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Inkjet printing of conductive polymer nanowire network on flexible substrates and its application in chemical sensing
Microelectronic Engineering 145 (2015) 143–148
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Inkjet printing of conductive polymer nanowire network on flexible substrates and its application in chemical sensing Edward Song a, Ryan P. Tortorich a, Tallis H. da Costa a, Jin-Woo Choi a,b,⇑ a b
School of Electrical Engineering and Computer Science, Louisiana State University, Baton Rouge, LA 70803, USA Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, LA 70803, USA
a r t i c l e
i n f o
Article history: Received 24 October 2014 Received in revised form 15 March 2015 Accepted 1 April 2015 Available online 7 April 2015 Keywords: Inkjet printing Polyaniline Nanowire Chemiresistive sensing
a b s t r a c t This work reports an inkjet printing technique for patterning a conducting polymer nanowire network on a flexible film for applications in chemical sensing. The novelty of this work is in the patterning capability of polymer nanowires to form a conducting path. Polyaniline nanowires were chemically synthesized in an aqueous solution and a surfactant was added to lower the surface tension which enabled the printing of the nanowires using a commercially available inkjet printer. The nanowire network-based patterns were printed on a flexible transparency film, and its morphology characterization, patterning ability as well as the electrical properties were investigated. Finally, as a proof-of-concept, a fully-printed chemical sensors were developed by using the proposed printing technique on flexible films. Two types of sensors were fabricated: a pH sensor and a hydrogen peroxide sensor. The results demonstrate that the developed sensors can be utilized as a low cost, disposable, and easily printable chemical sensors. The proposed technology may find applications in the development of a simple print-and-use biochemical sensing kit for potential use in point-of-care diagnostics. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Fabrication of a single-use and disposable sensor using various printing techniques has many advantages especially in chemo- and bio-sensors due to their low cost, mass producibility, and portability. Moreover, printed electronic materials are generally thin and flexible and therefore have potential applications in wearable and implantable electronics. Among the various printing techniques, an inkjet printing method can provide a quick, simple, and automated solution to developing disposable sensors [1]. Due to the many benefits that the nanomaterials offer especially in chemical and biological sensing, there have been numerous reports demonstrating the feasibility of inkjet printing of various nanomaterials including metal and oxide nanoparticles, carbon nanotubes [2], and even graphene sheets on a flexible substrate [3]. Inkjet printing of conducting polymer materials is also becoming a topic of great interest due to the unique properties of the conducting polymers that can be utilized in sensors and electronic devices [4,5]. However inkjet printing of conducting polymer nanowires to form highly interconnected network of 1-dimensional ⇑ Corresponding author at: School of Electrical Engineering and Computer Science, Louisiana State University, Baton Rouge, LA 70803, USA. Tel.: +1 225 578 8764. E-mail address: [email protected] (J.-W. Choi). http://dx.doi.org/10.1016/j.mee.2015.04.004 0167-9317/Ó 2015 Elsevier B.V. All rights reserved.
nanostructures have not been demonstrated so far. Nanowire network morphology is ideal for producing highly conductive patterns and is also desirable as a sensing material due to its large surface area and porosity leading to enhanced sensitivity and faster response time [6,7]. Polyaniline is one of the most widely studied conducting polymer and has found many applications as a sensing material for chemical and biological species. Polyaniline is a very interesting polymer due to its many unique properties. First, it is an intrinsically conducting polymer with a relatively high electrical conductivity (on the order of a few S/m) for a polymer material. Due to its ability to conduct electricity, it can be used for electronic sensor applications. Secondly, the conductivity of polyaniline is highly influenced by the pH of the environment to which it is exposed. Polyaniline exhibits maximum conductivity under a strongly acidic environment (low pH) and loses its conductivity at neutral or basic environment (pH 7 or higher). Therefore chemiresistive polyaniline-based pH sensors have been extensively studied and documented [8–10]. Inkjet printing of polyaniline nanoparticles and nanograins have been previously reported and the development of an ammonia sensor was demonstrated [4,11]. However, in order to form a well-conducting path for electron transport, an interweaved nanowire network is the desired solution for applications in resistive sensing. In this work, we demonstrate the capability of printing
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polyaniline nanowire network patterns on a transparency film to be used as chemical sensors. The novelties of this work are in the inkjet printing of conducting polymer nanowires and the development of fully-printable chemiresistive sensors.
