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One-step fabrication of chitosan–hematite nanotubes composite film and its biosensing for hydrogen peroxide
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Electrochemistry Communications 10 (2008) 123–126 www.elsevier.com/locate/elecom
One-step fabrication of chitosan–hematite nanotubes composite film and its biosensing for hydrogen peroxide Jingming Gong *, Lianyi Wang, Kun Zhao, Dandan Song
*
Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China Received 29 September 2007; received in revised form 24 October 2007; accepted 25 October 2007 Available online 1 November 2007
Abstract This work reports on a novel chitosan–hematite nanotubes composite film on a gold foil by a simple one-step electrodeposition method. The hybrid chitosan–hematite nanotubes (Chi–HeNTs) film exhibits strong electrocatalytic reduction activity for H2O2. Interestingly, two electrocatalytic reduction peaks are observed at 0.24 and 0.56 V (vs SCE), respectively, one controlled by surface wave and the other controlled by diffusion process. The Chi–HeNTs/Au electrode shows a linear response to H2O2 concentration ranging from 1 · 10 6 to 1.6 · 10 5 mol L 1 with a detection limit of 5 · 10 8 mol L 1 and a sensitivity as high as 1859 lA lM 1 cm 2. 2007 Elsevier B.V. All rights reserved. Keywords: Hematite nanotubes; Chitosan; Hydrogen peroxide; One-step electrodeposition; Biosensing
1. Introduction Detection of hydroxyl peroxide (H2O2) has received tremendous attention in sensor research for decades. So far, many techniques have been used to chemical, biological, clinical, environmental, and many other impressive applications [1,2]. Among them, amperometric detection is a most promising approach to achieve accurate, separate, and rapid H2O2 monitoring. However, direct electrochemical oxidation of H2O2 is usually accomplished at an applied potential of 0.6 V vs. Ag/AgCl [3]. At such a potential, oxidizable compounds (e.g., ascorbate, urate, etc.) that exist in biological fluids often severely interfere with the analysis. In recent years, enzyme (e.g., peroxidase and cytochrome C) electrodes have been fabricated for H2O2 detection with simplicity, high sensitivity, and selectivity [4–8], but their operational conditions are generally limited by the denatruation of enzymes. Prussian blue *
Corresponding authors. Tel./fax: +86 27 67867535. E-mail addresses: [email protected] (J. Gong), ddsong@ mail.ccnu.edu.cn (D. Song). 1388-2481/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.10.017
(PB) and its analogs have been explored in a variety of H2O2 determination [9–13]. Recently, supramolecular complex dentrimers-PB membranes and silver-DNA hybrid films have been developed as catalytic layers to detect H2O2 [14–16]. Despite these advances, it is still a great challenge to construct highly selective, sensitive, and stable biorecognition interfaces for determination of H2O2. In this work, we constructed a novel functional hybrid film of chitosan–hematite nanotubes on a gold electrode (labeled as Chi–HeNTs/Au) as a catalytic layer for H2O2 detection, taking advantage of hematite nanotubes as a catalyst and chitosan as an immobilization matrix. Hematite (a-Fe2O3), the most stable iron oxide under ambient conditions, is widely used in catalysts, pigments, and sensors [17]. In particular, tubelike a-Fe2O3 has been recently demonstrated to show novel properties with central hollow cores/outside walls and high surface areas [18–20]. Chitosan, a natural and biocompatible polymer, has been widely selected as the immobilization matrix for catalysts [21–23]. To the best of our knowledge, this work first reports on the fabrication of hybrid films composed of tubelike Fe2O3 nanostructures and biocompatible chitosan. Importantly,
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hematite nanotubes (HeNTs) entrapped in the composite film show strong electrocatalytic activity for reduction of H2O2. We believe that such a hybrid film is potentially useful in bio-electrochemistry and diagnostics applications.
electrode. For comparison, hematite nanoparticles-chitosan nanocomposite film was also fabricated (labeled as Chi–HeNPs/Au) by a similar one-step electrodeposition approach.
