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

Density enhancement of aligned single-walled carbon nanotube thin films on quartz substrates by sulfur-assisted synthesis

The density of the aligned single-walled carbon nanotubes (SWNTs) grown on quartz substrates is an important factor for the performance of fabricated electronic devices. It was discovered that the addition of a sulfur-containing compound (thiophene) to the reaction mixture improved the density of SWNTs by a factor of 2 or more, from approximately 2-4 SWNTs/microm to 6-8 SWNTs/microm under similar growth conditions. It was also observed that along with the increase in nanotube density, the cleanness of the samples improved as well. These effects were demonstrated over a large range of growth conditions, indicating that the addition sulfur makes the growth processes more favorable for the nucleation and growth of aligned SWNTs.

Wrapping nanotubes with micelles, hemimicelles, and cylindrical micelles

This work uses a simple model based on hydrophobic and hydrophilic forces to investigate the molecular dynamics that lead to the supramolecular self-assembly of surfactants around carbon nanotubes (CNTs). The effects of the concentration and the structure of surfactants are explored. The bead-based mesoscopic description spontaneously develops the several micellar morphologies that are known to wrap and solvate CNTs.

Color purity in polymer electrochromic window devices on indium-tin oxide and single-walled carbon nanotube electrodes

Dual polymer absorptive/transmissive electrochromic (EC) window devices have been assembled using the solution-processable and high-EC-contrast polymer PProDOT-(CH(2)OEtHx)(2) as the EC material, along with a non-color-changing electroactive polymer, poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA), as the counter electrode material. Indium-tin oxide (ITO) and highly transmissive single-walled carbon nanotube (SWNT) film coated glass electrodes are used as electrode substrates. The use of the EC/non-color-changing polymer combination allowed us to construct window devices that rapidly switch between magenta and highly transmissive (>95% T for ITO and approximately 79% T for SWNT) states with large optical modulation (>71% DeltaT for ITO and 66% DeltaT for SWNT). The devices showed effective coloration and bleaching: the lightness parameter (L*) changing from 67 to 95 for ITO (approximately 50-92 for SWNT), essentially reaching a diffuse white upon oxidation. The color modulates from highly pure magenta with a* = 28 (red hue) and b* = -28 (blue chroma) for ITO (a* = 40 and b* = -36 for SWNT) to nearly colorless with a* = 1 and b* = -1 for ITO (a* = -2 and b* = -3 for SWNT) devices. Increasing the switching voltage from 2.55 V up to 3.5 V resulted in faster SWNT-based window device performance.

Dispersions, novel nanomaterial sensors and nanoconjugates based on carbon nanotubes

Nanomaterials are structures with dimensions characteristically much below 100 nm. The unique physical properties (e.g., conductivity, reactivity) have placed these nanomaterials in the forefront of emerging technologies. Significant enhancement of optical, mechanical, electrical, structural, and magnetic properties are commonly found through the use of novel nanomaterials. One of the most exciting classes of nanomaterials is represented by the carbon nanotubes. Carbon nanotubes, including single-wall carbon nanotubes, multi-wall carbon nanotubes, and concentric tubes have been shown to possess superior electronic, thermal, and mechanical properties to be attractive for a wide range of potential applications They sometimes bunch to form "ropes" and show great potential for use as highly sensitive electronic (bio)sensors due to the very small diameter, directly comparable to the size of single analyte molecules and that every single carbon atom is in direct contact with the environment, allowing optimal interaction with nearby molecules. Composite materials based on integration of carbon nanotubes and some other materials to possess properties of the individual components with a synergistic effect have gained growing interest. Materials for such purposes include conducting polymers, redox mediators and metal nanoparticles. These tubes provide the necessary building blocks for electronic circuits and afford new opportunities for chip miniaturization, which can dramatically improve the scaling prospects for the semiconductor technologies and the fabrication of devices, including field-effect transistors and sensors. Carbon nanotubes are one of the ideal materials for the preparation of nanoelectronic devices and nanosensors due to the unique electrical properties, outstanding electrocatalytic properties, high chemical stability and larger specific surface area of nanotubes. Carbon nanotubes are attractive material for supercapacitors due to their unique one-dimensional mesoporous structure, high specific surface area, low resistivity and good chemical stability. Nanoscaled composite materials based on carbon nanotubes have been broadly used due to their high chemical inertness, non-swelling effect, high purity and rigidity. The integration of carbon nanotubes with organics, biomaterials and metal nanoparticles has led to the development of new hybrid materials and sensors. Hybrid nanoscale materials are well established in various processes such as organic and inorganic compounds, nucleic acid detachment, protein separation, and immobilization of enzymes. Those nanostructures can be used as the building blocks for electronics and nanodevices because uniform organic and metal coatings with the small and monodisperse domain sizes are crucial to optimize nanoparticle conductivity and to detect changes in conductivity and absorption induced by analyte adsorption on these surfaces. The highly ordered assembly of zero-dimensional and one-dimensional nanoparticles is not only necessary for making functional devices, but also presents an opportunity to develop novel collective properties.

