We have measured the Young’s modulus and thickness of ultrathin polyelectrolyte multilayer (PEM), polystyrene (PS), and poly(methyl methacrylate) (PMMA) films as a function of relative humidity. We demonstrate that PEMs undergo substantial swelling and plasticization in the presence of ambient water and that both the choice of polyelectrolytes and the pH of the deposition baths influence the response of a PEM film to changes in humidity. These responses were roughly linear in two of the PEM systems tested; however, a third system demonstrated evidence of antiplasticization and an abrupt swelling transition at intermediate humidity. This behavior is attributed to an internal structure rich in hydrogen-bonding sites. Overall, our results suggest that the chemical composition and molecular architecture of PEMs are responsible for differences in the environmental responsiveness of these materials. Both PS and PMMA films exhibited comparatively small but measurable swelling and plasticization in the presence of water; these effects were more pronounced in the case of the more hydrophilic PMMA. Our results offer insight into the material structure and internal chemical interactions that determine the properties and responsiveness of PEM systems.

Halloysite nanotube-filled epoxy composites were fabricated using spray-coating methods. The halloysite nanotubes (HNTs) were aligned by the hydrodynamic flow conditions at the spray nozzle, and the polymer viscosity helped to preserve this preferential orientation in the final coatings on the target substrates. Electron microscopy demonstrated a consistent trend of higher orientation degree in the nanocomposite coatings as viscosity increased. The nanoindentation mechanical performances of these coatings were studied using a Hysitron TriboIndenter device. Composites showed improvements up to ∼50% in modulus and ∼100% in hardness as compared to pure epoxy, and the largest improvements in mechanical performance correlated with higher alignment of HNTs along the plane-normal direction. Achieving this nanotube alignment using a simple spray-coating method suggests potential for large-scale production of multifunctional anisotropic nanocomposite coatings on a variety of rigid and deformable substrates.Keywords: alignment; halloysite; mechanical; orientation factor; thin-film coating

The influence of nanoparticle orientation on wear resistance of transparent composite coatings has been studied. Using a nozzle spray coating method, halloysite nanotubes (HNTs) were aligned in the in-plane and out-of-plane directions and in various randomly oriented states. Nanoscratching, falling sand, and Taber Abrasion tests were used to characterize the wear resistance at different length scales. Composites consistently displayed better wear resistance than pure epoxy. Samples with out-of-plane particle orientations exhibited better wear-resistant behavior than those with in-plane particle distributions. In nanoscratching tests, the out-of-plane orientation decreases the normalized scratch volume by as much as 60% compared to pure epoxy. In the falling sand and Taber Abrasion tests, out-of-plane aligned halloysite particles resulted in surfaces with smaller roughness based on stylus profilometry and SEM observations. The decrease in roughness values after these wear tests can be as large as 67% from pure epoxy to composites. Composites with higher out-of-plane particle orientation factors exhibited better light transmittance after sand impingements and other wear tests. This study suggests a useful strategy for producing material systems with enhanced mechanical durability and more durable optical properties.Keywords: alignment; composite; falling sand; HNTs; scratching; Taber Abrasion; transparent; wear;

Manipulating surface properties using chemistry and roughness has led to the development of advanced multifunctional surfaces. Here, in a nanostructured polymer film consisting of a hydrophilic reservoir of chitosan/carboxymethyl cellulose capped with various hydrophobic layers, we demonstrate the role of a third design factor, water permeation rate. We use this additional design criterion to produce antifogging coatings that readily absorb water vapor while simultaneously exhibiting hydrophobic character to liquid water. These zwitter-wettable films, produced via aqueous layer-by-layer assembly, consist of a nanoscale thin hydrophobic capping layer (chitosan/Nafion) that enables water vapor to diffuse rapidly into the underlying hydrophilic reservoir rather than nucleating drops of liquid water on the surface. We characterize these novel films using a quartz crystal microbalance with dissipation monitoring (QCM-D) and via depth-profiling X-ray photoelectron spectroscopy (XPS) in addition to extensive testing for fogging/antifogging performance.Keywords: antifogging coatings; condensation; layer-by-layer; surface science; wetting; zwitter-wettability

Multilayer films consisting of bovine submaxillary mucin (BSM) and poly(allylamine hydrochloride) (PAH) were prepared on various substrates using layer-by-layer assembly. The effects of both the assembly pH and ionic strength on multilayer characteristics were investigated by assessing film thicknesses (10–80 nm), surface wetting characteristics, and cell repulsion. Also, the dynamic assembly behavior was monitored using quartz crystal microbalance with dissipation monitoring (QCM-D) to further understand the effect of assembly pH on film characteristics. Assembly studies revealed that substantial amounts of BSM adhere to the outermost surface only at low pH conditions. The resulting multilayer films assembled at low pH conditions were found to exhibit hydrophilic and cell repellent behavior. In addition, it was found that batch-to-batch variations of the biopolymer BSM could dramatically alter properties.

