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nanohierarchical structures with superhydrophobicity
Materials Letters 78 (2012) 42–45
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
One step preparation of polyaniline micro/nanohierarchical structures with superhydrophobicity Haosen Fan, Ning Zhao ⁎, Hao Wang, Xiaofeng Li, Jian Xu ⁎ Beijing National Laboratory for Molecular Sciences, Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 20 December 2011 Accepted 13 March 2012 Available online 20 March 2012 Keywords: Hierarchical Microstructure Polyaniline Polymers Superhydrophobic
a b s t r a c t Polyaniline with a leaf-like hierarchical structure of 3 μm in length, 2 μm in width, 100 nm in thickness and crisscrossed nanofibers decorated on the surface, was successfully prepared in one step polymerization of aniline in the presence of sodium heptafluorobutyrate. The synthesized polyaniline shows superhydrophobicity with a water contact angle of 151° due to the combination of the low surface tension of fluorinated chain and the rough micro/nanostructure. Chemical structure and composition of the resultant PANI were characterized by FTIR, UV–vis, XRD and XPS. The formation mechanism of the hierarchical structure was investigated by monitoring the morphology development with the polymerization time. This method is facile and applicable for the large scale fabrication. Many potential applications can be expected based on the superhydrophobic PANI micro/nanostructures. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, micro/nanostructures of conducting polymers have been extensively investigated because of their important applications in organic conductors, actuators and electromagnetic interference shielding and so on [1]. Polyaniline (PANI) and its derivatives are important candidates among these conducting polymers due to their high conductivity and modifiable electrical properties by changing oxidation and protonation states [2,3]. Micro/nanostructures of PANI have been found in applications in molecular wires [4], chemical sensors [5], biosensors [6], artificial muscles [7], and dye-sensitized solar cells [8]. To date, a variety of synthetic approaches, such as interfacial polymerization [9], electrochemical method [10], hard-template [11], soft-template [12] and template-free method [13], have been developed. Hydrophobic modification of PANI microstructures has attracted considerable attention since surface wettability is one of the determinative factors for application [14]. Chiou and co-workers prepared superhydrophobic aligned PANI nanofibers on available substrates by chemical oxidative polymerization [15]. Zhu and co-workers reported the formation of superhydrophobic box-like PANI with the assistance of perfluorosebacic acid [16]. Herein, we demonstrated a one step polymerization for the large scale preparation of superhydrophobic leaf-like PANI in the presence of sodium heptafluorobutyrate (SHF). The hierarchical microstructure and the low surface tension of the fluorinated chain of SHF contribute to the superhydrophobicity. This method is ⁎ Corresponding authors. Fax: + 86 10 8261 9667. E-mail addresses: [email protected] (N. Zhao), [email protected] (J. Xu). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.053
facile and neither special instruments nor post treatment is needed. The structural characteristic, wettability, and formation mechanism of the leaf-like PANI are studied. This superhydrophobic hierarchically structured PANI may find many potential applications [5,6]. 2. Experimental section Aniline (Beijing Chemical Co.) was distilled under reduced pressure. Ammonium persulfate (APS, Sinopharm Chemical Reagent Co.), sodium heptafluorobutyrate (SHF, Aldrich), and other reagents were all A. R. grade and used as received without further treatment. A typical synthesis of the superhydrophobic leaf-like micro/ nanostructure of PANI was as follows: 92 μl aniline (1 mmol) and 0.236 g SHF (1 mmol) were dispersed in 50 ml of deionized water under magnetic stirring at room temperature for 5 min to obtain a uniform solution. Then aqueous solution of APS (1 mmol) was added to the above mixture in one portion. The resulting solution was stirred for 10 min to ensure complete mixing and then the reaction was allowed to proceed without agitation for 24 h at room temperature. Finally, the products were washed with water and methanol several times until the filtrate became colorless, and then dried in vacuum oven for 24 h. Morphology of the leaf-like PANI was examined by a Hitachi S4800 field emission scanning electron microscope (SEM) and a JEOL JEM2200FS transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectroscopy was recorded on a Bruker Equinox 55 spectrometer in the range of 400–4000 cm− 1. Ultraviolet and visible spectroscopy was measured in aqueous solution on a Shimadzu 1601PC UV–vis spectrophotometer. X-ray diffraction (XRD) pattern
