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Abstract
Microelectrode technologies have been widely used for a number of applications including optoelectronic and bioelectronics. In this study, we report highly conductive and highly reliable silver nanowire (AgNW)/poly(3,4,-ethylene dioxy thiophene): poly(styrenesulfonate) (PEDOT:PSS) composite microelectrodes fabricated by simple poly(ethylene glycol) photolithography. The electrical properties of AgNW/PEDOT:PSS were examined as functions of the AgNW concentration and layer number, and then compared with those of pure AgNWs. Importantly, the AgNW/PEDOT:PSS composite exhibited a high conductivity with a low sheet resistance of 1.22 Ω/□ as well as an excellent electrical standard deviation of 0.96 Ω/□ in a reliability test. We also demonstrated that these composite micropatterns were completely transferred from the glass to a flexible hydrogel by a direct transfer process. Moreover, the composite microelectrodes exhibited increases in the electrical resistance of only 11 and 24% after over 300 and 500 bending cycles, which were 65 and 90% enhancements compared to the single AgNW microelectrode, respectively. This novel approach could become a low-cost and efficient design for fabricating high-performance microelectrodes.
© 2017 Optical Society of America
1. Introduction
Recently, the development of highly conductive, reliable, and flexible microelectrodes has been crucial for advancing optoelectronics, bioelectronics, and optogenetics, such as organic light-emitting diodes (OLEDs), implantable optical-electrodes, retinal prostheses, and optical integrated circuits [1–6]. In particular, hydrogel-based microelectrodes, which are valuable for advanced bioelectronic and optogenetic interfaces, are challenging as they can improve the ability to sense and record biological events in vivo as well as be more mechanically compliant with target tissues [7,8]. To make fundamental progress in microelectrode fabrication techniques, it is necessary to develop both simple, low-cost, and highly efficient patterning methods, as well as bring together a new set of functional materials.
Our laboratory first reported a novel strategy for fabricating poly(3,4,-ethylenedioxythiophene) (PEDOT) patterns via poly(ethylene glycol) (PEG) photolithography for a flexible electrochromic device [9]. In a separate study, we constructed silver nanowire (AgNW) patterns on various substrates via PEG photolithography for a pressure-responsive sensor [10]. The PEDOT patterns in the former case were constructed based on covalent chemical bonds between PEG diacrylate and the underlying conjugated double bonds of PEDOT, while AgNW patterns in the latter case were formed based on physical adhesion resulting from the cross-linking of the PEG polymer chain and the AgNW network structures. In contrast to general strategies, such as photolithography with etching processes and printing techniques for conductive material patterns [7, 11–13], the PEG photolithographic method is simple, environmentally friendly, low cost, and an easily controllable process [14, 15].
It has been well-known that one-dimensional AgNWs are attractive due to their high electrical conductivity and flexibility for flexible optoelectronic devices. Compared to other nanostructures such as nanoparticles and nanoporous materials, AgNWs can offer direct and fast electron transport to the electron collecting electrode [16–21]. However, in spite of its effectiveness, there are critical limitations in the use of AgNW electrodes for highly efficient device fabrications. For example, their irregular, random network structures lead to poor reliability associated with a high degree of electrical deviation and also an inherent rough surface related to poor charge transport across the junctions between adjacent AgNWs [22–26].
In this paper, our objective was to fabricate and test silver nanowire (AgNW)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite microelectrodes prepared via simple PEG photolithography to take advantage of both the strengths of PEDOT:PSS and AgNWs. We chose to use PEDOT:PSS as a supporting agent of AgNW because PEDOT:PSS in aqueous dispersions is environmentally stable, highly conductive, and allows for aqueous solution-based processing [27]. Our experiments revealed that simple PEG photolithography can be employed to fabricate AgNW/PEDOT:PSS composite micropatterns. These AgNW/PEDOT:PSS composite microelectrodes outperformed pure AgNW microelectrodes by eliciting higher electrical conductivity, lower electrical standard deviations, and better mechanical durability.
