Interaction of Amino Acids and Single-Wall Carbon Nanotubes

Dec 16, 2011 - their unique physicochemical properties and wide-ranging ... between oxidized single-wall carbon nanotubes and three amino acids. A sim...
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Interaction of Amino Acids and Single-Wall Carbon Nanotubes Lingyu Piao,*,† Quanrun Liu,‡ and Yongdan Li‡ † ‡

National Center for Nanoscience and Technology, Beijing 100190, China Department of Catalysis Science and Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, China

bS Supporting Information ABSTRACT: In this article, we investigated the interactions between oxidized single-wall carbon nanotubes and three amino acids. A simple and environmental benign method to realize solubility of oxidized single-wall carbon nanotubes (OSWNT) in water was described. The amino acids used in this study include L-glycine (Gly), L-lysine (Lys), and L-phenylalanine (Phe). The OSWNT became soluble in water under ambient conditions and formed a stable suspension when amino acids (AA) were adsorbed on it. The interactions between OSWNT and three AA were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA). The results indicate that there is an increasing in the diameter of OSWNT after AA adsorption. The OSWNT with different diameters were separated as a result of AA adsorption. The smaller the diameter of OSWNT, the more the AA adsorption amount is. The adsorbed amount of different AA on OSWNT follows the trend: Lys > Phe > Gly. The Π Π stacking is an important factor to realizing adsorption of Phe zwitterions on the sidewall of OSWNT; but for Gly and Lys zwitterions, polar interaction is a determinant factor to realizing adsorption on the sidewall of OSWNT. The AA zwitterions were adsorbed on the surface of OSWNT by conjunct interaction of the Π Π stacking, polar interaction, hydrogen bond, and covalent bonding. Hydrogen bond and covalent bond, formed with oxygen containing groups, is dominant at the end of OSWNT. The catalysis property of OSWNT makes a noticeable reduction of decomposition temperature for AA adsorbed on OSWNT.

’ INTRODUCTION Single-wall carbon nanotubes (SWNTs) have rapidly become one of the most widely studied nanomaterials, primarily because their unique physicochemical properties and wide-ranging applications in molecular electronics, optoelectronics, drug delivery, and chemical and biological sensors.1 7 The unique electronic and optical properties of SWNTs, in conjunction with their size and mechanically robust nature, make SWNTs become crucial to the development of next-generation biosensing platforms.8 10 With regard to biomedical applications of SWNTs, the most important prerequisite is the development of methods to immobilize biomolecules on SWNTs. The adsorption and functionalization of biological molecules on SWNTs are common way to realize immobilization of biological molecules. SWNTs functionalization is important for above applications since it makes nanotubes compatible with different environments such as solutions, polymer matrices, and interfaces. Furthermore, it can also give them specific functions to address biological targets. So, the understanding of the interaction mechanism between SWNTs and biological molecules is mandatory for safe using of SWNTs in biological applications.11 16 Poenitzsch12 investigated the effect of electron-donating and electron-withdrawing groups for the peptide/SWNTs interactions combining Raman and scanning tunneling spectroscopy. Huang et al.13 advocated that bovine serum albumin protein could be covalently attached to r 2011 American Chemical Society

carbon nanotubes via diimide-activated amidation under ambient conditions. Afterward, the adsorption of biological molecules with different aromatic structures on SWNTs has also been demonstrated.14 16 However, the uncertainty about the interaction mechanism between biomolecules and SWNTs still exists. The structural complexity of macro-biomolecules is the main factor limiting our capabilities for understanding the interaction process. At the same time, the experimental evidence is still scarce. So, we need to provide more data and information to investigate the interaction mechanism between biomolecules and SWNTs. Amino acids (AA) are an elementary unit for composing biomolecules and can also reflect the common chemical properties of complicated biomolecules. So, the interaction between SWNTs and AA is very important for understanding the interaction mechanism between SWNTs and biomolecules. In this work, three AA were selected as example specimen for exploiting this interaction. They are L-glycine (Gly), L-lysine (Lys), and L-phenylalanine (Phe). Gly is the only kind of amino acid that has no chirality, Lys is a kind of alkali amino acid, and Phe is a kind of amino acid with an aromatic ring. The SWNTs have not been further functionalized except for purification and oxidization. Received: September 4, 2011 Revised: November 3, 2011 Published: December 16, 2011 1724

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Figure 2. Raman spectrum of OSWNT solid samples before and after adsorption: (a) OSWNT, (b) OSWNT-Phe0.15, (c) OSWNT-Gly0.15, and (d) OSWNT-Lys0.15.

