Molecular assembly of biomimetic microcapsules

Nanoscale self-assembly: concepts, applications and challenges

Self-assembly offers unique possibilities for fabricating nanostructures, with different morphologies and properties, typically from vapour or liquid phase precursors. Molecular units, nanoparticles, biological molecules and other discrete elements can spontaneously organise or form via interactions at the nanoscale. Currently, nanoscale self-assembly finds applications in a wide variety of areas including carbon nanomaterials and semiconductor nanowires, semiconductor heterojunctions and superlattices, the deposition of quantum dots, drug delivery, such as mRNA-based vaccines, and modern integrated circuits and nanoelectronics, to name a few. Recent advancements in drug delivery, silicon nanoelectronics, lasers and nanotechnology in general, owing to nanoscale self-assembly, coupled with its versatility, simplicity and scalability, have highlighted its importance and potential for fabricating more complex nanostructures with advanced functionalities in the future. This review aims to provide readers with concise information about the basic concepts of nanoscale self-assembly, its applications to date, and future outlook. First, an overview of various self-assembly techniques such as vapour deposition, colloidal growth, molecular self-assembly and directed self-assembly/hybrid approaches are discussed. Applications in diverse fields involving specific examples of nanoscale self-assembly then highlight the state of the art and finally, the future outlook for nanoscale self-assembly and potential for more complex nanomaterial assemblies in the future as technological functionality increases.

Three-Dimensional Cross-Linked Nb2O5 Polymorphs Derived from Cellulose Substances: Insights into the Mechanisms of Lithium Storage

Niobium pentoxide (Nb₂O₅)-based materials have been regarded as promising anodic materials for lithium-ion batteries due to their abundant crystalline phases and stable and safe lithium storage performances. However, there is a lack of systematic studies of the relationship among the crystal structures, electrochemical characteristics, and lithium storage mechanisms for the various Nb₂O₅ polymorphs. Herein, pure pseudohexagonal Nb₂O₅ (TT-Nb₂O₅), orthorhombic Nb₂O₅ (T-Nb₂O₅), tetragonal Nb₂O₅ (M-Nb₂O₅), and monoclinic Nb₂O₅ (H-Nb₂O₅) with three-dimensional interconnected structures are reported, which were synthesized via a hydrothermal method using the commercial filter paper as the structural template followed by specific annealing processes. Impressively, the TT- and T-Nb₂O₅ species both possess bronze-like phases with "room and pillar" structures, while M- and H-Nb₂O₅ ones are both in the Wadsley-Roth phases with crystallographic shear structures. Among the pristine Nb₂O₅ materials, H-Nb₂O₅ exhibits the highest initial specific capacity (270 mA h g^-1), while T-Nb₂O₅ performs with the lowest (197 mA h g^-1) at 0.02 A g^-1, meaning that crystallographic shear structures provide more lithium storage sites. TT-Nb₂O₅ realizes the best rate capability (207 mA h g^-1 at 0.02 A g^-1 and 103 mA h g^-1 at 4.0 A g^-1), indicating that the "room and pillar" crystal structures favor fast lithium storage. Electrochemical analyses reveal that the TT- and T-Nb₂O₅ phases with "room and pillar" crystal structures utilize a pseudocapacitive intercalation mechanism, while the M- and H-Nb₂O₅ phases with the Wadsley-Roth shear structures follow a typical battery-type intercalation mechanism. A fresh insight into the further understanding of the intercalation pseudocapacitance on the basis of the unit cells of the electrode materials and a meaningful guidance for crystalline structural design/construction of the electrode materials for the next-generation LIBs are thus provided.

A hierarchically structured anatase-titania/indium-tin-oxide nanocomposite as an anodic material for lithium-ion batteries

A nanotubular titania/ITO nanocomposite is synthesized, exhibiting enhanced electrochemical performance as an anodic material for lithium-ion batteries.

