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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    Impact of in vitro digestion on gastrointestinal fate and uptake of silver nanoparticles with different surface modifications
    Abdelkhaliq, Ashraf ; Zande, Meike van der; Undas, Anna K. ; Peters, Ruud J.B. ; Bouwmeester, Hans - \ 2020
    Nanotoxicology 14 (2020)1. - ISSN 1743-5390 - p. 111 - 126.
    bioavailability - in vitro digestion - Silver nanoparticles - single particle-ICP-MS - surface chemistry

    Nanomaterials, especially silver nanoparticles (AgNPs), are used in a broad range of products owing to their antimicrobial potential. Oral ingestion is considered as a main exposure route to AgNPs. This study aimed to investigate the impact of the biochemical conditions within the human digestive tract on the intestinal fate of AgNPs across an intestinal in vitro model of differentiated Caco-2/HT29-MTX cells. The co-culture model was exposed to different concentrations (250–2500 µg/L) of pristine and in vitro digested (IVD) AgNPs and silver nitrate for 24 h. ICP-MS and spICP-MS measurements were performed for quantification of total Ag and AgNPs. The AgNPs size distribution, dissolution, and particle concentration (mass- and number-based) were characterized in the cell fraction and in the apical and basolateral compartments of the monolayer cultures. A significant fraction of the AgNPs dissolved (86–92% and 48–70%) during the digestion. Cellular exposure to increasing concentrations of pristine or IVD AgNPs resulted in a concentration dependent increase of total Ag and AgNPs content in the cellular fractions. The cellular concentrations were significantly lower following exposure to IVD AgNPs compared to the pristine AgNPs. Transport of silver as either total Ag or AgNPs was limited (<0.1%) following exposure to pristine and IVD AgNPs. We conclude that the surface chemistry of AgNPs and their digestion influence their dissolution properties, uptake/association with the Caco-2/HT29-MTX monolayer. This highlights the need to take in vitro digestion into account when studying nanoparticle toxicokinetics and toxicodynamics in cellular in vitro model systems.

    Quantitative and Orthogonal Formation and Reactivity of SuFEx Platforms
    Gahtory, Digvijay ; Sen, Rickdeb ; Pujari, Sidharam ; Li, Suhua ; Zheng, Qinheng ; Moses, John E. ; Sharpless, K.B. ; Zuilhof, Han - \ 2018
    Chemistry-A European Journal 24 (2018)41. - ISSN 0947-6539 - p. 10550 - 10556.
    click chemistry - fluorine - sulfur - surface chemistry - surface modification

    The constraints of minute reactant amounts and the impossibility to remove any undesired surface-bound products during monolayer functionalization of a surface necessitate the selection of efficient, modular and orthogonal reactions that lead to quantitative conversions. Herein, we explore the character of sulfur–fluoride exchange (SuFEx) reactions on a surface, and explore the applicability for quantitative and orthogonal surface functionalization. To this end, we demonstrate the use of ethenesulfonyl fluoride (ESF) as an efficient SuFEx linker for creating “SuFEx-able” monolayer surfaces, enabling three distinct approaches to utilize SuFEx chemistry on a surface. The first approach relies on a di-SuFEx loading allowing dual functionalization with a nucleophile, while the two latter approaches focus on dual (CuAAC–SuFEx/SPOCQ–SuFEx) click platforms. The resultant strategies allow facile attachment of two different substrates sequentially on the same platform. Along the way we also demonstrate the Michael addition of ethenesulfonyl fluoride to be a quantitative surface-bound reaction, indicating significant promise in materials science for this reaction.

    Laccase-Mediated Grafting on Biopolymers and Synthetic Polymers : A Critical Review
    Slagman, Sjoerd ; Zuilhof, Han ; Franssen, Maurice C.R. - \ 2018
    ChemBioChem 19 (2018)4. - ISSN 1439-4227 - p. 288 - 311.
    biomass - enzyme catalysis - grafting - polymers - surface chemistry
    Laccase-mediated grafting on lignocelluloses has gained considerable attention as an environmentally benign method to covalently modify wood, paper and cork. In recent decades this technique has also been employed to modify fibres with a polysaccharide backbone, such as cellulose or chitosan, to infer colouration, antimicrobial activity or antioxidant activity to the material. The scope of this approach has been further widened by researchers, who apply mediators or high redox potential laccases and those that modify synthetic polymers and proteins. In all cases, the methodology relies on one- or two-electron oxidation of the surface functional groups or of the graftable molecule in solution. However, similar results can very often be achieved through simple deposition, even after extensive washing. This unintended adsorption of the active substance could have an adverse effect on the durability of the applied coating. Differentiating between actual covalent binding and adsorption is therefore essential, but proves to be challenging. This review not only covers excellent research on the topic of laccase-mediated grafting over the last five to ten years, but also provides a critical comparison to highlight either the lack or presence of compelling evidence for covalent grafting.
    High-resolution mass spectrometry for the analysis of interfacial kinetics of organic surface reactions
    Sen, Rickdeb - \ 2017
    Wageningen University. Promotor(en): H. Zuilhof. - Wageningen : Wageningen University - ISBN 9789463436243 - 308
    surface chemistry - unimolecular films - chemical reactions - analytical methods - mass spectrometry - oppervlaktechemie - unimoleculaire films - chemische reacties - analytische methoden - massaspectrometrie

    In this thesis, XPS and DART–HRMS have been used in close conjugation to supplement each other, since the latter is a relatively new addition to surface chemist’s repertoire that – after development – needed a firm comparison to build up a reputation of its own. The strength of our approach has been underlined by the high correlation between these two independent analytical techniques. Central to our approach has been the formation of mixed monolayers in case of aluminum oxide substrates. As presented in Chapters 2, 3 and 4, we have succeeded in the rapid formation of range stable, covalently bound mixed monolayers. The subsequent development of a general and fast analytical technique to determine the interfacial reaction kinetics, including the activation parameters DH‡ and DS‡, provided unparalleled insights. We have developed a “MS–ionizable tag” technique, which has been applied for the analysis of surface–bound organic reactions, to the best of our knowledge, for the first time.

    The Strain–Promoted Alkyne–Azide Cycloaddition (SPAAC) reaction was chosen as a model reaction given the fact that its kinetics had been well–studied in solution. As shown in Chapter 2, the microenvironment around the reactive surface group was carefully controlled by the length of the inert alkyl chains surrounding it. We observed a few interesting trends which could be of great interest to future surface chemists. First, the SPAAC reaction – which is a click reaction in solution – does not retain this nature on the surface (It does not proceed to full conversion and converges sluggishly to around 37% yield after significant temporal passage). A partially accessible microenvironment, where the motion of reactive groups is slightly restricted, was found to provide a high rate with the highest surface yield. In contrast, a freely accessible reactive moiety afforded a lower surface yield albeit with the highest overall rate. Finally, a buried microenvironment led to the highest overall rate albeit with a lower surface yield. As a corollary, for the surface–bound SPAAC reaction we can compare the partially accessible microenvironment to a marathon runner who is able to run further but at a pace slower than a sprinter (free microenvironment). This provides the surface chemist with a handle for tuning the monolayer as per her/his reaction goals.

