A Fully Integrated Wireless Flexible Ammonia Sensor Fabricated by

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A Fully Integrated Wireless Flexible Ammonia Sensor Fabricated by Soft Nano-Lithography Ning Tang, Cheng Zhou, Lihuai Xu, Yang Jiang, Hemi Qu, and Xuexin Duan ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b01690 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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A Fully Integrated Wireless Flexible Ammonia Sensor Fabricated by Soft Nano-Lithography Ning Tang, †,# Cheng Zhou, †,# Lihuai Xu, † Yang Jiang, † Hemi Qu, †,‡ and Xuexin Duan*,†,‡ †State

Key Laboratory of Precision Measuring Technology & Instruments, Tianjin University, Tianjin 300072, China

‡Nanchang

Institute for Microtechnology of Tianjin University, Tianjin 300072, China

KEYWORDS: flexible electronics, portable system, nanowire, ammonia, soft lithography ABSTRACT: Flexible ammonia (NH3) sensors based on one-dimensional nanostructures have attracted great attention due to their high flexibility and low-power consumption. However, it is still challenging to reliably and cost-effectively fabricate ordered nanostructure-based flexible sensors. Herein, a smartphone-enabled fully integrated system based on a flexible nanowire sensor was developed for real-time NH3 monitoring. Highly aligned, sub-100 nm nanowires on a flexible substrate fabricated by facile and low-cost soft lithography were used as sensitive elements to produce impedance response. The detection signals were sent to a smartphone and displayed on the screen in real time. This nanowire-based sensor exhibited robust flexibility and mechanical durability. Moreover, the integrated NH3 sensing system presented enhanced performance with a detection limit of 100 ppb, as well as high selectivity and reproducibility. The power consumption of the flexible nanowire sensor was as low as 3 μW. By using this system, measurements were carried out to obtain reliable information about the spoilage of foods. This smartphone-enabled integrated system based on a flexible nanowire sensor provided a portable and efficient way to detect NH3 in daily life.

Ammonia (NH3), a colorless and water-soluble gas, is a responsive, low-power-consumption sensors with low major air contaminant emitted from industry, agricultural usage of conductive materials.13 Furthermore, 1D 1,2 practices, and motor vehicles. The toxic gas not only acts nanostructures have better mechanical properties than as an indicator of environmental conditions but is also the corresponding bulk materials.14 By placing a flexible NH3 most common gas found in the decomposition of sensor with a low detection limit and high selectivity protein-rich foods.3,4 Thus, efficient monitoring of NH3 in directly on the surface of foods, freshness can be detected daily life is a promising approach to provide guaranteed at the onset of decay, in contrast to the visual appearance quality of life. Various traditional techniques for NH3 and classical olfactory recognition methods.15 Despite the detection have been proposed, including many advantages of nanoscale flexible gas sensors, spectrophotometry, chromatography, electrochemistry, and fabricating nanostructures on flexible substrates is acoustics.5–7 Though powerful and efficient, these methods challenging because most flexible substrates and materials suffer from the shortcomings of high equipment costs, are not compatible with conventional nanolithography time-consuming processes and poor portability and are techniques, such as focused ion beam and dip-pen suitable for laboratory use rather than inexpensive daily lithography.16–18 Various printing techniques, such as inkjet applications. Therefore, portable and well-performing printing and transfer printing, have been developed to sensors are greatly needed to monitor NH3 in daily life and fabricate nanostructures for gas sensing.19–21 However, the evaluate food quality in a timely manner. printing resolution is limited to the micrometer scale, and Different types of flexible sensors based on nanowires, the choice of ink material is limited due to the viscosity barrier.22 In addition, the transfer process may involve a such as highly sensitive chemical sensors,8 flexible strain 9 10 devices, and highly efficient energy generators, have chemical etching process requiring hazardous etchants or thermal processes that may cause damage to the supporting been developed since such sensors not only offer electrical functions but also have the ability to be compressed, bent, substrates and fabricated structures.23–25 Based on these 11,12 facts, the development of efficient approaches for the and conformed onto arbitrary shapes. In addition, the facile, low-cost, and damage-free fabrication of sub-100 intrinsically high surface-to-volume ratio of nanostructures enables the construction of highly sensitive, rapidly nm nanostructures on flexible substrates is meaningful for ACS Paragon Plus Environment