2. Experimental
2.1 Chemical and materials
Silver nanowires (AgNW, 1 wt%) with 20-40 nm diameters and 20-30 μm lengths as a dispersion solution in isopropyl alcohol were purchased from NANOPYXIS. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, 1.3 wt% dispersion in H2O), poly(ethylene glycol) diacrylate (PEG-DA, MW 575), were purchased from Aldrich. 4-(2-Hydroxy-ethoxy)phenyl-(2-hydroxy-2-propyl) ketone (Irgacure 2959) was provided by BASF. 3-Acryloxypropyltrichlorosilane was obtained from Gelst, Inc. (Morrisville, PA). Dimethyl sulfoxide (DMSO), toluene, and ethanol were purchased from Duksan Pure Chemicals Company. Phosphate-buffered saline (PBS) was purchased from Life Technologies.
2.2 Preparation of the AgNW/PEDOT:PSS composite film
The AgNW solutions (0.2, 0.6, 1 wt%) were spin-coated two or five times onto clean glass substrates at 5000 rpm for 40 s and annealed at 150 °C for 5 min. The PEDOT:PSS solution was diluted to 0.4 wt% with distilled water. The PEDOT:PSS solution was mixed with 5 wt% DMSO and stirred at room temperature for 24 h. The resulting PEDOT:PSS solution was spin-coated one time onto the pre-coated AgNW coated glass substrate at 5000 rpm for 40 s. The AgNW/PEDOT:PSS film was dried at room temperature for 1 h under ambient conditions.
2.3 Patterning of composite electrodes on glass and hydrogel substrates
PEG-DA (MW 575) was mixed in PBS containing 1% w/v photoinitiator (Irgacure 2959), which was dissolved in 70% v/v ethanol, to achieve a 60% w/v gel precursor solution. The PEG precursor solution was dropped onto the AgNW/PEDOT:PSS-coated glass substrate (2.5 cm × 2.5 cm) and then covered with a silane-treated glass [28]. AgNW/PEDOT-coated glass was exposed to a UV light source (INNO Cure 2000, 2.32 mW / cm2) through a photomask. The UV exposed region of the AgNW/PEDOT film was transferred to a silane-treated glass along with the PEG hydrogel. The AgNW/PEDOT film region not exposed to UV remained intact for AgNW/PEDOT patterns. The AgNW/PEDOT-patterned glass was washed carefully with distilled water to remove the unexposed PEG precursor solution. To transfer the composite microelectrode from glass to the hydrogel substrate, the hydrogel precursor solution was dropped onto the AgNW/PEDOT:PSS micro-patterned glass substrate and gelation was conducted by UV exposure. As a result, the AgNW/PEDOT composite micropatterns were cleanly transferred to the hydrogel substrate without any defects.
2.4 Characterization
The morphologies of the AgNW/PEDOT:PSS patterns were measured with a field emission scanning electron microscope (FE-SEM, Hitachi, model S-4200, Carl Zeiss, Merlin model). The sheet resistance measurements were performed with a sheet resistance tester (CMT-100S, Advanced Instrument Technology). Current-voltage (I-V) characteristics were determined using a two-point probe method from −1 V to 1 V with an electrochemical analysis device (PGSTAT204, Metrohm Autolab). The standard deviations were obtained from 100 data points of different areas on the sample for the reliability evaluation. The repeated tape test was carried out by attaching/detaching with a commercial adhesive (3M, Scotch 810 Magic Tape, USA). The AgNW/PEDOT:PSS pattern on the PEG hydrogel (1.5 cm × 1.5 cm) was bent around a diameter of 15 mm and subsequently unbent (bending test at 28% strain). This was repeated for 500 cycles. The sheet resistance of the sample was measured by a four-electrode system at every 100th cycle until 500 cycles were conducted. Each measurement was compared to the initial value. The LED bulb test was easily conducted before and after the bending cycle test. The potential was applied via an electrochemical analysis device using chronoamperometry (potential: 3 V).
