Genetically Modifiable Flagella as Templates for Silica Fibers: From Hybrid Nanotubes to 1D Periodic Nanohole Arrays

Bacterial flagellum is a protein nanotube that is helically self‐assembled from thousands of a protein subunit called flagellin. The solvent‐exposed domain of each flagellin on the flagella is genetically modifiable, in that a foreign peptide can be genetically inserted into this domain, leading to the high‐density display of this foreign peptide on the surface of flagella. In this work, wild‐type and genetically engineered flagella (inner diameter of ～2 nm and outer diameter of ～14 nm) detached from the surface of Salmonella bacterial cells are used as templates to site‐specifically form silica sheaths on the flagellar surface, resulting in the formation of double‐layered silica/flagella nanotubes. The flagella templates inside the silica/flagella nanotubes can be removed to obtain silica nanotubes by calcining the nanotubes at high temperature (550°C). Further calcination of the silica nanotubes at a higher temperature (800 °C) leads to the formation of a periodic nanohole array along the silica fibers with a center‐to‐center nanohole spacing of ～79 nm. It is demonstrated that the double‐layered silica‐flagella nanotubes can be used for selective CdTe quantum dot uptake into the inner channels or selective Au nanoparticle coating on the outer wall of the nanotubes due to the different chemistry between inner flagellum core (protein) and outer silica wall of the nanotubes. It is also found that flagella displaying different peptides result in different morphologies of the silica nanotubes. This work suggests that the monodisperse diameter and genetically tunable surface chemistry of the flagella can be exploited for the fabrication of silica nanotubes with uniform diameter and controllable morphologies as well as silica nanofibers decorated with periodic nanohole arrays.     

Hybrid “Golden Fleece”: Synthesis and Catalytic Performance of Uniform Carbon Nanofibers and Silica Nanotubes Embedded with a High Population of Noble‐Metal Nanoparticles

Carbon nanofibers produced by hydrothermal carbonization display remarkable reactivity and the capability for in situ loading with very fine noble‐metal nanoparticles of metals such as Pd, Pt, and Au. Large quantities of uniform carbon nanofibers embedded/confined with various kinds of noble‐metal nanoparticles can be easily prepared, resulting in the formation of the so‐called uniform and well‐defined “hybrid fleece” structures. In addition, a general method has been developed to synthesize uniform silica nanotubes embedded/confined with noble‐metal nanoparticles by using the “hybrid fleece” consisting of carbon nanofibers loaded with noble‐metal nanoparticles as a template. To the best of our knowledge, the filling of silica nanotubes with a dense population of noble‐metal nanoparticles has not been demonstrated so far. These hybrid carbon structures embedded with noble‐metal nanoparticles in a heterogeneous “fleece” geometry serve as excellent catalysts for a model reaction involving the conversion of CO to CO₂ at low temperatures.     

Hierarchical Self‐Assembly of Peptide‐Coated Carbon Nanotubes

Numerous applications, from molecular electronics to super‐strong composites, have been suggested for carbon nanotubes. Despite this promise, difficulty in assembling raw carbon nanotubes into functional structures is a deterrent for applications. In contrast, biological materials have evolved to self‐assemble, and the lessons of their self‐assembly can be applied to synthetic materials such as carbon nanotubes. Here we show that single‐walled carbon nanotubes, coated with a designed amphiphilic peptide, can be assembled into ordered hierarchical structures. This novel methodology offers a new route for controlling the physical properties of nanotube systems at all length scales from the nano‐ to the macroscale. Moreover, this technique is not limited to assembling carbon nanotubes, and could be modified to serve as a general procedure for controllably assembling other nanostructures into functional materials.     

Morphology of Polymer/Liquid‐Crystal Nanotubes: Influence of Confinement

Polymer/liquid‐crystal (LC) tubes consisting of an approximately 30 nm thick poly(methyl methacrylate) (PMMA) layer on the outside and a 5 to 10 nm thick discotic liquid‐crystalline layer on the inside of the tube walls have been prepared by wetting ordered porous alumina templates with a pore diameter of 400 nm. Decreasing the pore diameter to 60 nm results in a confinement‐induced transition from a wetting state to a non‐wetting state, and solid rods with a sequential morphology are obtained. The texture of the mesophase depends on the morphology type and the thermal history. Under certain conditions the LC mesophase exhibits a dominant, well‐ordered planar texture where the discotic columns are aligned with the long axes of the tubes. The controlled generation of one‐dimensional nano‐objects possessing mesoscopic fine structures and intrinsic anisotropy should be the first step towards a rational design of miniaturized building blocks.     

