Nisin adsorption to hydrophobic surfaces coated with the PEO-PPO-PEO triblock surfactant Pluronic F108

The adsorption and elution of the antimicrobial peptide nisin at hydrophobic, silanized silica surfaces coated with the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) surfactant Pluronic F108 were measured in situ, with ellipsometry. While such layers are known to inhibit protein adsorption, nisin was observed to adsorb in multilayer quantities, to an extent similar to its adsorption at uncoated, hydrophobic surfaces. The rates of nisin adsorption and elution were generally slower at F108-coated surfaces. And, the sequential adsorption of nisin, including two adsorption-elution cycles at each surface, showed greater differences in adsorption rates between the first and second adsorption cycles, when evaluated at identical mass density, for uncoated relative to F108-coated surfaces. These results indicate that nisin adsorption occurs via "entrapment" within the PEO brush layer at F108-coated surfaces, in this way slowing adsorption and spontaneous elution, and inhibiting post-adsorptive molecular rearrangements by reducing the lateral mobility of nisin. While F108-coated layers rejected adsorption of serum albumin, sequential adsorption experiments carried out with nisin and albumin showed a low level of albumin adsorption when nisin was present at the interface.     

Shifts in polystyrene particle surface charge upon adsorption of the Pluronic F108 surfactant

Electrical field-flow fractionation (ElFFF) and sedimentation field-flow fractionation (SdFFF) were used in combination to study the adsorption of the triblock polymeric surfactant, Pluronic F108 [(EO)129-(PO)56-(EO)129] to 200 nm polystyrene (PS) latex spheres. The SdFFF technique allowed an accurate determination of the mass of surfactant adsorbed on each particle from a solution of given concentration. To complement this isotherm study, we show that ElFFF can be used to measure fractional coverages of the formed electrically neutral surfactant layers on the charged PS particles. Through a combination of the two techniques it is possible to gain information about the structure of the adsorbate layer. Thus, when Pluronic F108 is taken up by the PS surface from solutions of low concentration, all three blocks appear to adhere to the surface as long as there is free space available. As the solution concentration increases and the fractional coverage reaches approximately 20%, the surface turns crowded enough to let the strongly adsorbing PPO blocks competitively displace the weakly adherent PEO blocks, which gradually rise to extend into the aqueous phase until the surface is fully saturated.     

Interactions between nonpolar surfaces coated with the nonionic surfactant n-dodecyl-beta-D-maltoside

The forces acting between nonpolar surfaces coated with the nonionic surfactant n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) were investigated at concentrations below and above the critical micelle concentration. The long-range and adhesive forces were measured with a bimorph surface force apparatus (MASIF). It was found that the effect of hydrodynamic interactions had to be taken into account for an accurate determination of the short-range static interactions. The results were compared with disjoining pressure versus thickness curves that were obtained earlier with a thin film pressure balance (TFPB). This comparison led to the conclusion that the charges observed at the air-water interface are not due to charged species present in the surfactant sample. In addition, it was observed that the stability of thin liquid films crucially depends on the surfactant's bulk concentration (c) and thus on the packing density in the adsorbed layer. The force barrier preventing removal of the surfactant layer from between two solid-liquid interfaces increases with increasing c, while for foam films it is the stability of the Newton black film that increases with c. Finally, the results obtained for beta-C(12)G(2) were compared with those obtained for the homologue n-decyl-beta-d-maltoside (beta-C(10)G(2)) as well as with those obtained for nonionic surfactants with polyoxyethylene moieties as polar groups.     

Improved micellar hydration and gelation characteristics of PEO-PPO-PEO triblock copolymer solutions in the presence of LiCl

