Plasma Activation of Carbon Nanotubes for Chemical Modification

Dec 23, 2000 - CSIRO Molecular Science, Bag 10, Clayton South, Victoria 3169, ... pyrolysis of FeC32N8H16 under Ar/H2 atmosphere at 800−1100 °C ...
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J. Phys. Chem. B 2001, 105, 618-622

ARTICLES Plasma Activation of Carbon Nanotubes for Chemical Modification Qidao Chen,† Liming Dai,* Mei Gao, Shaoming Huang, and Albert Mau CSIRO Molecular Science, Bag 10, Clayton South, Victoria 3169, Australia ReceiVed: September 19, 2000; In Final Form: NoVember 8, 2000

A novel approach for chemical modification of carbon nanotubes was developed, which involved radio frequency glow-discharge plasma activation, followed by chemical reactions characteristic of the plasmagenerated functional groups. For instance, amino-dextran chains have been immobilized onto acetaldehydeplasma-treated aligned carbon nanotubes through the formation of Schiff-base linkages, which were further stabilized by reduction with sodium cyanoborohydride. Using the same reaction, we have also chemically grafted periodate-oxidized dextran chains pre-labeled with fluorescein onto ethylenediamine-plasma-treated carbon nanotubes. The fluorescein labeling allows the surface immobilization reaction to be followed simply by photoluminescence measurements. The resulting polysaccharide-grafted carbon nanotubes are very hydrophilic, as demonstrated by X-ray photoelectron spectroscopic and air/water contact angle measurements.

Introduction Owing to their interesting electrical, magnetic, mechanical, and thermal properties, carbon nanotubes are promising as new materials for a variety of potential applications.1-4 The scope of using carbon nanotubes in practical devices and characterization of their physicochemical properties with conventional spectroscopic methods, however, has been largely hampered by their poor processability (e.g., insoluble and infusible). 5 Therefore, research on carbon nanotubes has recently been extended to include chemical modifications. For example, the tips of carbon nanotubes were shown to be more reactive than their sidewalls,6 and the treatment of carbon nanotubes with certain acids (e.g., refluxing in HNO3 and/or H2SO4) has been demonstrated to open the nanotube tips and to introduce -COOH and -OH groups at the opened ends.7 Besides, it has been demonstrated that carbon nanotube sidewalls can be covalently fluorinated within the temperature range between 250 and 400 °C8 or derivatized with certain highly reactive chemicals such as dichlorocarbene.9 Due to the rather harsh conditions involved, however, most of the above reactions caused the opening of the nanotube tips or detrimental damage to their sidewalls, or both. A particularly attractive option is the chemical modification of carbon nanotubes while largely retaining their structural integrity. In this regard, we have developed a novel approach for chemical modification of aligned carbon nanotubes by carrying out radio frequency glow-discharge plasma treatment, and subsequent reactions characteristic of the plasma-induced surface groups. In this paper, we report the surface immobilization of polysaccharide chains onto plasma-activated carbon nanotubes through the Schiff-base formation, followed by reductive stabilization of the Schiff-base linkage with sodium * Corresponding author. E-mail: [email protected]. † On leave of absence from the Department of Chemistry, Tsinghua University, Beijing 100084, China.

