Two-step sequential reaction catalyzed by layer-by-layer assembled urease and arginase multilayers

There is still plenty of room for layer-by-layer assembly for constructing nanoarchitectonics-based materials and devices

Nanoarchitectonics approaches can produce functional materials from tiny units through combination of various processes including atom/molecular manipulation, chemical conversion, self-assembly/self-organization, microfabrication, and bio-inspired procedures. Existing fabrication approaches can be regarded as fitting into the same concept. In particular, the so-called layer-by-layer (LbL) assembly method has huge potential for preparing applicable materials with a great variety of assembling mechanisms. LbL assembly is a multistep process where different components can be organized in planned sequences while simple alignment options provide access to superstructures, for example helical structures, and anisotropies which are important aspects of nanoarchitectonics. In this article, newly-featured examples are extracted from the literature on LbL assembly discussing trends for composite functional materials according to (i) principles and techniques, (ii) composite materials, and (iii) applications. We present our opinion on the present trends, and the prospects of LbL assembly. While this method has already reached a certain maturity, there is still plenty of room for expanding its usefulness for the fabrication of nanoarchitectonics-based materials and devices.

Polyelectrolyte Multilayers: An Overview on Fabrication, Properties, and Biomedical and Environmental Applications

Polyelectrolyte multilayers are versatile materials that are used in a large number of domains, including biomedical and environmental applications. The fabrication of polyelectrolyte multilayers using the layer-by-layer technique is one of the simplest methods to obtain composite functional materials. The properties of the final material can be easily tuned by changing the deposition conditions and the used building blocks. This review presents the main characteristics of polyelectrolyte multilayers, the fabrication methods currently used, and the factors influencing the layer-by-layer assembly of polyelectrolytes. The last section of this paper presents some of the most important applications of polyelectrolyte multilayers, with a special focus on biomedical and environmental applications.

Polyelectrolyte Multilayers Enhance the Dry Storage and pH Stability of Physically Entrapped Enzymes

Polyelectrolyte multilayers (PEMs) are attractive materials for immobilizing enzymes due to their unique ionic environment, which can prevent unfolding. Here, we demonstrated that the stability to dry storage and elevated pH were significantly enhanced when negatively charged nitroreductase (NfsB) was embedded in a PEM by depositing alternating layers of the enzyme and polycation (PC) onto porous silica particles. The PC strength (i.e., pK_a) and the surface charge of the film were varied to probe the effects that internal and surface chemistry had on the pH stability of the entrapped NfsB. All films showed enhanced activity retention at elevated pH (>6), and inactivation at reduced pH (<6) similar to NfsB in solution, indicating that the primary stabilizing effect of immobilization was achieved through ionic interactions between NfsB and the PC and not through changes to the surface charge of the NfsB. Additionally, films that were stored dry at 4 °C for 1 month retained full activity, while those stored at room temperature lost 30% activity. Remarkably, at 50 °C, above the NfsB melting temperature, 40% activity was retained after 1 month of dry storage. Our results suggest that internal film properties are significantly more important than surface charge, which had minor effects on activity. Specifically, immobilization with the weak PC, poly(l-lysine), increased the optimal pH and the activity of immobilized NfsB (which we attribute to greater permeability), relative to immobilization with the strong PC, poly(diallyldimethylammonium chloride). However, NfsB was leached from the PLL film to a greater extent. Overall, these observations demonstrate that internal ionic cross-linking is key to the stabilizing effects of PEMs and that the pH response can be tuned by controlling the number of cross-links (e.g., by changing the strength of the PC). However, this may be at the cost of reduced loading, illustrating the necessity of simultaneously optimizing enzyme loading, internal ionic cross-linking, and substrate transport.

