Ranking the affinity of aromatic residues for carbon nanotubes by using designed surfactant peptides

Immobilization of Cellulase Enzymes on Single-Walled Carbon Nanotubes for Recycling of Enzymes and Better Yield of Bioethanol Using Computer Simulations

The utilization of microbial cellulase enzymes for transforming plant biomass into biofuel or bioethanol, which can serve as a substitute for fossil fuel, is a subject of growing interest. Nonetheless, large-scale production of biofuel using cellulases is not economically feasible as the extraction of these enzymes from diverse microorganisms is an expensive process. To address this issue, immobilizing the enzyme to a substrate material, e.g., carbon nanotubes (CNTs), to recycle without a significant decline in its catalytic activity is a promising solution. Due to the hydrophobic nature of CNTs, we employed molecular docking and network analysis methodologies to identify potential CNT-binding sites on the outer surface of a wild-type cellulase enzyme, CelS. Classical molecular dynamics simulations of CNT-bound CelS through one of the selected binding sites resulted in negligible changes in the secondary structure of the enzyme and its catalytic domain, implying the least possible effect on the catalytic activity post-immobilization. Furthermore, our study reveals that while the unfolding near the CNT-binding region in CelS is more pronounced when the enzyme is interacting with a wider CNT, resulting in enhanced contact area and improved binding affinity, its impact on the overall CelS structure is relatively less significant when compared to thinner CNTs. Particularly, CNTs of diameter ∼12 Å can serve as a favorable option for substrate materials in cellulase immobilization. Our study also provides critical insights into the binding mechanisms between cellulase and CNTs, which could lead to the development of more efficient biocatalysts for biofuel production.

A Critical Review on Classified Excipient Sodium-Alginate-Based Hydrogels: Modification, Characterization, and Application in Soft Tissue Engineering

Alginates are polysaccharides that are produced naturally and can be isolated from brown sea algae and bacteria. Sodium alginate (SA) is utilized extensively in the field of biological soft tissue repair and regeneration owing to its low cost, high biological compatibility, and quick and moderate crosslinking. In addition to their high printability, SA hydrogels have found growing popularity in tissue engineering, particularly due to the advent of 3D bioprinting. There is a developing curiosity in tissue engineering with SA-based composite hydrogels and their potential for further improvement in terms of material modification, the molding process, and their application. This has resulted in numerous productive outcomes. The use of 3D scaffolds for growing cells and tissues in tissue engineering and 3D cell culture is an innovative technique for developing in vitro culture models that mimic the in vivo environment. Especially compared to in vivo models, in vitro models were more ethical and cost-effective, and they stimulate tissue growth. This article discusses the use of sodium alginate (SA) in tissue engineering, focusing on SA modification techniques and providing a comparative examination of the properties of several SA-based hydrogels. This review also covers hydrogel preparation techniques, and a catalogue of patents covering different hydrogel formulations is also discussed. Finally, SA-based hydrogel applications and future research areas concerning SA-based hydrogels in tissue engineering were examined.

Biomolecule-Directed Carbon Nanotube Self-Assembly

The strategy of combining biomolecules and synthetic components to develop biohybrids is becoming increasingly popular for preparing highly customized and biocompatible functional materials. Carbon nanotubes (CNTs) benefit from bioconjugation, allowing their excellent properties to be applied to biomedical applications. This study reviews the state-of-the-art research in biomolecule-CNT conjugates and discusses strategies for their self-assembly into hierarchical structures. The review focuses on various highly ordered structures and the interesting properties resulting from the structural order. Hence, CNTs conjugated with the most relevant biomolecules, such as nucleic acids, peptides, proteins, saccharides, and lipids are discussed. The resulting well-defined composites allow the nanoscale properties of the CNTs to be exploited at the micro- and macroscale, with potential applications in tissue engineering, sensors, and wearable electronics. This review presents the underlying chemistry behind the CNT-based biohybrid materials and discusses the future directions of the field.