2. Experimental
3. Results and discussion
Chitosan from crab shells (85% deacetylated) was purchased from Sigma. Chitosan solution was prepared by mixing chitosan flakes with water and gradually dropping HCl solution to the mixture to maintain the pH near 3. After removal of the undissolved part by a 0.45-lm syringe filter unit, its pH value was adjusted to about 5.0 using an aqueous NaOH solution. Hematite nanotubes (HeNTs) and nanoparticles (HeNPs) were prepared according to the previous literature [18]. Typically, hematite products were obtained by a hydrothermal treatment of a FeCl3 solution (0.02 M) in the presence of NH4H2PO4 (7.2 · 10 4 M) at 220 C. After 2 h, the spindle-like HeNPs were formed with a diameter of 70 nm and a length of 400 nm. When further prolonging the reaction time to 48 h, nanotubes were obtained. All other reagents were of analytical grade and were used without further purification. A CHI 660A electrochemical workstation (CHI, USA) was used for the electrochemical measurements. A threeelectrode system was used, which was composed of a saturated calomel reference electrode (SCE), a platinum coil counter electrode, and a modified or unmodified gold electrode (Au, Ø 1 mm) as the working electrode. All solutions were deaerated efficiently with nitrogen prior to the electrochemical experiments unless otherwise stated and N2 atmosphere was maintained over the solution during the experiments. The general morphology of the products was characterized by scanning electron microscopy (SEM, LEO, 1450VP). XPS measurement was performed on PHI 5600 (Physical Electronics, USA). Prior to modification, the bulk Au disk electrode was successively polished by 1.0, 0.3 and 0.05 lm alumina slurry, followed by the sonication in distilled water and ethanol for 5 min each. The as-prepared hematite nanotubes were dispersed into chitosan solution by sonication. Then, the electrode was immersed in the 1.5 mg/mL hematite nanotubes/chitosan dispersions by controlling the potential at 3.0 V vs SCE for 4 min. This concentration of 1.5 mg/ml is an optimized value and it exhibits the maximum value for the response of H2O2. H+ in the solution was reduced to H2 at Au electrode, and the pH near Au electrode surface gradually increased. The solubility of chitosan is pH-dependent. When the pH exceeds the pKa of chitosan (about 6.3), chitosan becomes insoluble [22] in water, and therefore the chitosan entrapped HeNTs will deposit onto the Au electrode surface. In addition, owing to the presence of hydroxyl and amino groups in chitosan, the chemical bonding possibly occurs between chitosan and Au substrate, which might further prompts the chitosan entrapped HeNTs deposited onto the gold
Fig. 1a and b shows the typical SEM images of the asprepared hematite nanotubes at different magnifications. It is observed that the hematite nanotubes have diameters of 80 nm and lengths up to 400 nm. Fig. 1c shows the SEM image for the composite film of Chi–HeNTs. Apparently, the hematite tubes are homogenously distributed in the chitosan matrix, indicating that hematite nanotubes indeed have been entrapped in chitosan. Fig. 1d shows the survey XPS spectrum of the Chi–HeNTs composite film. In the high-resolution Fe 2p spectrum (inset of Fig. 1d), two distinct peaks at binding energies of 725.5 eV for Fe 2p1/2 and 711.5 eV for Fe 2p3/2 with a shake-up satellite are observed. This is characteristic of Fe3+ in Fe2O3 [24]. These results confirm that Chi–HeNTs nanocomposite film has been formed onto the Au foil through the simple one-step electrodeposition process. Fig. 2 shows typical CVs of H2O2 at Chi–HeNTs/Au in the absence and presence of H2O2, respectively, at the scan rate of 50 mV/s in deoxygenized 0.1 M pH 7.4 PBS. The bare Au electrode shows no obvious voltammetric peaks within the potential range in 0.1 M PBS pH 7.4 (Fig. 2a). In comparison, Chi–HeNTs composite film modified Au gives a well-defined redox peak with an average formal potential (Em = (Epa + Epc)/2) of 0.20 V (vs. SCE). The cathodic and anodic peaks appear at 0.240 and 0.155 V vs. SCE, respectively (Fig. 2b). This agrees well with the previous results reported on the direct electron transfer of nanostructured Fe2O3 onto ITO [25,26]. The redox peaks can be ascribed to the redox reaction of a-Fe2O3 nanotubes entrapped in the chitosan composite film. With the addition of H2O2 into solution, the cathodic peak was enhanced greatly at ca. 0.240 V vs. SCE (Er1), and gradually increased with the further addition of H2O2. Meanwhile, the anodic peak current at E = 0.155 V decreased, and disappeared finally, showing characteristics of an electrocatalytic reduction of H2O2 at Chi–HeNTs/Au. Apparently, HeNTs entrapped in the composite film effectively mediate the reduction of H2O2. Very interestingly, in the cathodic process, another reduction peak appears at ca. 0.55 V vs. SCE (Er2), also gradually increased with the addition of H2O2. The CV curves of 0.23 mM H2O2 at bare Au and Chi–HeNTs/Au electrodes are shown in the inset of Fig. 3. The response at bare gold electrode is barely detectable, except the cathodic polarization of H2O2. In comparison, H2O2 exhibits a strong catalytic reduction behavior at the Chi–HeNTs/Au electrode, with two reduction peaks of 0.24 and 0.55 V (vs. SCE). Obviously, the presence of Chi–HeNTs composite film plays a crucial role in the reduction of H2O2.
