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PubMed2006

Comparison of the fouling release properties of hydrophobic fluorinated and hydrophilic PEGylated block copolymer surfaces: attachment strength of the diatom Navicula and the green alga Ulva.

To understand the role of surface wettability in adhesion of cells, the attachment of two different marine algae was studied on hydrophobic and hydrophilic polymer surfaces. Adhesion of cells of the diatom Navicula and sporelings (young plants) of the green macroalga Ulva to an underwater surface is mainly by interactions between the surface and the adhesive exopolymers, which the cells secrete upon settlement and during subsequent colonization and growth. Two types of block copolymers, one with poly(ethylene glycol) side-chains and the other with liquid crystalline, fluorinated side-chains, were used to prepare the hydrophilic and hydrophobic surfaces, respectively. The formation of a liquid crystalline smectic phase in the latter inhibited molecular reorganization at the surface, which is generally an issue when a highly hydrophobic surface is in contact with water. The adhesion strength was assessed by the fraction of settled cells (Navicula) or biomass (Ulva) that detached from the surface in a water flow channel with a wall shear stress of 53 Pa. The two species exhibited opposite adhesion behavior on the same sets of surfaces. While Navicula cells released more easily from hydrophilic surfaces, Ulva sporelings showed higher removal from hydrophobic surfaces. This highlights the importance of differences in cell-surface interactions in determining the strength of adhesion of cells to substrates.

Biopolymers↗
PubMed2006

Cell fouling resistance of polymer brushes grafted from ti substrates by surface-initiated polymerization: effect of ethylene glycol side chain length.

This paper presents a comparative study on the antifouling properties of poly(ethylene glycol) (PEG)-based polymer coatings prepared by surface-initiated polymerization (SIP). Three types of poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMEMA) polymer thin films of approximate 100 nm thickness were grafted from a catechol initiator that was immobilized on a Ti substrate. OEGMEMA monomers containing side chains of 4, 9, and 23 EG units were used in surface-initiated atom transfer radical polymerization (SI-ATRP) to form POEGMEMA-4, -9, and -23 polymer brushes. The chemical composition, thickness, and wettability of the polymer brushes were characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry, and static water contact angle measurements, respectively. The dependence of antifouling performance on EG side chain length was systemically tested and compared by 3T3 fibroblast cell adhesion assays. Results from 4-h cell culture experiments revealed the complete absence of cell attachment on all the grafted Ti substrates. Excellent cell fouling resistance continued with little dependence on EG side chain length up to three weeks, after which long-term antifouling performance depended on the EG chain length as the grafted samples reached confluent cell coverage in 7, 10, and 11 weeks for POEGMEMA-4, -9, and -23, respectively.

3T3 Cells↗
PubMed2007

Synergistic activity of hydrophilic modification in antibiotic polymers.

Quaternized poly(vinylpyridine) is known to kill up to 99% of drug-resistant gram-positive and -negative bacteria but shows minimal biocompatibility. We report enhanced bactericidal activity of vinylpyridine through copolymerization with hydroxyethyl methacrylate and poly(ethylene gycol) methyl ether methacrylate. Copolymers with increasing comonomer content were synthesized by radical polymerization and quaternized with hexylbromide. We assessed the effects of the changes in polymer composition on the bactericidal activity of the surface activity using a bioluminescent pathogenic strain of Escherichia coli (O157:H7). By recording the photoluminescence emitted by these bacteria in contact with the copolymers, it was shown that several of the copolymers possess better antibacterial efficiency than quaternized poly(vinylpyridine). Results indicate that several of the copolymers synthesized possess antibacterial activity approximately 20 times greater than the pure quaternized poly(vinylpyridine) homopolymer, while only containing 1 wt % hexylated pyridinium. This behavior is explained by the increased surface wettability of the copolymers containing lesser amounts of poly(vinylpyridine), as bactericidal behavior correlates to the hydrophilicity of the system as measured by contact angles. A hydrophilicity based design-paradigm can significantly improve both the efficacy and the biocompatibility of antibacterial materials.

