A Fully Integrated Wireless Flexible Ammonia Sensor Fabricated bySoft Nano-LithographyNing 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

*S Supporting Information

ABSTRACT: Flexible ammonia (NH3) sensors based on one-dimensionalnanostructures have attracted great attention due to their high flexibility and lowpower consumption. However, it is still challenging to reliably and cost-effectivelyfabricate ordered nanostructure-based flexible sensors. Herein, a smartphone-enabled fully integrated system based on a flexible nanowire sensor wasdeveloped for real-time NH3 monitoring. Highly aligned, sub-100 nm nanowireson a flexible substrate fabricated by facile and low-cost soft lithography were usedas sensitive elements to produce impedance response. The detection signals weresent to a smartphone and displayed on the screen in real time. This nanowire-based sensor exhibited robust flexibility and mechanical durability. Moreover, theintegrated NH3 sensing system presented enhanced performance with a detectionlimit of 100 ppb, as well as high selectivity and reproducibility. The powerconsumption of the flexible nanowire sensor was as low as 3 μW. By using thissystem, measurements were carried out to obtain reliable information about thespoilage of foods. This smartphone-enabled integrated system based on a flexible nanowire sensor provided a portable andefficient way to detect NH3 in daily life.

KEYWORDS: flexible electronics, portable system, nanowire, ammonia, soft lithography

Ammonia (NH3), a colorless and water-soluble gas, is amajor air contaminant emitted from industry, agricultural

practices, and motor vehicles.1,2 The toxic gas not only acts asan indicator of environmental conditions but is also the mostcommon gas found in the decomposition of protein-richfoods.3,4 Thus, efficient monitoring of NH3 in daily life is apromising approach to provide guaranteed quality of life.Various traditional techniques for NH3 detection have beenproposed, including spectrophotometry, chromatography,electrochemistry, and acoustics.5−7 Though powerful andefficient, these methods suffer from the shortcomings of highequipment costs, time-consuming processes and poor port-ability, and are only suitable for laboratory use rather thaninexpensive daily applications. Therefore, portable and well-performing sensors are greatly needed to monitor NH3 in dailylife and evaluate food quality in a timely manner.Different types of flexible sensors based on nanowires, such

as highly sensitive chemical sensors,8 flexible strain devices,9

and highly efficient energy generators,10 have been developed,since such sensors not only offer electrical functions but alsohave the ability to be compressed, bent, and conformed intoarbitrary shapes.11,12 In addition, the intrinsically high surface-to-volume ratio of nanostructures enables the construction ofhighly sensitive, rapidly responsive, low-power-consumptionsensors with low usage of conductive materials.13 Furthermore,1D nanostructures have better mechanical properties thancorresponding bulk materials.14 By placing a flexible NH3

sensor with a low detection limit and high selectivity directlyon the surface of foods, freshness can be detected at the onsetof decay, in contrast to the visual appearance and classicalolfactory recognition methods.15 Despite the many advantagesof nanoscale flexible gas sensors, fabricating nanostructures onflexible substrates is challenging because most flexiblesubstrates and materials are not compatible with conventionalnanolithography techniques, such as focused ion beam and dip-pen lithography.16−18 Various printing techniques, such asinkjet printing and transfer printing, have been developed tofabricate nanostructures for gas sensing.19−21 However, theprinting resolution is limited to the micrometer scale, and thechoice of ink material is limited due to the viscosity barrier.22

In addition, the transfer process may involve a chemicaletching process requiring hazardous etchants or thermalprocesses that may cause damage to the supporting substratesand fabricated structures.23−25 Based on these facts, thedevelopment of efficient approaches for the facile, low-cost,and damage-free fabrication of sub-100 nm nanostructures onflexible substrates is meaningful for developing NH3 sensorswith excellent sensing performance. As a collection oftechniques based on printing and molding, soft lithography isa good potential method to achieve the simple and

Received: December 28, 2018Accepted: February 22, 2019Published: February 22, 2019

