Extremely Deformable, Transparent, and High-Performance GasSensor Based on Ionic Conductive HydrogelJin Wu,*,† Zixuan Wu,† Songjia Han,† Bo-Ru Yang,† Xuchun Gui,† Kai Tao,*,‡ Chuan Liu,*,†
Jianmin Miao,§ and Leslie K. Norford∥
†State Key Laboratory of Optoelectronic Materials and Technologies and the Guangdong Province Key Laboratory of DisplayMaterial and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China‡The Ministry of Education Key Laboratory of Micro and Nano Systems for Aerospace, Northwestern Polytechnical University,Xi’an 710072, China§School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore∥Department of Architecture, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
*S Supporting Information
ABSTRACT: Fabrication of stretchable chemical sensorsbecomes increasingly attractive for emerging wearableapplications in environmental monitoring and health care.Here, for the first time, chemically derived ionic conductivepolyacrylamide/carrageenan double-network (DN) hydrogelsare exploited to fabricate ultrastretchable and transparent NO2and NH3 sensors with high sensitivity (78.5 ppm−1) and lowtheoretical limit of detection (1.2 ppb) in NO2 detection. Thehydrogels can withstand various rigorous mechanicaldeformations, including up to 1200% strain, large-rangeflexion, and twist. The drastic mechanical deformations donot degrade the gas-sensing performance. A facile solventreplacement strategy is devised to partially replace water withglycerol (Gly) molecules in the solvent of hydrogel, generating the water−Gly binary hydrogel with 1.68 times boostedsensitivity to NO2 and significantly enhanced stability. The DN-Gly NO2 sensor can maintain its sensitivity for as long as 9months. The high sensitivity is attributed to the abundant oxygenated functional groups in the well-designed polymer chainsand solvent. A gas-blocking mechanism is proposed to understand the positive resistance shift of the gas sensors. This worksheds light on utilizing ionic conductive hydrogels as novel channel materials to design highly deformable and sensitive gassensors.
KEYWORDS: gas sensor, stretchable, double-network hydrogel, transparent, water retention
1. INTRODUCTION
Flexible and stretchable electronic sensors have attractedconsiderable interest in the past decade because of theiremerging applications in health assessment, internet-of-things,electronic skin (e-skin), robotics, and environmental monitor-ing.1 Among various applications, gas sensing has becomeincreasingly important because air pollutants, including carbonoxides (COx), nitrogen oxides (NOx), sulfur dioxide (SO2),ammonia (NH3), and so forth, have endangered both theenvironment and the health of humans.2 For example, thehuman respiration system may be damaged by nitrogen dioxide(NO2) with the concentration higher than 1 ppm.3,4 NO2 alsoparticipates in many processes of forming hazardous acid rain,ozone (O3), and photochemical smog.3 As such, USEnvironmental Protection Agency (EPA) set a 1 h and theannual average NO2 standard of 100 and 53 ppb, respectively.5
Otherwise, NO2 is very useful in the synthesis of nitric acid,which is an important chemical in the production of fertilizers
for agriculture and explosives for military application.3,6 Inaddition, NOx, an important gut neurotransmitter, also plays asignificant role in regulating mucosal blood flow, intestinalmotility, and secretory functions.7,8 NOx can be used as abiomarker in the noninvasive detection of diseases such as lunginfections and bowel disease.6 NH3 is a toxic and colorless gasthat may cause respiratory tract irritation a at lowconcentration from 50 to 100 ppm.9,10 It can also be utilizedas a biomarker for disease diagnosis.9 Hence, development ofgas sensors that can precisely detect NO2 and NH3 at lowconcentrations can protect people from being harmed by thesetoxic gases and improve the accuracy of diagnosing the diseasesclosely related to the levels of these gaseous chemicals.Especially, if these gas sensors can work at a flexible/
Received: October 6, 2018Accepted: December 3, 2018Published: December 31, 2018
stretchable state, they can be used for emerging wearableapplications in health care and environmental safety.11,12 Forinstance, the flexible/stretchable gas sensors can be conform-ably attached on curved surfaces, such as clothes, bags, animals,and plants, for real-time gas monitoring.5