Denatured lysozyme-coated carbon nanotubes: a versatile biohybrid material

Carbon nanotubes (CNTs) are among the most versatile nanomaterials, but their exploitation is hindered by limited dispersibility, especially in aqueous solvents. Here, we show that AP-LYS, a highly cationic soluble derivative of denatured hen egg lysozyme, is a very effective tool for the unbundling and solubilisation of CNTs. AP-LYS proved to mediate the complete and stable dispersion of CNTs at protein: CNT ratios ≥1: 3 (w:w) in very mild conditions (10–20 minutes sonication in ammonium acetate buffer, pH 5.0). Electrophoretic mobility and ζ-potential measurements confirmed that dispersed CNTs were coated by the protein, whereas molecular docking was used to study the interactions between AP-LYS and CNTs. AP-LYS-coated CNTs proved to be a very effective microbial cell-flocculating agent with an efficiency similar to that of chitosan, one of the best available flocculating agents, thus suggesting that this hybrid could find industrial applications in the treatment of wastewaters contaminated by microbial cells, or to remove microbial cells after fermentation processes. Moreover, we exploited the low stability of AP-LYS-coated CNT dispersions in eukaryotic cell culture media to prepare scaffolds with an extracellular matrix-like rough surface for the cultivation of eukaryotic cells.

Tracing chirality, diameter dependence, and temperature-controlling of single-walled carbon nanotube non-covalent functionalization by biologically compatible peptide: insights from molecular dynamics simulations

Biological applications of single-walled carbon nanotubes (SWCNTs), including drug delivery, require their functionalization with various functional groups such as peptides. Recently, a biologically compatible peptide (named PW3 with the sequence of NH₂-Trp-Val-Trp-Val-Trp-Val-Lys-Lys-COOH) has been introduced as a good candidate for modification of carbon nanotubes due to its high affinity toward the exterior surface of these nano-carriers. In order to optimize the process of SWCNT peptide functionalization, the effects of chirality and diameter of SWCNTs as well as the temperature on PW3 adsorption were systematically investigated using molecular dynamics (MD) simulation. It was found that modification of chiral/zigzag SWCNT by PW3 peptide was more suitable compared with the armchair system due to the strong peptide-nanotube interactions and more water solubility at 310 K which can be well explained by microscopic structural investigations. Regarding the enhanced peptide-chiral nanotube interactions at the low temperature of 277 K, chiral nanotubes can be effective structures for SWCNT functionalization process at reduced temperatures. Our analysis indicated that disrupted PW3 and SWCNT hydration patterns and fewer internal interactions within the peptide could be responsible for the stronger peptide modification of SWCNT at higher temperatures. Additionally, "PW3/SWCNT" systems containing larger tube diameters formed more stable complexes owing to their effective surface area increment.

An investigation into non-covalent functionalization of a single-walled carbon nanotube and a graphene sheet with protein G:A combined experimental and molecular dynamics study

Investigation of non-covalent interaction of hydrophobic surfaces with the protein G (PrG) is necessary due to their frequent utilization in immunosensors and ELISA. It has been confirmed that surfaces, including carbonous-nanostructures (CNS) could orient proteins for a better activation. Herein, PrG interaction with single-walled carbon nanotube (SWCNT) and graphene (Gra) nanostructures was studied by employing experimental and MD simulation techniques. It is confirmed that the PrG could adequately interact with both SWCNT and Gra and therefore fine dispersion for them was achieved in the media. Results indicated that even though SWCNT was loaded with more content of PrG in comparison with the Gra, the adsorption of the PrG on Gra did not induce significant changes in the IgG tendency. Several orientations of the PrG were adopted in the presence of SWCNT or Gra; however, SWCNT could block the PrG-FcR. Moreover, it was confirmed that SWCNT reduced the α-helical structure content in the PrG. Reduction of α-helical structure of the PrG and improper orientation of the PrG-SWCNT could remarkably decrease the PrG tendency to the Fc of the IgG. Importantly, the Gra could appropriately orient the PrG by both exposing the PrG-FcR and also by blocking the fragment of the PrG that had tendency to interact with Fab in IgG.

