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Novel estradiol sensors based on carbon nanotube multilayer modified gold hair microelectrodes
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Chinese Chemical Letters 20 (2009) 1248–1250 www.elsevier.com/locate/cclet
Novel estradiol sensors based on carbon nanotube multilayer modified gold hair microelectrodes Jun Hui Xu a,b, Cheng Guo Hu a,*, Sheng Shui Hu a,b,* a
b
Department of Chemistry, Wuhan University, Wuhan 430072, China State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing 100080, China Received 3 February 2009
Abstract Multi-walled carbon nanotube multilayers were modified onto a newly proposed gold hair microelectrode via a simple layer-bylayer assembling method. The resulting electrode showed a sensitive oxidation response to estradiol with detection limit as low as 1.0 10 8 mol/L, foreseeing a promising approach to the fabrication of high-sensitive microsensors. # 2009 Sheng Shui Hu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Layer-by-layer assembly; Carbon nanotubes; Hair microsensors; Estradiol
Since the introduction by Decher [1], layer-by-layer (LBL) assembly has been proven to be a simple but powerful approach for constructing functional thin films, and is widely applied to the fabrication of highly selective and sensitive electrochemical sensors [2–4]. In most cases, the assembling process was carried out on planar conductive substrates, including glassy carbon, pyrolytic graphite, indium tin oxide or gold-sputtered matrix, which is initiated by the deposition of charged polyelectrolytes on these substrates and followed by oppositely charged species. However, rare works focused on the LBL assembly of functional materials on microelectrodes, an important type electrode platform possessing extensive applications in the field of electroanalysis, for instance, in vivo monitoring, microscopic exploration and microanalysis. Therefore, the surface functionalization of microelectrodes by LBL methods seems attractive for developing high-performance electrochemical microsensors. In our previous work, we proposed a simple chemical liquid deposition method for fabricating novel gold hair microelectrodes (GHME) based on a seeding-growth strategy [5]. Different from conventional carbon fiber or noble metal microelectrodes, GHME possesses a rough conductive surface comprising continuous gold nanoparticles, which favors the binding of functional polymers, especially polyelectrolytes containing abundant amino groups. In this work, we reported a routine method for the LBL assembling of multi-walled carbon nanotubes (MWNTs) on GHME by using polyethyleneimine as the crosslinker. The resulting microelectrode was successfully applied to the sensitive determination of estradiol, demonstrating a promising approach for functional electrochemical microsensors.
* Corresponding authors. E-mail addresses: [email protected] (C.G. Hu), [email protected] (S.S. Hu). 1001-8417/$ – see front matter # 2009 Sheng Shui Hu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2009.04.019
J.H. Xu et al. / Chinese Chemical Letters 20 (2009) 1248–1250
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The preparation of polyacrylic acid (PAA) functionalized MWNTs were modified from a previous literature [6]. Briefly, 50 mg as-received MWNTs (20–30 nm in diameter, 95%, Nanotimes Co., Chengdu, China) were dispersed into 500 mL 0.5 wt% PAA (Mw = 100,000, Acros) under ultrasonication. The resulting black suspension was centrifuged at 4000 rpm for 10 min. The supernatant was collected and filtrated with a disc membrane filter (pore size 200 nm). The black filtering cake was re-suspended in 100 mL water to form a PAA-MWNT solution of 0.5 mg/mL. Then, GHME (prepared according to our previous work [5]) was dipped in turn into 0.1% polyethyleneimine (PEI, Mw = 60,000, 50%, Acros) and PAA-MWNT solution, each for 20 min. This step was repeated for eight times to obtain an 8-layer MWNT film. After each assembly, GHME was washed with water and dried under nitrogen stream. The resulting electrode was denoted as MWNT/GHME. Electrochemical experiments were carried out with a CHI 660B electrochemical analyzer (Shanghai Chenhua Co., China). The surface morphology of GHMEs was characterized by SEM. It is clear that bare GHME possesses a rather rough surface comprising continuous gold nanoparticles (inset