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Inkjet printable polyaniline-gold dispersions
Thin Solid Films 519 (2011) 4351–4356
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Inkjet printable polyaniline-gold dispersions Natascha Lenhart a, Karl Crowley b, Anthony J. Killard c, Malcolm R. Smyth d, Aoife Morrin d,⁎ a
Hochschule Reutlingen/Reutlingen University, Alteburgstraße 150, 72762 Reutlingen, Germany Biomedical Diagnostics Institute, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland c Department of Applied Sciences, University of the West of England, Coldharbour Lane, Bristol BS16 1QY, UK d School of Chemical Sciences, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland b
a r t i c l e
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Article history: Received 16 June 2010 Received in revised form 10 February 2011 Accepted 11 February 2011 Available online 2 March 2011 Keywords: Polyaniline composite Hybrid Gold Oxidative polymerisation Inkjet printing
a b s t r a c t Inkjet printable polyaniline-gold (PANI-Au) hybrid dispersions were synthesised using gold salt as oxidant to simultaneously induce chemical oxidative polymerisation of aniline and reduction of HAuCl4 in bulk aqueous solution. By varying the amount of HAuCl4 used for the polymerisation, the size, morphology and population of the resulting gold particles embedded within the polymer were controllable. It was shown however, that the metallic gold particles contained within the resulting dispersions did not appear to affect the bulk conductivity of PANI, but rather that by varying the amount of HAuCl4 used, both the quality and printability of the resulting PANI-Au dispersion could be affected. PANI-Au synthesised using a ratio of 1:0.25 aniline: HAuCl4 was shown to be optimum as it resulted in dispersions with high polymer conjugation lengths and doping levels, as well as high conductivity and well-defined electrochemistry. This hybrid material was shown to be highly processable, and inkjet printing of this material was demonstrated on flexible substrate which resulted in high quality, printed films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polyaniline (PANI) is recognized as one of the most promising inherently conducting polymers (ICPs) to be discovered due to its highly reversible redox and pH switching properties, recent breakthroughs in ease of synthesis and wide range of potential applications in anti-corrosion coatings [1], electromagnetic shielding [2], rechargeable polymeric batteries, polymer photovoltaics and polymer actuators and sensors. Several techniques have been used to prepare thin films of pure PANI, e.g., spin-coating, casting, self-assembly and Langmuir–Blodgett (LB) methods [3]. Recently, printing methods such as gravure- [4], screen- [5] and inkjet- printing [6–9] are being employed to pattern PANI. An approach to render PANI as printable is to create a stable dispersion of the material in aqueous or mild organic media. A wide range of stable nanoparticulate forms of PANI have been reported in the literature recently to include spherical nanoparticles, nanofibres and nanotubes. Emerging from the literature is that the spherical nanoparticulate form is most suitable for printing applications. Nanofibres, for example, have the drawback of their inherent length, typically of the order of several microns [10], making them unsuitable for printing as they can become entangled and easily cause clogging of printhead nozzles. The inherent morphology of PANI nanoparticulates used for this research is spherical and of the order of 100 nm diameter
⁎ Corresponding author: Tel.: +353 1 700 6730; fax: +353 1 700 5309. E-mail address: [email protected] (A. Morrin). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.02.045
making them highly processable and hence amenable to inkjet printing deposition [6]. Compositing PANI with noble metals or carbon nanotubes to create hybrid functional materials has been shown to enhance sensing and catalytic capabilities [11,12], as compared to those of pure PANI. Specifically, incorporation of gold into PANI has become a popular and interesting aspect of composite synthesis as these resulting materials differ from pure PANI in respect of some physical and chemical properties [13]. Enhanced conductivities [14] and electrocatalysis [15,16] over pure PANI have been reported. For example, Zhang et al. showed that the electrical conductivity of a PANI-Au nanocomposite (synthesised in the presence of p-toluene sulphonic acid) was two orders of magnitude greater than that of the pure PANI [14]. Electrical bistability and memory characteristics have also recently been reported by using chemically synthesised PANI nanofibers embedded with Au [17] and other composite configurations [18,19]. A number of different approaches have been suggested for gold incorporation into the polymer [15,16,20]. The synthesis of PANI-Au composites was pioneered by Kang and co-workers in 1993 [21]. They published the first report on the spontaneous and sustained reduction and accumulation of metallic gold from a chloroauric acid solution by PANI. As well as using pre-formed PANI as the reducing agent for the metallic ions, several papers have reported using aniline as the reducing agent, wherein as the gold salt becomes reduced, the aniline undergoes oxidative polymerisation [22,23]. This approach has yielded a range of PANI morphologies including bulk and fibrillar [24] structures containing spherical particles or clusters of gold which have been reported to range in size from just 2 nm [25] to micrometer
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dimensions [26]. The size, monodispersity and population of the gold particles within the polymer depend heavily on the initial synthesis conditions used, i.e., the type and mass of each of the dopant, oxidant and monomer. Architectures such as Au-PANI core–shell particles have also been reported [27]. In this instance, surfactant and acetic acid were found to be critical for the formation of the stable core–shell structure. In addition to gold, other noble metal salts such as silver, platinum and palladium salts have been reported as suitable oxidants for PANI synthesis. However, PANI synthesis using silver nitrate for example was reported to be very slow, taking up to several weeks to polymerise the aniline depending on conditions used [28]. Combining precious metals and conducting organic polymers through compositing provides combinatorial hybrid materials, potentially possessing both the properties of the metallic component and the conducting polymer, as well as unique ensuing properties due to the compositing itself. This type of research is critical for driving innovation in materials research. Currently it is in its infancy, where the research is heavily focussed on demonstrating composite synthesis [29,30] with a few early reports on their application for devices such as non-volatile memory [17] and DNA hybridisation sensing [17,31]. Other anticipated future applications of these hybrid materials include noble metal deposition, electrocatalysis, neural tissue engineering, sensors, photovoltaic cells and memory devices [32]. For these composite materials to be truly suitable for low-cost commercial applications, it is important that they can be rendered processable. Exploiting an established oxidative polymerisation protocol for producing stable, processable nanodispersions of PANI was examined in this paper where the standard ammonium persulphate (APS) oxidant was replaced with HAuCl4 at a range of monomer:oxidant ratios to produce a range of hybrid PANI-Au dispersions. The particle size and populations of the gold microspheres and clusters, as well as the quality and processability of the PANI composite were shown to be influenced by the concentration of oxidant used. A ratio of 1:0.25 monomer:gold was found to be optimal for this type of synthesis where it yielded a stable, inkjet printable conductive PANI-Au composite, potentially suitable for application in electronic devices. 2. Materials and methods 2.1. Materials Aniline was purchased from Sigma-Aldrich and distilled before use. Ammonium peroxydisulfate (APS) (98.0% p.), sodium dodecyl sulfate (SDS) (99% p.), tetrachloroauric acid (HAuCl4) (≥ 49% Au), were purchased from Sigma-Aldrich. Dodecylbenzenesulfonic acid (DBSA) (90.0%, soft type mixture) was purchased from TCI Europe. Dialysis membrane (D9402), 12,000 Da molecular weight cut-off, was purchased from Sigma and soaked in deionised water at approximately 70 °C before use. The silver/silver chloride (Ag/AgCl) reference electrode was purchased from Bioanalytical Systems Ltd. (Cheshire, UK). Carbon paste no. C10903D14 was obtained from Gwent Electronic Materials Ltd. (Gwent, UK), silver conductive ink (Electrodag PF-410) and dielectric polymer ink (Electrodag 452 SS BLUE) were purchased from Acheson. Poly(ethylene) terephthalate substrates were Melinex ®films obtained from HiFi Industrial Film Ltd. (Dublin, Ireland).
