Diisocyanates as novel molecular binders for monolayer assembly of zeolite crystals on glass

Isocyanate groups readily form urethane linkages with surface hydroxy groups on glass and zeolites and this phenomenon was utilized in the assembly of monolayers of zeolite microcrystals on glass by employing diisocyanates as novel molecular binders.     

Layer-by-layer nanoassembly of polyelectrolytes using formamide as the working medium

Formamide, in its pure state, has been used as a working solvent for layer-by-layer (LbL) polyelectrolyte self-assembly. Polystyrene sulfonate (PSS) and polyallylamine hydrochloride (PAH) polyelectrolyte films were deposited onto planar substrates and colloidal particles. Film deposition was confirmed using quartz crystal microbalance and zeta potential measurements. Formamide was used as an alternative to the water-based working solvents commonly used for LbL self-assembly. Few LbL self-assembly studies using nonaqueous solvents have been reported. Most studies performed with nonaqueous solvents have required the addition of small volumes of water to dissolve the polyelectrolytes. Conversely, the high dielectric constant of pure formamide led to the dissolution and transport of PSS and PAH. Using formamide, it is possible to deposit nanometer thick polyelectrolyte films onto water-sensitive surfaces. Formamide can be thus be used for encapsulating water sensitive hydrogen storage materials within polyelectrolyte films.     

Layer-by-layer assembly of pH-responsive, compositionally controlled (co)polyelectrolytes synthesized via RAFT

Homo- and block copolyelectrolytes that have well-defined structures and are responsive to pH were synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization and employed to produce layer-by-layer (LBL) films. Acrylamido monomers with carboxylate, sulfonate, and amine functionality were utilized to provide both strong and weak homopolyelectrolytes and mixed strong/weak copolyelectrolyte systems. Multilayer films were prepared under specified conditions of pH and ionic strength and analyzed via atomic force microscopy and ellipsometry to study the effects of changes in the local molecular environment on film morphologies. The pH responsiveness and integrity of the multilayer assemblies were investigated by exposing films to solutions of varying pH in a fluid cell and performing in situ AFM analysis. The multilayer dimensions, morphology, and integrity were found to depend on the molecular architecture of the polyelectrolytes, with changes in segmental type and repeating unit distribution producing dramatic differences in film characteristics. These results suggest the possibility of producing LBL assemblies of precisely controlled dimensions and properties by specifically tailoring copolymer structure. To our knowledge, this is the first report of LBL assembly of RAFT-synthesized homo- and copolyelectrolyte multilayer complexes.     

Layer-by-layer assembly on stimuli-responsive microgels

From flat and rigid substrates to hard and rigid particles, the complexity of shape and size of templates have taken new dimensions with the advent of new class of soft nanomaterials obtained from more conventional polymeric structures. This review focuses mainly on the layer-by-layer assembly on soft and porous stimuli-responsive microgels with particular emphasis on recent advances and developments made in recent years.     

Layer-by-layer electrostatic self-assembly of single-wall carbon nanotube polyelectrolytes

We have used anionic and cationic single-wall carbon nanotube polyelectrolytes (SWNT-PEs), prepared by the noncovalent adsorption of ionic naphthalene or pyrene derivatives on nanotube sidewalls, for the layer-by-layer self-assembly to prepare multilayers from carbon nanotubes with polycations, such as poly(diallyldimethylammonium) or poly(allylamine hydrochloride) (PDADMA or PAH, respectively), and polyanions (poly(styrenesulfonate), PSS). This is a general and powerful technique for the fabrication of thin carbon nanotube films of arbitrary composition and architecture and allows also an easy preparation of all-SWNT (SWNT/SWNT) multilayers. The multilayers were characterized with vis-near-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), surface plasmon resonance (SPR) measurements, atomic force microscopy (AFM), and imaging ellipsometry. The charge compensation in multilayers is mainly intrinsic, which shows the electrostatic nature of the self-assembly process. The multilayer growth is linear after the initial layers, and in SWNT/polyelectrolyte films it can be greatly accelerated by increasing the ionic strength in the SWNT solution. However, SWNT/SWNT multilayers are much more inert to the effect of added electrolyte. In SWNT/SWNT multilayers, the adsorption results in the deposition of 1-3 theoretical nanotube monolayers per adsorbed layer, whereas the nominal SWNT layer thickness is 2-3 times higher in SWNT/polyelectrolyte films prepared with added electrolyte. AFM images show that the multilayers contain a random network of nanotube bundles lying on the surface. Flexible polyelectrolytes (e.g., PDADMA, PSS) probably surround the nanotubes and bind them together. On macroscopic scale, the surface roughness of the multilayers depends on the components and increases with the film thickness.     

