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Characterisation of polypyrrole nano-films for membrane-based sensors
Synthetic Metals 131 (2002) 161–165
Characterisation of polypyrrole nano-films for membrane-based sensors Usha Sree, Yojiro Yamamoto, Bhavana Deore, Hiroshi Shiigi, Tsutomu Nagaoka* Department of Applied Chemistry, Faculty of Engineering, Yamaguchi University, Tokiwadai, Ube 755-0001, Japan Received 10 May 2001; received in revised form 1 August 2002; accepted 5 August 2002
Abstract Thin polypyrrole nano-films were formed chemically by interfacial polymerisation. An interesting feature of this chemical synthesis is that compact ultra thin polypyrrole films of 50 nm thickness doped with sulphate are obtained by a simple technique, although, films grown in the presence of different dopants were thicker (100 nm). Oxidative polymerisation allowed reproducible synthesis of very thin films in a very short time. Assessment of the surface of the films by atomic force microscopy indicated a strong influence on experimental conditions such as pH, nature and size of the dopant ions. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymer; Interfacial polymerisation; Nano-films; Sensors
1. Introduction Conducting polymers has been fascinating many scientists worldwide with their interesting electronic properties and sensing capabilities. Innumerable applications of these polymers as such as thin films for batteries, sensors, ion selective electrodes and solid-state devices have been extensively reported [1–5]. Polypyrrole membranes have found applications also in medicine for the continuous delivery of drugs [6]. Thin films with desired properties and functionalities have been synthesised by various available methods, such as chemical oxidation, electrochemical synthesis, admicellar polymerisation and layer-by-layer deposition techniques [7–15]. However, a demand for improvement in quality of thin films has never ceased. Thick films have been produced electrochemically, but chemically produced films often show better features and characteristics. Chemical polymerisation has several advantages over conventional electrochemical deposition. Thin films formed by interfacial polymerisation can be easily collected onto any insulating or conducting substrate. Properties of the films deposited by this approach can be
* Corresponding author. E-mail address: [email protected] (T. Nagaoka).
controlled by several parameters such as pH, electrolyte, nature and size of dopant, solvent, and concentrations of pyrrole and dopant. Polypyrrole films of 3–4 mm thick have been produced chemically by interfacial polymerisation [16]. Here, we show that much thinner polypyrrole films can be chemically synthesised as freestanding membranes by using the interfacial polymerisation technique. In this study, we have characterised chemically grown polypyrrole nano-films that could eventually be used in fabrication of sensors as freestanding membranes. This model electroactive nanometre thick membrane can serve as a permselective ion gate membrane for molecular sensing and releasing of bioactive molecules in pharmaceutical and biomedical separations. Several mechanisms have been explained for formation of polypyrrole. For the chemical polymerisation, a monomer is oxidised in solution by added oxidant. In the electrochemical method, either radical monomer or oligomer nucleates on a electrode to start growing films. Additionally, monomer oxidation can be catalysed by metal oxides formed on the electrode, by protons at low pH, or by discharged electrolyte ions at a high potential. The resulting polymer is a combined result of nucleus growth, oligomer deposition, and polypyrrole moiety attachment from various oxidation routes. In this study, we focus on the use of the interfacial polymerisation technique to produce thin polypyrrole nano-films onto a glass slide or a stainless steel mesh.
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 1 7 9 - 0
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U. Sree et al. / Synthetic Metals 131 (2002) 161–165
Fig. 1. Preparation of thin polypyrrole film at interface.