Peptide Probe for Multiwalled Carbon Nanotubes: Electrophoretic Assessment of the Binding Interface and Evaluation of Surface Functionalization

Noncovalent interactions of peptides and proteins with carbon nanotubes play a key role in sensing, dispersion, and biocompatibility. Advances in these areas require that the forces which contribute to physical adsorption are understood in order that the carbon nanotubes present a degree of functionalization appropriate to the desired application. Affinity analyses of peptides are employed to evaluate the role of tryptophan and arginine residues in physical adsorption to carboxylated multiwalled carbon nanotubes. Peptides containing arginine and tryptophan, WR(W) _n, are used with affinity capillary electrophoresis to identify factors that lead to the formation of peptide-carbon nanotube complexes. The effects of changing the amino acid composition and residue length are evaluated by measuring dissociation constants. Electrostatic interactions contribute significantly to complexation, with the strongest interaction observed using the peptide WRWWWW and carboxylated carbon nanotube. Stronger interaction is observed when the tryptophan content is successively increased as follows: WR(W)₄ > WR(W)₃ > WR(W)₂ > WRW > WR. However, as observed with polytryptophan (W₅, W₄, W₃, and W₂), removing the arginine residue significantly reduces the interaction with carbon nanotubes. Increasing the arginine content to WRWWRW does not improve binding, whereas replacing the arginine residue in WRWWWW with lysine (WKWWWW) reveals that lysine also contributes to surface adsorption, but not as effectively as arginine. These observations are used to guide a search of the primary sequence of lysozyme to identify short regions in the peptide that contain a single cationic residue and two aromatic residues. One candidate peptide sequence (WMCLAKW) from this search is analyzed by capillary electrophoresis. The dissociation constant of carboxylated multiwalled carbon nanotubes is measured for the peptide, WMCLAKW, to demonstrate the utility of affinity capillary electrophoresis analysis.

Multifunctional, biocompatible and pH-responsive carbon nanotube- and graphene oxide/tectomer hybrid composites and coatings

Here we present a route for non-covalent functionalization of carboxylated multi-walled carbon nanotubes and graphene oxide with novel two-dimensional peptide assemblies. We show that self-assembled amino-terminated biantennary and tetraantennary oligoglycine peptides (referred to as tectomers) effectively coat carboxylated multi-walled carbon nanotubes and also strongly interact with graphene oxide due to electrostatic interactions and hydrogen bonding as the driving force, respectively. The resulting hybrids can be made into free-standing conducting composites or applied in the form of thin, pH-switchable bioadhesive coatings onto graphene oxide fibers. Monitoring of cell viability of pancreatic cell lines, seeded on those CNT hybrids, show that they can be used as two- and three-dimensional scaffolds to tissue engineer tumour models for studying ex vivo the tumour development and response to treatment. This highly versatile method in producing pH-responsive hybrids and coatings offers an attractive platform for a variety of biomedical applications and for the development of functional materials such as smart textiles, sensors and bioelectronic devices.

Noncovalent Protein and Peptide Functionalization of Single-Walled Carbon Nanotubes for Biodelivery and Optical Sensing Applications

The exquisite structural and optical characteristics of single-walled carbon nanotubes (SWCNTs), combined with the tunable specificities of proteins and peptides, can be exploited to strongly benefit technologies with applications in fields ranging from biomedicine to industrial biocatalysis. The key to exploiting the synergism of these materials is designing protein/peptide-SWCNT conjugation schemes that preserve biomolecule activity while keeping the near-infrared optical and electronic properties of SWCNTs intact. Since sp² bond-breaking disrupts the optoelectronic properties of SWCNTs, noncovalent conjugation strategies are needed to interface biomolecules to the nanotube surface for optical biosensing and delivery applications. An underlying understanding of the forces contributing to protein and peptide interaction with the nanotube is thus necessary to identify the appropriate conjugation design rules for specific applications. This article explores the molecular interactions that govern the adsorption of peptides and proteins on SWCNT surfaces, elucidating contributions from individual amino acids as well as secondary and tertiary protein structure and conformation. Various noncovalent conjugation strategies for immobilizing peptides, homopolypeptides, and soluble and membrane proteins on SWCNT surfaces are presented, highlighting studies focused on developing near-infrared optical sensors and molecular scaffolds for self-assembly and biochemical analysis. The analysis presented herein suggests that though direct adsorption of proteins and peptides onto SWCNTs can be principally applied to drug and gene delivery, in vivo imaging and targeting, or cancer therapy, nondirect conjugation strategies using artificial or natural membranes, polymers, or linker molecules are often better suited for biosensing applications that require conservation of biomolecular functionality or precise control of the biomolecule's orientation. These design rules are intended to provide the reader with a rational approach to engineering biomolecule-SWCNT platforms, broadening the breadth and accessibility of both wild-type and engineered biomolecules for SWCNT-based applications.

