Nisin antimicrobial activity and structural characteristics at hydrophobic surfaces coated with the PEO-PPO-PEO triblock surfactant Pluronic F108

The antimicrobial peptide nisin has been observed to preferentially locate at surfaces coated with the poly[ethylene oxide]-poly[propylene oxide]-poly[ethylene oxide] (PEO-PPO-PEO) surfactant Pluronic F108, to an extent similar to its adsorption at uncoated, hydrophobic surfaces. In order to evaluate nisin function following its adsorption to surfaces presenting pendant PEO chains, the antimicrobial activity of nisin-loaded, F108-coated polystyrene microspheres and F108-coated polyurethane catheter segments was evaluated against the Gram-positive indicator strain, Pediococcus pentosaceus. The retained biological activity of these nisin-loaded layers was evaluated after incubation in the presence and absence of blood proteins, for contact periods up to one week. While an increase in serum protein concentration reduced the retained activity on both bare hydrophobic and F108-coated materials, F108-coated surfaces retained more antimicrobial activity than the uncoated surfaces. Circular dichroism spectroscopy experiments conducted with nisin in the presence of F108-coated and uncoated, silanized silica nanoparticles suggested that nisin experienced conformational rearrangement at a greater rate and to a greater extent on bare hydrophobic surfaces relative to F108-coated surfaces. These results support the notion that immobilized, pendant PEO chains confer some degree of conformational stability to nisin while also inhibiting its exchange by blood proteins.     

Adsolubilization of 2-naphthol into adsorbed layers of triblock PEO-PPO-PEO copolymers on hydrophobic silica particles

Adsolubilization of 2-naphthol into an adsorbed layer of triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO, Pluronics) copolymers on hydrophobically modified silica particles has been investigated. Four kinds of Pluronics (P103, P105, P123, and F108) were employed in order to understand the effect of the hydrophobicity of the surfactants on the adsolubilization. The amount of the Pluronics adsorbed of the maximum/saturation adsorption level was increased with a decrease in the HLB value, suggesting that the more hydrophobic Pluronics (P103 and P123) adsorb preferentially onto the hydrophobic silica surface over the more hydrophilic Pluronics (P105 and F108). The greater adsorbed amount of the more hydrophobic surfactants resulted in a greater amount of 2-naphthol adsolubilized into the adsorbed Pluronics layers. In the case of simultaneous addition of the Pluronics and 2-naphthol, the amount adsolubilized into the adsorbed P123 and P103 layers increased in their low-surfactant-concentration regime, reached a maximum, and then decreased with a further increase in the Pluronics concentration. On the other hand, for both the P105 and F108 copolymers, a decrease in the adsolubilized amount was not observed over the whole range of copolymer concentration investigated. This difference is attributed to a difference in the hydrophobicity of the micellar aggregates in solution and of the adsorbed layers on the hydrophobic surface. When 2-naphthol was added after replacement of the Pluronics supernatant by a surfactant-free solution, the final decrease in the adsolubilization was insignificant for all the Pluronics. Indeed, the maximum amount of adsolubilization was comparable to the corresponding amount obtained in the case of simultaneous addition.     

Detection of nisin and fibrinogen adsorption on poly(ethylene oxide) coated polyurethane surfaces by time-of-flight secondary ion mass spectrometry (TOF-SIMS)

Stable, pendant polyethylene oxide (PEO) layers were formed on medical-grade Pellethane? and Tygon? polyurethane surfaces, by adsorption and gamma-irradiation of PEO-polybutadiene-PEO triblock surfactants. Coated and uncoated polyurethanes were challenged individually or sequentially with nisin (a small polypeptide with antimicrobial activity) and/or fibrinogen, and then analyzed with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Data reduction by robust principal components analysis (PCA) allowed detection of outliers, and distinguished adsorbed nisin and fibrinogen. Fibrinogen-contacted surfaces, with or without nisin, were very similar on uncoated polymer surfaces, consistent with nearly complete displacement or coverage of previously-adsorbed nisin by fibrinogen. In contrast, nisin-loaded PEO layers remained essentially unchanged upon challenge with fibrinogen, suggesting that the adsorbed nisin is stabilized within the pendant PEO layer, while the peptide-loaded PEO layer retains its ability to repel large proteins. Coatings of PEO loaded with therapeutic polypeptides on medical polymers have the potential to be used to produce anti-fouling and biofunctional surfaces for implantable or blood-contacting devices.     

