8890
Langmuir 2008, 24, 8890-8897
Surface Design of Carbon Nanotubes for Optimizing the Adsorption and Electrochemical Response of Analytes Chengguo Hu*,† and Shengshui Hu†,‡ Department of Chemistry, Wuhan UniVersity, Wuhan 430072, China, and State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing 10080, China ReceiVed October 24, 2007. ReVised Manuscript ReceiVed April 1, 2008 Carbon nanotubes (CNTs) from different sources were dissolved in water with high solubility by Congo red (CR) via strong noncovalent π-stacking interactions. The resulting CNTs were capable of forming uniform, compact, stable films on various substrates. This provided a chance to explore the relationship between the surface property of CNTs and the adsorptive behavior of analytes on CNTs without considering the influence of film structures or free additives. Electrochemical behaviors of several small biomolecules and glucose oxidase (GOD) on various CR-functionalized CNT films were examined. The results showed that both the hydrophobic structural defect sites and the hydrophilic oxygen-containing groups were the electroactive sites of CNTs, which was further proven by UV-vis and FTIR spectra. Moreover, the surface properties of CNTs could be conveniently designed by simple pretreatments for optimizing the adsorption and the electrochemical response of analytes. For instance, the hydrophobic defect sites created during the growth or the workup of CNTs were favorable to the adsorption and the electrochemical response of hydrophobic analytes, whereas the hydrophilic oxygen-containing groups produced by acid treatment facilitated the stable adsorption and the direct electrochemistry of redox proteins.
Introduction Carbon nanotubes (CNTs) have attracted much attention in the past decade because of their unique properties and promising applications in almost any aspect of nanotechnology, including electronic and optoelectronic, biomedical, pharmaceutical, energy, catalytic, analytical, and material fields.1 The special properties of small dimensions, good conductivity and biocompatibility, high stability, and modifiable sidewalls make CNTs ideal candidates for constructing high-performance sensors. Since the report by Britto et al.,2 numerous works have focused on the fabrication of electrochemical sensors by CNTs and their applications.3–5 The employment of CNTs generally improves the redox currents of both inorganic and organic species and reduces the overpotentials.6–14 The direct electron transfer of redox proteins on CNT films has also been widely reported.15–18 * Corresponding author. E-mail:
[email protected]. Tel: +86-2787218864. Fax: +86-27-68754067. † Wuhan University. ‡ Chinese Academy of Sciences. (1) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Springer-Verlag: New York, 2001. (2) Britto, P. J.; Santhanam, K. S. V.; Ajayan, P. M. Bioelectrochem. Bioenerg. 1996, 41, 121. (3) Li, N. Q.; Wang, J. X.; Li, M. X. ReV. Anal. Chem. 2003, 22, 19. (4) Wang, J. Electroanalysis 2005, 17, 7. (5) Gooding, J. J. Electrochim. Acta 2005, 50, 3049. (6) Luo, H. X.; Shi, Z. J.; Li, N. Q.; Gu, Z. N.; Zhuang, Q. K. Anal. Chem. 2001, 73, 915. (7) Wang, J.; Musameh, M.; Lin, Y. H. J. Am. Chem. Soc. 2003, 125, 2408. (8) Zhang, M. G.; Smith, A.; Gorski, W. Anal. Chem. 2004, 76, 5045. (9) Chen, R.; Huang, W.; Tong, H.; Wang, Z.; Cheng, J. Anal. Chem. 2003, 75, 6341. (10) Wu, K. B.; Fei, J. J.; Hu, S. S. Anal. Biochem. 2003, 318, 100. (11) Hrapovic, S.; Majid, E.; Liu, Y. L.; Male, K.; Luong, J. H. T. Anal. Chem. 2006, 78, 5504. (12) Profumo, A.; Fagnoni, M.; Merli, D.; Quartarone, E.; Protti, S.; Dondi, D.; Albini, A. Anal. Chem. 2006, 78, 4194. (13) Liu, G. D.; Lin, Y. H. Anal. Chem. 2006, 78, 835. (14) Chen, J.; Du, D.; Yan, F.; Ju, H. X.; Lian, H. Z. Chem.sEur. J. 2005, 11, 1467. (15) Cai, C. X.; Chen, J. Anal. Biochem. 2004, 332, 75. (16) Yan, Y. M.; Zheng, W.; Zhang, M. N.; Wang, L.; Su, L.; Mao, L. Q. Langmuir 2005, 21, 6560.