H. Fan et al. / Materials Letters 78 (2012) 42–45
was recorded on a Rigaku D/max 2400 diffractometer using Cu–Kα (λ = 1.5418 Å) radiation (40 kV, 200 mA). X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALab220i-XL electron spectrometer from VG Scientific. Al–Kα radiation was used as the X-ray source and operated at 300 W. Contact angle (CA) was measured with a homemade instrument at ambient circumstance with a water drop of 5 μl. Reported data are the average of 3 different measurements. 3. Results and discussion Typical SEM and TEM images of the obtained PANI at the polymerization time of 24 h are presented in Fig. 1. The synthesized PANI shows a uniform leaf-like morphology of about 3 μm in length, 2 μm in width, and 100 nm in thickness (Fig. 1a and b). The magnified SEM image in Fig. 1c indicates that the PANI leaf is covered with interlaced nanofibers of tens of nanometer in diameter. The TEM image of the PANI (Fig. 1d) further confirms the formation of the highly crisscrossed nanofibers. This special microstructure endows the obtained PANI a hierarchical morphology in micro and nano-scale. Water CA on the surface constructed by the leaf-like PANI was about 151°, as shown in the inset of Fig. 1a. This superhydrophobic property can be ascribed to the hierarchical microstructure and the low-surface energy of the dopant of SHF which contains perfluorinated chain. FTIR spectrum of the leaf-like PANI is shown in Fig. 2a which is in good accord with previously reported result [17]. The weak absorption at 3056 cm − 1 is attributed to the N\H stretching vibration. The major strong peaks at 1580 and 1508 cm − 1 are ascribed to the C_C stretching vibration of the quinonoid and benzenoid rings, respectively, and the peak at 1285 cm − 1 is to the C\N stretching vibration of the benzene ring. The peak at 846 cm − 1 can be assigned to the out-of-plane vibration in the 1, 4-disubstituted benzene ring. The vibration bands at 1144 and 1246 cm − 1 ascribed to the symmetric and asymmetric CF2 stretches were also observed, indicating the existence of SHF in the resultant PANI. Fig. 2b shows the UV–vis spectra of the PANI dispersed in deionized water. The peak centered at 408 nm is identical to that of the emeraldine base form of PANI [18]. Fig. 2c exhibits the XRD pattern of the obtained PANI. The broad
43
peak centered at 2θ = 21° may be ascribed to the periodicity parallel to the polymer chains, indicating the resulted PANI is amorphous [19]. XPS (Fig. 2d) was studied to analyze the chemical elements on the surface. The survey spectrum shows three major peaks at 532, 401, and 285 eV that correspond to O1s, N1s, and C1s photoemission. The peak at 685 eV of F1s confirms the immigration of the low surface tension moieties on the surface. The formation mechanism of the hierarchical structure was studied by monitoring the morphology change of PANI with the polymerization time, as shown in Fig. 3. Small square plates with side lengths of about 800 nm and featureless surface are formed after 10 min of reaction (Fig. 3a), and many fragments are also observed. As the reaction time increased to 30 min (Fig. 3b), square plates grow larger with a side length of about 1–2 μm. The surface is till glossy. With the polymerization time prolonging to 1 h (Fig. 3c), leaf-like microstructure with flat surface formed with a length of about 2.5 μm, width of about 1.5 μm. As the polymerization time extended to 3 h (Fig. 3d), both the size and thickness of the leaf-like microstructures further increase, but quite close to the final products (Fig. 1a). Interlaced nanofibers began to appear on the leaf surface. The results demonstrated that the formation of the leaf like PANI follows a step by step process, the later polymerization of aniline happens on the periphery of the existing PANI nuclei. As the wettability of the surface is determined largely by the surface morphology, the contact angle of the surfaces shown in Fig. 3 changed greatly. As shown in Fig. 4, CA increases with extending of reaction time. The different micro/nanostructures of PANI show the CA of 84.7, 122.1, 134.3 and 142.8° at the reaction time of 10 min, 30 min, 1 h and 3 h, respectively. The obtained PANI has a low CA of 84.7°at the beginning of the polymerization. This is mainly due to the low-surface energy of SHF. As the reaction time extends to 3 h, the CA reaches 142.8°. It is clear that the change of the contact angle is in according with the increase of the surface roughness of PANI with the polymerization time. The high CA of resulting leaflike PANI is attributed to the low-surface energy of hydrophobic perfluorinated carbon in the SHF as well as the rough micro/nanostructures of PANI.
