Aqueous Surfactant Suspensions Research Articles

Exploiting the Physiochemical Interactions between Single-Walled Carbon Nanotubes and Hydrogel Microspheres To Afford Chirally Pure Nanotubes

Single-walled carbon-nanotubes (SWNTs) exhibit unique electronic, optical, and mechanical properties; however, a narrow availability of chirally pure (single electronic structure) material can limit effective integration within novel devices and schemes. This work focuses on understanding, modeling, and advancing methodology used to generate preparative quantities of single-chirality SWNT: the iterative adsorption/desorption of SWNT from aqueous surfactant suspensions to/from hydrogel microspheres. Commercially available hydrogel microspheres (Sephacryl S200) were sorted by radius and exposed to SWNT, affording a direct correlation between microsphere surface area and quantity of SWNT adsorbed using differential absorbance spectroscopy. This relationship elucidates a SWNT/gel purification scheme interaction mechanism exclusively involving the gel surface. High-concentration surfactant was used to elute SWNT from the gel with desorption efficiencies dependent on both SWNT chirality and hydrogel microsphere radius, ranging from 25–45%. A thermodynamic model for SWNT desorption that accounts for hydrogel microsphere curvature effects is presented and suggests that (when compared with experimental data) SWNT with greater than ∼41% of their length adsorbed to a hydrogel surface bind irreversibly, while others are desorbed in the presence of high-concentration surfactant. These findings inspired the generation and application of mechanically fractured hydrogel microspheres (exhibiting greater surface area) for use as SWNT purification media in per-iteration quantities far less than traditional gels. A 10-iteration SWNT purification procedure demonstrated a marked improvement in process efficiency, as mechanically fractured gels afford a 10-fold reduction in gel-media use (the major expense of the process) while yielding equivalent SWNT purification.

Influence of endohedral water on diameter sorting of single-walled carbon nanotubes by density gradient centrifugation

Separation of single-walled carbon nanotubes (SWNT) by diameter is an important prerequisite for controlled experimental studies and efficient application of these systems. By comparing experimental data with molecular dynamics (MD) simulations, we demonstrate that water filling has a significant, tube-diameter dependent effect on the effective mass density of individual single-walled carbon nanotubes suspended in aqueous surfactant suspensions. We present a model for the effective density of the nanotube-surfactant complex in aqueous solution that permits a comprehensive description of its density across the entire, experimentally relevant range of SWNT diameters. Parameters for this model can be obtained from molecular dynamics simulations and/or experiment and help explain the subtle interplay of surfactant coverage and endohedral water in the separation of a particular diameter species of SWNT by gradient centrifugation.

Spectroscopy of single‐walled carbon nanotubes in aqueous surfactant dispersion

AbstractOptical studies of single‐walled carbon nanotubes (SWNTs) dispersed in aqueous surfactant suspensions have made significant progress since the discovery of their bright band‐gap fluorescence in the near‐infrared region [M. J. O'Connell et al., Science 297, 593–596 (2002)]. In this article we report the systematic investigation of the effect of chemical purification and oxidation on the spectroscopic properties of SWNTs. The SWNTs samples we prepared differ for the percentage of impurities present, the amount of defects introduced in their graphitic structure and/or the functional groups expressed on their surface. The generation of carboxylic groups on the nanotubes surface by oxidative treatment was validated by means of thermal gravimetric analysis and ATR/Fourier transform IR spectroscopies, as well as Raman spectroscopy. We have solubilized pristine, purified SWNTs (p‐SWNTs) and oxidized SWNTs (o‐SWNTS) in Milli‐Q water using sodium dodecylbenzene sulphonate (SDBS). While our Vis‐NIR Absorption spectra of p‐SWNTs and o‐SWNTS dispersions in aqueous SDBS solution show the loss of the resolution of the characteristic van Hove singularities, NIR Photoluminescence spectra are characterized by structured emission peaks.

Photophysical comparative study of amylose and polyvinyle pyrrolidone/single walled carbon nanotubes complex

Progressive addition of hydroxypropylated amylose (AmH), from 0.05 wt% to 4.5 wt%, to single-walled carbon nanotubes (SWNTs) in aqueous surfactant suspensions quenches the intrinsic near Infra-Red fluorescence of semiconducting SWNTs while dispersions obtained with a same amount of polyvinylpyrrolidone (PVP) remain luminescent. Near Infra-Red emission spectroscopy (fluorescence and Raman scattering) of the samples is used to characterize the supramolecular organization of these polymer/SWNT complexes. The SWNTs are found to be wrapped by the PVP chains and not by the AmH chains which rather form AmH/surfactant/SWNTs complexes. In PVP/SWNTs dispersion, the fluorescence line position and intensity are affected by dielectric screening. In the case of AmH/surfactant/SWNTs complex, dielectric screening plays also a role but quenching occurs above about 3 wt% of AmH. We attribute the quenching to the formation of a "composite like" microstructure by opposition to stabilized dispersion.

