was investigated using atomic force microscopy (AFM), shear birefringence, reversed phase high pressure liquid chromatography, attenuated total reflectance-Fourier transform infrared spectroscopy and fluorescence spectroscopy. The morphology and length distribution of the fibrils were determined using AFM and flow-induced birefringence, respectively. High pressure (= 50 MPa) microfluidization treatment of ß-lg induced ˜30% protein denaturation, accompanied by changes in secondary structure. Fibrils formed from high pressure treated ß-lg or WPI were similar in length to fibrils formed from non-pressure treated proteins. High pressure (= 50 MPa) microfluidization of fibrils formed from ß-lg or WPI resulted in their breakup into more uniformly sized and much shorter fibrils. Microfluidization pressures of up to 170 MPa resulted in slightly shorter fibrils but did not completely dissociate them.
    Interfacial re-arrangement in initial microbial adhesion to surfaces
    Busscher, H.J. ; Norde, W. ; Sharma, P.K. ; Mei, H.C. van der - \ 2010
    Current Opinion in Colloid and Interface Science 15 (2010)6. - ISSN 1359-0294 - p. 510 - 517.
    quartz-crystal microbalance - atomic-force microscopy - fibronectin-binding proteins - plate flow chamber - escherichia-coli - ionic-strength - staphylococcus-epidermidis - bacterial adhesion - streptococcus-mutans - parallel-plate
    Upon initial microbial adhesion to a surface multiple events occur that include interfacial re-arrangements in the region between an adhering organism and a surface Application of physico-chemical mechanisms to explain microbial adhesion to surfaces requires better knowledge of the interfacial re arrangement occurring immediately after adhesion than hitherto available
    Poisson analysis of streptococcal bond strengthening on stainless steel with and without salivary conditioning film
    Mei, Li ; Mei, H.C. van der; Ren, Y. ; Norde, W. ; Busscher, H.J. - \ 2009
    Langmuir 25 (2009)11. - ISSN 0743-7463 - p. 6227 - 6231.
    atomic-force microscopy - bacterial adhesion - surface - hydrophobicity - charge
    Poisson analysis of retract force-distance curves in atomic force microscopy (AFM) has yielded a new dimension to the decoupling of individual bond forces into a hydrogen bonding and nonspecific force component. Accordingly, bacterial adhesion forces have been decoupled into a hydrogen bonding and nonspecific Lifshitz-Van der Waals contribution. Due to the forced nature of AFM contact, the nonspecific force contribution has hitherto turned out to be repulsive in the analysis of bacterial adhesion forces on nonconducting surfaces. In this study, we present the results of a Poisson analysis of adhesion forces for streptococci adhering to a conducting surface. Adhesion forces measured between stainless steel, both in the absence and presence of an adsorbed salivary conditioning film, increased with increasing contact time between the streptococcal AFM probe and the surface. Concurrent with the increase in adhesion force, there was an increase in the number of minor force peaks in the retract force-distance curves. Poisson analyses of the adhesion forces indicated repulsive nonspecific Lifshitz-Van der Waals forces for streptococci adhering to saliva-coated stainless steel, but interestingly and for the first time, attractive nonspecific forces were revealed on stainless steel in the absence of a salivary conditioning film. We tentatively attribute this to attraction between the negatively charged streptococci and their positive image charges in the conducting material, which cannot develop in a nonconducting material or in the presence of a nonconductive protein layer on the stainless steel surface.