Article

pubs.acs.org/acssensorsCite This: ACS Sens. XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acssensors.8b01690ACS Sens. XXXX, XXX, XXX−XXX

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nondestructive fabrication of nanostructures.26−30 Such anapproach would simplify the preparation process and improvethe properties of gas sensors, leading to promising progress inNH3 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 polymernanowires were fabricated by a capillary filling-based softlithography technique on a polyethylene terephthalate (PET)substrate. The prepared nanowires were then integrated into afunctional flexible device. The mechanical properties of thenanoflexible device were tested by multiple bending experi-ments and simulations. The nanowire NH3 sensor showed alow limit of detection (LOD), fast response, high selectivity,and low power consumption. Furthermore, a watch-type devicebased on a flexible printed circuit board (FPCB) wasdeveloped to detect the sensor impedance and deliver thedata to a smartphone through Bluetooth. With the home-builtanalysis program, the real-time response of the nanowiresensor to NH3 could be displayed on the phone screen. Theproposed smartphone-integrated system based on a flexiblenanowire sensor has demonstrated its capability for theportable detection of trace NH3 in food spoilage monitoring.

■ EXPERIMENTAL SECTIONMaterials and Apparatus. Imprint resist mr-I T85 was

purchased from Micro-Resist. Conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) wasobtained 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 indiumnitrate hydrate was dissolved in water at a concentration of 0.8 mg/mL. Graphene oxide was obtained from Chengdu Organic ChemicalCompany and sonicated for 30 min to form a uniform and stabledispersion with a concentration of 0.5 mg/mL. Polydimethylsiloxane(PDMS) was prepared from Sylgard 184 silicone elastomer. Scanningelectron microscopy (SEM) and atomic force microscopy (AFM)imaging were carried out with an FEI FP 2031/12 Inspect F50 andDimension Icon (Bruker), respectively. Electrical measurements witha constant voltage of +1 V were performed with an AgilentTechnologies B1500A.Fabrication of Large-Area Nanowires. PDMS and mr-I T85

were used for the fabrication of the PDMS mold. The prepolymerbase and curing agent were mixed in a 10:1 (w/w) ratio, followed bydegassing to remove air bubbles and curing at 80 °C for 2 h. Then,the mr-I T85 solution was spin-coated onto the cured PDMS,followed by baking at 140 °C for 2 min in a hot oven. The compositePDMS 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 °C, and 2 bar ofpressure was applied to the system for 40 s. Upon cooling to 65 °C,the template was separated from the PDMS substrate. Thus, a PDMSmold containing well-defined parallel nanogrooves was prepared.Subsequently, the PDMS mold was placed in contact with an O2plasma-treated (20 mTorr; 20 sccm; 30 W; 30 s) PET substrate, and afew drops of water were applied immediately for good sealing. Afterwater evaporation, the material solution (PEDOT:PSS; zinc oxide;indium nitrate hydrate; graphene oxide) was dropped into the openend of the PDMS mold. After the solvent completely evaporated, thePDMS mold was gently removed from the PET.Interdigital Electrode Fabrication. Au interdigital electrodes

with ten fingers (N = 9) were evaporated on the PET through adesigned copper shadow mask in a top-contact configuration with apad length (L) of 8 mm and width (W) of 300 μm. The spacingbetween 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 theprinted circuit board is shown in the Supporting Information. Androidapplication programs were developed to receive real-time signals andplot the results on the screen. The “Bluetooth” button was used tosearch and link the FPCB with the smartphone. Then, themicrocontroller received commands to communicate with theresistance analyzer and simultaneously sent out feedback signalsfrom the flexible sensor to AD5933. At the same time, a coordinategraph on the smartphone screen displayed the response change in realtime. A resistance threshold in the food freshness detection programwas set. Once the measurement results exceeded the threshold, theconditions were considered to indicate food spoilage, and a warningmessage was displayed on the phone screen.

■ RESULTS AND DISCUSSIONFlexible Nanowire Fabrication and Characterization.