Figure 1. HRTEM images of OSWNT before and after AA adsorption: A, before adsorption; B, OSWNT-Gly0.15; C, OSWNT-Phe0.15; D, OSWNT-Lys0.15.

The purpose of this article is to investigate the interaction mechanism between the elementary unit of biomolecules and SWNTs. Here, we present a simple and environmental benign approach to obtain a stable SWNTs/AA suspension, which is transparent and stable for at least 200 days. Spectroscopy, thermal analysis, and chromatography together with morphological analysis have been used to investigate the interaction between AA molecules and oxidized SWNTs (OSWNT). We have proposed the adsorption mechanism of the three AA on OSWNT.

’ RESULTS AND DISCUSSION We have already confirmed that there are not any OSWNT present in aqueous suspension after centrifugation at 50 000g. However, supernatant of the OSWNT-Phe0.15 suspension after centrifugation at 50 000g consists primarily of long bundled OSWNT.16 Normally, AA existed in the form of zwitterions when it is dissolved in water. The OSWNT became soluble in water and formed a transparent suspension since the Phe zwitterions were adsorbed on OSWNT.16 The phenomenon is also the same for Gly and Lys adsorption. The photograph of OSWNT-AA supernatant after centrifugation is shown in Figure 2 of the Supporting Information. The OSWNT-AA supernatant was transparent, yet after 210 days at 277 K, there was no deposit. This means the interaction between the AA zwitterions and the OSWNT is stable. HRTEM. Figure 1 shows the HRTEM images of OSWNT samples before (Figure 1A) and after adsorption of Gly (Figure 1B), Phe (Figure 1C), and Lys (Figure 1D), respectively. We can see that OSWNT has an even diameter and clean sidewall before AA adsorption. From Figure 1B D, we can observe the coverage of AA on the sidewall of OSWNT in comparison with Figure 1A. We have confirmed that the element nitrogen existed in OSWNT-Phe0.15, OSWNT-Lys0.15, and OSWNT-Gly0.15 from the EDX spectrum (Figures 3 5, Supporting Information, respectively). In contrast, there is no

element nitrogen in OSWNT before adsorption of AA (Figure 6, Supporting Information). As indicated above, we reasoned that the AA zwitterions were adsorbed on OSWNT. The conclusion will be confirmed again by following Raman and FT-IR results. Morphological characterization is far less sensitive in exposing disorder in the carbon skeleton. Raman scattering is a better probe when the sidewall of SWNTs are largely altered. Raman Spectrum. Raman spectroscopy is commonly used for characterizing SWNTs because it is one of the most sensitive characterization tools for the nanostructure.17,18 Figure 2 shows the Raman spectrum of OSWNT solid samples before and after adsorption of AA. We have found that the OSWNT used in this work are mostly attributed to semiconducting tubes according to the literature.19 22 It is generally accepted that the sharp 1590 cm 1 component of the G band and the 150 210 cm 1 component of the radial breathing mode (RBM) are associated with the resonance of semiconducting SWNTs. The band in the range 100 350 cm 1 is attributed to the RBM of SWNTs. The RBM can be used to study the SWNTs diameter through its frequency.19,23 The spectrum has been normalized on the RBM of OSWNT at 216 cm 1 (inset of Figure 2). The strong peak at 216 cm 1 and weak peaks at 160 cm 1 and 252 cm 1 are presented in the spectrum of OSWNT. They correspond to 1.11 nm, 1.54 nm, and 0.94 nm tube diameters, respectively. From the inset of Figure 2, the larger diameter OSWNT (band at 160 cm 1) has been significantly affected by adsorption of AA. The RBM at 160 cm 1 of OSWNT has a remarkable enhancement of intensity and noticeable shift after AA adsorption (13 cm 1). A clear downshift for the peak from 160 to 153 cm 1, 147 cm 1, and 149 cm 1 was observed in OSWNT after the adsorption of Phe, Lys, and Gly, respectively. Correspondingly, the remarkable decline of intensity of RBM at 252 cm 1 was found, which shifted to lower frequency 248 cm 1 with the adsorption of Lys and Gly. The results mean that an increase in OSWNT diameter from bare OSWNT to OSWNTAA occurs. Liu has also found that the RBM of SWNTs has a downshift when alcohol dehydrogenase was adsorbed on SWNTs.24 The results suggest that at least a part of AA was adsorbed on the external wall of OSWNT and that the adsorption layer thickness of Lys on OSWNT is thicker than that of Gly and Phe. Therefore, the downshift of RMB at 150 cm 1 for Lys is most noticeable. The results may also be attributed to the 1725