J. Gong et al. / Electrochemistry Communications 10 (2008) 123–126
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Fig. 1. SEM images of (a, b) hematite nanotubes and (c) Chi–HeNTs nanocomposite film onto a gold foil. (d) XPS spectra of the Chi–HeNTs nanocomposite film onto Au foil. Inset shows the high-resolution spectrum of Fe 2p.
Fig. 2. Cyclic voltammograms of (a) bare Au, (b) the Chi–HeNTs/Au in the absence of H2O2 and presence of (c) 0.03 mM H2O2, (d) 0.13 mM H2O2, (e) 0.23 mM H2O2. Inset: CVs of 0.23 mM H2O2 at (a) bare Au electrode and (b) Chi–HeNTs/Au electrode. Electrolyte: deoxygenized 0.1 M PBS (pH 7.4), Scan rate, 50 mV/s.
Regarding the reduction peaks of H2O2 at Chi–HeNTs/ Au, the effect of potential scan rate was investigated clearly. For Er1, a plot of peak current against the scan rate is linear over the range of 10–100 mV/s with a linear regression equation, ipa/lA = 0.481 + 3.764v/V s 1 (r = 0.9931), revealing a surface wave. For Er2, the peak current is proportional to the square root of scan rate in the range of 5– 200 mV/s, ipa/lA = 1.494 34.61v1/2/V s 1, r = 0.9920, indicating a diffusion controlled process. For comparison, hematite nanoparticles (HeNPs)-chitosan nanocomposite film onto Au was also fabricated (Chi–HeNPs/Au). No obvious reduction peak was observed with only a polariza-
Fig. 3. Amperometric response of Chi–HeNTs/Au electrode with successive addition of H2O2 at 0.45 V vs. SCE. Inset A: linear relationship between catalytic current of H2O2 and its concentration. Inset B: amplification of curve B. Electrolyte: deoxygenized 0.1 M PBS (pH 7.4).