It is demonstrated that poly(allylamine hydrochloride)/poly(styrenesulfonate) (PAH/SPS) multilayer films can be successfully tailored for the capture and detection of small biomolecules in dilute concentrations. Based on in vitro results, these films could be potentially applied for rapid and high-throughput diagnosis of dilute biomarkers in serum or tissue. PAH presents functional amino groups that can be further reacted with desired chemistries in order to create customizable and specific surfaces for biomolecule capture. A variety of film assembly characteristics were tested (pH, molecular weight of PAH, and ionic strength) to tune the biotinylation and swelling behavior of these films to maximize detection capabilities. The resultant optimized biotinylated PAH/SPS 9.3/9.3 system was utilized in conjunction with quantum dots (Qdots) to capture and detect a dilute biomarker for prostate cancer, prostate-specific antigen (PSA). Compared to previous work, our system presents a good sensitivity for PSA detection within the clinically relevant range of 0.4–100 ng/mL.

X-ray photoelectron spectroscopy (XPS) depth profiling with C60+ sputtering was used to resolve the lithium-ion distribution in the nanometer-scale domain structures of block polymer electrolyte thin films. The electrolytes of interest are mixtures of lithium trifluoromethanesulfonate and lamellar-forming polystyrene–poly(oligo(oxyethylene)methacrylate) (PS–POEM) copolymer. XPS depth profiling results showed that the lithium-ion concentration was directly correlated with the POEM concentration. Furthermore, chemical state and atomic composition of the film were analyzed through the deconvolution of the C1s signal, indicating that the lithium ions appear to be uniformly distributed in the POEM domains. Overall, the unique capabilities of C60+ depth profiling XPS provide a powerful tool for the analysis of nanostructured polymer thin films in applications ranging from energy storage and generation to surface coatings and nanoscale templates.Keywords: block polymer; C60+; electrolyte; ion distribution; salt-doping; thin film; XPS depth profiling;

The layer-by-layer (LbL) assembly of thin films on surfaces has proven to be an extremely useful technology for uses ranging from optics to biomedical applications. Releasing these films from the substrate to generate so-called free-standing multilayer films opens a new set of applications. Current approaches to generating such materials are limited because they can be cytotoxic, difficult to scale up, or have undesirable side reactions on the material. In this work, a new sacrificial thin film system capable of chemically triggered dissolution at physiological pH of 7.4 is described. The film was created through LbL assembly of bovine submaxillary mucin (BSM) and the lectin jacalin (JAC) for a (BSM/JAC) multilayer system, which remains stable over a wide pH range (pH 3–9) and at high ionic strength (up to 5 M NaCl). This stability allows for subsequent LbL assembly of additional films in a variety of conditions, which could be released from the substrate by incubation in the presence of a competitive inhibitor sugar, melibiose, which selectively disassembles the (BSM/JAC) section of the film. This novel multilayer system was then applied to generate free-standing, 7 μm diameter, circular ultrathin films, which can be attached to a cell surface as a “backpack”. A critical thickness of about 100 nm for the (BSM/JAC) film was required to release the backpacks from the glass substrate, after incubation in melibiose solution at 37 °C for 1 h. Upon their release, backpacks were subsequently attached to murine monocytes without cytotoxicity, thereby demonstrating the compatibility of this mucin-based release system with living cells.

We report that the interlayer diffusion of polymer chains within heterostructured hydrogen-bonded multilayer films depends on the stacking order: polymers diffuse more when high pH stability polymer pairs are assembled on top of low pH stability polymer pairs. By varying the stacking sequence, the fraction of the film that is released from the substrate can be tuned to achieve sequential pH-programmed release of the multilayer film. Also, we show that a multifunctional freestanding film with tunable film thickness can be generated by appropriate stacking and subsequent thermal cross-linking.

