Self-assembly of composite nanotubes and their applications

Self-Assembling Hydrogel Structures for Neural Tissue Repair

Hydrogel materials have been employed as biological scaffolds for tissue regeneration across a wide range of applications. Their versatility and biomimetic properties make them an optimal choice for treating the complex and delicate milieu of neural tissue damage. Aside from finely tailored hydrogel properties, which aim to mimic healthy physiological tissue, a minimally invasive delivery method is essential to prevent off-target and surgery-related complications. The specific class of injectable hydrogels termed self-assembling peptides (SAPs), provide an ideal combination of in situ polymerization combined with versatility for biofunctionlization, tunable physicochemical properties, and high cytocompatibility. This review identifies design criteria for neural scaffolds based upon key cellular interactions with the neural extracellular matrix (ECM), with emphasis on aspects that are reproducible in a biomaterial environment. Examples of the most recent SAPs and modification methods are presented, with a focus on biological, mechanical, and topographical cues. Furthermore, SAP electrical properties and methods to provide appropriate electrical and electrochemical cues are widely discussed, in light of the endogenous electrical activity of neural tissue as well as the clinical effectiveness of stimulation treatments. Recent applications of SAP materials in neural repair and electrical stimulation therapies are highlighted, identifying research gaps in the field of hydrogels for neural regeneration.

A Bubble-Dragged Catalytic Polymer Microrocket

We report the bubble dragged microrocket consisting of functionalized multilayer polymer covered asymmetrically by platinum nanoparticles. The microrocket is pushed back during bubble growth over a small step and dragged forward over a big step during bubble explosion. Each bubble explosion induced a shock wave of gas which propagates in water at ultrafast speed. The bubble dragged microrocket can move along an approximate straight line instead of a fluctuating circle which is the trajectory of a bubble-pushed microrocket in most cases, which makes it a promising candidate for drug delivery and simulating rod-shaped bacteria.

A silver-nanoparticle/cellulose-nanofiber composite as a highly effective substrate for surface-enhanced Raman spectroscopy

A highly active surface-enhanced Raman scattering (SERS) substrate was developed by facile deposition of silver nanoparticles onto cellulose fibers of ordinary laboratory filter paper. This was achieved by means of the silver mirror reaction in a manner to control both the size of the silver nanoparticles and the silver density of the substrate. This paper-based substrate is composed of a particle-on-fiber structure with the unique three-dimensional network morphology of the cellulose matrix. For such a SERS substrate with optimized size of the silver nanoparticles (ca. 70 nm) and loading density of silver (17.28 wt %), a remarkable detection limit down to the sub-attomolar (1 × 10^-16 M) level and an enhancement factor of 3 × 10⁶ were achieved by using Rhodamine 6G as the analyte. Moreover, this substrate was applied to monitor the molecular recognition through multiple hydrogen bonds in between nucleosides of adenosine and thymidine. This low-cost, highly sensitive, and biocompatible paper-based SERS substrate holds considerable potentials for the detection and analyses of chemical and biomolecular species.

Engineering and delivery of nanocolloids of hydrophobic drugs

A lot of efforts have been devoted to engineering the delivery of hydrophobic drugs due to the high demand of chemotherapy against cancer. While early developed liposomes and polymeric nanoparticles did not meet the requirements of high drug loading efficiency, pure drug nanoparticles appeared to meet these together with high stability. Current drug delivery systems demand an improved performance over the whole aspects of stability, loading capacity, and therapeutic effects. As a result, both new techniques based on traditional methods and totally new procedures are under investigation. In this review, we focus on the evaluation of pure drug nanolloids fabricated by different engineering protocols with emphasis on the size and morphology, delivery and controlled release, and therapeutic effects of these drug nanocolloids.

Molecular Assembly of Polysaccharide-Based Microcapsules and Their Biomedical Applications

Advanced multifunctional microcapsules have revealed great potential in biomedical applications owing to their tunable size, shape, surface properties, and stimuli responsiveness. Polysaccharides are one of the most acceptable biomaterials for biomedical applications because of their outstanding virtues such as biocompatibility, biodegradability, and low toxicity. Many efforts have been devoted to investigating novel molecular design and efficient building blocks for polysaccharide-based microcapsules. In this Personal Account, we first summarize the common features of polysaccharides and the main principles of the design and fabrication of polysaccharide-based microcapsules, and further discuss their applications in biomedical areas and perspectives for future research.