3. Results and discussion
Figure 1 shows a schematic diagram of the fabrication of the AgNW/PEDOT:PSS composite micropatterns on glass and hydrogel substrates via consecutive solution processes. First, pristine AgNW solutions of various concentrations (0.2, 0.6, and 1.0 wt%) were spin-coated onto a glass substrate. Subsequently, a DMSO-doped PEDOT:PSS solution was directly spin-coated onto the pre-coated AgNW films to create composite films. Second, PEG photolithography was performed to construct the AgNW/PEDOT:PSS composite micropatterns. The PEG hydrogel formed on the UV exposed regions was peeled off with the AgNW/PEDOT:PSS composite film due to good adhesion between the PEG hydrogel and PEDOT:PSS. As a result, AgNW/PEDOT:PSS composite micropatterns were constructed on the non-UV-exposed region. These composite micropatterns were directly transferred from the glass to the hydrogel substrate via a second gelation. Figures 2(a) and 2(b) present representative images of the AgNW/PEDOT:PSS composite micropatterns with a width of 300 μm on the glass and hydrogel substrates. These images indicate that the conductive composite micropatterns could be easily created on the glass substrate via simple PEG photolithography with no need for multiple washing or etching steps, followed by complete transfer to hydrogel substrates via second gelation without any defects.
Fig. 1 Schematic illustration of the fabrication of AgNW/PEDOT:PSS composite micropatterns on the glass and hydrogel substrates via PEG photolithography and second gelation methods. i) The AgNW dispersion solution (0.2, 0.6, 1 wt%) was spin-coated on the glass substrate and the PEDOT:PSS solution was then overcoated on the AgNW-coated substrate. ii) The PEG photolithography process using UV exposure via a photomask. iii) Peeling off the PEG hydrogel layer of the UV-exposed AgNW/PEDOT:PSS region to construct the AgNW/PEDOT:PSS composite micropattern on the region not exposed to UV. iv) Transferring the AgNW/PEDOT pattern from the glass to the hydrogel substrate via the second PEG gelation.
Fig. 2 Photographic (left) and optical microscopic (right) images of AgNW/PEDOT:PSS micropatterns with a line width of 300 μm on the (A) glass and (B) hydrogel substrates.
Furthermore, the FE-SEM images as presented in Figs. 3(b), 3(d), and 3(e) clearly show the successful construction of the AgNW/PEDOT:PSS composite micropatterns on both the glass and hydrogel substrates. Importantly, the morphology and structure of the AgNW/PEDOT:PSS micropatterns were quite different from those of the pure AgNW micropatterns. In contrast to pure AgNW patterns that exhibit a randomly-crosslinked network with low coverage Fig. 3(a), AgNW/PEDOT:PSS composite patterns revealed that AgNW networks were over-coated and more interconnected by PEDOT:PSS Figs. 3(c) and 3(f).
Fig. 3 FE-SEM images of (A) pure AgNWs used in the preparation of composite micropatterns. (B and C) AgNW /PEDOT:PSS composite micropatterns (500 μm width) on the glass substrate. (D, E, and F) AgNW/PEDOT:PSS composite micropatterns (500 μm width) on the hydrogel substrate.
After confirming the successful construction of the composite micropatterns, we examined and compared the electrical properties of pure AgNW and AgNW/PEDOT:PSS micropatterns to verify the impact of PEDOT:PSS on AgNW. Figures 4(a) and 4(b) show the sheet resistance difference of AgNW and AgNW/PEDOT:PSS composite films at various AgNW concentrations. The spin coating of AgNW solutions was performed two or five times for the preparation of the initial, uniform AgNW films even at the low AgNW concentration. The nanowire density was easily controlled by adjusting the AgNW concentration as well as the number of AgNW coatings.
Fig. 4 Sheet resistance of single AgNW and AgNW/PEDOT:PSS composite films on a glass substrate (2.5 cm × 2.5 cm) when AgNWs were spin-coated (A) two and (B) five times; *p < 0.05. (C) The standard deviation values of single AgNW and AgNW/PEDOT:PSS composite films when AgNWs were spin-coated two and five times on the glass substrate. (D) Current-voltage (I-V) characteristics of the AgNW/PEDOT:PSS composite-micropatterns (500 μm in width and 1.5 cm in length) on both the glass and hydrogel substrates.
