Noncovalent sidewall functionalization of single-walled carbon nanotubes for protein immobilization

Diameter- and metallicity-selective enrichment of single-walled carbon nanotubes using polymethacrylates with pendant aromatic functional groups

Current methods for the synthesis of single-walled nanotubes (SWNTs) produce mixtures of semiconducting (sem-) and metallic (met-) nanotubes. Most approaches to the chemical separation of sem-/met-SWNTs are based on small neutral molecules or conjugated aromatic polymers, which characteristically have low separation/dispersion efficiencies or present difficulties in the postseparation removal of the polymer so that the resulting field-effect transistors (FETs) have poor performance. In this Full Paper, the use of three polymethacrylates with different pendant aromatic functional groups to separate cobalt-molybdenum catalyst (CoMoCAT) SWNTs according to their metallicity and diameters is reported. UV/Vis/NIR spectroscopy indicates that poly(methyl-methacrylate-co-fluorescein-o-acrylate) (PMMAFA) and poly(9-anthracenylmethyl-methacrylate) (PAMMA) preferentially disperse semiconducting SWNTs while poly(2-naphthylmethacrylate) (PNMA) preferentially disperses metallic SWNTs, all in dimethylforamide (DMF). Photoluminescence excitation (PLE) spectroscopy indicates that all three polymers preferentially disperse smaller-diameter SWNTs, particularly those of (6,5) chirality, in DMF. When chloroform is used instead of DMF, the larger-diameter SWNTs (8,4) and (7,6) are instead selected by PNMA. The solvent effects suggest that diameter selectivity and change of polymer conformation is probably responsible. Change of the polymer fluorescence upon interaction with SWNTs indicates that metallicity selectivity presumably results from the photon-induced dipole-dipole interaction between polymeric chromophore and SWNTs. Thin-film FET devices using semiconductor-enriched solution with PMMAFA have been successfully fabricated and the device performance confirms the sem-SWNTs enrichment with a highly reproducible on/off ratio of about 10(3).

Langmuir-Blodgett films of pyridyldithio-modified multiwalled carbon nanotubes as a support to immobilize hydrogenase

Pyridylthio-modified multiwalled carbon nanotubes (pythio-MWNTs) have been prepared by a reaction of the oxidized MWNTs with S-(2-aminoethylthio)-2-thiopyridine hydrochloride. The obtained pythio-MWNTs nanocomposites formed stable floating monolayers at the air-water interface, which were transferred onto substrate surfaces by the Langmuir-Blodgett (LB) method. Compositions and morphologies of the LB films were characterized by absorption, Raman, X-ray photoelectron spectra as well as by scan electron microscopy and atomic force microscopy. These pythio-MWNTs LB films were then used as a support to immobilize hydrogenase (H(2)ase) to form bionanocomposite of pythio-MWNTs-H(2)ase. Cyclic voltammograms for indium tin oxide electrode covered with the pythio-MWNTs-H(2)ase films were investigated in both Ar and H(2) saturated 0.05 M KCl electrolyte solutions at pH from 4.0 to 9.0. A reversible redox couple of [4Fe-4S](2+/1+) clusters of H(2)ase was recorded when the pH value was 6.0 and 9.0, with reduction and oxidation potentials appearing at about -0.70 and -0.35 V vs Ag/AgCl, respectively. It was revealed that the H(2)ase was of high catalytic activity and strong stability in the LB films of pythio-MWNTs-H(2)ase. Hence, we suggested that the present bionanocomposites could be used as heterogeneous biocatalyst to catalyze reversible reaction between protons and H(2), resulting in potential applications in biohydrogen evolution and H(2) biofuel cells.