Layer-by-layer self-assembled multilayers of redox polyelectrolytes and gold nanoparticles

Construction and characterization of structural and charge transport properties of electrostatically LbL self-assembled multilayers of gold nanoparticles and a viologen-based redox-active polyelectrolyte is reported.     

One-dimensional assembly of chalcogenide nanoclusters with bifunctional covalent linkers

Even though different approaches have been developed to achieve various 1D assemblies of nanocrystals, few studies have been done on the assembly of crystallographically well-defined chalcogenide nanoclusters. Here, by using bifunctional organic ligands as the directional linker, a series of one-dimensional assemblies of semiconducting chalcogenide nanoclusters have been prepared and characterized. The synthetic method allows for the preparation of differently sized tetrahedral nanoclusters that are joined together with organic linkers of different length and rigidity. Multiple linking modes between nanoclusters and organic ligands are revealed in four different assemblies that also exhibit size-dependent optical properties.     

Layer-by-layer assembly of polyoxometalates into microcapsules

The polyoxometalate (POM) chemistry world has been experiencing an unparalleled development of rapid synthesis of new compounds and slow development of POM-based functional materials and devices. Meanwhile, researchers in the microcapsule world, encouraged by the introduction of the layer-by-layer method, are pursuing good components for constructing functional capsule devices. Here, in view of the versatile properties that POM-based microcapsules may possess, various types of POM-polyelectrolyte composite microcapsules were constructed using the layer-by-layer method. Microscopy reveals that polyoxometalates form nanoparticles on the shell in the presence of cationic polyelectrolytes. These nanoparticles connected with polyelectrolytes constitute the shell and support the microcapsule from collapse after drying, and this is an interesting characteristic different from those of common composite and polyelectrolyte capsules. Fourier transform infrared (FTIR), UV-vis absorption, and X-ray photoelectron spectroscopy (XPS) were used to examine the properties of the POMs in the microcapsules. The obtained microcapsules exhibit higher thermal stability than polyelectrolyte microcapsules. Furthermore, the functions of POMs were maintained when they were assembled into microcapsules. It is proved that microcapsules bearing POMs with redox activity can provide a reduction environment, which can lead to the realization of in situ synthesis of materials, and that microcapsules with photoluminescent POMs as a component can also have a photoluminescent property, providing a way to develop functional capsule devices. This work may provide an opportunity to enrich both the polyoxometalate chemistry and the capsule field.     

Layer-by-layer assembly of metal-mediated multiporphyrin arrays

Two types of multiporphyrin arrays, mediated by PdCl4(2-) complex ions at the air-water interface, were alternately transferred onto solid supports to form three-dimensional organized multilayers by a layer-by-layer method.     

Layer-by-layer assembly of nacre-like nanostructured composites with antimicrobial properties

In a recent report, we have presented the layer-by-layer (LBL) assembly of a biomimetic nanostructured composite from Na(+)-montmorillonite clay nanosheets and poly(diallylmethylammonium chloride) (Tang, Z.; Kotov, N.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413). The structure, deformation mechanism, and mechanical properties of the material are very similar to those of natural nacre and lamellar bones. This fact prompts further investigation of these composites as potential bone implants. LBL assembly affords preparation of multifunctional composites, and here we demonstrate that not only mechanical strength, but also antibacterial activity, can be introduced in these implantable materials by alternating clay layers with starch-stabilized silver nanoparticles. The resulting composite showed excellent structural stability with no detectable levels of silver lost over a 1 month period. Evaluation of the antibacterial properties showed almost complete growth inhibition of E. coli over an 18 h period. The amount of silver eluted from the LBL composite over a 1 month period was determined to be only 0.5-3.0 microg/L. This concentration of silver did not prevent the growth of the mammalian tissue cultures. The LBL composite has shown biocompatibility with the human osteoblast cell line.     