Self‐Assembling Nanotubes Consisting of Rigid Cyclic γ‐Peptides

Self‐assembling cyclic peptide nanotubes (SPNs) have been extensively studied due to their potential applications in biology and material sciences. Cyclic γ‐peptides, which have a larger conformational space, have received less attention than the cyclic α‐ and β‐peptides. The self‐assembly of cyclic homo‐γ‐tetrapeptide based on cis‐3‐aminocyclohexanecarboxylic acid (γ‐Ach) residues, which can be easily synthesized by a one‐pot process is investigated. Fourier transform infrared (FTIR) and NMR analysis along with density functional theory (DFT) calculations indicate that the cyclic homo‐γ‐tetrapeptide, with a non‐planar conformation, can self‐assemble into nanotubes through hydrogen‐bond‐mediated parallel stacking. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) experiments reveal the formation of bundles of nanotubes in CH₂Cl₂/hexane, but individual nanotubes and bundles of only two nanotubes are obtained in water. The integration of TEG (triethylene glycol) monomethyl ether chains and cyclopeptide backbones may allow the control of width of single nanotubes.     

Dual‐Functionalized Polymer Nanotubes as Substrates for Molecular‐Probe and DNA‐Carrier Applications

Bifunctionalized polymer nanotubes have been fabricated using vapor‐deposition polymerization in FeCl₃‐adsorbed anodic aluminum oxide membranes followed by attachment of amine‐functionalized silica nanoparticles. The prepared bifunctionalized polymer nanotubes are applied as both a molecular probe and a DNA carrier by conjugating pyreneacetic acid with the amine groups and immobilizing DNA with the carboxylic acid groups on the surface. The number of amine functional groups on the nanotubes' surface can be measured by means of the photoluminescence intensity of pyreneacetic acid conjugated with amine groups, and the number of the residual carboxylic acid groups is calculated by titration with sodium hydroxide. Fourier‐transform infrared spectroscopy, X‐ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy have been performed to confirm the complete polymerization of the monomer and the attachment of photoluminescent molecules and single‐stranded DNA.     

Unique Necklace‐Like Phenol Formaldehyde Resin Nanofibers: Scalable Templating Synthesis,Casting Films,and Their Superhydrophobic Property

1D necklace‐like nanostructures have exhibited different potential applications due to their unique geometry and property. However, their macroscopic and controllable synthesis has been a challenge. Herein, a facile and scalable template‐directed hydrothermal process is reported to synthesize a series of necklace‐like phenol‐formaldehyde resin (PFR) wrapped nanocables. The 1D templates involved in the synthesis can be various, such as tellurium nanowires (TeNWs), silver nanowires, and carbon nanotubes. After removal of the TeNWs template, pure PFR necklace‐like nanofibers with different morphologies can be prepared. Owning to their multiscale roughness and formed 3D network structures, such necklace‐like PFR nanofibers can be further used as building blocks for constructing robust superhydrophobic coatings with excellent mechanical properties on various substrates.     

Preparation of Hollow Silica Nanospheres by Surface‐Initiated Atom Transfer Radical Polymerization on Polymer Latex Templates

Poly(vinylbenzyl chloride), (PVBC) latex particles of about 100 nm in size are prepared by emulsion polymerization. Silyl functional groups are introduced onto the PVBC‐nanoparticle templates via surface‐initiated atom transfer radical polymerization of 3‐(trimethoxysilyl)propyl methacrylate. The silyl groups are then converted into a silica shell, approximately 20 nm thick, via a reaction with tetraethoxysilane in ethanolic ammonia. Hollow silica nanospheres are finally generated by thermal decomposition of the PVBC template cores. Field‐emission scanning electron microscopy and field‐emission transmission electron microscopy are used to characterize the intermediate products and the hollow nanospheres. Fourier‐transform infrared spectroscopy results indicate that the polymer cores are completely decomposed.     

