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.