tion peak appeared for the reduction of H2O2 at Chi–HeNPs/Au. Undoubtedly, this distinct response for the reduction of H2O2 at Chi–HeNTs/Au can be attributed to the subtle microstructure of HeNTs entrapped in the film. For conventional film-type sensors based on HeNPs, the surface-to-volume ratio is relatively low as a result of large grain sizes. It has been reported that there exists a network of interconnected pores in the sensor films made by SnO2 nanoribbons [27], SnO2 nanowires [28], Fe2O3 nanotubes and nanorings [29,17]. These network of pores are favored for sensing applications with enhanced sensitivity. Thus, owing to their high surface areas, the central hollow cores/outside walls and the network of pores interconnected, HeNTs possess abundant active sites for
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the reduction of H2O2 and dramatically facilitate the reduction process of H2O2. This is very similar to the previous results for a-Fe2O3 nanotubes in electrochemical and gas sensors [29,30]. To examine the sensitivity of the as-prepared Chi– HeNTs/Au electrode, we investigated the amperometric response of H2O2 in the stirring pH 7.4 PBS solution. The applied potential was set at 0.45 V vs. SCE, where the amperometric response reaches the maximum value. Fig. 3 shows the typical current–time curves for the Chi– HeNTs/Au electrode with a successive addition of H2O2. Upon the addition of H2O2, the Chi–HeNTs/Au reaches the maximum steady-state response within 4 s, indicating a very rapid amperometric response. The linear relationship was obtained in the concentration range from 1 · 10 6 to 1.6 · 10 5 mol L 1 with a correlation coefficient of 0.9995 (inset A of Fig. 3) and a slope of 14.6 lA mM 1. The sensitivity is estimated to be 1859 lA mM 1 cm 2, which is higher than 773 lA mM 1 cm 2 at a silver-DNA hybrid nanoparticle modified electrode [16], and also higher than 742 mA mM 1 cm 2 at a PB complex membrane modified electrode [15]. The detection limit calculated for S/N of 3 is 5 · 10 8 mol L 1. Some coexisting interfering compounds in biological samples were used to evaluate the selectivity of the composite Chi–HeNTs film. Those coexisting oxidizable interfering compounds in biological fluids such as ascorbic acid, uric acid, and glucose brought negligible response in the amperometric detection of H2O2 (data not shown). 4. Conclusions In summary, we first successfully fabricated a composite film of Chi–HeNTs onto Au by a facile one-step electrodeposition approach and explored its sensing properties for H2O2. The composite Chi–HeNTs/Au film displays a strong catalytic reduction effect for H2O2, with a high sensitivity of 1859 lA lM 1 cm 2. Furthermore, it effectively eliminates the interference from those oxidizable compounds in biological fluids. Obviously, Chi–HeNTs composite film provides an accurate, specific and functional recognition surface for the detection of H2O2. We believe that this functional Chi–HeNTs/Au composite electrode is very attractive in the fields of catalysis, bio-electrochemistry and diagnostics. Such an easy one-step electrodeposition method can also be extended to construct other nanostructured functional surfaces.
References [1] P. Westbroek, E. Temmerman, P. Kiekens, Anal. Chim. Acta 385 (1999) 423. [2] L. Campanella, R. Roversi, M.P. Sammartino, M. Tomassetti, J. Pharm. Biomed. Anal. 18 (1998) 105. [3] C. Malitesta, F. Palmisano, L. Torsi, P. Zambonin, Anal. Chem. 62 (1990) 2735. [4] C. Lei, J. Deng, Anal. Chem. 68 (1996) 3344. [5] B. Wang, B. Li, Z. Wang, G. Xu, Q. Wang, S. Dong, Anal. Chem. 71 (1999) 1935. [6] J.M. Gong, X.Q. Lin, Microchem. J. 75 (2003) 51. [7] T. Gu, Y. Hasebe, Anal. Chim. Acta 525 (2004) 191. [8] C. Camacho, J.C. Matı´as, D. Garcı´a, B.K. Simpson, R. Villalonga, Electrochem. Commun. 