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developing NH3 sensors with excellent sensing performance. As a collection of techniques based on printing and molding, soft lithography is a good potential method to achieve the simple and nondestructive fabrication of nanostructures.26–30 Such an approach would simplify the preparation process and improve the properties of gas sensors, leading to promising progress in NH3 monitoring. Here, a smartphone-enabled fully integrated wireless system based on a flexible nanowire sensor was developed for real-time NH3 monitoring. Highly aligned conducting polymer nanowires were fabricated by a capillary filling-based soft lithography technique on a polyethylene terephthalate (PET) substrate. The prepared nanowires were then integrated into a functional flexible device. The mechanical properties of the nanoflexible device were tested by multiple bending experiments and simulations. The nanowire NH3 sensor showed a low limit of detection (LOD), fast response, high selectivity, and low power consumption. Furthermore, a watch-type device based on a flexible printed circuit board (FPCB) was developed to detect the sensor impedance and deliver the data to a smartphone through Bluetooth. With the home-built analysis program, the real-time response of the nanowire sensor to NH3 could be displayed on the phone screen. The proposed smartphone-integrated system based on a flexible nanowire sensor has demonstrated its capability for the portable detection of trace NH3 in food spoilage monitoring. EXPERIMENTAL SECTION Materials and Apparatus. Imprint resist mr-I T85 was purchased from Micro-Resist. Conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was obtained from Sigma-Aldrich as an aqueous suspension (~1.3 wt%). Zinc oxide (≤ 50 nm, 40 wt% in H2O) and indium nitrate hydrate (99.999% metal basis) were purchased from Aladdin. The indium nitrate hydrate was dissolved in water at a concentration of 0.8 mg/ml. Graphene oxide was obtained from Chengdu Organic Chemical Company and sonicated for 30 min to form a uniform and stable dispersion with a concentration of 0.5 mg/ml. Polydimethylsiloxane (PDMS) was prepared from Sylgard 184 silicone elastomer. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) imaging were carried out with an FEI FP 2031/12 Inspect F50 and Dimension Icon (Bruker), respectively. Electrical measurements with a constant voltage of +1 V were performed with an Agilent Technologies B1500A. Fabrication of Large-area Nanowires. PDMS and mr-I T85 were used for the fabrication of the PDMS mold. The prepolymer base and curing agent were mixed in a 10:1 (w/w) ratio, followed by degassing to remove air bubbles and curing at 80℃ for 2 hours. Then, the mr-I T85 solution was spin-coated onto the cured PDMS, followed by baking at 140℃ for 2 min in a hot oven. The composite PDMS substrate coated with mr-I T85 and a silicon template (width: 70 nm; height: 80 nm) were inserted into a stamping machine (769YP-30T), the temperature was raised to 135℃, and 2 bar of pressure was applied to the