Molecular dynamics simulation of non-covalent single-walled carbon nanotube functionalization with surfactant peptides

Non-covalent functionalized single-walled carbon nanotubes (SWCNTs) with improved solubility and biocompatibility can successfully transfer drugs, DNA, RNA, and proteins into the target cells. Theoretical studies such as molecular docking and molecular dynamics simulations in fully atomistic scale were used to investigate the hydrophobic and aromatic π-π-stacking interaction of designing four novel surfactant peptides for non-covalent functionalization of SWCNTs. The results indicated that the designed peptides have binding affinity towards SWCNT with constant interactions during MD simulation times, and it can even be improved by increasing the number of tryptophan residues. The aromatic content of the peptides plays a significant role in their adsorption in SWCNT wall. The data suggest that π-π stacking interaction between the aromatic rings of tryptophan and π electrons of SWCNTs is more important than hydrophobic effects for dispersing carbon nanotubes; nevertheless SWCNTs are strongly hydrophobic in front of smooth surfaces. The usage of aromatic content of peptides for forming SWCNT/peptide complex was proved successfully, providing new insight into peptide design strategies for future nano-biomedical applications.

Self-assembly of diphenylalanine backbone homologues and their combination with functionalized carbon nanotubes

The integration of carbon nanotubes (CNTs) into organized nanostructures is of great interest for applications in materials science and biomedicine. In this work we studied the self-assembly of β and γ homologues of diphenylalanine peptides under different solvent and pH conditions. We aimed to investigate the role of peptide backbone in tuning the formation of different types of nanostructures alone or in combination with carbon nanotubes. In spite of having the same side chain, β and γ peptides formed distinctively different nanofibers, a clear indication of the role played by the backbone homologation on the self-assembly. The variation of the pH allowed to transform the nanofibers into spherical structures. Moreover, the co-assembly of β and γ peptides with carbon nanotubes covalently functionalized with the same peptide generated unique dendritic assemblies. This comparative study on self-assembly using diphenylalanine backbone homologues and of the co-assembly with CNT covalent conjugates is the first example exploring the capacity of β and γ peptides to adopt precise nanostructures, particularly in combination with carbon nanotubes. The dendritic organization obtained by mixing carbon nanotubes and peptides might find interesting applications in tissue engineering and neuronal interfacing.

Liquid Chromatographic Analysis of the Interaction between Amino Acids and Aromatic Surfaces Using Single-Wall Carbon Nanotubes

Proteins have nonspecific adsorption capacities for solid surfaces. Although the nonspecific adsorption capacities are generally understood to be related to the hydrophobicity or charge density of the surfaces, little is known at the amino acid level about the interaction between proteins and polyaromatic surfaces such as carbon nanotubes, which have recently been used for biotechnology applications. In this study, we investigated the interaction between proteinogenic amino acids and carbon nanotubes using high-performance liquid chromatography on silica matrices coated by single-wall carbon nanotubes (SWCNTs). Among the amino acids used in this study, tryptophan, tyrosine, and phenylalanine showed exceptional affinity for the matrices. The characteristic affinities of these amino acids were ascribed to their unique interactions with the large polyaromatic surfaces of the SWCNTs. These results are useful for understanding and controlling protein adsorption onto aromatic surfaces.

Dewetting transition assisted clearance of (NFGAILS) amyloid fibrils from cell membranes by graphene

Clearance of partially ordered oligomers and monomers deposited on cell membrane surfaces is believed to be an effective route to alleviate many potential protein conformational diseases (PCDs). With large-scale all-atom molecular dynamics simulations, here we show that graphene nanosheets can easily and quickly win a competitive adsorption of human islet amyloid polypeptides (hIAPP22-28) NFGAILS and associated fibrils against cell membrane, due to graphene's unique two-dimensional, highly hydrophobic surface with its all-sp(2) hybrid structure. A nanoscale dewetting transition was observed at the interfacial region between the fibril (originally deposited on the membrane) and the graphene nanosheet, which significantly assisted the adsorption of fibrils onto graphene from the membrane. The π-π stacking interaction between Phe23 and graphene played a crucial role, providing the driving force for the adsorption at the graphene surface. This study renders new insight towards the importance of water during the interactions between amyloid peptides, the phospholipidic membrane, and graphene, which might shed some light on future developments of graphene-based nanomedicine for preventing/curing PCDs like type II diabetes mellitus.