9 (2007) 1655. [9] A.A. Karyakin, O.V. Gitelmacher, E.E. Karyakina, Anal. Chem. 67 (1995) 2419. [10] M.P. O’Halloran, M. Pravda, G.G. Guibault, Talanta 55 (2001) 605. [11] I.L. Mattors, L. Gorton, T. Ruzgas, A.A. Karyakin, Anal. Sci. 16 (2000) 795. [12] A.A. Karyakin, E.A. Puganova, I.A. Budashov, I.N. Kurochkin, E.E. Karyakina, V.A. Levchenko, V.N. Matveyenko, S.D. Varfolomeyev, Anal. Chem. 76 (2004) 474. [13] Y.Z. Xian, Y. Hu, F. Liu, Y. Xian, L.J. Feng, L.T. Jin, Biosens. Bioelectron. 22 (2007) 2827. [14] E. Bustos, J. Manri’quez, G. Orozco, L.A. Godinez, Langmuir 21 (2005) 3013. [15] S.G. Wu, T.L. Wang, C.Q. Wang, Z.Y. Gao, C.Q. Wang, Electroanalysis 19 (2007) 659. [16] S. Wu, H.T. Zhao, H.X. Ju, C.G. Shi, J.W. Zhao, Electrochem. Commun. 8 (2006) 1197. [17] X.L. Hu, J.C. Yu, J.M. Gong, Q. Li, G.S. Li, Adv. Mater. 19 (2007) 2324. [18] C.J. Jia, L.D. Sun, Z.G. Yan, L.P. You, F. Luo, X.D. Han, Y.C. Pang, Z. Zhang, C.H. Yan, Angew. Chem. Int. Ed. 44 (2005) 4328. [19] Z.Y. Sun, H.Q. Yuan, Z.M. Liu, B.X. Han, X.R. Zhang, Adv. Mater. 17 (2005) 2993. [20] J. Chen, X.L. Xu, W.Y. Li, X.L. Gou, Adv. Mater. 17 (2005) 582. [21] X.L. Luo, J.J. Xu, J.L. Wang, H.Y. Chen, Chem. Commun. (2005) 2169. [22] L.Q. Wu, A.P. Gadre, H.M. Yi, M.J. Kastantin, G.W. Rubloff, W.E. Bentley, G.F. Payne, R. Ghodssi, Langmuir 18 (2002) 8620. [23] Y.P. Shan, G.C. Yang, Y.T. Jia, J. Gong, Z.M. Su, L.Y. Qu, Electrochem. Commun. 9 (2007) 2224. [24] F.M. John, F.S. William, E.S. Peter, D.B. Kenneth, Handbook of Xray Photoelectron Spectroscopy, 1992. [25] K.J. Mckenzie, F. Marken, Pure Appl. Chem. 73 (2001) 1885. [26] F. Marken, D. Patel, C.E. Madden, R.C. Millward, S. Fletcher, New J. Chem. 26 (2002) 259. [27] M. Law, H. Kind, B. Messer, F. Kim, P. Yang, Angew. Chem. Int. Ed. 41 (2002) 2405. [28] Y. Wang, X. Jiang, Y. Xia, J. Am. Chem. Soc. 125 (2003) 16176. [29] J. Chen, L. Xu, W.Y. Li, X.L. Gou, Adv. Mater. 17 (2005) 582. [30] Z.Y. Sun, H.Q. Yuan, Z.M. Liu, B.X. Han, X.R. Zhang, Adv. Mater. 17 (2005) 2993.
Electrochemistry Communications 10 (2008) 123–126 www.elsevier.com/locate/elecom
One-step fabrication of chitosan–hematite nanotubes composite film and its biosensing for hydrogen peroxide Jingming Gong *, Lianyi Wang, Kun Zhao, Dandan Song
*
Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China Received 29 September 2007; received in revised form 24 October 2007; accepted 25 October 2007 Available online 1 November 2007
Abstract This work reports on a novel chitosan–hematite nanotubes composite film on a gold foil by a simple one-step electrodeposition method. The hybrid chitosan–hematite nanotubes (Chi–HeNTs) film exhibits strong electrocatalytic reduction activity for H2O2. Interestingly, two electrocatalytic reduction peaks are observed at 0.24 and 0.56 V (vs SCE), respectively, one controlled by surface wave and the other controlled by diffusion process. The Chi–HeNTs/Au electrode shows a linear response to H2O2 concentration ranging from 1 · 10 6 to 1.6 · 10 5 mol L 1 with a detection limit of 5 · 10 8 mol L 1 and a sensitivity as high as 1859 lA lM 1 cm 2. 2007 Elsevier B.V. All rights reserved. Keywords: Hematite nanotubes; Chitosan; Hydrogen peroxide; One-step electrodeposition; Biosensing
1. Introduction Detection of hydroxyl peroxide (H2O2) has received tremendous attention in sensor research for decades. So far, many techniques have been used to chemical, biological, clinical, environmental, and many other impressive applications [1,2]. Among them, amperometric detection is a most promising approach to achieve accurate, separate, and rapid H2O2 monitoring. However, direct electrochemical oxidation of H2O2 is usually accomplished at an applied potential of 0.6 V vs. Ag/AgCl [3]. At such a potential, oxidizable compounds (e.g., ascorbate, urate, etc.) that exist in biological fluids often severely interfere with the analysis. In recent years, enzyme (e.g., peroxidase and cytochrome C) electrodes have been fabricated for H2O2 detection with simplicity, high sensitivity, and selectivity [4–8], but their operational conditions are generally limited by the denatruation of enzymes. Prussian blue *