Existing flexible gas sensors are fabricated by integratinginorganic gas-sensitive materials including graphene, MoS2,carbon nanotubes, colloidal quantum dots (CQDs), and soforth on flexible substrates.5,11,13−19 However, this structuralengineering method demands additional flexible substrates andfabrication procedures.16 Furthermore, applications such as e-skin, soft robotics, and wearable devices require the sensors tosustain larger mechanical deformation beyond bending, forexample, stretching and twist.11,20 There are few reports onstretchable gas sensors fabricated by depositing graphene-based materials on stretchable substrates, such as polydime-thylsiloxane (PDMS) and Ecoflex.11,21 For example, Lee andco-workers reported a stretchable NO2 sensor using reducedgraphene oxide (RGO) that was assembled on polyurethane(PU) nanofibres.11 The nanohybrids are deposited on thestretchable PDMS substrate, realizing up to 50% strain.Besides, Ha and co-workers reported a micro-supercapacitorarray derived graphene gas sensor on a deformable Ecoflexsubstrate.21 A maximal biaxial 40% strain and a uniaxial 50%strain are enabled. However, such a structural engineeringapproach requires complicated experimental procedures tofabricate the multilayer-sensing devices with very limitedstretchability. For example, the largest tensile strain is only 50%for state-of-the-art stretchable gas sensors.11 In addition tostretchability, transparency is also an important attribute in e-skin, wearable circuitry, and flexible display applications.14,22,23
In skin electronics, the transparency of sensors is important forthe improvements of aesthetics in daily usage and the securityin military applications.24 Nevertheless, conventional gas-responsive materials, electrodes, and interconnects are usuallyopaque, which significantly decrease the transparency offabricated devices. A simple but effective alternative to imparthigh stretchability and transparency to gas sensors is thedevelopment of the sensing material that intrinsically possessesthese attributes. However, to the best of our knowledge, thismaterial has seldom been reported.Recently, ionic conductive hydrogels have attracted wide-
spread applications in soft electronics because of the goodstretchability, conductance, transparency, and biocompatibilityof the material.25−28 Especially, double-network (DN) hydro-gels that consisted of two kinds of interconnected polymernetworks exhibit better mechanical robustness, stretchability,and chemical functionalities than their single-network counter-parts.29−32 In the hydrogel, the hydrophilic polymer chains aredispersed in water. The polymer network and liquid phaseendow solid-like mechanical properties and flexibility to thehydrogel, respectively.33 Meanwhile, the movement of ions inliquid media endows conductivity to the hydrogel. Recently,many studies on graphene-based gas sensors reveal that theoxygen-containing functional groups such as −OH, SO3
−, and−NH2 on graphene play an important role in enhancing theadsorption of NO2 and NH3 molecules.34−38 Considering thevital role of oxygen-containing functional groups in improvingthe gas-sensing performance, herein, we employ chemicallyfunctionalized polyacrylamide (PAM)/carrageenan DN hydro-gel that contains abundant oxygenated groups to fabricateextremely stretchable, transparent, and sensitive NO2 and NH3gas sensors. Differing from a traditional electron-conductive
gas-sensing material, the ionic conductive DN hydrogelprovides remarkable advantages of extreme stretchability (upto 1200% strain), flexibility, and transparency. Therefore,additional structural engineering is bypassed when it is used tofabricate stretchable chemical sensors. In addition to highsensitivity, selectivity, and fast response, good stability is alsoan important prerequisite for the practical application of a gassensor.39 However, water loss induced instability is an intrinsicproblem of hydrogels.26 The water evaporation may degradethe stretchability, conductivity, and stability of hydrogel-baseddevices in the long term.26,40 In order to enhance the waterretaining ability of hydrogels, herein, a facile solventreplacement method is proposed to introduce hygroscopicglycerol (Gly) in the solvent of DN hydrogel. The chemicalmodification of hydrogel with Gly molecules not only brings1.68 times increased sensitivity in NO2 detection but alsoensures the stability of the gas sensors for as long as 9 months.
2. EXPERIMENTAL SECTION2.1. Materials and Synthesis of DN and DN-Gly Hydrogels.
All chemicals including acrylamide (AAm), κ-carrageenan, KCl, N-methylenebisacrylamide (MBA), and ammonium persulfate (AP)were purchased from Sigma-Aldrich. A one-pot polymerizationmethod was used to synthesize the DN hydrogel with the totalwater content of 82% (wt %). The AAm and carrageenan with theweight ratio of 15:3 were dissolved in deionized water at 95 °C. Then,MBA with the weight percent of 0.05% relative to AAm and KCl withthe weight percent of 6% relative to carrageenan were dissolved in thewater pot and stirred magnetically at 95 °C for 5 h. Subsequently, APwith the weight percent of 0.5% relative to AAm was added andstirred for 1 min. The solution was immediately poured into a glassdish with the length, width, and height of 11, 11, and 2 cm,respectively. The system was stored at 5 °C for 1 h for the formationof carrageenan. Then, the system was heated at 95 °C for 1 h for thecross-linking of PAM. Finally, the synthesized DN hydrogel was cutinto slices to investigate its properties.
