Electrochemical Biosensors Based on Layer‐by‐Layer Assemblies

Layer‐by‐layer (LBL) assemblies, which have undergone great progress in the past decades, have been used widely in the construction of electrochemical biosensors. The LBL assemblies provide a strategy to rationally design the properties of immobilized films and enhance the performance of biosensors. The following review focuses on the application of LBL assembly technique on electrochemical enzyme biosensors, immunosensors and DNA sensors.     

Spontaneous Birefringence in Layer‐by‐Layer Films of Chitosan and Azo Dye Sunset Yellow

Layer‐by‐layer (LBL) films consisting of layers of the azo dye Sunset Yellow alternated with chitosan display spontaneous birefringence, which is attributed to the film anisotropy imparted by the LBL method. This is unusual for azobenzene‐containing materials as they normally form films with randomly oriented molecules, presenting birefringence only due to photoinduced isomerization cycles. Spontaneous birefringence does not appear in cast films, but occurs for LBL films obtained under various experimental conditions.

Chemical structures of (a) Sunset Yellow and (b) chitosan.     

Molecularly Selective Regulation of Delivery Fluxes by Employing Supramolecular Interactions in Layer‐by‐Layer Films

The molecularly selective regulation of molecular fluxes in a biomaterial that delivers multiple chemical species simultaneously is still beyond the reach of materials scientists. A delivery material was developed by means of the layer‐by‐layer (LbL) technique. This material discriminatively regulates the delivery flux of bioactive small molecules, as represented by a peptide containing the RGD fragment and the chemotherapy drug doxorubicin (DOX). Molecularly selective flux regulations in LbL films are realized through fast, reversible supramolecular interactions between cyclodextrin and its guests. The mechanism underlining the delivery strategy is that supramolecular interactions promote molecular loading and slow down diffusion‐dependent release. In a preliminary survey of materials parameters, a maximum difference in cell viability between healthy human bronchial epithelial cells and cancer cells (A549) was realized.     

Biocatalytic Performance of pH‐Sensitive Magnetic Nanoparticles Derived from Layer‐by‐Layer Ionic Self‐Assembly of Chitosan with Glucoamylase

Based on the characteristics of polycations of chitosan and glucoamylase, which are oppositely charged, they were successfully alternatingly deposited onto the surface of aldehyde‐modified Fe₃O₄ nanoparticles by using a layer‐by‐layer ion exchange method to form magnetic carriers to construct multilayer films (designated as Fe₃O₄@(CS/GA)_n). The (CS/GA)_n film systems were endowed with the pH‐dependent properties of chitosan as well as the catalytic activity of glucoamylase. The changes in weight loss and surface chemistry, morphology, and magnetic sensitivity were monitored and verified by UV/Vis spectroscopy, zeta potential, TEM, and a vibrating sample magnetometer. Subsequently, the influence of the number of bilayers, storage stability, pH, temperature, and reusability of Fe₃O₄@(CS/GA)₅ biocatalysts on catalytic activity were investigated. The results from characterization and determination remarkably indicate that Fe₃O₄@(CS/GA)₅ presents excellent catalytic activity, storage stability, pH stability, and reusability in comparison with free enzyme. Fe₃O₄@(CS/GA)₅ retained >60 % of its initial activity at 65 °C over 6 h; the optimum temperature and pH also increased to the ranges of 45–65 °C and 2.5–3.5, respectively, and only 27 % activity was lost after 10 cycles. This new strategy simplifies the reaction protocol and improves encapsulation efficiency and catalytic activity for new potential applications in biotechnology.     

Magnetically Encoded Luminescent Composite Nanoparticles through Layer‐by‐Layer Self‐Assembly

Sensitive and rapid detection of multiple analytes and the collection of components from complex samples are important in fields ranging from bioassays/chemical assays, clinical diagnosis, to environmental monitoring. A convenient strategy for creating magnetically encoded luminescent CdTe@SiO₂@n Fe₃O₄ composite nanoparticles, by using a layer‐by‐layer self‐assembly approach based on electrostatic interactions, is described. Silica‐coated CdTe quantum dots (CdTe@SiO₂) serve as core templates for the deposition of alternating layers of Fe₃O₄ magnetic nanoparticles and poly(dimethyldiallyl ammonium chloride), to construct CdTe@SiO₂@n Fe₃O₄ (n=1, 2, 3, …?) composite nanoparticles with a defined number (n) of Fe₃O₄ layers. Composite nanoparticles were characterized by zeta‐potential analysis, fluorescence spectroscopy, vibrating sample magnetometry, and transmission electron microscopy, which showed that the CdTe@SiO₂@n Fe₃O₄ composite nanoparticles exhibited excellent luminescence properties coupled with well‐defined magnetic responses. To demonstrate the utility of these magnetically encoded nanoparticles for near‐simultaneous detection and separation of multiple components from complex samples, three different fluorescently labeled IgG proteins, as model targets, were identified and collected from a mixture by using the CdTe@SiO₂@n Fe₃O₄ nanoparticles.     