Furthermore, owing to their well-defined nanostructures, good chemical stability, and possible electrocatalytic activity toward some substances, CNTs are extensively used as carrier platforms of various electrochemical sensors.19–23 Despite having promising applications in practice, CNTs are commonly insoluble in most solvents and exist in the form of tangled network structures containing various impurities, which seriously hinder their processability and electroanalytical applications. In addition, the intact structure of pristine CNTs has been proven to have poor electrochemical performance.24–29 To overcome these deficiencies, raw CNTs are usually treated with oxidizing acids (e.g., nitric acid) to bring about their purification, surface modification, electrochemical activation, and solubilization in various solutions such as organic solvents6,30 and solutions of polymers7,8 or surfactants.9,10 The chemical oxidation treatments inevitably introduce many ionizable groups such as carboxyl groups on the surface of CNTs,31,32 which produce (17) Xu, Z. A.; Gao, N.; Chen, H. J.; Dong, S. J. Langmuir 2005, 21, 10808. (18) Zhao, F.; Wu, X.; Wang, M. K.; Liu, Y.; Gao, L. X.; Dong, S. J. Anal. Chem. 2004, 76, 4960. (19) Joshi, K. A.; Prouza, M.; Kum, M.; Wang, J.; Tang, J.; Haddon, R.; Chen, W.; Mulchandani, A. Anal. Chem. 2006, 78, 331. (20) Wildgoose, G. G.; Leventis, H. C.; Streeter, I.; Lawrence, N. S.; Wilkins, S. J.; Jiang, L.; Jones, T. G. J.; Compton, R. G. ChemPhysChem 2004, 5, 669. (21) Banks, C. E.; Crossley, A.; Salter, C.; Wilkins, S. J.; Compton, R. G. Angew. Chem. 2006, 45, 2533. (22) Yan, Y.; Zheng, W.; Su, L.; Mao, L. AdV. Mater. 2006, 18, 2639. (23) Gong, K. P.; Zhu, X. Z.; Zhao, R.; Xiong, S. X.; Mao, L. Q.; Chen, C. F. Anal. Chem. 2005, 77, 8158. (24) Li, J.; Cassell, A.; Delzeit, L.; Han, J.; Meyyappan, M. J. Phys. Chem. B 2002, 106, 9299. (25) Moore, R. R.; Banks, C. E.; Compton, R. G. Anal. Chem. 2004, 76, 2677. (26) Banks, C. E.; Moore, R. R.; Davies, T. J.; Compton, R. G. Chem. Commun. 2004, 6, 1804. (27) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 7, 829. (28) Sˇljukic´, B.; Banks, C. E.; Compton, R. G. Nano Lett. 2006, 6, 1556. (29) Chou, A.; Bocking, T.; Singh, N. K.; Gooding, J. J. Chem. Commun. 2005, 7, 842. (30) Qu, J. Y.; Shen, Y.; Qu, X. H.; Dong, S. J. Chem. Commun. 2004, 1, 34. (31) Hu, H.; Zhao, B.; Itkis, M. E.; Haddon, R. C. J. Phys. Chem. B 2003, 107, 13838. (32) Kuznetsova, A.; Popova, I.; Yates, J. T.; Bronikowski, M. J.; Huffman, C. B.; Liu, J.; Smalley, R. E.; Hwu, H. H.; Chen, J. G. G. J. Am. Chem. Soc. 2001, 123, 10699.
10.1021/la703330q CCC: $40.75 2008 American Chemical Society Published on Web 07/17/2008
Surface Design of Carbon Nanotubes Scheme 1. Molecular structure of Congo red
Langmuir, Vol. 24, No. 16, 2008 8891
would be very helpful for designing high-performance CNTbased electrochemical sensors.
Experimental Section
CNT-based electrochemical sensors with rather large backgrounds, especially in the extremely high or low potential ranges. This would generally lower the signal-to-noise ratio and the sensitivity of sensors. Moreover, because the suspension of CNTs in organic solvents lacks stability and solubility as a result of weak solvent-CNT interactions, it may produce irregular and uncontrollable film structures on electrode surfaces. The free dispersing additives in CNT composite films such as polymers or surfactants would seriously block the diffusion of analytes with large dimensions (e.g., proteins) in the films and lower the electrochemical response. In contrast to the extensive applications of CNTs in electroanalytical chemistry,3–5 only a few important works focus on the fundamentals of the electrochemical performance of CNTs.24–29 Rare works pay attention to the physical-chemical properties of electroactive sites on CNTs and the interactions of these sites with other species. This is probably due to the lack of dispersing methods that could effectively exclude the influences of several important factors, for instance, the solubility and stability of CNT solutions, the free additives, and the structural properties of CNT films. The stable adsorption of functional conjugated species on CNTs by strong π-stacking interactions provided a simple approach to the surface modification, solubilization, and functionalization of CNTs, with promising applications in electroanalytical chemistry.33,34 Compared with other dispersing methods of CNTs, the π-stacking method has the virtue of the absence of free additives, controllable surface coverage, a capacity for further surface modification, and higher solubility due to the stronger modifier-CNT interactions. Recently, we developed a noncovalent method for dissolving single-walled carbon nanotubes (SWNTs) in water with high solubility and stability on the basis of the strong π-stacking adsorption of a planar conjugated diazo dye, Congo red (CR) (Scheme 1), on the sidewall of SWNTs.35 The resulting watersoluble CNTs were able to form marvelously uniform and compact CNT films with nanosized network structures on various solid substrates when dried, which exhibited excellent electrochemical activity toward several important biomolecules.36,37 This method allows us to explore the physical-chemical properties of electroactive sites on CNTs and the interactions between analytes and CNTs without considering the influence of either film structures or free additives. In this work, we found that both the hydrophobic structural defect sites and the hydrophilic oxygencontaining groups were the electroactive sites of CNTs. In addition, the different physical-chemical properties of these sites could be conveniently utilized to control the interactions between analytes and CNTs and optimize the adsorption and electrochemical responses of analytes on CNT films. Understanding noncovalent interactions between absorbates and CNTs (33) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (34) Yan, Y. M.; Zhang, M. N.; Gong, K. P.; Su, L.; Guo, Z. X.; Mao, L. Q. Chem. Mater. 2005, 17, 3457. (35) Hu, C. G.; Chen, Z. L.; Shen, A. G.; Shen, X. C.; Li, J.; Hu, S. S. Carbon 2006, 44, 428. (36) Hu, C. G.; Chen, X. X.; Hu, S. S. J. Electroanal. Chem. 2006, 586, 77. (37) Hu, C. G.; Yang, C. H.; Hu, S. S. Electrochem. Commun. 2007, 9, 128.