Salt induced micellization of very hydrophilic PEO-PPO-PEO block copolymers in aqueous solutions

Symmetrical poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, triblock copolymers with 80% polyethylene oxide (PEO, the hydrophilic end blocks) and polypropylene oxide (PPO, the hydrophobic middle block) usually remain as molecularly dissolved at ambient temperature even at fairly high-concentrations (2 wt.% or more). However, the micellization is induced at lower concentration/temperature in the presence of salts. The results on salt induced micellization from four such hydrophilic copolymers Pluronic^® F38, F68, F88 and F108 obtained from several independent techniques are described. FTIR and fluorescence results provide essentially identical critical micelle temperatures (CMTs) showing marked decrease with increase in PPO molecular weight and in the presence of salt. These copolymers were weakly surface active and did not show a clear break point in surface tension concentration plot typical of surfactants. While addition of salt decreases the cloud point, no significant micelle growth was observed even at temperature close to cloud point (CP). Marked increased in solubilization of an oil dye was observed in presence of KCl. Different methods showed good agreement in temperature/salt-induced micellization of these hydrophilic copolymers.     

A robust approach to studying the adsorption of Pluronic F108 on nonporous membranes

A method for poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) desorption from synthetic nonporous polymeric membranes, using hexane:isopropanol treatment and subsequent colorimetric quantification, is described. The polymers polysulfone, poly(vinyldiene fluoride), and poly(ether imide) were used to fabricate solid adsorption matrices. The desorbed Pluronic F108 forms a color complex with ammonium ferrothiocyanate (NH4FeSCN) and is based on partitioning of a chromophore present in NH4FeSCN from an aqueous phase to a chloroform phase in the presence of Pluronic. The protocols for Pluronic desorption and detection are simple, sensitive, inexpensive, rapid, and reproducible over a wide range of Pluronic coating concentrations and membrane surface chemistries. A linear response over the concentration range from 3 to 130 microg ml(-1) is obtained. The adsorption isotherms for flat sheet membranes are also described and the Langmuir equation provides the best fit for the adsorption data obtained within the concentration range studied. The absence of any significant interference from certain proteins, vitamins, carbohydrates, plasma, and halogenated derivatives makes the assay equally suitable for the estimation of Pluronic F108 in the attendant Pluronic conjugates or in biomedical applications. Using nonporous hollow fine fibers and capillary membranes as model curved substrates we were also able to correlate an increase in the radius of curvature with a corresponding increase in the surface interfacial adsorption of Pluronic F108.     

Adsolubilization of 2-naphthol into adsorbed layer of PEO-PPO-PEO triblock copolymers on hydrophilic silica

Adsolubilization of 2-naphthol into an adsorbed layer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronics) on hydrophilic silica has been investigated. Four kinds of Pluronics (P103, P105, P123, and F108) were used in order to understand the effect of the hydrophobicity of surfactant on the adsolubilization. The order of the adsorption in the saturation level was found to be P123 approximately P103 > P105 > F108, meaning that Pluronics with higher hydrophobicity can adsorb preferentially to the silica surface. Indeed, this order was parallel to the order of the adsolubilization amount of 2-naphthol. In the case of co-addition of the Pluronics and 2-naphthol, the adsolubilization amount increased gradually at lower surfactant concentration regions, reached a maximum, and then decreased with increasing concentration of the Pluronics. The maximum amount appeared at critical polymolecular micelle concentration of each Pluronics. On the other hand, the final decrement was not observed when 2-naphthol was added after replacement of the Pluronics supernatant by the Pluronics free solution. These results suggest that adsolubilization behavior is influenced by the existence of the polymolecular micellar aggregates in the solution phase.     

