Surface Acoustic Wave DMMP Gas Sensor with a Porous Graphene/PVDF Molecularly Imprinted Sensing Membrane

micromachines

Article Surface Acoustic Wave DMMP Gas Sensor with a Porous Graphene/PVDF Molecularly Imprinted Sensing Membrane

Sheng Xu 1 , Rui Zhang 1,2, Junpeng Cui 1, Tao Liu 1, Xiuli Sui 1, Meng Han 3, Fu Zheng 4 and Xiaoguang Hu 1,*

1 School of Software and Communication, Tianjin Sino-German University of Applied Sciences, Tianjin 300350, China; [email protected] (S.X.); [email protected] (R.Z.); [email protected] (J.C.); [email protected] (T.L.); [email protected] (X.S.) 2 School of Electrical and Information Engineering, Tianjin University, Tianjin 300072, China 3 Deepinfar Ocean Technology Co., Ltd., Building #28, Tianjin Binhai Innovation Park, Tianjin 300000, China; [email protected] 4 Tianjin Key Laboratory of Film Electronic and Communicate Devices, School of Electrical and Electronic Engineering, Tianjin University of Technology, Tianjin 300384, China; [email protected] * Correspondence: [email protected]

Abstract: In this paper, surface acoustic wave (SAW) sensors containing porous graphene/PVDF (polyvinylidene fluoride) molecularly imprinted sensitive membrane for DMMP gas detection were investigated. A 433 MHz ST-cut quartz SAW resonator was used to convert gas concentration changes into frequency shifts by the sensors. The porous graphene/PVDF film was fabricated on the sensor’s surface by using the tape-casting method. DMMP molecules were adsorbed on the porous structure sensing film prepared by the 2-step method to achieve the specific recognition effect. The −1 sensitivity of the sensor could reach −1.407 kHz·ppm . The response time and recovery time of the SAW sensor with porous graphene/PVDF sensing membrane were about 4.5 s and 5.8 s at the

Citation: Xu, S.; Zhang, R.; Cui, J.; concentration of 10 ppm, respectively. The sensor has good anti-interference ability to most gases in Liu, T.; Sui, X.; Han, M.; Zheng, F.; the air. Hu, X. Surface Acoustic Wave DMMP Gas Sensor with a Porous Keywords: surface acoustic wave (SAW); molecularly imprinted; three-dimensional architec- Graphene/PVDF Molecularly ture graphene Imprinted Sensing Membrane. Micromachines 2021, 12, 552. https:// doi.org/10.3390/mi12050552 1. Introduction Academic Editor: Arezoo Emadi Chemical warfare agents (CWAs) [1] are fast-acting lethal compounds even at low concentration levels. It is necessary to develop CWAs detection technology with high sensi- Received: 15 April 2021 tivity, good selectivity, strong anti-interference ability, fast response and compatibility with Accepted: 10 May 2021 Published: 12 May 2021 the current Internet of things technology. Nerve gases [2] are the most dangerous agents of chemical warfare and mass destruction, can cause irreversible damage to the nervous

Publisher’s Note: MDPI stays neutral system within seconds and are fatal if exposure occurs for even a few minutes. Therefore, it with regard to jurisdictional claims in is very dangerous for researchers to directly study such gases’ sensing characteristics in the published maps and institutional affil- laboratory. Almost all studies examine simulants, which have no toxicity or less toxicity, iations. but similar functional groups, structures and properties as nerve gases. DMMP [3,4] is a well-known nerve agent simulator, specifically of Sarin. As a new and efficient recognition technology, molecularly imprinted technology [5,6] integrates materials science, polymer science, chemical engineering, biochemistry and other disciplines and can recognize specific analytes. Due to the characteristics of predeter- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. mination, recognition and practicability, it has been widely used in many fields, such as This article is an open access article chromatographic separation [7,8], solid-phase extraction [9,10], biomimetic sensing [11], distributed under the terms and enzyme catalysis [12], clinical drug analysis [13] and so on. In addition, sensing films conditions of the Creative Commons prepared by molecularly imprinted technology can also be integrated with electrochemical Attribution (CC BY) license (https:// sensors and microelectronic sensors for detecting trace analytes. creativecommons.org/licenses/by/ Recently, surface acoustic wave (SAW) sensors [14–16] have been extensively used in 4.0/). liquid [17,18] and gas [19–21] detection because of their low cost, high precision, miniatur-

Micromachines 2021, 12, 552. https://doi.org/10.3390/mi12050552 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, x FOR PEER REVIEW 2 of 10

