Static Magnetic Field Reduces Intracellular ROS Levels and Protects Cells Against Peroxide-Induced Damage: Suggested Roles for Catalase

A feature in neurodegenerative disorders is the loss of neurons, caused by several factors including oxidative stress induced by reactive oxygen species (ROS). In this work, static magnetic field (SMF) was applied in vitro to evaluate its effect on the viability, proliferation, and migration of human neuroblastoma SH-SY5Y cells, and on the toxicity induced by hydrogen peroxide (H2O2), tert-butyl hydroperoxide (tBHP), H2O2/sodium azide (NaN3) and photosensitized oxidations by photodynamic therapy (PDT) photosensitizers. The SMF increased almost twofold the cell expression of the proliferation biomarker Ki-67 compared to control cells after 7 days of exposure. Exposure to SMF accelerated the wound healing of scratched cell monolayers and significantly reduced the H2O2-induced and the tBHP-induced cell deaths. Interestingly, SMF was able to revert the effects of NaN3 (a catalase inhibitor), suggesting an increased activity of catalase under the influence of the magnetic field. In agreement with this hypothesis, SMF significantly reduced the oxidation of DCF-H2, indicating a lower level of intracellular ROS. When the redox imbalance was triggered through photosensitized oxidation, no protection was observed. This observation aligns with the proposed role of catalase in cellular proctetion under SMF.  Exposition to SMF should be further validated in vitro and in vivo as a potential therapeutic approach for neurodegenerative disorders.

The effect of the molecular structure of hydroxypropyl methylcellulose on the states of water, wettability, and swelling properties of cryogels prepared with and without CaO2

Hydroxypropyl methylcellulose (HPMC) belongs to the cellulose ether family that has hydroxyl groups substituted by hydrophobic methyl groups (DS) and hydrophilic hydroxypropyl groups (MS). Herein, the interactions between water molecules and cryogels prepared with HPMC in the presence and absence of a linear nonionic surfactant, as well as CaO₂ microparticles, which react with water producing O₂, were systematically investigated by sorption experiments and Time-Domain Nuclear Magnetic Resonance. Regardless of the DS and MS, most water molecules presented transverse relaxation time t2 typical of intermediate water and a small population of more tightly bound water. HPMC cryogels with the highest DS of 1.9 presented the slowest swelling rate of 0.519 ± 0.053 g_water/(g.s) and the highest contact angle values 85.250^o ± 0.004^o, providing the best conditions for a slow reaction between CaO₂ and water. The presence of surfactant favored hydrophobic interactions that allowed the polar head of the surfactant to be exposed to the medium, resulting in a higher swelling rate and lower contact angle values. The HPMC with the highest MS presented the fastest swelling rate and the lowest contact angle. These findings are relevant for the formulations and reactions, where tuning the swelling kinetics is crucial for the final application.

Poly(vinyl alcohol)/sodium alginate/magnetite composites: magnetic force microscopy for tracking magnetic domains

Hydrogels of poly(vinyl alcohol) (PVA)/sodium alginate (SA), and magnetic nanoparticles (MNPs) were prepared by solvent casting in the absence and in the presence of magnets, in order to obtain MNPs distributed randomly (PVA/SA-rMNP) and magnetically oriented MNPs (PVA/SA-gMNP) in the polymer matrix. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) techniques were used to evaluate the topography and to map the distribution of magnetic domains in the polymer matrix, respectively. The tip-surface distance (lift distance) of 50 nm during the MFM analyses facilitated the mapping of magnetic domains because the van der Waals forces were minimized. The magnetic signal stemming from clusters of MNPs were more easily identified than that from isolated MNPs. PVA and SA, PVA/SA, PVA/SA-rMNP, and PVA/SA-gMNP coatings with surface roughness (R_a) values of 3.8 nm, 28.7 nm, and 49.8 nm, respectively, were tested for the proliferation of mouse hippocampal HT-22 cells. While PVA/SA, PVA/SA-rMNP, and PVA/SA-gMNP coatings preserved cell viability >70% in comparison to the control (plastic plate) over 48 h, cell proliferation tended to decrease on surfaces with higher R_a values (PVA/SA-gMNP). These findings showed that the orientation of magnetic domains led to an increase of surface roughness, which decreased the viability of HT-22 cells. Thus, these results might be interesting for situations, where the control of cell proliferation is necessary.

