Research Article www.acsami.org
Wireless, Room Temperature Volatile Organic Compound Sensor Based on Polypyrrole Nanoparticle Immobilized Ultrahigh Frequency Radio Frequency Identification Tag Jaemoon Jun,‡ Jungkyun Oh,‡ Dong Hoon Shin, Sung Gun Kim, Jun Seop Lee, Wooyoung Kim, and Jyongsik Jang* School of Chemical and Biological Engineering, College of Engineering, Seoul National University (SNU), 599 Gwanangno, Gwanak-gu, Seoul 151-742, Korea S Supporting Information *
ABSTRACT: Due to rapid advances in technology which have contributed to the development of portable equipment, highly sensitive and selective sensor technology is in demand. In particular, many approaches to the modification of wireless sensor systems have been studied. Wireless systems have many advantages, including unobtrusive installation, high nodal densities, low cost, and potential commercial applications. In this study, we fabricated radio frequency identification (RFID)-based wireless sensor systems using carboxyl group functionalized polypyrrole (C-PPy) nanoparticles (NPs). The C-PPy NPs were synthesized via chemical oxidation copolymerization, and then their electrical and chemical properties were characterized by a variety of methods. The sensor system was composed of an RFID reader antenna and a sensor tag made from a commercially available ultrahigh frequency RFID tag coated with C-PPy NPs. The C-PPy NPs were covalently bonded to the tag to form a passive sensor. This type of sensor can be produced at a very low cost and exhibits ultrahigh sensitivity to ammonia, detecting concentrations as low as 0.1 ppm. These sensors operated wirelessly and maintained their sensing performance as they were deformed by bending and twisting. Due to their flexibility, these sensors may be used in wearable technologies for sensing gases. KEYWORDS: wireless, polypyrrole, sensor, volatile organic compound, RFID tag
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INTRODUCTION Recently, technologies to sense volatile organic compounds (VOCs) such as acetone (CH3COCH3), ethanol (C2H5OH), acetic acid (CH3COOH), and ammonia (NH3) with high sensitivity and fast response times have been studied.1−4 These technologies have a variety of potential applications such as home health care, home automation, and disaster prevention. In particular, NH3 is a widely used gas which contributes to the nutritional needs of terrestrial organisms by serving as a precursor for food and fertilizer.5,6 Additionally, NH3 is a biomarker that can be rapidly and noninvasively detected from breath analysis. Despite its usefulness, NH3 shows hazardous effects on the human respiratory system when its concentration exceeds the threshold limit value of 25 ppm.7−9 Therefore, costeffective and highly sensitive systems for detection of NH3 are needed across industrial and commercial areas. © XXXX American Chemical Society
Due to their small size in the range of 1−100 nm, sensor devices based on nanomaterials perform outstandingly well at detecting the target substances.10,11 During the past decade, a number of strategies have been proposed for enhancing sensor performance using nanomaterials such as metal oxides, metal nanowires, carbonaceous materials, and conducting polymer nanomaterials.12−18 In particular, conducting polymer nanomaterials have been used as sensor transducers as they have inherent electronic and mechanical transduction mechanisms, operate at room temperature, and are cost-effective.4,19−22 Although these conducting polymer materials have attractive properties, their use in commercial devices has been limited due Received: July 8, 2016 Accepted: November 9, 2016 Published: November 9, 2016 A
DOI: 10.1021/acsami.6b08344 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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step, poly(vinyl alcohol) (PVA) was dissolved in deionized water, and iron cations were obtained from ferric chloride (FeCl3) (Figure 1a). The pyrrole solution, which contained
