Carbon Nanotubes In Aqueous Media Research Articles

The Use of Electrochemical Impedance Spectroscopy as a Tool for the In-Situ Monitoring and Characterization of Carbon Nanotube Aqueous Dispersions.

So far, there is no validated technology for characterizing the dispersion and morphology state of carbon nanotubes (CNTs) aqueous dispersions during sonication. Taking advantage of the conductive nature of CNTs, the main hypothesis of the current study is that Electrochemical Impedance Spectroscopy (EIS) is an appropriate technique for the in-situ monitoring and qualification of the dispersion state of CNTs in aqueous media. To confirm our hypothesis, we monitored the Impedance |Z| during the sonication process as a function of type CNTs/admixtures used for the preparation of the aqueous solutions and of crucial process parameters, such as the applied sonication power and duration (i.e., sonication energy). For dispersions above the percolation threshold, a drop of |Z| by approximately seven orders of magnitude was observed, followed by a linear reduction. The dramatic change in |Z| is regarded as an indication of the formation of a conductive path or destruction of an existing one during sonication and can be used to characterize the dispersion and morphology state of CNTs. The results of the EIS provide, straightforwardly and reliably, the required information to create an optimum dispersion protocol for conductive CNT suspensions. The produced dispersions are part of research focusing on the manufacturing of cement-based composite materials with advanced thermoelectric functionalities for energy harvesting. Such dispersions are not only limited to energy harvesting applications but also to applications where functionalities are introduced through the use of conductive-based suspensions.

A facile, green fabrication of aqueous nanofluids containing hydrophilic functionalized carbon nanotubes toward improving heat transfer in a closed horizontal flow passage

In the present research, the synthesis of highly stable multiwalled carbon nanotubes in aqueous media is developed using a one-pot, covalent and green functionalization technique to improve the heat transfer and hydrodynamic behavior of a horizontal stainless-steel tube subjected to a uniform heat flow at its outer surface. Instead of using corrosive inorganic acids, the free radical grafting of gallic acids is used in this procedure. In this work, GA-functionalized multiwalled carbon nanotubes water-based colloidal suspensions (nanofluids) were prepared at three different weight concentrations. Various characterization techniques comprising Fourier transform infrared spectroscopy, Raman spectroscopy, High resolution transmission Electron Microscopy, and zeta-potential measurements were performed and confirmed the success of MWCNT functionalization. The thermophysical properties were evaluated experimentally and validated using empirical correlations available in the literature. To prove the colloidal suspension stability, ultraviolet-visible spectroscopy was used, and the results showed that nanofluid was stable for almost 60 days. When the GAMWCNT concentration was increased, there was a significant improvement in the thermal conductivity. Moreover, the nanofluids' dynamic viscosity experienced a slight increment up to 7.30% when the GAMWCNTs were loaded relative to distilled water. Following the confirmation of the experimental setup's analytical correlations, tests for the colloidal GAMWCNT suspension flowing through a heated horizontal tube were carried out in a fully developed turbulent state. A significant enhancement in the convective heat transfer coefficient was obtained, with only minor growth in the relative pumping power by 33.05 and 1.19%, respectively. More importantly, the reported positive performance index indicator for all the Reynolds numbers of ranges shows the possibility of using the synthesized GAMWCNT aqueous suspensions as an alternative working fluid in heat transfer systems.

Influence of Oxygen-Containing Functional Groups on the Environmental Properties, Transformations, and Toxicity of Carbon Nanotubes.

Carbon nanotubes (CNTs) have unique physical and chemical properties that drive their use in a variety of commercial and industrial applications. CNTs are commonly oxidized prior to their use to enhance dispersion in polar solvents by deliberately grafting oxygen-containing functional groups onto CNT surfaces. In addition, CNT surface oxides can be unintentionally formed or modified after CNTs are released into the environment through exposure to reactive oxygen species and/or ultraviolet irradiation. Consequently, it is important to understand the impact of CNT surface oxidation on the environmental fate, transport, and toxicity of CNTs. In this review, we describe the specific role of oxygen-containing functional groups on the important environmental behaviors of CNTs in aqueous media (e.g., colloidal stability, adsorption, and photochemistry) as well as their biological impact. We place special emphasis on the value of systematically varying and quantifying surface oxides as a route to identifying quantitative structure-property relationships. The role of oxygen-containing functional groups in regulating the efficacy of CNT-enabled water treatment technologies and the influence of surface oxides on other carbon-based nanomaterials are also evaluated and discussed.

