Covalent layer-by-layer assembly of redox active molecular multilayers on silicon (100) by photochemical thiol-ene chemistry

The fabrication of thin organic films covalently grafted onto silicon substrates is of significant interest, as they are expected to give access to a broad range of new materials for integration into microelectronic applications. Covalent layer-by-layer (LbL) assembly offers a high degree of freedom when designing such thin films. In this work an approach for the preparation of covalent redox active molecular multilayers on silicon (100) surfaces is presented using a highly branched decaallylferrocene and thiol-ene click chemistry. The multilayers are analyzed by ellipsometry, X-ray photoelectron sprectroscopy, and cyclic voltammetry. The results indicate that the multilayer growth is linear for at least sixteen layers and the density of ferrocenes per layer is in the range of 6 × 10?11 mol cm?2. Moreover, this method for LbL assembly is extended to surfaces which have been locally passivated by microcontact printing. By atomic force microscopy measurements it is possible to show that the covalent LbL deposition proceeds exclusively in the nonpassivated areas.     

Layer‐by‐Layer Hydrogen‐Bonded Polymer Films: From Fundamentals to Applications

Recent years have seen increasing interest in the construction of nanoscopically layered materials involving aqueous‐based sequential assembly of polymers on solid substrates. In the booming research area of layer‐by‐layer (LbL) assembly of oppositely charged polymers, self‐assembly driven by hydrogen bond formation emerges as a powerful technique. Hydrogen‐bonded (HB) LbL materials open new opportunities for LbL films, which are more difficult to produce than their electrostatically assembled counterparts. Specifically, the new properties associated with HB assembly include: 1) the ease of producing films responsive to environmental pH at mild pH values, 2) numerous possibilities for converting HB films into single‐ or two‐component ultrathin hydrogel materials, and 3) the inclusion of polymers with low glass transition temperatures (e.g., poly(ethylene oxide)) within ultrathin films. These properties can lead to new applications for HB LbL films, such as pH‐ and/or temperature‐responsive drug delivery systems, materials with tunable mechanical properties, release films dissolvable under physiological conditions, and proton‐exchange membranes for fuel cells. In this report, we discuss the recent developments in the synthesis of LbL materials based on HB assembly, the study of their structure–property relationships, and the prospective applications of HB LbL constructs in biotechnology and biomedicine.     

Construction of pegylated multilayer architectures via (strept)avidin/biotin interactions

Pegylated multilayer architectures were fabricated as films on planar substrates, as shells on colloidal particles, or as free-standing hollow capsules using layer-by-layer (LbL) self-assembly of biotinylated poly-l-lysine (PLL) and (strept)avidin. Poly(ethylene glycol) (PEG) was incorporated into the multilayer architectures by assembly with biotin-derivatized poly(l-lysine)-g-poly(ethylene glycol)(PPB). Stepwise growth of multilayers was followed by UV–vis spectroscopy and the formation of core–shells and hollow capsules characterized by means of confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM). Both absorbance and TEM data suggest that approximately two layers of FITC–avidin were adsorbed with each surface deposition. In contrast, use of unmodified PLL did not lead to formation of multilayer coatings, confirming that (strept)avidin–biotin interactions were responsible for film growth even in the presence of electrostatic repulsive forces between PLL and avidin and the steric hindrance of associated PEG chains. This technique provides new opportunities for the generation of robust films with tailored interfacial binding and transport properties.     

Nanoparticle syntheses using non-porous and porous polyelectrolyte multilayers for biological applications

Polyelectrolyte multilayers comprised of poly(allylamine hydrochloride) and poly(acrylic acid) were assembled by layer-by-layer technique for metal nanoparticle syntheses. Using weak polyelectrolytes in LbL process, it is readily available to tune the deposited film properties by simple changing of the dipping solution pH. The PAH/PAA multilayer systems exhibit different surface morphologies and functionalities depending on the assembly conditions. We have studied two distinctive PAH/PAA multilayer films to utilize them for nanoparticle synthesis. The reactive functional groups of the polyelectrolytes within the films were remained after the film deposition or reactivated by a simple pH stimulus, and therefore they were allowed to undergo further chemical reactions to synthesize Pd and Au nanoparticles. Synthesized metal nanoparticles were easily characterized by their optical properties including surface plasmon absorption. These metal nanoparticle-embedded multilayers may have great potentials for biomolecule sensing or catalytically active coatings.     