Micromachines 2021, 12, 552 2 of 10 Recently, surface acoustic wave (SAW) sensors [14–16] have been extensively used in liquid [17,18] and gas [19–21] detection because of their low cost, high precision, min- iaturization, and compatibility with semiconductor technology. The SAW sensor consists ization, and compatibility with semiconductor technology. The SAW sensor consists of a of a sensing film and a conversion element (SAW resonator or delay line). A sensing film sensing film and a conversion element (SAW resonator or delay line). A sensing film is the is the core of the SAW sensor. After it adsorbs the analyte to be measured, the SAW con- core of the SAW sensor. After it adsorbs the analyte to be measured, the SAW converter verter converts the adsorptions into frequency or phase shifts of the sensor to obtain the converts the adsorptions into frequency or phase shifts of the sensor to obtain the device’s device’s response. Due to the high sensitivity and selectivity, the polymer [22–24], semi- response. Due to the high sensitivity and selectivity, the polymer [22–24], semiconduc- conductor [25,26] and graphene [27,28] sensing films have mostly been used in SAW sen- tor [25,26] andsors. graphene [27,28] sensing films have mostly been used in SAW sensors. In this paper, weIn investigatedthis paper, we a surface investigated acoustic a surface wave (SAW) acoustic sensor wave with (SAW) porous sensor PVDF with porous layer sensing filmPVDF based layer on sensing molecularly film based imprinted on molecularly recognition imprinted technology recognition for simulant technology for sim- DMMP detection.ulant A DMMP 433 MHz detection. SAW one-port A 433 MHz resonator SAW one-port based on resonator ST-quartz based substrate on ST-quartz and substrate aluminum interdigitaland aluminum transducers interdigital (Al-IDTs) transducers was selected (Al-IDTs) to convertwas selected the changeto convert of gasthe change of concentration intogas concentration frequency shift. into frequency shift.

2. Materials and2. Materials Methods and Methods 2.1. Preparation of the SAW Sensor with a Porous Graphene/PVDF Sensing Layer 2.1. Preparation of the SAW Sensor with a Porous Graphene/PVDF Sensing Layer Figure1 is a schematic diagram of the fabrication process for the one-port SAW Figure 1 is a schematic diagram of the fabrication process for the one-port SAW res- · −1 resonator as a basedonator device. as a based The substratedevice. The was substrate ST-quartz was with ST-quartz a SAW with velocity a SAW of 3158 velocity m s of. 3158 m·s−1. The aluminumThe IDTs aluminum were fabricated IDTs were using fabricated a standard using UV-photolithography a standard UV-photolithography process. Each process. Each resonator consistedresonator of an consisted IDT with of 160 an pairsIDT with of electrodes 160 pairs of and electrodes two reflectors and two on reflectors both sides on both sides of IDTs with 270of IDTs electrodes. with 270 The electrodes. pitch and The acoustic pitch and apertures acoustic were apertures 1.8 µm were and 1.8 172 μmµm, and 172 μm, respectively. Moreover,respectively. the Moreover, resonant frequencythe resonant of freque the SAWncy of devices the SAW was devices approximately was approximately 434.9 MHz. The434.9 sensing MHz. films The were sensing coated films on were both coated reflectors’ on both IDTs reflectors’ on the SAW IDTs resonator on the SAW to resonator reduce the propagationto reduce loss.the propagation loss.

FigureFigure 1.1. SchematicSchematic diagram diagram of of the the fabrication fabrication process process for for the the surface surface acoustic acoustic wave wave (SAW) (SAW) sensor. sensor.

