Porous Nanofilm Biomaterials Via Templated Layer‐by‐Layer Assembly

Hydrogel‐like biomaterials are often too soft to support robust cell adhesion, yet methods to increase mechanical rigidity (e.g., covalent cross‐linking the gel matrix) can compromise bioactivity by suppressing the accessibility or activity of embedded biomolecules. Nanoparticle templating is reported here as a strategy toward porous, layer‐by‐layer assembled, thin polyelectrolyte films of sufficient mechanical rigidity to promote strong initial cell adhesion, and that are capable of high bioactive species loading. Latex nanoparticles are incorporated during layer‐by‐layer assembly, and following 1‐ethyl‐3‐[3‐dimethylaminopropyl]carbodiimide/N‐hydroxysulfosuccinimide (EDC‐NHS) cross‐linking of the polyelectrolyte film, are removed via exposure to tetrahydrofuran (THF). THF exposure results in only a partial reduction in film thickness (as observed by ellipsometry), suggesting the presence of internal pore space. The attachment, spreading, and metabolic activity of pre‐osteoblastic MC3T3‐E1 cells cultured on templated, cross‐linked films are statistically similar to those on non‐templated films, and much greater than those on non‐cross‐linked films. Laser scanning confocal microscopy and quartz crystal microgravimetry indicate a high capacity for bioactive species loading (ca. 10% of film mass) in nanoparticle templated films. Porous nanofilm biomaterials, formed via layer‐by‐layer assembly with nanoparticle templating, promote robust cell adhesion and exhibit high bioactive species loading, and thus appear to be excellent candidates for cell‐contacting applications.     

Tubular Titania Nanostructures via Layer‐by‐Layer Self‐Assembly

Nanostructured titania‐polyelectrolyte composite and pure anatase and rutile titania tubes were successfully prepared by layer‐by‐layer (LbL) deposition of a water‐soluble titania precursor, titanium(IV ) bis(ammonium lactato) dihydroxide (TALH) and the oppositely charged poly(ethylenimine) (PEI) to form multilayer films. The tube structure was produced by depositing inside the cylindrical pores of a polycarbonate (PC) membrane template, followed by calcination at various temperatures. The morphology, structure and crystal phase of the titania tubes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD) and UV‐vis absorbance measurements. The as‐prepared anatase titania tubes exhibit very promising photocatalytic properties, demonstrated by the degradation of the azodye methyl orange (MO) as a model molecule. They are also easily separated from the reaction system by simple filtration or centrifugation, allowing for straightforward recycling. The reported strategy provides a simple and versatile technique to fabricate titania based tubular nanostructures, which could easily be extended to prepare tubular structures of other materials and may find application in catalysis, chemical sensing, and nanodevices.     

Enhanced Photocatalytic Activity using Layer‐by‐Layer Electrospun Constructs for Water Remediation

Endocrine disruptors such as bisphenol A (BPA) are environmental pollutants that interfere with the body's endocrine system because of their structural similarity to natural and synthetic hormones. Due to their strong oxidizing potential to decompose such organic pollutants, colloidal metal oxide photocatalysts have attracted increasing attention for water detoxification. However, achieving both long‐term physical stability and high efficiency simultaneously with such photocatalytic systems poses many challenges. Here a layer‐by‐layer (LbL) deposition approach is reported for immobilizing TiO₂ nanoparticles (NPs) on a porous support while maintaining a high catalytic efficiency for photochemical decomposition of BPA. Anatase TiO₂ NPs ≈7 nm in diameter self‐assemble in consecutive layers with positively charged polyhedral oligomeric silsesquioxanes on a high surface area, porous electrospun polymer fiber mesh. The TiO₂ LbL nanofibers decompose approximately 2.2 mg BPA per mg of TiO₂ in 40 h of illumination (AM 1.5G illumination), maintaining first‐order kinetics with a rate constant (k) of 0.15 h^?1 for over 40 h. Although the colloidal TiO₂ NPs initially show significantly higher photocatalytic activity (k ≈ 0.84 h^?1), the rate constant drops to k ≈ 0.07 h^?1 after 4 h of operation, seemingly due to particle agglomeration. In the BPA solution treated with the multilayered TiO₂ nanofibers for 40 h, the estrogenic activity, based on human breast cancer cell proliferation, is significantly lower than that in the BPA solution treated with colloidal TiO₂ NPs under the same conditions. This study demonstrates that water‐based, electrostatic LbL deposition effectively immobilizes and stabilizes TiO₂ NPs on electrospun polymer nanofibers for efficient extended photochemical water remediation.     