Fig. 1. SEM (a,b,c) and TEM (d) images of the synthesized PANI at the polymerization time of 24 h.
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H. Fan et al. / Materials Letters 78 (2012) 42–45
b
Intensity (a.u.)
Intensity (a.u.)
a
3500 3000 2500 2000 1500 1000 500
300
Wavenumber (cm-1)
c
400
500
600
700
800
Wavelength (nm)
d
Intensity (a.u.)
Intensity (a.u.)
C1s
10
20
30
40
50
2θ(degree)
O1s F1s N1s
1000
800
600
400
200
Binding Energy (eV)
Fig. 2. FTIR (a), UV–vis (b), XRD (c) and XPS (d) spectra of the as-formed PANI.
4. Conclusion In summary, polyaniline hierarchical micro/nanostructures have been successfully prepared by the polymerization of aniline in the presence of sodium heptafluorobutyrate. The as-formed PANI shows a uniform leaf-like morphology with an approximate length of 3 μm, width of 2 μm and thickness of 100 nm, and covered by crisscrossed nanofibers
on the surface. Due to the low surface tension of sodium heptafluorobutyrate and the hierarchical structure, the obtained PANI exhibits superhydrophobic behavior with water contact angle above 150°. The leaflike PANI micro/nanostructures start from square flat plates, which grow in size and then are covered by nanofibers on the surface with the polymerization time. This method is facile and effective for the fabrication of PANI micro/nanostructures with superhydrophobicity.
Fig. 3. SEM images of the PANI synthesized at different polymerization times: (a) 10 min; (b) 30 min; (c) 1 h and (d) 3 h.
H. Fan et al. / Materials Letters 78 (2012) 42–45
References
140
Contact angle (degree)
45
130 120 110 100 90 80 30
60
90
120
150
180
Time (min) Fig. 4. Contact angle of the PANI micro/nanostructures as a function of the polymerization time.
Acknowledgments This work was supported by NSFC (grant no. 50821062) and MOST (2009CB930400). We acknowledge Beijing Municipal Commission of Education for the special fund for the Disciplines & Postgraduate Education Construction project.
[1] Liang L, Liu J, Windisch CF, Exarhos GJ, Lin YH. Angew Chem Int Ed Engl 2002;41: 3665–8. [2] MacDiarmid AG. Angew Chem Int Ed Engl 2001;40:2581–90. [3] Kang ET, Neoh KG, Tan KL. Prog Polym Sci 1998;23:277–324. [4] Akai T, Shimomura T, Ito K. Synth Met 2003;135:777–8. [5] Huang JX, Virji S, Weiller BH, Kaner RB. J Am Chem Soc 2003;125:314–5. [6] Zhang LJ, Peng H, Kilmartin PA, Soeller C, Travas-Sejdic J. Electroanalysis 2007;19: 870–5. [7] Spinks GM, Mottaghitalab V, Bahrami-Saniani M, Whitten PG, Wallace GG. Adv Mater 2006;18:637–40. [8] Tan SX, Zhai J, Wan MX, Meng QB, Li YL, Jiang L, et al. J Phys Chem B 2004;108: 18693–7. [9] Chen JY, Chao DM, Lu XF, Zhang WJ. Mater Lett 2007;61:1419–23. [10] Raj JA, Mathiyarasu J, Vedhi C, Manisankar P. Mater Lett 2010;64:895–7. [11] Martin CR. Chem Mater 1996;8:1739–46. [12] Han J, Song G, Guo R. Adv Mater 2007;19:2993–9. [13] Wei ZX, Zhang ZM, Wan MX. Langmuir 2002;18:917–21. [14] Zhu Y, Hu D, Wan MX, Jiang L, Wei Y. Adv Mater 2007;19:2092–6. [15] Chiou N-R, Lu C, Guan J, Lee LJ, Epstein AJ. Nat Nanotechnol 2007;2:354–7. [16] Zhu Y, Li JM, Wan MX, Jiang L. Polymer 2008;49:3419–23. [17] Yu XL, Fan HS, Wang H, Zhao N, Zhang XL, Xu J. Mater Lett 2011;65:2724–7. [18] Macdiarmid AG, Epstein AJ. Synth Met 1994;65:103–16. [19] Yu XL, Fan HS, Wang H, Zhao N, Zhang XL, Xu J. Mater Lett 2011;65:2812–5.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
One step preparation of polyaniline micro/nanohierarchical structures with superhydrophobicity Haosen Fan, Ning Zhao ⁎, Hao Wang, Xiaofeng Li, Jian Xu ⁎ Beijing National Laboratory for Molecular Sciences, Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 20 December 2011 Accepted 13 March 2012 Available online 20 March 2012 Keywords: Hierarchical Microstructure Polyaniline Polymers Superhydrophobic