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The Journal of Physical Chemistry C fact that the adsorption layer of Lys zwitterions on OSWNT is more symmetrical and uniform than that of Phe or Gly zwitterions. The band intensity at 252 cm 1of OSWNT (0.94 nm diameter) has a remarkable decline comparison with the band at 160 cm 1 OSWNT (1.54 nm diameter) after AA adsorption. Therefore, the thickness of OSWNT increased noticeably after AA adsorption for every OSWNT-AA0.15 sample. This changing of diameter is also most apparent for OSWNT-Lys0.15. As described by Smalley et al.,25 31 the chemical reactivity is related to the high strain caused by the critical curvature of the sidewall of SWNTs. So, the smaller the diameter of OSWNT, the higher the chemical reactivity and the more the AA adsorption is. The Π Π stacking interaction between SWNTs and aromatic rings induced a higher frequency shift of RBM and give a kind of mode hardening effect.15 This would be inevitable if the attachment of the Phe zwitterions would restrict the RBM of SWNTs. So, the position of RMB just only for OSWNT-Phe0.15 has an upshift from 215 cm 1 to 218 cm 1. For Lys and Gly, the situation is different. The AA are adsorbed on the sidewall of OSWNT in the form of zwitterions, and the Π electron of OSWNT is delocalized. So, the polar interaction between NH3+of amino acids and delocalized Π electrons on the sidewall of OSWNT may be a determinant factor to realize the adsorption of Lys and Gly on the sidewall of OSWNT. Of course, this ratiocination needs to be confirmed next by FT-IR results. From the above Raman results, the adsorption amount of Lys on OSWNT is likely to be the most among three AA. This conclusion needs to be confirmed by the following TGA result. This phenomenon is correlative with the intermolecular hydrogen bond of AA. The more intermolecular hydrogen bond can be formed between the Lys zwitterion because the Lys zwitterion has two amino groups. Likewise, this conclusion needs to be confirmed by the following FT-IR results. In the high-frequency region of 1300 1600 cm 1, there are two bands that are associated with the tangential C C stretching modes of SWNTs. The weak band at 1326 cm 1, the so-called D band, may arise from disorder of any kind in the aromatic Πdomain. The E2g band at 1581 cm 1 is called the G band. The G band is associated with carbon atom vibrations along the nanotube axis. The G band of the SWNTs stems from the perfect cylindrical symmetry of the carbon nanotube. The cylindrical symmetry of the SWNTs will be affected by specimen adsorption on the sidewall. So, the G band would be changed when the adsorption of AA zwitterions on their sidewall occurs. Previous experimental and theoretical work has shown that the G frequency is sensitive to charge transfer from dopant additions to SWNTs26,27 and that the removal of electrons from SWNTs (oxidizing) results in an upshift in the G band.23,27 The Raman result in this article is consistent with the above conclusions. The Raman spectrum has been normalized on the G band of OSWNT at 1581 cm 1 as shown in Figure 2. The G band has an upshift from 1581 cm 1 to 1586 cm 1, 1584, and 1586 due to adsorption of Phe, Gly, and Lys zwitterions, respectively. It means that AA zwitterions are an electron acceptor during the interaction process. This conclusion is completely consistent with theoretical analysis.28 30 When the sidewalls of SWNTs are covalently modified, the appearance of a prominent Raman peak at 1290 cm 1 due to the sp3 states of carbon demonstrates the disruption of the aromatic system of Π electrons, as established previously.31 35 However, the band at 1290 cm 1 is not presented in the Raman spectrum of the OSWNT-AA sample. So, the upshift of the G band of the OSWNT-AA sample in this

