J. Phys. Chem. C 2008, 112, 2857-2863
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Adsorption of L-Phenylalanine on Single-Walled Carbon Nanotubes Lingyu Piao,*,† Quanrun Liu,‡ Yongdan Li,‡ and Chen Wang† National Center for Nanoscience and Technology, Beijing 100080, People’s Republic of China, and Department of Catalysis Science and Technology, School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, People’s Republic of China ReceiVed: September 2, 2007; In Final Form: NoVember 20, 2007
Single-walled carbon nanotubes (SWNTs) became soluble in water and formed a stable solution when L-phenylalanine (Phe) was adsorbed. The adsorption selectivity of Phe zwitterions for larger diameter SWNTs was confirmed by analysis of Fourier transform infrared spectra and by differential thermogravimetric analysis. Enhanced adsorption of Phe on the oxidized single-walled carbon nanotubes (OSWNT) was observed in comparison with that of the purified single-walled carbon nanotubes (PSWNT). The Phe zwitterions are thought to adsorb on the surface of OSWNT by joint interaction of the π-π stacking, hydrogen bond, and part of the covalent bond. The π-π stacking is the dominant interaction in the sidewall of OSWNT without defects. The hydrogen bond and covalent bond formed with oxygen-containing groups becomes dominant on the end of OSWNT. For the PSWNT system, π-π stacking is an important factor to realize the adsorption of Phe zwitterions on the sidewall of PSWNT. The intermolecular hydrogen bond between Phe zwitterions is also formed when Phe zwitterions are adsorbed on the PSWNT.
Introduction Research about biological effects and applications of singlewalled carbon nanotubes (SWNTs) in biological and medical fields, which has a direct impact on health and environment, is being pursued.1-3 With regard to biomedical applications of SWNTs, the most important prerequisite is development of methods to immobilize biomolecules onto SWNTs.1,3,4 The immobilization process is complicated, and the interaction mechanism between biomolecules and the SWNTs is not clear yet.5 Huang et al.4 reported that bovine serum albumin (BSA) protein could be covalently attached to carbon nanotubes via diimide-activated amidation under ambient conditions. However, direct evidence to attest the covalent interaction has not yet been presented. Hydrophobic interactions, π-π stacking interactions, and hydrogen bond (i.e., weak interactions) were commonly considered to play an important role in biomolecules’ dispersion on SWNTs.1,6,7 But direct experimental data are not enough. The complexity of the immobilization process comes from structural complexity and various properties of both biomolecules and SWNTs (especially for the functionalized SWNTs). It was proposed that the biomolecules can be only adsorbed on functionalized SWNTs.8,9 On the other hand, available morphological characterization tools limit our capabilities for understanding the immobilization process. The characterization tools in the reported studies were mainly atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy.1,8,10-12 Amino acids are an elementary unit for composing biomolecules and can also reflect the common chemical properties of complicated biomolecules. So, the interaction between SWNTs and typical amino acids is very important for understanding the * Corresponding author: tel 86-10-82545519; fax 86-10-82545519; e-mail
[email protected]. † National Center for Nanoscience and Technology. ‡ Tianjin University.
interaction mechanism between SWNTs and biomolecules. L-Phenylalanine (Phe) was selected as an example specimen for exploiting this interaction in this work. The SWNTs have not been further functionalized except for normal purification and oxidization. Here, we first report a simple and green approach to obtain stable SWNTs/Phe solution. It is transparent and stable after 210 days. Spectroscopy, thermal analysis, and chromatography together with morphological analysis have been used to investigate the interaction between Phe molecules and SWNTs. We have proposed the adsorption mechanism of Phe on SWNTs from experimental results. Specific adsorption and controlled assembly of amino acids on SWNTs will be evaluated. Experimental Section SWNT Preparation. The SWNTs were prepared by decomposition of methane on Fe/MgO catalyst at 1123 K for 30 min. Purification and oxidation of SWNTs was carried out according to literature.13 At first, the starting SWNT samples were suspended in benzene for about 1 day and were filtered to extract benzene-soluble materials, such as fullerenes. After being washed with deionized water, the samples were treated in 20 wt % hydrochloric acid for 3 h at room temperature (RT) under stirring, followed by the acid dissolution of most of the Fe/ MgO catalyst particles. The samples were washed to remove any acidic residues. After filtration, the samples were dried at 373 K and calcined in the air at 673 K for 1 h to burn out amorphous carbon. The samples were then refluxed in 20 wt % hydrochloric acid at 373 K for 12 h to dissolve completely Fe/MgO catalyst particles. After being washed and dried, the air-dried samples were then calcined in the air at 673 K for 3 h again. The samples were frozen at 77 K for about 3 h. Then the samples were dried and calcined in the air at 673 K for 3 h to completely burn out the amorphous carbon. After this treatment, the samples were refluxed in 2 M nitric acid at 373 K for 20 h to oxidize and remove the exposed graphite particles.