    Room-temperature intermediate layer bonding for microfluidic devices
    Bart, J. ; Tiggelaar, R. ; Yang, M.L. ; Schlautmann, S. ; Zuilhof, H. ; Gardeniers, H. - \ 2009
    Lab on a Chip 9 (2009)24. - ISSN 1473-0197 - p. 3481 - 3488.
    atomic-force microscopy - silicon-wafers - gold nanoparticles - membrane valves - fabrication - surface - plasma - pumps - metallization - polymers
    In this work a novel room-temperature bonding technique based on chemically activated Fluorinated Ethylene Propylene (FEP) sheet as an intermediate between chemically activated substrates is presented. Surfaces of silicon and glass substrates are chemically modified with APTES bearing amine terminal groups, while FEP sheet surfaces are treated to form carboxyl groups and subsequently activated by means of EDC-NHS chemistry. The activation procedures of silicon, glass and FEP sheet are characterized by contact angle measurements and XPS. Robust bonds are created at room-temperature by simply pressing two amine-terminated substrates together with activated FEP sheet in between. Average tensile strengths of 5.9 MPa and 5.2 MPa are achieved for silicon-silicon and glass-glass bonds, respectively, and the average fluidic pressure that can be operated is 10.2 bar. Moreover, it is demonstrated that FEP-bonded microfluidic chips can handle mild organic solvents at elevated pressures without leakage problems. This versatile room-temperature intermediate layer bonding technique has a high potential for bonding, packaging, and assembly of various (bio-) chemical microfluidic systems and MEMS devices
    Multicompartment Nanoparticles Formed by a Heparin-Mimicking Block Terpolymer in Aqueous Solutions
    Uchman, M. ; Stepanek, M. ; Prochazka, K. ; Mountrichas, G. ; Pispas, S. ; Voets, I.K. ; Walther, A. - \ 2009
    Macromolecules 42 (2009)15. - ISSN 0024-9297 - p. 5605 - 5613.
    atomic-force microscopy - abc triblock copolymer - onion-type micelles - fluorescence correlation spectroscopy - diblock copolymers - selective solvents - light-scattering - poly(2-vinylpyridine)-block-poly(ethylene oxide) - wormlike micelles - star terpolymers
    A new amphiphilic block. terpolymer poly((sulfamate-carboxylate)isoprene)-block-polystyrene-block-poly(ethyl ene oxide), PISC230-PS52-PEO151, with a narrow molecular weight distribution (PDI = 1.05), was synthesized via the post. polymerization reaction of the anionically prepared precursor block terpolymer polyisoprene-block-polystyrene-block--poly(ethylene oxide) with chlorosulfonyl isocyanate. The formation and Structure of self-assemblies of the polyelectrolyte block terpolymer in dilute aqueous Solutions were studied by static and dynamic light scattering, atomic force and cryogenic transmission electron microscopy, fluorometry, and H-1 NMR spectroscopy. In acidic solutions, the terpolymers self-assemble into kinetically trapped multicompartment micelles, with the core consisting of discrete PS and PISC domains and PEO in the shell. If the solution pH is adjusted to the alkaline region, the multicompartment micelles undergo an irreversible transition to regular micelles, with a PS core and a mixed shell formed by PEO and PISC blocks.
    Local Probe Oxidation of Self-Assembled Monolayers on Hydrogen-Terminated Silicon
    Yang, M. ; Wouters, D. ; Giesbers, M. ; Schubert, U.S. ; Zuilhof, H. - \ 2009
    ACS Nano 3 (2009)10. - ISSN 1936-0851 - p. 2887 - 2900.