Precise 2D-Patterned Incompatible Catalysts for Reactions in One-Pot

Precise and direct two-dimensional (2D) printing of the incompatible polymer acid-base catalysts and their utility in one-pot two-step reactions were shown. Multistep catalytic reactions using incompatible catalysts in a one-pot reaction cascade requires special methods and materials to isolate the catalysts from each other. In general, this is a tedious process requiring special polymer architectures as the carrier for the catalysts to preserve the activity of otherwise incompatible catalysts. We propose the immobilization of incompatible polymer catalysts, such as polymer acid and base catalysts, on a substrate in variable sizes and amounts by precise 2D printing. The terpolymers with basic (4-vinylpyridine) and acidic (styrene sulfonic acid) functionalities and methacryloyl benzophenone as a UV cross-linking unit were used for 2D printing on poly(ethylene terephthalate) (PET). The printed meshes were immersed together in a reaction solution containing (dimethoxymethyl)benzene and ethyl cyanoformate, resulting in a two-step acid-base catalyzed cascade reaction; that is, deacetalization followed by carbon-building reaction. The time-dependent consumption of (dimethoxymethyl)benzene to the intermediate benzaldehyde and the product was monitored, and a kinetic model was developed to investigate the underlying reaction dynamics. The complexity of multistep Wolf-Lamb-type reactions was generally significantly decreased by using our approach because of the easy polymerization and immobilization procedure.

Analyzing protein-ligand and protein-interface interactions using high pressure

All protein function is based on interactions with the environment. Proteins can bind molecules for their transport, their catalytic conversion, or for signal transduction. They can bind to each other, and they adsorb at interfaces, such as lipid membranes or material surfaces. An experimental characterization is needed to understand the underlying mechanisms, but also to make use of proteins in biotechnology or biomedicine. When protein interactions are studied under high pressure, volume changes are revealed that directly describe spatial contributions to these interactions. Moreover, the strength of protein interactions with ligands or interfaces can be tuned in a smooth way by pressure modulation, which can be utilized in the design of drugs and bio-responsive interfaces. In this short review, selected studies of protein-ligand and protein-interface interactions are presented that were carried out under high pressure. Furthermore, a perspective on bio-responsive interfaces is given where protein-ligand binding is applied to create functional interfacial structures.

Enzyme Multilayers on Graphene-Based FETs for Biosensing Applications

Electrochemical sensors represent a powerful tool for real-time measurement of a variety of analytes of much significance to different areas, ranging from clinical diagnostics to food technology. Point-of-care devices which can be used at patient bedside or for online monitoring of critical parameters are of great importance in clinical daily routine. In this work, portable, low-cost electrochemical sensors for a fast and reliable detection of the clinically relevant analyte urea have been developed. The intrinsic pH sensitivity of reduced graphene oxide (rGO)-based field-effect transistors (FETs) was exploited to monitor the enzymatic hydrolysis of urea. The functionalization of the sensor platform using the layer-by-layer technique is especially advantageous for the immobilization of the biorecognition element provided that this approach preserves the enzyme integrity as well as the rGO surface. The great selectivity of the enzyme (urease) combined with the high sensitivity of rGO-based FETs result in the construction of urea biosensors with a limit of detection (LOD) of 1μM and a linear range up to 1mM. Quantification of Cu²⁺ with a LOD down to 10nM was performed by taking advantage of the specific inhibition of urease in the presence of heavy metals. These versatile biosensors offer great possibilities for further development of highly sensitive enzyme-based FETs for real-time, label-free detection of a wide variety of clinically relevant analytes.

Cascading reaction of arginase and urease on a graphene-based FET for ultrasensitive, real-time detection of arginine

Herein, a biosensor based on a reduced graphene oxide field effect transistor (rGO-FET) functionalized with the cascading enzymes arginase and urease was developed for the detection of L-arginine. Arginase and urease were immobilized on the rGO-FET sensing surface via electrostatic layer-by-layer assembly using polyethylenimine (PEI) as cationic building block. The signal transduction mechanism is based on the ability of the cascading enzymes to selectively perform chemical transformations and prompt local pH changes, that are sensitively detected by the rGO-FET. In the presence of L-arginine, the transistors modified with (PEI/urease(arginase)) multilayers showed a shift in the Dirac point due to the change in the local pH close to the graphene surface, produced by the catalyzed urea hydrolysis. The transistors were able to monitor L-arginine in the 10-1000 μM linear range with a LOD of 10 μM, displaying a fast response and a good long-term stability. The sensor showed stereospecificity and high selectivity in the presence of non-target amino acids. Taking into account the label-free, real-time measurement capabilities and the easily quantifiable, electronic output signal, this biosensor offers advantages over state-of-the-art L-arginine detection methods.