10.1021/jp077047s CCC: $40.75 © 2008 American Chemical Society Published on Web 02/05/2008
2858 J. Phys. Chem. C, Vol. 112, No. 8, 2008 Finally, followed by further ultrasonic treatment for 1 h, the samples were washed and dried in a vacuum oven. Thus the purified single-walled carbon nanotubes (PSWNT) were obtained. For oxidized single-walled carbon nanotube (OSWNT) sample preparation, the PSWNT samples were refluxed in concentrated hydrochloric acid and nitric acid (volume ratio 1:3) solution at 353 K for 3 h. Then the samples were washed with deionized water and dried in a vacuum oven to produce OSWNT. The flowchart of the purification procedure is shown in Supporting Information Figure 1. The purified and oxidized SWNTs are noted as PSWNT and OSWNT in this work, respectively. The content of the residue in PSWNT is less than 7 wt % by thermogravimetric analysis (TGA) in air. The PSWNT and OSWNT samples were calcined at 573 K for 3 h in air before adsorption experiments. SWNTs/Phe Solution Preparation. In a typical experiment, the PSWNT or OSWNT sample was mixed with 0.15 M Phe solution and suspension was sonicated for 1 h at RT. Phe is present as zwitterions in the solution. For supernatant preparation, the suspension of SWNTs and Phe was centrifugated at 50000g for 0.5 h. The supernatant (75%) was collected and characterized. For solid sample preparation, the PSWNT/Phe or OSWNT/Phe suspension after sonication was directly filtered. The solid was dried in a vacuum oven for 16 h at RT. The PSWNT or OSWNT samples after adsorption (either solid or liquid) were labeled as PSWNT-Phe 0.15 M or OSWNTPhe 0.15 M in this work, respectively. Characterization: High-Resolution Transmission and Scanning Electron Microscopy. HRTEM and SEM images were acquired by use of a TECHNAI G2 F20 transmission electron microscope (200 kV) and a Hitachi S-4800 scanning electron microscope (2 kV), respectively. One drop of PSWNT, PSWNTPhe 0.15 M, OSWNT, or OSWNT-Phe 0.15 M aqueous solution was placed on a Cu grid with holey carbon support film or silicon wafer for HRTEM and SEM imaging, respectively. RamanSpectroscopy. Raman spectra of the supernatant and solid samples were obtained by use of a Renishaw inVia Raman spectrometer equipped laser with 633 nm excitation and a charge-coupled device (CCD) detector. Each curve is the average result of the signals coming from 10 different positions in the samples. The curve of liquid sample was obtained on the Au surface. Fourier Transform Infrared Spectroscopy. All spectra of the supernatant and solid samples were collected by use of a PerkinElmer FT-IR spectroscope. An average of 64 scans was obtained with a resolution factor of 1 cm-1. UltraViolet-Visible Absorption Spectroscopy. Absorption spectra of Phe solution before and after adsorption were obtained by use of a Perkin-Elmer Lambda 950 UV-visible spectroscope. ThermograVimetric Analysis. TGA measurements of PSWNT and OSWNT solid samples before and after adsorption, to determine the content of impurity and adsorbed Phe, were made on a Perkin-Elmer Diamond thermogravimetric analyzer in flowing air. The analytical temperature was up to 1173 K at 10 K/min. High-Performance Liquid Chromatography. The PSWNT/ Phe or OSWNT/Phe suspension was filtered after sonication. The solid after filtration was washed and sonicated three times with deionized water. All filtrate was analyzed by HPLC. The Phe solution and filtrate were separated at 308 K on an Atlantisdc18 column (4.6 mm × 150 mm, 5 µm) with phosphate buffer that was adjusted pH to 4.4 with phosphoric acid, respectively. Detective wavelength was 224 nm. Flow rate was 1 mL/min, and 10 µL was injected every time.