to the lack of methods for integrating the sensor components into electronic circuits.23 Difficulties arising due to the properties of aggregates of these materials have also limited their use in sensor devices. Several approaches to integrating the electronic circuit with a uniform distribution of the sensor material have been proposed. One of the most promising approaches is to integrate the nanomaterials into the wireless network circuit as the sensors can be installed unobtrusively at a low cost with high nodal densities.23,24 This approach has potential commercial applications. Rapid advances in portable technologies, such as improvements to smart phones, have broadened the range of potential applications of wireless sensors.25−31 For instance, they are used for real-time monitoring of hospitalized patients, food freshness, military personnel, and to detect the presence of harmful gases such as carbon dioxide, nitrogen dioxide, oxygen, and flammables.32−34 Due to its simple architecture and broad detection range, the radio frequency identification (RFID)-based sensor is the most promising of the proposed wireless sensing systems.15,35−38 There are two types of RFID tag: the active type, which requires an on-board power supply, and the passive type, which does not.39,40 Although active RFID tags can be used as sensors at distances from the reader antenna greater than those of passive RFID tags, their large size and short life cycle possibly limit their applicability.41 The passive RFID tags are more attractive as they have a longer life span, are cost efficient, and small in size. We studied a wireless sensor system which uses a passive ultrahigh frequency (UHF)-RFID sensor tag with a distance of 50 cm.42 The proposed wireless sensor system is composed of two parts. The signal receiver is an RFID reader antenna connected to a network analyzer.23 The signal transfer is performed by carboxylated polypyrrole (C-PPy) nanoparticles bonded to a passive UHF-RFID tag to form a sensing tag. The C-PPy nanoparticles were synthesized from a water-soluble polymer and highly monodispersive metal cations. The C-PPy nanoparticles, which had an average size of 60 nm, were mixed with an aqueous solution of functional groups of modified pyrrole monomers.43 The sensing area was created using a condensation reaction to pattern the antenna with the C-PPy NPs.44 This method was chosen as it creates covalent bonds which help to maintain the stability of the nanoparticles in the sensor coating during exposure to gases. The presence of the CPPy NPs gave rise to a strong affinity between the PPy backbone and the NH3, as indicated by a shift in the reflectance of the RFID sensor. Furthermore, the reflectance of our wireless sensor varied with the NH3 concentration and enabled detection of low concentrations (0.1 ppm) of NH3 at room temperature. To our knowledge, no reports have described the fabrication of an RFID-based wireless sensor using the C-PPy NPs.
Figure 1. (a) Preparation of the carboxyl functional groups controlled by polypyrrole nanoparticles (C-PPy NPs) in an aqueous solution with polymer/metal cation complexes. (b) SEM image of the C-PPy NPs (inset: photograph of a Petri dish containing C-PPy NPs), and (c) low- and high-(inset) resolution TEM images of the C-PPy NPs.
pyrrole-3-carboxylic acid monomers, was then vigorously stirred into the PVA/iron cation complex aqueous solution. The iron cations acted as both an oxidizing agent and a forming agent for the PVA/iron cation complex during the polymerization. We could easily tune the number of carboxyl functional groups of C-PPy NPs using carboxylic acid modified monomers. We used C-PPy NPs with pyrrole monomer-topyrrole-3-carboxylic acid monomer weight ratios of 45:1 (CPPy_1), 30:1 (C-PPy_2), and 15:1 (C-PPy_3). A field-effect scanning electron microscopy (FE-SEM) image of the C-PPy NPs is shown in Figure 1b. Nearly all of the C-PPy NPs were nanospheres with an average diameter of 60 nm. The transmission electron microscopy (TEM) images in Figure 1c and the inset also revealed that C-PPy NPs were highly monodispersed, spherically shaped, and uniform in size. Our method enables rapid subkilogram-scale fabrication of a high quality nanoparticle product. For laboratory scale production, quantities on the order of kilograms are considered very large (inset of Figure 1b). To confirm that our solutions contained the expected ratios of the 3-carboxyl functional groups, we made Fourier-transform infrared (FT-IR) measurements of the C-PPy NPs, as shown in Figure 2. The green curves indicate pristine polypyrrole (PPy) NPs, which have peaks indicating C−C stretching vibrations in the pyrrole ring at 1554 cm−1,