Building Polyelectrolyte Multilayers with Calmodulin: A Neutron and X-ray Reflectivity Study

We have studied the formation and functional properties of polyelectrolyte multilayers where calmodulin (CaM) is used as a polyanion. CaM is known to populate distinct conformational states upon binding Ca²⁺ and small ligand molecules. Therefore, we have also probed the effects of Ca²⁺ ions and trifluoperazine (TFP) as ligand molecule on the interfacial structures. Multilayers with the maximum sequence PEI-(PSS-PAH)_x-CaM-PAH-CaM-PAH have been deposited on silicon wafers and characterized by X-ray and neutron reflectometry. From the analysis of all data, several remarkable conclusions can be drawn. When CaM is deposited for the second time, a much thicker sublayer is produced than in the first CaM deposition step. However, upon rinsing with PAH, very thin CaM-PAH sublayers remain. There are no indications that ligand TFP can be involved in the multilayer buildup due to strong CaM-PAH interactions. However, there is a significant increase in the multilayer thickness upon removal of Ca²⁺ ions from holo-CaM and an equivalent decrease in the multilayer thickness upon subsequent saturation of apo-CaM with Ca²⁺ ions. Presumably, CaM can still be toggled between an apo and a holo state, when it is embedded in polyelectrolyte multilayers, providing an approach to design bioresponsive interfaces.

Quorum-Quenching and Matrix-Degrading Enzymes in Multilayer Coatings Synergistically Prevent Bacterial Biofilm Formation on Urinary Catheters

Bacteria often colonize in-dwelling medical devices and grow as complex biofilm communities of cells embedded in a self-produced extracellular polymeric matrix, which increases their resistance to antibiotics and the host immune system. During biofilm growth, bacterial cells cooperate through specific quorum-sensing (QS) signals. Taking advantage of this mechanism of biofilm formation, we hypothesized that interrupting the communication among bacteria and simultaneously degrading the extracellular matrix would inhibit biofilm growth. To this end, coatings composed of the enzymes acylase and α-amylase, able to degrade bacterial QS molecules and polysaccharides, respectively, were built on silicone urinary catheters using a layer-by-layer deposition technique. Multilayer coatings of either acylase or amylase alone suppressed the biofilm formation of corresponding Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus. Further assembly of both enzymes in hybrid nanocoatings resulted in stronger biofilm inhibition as a function of acylase or amylase position in the layers. Hybrid coatings, with the QS-signal-degrading acylase as outermost layer, demonstrated 30% higher antibiofilm efficiency against medically relevant Gram-negative bacteria compared to that of the other assemblies. These nanocoatings significantly reduced the occurrence of single-species (P. aeruginosa) and mixed-species (P. aeruginosa and Escherichia coli) biofilms on silicone catheters under both static and dynamic conditions. Moreover, in an in vivo animal model, the quorum quenching and matrix degrading enzyme assemblies delayed the biofilm growth up to 7 days.

