Preparation and Organization of Nanoscale Polyelectrolyte‐Coated Gold Nanoparticles

The layer‐by‐layer (LbL) desposition of oppositely charged polyelectrolytes from adsorption solutions of different ionic strength onto ～7 nm diameter carboxylic acid‐derivatized gold nanoparticles has been studied. The polyelectrolyte‐modified nanoparticles were characterized by UV‐vis spectrophotometry, microelectrophoresis, analytical ultracentrifugation, and transmission electron microscopy. UV‐vis data showed that the peak plasmon absorption wavelength of the gold nanoparticles red‐shifted after each adsorption step, and microelectrophoresis experiments revealed a reversal in the surface charge of the nanoparticles following deposition of each layer. These data are consistent with the formation of polyelectrolyte layers on the nanoparticles. Analytical ultracentrifugation showed an increase in mean nanoparticle diameter on adsorption of the polyelectrolytes, confirming the formation of gold‐core/polyelectrolyte‐shell nanoparticles. Transmission electron microscopy studies showed no signs of aggregation of the polyelectrolyte‐coated nanoparticles. The adsorption of the polyelectrolyte‐coated gold nanoparticles onto oppositely charged planar supports has also been examined. UV‐vis spectrophotometry and atomic force microscopy showed increased amounts of nanoparticles were adsorbed with increasing ionic strength of the nanoparticle dispersions. This allows control of the nanoparticle surface loading by varying the salt content in the nanoparticle dispersions used for adsorption. The LbL strategy used in this work is expected to be applicable to other nanoparticles (e.g., semiconductors, phosphors), thus providing a facile means for their controlled surface modification through polyelectrolyte nanolayering. Such nanoparticles are envisaged to have applications in the biomedical and bioanalytical fields, and to be useful building blocks for the creation of advanced nanoparticle‐based films.     

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.     

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.     

Selective Ion Transport and Complexation in Layer‐by‐Layer Assemblies of p‐Sulfonato‐ calix[n]arenes and Cationic Polyelectrolytes

The first study of ion transport across self‐assembled multilayered films of p‐sulfonato‐calix[n]arenes and poly(vinyl amine) (PVA) is presented. The films are prepared by the alternate electrostatic layer‐by‐layer assembly of the anionic calixarenes and cationic PVA on porous polyacrylonitrile (PAN) supports. We use tetra‐p‐sulfonato‐calix[4]arene (calix4), hexa‐p‐sulfonato‐calix[6]arene (calix6), and octa‐p‐sulfonato‐calix[8]arene (calix8) as the calixarenes. Ultraviolet (UV) studies indicate that dipping solutions of pH 6.8, without a supporting electrolyte, are most suited for film preparation. Calix8 is adsorbed in higher concentrations per layer than calix6 or calix4, probably because desorption is less pronounced. The permeation rates, P_Rs, of monovalent alkali‐metal chlorides (Li, Na, K, Cs), magnesium chloride, divalent transition‐metal chlorides (Ni, Cu, Zn), trivalent lanthanide chlorides (La, Ce, Pr, Sm), and sodium sulfate across the calix4/PVA, calix6/PVA, and calix8/PVA membranes are studied and compared with the corresponding P_R values across a poly(styrene sulfonate) (PSS)/PVA multilayer membrane prepared under the same conditions. The P_R values of the alkali‐metal salts are between 4 and 17 × 10^–6 cm s^–1, those of magnesium chloride and the transition‐metal salts are 0.2–1.3 × 10^–6 cm s^–1, and those of the lanthanide salts are about 0.1 × 10^–6 cm s^–1. Possible origins for the large differences are discussed. Ion transport is first of all controlled by electrostatic effects such as Donnan rejection of di‐ and trivalent ions in the membrane, but metal‐ion complexation with the calixarene derivatives also plays a role. Complexation occurs especially between Li⁺ or Na⁺ and calix4, Mg²⁺, or Cu²⁺ and calix6, Cu²⁺, Zn²⁺, or the lanthanide ions and calix8. Divalent sulfate ions are found to replace the calixarene polyanions in the membrane. UV studies of the permeate solutions indicate that calix4 especially is displaced during sulfate permeation.     

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.     