a b s t r a c t Polyaniline with a leaf-like hierarchical structure of 3 μm in length, 2 μm in width, 100 nm in thickness and crisscrossed nanofibers decorated on the surface, was successfully prepared in one step polymerization of aniline in the presence of sodium heptafluorobutyrate. The synthesized polyaniline shows superhydrophobicity with a water contact angle of 151° due to the combination of the low surface tension of fluorinated chain and the rough micro/nanostructure. Chemical structure and composition of the resultant PANI were characterized by FTIR, UV–vis, XRD and XPS. The formation mechanism of the hierarchical structure was investigated by monitoring the morphology development with the polymerization time. This method is facile and applicable for the large scale fabrication. Many potential applications can be expected based on the superhydrophobic PANI micro/nanostructures. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, micro/nanostructures of conducting polymers have been extensively investigated because of their important applications in organic conductors, actuators and electromagnetic interference shielding and so on [1]. Polyaniline (PANI) and its derivatives are important candidates among these conducting polymers due to their high conductivity and modifiable electrical properties by changing oxidation and protonation states [2,3]. Micro/nanostructures of PANI have been found in applications in molecular wires [4], chemical sensors [5], biosensors [6], artificial muscles [7], and dye-sensitized solar cells [8]. To date, a variety of synthetic approaches, such as interfacial polymerization [9], electrochemical method [10], hard-template [11], soft-template [12] and template-free method [13], have been developed. Hydrophobic modification of PANI microstructures has attracted considerable attention since surface wettability is one of the determinative factors for application [14]. Chiou and co-workers prepared superhydrophobic aligned PANI nanofibers on available substrates by chemical oxidative polymerization [15]. Zhu and co-workers reported the formation of superhydrophobic box-like PANI with the assistance of perfluorosebacic acid [16]. Herein, we demonstrated a one step polymerization for the large scale preparation of superhydrophobic leaf-like PANI in the presence of sodium heptafluorobutyrate (SHF). The hierarchical microstructure and the low surface tension of the fluorinated chain of SHF contribute to the superhydrophobicity. This method is ⁎ Corresponding authors. Fax: + 86 10 8261 9667. E-mail addresses: [email protected] (N. Zhao), [email protected] (J. Xu). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.053
facile and neither special instruments nor post treatment is needed. The structural characteristic, wettability, and formation mechanism of the leaf-like PANI are studied. This superhydrophobic hierarchically structured PANI may find many potential applications [5,6]. 2. Experimental section Aniline (Beijing Chemical Co.) was distilled under reduced pressure. Ammonium persulfate (APS, Sinopharm Chemical Reagent Co.), sodium heptafluorobutyrate (SHF, Aldrich), and other reagents were all A. R. grade and used as received without further treatment. A typical synthesis of the superhydrophobic leaf-like micro/ nanostructure of PANI was as follows: 92 μl aniline (1 mmol) and 0.236 g SHF (1 mmol) were dispersed in 50 ml of deionized water under magnetic stirring at room temperature for 5 min to obtain a uniform solution. Then aqueous solution of APS (1 mmol) was added to the above mixture in one portion. The resulting solution was stirred for 10 min to ensure complete mixing and then the reaction was allowed to proceed without agitation for 24 h at room temperature. Finally, the products were washed with water and methanol several times until the filtrate became colorless, and then dried in vacuum oven for 24 h. Morphology of the leaf-like PANI was examined by a Hitachi S4800 field emission scanning electron microscope (SEM) and a JEOL JEM2200FS transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectroscopy was recorded on a Bruker Equinox 55 spectrometer in the range of 400–4000 cm− 1. Ultraviolet and visible spectroscopy was measured in aqueous solution on a Shimadzu 1601PC UV–vis spectrophotometer. X-ray diffraction (XRD) pattern
H. Fan et al. / Materials Letters 78 (2012) 42–45
