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Article Cite This: ACS Omega 2019, 4, 5586−5594
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Fabrication and Optimization of Conductive Paper Based on ScreenPrinted Polyaniline/Graphene Patterns for Nerve Agent Detection Hyunjae Yu,†,§ Hoseong Han,†,§ Jyongsik Jang,*,‡,∥,§ and Sunghun Cho*,†,§ †
School of Chemical Engineering, Yeungnam University, 280 Daehak-Ro, Gyeongsan 38541, Korea School of Chemical and Biological Engineering, College of Engineering, Seoul National University, 599 Gwanangno, Gwanakgu, Seoul 08826, Republic of Korea
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‡
ABSTRACT: In this work, a high-performance sensor capable of effectively detecting nerve gas, a type of chemical warfare agent, was realized by conductive paper with polyaniline (PANI) nanofiber and graphene sheet. To realize the high-performance nerve gas sensor, dimethyl methylphosphonate (DMMP) was used as a model of nerve gas, and the conductive paper sensor was used to detect DMMP at a concentration of parts per billion within a few seconds. Improvements in electrical properties and sensor performance of conductive papers were realized by the addition of optimized amounts of graphene (0.14 wt %) and polyethylene oxide (13.1 wt %). In addition, poly(vinylbutyral-co-vinyl alcohol-co-vinyl acetate) (P(VB-co-VA-co-VAc)) copolymer significantly improved the intermolecular forces between PANI nanofiber, graphene sheet, and cellulosic paper. Conductive patterns containing PANI nanofiber/graphene cofillers were fabricated into sensor electrodes of various sizes and shapes by screen printing. The prepared conductive papers were exposed under DMMP at various concentrations of at least 3 to at most 30 000 ppb. The conductive paper sensor containing PANI nanofiber/graphene cofillers exhibited a minimum detectable level of 3 ppb, a response time of 2 s, and a recovery time of 35 s, and the sensor realized a high life cycle. Furthermore, the conductive paper sensor demonstrated excellent selectivity to selectively detect DMMP from other harmful substances, such as methanol and chloroform. It is expected that the conductive paper sensor will be a very useful means to protect the safety of people when it is widely spread.
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INTRODUCTION With the development of industrial technology, the use of chemical substances has become more frequent, and the degree of exposure of human body to harmful chemicals has increased either directly or indirectly.1−7 When toxic chemicals are solid or liquid, they are visible and it is easy to detect and block them form human body. However, it is very difficult to detect and block the harmful substances in the form of gas because they are almost invisible.1−7 In order to reduce the damage caused by the harmful gases and effectively prevent them, it is necessary to develop a gas sensor having a high sensitivity to detect the leak of the harmful gases at an early stage. The VX gas used in Kim Jong-nam’s assassination that occurred in 2017 is classified as a nerve agent that is currently prohibited as a chemical warfare agent.3,4 In addition, when VX is inhaled into the body or contacted with the skin, a very small amount (several milligrams) can act as a lethal weapon.3,4 However, because the nerve gas cannot be used directly in detection experiments, dimethyl methylphosphonate © 2019 American Chemical Society
(DMMP), a model of nerve gases similar to VX, has been generally chosen as an analyte for nerve gas sensor researches.3,4 Conductive polymer (CP) with a conjugated system structure can be defined as a polymer that can generate electricity through delocalization of electrons after doping.1−10 Among the many CPs, polyaniline (PANI) is one of the most attractive candidates for chemical sensors because of its attractive properties, such as easy synthesis, excellent redox sensitivity, and excellent electrical conductivity of up to 103 S/ cm.4−10 Because of this excellent redox reactivity, PANI can be applied to sensors that realize various color and electrical changes. Therefore, various studies have been conducted on the synthesis of PANI materials for chemical sensors through chemical oxidation polymerization and electrochemical polyReceived: February 8, 2019 Accepted: March 11, 2019 Published: March 20, 2019 5586
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Figure 1. FE-SEM images of (a) PANI nanofiber, (b) graphene sheet, (c) PANI nanofiber/graphene cofiller (9.1:0.14, w/w).