Layer by layer assembly of a biocatalytic packaging film: lactase covalently bound to low-density polyethylene

Active packaging is utilized to overcome limitations of traditional processing to enhance the health, safety, economics, and shelf life of foods. Active packaging employs active components to interact with food constituents to give a desired effect. Herein we describe the development of an active package in which lactase is covalently attached to low-density polyethylene (LDPE) for in-package production of lactose-free dairy products. The specific goal of this work is to increase the total protein content loading onto LDPE using layer by layer (LbL) deposition, alternating polyethylenimine, glutaraldehyde (GL), and lactase, to enhance the overall activity of covalently attached lactase. The films were successfully oxidized via ultraviolet light, functionalized with polyethylenimine and glutaraldehyde, and layered with immobilized purified lactase. The total protein content increased with each additional layer of conjugated lactase, the 5-layer sample reaching up to 1.3 μg/cm2 . However, the increase in total protein did not lend to an increase in overall lactase activity. Calculated apparent Km indicated the affinity of immobilized lactase to substrate remains unchanged when compared to free lactase. Calculated apparent turnover numbers (kcat ) showed with each layer of attached lactase, a decrease in substrate turnover was experienced when compared to free lactase; with a decrease from 128.43 to 4.76 s(-1) for a 5-layer conjugation. Our results indicate that while LbL attachment of lactase to LDPE successfully increases total protein mass of the bulk material, the adverse impact in enzyme efficiency may limit the application of LbL immobilization chemistry for bioactive packaging use.

Probing aggregation and fibril formation of insulin in polyelectrolyte multilayers

Ultrathin films are useful for coating materials and controlling drug delivery processes. Here, we explore the use of polyelectrolyte multilayers as templates for the formation of two-dimensional protein networks, which represent biocompatible and biodegradable ultrathin films. In a first step, we have studied the lateral aggregation and amyloid fibril formation of bovine insulin that is adsorbed at and confined within planar polyelectrolyte multilayers, assembled with poly(diallyldimethylammonium chloride) (PDDA), poly(styrenesulfonic acid) (PSS), and hyaluronic acid (HA). Si-PDDA-PSS-(insulin-PSS)(x) and Si-PDDA-PSS-(insulin-HA)(x) multilayers (x=1-4) have been prepared and characterized in the fully hydrated state by using X-ray reflectometry, attenuated total reflection-Fourier transform infrared spectroscopy and confocal fluorescence microscopy. The obtained data demonstrate a successful build-up of the insulin-polyelectrolyte multilayers on silicon wafers that grow strongly in thickness upon insulin adsorption on PSS and HA layers. The secondary structure analysis of insulin, based on the vibrational amide I'-band, indicates an enhanced intermolecular β-sheet formation within the multilayers at 70°C and pD=2, i.e. at conditions that promote insulin amyloid fibrils rich in β-sheet contents. However, insulin that is confined between two polyelectrolyte layers rather forms amorphous aggregates as can be inferred from confocal fluorescence images. Remarkably, when insulin is deposited as the top-layer, a partial conversion into a two-dimensional fibrillar network can be induced by adding amyloid seeds to the solution. Thus, the results of this study illustrate the capability of polyelectrolyte multilayers as templates for the growth of protein networks.

Surface-supported multilayers decorated with bio-active material aimed at light-triggered drug delivery

