Antimicrobial behavior of polyelectrolyte-surfactant thin film assemblies

Mesostructured Electroactive Thin Films Through Layer-by-Layer Assembly of Redox Surfactants and Polyelectrolytes

Electroactive thin films are an important element in the devices devoted to energy conversion, actuators, and molecular electronics, among others. Their build-up by the layer-by-layer technique is an attractive choice since a fine control over the thickness and composition can be achieved. However, most of the assemblies described in the literature show a lack of internal order, and their thicknesses change upon oxidation-state alterations. In this work, we describe the formation of layer-by-layer assemblies of redox surfactants and polyelectrolytes that leads to the construction of mesoscale organized electroactive films. In contrast to thin films prepared with traditional redox polymers, here, the redox surfactant does not only allow the control of the film meso-organization (from 2D hexagonal to circular hexagonal phases) but it also allows the control of the number and position of the redox centers. Finally, these films show high stability and a negligible structural deformation under redox-state changes.

Surfactants as mesogenic agents in layer-by-layer assembled polyelectrolyte/surfactant multilayers: nanoarchitectured "soft" thin films displaying a tailored mesostructure

Interfacial supramolecular architectures displaying mesoscale organized components are of fundamental importance for developing materials with novel or optimized properties. Nevertheless, engineering the multilayer assembly of different building blocks onto a surface and exerting control over the internal mesostructure of the resulting film is still a challenging task in materials science. In the present work we demonstrate that the integration of surfactants (as mesogenic agents) into layer-by-layer (LbL) assembled polyelectrolyte multilayers offers a straightforward approach to control the internal film organization at the mesoscale level. The mesostructure of films constituted of hexadecyltrimethylammonium bromide, CTAB, and polyacrylic acid, PAA (of different molecular weights), was characterized as a function of the number of assembled layers. Structural characterization of the multilayered films by grazing-incidence small-angle X-ray scattering (GISAXS), showed the formation of mesostructured composite polyelectrolyte assemblies. Interestingly, the (PAA/CTA)n assemblies prepared with low PAA molecular weight presented different mesostructural regimes which were dependent on the number of assembled layers: a lamellar mesophase for the first bilayers, and a hexagonal circular mesophase for n ≥ 7. This interesting observation was explained in terms of the strong interaction between the substrate and the first layers leading to a particular mesophase. As the film increases its thickness, the prevalence of this strong interaction decreases and the supramolecular architecture exhibits a "bulk" mesophase. Finally, we demonstrated that the molecular weight of the polyelectrolyte has a considerable impact on the meso-organization for the (PAA/CTA)n assemblies. We consider that these studies open a path to new rational methodologies to construct "nanoarchitectured" polyelectrolyte multilayers.

Polyelectrolyte Multilayer Nanocoating Dramatically Reduces Bacterial Adhesion to Polyester Fabric

Bacterial adhesion to textiles is thought to contribute to odor and infection. Alternately exposing polyester fabric to aqueous solutions of poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) is shown here to create a nanocoating that dramatically reduces bacterial adhesion. Ten PDDA/PAA bilayers (BL) are 180 nm thick and only increase the weight of the polyester by 2.5%. The increased surface roughness and high degree of PAA ionization leads to a surface with a negative charge that causes a reduction in adhesion of Staphylococcus aureus by 50% when compared to uncoated fabric, after rinsing with sterilized water, because of electrostatic repulsion. S. aureus bacterial adhesion was quantified using bioluminescent radiance measured before and after rinsing, revealing 99% of applied bacteria were removed with a ten bilayer PDDA/PAA nanocoating. The ease of processing, and benign nature of the polymers used, should make this technology useful for rendering textiles antifouling on an industrial scale.

Note: Automatic layer-by-layer spraying system for functional thin film coatings

In this study, we have constructed an automatic spray machine for producing polyelectrolyte multilayer films containing various functional materials on wide substrates via the layer-by-layer (LbL) assembly technique. The proposed machine exhibits advantages in terms of automation, process speed, and versatility. Furthermore, it has several features that allow a fully automated spraying operation, such as various two-dimensional spraying paths, control of the flow rate and operating speed, air-assist fan-shaped twin-fluid nozzles, and an optical display. The robot uniformly sprays aqueous mixtures containing complementary (e.g., oppositely charged, capable of hydrogen bonding, or capable of covalent bonding) species onto a large-area substrate. Between each deposition of opposite species, samples are spray-rinsed with deionized water and blow-dried with air. The spraying, rinsing, and drying areas and times are adjustable by a computer program. Twenty-bilayer flame-retardant thin films were prepared in order to compare the performance of the spray-assisted LbL assembly with a sample produced by conventional dipping. The spray-coated film exhibited a reduction of afterglow time in vertical flame tests, indicating that the spray-LbL technique is a simple method to produce functional thin film coatings.