system for 40 s. Upon cooling to 65℃, the template was separated from the PDMS substrate. Thus, a PDMS mold containing well-defined parallel nanogrooves was prepared. Subsequently, the PDMS mold was placed in contact with an O2 plasma-treated (20 mTorr; 20 sccm; 30 W; 30 s) PET substrate, and a few drops of water were applied immediately for good sealing. After water evaporation, the material solution (PEDOT:PSS; zinc oxide; indium nitrate hydrate; graphene oxide) was dropped into the open end of the PDMS mold. After the solvent completely evaporated, the PDMS mold was gently removed from the PET. Interdigital Electrode Fabrication. Au interdigital electrodes with ten fingers (N = 9) were evaporated on the PET through a designed copper shadow mask in a top-contact configuration with a pad length (L) of 8 mm and width (W) of 300 μm. The spacing between adjacent fingers (D) was kept at approximately 500 μm. Resistance Detecting Based on a Smartphone. The smartphone-enabled sensing system was used as a portable device to record electrical responses. A schematic circuit diagram of the printed circuit board is shown in the Supporting Information. Android application programs were developed to receive real-time signals and plot the results on the screen. The ‘Bluetooth’ button was used to search and link the FPCB with the smartphone. Then, the microcontroller received commands to communicate with the resistance analyzer and simultaneously sent out feedback signals from the flexible sensor to AD5933. At the same time, a coordinate graph on the smartphone screen displayed the response change in real time. A resistance threshold in the food freshness detection program was set. Once the measurement results exceeded the threshold, the conditions were considered to indicate food spoilage, and a warning message was displayed on the phone screen. RESULTS AND DISCUSSION Flexible Nanowire Fabrication and Characterization. The direct fabrication of highly aligned nanowires on a flexible substrate and device integration on a flexible substrate is illustrated in Figure 1a. First, a soft nanomold containing well-defined parallel nanogrooves was prepared by thermal nanoimprint lithography (NIL) on a flat PDMS substrate (Figure S1 in the Supporting Information). After soft bonding to a plasma-treated PET substrate, the nanogrooves turned into parallel nanochannels. Then, a few drops of aqueous solution containing functional materials (e.g., PEDOT:PSS) were applied at the edges of the nanochannels. The aqueous solution easily filled the whole channels within a few seconds due to the large Laplace pressure (the detailed calculations are summarized in Figure S2 in the Supporting Information). Moreover, the improved surface affinity from O2 plasma treatment enhanced the adhesion between the deposited materials and PET substrate. Once the solvent had completely evaporated, the PDMS mold was gently removed from the PET, leaving the nanowires on the flexible substrate (Figure 1b). The interdigitated electrodes were then deposited by vacuum evaporation through a designed shadow mask; thus, a flexible chemiresistor consisting of highly aligned nanowires was successfully fabricated (Figure 1c).

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Figure 1. (a) Schematic illustration of the preparation process of flexible nanowire sensors; (b) Optical microscope image of the highly aligned PEDOT:PSS nanowires; (c) Picture of the integrated flexible device.

The fully aligned, ultralong PEDOT:PSS nanowires were extended to a relatively large area (>10 mm  10 mm) without obvious defects, as shown in Figure S3. The intervals between the nanowires were 5 μm and 3 μm. Thus, the number of nanowires in a distance of 8 mm (the pad length of the interdigital electrodes) was approximately 500 (n = 500). As measured from SEM and AFM height images (Figure 2a and 2b), the nanowires have an average width (w) of 70 ± 5 nm and height (h) of 60 ± 5 nm. Therefore, the resistance for n = 500 parallel nanowires is (1/N)(1/n)(1/σ)(l/wh) ≈ 0.26 MΩ, where N is the interval corresponding to the number of interdigitated electrodes, σ (1 S/cm) is the conductivity of the PEDOT:PSS solution, and l is the spacing between adjacent fingers (for calculation details, see the Supporting Information, Figure S3). Furthermore, the measured resistance of the flexible nanowire sensor, as shown in Figure 2c, is approximately 0.36 MΩ, which is quite close to the calculated value, demonstrating the good quality of the nanowires and reliability of this fabrication technique. With the use of only a printing or capillary molding procedure, most fabrication processes can be continued without cleanroom facilities once a mold is available.30 In fact, the yield of twenty flexible nanowire sensors prepared outside a cleanroom reached 95%. Thus, nanoscale soft lithography provides a facile and low-cost method to pattern nanostructures. The conformal contact between the PDMS mold and the target substrate is the key step to ensure high-quality nanopatterns. Due to the mild fabrication conditions and the lack of additional etching or transfer steps, this approach is highly suitable for the direct fabrication of large-area ordered nanowires on any substrate. The formation of aligned nanowires is based on capillary filling; thus, this method is suitable to pattern many different materials as well, providing a versatile nanofabrication approach with high material and substrate compatibility. To verify the material-independent capability, we fabricated nanowires on a PET substrate using three different types of materials: (1) a semiconductor (zinc oxide, ZnO), (2) a carbon material (graphene oxide, GO), and (3) an inorganic crystal (indium nitrate, In(NO3)3). Figure 2d-f confirms the high quality of the fabricated nanowires from these three materials. It is also worth noting that all these nanowires were fabricated from the same PDMS mold, indicating the good reusability of the mold. Furthermore, since most processes can be

