sensors Article

Development of a Room Temperature SAW Methane Gas Sensor Incorporating a Supramolecular Cryptophane A Coating Wen Wang 1, *, Haoliang Hu 1 , Xinlu Liu 1 , Shitang He 1 , Yong Pan 2 , Caihong Zhang 3 and Chuan Dong 3 Received: 21 November 2015; Accepted: 3 January 2016; Published: 7 January 2016 Academic Editor: Ha Duong Ngo 1

2 3

*

State Key Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences, No. 21, BeiSiHuan West Road, Beijing 100190, China; [email protected] (H.H.); [email protected] (X.L.); [email protected] (S.H.) State Key Laboratory of NBC Protection for Civilian, Yangfang, Changping District, Beijing 102205, China; [email protected] School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China; [email protected] (C.Z.); [email protected] (C.D.) Correspondence: [email protected]; Tel.: +86-10-8254-7803

Abstract: A new room temperature supra-molecular cryptophane A (CrypA)-coated surface acoustic wave (SAW) sensor for sensing methane gas is presented. The sensor is composed of differential resonator-oscillators, a supra-molecular CrypA coated along the acoustic propagation path, and a frequency signal acquisition module (FSAM). A two-port SAW resonator configuration with low insertion loss, single resonation mode, and high quality factor was designed on a temperature-compensated ST-X quartz substrate, and as the feedback of the differntial oscillators. Prior to development, the coupling of modes (COM) simulation was conducted to predict the device performance. The supramolecular CrypA was synthesized from vanillyl alcohol using a double trimerisation method and deposited onto the SAW propagation path of the sensing resonators via different film deposition methods. Experiential results indicate the CrypA-coated sensor made using a dropping method exhibits higher sensor response compared to the unit prepared by the spinning approach because of the obviously larger surface roughness. Fast response and excellent repeatability were observed in gas sensing experiments, and the estimated detection limit and measured sensitivity are ~0.05% and ~204 Hz/%, respectively. Keywords: surface acoustic wave; methane gas sensor; cryptophane A; room temperature; resonator-oscillator

1. Introduction Mining accidents with heavy casualties caused by methane gas explosions occur frequently, leading to huge economic losses. The most effective way to respond to such an issue is the early detection and monitoring of methane accumulation in mines or landfills. The current approaches for sensing methane gas include gas chromatography, electrochemical, optical, and semiconductor technologies. These techniques differ substantially in their approaches, and each have their own advantages and disadvantages [1–4]. Gas chromatography can perform an accurate quantitative analysis on methane gas, but, it is expensive and unsuitable for in situ monitoring which is essential in most cases [1]. Electrochemical methane gas sensors face greater challenges in real world applications because of the inertness of the methane molecule and their slow response [2]. The main challenge for optical methane sensors is that it is hard to find a suitable light source in the infrared range, and Sensors 2016, 16, 73; doi:10.3390/s16010073

