Single Polyelectrolyte Microcapsules Fabricated By Glutaraldehyde‐Mediated Covalent Layer‐By‐Layer Assembly

Summary: Single polyelectrolyte component microcapsules and multilayers, exemplified by poly(allylamine hydrochloride) (PAH), have been prepared using a method of glutaraldehyde (GA)‐mediated covalent layer‐by‐layer (LbL) assembly. The GA cross‐linking of the adsorbed PAH results in surfaces covered by reactive aldehyde groups, which can then react with PAH to result in another layer of covalently linked PAH. The repeated assembly of single polyelectrolyte in an LbL manner can be thus achieved. The PAH multilayers can grow linearly along with the layer number, and their thickness can be controlled at the nanometer scale, as verified by UV‐vis absorption spectrometry and ellipsometry. Single polyelectrolyte microcapsules are obtained after removal of the template cores at low pH. The morphology and integrity are confirmed by scanning force microscopy and confocal laser scanning microscopy.

Schematic illustration of the preparation of a single polyelectrolyte component microcapsule by GA‐mediated covalent LbL assembly.     

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 Assembled Nanotubes as Biomimetic Nanoreactors for Calcium Carbonate Deposition

Enzyme‐loaded magnetic polyelectrolyte multilayer nanotubes prepared by layer‐by‐layer assembly combined with the porous template could be used as biomimetic nanoreactors. It is demonstrated that calcium carbonate can be biomimetically synthesized inside the cavities of the polyelectrolyte nanotubes by the catalysis of urease, and the size of the calcium carbonate precipitates was controlled by the cavity dimensions. The metastable structure of the calcium carbonate precipitates inside the nanotubes was protected by the outer shell of the polyelectrolyte multilayers. These features may allow polyelectrolyte nanotubes to be applied in the fields of nanomaterials synthesis, controlled release, and drug delivery.

     

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.     

Melanin‐Containing Films: Growth from Dopamine Solutions versus Layer‐by‐Layer Deposition

Films formed by oxidation of dopamine are of interest for functionalisation of solid–liquid interfaces owing to their versatility. However, the ability to modulate the properties of such films, for example, permeability to ionic species and the absorption coefficient, is urgently needed. Indeed, melanin films produced by oxidation of dopamine absorb strongly over the whole UV/Vis part of the electromagnetic spectrum and are impermeable to anions even for a film thickness as low as a few nanometers. Herein we combine oxidation of dopamine to produce a solution containing dopamine–melanin particles and their alternating deposition with poly(diallyldimethylammonium chloride) to produce films which have nearly the same morphology as pure dopamine–melanin films but are less compact, more transparent and more permeable to ferrocyanide anions.     

Immobilization of Enzymes on PLGA Sub‐Micrometer Particles by Crosslinked Layer‐by‐Layer Deposition

Enzyme immobilization is of high interest for industrial applications. However, immobilization may compromise enzyme activity or stability due to the harsh conditions which have to be applied. The authors therefore present a new and improved crosslinked layer‐by‐layer (cLbL) approach. Two different model enzymes (acid phosphatase and β‐galactosidase) are immobilized under mild conditions on biocompatible, monodisperse, sub‐micrometer poly(lactide‐co‐glycolide) (PLGA) particles. The resulting PLGA enzyme systems are characterized regarding their size, surface charge, enzyme activity, storage stability, reusability, and stability under various conditions such as changing pH and temperature. The developed and characterized cLbL protocol can be easily adapted to different enzymes. Potential future uses of the technology for biomedical applications are discussed. PLGA‐enzyme particles are therefore injected into the blood circulation of zebrafish embryos in order to demonstrate the in vivo stability and activity of the designed system.

     

Bio‐Inspired Silicification on Patterned Surfaces Generated by Microcontact Printing and Layer‐by‐Layer Self‐Assembly

Micropatterns of silica were generated under biocompatible conditions by a combination of microcontact printing (μCP), layer‐by‐layer (LbL) self‐assembly, and biomimetic silicification. Quaternary amine‐containing poly(diallyl dimethyl ammonium chloride) induced polycondensation of silicic acid, resulting in spatioselective formation of silica micropatterns. Scale bar: 10 μm.

     

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).     