Materials. SWNTs (purity g90 wt %, average diameter ∼1.2 nm) and MWNTs (purity g95 wt %, average diameter ∼20 nm) (Timesnano Co., Chengdu, China) were produced by a chemical vapor deposition (CVD) method. SWNTs were acid treated by refluxing the as-received SWNTs in 2.6 M HNO3 for 48 h, treating with 2.0 M HCl for 12 h, and then thoroughly washing with water and drying at 60 °C in vacuum. The acid treatment of MWNTs was similar to that of SWNTs except for a different acid treatment of refluxing MWNTs in concentrated HNO3 for 15 h. The as-received CNTs and the acid-treated CNTs were denoted as r-CNTs and t-CNTs, respectively. Dopamine (DA), estradiol (ED), and glucose oxidase (GOD, from Aspergillus niger, 1060 U/mL) were purchased from Fluka Chemical Co. Stock solutions of 1.0 × 10-2 M DA and ED were prepared from redistilled water and absolute ethanol, respectively. Congo red (CR) and sodium dodecyl sulfate (SDS) were purchased from Shanghai Reagent Co. (Shanghai, China). All chemicals were of analytical-grade quality and used as received except for special statements. Apparatus. Electrochemical measurements were performed on an EG&G model 283 electrochemical workstation (Princeton Applied Research) controlled by M270 software. The electrode system contained a CR-functionalized CNT (CNT-CR) film-coated working electrode, a platinum wire counter electrode, and a potassium chloride (KCl) saturated calomel reference electrode (SCE). All of the potentials in this work were reported vs SCE. The transmission electron microscope (TEM) images were obtained on a JEM100CXII, UV-vis measurements were carried out on a Tu-1901 UV-vis spectrophotometer (Purkinje General Instrument Co. Ltd., Beijing, China), and atomic force microscopy (AFM) images were obtained on a PicoScan system (Molecular Imaging Inc.) in a tapping mode with commercially ultrasharpened Si3N4 tips (MAClever II, Molecular Imaging Inc.). Raman spectra were recorded on a Renishaw RM1000 confocal Raman system at an excitation of 514.5 nm. FTIR spectra were carried out on a Magna-IR 550 system (Nicolet) powered by Omnic 2.0 software. To produce optimal results, the original spectra were smoothed two times using an embedded smoothing tool in Omnic 2.0 by setting the smooth points to the largest number of 25 (i.e., 48.212 cm-1), and the baselines were corrected by another embedded tool in an automatic manner. Preparation of Water-Soluble Carbon Nanotubes. The preparation of CR-functionalized water-soluble CNTs has been described previously.35–37 Briefly, CNTs were mixed with CR at a certain weight ratio (e.g., 5/1 WCNTs/WCR) in an agate mortar and ground for 4 h with the addition of a little water, producing a greenish-black mixture. The grinding mixture of CNTs and CR was dissolved in water and centrifuged at 1000 rpm for 15 min. The supernatant was collected and washed with water on a polytetrafluoroethylene (PTFE) filter disk of 0.22 µm pore size to remove excess free CR in solution until the filtrate became colorless. The resulting CR-functionalized water-soluble CNTs were denoted as CNT-CRwater. CNT-CRwater prepared from as-received CNTs (r-CNTs) and acid-treated CNTs (t-CNTs) were denoted as r-CNT-CRwater and t-CNT-CRwater, respectively. A portion of the CR molecules adsorbed on CNTCRwater were removed by washing with N,N′-dimethyl formamide (DMF), producing CR-functionalized CNTs (CNT-CRDMF) with lower solubility. Fabrication of CNT-Modified Electrodes. A gold electrode (Au, 1.6 mm in diameter, BAS) was polished with a slush of 0.05 µm alumina (Al2O3), and washed with water, 1:1 HNO3, and ethanol with sonication, each for 3 min. Then, a certain volume of the CNT solutions was cast onto the clean electrode surface and air dried. The CNT-film-modified electrodes were pretreated by sweeping in supporting electrolytes in the desired potential range until stable voltammograms were obtained. This treatment could effectively eliminate the response of adsorbed CR, activate the surface of CNTs, and lower the background. The prepared electrodes were stored in the air when not in use. The volume of all CNT solutions used for
8892 Langmuir, Vol. 24, No. 16, 2008
Hu and Hu
Figure 1. TEM images of r-MWNTs (A) and r-SWNTs (C) in SDS solution as well as r-MWNT-CRwater (B) and r-SWNT-CRwater (D) in water. Samples for TEM imaging were loaded onto a carbon grid with a copper support by dropping the aqueous solution on the grid and drying. Arrows indicate the possible metal catalysts on r-MWNTs.
the modification of gold electrodes was 1.0 µL. The immobilization of GOD on CNT-CR films was achieved by immersing the modified electrodes in the as-received GOD solution (1060 U/mL) for 30 min and thoroughly rinsing with water. The procedures related to the electrochemical measurements and the electrochemical performances of the CNT-modified electrodes are presented in Supporting Information.
Results and Discussion Nafion,7 surfactants,9,10 and DMF6 are commonly used in electroanalytical chemistry to disperse CNTs in solutions. The prepared CNTs or CNT composite films have been successfully employed as the sensing materials for a variety of electrochemical sensors. Unfortunately, these dispersing methods more or less suffered from deficiencies such as low solubility and selectivity, especially for SWNTs. The presence of organic solvents or high concentrations of surfactants might also limit their applications under certain conditions (e.g., biological systems). Alternatively, the strong adsorption of conjugated functional molecules on CNTs via strong π-stacking interactions provided a promising method for dispersing CNTs in water with high solubility, stability, and selectivity and in the absence of free additives.33,34,38 Figure 1 represents the TEM images of as-received MWNTs and SWNTs in SDS solutions and CR-functionalized MWNTs and SWNTs in water. For as-received MWNTs, some dark nanoparticles are observed on only the surface of MWNTs (Figure 1A), suggesting that the dispersion of CNTs in SDS solutions by sonication cannot effectively remove the possible metal catalysts that are strongly bound to the surface of MWNTs. In contrast, the treatment of as-received MWNTs with CR produces water-soluble MWNTs with high purity (Figure 1B). In the case of SWNTs, the strongly bundled nanotube ropes are effectively exfoliated into small bundles or individual nanotubes, ensuring the formation of more subtle film structures (Figure 1C,D). Both cases are attributed to the highly selective and strong π-stacking interactions between CR and CNTs.35 Figure 2 shows the AFM images of the as-received MWNT films functionalized with CR and washed with water (r-MWNTCRwater) and the SWNT films (r-SWNT-CRwater). Generally, both MWNTs and SWNTs form uniform, compact films on graphite sheets (Supporting Information Figure S1), which are different from the loose, irregular structures of MWNT films prepared from suspensions in organic solvents6 but similar to the compact films of CNT composites with naked surfaces.7,10 In addition, it is surprising that MWNT and SWNT films seem to be made up of nanoparticles instead of tubular structures (Figure 2A,C), especially for SWNTs. The average diameters of MWNT and SWNT nanoparticles are about 150 and 80 nm, respectively, which are obviously larger than the diameters of about 20 and (38) Paloniemi, H.; Aaritalo, T.; Laiho, T.; Like, H.; Kocharova, N.; Haapakka, K.; Terzi, F.; Seeber, R.; Lukkari, J. J. Phys. Chem. B 2005, 109, 8634.