Carbon nanotubes randomly decorated with gold clusters: from nano2hybrid atomic structures to gas sensing prototypes

Carbon nanotube surfaces, activated and randomly decorated with metal nanoclusters, have been studied in uniquely combined theoretical and experimental approaches as prototypes for molecular recognition. The key concept is to shape metallic clusters that donate or accept a fractional charge upon adsorption of a target molecule, and modify the electron transport in the nanotube. The present work focuses on a simple system, carbon nanotubes with gold clusters. The nature of the gold-nanotube interaction is studied using first-principles techniques. The numerical simulations predict the binding and diffusion energies of gold atoms at the tube surface, including realistic atomic models for defects potentially present at the nanotube surface. The atomic structure of the gold nanoclusters and their effect on the intrinsic electronic quantum transport properties of the nanotube are also predicted. Experimentally, multi-wall CNTs are decorated with gold clusters using (1) vacuum evaporation, after activation with an RF oxygen plasma and (2) colloid solution injected into an RF atmospheric plasma; the hybrid systems are accurately characterized using XPS and TEM techniques. The response of gas sensors based on these nano(2)hybrids is quantified for the detection of toxic species like NO(2), CO, C(2)H(5)OH and C(2)H(4).

Noncovalent functionalization of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have attracted much attention on account of their potential to be transformed into new materials that can be employed to address a wide range of applications. The insolubility of the SWNTs in most solvents and the difficulties of handling these highly intractable carbon nanostructures, however, are restricting their real-life applications at the present time. To improve upon the properties of the SWNTs, low-cost and industrially feasible approaches to their modifications are constantly being sought by chemists and materials scientists. Together, they have shown that noncovalent functionalization of the SWNTs can do much to preserve the desired properties of the SWNTs while remarkably improving their solubilities. This Account describes recent advances in the design, synthesis, and characterization of SWNT hybrids and evaluates applications of these new hybrid materials based on noncovalently functionalized SWNTs. Their solubilization enables the characterization of these hybrids as well as the investigation of the properties of the SWNTs using solution-based techniques. Cognizant of the structural properties of the functional molecules on the SWNTs, we present some of the recent work carried out by ourselves and others under the umbrella of the following three subtopics: (i) aromatic small-molecule-based noncovalent functionalization, (ii) biomacromolecule-based noncovalent functionalization, and (iii) polymer-based noncovalent functionalization. Several examples for the applications of noncovalently functionalized SWNT hybrids in the fabrication of field-effect transistor (FET) devices, chemical sensors, molecular switch tunnel junctions (MSTJs), and photovoltaic devices are highlighted and discussed. The blossoming of new methods for the noncovalent functionalization of the SWNTs promises a new generation of SWNT hybrid-based integrated multifunctional sensors and devices, an outcome which is essential for the development of carbon nanotube chemistry that interfaces with physics, materials, biology, and medical science.

Species enrichment of SWNTs with pyrene alkylamide derivatives: is the alkyl chain length important?