Antifogging coatings with hydrophilic or even superhydrophilic wetting behavior have received significant attention due to their ability to reduce light scattering by film-like condensation. However, under aggressive fogging conditions, these surfaces may exhibit frost formation or excess and nonuniform water condensation, which results in poor optical performance of the coating. In this paper, we show that a zwitter-wettable surface, a surface that has the ability to rapidly absorb molecular water from the environment while simultaneously appearing hydrophobic when probed with water droplets, can be prepared by using hydrogen-bonding-assisted layer-by-layer (LbL) assembly of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA). An additional step of functionalizing the nano-blended PVA/PAA multilayer with poly(ethylene glycol methyl ether) (PEG) segments produced a significantly enhanced antifog and frost-resistant behavior. The addition of the PEG segments was needed to further increase the nonfreezing water capacity of the multilayer film. The desirable high-optical quality of these thin films arises from the nanoscale control of the macromolecular complexation process that is afforded by the LbL processing scheme. An experimental protocol that not only allows for the exploration of a variety of aggressive antifogging challenges but also enables quantitative analysis of the antifogging performance via real-time monitoring of transmission levels as well as image distortion is also described.Keywords: antifogging; antifrost; layer-by-layer; wetting; zwitter-wettability

Functional organic thin films often demand precise control over the nanometer-level structure. Interlayer diffusion of materials may destroy this precise structure; therefore, a better understanding of when interlayer diffusion occurs and how to control it is needed. X-ray photoelectron spectroscopy paired with C60+ cluster ion sputtering enables high-resolution analysis of the atomic composition and chemical state of organic thin films with depth. Using this technique, we explore issues common to the polyelectrolyte multilayer field, such as the competition between hydrogen bonding and electrostatic interactions in multilayers, blocking interlayer diffusion of polymers, the exchange of film components with a surrounding solution, and the extent and kinetics of interlayer diffusion. The diffusion coefficient of chitosan (M = ∼100 kDa) in swollen hydrogen-bonded poly(ethylene oxide)/poly(acrylic acid) multilayer films was examined and determined to be 1.4*10−12 cm2/s. Using the high-resolution data, we show that upon chitosan diffusion into the hydrogen-bonded region, poly(ethylene oxide) is displaced from the film. Under the conditions tested, a single layer of poly(allylamine hydrochloride) completely stops chitosan diffusion. We expect our results to enhance the understanding of how to control polyelectrolyte multilayer structure, what chemical compositional changes occur with diffusion, and under what conditions polymers in the film exchange with the solution.

Nanofluidic arrays containing high-aspect-ratio nanochannels were used as a platform for the deposition of all nanoparticle multilayers. LbL assembly of 6 nm titania and 15 nm silica nanoparticles resulted in conformal multilayers of uniform thickness throughout the nanochannels. These multilayers are inherently nanoporous with void volume fractions of about 0.5. Compared to unconfined assembly of the same materials on flat substrates, thinner multilayer films were observed for the case of deposition within confined channel geometries because of surface charge-induced electrostatic depletion of the depositing species. Additionally, systematic and reproducible bridging of the nanochannels occurred as multilayer assembly progressed, a phenomenon not seen in our earlier work involving polyelectrolytes. This behavior was attributed to relatively weak nanoparticle adsorption and the resulting formation of large aggregates. These results demonstrate a new route by which confined geometries can be coated and even bridged with a nanoporous multilayer without the need for calcination or other postassembly steps to introduce porosity into the conformal coating.Keywords: confined geometry; directed assembly; layer-by-layer assembly; nanochannel; nanofluidics; nanoparticle;

A novel “sink and etch” technique is used to generate stable surface nanoporosity in poly(methyl methacrylate). Layer-by-layer assembly is first used to conformally coat PMMA substrates with a uniform layer of silica nanoparticles. Thermal annealing is then applied to cause sinking and engulfment of the silica nanoparticles into the thermoplastic PMMA surface. By selectively etching away the layer of embedded silica nanoparticles, a conformal porous layer of inversely templated structure can be obtained in the PMMA surface. Characterization with atomic force microscopy shows that a variety of nanoporous surface morphologies can be achieved simply by controlling the duration and temperature of thermal annealing. The nanoporous surfaces consisting of either as assembled silica nanoparticles or templated inverse porosity in PMMA were compared in terms of their antireflective (AR) properties. Measuring AR properties provided a quantitative means to compare the stability of these porous AR surfaces before and after several cleaning cycles. Our results show that while both types of surface porosity can provide excellent AR properties (optimized for 300–400 nm), the porous layer generated by the “sink and etch” technique showed superior mechanical stability.