Figure 2. (a) Fabrication process of sensor patterns for DMMP sensor based on PANI/graphene composite. (b) Image of conductive paper containing sensor patterns actually printed. (c) Detection mechanism of DMMP gas using PANI.
CPs capable of rapid oxidation−reduction reaction is necessarily required. Given these facts, the development of PANI nanomaterials/graphene with optimized manufacturing conditions is expected to have a direct impact on the performance of DMMP sensors.6,10−15,27 Because paper materials are flexible and lightweight, they are important in the fields of flexible electronic devices,16−18 energy storage devices,19−21,27 nanofilters,22,23 and chemical sensors.24−26 In particular, conductive papers containing carbon nanotubes (CNTs) or graphene have been extensively studied for use as electrode materials in energy storage and sensor devices because of their high electrical conductivity. Moreover, these conductive papers have high portability. However, there are high costs and difficulties in producing solutions of CNTs with high dispersibility. Therefore, there are limitations in putting CNTs into practical use as cost-effective electrode materials capable of mass production. In order to utilize conductive papers as chemical sensors, rapid and reversible resistance change by oxidation−reduction reaction with analyte is essential.1,7,14,24−26 However, it can be predicted that the chemical sensors made of carbon materials will be less practical because they depend only on charge adsorption/desorption at the surface.14,24−26 Thus, there is a need to develop conductive paper-based DMMP sensors that combine PANI with good redox sensitivity and graphene with good electrical conductivity and large surface area.6,10−15,20,27 In this work, composites of PANI nanofibers and graphene sheets were prepared in the form of aqueous solutions, and the resulting aqueous solutions were prepared as sensor patterns for detecting DMMP through a screen-printing technique. The
merization. According to previous work, the sensitivities of PANI nanomaterials to DMMP are dependent on their morphological changes.4 However, when PANI is exposed to an analyte for a long time, it experiences expansion and chain scission because of volumetric changes, which limits the cycle life of PANI-based sensors.4−10 In addition, when powdered PANI is deposited on expensive platinum-based lithographic electrodes, the PANI transducers are detached from the substrate during adsorption/desorption of analytes. Therefore, it is required to realize the PANI-based sensors that are more structurally robust and reliable. Graphene, a two-dimensional electrode material, is one of the obvious candidates for improving charge transport in PANI sensors.6,11−13 Graphene sheets can induce strong π−π* interactions between the quinoid ring of PANI and the basal plane of the graphene sheet. This extends the conjugate length that allows the π-electrons of the PANI structure to be delocalized.6,10 These synergies can improve the properties of DMMP sensors based on PANI nanomaterials/graphenes. In addition, it has the advantage that the graphene sheets can prevent the swelling and collapse of PANI during the adsorption/desorption process of the analyte. Furthermore, the graphene sheet provides a surface area that is about 10− 100 times greater than that of the CP, so that a greater number of DMMP molecules can be adsorbed/desorbed within the electrode within the same time. However, because there are many portions not involved in adsorption and desorption of charges on the surface of the graphene sheet, graphene alone cannot exhibit a rapid change in electrical resistance.6,10−15,27 Therefore, when graphene is applied as a sensor, fusion with 5587
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with the −PO group of DMMP. This strong hydrogen bond promotes absorption of DMMP molecules inside the PANI structure. In addition, weak hydrogen bonds between the −OCH3 groups of DMMP and the PANI backbone could be additionally formed.4 Although H2O molecules can form intermolecular forces with PANI, H2O has little or no effect on the total amount of DMMP absorbed into the PANI structure at less than 30% of the relative humidity (RH).4−7 Conductive paper patterns were exposed to DMMP analytes at 25% RH. The absence of interference from the RH indicates that the interference of H2O molecules to the −NH bonds contained in the PANI backbone is reduced. Furthermore, the added graphene sheets enhance the electrical properties of PANI nanomaterials, and more DMMP molecules can be adsorbed and desorbed within the PANI/graphene composite because of the large theoretical surface area (2640 m2/g) of graphene.6,10−15,27 Figure 3 shows the FTIR spectra of PANI nanofibers and conductive paper samples prepared under different conditions.