Three similar amide-functionalized pyrene derivatives (N-methyl(18-Crown-6)pyrene-1-acetamide, N-methyl(18-Crown-6)pyrene-1-butyramide and N-methyl (18-Crown-6)pyrene-1-formamide), denoted as pa-18-C-6, pb-18-C-6 and pc-18-C-6, respectively, with different alkyl chain lengths between the amide groups and pyrene moieties have been successfully synthesized and for the first time, their separation effects on single-walled carbon nanotubes (SWNTs) according to their electronic structures (metallic/semiconducting) and diameter have been demonstrated. Resonant Raman spectroscopy and UV-vis-NIR show that all three pyrene derivatives are selective to metallic SWNTs (met-SWNTs). Resonant Raman scattering (RRS) spectra also indicate that two of the pyrene derivatives, pb-18-C-6 and pc-18-C-6, are selective to large-diameter (D>1 nm) met-SWNTs. The third compound, pa-18-C-6, is selective to small-diameter (D<1.03 nm) met-SWNTs due to the ketone-to-enol rearrangement resulting in higher strain force relaxation; the rearrangement was shown by Fourier-transform infrared spectroscopy data. For their discrimination between different relatively large-diameter semiconducting species, RRS spectra using a 785 nm laser show that pb-18-C-6 is selective to larger diameter ones while pa-18-C-6 and pc-18-C-6 have no selectivity. The selectivity for met-SWNTs in the solution is confirmed by a tenfold decline in thin film SWNT network resistivity (from 43 to 4.3 S m(-1)) after depletion of metallic nanotubes by a four-pass selection procedure using pa-18-C-6 and pc-18-C-6 surfactants alternately. Further, after the four-pass selection procedure, the conductivity of a network using the supernatant SWNTs changes less when heated in air compared to another network using the precipitated SWNTs; the high conductivity stability of the network with supernatant SWNTs further confirms the high content of met-SWNTs which are resistant to carrier doping.

Modifying the sorption properties of multi-walled carbon nanotubes via covalent functionalization

We demonstrate that the functionalization of carbon nanotubes dramatically alters their sorption characteristics. The effect of covalent functionalization of multiwalled carbon nanotubes (MWNTs) on the gas phase adsorption and desorption of polar and nonpolar organics is presented. Carboxylation and nitration led to the generation of polar functional groups on the nanotube surface. The derivatized nanotubes showed strong adsorption of polar analytes such as alcohols and relatively weaker adsorption for nonpolar and aromatic compounds. The breakthrough volume of ethanol increased by 300%, where as that of hexane decreased by 75% after functionalization. The functionalized MWNT also showed rapid desorption of the polar as well as nonpolar compounds.

Theoretical insights into the interaction mechanism between proteins and SWCNTs: adsorptions of tripeptides GXG on SWCNTs

Adsorptions of nine tripeptides GXG, ranging from negatively (D) and positively (K) charged, to hydrophilic (N and S), and to hydrophobic (G, V, F, W, and Y) residues, on the two cluster models (C(54)H(18) and C(54)) of (10,0) single-walled carbon nanotubes (SWCNTs) are systemically investigated with the MPWB1K and MP2 methods. The solvent effects are taken into account with the implicit CPCM model. The objective is to provide novel insights into the interaction mechanism between proteins and SWCNTs. Results reveal that the adsorption strength of two charged tripeptides is greatly affected by the solvent effect and the hydrogen saturability of the SWCNT models. In the gas phase, on the surface of C(54)H(18), GKG has the strongest adsorption (adsorption energy (AE): -29.3 kcal/mol at the MP2 level), whereas the adsorption of the negatively charged GDG is the strongest on C(54) (AE: -30.4 kcal/mol with MP2). However, because of strong solvation, the adsorptions of the charged residues (D and K) on both C(54)H(18) and C(54) surfaces in aqueous solution are either rather weak or even unbound. The two neutral hydrophilic residues (N and S) exhibit adsorptions on C(54)H(18) in the gas phase (AE: -3.3 and -4.2 kcal/mol), yet are unable to adsorb on SWCNTs in aqueous solution (AE: +0.3 kcal/mol at MP2+CPCM). The five hydrophobic residues present relatively strong adsorption on SWCNTs, especially for the three aromatic residues (GFG, GYG, and GWG), regardless of the CNT model and whether they are in the gas phase or solution. These results indicate that in general the aromatic groups of proteins would play a very important role on functionalizing CNTs, which basically supports the relevant experimental observations. In addition, the electron correlation is essential for adsorptions of GXG on pristine SWCNTs, and the three aromatic residues have the highest electron correlation effects. The present investigation provides strong evidence that for the functionalization of CNTs via proteins it is most likely that hydrophobic interaction and van der Waals are the dominant driving forces.