Multilayer thin films that consist of poly(vinyl alcohol) (PVA) and weak polyacids such as poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) were prepared by hydrogen bonding interactions. Both the degree of hydrolysis and molecular weight of PVA were investigated in terms of their influence on growth behavior and pH stability. Multilayer films containing PVA and PAA could be assembled successfully only by using partially hydrolyzed PVA and low-pH solutions. By comparing films containing PAA with those containing a more strongly interacting partner, PMAA, it was shown that the extent of PVA hydrolysis becomes significant only when weak hydrogen bonding pairs such as PVA and PAA were used. pH-triggered dissolution experiments demonstrated that the degree of hydrolysis can be used as an additional parameter by which to tune the pH stability of the film. Also, the presence of an abundance of free hydroxyl and carboxylic acid groups in the multilayer allowed enhanced pH stability to be obtained by thermal and chemical methods as well as numerous opportunities for postassembly functionalization.

We investigate the effect of block copolymer (BCP) molecular weight and connectivity on the morphology and time–temperature dependent swelling of thin film hydrogels created through layer-by-layer (LbL) assembly of BCP micelles with poly(acrylic acid). BCPs of poly(N,N-dimethylaminoethyl methacrylate) (D) and poly(propylene oxide) (P), a P–D diblock, a long D–P–D triblock, and a short D–P–D triblock copolymer, were compared in terms of their temperature response in solution and within micelle–polyelectrolyte multilayers (mPEMs). The critical micellization concentration and micellization temperature of the BCPs in solution, as well as the swelling transition temperature, Tstt, of the mPEMs, decreased with increasing P block length. AFM imaging of dry mPEMs shows regular dimpled surface structures that arise from surface relaxation of micelles. When the mPEMs are cooled below Tstt in water, the thin ∼200 nm films can swell reversibly between 3 and 6 times their dry thicknesses within 2 min. The degree of swelling (τ = wet thickness/dry thickness) increases with undercooling (ΔT = Tstt – T) and shows time dependencies related to ΔT and the constituent BCP connectivity. While the diblock films swell uncontrollably and lose integrity within 30 min at ΔT ≥ 6 °C, the triblock copolymer multilayers are able to sustain steady τ values (in the range of 4–10) under equivalent conditions. The differences in dynamic swelling behavior originating from BCP architecture have important implications in their utility as temperature responsive surfaces.

Mechanically durable, long-lasting antifog coatings based on polysaccharides were developed using a layer-by-layer (LBL) assembly process. The unique properties of these coatings are a result of a molecular-level blending of the polysaccharides, with multilayers containing chitosan and carboxymethyl cellulose providing the best overall properties. The antifog properties resulted from a strong interaction between the polar and H-bonding elements of the assembled polymers and water molecules and the concomitant formation of thin films of water. Environmental scanning electron microscopy (ESEM) studies confirmed that fogging coatings are decorated with light scattering, micrometer-sized droplets of water whereas antifogging coatings remain droplet free. To improve the mechanical durability of the multilayer films on substrates, the surface was modified via self-assembly of epoxy-functionalized silane molecules. Cross-linking chemistry was then applied to improve the mechanical robustness of the LBL films on various surfaces. These films were characterized using several techniques: optical profilometery (PL), spectroscopic ellipsometry (EL), contact angle goniometry (CA), and atomic force microscopy (AFM). The antifog properties of the films were evaluated by several tests under different environmental conditions. This work demonstrates that the unique water-adsorbing properties of polysaccharides can be exploited to create permanent antifog properties, which may be useful for various applications.

Automated spray-layer-by-layer (LbL) assembly was used to create highly reflective structurally colored thin films with high reflectance at near-UV light wavelengths. Reflectance peaks were tuned by fabricating alternating stacks of high (TiO2 nanoparticles) and low (SiO2 nanoparticles) refractive index materials using a non-quarter-wave design. Spray-assembled multilayer heterostructures fabricated with up to 840 individual polymer or nanoparticle deposition steps presented similar roughness and refractive index values compared to Bragg stacks obtained via immersion LbL assembly. Such complex multilayer heterostructures, however, could be fabricated in significantly shorter times; the time required to deposit a complete bilayer was only about 90 s, compared to 36 min for the immersion assembly of the same system. Optimization of the experimental parameters was performed to achieve uniform coatings and relatively smooth interfaces and surfaces. We observed that the spraying times of the nanoparticle and polymer solutions are the main parameters that determine the thickness, optical properties, and uniformity of the assembled films. Ellipsometry, atomic force microscopy (AFM), UV–vis spectroscopy, and transmission electron microscopy (TEM) were used to characterize the samples. The nanoparticle thin films were iridescent and presented relatively narrow peaks of high reflectance (∼90%) at visible and near-UV wavelengths of light.