Structure-Dependent Reactivity of Semiconducting Single-Walled Carbon Nanotubes with Benzenediazonium Salts

The addition of diazonium salts to single-walled carbon nanotubes (SWCNTs) in aqueous surfactant suspensions quenches the intrinsic near-infrared fluorescence of semiconducting SWCNTs through sidewall chemical reactions. Spectrally resolved fluorescence spectroscopy of mixed SWCNT samples has been used to measure structure-dependent relative reactivities in the initial stages of these reactions. For several 4-substituted benzenediazonium salts, Ar-R (Ar = N 2 (+)-C 6H 4 and R = Cl, NO 2, OMe), reactivities at pH 10 were found to be greatest for SWCNTs having the largest band gaps. The magnitude of this band gap dependence varies according to the R-group of the salt, with R = OMe showing the strongest variation. For R = OH, acidification of the sample to pH 5.5 results in reversal of the structural trend, as smaller band gap SWCNTs show slightly greater reactivities. The derivatization reactions observed here proceed concurrently, although at different rates, for semiconducting and metallic SWCNT species. These results therefore provide insight into the difficulties of separating metallic and semiconducting SWCNTs through selective reaction schemes and underscore the need for fluorescence spectroscopy to be used in assessing semiconducting SWCNT reactions.

Stable Luminescence from Individual Carbon Nanotubes in Acidic, Basic, and Biological Environments

Aqueous surfactant suspensions of single walled carbon nanotubes (SWNTs) are very sensitive to environmental conditions. For example, the photoluminescence of semiconducting SWNTs varies significantly with concentration, pH, or salinity. In most cases, these factors restrict the range of applicability of SWNT suspensions. Here, we report a simple strategy to obtain stable and highly luminescent individualized SWNTs at pH values ranging from 1 to 11, as well as in highly saline buffers. This strategy relies on combining SWNTs previously suspended in sodium dodecylbenzene sulfonate (SDBS) with biocompatible poly(vinyl pyrrolidone) (PVP), which can be polymerized in situ to entrap the SWNT-SDBS micelles. We present a model that accounts for the photoluminescence stability of these suspensions based on PVP morphological changes at different pH values. Moreover, we demonstrate the effectiveness of these highly stable suspensions by imaging individual luminescent SWNTs on the surface of live human embryonic kidney cells (HEK cells).

Femtosecond Transient Absorption Spectroscopy of Single-Walled Carbon Nanotubes and their Ultrafast Optical Switching Behavior

Two types of individualized single-walled nanotubes (SWNTs) in aqueous surfactant suspensions have been studied by femtosecond two-color absorption spectroscopy. A careful selection of pump and probe wavelengths allows for the determination of the “intrinsic” lifetimes of the lowest excited states which depend on the diameter of the SWNTs. Furthermore, a fast decay component in the ps to sub-ps regime was also observed and tentatively attributed to bundle relaxation. Likewise, this experimental approach can also assess spectral regions with decays faster than 1 ps, which is of great interest for all-optical switching devices near the optical telecommunication wavelength.

Exfoliation of single-wall carbon nanotubes in aqueous surfactant suspensions: A Raman study

Aqueous suspensions of bundles and individual single-wall carbon nanotubes were prepared with the help of surfactants. We study the changes in the Raman spectra of the suspensions with respect to powders, and of exfoliated tubes with respect to bundles. The radial breathing modes (RBM) upshift in suspensions because of the internal pressure of the liquid. By contrast, no shift is observed in the RBM spectra after exfoliation in the suspensions. However, we demonstrate a selectivity of the exfoliation process for tubes of small diameters.

Femtosecond transient absorption spectroscopy of single-walled carbon nanotubes in aqueous surfactant suspensions: Determination of the lifetime of the lowest excited state

Femtosecond one- and two-colour pump-probe spectroscopy of single-walled carbon nanotubes (SWNTs) individual in aqueous surfactant suspensions has been used to assess the "intrinsic" lifetime of the lowest excited states. We demonstrate that such measurements can be perturbed by several competing photophysical processes thus making lifetime deconvolution difficult. Furthermore we show how these effects, arising primarily from sample heterogeneity, can be reduced. Measurements of induced transients in the near IR yield lifetimes of (35 +/- 10) ps and (56 +/- 10) ps, for nanotubes having mean diameters of 0.95 and 1.2 nm, respectively. Furthermore, a fast decay component in the ps to sub-ps regime is also observed. We tentatively attribute this to relaxation in SWNT bundle components.

Bandgap-selective chemical doping of semiconducting single-walled carbon nanotubes

Bandgap-selective p doping of single-walled carbon nanotubes (SWCNTs) in aqueoussurfactant suspensions is reported. Nanotubes with narrower bandgaps are preferentiallydoped by hexacyanoferrate (III), as observed by optical absorption. The bandgapabsorption characteristics, which are controlled by the chemical potential, can bereversibly adjusted by controlling the oxidation potential of the dopants throughthe pH value. The origin of the selective modification of bandgap absorption iselectron transfer between the carbon nanotubes and the hexacyanoferrate (III),which preferentially depopulates the valence band in narrower gap nanotubes.