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RESULTS AND DISCUSSION Fabrication of the Materials. Two of the most desirable properties of sensor transducers for wireless sensor systems are a high surface-to-volume ratio and stability during the sensing process. We used a chemical oxidation procedure to synthesize C-PPy NPs that were highly uniform in distribution and size via copolymerization of pyrrole with pyrrole-3-carboxylic acid in aqueous solution.43 The C-PPy NPs can be tuned using functional groups. The carboxyl functional groups in the C-PPy NPs form covalent bonds on the aluminum pattern of the RFID antenna, which provide stability and flexibility. In the first B
DOI: 10.1021/acsami.6b08344 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figures 3b−d. The C 1s peak had four components. The positions that the components were centered at (284.6, 286, 287.5, and 288.85) indicate the presence of C−C, C−N, C− O−C, and O−CO or CO molecules, respectively. The strength of the peak associated with the O−CO or CO molecules in the carboxyl functional group increased as the ratio of carboxyl groups increased. The semiquantitative analysis was performed using internal standard (C−C in pyrrole rings). The ICO/IC−C ratio increased from 0.04 to 0.1, which demonstrates that it is possible to have control over the ratios of functional groups in C-PPy NPs. We present the additional data of C−C peaks about C-PPy NPs in Tables S3 and S4. Electrical Performance Measurements. Before testing the capabilities of the samples as wireless sensors, we made electrical performance measurements using an interdigitated array (IDA) substrate which was composed of a total of 40 2 μm alternating gold fingers. The spin-coating method was used to convert the C-PPy NPs into a close-packed crystalline array on the IDA substrate. Figure S1 describes an FE-SEM image of the C-PPy NP-based IDA sensor electrode, and we can see that it was coated with highly uniform, dense, close packed nanoparticles. To evaluate the electrical contact between the C-PPy NPs and the IDA substrates, current−voltage (I−V) curves were measured over voltages from −0.1 to 0.1 V, as shown in Figure 4a. The dV/dI values increased slightly at the electrodes, possibly due to the decreased conductivity of the C-PPy NPs. The nominal resistance values of the C-PPy NPs are 0.2564, 0.3125, and 0.4167 kΩ for C-PPy_1, C-PPy_2, and C-PPy_3, respectively. The carboxylic functional groups act as insulating materials. The linear relationships displayed in the I−V curves for each of the nanoparticle-coated substrates indicate excellent electrical contact (Ohmic contact), in contrast to the
Figure 2. Fourier-transform infrared (FT-IR) spectra of the solutions containing different ratios of carboxyl functional groups: C-PPy 1 (blue), C-PPy 2 (red), and C-PPy 3 (green).
conjugated C−N stretching at 1473 cm−1, and C−H planar vibrations at 1294 and 1195 cm−1. The same peaks were observed in all the samples, as they originated from the polypyrrole structure. However, the C-PPy NPs have distinctive peaks at 1700 and 3200 cm−1 from, respectively, the − CO and − OH bands of the carboxyl functional group.19,45 The intensities of these peaks slightly increased with the ratio of carboxyl functional groups in the C-PPy NPs using the internal standard at the C−C stretching band. To make up these results, we conducted an XPS analysis on C-PPy NPs. X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical bonds in the samples. Figure 3a displays that all of the samples had peaks at around 285, 399, and 532 eV, which are related to the C 1s, N 1s, and O 1s orbitals of the polypyrrole molecules. High-resolution XPS analysis of the C 1s peak confirmed the ratios of carboxylic functional groups present in the C-PPy samples, as shown in
Figure 3. X-ray photoelectron spectroscopy (XPS) analysis of (a) a fully scanned spectrum (0−1000 eV) and high-resolution spectra of the C 1s orbitals of the carboxyl functional groups: (b) C-PPy_1, (c) C-PPy_2, and (d) C-PPy_3. C
DOI: 10.1021/acsami.6b08344 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. (a) Current−voltage (I−V) curves for the coated interdigitated array (IDA) electrode with different ratios of the carboxyl functional groups. The normalized resistance changes are shown after (b) sequential exposure to acetic acid and (c) periodic exposure to 100 ppm acetic gas and with (d) a range of acetic acid concentrations. For panels a and d, the results for the different functional group ratios are C-PPy_1 (blue), CPPy_2 (red), and C-PPy_3 (green).