The direct fabrication of highly aligned nanowires on a flexiblesubstrate and device integration on a flexible substrate isillustrated in Figure 1a. First, a soft nanomold containing well-

defined parallel nanogrooves was prepared by thermalnanoimprint lithography (NIL) on a flat PDMS substrate(Figure S1 in the Supporting Information). After soft bondingto a plasma-treated PET substrate, the nanogrooves turnedinto parallel nanochannels. Then, a few drops of aqueoussolution containing functional materials (e.g., PEDOT:PSS)were applied at the edges of the nanochannels. The aqueoussolution easily filled the whole channels within a few secondsdue to the large Laplace pressure (the detailed calculations aresummarized in Figure S2 in the Supporting Information).Moreover, the improved surface affinity from O2 plasmatreatment enhanced the adhesion between the depositedmaterials and PET substrate. Once the solvent had completelyevaporated, the PDMS mold was gently removed from thePET, leaving the nanowires on the flexible substrate (Figure1b). The interdigitated electrodes were then deposited byvacuum evaporation through a designed shadow mask; thus, aflexible chemiresistor consisting of highly aligned nanowireswas successfully fabricated (Figure 1c).The fully aligned, ultralong PEDOT:PSS nanowires were

extended to a relatively large area (>10 mm × 10 mm) withoutobvious defects, as shown in Figure S3. The intervals betweenthe nanowires were 5 and 3 μm. Thus, the number ofnanowires in a distance of 8 mm (the pad length of theinterdigital electrodes) was approximately 500 (n = 500). Asmeasured from SEM and AFM height images (Figure 2a andb), the nanowires have an average width (w) of 70 ± 5 nm andheight (h) of 60 ± 5 nm. Therefore, the resistance for n = 500

Figure 1. (a) Schematic illustration of the preparation process offlexible nanowire sensors. (b) Optical microscope image of the highlyaligned PEDOT:PSS nanowires. (c) Picture of the integrated flexibledevice.

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parallel nanowires is (1/N)(1/n)(1/σ)(l/wh) ≈ 0.26 MΩ,where N is the interval corresponding to the number ofinterdigitated electrodes, σ (1 S/cm) is the conductivity of thePEDOT:PSS solution, and l is the spacing between adjacentfingers (for calculation details, see the Supporting Information,Figure S3). Furthermore, the measured resistance of theflexible nanowire sensor, as shown in Figure 2c, isapproximately 0.36 MΩ, which is quite close to the calculatedvalue, demonstrating the good quality of the nanowires andreliability of this fabrication technique.With the use of only a printing or capillary molding

procedure, most fabrication processes can be continuedwithout cleanroom facilities once a mold is available.30 Infact, the yield of 20 flexible nanowire sensors prepared outsidea cleanroom reached 95%. Thus, nanoscale soft lithographyprovides a facile and low-cost method to pattern nanostruc-tures. The conformal contact between the PDMS mold and thetarget substrate is the key step to ensure high-qualitynanopatterns. Due to the mild fabrication conditions and thelack of additional etching or transfer steps, this approach ishighly suitable for the direct fabrication of large-area orderednanowires on any substrate. The formation of alignednanowires is based on capillary filling; thus, this method issuitable to pattern many different materials as well, providing aversatile nanofabrication approach with high material andsubstrate compatibility. To verify the material-independentcapability, we fabricated nanowires on a PET substrate usingthree different types of materials: (1) a semiconductor (zincoxide, ZnO), (2) a carbon material (graphene oxide, GO), and(3) an inorganic crystal (indium nitrate, In(NO3)3). Figure2d−f confirms the high quality of the fabricated nanowiresfrom these three materials. It is also worth noting that all thesenanowires were fabricated from the same PDMS mold,indicating the good reusability of the mold. Furthermore,since most processes can be performed outside a cleanroom,the technique is accessible to most research laboratories.Bending and Fatigue Properties. The mechanical

properties of the flexible nanowire sensor were evaluated bymonitoring its resistance change under an applied voltage (+1V) when bending the sensor by a motor-controlled stepper.The stepper propelled one end of the flexible device back andforth with the other end clamped by two block clips for cyclingbetween bending and unbending states. The test system is

presented in Figure S4 in the Supporting Information. For theouter bending test, the bending radius was calculated using thefollowing equation:

π π=

−L

L L h LBending radius

2 (d / ) ( /12 )s2 2 2

(1)

where L, dL/L, and hs denote the initial length, the appliedstrain, and the PET thickness, respectively.31 Figure 3a shows

the outer bending results of the flexible chemiresistor underdifferent bending states (see also Figure S5 in the SupportingInformation, which shows all the I−V conductivity measure-ments). At the initial stage, the resistance of the sensorexhibited almost no change until it was bent to a radius of 3mm. The increase in resistance is most likely due to thecracking of the nanowires. However, the value of ΔR/R0 wasvery small (∼0.09), even when bending occurred below 2 mm.The outer bending fatigue test with a bending radius fixed at 5mm is displayed in Figure 3b. There was no change inresistance throughout 1200 bending cycles, demonstrating thesuperior durability and mechanical stability of the flexiblenanowire sensor. The results also verify that the fabricatednanowires have excellent adhesion to the flexible PETsubstrate 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, whichensure a direct path of electron transport and reduce the crossdefect density, thereby reducing the dependence of theresistance change on the strain.14 To shed further light onthe mechanical robustness of the flexible nanowire sensor, theinduced strain at the time of bending was calculated by usingthe finite element method simulation (Comsol Multiphysics5.3), as shown in Figure 3c. From the simulation, themaximum strains of the whole substrate are ∼4% and −4%when the curvature radius is 5 mm. Although the nanowires arelocated close to the top of the substrate and away from theneutral bending axis, the strain on the nanowires is relativelylow due to the relatively small size of the nanowires (sub-100nm in height), which results in most of the strain beingaccommodated by the PET substrate.32 Moreover, the strain inactive regions is relatively low, which explains the mechanicalrobustness of the device even under a large bending state.

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

Figure 3. (a) Response of the flexible nanowire device at differentouter bending radiuses. (b) Outer bending fatigue reliability at anouter bending radius of 5 mm. (c) Theoretical simulation of the strainfor a nanowire sensor bent to a 5 mm curvature radius.

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Another important factor to maintain device flexibility androbustness is the large-scale pattern of nanowires andinterdigital design of the electrodes. Due to these factors,even if the nanowires between some of the electrode stripes arecracked, the performance of the whole device will not begreatly affected.NH3 Sensing Performance of the Flexible Nanowire

Sensor. The real-time sensing results of the flexible nanowiresensor for different concentrations of NH3 are shown in Figure4a. Clearly, when contacting NH3 gas, the resistance of the

sensor increased immediately, followed by a region ofsaturation. The sensing mechanism of nanowires can beexplained by the p-type nature of PEDOT:PSS; thus, the holesin the valence band of conducting polymer can be depleted byan electron-donating gas (e.g., NH3), resulting in a significantincrease in resistance.33 The response of the nanowire NH3sensor was calculated to conduct a quantitative study. Theresponse is defined as

= −R R RResponse ( )/t 0 0 (2)

where R0 is the initially measured steady resistance and Rt isthe measured value in the presence of NH3. As shown in theinset in Figure 4a, the sensor response is linearly dependent onthe NH3 concentration in the range 0.75−6 ppm, greatly