    Harnessing the valuable insights gained from the SPAAC reaction, our concept of ionizable MS tag coupled with DART–HRMS was further extended to a more novel and yet unstudied interfacial reaction in Chapter 3. The Strain–Promoted Oxidation–Controlled cycloalkyne–1,2–Quinone (SPOCQ) cycloaddition was applied for the first time on a surface and afforded a quantitative yield for a free microenvironment in under 4 h. It is to be noted here, that for the first time a 100% (quantitative) metal–free click reaction was observed at a surface. This proved that our approach of engineering the microenvironment around the reactive site provides a distinct edge needed to attain quantitative yields. Quinones are hard to synthesize/store/use in solution given their high propensity to polymerize. However, we demonstrated that on the surface, quinones can be easily generated and stored over–extended period of time by a facile periodate oxidation. Auto–polymerization of surface–bound quinones is precluded by their tether and enforced distal separation by surrounding inert alkyl chains (3:1 ratio). The wider application of this interesting mixture has been further rigorously demonstrated in later chapters too. The bioorthogonality of the SPOCQ reaction coupled with its higher speed and its quantitative yields on the surface are definitely its most salient features.

    After studying strain–promoted click reactions on the surface (culminating for SPOCQ in quantitative conversion within 4 h), the question arose if DART–HRMS could also be used to reproducibly and precisely determine a different class of cycloadditions, for which we selected the interfacial inverse electron demand Diels–Alder (IEDDA) reaction as this reaction was reported to be really fast –at least for click reactions– in solution. This was studied in Chapter 4 extensively and we surpassed our previous kinetic record (SPOCQ) by obtaining a quantitative yield in a mere 15 min. The other interesting observation of this study was that reversing the reaction counterparts on the surface produced a discernible reaction rate difference. We found that one of the reactants when tethered in a particular stereochemistry (exo– form) gave the highest surface coverage (100%) within the shortest amount of time. This was also the first time that the effect of diastereomerism on interfacial reaction rates was studied.

    In Chapter 5, covalent modification of native non–activated mica has been carried out utilizing catechol linkers. Previous studies for mica modification produced poorly defined polymeric structures on the surface or required extensive and tedious organic synthesis. We have addressed both these issues head–on in this thesis. Well–defined and characterized ultrathin layers were constructed on mica using a catechol–based molecule involving a two–step synthesis. Mica being atomically flat provides an ideal surface upon which to study various phenomena by AFM and other forms of microscopy. However, most research until now was restricted to simply drop–casting the pre–fabricated moieties followed by studying their final structures. Our method now allows for the step–wise formation and characterization of these very interesting structures. Along with it, we also performed several click attachment chemistries on these ultrathin layers which can be harnessed by surface chemists to put various functional and structurally complex moieties on the surface. This opens the pathway for the attachment of more complex architectures on the surface with higher functionality along with the ability to study their formation in a step–wise controlled fashion.

    Overall, this thesis wishes to understand organic surface chemistry and several of its intricate mysteries. It clearly outlines several modification techniques and unravels interfacial kinetics of several interesting “metal–free click reactions”. It strives to rationalize the activation parameters in conjunction with classical organic chemistry and gives details on how surrounding “inert” alkyl chains can play a profound role in reaction rates. Lastly, we have striven to and achieved rapid and quantitative reactions on the surface by virtue of optimization of this microenvironment. Personally I believe, we have treaded on a road seldom traveled and unraveled a new understanding about molecular interactions on the ever–interesting and an infinitely–complex surface.

    Bioinspired nanopatterned surfaces via colloidal templating; a pathway for tuning wetting and adhesion
    Akerboom, Sabine - \ 2016
    Wageningen University. Promotor(en): Frans Leermakers, co-promotor(en): Marleen Kamperman. - Wageningen : Wageningen University - ISBN 9789462578470 - 198
    surface chemistry - surfaces - particles - water - nanotechnology - unimolecular films - adhesion - colloidal properties - oppervlaktechemie - oppervlakten - deeltjes - water - nanotechnologie - unimoleculaire films - adhesie - colloïdale eigenschappen

    We can learn from nature that, next to chemistry, surface structures can be used for tuning different functions of surfaces. In this thesis we present a novel fabrication method using colloidal templating on the air/water interface. Two distinct ways to obtain nanopatterned surfaces are described, namely (i) addition of PDMS on top of the colloidal monolayer and (ii) synthesis of polypyrrole around the particles of the monolayer. An increase in adhesion is found for the nanopatterned PDMS surfaces, and the contact angle of water on the nanopatterned polypyrrole surface is increased.

    Surface functionalization and analysis thereof by ambient mass spectrometry
    Manova, R.K. - \ 2014
    Wageningen University. Promotor(en): Han Zuilhof, co-promotor(en): Teris van Beek. - Wageningen : Wageningen University - ISBN 9789462571570 - 214
    biosensoren - detectie - biomarkers - allergenen - oppervlaktechemie - analytische methoden - synthese - unimoleculaire films - biosensors - detection - biomarkers - allergens - surface chemistry - analytical methods - synthesis - unimolecular films

    A challenge in the global healthcare is the lack of suitable diagnostic tools for early disease detection. One possible solution is the use of biosensors in diagnostic tests. By definition, a biosensor is a bioanalytical device that detects the presence of a compound (analyte) in the sample. The detection relies on the specific interactions between the ligands that are attached onto the biosensor surface and the analytes in the sample.

    This PhD dissertation is focused on developing an optimal protocol for attachment of ligands onto the biosensing surface. A step-wise approach was established for the versatile and reproducible modification and functionalization of a silicon nitride-based biosensor. This approach included the application of bioorthogonal copper-free reactions as a useful tool for oriented attachment of biomolecules. Additionally, a novel surface sensitive analytical method was developed for the identification of covalently bound molecules in monolayers. The method, which is fast and easy to apply, uses DART ionization coupled to a high-resolution mass spectrometer. The nm-thin layers were analysed, and interpretation rules for the obtained mass spectra were formulated. The method was applied in the identification of commercially available nm-thin coatings and biochips.