2. Experiments and results Fig. 1 shows the process of the interfacial polymerisation of pyrrole (partition coefficient of pyrrole in the chloroform/ water system; Kcw ¼ 0:951). In the first step, monomer (0.2 M) in the non-aqueous phase (chloroform) and dopant/oxidising agent (0.2 M) in the aqueous phase are allowed to form a film at the interface between the two immiscible phases. All the chemicals used were of reagent grade and were purchased from Wako Chemicals, Japan. A micro-porous stainless steel mesh (400 Mesh, Nilaco, Japan) or glass slide, acting as a substrate, was placed just below the interface. In the second step, monomer and dopant/oxidising agent dissolved in the two different phases, diffuse towards each other to yield a thin polymer skin at the interface. It has already been explained that the initial step in the formation of all films is a nucleation process [17]. During the nucleation period, pyrrole was being oxidised to form a film. Within a few minutes, a thin film started to appear at the interface. At this point, removing the solutions over and below the film quenched polymerisation. The film easily slides onto the placed mesh or glass slide; smooth and compact films could be obtained in a reproducible manner. We also found delaminated or folded patches of the film, which means that the film is mechanically integrated and flexible. It was seen that stirring inhibited both the nucleation and growth in the film formation. The collected film was allowed to dry prior to characterisation. Several different thin polypyrrole films were formed with varying dopant anions, such as sulphate (formed from persulphate), p-toluene sulfonate (p-TS) and lactic acid
(LA). The time required to obtain a virgin film with sulphate, p-TS and LA is 120, 60 and 30 s, respectively. The films showed visible colour differences: greyish black for sulphate, bluish green for p-TS and greenish black for LA. The separation of the film from the interface requires overcoming adhesion; wrinkles were suppressed with use of p-TS as dopant. The film thickness was measured using a scanning electron microscope (SEM S4700Y, Horiba, Japan). The film thickness was 50 nm for sulphate-doped film and 100 nm for both p-TS and LA doped films (Fig. 2), but the thickness also varied with polymerisation time. The electrical conductivity of the films measured by the standard four-probe technique was 1.6, 3.8 and 0.65 S/cm for sulphate, p-TS and LA doped films, respectively. In order to compare the film characteristics, UV-Vis spectroscopy (Shimadzu UV-2400, Japan), infrared spectroscopy (Horiba FT-710 Spectrometer, Japan) and atomic force microscopy (SPM 9500 Shimadzu, Japan) were used.
1
The partition coefficient was estimated by the shake-flask method. The partition coefficient of pyrrole in the octanol/water system is 0.75 (Hansch C. et al., 1995).
Fig. 2. SEM micrograph of virgin polypyrrole film.
U. Sree et al. / Synthetic Metals 131 (2002) 161–165
Fig. 3. UV-Vis absorption spectra of virgin polypyrrole films prepared by using different oxidising agents, (a) 0.1 M FeCl3, (b) 0.1 M (NH4)2S2O8 and (c) 0.1 M K4[Fe(CN)6].
The UV-Vis absorption spectra of polypyrrole films prepared by using different oxidising agents are shown in Fig. 3. A typical spectrum for the oxidised polypyrrole, showing p-p transition and polaron bands at 420 and 860 nm was obtained. Polymerisation with potassium ferricyanide (E0 ¼ þ0:36 V versus NHE) as an oxidising agent was slow and took nearly 20 h for the film to appear at the interface. On the other hand, use of ferric chloride (E0 ¼ þ0:77 V versus NHE) and ammonium persulphate (E0 ¼ þ2:01 V versus NHE) produced films at much faster rates. A film formed by using ferric chloride was very fragile to break easily into globular distortions on drying. Ammonium persulphate seems more suitable to obtain a clear, continuous and uniform conducting polypyrrole film. The FTIR spectra (Fig. 4) of the virgin polymer films with different dopants were recorded after vacuum drying overnight. Although, the region from 1000 to 1800 cm1 should contain both the pyrrole ring and dopant anion vibrations, polypyrrole vibrations dominated the FTIR spectra, masking
Fig. 4. FTIR spectra of polypyrrole films doped with 0.2 M anions, (a) p-toluene sulphonate, (b) sulphate and (c) lactate.
