Elucidating the Structure–Function Relationship of Poly(3,4

Article pubs.acs.org/JPCC

Elucidating the Structure−Function Relationship of Poly(3,4theylenedioxythiophene) Films to Advance Electrochemical Measurements Adam R. Meier, William A. Bahureksa, and Michael L. Heien* Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721, United States S Supporting Information *

ABSTRACT: Previous work has demonstrated the utility of poly(3,4-theylenedioxythiophene) (PEDOT) electrodes for electrochemical detection of neurochemicals. Although these electrodes have been implemented successfully, there is minimal data linking redox mechanisms, electron-transfer characteristics, and sensor lifetime to the electrode molecular composition. Common polymer electrodes are made from commercially available PEDOT:polystyrenesulfonate (PEDOT:PSS), which is easily processed but has slow electrontransfer kinetics and short electrochemical lifetimes. Here, we describe vapor-phase synthesized PEDOT:tosylate films that have a higher conductance and a much lower apparent capacitance than PEDOT:PSS (99 ± 8 versus 2390 ± 130 μF/cm2). Additionally, we show that the electron-transfer kinetics and electrochemical lifetime are both improved. To investigate the chemical causes of these improvements we used ultraviolet−visible absorbance and X-ray photoelectron spectroscopy (XPS). We discovered that the high density of PEDOT incorporated into PEDOT:tosylate films coupled with the lack of impurities and replacing the polymeric dopant (PSS) leads to both increased conductance and reduced film capacitance. This is most clearly demonstrated through the doping ratio of 3.80 ± 0.10 in vaporphase synthesized PEDOT:tosylate versus 0.20 ± 0.02 in PEDOT:PSS. Furthermore, the electrochemical lifetime of the films is dependent upon the amount of PEDOT present. XPS data was used to elucidate the mode of failure of these electrodes. This begins to illuminate the mechanism of electron transfer at conducting polymers electrodes. Understanding both the characteristics that improve the quality of conducting polymer electrodes and the mechanism of electron transfer therein is a crucial step in the wider adaptation of these materials in biosensor applications.

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and other brain functions and are redox active.21−23 In most applications aimed at neurotransmitter detection, PEDOT is used as a coating to improve the electrochemical performance of another electrode material, often platinum.24−26 However, there has been some investigation into using PEDOT-only electrodes as the working electrode material for these measurements.11,19 Unfortunately, most of these studies are application driven and there is little understood about the critical parameters that control the electrochemical properties and therefore how to improve said properties. Recently, we have shown how a carefully optimized synthesis of PEDOT:tosylate by oxidative chemical vapor deposition allowed for rapid electrochemical measurements (scan rates of 100 V/s) at a polymer electrode.19,27 The key to this measurement was reducing the apparent capacitance of

INTRODUCTION Applications of conducting polymers have grown considerably since they were first reported in the 1970s.1,2 Perhaps the most abundant and well-studied conducting polymer is poly(3,4theylenedioxythiophene) (PEDOT). Since it was first reported in 1997,3 applications of this material have grown to include antistatic coatings,3 solar cell interlayers,4,5 optoelectronic devices,6,7 and electrochemical biosensors.8−12 PEDOT is an attractive material for these applications because it is flexible, optically transparent, thermally stable, and biocompatible.8,12−15 Additionally, PEDOT films can be easily deposited and patterned making high-throughput processing possible. These features make PEDOT a promising alternative compared to traditional electrode materials such as carbon or noble metals.10,16−18 Of specific interest here are applications of PEDOT as a working electrode material for biosensor applications such as the electrochemical detection of biological analytes, including neurotransmitters.11,18−20 Neurotransmitters such as dopamine, norepinephrine, and serotonin are of interest because they have important physiological roles in regulating behavior, cognition, © XXXX American Chemical Society

Special Issue: Richard P. Van Duyne Festschrift Received: May 7, 2016 Revised: August 5, 2016