The porous graphene/PVDFThe porous graphene/PVDF sensing films sensing were fabricated films were as fabricated the sensing as the membrane. sensing membrane. First, 0.2 mmolFirst, of citric 0.2 acidmmol monohydrate of citric acid (CAM), monohydrate 0.02 mL (CAM), of dimethyl 0.02 mL methyl of dimethyl phosphonate methyl phospho- (DMMP), 8 mgnate of graphene(DMMP), 8 and mg of 0.6 graphene g of polyvinylidene and 0.6 g of polyvinylidene fluoride (PVDF) fluoride powder (PVDF) was powder was dispersed in 10dispersed mL of N, in N-dimethylformamide 10 mL of N, N-dimethylformamid (DMF) withe (DMF) the assistance with the of assistance ultrasonic of ultrasonic dispersion to formdispersion CAM/DMMP/PVDF/graphene/DMF to form CAM/DMMP/PVDF/graphene/DMF polymeric polymeric precursor solution.precursor solution. Then, 1 mL polymericThen, 1 mL precursor polymeric solution precursor was solution coated on was the coated SAW on resonator the SAW to resonator obtain the to obtain the porous film usingporous a tape-casting film using a method tape-casting [29], followedmethod [29], by afollowed drying treatmentby a drying at treatment 85 ◦C for at 85 °C for 30 min in an oven.30 min in an oven. The preparationThe process preparation of the process sensing of the film sensing on thefilm SAW on the resonator SAW resonator is shown is shown in in Figure Figure2a . The2a. graphene The graphene and recrystallized and recrystallized CAM CAM micro/nanoparticles micro/nanoparticles were were uniformly uniformly dispersed dispersed in theincured the cured graphene/PVDF/DMMP graphene/PVDF/DMMP membrane membrane (Figure (Figure2 (a1)).2(a1)). Next, Next, thethe SAW device was immersed in the concentrated NaHCO3 (0.01 mol/L) aqueous solution for 1.5 h at 25 °C. device was immersed in the concentrated NaHCO3 (0.01 mol/L) aqueous solution for 1.5 h at 25 ◦C.Due to to polymer polymer swelling, swelling, the the CAM CAM nanoparticles nanoparticles near nearthe surface the surface of the ofsensing the film will react with NaHCO3 at the interface between PVDF and graphene. Then, micro holes were sensing film will react with NaHCO3 at the interface between PVDF and graphene. Then, formed with the emission of CO2. At the same time, DMMP on the inner wall of the micro- micro holes were formed with the emission of CO2. At the same time, DMMP on the inner hole and the surface of the film was discharged with CO2 (Figure 2(a2)). Thus, the porous wall of the micro-hole and the surface of the film was discharged with CO2 (Figure2(a2)). Thus, the porous graphene/PVDF sensing films with DMMP molecular imprinting was formed. Finally, the devices were rinsed with ethanol and water to give the SAW sensors (Figure2(a3)). Micromachines 2021, 12, x FOR PEER REVIEW 3 of 10

Micromachines 2021, 12, 552 3 of 10 graphene/PVDF sensing films with DMMP molecular imprinting was formed. Finally, the devices were rinsed with ethanol and water to give the SAW sensors (Figure 2(a3)).

FigureFigure 2. 2. (a()a )Preparation Preparation process process diagram diagram of of the the sensing sensing film, ﬁlm, ( b) gas adsorption andand hydrogenhydrogen bond bondformation formation diagram. diagram.

2.2.2.2. Characterization Characterization and and Measurement Measurement TheThe morphologies morphologies of of devices devices were were characte characterizedrized using using scanning scanning electron electron microscopy microscopy (FE-SEM;(FE-SEM; Merlin Merlin Compact, Compact, Zeiss, Zeiss, Oberkochen, Oberkochen, Germany) Germany) and and atomic atomic force force microscopy microscopy (AFM;(AFM; Agilent Agilent 5500, 5500, Agilent Agilent Technologies, Technologies, Santa Santa Clara, Clara, CA, CA, USA). USA). The The crystalline crystalline phases phases ofof materials materials were were analyzed analyzed by by X-ray X-ray diffraction (XRD; D/max-2500/PC, Rigaku, Rigaku, Tokyo, Tokyo, Japan).Japan). The The responses responses of of the the SAW SAW resonators resonators and and sensors sensors were were measured measured by by a a network network analyzeranalyzer (ENA-E5071C, (ENA-E5071C, Keysight, Keysight, Santa Santa Rosa, Rosa, CA, CA, USA). USA). ForFor sensing sensing vapors, vapors, a aDMMP DMMP gas gas delivery delivery and and SAW SAW measurement measurement system system was was de- de- velopedveloped as as Figure Figure 3,3, which which was was included included (1) (1) standard standard N2 N2 source, source, (2) (2) massflow massflow controller controller (MFC)(MFC) for for the mixturemixture of of N2/ N2/ DMMP, DMMP, (3) (3) MFC MFC for for N2, N2, (4) heating(4) heating device, device, (5) DMMP (5) DMMP source, source,(6) prepared (6) prepared SAW sensor SAW insensor a chamber, in a chamber, and (7) networkand (7) network analyzer. analyzer. The nitrogen The nitrogen from the fromsource the was source divided was divided into two into paths, two onepaths, carrying one carrying the DMMP the DMMP and the and other the onlyother carrier only to the sensor surface. Both were precisely controlled by MFC. The gas flow rate of the carrier to the sensor surface. Both were precisely controlled by MFC. The gas flow rate of N2/DMMP path was maintained at 250 mL·min−1, and the flow rate of the pure nitrogen the N2/DMMP path was maintained at 250 mL·min−1, and the flow rate of the pure nitro- path can be adjusted. The flask containing DMMP is heated to 30 ◦C or 50 ◦C. Using the gen path can be adjusted. The flask containing DMMP is heated to 30 °C or 50 °C. Using molar volume at different temperatures and flow differences between two gas channels, the molar volume at different temperatures and flow differences between two gas chan- the concentration of the DMMP scan be achieved [30,31]. The SAW sensor was placed in a nels, the concentration of the DMMP scan be achieved [30,31]. The SAW sensor was placed sealed chamber with a volume of about 500 mL. A pump was connected to the chamber’s in a sealed chamber with a volume of about 500 mL. A pump was connected to the cham- outlet to realize the weak negative pressure in the test system, which could ensure the ber’s outlet to realize the weak negative pressure in the test system, which could ensure dynamic circulation of gas and the tightness of the system. Before testing, the SAW sensor, the dynamic circulation of gas and the tightness of the system. Before testing,◦ the SAW fixed on the test fixture, was placed in an N2 environment and preheated to 80 C for 1 h to sensor, fixed on the test fixture, was placed in an N2 environment and preheated to 80 °C remove any gas absorbed on the sensing film. for 1 h to remove any gas absorbed on the sensing film.