Layer-by-layer assembly of single-charged ions with a rigid polyampholyte

A novel method to produce multilayer films has been developed by layer-by-layer assembly of single-charged ions and a rigid polyampholyte containing unbalanced charges in each of its repeat units.     

Novel organosilicon dendrons as effective linkers for biomolecules binding on a glass surface

A new, flexible and selective protocol for the synthesis of new organofunctional silicon dendrons of the general formula EtOSi(CH₂CH₂SiMe₂CH₂CH₂SiMe₂R)₃ (where R is an amine or epoxy substituent) has been developed. The title compounds have been efficiently obtained via Karstedt platinum‐catalysed hydrosilylation and linked to a glass surface. Modified solid supports are successfully applied in immobilization of proteins, especially monoclonal antibodies against a wide range of antigens. Copyright © 2015 John Wiley & Sons, Ltd.     

Modelling layer-by-layer assembly of flexible polyelectrolytes

We have modelled the layer-by-layer assembly process of flexible polyelectrolytes on flat surfaces. The multilayer has a three-zone structure. An exponential growth is found for the first several layers, followed by a linear growth for subsequent layers evolving toward a steady state. While adjacent layers are highly interpenetrating, stratification can be seen for every four or more layers. The effects of surface charge density, bulk salt concentration, and solvent quality on the thickness and internal structure of the multilayer are also studied. Our results agree with experimental findings.     

Layer-by-layer self-assembly of single-walled carbon nanotubes with amine-functionalized weak polyelectrolytes for electrochemically tunable pH sensitivity

The layer-by-layer (LbL) assembly of carboxylated single-walled carbon nanotubes (SWCNT) is demonstrated to tune the electrochemical pH sensitivity of thin-film devices. The positively charged amine containing weak polyelectrolyte (wPE) is used as a counter species to control the proximal ions. The LbL assembly process is monitored by the quartz crystal microbalance, which results in the linear growth of a multilayer. The amount adsorbed is strongly dependent on the surface charge of previously deposited species. However, the thickness of the multilayer is determined by both the amount adsorbed and the coiling of polyelectrolyte chains. Indeed, electrical and structural characteristics of the (wPE/SWCNT) multilayer thin film are obtained according to the acid dissociation constants of amino groups in wPE. The electrochemical pH sensitivity in the physiological range demonstrates the effects of both charge carrier doping/trapping and proximal ions on the conductance of the SWCNT multilayer. Although doping/trapping shows the decreasing conductance, the proximal ion effect reveals the increasing conductance with pH in the basic region as a result of the p-type semiconducting nature of SWCNTs and the ability of wPE to capture hydrogen ions. This work sheds light on the applicability of nanostructured and/or engineered functional thin films of SWCNTs as chemical and biological sensors.     

Layer-by-layer assembly of ordinary and composite coordination multilayers

Coordination self-assembly of bishydroxamate-based metal-organic multilayers on gold employing a layer-by-layer (LbL) approach was investigated. It is shown that the solution chemistry of the participating metal ion has a marked influence on the composition and properties of the multilayers. Use of Ce4+ and particularly zirconium(IV) acetylacetonate (Zr(acac)4) solutions in the ion-binding step of multilayer construction leads to multilayers with a near-stoichiometric metal ion-to-ligand ratio, suggesting a structure close to that predicted by a simple coordination self-assembly scheme. On the other hand use of a ZrCl4 solution as the source of metal ions in the multilayer construction leads to a multilayer with greater thickness and a large excess of Zr(IV), evenly distributed between the organic layers. In the latter case, a ratio of ca. 1:2 between the excess Zr and oxygen, as well as long-term Zr4+ binding experiments showing deposition of ZrO2, suggest the formation of a zirconia-type nanophase between the bishydroxamate organic repeat units during multilayer self-assembly. Hence, while the multilayer prepared using Zr(acac)4 solution appears to represent a "true" coordination-based structure, the one prepared using ZrCl4 is best described as a composite organic-ceramic multilayer. Composite multilayers prepared in this way display different properties from those of the stoichiometric ones, such as improved dielectric behavior and higher stiffness. Even greater mechanical stability is obtained with multilayers constructed using alternate binding of ZrCl4 and Ce4+. The concept of LbL formation of coordination-based composite organic-ceramic structures may be useful in obtaining nanometer-scale structures with tunable properties.     