Figure 5. (a) Schematic diagram of the UHF-RFID sensor tag with carboxyl functional groups covalently bonded to the aluminum tag in the desired position. (b) Photograph of the proposed UHF-RFID-based gas sensor tag. (c) Field effect scanning electron microscopy image of the C-PPy_NP immobilized sensing area.
D
DOI: 10.1021/acsami.6b08344 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 6. Change in the reflectance properties of different C-PPy NP-based wireless sensors: (a) C-PPy_1, (b) C-PPy_2, and (c) C-PPy_3. (d) Relationship between the concentration of ammonia in the gas exposed to the sensor and the measured change in reflectance, where the reflection is calculated using ΔRe/Re0 = (Re − Re0)/Re0, where Re0 is the initial reflectance and Re is the reflectance after an exposure time of 2 min.
groups increased, the polymer/gas partition coefficient was enhanced, increasing the sensitivity of the sensors. The minimum detectable levels (MDL) of the C-PPy NPs were 0.1 and 1 ppm for NH3 and acetic acid, respectively. Figure 4c shows the resistance changes observed in the CPPy NPs after periodic exposure to 25 ppm of NH3 and 100 ppm of acetic acid at room temperature. The resistance varied in a similar way to the enhancement in sensing performance without retardation of the response and recovery times, as can be seen in Figure S2. This performance is superior to that of many other conducting polymer-based gas sensors at room temperature (Table S1). In Figure 4d, we plot the sensitivity (S) of the C-PPy NP sensors as a function of the NH3 and acetic acid concentrations. At concentrations less than 0.1 ppm, the sensitivity varied nonlinearly. In contrast, the sensitivity varied linearly over the concentrations of 1 to 100 ppm for NH3 and acetic acid. These results indicate that the C-PPy NP sensors can detect NH3 and acetic acid reversibly and reliably, even in very small concentrations. Additionally, Figure S3 shows the sensitivity of C-PPy NP sensors to various volatile organic compounds at 100 ppm. This result indicates that the sensitivity and selectivity were greatest for ammonia. Wireless Sensor Measurements. We fabricated wireless sensors using our C-PPy NP solutions and UHF-RFID tag antenna. The sensor systems were composed of an RFIDreader antenna, which was connected to a network analyzer, and an RFID tag antenna modified by sensing material.23 The C-PPy NPs were bonded to the passive UHF-RFID tags which contained a dipole antenna and an integrated circuit (IC) chip. Figure 5a shows the steps used to modify the UHF-RIFD tags to convert them into wireless gas sensors. In the first step, the UHF-RFID tag, apart from the section reserved for the sensing area, was covered with commercially available plastic tape. Then, we treated the tape-covered RFID tags with oxygen
nonlinearity observed in measurements of components which have poor electrical contact.13,19 The sensing performance of the C-PPy NP-based sensors was recorded in real time by measuring changes in the resistance to various gases. The conductivity of the C-PPy NPs was influenced by external stimuli such as oxidation and reduction reactions, swelling and doping, or dedoping of the sensor surfaces. Figure 4b exhibits the real-time responses of the C-PPy NPs on the IDA substrate to a range of concentrations of ammonia and acetic acid. To better understand the results, we calculated the normalized resistance change using ΔR /R 0 = (R − R 0)/R 0
(1)
where R and R0 are the real-time and initial resistances, respectively. The sensors rapidly responded to and recovered from exposure to gases. Interestingly, the electrical resistance of the C-PPy NP sensors were opposite for the two gases tested; the resistance gradually increased after exposure to NH3 and decreased after exposure to acetic acid. This results in the introduction of an electron-donating group (NH3) into the conducting polymer backbone as a p-type transducer, decreasing the charge carrier (electron−hole) concentration via a redox reaction and resulting in enhanced resistance. The carboxylic groups on the surface of the PPy act as additional active sites for ammonia interaction thorough hydrogen bonding. Therefore, different amount of carboxylic groups also affect the sensing performance during introduction of ammonia.46,47 On the other hand, the resistance was decreased when acetic acid was applied to the sensors because negatively charged counterions (CH3HOO−) were incorporated into the polymer to compensate for the positive charges in the polymer backbone.48 The emergence of polarons or bipolarons enhanced the conductivity. As the ratio of carboxylic functional E
DOI: 10.1021/acsami.6b08344 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 7. Photographs of the RFID tag sensor under different deformations: (a) flat, (b) twisting, and (c) rolling. (d) Normalized reflectance change under different deformations (flat, twisting, and rolling).