was recorded on a Rigaku D/max 2400 diffractometer using Cu–Kα (λ = 1.5418 Å) radiation (40 kV, 200 mA). X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALab220i-XL electron spectrometer from VG Scientific. Al–Kα radiation was used as the X-ray source and operated at 300 W. Contact angle (CA) was measured with a homemade instrument at ambient circumstance with a water drop of 5 μl. Reported data are the average of 3 different measurements. 3. Results and discussion Typical SEM and TEM images of the obtained PANI at the polymerization time of 24 h are presented in Fig. 1. The synthesized PANI shows a uniform leaf-like morphology of about 3 μm in length, 2 μm in width, and 100 nm in thickness (Fig. 1a and b). The magnified SEM image in Fig. 1c indicates that the PANI leaf is covered with interlaced nanofibers of tens of nanometer in diameter. The TEM image of the PANI (Fig. 1d) further confirms the formation of the highly crisscrossed nanofibers. This special microstructure endows the obtained PANI a hierarchical morphology in micro and nano-scale. Water CA on the surface constructed by the leaf-like PANI was about 151°, as shown in the inset of Fig. 1a. This superhydrophobic property can be ascribed to the hierarchical microstructure and the low-surface energy of the dopant of SHF which contains perfluorinated chain. FTIR spectrum of the leaf-like PANI is shown in Fig. 2a which is in good accord with previously reported result [17]. The weak absorption at 3056 cm − 1 is attributed to the N\H stretching vibration. The major strong peaks at 1580 and 1508 cm − 1 are ascribed to the C_C stretching vibration of the quinonoid and benzenoid rings, respectively, and the peak at 1285 cm − 1 is to the C\N stretching vibration of the benzene ring. The peak at 846 cm − 1 can be assigned to the out-of-plane vibration in the 1, 4-disubstituted benzene ring. The vibration bands at 1144 and 1246 cm − 1 ascribed to the symmetric and asymmetric CF2 stretches were also observed, indicating the existence of SHF in the resultant PANI. Fig. 2b shows the UV–vis spectra of the PANI dispersed in deionized water. The peak centered at 408 nm is identical to that of the emeraldine base form of PANI [18]. Fig. 2c exhibits the XRD pattern of the obtained PANI. The broad
43
peak centered at 2θ = 21° may be ascribed to the periodicity parallel to the polymer chains, indicating the resulted PANI is amorphous [19]. XPS (Fig. 2d) was studied to analyze the chemical elements on the surface. The survey spectrum shows three major peaks at 532, 401, and 285 eV that correspond to O1s, N1s, and C1s photoemission. The peak at 685 eV of F1s confirms the immigration of the low surface tension moieties on the surface. The formation mechanism of the hierarchical structure was studied by monitoring the morphology change of PANI with the polymerization time, as shown in Fig. 3. Small square plates with side lengths of about 800 nm and featureless surface are formed after 10 min of reaction (Fig. 3a), and many fragments are also observed. As the reaction time increased to 30 min (Fig. 3b), square plates grow larger with a side length of about 1–2 μm. The surface is till glossy. With the polymerization time prolonging to 1 h (Fig. 3c), leaf-like microstructure with flat surface formed with a length of about 2.5 μm, width of about 1.5 μm. As the polymerization time extended to 3 h (Fig. 3d), both the size and thickness of the leaf-like microstructures further increase, but quite close to the final products (Fig. 1a). Interlaced nanofibers began to appear on the leaf surface. The results demonstrated that the formation of the leaf like PANI follows a step by step process, the later polymerization of aniline happens on the periphery of the existing PANI nuclei. As the wettability of the surface is determined largely by the surface morphology, the contact angle of the surfaces shown in Fig. 3 changed greatly. As shown in Fig. 4, CA increases with extending of reaction time. The different micro/nanostructures of PANI show the CA of 84.7, 122.1, 134.3 and 142.8° at the reaction time of 10 min, 30 min, 1 h and 3 h, respectively. The obtained PANI has a low CA of 84.7°at the beginning of the polymerization. This is mainly due to the low-surface energy of SHF. As the reaction time extends to 3 h, the CA reaches 142.8°. It is clear that the change of the contact angle is in according with the increase of the surface roughness of PANI with the polymerization time. The high CA of resulting leaflike PANI is attributed to the low-surface energy of hydrophobic perfluorinated carbon in the SHF as well as the rough micro/nanostructures of PANI.