Molecular-level understanding of the adsorption mechanism of a graphite-binding peptide at the water/graphite interface

The association of proteins and peptides with inorganic material has vast technological potential. An understanding of the adsorption of peptides at liquid/solid interfaces on a molecular-level is fundamental to fully realising this potential. Combining our prior work along with the statistical analysis of 100+ molecular dynamics simulations of adsorption of an experimentally identified graphite binding peptide, GrBP5, at the water/graphite interface has been used here to propose a model for the adsorption of a peptide at a liquid/solid interface. This bottom-up model splits the adsorption process into three reversible phases: biased diffusion, anchoring and lockdown. Statistical analysis highlighted the distinct roles played by regions of the peptide studied here throughout the adsorption process: the hydrophobic domain plays a significant role in the biased diffusion and anchoring phases suggesting that the initial impetus for association between the peptide and the interface may be hydrophobic in origin; aromatic residues dominate the interaction between the peptide and the surface in the adsorbed state and the polar region in the middle of the peptide affords a high conformational flexibility allowing strongly interacting residues to maximise favourable interactions with the surface. Reversible adsorption was observed here, unlike in our prior work focused on a more strongly interacting surface. However, this reversibility is unlikely to be seen once the peptide-surface interaction exceeds 10 kcal mol(-1).

The role of basic residues in the adsorption of blood proteins onto the graphene surface

With its many unique properties, graphene has shown great potential in various biomedical applications, while its biocompatibility has also attracted growing concerns. Previous studies have shown that the formation of protein-graphene corona could effectively reduce its cytotoxicity; however, the underlying molecular mechanism remains not well-understood. Herein, we use extensive molecular dynamics simulations to demonstrate that blood proteins such as bovine fibrinogen (BFG) can absorb onto the graphene surface quickly and tightly to form a corona complex. Aromatic residues contributed significantly during this adsorption process due to the strong π−π stacking interactions between their aromatic rings and the graphene sp²-carbons. Somewhat surprisingly, basic residues like arginine, also played an equally or even stronger role during this process. The strong dispersion interactions between the sidechains of these solvent-exposed basic residues and the graphene surface provide the driving force for a tight binding of these basic residues. To the best of our knowledge, this is the first study with blood proteins to show that, in addition to the aromatic residues, the basic residues also play an important role in the formation of protein-graphene corona complexes.

Interfacing proteins with graphitic nanomaterials: from spontaneous attraction to tailored assemblies

Thiscritical reviewpresents a detailed overview of the chemico-physical principles ruling the non-covalent association between proteins and fullerene, carbon nanotubes and graphene towards the creation of fascinating and innovative hybrid materials for biotechnological applications.

Efficient simulations of the aqueous bio-interface of graphitic nanostructures with a polarisable model

To fully harness the enormous potential offered by interfaces between graphitic nanostructures and biomolecules, detailed connections between adsorbed conformations and adsorption behaviour are needed. To elucidate these links, a key approach, in partnership with experimental techniques, is molecular simulation. For this, a force-field (FF) that can appropriately capture the relevant physics and chemistry of these complex bio-interfaces, while allowing extensive conformational sampling, and also supporting inter-operability with known biological FFs, is a pivotal requirement. Here, we present and apply such a force-field, GRAPPA, designed to work with the CHARMM FF. GRAPPA is an efficiently implemented polarisable force-field, informed by extensive plane-wave DFT calculations using the revPBE-vdW-DF functional. GRAPPA adequately recovers the spatial and orientational structuring of the aqueous interface of graphene and carbon nanotubes, compared with more sophisticated approaches. We apply GRAPPA to determine the free energy of adsorption for a range of amino acids, identifying Trp, Tyr and Arg to have the strongest binding affinity and Asp to be a weak binder. The GRAPPA FF can be readily incorporated into mainstream simulation packages, and will enable large-scale polarisable biointerfacial simulations at graphitic interfaces, that will aid the development of biomolecule-mediated, solution-based graphene processing and self-assembly strategies.