2. Working principle Fig. 1 illustrates the basic concept of the use of inkjet printing technique for the development of a printable chemical sensors. First, a nanowire dispersed liquid is loaded into the printer cartridge to be printed on a substrate. Due to the nanowire morphology of the polyaniline material, the printed patterns will contain a randomly oriented network of conducting polymer nanowires which forms electrically conducting patterns as shown in Fig. 1(a) and (b). Since the conductivity of polyaniline is pH-dependent, a pH sensor can be developed by measuring the resistance across the polyaniline layer deposited between the two electrodes. This type of chemiresistive pH sensor can be implemented by the inkjet printing technique as illustrated in Fig. 1(b). A similar approach can be taken to develop a hydrogen peroxide sensor by including catalytic (silver) nanoparticles to the polyaniline nanowire network [12]. Fig. 1(c) depicts the configuration of the chemiresistive hydrogen peroxide sensor where the nanoparticles are evenly distributed throughout the network. When the catalytic reaction occurs between the nanoparticles and hydrogen peroxide, hydroxide ions (OH) are produced as the byproduct of the reaction which increases the local pH near polyaniline. Therefore, the resulting change in conductivity of polyaniline can be measured to determine the hydrogen peroxide concentration.
3. Experimental methods 3.1. Chemical synthesis of the polyaniline nanowires The method for the chemical synthesis of polyaniline nanowires is based on the technique developed by Kaner [13]. Briefly, 10 ml of 0.08 M aniline (73.5 ll) in 1 M HCl, and 10 ml of 0.02 M ammonium peroxydisulfate (APS, 45 mg) in 1 M HCl were prepared in two separate vials. The two solutions were rapidly and vigorously mixed together and left un-agitated overnight. The synthesized nanowires were rinsed by centrifuging the product at 13,000 RPM for 10 min followed by removing the aqueous liquid, adding the same volume of DI water, and gently agitating the centrifuge tube. This rinsing step was repeated twice to thoroughly remove the unreacted oxidants and acids from the liquid.
3.2. Printable ink preparation In order to print the polyaniline nanowires through the ink cartridge, the surface tension of the ink must be sufficiently low so that the ink droplets can be discharged through the nozzle. For lowering the surface tension of the nanowire suspension, 10 mg/ ml of sodium dodecylsulfate (SDS), which is a common surfactant, was added to the nanowire dispersion and gently stirred until SDS was completely dissolved. 3.3. Inkjet printing of conducting polymer patterns The printing process was performed using a commercially available inkjet printer (HP DeskJet D1520) which was used as received without further modification. A letter sized printable transparency film was used as a printing substrate. The black ink cartridge was thoroughly rinsed with water to remove any previously filled ink. Afterward, the prepared nanowire suspended ink was loaded into the cartridge by soaking the sponge inside the cartridge with the nanowire ink using a syringe. Then, the print head nozzle on the bottom of the cartridge was gently treated with ultrasonication in order to prevent the clogging of the nozzles. The print patterns were drawn using a standard Windows software (e.g. Microsoft Word and PowerPoint). The highest resolution print setting was selected for optimum printing performance. To further increase the number of nanowires per unit area, multiple layers of the same patterns were printed on a single substrate. 4. Results and discussion 4.1. Morphology characterization of the polyaniline nanowire network To verify the nanowire network morphology, the scanning electron microscopy images of the polyaniline nanowires immediately after chemical synthesis and after inkjet printing on a substrate were compared. Fig. 2(a) confirms the nanowire morphology of the chemically synthesized polyaniline using the rapid mixing technique. The dimensions of a typical nanowire are approximately 100–150 nm in diameter and a few micrometers in length. Fig. 2(b) shows the SEM image of the polyaniline nanowires on a transparency film deposited with inkjet printing technique and confirms that the nanowire network morphology of the polyaniline is preserved even after undergoing the printing process. 4.2. Electrical properties As evidenced by the images in Fig. 2, the randomly interweaved nanowire network structure forms many conducting paths throughout the printed area resulting in a low sheet resistance.
Fig. 1. Illustration of the processes for the inkjet printing of nanowires for the development of a fully-printed disposable chemical sensor.
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Fig. 2. The scanning electron microscope image of the polyaniline nanowires (a) as synthesized and (b) after inkjet printing on a transparency film (5 prints).
For characterizing the electrical conductivity of the printed patterns, a rectangular pattern with dimensions of 5 mm 25 mm was printed 1–25 times, and the sheet resistance of each rectangular strip was recorded. Fig. 3(a) shows the image of the printed polyaniline patterns from 1, 5, 10, 15, and 20 prints (from left to right) and Fig. 3(b) shows the measured sheet resistance as a function of the number of prints. After printing 25 times, the lowest sheet resistance achieved was 5 kX/h. The sheet resistance could not be measured for the single print case due to the lack of the interconnection formed between the nanowires. However after two or three layers of printing, a measureable sheet resistance was obtained. 4.3. Patterning ability characterization In order to develop reliable printed sensors, the inkjet printer must be able to print the designed patterns with a high resolution. Furthermore, for printing multiple layers on a single sheet of substrate, each print layer of the pattern must be well-aligned with other layers in order to preserve the resolution. Therefore in this section, the minimum line width, spacing, and its alignment capability are studied. For characterizing the minimum printable resolution, a set of parallel lines with incrementally varying widths and spacing between the lines were printed as shown in Fig. 4(a). Since the printed area was not clearly visible under the microscope with only a single print, the pattern was printed multiple times for better defined images. Fig. 4(b) and (c) show that the thinnest printable line and spacing that are clearly defined are approximately 200 lm and 240 lm,
Fig. 4. Minimum line and gap width achievable with inkjet patterning of polyaniline nanowire network (printed 5 times).
respectively, with 5 prints. As the number of print layer increases, the minimum resolution for the printable line and spacing also increases due to the misalignment between the layers as indicated in Fig. 5 below.