Corresponding authors. Tel./fax: +86 27 67867535. E-mail addresses: [email protected] (J. Gong), ddsong@ mail.ccnu.edu.cn (D. Song). 1388-2481/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.10.017
(PB) and its analogs have been explored in a variety of H2O2 determination [9–13]. Recently, supramolecular complex dentrimers-PB membranes and silver-DNA hybrid films have been developed as catalytic layers to detect H2O2 [14–16]. Despite these advances, it is still a great challenge to construct highly selective, sensitive, and stable biorecognition interfaces for determination of H2O2. In this work, we constructed a novel functional hybrid film of chitosan–hematite nanotubes on a gold electrode (labeled as Chi–HeNTs/Au) as a catalytic layer for H2O2 detection, taking advantage of hematite nanotubes as a catalyst and chitosan as an immobilization matrix. Hematite (a-Fe2O3), the most stable iron oxide under ambient conditions, is widely used in catalysts, pigments, and sensors [17]. In particular, tubelike a-Fe2O3 has been recently demonstrated to show novel properties with central hollow cores/outside walls and high surface areas [18–20]. Chitosan, a natural and biocompatible polymer, has been widely selected as the immobilization matrix for catalysts [21–23]. To the best of our knowledge, this work first reports on the fabrication of hybrid films composed of tubelike Fe2O3 nanostructures and biocompatible chitosan. Importantly,
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J. Gong et al. / Electrochemistry Communications 10 (2008) 123–126
hematite nanotubes (HeNTs) entrapped in the composite film show strong electrocatalytic activity for reduction of H2O2. We believe that such a hybrid film is potentially useful in bio-electrochemistry and diagnostics applications.
electrode. For comparison, hematite nanoparticles-chitosan nanocomposite film was also fabricated (labeled as Chi–HeNPs/Au) by a similar one-step electrodeposition approach.
2. Experimental
3. Results and discussion
Chitosan from crab shells (85% deacetylated) was purchased from Sigma. Chitosan solution was prepared by mixing chitosan flakes with water and gradually dropping HCl solution to the mixture to maintain the pH near 3. After removal of the undissolved part by a 0.45-lm syringe filter unit, its pH value was adjusted to about 5.0 using an aqueous NaOH solution. Hematite nanotubes (HeNTs) and nanoparticles (HeNPs) were prepared according to the previous literature [18]. Typically, hematite products were obtained by a hydrothermal treatment of a FeCl3 solution (0.02 M) in the presence of NH4H2PO4 (7.2 · 10 4 M) at 220 C. After 2 h, the spindle-like HeNPs were formed with a diameter of 70 nm and a length of 400 nm. When further prolonging the reaction time to 48 h, nanotubes were obtained. All other reagents were of analytical grade and were used without further purification. A CHI 660A electrochemical workstation (CHI, USA) was used for the electrochemical measurements. A threeelectrode system was used, which was composed of a saturated calomel reference electrode (SCE), a platinum coil counter electrode, and a modified or unmodified gold electrode (Au, Ø 1 mm) as the working electrode. All solutions were deaerated efficiently with nitrogen prior to the electrochemical experiments unless otherwise stated and N2 atmosphere was maintained over the solution during the experiments. The general morphology of the products was characterized by scanning electron microscopy (SEM, LEO, 1450VP). XPS measurement was performed on PHI 5600 (Physical Electronics, USA). Prior to modification, the bulk Au disk electrode was successively polished by 1.0, 0.3 and 0.05 lm alumina slurry, followed by the sonication in distilled water and ethanol for 5 min each. The as-prepared hematite nanotubes were dispersed into chitosan solution by sonication. Then, the electrode was immersed in the 1.5 mg/mL hematite nanotubes/chitosan dispersions by controlling the potential at 3.0 V vs SCE for 4 min. This concentration