The devil and holy water: protein and carbon nanotube hybrids

Integrating carbon nanotubes (CNTs) with biological systems to form hybrid functional assemblies is an innovative research area with great promise for medical, nanotechnology, and materials science applications. The specifics of molecular recognition and catalytic activity of proteins combined with the mechanical and electronic properties of CNTs provides opportunities for physicists, chemists, biologists, and materials scientists to understand and develop new nanomachines, sensors, or any of a number of other molecular assemblies. Researchers know relatively little about the structure, function, and spatial orientation of proteins noncovalently adsorbed on CNTs, yet because the interaction of CNTs with proteins depends strongly on the tridimensional structure of the proteins, many of these questions can be answered in simple terms. In this Account, we describe recent research investigating the properties of CNT/protein hybrids. Proteins act to solvate CNTs and may sort them according to diameter or chirality. In turn, CNTs can support and immobilize enzymes, creating functional materials. Additional applications include proteins that assemble ordered hierarchical objects containing CNTs, and CNTs that act as protein carriers for vaccines, for example. Protein/CNT hybrids can form bioscaffolds and can serve as therapeutic and imaging materials. Proteins can detect CNTs or coat them to make them biocompatible. One of the more challenging applications for protein/CNT hybrids is to make CNT substrates for cell growth and neural interfacing applications. The challenge arises from the structures' interactions with living cells, which poses questions surrounding the (nano)toxicology of CNTs and whether and how CNTs can detect biological processes or sense them as they occur. The surface chemistry of CNTs and proteins, including interactions such as π-π stacking interactions, hydrophobic interactions, surfactant-like interactions, and charge-π interactions, governs the wealth of structures, processes, and functions that appear when such different types of molecules interact. Each residue stars in one of two main roles, and understanding which residues are best suited for which type of interaction can lead to the design of new hybrids. Nonlocally, the peptide or protein primary, secondary, and tertiary structures govern the binding of proteins by CNTs. The conjugation of proteins with CNTs presents some serious difficulties both experimentally and culturally (such as bridging the "jargon barrier" across disciplines). The intersection of these fields lies between communities characterized by distinctly different approaches and methodologies. However, the examples of this Account illustrate that when this barrier is overcome, the exploitation of hybrid CNT-protein systems offers great potential.

Molecular dynamics study of carbon nanotube as a potential dual-functional inhibitor of HIV-1 integrase

HIV-1 integrase (IN) plays an important role in integrating viral DNA into human genome, which has been considered as the drug target for anti-AIDS therapy. The appearance of drug-resistance mutants urgently requires novel inhibitors that act on non-active site of HIV-1 IN. Nanoparticles have such unique geometrical and chemical properties, which inspires us that nanoparticles like nanotubes may serve as better HIV-1 IN inhibitors than the conventional inhibitors. To test this hypothesis, we performed molecular dynamics (MD) simulation to study the binding of a carbon nanotube (CNT) to a full-length HIV-1 IN. The results showed that the CNT could stably bind to the C-terminal domain (CTD) of HIV-1 IN. The CNT also induced a domain-shift which disrupted the binding channel for viral DNA. Further MD simulation showed that a HIV-1 IN inhibitor, 5ClTEP was successfully sealed inside the uncapped CNT. These results indicate that the CNT may serve as a potential dual-functional HIV-1 IN inhibitor, not only inducing conformation change as an allosteric inhibitor but also carrying small-molecular inhibitors as a drug delivery system.

Interactions between proteins and carbon-based nanoparticles: exploring the origin of nanotoxicity at the molecular level

The widespread application of nanomaterials has spurred an interest in the study of interactions between nanoparticles and proteins due to the biosafety concerns of these nanomaterials. In this review, a summary is presented of some of the recent studies on this important subject, especially on the interactions of proteins with carbon nanotubes (CNTs) and metallofullerenols. Two potential molecular mechanisms have been proposed for CNTs' inhibition of protein functions. The driving forces of CNTs' adsorption onto proteins are found to be mainly hydrophobic interactions and the so-called π-π stacking between CNTs' carbon rings and proteins' aromatic residues. However, there is also recent evidence showing that endohedral metallofullerenol Gd@C82 (OH)22 can be used to inhibit tumor growth, thus acting as a potential nanomedicine. These recent findings have provided a better understanding of nanotoxicity at the molecular level and also suggested therapeutic potential by using nanoparticles' cytotoxicity against cancer cells.