In this work, we report on the functionalization of layer-by-layer films with gold nanoparticles, microcapsules, and DNA molecules by spontaneous incorporation into the film. Exponentially growing films from biopolymers, namely, hyaluronic acid (HA) and poly-L-lysine (PLL), and linearly growing films from the synthetic polymers, namely, poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), were examined for the embedding. The studied (PLL/HA)(24)/PLL and (PAH/PSS)(24)/PAH films are later named HA/PLL and PSS/PAH films, respectively. The HA/PLL film has been found to be more efficient for both particle and DNA embedding than PSS/PAH because of spontaneous PLL transport from the interior of the whole HA/PLL film to the surface in order to make additional contact with embedded particles or DNA. DNA and nanoparticles can be immobilized in HA/PLL films, reaching loading capacities of 1.5 and 100 microg/cm(2), respectively. The capacities of PSS/PAH films are 5 and 12 times lower than that for films made from biopolymers. Polyelectrolyte microcapsules adsorb irreversibly on the HA/PLL film surface as single particles whereas very poor interaction was observed for PSS/PAH. This intrinsic property of the HA/PLL film is due to the high mobility of PLL within the film whereas the structure of the PSS/PAH film is "frozen in". Gold nanoparticles and DNA form micrometer-sized aggregates or patches on the HA/PLL film surface. The diffusion of nanoparticles and DNA into the HA/PLL film is restricted at room temperature, but DNA diffusion is triggered by heating to 70 degrees C, leading to homogeneous filling of the film with DNA. The film has not only a high loading capacity but also can be activated by "biofriendly" near-infrared (IR) laser light, thanks to the gold nanoparticle aggregates on the film surface. Composite HA/PLL films with embedded gold nanoparticles and DNA can be activated by light, resulting in DNA release. We assume that the mechanism of the release is dependent on the disturbance in bonding between "doping" PLL and DNA, which is induced by local thermal decomposition of the HA/PLL network in the film when the film is exposed to IR light. Remote IR-light activation of dextran-filled microcapsules modified by gold nanoparticles and integrated into the HA/PLL film is also demonstrated, revealing an alternative release pathway using immobilized light-sensitive carriers (microcapsules).

Native-like structure of proteins at a planar poly(acrylic acid) brush

Applying ATR-FTIR (attenuated total reflection Fourier transform infrared) and TIRF (total internal reflection fluorescence) spectroscopy, we have studied the secondary structure and aggregation properties of different proteins which are adsorbed at a poly(acrylic acid) (PAA) brush that covers a macroscopically large, planar surface. The PAA brush has been prepared on the surface of an ATR silicon crystal or a quartz plate. The preparation includes the deposition of a thin poly(styrene) film by spin-coating and the transfer of the diblock copolymer poly(styrene)-poly(acrylic acid) onto the hydrophobic film using the Langmuir-Schafer technique. It has been found that the proteins hen egg white lysozyme, bovine serum albumin, bovine alpha-lactalbumin, and bovine insulin adsorb spontaneously at a PAA brush at neutral pD values, albeit to different degrees. The secondary structure of the proteins was estimated from a decomposition of the amide I'-band in the observed ATR-FTIR spectra. Generally, the fractions of secondary structure elements recovered in this way were almost identical to those found when the proteins are native in solution. In addition, the tendency of insulin to form amyloid fibrils has also been tested when the protein is adsorbed at a planar PAA brush. Insulin is known to form amyloid fibrils in solution at low pH values and elevated temperatures. The experiments performed in this study suggest that a PAA brush does not promote fibril formation of insulin. Rather, insulin that is adsorbed at a PAA brush seems to be excluded from fibril formation pathways even at pD = 2 and 60 degrees C, where fibril formation of insulin is triggered in solution. Overall, the results of this study demonstrate that a planar PAA brush may serve as a mild environment for immobilized proteins.

Efficiency of a bienzyme sequential reaction system immobilized on polyelectrolyte multilayer-coated colloids

We assembled multilayer films of glucose oxidase (GOx) and horseradish peroxidase (HRP) coimmobilized together with polyelectrolyte layers on the surface of silica microparticles. The influence of different polyelectrolyte combinations on the immobilization and functionality of the enzymes was examined for several multilayer configurations. Precomplexation of the enzymes with a polyvinylpyridine-based polyamine allowed the stable adsorption of enzyme layers without affecting their catalytic activity. The efficiency of the sequential reaction between GOx and HRP on the surface of the colloids was quantitatively analyzed and rationalized in terms of the kinetic parameters of both enzymes and the reaction-diffusion kinetics of the system. In the optimized configuration, with GOx and HRP coimmobilized in the same layer, the overall rate of hydrogen peroxide conversion was around 2.5 times higher than for GOx and HRP in separate layers or for equivalent amounts of both enzymes free in solution.