CH1000 potentiostat (IJ Cambria Scientific Ltd, UK) with CH1000 software using cyclic voltammetry. Scanning electron microscopy (SEM) was performed with a Hitachi S3000N scanning electron microscope. An acceleration voltage of 20 kV was employed. UV measurements were carried out with a Varian Cary UV–Vis spectrophotometer (JVA Analytical Ltd., Ireland) where measurements were performed directly on the dispersions in quartz cuvettes by diluting the dispersions typically 50:1 with water. 2.3. Methods 2.3.1. Synthesis of PANI-APS and PANI-Au composites The synthesis of the PANI-APS was reported by Ngamna et al. [6]. Briefly, DBSA (3.4 g) was weighed out and made up to 40 ml with distilled water. The mixture was heated to 40 °C for 10 min to allow the DBSA to dissolve. The solution was then allowed to cool to room temperature. APS (0.36 g) was dissolved in 20 ml of the DBSA solution. The remaining DBSA solution was stirred at 400 rpm. Aniline (0.6 ml) was added to the solution, followed quickly by the APS solution. The mixture was left stirring for 2.5 h at room temperature. 20 ml SDS (0.05 M) was added to the dispersion and it was centrifuged for 30 min. The supernatant was dialyzed for 48 h against SDS (0.05 M) for 48 h, changing twice. PANI-Au was synthesised under same conditions as PANI using the oxidant HAuCl4 as a replacement for APS. Fig. 1 shows the spontaneous redox reaction that occurs between the anilinium ion and chloroauric acid to form metallic gold and the aniline radical cation which then polymerises to form polyaniline. Different molar ratios of aniline:HAuCl4 were prepared as summarized in Table 1. The concentration of DBSA used in all formulations was 0.25 M. 3. Results and discussion PANI is most commonly synthesised in the emeraldine salt form of PANI. Using APS as oxidant, the polymerisation reaction of aniline monomer on addition of oxidant, in the presence of DBSA typically takes approximately 30 min to reach completion, i.e., to change from the milky white colour of the DBSA-anilinium cation intermediate to a homogenous dark green colour, indicative of the presence of the emeraldine salt form of PANI. The rate of polymerisation increased substantially when using HAuCl4 as the oxidising agent at an equivalent concentration — the reaction took just 5 min to reach a dark green colour. This indicates that, under the conditions used, AuCl− 4 has a better capacity for the oxidation of the anilinium cation. This is contrary to the work of Zhang et al. [26] who reported that the rate of polymerisation of aniline was faster using APS as oxidant. In this report, camphor sulphonic acid (CSA) was used as the dopant acid, whereas for the synthesis reported here, DBSA was employed. The effect of the dopant on the rate of the polymerisation reaction is not yet fully understood. In terms of the oxidant, S2O2− ions are a more powerful oxidizing agent 8
2.2. Instrumentation Screen-printing of silver inter-digitated electrodes [8] and carbonpaste working electrodes [33] were performed on the DEK248 semiautomated screen-printer (DEK, UK), and are described elsewhere. Inkjet printing of PANI-Au was carried out using a Dimatix 2811 printer (Fuji Dimatix, United States) which is based on piezo jetting technology. No dilution/modification of the dispersions was required. Electroactivity and conductivity measurements were performed on a
Fig. 1. Reaction scheme showing the in-situ formation of gold and PANI. Aniline is oxidised with chloroauric acid to produce the aniline radical cation, metallic gold and hydrochloric acid. The aniline radical cations then form the polymer, polyaniline. DBSA in the polymerisation medium adjusts the starting acidity.
N. Lenhart et al. / Thin Solid Films 519 (2011) 4351–4356 Table 1 Ratio aniline:oxidant
Aniline [M]
Oxidant [M]
w/v % of resulting PANI composite
1:0.25 Aniline:APS 1:1 Aniline:HAuCl4 1:0.5 Aniline:HAuCl4 1:0.25 Aniline:HAuCl4 1:0.125 Aniline:HAuCl4 1:0.0625 Aniline:HAuCl4 1:0.03125 Aniline:HAuCl4
0.16 0.16 0.16 0.16 0.16 0.16 0.16
0.04 0.16 0.08 0.04 0.02 0.01 0.005
4.78 5.56 4.80 6.57 3.39 4.93
o o 2− 2− − than AuCl− 4 ions (E = 2.05 V S2O8 /SO4 , E = 1.002 V AuCl4 /Au(s) + − − 4Cl ) and so in this instance, it must be that the AuCl4 interaction with the anilinium cation is more synergistic for subsequent polymerisation. It may be that as very small amounts of the Au(III) are reduced initially to metallic particles, these particles can act as the nucleation sites for subsequent polymerisation to occur, thus reducing the activation energy required for this oxidation pathway to take place. The UV–Vis of the PANI-APS and the PANI-Au dispersions (Fig. 2) shows three typical PANI bands characteristic of the emeraldine salt form of the PANI. These experiments were performed directly on the dispersions in quartz cuvettes by diluting the dispersions typically 50:1 with water. The π–π* band appeared at 350 nm, the π–polaron band appeared at ~430 nm, and the localized polaron bands appeared in the range of 770–790 nm [7]. All spectra were normalised with respect to the π–π* band at 350 nm. The UV–Vis spectrum of the PANIAPS dispersion (Fig. 2a) was compared to that of the dispersions synthesised using HAuCl4 as oxidant. When APS was substituted by HAuCl4 the intensity of the π–polaron band at 430 nm increased, indicating a higher degree of doping in the resulting PANI-Au polymer. Additionally, increasing the monomer:Au ratio from 1:0.125 to 1:1 (Fig. 2b), also served to increase the relative intensity
Fig. 2. UV–Vis spectra of (a) PANI-APS and PANI-Au dispersions using a ratio of 1:0.25 aniline:oxidant and (b) PANI-Au dispersions synthesised using different molar ratios of aniline:HAuCl4. All spectra were normalised with respect to π–π* band at 350 nm.