Anti-Bacterial Agents↗
PubMed1998

The behavior of hydrocolloid coatings on vegetative materials

Coating vegetative materials by gelling agents is a process characterized by four different time scales. After wetting and penetration of the vegetative skin by the gum solution, adhesion of the viscous solution to the outer layer (skin) of the coated material is possible. The gelled film (coating) collapses during further drying and adheres to the vegetative tissue. Critical surface tensions of the solid object to be coated, its apparent and real roughness, wettability of the surfaces by the gum solution, the composition and polarity of the films designed to coat the solid, and the surface tension of gum coating solutions are among the critical properties that need to be explored and changed for a successful coating process. The critical surface tensions of garlic peel and gellan and alginate films (coatings) were evaluated by Zisman plots. Garlic skin has a low surface tension compared with those of synthetic films such as polystyrene and polyethylene. A spreading technique was used to determine the surface tension of the dry film and the solid garlic skin. Surface tension was divided into dispersive and polar components. The similarity between the coating solution and the object to be coated in values of dispersive and polar components influences the spreadability of the coating gum solutions. Better compatibility between the coated object and the coating films can be achieved by incorporating surface-active agents within the coating gum solution. From the compatibility requirements detailed above, it can be concluded that tailor-made hydrocolloid coatings for different vegetative materials can only be achieved by further exploring the chemical and physical properties of the coating solutions and the coated objects.

Journal Article↗
PubMed2006

CCN activation of pure and coated carbon black particles.

The CCN (cloud condensation nucleus) activation of pure and coated carbon black particles was investigated using the University of Vienna cloud condensation nuclei counter (Giebl, H.; Berner, A.; Reischl, G.; Puxbaum, H.; Kasper-Giebl, A.; Hitzenberger, R. J. Aerosol Sci. 2002, 33, 1623-1634). The particles were produced by nebulizing an aqueous suspension of carbon black in a Collison atomizer. The activation of pure carbon black particles was found to require higher supersaturations than predicted by calculations representing the particles as insoluble, wettable spheres with mobility equivalent diameter. To test whether this effect is an artifact due to heating of the light-absorbing carbon black particles in the laser beam, experiments at different laser powers were conducted. No systematic dependence of the activation of pure carbon black particles on laser power was observed. The observations could be modeled using spherical particles and an effective contact angle of 4-6 degrees of water at their surface. The addition of a small amount of NaCl to the carbon black particles (by adding 5% by mass NaCl to the carbon black suspension) greatly enhanced their CCN efficiency. The measured CCN efficiencies were consistent with Kohler theory for particles consisting of insoluble and hygroscopic material. However, coating the carbon black particles with hexadecanol (a typical film-forming compound with one hydrophobic and one hydrophilic end) efficiently suppressed the CCN activation of the carbon black particles.

Air Pollutants↗
PubMed2006

Fabrication and characterization of silica/titania nanotubes composite membrane with photocatalytic capability.

Silica/titania nanotubes composite membranes were successfully prepared from silica/titania sols using porous alumina support membranes. XRD patterns of the composite membranes confirmed that the embedding of amorphous silica into nanophase titania matrix helped to increase the thermal stability of titania which suppressed the phase transformation from anatase to rutile and decreased the size of titania particles. Ninety-five percent of the pore volume of composite membrane was located in mesopores with diameters ranging from 1.4 to 10 nm. The surface of these composite membranes exhibited extremely high affinity for water under UV irradiation with water contact angle decreased from 62 degrees to nearly 5 degrees within 80 min. By coupling membrane separation with photocatalysis technique, the removal efficiency of Direct Black 168 was improved remarkably; being 85% within 100 min, while the values were 66% and 73% with photocatalysis alone or membrane separation alone during the same time, respectively. Good photocatalytic activity and wettability of composite membrane under UV irradiation helped to obtain high permeate flux across the composite membrane. The silica/titania nanotubes composite membrane had the multifunctions of separation, degradation, and improvement of membrane flux in photooxidation of organic contaminants in wastewater.

Aluminum Oxide↗
PubMed2003

Two-level structured self-adaptive surfaces with reversibly tunable properties.

We report a route to fabricate two-level structured self-adaptive surfaces (SAS) of polymer materials. The first level of structure is built by a rough polymer film that consists of needlelike structures of micrometer size. The second level of structure is formed by the nanoscopic self-assembled domains of a demixed polymer brush irreversibly grafted onto the needles. By exposing the surface to solvents that are selective to one of the components of the brush, we reversibly tune the surface properties. The large-scale surface structure amplifies the response and enables us to control wettability, adhesion, and chemical composition of the surface over a wide range.

Journal Article↗
PubMed2005

Surface-catalyzed growth of polymethylene-rich copolymer films on gold.