Properties of Engineered and Fabricated Silks

Silk is a protein-based material which is predominantly produced by insects and spiders. Hundreds of millions of years of evolution have enabled these animals to utilize different, highly adapted silk types in a broad variety of applications. Silk occurs in several morphologies, such as sticky glue or in the shape of fibers and can, depending on the application by the respective animal, dissipate a high mechanical energy, resist heat and radiation, maintain functionality when submerged in water and withstand microbial settling. Hence, it's unsurprising that silk piqued human interest a long time ago, which catalyzed the domestication of silkworms for the production of silk to be used in textiles. Recently, scientific progress has enabled the development of analytic tools to gain profound insights into the characteristics of silk proteins. Based on these investigations, the biotechnological production of artificial and engineered silk has been accomplished, which allows the production of a sufficient amount of silk materials for several industrial applications. This chapter provides a review on the biotechnological production of various silk proteins from different species, as well as on the processing techniques to fabricate application-oriented material morphologies.

Bioinspired Hierarchical Nanofibrous Silver-Nanoparticle/Anatase-Rutile-Titania Composite as an Anode Material for Lithium-Ion Batteries

A new bioinspired hierarchical nanofibrous silver-nanoparticle/anatase-rutile-titania (Ag-NP/A-R-titania) composite was fabricated by employing a natural cellulose substance (e.g., commercial laboratory cellulose filter paper) as the structural scaffold template, which was composed of anatase-phase titania (A-titania) nanotubes with rutile-phase titania (R-titania) nanoneedles grown on the surfaces and further silver nanoparticles (AgNPs) immobilized thereon. As it was employed as an anode material for lithium-ion batteries (LIBs), high reversible capacity, enhanced rate performance, and excellent cycling stability were achieved as compared with those of the corresponding cellulose-substance-derived nanotubular A-titania, R-titania, heterogeneous anatase/rutile titania (A-R-titania) composite, and commercial P25 powder. This benefited from its unique porous cross-linked three-dimensional structure inherited from the initial cellulose substance scaffold, which enhances the sufficient electrode/electrolyte contact, relieves the severe volume change upon cycling, and improves the amount of lithium-ion storage; moreover, the high loading content of the silver component in the composite improves the electrical conductivity of the electrode. The structural integrity of the composite was maintained upon long-term charge/discharge cycling, indicating its significant stability.

Cellulose-Rich Nanofiber-Based Functional Nanoarchitectures

Surface self-assembly of functional molecules or nanoscale building blocks is an effective strategy for the syntheses of advanced materials. Natural cellulose-rich substances have unique macro-to-nano hierarchical structural features. The fabrication of nanoarchitectures, employing specific guest species on the surfaces of the fine structures of such substances, results in corresponding artificial nanomaterials that possess the chemical functionalities and physical properties of both sides. Metal oxide thin film coatings with nanometer precision on the nanofibers of bulk cellulose-rich substances not only yield replicas of nanostructured materials, but also make it possible for further assemblies of functional units on the surfaces. Hence, nanostructured metal oxides and further composites, as well as surface-functionalized cellulose-based composites are fabricated by employing cellulose-rich substances as templates or scaffolds. The three-dimensional cross-linked porous structures, with the high surface area of the resultant nanomaterials or composites, lead to superior performance when employed as photocatalysts, electrode materials, and sensing matrices, on which this report is focused.

A nanofibrous polypyrrole/silicon composite derived from cellulose substance as the anode material for lithium-ion batteries

A bio-inspired nanofibrous polypyrrole/silicon composite derived from cellulose substance was fabricated showing enhanced electrochemical performance as an anode material for LIBs.

Fluidized bed layer-by-layer microcapsule formation

Polymer microcapsules can be used as bioreactors and artificial cells; however, preparation methods for cell-like microcapsules are typically time-consuming, low yielding, and/or involve custom microfluidics. Here, we introduce a rapid (∼30 min per batch, eight layers), scalable (up to 500 mg of templates), and efficient (98% yield) microcapsule preparation technique utilizing a fluidized bed for the layer-by-layer (LbL) assembly of polymers, and we investigate the parameters that govern the formation of robust capsules. Fluidization in water was possible for particles of comparable diameter to mammalian cells (>5 μm), with the experimental flow rates necessary for fluidization matching well with the theoretical values. Important variables for polymer film deposition and capsule formation were the concentration of polymer solution and the molecular weight of the polymer, while the volume of the polymer solution had a negligible impact. In combination, increasing the polymer molecular weight and polymer solution concentration resulted in improved film deposition and the formation of robust microcapsules. The resultant polymer microcapsules had a thickness of ∼5.5 nm per bilayer, which is in close agreement with conventionally prepared (quiescent (nonflow) adsorption/centrifugation/wash) LbL capsules. The technique reported herein provides a new way to rapidly generate microcapsules (approximately 8 times quicker than the conventional means), while being also amenable to scale-up and mass production.