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Figure 1. Photograph of (A) aqueous suspension of OSWNT, (B) supernatant OSWNT with Phe after centrifugation, and (C) solution from panel B after 210 days.
Results and Discussion Figure 1 shows (A) the OSWNT aqueous suspension, (B) the supernatant of OSWNT/Phe after centrifugation at 50000g, and (C) the solution from panel B after 210 days. The deposit was separated from the dark suspension (Figure 1A) after 24 h. The OSWNT became soluble in water and formed a transparent solution since the Phe was adsorbed on OSWNT. The OSWNT/ Phe supernatant was transparent, yet after 210 days at 277 K there was no deposit. This means the interaction between Phe zwitterions and the OSWNT is stable. The concentration of OSWNT in this solution is about 0.12 mg/mL. The micromodality of Phe adsorbed on the OSWNT and PSWNT will be observed carefully by AFM, SEM, and HRTEM. SEM and HRTEM. Figure 2 shows the OSWNT samples before and after adsorption of Phe with SEM and HRTEM, respectively. The OSWNT bundles exist in the aqueous solution of the OSWNT before centrifugation as shown in Figure 2A. There were no OSWNT present in the aqueous solutions after centrifugation at 50000g by SEM observation (the image is not given). But from Figure 2B, supernatant of OSWNT/Phe after centrifugation at 50000g consists primarily of long bundled OSWNT. At the same time, the Phe zwitterions are adsorbed to the sidewall of OSWNT. We can observe distinctly the coverage of Phe zwitterions on the sidewall of OSWNT in Figure 2D in comparison with Figure 2C. From Figure 2C we can see that the OSWNT bundles existed in the aqueous solution. The boundary between the single OSWNT in the OSWNT bundle can be clearly seen. The boundary was not seen after the adsorption of Phe from Figure 2D. The OSWNT were covered with some materials. We have confirmed that the element nitrogen existed in this material from the EDX spectrum (Supporting Information Figure 2). However, the element nitrogen did not exist in OSWNT before adsorption of Phe (Supporting Information Figure 3). In addition, we also saw the boundary of OSWNT if Phe zwitterions were adsorbed on the internal surface of OSWNT. But here, we cannot see any boundary of OSWNT after adsorption of Phe. Similar behavior was observed for OSWNT and PSWNT, so only a photograph of OSWNT-Phe 0.15 M is given here. We reasoned that the Phe zwitterions were adsorbed on the external surface of OSWNT and PSWNT. Furthermore, it was found that Phe zwitterions were adsorbed on the end of OSWNT from Figure 2F. This image is blurry because of the rocking of OSWNTPhe 0.15 M. This nanotube was located outside the grid carbon film. Inspection of these images indicates that Phe zwitterions were adsorbed on the sidewall of OSWNT and PSWNT. In
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Figure 2. (A, B) SEM and (C-E) HRTEM images of the OSWNT: (A, C): before and (B, D, E) after adsorption.
Figure 3. Raman spectra of PSWNT and OSWNT solid samples before and after adsorption: (a) OSWNT, (b) OSWNT-Phe 0.15 M, (c) PSWNT, (d) PSWNT-Phe 0.15 M.