Hydroxypropyl methylcellulose-sugarcane bagasse adsorbents for removal of 17α-ethinylestradiol from aqueous solution and freshwater

Adsorbents made of hydroxypropyl methylcellulose (HPMC) and sugarcane bagasse (BG) microparticles were applied for the separation of 17α-ethinylestradiol (EE2) from aqueous solution in batch, and from aqueous solution and freshwater in fixed-bed columns. HPMC chains and BG microparticles were crosslinked by the esterification with citric acid. The adsorbents presented compression modulus values that increased from 208 ± 20 kPa (pure HPMC) to 917 ± 90 kPa, when the content of BG particles added to HPMC was 50 wt% (HPMC50BG). The porosity (~ 97%), specific surface area (1.16 ± 0.10 m²/g) and swelling degree (20 ± 1 g water/g) values were not affected by the addition of BG particles. The adsorption isotherms determined for EE2 on HPMC and on HPMC50BG fitted to the Langmuir and Freundlich models; the adsorption capacity of HPMC was slightly higher than that of composite HPMC50BG. Nevertheless, the addition of BG particles rendered outstanding mechanical reinforcement and dimensional stability to the adsorbents. The adsorption was driven by (i) hydrophobic interactions between EE2 methylene and aromatic groups and HPMC methyl groups, as evidenced by FTIR spectroscopy, and (ii) H bonds between HPMC and EE2 hydroxyl groups, as revealed by the adsorption enthalpy change (ΔH_ads) of - 45 kJ/mol. Column adsorption experiments of EE2 from aqueous solution on HPMC and HPMC50BG indicated adsorptive capacity (q₀) values of 8.06 mg/g and 4.07 mg/g, respectively. These values decreased considerably for the adsorption of EE2 from river water, probably due to the competition of EE2 with humic substances dissolved in natural water. The HPMC adsorbents could be recycled retaining up to 83% of the original efficiency.

Crosslinkers for polysaccharides and proteins: Synthesis conditions, mechanisms, and crosslinking efficiency, a review

Polysaccharides and proteins are important macromolecules for developing hydrogels devoted to biomedical applications. Chemical hydrogels offer chemical, mechanical, and dimensional stability than physical hydrogels due to the chemical bonds among the chains mediated by crosslinkers. There are many crosslinkers to synthesize polysaccharides and proteins based on hydrogels. In this review, we revisited the crosslinking reaction mechanisms between synthetic or natural crosslinkers and polysaccharides or proteins. The selected synthetic crosslinkers were glutaraldehyde, carbodiimide, boric acid, sodium trimetaphosphate, N,N'-methylene bisacrylamide, and polycarboxylic acid, whereas the selected natural crosslinkers included transglutaminase, tyrosinase, horseradish peroxidase, laccase, sortase A, genipin, vanillin, tannic acid, and phytic acid. No less important are the reactions involving click chemistry and the macromolecular crosslinkers for polysaccharides and proteins. Literature examples of polysaccharides or proteins crosslinked by the different strategies were presented along with the corresponding highlights. The general mechanism involved in chemical crosslinking mediated by gamma and UV radiation was discussed, with particular attention to materials commonly used in digital light processing. The evaluation of crosslinking efficiency by gravimetric measurements, rheology, and spectroscopic techniques was presented. Finally, we presented the challenges and opportunities to create safe chemical hydrogels for biomedical applications.

