Importance of aromatic content for peptide/single-walled carbon nanotube interactions

We have previously demonstrated that a designed amphiphilic peptide helix, denoted nano-1, coats and debundles single-walled carbon nanotubes (SWNTs) and promotes the assembly of these coated SWNTs into novel hierarchical structures via peptide-peptide interactions. The purpose of this study is to better understand how aromatic content impacts interactions between peptides and SWNTs. We have designed a series of peptides, based on the nano-1 sequence, in which the aromatic content is systematically varied. Atomic force microscopy measurements and optical absorption spectroscopy reveal that the ability to disperse individual SWNTs increases with increasing aromatic residues in the peptide. Altogether, the results indicate that pi-stacking interactions play an important role in peptide dispersion of SWNTs.     

Effect of electron-donating and electron-withdrawing groups on peptide/single-walled carbon nanotube interactions

Nano-1, a designed peptide, has been demonstrated to efficiently disperse individual single-walled carbon nanotubes (SWNTs) by folding into an amphiphilic alpha-helix wherein the phenylalanine (Phe) residues on the hydrophobic face of the helix interact via pi-stacking with the aromatic surface of the SWNT. In this study, the ability of electron-donating (hydroxyl) and electron-withdrawing (nitro) groups on the phenyl ring of Phe to affect the interactions between the peptide and SWNTs is examined by substituting the Phe residues in the nano-1 sequence with tyrosine and p-nitro-phenylalanine, respectively. Atomic force microscopy measurements and optical absorption spectroscopy revealed that the ability to disperse individual SWNTs increases with increasing electron density of the aromatic residue on the hydrophobic face of the amphiphilic helical peptides. Scanning tunneling spectroscopy (STS) and Raman analyses were used to examine the effect of noncovalent protein functionalization on the electronic properties of SWNTs. Small shifts in the Raman G band peak for the peptide/SWNT composites, as well as weak features that appear near the Fermi energy (Ef) in the STS dI/dV spectra of the peptide-coated SWNTs, are suggestive of a weak charge-transfer interaction between the peptides and the SWNTs.     

Tubular assembly of amphiphilic rigid macrocycle with flexible dendrons

An amphiphilic rigid macrocycle was shown to self-assemble into tubular aggregates that can solubilize SWNTs in aqueous solution through significant pi-pi interactions.     

Direct growth of shape-controlled nanocrystals on nanotubes via biological recognition

The new biological approach was examined to fabricate shape-controlled Ag nanocrystals grown directly on surfaces, inspired by nature that various shapes of nanocrystals are produced accurately and reproducibly in biological systems. Here we demonstrate the direct growth of hexagon-shaped Ag nanocrystals on sequenced peptide-coated nanotubes via biological recognition. When the peptide, Asn-Pro-Ser-Ser-Leu-Phe-Arg-Tyr-Leu-Pro-Ser-Asp, recognizing and effecting the Ag nanocrystal growth on the (111) face, was sequenced and incorporated onto template nanotube surfaces, the biomineralization of Ag ions on the nanotubes led the isotropic hexagon-shaped Ag nanocrystal coating under pH control of the growth solution. Multiple Ag nanocrystal shapes were observed when the peptide mineralized Ag ions without the template nanotubes, and therefore the template nanotube has a significant influence on regulating the majority of Ag nanocrystals into the hexagonal shape. This biological approach, using specific peptide sequences on surfaces to control nanocrystal shapes, may be developed as a simple and economical method to produce building blocks with desired physical properties for new generation of electronics, sensors, and optical devices.     

Preparation and characterization of individual peptide-wrapped single-walled carbon nanotubes

Two challenges for effectively exploiting the remarkable properties of single-walled carbon nanotubes (SWNTs) are the isolation of intact individual nanotubes from the raw material and the assembly of these isolated SWNTs into useful structures. In this study, we present atomic force microscopy (AFM) evidence that we can isolate individual peptide-wrapped SWNTs, possibly connected end-to-end into long fibrillar structures, using an amphiphilic alpha-helical peptide, termed nano-1. Transmission electron microscopy (TEM) and well-resolved absorption spectral features further corroborate nano-1's ability to debundle SWNTs in aqueous solution. Peptide-assisted assembly of SWNT structures, specifically in the form of Y-, X-, and intraloop junctions, was observed in the AFM and TEM images.     