cyanoborohydride (Schemes 1 and 2). Radio frequency glowdischarge plasma has been widely used for surface activation of various materials, ranging from organic polymers to inorganic ceramics and metals.10 Being very hydrophilic and biocompatible,11 the surface-bound polysaccharides (e.g., amino-dextran and periodate-oxidized dextran chains in the present case) will provide highly hydrated coatings with important implications for using the modified carbon nanotubes in biological systems. Experimental Section Materials. Iron(II) phthalocyanine (FeC32N8H16, designated as FePc) was purchased from Aldrich, while the aligned carbon nanotube films were prepared by pyrolysis of FeC32N8H16 under Ar/H2 atmosphere at 800-1100 °C as has been well described elsewhere.12 Acetaldehyde (Sigma) and ethylenediamine (H2NCH2CH2NH2, Aldrich) were used as received for plasma polymerization. Both amino-dextran (MW ) 70 000 g/mol, 23 NH2/mol) and fluorescein isothiocyanate (FITC)-labeled dextran (designated as dextran-FITC, MW ) 10 000 and ca. 2 FITC/ mol with main adsorption and emission peaks at ca. 480 and 515 nm, respectively) were purchased from Molecular Probes and used without further purification, as were sodium cyanoborohydride (NaBH3CN, Aldrich), sodium periodate (NaIO4, BDH Chemicals), and the BDH buffer solution (pH ) 7 at 20 °C). Water was purified through an ultrapure water system (Milli-Q plus, Millipore). Plasma Activation. While the as-synthesized aligned nanotube film on a quartz plate is often covered by a thin layer of amorphous carbon, aligned nanotubes free from the amorphous carbon layer have also been prepared by using Si wafers as the substrate. With the latter, direct (homogeneous) plasma treatment of the constituent nanotubes is feasible. To facilitate the plasma treatment for the amorphous-carbon-covered aligned nanotubes grown on a quartz plate, we inverted the nanotube film by transferring it from the quartz surface onto a Scotch tape that

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Plasma Activation of Carbon Nanotubes was first pressed on the amorphous carbon layer and then peeled off with the nanotube film. The transferred nanotubes were still aligned perpendicularly, allowing plasma treatment of individual nanotubes with minimized inter-tube interference. Plasma polymerization was performed on a custom-built plasma apparatus, with which the detailed procedures used for the plasma treatment have been previously reported.10 Briefly, acetaldehyde plasma polymerization was carried out at 200 kHz, 20 W, and a monomer pressure of 0.3 Torr for 5 min (the optimized conditions with the plasma reactor used in this study for maximizing aldehyde functionalities), whereas ethylenediamine vapor was plasma-polymerized at 0.3 Torr, 200 kHz, and 20 W for 1 min. Surface Immobilization. Immobilization of Amino-dextran. As shown in Scheme 1, amine groups along the amino-dextran backbone may react with the plasma-induced aldehyde surface groups to form a Schiff-base linkage, which can then be further stabilized through reduction to the amine linkage with NaBH3CN.13-15 In a typical experiment, the aligned carbon nanotube film freshly activated by acetaldehyde plasma was immersed into an amino-dextran solution (0.05 g/10 mL) containing a predetermined amount of NaBH3CN (typically, 0.15 g). The pH of the solution was buffered between 6 and 8 and the reaction mixture kept at room temperature (20 ˚C) for 24 h. The amino-dextran-grafted nanotube sample was then rinsed thoroughly with Milli-Q water prior to further analyses. SCHEME 1: Reaction Scheme for the Covalent Immobilization of Amino-dextran Chains onto Acetaldehyde-Plasma-Activated Carbon Nanotubes. (For reasons of clarity, only one of the many plasma-induced aldehyde surface groups is shown for an individual nanotube.)

J. Phys. Chem. B, Vol. 105, No. 3, 2001 619 SCHEME 2: Reaction Scheme for the Periodate Oxidation of Dextran-FITC, Followed by Covalent Immobilization onto Ethylenediamine-Plasma Activated Carbon Nanotubes. (For reasons of clarity, only two of the many plasma-induced amine surface groups are shown for an individual nanotube.)