Lipid, protein and poly(NIPAM) coated mesoporous silica nanoparticles for biomedical applications

In the past decade, mesoporous silica nanoparticles (MSNs) as nanocarriers have showed much potential in advanced nanomaterials due to their large surface area and pore volume. Especially, more and more MSNs based nanodevices have been designed as efficient drug delivery systems (DDSs) or biosensors. In this paper, lipid, protein and poly(NIPAM) coated MSNs are reviewed from the preparation, properties and their potential application. We also introduce the preparative methods including physical adsorption, covalent binding and self-assembly on the MSNs' surfaces. Furthermore, the interaction between the aimed cells and these molecular modified MSNs is discussed. We also demonstrate their typical applications, such as photodynamic therapy, bioimaging, controlled release and selective recognition in biomedical field.

Nanofibrous silicon/carbon composite sheet derived from cellulose substance as free-standing lithium-ion battery anodes

A bio-inspired nanofibrous Si/C composite sheet was fabricated and employed as self-supporting anode for Li-ion battery showing good electrochemical performances.

Antibacterial hybrid materials fabricated by nanocoating of microfibril bundles of cellulose substance with titania/chitosan/silver-nanoparticle composite films

Uniform ultrathin titania/chitosan composite films were coated on the cellulose microfibril bundles of natural cellulose substance (common commercial filter paper) by a layer-by-layer self-assembly process. Relying on the strong chelating ability of chitosan for metal ions, silver ions were abundantly adsorbed on the titania/chitosan composite film coated cellulose substance and were thereafter in situ reduced to silver nanoparticles (Ag-NPs) under UV irradiation. As such, hybrid cellulose/titania/chitosan/Ag-NP composite materials were obtained, which feature the same hierarchically fibrous structure as the initial cellulose substance. Meanwhile, the hybrid fibres in the composite materials exhibit a cable-like core-shell structure, displaying a cellulose microfibril bundle core as well as a nanometer-thick titania/chitosan/Ag-NP composite film shell with Ag-NPs (4-20 nm in diameter) well distributed. The antibacterial activities of the titania/chitosan/Ag-NP composite film coated cellulose materials were evaluated against both Gram-positive and Gram-negative bacteria, and the materials as such disinfected almost all the inoculated bacteria due to the intrinsic biocidal effect of titania composition, positively charged chitosan component and high loading content of Ag-NPs with small sizes, showing excellent antibacterial activities.

Protein capsules assembled via isobutyramide grafts: sequential growth, biofunctionalization, and cellular uptake

We report the sequential assembly of proteins via the alternating physical adsorption of human serum albumin (HSA) and chemical grafting with isobutyramide (IBAM) or bromoisobutyramide (BrIBAM) groups. This approach, performed on silica template particles, leads to the formation of noncovalent protein films with controlled growth at the nanometer scale. Further, after template removal, hollow protein capsules with tunable wall thicknesses and high mechanical stability are obtained. The use of BrIBAM, compared to IBAM grafts, leads to significantly thicker capsule walls, highlighting the influence of the bromine atoms in the assembly process, which is discussed in terms of a theoretical model of noncovalent interactions. Another feature of the process is the possibility to functionalize the HSA capsules with other biologically active macromolecules, including enzymes, polysaccharides, or DNA plasmids, demonstrating the versatility of this approach. We also report that BrIBAM-HSA and IBAM-HSA capsules display negligible cytotoxicity in vitro with HeLa cells and that their cellular uptake is dependent on the thickness of the capsule walls. These findings support the potential use of these protein capsules in tailored biological applications such as drug delivery.