screen-printing technique enabled the mass production of conductive patterns of various shapes and sizes. The compatibility of the PANI/graphene cofillers with the cellulosic paper was enhanced by introducing a poly(vinylbutyral-co-vinyl alcohol-co-vinyl acetate) (P(VB-co-VAco-VAc)) copolymer between cofillers and cellulose. To identify the reasons for the improved sensitivity, Fouriertransform infrared spectroscopy (FTIR) was utilized. The improved compatibility between the materials resulted in lower sensitivity (3 ppb), faster response time (2 s), and faster recovery time (35 s) for DMMP gas. Moreover, in order to further evaluate the actual sensor performance of PANI/ graphene-based conductive papers, reliability tests and selectivity tests from other harmful analytes were also conducted.
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RESULTS AND DISCUSSION Figure 1 shows field-emission scanning electron microscopy (FE-SEM) images of PANI nanofibers, graphene sheets, and PANI nanofiber/graphene sheet cofillers. The PANI nanofibers shown in Figure 1a were synthesized by chemical oxidative polymerization, with diameter and length of 40−50 nm and 0.6−1 μm, respectively.4 The graphene sheets used in this work were electrochemically exfoliated water-dispersible graphene (WDG), and the size of graphene sheets ranged from 1.0 to 3.0 μm (Figure 1b).18 Figure 1c represents that when PANI nanofibers are dispersed with graphene sheets, the PANI nanofibers can be well dispersed on the surface of the graphene sheet. As-prepared PANI/graphene cofillers were used as electrode materials for detecting DMMP, thereby enhancing the advantage of each material. An aqueous solution containing PANI nanofibers and graphene sheets was prepared in the presence of a polyethylene oxide (PEO) as a dispersing agent. Screen-printing technology could be used to produce electrodes of various shapes and sizes with thicknesses of about 10 μm (Figure 2a). Prior to screen printing, P(VB-co-VA-co-VAc) was coated on a cellulose substrate to lower the interfacial energy between PANI, graphene, and the cellulose substrate. The ether group (−O−), hydroxyl group (−OH), and carboxyl group (−COOH) of P(VB-co-VA-co-VAc) copolymer can form hydrogen bonds with the amine groups (−NH) and amine cations (−NH•+) of PANI, and the copolymer can also form hydrogen bonds with the ether (−O−) and hydroxyl groups (−OH) of cellulose. In addition, the P(VB-co-VA-co-VAc) forms dipole−dipole interactions with graphene sheets.18 Through such intermolecular forces, the P(VB-co-VA-co-VAc) copolymer improves the adhesion between the components and prevents delamination of the printed sensor patterns from the cellulosic paper. Therefore, it was observed that the electrode patterns printed on the cellulose not only had clear shapes but also maintained stable patterns even after repeated sensor measurements for a long time. The conductive paper sensor fabricated through the above process was connected to the current source meter, and the experiment was successfully carried out at various DMMP concentrations ranging from 3 ppb minimum concentration to 30 000 ppb maximum concentration. The PANI/graphene composite pastes made in the process described in Figure 2a were successfully used to produce conductive paper sensors of different sizes and shapes (Figure 2b). The mechanism for detecting DMMP molecules using PANI molecules is shown in Figure 2c. The −NH groups contained in the PANI backbone can form hydrogen bonds
Figure 3. FTIR spectra of PANI nanofibers and conductive paper samples.