Figure 2. AFM images of r-MWNT-CRwater (A, B) and r-SWNT-CRwater (C, D) films on graphite sheets in the topograph-flattened (A, C) and deflection-flattened (B, D) modes. Arrows indicate the possible individual nanotubes of MWNTs.
1.2 nm observed from the TEM images (Figure 1). In Figure 2B, we can see that the nanoparticles in MWNT films might consist of some individual nanotubes. This is supported by the fact that the diameter of the threadlike components in these nanoparticles is close to the diameter of MWNTs in the TEM images. Clearly, MWNTs form more regularly assembled structures than do SWNTs (Figure 2B,D), probably as a result of the more rigid structure of MWNTs. According to Bettinger,39 several groups of defects are generally formed during the growth or the workup of CNTs, including topological (ring sizes other than hexagons), rehybridization (sp2 and sp3 hybridization of carbon), incomplete bonding (vacancies), and doping defects. It has been proven that the introduction of defects onto the intact structure of CNTs results in a significant change in properties such as the chemical reactivity and electronic structure of CNTs. Compton et al. reported that the edge-planelike defect sites were the electroactive sites of CNTs and that the intact surfaces of CNTs were electrochemically inert, similar to the basal plane pyrolytic graphite.27 Gooding et al. further demonstrated the importance of oxygenated species at the ends of CNTs.29 Meanwhile, the annealing treatment of CNTs could eliminate the characteristic pseudocapacitance due to faradaic reactions of surface-bonded oxides and make the surfaces of CNTs more hydrophobic.40 In fact, the annealing treatment of CNTs effectively reduces surface functional groups such as quinone and carboxylic acid and leaves defect sites on the sidewall of CNTs.41 Wang’s work demonstrated that CNTs produced by chemical vapor deposition methods (CVD-CNTs) possessed better electrochemical performance than did ARC-CNTs as a result of the presence of a higher density of edge plane defects at CVDCNTs.42 Therefore, it is reasonable to assume that different (39) Bettinger, H. F. J. Phys. Chem. B 2005, 109, 6922. (40) Barisci, J. N.; Wallace, G. G.; Baughman, R. H. J. Electroanal. Chem. 2000, 488, 92. (41) Kuznetsova, A.; Mawhinney, D. B.; Naumenko, V.; Yates, J. T.; Liu, J.; Smalley, R. E. Chem. Phys. Lett. 2000, 321, 292. (42) Musameh, M.; Lawrence, N. S.; Wang, J. Electrochem. Commun. 2005, 7, 14.
Surface Design of Carbon Nanotubes
Figure 3. Cyclic voltammograms (CV) of 2.0 × 10-5 M dopamine (A) and 2.0 × 10-5 M estradiol (B) at r-MWNT-CRwater- (a), t-MWNTCRwater- (b), r-SWNT-CRwater- (c), t-SWNT-CRwater- (d), and r-SWNTCRDMF-modified (e) gold electrodes in 0.1 M phosphate buffer solution (pH 7.0). The concentration of modifier (1.0 µL) is 1.0 mg/mL for water-soluble MWNTs and 0.2 mg/mL for water-soluble SWNTs. Scan rate, 100 mV/s; accumulation condition, 400 s at an open circuit.
electroactive sites might exist on the surfaces of CNTs, which are responsible for the different adsorptive and electrochemical behaviors of analytes on different CNT films. Here, we try to explore the relationship between the physical-chemical properties of the electroactive sites and the electrochemical performance of CNTs by using several electrochemical probes, including dopamine, estradiol, and glucose oxidase (GOD). The electrochemical behaviors of dopamine (DA) and estradiol (ED) are found strongly dependent on the surface properties of CNTs (Figure 3). The corresponding voltammetric parameters are listed in Table 1. Similar to other CNT-based electrochemical sensors, the modification of electrode surfaces by CNT-CR could apparently improve the responses of DA and ED (Supporting Information Figure S2). Both species exhibit a similar or even higher sensitivity at SWNT films than at MWNT films, even when the mass ratio of modifiers (WMWNTs/WSWNTs) is 5/1. Thus, SWNTs can offer a much higher specific surface and utilization efficiency, but MWNTs cannot because the inner layers of MWNTs play negligible roles in the electroanalytical performance of the MWNT films. Figure 3A shows the signals of DA at various CR-functionalized CNT films. Compared with the response at as-received MWNTs (curve a), the electrochemical response of DA at acid-treated MWNTs is apparently improved (curves b), suggesting that DA preferably adsorbs on the oxygencontaining groups of MWNTs. Meanwhile, the acid treatment of SWNTs and the removal of adsorbed CR do not significantly improve the responses of DA at SWNTs films, especially for the reduction current (curves c-e).