addition to the sidewall, the adsorption of Phe zwitterions also occurred on the end of OSWNT. Morphological characterization is far less sensitive in exposing disorder in the carbon skeleton. Raman scattering is a better probe of disorder when the sidewall of SWNTs are largely altered. Raman Spectrum. Figure 3 shows the Raman spectra of PSWNT and OSWNT solid samples before and after adsorption of Phe. We have found from Figure 3 that the SWNTs used in this work are mostly attributed to semiconducting tubes according to literature.14-17 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 resonance of semiconducting SWNTs. The Raman spectra of PSWNT and OSWNT appear in two panels: 100-300 cm-1 (Figure 3A) and 1000-3500 cm-1 (Figure 3B). The band in the range 100-300 cm-1 is attributed to the RBM of SWNTs. The RBM can be correlated with the SWNT diameters.14,18 The strong peak at ∼216 cm-1 and weak peaks at 142, 159, and 251 cm-1 are presented in the spectra of PSWNT and OSWNT (Figure 3A). They correspond to 1.11, 1.74, 1.54, and 0.94 nm tube diameters, respectively. It is interesting to note that larger diameter PSWNT, with diameter more than 1.1 nm, have a noticeable shift after adsorption of Phe in Figure 3A. In contrast, the bands of smaller diameter PSWNT (corresponding to
vibration at 251.6 cm-1) have no shift after adsorption of Phe. This phenomenon also existed in the OSWNT samples. Detailed data about the RBM change of PSWNT-Phe 0.15 M and OSWNT-Phe 0.15 M are given in Supporting Information Table 1. The PSWNT peak at 142 cm-1 shifted to higher frequency and almost disappeared with the adsorption of Phe. A clear downshift for the peak from 160 to 153 cm-1 was observed in OSWNT after the adsorption of Phe. In the high-frequency region of 1300-1800 cm-1, there are two bands that are associated with the tangential C-C stretching modes of SWNTs. The stronger band around ∼1580 cm-1 is close to that of well-ordered graphitesthat is, the E2g band at 1582 cm-1 and is called the G band. The G band of the SWNTs, which is associated with its ordered sp2 hybridized carbon network, stems from the perfect cylindrical symmetry of the nanotube. The weak band at 1325 cm-1, the so-called D band, involves the scattering of an electron via phonon emission by the disordered sp2 network. Usually, the intensity of the D band is used to probe the degree of functionalization. It may arise from disorder of any kind in the aromatic π-domain. Although there is an obvious frequency change in RBM, the G band remains at almost the same frequency for PSWNT before and after adsorption of Phe. But the G band of OSWNT has 5 cm-1 shift to high frequency after Phe adsorption. The intensity ratio of the G band to D band (IG/ID) has significantly weakened
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Figure 4. FT-IR spectra of PSWNT solid samples before and after adsorption.
after adsorption of Phe on PSWNT and OSWNT. The reduction of IG/ID is primarily a result of weakening of G-mode intensity for either OSWNT-Phe 0.15 M or PSWNT-Phe 0.15 M. The RBM of PSWNT shifts to higher frequency after adsorption of Phe. Normally, the Raman band will shift to higher frequency due to the presence of an electron acceptor group. According to Raman spectra in the literature,19 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”. As one can see here, the adsorption of Phe zwitterions on the sidewall of PSWNT may cause the higher frequency shift of RBM. This would be inevitable if the attachment of the Phe zwitterions would restrict the radial breathing of SWNTs. Previous experimental and theoretical work has shown that doping SWNTs with either electron donors or acceptors would result in noticeable shifts in G mode.20,21 And the removal of electrons from SWNT (oxidizing) results in an upshift in G band.17,21 The cylindrical symmetry of the SWNTs will be affected by local π-π stacking, but the removing of electrons of the SWNTs would not be carried out during the π-π stacking process. So, the G band would have shift when the π-π stacking exists in the system. The G band has no shift for PSWNT before and after adsorption of Phe as observed here. We have only found a marked decline in G band for PSWNT from Figure 3B. Therefore, π-π stacking existed in the PSWNT-Phe 0.15 M, according to the change trend for both the RMB and the G band. Furthermore, the effect of Phe absorption is mainly observed on the larger diameter PSWNT. Taking into account the above Raman results and curvature of the larger diameter PSWNT, the π-π stacking between Phe and PSWNT could be the dominating interaction during the Phe adsorption process. The larger diameter OSWNT has been significantly affected by adsorption of Phe from Figure 3A. The RBM at 160 cm-1 of OSWNT has a noticeable shift after adsorption of Phe (7 cm-1). According to the above analysis, the Phe zwitterions play a contrary role during π-π stacking process between OSWNT and benzene ring of Phe zwitterions. It means that Phe zwitterions are an electron donor during the π-π stacking process for an unclear reason. This is likely correlative with the change of G band of the OSWNT-Phe 0.15 M sample. The noticeable intensity decline was observed in G band. At the same time, the G band has a shift to high frequency from 1581 cm-1 to 1586 cm-1. As mentioned above, removing electrons from SWNTs have resulted in an upshift of the G band. So, the upshift of G band of OSWNT-Phe 0.15 M sample in this case is probably associated with the formation of a polar chemical bond between Phe zwitterions and oxygen-containing