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Figure 3. (a) Preparation process diagram of the sensing film, (b) DMMP measurement system.

FigureFigure 3.3.( (aa)) PreparationPreparation processprocess diagram diagram of of the the sensing sensing film, film, ( b(b)) DMMP DMMP measurement measurement system. system. 3. Results and Discussion 3.3.3.1. ResultsResults Characterization andand DiscussionDiscussion and Measurement Results 3.1. Characterization and Measurement Results 3.1. CharacterizationFigure 4a is theand Measurementimage of a one-portResults SAW resonator fabricated by a standard inte- FigureFigure4 a4a is is the the image image of aof one-port a one-port SAW SAW resonator resonator fabricated fabricated by a standard by a standard integrated inte- circuitgrated manufacturing circuit manufacturing method. Figure method.4b,c shows Figure the 4b,c OM shows and SEM the imagesOM and of SEM IDTs, images which of IDTs, gratedwhich circuit confirm manufacturing that the IDTs method. finger Figure width 4b,c and shows gap are the equal OM and to 1.6SEM um, images respectively. of IDTs, Figure confirmwhich confirm that the that IDTs the finger IDTs widthfinger andwidth gap and are gap equal are toequal 1.6 um,to 1.6 respectively. um, respectively. Figure Figure4d is 4d is the cross-sectional SEM images of IDTs, showing that the thickness of aluminum the4d cross-sectionalis the cross-sectional SEM images SEM ofimages IDTs, showingof IDTs, thatshowing the thickness that the ofthickness aluminum of electrodesaluminum iselectrodes about 108.5 is nm. about 108.5 nm. electrodes is about 108.5 nm.

Figure 4. (a) One-port SAW resonators, (b) Optical microphotograph image of electrodes on bare SAW, (c) top view and (d) cross-sectional SEM images of interdigital transducers. FigureFigure 4. 4.(a ()a One-port) One-port SAW SAW resonators, resonators, (b) Optical (b) Optical microphotograph microphotograph image image of electrodes of electrodes on bare on bare SAW,SAW, (Figurec )(c top) top view 5a view is and the and ( dSEM) cross-sectional(d) imagescross-sectional of SEMgraphene/PVDF/CAM/DMMP imagesSEM images of interdigital of interdigital transducers. membrane,transducers. showing that the graphene and recrystallized CAM micro/nanoparticles were uniformly dispersed Figure5a is the SEM images of graphene/PVDF/CAM/DMMP membrane, showing in the Figurefilm. After 5a isimmersion the SEM in images the NaHCO of graphene/PVDF/CAM/DMMP3 solution, the recrystallized CAM membrane, precipitated showing thatinthat the the the form graphene graphene of carbon and and dioxide recrystallized recrystallized and formed CAM CAM micro/nanoparticlesa porous micro/nanoparticles structure on were the surface uniformly were of uniformly the dispersed sensing dispersed in the film. After immersion in the NaHCO3 solution, the recrystallized CAM precipitated filmin the (Figure film. 5b).After The immersion thickness of in the the sensing NaHCO film3 solution, was about the 2.47 recrystallized μm (Figure 5c). CAM Figure precipitated 6 in the form of carbon dioxide and formed a porous structure on the surface of the sensing shows AFM images of the Al IDTs, PVDF/DMMP film and porous graphene/PVDF sens- filmin the (Figure form5b). of Thecarbon thickness dioxide of the and sensing formed film a po wasrous about structure 2.47 µm on (Figure the surface5c). Figure of the6 sensing ing film. The roughness (RMS) of the IDT electrodes on one-port SAW resonator, showsfilm (Figure AFM images 5b). The of the thickness Al IDTs, PVDF/DMMP of the sensing film film and was porous about graphene/PVDF 2.47 μm (Figure sensing 5c). Figure 6 PVDF/DMMP /SAW, and porous graphene/PVDF /SAW was 1.14 nm, 10.9 nm and 119 film.shows The AFM roughness images (RMS) of the of the Al IDT IDTs, electrodes PVDF/DMMP on one-port film SAW and resonator, porous PVDF/DMMPgraphene/PVDF sens- nm, respectively. With the formation of the porous structure sensing film, the roughness /SAW,ing film. and porousThe roughness graphene/PVDF (RMS) /SAW of the was 1.14IDT nm,electrodes 10.9 nm andon 119one-port nm, respectively. SAW resonator, of the sensor surface increased significantly, which led to the S11 of the device decreasing WithPVDF/DMMP the formation /SAW, of the porousand porous structure graphene/PVDF sensing film, the /SAW roughness was of 1.14 the sensornm, 10.9 surface nm and 119 from −8.21 dB to −2.68 dB (Figure 7a). Moreover, after the sensing film was fabricated on increasednm, respectively. significantly, With which the formation led to the S of11 ofthe the porous device structure decreasing sensing from − film,8.21 dBthe toroughness of the sensor surface increased significantly, which led to the S11 of the device decreasing from −8.21 dB to −2.68 dB (Figure 7a). Moreover, after the sensing film was fabricated on