CTAB-modified carboxymethyl cellulose/bagasse cryogels for the efficient removal of bisphenol A, methylene blue and Cr(VI) ions: Batch and column adsorption studies

The simultaneous removal of organic and inorganic pollutants from water requires multifunctional adsorbents. Cryogels of carboxymethyl cellulose (CMC) and sugarcane bagasse (BG) were modified with cetyltrimethylammonium bromide (CTAB) micelles for the adsorption of methylene blue (MB), Cr(VI) ions and bisphenol A (BPA) separately, in binary or ternary aqueous mixtures. Batch adsorption studies of MB and Cr(VI) and BPA on the CMCBG-CTAB adsorbents indicated removal capacities of 100%, 70% and 95%, respectively. MB adsorbed as multilayers on the CMCBG walls by electrostatic interaction, whereas Cr(VI) and BPA adsorbed on the cationic CTAB micelles surface and hydrophobic core of CTAB micelles, respectively. The breakthrough curves obtained for pure adsorbates and their mixtures showed that the adsorption of Cr(VI) ions increased (i) ~ 3.5 times in binary mixture with BPA or in the ternary mixture, in comparison to pure Cr(VI) solution, and (ii) 1.4 times in binary mixture with MB molecules, due to synergistic effects. In the presence of Cr(VI) ions in binary or ternary mixtures, the adsorption of MB was dramatically reduced due to screening effects. The adsorption of BPA was not significantly affected by the presence of MB or Cr(VI). The adsorbents were recycled five times without significant efficiency loss.

Hydroxypropyl methylcellulose: Physicochemical properties and ocular drug delivery formulations

Hydroxypropyl methylcellulose (HPMC) is a cellulose ether widely used in drug formulations due to its biocompatibility, uncharged nature, solubility in water and thermoplastic behavior. Particularly for ocular and ophthalmic formulations, HPMC is applied as viscosity enhancer agent in eye drops, gelling agent in injections, and polymeric matrix in films, filaments and inserts. The different therapeutic approaches are necessary due to the complex anatomic structure of the eye. The natural ocular barriers and the low drug permeation into the circulatory system make the drug administration challenging. This review presents the eye anatomy and the usual local routes of drugs administration, which are facilitated by the physicochemical properties of HPMC. The relationship between chemical structure and physicochemical properties of HPMC is displayed. The different types of formulations (local application) including HPMC for ocular drug delivery are discussed with basis on recent literature reports and patents.

Preparation, Antimicrobial Properties, and Cytotoxicity of Acrylic Resins Containing Poly(diallyldimethylammonium chloride)

PURPOSE

To design and analyze the biologic properties (antibacterial and antifungal, as well as cytotoxicity) of a dental biomaterial based on incorporation of the biocide poly(diallyldimethylammonium chloride) (PDADMAC) into the masses of self- and thermopolymerizable acrylic resins.

MATERIALS AND METHODS

PDADMAC was diluted into tetrahydrofuran (4 wt%) and incorporated into self- and thermopolymerizable acrylic resins. PDADMAC inclusion was verified by measuring the contact angle with water droplets. Plain resins were used as controls. The antibacterial activity was evaluated against Staphylococcus aureus (ATCC 6538P) and Escherichia coli (ATCC 8739), and the antifungal activity was tested against Candida albicans (ATCC 10231) and Aspergillus niger (ATCC 16404). The cytotoxicity of substances leached from these materials was analyzed in human dental pulp stem cells using MTT reduction assay.

RESULTS

Reduction of contact angle confirmed the incorporation of PDADMAC in the resins. Both resins containing PDADMAC were more effective against Gram-positive and Gram-negative bacteria than their controls. The modified resins were also significantly more effective against Candida albicans than controls, but no resin was effective against Aspergillus niger. The cell viability of cultures submitted to substances leached from the PDADMAC resins was similar to that of cells cultured under ideal conditions.