Figure 2. Top-view SEM image (a) and side-view 3D AFM image (b) of the PEDOT:PSS nanowire; (c) I-V conductivity measurement of the flexible PEDOT:PSS nanowire sensor. SEM images of highly aligned nanowires (bright parts) based on different functional materials: (d) zinc oxide; (e) graphene oxide; and (f) indium nitrate; The insets are magnified SEM images of the nanowires.

Bending and Fatigue Properties. The mechanical properties of the flexible nanowire sensor were evaluated by monitoring its resistance change under an applied voltage (+1 V) when bending the sensor by a motor-controlled stepper. The stepper propelled one end of the flexible device back and forth with the other end clamped by two block clips for cycling between bending and unbending states. The test system is presented in Figure S4 in the Supporting Information. For the outer bending test, the bending radius was calculated using the following equation: 𝐿 𝐵𝑒𝑛𝑑𝑖𝑛𝑔 𝑅𝑎𝑑𝑖𝑢𝑠 = (1) 2𝜋 (𝑑𝐿/𝐿) ― (𝜋2ℎ2𝑠 /12𝐿2) where L, dL/L, and hs denote the initial length, the applied strain, and the PET thickness, respectively.31 Figure 3a shows the outer bending results of the flexible chemiresistor under different bending states (see also Figure S5 in the Supporting Information, which shows all the I-V conductivity measurements). At the initial stage, the resistance of the sensor exhibited almost no change until it was bent to a radius of 3 mm. The increase in resistance is most likely due to the cracking of the nanowires. However, the value of R/R0 was very small (~ 0.09), even when bending occurred below 2 mm. The outer bending fatigue test with a bending radius fixed at 5 mm is displayed in Figure 3b. There was no change in resistance throughout 1200 bending cycles, demonstrating the superior durability and mechanical stability of the flexible nanowire sensor. The results also verify that the fabricated nanowires have excellent adhesion to the flexible PET substrate and are not affected by multiple bending. The robustness of this nanodevice is mainly due to the 1D configuration and neat alignment of the nanowires, which ensure a direct path of electron transport and reduce the cross defect density, thereby reducing the dependence of the resistance change on the strain.14 To shed further light on the mechanical robustness of the flexible nanowire sensor, the induced strain at the time of bending was calculated by using the finite element method simulation

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(Comsol Multiphysics 5.3), as shown in Figure 3c. From the simulation, the maximum strains of the whole substrate are ~ 4% and - 4% when the curvature radius is 5 mm. Although the nanowires are located close to the top of the substrate and away from the neutral bending axis, the strain on the nanowires is relatively low due to the relatively small size of the nanowires (sub-100 nm in height), which results in most of the strain being accommodated by the PET substrate.32 Moreover, the strain in active regions is relatively low, which explains the mechanical robustness of the device even under a large bending state. Another important factor to maintain device flexibility and robustness is the large-scale pattern of nanowires and interdigital design of the electrodes. Due to these factors, even if the nanowires between some of the electrode stripes are cracked, the performance of the whole device will not be greatly affected.