In the spectrum of PANI nanofiber, characteristic peaks were found at wavenumbers 627, 832, 1005, 1098, 1123, 1236, 1294, 1480, 1579, and 3437 cm−1.4,6−8,28 The peak found at 627 cm−1 is because of the NH2 deformation of the aromatic amine in PANI. The peaks found at wavenumbers 832 and 1098 cm−1 represent the C−H in-plane bending and C−H out-of-plane bending, respectively, of the 1,4-disubstituted ring. The peaks found at wavenumbers 1294 and 3437 cm−1 refer to vibration because of C−N stretching and O−H stretching of the secondary aromatic amine, respectively. The peak at 1005 cm−1 is because of the symmetric stretching of SO3, which means that PANI nanofibers are well polymerized by ammonium persulfate (APS). The fact that the polymerized PANI nanofibers have a bipolaron structure is demonstrated by the peak at 1236 cm−1.28 A peak because of CC stretching of benzeneoid units (−NBN−) was observed at 1480 cm−1, and peaks because of CC stretching of quinoid units (−N QN−) were observed at wavenumbers 1123 and 1579 cm−1. The intensity ratio (IQ/IB) of −NQN− peak versus −NBN− peak is a very important criterion, which implies the doping level inside the PANI structure.4,6 The IQ/IB value of the pure PANI fiber was 0.78, indicating that the prepared PANI nanofiber was well formed in emeraldine salt form. PANI nanofibers were dispersed inside the PEO matrix and coated on cellulosic paper. As a result, new peaks were found at wavenumbers 698, 740, 1107, 1211, 1269, 2840, 2906, and 3281 cm−1.7,8,28 The peaks observed at 698, 1074, and 3281 cm−1 are related to cellulose, and these peaks originate from 5588
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Figure 4. (a) Sheet resistance and (b) electrical conductivity results as a function of graphene content. (c) Sheet resistance and (d) electrical conductivity results as a function of PEO content.
and the electrical conductivity increased until the graphene content reached 0.14 wt % (14 mg/mL) with respect to the weight of the total solution. However, after the graphene content increased to 0.28 wt % (28 mg/mL), the sheet resistance rapidly increased and the electrical conductivity decreased (Figure 4a,b). Even though graphene content was 0.28 wt %, the actual content of WDG paste dispersed in distilled water was 20 wt % (222 mg/mL). Considering this fact, a large amount of graphene sheets is contained within the conductive papers. According to previous studies, the amount of loaded graphene in water was about 1.5−25 mg/mL, implying that the critical coagulation concentration (CCC) of the graphene is usually formed at concentrations below 25 mg/ mL.29−32 At the concentration of graphene sheets above the CCC, the graphene sheets tend to cluster into graphitic structure because of the intensified van der Waals forces and π−π stacking between the graphene sheets.6,16,18,29−32 In addition, as the concentration of graphene increases, the size distribution of graphene is expected to increase. As the size distribution of graphene sheets increases, the size-dependent reorientation and self-diffusion in the graphene sheets occur more frequently.33 Therefore, as the concentration of graphene sheets increases, aggregation between the graphene sheets becomes easier. As the graphene agglomeration becomes easier, the irregular self-assembly of graphene sheets becomes noticeable.29−32 Therefore, as the graphene clusters increase, the contact resistance between the graphene sheets increases.31,32 Judging from these results, the CCC of graphene used in this experiment was between 0.14 and 0.28 wt %, and it is assumed that when the concentration of the graphene exceeds 0.14 wt %, the contact resistance between graphene sheets is greatly increased. Therefore, the optimum graphene content was found to be 0.14 wt %. The conductive paper containing 0.14 wt % graphene showed lower sheet resistance (5.13 ± 0.51 kΩ/sq) and higher electrical conductivity (0.195 ± 0.020 S/cm) than the other samples having different graphene contents. In addition, the conductive paper with 0.14 wt % graphene and P(VB-co-VA-co-VAc) exhibited lower sheet resistance (2.87 ± 0.29 kΩ/sq) and improved electrical conductivity (0.349 ± 0.035 S/cm) than the PANI/graphene sample. This demonstrates that the P(VB-co-VA-co-VAc)