Fig. 3. (a) Inkjet printed polyaniline nanowires on a transparency film with 1, 5, 10, 15, and 20 prints (from left to right), and (b) the measured sheet resistance vs. the number of prints.
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Fig. 5. Plot showing the minimum resolution for line and gap width (spacing) versus the number of prints.
stable current measurement between the two MWCNT-printed electrodes was obtained. Then, the device was immersed in a sample solution of a different pH value. Fig. 6(b) shows the conduction current response of the printed polyaniline material whose conductivity is dependent on the pH of the environment. As indicated in the plot, the conductivity of polyaniline decreases with increasing level of pH. This demonstrates that the printed polyaniline nanowires retain their electroactivity even after the inkjet printing process. A neutral pH solution such as pH 7 can also be used as a reference baseline to observe the pH-dependent conductivity characteristics. However, in this case, a longer response time can be expected since polyaniline becomes non-conducting at neutral pH, and regaining the conductivity by protonation may take longer time. In order for the polyaniline to fully recover its conductivity, a sufficient amount of protons must be adsorbed onto the polyaniline polymer chains to produce abundant mobile charge carriers.
5. Printed disposable chemiresistive sensors
5.2. A nanoparticle-based printed H2O2 sensor
As demonstrations for low cost printable chemical sensors, two types of printed sensors were developed: a pH sensor and a hydrogen peroxide (H2O2) sensor. Both sensors are of chemiresistive type that measure the change in resistance of the polyaniline material. The only difference between the two sensors in terms of the configuration is that the sensing area for the hydrogen peroxide sensor contains silver nanoparticles that are attached to the surfaces of polyaniline nanowires which act as catalysts for reacting with H2O2 species.
It has been reported that silver nanoparticles act as catalysts to undergo the following reactions with hydrogen peroxide [14,15]:
5.1. A fully-printed chemiresistive pH sensor Since the conductivity of polyaniline is pH-responsive, a simple chemiresistive pH sensor can be developed by printing a polyaniline layer between two electrodes. The low sheet resistance electrodes were formed by printing 15 layers of multi-walled carbon nanotubes (MWCNTs) on a transparency film using a similar technique [2]. The polyaniline nanowire-based sensing area was formed by printing 20 layers of the polyaniline ink to ensure highly interconnected network of nanowires. Therefore the entire pattern on the sensor can be fully printed using the inkjet printing method as illustrated in Fig. 6(a). The dimensions of the transparency film on which the nanomaterials were printed were approximately 2 cm 1 cm with a film thickness of 145 lm. The pH response of the printed device can be obtained by initializing the sensor with a strong acid in order to establish a maximum current and exposing the sensor to various pH buffer solutions. The sensor was initially exposed to pH 1 buffer until
H2 O2 þ e $ OHðadsÞ þ OH
ð1Þ
OHðadsÞ þ e $ OH
ð2Þ
2OH þ 2Hþ $ 2H2 O
ð3Þ
Based on this chemistry, the authors have demonstrated the use of silver nanoparticles in conjunction with polyaniline-based chemiresistive sensor for the detection hydrogen peroxide species [12]. As hydroxide ions are produced as a byproduct of the catalytic reaction at the surfaces of silver nanoparticles, the local pH near the vicinity of the polyaniline nanowire network is increased thereby reducing the conductivity of polyaniline. Therefore, by measuring the resistance change of the polyaniline-based chemiresistor, the concentration of H2O2 can be predicted. Furthermore, catalyst-based sensing offers enhanced selectivity due to a highly specific nature of catalysis. In this work, similar sensing mechanism is applied to the inkjetprinted sensor to test its feasibility. The polyaniline-based chemiresistor and the MWCNT-based electrodes were printed the same way as the pH sensor from the previous section. For depositing the silver catalysts, 3 ll of silver nanoparticle dispersed droplets were placed on top of the polyaniline area. The liquid dispersion of the nanoparticles were prepared by adding 1 mg/ml of silver nanoparticles (particle size < 100 nm, Sigma–Aldrich) and 1 mg/ml of SDS into deionized water, and sonicating the
Fig. 6. A fully-printed polyaniline-based pH sensor: (a) image of the sensor showing printed polyaniline nanowire network as a sensing area and MWCNT-printed electrodes as electrical contacts and (b) pH sensing results showing changes in conduction current through the polyaniline with respect to the initial current value (I0) for each pH sample solution.