Layer‐by‐Layer Spin Self‐Assembled Hole Injection Layers Containing a Perfluorinated Ionomer for Efficient Polymer Light‐Emitting Diodes

Deposition of hole injection layers including a perfluorinated ionomer has been demonstrated using layer‐by‐layer spin self‐assembly for enhanced device efficiency and lifetime in PLEDs. We show that the LBL spin self‐assembled thin films enable to control work functions of indium‐tin oxide anodes by changing the PFI concentration and that a resulting green‐emitting device has an enhanced luminescence efficiency and 18 times longer half lifetime than a device using a conventional HIL. We also fabricate a gradient of energy levels by the LBL self‐assembly of the PFI that results in a work function of 5.74 eV, which can be used to improve carrier injection even for an emitting layer whose ionization potential is over 5.7 eV.

     

Nitrogen‐Doped Carbon Capsules via Poly(ionic liquid)‐Based Layer‐by‐Layer Assembly

Layer‐by‐layer (LbL) assembly technique is applied for the first time for the preparation of nitrogen‐doped carbon capsules. This approach uses colloid silica as template and two polymeric deposition components, that is, poly(ammonium acrylate) and a poly (ionic liquid) poly(3‐cyanomethyl‐1‐vinylimidazolium bromide), which acts as both the carbon precursor and nitrogen source. Nitrogen‐doped carbon capsules are prepared successfully by polymer wrapping, subsequent carbonization and template removal. The as‐synthesized carbon capsules contain ≈7 wt% of nitrogen and have a structured specific surface area of 423 m² g⁻¹. Their application as supercapacitor has been briefly introduced. This work proves that LbL assembly methodology is available for preparing carbon structures of complex morphology.     

Layer‐by‐Layer Assembly of Polyelectrolyte and Nanoparticles,Monitored by Capillary Electrophoresis

Layer‐by‐layer (LBL) assembly is a versatile nanofabrication technique, and investigation of its kinetics is essential for understanding the assembly mechanism and optimizing the assembly procedure. In this work, the LBL assembly of polyelectrolyte and nanoparticles were monitored in situ by capillary electrophoresis (CE) for the first time. The assembly of poly(diallyldimethylammonium chloride) (PDDA), and gold nanoparticles (AuNPs) on capillary walls causes surface‐charge neutralization and resaturation, and thus yields synchronous changes in the electroosmotic flow (EOF). The EOF data show that formation of multilayers follows first‐order adsorption kinetics. On the basis of the fit results, influencing factors, including number of layers, concentration of materials, flow rate, and size of AuNPs, were investigated. The stability and robustness of the assembled coatings were also characterized by CE. It was found that degradation of PDDA layers follows first‐order chemical kinetics, while desorption of AuNPs takes place in a disorderly manner. The substrate strongly affects assembly of the underlying layer, while this effect is rapidly screened with increasing number of layers. Furthermore, we demonstrate that the EOF measuring step does not disturb LBL assembly, and the proposed method is reliable and rugged. This work not only studies in detail the LBL adsorption/desorption process of polyelectrolyte and nanoparticles, but also offers an alternative tool for monitoring multilayer buildup. It may also reveal the potential of CE in fields other than analytical separation.     

Thin‐Layer Spectroelectrochemistry on an Aqueous Microdrop

Here we report a technique to perform thin layer spectroelectrochemistry using an aqueous microdrop. The chemical systems used to demonstrate the aqueous microdrop technique were an absorption based ionic probe [Fe(CN)₆]^3?/4? and an emission based ionic probe [Ru(bpy)₃]^3+/2+. The ability of the technique to perform semi‐infinite linear diffusion spectroelectrochemistry on an aqueous microdrop has been previously demonstrated; in this work we were able to demonstrate spectroelectrochemical behavior consistent with the restricted diffusion in a thin layer cell by reducing the analyte volume and the optical path length. The thin‐layer diffusion behavior was illustrated by substantial reduction in peak‐to‐peak separations of the cyclic voltammograms and the significant decrease in electrolysis time compared to the semi‐infinite linear diffusion behavior.     

Layer‐by‐Layer Nano‐Encapsulation of Microbes: Controlled Cell Surface Modification and Investigation of Substrate Uptake in Bacteria

LbL nano self‐assembly coating of A. vinosum with different polyelectrolyte combinations is presented as an example to investigate substrate uptake in bacteria. The effects of surface charge and the formation of a physical barrier provides new insights in the contact mechanisms between the cell surface and insoluble elemental sulfur. Furthermore, uptake of sulfide by encapsulated cells was investigated. Growth experiments of coated cells showed that surface charge did neither affect sulfide uptake nor the contact formation between the cells and solid sulfur. However, increasing layers slowed or inhibited the uptake of sulfide and elemental sulfur. This work demonstrates how defining surface properties of bacteria has potential for microbiological and biotechnological applications.