In vitro assessments of nanomaterial toxicity

Nanotechnology has grown from a scientific interest to a major industry with both commodity and specialty nanomaterial exposure to global populations and ecosystems. Sub-micron materials are currently used in a wide variety of consumer products and in clinical trials as drug delivery carriers and imaging agents. Due to the expected growth in this field and the increasing public exposure to nanomaterials, both from intentional administration and inadvertent contact, improved characterization and reliable toxicity screening tools are required for new and existing nanomaterials. This review discusses current methodologies used to assess nanomaterial physicochemical properties and their in vitro effects. Current methods lack the desired sensitivity, reliability, correlation and sophistication to provide more than limited, often equivocal, pieces of the overall nanomaterial performance parameter space, particularly in realistic physiological or environmental models containing cells, proteins and solutes. Therefore, improved physicochemical nanomaterial assays are needed to provide accurate exposure risk assessments and genuine predictions of in vivo behavior and therapeutic value. Simpler model nanomaterial systems in buffer do not accurately duplicate this complexity or predict in vivo behavior. A diverse portfolio of complementary material characterization tools and bioassays are required to validate nanomaterial properties in physiology.

Noncovalent functionalization as an alternative to oxidative acid treatment of single wall carbon nanotubes with applications for polymer composites

We have created stable dispersions of single wall carbon nanotubes (SWNTs) in water by employing a noncovalent functionalization scheme that allows carboxylic acid moieties to be attached to the SWNT surface by a pi-pi stacking interaction. Pyrenecarboxylic acid (PCA) is noncovalently attached to the surface of SWNTs and affords highly uniform and stable aqueous dispersions. This method was developed to provide a noncovalent alternative to the commonly used oxidative acid treatment functionalization of carbon nanotubes. This alternative strategy avoids the damage to the carbon nanotube structure inherent to oxidative acid treatments. Carbon nanotubes are commonly functionalized with oxidative acid treatment schemes to create polymer-nanotube composites and improve the adhesion between the polymer and carbon nanotubes. Composites of SWNTs and polycarbonate were prepared and tested to determine the effect of PCA on the adhesion of the SWNTs to the polymer matrix. These tests confirmed that PCA improved the SWNT-polycarbonate adhesion and improved the dispersion of the SWNTs throughout the matrix. This study demonstrates that stable dispersions of SWNTs can be achieved without substantial cutting, introduction of defects, or covalent modification, by employing a simple and effective noncovalent functionalization with PCA.

Superior activity of structurally deprived enzyme-carbon nanotube hybrids in cationic reverse micelles