163
the anion vibrations. The strongest S–O stretching vibration for the p-TS anion was veiled by pyrrole absorptions coinciding at 1220 cm1. The bands in region of 1690 and 1710 cm1 were characteristic of the S-cis conjugated carbonyl group of p-TS anion. The stretching vibration of the C–O group for lactic acid was seen at 1292 cm1. By increasing the concentration of an oxidising agent, one can easily obtain an overoxidised polypyrrole film [16]. AFM images recorded for thin films formed with varying dopant anions provide some insight into their behaviour to show clearly the distinguishable characteristics (Fig. 5a). The film is basically a layer made of discrete polypyrrole particles that appear to have nucleated at many sites and then merged. The difference in the size of the granules of the three virgin films is a result of incorporation of the three different anions into the polymer. LA doped polypyrrole films showed regularly and linearly ordered rows of granules, whereas polypyrrole films doped with p-TS gave granules appearing more active on the surface (Fig. 5a(ii and iii)). Polypyrrole virgin films upon immersing in aqueous basic or acidic solutions led to some rearrangements of polymer chain packing. The mechanism mainly involves protonation and deprotonation of the polypyrrole ring. Earlier, it was assumed that injection and ejection of counter ions during the oxidation and reduction process compensates for charge neutrality on the film [18]. Later it became apparent that protons [19] and co-ions participate [20]. AFM images indeed showed the morphological changes in the polypyrrole surface on treatment with acidic and basic buffer solutions. Roughness parameters were determined using 3D surface tracer to indicate differences in nodular diameters (Table 1). In case of large anion (p-TS) doped film, treatment with acidic, neutral or basic buffer solutions does Table 1 Roughness parameters of fresh and pH treated films with various dopants Average granule diameter (nm)
RMSa (nm)
Raa (nm)
Sulphate Fresh film pH 1 pH 7 pH 11
143 26 88 54
4.1 7.7 6.8 4.1
3.3 6.6 5.1 3.2
p-TS Fresh film pH 1 pH 7 pH 11
85 56 60 71
5.5 3.5 3.2 3.6
4.0 2.7 2.1 2.5
Lactate Fresh film pH 1 pH 7 pH 11
60 62 80 69
4.8 2.1 4.9 5.1
6.3 1.6 3.8 3.9
Dopant
a Rq or RMS: root mean square roughness is defined as the standard deviation of the distribution of surface heights; Ra: arithmetic average height parameter is defined as the average absolute deviation of the roughness irregularities from the mean line over one sampling length [21].
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Fig. 5. (a) Polypyrrole films doped with (i) sulphate, (ii) p-toluene sulphonate, and (iii) lactate. Lactate doped films show linearly arranged rows of granules. The 1 mm 1 mm AFM images are recorded in dynamic mode and were obtained at a scan rate of 1 Hz. The vertical scale range is 0–90 nm. (b) p-Toluene sulphonate doped polypyrrole films treated with buffer solutions of (i) pH 1 (ii) pH 7 and (iii) pH 11. The size and arrangement of the granules differed with change in pH of the solution. The 1 mm 1 mm AFM images are recorded in dynamic mode and were obtained at a scan rate of 1 Hz. The vertical scale range is 0–90 nm.
not seem to cause complete dedoping but only a change in the granular diameter was observed. Granules have become smaller (average diameter of 56 nm) on treating with acidic pH solution (Fig. 5b(i)). A compact and suppressed granular
surface was observed for film treated with neutral pH buffer solution (Fig. 5b(ii)), while treatment in pH 11 solutions, granules are let loose and few of them appear to protrude out of the surface (Fig. 5b(iii)). Similar observations were seen
U. Sree et al. / Synthetic Metals 131 (2002) 161–165
for both lactate and sulphate-doped films. Optical spectra (UV-Vis and infrared) recorded for different pH values showed no remarkable changes. Only a small change in the intensity of the IR peaks and a slight shift in UV-Vis absorption bands were observed in each case as the pH changed from 1 to 11.
3. Conclusions Our work focused on an easy route to produce thin polypyrrole nano-films. By varying polymerisation time, the thickness of the films could be controlled. The spectral and surface properties of the films have clearly shown the ingress of dopant anions into polypyrrole. These films exhibited features, which were, generally similar to those of polypyrrole prepared by conventional techniques. This preparation method makes insertion of various functional groups to pyrrole films possible and will provide various applications in developing chemical and biological sensors.
Acknowledgements One of the authors, US, is thankful to VBL for awarding postdoctoral fellowship. Thanks to Koichi Nakaoka and Hiroshi Yano for analytical assistance. We appreciate help of Prof. Michael Higgins, Yamaguchi University, in preparation of the manuscript.