Arrays of silicon nanochannels were used to investigate the layer-by-layer (LbL) assembly of nanoparticle-containing multilayers in confined geometries. LbL assembly of poly(vinylsulfonic acid)/titania (PVS/TiO2) and poly(diallyldimethylammonium chloride)/silica (PDAC/SiO2) resulted in conformal, uniform thickness multilayers throughout the high-aspect-ratio nanochannels. These multilayers were also calcinated to form nanoporous coatings within the nanochannels. Confined and unconfined multilayers were compared using scanning electron microscopy (SEM), showing significantly lower bilayer thicknesses when layers were assembled in confined geometries. Deposition was found to essentially stop at a nanochannel gap size of 55 nm for the PVS/TiO2 system and at 210 nm for the PDAC/SiO2 system, even though the channels were not plugged. These results provide further evidence of surface charge-induced depletion of unadsorbed species during LbL assembly, which occurs because of the closely spaced walls with like surface charge inherent in confined geometry LbL. This approach offers a simple but powerful new pathway to functionalize nanochannels with nanoporous coatings made from LbL-assembled nanoparticle-containing multilayers. It has applications in the production of functionalized nanofluidic devices for advanced separations utilizing size, charge, and potentially chemical or biological selectivity.

Nanoparticles are indispensable ingredients of solution-processed optical, dielectric, and catalytic thin films. Although solution-based methods are promising low-cost alternatives to vacuum methods, they can have significant limitations. Coating uniformity, thickness control, roughness control, mechanical durability, and incorporation of a diverse set of functional organic molecules into nanoparticle thin films are major challenges. We have used the electrostatic layer-by-layer assembly technique to make uniform, conformal multistack nanoparticle thin films for optical applications with precise thickness control over each stack. Two particularly sought-after optical applications are broadband antireflection and structural color. The effects of interstack and surface roughness on optical properties of these constructs (e.g., haze and spectral response) have been studied quantitatively using a combination of Fourier-transform methods and atomic force microscopy measurements. Deconvoluting root-mean-square roughness into its large-, intermediate-, and small-scale components enables enhanced optical simulations. A 4-stack broadband antireflection coating (<0.5% average reflectance in the visible range, and 0.2% haze) composed of alternating high-index (n ≈ 1.96) and low-index (n ≈ 1.28) stacks has been made on glass substrate. Films calcinated at 550 °C endure a one-hour-long cloth cleaning test under 100 kPa normal stress.Keywords: antireflection; broadband; coating; interface; layer-by-layer; nanoparticle; roughness

Cellular “backpacks” are a new type of anisotropic, nanoscale thickness microparticle that may be attached to the surface of living cells creating a “bio-hybrid” material. Previous work has shown that these backpacks do not impair cell viability or native functions such as migration in a B and T cell line, respectively. In the current work, we show that backpacks, when added to a cell suspension, assemble cells into aggregates of reproducible size. We investigate the efficiency of backpack−cell binding using flow cytometry and laser diffraction, examine the influence of backpack diameter on aggregate size, and show that even when cell−backpack complexes are forced through small pores, backpacks are not removed from the surfaces of cells.

We have demonstrated the utility of hollow silica nanoparticles in fabricating conformal thin film nanoporous antireflection (AR) coatings on both poly(methyl methacrylate) (PMMA) and glass substrates. Layer-by-layer (LbL) assembly was successfully used to produce ultrathin AR coatings on planar and textured surfaces. Hollow silica nanoparticles were synthesized to extend the range of apparent refractive indices possible in an AR coating, enabling the design of both single index and graded index AR coatings on PMMA substrates. The diameter and shell thickness of the silica nanoparticles are the two independent, controllable parameters that we manipulated to tune the refractive index of the coating. The AR coatings reduced the minimum reflection of PMMA from 7% to 0.5%, while the maximum transmission increased from 92% to 98% at the optimized wavelength region that could be adjusted from the near UV into the visible. Cross sectional SEM showed that conformal coatings can be achieved on grooved PMMA Fresnel lenses. AFM was used to study surface topography on flat substrates.Keywords: hollow silica nanoparticles; layer-by-layer assembly; PMMA substrates; UV antireflection coating

Chitosan/silk fibroin multilayer thin films were assembled using layer-by-layer deposition. The resultant multilayer films contained nanofibers aligned parallel to the dipping direction. Fiber deposition and orientation was enabled uniquely by a judicious choice of solvent and drying conditions and layer-by-layer assembly with chitosan. The deposition of oriented nanofibers was found to be the result of a unique combination of layer-by-layer and Langmuir−Blodgett type processing. Fiber orientation was confirmed by fast Fourier transform (FFT) analysis of optical micrographs and atomic force microscopy (AFM). Bidirectional fiber alignment was realized by rotating the substrate between multilayer deposition steps. Infrared spectroscopy revealed that the silk fibroin adopted the silk II secondary structure in the deposited films. We anticipate that these anisotropic films are able to combine the biocompatibility of a natural polymer system with the mechanical strength of SF, two properties useful in many biological applications including scaffolds suitable for guiding cell attachment and spreading.