Layer-by-layer electrochemical assembly of poly(diphenylamine)/phosphotungstic acid as ascorbic acid sensor

A layer-by-layer (LbL) film assembly was constructed that comprises alternative layers of poly(diphenylamine) (PDPA) and phosphotungstic acid (PTA). First, a layer of oxidized PDPA (referred to as PDPA(+)) was deposited by electropolymerization. Then, a layer of negatively charged PTA was deposited on the PDPA(+) layer . This processes was repeated several times to obtain multilayer LbL film (PDPA/PTA)_n, where n is the number of double layers. The LbLs were characterized by UV-Vis spectroscopy, FT-IR spectroscopy and X- ray diffraction spectroscopy. The process of formation of the LbL assembly was monitored by electrochemical methods. Electrochemical studies revealed that this LbL film possesses a remarkable electrocatalytic activity towards oxidation of ascorbic acid in neutral aqueous medium. The enhanced electrocatalytic activity of (PDPA/PTA)_n LbL film is attributed to the existence of tungsten atoms in the interlayers of PDPA.     

Layer-by-layer assembly of microcapsules and their biomedical applications

Nanoengineered multifunctional capsules with tailored structures and properties are of particular interest due to their multifunctions and potential applications as new colloidal structures in diverse fields. Among the available fabrication methods, the layer-by-layer (LbL) assembly of multilayer films onto colloidal particles followed by selective template removal has attracted extensive attention due to its advantages of precise control over the size, shape, composition, wall thickness and functions of the obtained capsules. The past decade has witnessed a rapid increase of research concerning the new fabrication strategies, functionalization and applications of this kind of capsules, particularly in the biomedical fields such as drug delivery, biosensors and bioreactors. In this critical review, the very recent progress of the multilayer capsules is summarized. First, the advances in assembly of capsules by the LbL technique are introduced with focus on tailoring the properties of hydrogen-bonded multilayer capsules by cross-linking, and fabrication of capsules based on covalent bonding and bio-specific interactions. Then the fabrication strategies which can speed up capsule fabrication are reviewed. In the following sections, the multi-compartmental capsules and the capsules that can transform their shape under stimulus are presented. Finally, the biomedical applications of multilayer capsules with particular emphasis on drug carriers, biosensors and bioreactors are described (306 references).     

Fabrication of oriented zeolite L monolayer via covalent molecular linkers

For the first time, 1,4-diisocyanatobutane (DICB) was used as the covalent molecular linker in this study to prepare the uniformly oriented zeolite L monolayer with relatively high coverage degree and close packing degree. This could be ascribed to substantial amounts of DICB self-assemble and standing on the substrate surface instead of folding up into a U-shape. This point has been further verified by the quality of oriented zoelite L monolayers obtained from the procedure involving DICB, 1, 4-bis (triethoxysilyl) benzene (BTSEB) and 1,2-bis(trimethoxysilyl)ethane (BTMSE) as covalent molecular linkers.     

Growth of zeolite crystals with graphene oxide nanosheets

Growth of silicalite with graphene oxide (GO) nanosheets occurred via attachment of GO onto the silicalite surface, and entrapment of GO nanosheets inside single crystals. Electrically conductive composites were produced by calcination in nitrogen whereas silicalite crystals with slit-like mesopores of sizes 20-25 ? were obtained after GO burn-off.     

Layer-by-layer self-assembly of multiwalled carbon nanotube polyelectrolytes prepared by in situ radical polymerization

Anionic and cationic multiwalled carbon nanotube polyelectrolytes, prepared by covalent modification of multiwalled carbon nanotubes (MWCNTs) with poly(acrylic acid) and poly(acrylamide), were used for the layer-by-layer (LBL) self-assembly of MWCNTs on different substrates with polyelectrolytes, such as poly(diallyldimethylammonium chloride) and sodium poly(styrenesulfonate). Thermogravimetric analysis, Raman spectroscopy, and scanning electron microscopy (SEM) were used to demonstrate the modification of MWCNTs. Investigations using Fourier transform infrared spectroscopy, atomic force microscopy, SEM, and ultraviolet-visible spectroscopy proved this method to be practicable for preparing LBL films.