In the present work, we report the superior activity of hydrophobically adsorbed enzymes onto single-walled carbon nanotubes (SWNTs) in the reverse micelles of cationic surfactants. Horseradish peroxidase and soybean peroxidase adsorbed onto SWNTs endure a notable loss in secondary structure and catalytic activity. This structurally and functionally deformed enzyme-SWNT when confined in CTAB reverse micelles showed approximately 7-9-fold enhancement in activity compared to that was in water and also importantly approximately 1500-3500 times higher activity than that of the enzymes in aqueous-organic biphasic mixtures. The activation observed for this nanobiocomposite is due to the (i) possible localization of enzyme-SWNT hybrid at the micellar interface; (ii) facile transport of substrates across the microscopic interface of reverse micelles; and (iii) greater local concentration of substrates at the augmented interfacial space in the presence of SWNT. This interfacial localization of the SWNT-protein hybrid was tested using FITC-tagged protein (BSA) by fluorescence spectroscopy. FTIR and CD spectroscopy established that the enzyme notably loses its native structure as it gets adsorbed onto the CNTs. However, this loss in the secondary structure is neither aggravated nor recovered when the enzyme-SWNT resides at the reverse micellar interface. So, localization of the surface-active peroxidase-CNT hybrids at the interface is the main reason for significant enzyme activation. The generality of the activation of the enzyme-CNT hybrid by reverse micelles was tested using amphiphiles with varying headgroup sizes, where an overall enhancement in activity was observed with an increase in headgroup size. Activation of this nanobiocomposite would find utmost importance in material science as the activity of structurally deprived enzyme in reverse micelles surpassed (approximately 1.7-fold) even the activity of the native enzyme in water.

Polyaniline and poly(flavin adenine dinucleotide) doped multi-walled carbon nanotubes for p-acetamidophenol sensor

A conductive biocomposite film (MWCNTs-PANIFAD) which contains multi-walled carbon nanotubes (MWCNTs) along with the incorporation of poly(aniline) and poly(flavin adenine dinucleotide) co-polymer (PANIFAD) has been synthesized on gold and screen printed carbon electrodes by potentiostatic methods. The presence of MWCNTs in the MWCNTs-PANIFAD biocomposite film enhances the surface coverage concentration (Gamma) of PANIFAD and increases the electron transfer rate constant (k(s)) to 89%. Electrochemical quartz crystal microbalance studies reveal the enhancements in the functional properties of MWCNTs and PANIFAD present in MWCNTs-PANIFAD biocomposite film. Surface morphology of the biocomposite film has been studied using scanning electron microscopy and atomic force microscopy. The surface morphology results reveal that PANIFAD incorporated on MWCNTs. The MWCNTs-PANIFAD biocomposite film exhibits promising enhanced electrocatalytic activity towards the oxidation of p-acetamidophenol. The cyclic voltammetry has been used for the measurement of electroanalytical properties of p-acetamidophenol by means of PANIFAD, MWCNTs and MWCNTs-PANIFAD biocomposite film modified gold electrodes. The sensitivity value of MWCNTs-PANIFAD film (88.5 mA mM(-1)cm(-2)) is higher than the values which are obtained for PANIFAD (28.7 mA mM(-1)cm(-2)) and MWCNTs films (60.7 mA mM(-1)cm(-2)). Finally, the flow injection analysis (FIA) has been used for the amperometric detection of p-acetamidophenol at MWCNTs-PANIFAD film modified screen printed carbon electrode. The sensitivity value of MWCNTs-PANIFAD film (3.3 mA mM(-1)cm(-2)) in FIA is also higher than the value obtained for MWCNTs film (1.1 mA mM(-1)cm(-2)).

PEG branched polymer for functionalization of nanomaterials with ultralong blood circulation

Nanomaterials have been actively pursued for biological and medical applications in recent years. Here, we report the synthesis of several new poly(ethylene glycol) grafted branched polymers for functionalization of various nanomaterials including carbon nanotubes, gold nanoparticles (NPs), and gold nanorods (NRs), affording high aqueous solubility and stability for these materials. We synthesize different surfactant polymers based upon poly(gamma-glutamic acid) (gammaPGA) and poly(maleic anhydride-alt-1-octadecene) (PMHC18). We use the abundant free carboxylic acid groups of gammaPGA for attaching lipophilic species such as pyrene or phospholipid, which bind to nanomaterials via robust physisorption. Additionally, the remaining carboxylic acids on gammaPGA or the amine-reactive anhydrides of PMHC18 are then PEGylated, providing extended hydrophilic groups, affording polymeric amphiphiles. We show that single-walled carbon nanotubes (SWNTs), Au NPs, and NRs functionalized by the polymers exhibit high stability in aqueous solutions at different pH values, at elevated temperatures, and in serum. Moreover, the polymer-coated SWNTs exhibit remarkably long blood circulation (t(1/2) = 22.1 h) upon intravenous injection into mice, far exceeding the previous record of 5.4 h. The ultralong blood circulation time suggests greatly delayed clearance of nanomaterials by the reticuloendothelial system (RES) of mice, a highly desired property for in vivo applications of nanomaterials, including imaging and drug delivery.