Preparation of Protamine–Titania Microcapsules Through Synergy Between Layer‐by‐Layer Assembly and Biomimetic Mineralization

A novel approach combining layer‐by‐layer (LbL) assembly with biomimetic mineralization is proposed to prepare protamine–titiania hybrid microcapsules. More specifically, these microcapsules are fabricated by alternative deposition of positively charged protamine layers and negatively charged titania layers on the surface of CaCO₃ microparticles, followed by dissolution of the CaCO₃ microparticles using EDTA. During the deposition process, the protamine layer induces the hydrolysis and condensation of a titania precursor, to form the titania layer. Thereafter, the negatively charged titania layer allows a new cycle of deposition step of the protamine layer, which ensures a continuous LbL process. The morphology, structure, and chemical composition of the microcapsules are characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, and X‐ray photoelectron spectroscopy. Moreover, these protamine–titania hybrid microcapsules are first employed as the carrier for the immobilization of yeast alcohol dehydrogenase (YADH), and the encapsulated YADH displays enhanced recycling stability. This approach may open a facile, general, and efficient way to prepare organic–inorganic hybrid materials with different compositions and shapes.     

TiO2‐Based Composite Nanotube Arrays Prepared via Layer‐by‐Layer Assembly

A simple and versatile technique has been developed to prepare TiO₂ and TiO₂‐based composite (TiO₂–CdS and TiO₂–Au) nanotube arrays. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy‐dispersive X‐ray (EDX) analysis, X‐ray diffraction (XRD), thermogravimetric analysis (TGA), UV‐vis spectroscopy, and photoluminescence (PL) spectroscopy are used to characterize their morphology, structure, composition, and properties. The TiO₂–CdS nanotubes contained many TiO₂ and CdS quantum dots and exhibited a novel PL band in the blue‐wavelength range. The reported strategy will be useful for fabricating nanoparticle–nanoparticle composite nanostructure arrays, which are suitable for applications in catalysis, chemical sensors, nanoelectrodes, and nanodevices.     

Pseudobilayer Vesicle Formation via Layer‐by‐Layer Assembly of Hydrophobically Modified Polymers on Sacrificial Substrates

A bilayer of a hydrophobically modified polyelectrolyte, octadecyl poly(acrylamide) (PAAm), sandwiched between the layers of a hydrophilic polyelectrolyte, poly(ethyleneimine) (PEI), is prepared by the sequential electrostatic–hydrophobic–electrostatic‐interaction‐driven self‐assembly on planar and colloid substrates. This process results in a PEI/[PAAm]₂/PEI‐multilayer‐coated substrate. The removal of a PAA/PEI/[PAAm]₂/PEI‐multilayer‐coated decomposable colloidal template produces hollow capsules. Irregular hydrophobic domains of the [PAAm]₂ bilayer in the PEI/[PAAm]₂/PEI‐multilayer capsule are infiltrated with a lipid to obtain a uniform, distinct hydrophobic layer, imparting the capsule with a pseudobilayer vesicle structure.     