For the preparation of DN-Gly hydrogel, the as-synthesized DNhydrogel was soaked in pure Gly for 20 min at room temperature. Theweight of Gly in the container was 8 times more than that of DNhydrogel. The water in the DN hydrogel was partially replaced bysurrounding plenty Gly in the soaking process. After the hydrogel wasremoved from the container, it was placed on a filter paper for 10 minto remove extra Gly on the surface.
2.2. Characterization. The UV−vis spectra were acquired on aShimadzu UV-2501PC. The Fourier transform infrared (FTIR)spectra were obtained on a PerkinElmer Frontier spectrometer. Thetensile tests were implemented on an Instron machine (S6566) withthe speed of 5 mm/min to obtain the stress−strain curves. The X-rayphotoelectron spectroscopy (XPS) spectra were acquired on a PHI-5400 spectrometer (Physical Electronics, US) with an Al Kα (1486.6eV) X-ray source. Field emission scanning electron microscopy(SEM) (JSM 7600F, Japan) was deployed to characterize themorphology of hydrogel after freeze-drying.
2.3. Gas-Sensing Measurement. The gas-sensing test wascarried out in a gas chamber with electrical feedthroughs. Afterapplying a constant bias voltage of 1 V on the two electrodes of thechemiresistor, the resistance change of the sensor was monitored andrecorded using a Keithley 2602 SourceMeter. In a typical sensingmeasurement cycle, synthetic air with 72% relative humidity (RH)was introduced into the chamber to clean it. Subsequently, test gas(NO2 or NH3) balanced by synthetic air was introduced in thechamber for 300 s to record the response process. Finally, thesynthetic air was injected into the chamber for 300 s to record therecovery process. All gas flows and the concentration of test gas wereprecisely controlled by mass-flow controllers. For all results, thesensor output was measured three times to obtain the averageresponse value and the statistical error. For gas-sensing measurementsat a stretched state, the hydrogel was stretched to a desired strain in
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the first step; then two binder clips were used to fix the two ends ofthe hydrogel to an underlying glass slide; finally, the system was put inthe chamber for gas-sensing test.
3. RESULTS AND DISCUSSION
The highly stretchable and transparent PAM/carrageenan DNhydrogel was synthesized via a one-pot sol−gel process, inwhich an ionic cross-linked carrageenan network and acovalently cross-linked PAM were formed in situ (Figure1a). In the polymerization of PAM, MBA and AP wereemployed as a cross-linker and thermal initiator, respectively.All chemicals except AP were dissolved in a water pot andstirred magnetically at 95 °C for 5 h. Then, AP was added and
stirred for 1 min. After the system was kept at 5 °C for 1 h, thefirst network of carrageenan formed. Subsequently, the systemwas heated at 95 °C for 1 h for the cross-linking of the secondnetwork of PAM. The thermal induced polymerization of PAMhere differs from the previously reported photoinducedpolymerization method.25,32 The DN-Gly hydrogel wasprepared via a convenient solvent replacement process fromthe synthesized DN hydrogel. In brief, the as-synthesized DNhydrogel was soaked in pure Gly for the replacement of watermolecules with Gly molecules. After 20 min, the water in theDN hydrogel was partially replaced by surrounding Gly due tothe diffusion caused by concentration difference, leading to theformation of a water−Gly binary solvent. The hygroscopic Gly
Figure 1. (a) Schematic illustration of the synthesis of DN hydrogel via a one-pot polymerization method, followed by loading of Gly in hydrogelvia a solvent replacement strategy. (b−d) Schematics showing that NO2 molecules can form hydrogen bonds with Gly, PAM, and carrageenan,respectively, which facilitate the adsorption of NO2 molecules on the hydrogel.
Figure 2. (a) Transmittance spectra of DN and DN-Gly spectra. (b) FTIR spectra of DN and DN-Gly hydrogels. (c) Tensile stress−strain curvesof DN and DN-Gly hydrogels. (d) XPS spectrum of the DN hydrogel. (e,f) C 1s and N 1s XPS spectra of the DN hydrogel, respectively. (g,h)Photographs of 540° twisted and 120° bent DN-Gly hydrogel, respectively. (i,j) Photographs of the same DN-Gly hydrogel at 0 and 1200% strain,respectively. (k) Plots of the weight loss percentage of DN and DN-Gly hydrogels vs time at 25 °C and 70% RH (ambient air).