Langmuir, Vol. 24, No. 16, 2008 8893
The electrochemical behaviors of ED at CNT-CR films are much different (Figure 3B). The responses of ED are significantly enhanced at all CNT films, and the signal is more sensitive at r-MWNTs (curve a) than at t-MWNTs (curves b), with a lower oxidation overpotential. This is completely contrary to the traditional concept that acid treatment could generally improve the electrochemical performance of CNTs by creating many electroactive sites on the surfaces of CNTs.43 Similar to the case of MWNTs, acid treatment depresses the signal of ED at SWNTs (curves c and d). The removal of adsorbed CR from as-received SWNTs by DMF washing is able to improve the response of ED significantly (curve e). That is to say, the CR molecules adsorbed on CNTs indeed inhibit the electrochemical responses of analytes in solution, probably as a result of the blocking effect of adsorbed CR that makes the electroactive defects on CNTs difficult to access. The removal of adsorbed CR can make the surface properties of SWNT-CR films close to those of glassy carbon electrodes but the structural properties close to those of the network of nanotubes.36 In comparison with CNT composites comprising CNTs and other free additives such as polymers, the uncovered portion of CNT-CR provides more defect sites for the electrochemical reaction of analytes. This is supported by the much better electrochemical response of ofloxacin at CRfunctionalized MWNTs than at the commonly used MWNTNafion composite.44 In fact, it has been proven that the addition of trace cationic surfactants can further improve the electrochemical responses of hydrophobic species by enlarging the oxidation current and enhancing the antifouling capacity of CRfunctionalized MWNT-modified electrodes.37 Thus, the uncovered surface of CR-functionalized CNTs allows us to improve the electrochemical performance of CNT sensing films by further surface modifications. In most cases, the treatment of CNTs with HNO3 could create many oxygen-containing groups (e.g., carboxyl groups) on the surfaces of CNTs and could provide more reactive and hydrophilic adsorption sites for analytes, whereas the signals of ED at acidtreated CNTs (t-CNTs) are apparently lower than at as-received CNTs (r-CNTs). This result, together with the hydrophobic adsorption of ED on the hydrophobic surface of CNTs,37 demonstrates that the adsorption and electroactive sites of ED at CNTs are mainly the hydrophobic defect sites. Therefore, it is possible to adjust the surface properties of CNTs to suit the preferable adsorption and the enhanced responses of analytes. The improvement of ED signals on CNT-CR by DMF washing also indicates that the removal of CR from CNTs provides more reactive sites for ED (i.e., the adsorption sites of CR removed from CNT-CR by DMF washing should be mainly due to the hydrophobic defect sites). In a word, two types of defects might exist on the surface of CNTs for the adsorption of CR: The first one is the hydrophobic structural defects for the stable adsorption of CR on CNTs in water, and the second one is the hydrophilic oxygen-containing groups for the stable adsorption of CR even in DMF. Both cases are based on π-stacking interactions and might be enhanced by charge-transfer interactions,35 but the second one might be further strengthened by hydrogen bonding. To further verify the presence of different electroactive sites on CNTs, the electrochemical behaviors of GOD on various CNT films are investigated (Figure 4). Figure 4A shows the cyclic voltammograms (CV) of surface-confined GOD on CNT-CR films. No responses of GOD are observed at r-MWNTCRwater films (curve a), but a pair of redox peaks appears at (43) Papakonstantinou, P.; Kern, R.; Irvine, J.; McAdams, E.; McLaughlin, J.; McNally, T. Fullerenes, Nanotubes, Carbon Nanostruct. 2005, 13, 91. (44) Yang, C. H.; Xu, Y. X.; Hu, C. G.; Hu, S. S. Electroanalysis 2008, 20, 144.
8894 Langmuir, Vol. 24, No. 16, 2008
Hu and Hu
Table 1. Voltammetric Parameters Obtained from Figures 3 and 4 for Dopamine, Estradiol, and GOD at CR-Functionalized CNT Modified Electrodesa analytes
CNTs
dopamine
bare Au r-MWNT-CRwater t-MWNT-CRwater r-SWNT-CRwater t-SWNT-CRwater r-SWNT-CRDMF bare Au r-MWNT-CRwater t-MWNT-CRwater r-SWNT-CRwater t-SWNT-CRwater r-SWNT-CRDMF r-MWNT-CRwater t-MWNT-CRwater r-SWNT-CRwater t-SWNT-CRwater r-SWNT-CRDMF r-MWNT-CRwater t-MWNT-CRwater r-SWNT-CRwater t-SWNT-CRwater
estradiol
GODelectrode
GODsolution
a
Ip,a/µA
Ip,c/µA
∆Ep/V
0.157 0.154 0.150 0.148 0.154 0.155 0.559 0.499 0.522 0.535 0.539 0.525
0.096 0.114 0.124 0.121 0.124 0.133
0.069 0.826 1.366 1.510 1.678 2.104 0.062 7.148 4.430 6.591 5.257 11.93
0.065 0.543 0.838 1.079 0.949 0.929
0.061 0.041 0.026 0.027 0.030 0.022
0.127 0.134 0.137 0.135 0.139 0.144
-0.472 -0.468 -0.468 -0.459 -0.370 -0.357 -0.362 -0.355
-0.498 -0.494 -0.503 -0.495 -0.397 -0.395 -0.402 -0.395
0.426 0.508 1.809 0.549 0.542 1.760 2.260 2.963
0.515 0.576 2.129 0.611 0.528 1.383 2.634 3.375
0.026 0.026 0.035 0.036 0.027 0.038 0.040 0.040
-0.485 -0.481 -0.486 -0.477 -0.384 -0.376 -0.382 -0.375
Ep,a/V
Ep,c/V
E°′/V
The apparent formal potential (E°′) was defined as an average of the oxidation and the reduction peak potentials.