according to the published procedure,14,15 which is known to produce dialdehyde groups.16 The periodate-oxidized dextranFITC chains were then immobilized onto the ethylenediamineplasma-treated aligned carbon nanotubes by placing a piece of the freshly plasma-activated nanotube film (5 mm × 5 mm) at the bottom of a 1 cm quartz cell containing the periodateoxidized dextran-FITC solution (5 × 10-4 g/L) in the presence of NaBH3CN (2 mg). The pH of the solution was buffered between 6 and 8, as is in the case for amino-dextran immoblization. As a control, the immobilization of dextran-FITC was also carried out under the same conditions, but in the absence of NaBH3CN (vide infra). Characterization. Scanning electron micrographs (SEMs) were obtained using a Philips XL-30 FEG SEM unit at 5 kV, while transmission electron microscopic (TEM) images were made on a JEOL 200C TEM microscope at 200 kV. X-ray photoelectron spectra (XPS) were acquired using a Kratos Ultra Imaging XPS Spectrometer with nonmonochromatic Mg KR radiation at a power of 150 W. Air/water contact angles were measured on a modified Kernco G-II goniometer, equipped with a syringe incorporating a reversible plunger driven by a micrometer, using the Milli-Q water as the test liquid. Photoluminescence spectra were recorded on a luminescence spectrophotometer SL 50 from Perkin-Elmer. Results and Discussion

Immobilization of Dextran-FITC. Dextran-FITC was immobilized by reductive amination following oxidation by sodium periodate (Scheme 2). The periodate oxidation was performed

Plasma Activation. Figure 1a shows a typical SEM micrograph for the aligned carbon nanotubes as-synthesized on a Si wafer or those transfereed onto a Scotch tape from the quartz surface. By plasma polymerization (e.g., acetaldehyde), a concentric layer of (acetaldehyde) plasma polymer film was homogeneously deposited onto each of the constituent aligned

620 J. Phys. Chem. B, Vol. 105, No. 3, 2001

Figure 1. SEM micrographs of the aligned carbon nanotubes (a) before and (b) after the plasma polymerization of acetaldehyde. The insets show TEM images of an individual nanotube (a) before and (b) after being coated with a layer of the acetaldehyde-plasma-polymer. Note that the micrographs shown in (a) and (b) were not taken from the same spot due to technical difficulties.

carbon nanotubes (Figure 1b). The SEM image for the plasmapolymer-coated nanotubes given in Figure 1b shows the similar features as the aligned nanotube array of Figure 1a, but with a larger tubular diameter and smaller inter-tube distance due to the presence of the plasma coating. The TEM images of the constituent nanotube before and after the plasma treatment are given in the insets of Figure 1, parts a and b, respectively. Comparing the inset of Figure 1b with its counterpart in Figure 1a clearly shows the presence of a homogeneous plasma coating along the nanotube sidewall. The coating thickness was determined from the TEM image shown in the inset of Figure 1b to be 20-30 nm for this particular sample, but it can be varied in a controllable fashion by changing the plasma polymerization conditions (e.g., treatment time). To characterize the plasma coating, we carried out XPS spectroscopic and air/water contact angle measurements. Figure 2, parts a and b, shows the XPS survey spectra of the aligned carbon nanotube film before and after the plasma polymerization of acetaldehyde, respectively. As can be seen, Figure 2a shows the expected peak of the graphitic C at 284.7 eV, along with weak signals for N 1s (399 eV), O 1s (531 eV), and Fe 2p (708

Chen et al.

Figure 2. XPS survey and C 1s (inset) spectra of (a) an untreated carbon nanotube film on a Si wafer, (b) after the acetaldehyde plasma polymerization, (c) after the surface immobilization of amino-dextran chains.

eV). The low atomic ratio of O/C ) 0.016 deduced from the survey spectrum could indicate an incorporation of a trace amount of oxygen into the nanotube structure. However, the possibility with physically adsorbed oxygen cannot be ruled out since carbon nanotubes are known to be susceptible to oxygen adsorption even at pressures as low as 10-8 to 10-10 Torr,17 typical for the XPS measurements. In fact, the C 1s spectrum shown in the inset of Figure 2a is very similar to that of graphite (HOPG),12a,18a suggesting that the nanotube structure is free from oxygen. On the other hand, previous studies have demonstrated the presence of N in nanotubes prepared by pyrolysis of N-containing molecules18 (e.g., NiPc). Figure 2b shows a large increase in the atomic ratio of O/C from 0.016 to 0.250. The absence of a signal from N in Figure 2b indicates that the acetaldehyde plasma polymer coating is pinhole-free and thicker than the XPS probe depth (i.e., >10 nm).19 The corresponding high-resolution C 1s spectra given in the insets of Figure 2, parts a and b, show a significant increase in intensity at 288.0 eV attributable to sCdO groups upon the plasma polymerization of acetaldehyde, clearly indicating the introduction of aldehyde surface groups. An important check provided by the XPS C 1s spectra is that the amount of carboxylic groups (at 289 to 289.5 eV), if any, in the plasma layer is very low; consistent with previous findings under optimized conditions.13,20