The sheet resistances of pure AgNW and AgNW/PEDOT:PSS films were 38.2 Ω/□ and 27.6 Ω/□ at the low concentration of AgNWs (0.2 wt %) Fig. 4(a), indicating that the introduction of PEDOT:PSS could improve the electrical properties of the AgNW films. This observation was also noted at different AgNW concentrations. While less pronounced than the double layers of AgNWs, the analysis of the sheet resistance of the composite films with 5 layers of AgNWs demonstrated that PEDOT:PSS was also associated with a higher conductivity for the composite films Fig. 4(b). This might be because the incorporation of PEDOT:PSS filled the remaining empty space in the AgNW network structures and thus provided an electron pathway between non-crosslinked AgNW Fig. 3(c). In addition, the introduction of PEDOT:PSS led to a standard deviation approximately two times less than for pure AgNW films Fig. 4(c). These results were consistent with previous reports suggesting that AgNW/PEDOT:PSS composite films exhibited better electrical properties than pure AgNW films [18–21]. Figure 4(d) exhibits nearly similar I-V curves of the AgNW/PEDOT:PSS composite micropatterns on both the glass and hydrogel substrates, demonstrating that the composite micropatterns were completely transferred from the glass to the hydrogel substrates without any damage to the composite structures.
In Fig. 5(a), we investigated the adhesion stability of AgNW/PEDOT:PSS on hydrogel by the tape test. The sheet resistance of AgNW/PEDOT:PSS microelectrodes was slightly increased by approximately 29 and 62% after 30 and 60 times of tape test, while that of the pure AgNW microelectrodes was significantly increased approximately 498 and 635% after 30 and 60 times of tape test. These results reveal strong adhesion of composites patterns on hydrogel for practical handling in future applications.
Fig. 5 Electrical stability of pure AgNW and AgNW/PEDOT:PSS composite micropatterns (500 μm width) on the hydrogels during the (A) tape and (B) bending tests. The inset of (B) shows the photographs of the LED emission on the AgNW/PEDOT:PSS micropatterns on the hydrogel substrates before and after the bending cycle test.
Additionally, a bending test was performed to examine the flexibility and mechanical robustness of the AgNW/PEDOT:PSS composite microelectrodes. In this test, both pure AgNWs and the composite microelectrode on the hydrogel substrates bent at approximately 28% strain with subsequent stretching, which was then repeated for 500 cycles. Figure 5(b) reveals that the initial resistance of AgNW/PEDOT:PSS microelectrodes was slightly increased by approximately 11 and 24% after 300 and 500 cycles, while that of the pure AgNW microelectrodes was significantly increased approximately 31 and 235% after 300 and 500 cycles, respectively. The AgNW/PEDOT:PSS composite microelectrodes still possessed good electrical properties that were sufficient to turn on a light-emitting diode (LED) bulb, suggesting that there were no significant cracks or damage to the composite microelectrodes during the bending test. Overall, the results of both tape and bending tests indicated excellent mechanical stability of composite microelectrodes on hydrogel substrate.
4. Conclusion
In this study, the PEG photolithographic method, which is a solution-based simple process without aggressive etching steps, was employed to fabricate a AgNW/PEDOT:PSS composite microelectrode on a glass substrate. The resultant AgNW/PEODT:PSS composites exhibited higher conductivity with lower sheet resistance and lower electrical standard deviations than the pure AgNW material. Additionally, these AgNW/PEODT:PSS composite microelectrodes showed excellent mechanical durability. Our results may be valuable in the development of a highly-efficient micropattern process and highly-conductive composite micropatterns on biocompatible hydrogels. We anticipate that this patterning process for conductive composite micropatterns on biocompatible hydrogels will be helpful for the development of highly-efficient microelectrodes for next-generation optical healthcare systems.
Funding
National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2015R1C1A1A01054258).