165
References [1] E. Spila, S. Panero, B. Scrosati, Electrochim. Acta 43 (1998) 1651. [2] B. Deore, Z. Chen, T. Nagaoka, Anal. Chem. 72 (2000) 3989. [3] S. Brahim, D. Narinesingh, A. Guiseppi-Elie, Biosens. Bioelectron. 17 (2002) 53. [4] A. Hulanicki, A. Michalska, Electroanalysis 7 (1995) 692. [5] T.W. Lewis, G.M. Spinks, G.G. Wallace, A. Mazzoldi, D. De Rossi, Synth. Met. 122 (2001) 379. [6] K. Kontturi, P. Pentti, G. Sundholm, J. Electranal. Chem. 453 (1998) 231. [7] X. Hong, J.C. Tyson, J.S. Middlecoff, D.M. Collard, Macromolecules 32 (1999) 4232. [8] X. Chu, V. Chan, L.D. Schmidt, W.H. Smyrl, J. Appl. Phys. 77 (1995) 6658. [9] I. Berlot, P. Labbe´ , J.C. Moutet, Langmuir 16 (2000) 5814. [10] C.C. Hsueh, A. Brajter-Toth, Anal. Chem. 66 (1994) 2458. [11] D.M. Collard, M.A. Fox, J. Am. Chem. Soc. 113 (1991) 9414. [12] J.H. O’Haver, J.H. Harwell, E.A. O’Rear, Langmuir 10 (1994) 2588. [13] C.L. Lai, J.H. Harwell, E.A. O’Rear, Langmuir 11 (1995) 905. [14] E. Pigois-Landureau, Y.F. Nicolau, M. Delamar, Synth. Met. 72 (1995) 111. [15] Y.F. Nicolau, S. David, F. Genoud, M. Nechtschein, J.P. Travers, Synth. Met. 41/43 (1991) 1491. [16] Y. Lu, G. Shi, C. Li, Y. Liang, J. Appl. Polym. Sci. 70 (1998) 2169. [17] D.E. Raymond, D.J. Harrison, J. Electranal. Chem. 361 (1993) 65. [18] A.F. Diaz, Chem. Scripta 17 (1981) 145. [19] P.A. Christensen, A. Hammett, Electrochim. Acta 36 (1991) 1263. [20] M. Arca, M.V. Mirkin, A. Bard, J. Phys. Chem. 99 (1995) 5040. [21] E.S. Gadelmawla, et al., J. Mater. Proc. Tech. 123 (2002) 133– 145.
Characterisation of polypyrrole nano-films for membrane-based sensors Usha Sree, Yojiro Yamamoto, Bhavana Deore, Hiroshi Shiigi, Tsutomu Nagaoka* Department of Applied Chemistry, Faculty of Engineering, Yamaguchi University, Tokiwadai, Ube 755-0001, Japan Received 10 May 2001; received in revised form 1 August 2002; accepted 5 August 2002
Abstract Thin polypyrrole nano-films were formed chemically by interfacial polymerisation. An interesting feature of this chemical synthesis is that compact ultra thin polypyrrole films of 50 nm thickness doped with sulphate are obtained by a simple technique, although, films grown in the presence of different dopants were thicker (100 nm). Oxidative polymerisation allowed reproducible synthesis of very thin films in a very short time. Assessment of the surface of the films by atomic force microscopy indicated a strong influence on experimental conditions such as pH, nature and size of the dopant ions. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymer; Interfacial polymerisation; Nano-films; Sensors
1. Introduction Conducting polymers has been fascinating many scientists worldwide with their interesting electronic properties and sensing capabilities. Innumerable applications of these polymers as such as thin films for batteries, sensors, ion selective electrodes and solid-state devices have been extensively reported [1–5]. Polypyrrole membranes have found applications also in medicine for the continuous delivery of drugs [6]. Thin films with desired properties and functionalities have been synthesised by various available methods, such as chemical oxidation, electrochemical synthesis, admicellar polymerisation and layer-by-layer deposition techniques [7–15]. However, a demand for improvement in quality of thin films has never ceased. Thick films have been produced electrochemically, but chemically produced films often show better features and characteristics. Chemical polymerisation has several advantages over conventional electrochemical deposition. Thin films formed by interfacial polymerisation can be easily collected onto any insulating or conducting substrate. Properties of the films deposited by this approach can be
* Corresponding author. E-mail address: [email protected] (T. Nagaoka).