in Fig. 1A). These gold nanoparticles provide abundant active sites for the strong adsorption of PEI to form a positively charged initial layer, on which a negatively charged PAAMWNT layer could be further assembled via LBL methods. Thus, a homogeneous and dense three-dimensional network is formed on the GHME surface after the assembling of MWNTs (Fig. 1A). Fig. 1B shows cyclic voltammograms (CV) of 1.0 mmol/L K3Fe(CN)6 at different GHMEs. The bare GHME exhibits a pair of well-defined waves (dotted line). At MWNT/GHME, K3Fe(CN)6 shows a similar curve (solid line) except for a much higher background and more sensitive redox peak currents, demonstrating a larger surface area of MWNT/GHME compared with GHME. Moreover, the peak separation of K3Fe(CN)6 at MWNT/GHME is very small (0.080 V), revealing a fast electron transfer rate. Fig. 2A depicts the voltammetric response at MWNT/GHME in 1/15 mol/L phosphate buffer (PB, pH 7) in the absence and the presence of estradiol. In the potential range of 0–0.7 V, no redox peak is observed (dotted line) in blank PB after an open-circuit accumulation for 180 s. However, with the addition of 1.0 10 6 mol/L estradiol, a welldefined oxidation peak with high sensitivity is observed at 0.58 V (solid line). There is no corresponding reduction peak existing in the reverse potential scan, suggesting that the oxidation of estradiol is totally irreversible. In the following sweeping cycles, the oxidation peak completely disappears and a pair of new redox waves at about 0.1 V are observed, which are attributed to the electrochemical response of the oxidation product of estradiol. These results suggest the strong adsorption of estradiol and its product on the surface of MWNT/GHME. Since there is no apparent oxidation peak in the case of bare GHME (dashed line), the remarkable oxidation current of estradiol at MWNT/ GHME is essentially attributed to the unique characteristics of MWNTs. That is to say, MWNTs assembled on GHME exhibit excellent electrocatalytic activity for the oxidation of estradiol. The calibration curve of estradiol at MWNT/GHME under optimal working conditions is investigated by using linear sweep voltammetry (LSV) (Fig. 2B). The oxidation peak current is linearly related to the concentration of estradiol in the range of 2.0 10 8 to 2.0 10 6 mol/L (R = 0.9989) (inset of Fig. 2B) with a low detection limit of 1.0 10 8 mol/L. The relative standard deviation (RSD) for six parallel detections of 5.0 10 7 mol/L estradiol was
Fig. 1. (A) SEM of GHME (inset) and MWNT layers assembled on GHME; (B) CVof 1.0 mmol/L K3Fe(CN)6 at GHME (dotted line) and MWNT/ GHME (solid line).
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J.H. Xu et al. / Chinese Chemical Letters 20 (2009) 1248–1250
Fig. 2. (A) CV of 1.0 10 6 mol/L estradiol at bare GHME (dashed line) and MWNT/GHME (solid line); the dotted line shows the CV obtained at MWNT/GHME in blank solution; (B) LSV of estradiol at MWNT/GHME: (a) 0, (b) 0.02 10 6, (c) 0.05 10 6, (d) 0.1 10 6, (e) 0.2 10 6, (f) 0.5 10 6, (g) 1.0 10 6 and (h) 2.0 10 6 mol/L; the inset shows the plot of ip vs. the concentration of estradiol; scan rate, 0.05 V/s.
calculated as 4.8%, suggesting a good reproducibility. MWNT/GHME remained 94% of its initial current response when used daily for 1 week, indicating its good stability for long-term use. Interferences from several biomolecules on the determination of 5.0 10 7 mol/L estradiol were investigated. The results indicated that 100-fold glucose and 20fold of urea acid, ascorbic acid and dopamine showed no apparent influences, probably due to the much different oxidation potentials of these species in comparison with estradiol. Therefore, this estradiol sensor possessed fairly good analytical performances, demonstrating a promising routine for fabricating sensitive microsensors by LBL methods. Acknowledgments This research is supported by National Natural Science Foundation of China (Nos. 20805035 and 90817103). References [1] [2] [3] [4] [5] [6]
G. Decher, J.D. Hong, Macromol. Chem. Macromol. Symp. 46 (1990) 321. W. Zhao, J. Xu, H. Chen, Electroanalysis 18 (2006) 1737. J. Lutkenhaus, P. Hammond, Soft Matter 3 (2007) 804. D. Chen, G. Wang, J. Li, J. Phys. Chem. C 111 (2007) 2351. J. Xu, C. Hu, Y. Ji, S. Hu, Electrochem. Commun. 11 (2009) 764. A. Liu, T. Watanabe, I. Honma, J. Wang, H. Zhou, Biosens. Bioelectron. 15 (2006) 694.