LiCl-induced changes in the micellar hydration and gelation characteristics of aqueous solutions of the two triblock copolymers F127 (EO(100)PO(70)EO(100)) and P123 (EO(20)PO(70)EO(20)) (where EO represents the ethylene oxide block and PO represents the propylene oxide block) have been studied by small-angle neutron scattering (SANS) and viscometry. The effect of LiCl was found to be significantly different from those observed for other alkali metal chloride salts such as NaCl and KCl. This can be explained on the basis of the complexation of hydrated Li(+) ions with the PEO chains in the micellar corona region. The interaction between the chains and the ions is more significant in the case F127 because of its larger PEO block size, and therefore, micelles of this copolymer show an enhanced degree of hydration in the presence of LiCl. The presence of the hydrated Li(+) ions in the micellar corona increases the amount of mechanically trapped water there and compensates more than the water molecules lost through the dehydration of the PEO chains in the presence of the Cl(-) ions. The enhancement in micellar hydration leads to a decrease in the minimum concentration required for the F127 solution to form a room-temperature cubic gel phase from 18% to 14%. Moreover, for both copolymers, the temperature range of stability of the cubic gel phase also increases with increasing LiCl concentration, presumably because of the ability of the Li(+) ions to reduce micellar dehydration with increasing temperature. Viscosity studies on a poly(ethylene glycol) (PEG) homopolymer with a size equivalent to that of the PEO block in F127 (4000 g/mol) also suggest that the dehydrating effect of the Cl(-) ion on the PEG chain is compensated by its interaction with the hydrated Li(+) ions.     

beta-casein and symmetrical triblock copolymer (PEO-PPO-PEO and PPO-PEO-PPO) surface properties at the air-water interface

A comparison of beta-casein and symmetrical triblock copolymer (PEO-PPO-PEO and PPO-PEO-PPO) adsorption layer properties at the air-water interface has been carried out by bubble tensiometry and ellipsometry. It has been verified that the equation of state parameters (pi approximately gamma(y)) obtained from surface pressure (pi) and ellipticity in Brewster conditions (rhoB), which is proportional to the surface concentration (gamma) data, are the same as those obtained from dilational modulus epsilon and pi data. These two consistent approaches give further support to the theoretical model of block copolymers which has been previously developed for protein adsorption at fluid interfaces. It is shown that the interfacial behavior of the copolymer adsorption layer changes strongly as a function of the length of the hydrophilic and hydrophobic block sequences. The theoretical model may be used for the interpretation of the adsorption properties of the synthetic copolymers only when the size of the blocks is large enough. In the case of block copolymers, the coil is in a self-avoiding walk conformation (y = 3) whatever the temperature, while in the case of beta-casein, the polypeptide chain is partly collapsed at room temperature due to thermolabile noncovalent bonds. At the end of the first semidilute regime, there is clear evidence for a crossover toward a second semidilute regime for synthetic copolymers as well as for beta-casein but it is presently only partially characterized.     

Surface rheology of PEO-PPO-PEO triblock copolymers at the air-water interface: comparison of spread and adsorbed layers

The dilatational rheological properties of monolayers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)-type block copolymers at the air-water interface have been investigated by employing an oscillating ring trough method. The properties of adsorbed monolayers were compared to spread layers over a range of surface concentrations. The studied polymers were PEO26-PPO39-PEO26 (P85), PEO103-PPO40-PEO103 (F88), and PEO99-PPO65-PEO99 (F127). Thus, two of the polymers have similar PPO block size and two of them have similar PEO block size, which allows us to draw conclusions about the relationship between molecular structure and surface dilatational rheology. The dilatational properties of adsorbed monolayers were investigated as a function of time and bulk solution concentration. The time dependence was found to be rather complex, reflecting structural changes in the layer. When the dilatational modulus measured at different concentrations was replotted as a function of surface pressure, one unique master curve was obtained for each polymer. It was found that the dilatational behavior of spread (Langmuir) and adsorbed (Gibbs) monolayers of the same polymer is close to identical up to surface concentrations of approximately 0.7 mg/m2. At higher coverage, the properties are qualitatively alike with respect to dilatational modulus, although some differences are noticeable. Relaxation processes take place mainly within the interfacial layers by a redistribution of polymer segments. Several conformational transitions were shown to occur as the area per molecule decreased. PEO desorbs significantly from the interface at segmental areas below 20 A(2), while at higher surface coverage, we propose that segments of PPO are forced to leave the interface to form a mixed sublayer in the aqueous region.     

Formation and catalytic activity of spherical composites with surfaces coated with gold nanoparticles

Micelle-supported gold composites with a polystyrene core and a poly(4-vinyl pyridine)/Au shell are synthesized using NaBH(4) to reduce a mixture of micelle and HAuCl(4) in acidic aqueous solution (pH approximately 2). The template micelle with a polystyrene core and a poly(4-vinyl pyridine) shell is formed by self-assembly of block copolymer polystyrene-block-poly(4-vinyl pyridine). The gold nanoparticles coated onto the surfaces of the composites possess an average diameter of about 15 nm. The composites are applied to catalyze the reduction of p-nitrophenol in the presence of NaBH(4), and the results indicate that the kinetic constant of the reaction increases when the composite concentration and the reaction temperature increase. In addition, research results also indicate that composites with high content of gold show higher catalytic activity and higher catalytic efficiency.     