controlled by several parameters such as pH, electrolyte, nature and size of dopant, solvent, and concentrations of pyrrole and dopant. Polypyrrole films of 3–4 mm thick have been produced chemically by interfacial polymerisation [16]. Here, we show that much thinner polypyrrole films can be chemically synthesised as freestanding membranes by using the interfacial polymerisation technique. In this study, we have characterised chemically grown polypyrrole nano-films that could eventually be used in fabrication of sensors as freestanding membranes. This model electroactive nanometre thick membrane can serve as a permselective ion gate membrane for molecular sensing and releasing of bioactive molecules in pharmaceutical and biomedical separations. Several mechanisms have been explained for formation of polypyrrole. For the chemical polymerisation, a monomer is oxidised in solution by added oxidant. In the electrochemical method, either radical monomer or oligomer nucleates on a electrode to start growing films. Additionally, monomer oxidation can be catalysed by metal oxides formed on the electrode, by protons at low pH, or by discharged electrolyte ions at a high potential. The resulting polymer is a combined result of nucleus growth, oligomer deposition, and polypyrrole moiety attachment from various oxidation routes. In this study, we focus on the use of the interfacial polymerisation technique to produce thin polypyrrole nano-films onto a glass slide or a stainless steel mesh.
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 1 7 9 - 0
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U. Sree et al. / Synthetic Metals 131 (2002) 161–165
Fig. 1. Preparation of thin polypyrrole film at interface.
2. Experiments and results Fig. 1 shows the process of the interfacial polymerisation of pyrrole (partition coefficient of pyrrole in the chloroform/ water system; Kcw ¼ 0:951). In the first step, monomer (0.2 M) in the non-aqueous phase (chloroform) and dopant/oxidising agent (0.2 M) in the aqueous phase are allowed to form a film at the interface between the two immiscible phases. All the chemicals used were of reagent grade and were purchased from Wako Chemicals, Japan. A micro-porous stainless steel mesh (400 Mesh, Nilaco, Japan) or glass slide, acting as a substrate, was placed just below the interface. In the second step, monomer and dopant/oxidising agent dissolved in the two different phases, diffuse towards each other to yield a thin polymer skin at the interface. It has already been explained that the initial step in the formation of all films is a nucleation process [17]. During the nucleation period, pyrrole was being oxidised to form a film. Within a few minutes, a thin film started to appear at the interface. At this point, removing the solutions over and below the film quenched polymerisation. The film easily slides onto the placed mesh or glass slide; smooth and compact films could be obtained in a reproducible manner. We also found delaminated or folded patches of the film, which means that the film is mechanically integrated and flexible. It was seen that stirring inhibited both the nucleation and growth in the film formation. The collected film was allowed to dry prior to characterisation. Several different thin polypyrrole films were formed with varying dopant anions, such as sulphate (formed from persulphate), p-toluene sulfonate (p-TS) and lactic acid
(LA). The time required to obtain a virgin film with sulphate, p-TS and LA is 120, 60 and 30 s, respectively. The films showed visible colour differences: greyish black for sulphate, bluish green for p-TS and greenish black for LA. The separation of the film from the interface requires overcoming adhesion; wrinkles were suppressed with use of p-TS as dopant. The film thickness was measured using a scanning electron microscope (SEM S4700Y, Horiba, Japan). The film thickness was 50 nm for sulphate-doped film and 100 nm for both p-TS and LA doped films (Fig. 2), but the thickness also varied with polymerisation time. The electrical conductivity of the films measured by the standard four-probe technique was 1.6, 3.8 and 0.65 S/cm for sulphate, p-TS and LA doped films, respectively. In order to compare the film characteristics, UV-Vis spectroscopy (Shimadzu UV-2400, Japan), infrared spectroscopy (Horiba FT-710 Spectrometer, Japan) and atomic force microscopy (SPM 9500 Shimadzu, Japan) were used.
1
The partition coefficient was estimated by the shake-flask method. The partition coefficient of pyrrole in the octanol/water system is 0.75 (Hansch C. et al., 1995).
Fig. 2. SEM micrograph of virgin polypyrrole film.
U. Sree et al. / Synthetic Metals 131 (2002) 161–165
Fig. 3. UV-Vis absorption spectra of virgin polypyrrole films prepared by using different oxidising agents, (a) 0.1 M FeCl3, (b) 0.1 M (NH4)2S2O8 and (c) 0.1 M K4[Fe(CN)6].