We report the preparation of a new class of copolymer films that consist of polymethylene (PM) with a low percentage ( approximately 1-4%) of randomly distributed ethyl ester side groups, consistent with poly(methylene-co-ethyl acetate). The films are prepared through a surface-catalyzed polymerization on gold surfaces upon exposure to a dilute solution of diazomethane (DM) and ethyl diazoacetate (EDA) in ether at 0 degrees C. While EDA alone does not polymerize at gold surfaces but DM does decompose on gold to grow PM films, the combined presence of EDA and DM results in dramatic enhancements in film growth and promotes an alternative mechanism for propagation as compared with that for the PM homopolymerization. The rate of copolymer film growth is constant over a approximately 24 h period, consistent with a controlled polymerization in which chain terminations are minimized. Carefully controlled experiments indicate that chain propagation does not occur at the outer film-solvent interface, but more likely, at the film-metal interface, suggesting a catalyzed insertion mechanism that extends the chain and pushes the outer chain termini further away from the metal-polymer interface. The results also suggest that adsorbed intermediates of EDA function as co-catalysts to promote the propagation reaction. Of particular importance for materials modification is that the ester side chains of these copolymer films can be hydrolyzed to carboxylate groups that exhibit pH-dependent wettability.

Journal Article↗
PubMed2005

Electric field induced switching behaviors of monolayer-modified silicon surfaces: surface designs and molecular dynamics simulations.

Electric field induced switching behaviors of a series of low-density omega-carboxyalkyl modified H-Si(111) and the mixed omega-carboxyalkyl/alkyl covered H-Si(111) have been simulated by using molecular dynamics (MD) simulation techniques. The external electric fields may drive surface-confined molecules to reversibly change conformations between the all-trans (switching "on") and the mixed trans-gauche (switching "off") states. Such surfaces switch wettabilities between the hydrophilic state and the moderately hydrophobic state. It has been found in broad ranges of intensities of applied electric fields, -2.0 x 10(9) V/m < or = E(down) < or = 0 and 1.8 x 10(9) V/m < or = E(up) < or = 7.3 x 10(9) V/m, both the low-density (11.1%-33.3%) omega-carboxyalkyl and the mixed omega-carboxyalkyl/alkyl (in mole fraction of 0.4 < or = N(carboxyalkyl) : N(alkyl) < or = 3.0) monolayers covering H-Si(111) exhibit conformational switching in the aqueous medium. The critical intensity of the electric field, E(up) = 1.8 x 10(9) V/m, which is required to trigger the switches is observed by our MD simulations and further rationalized by a thermodynamical model. Some important factors in the control of switching performances, such as the steric hindrances, the formation of the electric double layer at the monolayer/electrolyte solution interface, the hydration effects of carboxylate anions, the components of surrounding electrolyte solutions, as well as the rigidity of surface-confined chains are elucidated. The lower ionic strength and additions of acetonitrile molecules in the surrounding aqueous solution can reduce the value of critical intensity of the electric field and hence facilitate the realization of switching. Some practical considerations in construction and optimum design of switching surfaces are also suggested.

Journal Article↗
PubMed2005

Coordination-based gold nanoparticle layers.

Gold nanoparticle (NP) mono- and multilayers were constructed on gold surfaces using coordination chemistry. Hydrophilic Au NPs (6.4 nm average core diameter), capped with a monolayer of 6-mercaptohexanol, were modified by partial substitution of bishydroxamic acid disulfide ligand molecules into their capping layer. A monolayer of the ligand-modified Au NPs was assembled via coordination with Zr4+ ions onto a semitransparent Au substrate (15 nm Au, evaporated on silanized glass and annealed) precoated with a self-assembled monolayer of the bishydroxamate disulfide ligand. Layer-by-layer construction of NP multilayers was achieved by alternate binding of Zr4+ ions and ligand-modified NPs onto the first NP layer. Characterization by atomic force microscopy (AFM), ellipsometry, wettability, transmission UV-vis spectroscopy, and cross-sectional transmission electron microscopy showed regular growth of NP layers, with a similar NP density in successive layers and gradually increased roughness. The use of coordination chemistry enables convenient step-by-step assembly of different ligand-possessing components to obtain elaborate structures. This is demonstrated by introducing nanometer-scale vertical spacing between a NP layer and the gold surface, using a coordination-based organic multilayer. Electrical characterization of the NP films was carried out using conductive AFM, emphasizing the barrier properties of the organic spacer multilayer. The results exhibit the potential of coordination self-assembly in achieving highly controlled composite nanostructures comprising molecules, NPs, and other ligand-derivatized components.

Gold↗
PubMed2005

Single-step covalent functionalization of polylactide surfaces.