Diameter-selective solubilization of single-walled carbon nanotubes by reversible cyclic peptides

We have utilized reversible cyclic peptides (RCPs)-peptides containing alternating l- and d-amino acids with N- and C-termini derivatized with thiol-containing groups allowing reversible peptide cyclization-to solubilize and noncovalently functionalize carbon single-walled nanotubes (SWNTs) in aqueous solution. Solubilization occurs through wrapping of RCPs around the circumference of a SWNT, followed by the formation of head-to-tail covalent bonds, yielding closed rings on the nanotubes. By controlling the length of the RCPs, we have demonstrated limited diameter-selective solubilization of the SWNTs as revealed by UV/vis/NIR and Raman spectroscopies, as well as atomic force microscopy.     

Self-organization of carbon nanotubes in evaporating droplets

Here, we report a simple and efficient way for organizing carbon nanotubes, in particular, single-wall carbon nanotubes (SWNTs) into ordered structures from their dilute solutions. It was found that drying a droplet of carbon nanotube solution at room temperature on a wettable surface such as glass or silica wafer led to redistribution, accumulation, and organization of carbon nanotubes along the perimeter of the droplet. Unlike the aggregation behaviors of colloid nanoparticles, anistropic carbon nanotubes tended to show two orientations in a ring deposit: one parallel to the outer perimeter of the ring and the other normal to it in the interior. Drying droplets of SWNT solutions at high temperatures exhibited a long-range ordered structure. In addition, droplet drying may cause size separation of carbon nanotubes and pattern formation through interactions between droplets. This result helps us not only to further understand fluid dynamics during the drying process but also to provide a promising and simple strategy for either assembling carbon nanotubes on a surface or organizing them into well-aligned films and fibers.     

De novo design, synthesis, and function of semiartificial myoglobin conjugated with coiled-coil two-alpha-helix peptides

The introduction of a flavin chromophore on the myoglobin (Mb) surface and an effective electron-transfer (ET) reaction through the flavin were successfully achieved by utilizing the self-assembly of heterostranded coiled-coil peptides. We have prepared a semiartificial Mb, named Mb-1alphaK, in which an amphiphilic and cationic alpha-helix peptide is conjugated at the heme propionate (Heme-1alphaK). Heme-1alphaK has a covalently bound iron-protoporphyrin IX (heme) at the N terminus of a 1alphaK peptide sequence. This sequence was designed to form a heterostranded coiled-coil in the presence of a counterpart amphiphilic and anionic 1alphaE peptide sequence in a parallel orientation. Two peptides, Fla(1)-1alphaE and Fla(31)-1alphaE, both incorporating a 10-methylisoalloxazine moiety as an artificial flavin molecule, were also prepared (Fla=2-[7-(10-methyl)isoalloxazinyl]-2-oxoethyl). Heme-1alphaK was successfully inserted into apomyoglobin to give Mb-1alphaK. Mb-1alphaK recognized the flavin-modified peptides and a two-alpha-helix structure was formed. In addition, an efficient ET from reduced nicotinamide adenine dinucleotide to the heme center through the flavin unit was observed. The ET rate was faster in the presence of Fla(1)-1alphaE than in the presence of Fla(31)-1alphaE or the equivalent molecule that has no peptide chain. These results demonstrate that the introduction of a functional chromophore on the Mb surface can be achieved by using specific peptide-peptide interactions. Moreover, the dependence of the ET rate on the position of the flavin indicated that the distance between the heme active site and the flavin chromophore was regulated by the three-dimensional structure of the designed polypeptide.     

Transformation of peptide nanotubes into a vesicle via fusion driven by stereo-complex formation

Two types of peptide nanotubes, one is prepared from an amphiphilic peptide having a right-handed helix segment and the other from that having a left-handed helix segment, are shown to transform the morphology into a vesicle by membrane fusion due to stereo-complex formation between these helical segments.     

Present and future applications of carbon nanotubes to analytical science

This article reviews the impact of carbon nanotubes on analytical science, and the main current and future applications of carbon nanotubes in this field. Given that it is necessary to solubilize carbon nanotubes for many applications, we consider the procedures developed to achieve this. The use of carbon nanotubes in analytical chemistry as a target analyte and as an analytical tool is also discussed. Chromatographic and electrophoretic methods used to separate and characterize carbon nanotubes are presented. The use of carbon nanotubes as an analytical tool in filters and membranes, as sorbent material for solid phase extraction, in electrochemical (bio)sensors, and in separation methods is discussed. It is clear that while nanotubes are being tested for use in many different fields, their truly enormous potential has yet to be realized in analytical chemistry.Dedicated to the memory of Wilhelm Fresenius     