Plasma Activation of Carbon Nanotubes

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TABLE 1: Composition of Carbon-Containing Surface Groups for the Acetaldhyde-Plasma-Activated Carbon Nanotubes (CNTs) before and after Grafting with Amino-dextran Chains sample

CHx % (285.0 eV)

C-O % (286.5 eV)

CdO % (288.0 eV)

plasma-CNTs amino-dextran-CNTs

66.47 57.19

17.23 33.16

16.30 9.65

The plasma-induced aldehyde surface groups were then used for the surface immobilization of amino-dextran chains onto the aligned carbon nanotubes. Immobilization of Amino-dextran. Figure 2c shows the XPS survey spectrum of the amino-dextran-immobilized carbon nanotubes. Also included in the inset of Figure 2c is the corresponding XPS C 1s spectrum. Comparing with Figure 2b, Figure 2c shows an increase in the O/C ratio from 0.250 to 0.367. This, together with the reappearance of the N signal in the survey spectrum of Figure 2c, clearly indicates the presence of the oxygen-rich amino-dextran coating. By using the values of 70 000 g/mol and 23 NH2/mol as the molecular weight and amine content for an amino-dextran chain (see, Experimental Section), we have calculated the N/C ratio to be 0.009. This value, being in the same order of magnitude, is slightly higher than the corresponding N/C ratio of 0.005 estimated from the XPS survey spectrum shown in Figure 2c. Likewise, the value of 0.367 for the O/C ratio determined by XPS is somewhat smaller than the corresponding O/C ratio of 0.824 calculated from the molecular structure of the pristine amino-dextran chains, indicating that the dehydrated polysaccharide coating is thinner than 10 nm (i.e., the typical XPS probe depth).15,19 Thus, the undercoated plasma layer was detected by the XPS measurements, as also indicated by the similarity in the hydrocarbon region of the XPS C 1s spectra (see, the insets of Figure 2, parts b and c). Numerical results from the curve-fitted XPS C 1s spectra shown in the insets of Figure 2, parts b and c are summarized in Table 1.21 As can be seen, the intended grafting reaction was also manifested by an increase in the percentage content of C-O from 17.23 to 33.16%, together with the concomitant decrease of the corresponding content for aldehyde group (i.e., sCdO) from 16.30 to 9.65%, upon grafting with the amino-dextran chains. The remaining 9.65% CdO component represents those aldehyde groups trapped inside of the (cross-linked) plasma coating that are inaccessible to the amino-dextran grafting chains. The intended covalent interfacial bonding was further tested by autoclaving, a process that has been proven a specific test for distinguishing the reduced Schiff-base linkage from physical adsorption.15 The XPS measurements on amino-dextran treated nanotubes with insufficient or no NaBH3CN reduction showed part or complete loss of the polysaccharide chains upon autoclaving in water at 121 °C for 20 min, whereas similar treatment for the NaBH3CNstabilized samples did not cause any obvious change in the XPS spectra. The resulting amino-dextran grafted nanotubes showed very low advancing (ACA), sessile (SCA), and receding (RCA) air/ water contact angles. For example, the ACA ) 155°, SCA ) 146°, and RCA ) 122° for an untreated aligned carbon nanotube film were found to be reduced to values of 90°, 78°, and 45°, respectively, by the acetaldehyde plasma treatment. Subsequent immobilization of amino-dextran chains led to such a highly hydrophilic surface that the contact angles could not be measured as the water drop for measuring the contact angles spread out instantaneously on the modified nanotube surface. Upon removal