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not only effectively inhibits the evaporation of water in thehydrogel (water retention) but also facilitates the adsorption ofNO2 and NH3 molecules on the hydrogel by forming hydrogenbonds with them (Figure 1b). In addition to the −OH on Gly,−NH2 on the polymer chain of PAM together with −OH andSO3
− on carrageenan can also promote the adsorption of NO2and NH3 molecules by forming hydrogen bonds with these gasmolecules, leading to an increased sensitivity in gas detection(Figure 1c,d).The transmittance spectra of DN and DN-Gly hydrogels
indicate that both of them are highly transparent (>75%transmittance for ≥600 nm wavelength) (Figure 2a). Theexcellent transparency of the hydrogels is advantageous forwearable applications that require visualization.22,23 The O−Hstretching peak at 3438 cm−1 and O−H bending peak at 1651cm−1 appeared in the FTIR spectra of both DN and DN-Glyhydrogels (Figure 2b). Note that the DN-Gly hydrogelexhibited higher intensity in both the O−H stretching peakand the O−H bending peak in comparison with theunmodified DN hydrogel, suggesting that Gly moleculeswere successfully modified on the DN hydrogel. After solventreplacement treatment, the DN-Gly hydrogel did not showdeteriorated stretchability, but displayed much highermechanical strength compared with the unmodified DNhydrogel (Figure 2c). Specifically, the elastic moduli of DNand DN-Gly hydrogels within the linear range (0−70% strain)were calculated to be 299 and 502 kPa, respectively. Note thatthe elastic modulus of DN-Gly hydrogel is also much higherthan that of carrageenan (72 kPa), PAM single-networkhydrogels (9 kPa), and many other PAM-based DN networks,demonstrating the enhanced mechanical toughness.23,32,41 Thehigh elastic modulus of DN-Gly hydrogel may be attributed tothe formation of hydrogen bonds between Gly and PAM, aswell as that between Gly and carrageenan.27 The hysteresisloop appeared in the loading−unloading curves is ascribed tothe energy dissipation in the stretching process.42 XPSelemental analysis demonstrates that the DN hydrogel has ahigh content of C (78.13%), small amounts of O (16.03%),some N (5.11%), and little S (0.74%) (Table S1). Althoughthe contents of K and Cl were too low to be detected in the
quantitative XPS elemental analysis, they could be discerned inthe XPS elemental spectra (Figure 2d−f). The SEM imagesshow that enormous microscale pores appear on the freeze-dried DN hydrogel (Figure S2). The porous system plays animportant role in facilitating gas adsorption by allowing gasmolecules to diffuse into the abundant pores.Both DN and DN-Gly hydrogels can withstand large and
rigorous mechanical deformations, including stretching strain,bending, and twist, without breaking their structures (Figures2g−j and S3−S5). For instance, the DN-Gly hydrogelremained intact when it was twisted 540°, bent 120°, andstretched up to 1200% strain, which is the highest value forreported gas-sensing materials so far.11,21 Detailed photographsof the two kinds of sensors at different strains are shown inFigures S3 and S4. The DN-Gly hydrogel remained conductiveeven when large deformations were applied, including 40%strain, 180° twist, and 120° flexion (Figure S5d−f). Theexcellent stretchability of DN and DN-Gly hydrogels isattributed to the efficient energy dissipation by unfolding thePAM chain and dissociating the double helices of carrageenanin the stretching process. Furthermore, the PAM network andcarrageenan can interact with each other by forming hydrogenbonds.29,32 The synergistic effect between the binary networkscould also dissipate energy.23 In addition, Gly molecules canform hydrogen bonds with both PAM and carrageenan, furtherenhancing the mechanical stretchability and toughness of theDN-Gly hydrogel.27 Investigation of the electromechanicalproperty of the DN hydrogel revealed that the resistance ofDN hydrogel nearly did not change with flexion and twistangles but increased linearly with tensile strain from 0 to 133%(Figure S6).The water loss induced long-term instability is an intrinsic
problem of hydrogels.33,40 The loss of water makes hydrogelsboth rigid and nonconductive in the long term, which mayseriously impair the performance of fabricated electronicdevices and thus hinder their practical applications. The DNhydrogel had the weight loss as high as 74% when it wasexposed to ambient air for 72 h (Figure 2k), whereas theweight loss decreased to as low as 28% after the hygroscopicGly was introduced in the DN-Gly hydrogel. Furthermore, the
Figure 3. NO2 sensing behaviors of the DN hydrogel sensor. (a) Dynamic response of the sensor to NO2 gas with decreased concentration from 1to 0.1 ppm. The white and shaded regions denoted “off” and “on” states of a test gas, respectively, in the chamber. (b) Quantitative responsevariation vs NO2 concentration. (c) Dynamic response of this sensor to NO2 with the low concentration of 100 ppb. (d) Relative resistance changeof the sensor as a function of time when exposed to 500 ppb NO2 in three experimental cycles. (e) Quantitative response variation of the sensor vsexperimental cycle. (f) Investigation of the response time and recovery time in the detection of 500 ppb NO2.