A strategy was developed to produce thin, biopolymer-based polyelectrolyte multilayer films, based on hyaluronic acid and chitosan, that are able to effectively bind B lymphocytes. These films explore CD44−hyaluronate interactions and provide a method to make surface-bound B cell arrays without the need for nonselective covalent chemistry. The rational design of these films using solution deposition variables, such as ionic strength and pH, allows one to maximize and fine tune this binding efficiency ex vivo. This work suggests two important conditions for successfully attaching B cells to hyaluronate-containing polyelectrolyte multilayer films: (1) hyaluronic acid is required for the proposed CD44-mediated binding mechanism, and (2) hyaluronic acid deposition conditions that favor loops and tails, such as low pH and with added salt, result in more available CD44 binding ligands and higher cell binding efficiency. Chitosan-terminated films prepared without NaCl in the deposition solutions and hyaluronic acid-terminated films prepared with salt, both under pH 3.0 assembly conditions, presented a similar high lymphocyte binding efficiency. In the former case, however, the binding strength was weaker due to a significant electrostatic contribution to the binding. Bioactive polyelectrolyte multilayers for selective binding of lymphocytes hold great promise in fields ranging from cell-based biosensors to immune system engineering.

In this article, we seek to enable large-scale, fully reversible, thermally induced volumetric changes in layer-by-layer (LbL) electrostatically self-assembled thin films through the incorporation of A-B-A triblock copolymers. Poly(N,N-dimethylaminoethyl methacrylate)-b-poly(propylene oxide)-b-poly(N,N-dimethylaminoethyl methacrylate) (abbreviated “PD-PPO-PD”) was used as a dual pH and temperature-responsive component in the electrostatic self-assembly of multilayer thin films. In solutions of this triblock copolymer with poly(N,N-dimethylaminoethyl methacrylate) (PD) weak polyelectrolyte end blocks, the dehydration temperature of the central poly(propylene oxide) (PPO) block was strongly dependent on solution pH, as shown by microdifferential scanning calorimetry (micro-DSC) and dye solubilization techniques. Multilayer films were then assembled with poly(acrylic acid) (PAA) or poly(4-styrene sulfonate) (PSS) as anionic binding partners at various pH values, where the triblock copolymer was incorporated within the film either as unimers or micelles. Using in situ ellipsometry, we showed that the polyanion type and the self-assembly pH were both critical parameters for constructing functional films, which change their swelling degree in response to temperature. In particular, strongly associated PD-PPO-PD/PSS multilayers lacked temperature sensitivity and maintained a constant swelling degree in a wide range of pH and temperature. In contrast, the temperature response of PD-PPO-PD/PAA films was strongly dependent on the self-assembly pH. Whereas swelling of PD-PPO-PD/PAA films constructed at pH ≤ 5 was independent of temperature, multilayers assembled at pH ≥ 6 showed fully reversible, three- to five-fold changes in film thickness in response to temperature cycling between 6 and 20 °C, enabled by the ability of PPO domains to transit reversibly between the swollen hydrated and collapsed dehydrated states. These nanocomposite coatings show highly responsive, reversible swelling transitions that can be useful for future biomedical and device applications.

A hybrid micro-/nanofluidic device which contains an array of parallel nanochannels has been employed to study polyelectrolyte multilayer (PEM) deposition in confined geometries. Layer-by-layer (LbL) assembly of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) at pH 4 and salt concentrations ranging from 0.1 to 1 M was used to conformally coat the nanochannel walls, systematically narrowing the channel width from 222 to 11 nm in the wet state. The thicknesses of confined multilayers were measured using SEM and these results were compared with those obtained on planar, unconfined surfaces. A procedure for direct measurement of the gap thickness using dc conductance was also developed. LbL assembly in the nanochannels resulted in lower bilayer thicknesses than those obtained on planar surfaces. This observation is attributed to the surface charge-induced depletion of unadsorbed polyelectrolytes within the channel. The ability to conformally coat the walls of the nanochannels with functional PEMs opens up new possibilities in the design of active nanochannel devices.