Fluorescence spectroscopy of single-walled carbon nanotubes in aqueous suspension

Optical studies of single-walled carbon nanotubes have advanced greatly through the recent discovery of near-infrared band gap photoluminescence from single-walled carbon nanotubes (SWNT) isolated in aqueous surfactant suspensions. This fluorescence emission has enabled the detection of many distinct optical transitions and their assignment to specific (n,m) semiconducting species of SWNT. The resulting set of precise transition energies presents a challenge to current theoretical models of nanotube electronic structure and a guide to nanotube researchers using resonance Raman spectroscopy. In the near future, structure-resolved fluorimetry should prove useful for revealing the quantitative (n,m) composition of mixed SWNT samples through sensitive, rapid, and nondestructive measurements. It will also permit detailed studies of many physical and chemical processes that vary with nanotube structure.

UV–VIS–NIR spectroscopy study of sensitivity of single-wall carbon nanotubes to chemical processing and Van-der-Waals SWNT/SWNT interaction. Verification of the SWNT content measurements by absorption spectroscopy

Spectrophotometric measurements of a large number of suspensions of SWNTs-containing soots in aqueous solutions of a surfactant with known weight/volume ratio have been carried out. SWNTs were purified by two methods, by gas-phase oxidation and by centrifugation of suspension. In contrast with gas-phase oxidation, centrifugation does not lead to chemical modification of SWNTs and to irreversible waste of substance. It was shown that gas-phase oxidation, as compared with centrifugation, weakly (by 5%) enriches samples (in which more than 60% of SWNTs were burned) by SWNTs with large diameters. Gas-phase oxidation of nanotubes produces no shift of the van Hove singularity peak positions of solitary nanotubes. It increases relative height of the van Hove bands of solitary nanotubes due to predominant combustion of defective nanotubes. The red shift of the van Hove singularity peaks can be used as a qualitative measure of agglomeration. Comparison of suspension spectra after powerful centrifugation at 160 000g (spectra of solitary nanotubes) shows that different catalysts of the arc discharge synthesis (Ni/Co, Ni/Y, Ni/La, Ni/Ce, Ni/Gd, Ni/Pr) produce almost the same diameter set of SWNTs, but with different distribution. A careful examination of the spectrophotometric method of determination of the abundance of SWNTs in aqueous surfactant suspensions with known weight/volume ratio was carried out.

Structure-assigned optical spectra of single-walled carbon nanotubes.

Spectrofluorimetric measurements on single-walled carbon nanotubes (SWNTs) isolated in aqueous surfactant suspensions have revealed distinct electronic absorption and emission transitions for more than 30 different semiconducting nanotube species. By combining these fluorimetric results with resonance Raman data, each optical transition has been mapped to a specific (n,m) nanotube structure. Optical spectroscopy can thereby be used to rapidly determine the detailed composition of bulk SWNT samples, providing distributions in both tube diameter and chiral angle. The measured transition frequencies differ substantially from simple theoretical predictions. These deviations may reflect combinations of trigonal warping and excitonic effects.

Electrochemical Oxidations of Several Organic Compounds in Aqueous Surfactant Suspensions Using Higher Valence Oxides as Intermediates

Copper(II) oxide and manganese(II) oxide were shown to be catalysts for the electro‐oxidation of 2‐chloroethyl ethyl sulfide, diethylsulfide, 2‐hydroxyethyl ethyl sulfide, and 1‐chlorobutane in aqueous suspensions containing cationic surfactants. These oxides were shown to act as catalysts by formation of copper(III), manganese(III), and manganese(IV) oxides which chemically oxidized the organic compounds. Analysis of the product suspensions showed the production of only the sulfoxide by oxidation of the 2‐chloroethyl ethyl sulfide and diethyl sulfide. Destruction of chlorobutane was catalyzed by both barium peroxide and copper(II) oxide. Volatile product(s) was trapped in a liquid nitrogen trap during electrolysis. These products included butane and butene.

The Electrodestruction of 1,2‐Dibromoethane by Anodic Oxidation in Aqueous Surfactant Suspensions in the Presence of Barium Peroxide

It was shown that electro‐oxidation of barium peroxide in aqueous sodium chloride solutions containing cationic surfactants formed a reactive intermediate which led to 89% destruction of 1,2‐dibromoethane (DBE) in 1 h under potentiostatic conditions. The 89% destruction was obtained under the following semioptimized conditions: 50 ml of 1.0M sodium chloride, 0.5 g barium peroxide, 2 ml of a 50% solution of dodecyltrimethylammonium chloride (DTAC), 6 mM DBE, 25°C, 0.9 V, graphite anode, and an initial pH of 5. Galvanostatic experiments (16.6 mA/cm2) using the same experimental conditions caused 86% destruction. The working hypothesis was that the barium peroxide was oxidized to barium superoxide, and the superoxide acted as a nucleophile displacing the halide. The resulting compound was then electrolytically and chemically oxidized. The products of the oxidation were bromide ions and barium carbonate. No oxidation occurred when hydrogen peroxide was used in place of barium peroxide. The surfactant was an aid in suspending the insoluble organic compounds and the barium peroxide. It also helped stabilize the superoxide ion.