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they also suffer from complicate sensor configurations and humidity interference [3–5]. As for the semiconductor methane gas sensors, their high operation temperature makes them unsuitable in mine Sensors 2016,due 16, 73to the risk of explosions [6–8]. 2 of 11 environments Therefore, the development of smart sensors with high sensitivity, fast response, excellent range, and they also suffer from complicate sensor configurations and humidity interference [3–5]. stability, andthe capable of roommethane temperature operation a necessary for methane gasthem sensing As for semiconductor gas sensors, theiris high operationlink temperature makes and monitoring. The birth of the so-called acoustic wave unsuitable in mine environments due to thesurface risk of explosions [6–8]. (SAW) sensor technology opens up a newTherefore, way for methane gas sensing. By means of sensitive materials with specific selectivity, the development of smart sensors with high sensitivity, fast response, excellent SAW stability, sensors and havecapable some of excellent features such as high-sensitivity, ambient-temperature operation, room temperature operation is a necessary link for methane gas sensing and monitoring. The birth of the so-called surface acoustic wave (SAW) technology opens up [9,10]. a simple packaging requirements, low cost, fast response, small size, sensor and large dynamic range new way for methane sensing. By means ofissensitive materials with specificoscillator selectivity,array, SAW and A typical SAW-based gas gas sensor configuration composed of a differential have some excellent features as high-sensitivity, operation, somesensors sensitive materials deposited insuch sensing channels on ambient-temperature the SAW propagation path ofsimple the SAW packaging requirements, low cost, fast response, small size, and large dynamic range [9,10]. A typical devices. The adsorption of the gas molecules to be analyzed by the sensitive interface modulates SAW-based gas sensor configuration is composed of a differential oscillator array, and some sensitive the SAW propagation properties. The corresponding change in velocity is read out by recording materials deposited in sensing channels on the SAW propagation path of the SAW devices. The the frequency is directly proportional thesensitive gas concentration. Obviously, the SAW adsorptionsignal, of the which gas molecules to be analyzed bytothe interface modulates the SAW device only plays a "quantification" role, while a qualitative assessment of the analyte is completed propagation properties. The corresponding change in velocity is read out by recording the frequency by suitable sensitive films. proportional Unfortunately, is not easy to find a good the sensitive material signal, which is directly to theitgas concentration. Obviously, SAW device onlycandidate plays a "quantification" role, while a qualitative assessment the analyte is completed suitable sensitive and for sensing methane until the supramolecular speciesofcryptophane A (CrypA)bywas synthesized films. Unfortunately, it is not easy to find a good sensitive candidate for cryptophane sensing methane confirmed to show excellent selectivity to methane gas. Asmaterial the smallest of the family, until the supramolecular species cryptophane A (CrypA) was synthesized and confirmed to show Cryp A was utilized as the sensitive interface and exhibits an amazing affinity towards methane gas excellent to methane gas. As thebetween smallest the of the cryptophane family, Cryp A was utilized molecules by selectivity supramolecular interactions host and methane molecules [11,12], which as the sensitive interface and exhibits an amazing affinity towards methane gas molecules by arises from size complementarity and efficient van der Waals interactions. Recently, a quartz crystal supramolecular interactions between the host and methane molecules [11,12], which arises from size microbalance (QCM)-based methane gas sensor configuration was presented employing CrypA as the complementarity and efficient van der Waals interactions. Recently, a quartz crystal microbalance sensitive interface methane [13,14], and superior selectivity,was a fast response and a low detection of 0.05% (QCM)-based gas sensor configuration presented employing CrypA as thelimit sensitive were interface achieved[13,14], at room However, QCM-based methane gaslimit sensor easilywere reaches andtemperature. superior selectivity, a fastthe response and a low detection of 0.05% saturation when applied methaneHowever, concentration is over 0.2%,methane that thisgas makes it difficult to meet the achieved at the room temperature. the QCM-based sensor easily reaches when of theunderground applied methane concentration is over 0.2%, thatpoint this makes it difficult to meet actualsaturation requirements methane gas alarms (the alarm methane gas concentration the actual requirements of underground methane gas alarms (the alarm point methane gas is usually 1%). concentration is usually 1%). Hence, to address this issue, the main contribution in this work is to develop a new methane to addressSAW this issue, the mainand contribution in this work is try to develop a newfast methane gas gas sensorHence, incorporating technology CrypA films, which to provide and accurate sensor incorporating SAW technology and CrypA films, which try to provide fast and accurate measurements in a larger dynamic range. The proposed sensor configuration consists of differential measurements in a larger dynamic range. The proposed sensor configuration consists of differential resonator-oscillators, and a sensitive coating on the sensing device surface, and a frequency signal resonator-oscillators, and a sensitive coating on the sensing device surface, and a frequency signal acquisition module (FSAM), as as shown A two-port two-portSAW SAW resonator is designed acquisition module (FSAM), shownininFigure Figure 1. 1. A resonator is designed on aon a temperature-compensated ST-X quartz substrate as the feedback of the differential oscillator. Lower temperature-compensated ST-X quartz substrate as the feedback of the differential oscillator. Lower insertion loss, high quality factor, and single resonation mode were achieved in the SAW devices insertion loss, high quality factor, and single resonation mode were achieved in the SAW devices developed byphotolithographic the photolithographic technique. sensing methane gas, CrypAwas wassynthesized synthesizedfrom developed by the technique. ForFor sensing methane gas, CrypA from vanillyl alcohol and deposited onto theofsurface of the SAW sensing SAW Different device. Different film vanillyl alcohol and deposited onto the surface the sensing device. film deposition deposition methods were applied for the CrypA coating to achieve higher sensor responses. The methods were applied for the CrypA coating to achieve higher sensor responses. The proposed proposed SAW sensor was exposed to various concentrations of methane gas at room temperature, SAW sensor was exposed to various concentrations of methane gas at room temperature, and and the resulting performance, measured in terms of sensitivity, detection limit, and repeatability, the resulting performance, measured in terms of sensitivity, detection limit, and repeatability, was was characterized experimentally. characterized experimentally.