groups of OSWNT. 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.22-24 However, the band at ∼1290 cm-1 is not presented in the Raman spectra of the OSWNT-Phe 0.15 M sample. It seems that the chemical bond is not mainly present in the sidewall of the OSWNT-Phe 0.15 M sample but in the end of the OSWNT-Phe 0.15 M sample, because the oxygencontaining group existed primarily on the end of OSWNT. This is the reason for no observable increase of intensity of D band in the OSWNT-Phe 0.15 M sample. Usually, the intensity of the D band is used to probe the degree of functionalization. The oxygen-containing group of OSWNT has played mainly an electron donor role in polar covalent bonds when approached by Phe zwitterions. So, from the Raman spectra, the larger diameter OSWNT has been significantly affected by adsorption of Phe zwitterions. The interaction between OSWNT and Phe is composed mainly of π-π stacking on the sidewall, and partly by covalent combination with those present at the end of OSWNT. FT-IR Spectra. FT-IR spectroscopy is useful for identifying the functional groups appended to the SWNTs.25 Next, we show how FT-IR spectroscopy of SWNTs before and after adsorption of Phe be used as an important complementary probe to Raman scattering. Figures 4 and 5 show the FT-IR spectra of PSWNT and OSWNT before and after adsorption of Phe zwitterions. All peaks about Phe were assigned according to the literature;26 detailed data are given in Supporting Information Table 2. The nanotube phonon modes at 1590-1630 and 1582 cm-1 were seen in FT-IR spectra of OSWNT and PSWNT.27,28 From Figure 4, the 1717 cm-1 band can be assigned to carbonyl (-Cd O) stretching of PSWNT.29,30 The carbonyl seems to come from the purification process of SWNTs. In Figure 5, the 1100 cm-1 band is located within the range for C-O- stretching modes in OSWNT. The bands at 1742 cm-1 were assigned to -CdO stretching vibrations in OSWNT. The ∼2900 cm-1 has been assigned as a CH2 stretching mode of OSWNTs.25,31 For OSWNT, the large FT-IR band observed at ∼3400 cm-1 was not attributed to asymmetrical stretching vibrations of trace water (adsorbed in the KBr pellet) in Figure 5. All samples were dried before analysis and the band at ∼3400 cm-1 is not present in other 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. From Figure 4, the vibration intensity of PSWNT-Phe 0.15 M is far weaker than that of Phe and PSWNT. Most of the
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Figure 5. FT-IR spectra of OSWNT solid samples before and after adsorption.
Figure 6. UV-vis spectra of PSWNT and OSWNT liquid samples before and after adsorption.
vibration bands in PSWNT-Phe 0.15 M are observed to disappear except for a few peaks at ∼698, 1399, 1581, 1622, and 1717 cm-1. The intensity of the band at 1717 cm-1 has noticeable reduction. For PSWNT-Phe 0.15 M, the bands from the benzene ring of Phe zwitterions have been remarkably altered. The vibrations due to the benzene ring detected at 1074, 700, 1560, and 1625 cm-1 are observed to disappear upon adsorption. It means that the Phe adsorption has an appreciable impact on benzene ring vibration. So, a π-π stacking interaction can be postulated between the benzene ring of Phe and the sidewall of PSWNT. The characteristic bands of -COO- at 1410, 1307, 1458, and 947 cm-1 in the Phe zwitterions have been influenced significantly. The band at 1410 cm-1 has a downshift to1399 cm-1 after Phe adsorption. Other bands at 1307, 1458, and 947 cm-1 have almost disappeared. The bands at 849 and 3066 cm-1, belonging to the -NH3+ in the Phe zwitterions, have also disappeared. This indicates that intermolecular hydrogen bond is formed between Phe zwitterions when Phe zwitterions are adsorbed on the PSWNT. The vibration band comes from -COO- and NH3+ of Phe zwitterions that have been reduced and disappeared because an intermolecular hydrogen bond is formed during adsorption of Phe. In Figure 5, the intensity of all bands in the OSWNT-Phe 0.15 M sample has lowered noticeably in comparison with Phe zwitterions and OSWNT. The vibration due to benzene ring detected at 1074 cm-1 is observed by a distortion of band shape after Phe adsorption. The vibrations at 1560 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 Phe zwitterions and OSWNT. The intensity of the carbonyl band at 1742 cm-1 for OSWNT has a noticeable