Protein interaction with a Pluronic-modified poly(lactic acid) Langmuir monolayer

Interaction of bovine serum albumin (BSA) with poly(lactic acid) (PLA) layers mixed with poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronic) at air/solution interfaces was studied by the Langmuir balance technique. Wettability of the mixed PLA-Pluronic system was characterized in the form of a transferred one-layer Langmuir-Blodgett film, and considerable hydrophilization was obtained for all of the Pluronics (6400, 6800, 10500, and 12700) applied here. The density of PEO chains in the monolayer and hence the coverage of PLA was controlled by the composition and the compression of the mixed monolayers. Tensiometric investigations revealed that a significant reduction of BSA adsorption/penetration was achieved by applying the Pluronic 6800 and 12700 with long PEO blocks for hydrophilization of PLA. Interaction of BSA with the modified PLA monolayer depended on the density and length of the PEO chains. The surface morphological characteristics of the films determined by atomic force microscopy were in good correlation with the results of BSA interaction. The average roughness of the polymer LB layer was high due to BSA penetration into the PLA film, while smooth surfaces with small roughness were obtained when the PLA layer was modified by Pluronic 6800.     

On the stability of the polymer brushes formed by adsorption of ionomer complexes on hydrophilic and hydrophobic surfaces

We have studied the effect of normal forces and shear forces on the stability and functionality of a polymer brush layer formed upon adsorption of polymeric micelles on hydrophilic and hydrophobic surfaces. The micelles consist of oppositely charged polyelectrolyte blocks (poly(acrylic acid) and poly(N-methyl 2-vinyl pyridinium iodide), and a neutral block (poly(vinyl alcohol)) or neutral grafts (poly(ethylene oxide)). The strength of the attachment of the micellar layers to various substrates was evaluated with Atomic Force Microscopy. Flow cell experiments allowed for the evaluation of long-term stability of coatings in lateral flow. Fixed angle optical reflectometry was used to quantify protein (BSA) adsorption on the micellar layers after their exposure to flow. The results show that adsorbed micellar layers are relatively weakly attached to hydrophobic surfaces and much stronger to hydrophilic surfaces, which has a significant impact on their stability. Adsorbed layers maintain their ability to suppress protein adsorption on hydrophilic surfaces but not on hydrophobic surfaces. Due to the relatively weak attachment to hydrophobic surfaces the structure of adsorbed layers may easily be disrupted by lateral forces, such that the complex coacervate-brush structure no longer exists.     

Small-angle neutron scattering study of adsorbed pluronic tri-block copolymers on laponite

The adsorption of selected poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) tri-block copolymers on synthetic clay particles (laponite) has been investigated. The adsorbed amount and distribution of polymer was determined as a function of relative block composition and size, using the technique of contrast variation small-angle neutron scattering. The pluronic molecules appear to adsorb via a preferential segregation of hydrophobic PPO segments at the surface, with hydrophilic PEO segments dangling into solution. The effect of the PPO segments is substantial with large increases in adsorbed amount and layer thickness as the anchor fraction decreases/PEO chain length increases. This is in direct contrast to the behavior observed for PEO homopolymer adsorption (of much higher molecular weights) where the adsorbed amount and layer thickness are smaller and change little with molecular weight.     

Non-fouling surfaces produced by gas phase pulsed plasma polymerization of an ultra low molecular weight ethylene oxide containing monomer

The pulsed plasma polymerization of low molecular weight molecules containing only one (ethylene oxide vinyl ether) and two (diethylene oxide vinyl ether) ethylene oxide units were investigated. The surface density of EO units retained in the polymer films increases sharply with decreasing average power input during deposition, particularly at very low plasma duty cycles. The protein adsorption properties of these plasma synthesized polymer were investigated using ¹²⁵I-labeled albumin and fibrinogen. Surprisingly effective, non-fouling surfaces were observed with films synthesized from the monomer containing two ethylene oxide units; however, the monomer containing only one EO unit gave surfaces that were not particularly effective in preventing protein adsorptions. The results obtained show that ultra short chain length PEO modified surfaces can be biologically non-fouling. This, in turn, has interesting consequences in terms of trying to identify the basic reason for the effectiveness of EO units in preventing biomolecule adsorptions on surfaces.     