Figure 3. Typical PL spectra of the periodate-oxidized dextran-FITC in the buffer solution (5 × 10-4 g/L, λex ) 480 nm) recorded in a 1 cm quartz cell, containing a piece of the ethylenediamine-plasma-treated aligned carbon nanotube film (5 mm × 5 mm) at the bottom, after the addition of NaBH3CN (2 mg) for (a) 0.1 h, (b) 0.5 h, (c) 1.5 h, (d) 4.5 h, (e) 14.5 h. The inset shows time-dependence of the PL peak intensity measured during the surface immobilization reaction with (a) and without (b) the addition of NaBH3CN.

from the substrate by scraping, the amino-dextran-grafted nanotubes became apparently soluble in water. The highly hydrated and biologically benign amino-dextran coating should facilitate the use of the aligned carbon nanotubes for many biologically related applications (e.g., as bio-sensors and artificial muscles).4,11 The glucose units within the surfacegrafted amino-dextran chains can be converted into dialdehyde moieties by periodate oxidation,14,15 thereby allowing for further chemical modification. Immobilization of Dextran-FITC. To acquire further evidence for the chemical immobilization of polysaccharides onto plasma-activated carbon nanotubes and to demonstrate the versatility of the Schiff-base formation for surface modification of carbon nanotubes, we carried out plasma treatment of the aligned carbon nanotubes with H2NCH2CH2NH2 vapor, followed by surface immobilization of fluorescein isothiocyanate (FITC)22labeled dextran (i.e., dextran-FITC). In so doing, we first oxidized dextran-FITC with NaIO4 in water14,15 and then grafted the periodate-oxidized dextran-FITC chains onto the ethylenediamine-plasma-treated carbon nanotubes in the presence of NaBH3CN (Scheme 2). The Schiff-base formation between aldehyde groups of the periodate-oxidized dextran-FITC and surface amine groups generated by the ethylenediamine-plasma was monitored by the photoluminescence (PL) spectra recorded in situ at various stages of the reaction. As seen in Figure 3, a continuous decrease in the main emission peak at 515 nm was observed for the PL spectrum of the periodate-oxidized dextranFITC solution, measured in a 1 cm quartz cell with a piece of the ethylenediamine-plasma-treated aligned carbon nanotube film (5 mm × 5 mm) at the bottom, after adding NaBH3CN (2 mg). The inset (curve a) of Figure 3 shows the change of the PL peak intensity as a function of time during the surface immobilization in the presence of NaBH3CN. Also included in the inset (curve b) of Figure 3 is the corresponding timedependence of the PL peak intensity for a control experiment that was carried out under the same conditions but without the addition of NaBH3CN. The effect of NaBH3CN on the PL peak intensity of the periodate-oxidized dextran-FITC solution at the