Construction of Polycation‐Based Non‐Viral DNA Nanoparticles and Polyanion Multilayers via Layer‐by‐Layer Self‐Assembly

Summary: The multilayers of polycation‐based non‐viral DNA nanoparticles and biodegradable poly(L ‐glutamic acid) (PGA) were constructed by a layer‐by‐layer (LbL) technique. Poly(ethyleneimine) (PEI) was used to condense DNA to develop non‐viral DNA nanoparticles. AFM, UV‐visible spectrometry, and TEM measurements revealed that the PEI‐DNA nanoparticles were successfully incorporated into the multilayers. The well‐structured, easily processed multilayers with the non‐viral DNA nanoparticles may provide a novel approach to precisely control the delivery of DNA, which may have great potential for gene therapy applications in tissue engineering, medical implants, etc.

A TEM image of the cross section of a (PGA/PEI‐DNA nanoparticle)₂₀ multilayer.     

Layer‐by‐Layer Approach to Superhydrophobic Zeolite Antireflective Coatings

Antireflection surfaces and coatings have attracted considerable interests because they can maximize light transmittance of the substrates. In this work, zeolite antireflective (ZAR) coatings are prepared via layer‐by‐layer (LBL) assembly of MFI ‐type zeolite silicalite‐1 and polyelectrolyte. A micro‐ and macroporous hierarchical structure was obtained which contributes to the antireflective property of the zeolite coatings. The light transmittance of the coating on quartz can achieve as high as 99.3% at 650 nm. Furthermore, a superhydrophobic ZAR coating can be obtained by chemical modification with 1H,1H,2H,2H–perfluorooctyl‐triethoxysilane. This work demonstrates that zeolites are excellent candidates as high transparent superhydrophobic coatings.     

Layer‐by‐layer assembly of weak‐strong copolymer polyelectrolytes: A route to morphological control of thin films

Multilayer films were assembled from a strong polyelectrolyte (poly(diallyldimethylammonium chloride), PDADMAC) and a copolymer containing both strongly charged styrene sulfonate moieties and weakly charged maleic acid moieties (poly(4‐styrenesulfonic acid‐co‐maleic acid), PSSMA). Growth of PSSMA/PDADMAC multilayers was linear, as characterized by UV‐vis spectroscopy and quartz crystal microgravimetry. The influence of both the pH of the PSSMA adsorption solutions and the ratio of SS:MA in the PSSMA on multilayer properties was investigated. Fourier transform infrared spectroscopy results showed that the ionization of carboxylic acid groups in PSSMA/PDADMAC multilayers did not vary significantly with changes in the PSSMA assembly pH. However, the multilayers showed different thicknesses, surface morphologies, and stability to post‐assembly pH treatment. We also demonstrate that PSSMA/PDADMAC multilayers are significantly more stable than PSSMA/PAH multilayers after post‐assembly pH treatment (i.e. the films remain intact when exposed to pH extremes). In addition, the surface morphology of two films (PSSMA 1:1 assembled at pH 5.8, post‐treated at pH 2 and PSSMA 3:1 assembled at pH 5.8, post‐treated at pH 11) changed significantly when the films were exposed to solutions of different pH and, in the former case, this change in film morphology was reversible. The porous morphology after treatment at pH 2 could be reversed to give a significantly smoother film after subsequent exposure to water for 24 h. Our results demonstrate that by the rational choice of the assembly pH of PSSMA, stable and pH‐responsive films can be obtained via the sequential assembly of PSSMA and PDADMAC. These films have potential in controlled release applications where film stability and pH‐responsive behavior are essential. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4341‐4351, 2007     

Photocatalytic Nanocomposite Films Fabricated by Layer‐by‐Layer Self‐assembly of TiO2 Nanoparticles and Lignosulfonates

Photocatalytic multilayer nanocomposite films composed of anatase TiO₂ nanoparticles and lignosulfonates (LS) were fabricated on quartz slides by the layer‐by‐layer (LBL) self‐assembly technique. X‐ray photoelectron spectroscopy (XPS), UV‐vis spectroscopy and atomic force microscopy (AFM) were used to characterize the TiO₂/LS multilayer nanocomposite films. Moreover, the photocatalytic properties (decomposition of methyl orange and bacteria) of multilayer nanocomposite films were investigated. XPS results indicated that the intensities of titanium and sulfur peaks increased with the LBL deposition process. A linear increase in absorbance at 280 nm was found by UV‐Vis spectroscopy, suggesting that stepwise multilayer growth occurs on the substrate and this deposition process is highly reproducible. AFM images showed that quartz slide was completely covered by TiO₂ nanoparticles when a 10‐bilayer multilayer film was formed. The decomposition efficiency of methyl orange by TiO₂/LS multilayer films under the same UV irradiation time increased linearly with the number of TiO₂ layers, and the results of decomposition of bacteria under UV irradiation showed that TiO₂/LS multilayer nanocomposite films exhibited excellent decomposition activity of bacteria (Escherichia coil).     