Effect of surface composition on the adsorption of photosystem I onto alkanethiolate self-assembled monolayers on gold

We have used self-assembled monolayers (SAMs) prepared from omega-terminated alkanethiols on gold to generate model surfaces and examine the effect of surface composition on the adsorption of Photosystem I (PSI), stabilized in aqueous solution by Triton X-100. Triton-stabilized PSI adsorbs to high-energy surfaces prepared from HO- and HO2C-terminated alkanethiols but does not adsorb to low-energy surfaces. The inhibition of PSI adsorption at low-energy surfaces is consistent with the presence of a layer of Triton X-100 that adsorbs atop the hydrophobic SAM and presents a protein-resistant poly(ethylene glycol) (PEG) surface. While the presence of the PEG surface prevents the adsorption of PSI, the displacement of the inhibiting layer of Triton X-100 by dodecanol, a more active surfactant, greatly enhances the adsorption of PSI. This inhibiting effect by Triton X-100 can be extended to other protein systems such as bovine serum albumin.     

Adsorption properties of ordered mesoporous silicas synthesized in the presence of block copolymer Pluronic F127 under microwave irradiation

Ordered mesoporous silicas (OMSs) were prepared at different temperatures by using tetraethyl orthosilicate (TEOS) as a silica source, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic F127) as a structure directing agent and sodium chloride as an additive under acidic conditions and microwave irradiation. The small angle X-ray diffraction patterns of these samples indicate the presence of ordered mesopores, while adsorption studies show that they possess high volumes of pores, bimodal pore size distributions and large pore sizes. There is an interesting change in the hysteresis loop of nitrogen adsorption isotherms with increasing temperature of hydrothermal treatment; a delayed desorption characteristic for cage-like mesostructures is observed for the OMS samples treated at 100 and 120?°C, while the hydrothermal treatment at 140 and 160?°C leads to the samples having hysteresis loops characteristic for channel-like materials.     

Heat inhibited reactions

Acyl transfer from p-nitrophenyl trimethylacetate to hydrogen peroxide in millimolar aqueous solutions of an amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer slows down as the temperature is raised due to partitioning of the hydrophobic ester into heat-induced micelles.     

Preparation of gold microparticles using halide ions in bulk block copolymer phases via photoreduction

Gold microparticles were prepared from the gold salt in the solid bulk phase of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer via a photoreduction process in the presence of halide ions. The shapes and sizes of the gold microparticles were found to be dependent on the types and amount of halide ions as well as the types of cations used due to the combined effects of the adsorption power and oxidative dissolution ability of the additives on gold surfaces. Gold nanorods were obtained when poly(ethylene oxide) was used instead of the block copolymer. This suggests that the poly(propylene oxide) (PPO) parts in the block copolymer are essential for the formation of gold microparticles, even though the degree of the direct interaction between the PPO blocks and gold salt is not significant.     

Surface modification of polyethylene terephthalate using PEO-polybutadiene-PEO triblock copolymers