Carbon-binding designer proteins that discriminate between sp2- and sp3-hybridized carbon surfaces

Robust and simple strategies to directly functionalize graphene- and diamond-based nanostructures with proteins are of considerable interest for biologically-driven manufacturing, biosensing, and bioimaging. Here, we identify a new set of carbon-binding peptides that vary in overall hydrophobicity and charge and engineer two of these sequences (Car9 and Car15) within the framework of E. coli thioredoxin 1 (TrxA). We develop purification schemes to recover the resulting TrxA derivatives in a soluble form and conduct a detailed analysis of the mechanisms that underpin the interaction of the fusion proteins with carbonaceous surfaces. Although equilibrium quartz crystal microbalance measurements show that TrxA::Car9 and TrxA::Car15 have similar affinities for sp(2)-hybridized graphitic carbon (Kd = 50 and 90 nM, respectively), only the latter protein is capable of dispersing carbon nanotubes. Further investigation by surface plasmon resonance and atomic force microscopy reveals that TrxA::Car15 interacts with sp(2)-bonded carbon through a combination of hydrophobic and π-π interactions but that TrxA::Car9 exhibits a cooperative mode of binding that relies on a combination of electrostatics and weaker π stacking. Consequently, we find that TrxA::Car9 binds equally well to sp(2)- and sp(3)-bonded (diamondlike) carbon particles whereas TrxA::Car15 is capable of discriminating between the two carbon allotropes. Our results emphasize the importance of understanding both bulk and molecular recognition events when exploiting the adhesive properties of solid-binding peptides and proteins in technological applications.

Modifying the electronic properties of single-walled carbon nanotubes using designed surfactant peptides

The electronic properties of carbon nanotubes can be altered significantly by modifying the nanotube surface. In this study, single-walled carbon nanotubes (SWCNTs) were functionalized noncovalently using designed surfactant peptides, and the resultant SWCNT electronic properties were investigated. These peptides have a common amino acid sequence of X(Valine)(5)(Lysine)(2), where X indicates an aromatic amino acid containing either an electron-donating or electron-withdrawing functional group (i.e. p-amino-phenylalanine or p-cyano-phenylalanine). Circular dichroism spectra showed that the surfactant peptides primarily have random coil structures in an aqueous medium, both alone and in the presence of SWCNTs, simplifying analysis of the peptide/SWCNT interaction. The ability of the surfactant peptides to disperse individual SWCNTs in solution was verified using atomic force microscopy and ultraviolet-visible-near-infrared spectroscopy. The electronic properties of the surfactant peptide/SWCNT composites were examined using the observed nanotube Raman tangential band shifts and the observed additional features near the Fermi level in the scanning tunneling spectroscopy dI/dV spectra. The results revealed that SWCNTs functionalized with surfactant peptides containing electron-donor or electron-acceptor functional groups showed n-doped or p-doped altered electronic properties, respectively. This work unveils a facile and versatile approach to modify the intrinsic electronic properties of SWCNTs using a simple peptide structure, which is easily adaptable to obtain peptide/SWCNT composites for the design of tunable nanoscale electronic devices.

Controlling self-assembly of engineered peptides on graphite by rational mutation

Self-assembly of proteins on surfaces is utilized in many fields to integrate intricate biological structures and diverse functions with engineered materials. Controlling proteins at bio-solid interfaces relies on establishing key correlations between their primary sequences and resulting spatial organizations on substrates. Protein self-assembly, however, remains an engineering challenge. As a novel approach, we demonstrate here that short dodecapeptides selected by phage display are capable of self-assembly on graphite and form long-range-ordered biomolecular nanostructures. Using atomic force microscopy and contact angle studies, we identify three amino acid domains along the primary sequence that steer peptide ordering and lead to nanostructures with uniformly displayed residues. The peptides are further engineered via simple mutations to control fundamental interfacial processes, including initial binding, surface aggregation and growth kinetics, and intermolecular interactions. Tailoring short peptides via their primary sequence offers versatile control over molecular self-assembly, resulting in well-defined surface properties essential in building engineered, chemically rich, bio-solid interfaces.