     

Ordered Poly(p‐phenylene)/Layered Double Hydroxide Ultrathin Films with Blue Luminescence by Layer‐by‐Layer Assembly

Lavender layers : A poly(p‐phenylene) anionic derivate and exfoliated Mg‐Al layered double hydroxide monolayers were assembled into ultrathin films with well‐defined blue fluorescence (see picture; the numbers indicate the number of bilayers), long‐range order, and high photostability. These films work as multiple quantum‐well structures for valence electrons.

     

Layer‐by‐Layer Assemblies of Catechol‐Functionalized TiO2 Nanoparticles and Porphyrins through Electrostatic Interactions

In the current work, we present the successful functionalization and stabilization of P‐25 TiO₂ nanoparticles by means of N1,N7‐bis(3‐(4‐tert‐butyl‐pyridium‐methyl)phenyl)‐4‐(3‐(3‐(4‐tert‐butyl‐pyridinium‐methyl)phenylamino)‐3‐oxopropyl)‐4‐(3,4‐dihydroxybenzamido)heptanediamide tribromide ( 1 ). The design of the latter is aimed at nanoparticle functionalization and stabilization with organic building blocks. On one hand, 1 features a catechol anchor to enable its covalent grafting onto the TiO₂ surface, and on the other hand, positively charged pyridine groups at its periphery to prevent TiO₂ agglomeration through electrostatic repulsion. The success of functionalization and stabilization was corroborated by thermogravimetric analysis, dynamic light‐scattering, and zeta potential measurements. As a complement to this, the formation of layer‐by‐layer assemblies, which are governed by electrostatic interactions, by alternate deposition of functionalized TiO₂ nanoparticles and two negatively charged porphyrin derivatives, that is, 5,10,15,20‐(phenoxyacetic acid)‐porphyrin ( 2 ) and 5,10,15,20‐(4‐(2‐ethoxycarbonyl)‐4‐(2‐phenoxyacetamido)heptanedioic acid)‐porphyrin ( 3 ), is documented. To this end, the layer‐by‐layer deposition is monitored by UV/Vis spectroscopy, scanning electron microscopy, ellipsometry, and profilometry techniques. The resulting assemblies are utilized for the construction and testing of novel solar cells. From stable and repeatable photocurrents generated during several “on‐off” cycles of illumination, we derive monochromatic incident photo‐to‐current conversion efficiencies of around 3 %.     

Nanoparticulate Hollow TiO2 Fibers as Light Scatterers in Dye‐Sensitized Solar Cells: Layer‐by‐Layer Self‐Assembly Parameters and Mechanism

Hollow structures show both light scattering and light trapping, which makes them promising for dye‐sensitized solar cell (DSSC) applications. In this work, nanoparticulate hollow TiO₂ fibers are prepared by layer‐by‐layer (LbL) self‐assembly deposition of TiO₂ nanoparticles on natural cellulose fibers as template, followed by thermal removal of the template. The effect of LbL parameters such as the type and molecular weight of polyelectrolyte, number of dip cycles, and the TiO₂ dispersion (amorphous or crystalline sol) are investigated. LbL deposition with weak polyelectrolytes (polyethylenimine, PEI) gives greater nanoparticle deposition yield compared to strong polyelectrolytes (poly(diallyldimethylammonium chloride), PDDA). Decreasing the molecular weight of the polyelectrolyte results in more deposition of nanoparticles in each dip cycle with narrower pore size distribution. Fibers prepared by the deposition of crystalline TiO₂ nanoparticles show higher surface area and higher pore volume than amorphous nanoparticles. Scattering coefficients and backscattering properties of fibers are investigated and compared with those of commercial P25 nanoparticles. Composite P25–fiber films are electrophoretically deposited and employed as the photoanode in DSSC. Photoelectrochemical measurements showed an increase of around 50 % in conversion efficiency. By employing the intensity‐modulated photovoltage and photocurrent spectroscopy methods, it is shown that the performance improvement due to addition of fibers is mostly due to the increase in light‐harvesting efficiency. The high surface area due to the nanoparticulate structure and strong light harvesting due to the hollow structure make these fibers promising scatterers in DSSCs.