Influence of NaCl on the behavior of PEO-PPO-PEO triblock copolymers in solution, at interfaces, and in asymmetric liquid films

The solution behavior of the polymeric surfactant Pluronic F127 (PEO(99)PPO(65)PEO(99)) and its adsorption behavior on aqueous-silica and aqueous-air interfaces, as well as the disjoining pressure isotherms of asymmetric films (silica/aqueous film/air) containing F127, are studied. The interfacial properties of adsorbed F127 layers (the adsorbed amount Gamma and the thickness h) as well as the aqueous wetting film properties [film thickness (h) and refractive indexes] were studied via ellipsometry. The solution properties of F127 were investigated using surface tensiometry and light scattering. The interactions between the air-water and silica-water interfaces were measured with a thin film pressure balance technique (TFB) and interpreted in terms of disjoining pressure as a function of the film thickness. The relations between the behaviors of the asymmetric films, adsorption at aqueous air, and aqueous silica interfaces and the solution behavior of the polymeric surfactant are discussed. Special attention is paid to the influence of the concentrations of F127 and NaCl. Addition of electrolyte lowers the critical micelle concentration, diminishes adsorption on silica, and increases the thickness of the asymmetric film.     

Influence of DNA adsorption and DNA/cationic surfactant coadsorption on the interaction forces between hydrophobic surfaces

The forces between hydrophobic surfaces with physisorbed DNA are markedly and irreversibly altered by exposure to DNA/cetyltrimethylammonium bromide (CTAB) mixtures. In this colloidal probe atomic force microscopy study of the interactions between a hydrophobic polystyrene particle and an octadecyltrimethylethoxysilane-modified mica surface in sodium bromide solutions, we measure distinct changes in colloidal forces depending on the existence and state of an adsorbed layer of DNA or CTAB-DNA complexes. For bare hydrophobic surfaces, a monotonically attractive approach curve and very large adhesion are observed. When DNA is adsorbed at low bulk concentrations, a long-range repulsive force dominates the approach, but on retraction some adhesion persists and DNA bridging is clearly observed. When the DNA solution is replaced with a CTAB-DNA mixture at relative low CTAB concentration, the length scale of the repulsive force decreases, the adhesion due to hydrophobic interactions greatly decreases, and bridging events disappear. Finally, when the surface is rinsed with NaBr solution, the length scale of the repulsive interaction increases modestly, and only a very tiny adhesion remains. These pronounced changes in the force behavior are consistent with CTAB-induced DNA compaction accompanied by increased DNA adsorption, both of which are partially irreversible.     

Structures and chromatographic characteristics of capsule-type silica gels coated with hydrophobic polymers in reversed-phase liquid chromatography

Summary Silicone polymer-coated silica gels modified with octadecyl and octyl groups (S/S-C₁₈, S/S-C₈), or “capsule-type silica gels” were developed as packing materials for reversed-phase liquid chromatography. They were obtained by coating the surface of totally porous silica gel with a homogeneous silicone polymer film, and thereafter modifying the coating polymer with octadecyl or octyl groups. Retaining the advantages of silica-based packings, they show strong resistance of alkali-like organic porous polymeric materials.     

Tailoring the structural,magnetic and antimicrobial activity of AgCrO2 delafossite at high annealing temperature

In the present study, we prepared AgCrO₂ delafossite at high annealing temperature 800 °C by flash auto-combustion technique which is an easy and a low-cost method. The objective of this study was to tailor the structural, magnetic and antimicrobial study of AgCrO₂ delafossite at this high annealing temperature (800 °C) which can be used in the fascinating applications. The sample was examined using X-ray diffraction pattern to confirm its single-phase hexagonal structure. Moreover, the morphology was analyzed by the field-emission scanning electron microscopy to clarify that AgCrO₂ delafossite was in the nanoscale range. The energy-dispersive X-ray analysis was performed to show the composition of the sample. The room-temperature magnetic hysteresis loop assured that the sample had superparamagnetic behavior. The investigated sample AgCrO₂ delafossite can be considered to have a strong antibacterial activity against Gram-positive and Gram-negative bacteria. However, by studying the effect of AgCrO₂ delafossite against fungi microorganism, no activity was detected using AgCrO₂ delafossite. The promising application of this study is to add AgCrO₂ delafossite to various drugs as an antibacterial agent.