Layer‐by‐Layer Assembly of 3D Tissue Constructs with Functionalized Graphene

Carbon‐based nanomaterials have been considered promising candidates to mimic certain structure and function of native extracellular matrix materials for tissue engineering. Significant progress has been made in fabricating carbon nanoparticle‐incorporated cell culture substrates, but only a limited number of studies have been reported on the development of 3D tissue constructs using these nanomaterials. Here, a novel approach to engineer 3D multilayer constructs using layer‐by‐layer (LbL) assembly of cells separated with self‐assembled graphene oxide (GO)‐based thin films is presented. The GO‐based structures are shown to serve as cell adhesive sheets that effectively facilitate the formation of multilayer cell constructs with interlayer connectivity. By controlling the amount of GO deposited in forming the thin films, the thickness of the multilayer tissue constructs could be tuned with high cell viability. Specifically, this approach could be useful for creating dense and tightly connected cardiac tissues through the co‐culture of cardiomyocytes and other cell types. In this work, the fabrication of stand‐alone multilayer cardiac tissues with strong spontaneous beating behavior and programmable pumping properties is demonstrated. Therefore, this LbL‐based cell construct fabrication approach, utilizing GO thin films formed directly on cell surfaces, has great potential in engineering 3D tissue structures with improved organization, electrophysiological function, and mechanical integrity.     

Layer‐by‐Layer Fabrication and Characterization of Gold‐Nanoparticle/Myoglobin Nanocomposite Films

Multilayer thin films of ～ 7 nm diameter gold nanoparticles (GNPs) linked with horse heart myoglobin (Mb) are fabricated, for the first time, by layer‐by‐layer (LbL) assembly on glass slides, and silicon and plastic substrates. The GNP/Mb nanocomposite films show sharp surface plasmon resonance (SPR) absorption bands that are used to follow the LbL growth of the film and to determine the kinetics of GNP adsorption on the Mb‐modified surface. The GNP/Mb nanocomposite films are characterized using atomic force microscopy, transmission electron microscopy, polarized UV‐vis spectroscopy, and spectroscopic ellipsometry. The GNPs in the multilayer films are spatially separated from one another, and interparticle interactions remain in the film, making it optically anisotropic. The GNP/Mb nanocomposite films are stable in air at temperatures up to 100 °C, and can withstand successive immersions in strongly acidic and basic solutions. The SPR absorption band of the GNP/Mb nanocomposite film in air exhibits a red‐shift in the wavelength maximum and an increase in the maximum absorbance relative to that in water. This result, which is in contrast to that observed with a GNP monolayer on an aminosilane‐functionalized substrate, suggests the shrinkage in air and swelling in water of Mb molecules embedded in the nanocomposite film.     

Ambient Layer‐by‐Layer ZnO Assembly for Highly Efficient Polymer Bulk Heterojunction Solar Cells

The use of metal oxide interlayers in polymer solar cells has great potential because metal oxides are abundant, thermally stable, and can be used in flexible devices. Here, a layer‐by‐layer (LbL) protocol is reported as a facile, room‐temperature, solution‐processed method to prepare electron transport layers from commercial ZnO nanoparticles and polyacrylic acid (PAA) with a controlled and tunable porous structure, which provides large interfacial contacts with the active layer. Applying the LbL approach to bulk heterojunction polymer solar cells with an optimized ZnO layer thickness of ≈25 nm yields solar cell power‐conversion efficiencies (PCEs) of ≈6%, exceeding the efficiency of amorphous ZnO interlayers formed by conventional sputtering methods. Interestingly, annealing the ZnO/PAA interlayers in nitrogen and air environments in the range of 60–300 °C reduces the device PCEs by almost 20% to 50%, indicating the importance of conformational changes inherent to the PAA polymer in the LbL‐deposited films to solar cell performance. This protocol suggests a new fabrication method for solution‐processed polymer solar cell devices that does not require postprocessing thermal annealing treatments and that is applicable to flexible devices printed on plastic substrates.     