Figure 1. The schematic and ofthe theSAW-based SAW-based methane sensor. Figure 1. The schematic andworking workingprinciple principle of methane gasgas sensor.

2. Technique Realization This section describes the realization of the physical structure of the CrypA-coated SAW Sensorssensor. 2016, 16, 73 3 of 10 methane 2.1. Two-Port SAW Resonator

2. Technique Realization

A two-port SAW resonator configuration was reproducibly on a temperatureThis section describes the realization of the physical structure developed of the CrypA-coated SAW compensated ST-X quartz substrate as the oscillation feedback, as shown in Figure 1. 1600 Å Al-strip methane sensor. was deposited onto ST-X quartz wafer by using the photolithographic process. A thin SiO2 with 200 2.1. Two-Port SAW Resonator Å thickness is deposited on the device surface to protect the electrodes in the CrypA deposition by using plasma enhanced chemical vapor depositionwas (PECVD), and the SiO2 coating amorphous and A two-port SAW resonator configuration reproducibly developed on aistemperaturecompensated ST-X substrate the oscillation feedback, as in Figure 1.between 1600 Å Al-strip porous to increase thequartz sensing contactasarea, which is benefitial forshown the interaction the CrypA deposited quartz wafer by using photolithographic process. A thin1.SiO and was methane gas.onto TheST-X design parameters of thethe SAW device are listed in Table Prior to200 theÅSAW 2 with thickness is deposited on the device surface to protect the electrodes in the CrypA deposition by using device development, the coupling of modes (COM) model was used for performance simulation. plasma enhanced chemical vapor deposition (PECVD), and the SiO2 coating is amorphous and porous to increase the sensing contact which is benefitial forSAW the interaction Table 1.area, Design parameters for the sensor chip.between the CrypA and methane gas. The design parameters of the SAW device are listed in Table 1. Prior to the SAW device Parameters Values Values development, the coupling of modes (COM) model was usedParameters for performance simulation.

Operation frequency (MHz) 300 Wavelength (λ: μm) 10.5 Table 1. Design parameters for the SAW sensor chip. IDT length (λ) 41 Gap between the reflectors and IDT (λ) 0.75/0.5 Reflector Parameters length (λ) 300 Length of the coating area (λ) Values Parameters Values 150 aperture (λ) 200300 Gap between the IDT and coating area (λ)10.5 5 Operation frequency (MHz) Wavelength (λ: µm) IDT length (λ) 41 Gap between the reflectors and IDT (λ) Reflector length (λ) 300 Length of the coating area (λ) y Gap for between thedevice, IDT and coating apertureadmittance (λ) 200 solution Using the typical matrix whole [Y] area [ 11(λ)

y21

y responseUsing S21 of the thetypical SAW admittance device can matrix be deduced byfor [15]: solution whole device, rYs “ r 11 y21 response S21 of the SAW device can be deduced by [15]: 2 y12 GinGout

S 21 

S21 “

0.75/0.5 y12 150 ] ,5the frequency

y22

y12 s, the frequency y22

(1)

?