Primary Cell Adhesion on RGD‐Functionalized and Covalently Crosslinked Thin Polyelectrolyte Multilayer Films

Polyelectrolyte multilayers (PEMs) are now widely used for biomedical applications. In this work, we investigated the primary osteoblast adhesion properties of PEMs of poly(L ‐lysine) (PLL), poly(L ‐glutamic acid) (PGA), poly(alginic acid) (Palg), and poly(galacturonic acid) (Pgal). In order to compensate for the poor adhesion of the as‐synthesized films, two kinds of film modifications were achieved: a purely physical modification by film crosslinking, and a chemical modification by grafting a arginine–glycine–aspartic acid (RGD) peptide to PGA. Crosslinking was performed using a water‐soluble carbodiimide in combination with N‐hydroxysulfosuccinimide (sulfo‐NHS) to induce amide formation. This reaction was followed by Fourier‐transform IR spectroscopy. For film functionalization, a 15‐amino‐acid peptide was grafted to PGA and deposited as the top layer of the film. PLL/PGA, PLL/Palg, and PLL/Pgal films were crosslinked or functionalized. The films were tested for both short‐term adhesion properties and long‐term proliferation of primary osteoblasts. Whereas the effect of film crosslinking on short‐term adhesion was moderate, it was much more important for the RGD‐functionalized films. On the other hand, the long‐term proliferation was the same or even higher for the crosslinked films as compared with the functionalized films. This effect was particularly enhanced for the PLL/Palg and PLL/Pgal films. Finally, we functionalized PLL/PGA that had been crosslinked prior to PGA‐RGD deposition. These architectures exhibited even higher short‐term adhesion and proliferation. These results clearly show the important role of the physical properties of the films, besides their chemical properties, for the modulation of primary cell‐adhesion behavior.     

Inside Front Cover: Freely Suspended Layer‐by‐Layer Nanomembranes: Testing Micromechanical Properties (Adv. Funct. Mater. 5/2005)

A study of the micromechanical properties of layer‐by‐layer nanomembranes composed of a center layer of gold nanoparticles is reported by Tsukruk and co‐workers on p. 771. The micro‐ and nanomechanical properties of these membranes are measured using a combination of resonance‐frequency tests, bulging tests, and point‐load nanodeflection experiments. These freely suspended nanomembranes (right) with an elastic modulus of 5–10 GPa are very robust and can sustain multiple significant deformations (left, image obtained by B. Rybak and P. Kladitis). They are sensitive to variations in pressure and therefore have potential applications in pressure and acoustic sensors. 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.     

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.     

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.     

Time Controlled Protein Release from Layer‐by‐Layer Assembled Multilayer Functionalized Agarose Hydrogels

Axons of the adult central nervous system exhibit an extremely limited ability to regenerate after spinal cord injury. Experimentally generated patterns of axon growth are typically disorganized and randomly oriented. Support of linear axonal growth into spinal cord lesion sites has been demonstrated using arrays of uniaxial channels, templated with agarose hydrogel, and containing genetically engineered cells that secrete brain‐derived neurotrophic factor (BDNF). However, immobilizing neurotrophic factors secreting cells within a scaffold is relatively cumbersome, and alternative strategies are needed to provide sustained release of BDNF from templated agarose scaffolds. Existing methods of loading the drug or protein into hydrogels cannot provide sustained release from templated agarose hydrogels. Alternatively, here it is shown that pH‐responsive H‐bonded poly(ethylene glycol)(PEG)/poly(acrylic acid)(PAA)/protein hybrid layer‐by‐layer (LbL) thin films, when prepared over agarose, provided sustained release of protein under physiological conditions for more than four weeks. Lysozyme, a protein similar in size and isoelectric point to BDNF, is released from the multilayers on the agarose and is biologically active during the earlier time points, with decreasing activity at later time points. This is the first demonstration of month‐long sustained protein release from an agarose hydrogel, whereby the drug/protein is loaded separately from the agarose hydrogel fabrication process.     

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.     

Fabrication of Organized Porphyrin‐Nanotube‐Attached Heat‐Sensitive Polyelectrolyte Capsules

A facile method of connecting fluorescent meso‐tetrakis(4‐sulfonatophenyl)porphine tetranion nanotubes to polyelectrolyte capsules is developed. Heat‐sensitive robust polyelectrolyte capsules consisting of poly(diallyldimethylammonium chloride) and poly(styrene sulfonate) multilayers have been fabricated using the conventional layer‐by‐layer technique. Supramolecular aggregation of porphyrin monomers to nanotubes is induced in the microenvironment of the capsules by sequential addition of salt and acid. Scanning electron microscopy, transmission electron microscopy, and atomic force microscopy images reveal satellite‐like structures consisting of a central capsule core with porphyrin nanotubes emerging radially from the capsule walls. The growth and the distribution of the porphyrin units have been monitored by UV‐vis spectroscopy, fluorescence spectroscopy, and confocal laser scanning microscopy. Changing the temperature alters the dimensions and the arrangement of the nanotubes on the capsule walls. Such an attachment of porphyrin tubes onto robust functional capsules should help in developing an artificial light‐harvesting system.     