Recent Progress on Bioinspired Self-Propelled Micro/Nanomotors via Controlled Molecular Self-Assembly

The combination of bottom-up controllable self-assembly technique with bioinspired design has opened new horizons in the development of self-propelled synthetic micro/nanomotors. Over the past five years, a significant advances toward the construction of bioinspired self-propelled micro/nanomotors has been witnessed based on the controlled self-assembly technique. Such a strategy permits the realization of autonomously synthetic motors with engineering features, such as sizes, shapes, composition, propulsion mechanism, and function. The construction, propulsion mechanism, and movement control of synthetic micro/nanomotors in connection with controlled self-assembly in recent research activities are summarized. These assembled nanomotors are expected to have a tremendous impact on current artificial nanomachines in future and hold potential promise for biomedical applications including drug targeted delivery, photothermal cancer therapy, biodetoxification, treatment of atherosclerosis, artificial insemination, crushing kidney stones, cleaning wounds, and removing blood clots and parasites.

Polymer Nanocomposites-A Comparison between Carbon Nanotubes, Graphene, and Clay as Nanofillers

Nanofilled polymeric matrices have demonstrated remarkable mechanical, electrical, and thermal properties. In this article we review the processing of carbon nanotube, graphene, and clay montmorillonite platelet as potential nanofillers to form nanocomposites. The various functionalization techniques of modifying the nanofillers to enable interaction with polymers are summarized. The importance of filler dispersion in the polymeric matrix is highlighted. Finally, the challenges and future outlook for nanofilled polymeric composites are presented.

Reversibly pH-Responsive Nanoporous Layer-by-Layer Microtubes

Nanoporous layer-by-layer (LbL) microtubes consisting of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) are prepared by LbL deposition in porous templates followed by postassembly acid treatment. The formation of the nanoporous structure is studied as a function of solution pH, treatment time, and number of layers. Pore formation is most effective at pH 1.8, requiring only 5 min to achieve a complete transition, and is shown to be reversible. Whereas the inner surface of the porous microtubes is rough, the outer surface is smoother and exhibited isolated pores, suggestive of an asymmetric porous structure.

Biodegradable protein-based rockets for drug transportation and light-triggered release

We describe a biodegradable, self-propelled bovine serum albumin/poly-l-lysine (PLL/BSA) multilayer rocket as a smart vehicle for efficient anticancer drug encapsulation/delivery to cancer cells and near-infrared light controlled release. The rockets were constructed by a template-assisted layer-by-layer assembly of the PLL/BSA layers, followed by incorporation of a heat-sensitive gelatin hydrogel containing gold nanoparticles, doxorubicin, and catalase. These rockets can rapidly deliver the doxorubicin to the targeted cancer cell with a speed of up to 68 μm/s, through a combination of biocatalytic bubble propulsion and magnetic guidance. The photothermal effect of the gold nanoparticles under NIR irradiation enable the phase transition of the gelatin hydrogel for rapid release of the loaded doxorubicin and efficient killing of the surrounding cancer cells. Such biodegradable and multifunctional protein-based microrockets provide a convenient and efficient platform for the rapid delivery and controlled release of therapeutic drugs.

Redox responsive nanotubes from organometallic polymers by template assisted layer by layer fabrication

Redox responsive nanotubes were fabricated by the template assisted layer-by-layer (LbL) assembly method and employed as platforms for molecular payload release. Positively and negatively charged organometallic poly(ferrocenylsilane)s (PFS) were used to construct the nanotubes, in combination with other polyions. During fabrication, multilayers of these polyions were deposited onto the inner pores of template porous membranes, followed by subsequent removal of the template. Anodized porous alumina and track-etched polycarbonate membranes were used as templates. The morphology, electrochemistry, composition and other properties of the obtained tubular structure were characterized by fluorescence microscopy, scanning (SEM) and transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy. Composite nanotubes, consisting of poly(acrylic acid) anions with PFS(+) and nanoparticles including fluorophore labelled dextran and decorated quantum dots, with PFS polyelectrolytes were also fabricated, broadening the scope of the structures. Cyclic voltammograms of PFS containing nanotubes showed similar redox responsive behaviour to thin LbL assembled films. Redox triggered release of labelled macromolecules from these tubular structures demonstrated application potential in controlled molecular delivery.