Monitoring phase transition behavior of Poly(N-vinylcaprolactam) via nanostructure-based functionalized carbon nanotubes

The ubiquitous behavior of thermoresponsive polymers (TRPs) in terms of their amphiphilicity and reversible behavior towards temperature make them to exhibit wide applications in biomedical science, biosensors and biotechnological fields. The electrical and surface properties of multiwalled carbon nanotubes (MWCNTs) have received immense attention due to their high flexibility to take any shape (tube, circles) to bind with different macromolecular moieties. Nonetheless, TRPs-MWCNTs interactions with variation in temperature are rarely explored. However, the low solubility of carbon nanotubes (CNTs) in molecular solvents limits their utility in biological process. Therefore, functionalization of CNTs was performed to enhance the dispersibility of CNTs in aqueous medium. In this present study, effect of functionalized carbon nanotubes (fCNTs) on the coil to globular conformation of poly (N-vinylcaprolactam) (PVCL) is explored. PVCL exhibits coil to globular transition due to its biocompatible nature with several additive in permissible range of temperature values. To investigate the lower critical solution temperature (LCST) of PVCL in presence of fCNTs, various biophysical techniques such as fluorescence spectroscopy, dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were employed. Furthermore, for better understanding of morphological changes of PVCL in fCNTs, Transmission electron microscopy (TEM) was assessed. The DSC and DLS results confirmed that LCST value of PVCL (32.0 °C) has dropped by 2 °C in presence of fCNTs due to hydrophobic interactions of polymer and fCNTs. This exclusive modification in LCST of PVCL will be sensitive in biological applications. We have studied the first time effect of fCNTs on the phase transition behavior of PVCL which will lead to its widespread applications in the field of biomedical sciences such as drug delivery systems.

Effect of Surfactant Stabilizers on the Distribution of Carbon Nanotubes in Aqueous Media

The results of a study on the homogeneity of suspensions are described considering the effect of different types of surfactant stabilizers and their concentrations on the uniform distribution of a carbon nanotubes (CNTs)-based modifying additive to construction materials in an aqueous medium. This problem was solved herein by using surfactants and ultrasound. The sonication treatment of CNTs particle agglomerates allowed for dispersing their globules and achieving a 15-20-fold decrease in their average size, for which it became possible to make better use of the CNTs as cement modifier. As a result of the experimental studies carried out, the effect of the surfactant type and concentration promoting uniform distribution of the CNTs in the bulk of the suspension and, correspondingly, in the structural composite matrix was revealed. The CNTs-based additive improved the physical-mechanical and operational characteristics of the material.

DNA-Assisted Dispersion of Carbon Nanotubes and Comparison with Other Dispersing Agents.

Separation and sorting of pristine carbon nanotubes (CNTs) from bundle geometry is a very challenging task due to the insoluble and nondispersive nature of CNTs in aqueous medium. Recently, many studies have been performed to address this problem using various organic and inorganic solutions, surfactant molecules, and biomolecules as dispersing agents. Recent experimental studies have reported the DNA to be highly efficient in dispersing CNTs from bundle geometry. However, there is no microscopic study and also quantitative estimation of the dispersion efficiency of the DNA. Using all-atom molecular dynamics simulation, we study the structure and stability of single-stranded DNA (ssDNA)-single-walled carbon nanotube (SWNT) (6,5) complex. To quantify the dispersion efficiency of various DNA sequences, we perform potential of mean forces (PMF) calculation between two bare SWNTs as well ssDNA-wrapped CNTs for different base sequences. From the PMF calculation, we find the PMF between two bare (6,5) SWNTs to be approximately -29 kcal/mol. For the ssDNA-wrapped SWNTs, the PMF reduces significantly and becomes repulsive. In the presence of ssDNA of different polynucleotide bases (A, T, G, and C), we present a microscopic picture of the ssDNA-SWNT (6,5) complex and also a quantitative estimate of the interaction strength between nanotubes from PMF calculation. From PMF, we show the sequence of dispersion efficiency for four different nucleic bases to be T > A > C > G. We have also presented a comparison of the dispersion efficiencies of ssDNA, flavin mononucleotide surfactant, and poly(amidoamine) (PAMAM) dendrimer by comparing their respective PMF values.

Evaluation of concentration and dispersion of functionalized carbon nanotubes in aqueous media by means of Low Field Nuclear Magnetic Resonance

Dispersions of functionalized carbon nanotubes (fx-CNTs) have been analyzed by means of Low Field Nuclear Magnetic Resonance measurements (LF-NMR). This technique showed that the presence of the fx-CNTs strongly influences the structuring of the water molecules. Thereupon, the water transversal relaxation rate (r2) can be used to get information about the concentration of the CNTs in aqueous dispersion and their aggregation tendency. Finally, the effect of the addition of an anionic polyelectrolyte, namely alginate, on aqueous dispersion of CNTs was explored by using LF-NMR, which revealed different dependence of r2 from CNT concentration, depending on the type and charge of fx-CNTs.