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DN-Gly hydrogel was still conductive when it was exposed toambient air for 72 h without encapsulation, demonstrating thesignificantly improved water retention capability and stability.In addition, the DN-Gly hydrogel did not shrink and hardenwhen exposed to 55% RH at 40 °C for 50 h and could stillwithstand large mechanical deformations, including 360° twistand 120° flexion without breaking or fracture (Figure S7).The gas-sensing performance of the DN hydrogel is
evaluated by monitoring its relative resistance variation (ΔR/R0 %) upon exposure to a test gas. Both the test gas exposuretime and air purging time were 300 s. The quantitativeresponse here is defined as the relative normalized resistancedifference between ΔR/R0 at the beginning of test gas “on” andthat at the end of test gas “on” for each experimental cycle.Figure 3a displays typical dynamic response curves when theDN hydrogel sensor was exposed to NO2 gas with a range ofdifferent concentrations (1−0.1 ppm). The sensor exhibited animmediately resistance increase upon exposure to NO2. Thepositive resistance variation implied that the movement of ions(K+ and Cl−) in the hydrogel was hampered by the NO2 gasmolecules dissolved in the hydrogel.43 The response decreasedmonotonically with reduced NO2 concentration in thereversible sensing (Figure S8). With decreased NO2 concen-tration, fewer NO2 molecules dissolved in the hydrogel andtherefore the occurrence of blocking effects was smaller. It wasworth noting that a remarkable linear relationship wasobserved between response and NO2 concentration, which isdesirable in practical application (Figure 3b).34 This DNhydrogel demonstrated its capability to detect NO2 with thelow concentration of 100 ppb (Figure 3c). When the DNsensor was exposed to 500 ppb NO2 repeatedly in threeconsecutive experimental cycles, a nearly constant response of18.2% with a small variation of 0.5% was observed,demonstrating the good repeatability (Figure 3d,e). Theresponse time t50 and recovery time t50 are defined as thetime required for a 50% signal change in the full magnitude ofresponse factor in a sensing cycle.15,35 Analysis of the detectionof 500 ppb NO2 in one cycle reveals the response and recoverytime of 10.1 and 46.8 s, respectively (Figure 3f). The responseand recovery time are comparable or shorter than the gas
sensors based on other materials, such as graphene-basedmaterials, MoS2, SnS2, phosphorene, and so forth.3,13,15,34,43,44
One distinct advantage of the DN hydrogel based NO2sensor is its ability to work under various mechanicaldeformations, which do not adversely affect the sensitivity ofgas detection (Figure 4a−d). Specifically, no obvious degradedresponse to 1 ppm NO2 was observed when the DN sensorwas twisted 180°, bent 180°, and stretched to 100% strain. Incontrast, the response increased from 35% to 82.5% withincreased strain from 0 to 100% (Figure 4c). The boostedresponse with strain may be resulted from the increasedinteraction surface area between gas molecules and hydrogelupon stretching.45 This assumption is further supported by theimmunity of this sensor to twist and flexion, which have littleeffect on the surface area exposed to NO2 gas. Furthermore,the unzipping of polymer chains and the alignment of ionicconduction paths in the stretching process may also increasethe response. These hydrogel-based stretchable gas sensorswith tunable sensitivity provide advantages over traditionalrigid gas sensors that only exhibit fixed sensitivity once thesensors are made.39,45 In practical applications, the deformablegas-sensing system can be calibrated by integrating a strainsensor to preclude the influence of strain on gas sensing.46
Importantly, chemical modification of DN hydrogel with Glycan significantly boost the sensitivity of NO2 detection, as thehydroxyl groups on Gly molecules can form hydrogen bondswith NO2 molecules, facilitating NO2 adsorption. For example,the DN and DN-Gly sensors exhibited quantitative responsesof 142 and 238%, respectively, to 5 ppm NO2, an indication of1.68 times increased response by chemical modification withGly (Figure 4e,f). The enhancement in sensitivity was alsoevidenced by detecting NO2 with other concentrations. Inaddition to sensitivity, the DN-Gly NO2 sensor also showedhigher recovery percentage compared with the DN sensor,demonstrating the feasibility of chemical modification inimproving the comprehensive sensing performance (FigureS9). The limit of detection (LOD) of a chemical sensor can bedetermined when the signal level is 3 times higher than thenoise level.47,48 Thereby, the theoretical LOD can be deducedby calculating the noise level (root-mean-square deviation) and
Figure 4. (a) Dynamic responses of the DN hydrogel sensor to 1 ppm NO2 at flat, 180° twisted, and 180° bent states, respectively. Insets arephotographs of the same DN hydrogel at corresponding deformations. (b) Dynamic responses of the DN sensor to 1 ppm NO2 at 0, 50, and 100%strain, respectively. (c) Quantitative response of the DN sensor vs tensile strain. (d) Comparison of the quantitative responses of the DN sensor to1 ppm NO2 at different deformations, including relaxed state, 180° twist, 180° flexion, and 50% strain. (e) Comparison of the dynamic responses ofDN and DN-Gly sensors to NO2 with reduced concentration from 5 to 0.5 ppm. (f) Quantitative responses of the DN and DN-Gly sensors vs NO2concentration.