Polyelectrolyte Blend Multilayers: A Versatile Route to Engineering Interfaces and Films

The ability to reliably engineer surfaces with nanoscale precision is a rapidly developing field of research with applications ranging from biosensing and biomedical materials to antifouling and corrosion protection. The layer‐by‐layer (LbL) approach is a widely utilized method for engineering surfaces, in part because of the large array of polymeric materials that can be integrated and the diverse range of functionality that these materials afford. Herein, we discuss the LbL deposition of multicomponent ‘blend' solutions to form polyelectrolyte blend multilayer films and coatings. This approach is a versatile platform for enhancing film stability, incorporating a wide range of functional materials, controlling film morphology and material properties, and increasing biological response, thereby expanding the range of potential applications.     

Short‐Time Tuning of the Biological Activity of Functionalized Polyelectrolyte Multilayers

This article demonstrates the tuning of the biological activity of a surface functionalized by a polyelectrolyte multilayer. The interaction of protein A with macrophages is used as the model system. The film consists of two polypeptides, poly(lysine) and poly(glutamic acid); each “build‐up” solution is a mixture of the respective D ‐ and L ‐enantiomers (d and l enantiomers). Cells are deposited on top of the film, and they produce tumor necrosis factor alpha (TNF‐α) as they come into contact with the protein. Depending upon the d/l‐enantiomer ratio of the polyelectrolyte solutions used for the film build‐up, and the embedding depth of the protein, the production of TNF‐α commences after a varying induction time and displays a transition from no‐production to full‐production, which takes place over a period of time that depends on the film's composition and embedding depth. Thus, it is shown that by changing these two parameters the timing of the protein's activity can be accurately tuned.     

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.     

Biofunctional Polyelectrolyte Multilayers and Microcapsules: Control of Non‐Specific and Bio‐Specific Protein Adsorption

Layers of the polyelectrolytes poly(allylamine hydrochloride) (PAH, polycationic) and poly(styrene sulfonate) (PSS, polyanionic) are consecutively adsorbed on flat silicon oxide surfaces, forming stable, ultrathin multilayer films. Subsequently, a final monolayer of the polycationic copolymer poly(L ‐lysine)‐graft‐poly(ethylene glycol) (PLL‐g‐PEG) is adsorbed onto the PSS‐terminated multilayer in order to impart protein resistance to the surface. The growth of each of the polyelectrolyte layers and the protein resistance of the resulting [PAH/PPS]_n(PLL‐g‐PEG) multilayer (n = 1–4) are followed quantitatively ex situ using X‐ray photoelectron spectroscopy and in situ using real‐time optical‐waveguide lightmode spectroscopy. In a second approach, the same type of [PAH/PSS]_n(PLL‐g‐PEG) multilayer coatings are successfully formed on the surface of colloidal particles in order to produce surface‐functionalized, hollow microcapsules after dissolution of the core materials (melamine formaldehyde (MF) and poly(lactic acid) (PLA; colloid diameters: 1.2–20 μm). Microelectrophoresis and confocal laser scanning microscopy are used to study multilayer formation on the colloids and protein resistance of the final capsule. The quality of the PLL‐g‐PEG layer on the microcapsules depends on both the type of core material and the dissolution protocols used. The greatest protein resistance is achieved using PLA cores and coating the polyelectrolyte microcapsules with PLL‐g‐PEG after dissolution of the cores. Protein adsorption from full serum on [PAH/PPS]_n(PLL‐g‐PEG) multilayers (on both flat substrates and microcapsules) decreases by three orders of magnitude in comparison to the standard [PAH/PPS]_n layer. Finally, biofunctional capsules of the type [PAH/PPS]_n(PLL‐g‐PEG/PEG‐biotin) (top copolymer layer with a fraction of the PEG chains end‐functionalized with biotin) are produced which allow for specific recognition and immobilization of controlled amounts of streptavidin at the surface of the capsules. Biofunctional multilayer films and capsules are believed to have a potential for future applications as novel platforms for biotechnological applications such as biosensors and carriers for targeted drug delivery.     

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.     