Capillary condensation, an often undesired natural phenomenon in nanoporous materials, was used advantageously as a universal functionalization strategy in nanoparticle thin films assembled layer-by-layer. Judicious choice of nanoparticle (and therefore pore) size allowed targeted capillary condensation of chemical vapors of both hydrophilic and hydrophobic molecules across film thickness. Heterostructured thin films with modulated refractive index profiles produced in this manner exhibited broadband antireflection properties with an average reflectance over the visible region of the spectrum of only 0.4%. Capillary condensation was also used to modify surface chemistry and surface energy. Photosensitive capillary-condensates were UV-cross-linked in situ. Undesired adventitious condensation of humidity could be avoided by condensation of hydrophobic materials such as poly(dimethyl siloxane).

The same near-ultraviolet (NUV) (300–400 nm) light that facilitates photo-degradation in polymers is also a key component in animal communication, particularly in insects and birds. We developed a nature-inspired 1D photonic crystal structure that has simultaneous narrow stop bands in the NUV and visible wavelength range, resulting in a NUV reflector with tunable structural color in the visible region of the spectrum. The samples were prepared by high precision deposition of alternating stacks of SiO2 and more densely packed TiO2nanoparticlesvia aqueous layer-by-layer (LbL) deposition. Multilayer thin films with eleven stacks achieved up to 75% reflectance with a 50 nm bandwidth and 65% reflectance with a 100 nm bandwidth for structural colors centered at 500 nm and 690 nm, respectively. Despite UV absorbance of the TiO2nanoparticles and the supporting glass substrate, all samples exhibited a narrow NUV stop-band centered in the 300–400 nm range with a reflectance over 65%. Upon introduction of a defect mode, we also demonstrated the creation of a narrow reflectance dip associated with a localized state in the photonic band gap. Multilayer-based structural colors with NUV reflectivity can be used as pigments that can promote prevention of photo-degradation in coatings, in addition to bird and insect repelling functionality.

The adhesion and proliferation of bacteria on abiotic surfaces pose challenges related to human infection, including subsequent formation of antibiotic-resistant biofilms in both healthcare and industrial applications. Although the design of antibacterial materials is a longstanding effort, the surface properties that modulate adhesion of viable bacteria—the critical first step in biofilm formation—have been difficult to decouple. This partial and limited success is due chiefly to two factors. First, bacteria cells exhibit multiple, complex adhesion mechanisms that vary with bacteria strain, rapid genetic mutations within a given strain, and mutable environmental stimuli such as nutrient levels and fluid velocities. Second, there exist only a limited number of studies that systematically characterize or vary the physical, chemical, and mechanical properties of potential antimicrobial materials. Here, we briefly review the dominant strategies for antimicrobial material surface design, including the advantages and limitations of approaches developed via synthetic and natural polymers. We then consider polyelectrolyte multilayers (PEMs) as a versatile materials platform to adopt and integrate these strategies, as well as to elucidate the individual contributions of tunable material properties that limit viable bacteria adhesion. Together, these findings suggest that PEMs can be tailored to leverage the key advantages of bacterial adhesion resistance, contact killing, and biocide leaching strategies for a wide range of antimicrobial surface applications.

Cationic contact-killing is an important strategy for creating antimicrobial surfaces that prevent viable bacteria attachment. Recent studies have shown that highly swollen, compliant surfaces resist bacterial attachment and a sufficient density of mobile cationic charge can effectively disrupt bacterial cell membranes. Polyelectrolyte multilayers (PEMs), a popular coating system for surface modification, have been used to kill bacteria through the incorporation of contact-killing or leaching biocides. In this work, we show that manipulation of multilayer assembly and postassembly conditions (e.g., pH) to expose mobile cationic charge can create antimicrobial PEMs without the addition of specific biocidal species. As a model system, we explored PEMs comprising poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (SPS) assembled at high pH and subsequently immersed in low pH solutions. This system undergoes a reversible pH-dependent swelling transition, and we demonstrate that antimicrobial functionality at physiological pH conditions can be turned on and off with suitable pH treatment. In both airborne and waterborne bacteria assays, the viability of two strains of Gram positive Staphylococcus epidermidis (S. epidermidis), one biofilm forming and one nonbiofilm forming, and two strains of Gram negative Escherichia coli (E. coli) was effectively reduced on SPS/PAH multilayers displaying accessible cationic charge. To generalize our results, the pH assembly conditions of PEMs comprising poly(acrylic acid) (PAA) and PAH were also modified to introduce antibacterial capabilities.