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sensitivity (slope of linearly fitted response vs gas concen-tration curve) (Supporting Information).2 Along this line, thesensitivities of DN and DN-Gly sensors are calculated to be33.2 and 78.5 ppm−1, respectively, and the LOD values of DNand DN-Gly sensors are calculated to be as low as 4.7 and 1.2ppb, respectively (Figure S10 and Tables S2−S4). Theexceptionally low LOD is attributed to the high sensitivityand the low noise level (0.05% for DN and 0.03% for DN-Gly), which may be related to the unique movement manner ofcharge carriers in the liquid phase. The LOD of this hydrogelsensor is not only much lower than the annual average NO2exposure limit recommended by US EPA (53 ppb), but is alsolower than that of most NO2 sensors based on other sensingmaterials (Table 1).5,13,16,49,50 For instance, the RGO/Cu2Ocomposite, single-walled carbon nanotubes (SWCNTs), andPbS CQDs only display the LOD of 64, 44, and 84 ppb,respectively, in NO2 detection.
16,49,50 Furthermore, the sensorsbased on these conventional materials are generally non-deformable and opaque. In addition, the sensitivity of ourhydrogel-based sensor is also very competitive when comparedwith other high-performance NO2 sensors, including graphene-related, MoS2, and CNT-based NO2 sensors.13,50,51 Itdemonstrates the remarkable advantages of ionic conductivehydrogel in fabricating ultrasensitive and deformable NO2sensors.In addition to NO2, the DN hydrogel also shows appreciable
response to NH3, a toxic and colorless gas with pungent odor(Figure 5). Similar to NO2 detection, the DN hydrogel alsoexhibited positive resistance variation upon exposure to NH3.This phenomenon reinforced the aforementioned mechanismthat the gas molecules blocked ion movement upon adsorptionand solvation. The DN hydrogel sensor was employed todetect NH3 in a wide concentration range (2−50 ppm). Theresponse increased monotonically from 3.5% to 50.4% withincreased NH3 concentration from 5 to 50 ppm (Figure 5a,b).A sensitivity of 1.3 ppm−1 was obtained by executing the linearfitting for the response versus NH3 concentration curves(Figure 5b). Notably, the sensor exhibited evident response toNH3 with the concentration as low as 2 ppm (Figure 5c).Although further diluted NH3 gas was not available for ourcurrent setup, careful analyses of the noise level andextrapolation of LOD reveal an exceptionally low theoreticalLOD of 0.22 ppm NH3 (Figure S11 and Tables S5 and S6).Both the sensitivity and LOD displayed by this NH3 sensor arecompetitive in comparison with those of state-of-the-art NH3sensors (Table S7). Similar to NO2, NH3 molecules can alsointeract with a large amount of oxygenated functional groupsincluding −OH, SO3
−, and −NH2 in the hydrogel viahydrogen bonds, promoting their adsorption. Importantly,this hydrogel-based NH3 sensor provides unique merits ofgood stretchability and high transparency. Furthermore, theresponse of this sensor to 40 ppm NH3 slightly increased withstrain (Figure 5d). From the dynamic response curves, the
response and recovery time were derived to be as short as 22.5and 13 s, respectively, which were competitive with many otherNH3 sensors.