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of the π–polaron band at 430 nm, indicating higher degrees of doping as increasing amounts of HAuCl4 were used. Moreover, when HAuCl4 was used in place of APS, the localised polaron band at ~ 780 nm shifted to longer wavelengths indicating higher conjugation lengths in polymers synthesised using AuHCl4 (Fig. 2a) [6]. There was no significant shift in wavelength observed however when varying amounts of HAuCl4 were used as oxidant (Fig. 2b). There was a noticeable shift in the baseline of the spectrum when HAuCl4 was used in place of APS as well as an upward shift in the baseline as increasing amounts of HAuCl4 was used. This was attributed to an increased scattering of reflected light by an increasing concentration of the metallic Au(0) particles within the dispersions as greater amounts of HAuCl4 were employed. No band attributable to the Au(0) was observed in the visible region due to this high baseline. The various PANI materials were deposited by drop-coating (3 μl) onto carbon-paste screen-printed electrodes and allowed to dry. Cyclic voltammetry was carried out on these modified electrodes in 1 M HCl in order to assess the electroactivity of the different PANI materials. It was observed that the voltammogram of the polymer synthesised from 1:0.25 was shown to be broadly similar to that of the previously reported PANI-APS material (Fig. 3) which has sharp, welldefined PANI redox peaks, showing rapid electron transfer between the different redox states of PANI [34]. The polymers synthesised from 1:1 and 1:0.0625 aniline-HAuCl4 ratios are also shown in Fig. 3. (All other voltammograms are available for viewing in the Supplementary data). It can be seen that due to the over-oxidation of PANI synthesised using a 1:1 aniline-HAuCl4 ratio, the resulting voltammogram possesses PANI redox peaks that are considerably broader than those of the other films and that the background capacitive current is high. The 1:0.0625 ratio resulted in a polymer dispersion that produced sharp, well-defined redox peaks, albeit with lower currents than the 1:0.25 ratio showing that the amount of electroactive PANI synthesised was considerably lower when very low amounts of gold salt was used. SEM was carried out on dried films cast onto polished glassy carbon plates from each of the dispersions synthesised using various ratios of aniline:HAuCl4. Fig. 4 shows the images of each of these films. It can be seen that as the amount of HAuCl4 used was decreased, the size and density of the gold clusters that were synthesised within the polymer decreased. The metallic Au clusters in Fig. 4(a), where a ratio of 1:1 aniline:HAuCl4 was used, are of the order of approximately 5 μm diameter and present as quite a high population. At a ratio of 1:0.5 (Fig. 4(b)), the Au clusters are still large, albeit with a lower population. Further decreasing the ratio to 1:0.25 (Fig. 4(c)), significantly smaller Au clusters and individual spherical particles (approx. 1000 nm diameter) are observed. These particles progressively decrease in population but appear to maintain the diameter of
Fig. 3. Cyclic voltammograms of carbon-paste screen-printed electrodes modified with the different PANI dispersions. Voltammograms were performed in 1 M HCl vs. Ag/AgCl at a scan rate of 500 mVs−1.
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Fig. 4. SEM images of PANI-Au films cast from dispersion onto glassy carbon plates. The ratio of aniline:HAuCl4 was varied for the syntheses of each of the dispersions to be (a) 1:1, (b) 1:0.5, (c) 1:0.25, (d) 1:0.125 and (e) 1:0.0625 aniline:HAuCl4. All images taken in backscattered electron (BSE) mode.
approx. 1000 nm as the ratio of aniline:HAuCl4 was reduced to 1:0.0625 (Fig. 4(e)). This trend is not surprising as the control over the particle size and population of gold nanodispersions is known to be dictated by the ratio of gold salt:reducing agent used. In addition, it has been reported previously that by using gold salts to oxidise aniline that the resulting size and population of the gold clusters is dependent on the initial aniline to metal ion ratio [25]. The underlying PANI morphology was also shown to vary as the volume of oxidant was decreased. In all instances, the nanoparticulate structure cannot be observed as the particles have coalesced together upon drying. At high concentrations of oxidant, the morphology of the PANI is rough whereas this decreased to result in a very smooth morphology of the PANI when the amount of oxidant used was reduced. All dispersions were inkjet printed without any modification to flexible PET using 10 consecutive prints (Fig. 5). The films were rinsed and allowed to dry at 70 °C for 20 min. The dispersions synthesised using higher ratios of gold salts (e.g., 1:1 and 1:0.5 aniline:HAuCl4) had prohibitively high viscosities (i.e., outside the range suitable for inkjet printing), as well as possessing large metallic gold clusters (Fig. 4). As a result, these materials did not print reliably. PANI-Au dispersions synthesised from ratios of 1:0.25 and 1:0.125 aniline:HAuCl4 had lower viscosities and smaller Au particles that should not limit printability (Fig. 4), resulting in high quality printed films. These dispersions resulted in the highest resolutions of all the printed films as these materials had much lower viscosities and the Au was observed as individual particles (1 μm diameter max.), as opposed to larger clusters. The surface tension of the dispersions was not measured but
is well within the range of 20–70 dyn cm−1, — the suitable range for inkjet printing. Given that the dispersions were heavily loaded with surfactant, clogging of the ejection nozzles in the printhead (20 μm in orifice diameter) with these micron-sized particles was prevented. It would be reasonable to expect that the thickness of the PANI bulk material within the films to be similar in thickness to those synthesised previously in a similar manner using APS as oxidant [7]. It had been reported that pure PANI films printed (10 prints) from these dispersions had thicknesses of approx. 350 nm. Therefore it would be reasonable to expect a similar thickness for the PANI component of these films. However, as the Au particles are of the order of 1 μm, these must protrude from the bulk film to heights up to 1–2 μm resulting in a highly heterogeneous composite morphology. Dispersions synthesised with even lower concentrations of HAuCl4 (1:0.0625 and 1:0.03125 aniline:HAuCl4) were not stable on the PET surface as when the films were rinsed, no continuous film of PANI remaining on the surface could be observed. This may be because the PANI formed during the oxidative polymerisation process in these dispersions possessed very short conjugation lengths and as such, the properties of the surfactant dominated the bulk film adhesion properties, and so was entirely removed from the surface upon washing. Fig. 6 shows the slope values of the current-to-voltage (i/V) curves of the inkjet printed PANI and PANI-Au films on inter-digitated electrodes. (It was not possible to calculate absolute resistances as the printed polymer films are soft and thus bulk film thickness could not be measured reliably). Thus the slopes of these plots (A/V), obtained by cycling the applied potential to the inkjet printed PANI composite-
N. Lenhart et al. / Thin Solid Films 519 (2011) 4351–4356
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Fig. 5. Fiducial camera images of inkjet printed films (10 prints) of the PANI-Au dispersions on PET.
Fig. 6. Histogram showing the measured slopes of the current-to-voltage plots for inkjet printed films of the different PANI composites inkjet printed onto inter-digitated carbon-paste electrodes (pitch spacing: 200 × 500 μm). Measurements based on the slope of the current-to-voltage curves (see inset) obtained when sweeping the potential between + 1 and −1 V and monitoring the current.
modified electrodes, were calculated (Fig. 6 inset) and plotted against each other. It was observed that the measured i/V (which is directly proportional to bulk conductivity if all dimension values of the samples remain equivalent) of the PANI-Au synthesised at a ratio of 1:0.25 was highest, approximately two-fold higher than that of PANIAPS when synthesised at the same ratio. If the presence of the gold in the films was a key factor attributed to this increase, it would have been expected that the measured i/V of the PANI-Au would increase with increasing amounts of gold oxidant used (see Fig. 4). However this was not the case. The highest i/V slope value was observed at a ratio of 1:0.25 aniline:HAuCl4. When higher amounts of HAuCl4 were employed, lower i/V values were observed which may be attributed to an over-oxidation of PANI in the presence of high amounts of strong oxidant. On the other hand, when lower molar ratios were used, the HAuCl4 becomes limiting and any polymerisation that took place probably resulted in short, oligomeric chains of aniline. This would explain why the i/V values of the inkjet printed films from these dispersions were observed to be so low. Using HAuCl4 at a ratio of 1:0.25 was found to be optimum for producing PANI with the highest i/V value. This was not attributed to the presence of the gold, but rather to the quality of the polymerisation that was induced using a ratio of 1:0.25 aniline:HAuCl4. This synergistic ratio produced the polymer with optimum doping levels and conjugation lengths to
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result in the highest relative conductivity. It can be seen from Table 1 that there was no significant difference in the w/v % between any of the dispersions, indicating that it was the quality (e.g., doping levels, conjugation lengths) and resulting redox state of the synthesised PANI that was critical for conductivity.