Layer‐by‐Layer Controlled Perovskite Nanocomposite Thin Films for Piezoelectric Nanogenerators

Perovskite nanoparticle‐based nanocomposite thin films strictly tailored using unconventional layer‐by‐layer (LbL) assembly in organic media for piezoelectric nanogenerators (NGs) are demonstrated. By employing sub‐20‐nm BaTiO₃ nanoparticles stabilized by oleic acid ligands (i.e., OA‐BTO_NPs) and carboxylic acid (COOH)‐functionalized polymers, such as poly(acrylic acid) (PAA), the resulting OA‐BTO_NP/PAA nanocomposite multilayers are prepared by exploiting the high affinity between the COOH groups of PAA and the BTO_NPs. The ferroelectric and piezoelectric performance of the (PAA/OA‐BTO_NP)_n thin films can be precisely controlled by altering the bilayer number, inserted polymer type, and OA‐BTO_NP size. It is found that the LbL assembly in nonpolar solvent media can effectively increase the quantity of adsorbed OA‐BTO_NPs, resulting in the dramatic enhancement of electric power output from the piezoelectric NGs. Furthermore, very low leakage currents are detected from the (PAA/OA‐BTO_NP)_n thin films for obtaining highly reliable power‐generating performance of piezoelectric NGs.     

Freely Suspended Layer‐by‐Layer Nanomembranes: Testing Micromechanical Properties

Freely suspended nanocomposite layer‐by‐layer (LbL) nanomembranes composed of a central layer of gold nanoparticles sandwiched between polyelectrolyte multilayers are fabricated via spin‐assisted LbL assembly. The diameter of the circular membranes is varied from 150 to 600 μm and the thickness is kept within the range of 25–70 nm. The micro‐ and nanomechanical properties of these membranes are studied using a combination of resonance‐frequency and bulging tests, and point‐load nanodeflection experiments. Our results suggest that these freely suspended nanomembranes, with a Young's modulus of 5–10 GPa are very robust and can sustain multiple significant deformations. They are very sensitive to minor variations in pressure, surpassing ordinary semiconductor and metal membranes by three to four orders of magnitude and therefore have potential applications as pressure and acoustic microsensors.     

Room‐Temperature Metallic Fusion‐Induced Layer‐by‐Layer Assembly for Highly Flexible Electrode Applications

To fabricate flexible electrodes, conventional silver (Ag) nanomaterials have been deposited onto flexible substrates, but the formed electrodes display limited electrical conductivity due to residual bulky organic ligands, and thus postsintering processes are required to improve the electrical conductivity. Herein, an entirely different approach is introduced to produce highly flexible electrodes with bulk metal–like electrical conductivity: the room‐temperature metallic fusion of multilayered silver nanoparticles (NPs). Synthesized tetraoctylammonium thiosulfate (TOAS)‐stabilized Ag NPs are deposited onto flexible substrates by layer‐by‐layer assembly involving a perfect ligand‐exchange reaction between bulky TOAS ligands and small tris(2‐aminoethyl)amine linkers. The introduced small linkers substantially reduce the separation distance between neighboring Ag NPs. This shortened interparticle distance, combined with the low cohesive energy of Ag NPs, strongly induces metallic fusion between the close‐packed Ag NPs at room temperature without additional treatments, resulting in a high electrical conductivity of ≈1.60 × 10⁵ S cm^?1 (bulk Ag: ≈6.30 × 10⁵ S cm^?1). Furthermore, depositing the TOAS–Ag NPs onto cellulose papers through this approach can convert the insulating substrates into highly flexible and conductive papers that can be used as 3D current collectors for energy‐storage devices.     