Carbon Nanotubes for Theranostics

The current technological development and application of Carbon Nanotubes (CNTs) unfailingly require the use of chemical tools. Pristine CNTs often need to be chemically processed or functionalized in order to exhibit the most suitable features and to optimize their handling in specific applications. Chemical functionalization stands therefore as a universal means to overcome CNTs insolubility in water, by rendering their surface hydrophilic, a necessary step towards promising biomedical outcomes.[1] As a matter of fact, water-dispersible CNTs may not trigger immunitary responses, and also may present high cell uptake along with very low cytotoxicity as compared to their not functionalized counterparts,[2]stressing out the crucial role of the affinity and stability of CNTs in aqueous media. Our recent efforts in the functionalization of carbon nanotubes are focused on the condition optimization to achieve a fast and versatile derivatization by introduction of appendages apt to increase dispersibility of the starting materials in polar solvent. The so obtained derivatives may be used as theranostic platform. [1] N. Saito, H. Haniu, Y. Usui, K. Aoki, K. Hara, S. Takanashi, M. Shimizu, N. Narita, M. Okamoto, S. Kobayashi, H. Nomura, H. Kato, N. Nishimura, S. Taruta, M. Endo, Chem. Rev. 2013, 11, 6040-6079. [2] S. Chen, S. Hu, E. F. Smith, P. Ruenraroengsak, A. J. Thorley, R. Menzel, A. E. Goode, M. P. Ryan, T. D. Tetley, A. E. Porter, M. S. P. Shaffer, Biomaterials 2014, 35, 4729-4862.

Recent developments concerning the dispersion of carbon nanotubes in surfactant/polymer systems by MD simulation

Carbon nanotubes (CNTs) hold the promise of delivering exceptional mechanical properties and multifunctional characteristics due to their unique physiochemical properties and prospective applications in various nanotechnologies. However, current techniques of CNTs fabrication cannot produce homogenous CNTs, and this prevents the widespread use of CNTs. Ever-increasing interest in applying CNTs in many different fields has led to continued efforts to develop dispersion and functionalization techniques. Techniques for separating bundles of CNTs into homogeneous dispersion are still under development. The preparation of effective dispersions of CNTs presents a major impediment to the extension and utilization of CNTs. CNTs intrinsically tend to bundle and/or aggregate. The prevention of such behavior has been explored by testing various techniques to improve the dispersibility of CNTs in a variety of solvents. There are mainly two approaches to obtain a good quality dispersion; chemical functionalization and physical interactions. The chemical functionalization technique has been found effective, but deteriorates the intrinsic properties of CNTs through the introduction of defects in the wall. Physical blending approaches with the ultrasound and high speed shearing have been proven capable of debundling CNTs and stabilizing individual CNTs while maintaining their integrity and intrinsic properties. Contemporary methods for dispersion of CNTs in aqueous media are discussed and most attention is paid to molecular dynamics simulation techniques and other physical techniques, as well as to the use of various surfactants and polymers.

On the vibrational characteristics of single- and double-walled carbon nanotubes containing ice nanotube in aqueous environment

The properties and behavior of carbon nanotubes (CNTs) in aqueous environment due to their considerable potential applications in nanobiotechnology and designing nanobiosensors have attracted the attention of researchers. In this study, molecular dynamics simulations are carried out to investigate the vibrational characteristics of single- and double-walled CNTs containing ice nanotubes (a new phase of ice) in vacuum and aqueous environments. The results demonstrate that formation of ice nanotubes inside the CNTs reduces the natural frequency of pure CNTs. Moreover, it is demonstrated that increasing the number of walls considerably reduces the sensitivity of frequency to the presence of ice nanotube inside CNT. Additionally, it is shown that increasing the length decreases the effect of ice nanotube on reducing the frequency. The calculation of natural frequency of CNTs in aqueous media demonstrates that the interaction of CNTs with water molecules considerably reduces the natural frequency up to 50 %. Finally, it is demonstrated that in the case of CNTs with one free end in aqueous environment, the CNT does not vibrate in its first mode, and its frequency is between the frequencies of first and second modes of vibration.