Layer-by-layer assembly through weak interactions and their biomedical applications

The surface design and control of substrates with nanometer- or micrometer-sized polymer films are of considerable interest for both fundamental and applied studies in the biomedical field because of the required surface properties. The layer-by-layer (LbL) technique was discovered in 1991 by Decher and co-workers for the fabrication of polymer multilayers constructed mainly through electrostatic interaction. The scope and applicability of this LbL assembly has been extended by introducing molecularly regular conformations of polymers or proteins by employing, for the first time, weak interactions such as van der Waals interactions and biological recognition. Since these weak interactions are the sum of the attractive or repulsive forces between parts of the same molecule, they allow macromolecules to be easily arranged into the most stable conformation in a LbL film. By applying this characteristic feature, the template polymerization of stereoregular polymers, stereoregular control of surface biological properties, drastic morphological control of biodegradable nano materials, and the development of three-dimensional cellular multilayers as a tissue model were successfully achieved. It is expected that LbL assembly using weak interactions will promote further interest into fundamental and applied studies on the design of surface chemistry in the biomedical field.     

Polyelectrolyte microcapsules for sustained delivery of water-soluble drugs

Polyelectrolyte capsules composed of weak polyelectrolytes are introduced as a simple and efficient system for spontaneous encapsulation of low molecular weight water-soluble drugs. Polyelectrolyte capsules were prepared by layer-by-layer (LbL) assembling of weak polyelectrolytes, poly(allylamine hydrochloride) (PAH) and poly(methacrylic acid) (PMA) on polystyrene sulfonate (PSS) doped CaCO₃ particles followed by core removal with ethylene-diaminetetraacetic acid (EDTA). The loading process was observed by confocal laser scanning microscopy (CLSM) using tetramethylrhodamineisothiocyanate labeled dextran (TRITC-dextran) as a fluorescent probe. The intensity of fluorescent probe inside the capsule decreased with increase in cross-linking time. Ciprofloxacin hydrochloride (a model water-soluble drug) was spontaneously deposited into PAH/PMA capsules and their morphological changes were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The quantitative study of drug loading was also elucidated which showed that drug loading increased with initial drug concentration, but decreased with increase in pH. The loaded drug was released in a sustained manner for 6 h, which could be further extended by cross-linking the capsule wall. The released drug showed significant antibacterial activity against E. coli. These findings indicate that such capsules can be potential carriers for water-soluble drugs in sustained/controlled drug delivery applications.     

Ultrathin, responsive polymer click capsules

We report a general click chemistry approach for the layer-by-layer assembly of ultrathin, polymer films on particles and the subsequent formation of polymer click capsules (CCs). Poly(acrylic acid) copolymers, synthesized with a minor component of either alkyne (PAA-Alk) or azide (PAA-Az) functionality, were alternately assembled on silica particles. The (PAA-Az/PAA-Alk)-coated particles were subsequently functionalized by exploiting the free alkyne click moieties present in the film upon exposure to an azide-modified rhodamine dye. Further, PAA CCs, obtained following removal of the silica particle template, were shown to exhibit pH-responsive behavior. This was demonstrated by reversible size changes of the CCs upon cycling between basic and acidic solutions. Polymer CCs are anticipated to find applications in various fields, including drug delivery and sensing.     

Controlled loading and release of methylene blue in layer-by-layer assembled polyelectrolyte films

The layer-by-layer (LbL) assembled multilayer films are widely used in the biomedical field for the controlled drug delivery. Here, multilayer films were assembled by LbL technique through alternating deposition of poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) on glass slides. Methylene blue (MB) was used as a model drug to investigate the loading and release ability of the multilayer films. The result showed that the loading rate and loading amount of MB were greatly influenced by ionic strength and pH value of the dye solution, and the release rate of MB was also controlled both by ionic strength and pH value of immersing solution. The result also testified that the films had good reversibility of drug loading and release. It suggested that the PDDA/PAA multilayer films had a potential application in drug delivery, especially in drug burst release.     