CONCLUSION

The incorporation of PDADMAC into the acrylic resins achieved the desired antibacterial effect, with no changes in the biocompatibility properties of the resins. Moreover, the modified resins were effective against Candida albicans, the most common fungus in the oral cavity. Thus, the incorporation of PDADMAC in biomaterials seems to be promising in dentistry.

Antimicrobial Coatings from Hybrid Nanoparticles of Biocompatible and Antimicrobial Polymers

Hybrid nanoparticles of poly(methylmethacrylate) synthesized in the presence of poly (diallyldimethyl ammonium) chloride by emulsion polymerization exhibited good colloidal stability, physical properties, and antimicrobial activity but their synthesis yielded poor conversion. Here we create antimicrobial coatings from casting and drying of the nanoparticles dispersions onto model surfaces such as those of silicon wafers, glass coverslips, or polystyrene sheets and optimize conversion using additional stabilizers such as cetyltrimethyl ammonium bromide, dioctadecyldimethyl ammonium bromide, or soybean lecithin during nanoparticles synthesis. Methodology included dynamic light scattering, determination of wettability, ellipsometry of spin-coated films, scanning electron microscopy, and determination of colony forming unities (log CFU/mL) of bacteria after 1 h interaction with the coatings. The additional lipids and surfactants indeed improved nanoparticle synthesis, substantially increasing the conversion rates by stabilizing the monomer droplets in dispersion during the polymerization. The coatings obtained by spin-coating or casting of the nanoparticles dispersions onto silicon wafers were hydrophilic with contact angles increasing with the amount of the cationic polymer in the nanoparticles. Against Escherichia coli and Staphylococcus aureus, bacteria cell counts were reduced by approximately 7 logs upon interaction with the coatings, revealing their potential for several biotechnological and biomedical applications.

Sustainable hydroxypropyl methylcellulose/xyloglucan/gentamicin films with antimicrobial properties

Hydroxypropyl methylcellulose (HPMC) and xyloglucan (XG) crosslinked with citric acid over a range of HPMC/XG weight ratios formed sustainable blend films characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, tensile tests, circular dichroism and determination of inhibitory activity against Staphylococcus aureus and Escherichia coli. Both in solution and in the crosslinked films, HPMC chains lost the original ordered conformation upon interacting with XG, giving rise to an entropic gain. The highest values of tensile strength (25MPa) and Young's modulus (689MPa) occurred for the 50:50 HPMC/XG blend films. In vitro loading of gentamicin sulfate (GS) in the films amounted to 0.18±0.05 -0.37±0.05g of GS per g polymer. At pH 7.4 and 37°C, the GS release kinetics from the films fitted with the Korsmeyer-Peppas model revealed a non-Fickian release mechanism with diffusional coefficient n∼0.7. The cross-linked films of HPMC, XG and their blends loaded with GS showed outstanding antibacterial activity against Staphylococcus aureus and Escherichia coli, disclosing their potential for novel biomedical applications.

Alginate/magnetite hybrid beads for magnetically stimulated release of dopamine

Hybrid beads composed of magnetite nanoparticles (MNP) and alginate (Alg) were synthesized and coded as Alg-MNP. They were incubated in dopamine (DOPA) solution (5 g/L), at pH 7.4 and 8 °C, during 12 h, promoting the DOPA loaded magnetic beads, coded as Alg-MNP/DOPA. The release of DOPA was further evaluated in the absence and the presence of external magnetic field (EMF) of 0.4 T. The products Alg-MNP and Alg-MNP/DOPA were characterized by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), Fourier transform infrared vibrational spectroscopy (FTIR), UV spectrophotometry, thermogravimetric analyses (TGA), inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyses and superconducting quantum interference device (SQUID) magnetometer. The magnetic and chemical properties of Alg-MNP beads were not affected by DOPA loading. The incorporation of DOPA into the beads depended on the pH and on the negative charge density. At pH 7.4 38% of DOPA were loaded into Alg-MNP beads, whereas at pH 2 or using neat Alg beads (lower charge density than Alg-MNP) the loading efficiency decreased to one third or less. In the absence of EMF, 24% of the loaded DOPA was released from Alg-MNP at pH 7.4 over a period of 26 h. The released amount increased to 33% under the stimulus of EMF. A model was proposed to explain the loading efficiency of charged drugs, as DOPA, into hybrid beads and the role played by EMF on delivery systems, where drug and matrix are oppositely charged. The results suggest that the alginate combined with magnetite nanoparticles is a promising system for release of DOPA in the presence of EMF.