Nanotubes protruding from poly(allylamine hydrochloride)-graft-pyrene microcapsules

Protrusion of one-dimensional nanotubes or nanorods from the poly(allylamine hydrochloride)-graft-pyrene (PAH-Py) microcapsules was discovered when the microcapsules were incubated in pH 0 and pH 2 solutions, respectively. Micelles assembled from deliberately synthesized PAH-Py polymers were also able to transform into one-dimensional structures, demonstrating the chemistry driven nature of the phenomenon. The one-dimensional nanotubes consisted of only 1-pyrenecarboxaldehyde with ordered π-π stacking, and showed a helical structure and anisotropic property. The hydrolysis of Schiff base and its rate at different pH values (10 times slower at pH 0 than at pH 2) played a key role in determining the final nanostructures, and the linear PAH directed the regular building up process especially for the nanotubes. Hollow capsules budded with nanotubes or nanorods mimicking the cellular protrusion of filopodia were successfully prepared by tuning the incubation pH and time. These results and the proposed mechanism open new opportunities for design of novel micronanostructures and materials for nanoscience, and biological and other advanced technologies.

Development of Cubosomes as a Cell-Free Biosensing Platform

The parallel between the lipidic microenvironments of the inverse bicontinuous cubic phase and the biological membrane distinguishes cubic phases as an attractive option for development of cell-free biosensors containing protein or glycolipid receptors. Herein we describe a novel strategy toward the creation of a biosensing platform derived from the surface attachment of a colloidally stable inverse cubic structure (cubosomes). We report the preparation of cubosomes composed of the amphiphile phytantriol, the membrane glycolipid receptor monosialoganglioside-GM1 and the biotin-functionalized amphiphile 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[biotinyl(polyethyleneglycol)-2000] (bDSPE). The tethering of cubosomes to the various surfaces was mediated through bDSPE binding to streptavidin- and avidin-modified surfaces. Allylamine plasma polymer surface modification enhanced the surface immobilization of avidin, which increased the density of bound cubosomes. The resultant polymer–protein–cubosome complex was imaged by cryo-transmission electron microscopy analysis and the cubosome structure was impressively preserved within the complex. Cholera toxin binding to cubosomes containing GM1 was used to assess the performance of the cubosomes, subsequent to surface attachment, via a modified enzyme-linked immunosorbent assay. Specific immobilization of complex protein–receptor–cubosome systems paves the way for development of a structurally complex, heterogeneous platform for sensing applications.

Sensing-applications of surface-based single vesicle arrays

A single lipid vesicle can be regarded as an autonomous ultra-miniaturised 3D biomimetic "scaffold" (Ø≥13 nm) ideally suited for reconstitution and interrogation of biochemical processes. The enclosing lipid bilayer membrane of a vesicle can be applied for studying binding (protein/lipid or receptor/ligand interactions) or transmembrane events (membrane permeability or ion channel activation) while the aqueous vesicle lumen can be used for confining few or single macromolecules and probe, e.g., protein folding, catalytic pathways of enzymes or more complex biochemical reactions, such as signal transduction cascades. Immobilisation (arraying) of single vesicles on a solid support is an extremely useful technique that allows detailed characterisation of vesicle preparations using surface sensitive techniques, in particular fluorescence microscopy. Surface-based single vesicle arrays allow a plethora of prototypic sensing applications in a high throughput format with high spatial and high temporal resolution. In this review we present a series of applications of single vesicle arrays for screening/sensing of: membrane curvature dependent protein-lipid interactions, bilayer tension, reactions triggered in the vesicle lumen, the activity of transmembrane protein channels and biological membrane fusion reactions.

LbL multilayer capsules: recent progress and future outlook for their use in life sciences

In this review we provide an overview of the recent progress in designing composite polymer capsules based on the Layer-by-Layer (LbL) technology demonstrated so far in material science, focusing on their potential applications in medicine, drug delivery and catalysis. The benefits and limits of current systems are discussed and the perspectives on emerging strategies for designing novel classes of therapeutic vehicles are highlighted.

Surface-active monomer as a stabilizer for polyurea nanocapsules synthesized via interfacial polyaddition in inverse miniemulsion

A surface-active monomer, polyisobutylene-succinimide pentamine (Lubrizol U), was used as a stabilizer for synthesizing polyurea nanocapsules with aqueous core via polyaddition at inverse miniemulsion droplet interface. Because of the presence of amine groups in the Lubrizol molecule, it is covalently incorporated into the polymeric interfacial layer after reaction, resulting in more compact (less permeable) capsule shell. The influence of the stabilizer and the monomer concentration on the shell thickness, colloidal stability, average capsule size, and capsule size polydispersity were examined in detail. Different materials, such as a water-soluble fluorescent dye and aqueous dispersion of magnetite nanoparticles with 10 nm in size, were used as inner phase of the polyurea capsules. The encapsulation efficiency was studied using fluorescein as a marker. As an example for biomedical application, the fluorescein-containing capsules were utilized in cell uptake experiments and visualized using fluorescence microscopy.