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−2.68 dB (Figure7a). Moreover, after the sensing ﬁlm was fabricated on the reﬂective grating,the reflective the resonant grating, frequency the resonant of the frequenc sensory showed of the sensor a negative showed frequency a negative shift frequency of about 294theshift kHzreflective of about dueto grating,294 the kHz mass-loading the due resonant to the effectmass-loading frequenc [32]. y of effect the sensor [32]. showed a negative frequency shift of about 294 kHz due to the mass-loading effect [32].

FigureFigure 5.5. (a()a SEM) SEM image image of graphene/PVDF/CAM/DMMP of graphene/PVDF/CAM/DMMP film, (b film,) SEM (b image) SEM of image porous of gra- porous Figure 5. (a) SEM image of graphene/PVDF/CAM/DMMP film, (b) SEM image of porous gra- graphene/PVDFFigurephene/PVDF 5. (a) SEM film, film, image (c) thickness (c) of thickness graphene/PVDF/CAM/DMMP of graphene/PVDF of graphene/PVDF sensing sensing film, film. film.( b) SEM image of porous graphene/PVDF film, (c) thickness of graphene/PVDF sensing film.

Figure 6. AFM image of (a) Al IDTs, (b) PVDF film and (c) porous graphene/PVDF sensing film. FigureFigure 6.6. AFMAFM imageimage ofof ((aa)) AlAl IDTs,IDTs, ((bb)) PVDFPVDF ﬁlmfilm and and ( (cc)) porous porous graphene/PVDF graphene/PVDF sensingsensing ﬁlm.film.

