Surfactant Concentration Dependent Spectral Effects of Oxygen and Depletion Interactions in Sodium Dodecyl Sulfate Dispersions of Carbon Nanotubes

Quenching of optical absorbance spectra for carbon nanotubes (CNTs) dispersed in sodium dodecyl sulfate (SDS) has been observed to be more pronounced at higher concentrations of the surfactant. The protonation-based quenching behavior displays wavelength dependence, affecting larger diameter nanotube species preferentially. Although absorbance may be recovered by hydroxide addition, pH measurements suggest that hydrolysis of SDS does not play a major role in the short term quenching behavior at high SDS concentrations. The degree of quenching is observed to correlate well with an increase in attractive depletion as SDS concentration is increased, while the extent of depletion is found to depend heavily on the concentration of preparation in comparison to the final SDS concentration. Attractive depletion in SDS is also found to be preferential for CNTs of larger diameter. It is proposed that depletion enhances the quenching effect due to close association of CNT-SDS complexes providing higher SDS densities on the CNT surface, leading to further oxidation. In addition, the quenching behavior in SDS is found to strongly suppress the optical and Raman signal from metallic nanotube species even at high pH. Displacement of SDS by sodium deoxycholate as a secondary surfactant is able to reverse the effects of protonation of metallic species, whereas hydroxide addition is only partially effective

Electrochemical and Computational Studies on Intramolecular Dissociative Electron Transfer in β‑Peptides

The preparation of the β-peptides PCB-β³Val-β³Ala-β³Leu-NHC­(CH₃)₂OO<i>t</i>Bu and PCB-(β³Val-β³Ala-β³Leu)₂-NHC­(CH₃)₂OO<i>t</i>Bu, with a specific donor and acceptor at each terminus, is described. Circular dichroism, 2D NMR, and density functional theory calculations confirmed that PCB-(β³Val-β³Ala-β³Leu)₂-NHC­(CH₃)₂OO<i>t</i>Bu adopts a 14-helix conformation, whereas PCB-β³Val-β³Ala-β³Leu-NHC­(CH₃)₂OO<i>t</i>Bu has an ill-defined secondary structure. The electron-transfer rate constants in the two peptides were found to be 2580 and 9.8 s^–1 respectively. Computational simulations based on Marcus theory coupled to constrained density functional theory provide clear theoretical evidence that different electron-transport pathways occur in the two peptides due to their different conformations: sequential hopping within PCB-(β³Val-β³Ala-β³Leu)₂-NHC­(CH₃)₂OO<i>t</i>Bu and superexchange within PCB-β³Val-β³Ala-β³Leu-NHC­(CH₃)₂OO<i>t</i>Bu. Electron population analysis provides the first clear theoretical evidence that amide groups can act as hopping sites in long-range electron transfer

Unraveling the Interplay of Backbone Rigidity and Electron Rich Side-Chains on Electron Transfer in Peptides: The Realization of Tunable Molecular Wires

Electrochemical studies are reported on a series of peptides constrained into either a 3₁₀-helix (<b>1</b>–<b>6</b>) or β-strand (<b>7</b>–<b>9</b>) conformation, with variable numbers of electron rich alkene containing side chains. Peptides (<b>1</b> and <b>2</b>) and (<b>7</b> and <b>8</b>) are further constrained into these geometries with a suitable side chain tether introduced by ring closing metathesis (RCM). Peptides <b>1</b>, <b>4</b> and <b>5</b>, each containing a single alkene side chain reveal a direct link between backbone rigidity and electron transfer, in isolation from any effects due to the electronic properties of the electron rich side-chains. Further studies on the linear peptides <b>3</b>–<b>6</b> confirm the ability of the alkene to facilitate electron transfer through the peptide. A comparison of the electrochemical data for the unsaturated tethered peptides (<b>1</b> and <b>7</b>) and saturated tethered peptides (<b>2</b> and <b>8</b>) reveals an interplay between backbone rigidity and effects arising from the electron rich alkene side-chains on electron transfer. Theoretical calculations on β-strand models analogous to <b>7</b>, <b>8</b> and <b>9</b> provide further insights into the relative roles of backbone rigidity and electron rich side-chains on intramolecular electron transfer. Furthermore, electron population analysis confirms the role of the alkene as a “stepping stone” for electron transfer. These findings provide a new approach for fine-tuning the electronic properties of peptides by controlling backbone rigidity, and through the inclusion of electron rich side-chains. This allows for manipulation of energy barriers and hence conductance in peptides, a crucial step in the design and fabrication of molecular-based electronic devices

Separation of Double-Walled Carbon Nanotubes by Size Exclusion Column Chromatography

In this report we demonstrate the separation of raw carbon nanotube material into fractions of double-walled (DWCNTs) and single-walled carbon nanotubes (SWCNTs). Our method utilizes size exclusion chromatography with Sephacryl gel S-200 and yielded two distinct fractions of single- and double-walled nanotubes with average diameters of 0.93 ± 0.03 and 1.64 ± 0.15 nm, respectively. The presented technique is easily scalable and offers an alternative to traditional density gradient ultracentrifugation methods. CNT fractions were characterized by atomic force microscopy and Raman and absorption spectroscopy as well as transmission electron microscopy