Nanomaterial processing using self-assembly-bottom-up chemical and biological approaches

Nanotechnology is touted as the next logical sequence in technological evolution. This has led to a substantial surge in research activities pertaining to the development and fundamental understanding of processes and assembly at the nanoscale. Both top-down and bottom-up fabrication approaches may be used to realize a range of well-defined nanostructured materials with desirable physical and chemical attributes. Among these, the bottom-up self-assembly process offers the most realistic solution toward the fabrication of next-generation functional materials and devices. Here, we present a comprehensive review on the physical basis behind self-assembly and the processes reported in recent years to direct the assembly of nanoscale functional blocks into hierarchically ordered structures. This paper emphasizes assembly in the synthetic domain as well in the biological domain, underscoring the importance of biomimetic approaches toward novel materials. In particular, two important classes of directed self-assembly, namely, (i) self-assembly among nanoparticle-polymer systems and (ii) external field-guided assembly are highlighted. The spontaneous self-assembling behavior observed in nature that leads to complex, multifunctional, hierarchical structures within biological systems is also discussed in this review. Recent research undertaken to synthesize hierarchically assembled functional materials have underscored the need as well as the benefits harvested in synergistically combining top-down fabrication methods with bottom-up self-assembly.

ZnO nanowire array-templated LbL self-assembled polyelectrolyte nanotube arrays and application for charged drug delivery

Vertically oriented and robust polyelectrolyte nanotube arrays with high density, large area and high uniformity were successfully grown on substrates by a ZnO nanowire array-templated layer-by-layer (LbL) self-assembly approach for the first time, and were further used to deliver charged drugs, showing that they not only possess pH-responsive loading property, but also significantly enhance the loading capacity and sustained release time. This work could be extended to fabricate polyelectrolyte nanotube arrays with different polyelectrolyte combinations, including weak polyelectrolyte/weak polyelectrolyte, weak polyelectrolyte/strong polyelectrolyte and strong polyelectrolyte/strong polyelectrolyte. With the great versatility to use various substrates and building blocks, the polyelectrolyte nanotube arrays may have great potential for broad applications such as biosensor arrays, bioreactor arrays and optoelectronics.

Protein-based nanotubes for biomedical applications

This review presents highlights of our latest results of studies directed at developing protein-based smart nanotubes for biomedical applications. These practical biocylinders were prepared using an alternate layer-by-layer (LbL) assembly of protein and oppositely charged poly(amino acid) into a nanoporous polycarbonate (PC) membrane (pore diameter, 400 nm), with subsequent dissolution of the template. The tube wall typically comprises six layers of poly-L-arginine (PLA) and human serum albumin (HSA) [(PLA/HSA)(3)]. The obtained (PLA/HSA)(3) nanotubes (NTs) can be dispersed in aqueous medium and are hydrated significantly. Several ligands for HSA, such as zinc(II) protoporphyrin IX (ZnPP), were bound to the HSA component in the cylindrical wall. Similar NTs comprising recombinant HSA mutant, which has a strong binding affinity for ZnPP, captured the ligand more tightly. The Fe(3)O(4)-coated NTs can be collected easily by exposure to a magnetic field. The hybrid NTs bearing a single avidin layer as an internal wall captured biotin-labeled nanoparticles into the central channel when their particle size is sufficiently small to enter the pores. The NTs with an antibody surface interior entrapped human hepatitis B virus with size selectivity. It is noteworthy that the infectious Dane particles were encapsulated completely into the hollows. Other HSA-based NTs having an α-glucosidase inner wall hydrolysed a glucopyranoside to yield α-D-glucose. A perspective of the practical use of the protein-based NTs is also described.

Reshaping elastic nanotubes via self-assembly of surface-adhesive nanoparticles

Elastic sheets with macroscopic dimensions are easy to deform by bending and stretching. Yet shaping nanometric sheets by mechanical manipulation is hard. Here we show that nanoparticle self-assembly could be used to this end. We demonstrate that spherical nanoparticles adhering to the outer surface of an elastic nanotube can self-assemble into linear structures: rings or helices on stretchable nanotubes, and axial strings on nanotubes with high rigidity to stretching. These self-assembled structures are inextricably linked to a variety of deformed nanotube profiles, which can be controlled by tuning the concentration of nanoparticles, the nanoparticle-nanotube diameter ratio and the elastic properties of the nanotube. Our results open the possibility of designing nanoparticle-laden tubular nanostructures with tailored shapes, for potential applications in materials science and nanomedicine.