reduction. The vibration at 1742 cm-1 was downshifted to 1729 cm-1 after Phe adsorption. The band at 1742 cm-1 is normally present in the free -COOH group and it will downshift to ∼1720 cm-1 when a hydrogen bond is formed.32 This means the hydrogen bond was formed between OSWNT and Phe. In addition, the band of OSWNT at 3418 cm-1 shifted to higher frequency, 3432 cm-1, after adsorption. The formation of the imide -C(dO)NH linkages for OSWNT-Phe 0.15 M, as a result of the condensation reaction of Phe zwitterions with OSWNT, was established by observing the FT-IR spectra since the presence of medium-intensity bands at ∼3430, 1627, and 1340 cm-1 due to N-H, CdO, and C-N stretches. It is a typical FT-IR band for secondary amides.33 The HPLC analysis of the solution, which was obtained by washing an OSWNT-Phe 0.15 M sample, confirms that the imido bond in the FT-IR spectra is not rooted in aggregation of Phe zwitterions (Supporting Information Figure 4). It just exists between Phe zwitterions and OSWNT, because no polymer of Phe zwitterions was formed in washing solution from HPLC results. On the basis of FT-IR and Raman results, we have reasoned that π-π stacking is an important factor in realizing adsorption of Phe zwitterions on the sidewall of PSWNT and OSWNT. Intermolecular hydrogen bonding between Phe zwitterions was also formed when Phe zwitterions were adsorbed on the PSWNT. The Phe zwitterions were adsorbed on the surface of
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Figure 7. DTG curve of PSWNT and OSWNT solid samples before and after adsorption: (a) PSWNT, (b) PSWNT-Phe 0.15 M, (c) OSWNT, and (d) OSWNT-Phe 0.15 M.
OSWNT by interaction of the π-π stacking, hydrogen bond, and partly from covalent bonding. The π-π stacking is the leading interaction in the sidewall of OSWNT without defects. Hydrogen bonds and covalent bonds, formed with oxygencontaining groups, are dominant at the end of OSWNT. UV-Visible Spectra and TGA Results. As shown in Figure 6A, the typical adsorption peaks for OSWNT and PSWNT appeared at 267 and 257 nm, respectively. The characterization of all samples was carried out under the same conditions. From Figure 6B, the concentration of Phe solution was lowered in the sequence Phe 0.15 M, PSWNT-Phe 0.15 M, and OSWNTPhe 0.15 M. This means that the adsorbed amount of Phe zwitterions adsorbed on OSWNT is more than that on PSWNT and is consistent with the result of FT-IR spectra. The result will be further confirmed by TGA result. The bands at 267 and 257 nm were submerged under the absorbed bands of Phe after adsorption.
Piao et al. Figure 7 shows the DTG results (differential curve of thermogravimetric analysis) of PSWNT and OSWNT before and after adsorption of Phe. The adsorbed amount of Phe on PSWNT is around 15 wt % according to the amount of weight lost before 673 K. The adsorbed amount of Phe on OSWNT is about 33 wt %. It further confirms the conclusion from UVvis spectra that the adsorbed amount of Phe on OSWNT is more than that of PSWNT. It also indicates that oxygen-containing groups play an important role during the adsorption process. From Figure 7, the initial decomposition temperature of Phe crystal is about 450 K. The burning temperature of OSWNT or PSWNT is higher than 700 K in air. The decomposition of Phe zwitterions occurs at lower temperatures when the Phe zwitterions are adsorbed on PSWNT and OSWNT. The initial decomposition temperatures of PSWNT-Phe 0.15 M and OSWNT-Phe 0.15 M are about 380 and 375 K, respectively. The decomposition temperature of these samples has about 75 K noticeable reduction compared with pure Phe crystal. The activation energy of Phe decomposition reaction was reduced in the PSWNT-Phe 0.15 M and OSWNT-Phe 0.15 M samples. So, the initial decomposition temperature of Phe has been remarkably reduced. As the catalyst, the SWNTs have catalyzed the decomposition reaction of Phe by reducing activation energy of the reaction when Phe zwitterions are adsorbed on PSWNT or OSWNT. The catalytic property of the PSWNT or OSWNT seems to come from their excellent electrical transportation properties. The peaks of weight lost for PSWNT and OSWNT with different diameters is mixed together since the PSWNT and OSWNT with different diameters are entangled with each other. They are appeared at 873 and 748∼890 K region for PSWNT and OSWNT from Figure 7, spectra a and c, respectively. The peaks of weight lost at 873 K (for PSWNT) were split into two shoulder peaks (854 and 888 K) after adsorption of Phe. And, the peaks of weight lost at 748∼890 K (for OSWNT) were split into two unattached peaks (669 and 858 K) in Figure 7c,d after adsorption of Phe. The peak at 669 K should belong to the smaller diameter OSWNT-Phe 0.15 M
Figure 8. Illustration of a possible mechanism accounting for Phe zwitterions adsorption on OSWNT. (Top panel) 3D model of Phe zwitterion adsorption on OSWNT. Red, oxygen atom; blue, nitrogen atom; gray, carbon atom; white, hydrogen atom.