    atomic-force microscopy - covalently attached monolayers - linked organic monolayers - extremely mild attachment - field-induced oxidation - alkyl monolayers - constructive nanolithography - anodization lithography - chemical-modification - molecular simulation
    Local probe oxidation experiments by conductive AFM have been performed on a hexadecyl monolayer and a N-hydroxysuccinimide (NHS)-ester-functionalized undecyl (NHS-UA) monolayer assembled on hydrogen-terminated (i.e., unoxidized) silicon. The oxidation conditions for the mild oxidation of the top terminal groups of monolayers and the deep oxidation of the underlying silicon into silicon oxide were investigated. The results show that the bias threshold for the AFM tip-induced oxidation of the top groups of monolayers on oxide-free silicon can be reduced by 2 V for the methyl-terminated hexadecyl monolayer and even by 3.5 V for the active NHS-ester-terminated undecyl monolayer, in comparison to a methyl-terminated octadecyl trichlorosilane (OTS) monolayer on oxidized silicon. Upon such local mild oxidation, the active NHS ester group of the NHS-UA monolayer is selectively cleaved off to generate carboxyl-containing monolayer nanopatterns, opening further possibilities for subsequent patterned multifunctionalization
    Streptococcus mutans and Streptococcus intermedius adhesion to fibronectin films are oppositely influenced by ionic strength
    Busscher, H.J. ; Belt-Gritter, B. van de; Dijkstra, R.J.B. ; Norde, W. ; Mei, H.C. van der - \ 2008
    Langmuir 24 (2008)19. - ISSN 0743-7463 - p. 10968 - 10973.
    atomic-force microscopy - mitis strains - dlvo theory - surfaces
    Bacterial adhesion to protein-coated surfaces is mediated by an interplay of specific and nonspecific interactions. Although nonspecific interactions are ubiquitously present, little is known about the physicochemical mechanisms of specific interactions. The aim of this paper is to determine the influence of ionic strength on the adhesion of two streptococcal strains to fibronectin films. Streptococcus mutans LT11 and Streptococcus intermedius NCTC11324 both possess antigen I/II with the ability to bind fibronectin from solution, but S. intermedius binds approximately 20 x less fibronectin than does the S. mutans strain under identical conditions. Both strains as well as fibronectin films are negatively charged in low ionic strength phosphate buffered saline (PBS, 10 x diluted), but bacteria appear uncharged in high ionic strength PBS. Physicochemical modeling on the basis of overall cell surface properties (cell surface hydrophobicity and zeta potentials) demonstrates that both strains should favor adhesion to fibronectin films in a high ionic strength environment as compared to in a low ionic strength environment, where electrostatic repulsion between equally charged surfaces is dominant. Adhesion of S. intermedius to fibronectin films in a parallel plate flow chamber was completely in line with this modeling, while in addition atomic force microscopy (AFM) indicated stronger adhesion forces upon retraction between fibronectin-coated tips and the cell surfaces in high ionic strength PBS than in low ionic strength PBS. Thus, the dependence of the interaction on ionic strength is dominated by the overall negative charge on the interacting surfaces. Adhesion of S. mutans to fibronectin films, however, was completely at odds with theoretical modeling. and the strain adhered best in low ionic strength PBS. Moreover, AFM indicated weaker repulsive forces upon approach between fibronectin-coated tips and the cell surfaces in low ionic strength PBS than in high ionic strength PBS. This indicated that the dependence of the interaction on ionic strength is dominated by electrostatic attraction between oppositely charged, localized domains on the interacting surfaces, despite their overall negative charge. In summary, this study shows that physicochemical modeling of bacterial adhesion to protein-coated surfaces is only valid provided the number of specific interaction sites on the cell surfaces is low. such as on S. intermedius NCTC11324. Nonspecific interactions are dominated by specific interactions if the number of specific interaction sites is large, Such as on S. mutans LT11. Its ionic strength dependence indicates that the specific interaction is electrostatic in nature and operative between oppositely charged domains on the interacting surfaces, despite the generally overall negatively charged character of the surfaces.
    A Review of Analytical Methods for the Identification and Characterization of Nano Delivery Systems in Food
    Luykx, D.M.A.M. ; Peters, R.J.B. ; Ruth, S.M. van; Bouwmeester, H. - \ 2008
    Journal of Agricultural and Food Chemistry 56 (2008)18. - ISSN 0021-8561 - p. 8231 - 8247.