Poly(ϵ‐caprolactone)‐Functionalized Carbon Nanotubes and Their Biodegradation Properties

Biodegradable poly(?‐caprolactone) (PCL) has been covalently grafted onto the surfaces of multiwalled carbon nanotubes (MWNTs) by the “grafting from” approach based on in‐situ ring‐opening polymerization of ?‐caprolactone. The grafted PCL content can be controlled easily by adjusting the feed ratio of monomer to MWNT‐supported macroinitiators (MWNT‐OH). The resulting products have been characterized with Fourier‐transform IR (FTIR), NMR, and Raman spectroscopies, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). After PCL was coated onto MWNT surfaces, core/shell structures with nanotubes as the “hard” core and the hairy polymer layer as the “soft” shell are formed, especially for MWNTs coated with a high density of polymer chains. Such a polymer shell promises good solubility/dispersibility of the MWNT–PCL nanohybrids in low‐boiling‐point organic solvents such as chloroform and tetrahydrofuran. Biodegradation experiments have shown that the PCL grafted onto MWNTs can be completely enzymatically degraded within 4 days in a phosphate buffer solution in the presence of pseudomonas (PS) lipase, and the carbon nanotubes retain their tubelike morphologies, as observed by SEM and TEM. The results present possible applications for these biocompatible PCL‐functionalized CNTs in bionanomaterials, biomedicine, and artificial bones.     

Synthesis and Characterization of Silica Nanotubes with Radially Oriented Mesopores

Hierarchical silica nanotubes with radially oriented mesoporous channels perpendicular to the central axis of the tube were synthesized by using self‐assembled chiral anionic surfactant, co‐structure directing agent (CSDA) and silica precursor. The average inner diameter and the wall thickness were ∼94, ∼62, and ∼62 nm and to ∼27, ∼33, and ∼45 nm, respectively, by manipulating the synthesis gel composition, while the diameter of the wall mesopores was kept constant at ∼4 nm. These materials with such a unique structure were produced only with chiral surfactant and achiral or racemic surfactant did not give rise to mesoporous silica nanotubes. The existence of helicity in the lipid bilayer template was confirmed by means of circular dichroism spectroscopy. The mesoporous penetrating from outside to inside of silica nanotubes are thought to originate from the initial formation of self‐assembled lipid tubes with helical bilayers, which in turn re‐assemble to form the mesopores in the wall of the nanotubes upon addition of co‐structure directing agent and silica source.     

Self‐Assembly of Polyaniline—From Nanotubes to Hollow Microspheres

By simply changing the molar ratio of the dopant to monomer, the morphology of salicylic acid (SA)‐doped polyaniline (PANI) can be changed from one‐dimensional nanotubes (～ 109–150 nm in diameter) to three‐dimensional hollow microspheres (～ 1.5–3.1 μm in diameter) via a self‐assembly process. Freeze–fracture electron microscopy (FFEM) proved that hollow spherical micelles composed of SA/aniline act as templates in the formation of either nanotubes or hollow spheres. FTIR and X‐ray diffraction measurements suggest that the hydrogen bond of the –OH group of SA with the amine group of PANI might be a driving force for self‐assembling hollow microspheres, while the hydrogen bond through hydrogen and oxygen of the adjacent SA doped on the polymer chains results in short‐range order of the counter‐ions along the polymer chain in the nanotubes.     

Synthesis and Applications of Stimuli‐Responsive DNA‐Based Nano‐ and Micro‐Sized Capsules

Stimuli‐responsive, drug‐loaded, DNA‐based nano‐ and micro‐capsules attract scientific interest as signal‐triggered carriers for controlled drug release. The methods to construct the nano‐/micro‐capsules involve i) the layer‐by‐layer deposition of signal‐reconfigurable DNA shells on drug‐loaded microparticles acting as templates, followed by dissolution of the core templates; ii) the assembly of three‐dimensional capsules composed of reconfigurable DNA origami units; and iii) the synthesis of stimuli‐responsive drug‐loaded capsules stabilized by DNA?polymer hydrogels. Triggers to unlock the nano‐/micro‐capsules include enzymes, pH, light, aptamer?ligand complexes, and redox agents. The capsules are loaded with fluorescent polymers, metal nanoparticles, proteins or semiconductor quantum dots as drug models, with anti‐cancer drugs, e.g., doxorubicin, or with antibodies inhibiting cellular networks or enzymes over‐expressed in cancer cells. The mechanisms for unlocking the nano‐/micro‐capsules and releasing the drugs are discussed, and the applications of the stimuli‐responsive nano‐/micro‐capsules as sense‐and‐treat systems are addressed. The scientific challenges and future perspectives of nano‐capsules and micro‐capsules in nanomedicine are highlighted.     