     

Interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 and the origin of surface charges at the air/water interface

The interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 were investigated using two techniques and three different types of surfaces. As nonpolar surfaces, the air/water interface, silanated negatively charged glass, and thiolated uncharged gold surfaces were chosen. The interactions between the air/water interfaces were measured with a thin film pressure balance in terms of disjoining pressure as a function of film thickness. The interactions between the solid/liquid interfaces were determined using a bimorph surface force apparatus. The influence of the nature of the surface on the interaction forces was investigated at surfactant concentrations below and above the cmc. The adsorption of the nonionic surfactant on the uncharged thiolated surface does not, as expected, lead to any buildup of a surface charge. On the other hand, adsorption of C12E6 on the charged silanated glass and the charged air/water interface results in a lowering of the surface charge density. The reduction of the surface charge density on the silanated glass surfaces is rationalized by changes in the dielectric permittivity around the charged silanol groups. The reason for the surface charge observed at the air/water interface as well as its decrease with increasing surfactant concentration is discussed and a new mechanism for generation of OH- ions at this particular interface is proposed.     

Drop impact characteristics and structure effects of hydrophobic surfaces with micro- and/or nanoscaled structures

We report the drop impact characteristics on four hydrophobic surfaces with different well-scale structures (smooth, nano, micro, and hierarchical micro/nano) and the effects of those structures on the behavior of water drops during impact. The specimens were fabricated using silicon wet etching, black silicon formation, or the combination of these methods. On the surfaces, the microstructures form obstacles to drop spreading and retracting, the nanostructures give extreme water-repellency, and the hierarchical micro/nanostructures facilitate drop fragmentation. The maximum spreading factor (D*(max)) differed among the structures. On the basis of published models of D*(max), we interpret the results of our experiment and suggest reasonable explanations for these differences. Especially, the micro/nanostructures caused instability of the interface between liquid and air at Weber number We > ~80 and impacting drops fragmented at We > ~150.     

Influence of the size of fixed-rigidity spheres on the structural and energy characteristics of hydrophobic hydration

Monte Carlo simulations of molecular configurations of aqueous solutions of spherical particles with a special potential of solute—water interaction were carried out. The influence of the particle size on the properties of hydration shells was investigated. Two regimes of hydrophobic hydration with a crossover point at 0.4 nm were found. Hydration of smaller particles causes insignificant changes in the properties of water. Particles larger than 0.4 nm break the liquid water structure. Breaking effects are more pronounced in the first hydration shell of particles. Dewetting of hard sphere surfaces predicted by the LCW phenomenological theory has peculiarities in the case of hydration of fixed-rigidity spheres. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1777–1786, September, 2008.     

Two-dimensional surface properties of PEO-PPO-PEO triblock copolymer film at the air/water interface in the absence and presence of Tyr-Phe dipeptide, Val-Tyr-Val tripeptide, SDS and stearic acid

Two-dimensional surface properties of PEO-PPO-PEO triblock copolymer film (Mol.Wt. 2800) in the absence and presence of Tyr-Phe dipeptide, Val-Tyr-Val tripeptide, sodium dodecylsulfate and stearic acid have been investigated for the first time at the air/water interface using Langmuir film balance technique. It is observed that the above polymer forms fairly stable film at the air/water interface. There are no significant changes observed in the surface pressure-area (π-A) isotherms of polymer in the presence of SDS. However, more expanded film was formed in presence of SDS since the solubility of the polymer is more in SDS and the polymer network is disturbed in presence of SDS, which results in the increase in surface area of the polymer films. In the presence of dipeptide and tripeptide, the surface area of the polymer film decreased with a slight increase in the surface pressure indicating the binding of these peptides to polymer, which enhances the stability of the polymer film. Thermodynamic studies on the change in surface area (ΔA) and excess free energy of mixing (ΔG(mix)(E)) associated with the formation of the mixed film suggest the occurrence of a thermodynamically unstable mixed film. The presence of SDS slightly decreases the formation of mixed film of stearic acid with triblock copolymer and peptides due to the solubilization of these compounds in SDS. However, the hydrophobicity of the polymer films increases in the presence of stearic acid, leading to the increase in surface pressure. The positive deviation of ΔA and the positive ΔG(mix)(E) values show the non-ideality and incompatibility of thermodynamically unstable mixed films. The thermodynamic results suggest that the stability and compatibility of the polymer, peptides and their mixed films with stearic acid in the presence of SDS are decreased, which is in good agreement with the results obtained for other polymeric systems.     