    Colloids at liquid interfaces: dynamics and organization
    Ershov, D.S. - \ 2014
    Wageningen University. Promotor(en): Jasper van der Gucht, co-promotor(en): Martien Cohen Stuart. - Wageningen : Wageningen University - ISBN 9789461738943 - 127
    colloïden - oppervlaktechemie - grensvlak - oppervlakteverschijnselen - capillairen - vloeistoffen (liquids) - colloids - surface chemistry - interface - surface phenomena - capillaries - liquids

    This thesis deals with spherical microparticles trapped at liquid interfaces. It focuses on two aspects of their behavior: firstly, the effect of the curvature of a liquid interface on interparticle interactions and their organization; secondly, the mobility of particles at visco-elastic interfaces.

    In Chapter 2of this thesis we showed that it is possible to induce capillary interactions between spherical microparticles with homogeneous surface chemistry by tailoring the curvature of the liquid interface. If the interfacial curvature is anisotropic, the constraint of constant contact angle along the contact line can only be satisfied if the interface is deformed locally. These deformations create excess surface area, which changes when two particles approach each other. This leads to a change in the surface free energy, which manifests itself as a capillary interaction between the particles.

    To study the effect of curvature on the interactions between particles, we created oil-water interfaces of different shape (ellipsoid, dumbbell, torus and squares) and added spherical negatively charged particles that adsorbed at these interfaces. On all these interfaces, we observed quadrupolar capillary interactions that organized the particles into square lattices. The order of this organization increased with increasing curvature anisotropy, indicating that capillary interactions are stronger as well. By contrast, on flat interfaces or on spherical droplets with homogeneous curvature, no attractive interaction was observed and only at very high surface coverage did the particles order in a hexagonal lattice, as a result of repulsive interactions.

    In Chapter 3we studied the interface deformations around particles at curved interfaces and the resulting capillary interactions theoretically. We used the finite element method to solve the Young-Laplace equation for the shape of the interface around a particle and calculated the interaction potential between the particles numerically.

    The main finding of these calculations is that for an anisotropically curved interface, with two different local principal curvatures, the particle deforms the interface in two ways simultaneously: concave deformation along one principal direction and convex – along the other, thus creating a deformation field with quadrupolar symmetry. Two particles with such deformations interact favorably only if the overlapping deformations are similar (concave-concave, convex-convex), which occurs when they approach each other along one of the two principal directions. Since the two local principal directions are always perpendicular, particles interacting along them will tend to arrange into a square pattern.

    As a consequence of the quadrupolar deformation field, two particles approaching each other along a line forming 45 degrees with the principal axes will repel each other (which is confirmed by our observations), because in this case the deformation fields overlap with four different “petals” (2 pairs of concave-convex), and the excessive surface area doesn’t reduce upon approaching, but increases. A system of two particles oriented at an angle with respect to the principal axis is therefore subjected to a torque rotating the axis of the system so that it gets aligned with one of the two principal directions. The torque magnitude reaches its maximum when the system’s axis is at an angle of 45 degrees with respect to the principal direction and decreases to 0 when the axis is aligned with one of the principal directions.

    The family of interaction potentials we obtained allows for calculating the minimum deviatoric curvature needed to initialize capillary interactions strong enough to compete with thermal energy, so that a stable organization can be expected. The calculated value was very close to the deviatoric curvature where ordering was observed experimentally in Chapter 2.

    In Chapter 4we studied the mobility of 3 mm polystyrene particles in a monolayer of 1.5 mm core-shell microparticles deposited at flat air-water interfaces; all the particles present in the system were stabilized by negative charges.

    In this exploratory chapter we made an attempt to characterize the mechanical properties of such monolayers by analyzing the mobility of the larger tracer particles in the monolayer. With increasing particle density of the monolayer, we observed that the mean-square displacement of the tracer particles was reduced, which can be interpreted as an increase of the viscosity of the monolayer. At very high densities the motion of the particles became subdiffusive and confined, pointing at elasticity of the monolayer. We also studied correlated movements between neighboring particles in an attempt to do two-point interfacial microrheology. A comparison between the one-point and two-point methods shows clear indications of heterogeneous dynamics of the tracer particles. Our results therefore call for a further development of two-point microrheology at interfaces.

    In Chapter 5we used tracer particles to study the properties of thin cross-linked actin networks deposited at the surface of oil droplets. These networks are a model system for the intracellular actin cortex. We used the generalized Stokes-Einstein relation to extract the complex frequency-dependent shear modulus of such networks from the movement of the added tracer particles. We studied the effects of the length of actin filaments and the cross-linker concentration on the mechanical properties of these layers.

    The advantage of this system is that actin networks are freely accessible from the water phase, and therefore can be subjected to in-situ addition of cross-linkers, enzymes or other chemicals of interest. Using this, we managed to show strong stiffening after addition of myosin motor proteins and ATP, which we ascribed to contraction of the actin-myosin network.

    Colloids in ultra-low dielectric media : surface forces and self-assembly
    Banerjee, S. - \ 2013
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Mieke Kleijn. - S.l. : s.n. - ISBN 9789461736857 - 257
    schoonmaken - was - kooldioxide - oppervlaktechemie - oppervlakte-interacties - oplosmiddelen - cleaning - laundry - carbon dioxide - surface chemistry - surface interactions - solvents

    This thesis aims at gaining fundamental insight on colloidal interactions in two types of apolar media, namely, liquid CO2 and (as a model for liquid CO2) n-hexane. The other components playing major roles are surfactants and water. The background of the work was to address the challenges met in the use of liquid CO2 as a dry-cleaning solvent, in particular the insufficient removal of particulate soil.

    Since the dielectric constant of liquid CO2 is extremely low (1.6 at 60 bar and 10 °C), it has a low individual Hamaker constant. This in turn leads to a many orders of magnitude higher van der Waals force between interacting surfaces through liquid CO2 as opposed to traditionally employed solvents like perchloroethylene (PERC), which are toxic and environmentally unfriendly. Both the absence of charge on surfaces and the high Van der Waals force mean that a high solvodynamic force (high Reynolds number) is required to dislodge particles from fabrics.

    The situation becomes worse in the presence of water (without a suitable surfactant), which is a minor component in any dry-cleaning formulation. Our atomic force microscopy results indicate that water-mediated capillary bridges can lead to higher adhesion forces between the interacting surfaces. The roughness and softness of the surfaces were found to affect the kinetics, magnitude and range of the interaction force.

    Further, we have shown that using suitable surfactants these forces can be reduced. Following a systematic selection approach based on the hypothesis that a hydrocarbon surfactant for liquid CO2 should have a low molecular weight and a branched t-butyl tail in its alkyl part, Igepal CA520 was chosen. This surfactant has an ethylene oxide (EO) chain as a headgroup (CiEOj type surfactant). The surfactant solubility was tested first in the model solvent, followed by measuring its cloud point in the liquid CO2 system, which showed that the surfactant is soluble at ~ 50 bar and 5 - 10 °C (CO2 dry-cleaning conditions). Furthermore, we found that Igepal CA520 was surface active at the water - liquid CO2 interface. Igepal CA520 was further tested in a pilot scale dry-cleaning apparatus, where it showed marked improvement in detergency of particulate soil.