Figure 4. CV of surface-confined GOD in 0.1 M phosphate buffer (pH 7.0) (A) and 530 U/mL GOD in 0.05 M phosphate buffer (pH5.7) (B) at CNT-CR films: (a) r-MWNT-CRwater, (b) t-MWNT-CRwater, (c) r-SWNT-CRwater, and (d) t-SWNT-CRwater. The accumulation condition of Figure 4B is 10 min at an open circuit. Other conditions are the same as for Figure 3.
t-MWNT-CRwater films (curve b), similar to the direct electron transfer of GOD on other CNT films.15 Different from the response at r-MWNT-CRwater, GOD exhibits a pair of well-defined redox peaks at r-SWNT-CRwater, with both peak potentials and peak currents close to those at t-MWNT-CRwater (curve c). Obviously, the acid treatment can improve the response of GOD at SWNTs (curve d). As a redox protein,
GOD has abundant amino groups on the surface, which might be responsible for the stable adsorption of GOD on t-MWNTs, properly through the hydrogen bonding between hydrophilic groups of GOD and oxygen-containing groups on t-MWNTs. Therefore, the oxygen-containing groups on CNTs should mainly account for the stable adsorption and the direct electron transfer of GOD on CNTs, especially for MWNTs. Figure 4B shows the CV of GOD in solution at different CNT films. Surprisingly, GOD exhibits direct electron transfer on all CNT films. The acid treatment also apparently improves the response of GOD. The redox response of GOD at r-MWNT-CRwater films completely disappeared when the electrode was transferred to blank solutions (not shown). As for SWNTs, the influence of acid treatment on the electrochemical response of GOD in solution is not as strong as that of surface-confined GOD (curves c and d). These results verify that the hydrophobic defect sites of CNTs are also the electroactive sites for the direct electron transfer of GOD, which are the reversible adsorption sites of GOD, particularly for MWNTs. Thus, both the hydrophobic structural defects and the oxygen-containing groups on CNTs are the electroactive sites for GOD. The oxygen-containing groups guarantee the stable adsorption of GOD on CNTs, perhaps through the multiple hydrogen bonds between the amino groups on GOD and the oxygen-containing groups on acid-treated CNTs. The removal of CR by DMF washing exerts little influence on the electrochemical responses of surfaceconfined GOD (Table 2) as a result of the presence of multiple adsorption sites on GOD. Actually, the stable adsorption of proteins on CNTs has been proposed as an effective method of achieving the surface modification and the solubilization of CNTs in water.45,46 Eklund et al.47 systematically investigated the effect of acid treatment on the structural properties of SWNTs by Raman spectra. They found that the treatment of SWNTs by dilute nitric acid (3 M) had a significant impact on the wall structure of SWNTs by creating defect sites that could be effectively (45) Karajanagi, S. S.; Yang, H.; Asuri, P.; Sellitto, E.; Dordick, J. S.; Kane, R. S. Langmuir 2006, 22, 1392. (46) Lin, Y.; Allard, L. F.; Sun, Y. P. J. Phys. Chem. B 2004, 108, 3760. (47) Furtado, C. A.; Kim, U. J.; Gutierrez, H. R.; Pan, L.; Dickey, E. C.; Eklund, P. C. J. Am. Chem. Soc. 2004, 126, 6095.
Surface Design of Carbon Nanotubes
Langmuir, Vol. 24, No. 16, 2008 8895
Table 2. FTIR Data Deduced from Figure 5 for Various SWNT Samples FTIR absorption (position and strength) CNTs
CR
SWNTs
σ/cm-1
A1580a
r-SWNTs t-SWNTs r-SWNT-CRwater t-SWNT-CRwater r-SWNT-CRDMF
1569.7 1581.0 1593.2 1584.9 1580.9
0.0160 0.0127 0.0096 0.0227 0.0366
σ/cm-1
1041.9 1042.4 1035.9
A1040a
0.0033 0.0069 0.0051
COOH A1040/A1580 (%)b
σ/cm-1
A1730a
A1730/A1580 (%)b
34.21 30.48 13.99
1735.0 1734.8 1726.5 1727.4 1726.5
0.0042 0.0047 0.0013 0.0051 0.0048
26.44 37.17 14.02 22.33 12.98
a The number in the footnote was used just to indicate the general position of peaks with a similar origin. b The strongest IR absorption located at around 1580 cm-1 was employed as a standard for evaluating the relative density of surface functionalities on various CNTs.
Figure 5. Raman spectra of MWNT (A) and SWNT (B) samples.
eliminated by annealing treatment. Zhang’s work48 indicated that the treatment of SWNTs with 2.6 M nitric acid produced many oxygen-containing groups (mainly carboxyl groups) on the walls of SWNTs. They also proposed a multistep process for the chemical oxidation of SWNTs by nitric acid on the basis of FTIR spectra, including the initial attack on the original existing active sites such as the -CH2 and -CH groups and heptatomic rings, with the following electrophilic addition at hexatomichexatomic boundaries generating more active sites or rather new defects and the final breaking of the graphene structure around the already generated active sites under strong oxidation conditions. Here, the influence of acid treatment on the structural properties of CNTs is examined by Raman spectra (Figure 5). Clearly, there exist two characteristic scatterings in the Raman spectra of MWNTs and SWNTs. The first one located between 1530 and 1560 cm-1 is ascribed to the tangential C-C stretching vibrational mode. The most intense band at ∼1590 cm-1 is (48) Zhang, J.; Zou, H. L.; Qing, Q.; Yang, Y. L.; Li, Q. W.; Liu, Z. F. J. Phys. Chem. B 2003, 107, 3712.