Nano aptasensor for protective antigen toxin of anthrax

We demonstrate a highly sensitive nano aptasensor for anthrax toxin through the detection of its polypeptide entity, protective antigen (PA toxin) using a PA toxin ssDNA aptamer functionalized single-walled carbon nanotubes (SWNTs) device. The aptamer was developed in-house by capillary electrophoresis systematic evolution of ligands by exponential enrichment (CE-SELEX) and had a dissociation constant (K(d)) of 112 nM. The aptasensor displayed a wide dynamic range spanning up to 800 nM with a detection limit of 1 nM. The sensitivity was 0.11 per nM, and it was reusable six times. The aptasensor was also highly selective for PA toxin with no interference from human and bovine serum albumin, demonstrating it as a potential tool for rapid and point-of-care diagnosis for anthrax.

Single-walled carbon nanotube/lyotropic liquid crystal hybrid materials fabricated by a phase separation method in the presence of polyelectrolyte

We present a detailed study on the incorporation of single-walled carbon nanotubes (SWNTs) into lyotropic liquid crystals (LLC) by phase separation in the presence of polyelectrolytes. Two cases were studied in this work: (i) incorporation of SWNTs into the LLC phase formed by an anionic surfactant sodium dodecyl sulfate (SDS) in the presence of an anionic polyelectrolyte poly(sodium styrenesulfonate) (PSS); (ii) incorporation of SWNTs into the LLC phase formed by a cationic surfactant cetyltrimethylammonium bromide (CTAB) in the presence of a cationic polyelectrolyte poly(diallydimethylammonium chloride) (PDADMAC). The SWNTs/LLC composites were characterized by polarized optical microscopy (POM) observations and small-angle X-ray scattering (SAXS) measurements. In both systems, the surfactant phase was condensed into a hexagonal lattice by the polyelectrolyte within the investigated concentration range. Several factors that can influence the property of SWNTs/LLC composite were examined, including concentration of surfactants and polyelectrolytes and temperature. Aggregated SWNTs were not observed, indicating that SWNTs were well dispersed in the LLC phases. SAXS measurements showed the lattice parameter of the host LLC phase changed upon varying the mixing ratio of polyelectrolyte to ionic surfactant. The SWNTs/LLC hybrids showed considerable stability against temperature rise in both systems, and desorption of surfactant from SWNTs was not observed at higher temperature.

Driving Forces and Consequences of the Adsorption of Proteins to Carbon Nanotubes

Different strategies used to biofunctionalize CNTs with proteins, from direct physical adsorption on pristine CNTs to chemical treatments to achieve covalent interaction, are described. The discussion is focused on the consequences of the adsorption process on the structure and properties of both proteins and CNTs. On this base, recent developments in CNTs-proteins based biosensors (electrochemical and optical) and drug delivery systems are reviewed.

In situ surface plasmon resonance investigation of the assembly process of multiwalled carbon nanotubes on an alkanethiol self-assembled monolayer for efficient protein immobilization and detection

In situ surface plasmon resonance (SPR) was used to study the assembly process of multiwalled carbon nanotubes (MWCNTs) quantitatively on an alkanethiol self-assembled monolayer (SAM) surface, showing that MWCNTs can follow the Langmuir adsorption kinetics to assemble spontaneously whereas the assembly temperature has an essential influence on the assembly kinetics and the surface distribution of MWCNTs. To further in situ investigate protein attachment on the MWCNT surface and its sensing application quantitatively, goat IgG was immobilized by three strategies: direct adsorption, covalent binding, and 1-pyrenebutanoic acid, succinimidyl ester (PBSE)-assisted attachment, of which the covalent binding approach provides the best protein loading capacity. The SPR label-free detection of anti-goat IgG demonstrates excellent performance with high sensitivity, good specificity, and rapid response in comparison to that with a plain substrate without MWCNT assembly reported in our previous work. This is contributed by the 3D MWCNT assembly matrix providing a high probe immobilization capability and superb accessibility for the target to enhance its sensing performance significantly.

Carbon nanostructure-based field-effect transistors for label-free chemical/biological sensors

Over the past decade, electrical detection of chemical and biological species using novel nanostructure-based devices has attracted significant attention for chemical, genomics, biomedical diagnostics, and drug discovery applications. The use of nanostructured devices in chemical/biological sensors in place of conventional sensing technologies has advantages of high sensitivity, low decreased energy consumption and potentially highly miniaturized integration. Owing to their particular structure, excellent electrical properties and high chemical stability, carbon nanotube and graphene based electrical devices have been widely developed for high performance label-free chemical/biological sensors. Here, we review the latest developments of carbon nanostructure-based transistor sensors in ultrasensitive detection of chemical/biological entities, such as poisonous gases, nucleic acids, proteins and cells.