The sensor was exposed to concentrations of NH3 ranging from 0.1 to 25 ppm. The reflection of the radio wave decreased with the NH3 concentrations due to increasing resistance on the sensor surface. As mentioned above, an electron-donating group (NH3) into the pyrrole backbone (p-type transducer) decreased the charge carrier (electron−hole) concentration. The increasing resistance of the sensor tag leads to impedance mismatches between the IC chip and antenna without gas exposure, and then the RFID tag decreases the radar cross section, resulting in diminished reflection.49,42 On the other hand, the increasing reflection was displayed when the sensor tag was exposed to acetic acid. The resistance of the C-PPy NPs decreased because of interactions with negatively charged counterions (CH3HOO−) (Figure S4). In addition, similar to the IDA-based sensing system, the sensitivity of the RFID sensor increased with an increasing ratio of carboxylic functional group concentrations on the C-PPy NPs (0.1 ppm for C-PPy_3, 1 ppm for C-PPy_2, and 5 ppm for C-PPy_1). This result is higher value compared to those of many other RFID-based ammonia sensors (Table S1). Additionally, we performed the control experiment using the UHF-RFID tag without a sensing area (Figure S7). As shown in Figure 6d, the percentage of signal reflected back (normalized reflection change) by the sensor increased with the ratio of carboxyl functional groups with 14.2, 7.35, and 5.8% reflected back for C-PPy_3, C-PPy_2, and C-PPy_1, respectively, in the case of a concentration of 25 ppm. The normalized reflection change is expressed as (Re − Re0)/Re0, where Re is the reflection after 2 min of acetic acid exposure and Re0 is the initial reflectance. These results indicate that the sensitivity of the wireless sensors increased with the ratio of carboxylic functional groups on the surface. These responses are similar to those observed in the case of the IDA substrate
plasma and 3-aminopropyltrimethoxysilane (APS) to create a specific functionalized region on the UHF-RFID tag. The CPPy NPs were then bound to the functionalized region of the antenna pattern through covalent bonding of the amino groups in the antenna pattern to the carboxylic groups in the C-PPy NPs. Symbolically, the procedure is described as: condensation: C − PPy NPs − COOH + H 2N(CH 2)3 Si (O)3 − RFID pattern
→C − PPy NPs − CONH(CH 2)3 Si(O)3 − RFID pattern (2)
The covalent anchoring has the advantages of improving the stability of the sensor and enabling the formation of efficient electrical pathways during wireless sensing. To initiate the wireless sensing, the RFID reader antenna, which was connected to the network analyzer, emitted an interrogation signal (Figure S8). The emitted electromagnetic field activated the UHF-RFID wireless sensor. Then, the signal was reflected back to the RFID reader antenna, a process known as backscattering.23,33,42 The response was monitored in real time by the network analyzer. When the UHF-RFID wireless sensor was exposed to the gases, the resistance changes in the chemiresponsive materials caused impedance mismatches between the dipole tag antenna and the IC chip. As a result, the network analyzer detected changes in the backscattering signal. Figures 6a−c displays the change in the level of backscatter power from the tag of the different RFID sensor tags (CPPy_1, C-PPy_2, and C-PPy_3, respectively), which was used to determine the responses of the wireless sensors to the gas concentration at a distance of 50 cm. The reflection change is expressed as power of the radio waves reflected (Re, in dB) versus the frequency, as measured by the network analyzer. F