Fig. 1. SEM (a,b,c) and TEM (d) images of the synthesized PANI at the polymerization time of 24 h.
44
H. Fan et al. / Materials Letters 78 (2012) 42–45
b
Intensity (a.u.)
Intensity (a.u.)
a
3500 3000 2500 2000 1500 1000 500
300
Wavenumber (cm-1)
c
400
500
600
700
800
Wavelength (nm)
d
Intensity (a.u.)
Intensity (a.u.)
C1s
10
20
30
40
50
2θ(degree)
O1s F1s N1s
1000
800
600
400
200
Binding Energy (eV)
Fig. 2. FTIR (a), UV–vis (b), XRD (c) and XPS (d) spectra of the as-formed PANI.
4. Conclusion In summary, polyaniline hierarchical micro/nanostructures have been successfully prepared by the polymerization of aniline in the presence of sodium heptafluorobutyrate. The as-formed PANI shows a uniform leaf-like morphology with an approximate length of 3 μm, width of 2 μm and thickness of 100 nm, and covered by crisscrossed nanofibers
on the surface. Due to the low surface tension of sodium heptafluorobutyrate and the hierarchical structure, the obtained PANI exhibits superhydrophobic behavior with water contact angle above 150°. The leaflike PANI micro/nanostructures start from square flat plates, which grow in size and then are covered by nanofibers on the surface with the polymerization time. This method is facile and effective for the fabrication of PANI micro/nanostructures with superhydrophobicity.
Fig. 3. SEM images of the PANI synthesized at different polymerization times: (a) 10 min; (b) 30 min; (c) 1 h and (d) 3 h.
H. Fan et al. / Materials Letters 78 (2012) 42–45
References
140
Contact angle (degree)
45
130 120 110 100 90 80 30
60
90
120
150
180
Time (min) Fig. 4. Contact angle of the PANI micro/nanostructures as a function of the polymerization time.
Acknowledgments This work was supported by NSFC (grant no. 50821062) and MOST (2009CB930400). We acknowledge Beijing Municipal Commission of Education for the special fund for the Disciplines & Postgraduate Education Construction project.
[1] Liang L, Liu J, Windisch CF, Exarhos GJ, Lin YH. Angew Chem Int Ed Engl 2002;41: 3665–8. [2] MacDiarmid AG. Angew Chem Int Ed Engl 2001;40:2581–90. [3] Kang ET, Neoh KG, Tan KL. Prog Polym Sci 1998;23:277–324. [4] Akai T, Shimomura T, Ito K. Synth Met 2003;135:777–8. [5] Huang JX, Virji S, Weiller BH, Kaner RB. J Am Chem Soc 2003;125:314–5. [6] Zhang LJ, Peng H, Kilmartin PA, Soeller C, Travas-Sejdic J. Electroanalysis 2007;19: 870–5. [7] Spinks GM, Mottaghitalab V, Bahrami-Saniani M, Whitten PG, Wallace GG. Adv Mater 2006;18:637–40. [8] Tan SX, Zhai J, Wan MX, Meng QB, Li YL, Jiang L, et al. J Phys Chem B 2004;108: 18693–7. [9] Chen JY, Chao DM, Lu XF, Zhang WJ. Mater Lett 2007;61:1419–23. [10] Raj JA, Mathiyarasu J, Vedhi C, Manisankar P. Mater Lett 2010;64:895–7. [11] Martin CR. Chem Mater 1996;8:1739–46. [12] Han J, Song G, Guo R. Adv Mater 2007;19:2993–9. [13] Wei ZX, Zhang ZM, Wan MX. Langmuir 2002;18:917–21. [14] Zhu Y, Hu D, Wan MX, Jiang L, Wei Y. Adv Mater 2007;19:2092–6. [15] Chiou N-R, Lu C, Guan J, Lee LJ, Epstein AJ. Nat Nanotechnol 2007;2:354–7. [16] Zhu Y, Li JM, Wan MX, Jiang L. Polymer 2008;49:3419–23. [17] Yu XL, Fan HS, Wang H, Zhao N, Zhang XL, Xu J. Mater Lett 2011;65:2724–7. [18] Macdiarmid AG, Epstein AJ. Synth Met 1994;65:103–16. [19] Yu XL, Fan HS, Wang H, Zhao N, Zhang XL, Xu J. Mater Lett 2011;65:2812–5.