CH2 bending, C−O−C stretching of the pyranose ring, and CH2 stretching vibrations, respectively.8,28 In addition, the peak of the O−H stretching vibration mode observed at 3440 cm−1 is significantly improved, which is also related to the alcohol groups of cellulose. Peaks observed at 1269, 2840, 2906, and 3033 cm−1 are related to PEO.7,28 Of these peaks, the peak at 1269 cm−1 originates from the C−O−C stretching of the ether groups, and peaks at 2840 and 2906 cm−1 are associated with −CH2 symmetric stretching. The peak corresponding to −CH2 asymmetric stretching was found at 3033 cm−1. Peaks for out-of-plane bending of C−H, C−N stretching of secondary aromatic amine, CC stretching of −NBN−, and CC stretching vibrations of −NQ N−shifted to shorter wavenumbers of 740, 1211, 1475, and 1574 cm−1, respectively. These blue shifts are thought to originate from the PEO. However, in graphene-doped conductive papers, peaks corresponding to CC stretching of −NBN− and CC stretching of −NQN− vibrations shifted to high wavenumbers of 1480 and 1581 cm−1, respectively. Moreover, IQ/IB (0.85) was also higher than that before the addition of graphene (0.74). It is considered that the graphene with high electrical conductivity extends the conjugation length inside the PANI, and the doping level of PANI is improved because of the increased conjugation length.6,10,14,15 In addition, this tendency was more pronounced in samples using cellulosic paper coated with P(VBco-VA-co-VAc) copolymer. The peaks for CC stretching vibrations of −NBN− and−NQN− units in the spectrum of conductive paper coated with P(VB-co-VA-coVAc) copolymer were further red-shifted to 1482 and 1583 cm−1, respectively. This result suggests that P(VB-co-VA-coVAc) copolymer improves the adhesion between cellulosic paper, PANI, and graphene.4,6,18 Furthermore, the peak intensity corresponding to the O−H stretching vibration was significantly increased after the introduction of P(VB-co-VA-coVAc) copolymer. This well demonstrates the hypothesis that improved adhesion and improved doping levels are caused by strong hydrogen bonding between the components. Figure 4 summarizes the sheet resistance and electrical conductivity results of conductive papers containing different contents of graphene and PEO. The sheet resistance decreased 5589
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Figure 5. Current (I) change amount characteristic of the conductive paper samples according to the change in the applied voltage V: 10 mV/s): (a) conductive paper with 8.2 wt % PEO, (b) conductive paper containing 13.1 wt % PEO.
Figure 6. (a) Real-time sensitivity and (b) real-time voltage variation of conductive paper patterns on cyclic exposure to DMMP (3−30 000 ppb) and N2 stream. (c) Real-time sensitivity and (d) real-time voltage variation of conductive paper patterns at periodic exposure to 300 ppb DMMP and N2 streams.
the resistance value inside the electrode, which means that a reliable current generation is achieved inside the conductive paper.2−4,6 The reciprocal value of dI/dV was lowest when conductive papers containing only PANI nanofibers were measured. The dI/dV of the PANI nanofiber/P(VB-co-VA-coVAc)/graphene sample was higher than that of the PANI nanofiber/graphene sample. This tendency was observed in conductive papers containing 8.2 and 13.1 wt % PEO (Figure 5a,b). Assuming that the cellulose substrate used for the conductive paper is unchanged, the contact resistance of the conductive paper mainly depends on the charge transport inside the PANI nanofiber and the graphene sheet.6,10,14,15 Furthermore, the contact resistance between the PANI nanofiber/graphene sheet composite and the cellulosic paper can be reduced through improved intermolecular forces caused by P(VB-co-VA-co-VAc) copolymer, as aforementioned in Figure 2.8,9,18 In addition, it was confirmed that the addition of the PEO dispersion medium with the proper content (13.1 wt %) minimizes the aggregation between the conductive materials and contributes to the reduction of the resistance inside the electrode, as evidenced in Figure 4 (Figure 5b). Considering these results, the graphene sheet reduces the electrical resistance inside the conductive patterns, and the contacts between the PANI nanofiber, graphene, and cellulosic paper are significantly improved by the PEO binder and P(VBco-VA-co-VAc) copolymer.