The initial step of thrombus formation on blood-contacting biomaterials is known to be adsorption of blood proteins followed by platelet adhesion. It is generally accepted that surface modification of the biomaterials with poly(ethylene oxide) (PEO) substantially reduces protein adsorption and cell adhesion. Dacron® (polyethylene terephthalate) fabric, which is one of the biomaterials commonly used in blood-contacting devices, was grafted with PEO. A simple two-step procedure for covalent grafting of PEO onto the surface of Dacron® fabric was used. The surface was first treated with PEO-polybutadiene-PEO (PEO-PB-PEO) triblock copolymer, to introduce a layer of double bonds onto the surface. The Dacron® surface was then exposed to a solution of Pluronic® F108 (PF108), a commercially available PEO-poly(propylene oxide)-PEO (PEO-PPO-PEO) triblock copolymer. The surface with two adsorbed layers of PEO-PB-PEO and PF108 was γ-irradiated in the presence of PF108 in the bulk solution for a total radiation dose of 0.8 Mrad. The bulk concentrations of PEO-PB-PEO and PF108 were varied to maximize the efficiency of PEO grafting. Fibrinogen adsorption on PEO-grafted surfaces was reduced more than 90%, compared with that on control surfaces, irrespective of the bulk concentrations of polymers used for grafting. Platelet adhesion was also reduced substantially by PEO grafting. Only a few round platelets were able to adhere to the PEO-grafted surface, while the control surface was fully covered with aggregates of activated platelets. PEO grafting on polyethylene terephthalate using PEO-PB-PEO and PEO-PPO-PEO block copolymers is a simple approach that can be used for various other biomaterials.     

Microenvironment in the corona region of triblock copolymer micelles: temperature dependent solvation and rotational relaxation dynamics of coumarin dyes

Dynamic Stokes' shift and fluorescence anisotropy measurements using coumarin-153 (C153) and coumarin-151 (C151) as the fluorescence probes have been carried out in aqueous poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123) and poly(ethylene oxide)100-poly(propylene oxide)70-poly(ethylene oxide)100 (F127) block copolymer micelles with an aim to understand the water structures and dynamics in the micellar corona region. It has been established that the probes reside in the micellar corona region. It is indicated that the corona regions of P123 and F127 micelles are relatively less hydrated than the Palisade layers of neutral micelles like Triton-X-100 and Brij-35. From the appraisal of total Stokes' shift values for the probes in the two block copolymer micelles, it is inferred that the F127 micelle is more hydrated than the P123 micelle. It is observed that the dynamic Stokes' shift values for both of the probes remain more or less similar at all the temperatures studied in the P123 micelle. For C153 in F127, however, the observed Stokes' shift is seen to decrease quite sharply with temperature, though it remains quite similar for C151. Moreover, the fraction of the unobserved initial dynamic Stokes' shift is appreciably higher for both the probes in the F127 micelle compared to that in P123. Over the studied temperature range of 293-313 K, the spectral shift correlation function is described adequately by a bi-exponential function. Rotational relaxation times for C153 in both the micelles show a kind of transition at around 303 K. These results have been rationalized assuming collapse of the poly(ethylene oxide) (PEO) blocks and formation of water clusters in the corona region due to dehydration of poly(ethylene oxide) blocks with an increase in temperature. A dissimilar probe location has been inferred for the differences in the results with C153 and C151 probes in F127. Comparison of the microviscosity and the hydration of the block copolymer micelles has also been made with those of the other commonly used neutral micelles, for a better understanding of the results in the block copolymer micelles.     

Association of humic acid with metal (hydr)oxide-coated sands at solid-water interfaces

Mineral-bound humic acid (HA) can significantly modify the physicochemical properties of the mineral surfaces and vice versa, thereby influencing the fate and transport of organic pollutants in the subsurface. The effect of various mineral surfaces on the adsorption-desorption of dissolved bulk, terrestrial HA was evaluated using three model sorbents [uncoated, alpha-FeO(OH)-coated, and Al2O3-coated sands] at two equilibrium pH values. The results of SEM/EDS and XPS analyses revealed relatively uniform and stable metal (hydr)oxide coatings on quartz surface and the presence of the HA coating. Strong hysteresis effects were observed for both metal (hydr)oxide-coated sands whereas a weaker hysteresis effect was observed for uncoated sand, suggesting that the adsorption-desorption of HA to model sorbents is dependent on the affinity of chemical interactions between the HA and surface composition of model sorbents. Adsorption of HA molecules onto metal (hydr)oxide-coated sands can be attributed to ligand exchange for lower molecular weight (MW) HA fractions and hydrophobic interaction for higher MW HA fractions, illustrating that both kinetic and fractional adsorption-desorption of HA subcomponents are important considerations.     