X-ray reflectivity measurements of layer-by-layer films at the solid/liquid interface

In this Letter, we present a method for the decoration of layer-by-layer (LbL) structures by heavy metal ions, which allows X-ray reflectivity (XRR) measurements at the solid/water interface. The improved contrast has allowed us to obtain well-structured X-ray reflectivity curves from samples at the liquid/solid interface that can be used for the film structure modeling. The developed technique was also used to follow the formation of complexes between DNA and the LbL multilayer. The XRR data are confirmed by independent null-ellipsometric measurements at the solid/liquid interface on the very same architectures.

A survey of the 2001 to 2005 quartz crystal microbalance biosensor literature: applications of acoustic physics to the analysis of biomolecular interactions

The widespread exploitation of biosensors in the analysis of molecular recognition has its origins in the mid-1990s following the release of commercial systems based on surface plasmon resonance (SPR). More recently, platforms based on piezoelectric acoustic sensors (principally 'bulk acoustic wave' (BAW), 'thickness shear mode' (TSM) sensors or 'quartz crystal microbalances' (QCM)), have been released that are driving the publication of a large number of papers analysing binding specificities, affinities, kinetics and conformational changes associated with a molecular recognition event. This article highlights salient theoretical and practical aspects of the technologies that underpin acoustic analysis, then reviews exemplary papers in key application areas involving small molecular weight ligands, carbohydrates, proteins, nucleic acids, viruses, bacteria, cells and lipidic and polymeric interfaces. Key differentiators between optical and acoustic sensing modalities are also reviewed.

The effect of the layer structure on the activity of immobilized enzymes in ultrathin films

The molecular engineering capability of the layer-by-layer (LbL) method for fabricating thin films has been exploited in order to immobilize glucose oxidase (GOD) in films with alternating layers of chitosan. Chitosan was proven a good scaffolding material, as GOD molecules preserved their catalytic activity towards glucose oxidation. Using electrochemical measurements, we showed that chitosan/GOD LbL films can be used to detect glucose with a limit of detection of 0.2 mmol l-1 and an activity of 40.5 microA mmol-1 L microg-1, which is highly sensitive when compared to other sensors in previous reports in the literature. The highest sensitivity of the LbL film was achieved when only the top layer contained GOD, thus indicating that GOD in inner layers did not contribute to glucose oxidation, probably because it hampers analyte diffusion and electron transport through the deposited layers. This may be explained by the dense packing of GOD molecules in the LbL films with chitosan, as inferred from estimates of the amount of GOD adsorbed per layer using a quartz crystal microbalance.

Layer-by-Layer Enzyme/Polyelectrolyte Films as a Functional Protective Barrier in Oxidizing Media

The influence of a catalase (Cat) layer located at different depths in the layer-by-layer hemoglobin/polystyrene sulfonate films with an (Hb/PSS)(20)(-)(x)/(Cat/PSS)/(Hb/PSS)(x) (x = 0-20) architecture on kinetics of hemoglobin degradation under treatment with hydrogen peroxide solutions of different concentrations and features of H(2)O(2) decay in surrounding solutions has been studied. While assembled on the top of the multilayers, the catalase layer shows the highest activity in hydrogen peroxide decomposition. Hemoglobin in such films retains its nativity for a longer period of time. The effect of catalase layers is compared with that of protamine, horseradish peroxidase, and inactivated catalase. Positioning an active layer with catalytic properties as an outer layer is the best protection strategy for layer-by-layer assembled films in aggressive media.

Preparation of ultrathin self-standing polyelectrolyte multilayer membranes at physiological conditions using pH-responsive film segments as sacrificial layers

A new system to obtain ultrathin self-standing polyelectrolyte multilayer membranes at physiological conditions is introduced. On the surface of a substrate, a hybrid film structure composed of two compartments, (1) a pH-responsive film segment formed via hydrogen bonds and (2) a polyelectrolyte multilayer film on top of 1, was assembled. The pH-responsive polymer multilayer segments disintegrate at a neutral pH and release self-standing polyelectrolyte multilayer films. The obtained self-supporting polyelectrolyte multilayer membranes had thicknesses of 55 to several hundred nanometers and areas of a few square centimeters, approximately. The preparation method introduced here avoids harsh release conditions and thus broadens the choice of materials that can be incorporated into the self-standing film.