    The interfacial behaviour of the surfactant - water - liquid CO2 system was also studied using the self-consistent field theory of Scheutjens and Fleer (SF-SCF). We showed that the interfacial tension of bare water - CO2 interface decreases with increasing pressure and becomes invariant of pressure beyond the saturated vapour pressure. The water contact angle on a hydrophilic surface in CO2 increases with increasing pressure. The first phenomenon was explained from the increasing Gibbs excess of CO2 at the water - vapour interface. The increase in contact angle was shown to result from the adsorption of CO2 on the -OH populated surfaces with increasing pressure. The model further predicted complete wetting of the water - vapour interface by a CO2 layer, in line with the fact that the system conditions were chosen not far from criticality.

    The model was further extended to describe and predict the interfacial and bulk properties of the liquid CO2/surfactant/water system. The experimental water - CO2 interfacial tension data and the SF-SCF modeling of the Igepal/water/liquid CO2 system indicated that Igepal adsorbs at the water - liquid CO2 interface. The model also predicted the formation of reverse micelles both at the three-phase (water/liquid CO2/gaseous CO2) coexistence (at P/Psat = 1 ) and for P/Psat > 1. With increasing pressure the critical reverse micellar concentration (CRMC) increases and the aggregation number at the CRMC decreases. A higher pressure leads to a stronger stopping mechanism for reverse micellization due to the better solvation (better solvency power of liquid CO2) of the surfactant tails by CO2.

    Apart from the bulk phase behaviour, the presence of the surfactant gives rise to interesting wetting phenomena at the water - vapour interface. Partial wetting by CO2 was noted, followed by a re-entrant wetting transition as the surfactant concentration in the bulk water phase was increased.

    The theoretical phase behaviour was validated by small angle X-ray scattering experiments (SAXS) on Igepal/water/liquid CO2 ternary systems. The SAXS results indicated conclusively the presence of self-assembled structures. The role of water in driving the self-assembly has been mapped and it was concluded that water acts as a mesogen (promoter of liquid crystals) in CiEOj type surfactant - liquid CO2 systems. In the absence of water, at a particular range of surfactant concentrations, the system contains isotropic reverse micellar mesophases (often termed as L2) and with the addition of water L2 undergoes a phase transition to a lamellar phase, Lα. The lamellar repeat distance increases with increasing water content. Following these findings, a partial phase diagram in liquid CO2 has been generated. Comparing the phase diagrams of Igepal CA520/water in liquid CO2 and in n-hexane it is clear that water plays similar roles in the two systems. Based on this we also conclude that n-hexane is a good model for liquid CO2.

    The model was used to arrive at design guidelines for surfactants for liquid CO2. It is interesting to note that the stability window for reverse micelles in liquid CO2 is rather narrow with respect to the two key Flory-Huggins interaction parameters, namely the χC3Dand χOW, characterizing the interaction between methyl groups in the surfactant tail and CO2, and between the head group oxygen and water, respectively. The first interaction parameter dominates the stopping mechanism for micellization, while the latter determines the driving force for this process. The fact that this window is narrow essentially points out the difficulties involved in designing amphiphiles for liquid CO2. The design criteria emerging from modeling are based on the numerical results that for smaller molecules (< C10), branching is important and for longer molecules (C10 and above), CH3 type interactions are more important over branching.

    Apart from CO2 dry-cleaning, the knowledge gained in this thesis can be beneficial to many other environmentally friendly industrial processes involving liquid CO2, such as enhanced oil recovery and extraction of polar compounds. The outcome of this thesis can also be extended to alleviate the problems associated with the geological storage of CO2 at high pressure under the ocean floor (deep saline aquifers).

    Oriented antibodies as versatile detection element in biosensors
    Trilling, A.K. - \ 2013
    Wageningen University. Promotor(en): Han Zuilhof, co-promotor(en): Jules Beekwilder. - S.l. : s.n. - ISBN 9789461735683 - 168
    biosensoren - oppervlakten - oppervlaktechemie - oriëntatie - antilichamen - lama's - antistoftesten - mycobacterium tuberculosis - unimoleculaire films - immobilisatie - biosensors - surfaces - surface chemistry - orientation - antibodies - llamas - antibody testing - mycobacterium tuberculosis - unimolecular films - immobilization

    The aim of this thesis is to explore orientation of detection elements on biosensor

    surfaces. To this end, different strategies were combined such as surface chemistry and protein functionalization, with the aim to generate a platform for oriented immobilization of antibodies

    in biosensors. Chapter 2 investigates the formation of organic monolayers on

    oxide-free copper. Detailed studies were performed to characterize the monolayers

    and proof its quality. Apart from being the first oxide-free monolayers on copper

    reported thus far, further functionalization was successfully investigated.

    Chapter 3 gives an overview about approaches used to orient antibodies on surfaces.

    It also summarizes methods used to characterize the orientation of immobilized

    antibodies in a more direct manner.

    In chapter 4 a set of detection elements for tuberculosis bacteria is described. These

    are variable domains of llama heavy-chain antibodies, known as VHH proteins. A

    number of VHHs, selected by phage display, were expressed by Escherichia coli bacteria

    and characterized for binding towards Mycobacterium tuberculosis bacteria.

    Specificity of VHHs was investigated and the antigen was identified.

    In chapter 5 the impact of orientation on the analyte binding capacity was studied

    by SPR as model biosensor. Established techniques (NH2 coupling, biotyinylation)

    were used to immobilize VHHs, and a comparison between oriented and random

    immobilized VHHs was made. The effect of molecular weight, epitope number and

    affinity of the target analyte was investigated.

    In chapter 6, a novel coupling chemistry was used to immobilize VHHs, and in this case the same chemistry could be used for oriented and random immobilization. VHHs were engineered and functionalized with a non-natural amino acid to bear either one or five azide groups. Azide groups served as unique chemical handles on the VHHs and were used to click proteins onto a cyclooctyne-modified surface in an oriented and random approach. Spectacular effects on biosensor sensitivity were observed when VHHs were immobilized in an oriented manner.

    Finally, in chapter 7, the main results of this thesis are summarized and remaining

    problems as well as ideas for future research are discussed.