DOI: 10.1021/acsami.6b08344 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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in length, and 50 nm in thickness on a 50 nm Cr adhesion layer) was patterned on a glass substrate through a photolithographic process. The spin-coating method at a rate of 1000 rpm was used for a duration of 60 s to obtain a uniformly coated electrode array. To reduce the resistance between the IDA electrode and the C-PPy NPs, the coated electrode array was stored in an inert atmosphere for 24 h at room temperature. To measure the influence of NH3 gas exposure on the electrical properties of the C-PPy NP-coated IDA electrodes, they were placed in a vacuum chamber which was custom designed for gas sensing with a vapor inlet/outlet pressure of 100 Torr. Various gas concentrations of NH3 (0.1−100 ppm) were controlled by a mass flow controller (KNH Instruments, Korea) system. The real-time resistance monitoring was conducted with a constant applied current of 10−6 A (defined as ΔR/R0 = (R − R0)/R0, where R and R0 are the real-time resistance and the initial resistance, respectively). After the sensor electrode had been exposed to gases for several minutes, compressed inert gas was introduced to the vacuum chamber to remove any molecules that had become attached to the C-PPy NPs. This step refreshed the electrode, enabling its reuse and hence repeated measurements of the sensor performance. Fabrication of the C-PPy NP-Based UHF-RFID Smart Sensor. To attach the C-PPy NPs firmly along the line of the UHF-RFID tag, the RFID tag was treated with APS. The passive UHF-RFID tags were composed of an antenna pattern and microcontroller integrated circuit (IC) chip (EPC global Class-1 Generation-2 (GEN2) protocol) on the plastic substrate ($2 US dollars by the piece) (EMPO Corp., Korea). The passive UHF-RFID reader antenna is MT-242025 (ThingMagic Corp., United States). The frequency range of the reader antenna was 865−956 MHz. The passive UHF-RFID tag (nominal frequency: ∼904 MHz) was wrapped with commercially available plastic tape everywhere apart from at the desired position of the C-PPy NP coating. To form the oxygen functional groups, O2 plasma treatment was used. The tape-wrapped UHF-RFID tag was soaked in 5 wt % APS aqueous solution, and the solution was rotated for 6 h at a constant slow rate. After the stirring treatment, the tag was removed from the solution and dried at room temperature. A glass bath was glued onto the section of the RFID tag that had been treated with APS, and then the C-PPy NPs and DMT-MM aqueous solutions were introduced to the glass bath simultaneously. The sensor was left to dry in standard atmospheric conditions for 24 h. Characterization. FE-SEM and high resolution transmission electron microscopy images were obtained using a JEOL 6700 and a JEOL JEM-200CX, respectively (JEOL Ltd., Japan). TEM images were acquired by a JEM-2100 (JEOL Ltd., Japan) installed at the National Center for Interuniversity Research Facilities at Seoul National University. XPS spectra were recorded using an M16XHF-SRA (Mac Science Co., Japan). The four-probe method was used to measure the electrical conductivity at ambient temperature with a source meter using SMU 2400 (Keithly Instruments Inc., United States).
sensor electrode. Additionally, the sensing performance of the C-PPy-based UHF-RFID wireless sensor was significantly lower than that of other materials-based RFID sensor (Table S2). The flexibility of the sensor substrate enabled consistent sensing performance as the sensor was deformed in a number of ways, such as by bending, rolling, or twisting. Figure 7d displays the change in the resonance reflection, which has a similar wave of modulation with deformation such as rolling and twisting. We tested the performance of the sensor as it was bent through a range of folding angles, which subjected the substrate to compressive forces, as shown in Figure S5. Due to the covalent bonds between the C-PPy NPs and the RFID tag, the bending did not have a significant effect on the sensor tag. This indicates that our polymer nanoparticle-based sensors have promising potential for applications as wearable and implantable wireless sensor systems.