referred to as the graphite (G) band. The second scattering with a frequency between 1250 and 1450 cm-1 is denoted the D band because it is related to the scattering from defects present in CNTs. There are also several weak peaks at around 200 cm-1 that are associated with the radical breathing mode (RBM) (R band) on the Raman spectra of SWNTs. As shown in Figure 5A, no discernible changes can be observed in the spectra of MWNTs by acid treatment, as reflected by a neglectable increase in the D/G band intensity ratio (ID/IG) from 1.215 to 1.243. This might be explained by the multilayer structure of MWNTs (i.e., the inner layers of MWNTs might be exposed and act as the new outer layers when the old ones are deconstructed during the modest acid treatment). In contrast, the acid treatment has been proven to obviously influence the structural properties of SWNTs, as supported by the significant increase in ID/IG from 0.083 to 0.220 (Figure 5B). Therefore, the acid treatment certainly destroys the intact structures of CNTs and introduces defects onto the sidewalls. The creation of oxygen-containing groups on acid-treated CNTs and the adsorptive behaviors of CR on CNTs are proven by the FTIR spectra (Figure 6). The corresponding FTIR data are listed in Table 2. Figure 6A shows the FTIR spectra of various MWNTs samples. The IR absorptions of MWNTs at 1730 and 1215 cm-1 are enhanced with acid treatment. The former absorption is ascribed to carboxyl groups (COOH), and the latter is related to the C-O bond.35 The IR features of CR are observed on t-MWNTCRwater, as reflected by the appearance of absorptions at 827 and 756 cm-1 and the remarkably enhanced intensity of absorptions at around 1054 and 1183 cm-1.35 The rather weak IR characteristics of CR suggest that the amount of CR adsorbed on t-MWNT-CRwater is small. Figure 6B shows the FTIR spectra of various SWNTs samples. With the acid treatment, many carboxyl groups are produced on the surface of SWNTs, as reflected by the increase from 26.44 to 37.17% of the relative intensity of the peak at 1724 cm-1. Similar results are observed for r-SWNTCRwater and t-SWNT-CRwater, with a similar increase in the peak intensity ratio. Meanwhile, stronger IR features of CR appear at SWNTs than at MWNTs, suggesting the adsorption of more CR molecules on SWNTs. Compared with the spectra of CR and SWNTs, no new absorptions are observed for CR-functionalized SWNTs, demonstrating the noncovalent interactions between CR and CNTs.35 As can be seen from Table 2, the amount of CR adsorbed on as-received SWNTs is close to that on acidtreated SWNTs, resembling the small differences in the electrochemical behaviors of dopamine, estradiol, or GOD on these two kinds of SWNTs. That is to say, the creation of more surface functionalities by acid treatment exerts little influence on the adsorption and the electrochemical responses of analytes on SWNTs. This conclusion was verified by the similar adsorption and electrochemical responses of CR at r-SWNT-CRwater and t-SWNT-CRwater films (Supporting Information Figure S3 and Table S1). Consistent with the visual observation, some CR
8896 Langmuir, Vol. 24, No. 16, 2008
Figure 6. FITR spectra of various MWNT (A) and SWNT (B) samples (smoothed and baseline corrected, KBr wafer).
Figure 7. UV-vis spectra in water of t-SWNT-CRwater (a), r-SWNTCRwater (b), r-SWNT-CRDMF (c), and CR desorbed from r-SWNT-CRwater (d) and t-SWNT-CRwater (e) by DMF washing. The concentration of CNTs in curve a-c is 8.0 µg/mL. The concentration of water-soluble SWNTs used to prepare desorbed CR in curves d and e is equal to 24.0 µg/mL.
molecules adsorbed on r-SWNT-CRwater can be removed by DMF washing, resulting in the significantly depressed IR features of CR at 1042, 826, and 758 cm-1 on r-SWNT-CRDMF. Figure 7 represents the UV-vis absorption of water-soluble CNTs and CR. Two absorptions appear at 245 and 510 nm for t-SWNT-CRwater (curve a). The absorption at 245 nm remains unchanged for r-SWNT-CRwater, but the strength of absorption at 510 nm is depressed (curve b). When adsorbed CR is partially removed by DMF washing, the absorption at 245 nm for r-SWNTCRDMF remains stable, but that at 510 nm further decreases (curve c), demonstrating the attribution of the absorption at 510 nm to CR. Because the intensity of the peak at 245 nm in curves a-c is much stronger than the highest absorption of CR at 510 nm
Hu and Hu
in this wavelength range, this peak should arise from an internal π f π* electron transition (HOMO f LUMO) absorption of SWNTs instead of CR, which can be generally observed for SWNTs in the SDS aqueous solution49 or the dry SWNT films.50,51 To explore the nature of CR adsorption on CNTs further, the extent of removal of CR from CNTs by DMF is examined. Curves d and e stand for the UV-vis spectra of CR desorbed from SWNT-CRwater by DMF washing. The absorptions located at 237, 342, and 499 nm are related to the short-wavelength Φ-Φ* transitions, the long-wavelength π-π* transitions of the aromatic rings, and the n-π* transitions from the free-electron pairs of the N atoms of the azo group, respectively.52 It is clear that more CR molecules are desorbed from as-received SWNTs (curve d) than from acid-treated SWNTs (curve e), confirming that the oxygencontaining groups are responsible for the stable adsorption of CR on CNTs in DMF. The important role of defect sites of CNTs in the stable adsorption of CR on CNTs was supported by the fact that the removal of CR from the surface of SWNTs by DMF could significantly enhance the characteristic redox peaks of CNTs (Supporting Information Figure S3 and Table S1). The relative mass ratio of adsorbed CR to SWNTs, estimated from the UV-vis absorption at 342 nm