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case is probably associated with the formation of a polar interaction between AA zwitterions and sidewall of OSWNT. From Raman spectrum, we have found that a part of AA was adsorbed on the external wall of OSWNT. The smaller the diameter of OSWNT, the higher the chemical reactivity and the more the AA adsorption amount is. The adsorption layer of Lys on OSWNT is thicker and more uniform than that of Gly and Phe. The Π Π stacking and polar interaction are important factors to realizing adsorption of Phe, Lys, and Gly zwitterions on the sidewall of OSWNT. FT-IR Spectrum. FT-IR spectrum is useful for identifying the functional groups appended to the SWNTs.36 Next, we show how the FT-IR spectrum of OSWNT before and after adsorption of AA can be used as an important complementary probe to Raman spectrum. Figure 3 shows the FT-IR spectrum of OSWNT-Phe0.15, OSWNT-Gly0.15, and OSWNT-Lys0.15 samples. All peaks about AA were assigned according to the literature.37 39 The detail assignments of FT-IR vibration of amino acids and OSWNT used in this work have been supplied in the Tables 2 and 3 of the Supporting Information. The 1100 1200 cm 1 band is located within the range for C O stretching modes of the hydroxyl group in OSWNT. The phonon mode of SWNTs at 1590 1630 cm 1 was seen in Figure 3.40,41 The band at 1742 cm 1 is assigned to the CdO stretching vibration of the carbonyl group in OSWNT. In addition, the spectrum of OSWNT showed bands around 2925 and 2854 cm 1 attributed to asymmetric and symmetric CH stretching, respectively.36,42 The broad band observed at ∼3400 cm 1 was not attributed to asymmetrical stretching vibrations of trace water (adsorbed in the KBr pellet). All samples were dried before analysis, and the band at ∼3400 cm 1 is not present in pure AA samples besides OSWNT. This broad band is assigned to contributions from a variety of OH stretching modes. The width indicates that several different OH containing groups are present in various chemical environments. In Figure 3, the intensity of all bands in the OSWNT-AA 0.15 sample has lowered noticeably in comparison with AA zwitterions and OSWNT. In the Figure 3A, we can see that the characteristic bands of COO in the Lys zwitterions at 1615 cm 1, 1349 cm 1, 1324 cm 1, and 956 cm 1 have been affected noticeably by interaction with OSWNT. The bands at ∼1320 cm 1 and ∼960 cm 1 have almost disappeared after AA adsorption. Moreover, similar features were observed for the OSWNT-Gly0.15 sample. The bands at 888 cm 1 and 3112 cm 1 and at 863 cm 1 and 3100 cm 1, belonging to the stretching vibration of NH in the Gly and Lys zwitterions, respectively, have also reduced remarkably after adsorption. The reason for the above phenomenon is possibly consisting of two factors. The first one is the intermolecular hydrogen bond between AA zwitterions that was formed. It can be reflected by an upshift of the band from 1615 cm 1 to 1628 cm 1, and 1630 cm 1 to 1634 cm 1. The other characteristic band of COO in Lys and Gly also has an obvious upshift, for example, 1324 cm 1and 1349 cm 1, and the second factor is a polar interaction exisiting between the sidewall of OSWNT and Lys or Gly. The polar interaction between NH3+ of amino acids (Lys and Gly) and Π electrons of OSWNT reduce the bending vibration of NH in NH3+. It can be improved by reducing the 1511 cm 1 and 1500 cm 1 bands. Furthermore, the above results also suggest the more intermolecular hydrogen bond can be formed between the Lys zwitterion because the Lys zwitterion has two amino groups. So, the OSWNT-Lys0.15 is 1726