Redox properties of the ferricyanide ion on electrodes coated with layer-by-layer thin films composed of polysaccharide and poly(allylamine)

Polyelectrolyte multilayer thin films were prepared by an alternate deposition of poly(allylamine hydrochloride) (PAH) and anionic polysaccharides {carboxymethylcellulose (CMC) and alginic acid (AGA)} on the surface of a gold (Au) disk electrode, and the binding of ferricyanide [Fe(CN)(6)](3)(-) and hexaammine ruthenium ions [Ru(NH(3))(6)](3+) to the films was evaluated. Poly(acrylic acid) (PAA) was also employed as a reference polyanion bearing carboxylate side chains. A quartz-crystal microbalance study showed that PAH-CMC and PAH-AGA multilayer films grow exponentially as the number of depositions increases. The thicknesses of five bilayers of (PAH-CMC)(5) and (PAH-AGA)(5) films were estimated to be 150 +/- 20 and 90 +/- 15 nm, respectively, in the dry state. The PAH/polysaccharide multilayer film-coated Au electrodes exhibited a redox response to the [Fe(CN)(6)](3)(-) ion dissolved in solution, irrespective of the sign of the surface charge of the film, suggesting the high permeability of the films to the [Fe(CN)(6)](3)(-) ion. In contrast, the PAH-PAA film-coated Au electrodes exhibited a redox response only when the outermost surface of the film was covered with a positively charged PAH layer. However, the permeation of the [Ru(NH(3))(6)](3+) cation was severely suppressed for all of the multilayer films. It was possible to confine the [Fe(CN)(6)](3)(-) ion in the films by immersing the film-coated electrodes in a 1 mM [Fe(CN)(6)](3)(-) solution for 15 min. Thus, the [Fe(CN)(6)](3)(-)-confined electrodes exhibited a cyclic voltammetric response in the [Fe(CN)(6)](3)(-) ion-free buffer solution. The loading of the [Fe(CN)(6)](3)(-) ion in the films was higher when the surface charge of the film was positive and increased with increasing film thickness. It was also found that the [Fe(CN)(6)](3)(-) ion confined in the films serves as an electrocatalyst that oxidizes ascorbic acid in solution.

Stimuli-sensitive thin films prepared by a layer-by-layer deposition of 2-iminobiotin-labeled poly(ethyleneimine) and avidin

Layered thin films composed of avidin and 2-iminobiotin-labeled poly(ethyleneimine) (ib-PEI) were prepared by a layer-by-layer deposition of avidin and ib-PEI on a solid surface, and the disintegration induced by changing environmental pH and adding biotin in the solution was studied. The avidin/ib-PEI layered film could be deposited only from the solutions of pH 10-12. The film did not form in pH 9 or more acidic media because of a low affinity of protonated 2-iminobiotin residues in ib-PEI to avidin. The avidin/ib-PEI layered films were stable in pH 8-12 solutions, while in pH 5-7 media the film decomposed spontaneously as a result of the protonation to 2-iminobiotin residues in ib-PEI. The avidin/ib-PEI films were disintegrated also upon addition of biotin and analogues in the solution owing to the preferential binding of biotin or analogues to the binding site of avidin. The decomposition rate was arbitrarily controlled by changing the type of stimulant (biotin or analogues) and its concentration. The avidin/ib-PEI films were disintegrated rapidly by addition of 10(-)(5) M of biotin or desthiobiotin, while the rate was slower upon adding the same concentration of lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid. On the other hand, the film was fully decomposed within 1 min in the 10(-)(3) M lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid solution. Thus, the decomposition rate is highly dependent on the concentration of the stimulants. It was observed that the stimuli-induced decomposition of the films is slow at pH 12, in contrast to a rapid decomposition in pH 8 medium due to a low affinity of the protonated 2-iminobiotin to avidin. The present system may be useful for constructing stimuli-sensitive devices that can release drug or other functional molecules.