and the IR absorption at around 1040 cm-1, is 34.47 and 38.69 wt % for t-SWNTCRwater and r-SWNT-CRwater, respectively. However, the removal ratio of adsorbed CR by DMF washing is estimated to be 28.83 and 49.45% for t-SWNT-CRwater and r-SWNT-CRwater, respectively. Therefore, the amount of CR adsorbed on r-SWNTs and t-SWNTs is similar, but more CR is desorbed from r-SWNTs by DMF washing. The removal of CR from the hydrophobic defect sites of CNTs is probably due to the replacement of adsorbed CR by DMF via strong adsorption interactions such as dipole interactions, which was responsible for the stable suspension of CNTs in amino-based solvents such as DMF.47 Additional hydrogen bonds between the amino groups of CR and the oxygen-containing groups on CNTs might well explain the stable adsorption of CR on CNTs even in DMF. It is clear from the above discussions that two different electroactive sites might exist on the surface of CNTs for the adsorption and electrochemical reactions of analytes (i.e., the hydrophobic structural defects and the hydrophilic oxygencontaining groups (Scheme 2)). For hydrophobic species such as estradiol, they preferably adsorb on the hydrophobic defect sites and exhibit their electrochemical responses (Scheme 2A). Although the acid treatment can produce more electroactive sites on the surface of CNTs, the introduced hydrophilic polar oxygencontaining groups obstruct the adsorption and the electrochemical reaction of these hydrophobic species (Scheme 2B). In contrast, the weak hydrophobic interaction between CNTs and hydrophilic species such as proteins leads to the weak, reversible adsorption of proteins on CNTs (Scheme 2C), and the oxygen-containing groups on acid-treated CNTs provide additional binding sites for the stable adsorption of proteins by possible hydrogen bonding and sometimes electrostatic interactions (Scheme 2D). Similar to the basal plane pyrolytic graphite, the intact surfaces of CNTs are electrochemically inert even for K3Fe(CN)6 (Scheme 2E).27,29 Recently published works have proven that various conjugated (49) Pichler, T.; Knupfer, M.; Golden, M. S.; Fink, J.; Rinzler, A.; Smalley, R. E. Phys. ReV. Lett. 1998, 80, 4729. (50) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555. (51) Itkis, M. E.; Niyogi, S.; Meng, M. E.; Hamon, M. A.; Hu, H.; Haddon, R. C. Nano Lett. 2002, 2, 155. (52) Neumann, B.; Pollmann, P. Phys. Chem. Chem. Phys. 2001, 3, 4508.
Surface Design of Carbon Nanotubes Scheme 2. Schematic Representations of the Adsorption and Electrochemical Reactions of Estradiol (A, B) and Redox Proteins (C, D) on Hydrophobic Structural Defects (A, C) and Hydrophilic Oxygen-Containing Groups (B, D) of CNTsa
Langmuir, Vol. 24, No. 16, 2008 8897
hydrogen bonding46). The combination of these interactions can form various adsorbing states of species on CNTs, which well explains the surface modification of CNTs by various noncovalent methods. Understanding interactions between adsorbates and CNTs might help guide the surface modification of CNTs and promising practical applications.
Conclusions
a Process E indicates the electrochemical inertness of the intact structure of CNTs even for K3Fe(CN)6.
organic molecules33,34,38,53 or polymers54–56 could stably adsorb on the surface of CNTs via π-stacking interactions to achieve the surface modification and the solubilization of CNTs in solutions, especially for organic dyes.33,34,38 In addition, previous reports have demonstrated that the charge-transfer interactions play important roles in the stable adsorption of amino-containing species on CNTs.38 Thus, at least five special interactions might exist between the adsorbates and CNTs (i.e., hydrophobic,37 electrostatic,57 π-π,54 and charge-transfer interactions38 and (53) Chen, J.; Collier, C. P. J. Phys. Chem. B 2005, 109, 7605. (54) Yang, D. Q.; Rochette, J. F.; Sacher, E. J. Phys. Chem. B 2005, 109, 4481. (55) Sinani, V. A.; Gheith, M. K.; Yaroslavov, A. A.; Rakhnyanskaya, A. A.; Sun, K.; Mamedov, A. A.; Wicksted, J. P.; Kotov, N. A. J. Am. Chem. Soc. 2005, 127, 3463. (56) Chen, J.; Liu, H.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker, G. C. J. Am. Chem. Soc. 2002, 124, 9034. (57) Chen, J.; Rao, A. M.; Lyuksyutov, S.; Itkis, M. E.; Hamon, M. A.; Hu, H.; Cohn, R. W.; Eklund, P. C.; Colbert, D. T.; Smalley, R. E.; Haddon, R. C. J. Phys. Chem. B 2001, 105, 2525.
The strong π-stacking adsorption of Congo red on CNTs from various sources (e.g., as-received or acid-treated MWNTs or SWNTs) provided a simple method for optimizing the electroanalytical performances of CNT sensing films by adjusting the surface properties of CNTs via introducing functional sites of different properties (e.g., the hydrophobic defects or the oxygencontaining groups). Electrochemical and spectroscopic investigations demonstrated that species with different properties preferably adsorbed on the specific functional sites of CNTs and exhibited sensitive electrochemical responses. For instance, hydrophobic estradiol preferably adsorbed on the hydrophobic defects whereas the oxygen-containing groups were responsible for the stable adsorption and the direct electron transfer of GOD on CNTs. Acknowledgment. This work was supported by the National Natural Science Foundation of China (nos. 30770549 and 60571042) and the Scientific Research Starting Foundation of Wuhan University (no. 502-265145). We also thank Professor Zhiling Zhang and Dr. Lei Bao for their helpful discussions on AFM images. Supporting Information Available: Detailed information on the electrochemical measurements and the AFM images of CNT-CR at low amplification. This material is available free of charge via the Internet at http://pubs.acs.org. LA703330Q