An SFG study of interfacial amino acids at the hydrophilic SiO2 and hydrophobic deuterated polystyrene surfaces

Sum frequency generation (SFG) vibrational spectroscopy was employed to characterize the interfacial structure of eight individual amino acids--L-phenylalanine, L-leucine, glycine, L-lysine, L-arginine, L-cysteine, L-alanine, and L-proline--in aqueous solution adsorbed at model hydrophilic and hydrophobic surfaces. Specifically, SFG vibrational spectra were obtained for the amino acids at the solid-liquid interface between both hydrophobic d(8)-polystyrene (d(8)-PS) and SiO(2) model surfaces and phosphate buffered saline (PBS) at pH 7.4. At the hydrophobic d(8)-PS surface, seven of the amino acids solutions investigated showed clear and identifiable C-H vibrational modes, with the exception being l-alanine. In the SFG spectra obtained at the hydrophilic SiO(2) surface, no C-H vibrational modes were observed from any of the amino acids studied. However, it was confirmed by quartz crystal microbalance that amino acids do adsorb to the SiO(2) interface, and the amino acid solutions were found to have a detectable and widely varying influence on the magnitude of SFG signal from water at the SiO(2)/PBS interface. This study provides the first known SFG spectra of several individual amino acids in aqueous solution at the solid-liquid interface and under physiological conditions.     

Fast and high-resolution ion chromatography at high pH on short columns packed with 1.8 microm surfactant coated silica reverse-phase particles

Rapid ion chromatographic separations of small inorganic anions are performed on columns packed with high-pH resistant Zorbax Extend-C18 1.8 microm silica particles. Seven anions (iodate, chloride, nitrite, bromide, nitrate, phosphate, sulphate) are separated with 1.3 and 2 cm long x 0.46 cm I.D. C18 columns coated with the surfactant didodecyldimethylammonium bromide (DDAB). A 40 s separation is achieved at 2 mL/min with a 2.5 mM 4-hydroxybenzoic acid eluent at pH 10. Finally, the DDAB removal procedure is improved to eliminate the pressure build-up caused by precipitation of the surfactant in the column upon uncoating.     

Interactions between protein coated particles and polymer surfaces studied with the rotating particles probe

Nonspecific interactions between proteins and polymer surfaces have to be minimized in order to control the performance of biosensors based on immunoassays with particle labels. In this paper we investigate these nonspecific interactions by analyzing the response of protein coated magnetic particles to a rotating magnetic field while the particles are in nanometer vicinity to a polymer surface. We use the fraction of nonrotating (bound) particles as a probe for the interaction between the particles and the surface. As a model system, we study the interaction of myoglobin coated particles with oxidized polystyrene surfaces. We measure the interaction as a function of the ionic strength of the solution, varying the oxidation time of the polystyrene and the pH of the solution. To describe the data we propose a model in which particles bind to the polymer by crossing an energy barrier. The height of this barrier depends on the ionic strength of the solution and two interaction parameters. The fraction of nonrotating particles as a function of ionic strength shows a characteristic shape that can be explained with a normal distribution of energy barrier heights. This method to determine interaction parameters paves the way for further studies to quantify the roles of protein coated particles and polymers in their mutual nonspecific interactions in different matrixes.     

The effect of sodium dodecyl sulfate and Pluronic F127 on the electrophoretic separation of protein and polypeptide test mixtures at acid pH

Using a test mixture consisting of standard proteins (cytochrome c, chymotrypsinogen A, hen egg albumin, bovine serum albumin, aldolase, catalase and ferritin) and synthetic polypeptides (polylysine, polyaspartic, polyglutamic acid and polyproline) it was revealed that using sodium dodecyl sulfate (SDS) as background electrolyte modifier at acid pH (2.5) allows selective separation of highly positively charged polypeptides (polylysine) provided that their relative molecular mass is sufficiently low (3300 Da). The altered elution sequence of standard proteins as compared to a separation done without SDS may help their identification. Addition of Pluronic F127 offers clear-cut separations of standard proteins up to a relative molecular mass of 5 x 10(4) Da and allows to reveal protein/polypeptide microheterogeneity where applicable. None of the systems tested is suitable for the separation of acidic polypeptides and polyproline.