Figure 7. (a) Frequency characteristics of the SAW resonators (red) and SAW sensors with sensing FigureFigurefilms (blue), 7.7. ((aa)) FrequencyFrequency(b) TCF of characteristicsthecharacteristics SAW resonators of the (red)SAW and resonators SAW sensors (red) and with SAW sensing sensors films with (blue). sensing sensing films (blue), (b) TCF of the SAW resonators (red) and SAW sensors with sensing films (blue). filmsfilms (blue),(blue), ((bb)) TCFTCF ofof thethe SAWSAW resonators resonators (red) (red) and and SAW SAW sensors sensors with with sensing sensing films films (blue). (blue). The temperature coefficient of frequency (TCF), which is an important parameter to characterizeTheThe temperature the temperature coefficient coefficient stability of of frequency frequency of the SAW (TCF), (TCF), sensor, which which was is isobserved an an important important in the parameter range parameter of 25– to characterize the temperature stability of the SAW sensor, was observed in the range of 25– tocharacterize100 characterize °C. As shown the the temperature in temperature Figure 7b, stability the stability preparation of the of SAW the SAWof sensor, the sensor,sensing was observed was film observed did innot the lead in range the to signifi- rangeof 25– 100 °C. As◦ shown in Figure 7b, the preparation of the sensing film did not lead to signifi- of100cant25–100 °C. changes As Cshown. in As the shownin temperature Figure in 7b, Figure the stability7 preparationb, the of preparation the device. of the sensingThe of the TCF sensingfilm of the did bare filmnot SAWlead did nottoresonator signifi- lead tocantand significant changesSAW sensor in changes the with temperature inporous the temperature graphene/PVDF stability of the stability device. sensing of The the membrane TCF device. of the Thewas bare TCF − SAW1.4305 of resonator the ppm/°C bare and SAW sensor with porous graphene/PVDF sensing membrane was −1.4305 ppm/°C SAWandand SAW− resonator1.5624 sensor ppm/°C. and with SAW porous sensor graphene/PVDF with porous graphene/PVDF sensing membrane sensing was membrane−1.4305 ppm/°C was ◦ ◦ −and1.4305 −1.5624 ppm/ ppm/°C.C and −1.5624 ppm/ C. 3.2. Static Responses of DMMP Sensors 3.2.3.2. Static Responses ofof DMMPDMMP SensorsSensors 3.2. StaticFigure Responses 8 shows of the DMMP static responsesSensors of the SAW gas sensor with different DMMP con- FigureFigure8 8 showsshows the static static responses responses of of the the SAW SAW gas gas sensor sensor with with different different DMMP DMMP con- centrationsFigure 8in shows the range the static from responses 0 to 10 ppm. of the The SAW sensor gas sensorwas tested with more different than DMMP 10 times con- at concentrations in the range from 0 to 10 ppm. The sensor was tested more than 10 times at centrationseach concentration in the range value. from The 0 raw to 10 data, ppm. statis Thetical sensor analysis was andtested residual more thanplots 10 are times shown at eacheach concentrationconcentration value. value. The The raw raw data, data, statistical statistical analysis analysis and and residual residual plots plots are are shown shown in eachin Table concentration S1 and Figure value. S2. The The raw response data, statis of thetical SAW analysis sensor and decreased residual plotsrapidly are with shown in- Tablein Table S1 andS1 and Figure Figure S2. The S2. responseThe response of the of SAW the sensorSAW sensor decreased decreased rapidly rapidly with increasing with in- increasing Table DMMPS1 and Figureconcentration S2. The in response the range of from the 0SAW to 10 sensor ppm. The decreased sensitivity rapidly of the with sensor, in- creasingwhich can DMMP be obtained concentration from the in slope the range of the from linear 0 tofitting 10 ppm. (solid The black sensitivity line in Figure of the sensor,8), was which can be obtained from the slope of the linear fitting (solid black line in Figure 8), was

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DMMP concentration in the range from 0 to 10 ppm. The sensitivity of the sensor, which can be obtained from the slope of the linear ﬁtting (solid black line in Figure8), was about −about1.407 kHz/ppm−1.407 kHz/ppm (R = 0.99346). (R = 0.99346). The linear-regression The linear-regression equation, parameters, equation, statistics parameters, and statistics R-valueand R-value are shown are inshown Figure in S3. Figure S3.