Preparation of micelle-encapsulated single-wall and multi-wall carbon nanotubes with amphiphilic hyperbranched polymer

The hyperbranched polyester (Boltorn^TM H20) was modified by maleic anhydride and then polystyrene (H20-MAh-PSt) to form amphiphilic micelles in water. The single-wall and multi-wall carbon nanotubes (SWCNTs and MWCNTs, respectively) were encapsulated in the formed micelles through non-covalent interactions. The formed structures were confirmed by FTIR, NMR, GPC, and XPS analysis. The dispersion and aggregation behaviors were observed by TEM and UV-vis and Raman spectroscopic analysis. The results showed that the dispersion performance of the obtained micelle-encapsulated carbon nanotubes in water was greatly improved compared to the pure carbon nanotubes. From the TEM observation, the individual SWCNT structure and the uniform polymer coating around the surface of SWCNT were seen after crosslinking. The Raman spectroscopic measurements also demonstrated that for the crosslinked samples, no effect occurred associated with concentration-dependent carbon nanotube aggregation.     

两亲性聚合物修饰碳纳米管研究进展

随着两亲性共聚物在越来越多的领域显示出其功能,它的重要理论和应用意义正为人们所认识。用它来修饰碳纳米管,将会赋予碳纳米管更加优异的性能,因此为碳纳米管的研究提供了一个方向。本文综述了两亲性聚合物修饰碳纳米管表面的研究进展,从化学修饰和物理吸附、包裹两种修饰手段入手,着重介绍了两亲性聚合物化学修饰碳纳米管的方法和研究状况。最后,对修饰后的两亲性聚合物碳纳米管的分散性和自组装行为进行了综述。     

破碎-絮凝法分离细长碳纳米管与碳纤维

根据碳纤维与细长碳纳米管耐磨性能与絮凝沉降性能的差异，提出了一种有效分离细长碳纳米管与碳纤维的物理方法——破碎-絮凝法.该方法包括研磨破碎、液相分散、絮凝沉降、过滤分离等步骤，可高效去除混杂于细长碳纳米管样品中的碳纤维，同时还可去除螺旋状碳纤维及细小碳颗粒等易悬浮杂质.纯化过程对细长碳纳米管无损伤.用电子显微镜和热重分析表征了纯化效果，并初步分析了纯化机理.     

Unilamellar Vesicles from Amphiphilic Graphene Quantum Dots

Graphene quantum dots (GQDs) have attracted considerable interest due to their unique physicochemical properties and various applications. For the first time it is shown that GQDs surface‐functionalized with hydrocarbon chains (i.e., amphiphilic GQDs) self‐assemble into unilamellar spherical vesicles in aqueous solution. The amphiphilic GQD vesicles exhibit multicolor luminescence that can be readily exploited for membrane studies by fluorescence spectroscopy and microscopy. The GQD vesicles were used for microscopic analysis of membrane interactions and disruption by the peptide beta‐amyloid.     

Interactions between polymers and carbon nanotubes: a molecular dynamics study

We used force-field-based molecular dynamics to study the interaction between polymers and carbon nanotubes (CNTs). The intermolecular interaction energy between single-walled carbon nanotubes and polymers was computed, and the morphology of polymers adsorbed to the surface of nanotubes was investigated. Furthermore, the "wrapping" of nanotubes by polymer chains was examined. It was found that the specific monomer structure plays a very important role in determining the strength of interaction between nanotubes and polymers. The results of our study suggest that polymers with a backbone containing aromatic rings are promising candidates for the noncovalent binding of carbon nanotubes into composite structures. Such polymers can be used as building blocks in amphiphilic copolymers to promote increased interfacial binding between the CNT and a polymeric matrix.     