4. Conclusion A range of PANI-Au hybrid dispersions were synthesised whereby the properties of each of the resulting dispersions were controlled by varying the amount of gold salt used as oxidant. All synthesised PANI dispersions contained metallic gold, formed by the reduction of the gold salt during the oxidative polymerisation of aniline. The population and morphology of the metallic gold was controlled by the initial concentration of HAuCl4. It was shown that the presence of metallic gold did not affect the conductivity of the resulting polymer films as the percolation threshold of the gold was not reached. However, the amount of gold salt used for the polymerisation was shown to affect the quality of the polymer material itself. It was shown that by using a ratio of 1:0.25 aniline:HAuCl4, an optimum form of PANI-Au was produced with inherently high conjugation lengths, doping levels, conductivity and well-defined redox behaviour (as compared to PANI synthesised using APS as oxidant). In addition, this material was shown to be inkjet printable, resulting in conductive, homogeneously printed films suitable for printed electronic applications. These films will be assessed in the future for their potential in printed sensor applications. It is envisaged that the functionality of the metallic gold could be exploited for the attachment of biomolecules to produce processable bio-mediator composites for the development of printable electrochemical biosensor platforms.
Acknowledgements The authors would like to acknowledge the support of Science Foundation Ireland DiaMonD UREKA programme hosted by the Biomedical Diagnostics Institute.
References [1] S. Sathiyanarayanan, R. Jeyaram, S. Muthukrishnan, G. Venkatachari, J. Electrochem. Soc. 156 (2009) C127. [2] J.D. Sudha, S. Sivakala, R. Prasanth, V.L. Reena, P.R. Nair, Compos. Sci. Technol. 69 (2009) 358. [3] H. Aoki, N. Tanaka, K. Tub, K. Hosoya, J. Sep. Sci. 32 (2009) 341. [4] J.W. Lee, K.K.I. Mun, Y.T. Yoo, Progr. Org. Coat. 64 (2009) 98. [5] E. Gill, A. Arshak, K. Arshak, O. Korostynska, Sensors 7 (2007) 3329. [6] O. Ngamna, A. Morrin, A.J. Killard, S.E. Moulton, M.R. Smyth, G.G. Wallace, Langmuir 23 (2007) 8569. [7] A. Morrin, O. Ngamna, E. O'Malley, N. Kent, S.E. Moulton, G.G. Wallace, M.R. Smyth, A.J. Killard, Electrochim. Acta 53 (2008) 5092. [8] K. Crowley, A. Morrin, A. Hernandez, E. O'Malley, P.G. Whitten, G.G. Wallace, M.R. Smyth, A.J. Killard, Talanta 77 (2008) 710. [9] J. Cho, K.H. Shin, and J. Jang, Thin Solid Films 518 5066. [10] D. Li, J.X. Huang, R.B. Kaner, Acc. Chem. Res. 42 (2009) 135. [11] A. Drelinkiewicz, M. Hasik, M. Kloc, Catal. Lett. 64 (2000) 41. [12] X.L. Luo, A.J. Killard, A. Morrin, M.R. Smyth, Anal. Chim. Acta 575 (2006) 39. [13] J.H. Shu, W. Qiu, S.Q. Zheng, Prog. Chem. 21 (2009) 1015. [14] L. Zhang, H. Peng, P.A. Kilmartin, C. Soeller, R. Tilley, J. Travas-Seidic, Macromol. Rapid Commun. 29 (2008) 598. [15] J.X. Song, J.H. Yuan, F. Li, D.X. Han, J.F. Song, L. Niu, J. Solid State Electrochem. 14 (2010) 1915. [16] C.C. Hung, T.C. Wen, Y. Wei, Mater. Chem. Phys. 122 (2010) 392. [17] R.J. Tseng, J.X. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Nano Lett. 5 (2005) 1077. [18] R.J. Tseng, C.O. Baker, B. Shedd, J.X. Huang, R.B. Kaner, J.Y. Ouyang, Y. Yang, Appl. Phys. Lett. 90 (2007). [19] D. Wei, J.K. Baral, R. Osterbacka, A. Ivaska, J. Mater. Chem. 18 (2008) 1853. [20] M. Guerra-Balcazar, D. Morales-Acosta, F. Castaneda, J. Ledesma-Garcia, L.G. Arriaga, Electrochem. Commun. 12 (2010) 864. [21] E.T. Kang, Y.P. Ting, K.G. Neoh, K.L. Tan, Polymer 34 (1993) 4994. [22] K. Mallick, M.J. Witcomb, A. Dinsmore, M.S. Scurrell, Macromol. Rapid Commun. 26 (2005) 232. [23] Z.Q. Peng, L.M. Guo, Z.H. Zhang, B. Tesche, T. Wilke, D. Ogermann, S.H. Hu, K. Kleinermanns, Langmuir 22 (2006) 10915. [24] K. Mallick, M.J. Witcomb, M.S. Scurrell, A.M. Strydom, Gold Bull. 41 (2008) 246. [25] S.K. Pillalamarri, F.D. Blum, A.T. Tokuhiro, M.F. Bertino, Chem. Mater. 17 (2005) 5941. [26] X.T. Zhang, V. Chechik, D.K. Smith, P.H. Walton, A.K. Duhme-Klair, Macromolecules 41 (2008) 3417. [27] Z.J. Wang, J.H. Yuan, D.X. Han, L. Niu, A. Ivaska, Nanotechnology 18 (2007). [28] N.V. Blinova, J. Stejskal, M. Trchova, I. Sapurina, G. Ciric-Marjanovic, Polymer 50 (2009) 50. [29] M. Gniadek, E. Bak, Z. Stojek, and M. Donten, J. Solid State Electrochem. 14 1303. [30] J. Wang, K.G. Neoh, E.T. Kang, J. Coll. Inter. Sci. 239 (2001) 78. [31] S.J. Tian, J.Y. Liu, T. Zhu, W. Knoll, Chem. Mater. 16 (2004) 4103. [32] P.S. Kishore, B. Viswanathan, T.K. Varadarajan, Nanoscale Res. Lett. 3 (2008) 14. [33] K. Grennan, A.J. Killard, M.R. Smyth, Electroanalysis 13 (2001) 745. [34] A. Morrin, O. Ngamna, A.J. Killard, S.E. Moulton, M.R. Smyth, G.G. Wallace, Electroanalysis 17 (2005) 423.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Inkjet printable polyaniline-gold dispersions Natascha Lenhart a, Karl Crowley b, Anthony J. Killard c, Malcolm R. Smyth d, Aoife Morrin d,⁎ a
Hochschule Reutlingen/Reutlingen University, Alteburgstraße 150, 72762 Reutlingen, Germany Biomedical Diagnostics Institute, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland c Department of Applied Sciences, University of the West of England, Coldharbour Lane, Bristol BS16 1QY, UK d School of Chemical Sciences, National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland b
a r t i c l e
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Article history: Received 16 June 2010 Received in revised form 10 February 2011 Accepted 11 February 2011 Available online 2 March 2011 Keywords: Polyaniline composite Hybrid Gold Oxidative polymerisation Inkjet printing
a b s t r a c t Inkjet printable polyaniline-gold (PANI-Au) hybrid dispersions were synthesised using gold salt as oxidant to simultaneously induce chemical oxidative polymerisation of aniline and reduction of HAuCl4 in bulk aqueous solution. By varying the amount of HAuCl4 used for the polymerisation, the size, morphology and population of the resulting gold particles embedded within the polymer were controllable. It was shown however, that the metallic gold particles contained within the resulting dispersions did not appear to affect the bulk conductivity of PANI, but rather that by varying the amount of HAuCl4 used, both the quality and printability of the resulting PANI-Au dispersion could be affected. PANI-Au synthesised using a ratio of 1:0.25 aniline: HAuCl4 was shown to be optimum as it resulted in dispersions with high polymer conjugation lengths and doping levels, as well as high conductivity and well-defined electrochemistry. This hybrid material was shown to be highly processable, and inkjet printing of this material was demonstrated on flexible substrate which resulted in high quality, printed films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polyaniline (PANI) is