Antibacterial Activity, in Vitro Cytotoxicity, and Cell Cycle Arrest of Gemini Quaternary Ammonium Surfactants

Twelve gemini quaternary ammonium surfactants have been employed to evaluate the antibacterial activity and in vitro cytotoxicity. The antibacterial effects of the gemini surfactants are performed on Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) with minimum inhibitory concentrations (MIC) ranging from 2.8 to 167.7 μM. Scanning electron microscopy (SEM) analysis results show that these surfactants interact with the bacterial cell membrane, disrupt the integrity of the membrane, and consequently kill the bacteria. The data recorded on C6 glioma and HEK293 human kidney cell lines using an MTT assay exhibit low half inhibitory concentrations (IC50). The influences of the gemini surfactants on the cell morphology, the cell migration ability, and the cell cycle are observed through hematoxylin-eosin (HE) staining, cell wound healing assay, and flow cytometric analyses, respectively. Both the values of MIC and IC50 decrease against the growth of the alkyl chain length of the gemini surfactants with the same spacer group. In the case of surfactants 12-s-12, the MICs and IC50s are found to decrease slightly with the spacer chain length changing from 2 to 8 and again to increase at higher spacer length (s = 10-12). All of the gemini surfactants show great antibacterial activity and cytotoxicity, and they might exhibit potential applications in medical fields.

Synthesis of polyhydroxyurethanes from di(trimethylolpropane) and their application to quaternary ammonium chloride-functionalized films

Polyhydroxyurethanes synthesized using non-isocyanate and non-phosgene derivatives afford quaternary ammonium chloride (QAC)-functionalized films after chloroacetylation and quaternization with cross-linkable amines.

Cationic antimicrobial polymers and their assemblies

Cationic compounds are promising candidates for development of antimicrobial agents. Positive charges attached to surfaces, particles, polymers, peptides or bilayers have been used as antimicrobial agents by themselves or in sophisticated formulations. The main positively charged moieties in these natural or synthetic structures are quaternary ammonium groups, resulting in quaternary ammonium compounds (QACs). The advantage of amphiphilic cationic polymers when compared to small amphiphilic molecules is their enhanced microbicidal activity. Besides, many of these polymeric structures also show low toxicity to human cells; a major requirement for biomedical applications. Determination of the specific elements in polymers, which affect their antimicrobial activity, has been previously difficult due to broad molecular weight distributions and random sequences characteristic of radical polymerization. With the advances in polymerization control, selection of well defined polymers and structures are allowing greater insight into their structure-antimicrobial activity relationship. On the other hand, antimicrobial polymers grafted or self-assembled to inert or non inert vehicles can yield hybrid antimicrobial nanostructures or films, which can act as antimicrobials by themselves or deliver bioactive molecules for a variety of applications, such as wound dressing, photodynamic antimicrobial therapy, food packing and preservation and antifouling applications.

Mechanism of Coomassie Brilliant Blue G-250 binding to cetyltrimethylammonium bromide: an interference with the Bradford assay

The Bradford protein assay is a popular method because of its rapidity, sensitivity, and relative specificity. This method is subject to some interference by nonprotein compounds. In this study, we describe the interference of cetyltrimethylammonium bromide (CTAB) with the Bradford assay. This interference is based on the interaction of Coomassie Brilliant Blue G-250 (CBB) with this cationic detergent. This study suggests that both electrostatic and hydrophobic interactions are involved in the interaction of CTAB and CBB. The anionic and neutral forms of CBB bind to CTAB by electrostatic attraction, which accelerates hydrophobic interactions of these CBB forms and the hydrophobic tail of CTAB. Consequently, the hydrophobic regions of the dominant free cationic form of CBB dye compete for the tail of CTAB with two other forms of the dye and gradually displace the primary hydrophobic interactions and rearrange the primary CBB-CTAB complex. This interaction of CTAB and CBB dye produces a primary 650-nm-absorbing complex that then gradually rearranges to a complex that shows an absorbance shoulder at 800-950 nm. This study conclusively shows a strong response of CBB to CTAB that causes a time-dependent and nearly additive interference with the Bradford assay. This study also may promote an application of CBB for CTAB quantification.