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Fig. 7. A chemiresistive hydrogen peroxide sensor: (a) image of the sensor showing printed polyaniline nanowire network modified with catalytic silver nanoparticles and (b) the conduction current response of the sensor when exposed to various concentrations of H2O2.
Fig. 8 shows the calibration curve for the printed H2O2 sensor. The data points were obtained by measuring the value of the conduction currents 3 min after the sample injection time. The calibration curve indicates a linear response over a wide range of concentration and, although the limit of detection is higher than other reported H2O2 sensors, the proposed sensor demonstrates a cheap and simple way to manufacture a disposable and easily printable H2O2 sensor. 6. Conclusions
Fig. 8. Calibration curve for the printed polyaniline/silver nanoparticle-based H2O2 sensor.
mixture for at least 3 h. It is conceivable to have a premixed ink solution containing both polyaniline nanowires and silver nanoparticles where both materials could be printed simultaneously to simplify the processing steps. Work is currently underway to develop a liquid suspension of highly concentrated silver nanoparticles and polyaniline nanowires in order to simultaneously print the nanoparticles and polyaniline nanowires using an inkjet printer. Fig. 7(a) shows the printed H2O2 sensor showing the printed carbon nanotube electrodes and the printed polyaniline-based sensing area on top of which silver catalyst were deposited. Fig. 7(b) shows the conduction current response of the sensor when exposed to various amounts of H2O2 species. For small concentrations of H2O2, the rate of decrease in the conduction current is small due to the limited production of hydroxide ions. However, as the concentration of H2O2 is increased, larger quantities of hydroxide ions are being produced resulting in a more rapid change in the current. The catalytic reaction continues to produce the pH-altering byproducts as long as there are target species present near the catalysts, which explains the steady decrease in the current measurements. In our previous work, rather than waiting until the current response stabilizes, we have suggested a method to utilize the slope of the decreasing current to estimate the concentration of the H2O2 species [12]. For example, a larger concentration of H2O2 causes faster current drop which leads to a steeper slope as shown in Fig. 7(b). Therefore, using the slope information can be one possible solution to determining the concentration from the measured current response.
A novel approach to patterning a conducting polymer nanowire network using an inkjet printing technique has been proposed and demonstrated. The nanowire network morphology, highly conductive nature and pH-sensitive properties of the polyaniline nanowires were preserved even after printing them on a substrate. The minimum printable resolution was characterized and found to be approximately 200 lm. As a proof-of-concept demonstration, the inkjet-printed chemical sensor was developed for applications in pH and hydrogen peroxide sensing. The sensing results indicated their feasibility to be used as a low cost, fully printed, and simple to fabricate chemical sensors. This technology can potentially be applied to a print-and-use diagnostic kit for point-of-care test systems. Acknowledgements This work was supported in part by the Fund for Innovation in Engineering Research, Economic Development Assistantship from Louisiana State University, IEEE Charles LeGeyt Fortescue Graduate Scholarship, and the CAPES Science without Borders Scholarship from the Ministry of Education of Brazil. References [1] R.P. Tortorich, J.-W. Choi, Nanomaterials 3 (3) (2013) 453–468. [2] R.P. Tortorich, E. Song, J.-W. Choi, J. Electrochem. Soc. 161 (2) (2014) B3044– B3048. [3] F. Torrisi, T. Hasan, W. Wu, Z. Sun, A. Lombardo, T.S. Kulmala, G.-W. Hsieh, S. Jung, F. Bonaccorso, P.J. Paul, D. Chu, A.C. Ferrari, ACS Nano 6 (4) (2012) 2992– 3006. [4] K. Crowley, A. Morrin, A. Hernandez, E. O’Malley, P.G. Whitten, G.G. Wallace, M.R. Smyth, A.J. Killard, Talanta 77 (2) (2008) 710–717. [5] A. Morrin, O. Ngamna, E. O’Malley, N. Kent, S.E. Moulton, G.G. Wallace, M.R. Smyth, A.J. Killard, Electrochim. Acta 53 (16) (2008) 5092–5099. [6] A. Mulchandani, N.V. Myung, Curr. Opin. Biotechnol. 22 (4) (2011) 502–508. [7] D. Li, J. Huang, R.B. Kaner, Acc. Chem. Res. 42 (1) (2009) 135–145. [8] J. Wang, S. Chan, R.R. Carlson, Y. Luo, G. Ge, R.S. Ries, J.R. Heath, H.-R. Tseng, Nano Lett. 4 (9) (2004) 1693–1697.
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[9] E. Song, J.-W. Choi, Microelectron. Eng. 116 (2014) 26–32. [10] E. Song, J.-W. Choi, Nanomaterials 3 (3) (2013) 498–523. [11] K. Crowley, E. O’Malley, A. Morrin, M.R. Smyth, A.J. Killard, Analyst 133 (3) (2008) 391–399. [12] E. Song, J.-W. Choi, J. Micromech. Microeng. 24 (6) (2014) 065004 (7pp).