Chinese Chemical Letters 20 (2009) 1248–1250 www.elsevier.com/locate/cclet
Novel estradiol sensors based on carbon nanotube multilayer modified gold hair microelectrodes Jun Hui Xu a,b, Cheng Guo Hu a,*, Sheng Shui Hu a,b,* a
b
Department of Chemistry, Wuhan University, Wuhan 430072, China State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Beijing 100080, China Received 3 February 2009
Abstract Multi-walled carbon nanotube multilayers were modified onto a newly proposed gold hair microelectrode via a simple layer-bylayer assembling method. The resulting electrode showed a sensitive oxidation response to estradiol with detection limit as low as 1.0 10 8 mol/L, foreseeing a promising approach to the fabrication of high-sensitive microsensors. # 2009 Sheng Shui Hu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Layer-by-layer assembly; Carbon nanotubes; Hair microsensors; Estradiol
Since the introduction by Decher [1], layer-by-layer (LBL) assembly has been proven to be a simple but powerful approach for constructing functional thin films, and is widely applied to the fabrication of highly selective and sensitive electrochemical sensors [2–4]. In most cases, the assembling process was carried out on planar conductive substrates, including glassy carbon, pyrolytic graphite, indium tin oxide or gold-sputtered matrix, which is initiated by the deposition of charged polyelectrolytes on these substrates and followed by oppositely charged species. However, rare works focused on the LBL assembly of functional materials on microelectrodes, an important type electrode platform possessing extensive applications in the field of electroanalysis, for instance, in vivo monitoring, microscopic exploration and microanalysis. Therefore, the surface functionalization of microelectrodes by LBL methods seems attractive for developing high-performance electrochemical microsensors. In our previous work, we proposed a simple chemical liquid deposition method for fabricating novel gold hair microelectrodes (GHME) based on a seeding-growth strategy [5]. Different from conventional carbon fiber or noble metal microelectrodes, GHME possesses a rough conductive surface comprising continuous gold nanoparticles, which favors the binding of functional polymers, especially polyelectrolytes containing abundant amino groups. In this work, we reported a routine method for the LBL assembling of multi-walled carbon nanotubes (MWNTs) on GHME by using polyethyleneimine as the crosslinker. The resulting microelectrode was successfully applied to the sensitive determination of estradiol, demonstrating a promising approach for functional electrochemical microsensors.
* Corresponding authors. E-mail addresses: [email protected] (C.G. Hu), [email protected] (S.S. Hu). 1001-8417/$ – see front matter # 2009 Sheng Shui Hu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2009.04.019
J.H. Xu et al. / Chinese Chemical Letters 20 (2009) 1248–1250
1249
The preparation of polyacrylic acid (PAA) functionalized MWNTs were modified from a previous literature [6]. Briefly, 50 mg as-received MWNTs (20–30 nm in diameter, 95%, Nanotimes Co., Chengdu, China) were dispersed into 500 mL 0.5 wt% PAA (Mw = 100,000, Acros) under ultrasonication. The resulting black suspension was centrifuged at 4000 rpm for 10 min. The supernatant was collected and filtrated with a disc membrane filter (pore size 200 nm). The black filtering cake was re-suspended in 100 mL water to form a PAA-MWNT solution of 0.5 mg/mL. Then, GHME (prepared according to our previous work [5]) was dipped in turn into 0.1% polyethyleneimine (PEI, Mw = 60,000, 50%, Acros) and PAA-MWNT solution, each for 20 min. This step was repeated for eight times to obtain an 8-layer MWNT film. After each assembly, GHME was washed with water and dried under nitrogen stream. The resulting electrode was denoted as MWNT/GHME. Electrochemical experiments were carried out with a CHI 660B electrochemical analyzer (Shanghai Chenhua Co., China). The surface morphology of GHMEs was characterized by SEM. It is clear that bare GHME possesses a rather rough surface comprising continuous gold nanoparticles (inset in Fig. 1A). These gold nanoparticles provide abundant active sites for the strong adsorption of PEI to form a positively