    Mechanic studies of monolayer formation on H-Si(111) surfaces
    Rijksen, B.M.G. - \ 2012
    Wageningen University. Promotor(en): Han Zuilhof. - S.l. : s.n. - ISBN 9789461732002 - 167
    oppervlaktechemie - unimoleculaire films - alkenen - alkynen - waterstof - silicium - surface chemistry - unimolecular films - alkenes - alkynes - hydrogen - silicon

    Covalently attached organic monolayers on silicon surfaces form thermally and chemically stable platforms for (bio)functionalization of the surface. Recent advances in monolayer formation – yielding increases in monolayer quality and the complete exclusion of oxygen at modified surfaces – have paved the way for the future development of biosensors, photovoltaics, and molecular electronic devices. Despite these successful innovations in monolayer formation (including patterning and functionalization) over the last two decades, the actual knowledge of the processes at the silicon surface still lags behind. Yet, a good understanding of the mechanisms of initiation and propagation will help in finding new tunable parameters for further speed-up, and new strategies for attaching interesting biomolecules to the silicon surface.

    In the search for even faster synthetic methods to build monolayers of even higher quality, this thesis presented a combined systematic experimental and theoretical study of the mechanisms underlying monolayer formation.

    A detailed overview of the current knowledge regarding the mechanisms that underlie monolayer formation onto hydrogen-terminated silicon (H-Si) is presented in Chapter 2. The focus of this chapter is mainly directed to H-Si(111) and H-Si(100) surfaces, where silyl radicals play a key role in the formation of Si-C bonds that link the monolayers to the surface. These radicals also readily react with oxygen leading to oxidation of the surface. Several initiation mechanisms that induce the formation of these radicals are discussed, along with supporting theoretical and experimental modeling studies.

    The radical cation initiation mechanism is studied in more detail in Chapter 3. This chapter describes how radical cations of low molecular-weight silicon model compounds, which were synthesized to represent the H-Si surface, were obtained by photo-induced electron transfer. The stability and the nature of the radical cations were investigated with lifetime and secondary electron transfer studies. This chapter shows that radical cation initiation at the silicon surface is feasible. However, given the differences in reactivity between oxygen-centered and carbon-centered nucleophiles, this reaction is likely to only play a significant role in the initiation steps of monolayer formation, and not in the propagation of the Si-C bond formation.

    Chapter 4, describes the experimental and theoretical study of the radical chain mechanism, and in particular the effect of stabilization of the b-carbon radical. The radical reactivity was studied by performing competition reactions of precursors (alkenes, alkynes, etc.) with the tris(trimethylsilyl)silyl radical, and via high-level theoretical calculations on a theoretical Si4-model to obtain the activation barrier and overall free energy changes.

    Based on the insights obtained in the research described in Chapters 3 and 4, Chapter 5 describes in a combined experimental and theoretical study a significant improvement of surface coverage and speed-up of monolayer formation on H-Si.

    Modification of inorganic surface with 1-alkenes and 1-alkynes
    Maat, J. ter - \ 2012
    Wageningen University. Promotor(en): Han Zuilhof. - S.l. : s.n. - ISBN 9789461732019 - 160
    oppervlaktechemie - alkenen - alkynen - surface chemistry - alkenes - alkynes

    Surface modification is important because it allows the tuning of surface properties, thereby enabling new applications of a material. It can change physical properties such as wettability and friction, but can also introduce chemical functionalities and binding specificity. Several techniques are available to modify the surface. Of these, organic monolayers have the advantages that they are easily tunable, fairly stable, and do not change the structural properties of the surface.

    In the last two decades studies on the coupling of unsaturated hydrocarbons to inorganic surfaces have emerged. These compounds (also referred to as 1-alkenes and 1-alkynes) form covalently coupled monolayers on a variety of substrates, which is shortly reviewed in Chapter 1. This class of surface modification also forms the basis of the studies described in the following chapters.

    In Chapter 2, a method for the direct patterning of 1-alkynes onto hydrogen-terminated silicon is presented. The method combines microcontact printing with visible light illumination through the stamp. Since the surface modification is clearly enhanced by the illumination, the method was named light-enhanced microcontact printing (LE-µCP). It results in the local formation of an alkenyl monolayer at the areas where the stamp is in contact with the surface. The method is compatible with functional inks and also allows the preparation of chemically heterogeneous surfaces by backfilling of the uncontacted areas with a second functional ink.

    In Chapter 3, a method is introduced to photochemically modify fused silica substrates with 1-alkenes. This yields highly hydrophobic surfaces with high thermal stability, whereas the adsorbed layer provides proper chemical passivation of the underlying surface. The alkenes initially bind to the surface hydroxyl groups in Markovnikov fashion, but at prolonged reaction times oligomerization takes place. Since the reaction is photochemically initiated, it enables the use of photolithography to constructively pattern the silica surfaces. Because of this, the newly developed method forms a valuable addition to the existing modification methods.

    The method developed in Chapter 3 is applied in Chapter 4 to locally furnish silica surfaces with a functional linker. This has allowed the selective attachment of single-stranded DNA onto the modified areas. In addition to plain surfaces, the surface reaction is also demonstrated on onto curved, enclosed surfaces, i.e. the inner surface of a microchannel. The surface-bound DNA has been selectively and reversibly hybridized with the complementary DNA. These experiments show that ~ 7 ´1011 fluorescently labeled DNA molecules can be hybridized per cm2. By furnishing target compounds with the complementary DNA strand, this hybridization approach allows the selective, localized binding of proteins, antibodies and other biomolecules to the surface.

    In Chapter 5, a new method for the organic modification of porous anodic alumina (PAA) is presented, which is based on the reaction of terminal alkynes with the alumina surface. The reaction results in the formation of a monolayer within several hours at 80 °C and is dependent on both oxygen and light. These monolayers are well-defined and consist of an oxidation product of the 1-alkyne, i.e. its a-hydroxy carboxylate. The obtained monolayers are fairly stable in water and at elevated temperatures. Modification with 1,15-hexadecadiyne results in a surface with available alkyne endgroups, which can be used for further surface chemistry.

    In Chapter 6, the biofunctionalization of PAA is explored. To this aim, lactosyl-terminated surfaces are prepared and the subsequent adsorption of peanut agglutinin (PNA) is studied. The PNA binds selectively and reversibly to these surfaces. Moreover, PNA adsorption is higher on surfaces that expose the b-lactoside than on those that display the a-anomer, which is attributed to surface-associated steric hindrance. The adsorption of the pathogens Neisseria gonorrhoeae and Candida albicans onto the lactosylated PAA surfaces is also investigated. Whereas quantification of N. gonorrhoeae adsorption is hindered by high background staining, C. albicans shows increased colonization onto lactosylated surfaces. Thus, this chapter shows that aluminum oxide surfaces can be modified to induce selective adsorption of proteins and microorganisms.

    The studies in this thesis show that there is much to be explored in the surface modification of inorganic surfaces. Future studies could focus on the mechanism of the coupling reaction, but also on the reactivity of 1-alkenes and 1-alkynes towards other relevant inorganic materials. In addition, the surface modification with living cells and biofilms is still largely unexplored and may be a research topic of prime interest for the coming years!