    desorption electrospray-ionization - field-flow-fractionation - mobility-mass-spectrometry - photon-correlation spectroscopy - thin-layer-chromatography - solid lipid nanoparticles - atomic-force microscopy - small-angle scattering - protamine-oligonucleotide-nan
    Detection and characterization of nano delivery systems is an essential part of understanding the benefits as well as the potential toxicity of these systems in food. This review gives a detailed description of food nano delivery systems based on lipids, proteins, and/or polysaccharides and investigates the current analytical techniques that can be used for the identification and characterization of these delivery systems in food products. The analytical approaches have been subdivided into three groups; separation techniques, imaging techniques, and characterization techniques. The principles of the techniques together with their advantages and drawbacks, and reported applications concerning nano delivery systems, or otherwise related compounds are discussed. The review shows that for a sufficient characterization, the nano delivery systems need to be separated from the food matrix, for which high-performance liquid chromatography or field flow fractionation are the most promising techniques. Subsequently, online photon correlation spectroscopy and mass spectrometry seem to be a convenient combination of techniques to characterize a wide variety of nano delivery systems
    Reduction of protein adsorption to a solid surface by a coating composed of polymeric micelles with a glass-like core
    Hofs, P.S. ; Brzozowska, A.M. ; Keizer, A. de; Norde, W. ; Cohen Stuart, M.A. - \ 2008
    Journal of Colloid and Interface Science 325 (2008)2. - ISSN 0021-9797 - p. 309 - 315.
    atomic-force microscopy - block-copolymers - assembled monolayers - polyethylene oxide - kinetics - brushes - silica - resistance - fibrinogen - interface
    Adsorption studies by optical reflectometry show that complex coacervate core micelles (C3Ms) composed of poly([4-(2-amino-ethylthio)-butylene] hydrochloride)49-block-poly(ethylene oxide)212 and poly([4-(2-carboxy-ethylthio)-butylene] sodium salt)47-block-poly(ethylene oxide)212 adsorb in equal amounts to both silica and cross-linked 1,2-polybutadiene (PB). The C3Ms have an almost glass-like core and atomic force microscopy of a dried layer of adsorbed C3Ms shows densely packed flattened spheres on silica, which very probably are adsorbed C3Ms. Experiments were performed with different types of surfaces, solvents, and proteins; bare silica and cross-linked 1,2-PB, NaNO3 and phosphate buffer, and lysozyme, bovine serum albumin, ß-lactoglobulin, and fibrinogen. On the hydrophilic surface the coating reduces protein adsorption >90% in 0.1 M phosphate buffer, whereas the reduction on the coated hydrophobic surface is much lower. Reduction is better in phosphate buffer than in NaNO3, except for the positively charged lysozyme, where the effect is reversed.
    Gentle Immobilization of Nonionic Polymersomes on Solid Substrates
    Li, F. ; Ketelaar, T. ; Cohen Stuart, M.A. ; Sudhölter, E.J.R. ; Leermakers, F.A.M. ; Marcelis, A.T.M. - \ 2008
    Langmuir 24 (2008)1. - ISSN 0743-7463 - p. 76 - 82.
    atomic-force microscopy - block-copolymers - drug-delivery - vesicles - morphology - solubilization - curvature - stability - blends
    Vesicles from Pluronic L121 (PEO5-PPO68-PEO5) triblock copolymers were first stabilized by a permanent interpenetrating polymer network and then gently immobilized onto a glass or mica surface. Fluorescence-labeled micrometer-sized vesicles were visualized with confocal laser scanning microscopy, and smaller sized capsules, around 100 nm, were probed by liquid atomic force microscopy. The immobilized vesicles were weakly attached to a negatively charged surface via negatively charged polyelectrolytes in combination with Mg2+ ions and can be reversibly detached from the surface by slightly elevated temperatures. To illustrate that the immobilized vesicles remain responsive to external stimuli, we show that it is possible to transform their shape from spherical to cylindrical by introducing a second Pluronic, namely, P123 (PEO20-PPO70-PEO20). The detailed transition process has been recorded in real time by confocal laser scanning microscopy. Electron microscopy studies confirmed that a similar morphology change also occurs in the bulk.
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