Figure 3. (a) The response of the flexible nanowire device at different outer bending radiuses; (b) Outer bending fatigue reliability at an outer bending radius of 5 mm; (c) Theoretical simulation of the strain for a nanowire sensor bent to a 5 mm curvature radius.

NH3 Sensing Performance of the Flexible Nanowire Sensor. The real-time sensing results of the flexible nanowire sensor for different concentrations of NH3 are shown in Figure 4a. Clearly, when contacting NH3 gas, the resistance of the sensor increased immediately, followed by a region of saturation. The sensing mechanism of nanowires can be explained by the p-type nature of PEDOT:PSS; thus, the holes in the valence band of conducting polymer can be depleted by an electron-donating gas (e.g., NH3), resulting in a significant increase in resistance.33 The response of the nanowire NH3 sensor was calculated to conduct a quantitative study. The response is defined as: 𝑅𝑒𝑠𝑝𝑜𝑛𝑠𝑒 = (𝑅𝑡 ― 𝑅0)/𝑅0 (2) where R0 is the initially measured steady resistance and Rt is the measured value in the presence of NH3. As shown in the inset in Figure 4a, the sensor response is linearly dependent on the NH3 concentration in a range of 0.75-6 ppm, greatly simplifying use in practical terms. Through the fitting result, we obtained a response slope of 0.2524 ppm-1 with a fitting quality r2 = 0.9834. The theoretical detection limit (for a signal-to-noise ratio of 3) for NH3 is approximately 100 ppb (for calculation details, see the Supporting Information). Here, t90,res and t10,rec were used to

characterize the response and recovery time, in which t90,res is defined as the time to reach 90% of the maximum response, and t10,rec is defined as the time interval over which the sensor response drops to 10% of the stabilized response in NH3 gas. For the analysis of 3 ppm NH3, t90,res and t10,rec were calculated as 70 s and 276 s, respectively, for this flexible chemiresistor (the detailed data are summarized in Figure S6 in the Supporting Information). As a comparison, a thin-film PEDOT:PSS sensor was prepared to detect NH3 at the same concentration. The proposed nanowire sensor presents several distinct advantages, including fast response and enhanced sensitivity, as shown in Figure S7. The low detection limit and rapid response of this sensor are attributed to the high surface-to-volume ratio of the PEDOT:PSS nanowires, which ensures excellent accessibility to NH3 gas molecules. The performance of the nanowire sensor and a comparison with other NH3 sensors are summarized in Table S1 (Supporting Information). In addition to a much simpler fabrication process and sensor structure, the sensor in this work also presents better performance. Reproducibility and selectivity are two key factors for practical sensing applications. Figure 4b displays the dynamic responses of the flexible nanowire sensor to the same NH3 concentration. Stable performance indicates good reproducibility for a single device. For sample-to-sample reproducibility, statistics were performed for flexible nanowire sensors from ten different devices. The distributions of the responsivity at a moderate concentration (0.75 ppm) and sensitivity (defined as the slope of the linear relationship of responsivity versus NH3 concentration) are illustrated in Figure 4c. Approximately 90% of the samples exhibited a responsivity of more than 0.8 at 0.75 ppm NH3, and approximately 60% of the nanowire gas sensors showed a sensitivity over 0.2 ppm-1. The high repeatability of the sensing performance from different devices is mainly due to the reusability of the PDMS mold, thus ensuring a high degree of consistency of the nanowire structures. To check the sensing selectivity, different interference volatile organic compounds (VOCs) were measured at room temperature as well (Figure S8 in Supporting Information). Based on the physical properties of these VOCs, summarized in Table S2, the corresponding concentrations of VOCs at different flow rates were calculated (for calculation details, see the Supporting Information). The flexible chemiresistor sensor presents a preferential response to NH3 and a negligible response to other VOCs (Figure 4d). Such high selectivity is due to the sensing mechanism of the PEDOT:PSS nanowire sensor to VOCs, which is caused by weak physical interactions involving absorbing and swelling.34,35 At ambient temperature, the conducting polymer is in its glassy state, which results in a low absorption and swelling level; thus, the contribution of these two interactions to the overall resistance change is minor.36 However, as mentioned above, the interaction between NH3 and PEDOT:PSS is considered to be electron transport, which is dominant at ambient temperature. The high selectivity of nanoscale PEDOT:PSS to NH3 will minimize the sensor noise in practical sensing environments where VOCs are usually