A single-step, nondestructive, and versatile technique for the grafting and chemical surface modification of biodegradable polymers such as polylactide is described. The substrates are subjected to the vapor phase of any of three investigated vinyl monomers: acrylamide, maleic anhydride, and N-vinylpyrrolidone, and grafting is induced by photoinitiation of benzophenone under solvent free conditions. The modified surfaces exhibit higher wettability, and the grafting is verified by X-ray photoelectron spectroscopy, attenuated total reflection Fourier-transform IR, contact-angle measurements, and scanning electron microscopy. The graft-chain pendant groups remain functional and can subsequently be modified so that a tailor-made surface with desired properties may be achieved.

Acrylamide↗
PubMed2005

Observing capillarity in hydrophobic silica nanotubes.

The development of template-synthesized silica nanotubes has created a unique opportunity for studying confined fluids by providing nanometer-scale containers in which the inner diameter (i.d.) and surface chemistry can be systematically and independently varied. An interesting question to be answered is the following: do solvents wet nanometer-scale tubes in the same way they wet ordinary capillaries? To answer this question, we have conducted studies to explore the wettability of the hydrophobic interiors of individual nanotubes. In these studies, single nanotubes with i.d.'s of either 30 or 170 nm were investigated over a range of water/methanol mixtures. These studies provide a direct route for comparing wetting phenomena in nanotubes with conventional macroscopic theories of capillarity. Our observations reveal four important aspects of capillary wetting in the 30-170 nm regime, a size range where the application of the Young-Laplace theory has not been experimentally investigated for hydrophobic pores. They are (i) a sharp transition between wetting and nonwetting conditions induced by addition of a cosolvent, (ii) invariance of this transition between nanotubes of 30 and 170 nm pore diameter, (iii) failure of the Young-Laplace equation to accurately predict the cosolvent's (methanol) mol fraction where the transition occurs, and (iv) reversibility of the observed wetting. The first two aspects conform to conventional capillarity (Young-Laplace), but the latter two do not. These measurements were complemented with ensemble experiments. The difference between theory and experiment is likely due to reliance on macroscopic values of contact angles or to liquid-phase instability within the hydrophobic pore.

Journal Article↗
PubMed2005

Branched coordination multilayers on gold.

A C3-symmetric tridentate hexahydroxamate ligand molecule was specially synthesized and used for coordination self-assembly of branched multilayers on Au surfaces precoated with a self-assembled monolayer (SAM) of ligand anchors. Layer-by-layer (LbL) growth of multilayers via metal-organic coordination using Zr4+ ions proceeds with high regularity, adding one molecular layer in each step, as shown by ellipsometry, wettability, UV-vis spectroscopy, and atomic force microscopy (AFM). The branched multilayer films display improved stiffness, as well as a unique defect self-repair capability, attributed to cross-linking in the layers and lateral expansion over defects during multilayer growth. Transmetalation, i.e., exposure of Zr4+-based assemblies to Hf4+ ions, was used to evaluate the cross-linking. Conductive atomic force microscopy (AFM) was used to probe the electrical properties of the multilayers, revealing excellent dielectric behavior. The special properties of the branched layers were emphasized by comparison with analogous multilayers prepared similarly using linear (tetrahydroxamate) ligand molecules. The process of defect annihilation by bridging over defective areas, attributed to lateral expansion via the excess bishydroxamate groups, was demonstrated by introduction of artificial defects in the anchor monolayer, followed by assembly of two layers of either the linear or the branched molecule. Analysis of selective binding of Au nanoparticles (NPs) to unblocked defects emphasized the superior repair mechanism in the branched layers with respect to the linear ones.

Journal Article↗
PubMed2006

A class of supported membranes: formation of fluid phospholipid bilayers on photonic band gap colloidal crystals.

We report the formation of a new class of supported membranes consisting of a fluid phospholipid bilayer coupled directly to a broadly tunable colloidal crystal with a well-defined photonic band gap. For nanoscale colloidal crystals exhibiting a band gap at the optical frequencies, substrate-induced vesicle fusion gives rise to a surface bilayer riding onto the crystal surface. The bilayer is two-dimensionally continuous, spanning multiple beads with lateral mobilities which reflect the coupling between the bilayer topography and the curvature of the supporting colloidal surface. In contrast, the spreading of vesicles on micrometer scale colloidal crystals results in the formation of bilayers wrapping individual colloidal beads. We show that simple UV photolithography of colloidal crystals produces binary patterns of crystal wettabilities, photonic stopbands, and corresponding patterns of lipid mono- and bilayer morphologies. We envisage that these approaches will be exploitable for the development of optical transduction assays and microarrays for many membrane-mediated processes, including transport and receptor-ligand interactions.