Stiff and Transparent Multilayer Thin Films Prepared Through Hydrogen‐Bonding Layer‐by‐Layer Assembly of Graphene and Polymer

Due to their exceptional orientation of 2D nanofillers, layer‐by‐layer (LbL) assembled polymer/graphene oxide thin films exhibit unmatched mechanical performance relative to any conventionally produced counterparts with similar composition. Unprecedented mechanical property improvement, by replacing graphene oxide with pristine graphene, is demonstrated in this work. Polyvinylpyrrolidone‐stabilized graphene platelets are alternately deposited with poly(acrylic acid) using hydrogen bonding assisted LbL assembly. Transmission electron microscopy imaging and the Halpin‐Tsai model are used to demonstrate, for the first time, that intact graphene can be processed from water to generate polymer nanocomposite thin films with simultaneous parallel‐alignment, high packing density, and exfoliation. A multilayer thin film with only 3.9 vol% of highly exfoliated, and structurally intact graphene, increases the elastic modulus (E) of a polymer multilayer thin film by 322% (from 1.41 to 4.81 GPa), while maintaining visible light transmittance of ≈90%. This is one of the greatest improvements in elastic modulus ever reported for a graphene‐filled polymer nanocomposite with a glassy (E > 1 GPa) matrix. The technique described here provides a powerful new tool to improve nanocomposite properties (mechanical, gas transport, etc.) that can be universally applied to a variety of polymer matrices and 2D nanoplatelets.     

Designing a New Generation of Proton‐Exchange Membranes Using Layer‐by‐Layer Deposition of Polyelectrolytes

All fuel cells utilizing the membrane‐electrode assembly have their ion‐conductive membrane sandwiched between bipolar plates. Unfortunately, applying conventional techniques to isolated polyelectrolyte membranes is challenging and difficult. A more practical alternative is to use the layer‐by‐layer assembly technique to fabricate a membrane‐electrode assembly that is technologically relatively simple, economic, and robust. The process presented here paves the way to fabricate ion‐conductive membranes tailored for optimum performance in terms of controlled thickness, structural morphology, and catalyst loading. Composite membranes are constructed through the layered assembly of ionically conductive multilayer thin films atop a porous polycarbonate membrane. Under ambient conditions, a fuel cell using a poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) composite membrane delivers a maximum power density of 16.5 mW cm^–2 at a relative humidity of 55 %, which is close to that of some commercial fuel cells operating under the same conditions. Further optimization of these systems may lead to new, ultrathin, flexible fuel cells for portable power and micropower applications.     

Covalent Layer‐by‐Layer Assembly of Conjugated Polymers and CdSe Nanoparticles: Multilayer Structure and Photovoltaic Properties

Hybrid thin films of conjugated polymers and CdSe nanoparticles have been fabricated by using a layer‐by‐layer (LbL) approach driven by covalent coupling reactions. This method permits facile covalent crosslinking of the polymer/nanoparticle interlayers in common organic solvents, which provides a general route for preparing robust and uniform functional thin films. The deposition process is linearly related to the number of bilayers as monitored by UV‐vis absorption spectroscopy and ellipsometry. Characterization of the multilayer structures has been carried out by fluorescence spectroscopy, X‐ray photoelectron spectroscopy (XPS), and grazing‐angle Fourier‐transform infrared spectroscopy (FTIR). Techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM) have also been used. A preliminary application of the hybrid films in the development of organic photovoltaics is presented. Upon illumination with white light at 10 mW cm^–2, the self‐assembled multilayer films exhibit steady photocurrent responses with an overall optical‐to‐electrical power conversion efficiency of 0.71 %.     

Electrically Conductive Thin Films Prepared from Layer‐by‐Layer Assembly of Graphite Platelets

Layer‐by‐layer (LBL) assembly of carbon nanoparticles for low electrical contact resistance thin film applications is demonstrated. The nanoparticles consist of irregularly shaped graphite platelets, with acrylamide/ββ‐methacryl‐oxyethyl‐trimethyl‐ammonium copolymer as the cationic binder. Nanoparticle zeta (ζζ) potential and thereby electrostatic interactions are varied by altering the pH of graphite suspension as well as that of the binder suspension. Film thickness as a function of zeta potential, immersion time, and the number of layers deposited is obtained using Monte Carlo simulation of the energy dispersive spectroscopy measurements. Multilayer film surface morphology is visualized via field‐emission scanning electron microscopy and atomic‐force microscopy. Thin film electrical properties are characterized using electrical contact resistance measurements. Graphite nanoparticles are found to self‐assemble onto gold substrates through two distinct yet overlapping mechanisms. The first mechanism is characterized by logarithmic carbon uptake with respect to the number of deposition cycles and slow clustering of nanoparticles on the gold surface. The second mechanism results from more rapid LBL nanoparticle assembly and is characterized by linear weight uptake with respect to the number of deposition cycles and a constant bilayer thickness of 15 to 21 nm. Thin‐film electrical contact resistance is found to be proportional to the thickness after equilibration of the bilayer structure. Measured values range from 1.6 mΩ cm^?2 at 173 nm to 3.5 mΩ cm^?2 at 276 nm. Coating volume resistivity is reduced when electrostatic interactions are enhanced during LBL assembly.     