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Figure 3. FT-IR spectrum of OSWNT solid samples before and after AA adsorption.

the most stable due to the extended network of intermolecular hydrogen bonds, and this conclusion is perfectly consistent with Raman results and theoretical analysis.28 The vibrations at 1560 cm 1 and 1625 cm 1 come from the benzene ring of Phe zwitterions shifted to high frequency upon adsorption. It means that the Phe adsorption has an appreciable impact on benzene ring vibration. The Π Π stacking interaction can be postulated between the benzene ring of Phe zwitterions and the Π electrons of OSWNT. This is also in agreement with the supposal rooted in Raman results. The stretching vibration due to C O of hydroxyl group in OSWNT at ∼1100 cm 1 was detected. The distortion of the band shape is observed after AA adsorption. In addition, the intensity of the carbonyl band at 1742 cm 1 for OSWNT has a noticeable reduction after AA adsorption. Concretely, the

vibration at 1742 cm 1 was downshifted to 1737 cm 1, 1729 cm 1, and 1726 cm 1 after Gly, Phe, and Lys adsorption, respectively. The band at 1742 cm 1 is normally present in the free COOH group, and it will downshift when a hydrogen bond is formed.43 The above results mean that the hydrogen bond was formed between COO of the OSWNT and NH3+ of AA. The hydrogen bonds formed with oxygen-containing groups are dominant at the end of OSWNT. The bands at 1615 cm 1, 1630 cm 1, and 1625 cm 1 are assigned to the CdO stretching vibration of the COO group in Lys, Gly, and Phe, respectively. The intensity of the above vibration increased visibly after AA adsorption. It is obvious that the increasing of intensity is not caused by the formation of an intermolecular hydrogen bond described above. It is probable that the new kind of CdO bond is formed, that is, 1727

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The Journal of Physical Chemistry C the CdO stretching vibration of amide bond ( C (dO) NHgroup) in this system. Furthermore, the bands at 1408 cm 1, 1393 cm 1, and 1410 cm 1 are assigned to C N stretching vibration of Lys, Gly, and Phe, respectively. The downshift for the bands from 1408 cm 1 to 1385 cm 1, 1393 cm 1 to 1383 cm 1, and 1410 cm 1 to 1384 cm 1 can be clearly seen after adsorption. At the same time, new C N stretching vibrations appeared at 1404 cm 1, 1406 cm 1, and 1410 cm 1 for OSWNT-Lys 0.15, OSWNT-Gly 0.15, and OSWNT-Phe 0.15, respectively. The last C N stretching vibration is assigned to the C N stretching vibration of the amide bond.44 Finally, the

Figure 4. DTG curves of OSWNT solid samples before and after adsorption: (a) OSWNT-Phe0.15, (b) OSWNT-Gly0.15, (c) OSWNTLys0.15, (d) OSWNT, (e) Lys, (f) Gly, and (g) Phe.

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band of OSWNT at 3418 cm 1 shifted to higher frequencies, 3425 cm 1, 3432 cm 1, and 3433 cm 1, after Gly, Phe, and Lys adsorption, respectively. It is a typical FT-IR band for secondary amides;44 so, the formation of the amide C (dO) NH linkages in OSWNT-AA 0.15, as a result of the condensation reaction of the AA zwitterions with oxygen-containing groups of OSWNT, was confirmed by results of FT-IR spectrum. On the basis of FT-IR and Raman results, we have reasoned that the AA zwitterions were adsorbed on the surface of OSWNT by a conjunct interaction of the polar interaction, Π Π stacking, hydrogen bond, and covalent bonding. The Π Π stacking and polar interaction are important factors to realizing the adsorption of Phe, Lys, and Gly zwitterions on the sidewall of OSWNT. Hydrogen and covalent bonds, formed with oxygen containing groups, is dominant at the end of OSWNT. The intermolecular hydrogen bonding between AA zwitterions was also formed when AA zwitterions were adsorbed on the OSWNT. The more intermolecular hydrogen bond can be formed between the Lys zwitterion because the Lys zwitterion has two amino groups. TGA Results. Figure 4 shows the DTG results (differential curve of TGA) of pure AA and OSWNT-AA0.15. The adsorption amount of Lys, Phe, and Gly on OSWNT is about 33 wt %, 28 wt %, and 16 wt %, respectively (mass percent), and it consists of chemical and physical adsorbed AA. The adsorption amount of Lys on OSWNT is the most among three AA. Therefore, the supposal from Raman results is confirmed by TGA results. In Figure 4e g, the initial decomposition temperature of a Phe, Gly, and Lys crystal is about 480 550 K. The decomposition reaction of Phe, Gly, and Lys zwitterions occurs at lower temperatures when the AA zwitterions are adsorbed on OSWNT. The initial decomposition temperatures of OSWNT-Phe0.15,