The antimicrobial activity of free and immobilized poly (diallyldimethylammonium) chloride in nanoparticles of poly (methylmethacrylate)

Background

Several cationic polymers exhibit a useful antimicrobial property, however the structure–activity relationship still requires a more complete investigation. The main objective of this work is the comparison between the antimicrobial activity and toxicity of free and immobilized poly (diallyldimethylammonium) chloride (PDDA) in biocompatible poly (methylmethacrylate) (PMMA) nanoparticles (NPs).

Results

NPs synthesis by emulsion polymerization is performed over a range of [PDDA] at two methylmethacrylate (MMA) concentrations. The PMMA/PDDA dispersions are characterized by dynamic light-scattering for sizing, polydispersity and zeta-potential analysis, scanning electron microscopy (SEM), plating plus colony forming unities (CFU) counting for determination of the minimal microbicidal concentrations (MMC) against Escherichia coli, Staphylococcus aureus and Candida albicans and hemolysis evaluation against mammalian erythrocytes. There is a high colloidal stability for the cationic PMMA/PDDA NPs over a range of [PDDA]. NPs diverse antimicrobial activity against the microorganisms reduces cell viability by eight-logs (E. coli), seven-logs (S. aureus) or two-logs (C. albicans). The NPs completely kill E. coli over a range of [PDDA] that are innocuous to the erythrocytes. Free PDDA antimicrobial activity is higher than the one observed for PDDA in the NPs. There is no PDDA induced-hemolysis at the MMC in contrast to the hemolytic effect of immobilized PDDA in the NPs. Hemolysis is higher than 15 % for immobilized PDDA at the MMC for S. aureus and C. albicans.

Conclusions

The mobility of the cationic antimicrobial polymer PDDA determines its access to the inner layers of the cell wall and the cell membrane, the major sites of PDDA antimicrobial action. PDDA freedom does matter for determining the antimicrobial activity at low PDDA concentrations and absence of hemolysis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-015-0123-3) contains supplementary material, which is available to authorized users.

Synthesis and characterization of xanthan-hydroxyapatite nanocomposites for cellular uptake

In this work xanthan-nanohydroxyapatite (XnHAp) and its equivalent strontium substituted (XnHApSr) were synthesized by the precipitation of nanohydroxyapatite in xanthan aqueous solution, characterized and compared to conventional hydroxyapatite particles (HAp). XnHAp and XnHApSr were less crystalline than HAp, as revealed by X-ray diffraction. Xanthan chains enriched the surface of XnHAp and XnHApSr particles, increasing the zeta potential values from -(7±1)mV, determined for HAp, to -(17±3)mV and -(25±3)mV, respectively. This effect led to high colloidal stability of XnHAp and XnHApSr dispersions and acicular particles (140±10)nm long and (8±2)nm wide, as determined by scanning electron microscopy and atomic force microscopy. XnHAp and XnHApSr particles were added to xanthan hydrogels to produce compatible nanocomposites (XCA/XnHAp and XCA/XnHApSr). Dried nanocomposites presented surface energy, Young's modulus and stress at break values comparable to those determined for bare xanthan matrix. Moreover, adding XnHAp or XnHApSr nanoparticles to xanthan hydrogel did not influence its porous morphology, gel content and swelling ratio. XCA/XnHAp and XCA/XnHApSr composites proved to be suitable for osteoblast growth and particularly XCA/XnHapSr composites induced higher alkaline phosphatase activity.