Robust Microcapsules with Controlled Permeability from Silk Fibroin Reinforced with Graphene Oxide

Robust and stable microcapsules are assembled from poly‐amino acid‐modified silk fibroin reinforced with graphene oxide flakes using layer‐by‐layer (LbL) assembly, based on biocompatible natural protein and carbon nanosheets. The composite microcapsules are extremely stable in acidic (pH 2.0) and basic (pH 11.5) conditions, accompanied with pH‐triggered permeability, which facilitates the controllable encapsulation and release of macromolecules. Furthermore, the graphene oxide incorporated into ultrathin LbL shells induces greatly reinforced mechanical properties, with an elastic modulus which is two orders of magnitude higher than the typical values of original silk LbL shells and shows a significant, three‐fold reduction in pore size. Such strong nanocomposite microcapsules can provide solid protection of encapsulated cargo under harsh conditions, indicating a promising candidate with controllable loading/unloading for drug delivery, reinforcement, and bioengineering applications.     

Drug delivery: Layer‐By‐Layer‐Assembled Capsules and Films for Therapeutic Delivery (Small 17/2010)

Polymeric materials formed via layer‐by‐layer (LbL) assembly have promise for use as drug delivery vehicles. These multilayered materials, both as capsules and thin films, can encapsulate a high payload of toxic or sensitive drugs, and can be readily engineered and functionalized with specific properties. This review highlights important and recent studies that advance the use of LbL‐assembled materials as therapeutic devices. It also seeks to identify areas that require additional investigation for future development of the field. A variety of drug‐loading methods and delivery routes are discussed. The biological barriers to successful delivery are identified, and possible solutions to these problems are discussed. Finally, state‐of‐the‐art degradation and cargo release mechanisms are also presented.     

Biomimetic Extracellular Environment Based on Natural Origin Polyelectrolyte Multilayers

Surface modification of biomaterials is a well‐known approach to enable an adequate biointerface between the implant and the surrounding tissue, dictating the initial acceptance or rejection of the implantable device. Since its discovery in early 1990s layer‐by‐layer (LbL) approaches have become a popular and attractive technique to functionalize the biomaterials surface and also engineering various types of objects such as capsules, hollow tubes, and freestanding membranes in a controllable and versatile manner. Such versatility enables the incorporation of different nanostructured building blocks, including natural biopolymers, which appear as promising biomimetic multilayered systems due to their similarity to human tissues. In this review, the potential of natural origin polymer‐based multilayers is highlighted in hopes of a better understanding of the mechanisms behind its use as building blocks of LbL assembly. A deep overview on the recent progresses achieved in the design, fabrication, and applications of natural origin multilayered films is provided. Such films may lead to novel biomimetic approaches for various biomedical applications, such as tissue engineering, regenerative medicine, implantable devices, cell‐based biosensors, diagnostic systems, and basic cell biology.     

Automated Buildup of Biomimetic Films in Cell Culture Microplates for High‐Throughput Screening of Cellular Behaviors

An automatic method is established for layer‐by‐layer (LbL) assembly of biomimetic coatings in cell culture microplates using a commercial liquid‐handling robot. Highly homogeneous thin films are formed at the bottom of each microwell. The LbL film‐coated microplates are compatible with common cellular assays, using microplate readers and automated microscopes. Cellular adhesion is screened on crosslinked and peptide‐functionalized LbL films and stem cell differentiation in response to increasing doses of bone morphogenetic proteins (2, 4, 7, 9). This method paves the way for future applications of LbL films in cell‐based assays for regenerative medicine and high‐throughput drug screening.     