Figure 5. Illustration of an interaction mechanism accounting for AA zwitterion adsorption on OSWNT. 1728

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The Journal of Physical Chemistry C OSWNT-Lys0.15, and OSWNT-Gly0.15 are about 375 K, 395K, and 390K, respectively. The temperature has about 30 60 K noticeable reduction compared with that of pure AA crystal. It means that activation energy of an AA decomposition reaction was reduced in the OSWNT-AA0.15 sample. The reason is likely that the OSWNT as the catalyst has catalyzed the decomposition reaction of AA. The catalysis property of the OSWNT seems to come from their excellent electrical transportation property. At the same time, the decomposition temperature of OSWNT has a remarkable reduction after adsorption due to the decomposition of AA. The peaks of weight lost for OSWNT with different diameters is mixed together since the OSWNT with different diameters are entangled with each other. They appeared at 748 and 890 K for OSWNT, respectively. The decomposition temperature is similar to those of refs 45 47. However, the decomposition temperature of OSWNT-AA0.15 has reduced from 750 K, 890 to 650 K, and 850 K, respectively. The peaks of weight lost at 748 890 K of OSWNT-AA0.15 were split into three unattached peaks (669 718 K, 845 858 K, and 1028 1043 K) in Figure 4a c. The peak at 669 718 K should belong to that part of a smaller diameter OSWNT-AA0.15 in view of their stable property. The above results indicate that the OSWNT with different diameter were separated as a result of AA adsorption. The supposal from Raman and FT-IR results are confirmed by TGA data. The adsorption amount of Lys on OSWNT is the most among the three AA. The catalysis property of OSWNT makes a noticeable reduction of decomposition temperature for OSWNT-AA. The OSWNT with different diameter were separated as a result of AA adsorption. Adsorption Moles. We put forward a mechanism according to all of the above characterization results and analysis for adsorption process. The illustration of the interaction mechanism accounting for AA zwitterion adsorption on OSWNT was shown in Figure 5. The OSWNT and its suspension in water were drawn in Figure 5A, and the 3D model of Phe, Lys, and Gly zwitterion adsorption on OSWNT were drawn in Figure 5B D, respectively. The Phe zwitterions are adsorbed on the external surface of OSWNT by Π Π stacking interaction, and Lys and Gly zwitterions are mainly adsorbed on the sidewall by polar interaction. The intermoleculer hydrogen bond exists between three AA zwitterions adsorbed on the external surface of OSWNT. From Figure 5B D, the AA zwitterions are adsorbed on the end of the OSWNT by amide and hydrogen bonds. Finally, the transparent and stable OSWNT/AA suspension was obtained due to AA adsorption (see the Figure 5E G).