We demonstrate a technique for synthesizing substrate-bound arrays of submicrometer-sized reversibly swellable tubes by using porous templates. The sacrificial template approach allows straightforward control over the tube length, diameter, and lateral arrangement of the resultant surface-bound nanotubes. We also explored methods for varying the tube opening structure by altering the pore shape at the surface of the template. A specific PEM system composed of poly(allylamine hydrochloride) and poly(acrylic acid) was chosen as the building block for the nanotube arrays because of its ability to undergo pH-triggered swelling−deswelling transitions. The activation of this transition results in dramatic changes in the length and diameter of the nanotubes as characterized in situ via confocal laser scanning microscopy (CLSM). The pH-driven reversible swelling−deswelling and nanoporosity behavior observed with planar films and nanotubes of this PEM system is a direct consequence of the breaking and reforming of ionic cross-links.

In situ synthesis of inorganic clusters in polyelectrolyte multilayers (PEMs) has typically relied on free carboxylic acid groups as the binding sites, limiting the scheme to the loading of cationic precursor reagents. The use of amine groups for similar purposes has not been demonstrated because of the challenge of incorporating free amine groups that are not paired with the oppositely charged groups residing on the polyanion of the PEM. In this paper, we use specific PEM assembly conditions to produce ultrathin conformal films of PAH/PAA (poly(allylamine hydrochloride)/poly(acrylic acid)) and PAH/PSS (poly(styrene sulfonate)) that upon suitable postassembly treatment undergo substantial molecular rearrangements that generate free amine groups in the films. These PEMs are capable of binding anionic precursors including complexes widely used for the synthesis of gold nanoparticles. On the basis of our understanding of the gold binding mechanisms, we demonstrate systematic control over the size and spatial distribution of gold nanoparticles in the films by changing the PEM assembly and postassembly treatment conditions.

We have measured the Young’s modulus and thickness of ultrathin polyelectrolyte multilayer (PEM), polystyrene (PS), and poly(methyl methacrylate) (PMMA) films as a function of relative humidity. We demonstrate that PEMs undergo substantial swelling and plasticization in the presence of ambient water and that both the choice of polyelectrolytes and the pH of the deposition baths influence the response of a PEM film to changes in humidity. These responses were roughly linear in two of the PEM systems tested; however, a third system demonstrated evidence of antiplasticization and an abrupt swelling transition at intermediate humidity. This behavior is attributed to an internal structure rich in hydrogen-bonding sites. Overall, our results suggest that the chemical composition and molecular architecture of PEMs are responsible for differences in the environmental responsiveness of these materials. Both PS and PMMA films exhibited comparatively small but measurable swelling and plasticization in the presence of water; these effects were more pronounced in the case of the more hydrophilic PMMA. Our results offer insight into the material structure and internal chemical interactions that determine the properties and responsiveness of PEM systems.

We demonstrate that functional polyelectrolyte multilayer (PEM) patches can be attached to a fraction of the surface area of living, individual lymphocytes. Surface-modified cells remain viable at least 48 h following attachment of the functional patch, and patches carrying magnetic nanoparticles allow the cells to be spatially manipulated using a magnetic field. The patch does not completely occlude the cellular surface from the surrounding environment; this approach allows a functional payload to be attached to a cell that is still free to perform its native functions, as suggested by preliminary studies on patch-modified T-cell migration. This approach has potential for broad applications in bioimaging, cellular functionalization, immune system and tissue engineering, and cell-based therapeutics where cell−environment interactions are critical.

The same near-ultraviolet (NUV) (300–400 nm) light that facilitates photo-degradation in polymers is also a key component in animal communication, particularly in insects and birds. We developed a nature-inspired 1D photonic crystal structure that has simultaneous narrow stop bands in the NUV and visible wavelength range, resulting in a NUV reflector with tunable structural color in the visible region of the spectrum. The samples were prepared by high precision deposition of alternating stacks of SiO2 and more densely packed TiO2nanoparticlesvia aqueous layer-by-layer (LbL) deposition. Multilayer thin films with eleven stacks achieved up to 75% reflectance with a 50 nm bandwidth and 65% reflectance with a 100 nm bandwidth for structural colors centered at 500 nm and 690 nm, respectively. Despite UV absorbance of the TiO2nanoparticles and the supporting glass substrate, all samples exhibited a narrow NUV stop-band centered in the 300–400 nm range with a reflectance over 65%. Upon introduction of a defect mode, we also demonstrated the creation of a narrow reflectance dip associated with a localized state in the photonic band gap. Multilayer-based structural colors with NUV reflectivity can be used as pigments that can promote prevention of photo-degradation in coatings, in addition to bird and insect repelling functionality.