The UV-Vis absorption spectra of polypyrrole films prepared by using different oxidising agents are shown in Fig. 3. A typical spectrum for the oxidised polypyrrole, showing p-p transition and polaron bands at 420 and 860 nm was obtained. Polymerisation with potassium ferricyanide (E0 ¼ þ0:36 V versus NHE) as an oxidising agent was slow and took nearly 20 h for the film to appear at the interface. On the other hand, use of ferric chloride (E0 ¼ þ0:77 V versus NHE) and ammonium persulphate (E0 ¼ þ2:01 V versus NHE) produced films at much faster rates. A film formed by using ferric chloride was very fragile to break easily into globular distortions on drying. Ammonium persulphate seems more suitable to obtain a clear, continuous and uniform conducting polypyrrole film. The FTIR spectra (Fig. 4) of the virgin polymer films with different dopants were recorded after vacuum drying overnight. Although, the region from 1000 to 1800 cm1 should contain both the pyrrole ring and dopant anion vibrations, polypyrrole vibrations dominated the FTIR spectra, masking
Fig. 4. FTIR spectra of polypyrrole films doped with 0.2 M anions, (a) p-toluene sulphonate, (b) sulphate and (c) lactate.
163
the anion vibrations. The strongest S–O stretching vibration for the p-TS anion was veiled by pyrrole absorptions coinciding at 1220 cm1. The bands in region of 1690 and 1710 cm1 were characteristic of the S-cis conjugated carbonyl group of p-TS anion. The stretching vibration of the C–O group for lactic acid was seen at 1292 cm1. By increasing the concentration of an oxidising agent, one can easily obtain an overoxidised polypyrrole film [16]. AFM images recorded for thin films formed with varying dopant anions provide some insight into their behaviour to show clearly the distinguishable characteristics (Fig. 5a). The film is basically a layer made of discrete polypyrrole particles that appear to have nucleated at many sites and then merged. The difference in the size of the granules of the three virgin films is a result of incorporation of the three different anions into the polymer. LA doped polypyrrole films showed regularly and linearly ordered rows of granules, whereas polypyrrole films doped with p-TS gave granules appearing more active on the surface (Fig. 5a(ii and iii)). Polypyrrole virgin films upon immersing in aqueous basic or acidic solutions led to some rearrangements of polymer chain packing. The mechanism mainly involves protonation and deprotonation of the polypyrrole ring. Earlier, it was assumed that injection and ejection of counter ions during the oxidation and reduction process compensates for charge neutrality on the film [18]. Later it became apparent that protons [19] and co-ions participate [20]. AFM images indeed showed the morphological changes in the polypyrrole surface on treatment with acidic and basic buffer solutions. Roughness parameters were determined using 3D surface tracer to indicate differences in nodular diameters (Table 1). In case of large anion (p-TS) doped film, treatment with acidic, neutral or basic buffer solutions does Table 1 Roughness parameters of fresh and pH treated films with various dopants Average granule diameter (nm)
RMSa (nm)
Raa (nm)
Sulphate Fresh film pH 1 pH 7 pH 11
143 26 88 54
4.1 7.7 6.8 4.1
3.3 6.6 5.1 3.2
p-TS Fresh film pH 1 pH 7 pH 11
85 56 60 71
5.5 3.5 3.2 3.6
4.0 2.7 2.1 2.5
Lactate Fresh film pH 1 pH 7 pH 11
60 62 80 69
4.8 2.1 4.9 5.1
6.3 1.6 3.8 3.9
Dopant
a Rq or RMS: root mean square roughness is defined as the standard deviation of the distribution of surface heights; Ra: arithmetic average height parameter is defined as the average absolute deviation of the roughness irregularities from the mean line over one sampling length [21].