Single‐Crystalline LiMn2O4 Nanotubes Synthesized Via Template‐Engaged Reaction as Cathodes for High‐Power Lithium Ion Batteries

Single‐crystalline nanotubes of spinel LiMn₂O₄ with a diameter of about 600 nm, a wall thickness of about 200 nm and a length of 1–4 μm have been synthesized via a template‐engaged reaction using β‐MnO₂ nanotubes as a self‐sacrifice template. In this fabrication, a minimal structural reorganization can be responsible for the chemical transformation from [001]‐oriented β‐MnO₂ template to [110]‐oriented LiMn₂O₄. Galvanostatic charge/discharge measurements indicate that the nanotubes exhibit superior high‐rate capabilities and good cycling stability. About 70% of its initial capacity can be retained after 1500 cycles at 5 C rate. Importantly, the tubular nanostructures and the single‐crystalline nature of the most LiMn₂O₄ nanotubes are also well preserved after prolonged charge/discharge cycling at a relatively high current density, indicating good structural stability of the single‐crystalline nanotubes during lithium intercalation/deintercalation process. As is confirmed from Raman spectra analyses, no evident microstructural changes occur upon long‐term cycling. These results reveal that single‐crystalline nanotubes of LiMn₂O₄ will be one of the most promising cathode materials for high‐power lithium ion batteries.     

Swelling‐Agent‐Free Synthesis of Siliceous and Functional Mesocellular Foam‐Like Mesophases by Using a Carboxy‐Terminated Triblock Copolymer

Swelling‐agent‐free synthesis of mesocellular foam (MCF)‐like silica mesophases by a pH‐dependent structural transformation using carboxy‐terminated triblock copolymer Pluronic P123 has been discovered. The structural properties of the MCF‐like silica materials can be modulated by controlled calcination or post‐synthesis treatment with sulfuric acid, and either closed‐cell or open‐cell mesostructures have been prepared. The MCF‐like silica mesophases have also been applied as hard templates to prepare MCF‐like carbon materials via a nanocasting route. Furthermore, the swelling‐agent‐free synthesis has been found to be less sensitive to the presence of organosilanes, and the cocondensation syntheses of functional MCF‐like materials with carboxyethyl, iodopropyl, or mercaptopropyl groups have also been demonstrated.     

Probing the Dynamic Nature of Self‐Assembling Cyclic Peptide–Polymer Nanotubes in Solution and in Mammalian Cells

Self‐assembling cyclic peptide–polymer nanotubes have emerged as a fascinating supramolecular system, well suited for a diverse range of biomedical applications. Due to their well‐defined diameter, tunable peptide anatomy, and ability to disassemble in situ, they have been investigated as promising materials for numerous applications including biosensors, antimicrobials, and drug delivery. Despite this continuous effort, the underlying mechanisms of assembly and disassembly are still not fully understood. In particular, the exchange of units between individual assembled nanotubes has been overlooked so far, despite its knowledge being essential for understanding their behavior in different environments. To investigate the dynamic nature of these systems, cyclic peptide–polymer nanotubes are synthesized, conjugated with complementary dyes, which undergo a Förster resonance energy transfer (FRET) in close proximity. Model conjugates enable to demonstrate not only that their self‐assembly is highly dynamic and not kinetically trapped, but also that the self‐assembly of the conjugates is strongly influenced by both solvent and concentration. Additionally, the versatility of the FRET system allows studying the dynamic exchange of these systems in mammalian cells in vitro using confocal microscopy, demonstrating the exchange of subunits between assembled nanotubes in the highly complex environment of a cell.     

Fabrication of a Memory Chip by a Complete Self‐Assembly Process Using State‐of‐the‐Art Multilevel Cell (MLC) Technology

Using a two bit molecular switch, an ultra‐dense memory chip has been built following a fully automated fabrication process. Well‐ordered templates are grown naturally using a well‐defined protocol of temperature variation. This template is so designed that molecules are adsorbed selectively only into particular sites whenever they are bombarded on the template through an e‐beam evaporator for a particular time. The technique is a generalized protocol that has been used to grow atomic‐scale templates by proper tuning of basic global parameters like temperature and evaporation time. Tuning of the basic template parameters is also demonstrated here, and has been used to scale down parameter values following the same route. Tuning the junction profile should allow selective adsorption of more complicated multi‐level switches in future. Therefore, a fairly simple technology has been established that addresses one of the most fundamental issues of continuous miniaturization, i.e., simultaneous automated growth of thousands of atomically precise single molecular devices.     