Molecular assembly and application of biomimetic microcapsules

The assembly of multifunctional biomimetic microcapsules at the molecular level is of tremendous interest for biophysical research and the biomedical field. Among the available molecular assembly techniques, layer-by-layer assembly has attracted extensive attention for the fabrication of biomimetic microcapsules because it possesses engineered features including size, shape, thickness, composition and permeation, and the capability of incorporating different types of biomolecules. In this tutorial review, we highlight how biomimetic microcapsules can be fabricated by directly applying lipids and proteins as assembly pairs and how layer-by-layer assembled polyelectrolyte microcapsules can be interfaced with biological components such as phospholipid membranes and proteins. The applications of these biomimetic microcapsules in drug delivery, biosensors, and hybrid nanodevices are also addressed.

Proton gradients produced by glucose oxidase microcapsules containing motor F0F1-ATPase for continuous ATP biosynthesis

Glucose oxidase (GOD) microcapsules held together by cross-linker, glutaraldehyde (GA), are fabricated by the layer-by-layer (LbL) assembly technique. The lipid bilayer containing CF(0)F(1)-ATPase was coated on the outer shell of GOD microcapsules. Driven under the proton gradients produced by catalysis of GOD microcapsules for glucose, ATP is synthesized from ADP and inorganic phosphate catalyzed by the ATPase rotary catalysis. The results show here that ATPase reconstituted on the GOD microcapsules retains its catalytic activity.

Polymeric microcapsules for synthetic applications

For decades scientists have been working on closed systems for transportation, catalysis and protection, which are inspired by natural cells. Only recently polymer based systems have emerged for these systems, since they are more robust, give protection from the environment and give a more stable membrane. Various methods have been developed to prepare polymer based capsules. They can be made by self-assembly, templating, in situ polymerization or precipitation. Their application has been explored in various areas e.g. drug delivery, diagnostics, sensors and nano reactors. Considering the output in this field has substantially grown, more developments can be expected from this latter application.

Hydrothermal-induced structure transformation of polyelectrolyte multilayers: from nanotubes to capsules

The assembled polyelectrolyte nanotubes composed of poly(styrenesulfonate) and poly(allylamine hydrochloride) multilayers by using the layer-by-layer assembly combined with the porous template method can be transformed into capsules by a high-temperature treatment. Scanning electron microscopy and confocal laser scanning microscopy images revealed the whole transition process. The structure transformation of polyelectrolyte multilayers after annealing can be initiated by the input of thermal energy which leads to a breakage of ion pairs between oppositely charged polyelectrolyte groups. The transition process from tubes to capsules is supposed to be driven by the Raleigh instability and leads to the generated polyelectrolyte capsules with different sizes.

Surface-based lipid vesicle reactor systems: fabrication and applications

Over the last ten years there has been a strong (bio)technological drive for the development of miniaturised reaction systems, motivated mainly by the need to reduce sample consumption and parallelise. Self-assembled soft-matter containers have naturally evolved to handle small volumes and could provide viable fluidic solutions especially in niche areas where ultra-miniaturisation, biocompatibility or cost are of critical importance. Here we focus on nanocontainers that are made of lipids and are immobilised on surfaces. We will highlight the most prominent contributions to date on the fabrication and the applications of surface-based vesicle systems as miniaturised reactors. Emphasis will be put on single-vesicle experiments.

Immobilization of glucose oxidase onto gold nanoparticles with enhanced thermostability

Immobilized proteins and enzymes were widely investigated in medical field as well as in food and environmental fields. In this paper, glucose oxidase (GOD) monolayer was covalently immobilized on the surface of gold nanoparticles (AuNPs) to fabricate bioconjugate complex. The citrate-stabilized AuNPs were first functionalized by a carboxyl-terminated alkanethiol and the terminal carboxyl groups were subsequently bonded with side-chain amino groups of protein surface through EDC/NHS coupling reaction. The enzyme activity assays of the obtained bioconjugates display an enhanced thermostability and similar pH-dependence behavior in contrast with that of free enzyme. Such GOD/AuNPs bioconjugates can be considered as a catalytic nanodevice to construct nanoreactor based on glucose oxidation reaction for biotechnological purpose.