Freestanding Nanostructures via Layer‐by‐Layer Assembly

Freestanding flexible nanocomposite structures fabricated by layer‐by‐layer (LbL) assembly are promising candidates for many potential applications, such as in the fields of thermomechanical sensing, controlled release, optical detection, and drug delivery. In this article, we review recent advances in the fabrication and characterization of different types of freestanding LbL structures in air and at air/liquid and liquid/liquid interfaces, including micro‐ and nanocapsules, microcantilevers, freely suspended membranes, encapsulated nanoparticle arrays, and sealed‐cavity arrays. Several recently developed fabrication techniques, such as spin‐assisted coating, dipping, and micropatterning, make the assembly process more efficient and impart novel physical properties to the freestanding films.     

Immobilization of liposomes in nanostructured layer-by-layer films containing dendrimers

Artificial vesicles or liposomes composed of lipid bilayers have been widely exploited as building blocks for artificial membranes, in attempts to mimic membrane interaction with drugs and proteins and to investigate drug delivery processes. In this study we report on the immobilization of liposomes of 1,2-dipalmitoyl-sn-Glycero-3-[Phospho-rac-(1-glycerol)] (Sodium Salt) (DPPG) in layer-by-layer (LbL) films, alternated with poly(amidoamine) G4 (PAMAM) dendrimer layers. The average size of the liposomes in solution was 120 nm as determined by dynamic light scattering, with their spherical shape being inferred from scanning electron microscopy (SEM) in cast films. LbL films containing up to 20 PAMAM/DPPG bilayers were assembled onto glass and/or silicon wafer substrates. The growth of the multilayers was achieved by alternately immersing the substrates into the PAMAM and DPPG solutions for 5 and 10 min, respectively. The formation of PAMAM/DPPG liposome multilayers and its ability to interact with BSA were confirmed by Fourier transform infrared spectroscopy (FTIR). The structural features and film thickness were obtained using X-ray diffraction and surface plasmon resonance (SPR).     

Poly(Methacrylic Acid) Polymer Hydrogel Capsules: Drug Carriers,Sub‐compartmentalized Microreactors,Artificial Organelles

Multilayered polymer capsules attract significant research attention and are proposed as candidate materials for diverse biomedical applications, from targeted drug delivery to microencapsulated catalysis and sensors. Despite tremendous efforts, the studies which extend beyond proof of concept and report on the use of polymer capsules in drug delivery are few, as are the developments in encapsulated catalysis with the use of these carriers. In this Concept article, the recent successes of poly(methacrylic acid) hydrogel capsules as carrier vessels for delivery of therapeutic cargo, creation of microreactors, and assembly of sub‐compartmentalized cell mimics are discussed. The developed technologies are outlined, successful applications of these capsules are highlighted, capsules properties which contribute to their performance in diverse applications are discussed, and further directions and plausible developments in the field are suggested.     

Unconventional layer-by-layer assembly: surface molecular imprinting and its applications

Layer-by-layer assembly (LbL) is a rich, versatile, and powerful technique for fabricating multilayer thin films with controlled architecture and functions. Singly charged, uncharged, or water-repellent molecules cannot be used directly in conventional LbL assembly. This problem can be solved with unconventional LbL methods, by employing the preassembly of building blocks in solution and the use of these assemblies for LbL formation at the interface. This Concept summarizes different methods of unconventional LbL assembly, including electrostatic complex formation, hydrogen-bonded complexes, block-copolymer micelles, and π-π interaction complexes. These preassembly treatments endow the building blocks with enhanced abilities for advanced functionality, in particular, surface molecular imprinting, a new concept emerging from unconventional LbL. Molecular imprinting approaches are thus conceptually described based on different types of interactions and their great potential in applications is demonstrated by examples such as selective surface patterning and selective filtration.     