Specific ion-dependent attraction and phase behavior of polymer-coated colloids

This paper presents results demonstrating the role of temperature and specific ions in mediating attraction between polymer-coated colloids and determining their equilibrium phase behavior. In particular, theoretical predictions of continuum van der Waals attraction between poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO)-coated polystyrene colloids are used to explain measured temperature and specific ion-dependent fluid-gel transitions in dispersions of these particles. Building on previous studies of PEO-PPO-PEO-coated polystyrene colloids dispersed in aqueous NaCl media, this work reports rheologically measured fluid-gel transitions as a function of temperature and NaCl/MgSO4 composition. Adhesive-sphere predictions of percolation thresholds are fit to measured fluid-gel data by allowing the adsorbed copolymer layer thickness as a single adjustable parameter. This allows the attraction between the PEO-PPO-PEO layers to be interpreted as a function of temperature and NaCl/MgSO4 composition. Quantitative predictions of a polymeric van der Waals attraction associated with the layer collapse in diminishing solvent conditions provides a simple mechanism for explaining the measured fluid-gel data as a dynamic percolation transition. Ultimately, this work identifies the importance of continuum polymeric van der Waals attraction for explaining specific ion-dependent phenomena.     

INTERACTIONS BETWEEN POLY(ETHYLENE OXIDE) COATED SURFACES AND BETWEEN SUCH SURFACES AND PROTEINS

Surfaces coated with poly(ethylene oxide) containing nonionic polymers are of interest in medical applications due to, among other things, the low adsorption of proteins on such surfaces. In this paper we have studied the interfacial properties of surfaces coated with PEO by measuring the forces acting between two such surfaces in water and across a protein solution as well as between one such surface and a surface carrying adsorbed proteins. One type of surface coating was a graft copolymer of poly(ethylene imine) and poly(ethylene oxide) where the cationic poly(ethylene imine) group anchored the polymer to negatively charged mica surfaces. Three different ways to prepare this coating was used and compared. It was found that this coating was not stable in the presence of lysozyme, a small positively charged protein, when the PEO graft density was low. The other type of coating was obtained by adsorbing ethyl(hydroxyethyl)-cellulose onto hydrophobised mica surfaces. The driving force for adsorption is in this case the hydrophobic interaction between nonpolar segments of the polymer and the surface. The EHEC coating was stable in the presence of lysozyme and the interactions between adsorbed layers of lysozyme and EHEC coated surfaces are purely repulsive due to long-range steric forces.     

Continuous synthesis of CdSe(x)Te(1-x) nanocrystals: chemical composition gradient and single-step capping

Thermosensitive poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer, Pluronic F68, containing a hydrophobic unit, oligo-(lactic acid)(oligo-LA) or oligo-caprolactone (oligo-CL), 2-META and RGD as side groups was successfully synthesized and characterized by (1)H NMR, FTIR, and elemental analysis. Their aqueous solution displayed special gel-sol-gel phase transition behavior with increasing temperature from 10 to 70°C, when the polymer concentration was above critical micelle concentration (CMC). The gel-sol phase diagram was investigated using tube inversion method, rheological measurement, and dynamic light scattering. Based on these results, the gelation properties of modified F68 were affected by several factors such as the composition of the substituents, chain length of oligo L-LA or oligo ε-CL, and the concentration of the polymer solutions. The unique phase transition behavior with temperature was observed by modified F68 triblock copolymer, composed of the PPO blocks core and the PEO blocks shell in aqueous solution. This phenomenon was elucidated using (1)H NMR data; the alteration of hydrophobic interaction and chain mobility led to the formation of transparent gel, coexistence of gel-sol, and opaque gel. These hydrogels may be useful in drug delivery and tissue engineering.