622 J. Phys. Chem. B, Vol. 105, No. 3, 2001 reaction pH between 6 and 8 was checked to be insignificant. Hence, the time evolution of the PL peak intensity associated with curve a in the inset of Figure 3 clearly indicates a continuous decrease of the fluorescein moieties in the solution phase, from which the PL spectra were recorded, due to surface immobilization of the periodate-oxidized dextran-FITC chains onto the plasma-treated nanotube array placed at the bottom of the quartz cell. In contrast, curve b in the inset of Figure 3 did not show the occurrence of the surface immobilization reactions the slight decrease in the PL peak intensity seen at the initial stage indicating, most probably, a physical adsorption process that rapidly reached to an equilibrium state. Therefore, Figure 3 clearly shows, once again, the successfulness of the Schiffbase formation for chemical grafting of polysaccharide chains onto plasma-treated carbon nanotubes in the presence of NaBH3CN. Furthermore, our preliminary results indicated that certain nondeposition plasma treatments (e.g., NH3 plasma) could also be used to activate the carbon nanotubes for chemical modifications. Conclusions In summary, we have demonstrated a novel approach for chemical modification of carbon nanotubes through plasma activation and subsequent reactions characteristic of the plasmainduced surface functionalities. Amino-dextran chains were immobilized onto acetaldehyde-plasma-treated aligned carbon nanotubes through the Schiff-base formation and reductive stabilization with sodium cyanoborohydride, while periodateoxidized dextran-FITC chains were chemically grafted onto ethylenediamine-plasma-treated carbon nanotubes via the same reaction. The resultant materials possess a highly hydrophilic surface with the integrity of the nanotube structure being largely maintained. Owing to the highly generic nature characteristic of the plasma treatment/polymerization,10 together with the versatile Schiff-base formation between aldehyde surfaces and amino-containing polysaccharides (or vice versa), the methodology developed in this study could be regarded as a general approach toward the surface modification of carbon nanotubes for many potential applications ranging from composite materials to biomedical devices. Acknowledgment. Q.C. is grateful for a visiting fellowship from the Department of Industry, Science and Technology (DIST), Australia, and the support from Tsinghua University. We thank J. V. Ward and M. H. Greaves for assistance with the SEM work, T. R. Gengenbach and N. Brack with the XPS analyses, and K. McLean and H. Griesser with the plasma treatment. References and Notes (1) (a) Yakobson, B. I.; Smalley, R. E. Am. Sci. 1997, 85, 325. (b) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. Science of Fullerenes and Carbon Nanotubes; Academic Press: New York, 1996. (2) De Heer, W. A.; Bonard, J.-M.; Fauth, K.; Chaˆtelain, A.; Forro´, L.; Ugarte, D. AdV. Mater. 1997, 9, 87 and references therein. (3) (a) Ajayan, P. M. Chem. ReV. 1999, 99, 1787. (b) Dai, H.; Kong, J.; Zhou, C.; Franklin, N.; Tombler, T.; Cassell, A.; Fan, S.; Chapline, M. J. Phys. Chem. B 1999, 103, 11246. (4) Baughman, R. H.; Changxing, C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, B. M.; Wallace, G. G.; Mazzoldi, A.; De Rossi, D.;