References and links
1. S. Choi, H. Lee, R. Ghaffari, T. Hyeon, and D.-H. Kim, “Recent Advances in Flexible and Stretchable Bio-Electronic Devices Integrated with Nanomaterials,” Adv. Mater. 28(22), 4203–4218 (2016). [CrossRef] [PubMed]
2. G. Eda, W. Ji, F. Xia, and H. Zhao, “Feature issue introduction: two-dimensional materials for photonics and optoelectronics,” Opt. Mater. Express 6(7), 2458–2459 (2016). [CrossRef]
3. S.-H. Chung and H. Y. Noh, “Color-tunable stacked organic light-emitting diode with semi-transparent metal electrode,” Opt. Mater. Express 6(9), 2834–2840 (2016). [CrossRef]
4. W. Honda, S. Harada, T. Arie, S. Akita, and K. Takei, “Wearable, human-interactive, health-monitoring, wireless devices fabricated by macroscale printing techniques,” Adv. Mater. 24(22), 3299–3304 (2014).
5. R. Kim, S. Joo, H. Jung, N. Hong, and Y. Nam, “Recent trends in microelectrode array technology for in vitro neural interface platform,” Biomed. Eng. Lett. 4(2), 129–141 (2014). [CrossRef]
6. V. Gautam, D. Rand, Y. Hanein, and K. S. Narayan, “A polymer optoelectronic interface provides visual cues to a blind retina,” Adv. Mater. 26(11), 1751–1756 (2014). [CrossRef] [PubMed]
7. Y. Ahn, H. Lee, D. Lee, and Y. Lee, “Highly conductive and flexible silver nanowire-based microelectrodes on biocompatible hydrogel,” ACS Appl. Mater. Interfaces 6(21), 18401–18407 (2014). [CrossRef] [PubMed]
8. S. Sekine, Y. Ido, T. Miyake, K. Nagamine, and M. Nishizawa, “Conducting polymer electrodes printed on hydrogel,” J. Am. Chem. Soc. 132(38), 13174–13175 (2010). [CrossRef] [PubMed]
9. D. Kim, J. Kim, Y. Ko, K. Shim, J. H. Kim, and J. You, “A Facile Approach for Constructing Conductive Polymer Patterns for Application in Electrochromic Devices and Flexible Microelectrodes,” ACS Appl. Mater. Interfaces 8(48), 33175–33182 (2016). [CrossRef] [PubMed]
10. Y. Ko, J. Kim, D. Kim, Y. Yamauchi, J. H. Kim, and J. You, “Simple, toxic-free photopatterning of highly conductive silver nanowires on hydrogels for soft electronics,” Sci. Rep. 7, 2282 (2017).
11. C. Acikgoz, M. A. Hempenius, J. Huskens, and G. J. Vancso, “Polymers in conventional and alternative lithography for the fabrication of nanostructures,” Eur. Polym. J. 47(11), 2033–2052 (2011). [CrossRef]
12. B.-H. Chen, S.-Y. Kao, C.-W. Hu, M. Higuchi, K.-C. Ho, and Y.-C. Liao, “Printed multicolor high-contrast electrochromic devices,” ACS Appl. Mater. Interfaces 7(45), 25069–25076 (2015). [CrossRef] [PubMed]
13. J. Kim, J. You, B. Kim, T. Park, and E. Kim, “Solution processable and patternable poly(3,4-alkylenedioxythiophene)s for large-area electrochromic films,” Adv. Mater. 23(36), 4168–4173 (2011). [CrossRef] [PubMed]
14. A. Revzin, R. J. Russell, V. K. Yadavalli, W.-G. Koh, C. Deister, D. D. Hile, M. B. Mellott, and M. V. Pishko, “Fabrication of poly(ethylene glycol) hydrogel microstructures using photolithography,” Langmuir 17(18), 5440–5447 (2001). [CrossRef] [PubMed]
15. W.-G. Koh, A. Revzin, and M. V. Pishko, “Poly(ethylene glycol) hydrogel microstructures encapsulating living cells,” Langmuir 18(7), 2459–2462 (2002). [CrossRef] [PubMed]