A facile approach for the fabrication of highly stable superhydrophobic cotton fabric with multi-walled carbon nanotubes-azide polymer composites

Homogeneous dispersion and functionalization of pristine multiwalled carbon nanotubes (MWNTs) in various organic solvents was achieved by a simple ultrasonic process in the presence of an azide copolymer, poly(4-azidophenyl methacrylate-co-methyl acrylate)(P(APM-co-MA)). The copolymes were noncovalently attached to the surface of the MWNTs via pi-pi interactions to form MWNT-P(APM-co-MA) composites. The composites were characterized by transmission electron microscopy, thermogravimetric analysis, Raman spectra and UV-vis spectra. The solution dispersion of the MWNT-P(APM-co-MA) composites were used to prepare superhydrophobic cotton fabric by a facile dip-coating approach. MWNTs were covalently attached to the surface of the cotton fabric through the chemical reactions between the azide groups of P(APM-co-MA) with both MWNTs and cotton fibers. The reactions are based on UV-activated nitrene chemistry. Owing to the nanoscale roughness introduced by the attachment of MWNTs, the cotton fabric surface was transformed from hydrophilic to superhydrophobic with an apparent water contact angle of 154 degrees . Since MWNTs were covalently attached on the surface of the cotton fabric, the superhydrophobicity possesses high stability and chemical durability.

Biological semiconductor based on electrical percolation

We have developed a novel biological semiconductor (BSC) based on electrical percolation through a multilayer three-dimensional carbon nanotube-antibody bionanocomposite network, which can measure biological interactions directly and electronically. In electrical percolation, the passage of current through the conductive network is dependent upon the continuity of the network. Molecular interactions, such as binding of antigens to the antibodies, disrupt the network continuity causing increased resistance of the network. A BSC is fabricated by immobilizing a prefunctionalized single-walled carbon nanotubes (SWNTs)-antibody bionanocomposite directly on a poly(methyl methacrylate) (PMMA) surface (also known as plexiglass or acrylic). We used the BSC for direct (label-free) electronic measurements of antibody-antigen binding, showing that, at slightly above the electrical percolation threshold of the network, binding of a specific antigen dramatically increases the electrical resistance. Using anti-staphylococcal enterotoxin B (SEB) IgG as a "gate" and SEB as an "actuator", we demonstrated that the BSC was able to detect SEB at concentrations of 1 ng/mL. The new BSCs may permit assembly of multiple sensors on the same chip to create "biological central processing units (CPUs)" with multiple BSC elements, capable of processing and sorting out information on multiple analytes simultaneously.

Facile synthesis of high-quality graphene nanoribbons

Graphene nanoribbons have attracted attention because of their novel electronic and spin transport properties, and also because nanoribbons less than 10 nm wide have a bandgap that can be used to make field-effect transistors. However, producing nanoribbons of very high quality, or in high volumes, remains a challenge. Here, we show that pristine few-layer nanoribbons can be produced by unzipping mildly gas-phase oxidized multiwalled carbon nanotubes using mechanical sonication in an organic solvent. The nanoribbons are of very high quality, with smooth edges (as seen by high-resolution transmission electron microscopy), low ratios of disorder to graphitic Raman bands, and the highest electrical conductance and mobility reported so far (up to 5e(2)/h and 1,500 cm(2) V(-1) s(-1) for ribbons 10-20 nm in width). Furthermore, at low temperatures, the nanoribbons show phase-coherent transport and Fabry-Perot interference, suggesting minimal defects and edge roughness. The yield of nanoribbons is approximately 2% of the starting raw nanotube soot material, significantly higher than previous methods capable of producing high-quality narrow nanoribbons. The relatively high-yield synthesis of pristine graphene nanoribbons will make these materials easily accessible for a wide range of fundamental and practical applications.