Catalytic activity of lipase immobilized onto ultrathin films of cellulose esters

Ultrathin (approximately 2.0 nm) films of cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB) supported on Si wafers have been prepared by adsorption and characterized by means of ellipsometry, atomic force microscopy (AFM), and contact angle measurements. CA, CAP, and CAB ultrathin films were characterized in air just after their formation and after annealing under reduced pressure at temperature higher than the corresponding melt temperature. Upon annealing, CA, CAP, and CAB ultrathin films became smoother and more hydrophobic, evidencing molecular reorientation at the solid-air interface. CA, CAP, and CAB films were used as supports for the immobilization of lipase. The adsorption of lipase onto annealed films was more pronounced than that onto untreated films, showing the strong affinity of lipase for the more hydrophobic substrates. Enzymatic activity was evaluated by a standard procedure, namely, (spectrophotometric) measurement of p-nitrophenol, the product formed from the hydrolysis of p-nitrophenyl dodecanoate (p-NPD). Lipase immobilized onto hydrophobic films exhibited higher activity than that of free lipase and could be recycled three times while retaining relatively high activity (loss of ca. 30% of original enzymatic activity). The effect of storing time on the activity of immobilized lipase was studied. Compared with free lipase, that immobilized onto more hydrophobic films retained 70% activity after 1 month. More importantly, the latter level of activity is similar to that of free lipase. However, lipase immobilized onto more hydrophilic films retained 50% and 30% activity after 20 and 30 days, respectively. These results are explained in terms of surface wettability and the contribution of the interactions between the polar residues of lipase and the glucopyranosyl moieties of cellulose ester to maintain the natural conformation of immobilized enzyme.

Adsorption behavior and activity of horseradish peroxidase onto polysaccharide-decorated particles

The adsorption behavior of horseradish peroxidase (HRP) onto hybrid particles of poly(methylmethacrylate) (PMMA) and carboxymethylcellulose (CMC) was investigated by means of spectrophotometry. Dispersions of PMMA/CMC particles were characterized by light scattering, zeta potential measurements and scanning electron microscopy before and after HRP adsorption. HRP adsorbed irreversibly onto PMMA/CMC particles; the adsorption isotherm showed an initial step and an adsorption plateau. The enzymatic activity of free HRP and immobilized HRP (plateau region) was monitored by means of spectrophotometry as a function of storing time. Upon adsorbing HRP there is little (up to 20%) or no reduction of enzymatic activity in comparison to that observed for free HRP in solution. After storing free HRP and HRP-covered PMMA/CMC particles for 18 days the level of enzymatic activity is kept. HRP-covered PMMA/CMC particles dispersions, which were dried and re-dispersed, retained 50% of their catalytic properties. These interesting findings were discussed in the light of a beneficial effect of a hydrated microenvironment for maintenance of enzyme conformation and activity.

Dewetting and surface properties of ultrathin films of cellulose esters

Surface properties of ultrathin films of cellulose esters deposited onto silicon wafers have been investigated by means of contact angle measurements and atomic force microscopy (AFM). Cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB) films adsorbed or spin-coated onto Si wafers were annealed up to one week. Film stability was monitored by AFM. Dewetting has been observed for CA and CAP. Only CAB films with lower degree of esterification presented dewetting, CAB films with high degree of butyrate were stable even after one week annealing. Surface energy of CA, CAP, and CAB was indirectly determined by contact angle measurements using drops of water, formamide and diiodomethane. The surface energy decreased as the size of alkyl ester group or the degree of esterification increased because van der Waals interactions became weaker. Effective Hamaker constant A(eff) was calculated for CA, CAP, and CAB onto Si wafers in air. Negative values of A(eff) were found for CA, CAP, and lower butyrate content CAB, which are related to instability and agree with dewetting phenomena observed by AFM. In contrast, a positive A(eff) was determined for higher butyrate content CAB, corroborating with experimental observations.