2,9,51,52
The sensing mechanism of the ionic conductive hydrogel isdifferent from the conventional charge-transfer mechanism thatis well known for carbon and semiconducting materials.4,45 Fortraditional metal oxides and graphene-based gas sensors, theresistance variation depends on both the electron-with-drawing/donating nature of analytes and the charge carriertype (electrons or holes) in the sensing materials.45,47 Forexample, the electron-withdrawing graphene with holes in theconduction band displays decreased and increased resistanceupon exposure to oxidizing NO2 and reducing NH3,respectively.45,47 However, our hydrogel-based gas sensorsexhibited a positive resistance variation regardless of the typeof gas. It indicates that the blocking effect plays the key role inchanging the resistance of hydrogel upon gas adsorption, whichis similar to that of recently reported metallic Ti3C2Tx MXene-based gas sensor.43 In the absence of NO2/NH3 gas, K
+ andCl− ions in the hydrogel could move freely, leading to a lowresistance level (Figure 6a, left). When introduced, NO2/NH3molecules diffused into the hydrogel and hindered themovements of ions, increasing the resistance (Figure 6a,right). The excellent sensitivity in NO2 and NH3 detection isattributed to the formation of hydrogen bonds between thesegas molecules and a large number of oxygenated functionalgroups including −OH, SO3
−, and −NH2 on the polymer
Table 1. Comparison between Different NO2 Sensing Materials in Terms of LOD, Response, Stretchability, and Transparencya
aThe responses refer to ΔR/R0 (%) or ΔG/G0 (%) in this table.
Figure 5. NH3 detection characteristics. (a) Relative resistancechange of the DN sensor vs time upon exposure to 5−50 ppm NH3.(b) Quantitative response vs NH3 concentration. (c) Dynamicresponse of the DN sensor to 2 ppm NH3. (d) Dynamic responses ofthe DN sensor to 40 ppm NH3 at 0 and 50% strain, respectively. Theresponse and recovery time were derived to be 22.5 and 13 s,respectively, from the dynamic response curve at 50% strain.
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chains of PAM and carrageenan. The formation of hydrogenbonds promoted the adsorption of NO2 and NH3 on thehydrogels. The abundant −OH groups on Gly moleculesfurther enhanced the sensitivity of DN-Gly sensor. In additionto hydrogen bond, other physical interactions such as van derWaals force also existed. These weak physical interactions ledto a reversible sensing process. Upon adsorption, the gasmolecules dissolved in water or Gly−water binary solvent. ForNO2 detection, some NO2 molecules reacted with water,producing HNO3 and NO. The unreacted NO2 and newlygenerated NO molecules blocked the diffusion pathways of K+
and Cl− ions, leading to the decreased charge carrier mobilityand increased resistance of hydrogel. On the contrary, whenthe NO2 concentration suddenly decreased in the atmosphere,the dissolved NO2 and NO molecules left the hydrogel anddiffused into the air because of the concentration difference.Because the blocking effect was removed, the resistance ofhydrogel recovered. As oxygen is not involved in the NO2sensing process, this NO2 sensor may also work in anaerobicenvironments.3 This is advantageous over traditional metaloxide chemiresistors, which must be operated in the presenceof oxygen.3 For NH3 detection, some NH3 molecules may beoxidized to NO2 molecules.53 The unreacted NH3 moleculestogether with newly generated NO2 molecules impeded thetransport of ions in the hydrogel, leading to an elevatedresistance level as well.The selectivity of the gas sensors was investigated by
exposing them to common interference gases, including 100ppm CO2, saturated ethanol, methanol, toluene, and acetonevapors and 72% RH (Figures 6b and S12). The magnitude ofresponses of this DN hydrogel sensor toward theseinterference gaseous chemicals was less than 7.5%, which wasmuch smaller than that to NO2 (100% for 2 ppm NO2) andNH3 (50.4% for 50 ppm NH3), indicating the good selectivity.Because of the gas blocking effect, the sensor exhibited positiveresistance variation toward all other measured gases except
water. Water molecules could increase the volume of solventupon adsorption and condensation and thus provide morespace to facilitate the ion transport. Hence, the resistance ofhydrogel shifted negatively when exposed to 72% RH (FigureS12f).54 The response of the DN hydrogel sensor to 72% RH(7.5%) is much smaller than that to 2 ppm NO2 (100%),indicating the good selectivity relative to humidity. To furthereliminate the influence of humidity, the DN-Gly hydrogelbased gas sensor may be encapsulated by a porous andhydrophobic membrane in future work. This kind ofmembrane allows for selective passage of NO2/NH3 gas byblocking the passage of interference gaseous chemicals, likewater.55,56 Thus, it is widely utilized in amperometric gassensors.55