    Colloids and interfaces in life sciences and bionanotechnology
    Norde, Willem - \ 2011
    CRC Press - ISBN 9781439817186 - 466 p.
    colloids - colloidal properties - interface - surface tension - emulsions - foams - rheological properties - textbooks - surface chemistry

    Colloidal systems occur everywhere-in soils, seawater, foodstuff, pharmaceuticals, paints, blood, biological cells, and microorganisms. Colloids and Interfaces in Life Sciences and Bionanotechnology, Second Edition, gives a concise treatment of physicochemical principles determining interrelated colloidal and interfacial phenomena. New in the Second Edition: New topics, including phase separations in polymer systems, electrokinetics of charged permeable surface coatings, and polymer brush coatings to control adsorption and adhesion of particles. Emphasis on inter-particle interactions and surface phenomena in (bio)nanotechnology. Full solutions to over 100 updated and additional exercises are presented in the Appendix. Focusing on physicochemical concepts that form the basis of understanding colloidal and interfacial phenomena-rather than on experimental methods and techniques-this book is an excellent primer for students and scientists interested in colloidal and interfacial phenomena, their mutual relations and connections, and the fascinating role they play in natural and man-made systems.

    Covalent functionalization of silicon nitride surfaces for anti-biofouling and bioselective capture
    Nguyen, A.T. - \ 2011
    Wageningen University. Promotor(en): Cees van Rijn; Han Zuilhof, co-promotor(en): Jos Paulusse. - [s.l.] : S.n. - ISBN 9789461730084 - 141
    microfiltratie - ongewenste aangroei van levende (micro)organismen - aangroeiwerende middelen - oppervlaktechemie - microfiltration - biofouling - antifouling agents - surface chemistry

    Microsieves – microengineered membranes – have been introduced in microfiltration technology as a new generation of inorganic membranes. The thin membranes are made of silicon nitride (SixN4), which gives the membranes outstanding features, such as chemical inertness and high mechanical strength. Microsieves have very well-defined pore size and pore shape, with an extremely homogeneous size distribution and high porosity. As a result, high-flux performance and excellent selectivity may be achieved. However, biofouling issues exert limitations on the application of microsieves in filtration and diagnostics. Surface functionalization was found to be a feasible way to minimize biofouling, but also to achieve biorecognition in microbiological applications. The aim of this thesis is to improve microsieve performance in biological applications by means of surface functionalization with organic coatings for protein repellence and selective capture of microorganisms.
    In this thesis, SixN4 surfaces were functionalized with organic monolayers via stable Si C and N-C linkages. Coatings to render SixN4 surfaces protein repellent were studied in depth by two approaches: grafting of ethylene oxide monolayers onto the surface (Chapter 2); and grafting of zwitterionic polymers from the surface (Chapter 3). UV induced surface modification with oligo(ethylene oxide) chains with three (EO3) and six (EO6) units and the detailed characterization of these modified surfaces are described in Chapter 2. Successful attachment of EO3 and EO6 on SixN4 surfaces was achieved. The modified surfaces exhibit excellent protein repellence in bovine serum albumin (BSA) solution (~ 94%), but only moderate (~ 80%) protein repulsion was observed in fibrinogen (FIB) solution. This observation motivated the study towards grafting zwitterionic polymer brushes from SixN4 surfaces for improved protein repellence. A new method to grow zwitterionic polymers from monolayers containing tertiary bromides, via atom transfer radical polymerization (ATRP) was developed. The zwitterionic polymer coated surfaces showed excellent protein repellence in FIB solution (> 99%), while exhibiting very stable performance in PBS during one week, i.e., unchanged thickness, no hydrolysis of the polymers occurred and protein repellence in FIB solution remained constant.
    The use of microsieves as detection platform for microorganisms was explored in Chapter 4. Microorganisms can be caught by microsieves whose pore sizes are smaller than the microorganisms while allowing an easy flow-through of other components. However, detection capacity of microsieves is severely hampered by fouling issues. To avoid this problem, the use of microsieves with pore sizes larger than the microorganisms, in combination with immobilized antibodies was investigated in Chapter 4. Anti Salmonella antibodies were immobilized onto epoxide monolayers on microsieve surfaces by reaction with the primary amines present in the antibody. The antibody-coated microsieves showed excellent detection of Salmonella with high sensitivity and selectivity, significantly improving detection efficiency in crude biological samples, and reducing analysis times.
    The capture efficiency of Salmonella in milk samples was, however, found to be lower than that achieved in buffered solution. Most likely, this is due to nonspecific adsorption of milk proteins on the antibody-coated microsieves. In addition, the use of a blocking solution before incubation with microorganism solution remained an essential step in order to avoid the occurrence of interfering background fluorescence. In order to minimize these problems, the incorporation of antibodies on top of protein-repellent zwitterionic polymers coated on SixN4 surfaces was studied in Chapter 5. Anti-Salmonella antibodies were immobilized on zwitterionic polymer brushes coated SixN4 surfaces through the bromide moieties retained at the end of the polymer chain after ATRP. Antibody-functionalized zwitterionic polymers adsorbed only minimal amounts of FIB, indicating excellent protein repellence of the modified surfaces. Moreover, anti-Salmonella antibodies immobilized onto zwitterionic surfaces exhibit highly selective capture and improved sensitivity, as compared to antibodies on epoxide coated surfaces. This achievement offers a new approach that enables highly sensitive and selective detection of microorganism, while minimizing nonspecific adsorption of proteins that are not of interest.
    In Chapter 6, an overview is given of the most important findings presented in the thesis. Recommendations, as well as additional ideas on how to bring this research into industrial application are discussed.

