Single-walled carbon nanotubes (SWNTs) are potential materials for future nanoelectronics. Since the electronic and optical properties of SWNTs strongly depend on tube diameter and chirality, obtaining SWNTs with narrow (n,m) chirality distribution by selective growth or chemical separation has been an active area of research. Here, we demonstrate that a new, bimetallic FeRu catalyst affords SWNT growth with narrow diameter and chirality distribution in methane CVD. At 600 °C, methane CVD on FeRu catalyst produced predominantly (6,5) SWNTs according to UV−vis−NIR absorption and photoluminescence excitation/emission (PLE) spectroscopic characterization. At 850 °C, the dominant semiconducting species produced are (8,4), (7,6), and (7,5) SWNTs, with much narrower distributions in diameter and chirality than materials grown by other catalysts. Further, we show that narrow diameter/chirality growth combined with chemical separation by ion exchange chromatography (IEC) greatly facilitates achieving single (m,n) SWNT samples, as demonstrated by obtaining highly enriched (8,4) SWNTs with near elimination of metallic SWNTs existing in the as-grown material.

There is great interest in using single-walled carbon nanotubes (SWNTs) as nanoscale probes and sensors in biological electronics and optical devices because the electronic and optical properties of SWNTs are extremely sensitive to the surrounding environments. [1][2][3][4][5] For the applications of SWNTs-based sensors in a biological environment, an immediate question is how the sensors respond to the biological conditions such as pH, 5c glucose, various ions, and proteins. This study requires a well-controlled modification of SWNT surfaces to obtain interfaces that are sensitive to these variables. 6 The exploration in this exciting area is still not in full blossom, partially due to the difficulty in preparing water-soluble SWNTs while maintaining the SWNT electronic structure intact. 4 In light of recent great progress in solubilization of SWNTs in various solvents by polymer wrapping and sidewall functionalization, 3a,4,5b,7-10 a better controlled modification of SWNT surfaces may be realized soon. In this work, we report a facile chemical routine to prepare water-soluble SWNTs that still retain their van Hove singularities after oxidative treatment. 7 The solubility in water for as-treated SWNTs with modified surfaces provides us with a unique opportunity to reveal the relationship of their electronic and optical properties with pH. Here we observe that after surface modification with carboxylate groups, the optical absorption of asprepared water-soluble semiconducting SWNTs (Tube@Rice and HiPco) reversibly responds to the pH change.Purified pristine Tube@Rice SWNTs suspended in toluene were purchased from Rice University. Raw HiPco SWNTs were purchased from Carbon Nanotechnologies, Inc., and were purified by the method described in ref 11. Because similar results were obtained with these two types of SWNTs, we reported the results here only for Tube@Rice SWNTs for the sake of clarity.The facile routine for preparation of water-soluble SWNTs was a modification of the acid oxidative method developed in Smalley's group. 7 In a typical experiment, 14 mg of SWNTs were added into 5 mL of a 9:1 concentrated H 2 SO 4 /30% H 2 O 2 aqueous solution. The mixture was stirred for 30 min. After the reaction, 15 mL of the 9:1 concentrated H 2 SO 4 /30% H 2 O 2 solution was added into the mixture. Then the mixture was divided into six aliquots in test tubes. Each aliquot was placed in an ultrasonic bath (Branson model 1510) and was sonicated for a different period of time, ranging from 0 to 5.0 min. Each resulting SWNT dispersion was diluted using 250 mL of distilled water and then was filtered through a 0.4 µm Millipore polycarbonate filter membrane. The resulting six SWNT mats were continuously washed using 10 mM NaOH solution and distilled water until the pH of the filtrates was 7. Then the wet SWNT mats were separated from the filters by dispersing them in distilled water. Six aqueous solutions of the SWNTs (0.03 mg/ mL) were prepared by sonication for 1-2 min. No tube precipitation was observed from these solutio...

The dynamics of water at the surface of artificial membranes composed of aligned multibilayers of the phospholipid dilauroyl phosphatidylcholine (DLPC) are probed with ultrafast polarization selective vibrational pump-probe spectroscopy. The experiments are performed at various hydration levels, x = 2 -16 water molecules per lipid at 37 °C. The water molecules are ~1 nm above or below the membrane surface. The experiments are conducted on the OD stretching mode of dilute HOD in H 2 O to eliminate vibrational excitation transfer. The FT-IR absorption spectra of the OD stretch in the DLPC bilayer system at low hydration levels shows a red-shift in frequency relative to bulk water, which is in contrast to the blue shift often observed in systems such as water nanopools in reverse micelles. The spectra for x = 4 -16 can be reproduced by a superposition of the spectra for x = 2 and bulk water. IR Pump-probe measurements reveal that the vibrational population decays (lifetimes) become longer as the hydration level is decreased. The population decays are fit well by biexponential functions. The population decays, measured as a function of the OD stretch frequency, suggest the existence of two major types of water molecules in the interfacial region of the lipid bilayers. One component may be a clathrate-like water cluster near the hydrophobic choline group and the other may be related to the hydration water molecules mainly associated with the phosphate group. As the hydration level increases, the vibrational lifetimes of these two components decrease, suggesting a continuous evolution of the hydration structures in the two components associated with the swelling of the bilayers. The agreement of the magnitudes of the two components obtained from IR spectra with those from vibrational lifetime measurements further supports the two component model. The vibrational population decay fitting also gives an estimation of the number of phosphate-associated water molecules and choline-associated water molecules from, which range from 1 to 4, and 1 to 12, respectively as x increases from 2 to 16. Time dependent anisotropy measurements yield the rate of orientational relaxation as a function of x. The anisotropy decay is biexponential. The fast component is almost independent of x, and is interpreted as small orientational fluctuations that occur without hydrogen bond rearrangement. The slower component becomes very long as the hydration level decreases. This component is a measure of the rate of complete orientational randomization, which requires hydrogen bond rearrangement and is discussed in terms of a jump reorientation model.