High-performance semiconducting nanotube inks: progress and prospects

While the potential for high mobility printed semiconducting nanotube inks has been clear for over a decade, a myriad of scientific and technological issues has prevented commercialization and practical use. One of the most challenging scientific problems has been to understand the relationship between the pristine, individual nanotube mobility (known to be in the 10,000 cm(2)/V·s range) and the as-deposited random network mobility (recently demonstrated in the 100 cm(2)/V·s range). An additional significant scientific hurdle has been to understand, manage, and ultimately eliminate the effects of metallic nanotubes on the network performance, specifically the on/off ratio. Additional scientific progress is important in understanding the dependence of nanotube length, diameter, and density on device performance. Finally, the development of ink formulations that are of practical use in manufacturing is of paramount importance, especially with regard to drying time and uniformity, and ultimately, the issue of scalability and cost must be addressed. Many of these issues have recently been investigated from a phenomenological point of view, and a comprehensive understanding is beginning to emerge. In this paper, we present an overview of solution-based printed carbon nanotube devices and discuss long-term technology prospects. While significant technical challenges still remain, it is clear that the prospects for the use of nanotube ink in a myriad of systems is feasible given their unmatched mobility and compatibility with heterogeneous integration into a variety of applications in printed and flexible electronics.

The potential of perylene bisimide derivatives for the solubilization of carbon nanotubes and graphene

Carbon nanotubes and graphene are outstanding materials of the 21st century with a broad spectrum of applications. However, major challenges are faced such as the intrinsically low solubility of both sp2 carbon allotropes. To overcome this hurdle the potential of noncovalent functionalization is summarized with a special focus on the establishment of the perylene bisimide unit as aromatic anchor to the graphitic surface. Rational surfactant design is unmasked as the key to solubilization of the carbon allotropes, while at the same time tailoring their surface properties, or even electronic properties in a fully reversible fashion.

Probing diameter-selective solubilisation of carbon nanotubes by reversible cyclic peptides using molecular dynamics simulations

Molecular dynamics simulations are used to explore the encapsulation behaviour of reversible cyclic peptides when adsorbed onto single-walled carbon nanotubes (CNTs) in aqueous solution. Our findings suggest that CNT encapsulation via cyclisation of a single peptide chain is relatively less likely, compared with encapsulation via two-chain complexes. These two-chain complexes comprise pairings of the motifs identified for single-chain adsorption. Our simulation data are compared with existing experimental findings [A. Ortiz-Acevedo et al., J. Am. Chem. Soc., 2005, 127, 9512], for relevant CNT diameters, and are found to be consistent with the experimental results. Our data help to explain the limited diameter selectivity reported by Ortiz-Acevedo et al. These findings should help in the optimisation and future design of peptides capable of enhanced selectivity for specific CNT diameters.

Influence of alternating L-/D-amino acid chiralities and disulfide bond geometry on the capacity of cysteine-containing reversible cyclic peptides to disperse carbon nanotubes

Although single-walled carbon nanotubes (SWNTs) have exciting properties and potential applications, their hydrophobic nature makes them difficult to purify and manipulate. To fully realize the potential of SWNTs, strategies for the effective dispersion, separation, and organization of these materials must be devised. In this article, work involving the recent design and characterization of reversible cyclic peptides (RCPs) and RCP/SWNT composites will be described. The peptides in this work contain alternating L- and D-amino acid sequences, as well as N- and C-terminal cysteine residues (RCP-Cys) that allow for their covalent closure around the circumference of individual SWNTs. When RCPs are oxidized in the presence of SWNTs, dispersions are produced that are stable against dilution by dialysis without the formation of aggregates. The reported studies using Raman spectroscopy and UV/Vis/NIR were focused on answering the questions (1) does the chirality of the disulfide bond impact the capacity of the RCP-Cys to disperse SWNTs, and (2) is the alternating chirality of the amino acids in the RCP-Cys peptides important for SWNT dispersion. It was found that though Cys-containing RCPs are indeed able to disperse SWNTs, the chirality of the Cys residues on the N- and C- termini does not have a significant influence on the dispersed SWNT population. However, there is a large decrease in the dispersability by RCP-Cys when the alternating L/D-chiral pattern of amino acids is replaced with all L-amino acids.