Amphiphilic Peptide–Polymer Conjugates with Side‐Conjugation

Polymers conjugated to the exterior of a protein mediate its interactions with surroundings, enhance its processability and can be used to direct its macroscopic assemblies. Most studies to date have focused on peptide–polymer conjugates based on hydrophilic polymers. Engineering amphiphilicity into protein motifs by covalently linking hydrophobic polymers has the potential to interface peptides and proteins with synthetic polymers, organic solvents, and lipids to fabricate functional hybrid materials. Here, we synthesized amphiphilic peptide–polymer conjugates in which a hydrophobic polymer is conjugated to the exterior of a heme‐binding four‐helix bundle and systematically investigated the effects of the hydrophobicity of the conjugated polymer on the peptide structure and the integrity of the heme‐binding pocket. In aqueous solution with surfactants present, the side‐conjugated hydrophobic polymers unfold peptides and may induce an α‐helix to β‐sheet conformational transition. These effects decrease as the polymer becomes less hydrophobic and directly correlate with the polymer hydrophobicity. Upon adding organic solvent to solubilize the hydrophobic polymers, however, the deleterious effects of hydrophobic polymers on the peptide structures can be eliminated. Present studies demonstrate that protein structure is sensitive to the local environment. It is feasible to dissolve amphiphilic peptide–polymer conjugates in organic solvents to enhance their solution processability while maintaining the protein structures.

     

Structural control of peptide-coated gold nanoparticle assemblies by the conformational transition of surface peptides

Gold nanoparticles having peptide chains on the surfaces have been prepared yb ring-opening polymerization of gamma-methyl L-glutamate N-carboxyanhydride with fixed amino groups on the nanoparticle surface as an initiator. The number of peptide chains on the surface was adjusted to ca. 2 molecules per gold nanoparticle by controlling the number of fixed amino groups on the surface. The peptide chains on the surface were partially saponified to obtain poly(gamma-methyl L-glutamate-co-L-glutamic acid) with 28 mol% of glutamic acid residues. The number-average molecular weight of the peptide was 73,000. We described structural control of the peptide-coated gold nanoparticle assembly by conformational transition of the surface peptides. In deionized water, the peptide chains on the nanoparticle took a random coil conformation, and the individual nanoparticles existed in dispersed globular species. On the other hand, the peptide chains on the nanoparticle took an alpha-helical conformation in trifluoroethanol. Under this condition, the alpha-helical peptide chains on distinct gold nanoparticles connected the nanoparticles to form a fibril assembly owing to the dipole-dipole interaction between the surface peptide chains. The morphology of the peptide-coated gold nanoparticle assembly could be controlled by the conformational transition of surface peptides, which was attended by solution composition changes.     

Noncovalent functionalization of single-walled carbon nanotubes with water-soluble porphyrins

We have employed water-soluble porphyrin molecules [meso-(tetrakis-4-sulfonatophenyl) porphine dihydrochloride] to solubilize single-walled carbon nanotubes (SWNTs), resulting in aqueous solutions that are stable for several weeks. The porphyrin-nanotube complexes have been characterized with absorption and fluorescence spectroscopy and with AFM. We find that the porphyrin/SWNT interaction is selective for the free base form, and that this interaction stabilizes the free base against protonation to the diacid. Under mildly acidic conditions nanotube-mediated J-aggregates form, which are unstable in solution and result in precipitation of the nanotubes over the course of a few days. Porphyrin-coated SWNTs can be precisely aligned on hydrophilic poly(dimethylsiloxane) (PDMS) surfaces by combing SWNT solution along a desired direction and then transferred to silicon substrates by stamping. Parallel SWNT patterns have been fabricated in this manner.     

Dynamic mechanism of collagen-like peptide encapsulated into carbon nanotubes

Carbon nanotubes (CNTs)-based devices and their applications have received more and more attention, and several biomolecules have been found to be encapsulated into the inner space of the CNTs spontaneously. In this work, the molecular dynamics simulations demonstrate that a collagen-like peptide with a hydrophobic center and hydrophilic surfaces could be inserted into CNTs spontaneously but slowly. Then the dynamic mechanism of the encapsulation process was investigated by a series of steered molecular dynamics simulations. The van der Waals interaction between the peptide and the carbon nanotubes was proved to be a positive factor for this insertion process, whereas the major resistance of this process is attributed to the repelling of the trapped water molecules and the breaking of the hydrogen-bond networks of water molecules around the peptide. Because of the synthetical effect of these interactions, there is an optimal tube size corresponding to effective encapsulation of the protein into the CNTs for a given kind of protein molecule.     

两亲分子对碳纳米管的分散稳定作用

综述了近年来国内外对碳纳米管在两亲分子水溶液中的分散作用研究, 从表面活性剂、聚合物和生物大分子三方面, 分别阐述了用非成键法对碳纳米管进行分散的不同机理. 离子型表面活性剂或聚电解质主要靠亲水基团之间的静电斥力阻止碳纳米管之间的聚集, 而非离子型表面活性剂或大分子则主要靠亲水基团所产生的空间位阻使分散体系保持稳定.