Copper‐Assisted Weak Polyelectrolyte Multilayer Formation on Microspheres and Subsequent Film Crosslinking

The formation of weak polyelectrolyte films on planar and spherical supports has recently evoked major interest, as such coatings allow novel material properties to be tunable by pH and salt adjustment of the polyelectrolyte deposition conditions. We report on the build up of multilayers of the weak polyelectrolytes poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) on submicrometer‐sized polystyrene (PS) and silica colloid spheres (～ 500 nm) with the aid of copper ion templating. The copper ions complex to the carboxylate groups of PAA, facilitating the formation of PAA/PAH multilayers on the particles. Regular growth of the layers on the colloid spheres with each polyelectrolyte deposition step was confirmed by microelectrophoresis, single‐particle light scattering (SPLS), and transmission electron microscopy (TEM), with an average bilayer thickness of ～ 3 nm. The polyelectrolyte multilayer‐coated particles formed stable colloidal dispersions, with ζ‐potentials ranging from 30 mV (PAH outer layer) and –50 mV (PAA outer layer). Complementary quartz‐crystal microbalance and UV‐vis spectrophotometry studies on PAA/PAH multilayers formed on planar supports were performed to examine the film formation and the role of copper ion binding to the layers. PAA/PAH multilayers formed on colloid particles were also chemically crosslinked by using the activator 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide (EDC). The degree of film crosslinking could be readily controlled by varying the concentration of EDC employed. Following solvent decomposition of the template particles coated with crosslinked PAA/PAH multilayers, intact hollow polymer capsules were obtained. These capsules were found to be impenetrable to polystyrene.     

Biomaterials by Design: Layer‐By‐Layer Assembled Ion‐Selective and Biocompatible Films of TiO2 Nanoshells for Neurochemical Monitoring

Titania nanoshells with an external diameter of 10–30 nm and a wall thickness of 3–5 nm were prepared by dissolving the silver cores of Ag@TiO₂ nanoparticles in a concentrated solution of ammonium hydroxide. The nanoshells were assembled layer‐by‐layer (LBL), with negatively charged poly(acrylic acid) (PAA) to produce coatings with a network of voids and channels in the interior of the film. The diameter of the channels in the titania shells was comparable to the thickness of the electrical double layer in porous matter (0.3–30 nm). The prepared nanoparticulate films demonstrated strong ion‐sieving properties due to the exclusion of some ions from the diffuse region of the electrical double layer. The permeation of ions could be tuned effectively by the pH and ionic strength of a solution between “open” and “closed” states. The ion‐separation effect was utilized for the selective determination of one of the most important neurotransmitters, dopamine, on a background of ascorbic acid. Under physiological conditions, the negative charge on the surface of TiO₂ facilitated the permeation of positively charged dopamine through the LBL film to the electrode, preventing the access of the negatively charged ascorbic acid. The deposition of the nanoshell/polyelectrolyte film resulted in a significant improvement to the selectivity of dopamine determination. The prepared nanoshell films were also found to be compatible with nervous tissue secreting dopamine. Although the obtained data demonstrated the potential of TiO₂ LBL films for implantable biomedical devices for nerve tissue monitoring, the problem of electrode poisoning by the by‐products of dopamine reduction has yet to be resolved.     

Flexible and Robust 2D Arrays of Silver Nanowires Encapsulated within Freestanding Layer‐by‐Layer Films

Freestanding layer‐by‐layer (LbL) films encapsulating controlled volume fractions (? = 2.5–22.5 %) of silver nanowires are fabricated. The silver nanowires are sandwiched between poly(allylamine hydrochloride)/poly(styrene sulfonate) (PAH/PSS) films resulting in nanocomposite structures with a general formula of (PAH/PSS)₁₀PAH Ag(PAH/PSS)₁₀PAH. The Young's modulus, toughness, ultimate stress, and ultimate strain are evaluated for supported and freestanding structures. Since the diameter of the nanowires (73 nm) is larger than the thickness of the LbL films (total of about 50 nm), a peculiar morphology is observed with the silver nanowires protruding from the planar LbL films. Nanowire‐containing LbL films possess the ability to sustain significant elastic deformations with the ultimate strain reaching 1.8 %. The Young's modulus increases with increasing nanowire content, reaching about 6 GPa for the highest volume fraction, due to the filler reinforcement effect commonly observed in composite materials. The ultimate strengths of these composites range from 60–80 MPa and their toughness reaches 1000 kJ m^–3 at intermediate nanowire content, which is comparable to LbL films reinforced with carbon nanotubes. These robust freestanding 2D arrays of silver nanowires with peculiar optical, mechanical, and conducting properties combined with excellent micromechanical stability could serve as active elements in microscopic acoustic, pressure, and photothermal sensors.