copolymer improves the adhesion between PANI nanofibers, graphene sheets, and cellulosic paper substrates, leading to increased charge transport properties of the conductive papers. Moreover, when the thickness of the thin film was 10 μm (PANI/copoly/graphene-2 and PANI/graphene-2), the surface resistance was significantly lower and the electric conductivity was higher than 6 μm (PANI/copoly/graphene1 and PANI/graphene-1). This means that electrically conductive passageways can be formed, in which electrons can be delocalized within a sample having an appropriate thickness.6,10,14,15 Figure 4b,c summarize the sheet resistance and electrical conductivity of the conductive paper according to the content of PEO as a dispersing agent. As the content of PEO increased, the sheet resistance decreased and electrical conductivity increased. This demonstrates that the PEO has a direct effect on resolving the aggregation of the graphene sheet and PANI nanofibers and improving dispersibility.7 Moreover, it was confirmed that the sample containing P(VB-co-VA-coVAc) copolymer had a lower sheet resistance than the sample without P(VB-co-VA-co-VAc). The optimum weight fractions of PANI nanofiber, graphene sheet, and PEO required to implement the conductive paper pattern electrode optimized for DMMP sensing were 9.1, 0.14, and 13.1 wt %, respectively. Figure 5 shows the current−voltage characteristics of conductive paper consisting of PANI nanofibers, PANI nanofibers/graphene, and PANI nanofibers/P(VB-co-VA-coVAc)/graphene. The reciprocal of the linear dI/dV (slope) is 5590
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Figure 7. (a) Plots of sensing response versus DMMP concentration for conductive papers. (b) Sensitivity comparison graph of conductive papers based on PANI/P(VB-co-VA-co-VAc)/graphene sample for different analytes at 300 ppb.
method is suitable for implementing a DMMP sensor with a high cycle life. Also, the sensitivity of conductive papers at the same concentration of 300 ppb was lowest when PANI nanofibers were included. PANI/P(VB-co-VA-co-VAc)/graphene sample exhibited higher sensitivity than the PANI/ graphene sample. From these results, it was confirmed that the addition of graphene sheet and the improved intermolecular force between the components by the P(VB-co-VA-co-VAc) copolymer directly enhance the resulting sensor performances. To study the linear relationship between the sensitivities and concentrations, calibration curves of the conductive papers are represented in Figure 7a.11−13,34 In the calibration curves of samples, the ΔR/R0 increased linearly with the DMMP concentration varying from 3 to 30 000 ppb. This indicates that more DMMP molecules are absorbed onto the surface of conductive papers, resulting in the increased sensitivity of the conductive papers.34 The calibration sensitivities increased in the following order: PANI < PANI/graphene < PANI/copoly/ graphene. It has been found that the significant improvement in the sensitivity of the conductive paper is because of the synergistic effect between PANI nanofibers, graphene sheets, and P(VB-co-VA-co-VAc) copolymer. From the above analysis, it was confirmed that there is a good linearity relationship between the concentration of DMMP and the sensitivity. Selectivity of conductive paper based on PANI/P(VB-co-VAco-VAc)/graphene sample was assessed by exposing the conductive paper to other analytes, such as methanol, ethanol, ammonia, chloroform, and nitrogen dioxide (Figure 7b). Changes in sensitivity depending on the types of hazardous analytes depend on the inherent dipole moment (μ) of the analytes.3,4 The μ values for DMMP, methanol, ethanol, ammonia, chloroform, and nitrogen dioxide were 3.62, 1.7, 1.69, 1.47, 1.04, and 0.32 D, respectively. Molecules with a relatively high polarity, such as DMMP, cause a large resistance change, whereas molecules with lower polarities than DMMP, such as methanol, ethanol, ammonia, chloroform, and nitrogen dioxide, showed only small resistance changes. In addition, the CH3O-group of DMMP acts as a weak hydrogen-bonding receptor, promoting the hydrogen-bonding interaction between the PANI structure and DMMP.4 Considering these facts, the manufactured conductive paper sensor realizes excellent selectivity.