Chen et al. Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Science 1999, 284, 1340. (5) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (6) See, for example: Dai, L. Polym. AdV. Technol. 1999, 10, 357 and references therein. (7) See, for example: (a) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (b) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C. Science 1998, 282, 95. (c) Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y.; Itkis, M. E.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. AdV. Mater. 1999, 11, 834. (d) Sloan, J.; Hammer, J.; Zwiefka-Sibley, M.; Green, M. L. H. Chem. Commun. 1998, 347. (8) See, for example: (a) Nakajima, T.; Kasamatsu, S.; Matsuo, Y. Eur. J. Solid State Inorg. Chem. 1996, 33, 831. (b) Mickelson, E. T.; Huffman, C. B.; Rinzler, A. G.; Smalley, R. E.; Hauge, R. H. Chem. Phys. Lett. 1998, 296, 188. (c) Mickelson, E. T.; Chiang, I. W.; Zimmerman, J. L.; Boul, P. J.; Lozano, J.; Liu, J.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. B 1999, 103, 4318. (9) (a) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (b) Chen, Y.; Haddon, R. C.; Fang, S.; Rao, A. M.; Eklund P. C.; Lee, W. H.; Dichey, E. C.; Grulke, E. A.; Pendergrass, J. C.; Chavan, A.; Haley, B. E.; Smalley, R. E. J. Mater. Res. 1998, 13, 2423. (10) See, for example: (a) Dai, L. J. Macromol. Sci, ReV. Macromol. Chem. Phys. C 1999, 39, 237 and references therein. (b) Dai, L.; Griesser, H. J.; Mau, A. W. H. J. Phys. Chem. 1997, 101, 9548. (c) Chen, Q.; Dai, L. Appl. Phys. Lett. 2000, 76, 2719. (11) Dai, L.; Mau, A. W. H. J. Phys. Chem. B 2000, 104, 1891. (12) See, for example: (a) Huang, S.; Dai, L.; Mau, A. W. H. J. Phys. Chem. B 1999, 103, 4223. (b) Yang, Y.; Huang, S.; He, H.; Mau, A. W. H.; Dai, L. J. Am. Chem. Soc. 1999, 121, 10832. (c) Huang, S.; Mau, A. W. H.; Turney, T.; White, P.; Dai, L. J. Phys. Chem. B 2000, 104, 2193. (d) Li, D.-C.; Dai, L.; Huang, S.; Mau, A. W. H.; Wang, Z. L. Chem. Phys. Lett. 2000, 316, 349. (e) Gao, M.; Huang, S.; Dai, L.; Wallace, G.; Gao, R. P.; Wang, Z. L. Angew. Chem., Int. Ed. 2000, 39, 3664. (f) Gao, R. P.; Wang, Z. L.; Bai, Z. G.; de Heer, W. A.; Dai, L.; Gao, M. Phys. ReV. Lett. 2000, 85, 622. (13) Gong, X.; Dai, L.; Griesser, H. J.; Mau, A. W. H. J. Polym. Sci. Part B, Polym. Phys. 2000, 38, 2323. (14) Dai, L.; Zientek, P.; StJohn, H. A. W.; Pasic, P.; Chatelier, R. C.; Griesser, H. J. In Surface Modification of Polymeric Biomaterials; Ratner, B. D., Castner, D. G., Eds.; Plenum Press: New York, 1996. (15) Dai, L.; StJohn, H. A. W.; Bi, J.; Zientek, P.; Chatelier, R. C.; Griesser, H. J. Surf. Interface Anal. 2000, 29, 46. (16) (a) Dyer, J. R. In Methods of Biochemical Analysis; Glick, D., Ed.; Interscience Publishers: New York, 1960. (b) Guthrie, R. D. AdV. Carbohydr. Chem. 1961, 16, 105. (17) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (18) (a) Yudasaka, M.; Kikuchi, R.; Ohki, Y.; Yoshimura, S. Carbon 1997, 35, 195. (b) Terrones, M.; Grobert, N.; Olivares, J.; Zhang, J. P.; Terrones, H.; Kordatos, K.; Hsu, W. K.; Hare, P. P.; Townsend, P. D.; Prassides, K.; Cheetham, A. K.; Kroto, H. W.; Walton, D. R. M. Nature 1997, 388, 52. (19) Methods of Surface Analysis: Techniques and Applications; Walls, J. M., Ed.; Cambridge University Press: Cambridge, 1990. (20) Gong, X.; Griesser, H. J. Plasmas Polym. 1997, 2, 261. (21) Curve fitting of the XPS C 1s spectra was carried out using a Gaussian/Lorentzian product function (Hughes, A. E.; Sexton, B. A. J. Electron Spectrosc. Relat. Phenom. 1988, 46, 31) and a nonlinear Shirley background correction (Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129). The slightly higher O/C ratios derived from the C 1s XPS peaks than those calculated from survey spectra are not inconsistent with other plasma surfaces due, most probably, to the cross-linked structures (see, for example: Qiu, Y. X.; Klee, D.; Plsu¨ter, W.; Severich, B.; Ho¨cker, H. J. Appl. Polym. Sci. 1996, 61, 2373). As the C 1s spectrum given in the inset of Figure 2a indicated no chemical incorporation of oxygen into the carbon nanotube structure (see text), the corresponding curve fitting to O-containing components was not performed. (22) (a) Chatelier, R. C.; Gengenbach, T. R.; Vasic, Z. R.; Griesser, H. J. J. Biomater. Sci. Polym. Ed. 1995, 7, 601. (b) Griesser, H. J.; Chatelier, R. C. J. Appl. Polym.: Appl. Polym. Symp. 1990, 46, 361.