Solvation of alanine and histidine functionalized carbon nanotubes in aqueous media: A Monte Carlo simulation study

In this research, the effect of adsorption of two amino acids on the solvation of carbon nanotubes is investigated. The complexes of β-Alanine and Histidine with armchair single wall carbon nanotube are studied by density functional theory. Then, the outcomes of Monte Carlo simulations of these nanotubes immersed in water to look at the effects of nanotube type on the solvation in water are reported. It is found that the binding energy of the interaction of β-Alanine with nanotube is larger than Histidine. The results of computer simulation in aqueous solution indicate that amino acid functionalization increases the intermolecular interactions of carbon nanotube and water. Computed solvation free energies are in this order: CNTAlanine>CNTHistidine>pure CNT.

Electrochemical characterization of oligonucleotide-carbon nanotube functionalized using different strategies

Carbon nanotubes (CNTs) were functionalized with a single-strand deoxyoligonucleotide oligomer (DNO) using two different methods and characterized using different electrochemical techniques to finally evaluate their ability of the DNO to hybridize. The DNO immobilization methods were: direct adsorption (CNT-DNOADS) and covalent attachment (CNT-DNOCOV). The electrochemical behavior of both systems was characterized using cyclic voltammetry, differential pulse voltammetry and scanning electrochemical microscopy. These systems were compared using a bare glassy carbon (GC) electrode and an electrode containing oxidized CNTs without DNO (CNTCOOH). Our results indicate that the direct physical adsorption of DNO improves the dispersion of CNTs in aqueous media and therefore, the film generated over the electrode is more homogeneous than the films created with CNTCOOH and CNT-DNOCOV. The results demonstrated the existence of two different spatial conformations of the oligonucleotide on the CNT depending on the type of functionalization. The modified electrodes to hybridization studies were tested using the well-known DNA intercalation agent methylene blue. Thus, both conformations allowed for the detection of the hybridization process between DNO and its complementary strand; however, using CNT-DNOADS yielded a higher response.

Protein imprinted ionic liquid polymer on the surface of multiwall carbon nanotubes with high binding capacity for lysozyme.

In this research, ionic liquid as functional monomer to prepare molecularly imprinted polymers for protein recognition was for the first time demonstrated, in which, 1-vinyl-3-butylimidazolium chloride was selected as functional monomer, acrylamide as co-functional monomer and lysozyme (Lyz) as template protein to synthesize imprinted polymers on the surface of multiwall carbon nanotubes in aqueous medium. The results indicated that ionic liquid was helpful to improve binding capacity of imprinted polymers, which had a maximum binding capacity of 763.36 mg/g in the optimum adsorption conditions. The prepared imprinted polymers had a fast adsorption rate and a much higher adsorption capacity than the corresponding non-imprinted polymers, with the difference in adsorption capacity up to 258.31 mg/g. The obtained polymer was evaluated by Lyz, bovine serum albumin (BSA), bovine hemoglobin (BHb), equine myoglobin (MB) and cytochrome c (Cyt c). The selectivity factor (β) for Lyz/BSA, Lyz/Mb, Lyz/BHb, and Lyz/Cyt c were 7.17, 2.12, 1.76 and 1.57, respectively, indicating the imprinted polymers had a good selectivity. Easy preparation of the imprinted polymers as well as high ability and selectivity to adsorb template proteins makes this polymer attractive and broadly applicable in biomacromolecular separation, biotechnology and sensors.

Effects of surface modification on the dispersion and thermal conductivity of CNT/water nanofluids

In the present work, effects of different surface modification methods (surfactant, acid, base, amide, sulfate) on multi walled carbon nanotubes (CNTs) are studied. The dispersion stability of CNTs in aqueous media was confirmed and the effects of the type of treatment on the thermal conductivity of CNT/water nanofluids were investigated. The surface of the CNTs was modified with acid mixtures (H2SO4–HNO3), potassium persulfate (KPS), tetrahydrofuran (THF), octadecylamine and sodium dodecyl sulfate (SDS). UV–visible spectral data indicate that the CNTs treated first with the acid mixture and then with KPS show the best dispersion stability. The basic treatment and SDS treated CNT/water nanofluids (SDS-KCNT/water) showed the highest conductivity of 0.765W/mK which increases 24.9% of water as a base fluid conductor.