recognized as one of the most promising inherently conducting polymers (ICPs) to be discovered due to its highly reversible redox and pH switching properties, recent breakthroughs in ease of synthesis and wide range of potential applications in anti-corrosion coatings [1], electromagnetic shielding [2], rechargeable polymeric batteries, polymer photovoltaics and polymer actuators and sensors. Several techniques have been used to prepare thin films of pure PANI, e.g., spin-coating, casting, self-assembly and Langmuir–Blodgett (LB) methods [3]. Recently, printing methods such as gravure- [4], screen- [5] and inkjet- printing [6–9] are being employed to pattern PANI. An approach to render PANI as printable is to create a stable dispersion of the material in aqueous or mild organic media. A wide range of stable nanoparticulate forms of PANI have been reported in the literature recently to include spherical nanoparticles, nanofibres and nanotubes. Emerging from the literature is that the spherical nanoparticulate form is most suitable for printing applications. Nanofibres, for example, have the drawback of their inherent length, typically of the order of several microns [10], making them unsuitable for printing as they can become entangled and easily cause clogging of printhead nozzles. The inherent morphology of PANI nanoparticulates used for this research is spherical and of the order of 100 nm diameter
⁎ Corresponding author: Tel.: +353 1 700 6730; fax: +353 1 700 5309. E-mail address: [email protected] (A. Morrin). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.02.045
making them highly processable and hence amenable to inkjet printing deposition [6]. Compositing PANI with noble metals or carbon nanotubes to create hybrid functional materials has been shown to enhance sensing and catalytic capabilities [11,12], as compared to those of pure PANI. Specifically, incorporation of gold into PANI has become a popular and interesting aspect of composite synthesis as these resulting materials differ from pure PANI in respect of some physical and chemical properties [13]. Enhanced conductivities [14] and electrocatalysis [15,16] over pure PANI have been reported. For example, Zhang et al. showed that the electrical conductivity of a PANI-Au nanocomposite (synthesised in the presence of p-toluene sulphonic acid) was two orders of magnitude greater than that of the pure PANI [14]. Electrical bistability and memory characteristics have also recently been reported by using chemically synthesised PANI nanofibers embedded with Au [17] and other composite configurations [18,19]. A number of different approaches have been suggested for gold incorporation into the polymer [15,16,20]. The synthesis of PANI-Au composites was pioneered by Kang and co-workers in 1993 [21]. They published the first report on the spontaneous and sustained reduction and accumulation of metallic gold from a chloroauric acid solution by PANI. As well as using pre-formed PANI as the reducing agent for the metallic ions, several papers have reported using aniline as the reducing agent, wherein as the gold salt becomes reduced, the aniline undergoes oxidative polymerisation [22,23]. This approach has yielded a range of PANI morphologies including bulk and fibrillar [24] structures containing spherical particles or clusters of gold which have been reported to range in size from just 2 nm [25] to micrometer
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dimensions [26]. The size, monodispersity and population of the gold particles within the polymer depend heavily on the initial synthesis conditions used, i.e., the type and mass of each of the dopant, oxidant and monomer. Architectures such as Au-PANI core–shell particles have also been reported [27]. In this instance, surfactant and acetic acid were found to be critical for the formation of the stable core–shell structure. In addition to gold, other noble metal salts such as silver, platinum and palladium salts have been reported as suitable oxidants for PANI synthesis. However, PANI synthesis using silver nitrate for example was reported to be very slow, taking up to several weeks to polymerise the aniline depending on conditions used [28]. Combining precious metals and conducting organic polymers through compositing provides combinatorial hybrid materials, potentially possessing both the properties of the metallic component and the conducting polymer, as well as unique ensuing properties due to the compositing itself. This type of research is critical for driving innovation in materials research. Currently it is in its infancy, where the research is heavily focussed on demonstrating composite synthesis [29,30] with a few early reports on their application for devices such as non-volatile memory [17] and DNA hybridisation sensing [17,31]. Other anticipated future applications of these hybrid materials include noble metal deposition, electrocatalysis, neural tissue engineering, sensors, photovoltaic cells and memory devices [32]. For these composite materials to be truly suitable for low-cost commercial applications, it is important that they can be rendered processable. Exploiting an established oxidative polymerisation protocol for producing stable, processable nanodispersions of PANI was examined in this paper where the standard ammonium persulphate (APS) oxidant was replaced with HAuCl4 at a range of monomer:oxidant ratios to produce a range of hybrid PANI-Au dispersions. The particle size and populations of the gold microspheres and clusters, as well as the quality and processability of the PANI composite were shown to be influenced by the concentration of oxidant used. A ratio of 1:0.25 monomer:gold was found to be optimal for this type of synthesis where it yielded a stable, inkjet printable conductive PANI-Au composite, potentially suitable for application in electronic devices. 2. Materials and methods 2.1. Materials Aniline was purchased from Sigma-Aldrich and distilled before use. Ammonium peroxydisulfate (APS) (98.0% p.), sodium dodecyl sulfate (SDS) (99% p.), tetrachloroauric acid (HAuCl4) (≥ 49% Au), were purchased from Sigma-Aldrich. Dodecylbenzenesulfonic acid (DBSA) (90.0%, soft type mixture) was purchased from TCI Europe. Dialysis membrane (D9402), 12,000 Da molecular weight cut-off, was purchased from Sigma and soaked in deionised water at approximately 70 °C before use. The silver/silver chloride (Ag/AgCl) reference electrode was purchased from Bioanalytical Systems Ltd. (Cheshire, UK). Carbon paste no. C10903D14 was obtained from Gwent Electronic Materials Ltd. (Gwent, UK), silver conductive ink (Electrodag PF-410) and dielectric polymer ink (Electrodag 452 SS BLUE) were purchased from Acheson. Poly(ethylene) terephthalate substrates were Melinex ®films obtained from HiFi Industrial Film Ltd. (Dublin, Ireland).