Figure 6a represents the real-time sensitivity changes when the conductive papers are periodically exposed to DMMP and N2 streams at concentrations of at least 3 ppb up to 30 000 ppb. The minimum detectable level of DMMP, which can be detected with conductive papers, was 3 ppb. Conductive papers responded within 2 s of exposure to DMMP and it took 35 s to recover to the background signal value. Sensitivity values of conductive papers at every concentration of DMMP increased in the order of PANI < PANI/graphene < PANI/ P(VB-co-VA-co-VAc)/graphene, which were in good agreement with the results in Figure 4. The improved signal values at DMMP exposure are related to the improved charge transport properties of the conductive paper. The graphene sheet can induce a strong π−π* interaction between the quinoid ring of PANI and the basal plane of the graphene sheet, which results in an extended conjugation length within the PANI structure that can delocalize π-electrons.6,10,14,15 These synergies can improve the properties of DMMP sensors based on PANI nanofiber/graphene. It is believed that the improved charge transport properties are closely related to the enhancement of electrical properties by the addition of graphene sheet as well as the intensified intermolecular forces between the components caused by P(VB-co-VA-co-VAc) copolymer.18 When the DMMP gas is adsorbed on the conductive paper, the normalized sensitivity (ΔR/R0) increases by operating with the reduction voltage. On the contrary, when the DMMP gas is desorbed, the ΔR/R0 is reduced by operating the oxidation voltage (Figure 6b). If the PANI is not exposed to DMMP, the PANI becomes oxidized.3,4 Therefore, the R value of oxidized PANI is almost the same as the initial resistance value R0, and the value of ΔR/R0 becomes smaller. On the other hand, when PANI is exposed to DMMP gas, the measured resistance difference ΔR = R − R0 becomes larger and the value of ΔR/R0 value also increases.3,4 For these reasons, the real-time variations of ΔR/R0 and V show the opposite tendency, whereas the real-time variation of R follows the same trend as V. The reliability of a conductive paper sensor is a measure of whether it can maintain high sensitivities even after repetitive sensing cycles. Hence, this cycling stability is a very important part in actual sensor application. To realize this, we measured the real-time sensitivity and voltage change when conductive papers were periodically exposed to DMMP and N2 streams of 300 ppb (Figure 6c,d). When the reduction voltage was applied to the conductive paper, the electrical resistance was increased because of the adsorption of DMMP. On the other hand, when the oxidation voltage was applied, the electrical resistance was decreased because of desorption of DMMP. The manufactured conductive paper patterns retained their initial sensing intensities well after the repetitive sensing experiments for 1300 s, which means that the screen-printing
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CONCLUSIONS In this study, it was possible to mass-produce conductive patterns containing various shapes and sizes of PANI/graphene composites through screen-printing technology. The electrical properties of conductive papers could be optimized by identifying an optimal graphene content (0.28 wt %) and an optimal PEO content (13.1 wt %). The electrical conductivity 5591
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deposited P(VB-co-VA-co-VAc) copolymer film was about 5 μm. The PANI/graphene composite pastes prepared in the previous step were fabricated into patterns with different sizes and various shapes onto the cellulosic paper coated by P(VBco-VA-co-VAc) using a screen printer (SM-S320, Sunmechanix, Seoul, Korea). As-prepared conductive patterns were dried at 60 °C for 10 min. The thicknesses of the once- and twicedeposited conductive patterns were about 6 and 10 μm, respectively. Detection of DMMP Using Conductive Papers. The conductive paper-based sensor device was installed in a 350 mL vacuum chamber with an electrical feed-through equipped with a gas inlet and outlet. The pressure was maintained at approximately 10 Torr. The nitrogen (N2) stream was passed through an external bubbler in a vessel containing DMMP and then flowed into the