Layer by Layer coatings assembled through dipping, vertical or horizontal spray for cotton flame retardancy

Silica-based assemblies have been deposited on cotton fibres through Layer by Layer technique in order to enhance their flame retardant properties. To this aim, three different deposition procedures (namely, dipping, vertical and horizontal sprays) have been considered and compared. The resulting morphologies of the deposited assemblies have been thoroughly investigated by scanning electron microscopy (SEM) and elemental analysis. SEM observations have demonstrated that only the horizontal spray allows obtaining the deposition of a very homogeneous silica coating when compared to vertical spray or dipping. As a consequence, horizontal spray has proved to ensure the best flame resistance, promoting a substantial increase of the total burning time and final residue, as assessed by flammability tests. Furthermore, cone calorimetry measurements have shown a remarkable increase of the time to ignition, and a significant decrease of heat release rate and total heat release for the fabrics treated by horizontal spray.

Self-assembled poly(ethylene glycol)-co-acrylic acid microgels to inhibit bacterial colonization of synthetic surfaces

We explored the use of self-assembled microgels to inhibit the bacterial colonization of synthetic surfaces both by modulating surface cell adhesiveness at length scales comparable to bacterial dimensions (∼1 μm) and by locally storing/releasing an antimicrobial. Poly(ethylene glycol) [PEG] and poly(ethylene glycol)-co-acrylic acid [PEG-AA] microgels were synthesized by suspension photopolymerization. Consistent with macroscopic gels, a pH dependence of both zeta potential and hydrodynamic diameter was observed in AA-containing microgels but not in pure PEG microgels. The microgels were electrostatically deposited onto poly(l-lysine) (PLL) primed silicon to form submonolayer surface coatings. The microgel surface density could be controlled via the deposition time and the microgel concentration in the parent suspension. In addition to their intrinsic antifouling properties, after deposition, the microgels could be loaded with a cationic antimicrobial peptide (L5) because of favorable electrostatic interactions. Loading was significantly higher in PEG-AA microgels than in pure PEG microgels. The modification of PLL-primed Si by unloaded PEG-AA microgels reduced the short-term (6 h) S. epidermidis surface colonization by a factor of 2, and the degree of inhibition increased when the average spacing between microgels was reduced. Postdeposition L5 peptide loading into microgels further reduced bacterial colonization to the extent that, after 10 h of S. epidermidis culture in tryptic soy broth, the colonization of L5-loaded PEG-AA microgel-modified Si was comparable to the very small level of colonization observed on macroscopic PEG gel controls. The fact that these microgels can be deposited by a nonline-of-sight self-assembly process and hinder bacterial colonization opens the possibility of modifying the surfaces of topographically complex biomedical devices and reduces the rate of biomaterial-associated infection.

Clay-chitosan nanobrick walls: completely renewable gas barrier and flame-retardant nanocoatings

Thin films prepared via a layer-by-layer (LbL) assembly of renewable materials exhibit exceptional oxygen barrier and flame-retardant properties. Positively charged chitosan (CH), at two different pH levels (pH 3 and pH 6), was paired with anionic montmorillonite (MMT) clay nanoplatelets. Thin-film assemblies prepared with CH at high pH are thicker, because if the low polymer charge density. A 30-bilayer (CH pH 6-MMT) nanocoating (~100 nm thick) reduces the oxygen permeability of a 0.5-mm-thick polylactic acid film by four orders of magnitude. This same coating system completely stops the melting of a flexible polyurethane foam, when exposed to direct flame from a butane torch, with just 10 bilayers (~30 nm thick). Cone calorimetry confirms that this coated foam exhibited a reduced peak heat-release rate, by as much as 52%, relative to the uncoated control. These environmentally benign nanocoatings could prove beneficial for new types of food packaging or a replacement for environmentally persistent antiflammable compounds.

Influence of clay concentration on the gas barrier of clay-polymer nanobrick wall thin film assemblies

The influence of the clay deposition suspension concentration on gas barrier thin films of sodium montmorillonite (MMT) clay and branched polyethylenimine (PEI), created via layer-by-layer assembly, was investigated. Films grown with MMT suspension concentrations ranging from 0.05 to 2.0 wt % were analyzed for their growth as a function of deposited polymer-clay bilayers (BL) and their thickness, clay concentration, transparency, nanostructure, and oxygen barrier as a function of the suspension concentration. The film thickness doubles and the visible light transmission decreases less than 5% as a function of MMT concentration for 20-BL films. Atomic force and transmission electron microscope images reveal a highly aligned nanobrick wall structure, with quartz crystal microbalance measurements revealing a slight increase in the film clay concentration as the MMT suspension concentration increases. The oxygen transmission rate (OTR) through these 20-BL composites, deposited on a 179 μm poly(ethylene terephthalate) film, decreases exponentially as a function of the MMT clay concentration. A 24-BL film created with 2.0 wt % MMT has an OTR below the detection limit of commercial instrumentation (<0.005 cc/m(2)·day·atm). This study demonstrates an optimal clay suspension concentration to use when creating LbL barrier films, which minimizes deposition steps and the overall processing time.