CH1000 potentiostat (IJ Cambria Scientific Ltd, UK) with CH1000 software using cyclic voltammetry. Scanning electron microscopy (SEM) was performed with a Hitachi S3000N scanning electron microscope. An acceleration voltage of 20 kV was employed. UV measurements were carried out with a Varian Cary UV–Vis spectrophotometer (JVA Analytical Ltd., Ireland) where measurements were performed directly on the dispersions in quartz cuvettes by diluting the dispersions typically 50:1 with water. 2.3. Methods 2.3.1. Synthesis of PANI-APS and PANI-Au composites The synthesis of the PANI-APS was reported by Ngamna et al. [6]. Briefly, DBSA (3.4 g) was weighed out and made up to 40 ml with distilled water. The mixture was heated to 40 °C for 10 min to allow the DBSA to dissolve. The solution was then allowed to cool to room temperature. APS (0.36 g) was dissolved in 20 ml of the DBSA solution. The remaining DBSA solution was stirred at 400 rpm. Aniline (0.6 ml) was added to the solution, followed quickly by the APS solution. The mixture was left stirring for 2.5 h at room temperature. 20 ml SDS (0.05 M) was added to the dispersion and it was centrifuged for 30 min. The supernatant was dialyzed for 48 h against SDS (0.05 M) for 48 h, changing twice. PANI-Au was synthesised under same conditions as PANI using the oxidant HAuCl4 as a replacement for APS. Fig. 1 shows the spontaneous redox reaction that occurs between the anilinium ion and chloroauric acid to form metallic gold and the aniline radical cation which then polymerises to form polyaniline. Different molar ratios of aniline:HAuCl4 were prepared as summarized in Table 1. The concentration of DBSA used in all formulations was 0.25 M. 3. Results and discussion PANI is most commonly synthesised in the emeraldine salt form of PANI. Using APS as oxidant, the polymerisation reaction of aniline monomer on addition of oxidant, in the presence of DBSA typically takes approximately 30 min to reach completion, i.e., to change from the milky white colour of the DBSA-anilinium cation intermediate to a homogenous dark green colour, indicative of the presence of the emeraldine salt form of PANI. The rate of polymerisation increased substantially when using HAuCl4 as the oxidising agent at an equivalent concentration — the reaction took just 5 min to reach a dark green colour. This indicates that, under the conditions used, AuCl− 4 has a better capacity for the oxidation of the anilinium cation. This is contrary to the work of Zhang et al. [26] who reported that the rate of polymerisation of aniline was faster using APS as oxidant. In this report, camphor sulphonic acid (CSA) was used as the dopant acid, whereas for the synthesis reported here, DBSA was employed. The effect of the dopant on the rate of the polymerisation reaction is not yet fully understood. In terms of the oxidant, S2O2− ions are a more powerful oxidizing agent 8
2.2. Instrumentation Screen-printing of silver inter-digitated electrodes [8] and carbonpaste working electrodes [33] were performed on the DEK248 semiautomated screen-printer (DEK, UK), and are described elsewhere. Inkjet printing of PANI-Au was carried out using a Dimatix 2811 printer (Fuji Dimatix, United States) which is based on piezo jetting technology. No dilution/modification of the dispersions was required. Electroactivity and conductivity measurements were performed on a
Fig. 1. Reaction scheme showing the in-situ formation of gold and PANI. Aniline is oxidised with chloroauric acid to produce the aniline radical cation, metallic gold and hydrochloric acid. The aniline radical cations then form the polymer, polyaniline. DBSA in the polymerisation medium adjusts the starting acidity.
N. Lenhart et al. / Thin Solid Films 519 (2011) 4351–4356 Table 1 Ratio aniline:oxidant
Aniline [M]
Oxidant [M]
w/v % of resulting PANI composite
1:0.25 Aniline:APS 1:1 Aniline:HAuCl4 1:0.5 Aniline:HAuCl4 1:0.25 Aniline:HAuCl4 1:0.125 Aniline:HAuCl4 1:0.0625 Aniline:HAuCl4 1:0.03125 Aniline:HAuCl4
0.16 0.16 0.16 0.16 0.16 0.16 0.16
0.04 0.16 0.08 0.04 0.02 0.01 0.005
4.78 5.56 4.80 6.57 3.39 4.93
o o 2− 2− − than AuCl− 4 ions (E = 2.05 V S2O8 /SO4 , E = 1.002 V AuCl4 /Au(s) + − − 4Cl ) and so in this instance, it must be that the AuCl4 interaction with the anilinium cation is more synergistic for subsequent polymerisation. It may be that as very small amounts of the Au(III) are reduced initially to metallic particles, these particles can act as the nucleation sites for subsequent polymerisation to occur, thus reducing the activation energy required for this oxidation pathway to take place. The UV–Vis of the PANI-APS and the PANI-Au dispersions (Fig. 2) shows three typical PANI bands characteristic of the emeraldine salt form of the PANI. These experiments were performed directly on the dispersions in quartz cuvettes by diluting the dispersions typically 50:1 with water. The π–π* band appeared at 350 nm, the π–polaron band appeared at ~430 nm, and the localized polaron bands appeared in the range of 770–790 nm [7]. All spectra were normalised with respect to the π–π* band at 350 nm. The UV–Vis spectrum of the PANIAPS dispersion (Fig. 2a) was compared to that of the dispersions synthesised using HAuCl4 as oxidant. When APS was substituted by HAuCl4 the intensity of the π–polaron band at 430 nm increased, indicating a higher degree of doping in the resulting PANI-Au polymer. Additionally, increasing the monomer:Au ratio from 1:0.125 to 1:1 (Fig. 2b), also served to increase the relative intensity
Fig. 2. UV–Vis spectra of (a) PANI-APS and PANI-Au dispersions using a ratio of 1:0.25 aniline:oxidant and (b) PANI-Au dispersions synthesised using different molar ratios of aniline:HAuCl4. All spectra were normalised with respect to π–π* band at 350 nm.