Hollow chitosan-alginate multilayer microcapsules as drug delivery vehicle: doxorubicin loading and in vitro and in vivo studies

We report here the loading of the antitumor drug doxorubicin (DOX) in preformed multilayer microcapsules and its application in tumor treatment assayed by in vitro cell culture and in vivo animal experiments. The microcapsules, consisting completely of polysaccharides, were fabricated by deposition of oppositely charged chitosan and alginate onto carboxylmethyl cellulose (CMC)-doped CaCO(3) colloidal particles in a layer-by-layer fashion, followed by cross-linking with glutaraldehyde and decomposition of the cores by disodium ethylenediaminetetraacetic acid. The microcapsules as prepared contain negatively charged CMC-either in a free state or very possibly coupled with the excess chitosan of the first layer. They showed a strong ability to accumulate the positively charged DOX with a factor of tens to hundreds; that is, the drug concentration within the microcapsules was hundreds of times higher than the feeding concentration. Confocal microscopy and transmission electron microscopy revealed homogeneous distribution of the drug. The encapsulated DOX could be released again, following a diffusion-controlled model at the initial stage. In vitro experiments showed that the encapsulated drug can effectively induce the apoptosis of HepG2 tumor cells, as shown by various microscopy techniques after acridine orange, Hoechst 33342, and osmium tetraoxide staining. By seeding the HepG2 hepatoma cells into BALB/c/nu mice, tumors were created for the experimental studies. The results showed that the encapsulated DOX had better efficacy than that of the free drug in terms of tumor inhibition in a 4-week in vivo culture period.

Composite interfacial layers containing micro-size and nano-size particles

Surface layers of micro- and nanoparticles at fluid/liquid interfaces in absence and presence of surfactants are of large importance in the process of re-discovering Pickering systems, i.e. emulsions and foams stabilized by particles. The surface pressure/area isotherms of such layers can provide information about the properties of the used particles (dimensions, interfacial contact angles), the structure of interfacial layers, the interactions between the particles as well as about relaxation processes within the layers. For a correct description of Pi-A isotherms of composite surface layers containing particles the significant difference in size of these particles to that of solvent and surfactant molecules should be taken into account. Corresponding equations can be derived on the basis of the two-dimensional solution theory. The gained equations provide satisfactory agreement with experimental data and predict realistic values for the area of particles at the interface. Also equations of state and of the dilational elasticity for composite surface layers containing particles can be obtained in the framework of the presented methodology.

pH-Triggered softening of crosslinked hydrogen-bonded capsules

We report here the first AFM single capsule mechanical measurements on hydrogen-bonded polymeric multilayer microcapsules made of poly(vinylpyrrolidone)/poly(methacrylic acid) (PVPON/PMAA) and of poly(-vinylpyrrolidone-co-NH-20)/poly(methacrylic acid) (PVPON-co-NH-20/PMAA), as well as of capsules derived from these systems chemical crosslinking. The stiffness of the non-crosslinked hydrogen-bonded capsules was found to be proportional to the square of the wall thickness which is in agreement with previous observations on other multilayer capsules and continuum mechanical theory. The found elastic modulus of 610 MPa for low pH (= 2) is typical for a highly stable, glass-like structure similar to electrostatically bound multilayers. At pH > 6, (PMAA) capsules obtained through chemical crosslinking of hydrogen-bonded (PVPON/PMAA) multilayers, or crosslinked (PVPON-co-NH-20/PMAA) capsules showed a sharp hundreds-fold stiffness decrease to ∼1 mN/m which was orders of magnitude lower than those reported earlier for polymeric multilayer systems. pH-Triggered softening was reversible and highly reproducible. Softening of both (PMAA) and (PVPON-co-NH-20/PMAA) crosslinked capsules resulted from increased PMAA ionization, and additional dissociation of intermolecular hydrogen bonds occurred in the case of (PVPON-co-NH-20/PMAA) crosslinked system.