Polymer assemblies for controlled delivery of bioactive molecules from surfaces

Localized delivery of bioactive compounds from surfaces of biomedical devices affords significant therapeutic benefits, and often relies on the capability of surface coatings to provide spatial and temporal control over release rate. The layer-by-layer technique presents a unique means to construct surface coatings that can conform to a variety of biomaterial surfaces and serve as matrices enabling controlled delivery of bioactive molecules from surfaces. The versatility of layer-by-layer assembly enables construction of surface coatings of diverse chemistry and internal architecture with controlled release properties. This review focuses on recent developments in constructing such layered matrices using linear polymers, polymer nanoparticles and block copolymer micelles, including micelles with stimuli-responsive cores, as film building blocks and in controlling release rate of therapeutics from these matrices via degradation, application of pH, ionic strength, temperature, light, electric field and chemical or biological stimuli. Challenges and opportunities associated with fabrication of stratified multilayer films capable of multi-stage delivery of multiple drugs are also discussed.

Stimuli-responsive LbL capsules and nanoshells for drug delivery

Review of basic principles and recent developments in the area of stimuli responsive polymeric capsules and nanoshells formed via layer-by-layer (LbL) is presented. The most essential attributes of the LbL approach are multifunctionality and responsiveness to a multitude of stimuli. The stimuli can be logically divided into three categories: physical (light, electric, magnetic, ultrasound, mechanical, and temperature), chemical (pH, ionic strength, solvent, and electrochemical) and biological (enzymes and receptors). Using these stimuli, numerous functionalities of nanoshells have been demonstrated: encapsulation, release including that inside living cells or in tissue, sensors, enzymatic reactions, enhancement of mechanical properties, and fusion. This review describes mechanisms and basic principles of stimuli effects, describes progress in the area, and gives an outlook on emerging trends such as theranostics and nanomedicine.

Structure-activity relationship for quaternary ammonium compounds hybridized with poly(methyl methacrylate)

Hybrid films from poly (methylmethacrylate) (PMMA) and dioctadecyldimethylammonium bromide (DODAB), cetyltrimethylammonium bromide (CTAB), or tetrapropylammonium bromide (TPAB) were characterized by determination of wettability, ellipsometry, atomic force microscopy, active compounds diffusion to water, X-ray photoelectron spectroscopy (XPS) with determination of atomic composition on the films surface, and biocidal activity against Pseudomonas aeruginosa or Staphylococcus aureus. QAC mobility in the films increased from DODAB to CTAB to TPAB. Diffusion and optimal hydrophobic-hydrophilic balance imparted the highest bioactivity to CTAB. DODAB sustained immobilization at the film surface killed bacteria upon contact. TPAB ability to diffuse was useless because of its unfavorable hydrophobic-hydrophilic balance for bioactivity.

Strong response of multilayer polyelectrolyte films to cationic surfactants

The influence of common cationic surfactants on the physical properties of differently composed polyelectrolyte films prepared by the layer-by-layer (LbL) technology was investigated. Free-standing polyelectrolyte films as microcapsules showed a fast, strong response to the addition of less than 1 mM cationic surfactant cetyltrimethylammonium bromide (CeTAB). As a function of the polyelectrolyte composition, the behavior of the capsules varied from negligible changes to complete disintegration via strong swelling. The response of microcapsules consisting of (poly(allylamine hydrochloride)(PAH)/poly(styrene sulfonate)(PSS))(4) was associated with a 5-fold volume increase, a fast switch of permeability, and in the case of fluorescently labeled films a 4-fold increase in fluorescence intensity. The kinetics and strengths of the interaction process were investigated by confocal laser scanning microscopy (CLSM) and fluorescence spectroscopy. Also, the relative stabilities of the polycation/polyanion and surfactant/polyanion complexes were determined. A mechanism was suggested to explain the interactions between the cationic surfactants and polyelectrolyte capsules. The strong response can be exploited in potential applications such as the triggered release of drugs or other encapsulated materials, the fluorescence-based detection of cationic detergents, and a switchable stopper in microchannels. However, the high sensitivity of LbL films to traces of cationic surfactants can also limit their applicability to the encapsulation of drugs or other materials because pharmaceutical or technical formulations often contain cationic surfactants as preservatives such as benzalkonium salts (BAC). It was demonstrated that undesired capsule opening can be effectively prevented by cross-linking the polyelectrolyte multilayers.