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of the π–polaron band at 430 nm, indicating higher degrees of doping as increasing amounts of HAuCl4 were used. Moreover, when HAuCl4 was used in place of APS, the localised polaron band at ~ 780 nm shifted to longer wavelengths indicating higher conjugation lengths in polymers synthesised using AuHCl4 (Fig. 2a) [6]. There was no significant shift in wavelength observed however when varying amounts of HAuCl4 were used as oxidant (Fig. 2b). There was a noticeable shift in the baseline of the spectrum when HAuCl4 was used in place of APS as well as an upward shift in the baseline as increasing amounts of HAuCl4 was used. This was attributed to an increased scattering of reflected light by an increasing concentration of the metallic Au(0) particles within the dispersions as greater amounts of HAuCl4 were employed. No band attributable to the Au(0) was observed in the visible region due to this high baseline. The various PANI materials were deposited by drop-coating (3 μl) onto carbon-paste screen-printed electrodes and allowed to dry. Cyclic voltammetry was carried out on these modified electrodes in 1 M HCl in order to assess the electroactivity of the different PANI materials. It was observed that the voltammogram of the polymer synthesised from 1:0.25 was shown to be broadly similar to that of the previously reported PANI-APS material (Fig. 3) which has sharp, welldefined PANI redox peaks, showing rapid electron transfer between the different redox states of PANI [34]. The polymers synthesised from 1:1 and 1:0.0625 aniline-HAuCl4 ratios are also shown in Fig. 3. (All other voltammograms are available for viewing in the Supplementary data). It can be seen that due to the over-oxidation of PANI synthesised using a 1:1 aniline-HAuCl4 ratio, the resulting voltammogram possesses PANI redox peaks that are considerably broader than those of the other films and that the background capacitive current is high. The 1:0.0625 ratio resulted in a polymer dispersion that produced sharp, well-defined redox peaks, albeit with lower currents than the 1:0.25 ratio showing that the amount of electroactive PANI synthesised was considerably lower when very low amounts of gold salt was used. SEM was carried out on dried films cast onto polished glassy carbon plates from each of the dispersions synthesised using various ratios of aniline:HAuCl4. Fig. 4 shows the images of each of these films. It can be seen that as the amount of HAuCl4 used was decreased, the size and density of the gold clusters that were synthesised within the polymer decreased. The metallic Au clusters in Fig. 4(a), where a ratio of 1:1 aniline:HAuCl4 was used, are of the order of approximately 5 μm diameter and present as quite a high population. At a ratio of 1:0.5 (Fig. 4(b)), the Au clusters are still large, albeit with a lower population. Further decreasing the ratio to 1:0.25 (Fig. 4(c)), significantly smaller Au clusters and individual spherical particles (approx. 1000 nm diameter) are observed. These particles progressively decrease in population but appear to maintain the diameter of
Fig. 3. Cyclic voltammograms of carbon-paste screen-printed electrodes modified with the different PANI dispersions. Voltammograms were performed in 1 M HCl vs. Ag/AgCl at a scan rate of 500 mVs−1.
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Fig. 4. SEM images of PANI-Au films cast from dispersion onto glassy carbon plates. The ratio of aniline:HAuCl4 was varied for the syntheses of each of the dispersions to be (a) 1:1, (b) 1:0.5, (c) 1:0.25, (d) 1:0.125 and (e) 1:0.0625 aniline:HAuCl4. All images taken in backscattered electron (BSE) mode.
approx. 1000 nm as the ratio of aniline:HAuCl4 was reduced to 1:0.0625 (Fig. 4(e)). This trend is not surprising as the control over the particle size and population of gold nanodispersions is known to be dictated by the ratio of gold salt:reducing agent used. In addition, it has been reported previously that by using gold salts to oxidise aniline that the resulting size and population of the gold clusters is dependent on the initial aniline to metal ion ratio [25]. The underlying PANI morphology was also shown to vary as the volume of oxidant was decreased. In all instances, the nanoparticulate structure cannot be observed as the particles have coalesced together upon drying. At high concentrations of oxidant, the morphology of the PANI is rough whereas this decreased to result in a very smooth morphology of the PANI when the amount of oxidant used was reduced. All dispersions were inkjet printed without any modification to flexible PET using 10 consecutive prints (Fig. 5). The films were rinsed and allowed to dry at 70 °C for 20 min. The dispersions synthesised using higher ratios of gold salts (e.g., 1:1 and 1:0.5 aniline:HAuCl4) had prohibitively high viscosities (i.e., outside the range suitable for inkjet printing), as well as possessing large metallic gold clusters (Fig. 4). As a result, these materials did not print reliably. PANI-Au dispersions synthesised from ratios of 1:0.25 and 1:0.125 aniline:HAuCl4 had lower viscosities and smaller Au particles that should not limit printability (Fig. 4), resulting in high quality printed films. These dispersions resulted in the highest resolutions of all the printed films as these materials had much lower viscosities and the Au was observed as individual particles (1 μm diameter max.), as opposed to larger clusters. The surface tension of the dispersions was not measured but
is well within the range of 20–70 dyn cm−1, — the suitable range for inkjet printing. Given that the dispersions were heavily loaded with surfactant, clogging of the ejection nozzles in the printhead (20 μm in orifice diameter) with these micron-sized particles was prevented. It would be reasonable to expect that the thickness of the PANI bulk material within the films to be similar in thickness to those synthesised previously in a similar manner using APS as oxidant [7]. It had been reported that pure PANI films printed (10 prints) from these dispersions had thicknesses of approx. 350 nm. Therefore it would be reasonable to expect a similar thickness for the PANI component of these films. However, as the Au particles are of the order of 1 μm, these must protrude from the bulk film to heights up to 1–2 μm resulting in a highly heterogeneous composite morphology. Dispersions synthesised with even lower concentrations of HAuCl4 (1:0.0625 and 1:0.03125 aniline:HAuCl4) were not stable on the PET surface as when the films were rinsed, no continuous film of PANI remaining on the surface could be observed. This may be because the PANI formed during the oxidative polymerisation process in these dispersions possessed very short conjugation lengths and as such, the properties of the surfactant dominated the bulk film adhesion properties, and so was entirely removed from the surface upon washing. Fig. 6 shows the slope values of the current-to-voltage (i/V) curves of the inkjet printed PANI and PANI-Au films on inter-digitated electrodes. (It was not possible to calculate absolute resistances as the printed polymer films are soft and thus bulk film thickness could not be measured reliably). Thus the slopes of these plots (A/V), obtained by cycling the applied potential to the inkjet printed PANI composite-
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Fig. 5. Fiducial camera images of inkjet printed films (10 prints) of the PANI-Au dispersions on PET.
Fig. 6. Histogram showing the measured slopes of the current-to-voltage plots for inkjet printed films of the different PANI composites inkjet printed onto inter-digitated carbon-paste electrodes (pitch spacing: 200 × 500 μm). Measurements based on the slope of the current-to-voltage curves (see inset) obtained when sweeping the potential between + 1 and −1 V and monitoring the current.
modified electrodes, were calculated (Fig. 6 inset) and plotted against each other. It was observed that the measured i/V (which is directly proportional to bulk conductivity if all dimension values of the samples remain equivalent) of the PANI-Au synthesised at a ratio of 1:0.25 was highest, approximately two-fold higher than that of PANIAPS when synthesised at the same ratio. If the presence of the gold in the films was a key factor attributed to this increase, it would have been expected that the measured i/V of the PANI-Au would increase with increasing amounts of gold oxidant used (see Fig. 4). However this was not the case. The highest i/V slope value was observed at a ratio of 1:0.25 aniline:HAuCl4. When higher amounts of HAuCl4 were employed, lower i/V values were observed which may be attributed to an over-oxidation of PANI in the presence of high amounts of strong oxidant. On the other hand, when lower molar ratios were used, the HAuCl4 becomes limiting and any polymerisation that took place probably resulted in short, oligomeric chains of aniline. This would explain why the i/V values of the inkjet printed films from these dispersions were observed to be so low. Using HAuCl4 at a ratio of 1:0.25 was found to be optimum for producing PANI with the highest i/V value. This was not attributed to the presence of the gold, but rather to the quality of the polymerisation that was induced using a ratio of 1:0.25 aniline:HAuCl4. This synergistic ratio produced the polymer with optimum doping levels and conjugation lengths to
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result in the highest relative conductivity. It can be seen from Table 1 that there was no significant difference in the w/v % between any of the dispersions, indicating that it was the quality (e.g., doping levels, conjugation lengths) and resulting redox state of the synthesised PANI that was critical for conductivity.