An Optimized and General Synthetic Strategy for Fabrication of Polymeric Carbon Nitride Nanoarchitectures

Nanostructured covalent carbon nitride (CN) holds great promise for artificial photosynthesis, but its nanotexturation using templating methods is restricted by the weak binding affinities of neutral silica templates towards basic precursors that are kinetically difficult to diffuse into the nanopores of the templates. This weak affinity leads to an incomplete inclusion of the CN precursors into the nanostructured silica templates, and consequently, yields a defective replica of the parent porous structures. Here, this issue is addressed through the development of an innovative synthetic strategy to facilitate the sufficient inclusion of CN precursors in silica templates, by taking advantage of the surface acidification of silica and sonication‐promoted insertion. The ordered mesoporous CN (ompg‐CN) fabricated using SBA‐15 mesozeolite as the template has been demonstrated to show a better 2D mesoporous hexagonal framework, larger surface area, and higher photocatalytic activity than that synthesized by the traditional method. This innovative strategy can in general be expanded to other silica templates with various nanostructures, enabling the creation of stable polymeric CN nanostructures with maximized material and structure functions.     

Hexahistidine‐Tagged Single‐Walled Carbon Nanotubes (His6‐tagSWNTs): A Multifunctional Hard Template for Hierarchical Directed Self‐Assembly and Nanocomposite Construction

While a hexahistidine affinity tag can be introduced at protein termini or internal sites by standard molecular biology procedures for purification, immobilization, or labeling of proteins, here the versatility of this concept is exploited for the chemical preparation of novel hexahistidine‐tagged single‐walled carbon nanotubes (His₆‐tagSWNTs), a novel hard template useful for solubilizing, assembling, processing, and interfacing SWNTs in aqueous conditions. Water‐soluble and exfoliated His₆‐tagSWNTs are prepared and fully characterized. This functional molecular module is able to interact via robust physisorption (π?π stacking) with the sidewall of SWNTs and combines the versatility of small, water‐soluble reporters (His₆) for hierarchical directed self‐assembly (HDSA) and construction of nanocomposites. It is demonstrated that metal coordination bonds with Ni(II) can be used for the supramolecular self assembly of His₆‐tagSWNTs, generating complex reticulated networks and architectures. The His₆‐tagSWNTs hard template nanohybrid is further utilized for directed self‐assembly with silica nanoparticles. The versatility of the novel hybrids opens a new era for the rational design, smart (bio)functionalization, processing, interfacing, and self assembling of carbon nanotubes for the construction of multicomposites and more complex systems with controllable spatial organization and programmable properties for a wide range of applications in biology, nanoelectronics, and catalysis.     

制备纳米多孔材料的模板自组装技术

纳米自组装技术的突出优点是:通过改变相应模板的形状和大小可以实现对不同材料形状、结构和大小的预先控制,从而拓展了它的应用范围。本文主要阐述了纳米多孔材料模板自组装技术的原理和工艺流程,介绍了这种技术的几种典型方法的最新进展,并比较了各种方法的优劣,同时展示了它的应用前景。     

Mosaic,Single‐Crystal CaCO3 Thin Films Fabricated on Modified Polymer Templates

Mosaic, single‐crystal CaCO₃ thin films have been prepared on modified poly(ethylene terephthalate) (PET) templates. Surface modification of PET through the introduction of carboxylic acid groups (COOH‐PET), and the subsequent physical and chemical adsorption of poly(allylamine hydrochloride) (PAH) at pH 8 (PAH8‐PET) and pH 11 (PAH11‐PET), afford template surfaces that influenced the phase transition of an amorphous CaCO₃ (ACC) films during crystallization in air. Macroscopic ACC thin films are prepared on modified PET films in the presence of poly(acrylic acid). Polycrystalline, spherulitic vaterite (CaCO₃) films are observed to form on native PET and PAH11‐PET, while mosaic, single‐crystal calcitic (CaCO₃) films form on COOH‐PET and PAH8‐PET templates. These results confirm that single‐crystal CaCO₃ growth patterns are dependent on the surface characteristics of the PET template. We infer therefore, that the nucleation and growth of ceramic films on polymeric templates can be controlled by chemical modification of the polymeric template surface, and by the subsequent attachment of ionic polyelectrolytes.