Adsorption of L-Phenylalanine on SWNTs in view of their stability. Therefore, SWNTs with different diameters were separated as a result of Phe adsorption. It also confirms the results of Raman spectra from another perspective that Phe zwitterions are adsorbed on the larger diameter PSWNT or OSWNT selectively. The selective adsorption would result in larger diameter SWNTs departed from the smaller diameter SWNTs. Therefore, the peaks of weight lost for PSWNT and OSWNT are split into two parts after Phe adsorption. Adsorption Moles. The illustration of a possible mechanism accounting for Phe zwitterions’ adsorption on SWNTs is shown in Figure 8. We proposed the mechanism according to all the above characterization results and analysis for adsorption process. The 3D model of Phe zwitterions adsorption on OSWNT is located on the top of Figure 8. The adsorption performance of Phe on PSWNT was similar to that of OSWNT except for those at the end of OSWNT. The planar drawing of Phe adsorption on the sidewall of PSWNT or OSWNT is drawn in Figure 8A, and the planar drawing of Phe adsorption on the end of OSWNT is drawn in Figure 8B. On the sidewall of OSWNT or PSWNT, the Phe zwitterions adsorbed on the external surface of SWNTs by π-π stacking interaction from Figure 8A. The intermolecular hydrogen bond exists between the Phe zwitterions adsorbed on the external surface of PSWNT or OSWNT. From Figure 8B, the Phe zwitterions are adsorbed on the end of the OSWNT by imido bond and hydrogen bond. Conclusions We present a systematic investigation about the adsorption mechanism of Phe zwitterions on SWNTs. Spectroscopy, thermal analysis, and chromatography together with morphological analysis have been used to investigate the interaction between Phe zwitterions and SWNTs. SWNTs became soluble in water and formed a stable solution when the Phe zwitterions were adsorbed. The adsorption selectivity of Phe zwitterions for larger-diameter SWNTs is confirmed by analysis of FT-IR spectra and DTG data. The Phe zwitterions are mainly adsorbed on the PSWNT and OSWNT with larger diameters. Enhanced adsorption of Phe on the oxidized SWNTs (OSWNT) is observed in comparison with that of the purified SWNTs (PSWNT). The Phe zwitterions are inclined to adsorb on the surface of OSWNT by joint interaction of the π-π stacking, hydrogen bond, and part of covalent bond. The π-π stacking is the dominant interaction in the sidewall of OSWNT without defects. The oxygen-containing groups play an important role during adsorption process on OSWNT. The hydrogen and covalent bonds formed with oxygen-containing groups become dominant on the end of OSWNT. For the PSWNT system, π-π stacking is an important factor to realize the adsorption of Phe zwitterions on the sidewall of PSWNT. The intermolecular hydrogen bond between Phe zwitterions is also formed when Phe zwitterions are adsorbed on the PSWNT. Acknowledgment. We are grateful for financial support from the National Natural Science Foundation of China (NSFC 50602008) and the Ministry of Science and Technology of China (2005CB724700). Supporting Information Available: Detailed data about the purification process; Raman results of PSWNT, OSWNT, PSWNT-Phe 0.15 M, and OSWNT-Phe 0.15 M; FT-IR spectra of L-phenylalanine; HPLC results for PSWNT and OSWNT before and after adsorption; and EDX spectra of
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