Adsorption behavior and activity of hexokinase

The study on the adsorption of hexokinase (HK) onto silicon wafers was carried out by means of in situ ellipsometry and atomic force microscopy in the liquid. The thickness values of the adsorbed HK layer determined by both techniques were in excellent agreement and evidenced HK monolayer formation. The adsorption of HK onto Si wafers was favored at low ionic strength, indicating that the adsorption is mainly driven by electrostatic forces, since salt screens not only the segment-segment repulsion but also the segment-surface attraction when the salt concentration increases. The enzymatic activity of free HK and of adsorbed HK was measured as a function of time. Free HK in solution lost activity upon storage. Contrarily, adsorbed HK kept its activity level even after 48 h storage at room temperature. This outstanding behavior was attributed to specific orientation of the HK active site to the solution.

Hybrid particles of polystyrene and carboxymethyl cellulose as substrates for copper ions

The synthesis of hybrid particles was carried out by emulsion polymerization of styrene in complexes formed by carboxymethyl cellulose (CMC), a polyanion, and a cationic surfactant, cetyltrimethylammonium bromide (CTAB). CMC chains with variable molecular weights and degrees of substitution were tested. The polymerization condition chosen was that corresponding to CMC chains fully saturated with CTAB and to the onset of pure surfactant micelle formation, namely, at the critical aggregation concentration. The hybrid particles were characterized by zeta potential and light scattering measurements. The period of colloidal stability in the ionic strength of 2.0 mol L(-)(1) NaCl was observed visually. Upon increasing the CMC chain length, the particle characteristics remained practically unchanged, but the colloid stability was increased. The increase in the CMC degree of substitution led to particles with more negative zeta potential values. The adsorption of copper ions (Cu(2+)) on the surface of hybrid particles could be described by the Langmuir model, as determined by potentiometric measurements. The increase in the mean zeta potential values and X-ray absorption near-edge spectra evidenced the immobilization of Cu(2+) ions on the hybrid particles.

The effect of water or salt solution on thin hydrophobic films

Hydrophobic films of polystyrene synthesized in bulk (PS) and by emulsion polymerization in the presence of the cationic surfactant cetyltrimethylammonium bromide (PS-CTAB) or the anionic surfactant sodium dodecyl sulfate (PS-SDS) were characterized by means of ellipsometry, contact angle measurements, and atomic force microscopy. Thin (approximately 65 nm) and thick (approximately 300 nm) films were spin-coated on hydrophilic silicon wafers. PS films presented scarcely tiny holes, while PS-CTAB and PS-SDS films presented holes and protuberances. The former were attributed to dewetting effects and the latter to surfactant clusters. The films were exposed to water or to a 0.1 mol/L NaCl solution for 24 h. Ex situ measurements evidenced strong topographic alterations after the exposure to the fluid. A model based on the diffusion of water (or electrolyte) molecules to the polymer/silcon dioxide interface through holes or defects on the film edges was proposed to explain the appearance of wrinkles and protuberances. In situ ellipsometric measurements were performed and compared with simulations, which considered either a water layer between a polymer and a silcon dioxide layer or an air layer between a polymer and water (medium). In the case of thin PS films, the ellipsometric angles evidenced a very thin (0.5-1.0 nm) air layer between water and the PS films. Upon increasing the PS film thickness, no air layer could be observed by ellipsometry. Regardless of the thickness, the ellipsometric data obtained for PS-CTAB and PS-SDS films did not indicate the presence of an air layer between them and the aqueous media. The dramatic changes in the topography of PS, PS-CTAB, and PS-SDS after immersion in salt solution were explained with proposed models. From a practical point of view, this study is particularly relevant because many hydrophobic polymers are used as substrates for biomedical purposes, where the physiological ionic strength is 0.15 mol/L NaCl.