    Reduction of protein adsorption on surfaces coated with Complex Coacercate Core Micelles
    Brzozowska, A.M. - \ 2010
    Wageningen University. Promotor(en): Martien Cohen Stuart; Willem Norde, co-promotor(en): Arie de Keizer. - [S.l. : S.n. - ISBN 9789085856832 - 246
    micellen - afdeklagen - eiwitten - adsorptie - oppervlaktechemie - micelles - coatings - proteins - adsorption - surface chemistry
    The structure and formation of Ionomer Complexes (ICs) consisting of linear polyelectrolytes
    (C3Ms) has been extensively studied in the past years. Recently, these structures were also
    considered for several applications. This thesis deals with the possible application of ICs as a
    surface coating suppressing protein adsorption independent of the properties of the native
    surface. To reach this goal it was crucial to understand the principles governing the adsorption
    and the stability of the IC layers on solid surfaces, as well as their interactions with proteins.
    Therefore, this research was limited to well defined model substrates: silica (model hydrophilic
    surface), polystyrene (model hydrophobic surface), and polysulfone (a surface mimicking
    polymeric membrane material), and model proteins: β-lactoglobulin, bovine serum albumin,
    fibrinogen, and lysozyme. The ultimate goal, however, was to apply the coating on surfaces of
    membranes used in water purification to suppress biofilm growth. We have observed that
    reduction of protein adsorption by coating formed by regular C3Ms is not satisfactory due to
    relatively low density of the polymer brushes formed on the coated surface. We increased the
    grafting density, and hence significantly improved the reduction of protein adsorption, by
    introducing grafted block and grafted copolymers into the micelles. In our work we discuss an
    influence of various factors, i.e. physical-chemical properties of the native surfaces, lengths of the
    charged blocks, distribution of the grafts along the backbone of the copolymer, salt concentration
    etc., on the performance of the formed coatings. In the final part we focus on the mechanical
    stability of the coatings formed with ICs, and discuss their applicability as a membrane surface
    coating.
    Van eiwitten in platland tot bionanotechnologie in Wageningen
    Norde, W. - \ 2010
    Wageningen : Wageningen Universiteit - ISBN 9789085855767 - 28
    eiwitten - oppervlakten - grensvlak - nanotechnologie - bionanotechnologie - oppervlaktechemie - oppervlakteverschijnselen - proteins - surfaces - interface - nanotechnology - bionanotechnology - surface chemistry - surface phenomena
    Modification of silicon nitride and silicon carbide surfaces for food and biosensor applications
    Rosso, M. - \ 2009
    Wageningen University. Promotor(en): Han Zuilhof; Remko Boom, co-promotor(en): Karin Schroen. - [S.l. : S.n. - ISBN 9789085853794 - 221
    organische verbindingen - unimoleculaire films - microfiltratie - nanotechnologie - oppervlaktechemie - oppervlakteverschijnselen - organic compounds - unimolecular films - microfiltration - nanotechnology - surface chemistry - surface phenomena
    Silicon-rich silicon nitride (SixN4, x > 3) is a robust insulating material widely used for the coating of microdevices: its high chemical and mechanical inertness make it a material of choice for the reinforcement of fragile microstructures (e.g. suspended microcantilevers, micro-fabricated membranes-“microsieves”) or for the coating of the exposed surfaces of sensors (field-effect transistors, waveguide optical detectors). To a more limited extent, silicon carbide (SiC) can find similar applications, and this material also starts to be more and more applied in coating and sensor technologies.
    In all these applications, control over the surface properties of inorganic materials is crucial, for example to avoid blockage of membranes during filtration, or to provide sensor surfaces with specific (bio-)recognition properties. In this thesis, a variety of methods is developed to obtain and study robust functional coatings on SixN4 and SiC. These enable a whole new range of applications involving biocompatible and bio-specific surfaces, while retaining the bulk mechanical, structural, electrical or optical properties of the inorganic substrates.
    Chapter 2 and 3 of the thesis give an overview of the great potential of covalent organic monolayers: Chapter 2 presents the formation of alkylthiol, alkylsilane and alkene monolayers, as well as a number of applications in biocompatible surfaces, micro- and nanopatterning of surfaces and sensing. The emphasis of this review chapter is put on the possible combinations of the bulk properties of inorganic materials (electrical, optical, structural) and the surface properties of organic monolayers (wettability, biospecificity, biorepellence). Chapter 3 is focused on biorepellent surfaces in the field of filtration with microfabricated membranes. Indeed, silicon nitride microsieves, despite their high permeability and structural homogeneity, are prone to pore blocking, when submitted to biological solutions. The chapter gives a review of the available surface modification techniques involving organic coatings that can minimize or even prevent this surface contamination. These coatings involve highly hydrophilic oligomers and polymers, which have been widely explored for organic surfaces. Covalent organic monolayers formed onto inorganic surfaces can extend the applications of these biorepellent coatings to microdevices like SixN4 microsieves (as also discussed in Chapters 7 and 8)
    Chapter 4 and 5 present the thermal functionalization with highly stable alkene-based organic monolayers of the surfaces of silicon-rich silicon nitride (Chapter 4) and silicon carbide (Chapter 5). This work was motivated by the substantial knowledge of similar monolayer formation on silicon surfaces1,2 and the initial success of simple functionalizations on silicon nitride.3 The strong covalent attachment of the coating molecules with the substrates makes the obtained hybrid structures much more resistant to chemical degradation than other types of monolayers on these substrates. The reaction proceeds via attachment of the terminal double bond of alkenes with the surface groups (Si-H in the case of silicon nitride surfaces or –OH for silicon carbide surfaces). Besides methyl-terminated surfaces, functional coatings can be obtained by the use of bi-functionalized alkenes (Figure 1), also allowing further surface reactions and the attachment of bio-recognition elements, through covalent attachment of diverse chemical (carboxylic acid, amine) or biological (oligo-peptides, protein) moieties.

    Figure 1. Modification of SiC and Si¬xN4 surfaces with alkyl monolayers
    Chapter 6 describes a modification of this method, where UV irradiation is used instead of heat to initiate the modification of both silicon nitride and silicon carbide. For both materials, this method allows the grafting of heat-sensitive compounds, needs less starting material (using only a liquid film) and provides monolayers with higher quality (as e.g. indicated by grafting density and stability) and higher reproducibility. Here again the attachment of diverse functionalities is possible, via formation of activated esters. After hydrolysis and activation of such grafted ester, amines can be attached in high yield (> 80 %), as demonstrated using X-ray photoelectron spectroscopy (XPS). Besides the homogeneous modification of plain surfaces, this method also opens the way to surface patterning of silicon nitride and silicon carbide and the modification of mechanically sensitive microfabricated devices.
    In Chapters 4 to 6, the chemical functionalizations are studied using X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), atomic force microscopy (AFM), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and static water contact angles. Si-C bonds are formed preferentially upon reaction of SixN4 surfaces with alkenes, similarly to what is reported for pure silicon surfaces, albeit that no measurement could totally exclude the presence of C-N bonds. The wet etching of SiC yields hydroxyl-terminated surfaces, and an IRRAS study reveals the attachment of alkenes via a Markovnikov-type addition (O-C bond formed on the second carbon of the double bond). The stability of these monolayers is reported in acidic and basic conditions, and it was shown that UV initiation yields even more stable monolayers, probably due to some cross-linking of the alkyl chains.
    Chapter 7 explores the biorepellence of UV-initiated monolayers on silicon nitride surfaces Oligomers of ethylene glycols (3 or 6 units: methoxy-tri(ethylene oxide) undec-1-ene (CH3O(CH2CH2O)3(CH2)9CH=CH2; EO3, and methoxy-hexa(ethylene oxide) undec-1-ene (CH3O(CH2CH2O)6(CH2)9CH=CH2; EO6) are attached on the silicon nitride surfaces. The adsorption of two proteins, bovine serum albumin (BSA) and fibrinogen is used to test the biorepellence of the monolayers, in comparison with bare oxidized silicon nitride. Both proteins adsorb readily onto bare SixN4 surfaces, with adsorbed amounts of 1.25 and 2.7 mg.m-2 for BSA and fibrinogen, respectively, of which more than 80 % is irreversibly bound. In contrast to this, when oligomers are attached to the surface, this adsorption decreases to under the detection limit of the method used for this experiment (optical reflectometry). The ex situ study of surfaces with AFM and water contact angles also indicates that some of the monolayers completely prevent the adsorption of proteins.