Colloids↗
PubMed2006

Photoreversibly switchable superhydrophobic surface with erasable and rewritable pattern.

We report the facile fabrication of a functional nanoporous multilayer film with wettability that is reversibly tunable between superhydrophobicity and superhydrophilicity with UV/visible irradiation. Our approach controls surface roughness with an electrostatic self-assembly process and makes use of the photoresponsive molecular switching of fluorinated azobenzene molecules. Selective UV irradiation onto the nanostructured substrate was used to realize substrates with erasable and rewritable patterns of extreme wetting properties. Our findings will open up new avenues for external stimuli-responsive smart surfaces.

Journal Article↗
PubMed2002

Inorganic and organic clays as carriers for controlled release of the herbicide hexazinone.

The risk of ground water contamination resulting from rapid leaching of highly soluble pesticides can be minimized through the application of the pesticide adsorbed on a matrix or carrier, which limits the amount of pesticide immediately available for undesirable losses. The use of natural materials for this purpose is of special interest in terms of economy and sustainability. In this work the adsorption of the herbicide hexazinone by two montmorillonites saturated with various inorganic and organic cations was determined and the ability of the two clays displaying the highest adsorption capacities [Fe(3+)-saturated Wyoming montmorillonite, (Fe-SW) and hexadecyltrimethylammonium-saturated Arizona montmorillonite (HDTMA-SA)] to act as carriers for slow release of hexazinone and to reduce herbicide leaching losses was evaluated. Hexazinone formulations based on Fe-SW and HDTMA-SA displayed slow release properties in water and soil/water suspensions, reduced herbicide leaching in soil columns, and maintained herbicidal activity, as compared with the currently available commercial hexazinone formulation (wettable powder). Loosely bound hexazinone-HDTMASA formulations, which led to the slowest breakthrough of hexazinone in soil columns along with the greatest amounts of herbicide released from the clay particles, displayed the most interesting characteristics for their use as slow release formulations and to prevent ground water contamination.

Adsorption↗
PubMed2002

Preparation of plastic and biopolymer multilayer films by plasma source ion implantation.

The plasma source ion implantation (PSII) technique was used to improve the adhesion between linear low-density polyethylene (LLDPE) and biopolymer. LLDPE was treated with the PSII using O(2) or CF(4) gas to modify its surface. After modification, chitosan or corn zein was used for coating on LLDPE. Wettability of the LLDPE surface was evaluated with a contact angle meter by the sessile drop method. X-ray photoelectron spectroscopy (XPS) was used to analyze the LLDPE surface. Before and after treatment, in the case of LLDPE treated with O(2) PSII, oxygen-containing functional groups were formed on the implanted surface. In the CF(4) PSII treated LLDPE, it was observed that the fluorine concentration on the surface of LLDPE remarkably increased and hydrophobic groups were formed by chemical reaction. Bilayer films coated with chitosan or corn zein showed 10 times lower oxygen permeability. Tensile strength of multilayer films was decreased a little compared with that of LLDPE. The plastic and biopolymer multilayer films have potential for food packaging application because of their O(2) gas barrier property and easy recyclability of the multilayer film.

Chemical Phenomena↗
PubMed2002

Chlorpyrifos decline curves and residue levels from different commercial formulations applied to oranges.

Residue levels and degradation rates of chlorpyrifos in orange fruits, orange leaves, and soil were investigated by using three different formulation types, that is, emulsifiable concentrate (EC), wettable granules (WG), and microencapsulates (ME). The pesticide degradation was studied for a period 131 days in orange fruits and for 161 days in orange leaves and soil. The experimental data were used to establish a mathematical model for the decline curves of chlorpyrifos residues as a function of time and to determine the relevant parameters describing such a process. Field trials showed a different degradation rate for EC and WG formulations as compared to ME formulation. For the first two formulations, the dissipation of chlorpyrifos in orange fruits was fast during the first phase and became much slower during the later period. Residue levels of chlorpyrifos from ME remained almost constant for approximately 65 days and then began to decrease. A similar behavior was observed for the three chlorpyrifos formulations on orange leaves and soil. Although microencapsulation of pesticides leads to improved handling safety, additional risks for the consumers, the agriculture workers, and the environment should be taken into account due to prolonged persistence of high residue levels in fruits as well as in leaves and soil.

Chlorpyrifos↗