’ CONCLUSIONS The OSWNT-AA sample prepared by the simple and green method particularly show solubility and stability in water. OSWNT became soluble in water and formed a stable suspension when the AA zwitterions were adsorbed on it. The preparation process of the OSWNT-AA aqueous suspension is not involved with any organic solvents. We present a systematic investigation about the adsorption mechanism of three AA zwitterions on OSWNT. Spectroscopy and thermal analysis together with morphological analysis have been used to investigate the interaction between AA zwitterions and the OSWNT. There is an increase in the diameter of OSWNT from bare OSWNT to OSWNT-AA due to AA adsorption. It means that at

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least a part of AA was adsorbed on the external wall of OSWNT. The smaller the diameter of OSWNT, the higher the chemical reactivity, and the more the AA adsorption amount is. The OSWNT with different diameter were separated as a result of AA adsorption. The adsorption layer thickness of Lys on OSWNT is thicker and more uniform than that of Gly and Phe. The AA zwitterions were adsorbed on the surface of OSWNT by a conjunct interaction of the Π Π stacking, polar interaction, hydrogen bond, and covalent bonding. The Π Π stacking is an important factor to realizing the adsorption of Phe zwitterions on the sidewall of OSWNT. For Gly and Lys zwitterions, polar interaction is a determinant factor to realizing adsorption on the sidewall of OSWNT. Hydrogen and covalent bonds, formed with oxygen containing groups, are dominant at the end of OSWNT. The intermolecular hydrogen bonding between AA zwitterions was also formed when AA zwitterions were adsorbed on the OSWNT. The more intermolecular hydrogen bond can be formed between the Lys zwitterion because the Lys zwitterion has two amino groups. The catalysis property of OSWNT makes a noticeable reduction of decomposition temperature for AA adsorbed on OSWNT. The above conclusions are identical with a previous theoretical model done by other researchers. We deduce that the specific adsorption of AA can be realized according to the above analysis. Further studies on the specific adsorption of AA will be summarized in our next work.

’ MATERIALS AND METHODS SWNTs Preparation. The SWNTs were synthesized, purified, and oxidized according to previous work.15 The flowchart of the purification and oxidation procedure is shown in Figure 1 of the Supporting Information. The OSWNT sample was calcined at 573 K for 3 h in air before adsorption experiments. SWNTs/AA Suspension Preparation. In a typical experiment, the OSWNT sample was mixed with 0.15 M AA suspension, and the suspension was sonicated for 1 h at room temperature (RT). Then, the suspension was centrifugated at 50 000g for 0.5 h. The supernatant (75%) was collected and characterized. For solid sample preparation, the OSWNT/AA suspension after sonication was directly filtered. The solid sample was dried in a vacuum oven for 16 h at RT. The OSWNT samples after adsorption (either solid or liquid) were labeled as OSWNTLys0.15, OSWNT-Phe0.15, and OSWNT-Gly0.15, respectively. Characterization. High-Resolution Transmission Electron Microscopy (HRTEM) and Scanning Electron Microscopy (SEM). HRTEM and SEM images were acquired using a TECHNAI G2 F20 transmission electron microscope (200 kV) and a Hitachi S-4800 scanning electron microscope (5 kV), respectively. For sample preparation, one drop of supernatant of OSWNT or OSWNT-Lys0.15 was placed on a Cu grid with holey carbon support film or silicon wafer for HRTEM and SEM imaging, respectively. Images were acquired from at least ten different areas on each substrate to ensure that the data were representative of the sample. Raman Spectroscopy. Raman spectrum of the solid samples were taken using a Renishaw inVia Raman spectrometer, which equipped a laser with a 633 nm excitation and a charge-coupled device (CCD) detector. Raman analyses were acquired from ten different areas on each sample, and each curve is the average result of the signals coming from ten different positions. Fourier Transform Infrared Spectroscopy (FT-IR). All FT-IR spectra of the solid samples were collected using a Perkin- Elmer 1729

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The Journal of Physical Chemistry C FT-IR spectrometer. Typical spectrum acquisition parameters included an average of 64 scans and resolution factor of 1 cm 1. Thermogravimetric Analysis (TGA). TGA curves of solid samples were acquired using a Perkin-Elmer Diamond thermogravimetric analyzer in flowing air. Data were collected from 298 K to 1173 K at 10 K/min.

’ ASSOCIATED CONTENT

bS Supporting Information. The purification process, photograph of supernatant OSWNT after AA adsorption, Raman results of OSWNT and OSWNT-AA 0.15 M, EDX spectrum of OSWNT-AA 0.15 M. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*Phone: 86-10-8254 5653. E-mail: [email protected].

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