Structured thin films as functional components within biosensors

This review provides an introduction to the field of thin films formed by Langmuir-Blodgett or self-assembly techniques and discusses applications in the field of biosensors. The review commences with an overview of thin films and methods of construction. Methods covered will include Langmuir-Blodgett film formation, formation of self-assembled monolayers such as gold-thiol monolayers and the formation of multilayers by the self-assembly of polyelectrolytes. The structure and forces governing the formation of the materials will also be discussed. The next section focussed on methods for interrogating these films to determine their selectivity and activity. Interrogation methods to be covered will include electrochemical measurements, optical measurements, quartz crystal microbalance, surface plasmon resonance and other techniques. The final section is dedicated to the functionality of these films, incorporation of biomolecules within these films and their effect on film structure. Species for incorporation will include antibodies, enzymes, proteins and DNA. Discussions on the location, availability, activity and stability of the included species are included. The review finishes with a short consideration of future research possibilities and applications of these films.

Nanoscale neuro-integrative coatings for neural implants

Silicon microelectrode arrays (Si MEAs) have great potential in enabling chronic in vivo recording of neural activity, but this potential has been hampered by scar tissue formation at the site of implantation. In this study, we report the fabrication and characterization of nanoscale coatings that have the potential of enhancing the biocompatibility of Si electrodes. We use electrostatic layer-by-layer (LbL) assembly to prepare nanoscale bioactive coatings on silicon substrates. We use the response of chick cortical neurons to these coatings to assess potential improvement in biocompatibility in vitro. The coatings are built on oxide covered silicon wafers by alternating polycations, polyethyleneimine (PEI) or chitosan (CH), with polyanions, either gelatin or laminin (LN). We use quartz crystal microbalance (QCM) to characterize the coatings. Our analysis confirms that we achieved approximately 30-110 angstroms scale coatings via LbL assembly. In contrast to bare oxide covered silicon, coated substrates had significantly enhanced chick cortical neuron adhesion and differentiation, with multilayers of PEI-LN showing the greatest improvement. The multilayers of PEI-LN were stable for at least 7 days in physiological conditions, as determined by an enzyme-linked immunosorbent assay (ELISA). In addition, impedance spectroscopy confirmed that multilayers of PEI and LN did not increase the magnitude of impedance of Si MEAs at the biologically relevant frequency of 1 kHz. Our study demonstrates that electrostatic LbL assembly enables nanoscale bioactive coatings, and that PEI-LN multilayers significantly enhance cortical neuronal attachment and differentiation in vitro with no deleterious effects on impedance of the electrodes. Such well-controlled nanoscale coatings have the potential to significantly impact the compatibility and performance of Si MEAs in vivo.

Disintegration of Layer-by-Layer Assemblies Composed of 2-Iminobiotin-Labeled Poly(ethyleneimine) and Avidin

A layer-by-layer thin film composed of avidin and 2-iminobiotin-labeled poly(ethyleneimine) (ib-PEI) was prepared and their sensitivity to the environmental pH and biotin was studied. The avidin/ib-PEI multilayer assemblies were stable at pH 8-12, whereas the assemblies were decomposed at pH 5-6 due to the low affinity of the protonated iminobiotin residue to avidin. The avidin/ib-PEI assemblies can be disintegrated upon addition of biotin and analogues in the solution as a result of the preferential binding of biotin or analogues to the binding site of avidin. The decomposition rate was arbitrarily controlled by changing the type of stimulant (biotin or analogues) and its concentration. The avidin/ib-PEI assemblies were disintegrated rapidly by the addition of biotin or desthiobiotin, whereas the rate of decomposition was rather slow upon addition of lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid. The present system may be useful for constructing the stimuli-sensitive devices that can release drug or other functional molecules.