The stability is vital for the life time and accuracy of apractical gas sensor.39 The water evaporation inducedinstability is a common concern for many hydrogel-baseddevices.26 Here, we exploited chemically modified DN-Glyhydrogel to improve the stability and thus prolong the life timeof hydrogel-based sensors significantly (Figure 6c). Specifically,the DN hydrogel became nonconductive after 72 h andtherefore could not be employed as a chemiresistor thereafterif without rehydration. Although rehydration of the DNhydrogel can restore its conductivity and gas-sensing ability insome degree, the gas-sensing performance degraded withprolonged time. In contrast, the DN-Gly sensor did not showappreciable degradation in its sensing performance even after 9months. The remarkably improved stability of the DN-Glysensor is attributed to the formation of hydrogen bondsbetween the hygroscopic Gly molecules and water molecules,which boosted the water retention capability of the DN-Glyhydrogel (Figure 6d).Recently, ingestible sensing capsules have attracted increas-
ing attention as they allow for the gathering of a wealth ofinvaluable information related to our health, nutrition, andphysical state in a noninvasive and cost-effective manner.7,8,57
Figure 6. (a) Scheme illustrating the working mechanism of the NO2 sensor. (b) Comparison of the quantitative responses of the DN and DN-Glysensors to different gaseous chemicals, including 100 ppm CO2, saturated ethanol, methanol, toluene, and acetone vapors, 72% RH, 50 ppm NH3,and 2 ppm NO2. (c) Plots of the quantitative responses to 2 ppm NO2 vs service time for the three different kinds of sensors, which are DN sensorswithout (black) and with (red) rehydration and DN-Gly sensor without rehydration (blue). The DN-Gly sensors showed stable response to NO2within 9 months, demonstrating remarkably enhanced stability compared with DN sensors. (d) Schematic showing the excellent water retentionability of DN-Gly sensor is attributed to the formation of hydrogen bonds between Gly and water molecules. Meanwhile, the enhanced sensitivityof DN-Gly to NO2 is ascribed to the formation of hydrogen bonds between Gly and NO2 molecules.
ACS Applied Materials & Interfaces Research Article
Various important intestinal gases including CO2, H2, andmethane (CH4) have been monitored by the ingestible sensorin situ and in real time to understand the functionality of thegut in the digestion process.8 However, NO2, an important gutneurotransmitter, has yet to be monitored in situ. Attributingto the exceptional flexibility, biocompatibility, and highsensitivity, the ionic hydrogel-based gas sensors may beintegrated in ingestible sensing capsules to monitor theintestinal NOx gas in future work.
4. CONCLUSIONSIn summary, we have successfully fabricated ultrastretchablegas sensors based on an ionic conductive DN hydrogel, whichwas found to be highly sensitive to NO2 and NH3 gases atroom temperature. The DN-Gly hydrogel sensors exhibit highsensitivity (78.5 ppm−1), selectivity, linearity, and exceptionallylow theoretical LOD (1.2 ppb) in NO2 detection. Theexcellent sensing properties are attributed to the interactionbetween gas molecules and enormous oxygenated functionalgroups covered on the hydrogel. The gas-sensing mechanism isdominated by the blocking effect brought by the dissolved gasmolecules on the movement of conductive ions. This isdifferent from conventional charge-transfer and electrondepletion mechanisms that occurred on traditional gas-sensingmaterials.45 The room-temperature sensing operation bringslow energy consumption and thermal safety.15 In addition tosuperior gas-sensing performance in comparison withpreviously reported gas-sensing materials, this ionic hydrogelprovides unique advantages of extreme mechanical deform-ability and high transparency. For example, this hydrogel canbe elongated up to 1200% strain, which is 24 times higher thanthe maximal strain (50%) achieved by the state-of-the-art gas-sensing system.11,21 Furthermore, the mechanical deformationsdid not exert a negative effect on the gas-sensing performance.Note that the introduction of Gly molecules in the solvent ofhydrogel via a facile solvent replacement strategy not onlysignificantly enhanced the water retention ability and stabilityof gas sensors, but also boosted the NO2 sensitivity 1.68 times.The response of the DN-Gly hydrogel to NO2 did notdeteriorate even after 9 months. This work may open the doorto the fabrication of highly deformable and high-performancegas sensors using a family of ionic conductive hydrogels.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b17437.
XPS spectra and elemental analysis of DN hydrogel;SEM image of the DN hydrogel; digital imagesdisplaying the DN and DN-Gly hydrogels stretched todifferent strains; photographs showing the deformability,conductance, and water retention capability of the DNhydrogel; investigation of the electromechanical prop-erty of the DN hydrogel; response curves of the DN andDN-Gly hydrogel sensors to NO2 with differentconcentrations; plots of the signal recovery percentagesof the DN and DN-Gly sensors versus NO2 concen-trations; calculation of sensitivity, noise level and LOD;comparison between different NH3 sensing materials interms of response, stretchability, and transparency; anddynamic responses of the DN hydrogel sensor todifferent gaseous chemicals (PDF)
■ ACKNOWLEDGMENTSJ.W. acknowledges the financial support from the NationalNatural Science Foundation of China (61801525) and theGuangdong Natural Science Funds grant (2018A030313400).
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