16. B. S. Kim, K.-Y. Shin, J. B. Pyo, J. Lee, J. G. Son, S.-S. Lee, and J. H. Park, “Reversibly stretchable, optically transparent radio-frequency antennas based on wavy ag nanowire networks,” ACS Appl. Mater. Interfaces 8(4), 2582–2590 (2016). [CrossRef] [PubMed]
17. M. Marus, A. Hubarevich, R. J. W. Lim, H. Huang, A. Smirnov, H. Wang, W. Fan, and X. W. Sun, “Effect of silver nanowire length in a broad range on optical and electrical properties as a transparent conductive film,” Opt. Mater. Express 7(3), 1105–1112 (2017). [CrossRef]
18. H. Ataee-Esfahani, M. Imura, and Y. Yamauchi, “All-metal mesoporous nanocolloids: solution-phase synthesis of core-shell Pd@Pt nanoparticles with a designed concave surface,” Angew. Chem. Int. Ed. Engl. 52(51), 13611–13615 (2013). [CrossRef] [PubMed]
19. H. Wang, S. Ishihara, K. Ariga, and Y. Yamauchi, “All-metal layer-by-layer films: bimetallic alternate layers with accessible mesopores for enhanced electrocatalysis,” J. Am. Chem. Soc. 134(26), 10819–10821 (2012). [CrossRef] [PubMed]
20. V. Malgras, H. Ataee-Esfahani, H. Wang, B. Jiang, C. Li, K. C.-W. Wu, J. H. Kim, and Y. Yamauchi, “Nanoarchitectures for Mesoporous Metals,” Adv. Mater. 28(6), 993–1010 (2016). [CrossRef] [PubMed]
21. P. Poudel and Q. Qiao, “One dimensional nanostructure/nanoparticle composites as photoanodes for dye-sensitized solar cells,” Nanoscale 4(9), 2826–2838 (2012). [CrossRef] [PubMed]
22. S. Kim, S. Y. Kim, J. Kim, and J. H. Kim, “Highly reliable AgNW/PEDOT:PSS hybrid films: efficient methods for enhancing transparency and lowering resistance and haziness,” J. Mater. Chem. C Mater. Opt. Electron. Devices 2(28), 5636–5643 (2014). [CrossRef]
23. Y.-S. Liu, J. Feng, X.-L. Ou, H.-F. Cui, M. Xu, and H.-B. Sun, “Ultrasmooth, highly conductive and transparent PEDOT:PSS/silver nanowire composite electrode for flexible organic light-emitting devices,” Org. Electron. 31, 247–252 (2016). [CrossRef]
24. D. Y. Choi, H. W. Kang, H. J. Sung, and S. S. Kim, “Annealing-free, flexible silver nanowire-polymer composite electrodes via a continuous two-step spray-coating method,” Nanoscale 5(3), 977–983 (2013). [CrossRef] [PubMed]
25. W. Gaynor, G. F. Burkhard, M. D. McGehee, and P. Peumans, “Smooth nanowire/polymer composite transparent electrodes,” Adv. Mater. 23(26), 2905–2910 (2011). [CrossRef] [PubMed]
26. S. Kim, S. Y. Kim, M. H. Chung, J. Kim, and J. H. Kim, “A one-step roll-to-roll process of stable AgNW/PEDOT:PSS solution using imidazole as a mild base for highly conductive and transparent films: optimizations and mechanisms,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(22), 5859–5868 (2015). [CrossRef]
27. J. Kim, J. Lee, J. You, M.-S. Park, M. S. A. Hossain, Y. Yamauchi, and J. H. Kim, “Conductive polymers for next-generation energy storage systems: recent progress and new functions,” Mater. Horiz. 3(6), 517–535 (2016). [CrossRef]
28. J. You, D.-S. Shin, D. Patel, Y. Gao, and A. Revzin, “Multilayered heparin hydrogel microwells for cultivation of primary hepatocytes,” Adv. Healthc. Mater. 3(1), 126–132 (2014). [CrossRef] [PubMed]