Molecular diagnostic and drug delivery agents based on aptamer-nanomaterial conjugates

Recent progress in an emerging area of designing aptamer and nanomaterial conjugates as molecular diagnostic and drug delivery agents in biomedical applications is summarized. Aptamers specific for a wide range of targets are first introduced and compared to antibodies. Methods of integrating these aptamers with a variety of nanomaterials, such as gold nanoparticles, quantum dots, carbon nanotubes, and superparamagnetic iron oxide nanoparticles, each with unique optical, magnetic, and electrochemical properties, are reviewed. Applications of these systems as fluorescent, colorimetric, magnetic resonance imaging, and electrochemical sensors in medical diagnostics are given, along with new applications as smart drug delivery agents.

Inhibition of proliferation and differentiation of mesenchymal stem cells by carboxylated carbon nanotubes

Multipotent mesenchymal stem cells (MSCs) have attracted substantial attention in stem cell therapy and tissue engineering due to their ability to be cultured for successive passages and multilineage differentiation. Carbon nanotubes (CNTs) have been proposed to be used as potential biomedical structures for bone formation. Therefore, it is important to study the mechanisms of interaction between MSCs and CNTs. We demonstrated that carboxylated single-walled carbon nanotubes (SWCNTs) and carboxylated multiwalled carbon nanotubes (MWCNTs) inhibited the proliferation, osteogenic differentiation, adipogenic differentiation, and mineralization of MSCs. Oxidative stress assay indicated that reactive oxygen species (ROS) may not be responsible for the observed cytotoxicity of carboxylated CNTs. Quantitative real-time polymerase chain reaction (Q-PCR) experiments confirmed that the expression of osteoblast specific genes and adipocyte differentiation specific genes was greatly attenuated during the differentiation of MSCs in the presence of carboxylated CNTs. TEM images revealed that CNTs might interact with proteins located on the cell membrane or in the cytoplasm, which have a further impact on subsequent cellular signaling pathways. Q-PCR results and Western blot analysis together verified that the inhibition of proliferation and osteogenic differentiation of MSCs may be modulated through a Smad-dependent bone morphogenetic protein (BMP) signaling pathway.

Enhanced thermoelectric performance of single-walled carbon nanotubes/polyaniline hybrid nanocomposites

Hybrid nanocomposites containing carbon nanotubes (CNTs) and ordered polyaniline (PANI) have been prepared through an in situ polymerization reaction using a single-walled nanotube (SWNT) as template and aniline as reactant. TEM, SEM, XRD, and Raman analyses show that the polyaniline grew along the surface of CNTs forming an ordered chain structure during the SWNT-directed polymerization process. The SWNT/PANI nanocomposites show both higher electrical conductivity and Seebeck coefficient as compared to pure PANI, which could be attributed to the enhanced carrier mobility in the ordered chain structures of the PANI. The maximum electrical conductivity and Seebeck coefficient of composites reach 1.25 x 10(4) S m(-1) and 40 microV K(-1), respectively, and the maximum power factor is up to 2 x 10(-5) W m(-1) K(-2), more than 2 orders of magnitude higher than the pure polyaniline. This study suggests that constructing highly ordered chain structure is a novel and effective way for improving the thermoelectric properties of conducting polymers.

Clustering of stimuli on single-walled carbon nanotube bundles enhances cellular activation

Functionalized single-walled carbon nanotube bundles (f-bSWNT) adsorbed with T-cell-stimulating antibodies are shown to enhance both the kinetics and magnitude of T cell stimulation compared to the same concentration of free antibodies in solution. This enhancement is unique to f-bSWNT compared to other artificial substrates with high surface area and similar chemistry. We explored the origins of this enhanced activity with FRET microscopy and found the preferential formation of large antibody stimuli clusters (5 to 6 microm) on the surface of functionalized versus untreated nanotubes. This highlights the important aspect that antigen clusters can be formed on f-bSWNT, impacting the potency of the T cell stimulus. Clustering of T cell antigens on artificial substrates impacts the avidity of interaction with cells facilitating rapid stimulation dynamics and an overall greater magnitude of response. These findings support the use of chemically treated nanotube bundles as an efficient substrate for the presentation of antigens and point to their potential in clinical applications involving artificial antigen-presentation for ex vivo T cell expansion in adoptive immunotherapy.