    Figure 2. Biorepellent behavior of oligoethylene oxide coated SixN4 surfaces
    Chapter 8 describes the applications of the biorepellent coatings used in Chapter 7 (EO6) to silicon nitride microsieves, in order to improve the filtration of biological solutions and liquid food products. The EO6 coatings are successfully formed on microfabricated membranes with pore diameters of 0.45 micrometer, using the UV-initiated monolayer formation described in Chapter 6. This work shows that these coatings could be applied without loss of permeability due to wettability or pore blocking. Moreover, AFM showed that these coatings significantly decrease the adsorption of proteins on the surface between the pores.
    Chapter 9 describes an alternative functionalization technique for inorganic surfaces, namely the use of plasma oxidation of alkyl monolayers to reproducibly form aldehydes (among other oxidized species) onto surfaces. The method described here for silicon and silicon nitride surfaces, is developed for the functionalization of sensitive devices and substrates. The formation of methyl-terminated alkyl monolayers from linear terminal alkenes is one of the easiest to perform, since linear monofunctional alkenes are readily available, their purification is easy (distillation) and their grafting conditions are very flexible (liquid state, heat-resistant, UV-resistant > 250 nm). Once these stable monolayers are formed, a short plasma treatment (0.5 to 2 s) is able to form oxidized functionalities within the top few angstroms of the surface, while the underlying alkyl chains retain their initial packing and insulation properties of the inorganic substrate. The grafting of gold nanoparticles shows that micron-sized patterns can be formed using a soft contact mask to protect a limited area of the monolayer. Alternatively, the aldehydes can be used to attach biotin and avidin onto SixN4 surfaces. The selective adsorption of biotinylated BSA onto the avidin-modified surfaces shows that the plasma treatment of methyl-terminated monolayers is a fast and efficient method to produce surfaces displaying high specific biochemical interactions.
    In the chapter 10, some of the most striking effects that are described in the previous chapters are put into a wider perspective. Especially the formation and stability of monolayers is discussed, also in relation to biofunctionalization, biorepellence, and opportunities for surface engineering are proposed.
    Ketens en grenzen
    Fleer, G.J. - \ 2007
    Wageningen : Wageningen Universiteit - 34
    polymeren - colloïdale eigenschappen - oppervlaktechemie - oppervlakteverschijnselen - polymers - colloidal properties - surface chemistry - surface phenomena
    Protein/polysaccharide complexes at air/water interfaces
    Ganzevles, R.A. - \ 2007
    Wageningen University. Promotor(en): Martien Cohen Stuart; Fons Voragen, co-promotor(en): Harmen de Jongh; Ton van Vliet. - [S.l.] : S.n. - ISBN 9789085046141 - 151
    polysacchariden - eiwitten - adsorptie - oppervlaktechemie - oppervlakteverschijnselen - polysaccharides - proteins - adsorption - surface chemistry - surface phenomena
    KEYWORDS:protein, polysaccharide,b‑lactoglobulin, pectin, electrostatic interaction, complex coacervation, adsorption, air/water interface, oil/water interface, surface pressure, surface rheology, spectroscopy

    Proteins are often used to create and stabilise foams and emulsions and therefore their adsorption behaviour to air/water and oil/water interfaces is extensively studied. Interaction of protein and polysaccharides in bulk solution can lead to the formation of soluble or insoluble complexes. The aim of this thesis was to understand the influence of (attractive and non-covalent) protein/polysaccharide interaction on adsorption behaviour at air/water interfaces (and oil/water interfaces) in terms of adsorption kinetics, and rheological and spectroscopic characterisation of the adsorbed layers. The approach was to first identify the relevant parameters (like charge density, charge distribution or molecular weight of the ingredients) in the mixed protein/polysaccharide adsorption process. Subsequently, for each parameter a range of ingredients was selected/prepared allowing variation of only this single parameter. After investigation of the phase behaviour in bulk solution of the different protein/polysaccharide mixtures to be used, the role of each parameter in mixed protein/polysaccharide adsorption was studied. The parameters most thoroughly assessed were: protein/polysaccharide mixing ratio, polysaccharide charge density and molecular weight and the sequence of adsorption. The majority of the measurements were performed withb‑lactoglobulin (in combination with various polysaccharides e.g. pectin or carboxylated pullulan) at air/water interfaces, at standard conditions of pH 4.5 and low ionic strength (< 10 mM). In addition, experiments were performed at higher ionic strengths, different pH's, with different proteins or at an oil/water interface, to extend the insight in mixed protein/polysaccharide adsorption. This results obtained lead to a generic mechanistic model of mixed protein/polysaccharide adsorption.

    In conclusion, protein/polysaccharide interaction can be exploited to control protein adsorption at air/water interfaces. Any parameter affecting protein/polysaccharide interaction (e.g. ingredient parameters like polysaccharide molecular weight, charge density and distribution or system parameters like charge ratio, pH and ionic strength) may be varied to obtain the desired adsorption kinetics, surface rheological behaviour, or net charge of the surface layer. The choice of simultaneous protein/polysaccharide adsorption (in the form of complexes) versus sequential adsorption (first the protein, than the polysaccharide) provides an extra control parameter regarding the functionality of mixed adsorbed layers.
    Faradaic and adsorption-mediated depolarization of electric double layers in colloids
    Duval, J.F.L. - \ 2003
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Herman van Leeuwen. - Wageningen : s.n. - ISBN 9789058089403 - 238
    colloïden - elektrokinetische potentiaal - oppervlakteverschijnselen - oppervlaktechemie - colloids - electrokinetic potential - surface phenomena - surface chemistry - cum laude
    cum laude graduation (with distinction)
    Colloids and interfaces in life sciences
    Norde, W. - \ 2003
    New York; Basel : Marcel Dekker - ISBN 9780824709969 - 433
    colloïden - colloïdale eigenschappen - grensvlak - oppervlaktespanning - emulsies - schuim - reologische eigenschappen - studieboeken - oppervlaktechemie - colloids - colloidal properties - interface - surface tension - emulsions - foams - rheological properties - textbooks - surface chemistry
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