[13] H.D. Tran, Y. Wang, J.M. D’Arcy, R.B. Kaner, ACS Nano 2 (9) (2008) 1841–1848. [14] C.M. Welch, C.E. Banks, A.O. Simm, R.G. Compton, Anal. Bioanal. Chem. 382 (1) (2005) 12–21. [15] W. Zhao, H. Wang, X. Qin, X. Wang, Z. Zhao, Z. Miao, L. Chen, M. Shan, Y. Fang, Q. Chen, Talanta 80 (2) (2009) 1029–1033.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Inkjet printing of conductive polymer nanowire network on flexible substrates and its application in chemical sensing Edward Song a, Ryan P. Tortorich a, Tallis H. da Costa a, Jin-Woo Choi a,b,⇑ a b
School of Electrical Engineering and Computer Science, Louisiana State University, Baton Rouge, LA 70803, USA Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, LA 70803, USA
a r t i c l e
i n f o
Article history: Received 24 October 2014 Received in revised form 15 March 2015 Accepted 1 April 2015 Available online 7 April 2015 Keywords: Inkjet printing Polyaniline Nanowire Chemiresistive sensing
a b s t r a c t This work reports an inkjet printing technique for patterning a conducting polymer nanowire network on a flexible film for applications in chemical sensing. The novelty of this work is in the patterning capability of polymer nanowires to form a conducting path. Polyaniline nanowires were chemically synthesized in an aqueous solution and a surfactant was added to lower the surface tension which enabled the printing of the nanowires using a commercially available inkjet printer. The nanowire network-based patterns were printed on a flexible transparency film, and its morphology characterization, patterning ability as well as the electrical properties were investigated. Finally, as a proof-of-concept, a fully-printed chemical sensors were developed by using the proposed printing technique on flexible films. Two types of sensors were fabricated: a pH sensor and a hydrogen peroxide sensor. The results demonstrate that the developed sensors can be utilized as a low cost, disposable, and easily printable chemical sensors. The proposed technology may find applications in the development of a simple print-and-use biochemical sensing kit for potential use in point-of-care diagnostics. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Fabrication of a single-use and disposable sensor using various printing techniques has many advantages especially in chemo- and bio-sensors due to their low cost, mass producibility, and portability. Moreover, printed electronic materials are generally thin and flexible and therefore have potential applications in wearable and implantable electronics. Among the various printing techniques, an inkjet printing method can provide a quick, simple, and automated solution to developing disposable sensors [1]. Due to the many benefits that the nanomaterials offer especially in chemical and biological sensing, there have been numerous reports demonstrating the feasibility of inkjet printing of various nanomaterials including metal and oxide nanoparticles, carbon nanotubes [2], and even graphene sheets on a flexible substrate [3]. Inkjet printing of conducting polymer materials is also becoming a topic of great interest due to the unique properties of the conducting polymers that can be utilized in sensors and electronic devices [4,5]. However inkjet printing of conducting polymer nanowires to form highly interconnected network of 1-dimensional ⇑ Corresponding author at: School of Electrical Engineering and Computer Science, Louisiana State University, Baton Rouge, LA 70803, USA. Tel.: +1 225 578 8764. E-mail address: [email protected] (J.-W. Choi). http://dx.doi.org/10.1016/j.mee.2015.04.004 0167-9317/Ó 2015 Elsevier B.V. All rights reserved.
nanostructures have not been demonstrated so far. Nanowire network morphology is ideal for producing highly conductive patterns and is also desirable as a sensing material due to its large surface area and porosity leading to enhanced sensitivity and faster response time [6,7]. Polyaniline is one of the most widely studied conducting polymer and has found many applications as a sensing material for chemical and biological species. Polyaniline is a very interesting polymer due to its many unique properties. First, it is an intrinsically conducting polymer with a relatively high electrical conductivity (on the order of a few S/m) for a polymer material. Due to its ability to conduct electricity, it can be used for electronic sensor applications. Secondly, the conductivity of polyaniline is highly influenced by the pH of the environment to which it is exposed. Polyaniline exhibits maximum conductivity under a strongly acidic environment (low pH) and loses its conductivity at neutral or basic environment (pH 7 or higher). Therefore chemiresistive polyaniline-based pH sensors have been extensively studied and documented [8–10]. Inkjet printing of polyaniline nanoparticles and nanograins have been previously reported and the development of an ammonia sensor was demonstrated [4,11]. However, in order to form a well-conducting path for electron transport, an interweaved nanowire network is the desired solution for applications in resistive sensing. In this work, we demonstrate the capability of printing
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polyaniline nanowire network patterns on a transparency film to be used as chemical sensors. The novelties of this work are in the inkjet printing of conducting polymer nanowires and the development of fully-printable chemiresistive sensors.