of 1.5 mg/ml is an optimized value and it exhibits the maximum value for the response of H2O2. H+ in the solution was reduced to H2 at Au electrode, and the pH near Au electrode surface gradually increased. The solubility of chitosan is pH-dependent. When the pH exceeds the pKa of chitosan (about 6.3), chitosan becomes insoluble [22] in water, and therefore the chitosan entrapped HeNTs will deposit onto the Au electrode surface. In addition, owing to the presence of hydroxyl and amino groups in chitosan, the chemical bonding possibly occurs between chitosan and Au substrate, which might further prompts the chitosan entrapped HeNTs deposited onto the gold
Fig. 1a and b shows the typical SEM images of the asprepared hematite nanotubes at different magnifications. It is observed that the hematite nanotubes have diameters of 80 nm and lengths up to 400 nm. Fig. 1c shows the SEM image for the composite film of Chi–HeNTs. Apparently, the hematite tubes are homogenously distributed in the chitosan matrix, indicating that hematite nanotubes indeed have been entrapped in chitosan. Fig. 1d shows the survey XPS spectrum of the Chi–HeNTs composite film. In the high-resolution Fe 2p spectrum (inset of Fig. 1d), two distinct peaks at binding energies of 725.5 eV for Fe 2p1/2 and 711.5 eV for Fe 2p3/2 with a shake-up satellite are observed. This is characteristic of Fe3+ in Fe2O3 [24]. These results confirm that Chi–HeNTs nanocomposite film has been formed onto the Au foil through the simple one-step electrodeposition process. Fig. 2 shows typical CVs of H2O2 at Chi–HeNTs/Au in the absence and presence of H2O2, respectively, at the scan rate of 50 mV/s in deoxygenized 0.1 M pH 7.4 PBS. The bare Au electrode shows no obvious voltammetric peaks within the potential range in 0.1 M PBS pH 7.4 (Fig. 2a). In comparison, Chi–HeNTs composite film modified Au gives a well-defined redox peak with an average formal potential (Em = (Epa + Epc)/2) of 0.20 V (vs. SCE). The cathodic and anodic peaks appear at 0.240 and 0.155 V vs. SCE, respectively (Fig. 2b). This agrees well with the previous results reported on the direct electron transfer of nanostructured Fe2O3 onto ITO [25,26]. The redox peaks can be ascribed to the redox reaction of a-Fe2O3 nanotubes entrapped in the chitosan composite film. With the addition of H2O2 into solution, the cathodic peak was enhanced greatly at ca. 0.240 V vs. SCE (Er1), and gradually increased with the further addition of H2O2. Meanwhile, the anodic peak current at E = 0.155 V decreased, and disappeared finally, showing characteristics of an electrocatalytic reduction of H2O2 at Chi–HeNTs/Au. Apparently, HeNTs entrapped in the composite film effectively mediate the reduction of H2O2. Very interestingly, in the cathodic process, another reduction peak appears at ca. 0.55 V vs. SCE (Er2), also gradually increased with the addition of H2O2. The CV curves of 0.23 mM H2O2 at bare Au and Chi–HeNTs/Au electrodes are shown in the inset of Fig. 3. The response at bare gold electrode is barely detectable, except the cathodic polarization of H2O2. In comparison, H2O2 exhibits a strong catalytic reduction behavior at the Chi–HeNTs/Au electrode, with two reduction peaks of 0.24 and 0.55 V (vs. SCE). Obviously, the presence of Chi–HeNTs composite film plays a crucial role in the reduction of H2O2.
J. Gong et al. / Electrochemistry Communications 10 (2008) 123–126
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Fig. 1. SEM images of (a, b) hematite nanotubes and (c) Chi–HeNTs nanocomposite film onto a gold foil. (d) XPS spectra of the Chi–HeNTs nanocomposite film onto Au foil. Inset shows the high-resolution spectrum of Fe 2p.
Fig. 2. Cyclic voltammograms of (a) bare Au, (b) the Chi–HeNTs/Au in the absence of H2O2 and presence of (c) 0.03 mM H2O2, (d) 0.13 mM H2O2, (e) 0.23 mM H2O2. Inset: CVs of 0.23 mM H2O2 at (a) bare Au electrode and (b) Chi–HeNTs/Au electrode. Electrolyte: deoxygenized 0.1 M PBS (pH 7.4), Scan rate, 50 mV/s.