FigureFigure 8. 8.Responses Responses of the of sensorthe sensor to different to different concentrations concentrations of DMMP of in DMMP nitrogen. in nitrogen. 3.3. Dynamic Responses and Sensing Mechanism 3.3. TheDynamic frequency Responses shift of and the Sensing SAW sensors Mechanism based on the electric loading effect can be correlatedThe withfrequency the sheet shift conductivity of the SAW and the sensors surface capacitancebased on perthe lengthelectric of theloading sensing effect can be filmcorrelated using the with following the sheet relationship conductivity [33,34]: and the surface capacitance per length of the sensing film using the following relationship" [33,34]: # ∆ f K2 σ2 = − ∆ S 2 2 2 2 2 f0 ΔfK2 σS + v0CS σ =− ΔS f 2 σ 222+ vC 0 2 SS0 where v0 is the effective SAW velocity of the device, K is the electromechanical coupling coefficient of the piezoelectric substrate, σS is the sheet conductivity,2 and CS is the surface where v 0 is the effective SAW velocity of the device, K is the electromechanical cou- capacitance per length. In the process of adsorbing DMMP gas, CS, which is determined by σ thepling structure coefficient of the device,of the remainspiezoelectric constant. substrate, The porous grapheneS is the andsheet PVDF conductivity, film could and C S is both absorb DMMP gas and resulted in a decrease of resonant frequency due to the shift of the surface capacitance per length. In the process of adsorbing DMMP gas, C S , which is the sheet conductivity. The sheet conductivity of porous graphene/PVDF film was about 6.1determined× 10−3 S/cm by under the structure pure N2 and of 2.7the× device,10−3 S/cm remains under 10constant. ppm DMMP The gas.porous graphene and PVDFAs shownfilm could in Figure both9, theabsorb dynamic DMMP responses gas and of the resulted prepared in SAW a decrease sensors with of resonant porous frequency PVDFdue sensingto the filmshift to fastof DMMPthe sheet concentration conductivity (0, 3, 5,. 10The ppm) sheet changes conductivity were investigated. of porous gra- Thephene/PVDF blue bubble film was was the DMMP about concentration–time6.1 × 10−3 S/cm under curve. pure The N2 gas and concentration 2.7 × 10−3 was S/cm under 10 determinedppm DMMP by thegas. ratio of nitrogen and DMMP, while 0 ppm is pure nitrogen with equal gas flow. The response time and recovery time were defined as the time required for the As shown in Figure 9, the dynamic responses of the prepared SAW sensors with po- frequency shift to achieve 10% and 90% of the change for the adsorption and desorption processed,rous PVDF respectively sensing film [35]. to It fast can DMMP be seen fromconcentration Figure9 that (0, the 3, response5, 10 ppm) time changes of the were inves- sensortigated. was The clearly blue faster bubble than was the the recovery DMMP time. concen For thetration–time concentration curve. of 3 ppm, The 5gas ppm concentration andwas 10 determined ppm, the response by the timeratio was of nitrogen about 2.9 s,and 3.6 DMMP, s, and 4.5 while s, and 0 the ppm recovery is pure time nitrogen with wasequal about gas 3.7flow. s, 4.9 The s, 5.8response s, respectively. time and When recovery DMMP time gas were was introduced defined as into the the time required sensitivefor the frequency membrane, theshift molecularly to achieve imprinted 10% and sensitive 90% of membrane the change could for quickly the adsorption capture and de- the analyte. Moreover, the hydrogen bond structure between DMMP and PVDF needs a sorption processed, respectively [35]. It can be seen from Figure 9 that the response time certain gas flux to desorb when the gas concentration decreased. Figure2b shows the gas adsorptionof the sensor and hydrogenwas clearly bond faster formation than diagram.the recovery In the time. multilayer For the sensing concentration film, PVDF of 3 ppm, 5 wasppm used and to 10 protect ppm, the the metal response interdigital time electrode, was about and 2.9 porous s, 3.6 graphene s, and 4.5 could s, and effectively the recovery time increasewas about the specific 3.7 s, 4.9 surface s, 5.8 area s, respectively. of the sensing film When and DMMP increased gas the adsorptionwas introduced of the gas into the sensi- totive be measured.membrane, When the the molecularly sensitive membrane imprinted adsorbs sensitive DMMP membrane gas, DMMP could molecules quickly pass capture the analyte. Moreover, the hydrogen bond structure between DMMP and PVDF needs a certain gas flux to desorb when the gas concentration decreased. Figure 2b shows the gas adsorption and hydrogen bond formation diagram. In the multilayer sensing film, PVDF was used to protect the metal interdigital electrode, and porous graphene could effectively increase the specific surface area of the sensing film and increased the adsorption of the gas to be measured. When the sensitive membrane adsorbs DMMP gas, DMMP molecules pass through the porous structure, the O atom in the P = O group of DMMP and the H atom of PVDF form an H-bond [36] structure, which achieves specific recognition. The

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synergetic effect between PVDF and porous graphene was proven by the enhancement of sensitivity. through the porous structure, the O atom in the P = O group of DMMP and the H atom of PVDFsynergetic form an effect H-bond between [36] structure, PVDF which and porous achieves graphene speciﬁc recognition. was proven The synergetic by the enhancement of effectsensitivity. between PVDF and porous graphene was proven by the enhancement of sensitivity.

Figure 9. Dynamic response of the graphene/PVDF SAW sensor for DMMP.

3.4. Selectivity and Stability Figure 9. 9.Dynamic Dynamic response response of the of graphene/PVDF the graphene/PVDF SAW sensor SAW for DMMP. sensor for DMMP. 3.4. SelectivityFigure and10 shows Stability the selectivity of the SAW DMMP sensor. The SAW sensor did not have3.4. FigureSelectivity a marked 10 shows and response theStability selectivity to pure of N the2, O SAW2 and DMMP Ar. Although sensor. The the SAW responses sensor did could not be interfaced by pure CO2 and ethanol gases, it was also far less than DMMP of 3 ppm. These results have aFigure marked 10 response shows to the pure selectivity N2,O2 and Ar. of Althoughthe SAW the DMMP responses sensor. could be The interfaced SAW sensor did not suggestby pure CO that2 and due ethanol to the gases, preparation it was also farof lessmo thanlecularly DMMP imprinted of 3 ppm.These sensitive results membrane, the have a marked response to pure N2, O2 and Ar. Although the responses could be interfaced sensorsuggest had that duestrong to the selectivity preparation for of DMMP. molecularly Furthe imprintedrmore, sensitive as shown membrane, in Figure the 11, the response by pure CO2 and ethanol gases, it was also far less than DMMP of 3 ppm. These results frequenciessensor had strong for selectivitydifferent for DMMP DMMP. concentrations Furthermore, as shown were in recorded Figure 11, thefor response 15 days to verify the frequenciessuggest that for differentdue to the DMMP preparation concentrations of mo werelecularly recorded imprinted for 15 days tosensitive verify the membrane, the stabilitysensor had of of the strongthe sensors. sensors. selectivity When When the for time the DMMP. of time storage ofFurthe increased,storagermore, increased, the as frequency shown the shiftsin frequency Figure of the 11, theshifts response of the porousfrequencies graphene/PVDF graphene/PVDF for different SAW SAW sensorDMMP sensor were concentrations nearly were constant. nearly wereconstant. recorded for 15 days to verify the stability of the sensors. When the time of storage increased, the frequency shifts of the porous graphene/PVDF SAW sensor were nearly constant.