4. Conclusion A range of PANI-Au hybrid dispersions were synthesised whereby the properties of each of the resulting dispersions were controlled by varying the amount of gold salt used as oxidant. All synthesised PANI dispersions contained metallic gold, formed by the reduction of the gold salt during the oxidative polymerisation of aniline. The population and morphology of the metallic gold was controlled by the initial concentration of HAuCl4. It was shown that the presence of metallic gold did not affect the conductivity of the resulting polymer films as the percolation threshold of the gold was not reached. However, the amount of gold salt used for the polymerisation was shown to affect the quality of the polymer material itself. It was shown that by using a ratio of 1:0.25 aniline:HAuCl4, an optimum form of PANI-Au was produced with inherently high conjugation lengths, doping levels, conductivity and well-defined redox behaviour (as compared to PANI synthesised using APS as oxidant). In addition, this material was shown to be inkjet printable, resulting in conductive, homogeneously printed films suitable for printed electronic applications. These films will be assessed in the future for their potential in printed sensor applications. It is envisaged that the functionality of the metallic gold could be exploited for the attachment of biomolecules to produce processable bio-mediator composites for the development of printable electrochemical biosensor platforms.
Acknowledgements The authors would like to acknowledge the support of Science Foundation Ireland DiaMonD UREKA programme hosted by the Biomedical Diagnostics Institute.
References [1] S. Sathiyanarayanan, R. Jeyaram, S. Muthukrishnan, G. Venkatachari, J. Electrochem. Soc. 156 (2009) C127. [2] J.D. Sudha, S. Sivakala, R. Prasanth, V.L. Reena, P.R. Nair, Compos. Sci. Technol. 69 (2009) 358. [3] H. Aoki, N. Tanaka, K. Tub, K. Hosoya, J. Sep. Sci. 32 (2009) 341. [4] J.W. Lee, K.K.I. Mun, Y.T. Yoo, Progr. Org. Coat. 64 (2009) 98. [5] E. Gill, A. Arshak, K. Arshak, O. Korostynska, Sensors 7 (2007) 3329. [6] O. Ngamna, A. Morrin, A.J. Killard, S.E. Moulton, M.R. Smyth, G.G. Wallace, Langmuir 23 (2007) 8569. [7] A. Morrin, O. Ngamna, E. O'Malley, N. Kent, S.E. Moulton, G.G. Wallace, M.R. Smyth, A.J. Killard, Electrochim. Acta 53 (2008) 5092. [8] K. Crowley, A. Morrin, A. Hernandez, E. O'Malley, P.G. Whitten, G.G. Wallace, M.R. Smyth, A.J. Killard, Talanta 77 (2008) 710. [9] J. Cho, K.H. Shin, and J. Jang, Thin Solid Films 518 5066. [10] D. Li, J.X. Huang, R.B. Kaner, Acc. Chem. Res. 42 (2009) 135. [11] A. Drelinkiewicz, M. Hasik, M. Kloc, Catal. Lett. 64 (2000) 41. [12] X.L. Luo, A.J. Killard, A. Morrin, M.R. Smyth, Anal. Chim. Acta 575 (2006) 39. [13] J.H. Shu, W. Qiu, S.Q. Zheng, Prog. Chem. 21 (2009) 1015. [14] L. Zhang, H. Peng, P.A. Kilmartin, C. Soeller, R. Tilley, J. Travas-Seidic, Macromol. Rapid Commun. 29 (2008) 598. [15] J.X. Song, J.H. Yuan, F. Li, D.X. Han, J.F. Song, L. Niu, J. Solid State Electrochem. 14 (2010) 1915. [16] C.C. Hung, T.C. Wen, Y. Wei, Mater. Chem. Phys. 122 (2010) 392. [17] R.J. Tseng, J.X. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Nano Lett. 5 (2005) 1077. [18] R.J. Tseng, C.O. Baker, B. Shedd, J.X. Huang, R.B. Kaner, J.Y. Ouyang, Y. Yang, Appl. Phys. Lett. 90 (2007). [19] D. Wei, J.K. Baral, R. Osterbacka, A. Ivaska, J. Mater. Chem. 18 (2008) 1853. [20] M. Guerra-Balcazar, D. Morales-Acosta, F. Castaneda, J. Ledesma-Garcia, L.G. Arriaga, Electrochem. Commun. 12 (2010) 864. [21] E.T. Kang, Y.P. Ting, K.G. Neoh, K.L. Tan, Polymer 34 (1993) 4994. [22] K. Mallick, M.J. Witcomb, A. Dinsmore, M.S. Scurrell, Macromol. Rapid Commun. 26 (2005) 232. [23] Z.Q. Peng, L.M. Guo, Z.H. Zhang, B. Tesche, T. Wilke, D. Ogermann, S.H. Hu, K. Kleinermanns, Langmuir 22 (2006) 10915. [24] K. Mallick, M.J. Witcomb, M.S. Scurrell, A.M. Strydom, Gold Bull. 41 (2008) 246. [25] S.K. Pillalamarri, F.D. Blum, A.T. Tokuhiro, M.F. Bertino, Chem. Mater. 17 (2005) 5941. [26] X.T. Zhang, V. Chechik, D.K. Smith, P.H. Walton, A.K. Duhme-Klair, Macromolecules 41 (2008) 3417. [27] Z.J. Wang, J.H. Yuan, D.X. Han, L. Niu, A. Ivaska, Nanotechnology 18 (2007). [28] N.V. Blinova, J. Stejskal, M. Trchova, I. Sapurina, G. Ciric-Marjanovic, Polymer 50 (2009) 50. [29] M. Gniadek, E. Bak, Z. Stojek, and M. Donten, J. Solid State Electrochem. 14 1303. [30] J. Wang, K.G. Neoh, E.T. Kang, J. Coll. Inter. Sci. 239 (2001) 78. [31] S.J. Tian, J.Y. Liu, T. Zhu, W. Knoll, Chem. Mater. 16 (2004) 4103. [32] P.S. Kishore, B. Viswanathan, T.K. Varadarajan, Nanoscale Res. Lett. 3 (2008) 14. [33] K. Grennan, A.J. Killard, M.R. Smyth, Electroanalysis 13 (2001) 745. [34] A. Morrin, O. Ngamna, A.J. Killard, S.E. Moulton, M.R. Smyth, G.G. Wallace, Electroanalysis 17 (2005) 423.