2. Working principle Fig. 1 illustrates the basic concept of the use of inkjet printing technique for the development of a printable chemical sensors. First, a nanowire dispersed liquid is loaded into the printer cartridge to be printed on a substrate. Due to the nanowire morphology of the polyaniline material, the printed patterns will contain a randomly oriented network of conducting polymer nanowires which forms electrically conducting patterns as shown in Fig. 1(a) and (b). Since the conductivity of polyaniline is pH-dependent, a pH sensor can be developed by measuring the resistance across the polyaniline layer deposited between the two electrodes. This type of chemiresistive pH sensor can be implemented by the inkjet printing technique as illustrated in Fig. 1(b). A similar approach can be taken to develop a hydrogen peroxide sensor by including catalytic (silver) nanoparticles to the polyaniline nanowire network [12]. Fig. 1(c) depicts the configuration of the chemiresistive hydrogen peroxide sensor where the nanoparticles are evenly distributed throughout the network. When the catalytic reaction occurs between the nanoparticles and hydrogen peroxide, hydroxide ions (OH) are produced as the byproduct of the reaction which increases the local pH near polyaniline. Therefore, the resulting change in conductivity of polyaniline can be measured to determine the hydrogen peroxide concentration.
3. Experimental methods 3.1. Chemical synthesis of the polyaniline nanowires The method for the chemical synthesis of polyaniline nanowires is based on the technique developed by Kaner [13]. Briefly, 10 ml of 0.08 M aniline (73.5 ll) in 1 M HCl, and 10 ml of 0.02 M ammonium peroxydisulfate (APS, 45 mg) in 1 M HCl were prepared in two separate vials. The two solutions were rapidly and vigorously mixed together and left un-agitated overnight. The synthesized nanowires were rinsed by centrifuging the product at 13,000 RPM for 10 min followed by removing the aqueous liquid, adding the same volume of DI water, and gently agitating the centrifuge tube. This rinsing step was repeated twice to thoroughly remove the unreacted oxidants and acids from the liquid.
3.2. Printable ink preparation In order to print the polyaniline nanowires through the ink cartridge, the surface tension of the ink must be sufficiently low so that the ink droplets can be discharged through the nozzle. For lowering the surface tension of the nanowire suspension, 10 mg/ ml of sodium dodecylsulfate (SDS), which is a common surfactant, was added to the nanowire dispersion and gently stirred until SDS was completely dissolved. 3.3. Inkjet printing of conducting polymer patterns The printing process was performed using a commercially available inkjet printer (HP DeskJet D1520) which was used as received without further modification. A letter sized printable transparency film was used as a printing substrate. The black ink cartridge was thoroughly rinsed with water to remove any previously filled ink. Afterward, the prepared nanowire suspended ink was loaded into the cartridge by soaking the sponge inside the cartridge with the nanowire ink using a syringe. Then, the print head nozzle on the bottom of the cartridge was gently treated with ultrasonication in order to prevent the clogging of the nozzles. The print patterns were drawn using a standard Windows software (e.g. Microsoft Word and PowerPoint). The highest resolution print setting was selected for optimum printing performance. To further increase the number of nanowires per unit area, multiple layers of the same patterns were printed on a single substrate. 4. Results and discussion 4.1. Morphology characterization of the polyaniline nanowire network To verify the nanowire network morphology, the scanning electron microscopy images of the polyaniline nanowires immediately after chemical synthesis and after inkjet printing on a substrate were compared. Fig. 2(a) confirms the nanowire morphology of the chemically synthesized polyaniline using the rapid mixing technique. The dimensions of a typical nanowire are approximately 100–150 nm in diameter and a few micrometers in length. Fig. 2(b) shows the SEM image of the polyaniline nanowires on a transparency film deposited with inkjet printing technique and confirms that the nanowire network morphology of the polyaniline is preserved even after undergoing the printing process. 4.2. Electrical properties As evidenced by the images in Fig. 2, the randomly interweaved nanowire network structure forms many conducting paths throughout the printed area resulting in a low sheet resistance.
Fig. 1. Illustration of the processes for the inkjet printing of nanowires for the development of a fully-printed disposable chemical sensor.
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Fig. 2. The scanning electron microscope image of the polyaniline nanowires (a) as synthesized and (b) after inkjet printing on a transparency film (5 prints).