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CONCLUSION We fabricated wireless sensors using carboxyl functional groups controlled by C-PPy NPs and passive UHF-RFID tags. To our knowledge, this is the first demonstration of the manufacture of C-PPy NPs for wireless sensor transducers at room temperature. The C-PPy NPs were synthesized from a highly uniform and monodispersed aqueous solution using a chemical oxidation polymerization technique. The UHF-RFID-based wireless sensor system was fabricated by covalently bonding the C-PPy NPs to the UHF-RFID tag. The stability of the covalent bonds gave rise to advantages during ammonia exposure and deformation of the sensor. The sensors investigated in this paper were highly sensitive, detecting ammonia in concentrations as low as 0.1 ppm, operated at wireless system (50 cm) without a battery, and were flexible, maintaining their sensing performance as they were bent through a range of angles. Thus, this study demonstrated an effective way to fabricate UHFRFID-based wireless sensors using conducting polymer NPs.
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MATERIALS AND METHODS
Materials. PVA (MW 9000), dodecyltrimethylammonium bromide (DTAB), ferric chloride (FeCl3) (97%), pyrrole (98%), and (3aminopropyl)triethoxysilane (APS) were purchased from SigmaAldrich Co. (St. Louis, United States) and used without purification. Pyrrole-3-carboxylic acid hydrate (95%) and 4-(4,6-dimethoxy-1,3,5triazin-2-yl)-4-methyl morpholinium chloride (DMT-MM) were purchased from Acros Organics (United States) and Fluka (Buchs, Switzerland), respectively. Preparation of Carboxylated Pyrrole Nanoparticles. A microemulsion method was used to obtain the C-PPy NPs. To synthesize the C-PPy NPs, PVA was dissolved in distilled water to make 1 wt % PVA aqueous solution. This solution was stirred for 12 h at 60 °C with a uniform stirring rate. DTAB (0.2 g) and a 7 M aqueous FeCl3 solution (0.5 mL) were injected into the PVA solution, which was then stirred for 1 h at a rate of 1000 rpm. Subsequently, a pipet was used to add the mixture of pyrrole and pyrrole-3-carboxylic acid hydrate monomers for different molar ratios (6:0.4 mM for C-PPy_3, 6:0.2 mM for C-PPy_2, and 45:0.1 mM for C-PPy_1) with stirring at 1000 rpm for 2 h. This solution was centrifuged to remove reagents without removing resultants and then diluted with distilled water and stirred for 4 h at a rate of 400 rpm at a temperature of 60 °C. After this process had been repeated four times, the solid C-PPy NPs were placed in a 60 °C oven to dry. Electrical Measurement of the C-PPy NPs on the IDA Substrate. To measure the electrical properties of the polymer coating, the aqueous C-PPy NP solution was sonicated and dropcasted onto an IDA electrode. The microarray consisting of a gold IDA electrodes with 40 fingers (2 μm in width, 2 μm in spacing, 4000 μm
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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/acsami.6b08344. (1) Schematic diagram of IDA electrode substrate, (2) response and recovery times of electrodes after exposure to VOCs, (3) sensing performance histogram of CPPy_3, (4) measured change in reflectance of wireless sensor, (5) resistance change with different bending angles, (6) comparison of sensor performance with other conducting polymer-based nanomaterials, (7) comparison of RFID tag-based wireless sensor performance with other materials (PDF) G
DOI: 10.1021/acsami.6b08344 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Author
*Tel: 82-2-880-8348; Fax: +82-2-888-7295; E-mail: jsjang@ plaza.snu.ac.kr. Author Contributions ‡
J. Jun and J. Oh contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Development Fund of Seoul National University funded by Dongjin Semichem Co., Korea (Grant 0458-20130066).
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