Figure 10. 10.Selectivity Selectivity of the of SAW the SAW DMMP DMMP sensor. sensor.

Figure 10. Selectivity of the SAW DMMP sensor.

MicromachinesMicromachines 20212021, 12, 12, x, 552FOR PEER REVIEW 8 of 10 8 of 10

FigureFigure 11. 11.(a ()a Stability) Stability of the of proposedthe proposed SAW sensor;SAW sensor; (b) frequency (b) frequency response curves response of the curves sensor forof the sensor variousfor various DMMP DMMP concentrations; concentrations; (c) stability (c) of stability the sensor of storedthe sensor at ambient stored conditions. at ambient conditions. 4. Conclusions 4. Conclusions In this paper, a DMMP gas sensor based on the SAW device with a porous graphene/PVDF molecularlyIn this imprinted paper, a sensing DMMP membrane gas sensor was developed.based on the The sensingSAW device membrane with with a porous gra- aphene/PVDF porous structure molecularly could improve imprinted the adsorption sensing capacity membrane of DMMP was gas. developed. Due to the The sensing molecularlymembrane imprintedwith a porous recognition structure technology could usedimpr inove the the sensing adsorption film, the capacity SAW sensor, of DMMP gas. whichDue to had the an molecularly excellent anti-interference imprinted recognition ability, could technology detect DMMP used specifically. in the sensing The film, the sensitivity of the sensor could reach −1.407 kHz·ppm−1. The response time of the SAW sensorSAW sensor, with porous which graphene/PVDF had an excellent molecularly anti-inter imprintedference sensing ability, membrane could detect was faster DMMP specif- thanically. 4.5 The s, and sensitivity the recovery of the time sensor was faster could than reach 5.8 s − at1.407 the concentrationkHz·ppm−1. ofThe 10 response ppm. time of Furthermore,the SAW sensor the developed with porous SAW graphene/PVDF sensor could also molecularly be used for real-timeimprinted and sensing online membrane monitoringwas faster ofthan nerve 4.5 gas. s, and the recovery time was faster than 5.8 s at the concentration of 10 ppm. Furthermore, the developed SAW sensor could also be used for real-time and online Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/mi12050552/s1monitoring of nerve, Figure gas. S1: Responses of the bare SAW and sensor with sensitivity film for TCF, Figure S2: Residual plots for the data of Figure8, Table S1: Statistical Analysis of the ExperimentalSupplementary Data. Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Responses of the bare SAW and sensor with sensitivity film for TCF, Figure S2: Residual plots for Author Contributions: Conceptualization, S.X. and R.Z.; methodology, X.H.; software, X.S. and T.L.; validation,the data of S.X. Figure andJ.C.; 8, Table formal S1: analysis, Statisti R.Z.;cal Analysis investigation, of the M.H.; Experimental resources, X.H.; Data. data curation, S.X.;Author writing—original Contributions: draft Conceptualization, preparation, F.Z., S.X. S.X. and J.C.;and writing—reviewR.Z.; methodology, and editing, X.H.; S.X. software, All X.S. and authors have read and agreed to the published version of the manuscript. T.L.; validation, S.X. and J.C.; formal analysis, R.Z.; investigation, M.H.; resources, X.H.; data cura- Funding:tion, S.X.;This writing—original research was funded draft by preparation, the Natural ScienceF.Z., S.X. Foundation and J.C.; of writing—review Tianjin, grant number and editing, S.X.; 20JCYBJC00280.All authors have read and agreed to the published version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Funding: This research was funded by the Natural Science Foundation of Tianjin, grant number References 20JCYBJC00280. 1. Kloske, M.; Witkiewicz, Z. Novichoks—TheConflicts of Interest: A group The of organophosphorusauthors declare no chemical conflicts warfare of interest. agents. Chemosphere 2019, 221, 672–682. [CrossRef][PubMed] 2. Zhao, S.; Zhu, Y.; Xi, H.; Han, M.; Li, D.; Li, Y.; Zhao, H. 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