For characterizing the electrical conductivity of the printed patterns, a rectangular pattern with dimensions of 5 mm 25 mm was printed 1–25 times, and the sheet resistance of each rectangular strip was recorded. Fig. 3(a) shows the image of the printed polyaniline patterns from 1, 5, 10, 15, and 20 prints (from left to right) and Fig. 3(b) shows the measured sheet resistance as a function of the number of prints. After printing 25 times, the lowest sheet resistance achieved was 5 kX/h. The sheet resistance could not be measured for the single print case due to the lack of the interconnection formed between the nanowires. However after two or three layers of printing, a measureable sheet resistance was obtained. 4.3. Patterning ability characterization In order to develop reliable printed sensors, the inkjet printer must be able to print the designed patterns with a high resolution. Furthermore, for printing multiple layers on a single sheet of substrate, each print layer of the pattern must be well-aligned with other layers in order to preserve the resolution. Therefore in this section, the minimum line width, spacing, and its alignment capability are studied. For characterizing the minimum printable resolution, a set of parallel lines with incrementally varying widths and spacing between the lines were printed as shown in Fig. 4(a). Since the printed area was not clearly visible under the microscope with only a single print, the pattern was printed multiple times for better defined images. Fig. 4(b) and (c) show that the thinnest printable line and spacing that are clearly defined are approximately 200 lm and 240 lm,
Fig. 4. Minimum line and gap width achievable with inkjet patterning of polyaniline nanowire network (printed 5 times).
respectively, with 5 prints. As the number of print layer increases, the minimum resolution for the printable line and spacing also increases due to the misalignment between the layers as indicated in Fig. 5 below.
Fig. 3. (a) Inkjet printed polyaniline nanowires on a transparency film with 1, 5, 10, 15, and 20 prints (from left to right), and (b) the measured sheet resistance vs. the number of prints.
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Fig. 5. Plot showing the minimum resolution for line and gap width (spacing) versus the number of prints.
stable current measurement between the two MWCNT-printed electrodes was obtained. Then, the device was immersed in a sample solution of a different pH value. Fig. 6(b) shows the conduction current response of the printed polyaniline material whose conductivity is dependent on the pH of the environment. As indicated in the plot, the conductivity of polyaniline decreases with increasing level of pH. This demonstrates that the printed polyaniline nanowires retain their electroactivity even after the inkjet printing process. A neutral pH solution such as pH 7 can also be used as a reference baseline to observe the pH-dependent conductivity characteristics. However, in this case, a longer response time can be expected since polyaniline becomes non-conducting at neutral pH, and regaining the conductivity by protonation may take longer time. In order for the polyaniline to fully recover its conductivity, a sufficient amount of protons must be adsorbed onto the polyaniline polymer chains to produce abundant mobile charge carriers.
5. Printed disposable chemiresistive sensors
5.2. A nanoparticle-based printed H2O2 sensor
As demonstrations for low cost printable chemical sensors, two types of printed sensors were developed: a pH sensor and a hydrogen peroxide (H2O2) sensor. Both sensors are of chemiresistive type that measure the change in resistance of the polyaniline material. The only difference between the two sensors in terms of the configuration is that the sensing area for the hydrogen peroxide sensor contains silver nanoparticles that are attached to the surfaces of polyaniline nanowires which act as catalysts for reacting with H2O2 species.
It has been reported that silver nanoparticles act as catalysts to undergo the following reactions with hydrogen peroxide [14,15]:
5.1. A fully-printed chemiresistive pH sensor Since the conductivity of polyaniline is pH-responsive, a simple chemiresistive pH sensor can be developed by printing a polyaniline layer between two electrodes. The low sheet resistance electrodes were formed by printing 15 layers of multi-walled carbon nanotubes (MWCNTs) on a transparency film using a similar technique [2]. The polyaniline nanowire-based sensing area was formed by printing 20 layers of the polyaniline ink to ensure highly interconnected network of nanowires. Therefore the entire pattern on the sensor can be fully printed using the inkjet printing method as illustrated in Fig. 6(a). The dimensions of the transparency film on which the nanomaterials were printed were approximately 2 cm 1 cm with a film thickness of 145 lm. The pH response of the printed device can be obtained by initializing the sensor with a strong acid in order to establish a maximum current and exposing the sensor to various pH buffer solutions. The sensor was initially exposed to pH 1 buffer until
H2 O2 þ e $ OHðadsÞ þ OH
ð1Þ
OHðadsÞ þ e $ OH
ð2Þ
2OH þ 2Hþ $ 2H2 O
ð3Þ
Based on this chemistry, the authors have demonstrated the use of silver nanoparticles in conjunction with polyaniline-based chemiresistive sensor for the detection hydrogen peroxide species [12]. As hydroxide ions are produced as a byproduct of the catalytic reaction at the surfaces of silver nanoparticles, the local pH near the vicinity of the polyaniline nanowire network is increased thereby reducing the conductivity of polyaniline. Therefore, by measuring the resistance change of the polyaniline-based chemiresistor, the concentration of H2O2 can be predicted. Furthermore, catalyst-based sensing offers enhanced selectivity due to a highly specific nature of catalysis. In this work, similar sensing mechanism is applied to the inkjetprinted sensor to test its feasibility. The polyaniline-based chemiresistor and the MWCNT-based electrodes were printed the same way as the pH sensor from the previous section. For depositing the silver catalysts, 3 ll of silver nanoparticle dispersed droplets were placed on top of the polyaniline area. The liquid dispersion of the nanoparticles were prepared by adding 1 mg/ml of silver nanoparticles (particle size < 100 nm, Sigma–Aldrich) and 1 mg/ml of SDS into deionized water, and sonicating the
Fig. 6. A fully-printed polyaniline-based pH sensor: (a) image of the sensor showing printed polyaniline nanowire network as a sensing area and MWCNT-printed electrodes as electrical contacts and (b) pH sensing results showing changes in conduction current through the polyaniline with respect to the initial current value (I0) for each pH sample solution.