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ACS Sensors present. The sensing performance of the flexible nanowire sensor was also tested with respect to bending status. The dynamic responses of the sensor at different bending radiuses are shown in Figure 4e and Figure S9. The response values and excellent reversibility of the sensor are fully retained at these bending angles. Figure 4f presents the real-time results of a fatigue test after several bending and relaxing processes. No significant drop in response values was observed, indicating that the flexible aligned nanowire sensor in our work exhibits excellent mechanical durability and robustness while maintaining good sensing performance.

the schematic diagrams in Figure 5 and Figure S10, in the presence of NH3, the impedance signals generated by the portable sensing device were sent wirelessly to the smartphone through the Bluetooth module, and the sensing data were real-time plotted on the screen (see also Supplemental Video 1). This fully integrated sensing system based on flexible PEDOT:PSS nanowires demonstrates the accurate determination of NH3 in a real applications.

Figure 5. Basic diagram of the smartphone-enabled integrated wireless system, including a watch-type sensing device (green dashed box) and a smartphone part (red dashed box). The watch-type sensing device consists of a (1) LT1763 chip, (2) STM 32 microcontroller, (3) AD5933 chip, and (4) Bluetooth module connector. The red dashed box (5) indicates the location of the flexible nanowire sensor. The smartphone part illustrates the real-time sensing window of APP for NH3 monitoring.

Figure 4. (a) Real-time response of the flexible nanowire gas sensor to NH3; The inset figure shows a linear correlation of the response values as a function of NH3 concentration; (b) Repeatability tests of a single nanowire gas sensor towards different concentrations of NH3; (c) Distributions of the responsivity at 0.75 ppm NH3 and the sensitivity of ten different sensors; (d) Responsivity and sensitivity to different gases (NH3 (0.75 ppm), ethanol (15 ppm), IPA (11.2 ppm), p-xylene (2.3 ppm), n-heptane (11.9 ppm), n-hexane (39.8 ppm), and acetone (60 ppm)); Response curves of the flexible nanowire gas sensor to 1.5 ppm NH3 when tested (e) under different bending radiuses, and (f) before and after bending 500 and 1000 times (bending radius = 15 mm).

NH3 Sensing Based on Smartphone-Enabled System. The flexible nanowire sensor was integrated with other electronic components on a FPCB (also known as “hard-soft” integration) to compose a portable electronic system that bridges the technology gap between signal generation by flexible electronics and data transmission using readily integrated circuit components.37–40 The smartphone-enabled sensing system was used as a portable device to monitor the real-time response to NH3. An Android application program (detailed information can be found in the Supporting Information) was developed and installed on a smartphone to receive and process the sensing signals from the flexible nanowires. As shown in

Slight spoilage of protein-rich foods is not easy to notice; however, such spoilage is indeed harmful to human health. NH3 is produced naturally from decomposition processes of organic matter, and the concentration of this corrosive gas could be regarded as a biomarker for the freshness of foods.41,42 The integrated NH3 system was applied to monitor the spoilage of salmon stored in a refrigerator in ambient air by packing the wireless system together with the salmon. Compared with the salmon stored in the refrigerator, the response of the sample placed in ambient air for 12 hours changed significantly, while a slight color change only started to occur after 36 hours (Figure 6a). The smartphone receives electrical signals from the smart packaging and displays an alarm when the food is spoiled (Figure 6b; see also Supplemental Video 2). The power consumption of the flexible nanowire sensor in this sensing system is calculated to be less than 3 μW, which is much lower than that of most commercially available NH3 sensors (e.g., sensors from Figaro Engineering Inc.).43 The low cost, flexibility, low power consumption, and high sensitivity of the nanowire sensor benefit its usage to guarantee food quality. Collectively, these results demonstrate that this smartphone-enabled NH3 detection system, integrating a miniaturized nanowire sensor and portable circuits, has good sensing performance and can thus potentially meet the request for portable and convenient monitoring of food quality in daily storage and supply chains.