simplifying use in practical terms. Through the fitting result, weobtained a response slope of 0.2524 ppm−1 with a fittingquality r2 = 0.9834. The theoretical detection limit (for asignal-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 andrecovery time, in which t90,res is defined as the time to reach90% of the maximum response, and t10,rec is defined as the timeinterval over which the sensor response drops to 10% of thestabilized response in NH3 gas. For the analysis of 3 ppm ofNH3, t90,res and t10,rec were calculated as 70 and 276 s,respectively, for this flexible chemiresistor (the detailed dataare summarized in Figure S6 in the Supporting Information).As a comparison, a thin-film PEDOT:PSS sensor was preparedto detect NH3 at the same concentration. The proposednanowire sensor presents several distinct advantages, includingfast response and enhanced sensitivity, as shown in Figure S7.The low detection limit and rapid response of this sensor areattributed to the high surface-to-volume ratio of thePEDOT:PSS nanowires, which ensures excellent accessibilityto NH3 gas molecules. The performance of the nanowiresensor and a comparison with other NH3 sensors aresummarized in Table S1 (Supporting Information). In additionto a much simpler fabrication process and sensor structure, thesensor in this work also presents better performance.Reproducibility and selectivity are two key factors for

practical sensing applications. Figure 4b displays the dynamicresponses of the flexible nanowire sensor to the same NH3concentration. Stable performance indicates good reproduci-bility for a single device. For sample-to-sample reproducibility,statistics were performed for flexible nanowire sensors from tendifferent devices. The distributions of the responsivity at amoderate concentration (0.75 ppm) and sensitivity (defined asthe slope of the linear relationship of responsivity versus NH3concentration) are illustrated in Figure 4c. Approximately 90%of the samples exhibited a responsivity of more than 0.8 at 0.75ppm of NH3, and approximately 60% of the nanowire gassensors showed a sensitivity over 0.2 ppm−1. The highrepeatability of the sensing performance from different devicesis mainly due to the reusability of the PDMS mold, thusensuring a high degree of consistency of the nanowirestructures.To check the sensing selectivity, different interference

volatile organic compounds (VOCs) were measured at roomtemperature as well (Figure S8 in Supporting Information).Based on the physical properties of these VOCs, summarizedin Table S2, the corresponding concentrations of VOCs atdifferent flow rates were calculated (for calculation details, seethe Supporting Information). The flexible chemiresistor sensorpresents a preferential response to NH3 and a negligibleresponse to other VOCs (Figure 4d). Such high selectivity isdue to the sensing mechanism of the PEDOT:PSS nanowiresensor to VOCs, which is caused by weak physical interactionsinvolving absorbing and swelling.34,35 At ambient temperature,the conducting polymer is in its glassy state, which results in alow absorption and swelling level; thus, the contribution ofthese two interactions to the overall resistance change isminor.36 However, as mentioned already, the interactionbetween NH3 and PEDOT:PSS is considered to be electrontransport, which is dominant at ambient temperature. The highselectivity of nanoscale PEDOT:PSS to NH3 will minimize thesensor noise in practical sensing environments where VOCsare usually present.

Figure 4. (a) Real-time response of the flexible nanowire gas sensor toNH3; The inset figure shows a linear correlation of the responsevalues as a function of NH3 concentration. (b) Repeatability tests of asingle nanowire gas sensor toward different concentrations of NH3.(c) Distributions of the responsivity at 0.75 ppm of NH3 and thesensitivity of ten different sensors. (d) Responsivity and sensitivity todifferent 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), andacetone (60 ppm)). Response curves of the flexible nanowire gassensor to 1.5 ppm of NH3 when tested (e) under different bendingradiuses, and (f) before and after bending 500 and 1000 times(bending radius = 15 mm).

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The sensing performance of the flexible nanowire sensor wasalso tested with respect to bending status. The dynamicresponses of the sensor at different bending radiuses are shownin Figure 4e and Figure S9. The response values and excellentreversibility of the sensor are fully retained at these bendingangles. Figure 4f presents the real-time results of a fatigue testafter several bending and relaxing processes. No significantdrop in response values was observed, indicating that theflexible aligned nanowire sensor in our work exhibits excellentmechanical durability and robustness while maintaining goodsensing performance.NH3 Sensing Based on Smartphone-Enabled System.