Assembling nanotubes and nanofibres: Cooperativeness in sepiolite–carbon nanotube materials

The dispersibility and stability of carbonaceous materials in water is a long-awaited feature expected to boost the use carbon nanotubes (CNT) and other carbon allotropes to an industrial scale. However, wide use of these materials relies on a deep understanding of the underlying mechanisms that govern its stability in aqueous media. This work focuses on a new approach to effectively disperse multi-walled carbon nanotubes (MWCNT) in water making use of clay mineral sepiolite. Such co-dispersion approach is based on a mechanism similar to steric stabilization. It is conducted under ultrasonic irradiation in the presence of the fibrous clay mineral and MWCNT. This strategy, highly dependent on the rheological properties of the co-dispersant, opens a clear pathway to use CNT in aqueous media without the use of surfactants or oxidative treatments, responsible for extensively damaging the CNT outer walls. To assess the feasibility of the proposed approach, MWCNT were dispersed in water under different conditions and the role of sepiolite fibers discussed. Finally, hybrid buckypapers were prepared from stable sepiolite/MWCNT suspensions and used to prepare a horseradish peroxidase biosensor and a PVA:sepiolite:CNT nanocomposite without compromising the carbon nanotubes integrity and performance.

A tailored RAFT copolymer for the dispersion of single walled carbon nanotubes in aqueous media

A new rational approach to the dispersion of SWCNT in aqueous media with amphiphilic RAFT block copolymers is proposed.

PH responsive MWCNT–star terpolymer nanohybrids

We present a unique multisegmented, heteroarm star block terpolymer that can be used as an effective dispersing agent for multiwalled carbon nanotubes in aqueous media. The (polystyrene)22(poly(2-vinylpyridine)-b-poly(acrylic acid))22, PS22(P2VP-b-PAA)22, star terpolymer exhibits a pH responsive self-assembly behavior in aqueous solution, due to the ampholytic nature of its diblock copolymer arms. Depending on the pH of the medium, two different self-assemblies were observed. At a low pH (pH 2), core–shell unimolecular micelles, with a hydrophobic PS core and P2VP, PAA segments in the shell, as well as multicore large compound micelles were formed. At pH 8.5, the star was transformed from a bis-hydrophilic into a bis-hydrophobic state, leading to a network-like intermicellar assembly. This star terpolymer was capable of dispersing MWCNTs, through non-covalent interactions, thus leading to stable MWCNT–star nanohybrids in water. More importantly, the resulted MWCNT–star nanohybrids adopted the “smart” pH responsive properties of the physisorbed stars, i.e. the nanohybrids can be positively or negatively charged at low and high pH respectively, while at an intermediate pH window, phase separation occurs due to neutralization of the oppositely charged segments.

Controlled reversible debundling of single-walled carbon nanotubes by photo-switchable dendritic surfactants

Stimulus responsive surfactants based on dendritic glycerol azobenzene conjugates were used to solubilize and debundle single-walled carbon nanotubes in aqueous media. Their debundling property as well as their reaggregation behavior upon irradiation with light was examined and light triggered reversible bundling and precipitation are shown.

Size characterization of the associations between carbon nanotubes and humic acids in aqueous media by asymmetrical flow field-flow fractionation combined with multi-angle light scattering

This work focuses on the influence of humic acids (HAs) on the fate of carbon nanotubes (CNTs) in aqueous media. This influence was demonstrated by mixing CNT powder with HAs in aqueous solution in varying concentrations. The aqueous media containing HAs and CNTs were size-characterized by asymmetrical flow field-flow fractionation (AsFlFFF) coupled with multi-angle light scattering (MALS). This coupling yielded information concerning the size distribution of single- and multi-walled CNTs (SWCNTs and MWCNTs) and HAs under different physico-chemical conditions that can occur in environmental water. HAs can disperse individual CNTs in aqueous media. However, the difference in the physical structure between SWCNTs and MWCNTs leads to significant differences in the quantity of HA that can adsorb onto the nanotube surface and in the stability of the CNT/HA complex. Compared with MWCNTs, SWCNTs suspended in HAs are less affected by changing ionic strength with respect to stability and the amount suspended.

Graphene Oxide-Assisted Dispersion of Pristine Multiwalled Carbon Nanotubes in Aqueous Media

Graphene oxide (GO) sheets, considered as “soft” two-dimensional macromolecules containing multiple aromatic regions and hydrophilic oxygen groups, can adsorb the pristine multiwalled carbon nanotubes (MWNTs) through the π-stacking interaction, thus causing pristine MWNTs to stably disperse and fractionate in aqueous media. The water-soluble and insoluble complexes can be obtained by changing the initial proportion of MWNTs to GO sheets, which are important for noncovalent approaches toward solubilizing CNTs and open a new way for GO applications in colloidal chemistry. Solubility results indicate that the GO sheets are prone to stabilize MWNTs with larger diameters. When the diameters of CNTs decrease to a critical value, the π-stacking interactions are weakened by large surface energy among CNTs themselves; the CNTs are inclined to form bundles or twisted structures instead of CNT−GO complexes. This can be probably developed into a practical method to fractionate MWNTs with different outer diameters in ...