Surface Imprinting in Layer‐by‐Layer Nanostructured Films

In this Full Paper, we develop a novel approach for the generation of stable molecularly imprinted sites in polymeric films by combining the layer‐by‐layer (LbL) technique and photochemical crosslinking of the layered structure. After photo‐crosslinking, the imprinted films show high reproducibility and rapid loading and unloading of imprinted sites by the template molecules. Moreover, the competitive adsorption of template molecules and redox labels into the imprinted film using electrochemical methods indicates that the imprinted film has higher affinity for template molecules. We believe this approach may have some advantages over traditional ways of preparing imprinted sites in polymer matrices and it may open a new avenue for the functionalization of LbL films.     

Layer‐by‐Layer Growth of Multicomponent Colloidal Crystals Over Large Areas

Self‐assembly of different sized colloidal particles into multicomponent crystals results in novel material properties compared to the properties of the individual components alone. The formation of binary and, for the first time, ternary colloidal crystals through a simple and inexpensive confined‐area evaporation‐induced layer‐by‐layer (LBL) assembly method is reported. The proposed method produces high quality multicomponent colloidal crystal films over a broad range of particle size‐ratios and large surface areas (cm²) from silica/polystyrene colloidal suspensions of low concentration. By adjusting the size‐ratio and concentration of the colloidal particles, complex crystals of tunable stoichiometries are fabricated and their structural characteristics are further confirmed with reported crystal analogues. In addition, complex structures form as a result of the interplay of the template layer effect, the surface forces exerted by the meniscus of the drying liquid, the space filling principle, and entropic forces. Thus, this LBL approach is a versatile way to grow colloidal crystals with binary, ternary, or more complex structures.     

Fabrication of a “Soft” Membrane Electrode Assembly Using Layer‐by‐Layer Technology

A new type of thin‐film electrode that does not utilize conducting polymers or traditional metal or chemical vapor deposition methods has been developed to create ultrathin flexible electrodes for fuel cells. Using the layer‐by‐layer (LbL) technique, carbon–polymer electrodes have been assembled from polyelectrolytes and stable carbon colloidal dispersions. Thin‐film LbL polyelectrolyte–carbon electrodes (LPCEs) have been successfully assembled atop both metallic and non‐metallic, porous and non‐porous substrates. These electrodes exhibit high electronic conductivities of 2–4 S cm^–1, and their porous structure provides ionic conductivities in the range of 10^–4 to 10^–3 S cm^–1. The electrodes show remarkable stability towards oxidizing, acidic, or delaminating basic solutions. In particular, an LPCE consisting of poly(diallyldimethyl ammonium chloride)/poly(2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid)/carbon–platinum assembled on a porous stainless steel support yields an open‐circuit potential similar to that of a pure platinum electrode. With LbL carbon–polymer electrodes, the membrane‐electrode assembly (MEA) in a fuel cell can be made several times thinner, assume multiple geometries, and hence be more compact. The mechanism for LPCE deposition, electrode structure, and miniaturization will be presented and discussed, and demonstrations of the LbL electrodes in a traditional Nafion‐based proton fuel cell and the first demonstration of a thin‐film hydrogen–air “soft” fuel cell fully constructed using multilayer assembly are described.