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U. Sree et al. / Synthetic Metals 131 (2002) 161–165
Fig. 5. (a) Polypyrrole films doped with (i) sulphate, (ii) p-toluene sulphonate, and (iii) lactate. Lactate doped films show linearly arranged rows of granules. The 1 mm 1 mm AFM images are recorded in dynamic mode and were obtained at a scan rate of 1 Hz. The vertical scale range is 0–90 nm. (b) p-Toluene sulphonate doped polypyrrole films treated with buffer solutions of (i) pH 1 (ii) pH 7 and (iii) pH 11. The size and arrangement of the granules differed with change in pH of the solution. The 1 mm 1 mm AFM images are recorded in dynamic mode and were obtained at a scan rate of 1 Hz. The vertical scale range is 0–90 nm.
not seem to cause complete dedoping but only a change in the granular diameter was observed. Granules have become smaller (average diameter of 56 nm) on treating with acidic pH solution (Fig. 5b(i)). A compact and suppressed granular
surface was observed for film treated with neutral pH buffer solution (Fig. 5b(ii)), while treatment in pH 11 solutions, granules are let loose and few of them appear to protrude out of the surface (Fig. 5b(iii)). Similar observations were seen
U. Sree et al. / Synthetic Metals 131 (2002) 161–165
for both lactate and sulphate-doped films. Optical spectra (UV-Vis and infrared) recorded for different pH values showed no remarkable changes. Only a small change in the intensity of the IR peaks and a slight shift in UV-Vis absorption bands were observed in each case as the pH changed from 1 to 11.
3. Conclusions Our work focused on an easy route to produce thin polypyrrole nano-films. By varying polymerisation time, the thickness of the films could be controlled. The spectral and surface properties of the films have clearly shown the ingress of dopant anions into polypyrrole. These films exhibited features, which were, generally similar to those of polypyrrole prepared by conventional techniques. This preparation method makes insertion of various functional groups to pyrrole films possible and will provide various applications in developing chemical and biological sensors.
Acknowledgements One of the authors, US, is thankful to VBL for awarding postdoctoral fellowship. Thanks to Koichi Nakaoka and Hiroshi Yano for analytical assistance. We appreciate help of Prof. Michael Higgins, Yamaguchi University, in preparation of the manuscript.
165
References [1] E. Spila, S. Panero, B. Scrosati, Electrochim. Acta 43 (1998) 1651. [2] B. Deore, Z. Chen, T. Nagaoka, Anal. Chem. 72 (2000) 3989. [3] S. Brahim, D. Narinesingh, A. Guiseppi-Elie, Biosens. Bioelectron. 17 (2002) 53. [4] A. Hulanicki, A. Michalska, Electroanalysis 7 (1995) 692. [5] T.W. Lewis, G.M. Spinks, G.G. Wallace, A. Mazzoldi, D. De Rossi, Synth. Met. 122 (2001) 379. [6] K. Kontturi, P. Pentti, G. Sundholm, J. Electranal. Chem. 453 (1998) 231. [7] X. Hong, J.C. Tyson, J.S. Middlecoff, D.M. Collard, Macromolecules 32 (1999) 4232. [8] X. Chu, V. Chan, L.D. Schmidt, W.H. Smyrl, J. Appl. Phys. 77 (1995) 6658. [9] I. Berlot, P. Labbe´ , J.C. Moutet, Langmuir 16 (2000) 5814. [10] C.C. Hsueh, A. Brajter-Toth, Anal. Chem. 66 (1994) 2458. [11] D.M. Collard, M.A. Fox, J. Am. Chem. Soc. 113 (1991) 9414. [12] J.H. O’Haver, J.H. Harwell, E.A. O’Rear, Langmuir 10 (1994) 2588. [13] C.L. Lai, J.H. Harwell, E.A. O’Rear, Langmuir 11 (1995) 905. [14] E. Pigois-Landureau, Y.F. Nicolau, M. Delamar, Synth. Met. 72 (1995) 111. [15] Y.F. Nicolau, S. David, F. Genoud, M. Nechtschein, J.P. Travers, Synth. Met. 41/43 (1991) 1491. [16] Y. Lu, G. Shi, C. Li, Y. Liang, J. Appl. Polym. Sci. 70 (1998) 2169. [17] D.E. Raymond, D.J. Harrison, J. Electranal. Chem. 361 (1993) 65. [18] A.F. Diaz, Chem. Scripta 17 (1981) 145. [19] P.A. Christensen, A. Hammett, Electrochim. Acta 36 (1991) 1263. [20] M. Arca, M.V. Mirkin, A. Bard, J. Phys. Chem. 99 (1995) 5040. [21] E.S. Gadelmawla, et al., J. Mater. Proc. Tech. 123 (2002) 133– 145.