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Fig. 7. A chemiresistive hydrogen peroxide sensor: (a) image of the sensor showing printed polyaniline nanowire network modified with catalytic silver nanoparticles and (b) the conduction current response of the sensor when exposed to various concentrations of H2O2.
Fig. 8 shows the calibration curve for the printed H2O2 sensor. The data points were obtained by measuring the value of the conduction currents 3 min after the sample injection time. The calibration curve indicates a linear response over a wide range of concentration and, although the limit of detection is higher than other reported H2O2 sensors, the proposed sensor demonstrates a cheap and simple way to manufacture a disposable and easily printable H2O2 sensor. 6. Conclusions
Fig. 8. Calibration curve for the printed polyaniline/silver nanoparticle-based H2O2 sensor.
mixture for at least 3 h. It is conceivable to have a premixed ink solution containing both polyaniline nanowires and silver nanoparticles where both materials could be printed simultaneously to simplify the processing steps. Work is currently underway to develop a liquid suspension of highly concentrated silver nanoparticles and polyaniline nanowires in order to simultaneously print the nanoparticles and polyaniline nanowires using an inkjet printer. Fig. 7(a) shows the printed H2O2 sensor showing the printed carbon nanotube electrodes and the printed polyaniline-based sensing area on top of which silver catalyst were deposited. Fig. 7(b) shows the conduction current response of the sensor when exposed to various amounts of H2O2 species. For small concentrations of H2O2, the rate of decrease in the conduction current is small due to the limited production of hydroxide ions. However, as the concentration of H2O2 is increased, larger quantities of hydroxide ions are being produced resulting in a more rapid change in the current. The catalytic reaction continues to produce the pH-altering byproducts as long as there are target species present near the catalysts, which explains the steady decrease in the current measurements. In our previous work, rather than waiting until the current response stabilizes, we have suggested a method to utilize the slope of the decreasing current to estimate the concentration of the H2O2 species [12]. For example, a larger concentration of H2O2 causes faster current drop which leads to a steeper slope as shown in Fig. 7(b). Therefore, using the slope information can be one possible solution to determining the concentration from the measured current response.
A novel approach to patterning a conducting polymer nanowire network using an inkjet printing technique has been proposed and demonstrated. The nanowire network morphology, highly conductive nature and pH-sensitive properties of the polyaniline nanowires were preserved even after printing them on a substrate. The minimum printable resolution was characterized and found to be approximately 200 lm. As a proof-of-concept demonstration, the inkjet-printed chemical sensor was developed for applications in pH and hydrogen peroxide sensing. The sensing results indicated their feasibility to be used as a low cost, fully printed, and simple to fabricate chemical sensors. This technology can potentially be applied to a print-and-use diagnostic kit for point-of-care test systems. Acknowledgements This work was supported in part by the Fund for Innovation in Engineering Research, Economic Development Assistantship from Louisiana State University, IEEE Charles LeGeyt Fortescue Graduate Scholarship, and the CAPES Science without Borders Scholarship from the Ministry of Education of Brazil. References [1] R.P. Tortorich, J.-W. Choi, Nanomaterials 3 (3) (2013) 453–468. [2] R.P. Tortorich, E. Song, J.-W. Choi, J. Electrochem. Soc. 161 (2) (2014) B3044– B3048. [3] F. Torrisi, T. Hasan, W. Wu, Z. Sun, A. Lombardo, T.S. Kulmala, G.-W. Hsieh, S. Jung, F. Bonaccorso, P.J. Paul, D. Chu, A.C. Ferrari, ACS Nano 6 (4) (2012) 2992– 3006. [4] K. Crowley, A. Morrin, A. Hernandez, E. O’Malley, P.G. Whitten, G.G. Wallace, M.R. Smyth, A.J. Killard, Talanta 77 (2) (2008) 710–717. [5] A. Morrin, O. Ngamna, E. O’Malley, N. Kent, S.E. Moulton, G.G. Wallace, M.R. Smyth, A.J. Killard, Electrochim. Acta 53 (16) (2008) 5092–5099. [6] A. Mulchandani, N.V. Myung, Curr. Opin. Biotechnol. 22 (4) (2011) 502–508. [7] D. Li, J. Huang, R.B. Kaner, Acc. Chem. Res. 42 (1) (2009) 135–145. [8] J. Wang, S. Chan, R.R. Carlson, Y. Luo, G. Ge, R.S. Ries, J.R. Heath, H.-R. Tseng, Nano Lett. 4 (9) (2004) 1693–1697.
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