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Vapor-Phase PEDOT:Tosylate Film Synthesis. Vaporphase synthesis of PEDOT:tosylate films was carried out as described in Meier et al.19 TOPAS 5013L Cyclic Olefin Copolymer (Topas Advanced Polymers Inc., Florence, KY, United States) substrates (Ø 50 mm × 2 mm wafers) were generated via injection molding. A solution of 20% w/v iron(III) tosylate in dried n-butanol and an equimolar amount of anhydrous pyridine was prepared and filtered using a 0.45 μm nylon syringe filter. PEDOT:tosylate films were deposited by spin coating 1 mL of this solution using a CEE Model 100 Spin Coating System (Brewer Science, Rolla, MO, United States) onto TOPAS substrates at 1500 rpm for 15 s with an acceleration of 500 rpm/s. The coated substrates were placed in a glass Petri dish. Ethylenedioxythiophene (EDOT) monomer (∼100 μL) was deposited at the bottom of the dish, and the dish was placed in a drying oven with an internal temperature of 55 °C to evaporate the monomer. The dish containing the substrates was removed after 20 min, and the substrates were removed immediately. During this time, the films changed from yellow to blue-green, indicating successful deposition of the PEDOT film. PEDOT:tosylate-coated substrates were rinsed with 18.2 MΩ·cm water to remove residual iron(III) tosylate and then dried using N2. This rinse step yielded a blue film. Substrates were placed in a 70 °C oven to dry overnight. PEDOT:PSS Film Synthesis. Glass slides were cleaned with Alconox and ethanol, rinsed with 18.2 MΩ·cm water, and dried under N2. A thin adhesion layer composed of poly-3,4ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) was spin-coated onto these substrates at 5000 rpm for 60 s at an acceleration of 5000 rpm/s and baked on a hot plate at 200 °C for 10 min. This baking step yielded a nonconducting adhesion layer ( 0.75). This indicates that this phenomenon is solely a function of charge transfer and is independent of time spent in solution or application of the waveform. Interestingly, when the total charged passed (i.e., the sum of the integrals of the oxidation peaks for both voltammograms) is measured, the PEDOT:tosylate electrodes transferred 3.2 ± 0.3 (±SEM, n = 5) times more electrons than the PEDOT:PSS had by the time they had each lost 25% function (PEDOT:tosylate did not decay below 50% during the time course of the experiment). This would indicate that the lifetime of the material is limited by the total charge transferred, which correlates with the density of PEDOT in the film. Recall that Figure 1 (vide supra) showed that the PEDOT:tosylate has approximately 3 times more PEDOT than the PEDOT:PSS. This supports the conclusion that the electrochemical lifetime of PEDOT electrodes is determined by the total charge transferred by the electrode, which is correlated to the amount of PEDOT in the electrode. Therefore, the electrochemical lifetime of a PEDOT-based sensor platform could be extended by using a PEDOT-rich film.

carboxylic acid and dopamine at PEDOT:PSS and PEDOT:tosylate are shown in Figure 4C,D. For the analytes ferrocene carboxylic acid and potassium ferricyanide, we used the Nicholson method to calculate the heterogeneous electrontransfer rate constants (k0) for these molecules at PEDOT electrodes.42 For ferrocene carboxylic acid, k0 = 6.2 ± 0.3 cm/s at PEDOT:tosylate and k0 = 4.6 ± 0.2 cm/s at PEDOT:PSS (±SEM, n = 5 electrodes). Similarly for potassium ferricyanide, k0 = 4.1 ± 0.7 cm/s at PEDOT:tosylate and k0 = 2.1 ± 0.2 cm/s at PEDOT:PSS (±SEM, n = 4 electrodes). PEDOT:tosylate electrodes exhibited significantly faster kinetics than PEDOT:PSS for both ferrocene carboxylic acid and potassium ferricyanide (Student’s t test, p < 0.005 and p < 0.05, respectively). For the neurochemicals we tested, the ΔEp values were too large to use with the Nicholson method. However, ΔEp is generally smaller with the PEDOT:tosylate electrodes, indicating an extension of the observed faster electron-transfer kinetics to neurotransmitter molecules. Additionally, the shape of the cyclic voltammograms is more ideal, indicating better electrochemical behavior.36 This data demonstrates that the increased conductance and low capacitance combine to give better electron-transfer properties for electroactive neurotransmitters. Electrochemical Lifetime of PEDOT Electrodes. To this point, we have shown that vapor-phase synthesized PEDOT:tosylate electrodes have higher conductance, lower capacitance, and better electron-transfer properties than commercially available PEDOT:PSS. However, potential biosensor applications would require that this material is stable over an extended time frame. To evaluate the electrochemical lifetime of these materials we made successive measurements of ferrocene carboxylic acid using cyclic voltammetry over a time period of 90 min (Figure 5). This analyte was chosen over the neurotransmitters that we tested because it is stable at room temperature over this time period. We observed the decay in F

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these experiments. While there is some loss in function of the PEDOT:tosylate electrodes, as demonstrated in Figure 5B,D, the PEDOT peaks are still present in the XPS spectra, indicating that the polymer is still present. The dopant may be replaced by another, more abundant, anion in solution such as chloride or sulfate. This again supports the conclusion that the structure may become consolidated through use but still maintains its function. Regardless, the persistence of function indicates the vapor-synthesized PEDOT:tosylate electrodes are better for long-term measurement applications. Ultimately, the improved electrical and electrochemical properties demonstrated here result in faster electrochemistry, more robust sensors, and longer sensor lifetime. Using XPS we showed that the high PEDOT content in the vapor-phase synthesized PEDOT:tosylate films and the high doping ratio impart these desirable characteristics. We can use this information to inform the design of devices, including biosensors, which incorporate PEDOT as either a conducting layer or an electrode material.