vacuum chamber at a flow rate of 2000 sccm. The concentration of DMMP varied from 3 to 30 000 ppb by adjusting the ratio of DMMP to N2 gas. Conductive paper was exposed to circulating DMMP/N2 exposure using an MFC (KNH Instruments, Pucheon, Korea). After PANI nanomaterials were exposed to DMMP vapor for 2 min, DMMP vapor was replaced with compressed N2 for 2 min; this process was repeated several times. The sensitivities of the conductive paper electrodes caused by real-time irradiation of DMMP at a constant applied current of 10−6 A were evaluated. Real-time changes in the voltage and resistance of the conductive paper were recorded at room temperature using a current source meter (Keithley 2400, Keithley Co., Cleveland, OH, USA). The normalized resistance changes (ΔR/R0) of the conductive papers were recorded in real time as they reached a saturated signal value after exposure to DMMP. The ΔR/R0 of the conductive paper sensor can be expressed as equation ΔR/ R0 = (R − R0)/R0, where R0 and R refer to the initial resistance and the measured real-time resistance, respectively.3,4,6 The response time is defined as the time required for the conductance to reach 90% of the equilibrium value after gases were injected. The recovery time refers to the time necessary for a sensor to attain a conductance of 10% above its original value in air.3,4,6 Instrumentation. Morphological images of the PANI nanofiber, graphene sheet, and PANI/graphene cofiller were recorded on an FE-SEM (S-4800, HITACHI, LTD, Hitachi, Japan). Electrical properties and sensing performances of the conductive papers were measured using a Keithley 2400 current source meter (Keithley Co., Cleveland, OH, USA) equipped with a WBCS 3000 potentiostat (WonA Tech, Seoul, Korea). The surface resistance values of the produced conductive papers were expressed in Ω/sq. The conductivity values were estimated according to the 4-point probe method equation σ (S cm−1) = 1/ρ = (ln 2)/(πt)1/R, where ρ, R, and t indicate the static resistivity, sheet resistivity, and thickness of the sample, respectively.4,6 FTIR spectra of conductive papers were recorded on Frontier FTIR spectrometers (PerkinElmer Inc., Waltham, MA, USA).
of conductive papers could be even intensified by coating PANI/graphene composite on the cellulosic paper treated with the P(VB-co-VA-co-VAc) copolymer. As-prepared conducting paper composed of PANI nanofibers, graphene sheets, PEO, and P(VB-co-VA-co-VAc) copolymer was exposed to DMMP at various concentrations ranging from 3 to 30 000 ppb using a sensor device composed of a current source meter and a massflow controller (MFC). The fabricated conductive papers showed the highest sensitivity of 3 ppb, the response time of 2 s, and the recovery time of 35 s. In addition, when the conductive papers were exposed to DMMP of 300 ppb for a long time, the sensitivity of the conductive paper sensor was shown to maintain the initial intensity steadily over a long period of time. This confirms that the reliability of the conductive paper sensor is high. It is also shown that the conductive paper sensor has excellent selectivity because the conductive paper was very sensitive only to DMMP, unlike the case where the conductive paper was exposed to other harmful substances. The above excellent sensor performance is closely related to the improved electrical properties and enlarged surface area with the addition of the graphene sheets and the enhanced intermolecular forces inside the conductive paper by the P(VB-co-VA-co-VAc) copolymer. Accordingly, the results of this research are expected to contribute to the spread of high-performance nerve gas sensors capable of mass production at low cost.
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EXPERIMENTAL SECTION Materials. Aniline (99.0%), APS (98.0%), (P(VB-co-VA-coVAc), Mw: 70 000−100 000), and PEO (Mw: 100 000) were purchased from Sigma-Aldrich (St. Louis, USA). Hydrochloric acid (HCl, 35.0−37.0%) was purchased from Daejung Chemical & Metals Co. Ltd. (Siheung, Korea). Distilled water was used as a solvent for the polymerization of aniline. WDG paste (1.5 wt % with respect to distilled water) was obtained from MExplorer Co. Ltd. (Ansan, Korea). The graphene sheet included in the WDG paste has an average thickness of