Solvent-dependent fluorescence property of multi-walled carbon nanotubes noncovalently functionalized by pyrene-derivatized polymer

Multi-walled carbon nanotubes (MWCNTs) were supramolecularly functionalized by pyrene-derivatized hydrolyzed poly(styrene-co-maleic anhydride) (pyrene-HSMA) in aqueous solution. The MWCNTs/pyrene-HSMA conjugates were found to exhibit a solvent-dependent fluorescence property by photoluminescence measurements. The fluorescence intensity of the MWCNTs/pyrene-HSMA conjugate was used as a probe to estimate the interaction between polymer and MWCNTs in solvents. The polymer conformation around the MWCNTs was demonstrated by atomic force microscopy. We suggested that pyrene-HSMA functionalized MWCNTs in a non-wrapping mode, and that the conformation change of the polymer around the MWCNTs resulted in the solvent-dependent fluorescence property. The dispersion state of the MWCNTs/pyrene-HSMA conjugate in various solvents was also investigated by transmission electron microscopy. The fluorescence responses of the MWCNTs/pyrene-HSMA conjugate to pH value and temperature were studied as well.

Single-walled carbon nanotubes decorated with a pyrene-fluorenevinylene conjugate

Single-walled carbon nanotubes are noncovalently functionalized using a pyrene-fluorenevinylene dye and the resulting nanohybrids are isolated from the free molecules. The tubes modified by means of this noncovalent approach show enhanced solubility in organic media. The structure and morphology of this hybrid material are fully characterized using absorption, infrared and Raman spectroscopies as well as atomic force and scanning electron microscopies. Steady state fluorescence measurements reveal that significant quenching of the pyrene derivative excited state takes place through an energy transfer mechanism.

Mechanism of the coupling of diazonium to single-walled carbon nanotubes and its consequences

On the tube: The coupling of diazonium ions onto single-walled carbon nanotubes is shown to proceed through a radical chain reaction by kinetic analysis of the absorption peak drop (see picture). Radical species are also revealed by ESR. Metallic (m) nanotubes play a special catalytic role in the functionalization of semiconducting (sc) nanotubes.Due to its simplicity and versatility, diazonium coupling is the most widely used method for carbon nanotube (CNT) functionalization to increase CNT processability and add new functionalities. Yet, its mechanism is so far mostly unknown. Herein, we use kinetic analysis to shed light on this complex mechanism. A free-radical chain reaction is revealed by absorption spectroscopy and ESR. Metallic CNTs are shown to play an unexpected catalytic role. The step determining the selectivity towards metallic CNTs is identified by a Hammett correlation. A mechanistic model is proposed that predicts reactivity and selectivity as a function of diazonium electrophilicity and metallic-to-semiconducting CNT ratio, thus opening perspectives of controlled high-yield functionalization and purification.

Quantum electronic stability in selective enrichment of carbon nanotubes

We have studied the structural and electronic stability of a helical ribbon of flavin mononucleotide wrapping around single-walled carbon nanotubes using first-principles density-functional calculations. The helical ribbon is formed through hydrogen bonding between adjacent uracil moieties of the isoalloxazine ring and stabilized through concentric pi-pi interactions. The electronic structure calculations reveal quantum electronic stability associated with lattice registry and band alignment between the helical assembly and the (8,6) nanotube. The electronic stability plays an essential role in the experimentally observed highly selective enrichment of specific chirality tubes.