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# These authors contributed equally. Notes The authors declare no competing financial interest.

Figure 6. (a) Test results for two groups of salmon samples, which were stored in the refrigerator and the ambient air, respectively; (b) Food spoilage detection using the smartphone-enabled packaging system.

CONCLUSIONS In summary, a sensitive and low-power-consumption integrated wireless portable system based on a flexible nanowire sensor was designed and fabricated to detect NH3. The nanowire sensor was fabricated by a capillary filling-based soft lithography technique. Owing to the facile and mild fabrication conditions, this method provides a versatile nanofabrication approach with high material and substrate compatibility. Using this technique, we demonstrated dense and ordered PEDOT:PSS, ZnO, GO, and In(NO3)3 nanowires on a PET substrate. The experimental results suggest that the flexible device exhibits excellent mechanical bendability and durability without an obvious decrease in performance even after 1200 bending cycles. Moreover, the sensor shows good sensitivity and selectivity to NH3, and the power consumption is as low as 3 μW. The smartphone-enabled portable integrated sensing system shows good performance in NH3 detection and protein-rich food spoilage monitoring. Considering the low cost, easy implementation, and low power consumption of this device, it is believed that the smartphone-enabled NH3 detection system based on our flexible nanowire sensor will help pave the way for NH3 monitoring in daily life and intelligent food detection in storage and supply chains. ASSOCIATED CONTENT Supporting Information. Composite PDMS stamp, PEDOT:PSS nanowire fabrication, SEM result of large-area nanowires, Outer bending test system, I-V Conductivity Measurement, Limit of detection (LOD) calculation, Response and recovery time, Sensing performance of thin-film based sensors, Sensing performance of conducting polymer based sensors, Dynamic response to VOCs, Dynamic response to VOCs under different bending radiuses, Schematic circuit diagram, Source code download, Real-time detection using smartphone-enable watch-type sensing system (video 1), spoilage detection (video 2). This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * Tel. /Fax: +86 2227401002. E-mail: [email protected]. ORCID Xuexin Duan: 0000-0002-7550-3951. Author Contributions

ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (NSFC No. 61674114, 91743110, 21861132001), National Key R&D Program of China (2017YFF0204600), Tianjin Applied Basic Research and Advanced Technology (17JCJQJC43600), the Foundation for Talent Scientists of Nanchang Institute for Microtechnology of Tianjin University, and the 111 Project (B07014). REFERENCES (1) Li, X.; Li, X.; Li, Z.; Wang, J.; Zhang, J. WS2 Nanoflakes Based Selective Ammonia Sensors at Room Temperature. Sensors Actuators, B Chem. 2017, 240, 273– 277. (2) Wang, Y.; Liu, J.; Cui, X.; Gao, Y.; Ma, J.; Sun, Y.; Sun, P.; Liu, F.; Liang, X.; Zhang, T.; et al. NH3 Gas Sensing Performance Enhanced by Pt-Loaded on Mesoporous WO3. Sensors Actuators, B Chem. 2017, 238, 473–481. (3) Chung, W. Y.; Le, G. T.; Tran, T. V.; Nguyen, N. H. Novel Proximal Fish Freshness Monitoring Using Batteryless Smart Sensor Tag. Sensors Actuators, B Chem. 2017, 248, 910–916. (4) Guo, Y.; Wang, T.; Chen, F.; Sun, X.; Li, X.; Yu, Z.; Wan, P.; Chen, X. Hierarchical Graphene– polyaniline Nanocomposite Films for High-Performance Flexible Electronic Gas Sensors. Nanoscale 2016, 8, 1–5. (5) Dong, X.; Cheng, X.; Zhang, X.; Sui, L.; Xu, Y.; Gao, S.; Zhao, H. A Novel Coral-Shaped Dy2O3 Gas Sensor for High Sensitivity NH 3 Detection at Room Temperature. Nanoscale 2018, 255, 1308–1315. (6) Meng, W.; Dai, L.; Zhu, J.; Li, Y.; Meng, W.; Zhou, H.; Wang, L. A Novel Mixed Potential NH3 Sensor Based on TiO2 @WO3 Core–shell Composite Sensing Electrode. Electrochim. Acta 2016, 193, 302–310. (7) Tang, Y.; Ao, D.; Li, W.; Zu, X.; Li, S.; Fu, Y. Q. NH3 Sensing Property and Mechanisms of Quartz Surface Acoustic Wave Sensors Deposited with SiO2, TiO2, and SiO2-TiO2 Composite Films. Sensors Actuators, B Chem. 2018, 254, 1165–1173. (8) Yi, J.; Lee, J. M.; Park, W. Il. Vertically Aligned ZnO Nanorods and Graphene Hybrid Architectures for High-Sensitive Flexible Gas Sensors. Sensors Actuators, B Chem. 2011, 155, 264–269. (9) Zhu, H.; Wang, X.; Liang, J.; Lv, H.; Tong, H.; Ma, L.; Hu, Y.; Zhu, G.; Zhang, T.; Tie, Z.; et al. Versatile Electronic Skins for Motion Detection of Joints Enabled by Aligned Few-Walled Carbon Nanotubes in Flexible Polymer Composites. Adv. Funct. Mater. 2017, 27, 1–7. (10) Zhu, G.; Yang, R.; Wang, S.; Wang, Z. L. Flexible High-Output Nanogenerator Based on Lateral ZnO Nanowire Array. Nano Lett. 2010, 10, 3151–3155. (11) Yao, S.; Swetha, P.; Zhu, Y. Nanomaterial-Enabled Wearable Sensors for Healthcare. Adv. Healthc. Mater. 2018, 7, 1–27.

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(40) Tang, N.; Mu, L.; Qu, H.; Wang, Y.; Duan, X.; Reed, M. A. Smartphone-Enabled Colorimetric Trinitrotoluene Detection Using Amine-Trapped Polydimethylsiloxane Membranes. ACS Appl. Mater. Interfaces 2017, 9, 14445-14452. (41) Matindoust, S.; Farzi, A.; Baghaei Nejad, M.; Shahrokh Abadi, M. H.; Zou, Z.; Zheng, L. R. Ammonia Gas Sensor Based on Flexible Polyaniline Films for Rapid Detection of Spoilage in Protein-Rich Foods. J. Mater. Sci. Mater. Electron. 2017, 28, 7760–7768. (42) Tiggemann, L.; Ballen, S. C.; Bocalon, C. M.; Graboski, A. M.; Manzoli, A.; Steffens, J.; Valduga, E.; Steffens, C. Electronic Nose System Based on Polyaniline Films Sensor Array with Different Dopants for Discrimination of Artificial Aromas. Innov. Food Sci. Emerg. Technol. 2017, 43, 112–116. (43) Panes-Ruiz, L. A.; Shaygan, M.; Fu, Y.; Liu, Y.; Khavrus, V. O.; Oswald, S.; Gemming, T.; Baraban, L.; Bezugly, V.; Cuniberti, G. Towards Highly Sensitive and Energy Efficient Ammonia Gas Detection with Modified Single-Walled Carbon Nanotubes at Room Temperatures. ACS Sensors 2017, 3, 79-86.

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