To further understand the mechanism by which the function of these electrodes is lost, we used XPS to observe the molecular changes in sulfur and carbon occurring in the polymer electrodes. We collected XPS spectra of the electrodes used in the experiment in Figure 5 (90 min of oxidizing ferrocene carboxylic acid). Looking first at the whole spectra from PEDOT:PSS (Figure 6A) and PEDOT:tosylate (Figure

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CONCLUSION

The results presented here describe the structure−function relationship of PEDOT-based electrode materials for electrochemical measurements. Our synthetic method based on vaporphase synthesis results in PEDOT:tosylate thin-films that have a higher conductance and lower apparent capacitance than commercially available PEDOT:PSS. Using UV−vis spectroscopy and XPS, we showed that this can be owed to a higher content of PEDOT in the films, fewer impurities, and a better dopant choice and ratio. Using photolithography, these films were patterned into electrodes and used to measure both electrochemical standards and neurotransmitters. Generally, we observed faster electron-transfer kinetics using PEDOT:tosylate electrodes. Lastly, we showed that the electrochemical lifetime of these electrodes is determined by the total charge transferred by the electrode. The lifetime can be extended by using PEDOT-rich films such as the PEDOT:tosylate described here. The observations made in this work also start to elucidate the charge-transfer mechanisms at PEDOT electrodes. We observed more ideal electrochemical behavior at the PEDOT:tosylate electrodes which we have shown to be relatively more metallic in nature compared to PEDOT:PSS. Considering this we can arrive at the conclusion that creating films with lower barriers to charge transfer and a higher density of states improves their electrochemical properties. In summary, this work demonstrates that synthesizing lowcapacitance, PEDOT-dense films results in better electrode materials toward biosensor applications. The use of vapor-phase synthesis to enable better electrical and electrochemical properties coupled with the extended electrochemical lifetime are all desirable when considering PEDOT-based materials for creating effective measurement devices. Furthermore, this material has great potential to replace commercial PEODT:PSS for solid-state applications such as solar cells, organic LEDs and displays, and biosensors, all of which could benefit from the greatly reduced capacitance demonstrated here. The observed properties arise from careful consideration of doping and synthesis conditions to yield a high-conductance, lowcapacitance film. These properties can be tuned depending upon the desired application.

Figure 6. XPS spectra of PEDOT:PSS (red) and PEDOT:tosylate (blue) electrodes after 90 min of use. (A) Full XPS spectrum of PEDOT:PSS and (B) full XPS spectrum of PEDOT:tosylate. (C) Sulfur-2p region from XPS spectrum of PEDOT:PSS and (D) sulfur2p region from XPS spectrum of PEDOT:tosylate. The structures for the peaks in the sulfur region are shown. PEDOT:PSS (C) has a loss of characteristic PEDOT peaks (indicated by †) when compared to PEDOT:tosylate (D) which only has a reduction of the dopant peak (indicated by *). Fits are shown in Figure S5.

6B), we can see substantial differences compared to the unused electrodes (recall Figure 3A,B). The most stark difference is the substantial reduction of the carbon-1s peak in the PSS spectrum. Further examination of this region revealed that the peak at 286 eV, which is indicative of ether (C−O−C) bonding moieties, was lost completely in the PEDOT:PSS spectrum; this feature is still present in the PEDOT:tosylate spectrum (Figure S4). Recall the structure of PEDOT from Scheme 1; the only ether bonds present are in the ring on the EDOT monomer. Loss of this feature would indicate reduction or loss in the conducting polymer chains. This supports the conclusion that the loss of function of the PEDOT:PSS electrodes is due to degradation of the conducting polymer chains. We also wanted to examine the sulfur-2p region because this region contains reporters for both the PEDOT at 164−165 eV and the sulfonate on the dopant molecules at 168.5 eV. We see that the dominant shape of the spectra remains for both the PEDOT:PSS (Figure 6C) at 168.5 eV and the PEDOT:tosylate (Figure 6D) at 164−165 eV (compare to Figure 3C,D). However, the PEDOT:PSS spectra shows no detected peaks in the 164−165 eV, indicating that the PEDOT is no longer present. This would explain the complete loss of function observed in the PEDOT:PSS films shown in Figure 5A,C. Conversely, the PEDOT:tosylate spectra shows a decrease in the peak at 168.5 eV, meaning that less of the tosylate dopant is longer present. This is unsurprising because tosylate carries a charge and is soluble in the aqueous electrolyte solution used in G

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04622. Additional schematic of electrodes and electrochemical setup, XPS data and fits, and cyclic voltammetry data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 520-621-6293. Fax: 520621-8407. Notes

The authors declare the following competing financial interest(s): Dr. Michael Heien declares an actual or potential financial conflict of interest and is co-founder/equity holder in Knowmad Technologies, a licensee of University of Arizona (UA) intellectual property used in this research. This relationship has been disclosed to the UA Institutional Review Committee and is managed by a Financial Conflict of Interest Management Plan.

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ACKNOWLEDGMENTS The authors thank Paul Lee for technical expertise, XPS experiments, and lithography materials and Oliver L. A. Monti for helpful comments. We thank University of Arizona and NSF 1450767 (M.L.H.) for funding this work.

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DOI: 10.1021/acs.jpcc.6b04622 J. Phys. Chem. C XXXX, XXX, XXX−XXX