Regarding the reduction peaks of H2O2 at Chi–HeNTs/ Au, the effect of potential scan rate was investigated clearly. For Er1, a plot of peak current against the scan rate is linear over the range of 10–100 mV/s with a linear regression equation, ipa/lA = 0.481 + 3.764v/V s 1 (r = 0.9931), revealing a surface wave. For Er2, the peak current is proportional to the square root of scan rate in the range of 5– 200 mV/s, ipa/lA = 1.494 34.61v1/2/V s 1, r = 0.9920, indicating a diffusion controlled process. For comparison, hematite nanoparticles (HeNPs)-chitosan nanocomposite film onto Au was also fabricated (Chi–HeNPs/Au). No obvious reduction peak was observed with only a polariza-
Fig. 3. Amperometric response of Chi–HeNTs/Au electrode with successive addition of H2O2 at 0.45 V vs. SCE. Inset A: linear relationship between catalytic current of H2O2 and its concentration. Inset B: amplification of curve B. Electrolyte: deoxygenized 0.1 M PBS (pH 7.4).
tion peak appeared for the reduction of H2O2 at Chi–HeNPs/Au. Undoubtedly, this distinct response for the reduction of H2O2 at Chi–HeNTs/Au can be attributed to the subtle microstructure of HeNTs entrapped in the film. For conventional film-type sensors based on HeNPs, the surface-to-volume ratio is relatively low as a result of large grain sizes. It has been reported that there exists a network of interconnected pores in the sensor films made by SnO2 nanoribbons [27], SnO2 nanowires [28], Fe2O3 nanotubes and nanorings [29,17]. These network of pores are favored for sensing applications with enhanced sensitivity. Thus, owing to their high surface areas, the central hollow cores/outside walls and the network of pores interconnected, HeNTs possess abundant active sites for
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the reduction of H2O2 and dramatically facilitate the reduction process of H2O2. This is very similar to the previous results for a-Fe2O3 nanotubes in electrochemical and gas sensors [29,30]. To examine the sensitivity of the as-prepared Chi– HeNTs/Au electrode, we investigated the amperometric response of H2O2 in the stirring pH 7.4 PBS solution. The applied potential was set at 0.45 V vs. SCE, where the amperometric response reaches the maximum value. Fig. 3 shows the typical current–time curves for the Chi– HeNTs/Au electrode with a successive addition of H2O2. Upon the addition of H2O2, the Chi–HeNTs/Au reaches the maximum steady-state response within 4 s, indicating a very rapid amperometric response. The linear relationship was obtained in the concentration range from 1 · 10 6 to 1.6 · 10 5 mol L 1 with a correlation coefficient of 0.9995 (inset A of Fig. 3) and a slope of 14.6 lA mM 1. The sensitivity is estimated to be 1859 lA mM 1 cm 2, which is higher than 773 lA mM 1 cm 2 at a silver-DNA hybrid nanoparticle modified electrode [16], and also higher than 742 mA mM 1 cm 2 at a PB complex membrane modified electrode [15]. The detection limit calculated for S/N of 3 is 5 · 10 8 mol L 1. Some coexisting interfering compounds in biological samples were used to evaluate the selectivity of the composite Chi–HeNTs film. Those coexisting oxidizable interfering compounds in biological fluids such as ascorbic acid, uric acid, and glucose brought negligible response in the amperometric detection of H2O2 (data not shown). 4. Conclusions In summary, we first successfully fabricated a composite film of Chi–HeNTs onto Au by a facile one-step electrodeposition approach and explored its sensing properties for H2O2. The composite Chi–HeNTs/Au film displays a strong catalytic reduction effect for H2O2, with a high sensitivity of 1859 lA lM 1 cm 2. Furthermore, it effectively eliminates the interference from those oxidizable compounds in biological fluids. Obviously, Chi–HeNTs composite film provides an accurate, specific and functional recognition surface for the detection of H2O2. We believe that this functional Chi–HeNTs/Au composite electrode is very attractive in the fields of catalysis, bio-electrochemistry and diagnostics. Such an easy one-step electrodeposition method can also be extended to construct other nanostructured functional surfaces.
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