Biosensor system-on-a-chip including CMOS-based signal processing circuits and 64 carbon nanotube-based sensors for the detection of a neurotransmitter

We developed a carbon nanotube (CNT)-based biosensor system-on-a-chip (SoC) for the detection of a neurotransmitter. Here, 64 CNT-based sensors were integrated with silicon-based signal processing circuits in a single chip, which was made possible by combining several technological breakthroughs such as efficient signal processing, uniform CNT networks, and biocompatible functionalization of CNT-based sensors. The chip was utilized to detect glutamate, a neurotransmitter, where ammonia, a byproduct of the enzymatic reaction of glutamate and glutamate oxidase on CNT-based sensors, modulated the conductance signals to the CNT-based sensors. This is a major technological advancement in the integration of CNT-based sensors with microelectronics, and this chip can be readily integrated with larger scale lab-on-a-chip (LoC) systems for various applications such as LoC systems for neural networks.

Influence of biomacromolecules and humic acid on the aggregation kinetics of single-walled carbon nanotubes

The initial aggregation kinetics of single-walled carbon nanotubes (SWNTs) were studied using time-resolved dynamic light scattering. Aggregation of SWNTs was evaluated in the presence of natural organic matter [Suwannee River humic acid (SRHA)], polysaccharide (alginate), protein [bovine serum albumin (BSA)], and cell culture medium [Luria-Bertani (LB) broth] with varying solution concentrations of monovalent (NaCl) and divalent (CaCl(2)) salts. Increasing salt concentration and adding divalent calcium ions induced SWNT aggregation by screening electrostatic charge and thereby suppressing electrostatic repulsion, similar to observations with aquatic colloidal particles. The presence of biomacromolecules significantly retarded the SWNT aggregation rate. BSA protein molecules were most effective in reducing the rate of aggregation followed by SRHA, LB, and alginate. The slowing of the SWNT aggregation rate in the presence of the biomacromolecules and SRHA can be attributed to steric repulsion originating from the adsorbed macromolecular layer. The remarkably enhanced SWNT stability in the presence of BSA, compared to that with the other biomacromolecules and SRHA, is ascribed to the BSA globular molecular structure that enhances steric repulsion. The results have direct implications for the fate and behavior of SWNTs in aquatic environments and biological media.

Asymmetries in transition metal XPS spectra: metal nanoparticle structure, and interaction with the graphene-structured substrate surface

Transition-metal XPS spectra have traditionally been considered to possess a natural asymmetry, extending to the high-binding-energy side. This is based on the fact that these spectra have generally been found experimentally to have such an asymmetry, as well as on the confirmation of asymmetry offered by the Doniach-Sunjić equation, an equation based on the proposal that the conduction electron scattering amplitude for interband absorption or emission in metals, at the Fermi level, is a singularity. Our discovery that metal nanoparticles, prepared under vacuum and characterized without exposure to air, have symmetric peaks, which become asymmetric with time, informed us that these peak asymmetries have other sources. On the basis of our belief that all metal spectra are composed of symmetric peaks, where the asymmetries are attributed to overlapping minor peaks that are consistent with known physical and chemical phenomena associated with that metal, we have shown that, for the metals that we have studied, these asymmetries contain much information, otherwise unavailable, on the structures, contaminants, oxidation, and interfacial interactions of nanoparticle surfaces. The existence of this information has been demonstrated for several metals, and its value is shown by its use in explaining the strong interfacial bonding of the nanoparticles with substrates having graphene structures. A possible future research direction is offered in the field of metal-metal interactions in nanoparticle alloys.