Layer-by-Layer Assemblies in Nanoporous Templates: Nano-Organized Design and Applications of Soft Nanotechnology

The synergistic combination of layer-by-layer (LbL) assembly and nanoporous membrane templating has greatly facilitated the creation of complex and functional nanotubular structures. The approach takes advantage of both the new properties conferred by assembling diverse LbL building blocks and the tight dimensional control offered by nanotemplating to enable new functionalities that arise from the highly anisotropic "one-dimensional" LbL-nanotube format. In this review, we aim to convey the key developments and provide a current snap-shot of such templated LbL nanoarchitectures. We survey recent developments that have enabled the assembly of polymers, biomolecules and inorganic nanoparticles "à la carte", via electrostatic, covalent and specific (bio)recognition interactions. We also discuss the emerging mechanistic understanding of the LbL assembly process within the nanopore environment. Finally, we present a diverse range of LbL nanotube "devices" to illustrate the versatility of the nanotemplated LbL toolbox for generating functional soft nanotechnology.

Growth mechanism of confined polyelectrolyte multilayers in nanoporous templates

We investigate the mechanism of polyelectrolyte multilayer (PEM) assembly in nanoporous templates with a view to synthesizing nanotubes or nanowires under optimal conditions. For this purpose, we focus on the effect of parameters related to the geometrical constraints (pore diameter), the size of the macromolecules (their molar mass and the ionic strength), and the interaction between the pore walls and the adsorbed chains (modulated by the ionic strength). Our results reveal the existence of two regimes in the mechanism of PEM growth: (i) the first regime is comparable to that observed on flat substrates, including the influence of ionic strength and (ii) the second regime, which is slower in terms of kinetics, results from the interconnection established between polyelectrolyte chains across the pores and leads to the formation of a dense gel. As a consequence, the diffusion of polyelectrolytes in nanopores becomes the controlling factor of PEM growth in this second regime. The dense gel, owing to its peculiar structure, enhances the formation of nanowires or of partially occluded nanotubes in some cases, depending on initial pore dimensions.

By what means should nanoscaled materials be constructed: molecule, medium, or human?

There is great potential in nanoscale science and technology, and construction of macrosized materials and systems possessing nanoscale structural features is a crucial factor in the everyday application of nanoscience and nanotechnology. Because nanoscale substances are often constructed through self-assembly of unit molecules and nanomaterials, control of the self-assembly process is required. In order to establish general guidelines for the fabrication of materials with nanoscale structural characteristics, i.e., nanoscaled materials, we introduce here examples of recent research in related fields categorised as: (i) self-assembled structures with forms generally determined by intrinsic interactions between molecules and/or unit nanomaterials, (ii) self-assemblies influenced by their surrounding media, especially interfacial environments, (iii) modulation of self-assembly by artificial operation or external stimuli. Examples are not limited to organic molecules, which are often regarded as the archetypal species in self-assembly chemistry, and many examples of inorganic assemblies and hybrid structures are included in this review.

Molecular capture in protein nanotubes

We describe molecular capturing properties of protein nanotubes with a controllable ligand binding affinity and size selectivity. These practical biocylinders were prepared using an alternating layer-by-layer (LbL) assembly of protein and oppositely charged poly(amino acid) into the nanoporous polycarbonate (PC) membrane (pore diameter, 400 nm), with subsequent dissolution of the template. The tube wall typically comprises six layers of poly-L-arginine (PLA) and human serum albumin (HSA) [(PLA/HSA)(3)]. Use of high molecular weight PLA (M(w) = ca. 70 000) yielded robust nanotubes, which are available as lyophilized powder. The (PLA/HSA)(3) nanotubes swelled considerably in water, although the outer diameter was almost unaltered. Uranyl ion, 3,3'-diethylthiacarbocyanine iodide, and zinc(II) protoporphyrin IX (ZnPP) were bound to the HSA component in the cylinder wall. Similar nanotubes comprising recombinant HSA mutant [rHSA(His)], which has a strong binding affinity for ZnPP, captured this ligand more tightly. Furthermore, addition of excess myristic acid released ZnPP from the tubes through a ligand replacement reaction. The hybrid nanotubes bearing a single avidin layer as an internal surface captured FITC-biotin efficiently. Biotin-labeled nanoparticles are also incorporated into the tubes when their particle size is sufficiently small to enter the pores. Subsequent TEM observation revealed a line of loaded nanoparticles (100 nm) in the one-dimensional space interior.

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.