N-halamine biocidal coatings via a layer-by-layer assembly technique

Two N-halamine copolymer precursors, poly(2,2,6,6-tetramethyl-4-piperidyl methacrylate-co-acrylic acid potassium salt) and poly(2,2,6,6-tetramethyl-4-piperidyl methacrylate-co-trimethyl-2-methacryloxyethylammonium chloride) have been synthesized and successfully coated onto cotton fabric via a layer-by-layer (LbL) assembly technique. A multilayer thin film was deposited onto the fiber surfaces by alternative exposure to polyelectrolyte solutions. The coating was rendered biocidal by a dilute household bleach treatment. The biocidal efficacies of tested swatches composed of treated fibers were evaluated against Staphylococcus aureus and Escherichia coli. It was determined that chlorinated samples inactivated both S. aureus and E. coli O157:H7 within 15 min of contact time, whereas the unchlorinated control samples did not exhibit significant biocidal activities. Stabilities of the coatings toward washing and ultraviolet light exposure have also been studied. It was found that the stability toward washing was superior, whereas the UVA light stability was moderate compared to previously studied N-halamine moieties. The layer-by-layer assembly technique can be used to attach N-halamine precursor polymers onto cellulose surfaces without using covalently bonding tethering groups which limit the structure designs. In addition, ionic precursors are very soluble in water, thus promising for biocidal coatings without the use of organic solvents.

Super gas barrier of transparent polymer-clay multilayer ultrathin films

Flexible and transparent polymeric "superbarrier" packaging materials have become increasingly important in recent years. Layer-by-layer assembly offers a facile technique for the fabrication of layered, polymer-clay superbarrier thin films. At only 51 nm thick, these nanocomposite thin films, comprised of 12 polymer and 4 clay layers, exhibit an oxygen permeability orders of magnitude lower than EVOH and SiOx. Coupling high flexibility, transparency, and barrier protection, these films are good candidates for a variety packaging applications.

Flame retardant behavior of polyelectrolyte-clay thin film assemblies on cotton fabric

Cotton fabric was treated with flame-retardant coatings composed of branched polyethylenimine (BPEI) and sodium montmorillonite (MMT) clay, prepared via layer-by-layer (LbL) assembly. Four coating recipes were created by exposing fabric to aqueous solutions of BPEI (pH 7 or 10) and MMT (0.2 or 1 wt %). BPEI pH 10 produces the thickest films, while 1 wt % MMT gives the highest clay loading. Each coating recipe was evaluated at 5 and 20 bilayers. Thermogravimetric analysis showed that coated fabrics left as much as 13% char after heating to 500 degrees C, nearly 2 orders of magnitude more than uncoated fabric, with less than 4 wt % coming from the coating itself. These coatings also reduced afterglow time in vertical flame tests. Postburn residues of coated fabrics were examined with SEM and revealed that the weave structure and fiber shape in all coated fabrics were preserved. The BPEI pH 7/1 wt % MMT recipe was most effective. Microcombustion calorimeter testing showed that all coated fabrics reduced the total heat release and heat release capacity of the fabric. Fiber count and strength of uncoated and coated fabric are similar. These results demonstrate that LbL assembly is a relatively simple method for imparting flame-retardant behavior to cotton fabric. This work lays the foundation for using these types of thin film assemblies to make a variety of complex substrates (foam, fabrics, etc.) flame resistant.

Note: Influence of rinsing and drying routines on growth of multilayer thin films using automated deposition system

A versatile, high speed robot for layer-by-layer deposition of multifunctional thin films, which integrates concepts from previous dipping systems, has been designed with dramatic improvements in software, positioning, rinsing, drying, and waste removal. This system exploits the electrostatic interaction of oppositely charged species to deposit nanolayers (1-10 nm thick) from water onto the surface of a substrate. Dip times and number of deposited layers are adjustable through a graphical user interface. In between dips the system spray rinses and dries the substrate by positioning it in the two-tiered rinse-dry station. This feature significantly reduces processing time and provides the flexibility to choose from four different procedures for rinsing and drying. Assemblies of natural montmorillonite clay and polyethylenimine are deposited onto 175 microm poly(ethylene terephthalate) film to demonstrate the utility of this automated deposition system. By altering the type of rinse-dry procedure, these clay-based assemblies are shown to exhibit variations in film thickness and oxygen transmission rate. This type of system reproducibly deposits films containing 20 or more layers and may also be useful for other types of coatings that make use of dipping.