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. 2007 Nov;45(13):2463-2470.
doi: 10.1016/j.carbon.2007.08.035.

Tocopheryl Polyethylene Glycol Succinate as a Safe, Antioxidant Surfactant for Processing Carbon Nanotubes and Fullerenes

Aihui Yan  1 , Annette Von Dem Bussche, Agnes B Kane, Robert H Hurt
Affiliations

Tocopheryl Polyethylene Glycol Succinate as a Safe, Antioxidant Surfactant for Processing Carbon Nanotubes and Fullerenes

Aihui Yan et al. Carbon N Y. 2007 Nov.

Abstract

This work investigates the physical interactions between carbon nanomaterials and tocopheryl polyethylene glycol succinate (TPGS). TPGS is a synthetic amphiphile that undergoes enzymatic cleavage to deliver the lipophilic antioxidant, alpha-tocopherol (vitamin E) to cell membranes, and is FDA approved as a water-soluble vitamin E nutritional supplement and drug delivery vehicle. Here we show that TPGS 1000 is capable of dispersing multi-wall and single-wall carbon nanotubes in aqueous media, and for multiwall tubes is more effective than the commonly used non-ionic surfactant Triton X-100. TPGS is also capable of solubilizing C(60) in aqueous phases by dissolving fullerene in the core of its spherical micelles. Drying of these solutions leads to fullerene/TPGS phase separation and the self-assembly of highly ordered asymmetric nanoparticles, with fullerene nanocrystals attached to the hydrophobic end of crystalline TPGS nanobrushes. The article discusses surface charge, colloidal stability, and the potential applications of TPGS as a safe surfactant for "green" processing of carbon nanomaterials.

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Figures

FIGURE 1
FIGURE 1
The structure, hydrolysis, and antioxidant function of TPGS. PUFA are polyunsaturated fatty acids that are vulnerable to peroxidation, leading to free radical propagation reactions and cell membrane damage [27].
FIGURE 2
FIGURE 2
Effectiveness of TPGS and Triton in dispersing MWNTs. Digital photos of 15 ug/mL MWNT suspensions after 2-hour sonication and 15-hour settling. TPGS studied achieves complete dispersion (“CD”) above 3.8 ug-TPGS/mL. Triton leaves some MWNTs undispersed and visible at the top and bottom surfaces at all concentrations studied (see arrows). Note that 3.8 ug-TPGS/ml is 2.5 uM, while 6 ug-Triton/ml is 10 uM.
FIGURE 3
FIGURE 3
Solubility of C60 in surfactant solutions probed by UV-Vis light absorption. A,B UV-Vis absorption spectra: (A) in TPGS solutions (note: 25 wt-% solution was diluted by 5x prior to spectral measurement); (B) in Triton solutions. C,D: Quantitative optical absorbance at various characteristic wavelengths for C60 absorption as a function of surfactant concentration in aqueous solution. Stars mark characteristic fullerene peaks at 331, 358, 379, and 469 nm [43].
FIGURE 4
FIGURE 4
HRTEM images of carbon nanotubes in TPGS solutions after 2-hour sonication. (A–D) 15 ug/mL MWNTs (A,C) in water or (B,D) in 3.8 ug/mL TPGS solution. Scale bar: 5 nm. (E,F) 15 ug/mL SWNTs (E) in water, (F) in 15 ug/mL TPGS solution. Scale bar: 20 nm.
FIGURE 5
FIGURE 5
HRTEM images of unique TPGS/C60 nanostructures following various degrees of drying from 3.8 ug/mL TPGS/C60 solutions. A,B: 20-hour air drying; (arrow shows appearance of fullerene nanocrystals upon e-beam exposure), C,D: faceted cubic particles after 3-day air drying; and (E) unique asymmetric nanostructures after 1-week air drying.
FIGURE 6
FIGURE 6
The ED pattern of TPGS/C60 nanostructures after 3-day air drying. The rings are assigned to fcc-structure of nC60, but the (111) ring is not observed due to its very large d value, 0.82 nm.
FIGURE 7
FIGURE 7
Zeta potentials of nanotube/surfactant suspensions at: 15 ug-CNT/mL; A: MWNT; B: SWNT.
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