charged initial layer, on which a negatively charged PAAMWNT layer could be further assembled via LBL methods. Thus, a homogeneous and dense three-dimensional network is formed on the GHME surface after the assembling of MWNTs (Fig. 1A). Fig. 1B shows cyclic voltammograms (CV) of 1.0 mmol/L K3Fe(CN)6 at different GHMEs. The bare GHME exhibits a pair of well-defined waves (dotted line). At MWNT/GHME, K3Fe(CN)6 shows a similar curve (solid line) except for a much higher background and more sensitive redox peak currents, demonstrating a larger surface area of MWNT/GHME compared with GHME. Moreover, the peak separation of K3Fe(CN)6 at MWNT/GHME is very small (0.080 V), revealing a fast electron transfer rate. Fig. 2A depicts the voltammetric response at MWNT/GHME in 1/15 mol/L phosphate buffer (PB, pH 7) in the absence and the presence of estradiol. In the potential range of 0–0.7 V, no redox peak is observed (dotted line) in blank PB after an open-circuit accumulation for 180 s. However, with the addition of 1.0 10 6 mol/L estradiol, a welldefined oxidation peak with high sensitivity is observed at 0.58 V (solid line). There is no corresponding reduction peak existing in the reverse potential scan, suggesting that the oxidation of estradiol is totally irreversible. In the following sweeping cycles, the oxidation peak completely disappears and a pair of new redox waves at about 0.1 V are observed, which are attributed to the electrochemical response of the oxidation product of estradiol. These results suggest the strong adsorption of estradiol and its product on the surface of MWNT/GHME. Since there is no apparent oxidation peak in the case of bare GHME (dashed line), the remarkable oxidation current of estradiol at MWNT/ GHME is essentially attributed to the unique characteristics of MWNTs. That is to say, MWNTs assembled on GHME exhibit excellent electrocatalytic activity for the oxidation of estradiol. The calibration curve of estradiol at MWNT/GHME under optimal working conditions is investigated by using linear sweep voltammetry (LSV) (Fig. 2B). The oxidation peak current is linearly related to the concentration of estradiol in the range of 2.0 10 8 to 2.0 10 6 mol/L (R = 0.9989) (inset of Fig. 2B) with a low detection limit of 1.0 10 8 mol/L. The relative standard deviation (RSD) for six parallel detections of 5.0 10 7 mol/L estradiol was
Fig. 1. (A) SEM of GHME (inset) and MWNT layers assembled on GHME; (B) CVof 1.0 mmol/L K3Fe(CN)6 at GHME (dotted line) and MWNT/ GHME (solid line).
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J.H. Xu et al. / Chinese Chemical Letters 20 (2009) 1248–1250
Fig. 2. (A) CV of 1.0 10 6 mol/L estradiol at bare GHME (dashed line) and MWNT/GHME (solid line); the dotted line shows the CV obtained at MWNT/GHME in blank solution; (B) LSV of estradiol at MWNT/GHME: (a) 0, (b) 0.02 10 6, (c) 0.05 10 6, (d) 0.1 10 6, (e) 0.2 10 6, (f) 0.5 10 6, (g) 1.0 10 6 and (h) 2.0 10 6 mol/L; the inset shows the plot of ip vs. the concentration of estradiol; scan rate, 0.05 V/s.
calculated as 4.8%, suggesting a good reproducibility. MWNT/GHME remained 94% of its initial current response when used daily for 1 week, indicating its good stability for long-term use. Interferences from several biomolecules on the determination of 5.0 10 7 mol/L estradiol were investigated. The results indicated that 100-fold glucose and 20fold of urea acid, ascorbic acid and dopamine showed no apparent influences, probably due to the much different oxidation potentials of these species in comparison with estradiol. Therefore, this estradiol sensor possessed fairly good analytical performances, demonstrating a promising routine for fabricating sensitive microsensors by LBL methods. Acknowledgments This research is supported by National Natural Science Foundation of China (Nos. 20805035 and 90817103). References [1] [2] [3] [4] [5] [6]
G. Decher, J.D. Hong, Macromol. Chem. Macromol. Symp. 46 (1990) 321. W. Zhao, J. Xu, H. Chen, Electroanalysis 18 (2006) 1737. J. Lutkenhaus, P. Hammond, Soft Matter 3 (2007) 804. D. Chen, G. Wang, J. Li, J. Phys. Chem. C 111 (2007) 2351. J. Xu, C. Hu, Y. Ji, S. Hu, Electrochem. Commun. 11 (2009) 764. A. Liu, T. Watanabe, I. Honma, J. Wang, H. Zhou, Biosens. Bioelectron. 15 (2006) 694.