The flexible nanowire sensor was integrated with otherelectronic components on a FPCB (also known as “hard-soft” integration) to compose a portable electronic system thatbridges the technology gap between signal generation byflexible electronics and data transmission using readilyintegrated circuit components.37−40 The smartphone-enabledsensing system was used as a portable device to monitor thereal-time response to NH3. An Android application program(detailed information can be found in the SupportingInformation) was developed and installed on a smartphoneto receive and process the sensing signals from the flexiblenanowires. As shown in the schematic diagrams in Figure 5

and Figure S10, in the presence of NH3, the impedance signalsgenerated by the portable sensing device were sent wirelesslyto the smartphone through the Bluetooth module, and thesensing data were real-time plotted on the screen (see alsoVideo 1). This fully integrated sensing system based on flexiblePEDOT:PSS nanowires demonstrates the accurate determi-nation of NH3 in a real applications.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 oforganic matter, and the concentration of this corrosive gascould be regarded as a biomarker for the freshness offoods.41,42 The integrated NH3 system was applied to monitorthe spoilage of salmon stored in a refrigerator in ambient air bypacking the wireless system together with the salmon.Compared with the salmon stored in the refrigerator, theresponse of the sample placed in ambient air for 12 h changedsignificantly, while a slight color change only started to occurafter 36 h (Figure 6a). The smartphone receives electrical

signals from the smart packaging and displays an alarm whenthe food is spoiled (Figure 6b; see also Video 2). The powerconsumption of the flexible nanowire sensor in this sensingsystem is calculated to be less than 3 μW, which is much lowerthan 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 ofthe nanowire sensor benefit its usage to guarantee food quality.Collectively, these results demonstrate that this smartphone-enabled NH3 detection system, integrating a miniaturizednanowire sensor and portable circuits, has good sensingperformance, and can thus potentially meet the request forportable and convenient monitoring of food quality in dailystorage and supply chains.

■ CONCLUSIONSIn summary, a sensitive and low-power-consumption inte-grated wireless portable system based on a flexible nanowiresensor was designed and fabricated to detect NH3. Thenanowire sensor was fabricated by a capillary filling-based softlithography technique. Owing to the facile and mild fabricationconditions, this method provides a versatile nanofabricationapproach with high material and substrate compatibility. Usingthis technique, we demonstrated dense and orderedPEDOT:PSS, ZnO, GO, and In(NO3)3 nanowires on a PETsubstrate. The experimental results suggest that the flexibledevice exhibits excellent mechanical bendability and durabilitywithout an obvious decrease in performance even after 1200bending cycles. Moreover, the sensor shows good sensitivityand selectivity to NH3, and the power consumption is as low as3 μW. The smartphone-enabled portable integrated sensingsystem shows good performance in NH3 detection andprotein-rich food spoilage monitoring. Considering the lowcost, easy implementation, and low power consumption of thisdevice, it is believed that the smartphone-enabled NH3detection system based on our flexible nanowire sensor willhelp pave the way for NH3 monitoring in daily life andintelligent food detection in storage and supply chains.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acssen-sors.8b01690.

Composite PDMS stamp, PEDOT:PSS nanowirefabrication, SEM result of large-area nanowires, Outerbending test system, I−V Conductivity Measurement,Limit of detection (LOD) calculation, Response and

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

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

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recovery time, Sensing performance of thin-film basedsensors, Sensing performance of conducting polymerbased sensors, Dynamic response to VOCs, Dynamicresponse to VOCs under different bending radiuses,Schematic circuit diagram, Source code download(PDF)Video 1: Real-time detection using smartphone-enablewatch-type sensing system (AVI)Video 2: Spoilage detection (AVI)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: xduan@tju.edu.cn. Tel. /Fax: +86 2227401002.ORCIDXuexin Duan: 0000-0002-7550-3951Author Contributions#N.T. and C.Z. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors gratefully acknowledge financial support from theNational Natural Science Foundation of China (NSFC No.61674114, 91743110, 21861132001), National Key R&DProgram of China (2017YFF0204600), Tianjin Applied BasicResearch and Advanced Technology (17JCJQJC43600), theFoundation for Talent Scientists of Nanchang Institute forMicrotechnology of Tianjin University, and the 111 Project(B07014).

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