Differential modulation of the active site environment of human carbonic anhydrase XII by cationic quantum dots and polylysine

Due to prevalence of negative charges on the protein surface, opposite to the active site pocket of human carbonic anhydrase XII (hCA XII), both positively charged CdTe quantum dots (Qds(+)) and polylysine electrostatically interact with the enzyme, and such interaction does not influence the catalytic activity of the enzyme. However, both these cationic macromolecules differently modulate the active site environment of the enzyme. The steady-state kinetic data revealed that whereas polylysine exhibited no influence on dansylamide (DNSA) dependent inhibition of the enzyme, Qds(+) overcame such an inhibitory effect, leading to almost 70% restoration of the catalytic activity of the enzyme. We provide evidence that DNSA remains bound to the enzyme upon interaction with both polylysine and Qds(+). Arguments are presented that the above differential feature of polylysine and Qds(+) on hCA XII is encoded in the "rigidity" versus "flexibility" of these cationic macromolecules.

Novel polyaromatic-terminated transition metal complexes for the functionalization of carbon surfaces

In order to investigate the process of noncovalent adsorption on glassy carbon surfaces, two terpyridine ligands 4-pyren-1-yl-N-[5-([2,2';6',2'']terpyridin-4'-yloxy)-pentyl]-butyramide (tpy~py) and N-[5-([2,2';6',2'']terpyridin-4'-yloxy)-pentyl]-2-naphthamide (tpy~nap) as well as the homoleptic cobalt(II) complexes of these ligands (Co(tpy~py)(2)(PF(6))(2) and Co(tpy~nap)(2)(PF(6))(2)) were synthesized. Electrochemical measurements in solution were used to characterize the transport behavior of these complexes and to verify that the polyaromatic portion of each ligand did not dramatically influence the electronic properties of the transition metal complex. The adsorption of the cobalt complexes above on glassy carbon electrode surfaces was then examined using cyclic voltammetry and was found to be well described by Langmuir or Frumkin isotherms. The free energy of adsorption for Co(tpy~py)(2)(PF(6))(2) was considerably larger than that for Co(tpy~nap)(2)(PF(6))(2): -41 versus -30 kJ/mol.

Reinforced thermoplastic polyimide with dispersed functionalized single wall carbon nanotubes

Molecular pi-complexes were formed from pristine HiPCO single- wall carbon nanotubes (SWCNTs) and 1-pyrene- N-(4-N'-(5-norbornene-2,3-dicarboxyimido)phenyl butanamide, 1. Polyimide films were prepared with these complexes as well as uncomplexed SWCNTs and the effects of nanoadditive addition on mechanical, thermal, and electrical properties of these films were evaluated. Although these properties were enhanced by both nanoadditives, larger increases in tensile strength and thermal and electrical conductivities were obtained when the SWCNT/1 complexes were used. At a loading level of 5.5 wt %, the T(g) of the polyimide increased from 169 to 197 degrees C and the storage modulus increased 20-fold (from 142 to 3045 MPa). The addition of 3.5 wt % SWCNT/1 complexes increased the tensile strength of the polyimide from 61.4 to 129 MPa; higher loading levels led to embrittlement and lower tensile strengths. The electrical conductivities (DC surface) of the polyimides increased to 1 x 10(-4) Scm(-1) (SWCNT/1 complexes loading level of 9 wt %). Details of the preparation of these complexes and their effects on polyimide film properties are discussed.

Structural and electronic properties of carbon nanotube-reinforced epoxy resins

Nanocomposites of cured epoxy resin reinforced by single-walled carbon nanotubes exhibit a plethora of interesting behaviors at the molecular level. We have employed a combination of force-field-based molecular mechanics and first-principles calculations to study the corresponding binding and charge-transfer behavior. The simulation study of various nanotube species and curing agent configurations provides insight into the optimal structures in lieu of interfacial stability. An analysis of charge distributions of the epoxy functionalized semiconducting and metallic tubes reveals distinct level hybridizations. The implications of these results for understanding dispersion mechanism and future nano reinforced composite developments are discussed.