Hyperthin Membranes for Gas Separations via Layer‐by‐Layer Assembly

Thin film formation via the Layer‐by‐Layer method is now a well‐established and broadly used method in materials science. We have been keenly interested in exploiting this technique in the area of gas separations. Specifically, we have sought to create hyperthin (<100 nm) polyelectrolyte‐based membranes that have practical potential for the separation of CO₂ from N₂ (flue gas) and H₂ from CO₂ (syngas). In this personal account, we summarize recent studies that have been aimed at measuring the influence of a variety of factors that can affect the permeability and permeation selectivity of hyperthin polyelectrolyte multilayers (PEMs).     

A Novel Sensor Based on Layer‐by‐Layer Hybridized Phosphomolybdate and Poly(ferrocenylsilane) on a Cysteamine Modified Gold Electrode

A novel sensor have been constructed by layer‐by‐layer hybridizing phosphomolybdate (POM) and poly(ferrocenylsilane) (PFS) on a cysteamine modified gold electrode. The properties and performance of the sensor have been measured by electrochemistry and atomic force microscopy in detail. The results showed that the constructed multilayers modified gold electrode combined the properties of POM and PFS, and exhibited good electrocatalytic ability to a series of inorganic ions, including BrO , IO , NO , Fe³⁺, ascorbic acid and SO . The well catalytic activity of the sensor was ascribed to the porous structure of hybrid POM‐PFS multilayer. The resulted sensor exhibited extremely fast amperometric response, low detection limit, high selectivity and wide linear range to these analyses.     

New nanohybrids of poly(ε‐caprolactone) and a modified Mg/Al hydrotalcite: Mechanical and thermal properties

Novel composites based on poly(ε‐caprolactone) (PCL) and an organically modified layer double hydroxide (LDH) obtained using the melt‐extrusion technique have been characterized through structural, thermal, and mechanical analyses. Although exfoliation has not been achieved and despite the very low content of filler (from 1 to 3% by weight), significant enhancements are obtained in the physical and mechanical properties of the composites with respect to neat PCL. As a consequence, LDHs can substitute other nanofillers, in particular, cationic clays for polymeric matrices. They can be modified by a large number of organic anions, generally more numerous than the cationic ones, and can be mixed in very simple ways with polymers. This makes such nanofillers suitable to obtain new hybrid materials for a series of applications, from active food packaging to intelligent materials for biomedical device, for example, controlled drug release. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 945–954, 2007     

Kinetically Controlled Layer‐by‐Layer Stacking of Metal Oxide 2D Nanosheets

An efficient chemical way to finely control the layer‐by‐layer stacking of inorganic nanosheets (NS) is developed by tuning the type and composition of intercalant ion, and the reaction temperature for restacking process. The finely controlled stacking of NS relies on a kinetic control of the self‐assembly of NS in the presence of coordinating organic cations. A critical role of organic cations in this assembly highlights the importance of the appropriate activation energy. Of prime importance is that a fine‐control of the interstratification of 2D NS is highly effective not only in tailoring its pore structure but also in enhancing its electrode activity. The present study clearly demonstrates that the kinetically controlled restacking of NS provides a facile and powerful method to tailor their stacking number and functionality.     

Preconcentration and Determination of a Phospholipid at a Surface Modified by Layer‐by‐Layer Assembly

Electrochemical oxidation of a phospholipid, phosphatidylcholine (PC), was accomplished at a 4‐aminothiophenol (ATP)‐modified gold electrode coated with a layer‐by‐layer assembly of an electrochemical catalyst (dirhodium phosphomolybdic acid), a trapping agent for PC (a cyclophane, CP, derivative, 1,4‐xylylene‐1,4‐phenylene‐diacetate), and a spacer (generation‐4 polyamidoamine dendrimer, PAMAM). The layer‐by‐layer assembly process and the trapping of PC was verified by quartz crystal microbalance measurements; Au|ATP|CP|PAMAM|CP trapped (1.5±0.4)×10^?9 mol cm^?2 of PC. The electrocatalytic oxidation of PC yielded a current that varied linearly with concentration over the range 1–50 μM; the R² value was 0.996.     

Theranostic Layer‐by‐Layer Nanoparticles for Simultaneous Tumor Detection and Gene Silencing

Layer‐by‐layer nanoparticles (NPs) are modular drug delivery vehicles that incorporate multiple functional materials through sequential deposition of polyelectrolytes onto charged nanoparticle cores. Herein, we combined the multicomponent features and tumor targeting capabilities of layer‐by‐layer assembly with functional biosensing peptides to create a new class of nanotheranostics. These NPs encapsulate a high weight percentage of siRNA while also carrying a synthetic biosensing peptide on the surface that is cleaved into a urinary reporter upon exposure to specific proteases overexpressed in the tumor microenvironment. Importantly, this biosensor reports back on a molecular signature characteristic to metastatic tumors and associated with poor prognosis, MMP9 protease overexpression. This nanotheranostic mediates noninvasive urinary‐based diagnostics in mouse models of three different cancers with simultaneous gene silencing in flank and metastatic mouse models of ovarian cancer.