Exceptional Flame Resistance and Gas Barrier with Thick Multilayer Nanobrick Wall Thin Films

Layer‐by‐layer (LbL) assembly is a powerful and versatile technique to deposit functional thin films, but often requires a large number of deposition steps to achieve a film thick enough to provide a desired property. By incorporating amine salts into the cationic polyelectrolyte and its associated rinse, LbL clay‐containing nanocomposite films can achieve much greater thickness (>1 μm) with relatively few deposition cycles (≤6 bilayers). Amine salts interact with nanoclays, causing nanoplatelets to deposit in stacks rather than as individual platelets. This technique appears to be universal, exhibiting thick growth with multiple types of nanoclay, including montmorillonite and vermiculite (VMT), and a variety of amine salts (e.g., hexylamine and diethanolamine). The characteristic order found in LbL‐assembled films is maintained despite the incredible thickness. Films assembled in this manner achieve oxygen transmission rates below 0.009 cc m⁻² d⁻¹ atm⁻¹ with just 6 bilayers (BLs) of chitosan/VMT deposited. These thick clay‐based thin films also impart exceptional flame resistance. A 2‐BL film renders a 3.2 mm polystyrene plate self‐extinguishing, while an 8‐BL film (3.9 μm thick) prevents ignition entirely. This ability to generate much thicker clay‐based multilayers with amine salts opens up tremendous potential for these nanocoatings in real world applications.     

Controlled fabrication of gold nanoparticles biomediated by glucose oxidase immobilized on chitosan layer-by-layer films

The control of size and shape of metallic nanoparticles is a fundamental goal in nanochemistry, and crucial for applications exploiting nanoscale properties of materials. We present here an approach to the synthesis of gold nanoparticles mediated by glucose oxidase (GOD) immobilized on solid substrates using the Layer-by-Layer (LbL) technique. The LbL films contained four alternated layers of chitosan and poly(styrene sulfonate) (PSS), with GOD in the uppermost bilayer adsorbed on a fifth chitosan layer: (chitosan/PSS)₄/(chitosan/GOD). The films were inserted into a solution containing gold salt and glucose, at various pHs. Optimum conditions were achieved at pH 9, producing gold nanoparticles of ca. 30 nm according to transmission electron microscopy. A comparative study with the enzyme in solution demonstrated that the synthesis of gold nanoparticles is more efficient using immobilized GOD.     

Self‐Assembled Metal–Phenolic Networks on Emulsions as Low‐Fouling and pH‐Responsive Particles

Interfacial self‐assembly is a powerful organizational force for fabricating functional nanomaterials, including nanocarriers, for imaging and drug delivery. Herein, the interfacial self‐assembly of pH‐responsive metal–phenolic networks (MPNs) on the liquid–liquid interface of oil‐in‐water emulsions is reported. Oleic acid emulsions of 100–250 nm in diameter are generated by ultrasonication, to which poly(ethylene glycol) (PEG)‐based polyphenolic ligands are assembled with simultaneous crosslinking by metal ions, thus forming an interfacial MPN. PEG provides a protective barrier on the emulsion phase and renders the emulsion low fouling. The MPN‐coated emulsions have a similar size and dispersity, but an enhanced stability when compared with the uncoated emulsions, and exhibit a low cell association in vitro, a blood circulation half‐life of ≈50 min in vivo, and are nontoxic to healthy mice. Furthermore, a model anticancer drug, doxorubicin, can be encapsulated within the emulsion phase at a high loading capacity (≈5 fg of doxorubicin per emulsion particle). The MPN coating imparts pH‐responsiveness to the drug‐loaded emulsions, leading to drug release at cell internalization pH and a potent cell cytotoxicity. The results highlight a straightforward strategy for the interfacial nanofabrication of pH‐responsive emulsion–MPN systems with potential use in biomedical applications.     

Hybrid organic-inorganic nanostructures fabricated from layer-by-layer self-assembled multilayers of hyperbranched polyglycerols and phosphorus dendrimers

Multilayer thin films of cationic phosphorous dendrimers and anionic hyperbranched polyglycerols were fabricated by electrostatic layer-by-layer (LbL) self-assembly (SA). The film formation was monitored by surface plasmon resonance (SPR) spectroscopy and UV-visible spectroscopy, and it was found that the stepwise, alternating deposition results in a linear growth up to four bilayers. Hybrid organic-TiO2 nanostructures were generated by exposing the supramolecular multilayers to TiCl4 precursors. The amounts of TiO2 incorporated inside the scaffolds could be tuned by controlling the porosity of the multilayers with the addition of a small amount of salts. The resulting hybrid films exhibit characteristic photoluminescence (PL) properties.