11)(G (Gin  y´2y inyG22out )  y12 y 21 12out G

pGin ` y11 qpGout ` y22 q ´ y12 y21

(1)

Here, Gin and Gout are the input and output impedance, respectively. Using Equation (1), the SAW the inputwas and simulated output impedance, respectively. Using Equation (1), the SAW out areAl-strip resonatorHere, withG1600 utilizing the corresponding structure parameters in andÅGthick resonator with 1600 Å thick Al-strip was simulated utilizing the corresponding structure parameters listed in Table. 1. The simulated frequency response of the SAW resonator is depicted in Figure 2. in Table simulated frequency response of the resonator is depicted Figure 2. Lowlisted insertion loss1.ofThe 4.5 dB, high quality factor of ~3500 andSAW single resonation mode in were achieved. Low insertion loss of 4.5 dB, high quality factor of ~3500 and single resonation mode were achieved. Then, the fabricated SAW resonator (Figure 2) was characterized by using a network analyzer, and Then, the fabricated SAW resonator (Figure 2) was characterized by using a network analyzer, and in in comparison with the simulation. The measured result agrees well with the simulation. The comparison with the simulation. The measured result agrees well with the simulation. The resonant resonant frequency of the developed SAW device is measured as 299.4 MHz. frequency of the developed SAW device is measured as 299.4 MHz.

Figure 2. The measured andsimulated simulated frequency frequency response ofof thethe SAW resonator. Figure 2. The measured and response SAW resonator.

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2.2. Differential Resoantor-Oscillator 2.2. Differential Resoantor-Oscillator The fabricated SAW sensor chip was loaded into a standard metal base (seen in the inset of Sensors 2016, 16, 73 4 of 10 Figure As the feedback in the oscillation path, alla transducers of the SAW were The2).fabricated SAW sensor chip was loaded into standard metal base (seenresonators in the inset of connected by each oscillator circuit which was composed of an amplifier (BGA2817), phase shifter, Figure 2). As the feedback in the oscillation path, all transducers of the SAW resonators were Differential Resoantor-Oscillator mixer 2.2. (UPC2758) LPF and so on, as shown in Figureof3.an The output of the each oscillator was connected by eachand oscillator circuit which was composed amplifier (BGA2817), phase shifter, mixed (UPC2758) to obtain aand different frequency in the MHz range and reduce theofinfluence the of thermal mixer LPF and so on, shown ininto Figure 3. The output the each oscillator was The fabricated SAW sensor chipas was loaded a standard metal base (seen in theof inset expansion of2).theAs substrate. mixed oscillation frequency signal was by a Figure the feedback in the oscillation path,range all transducers of the resonators were mixed to obtain apiezoelectric different frequency in The the MHz and reduce theSAW influence of picked the thermal connected bypiezoelectric each oscillator circuit which was composed of an amplifier (BGA2817), phase FPGA-based and plottedsubstrate. in real-time by a self-made interface display program. Usually,bythe expansion of FSAM, the The mixed oscillation frequency signal was shifter, picked a mixer (UPC2758) and LPF and so on, as shown in Figure 3. The output of the each oscillator was mixed frequency stability the oscillator significantly the detection limit of theprogram. gas sensor. Therefore, FPGA-based FSAM,ofand plotted inaffects real-time by a self-made interface display Usually, the to obtain a different frequency in the MHz range and reduce the influence of the thermal expansion of an experiment was toaffects measure the frequency stability of of the frequency stability of conducted the oscillator significantly the detection limit thedeveloped gas sensor.differential Therefore, the piezoelectric substrate. The mixed oscillation frequency signal was picked by a FPGA-based FSAM, resonator-oscillator at room temperature (20 controlled by anofincubator. To improve the an experiment conducted to measure the°C) frequency stability thefrequency developed differential and plottedwas in real-time by a self-made interface display program. Usually, the stability of frequency stability, the oscillation was conducted at the frequency point corresponding to the lowest resonator-oscillator at room temperature (20 limit °C) ofcontrolled by Therefore, an incubator. To improve the the oscillator affects significantly the detection the gas sensor. an experiment was insertion loss by a strategically phase modulation [15]. The measured short-term (in seconds) frequency stability, the oscillation wasstability conducted the frequency point corresponding at toroom the lowest conducted to measure the frequency of theatdeveloped differential resonator-oscillator ˝ C) frequency stability of the oscillator without a sensor coating is ±1.5 Hz/s, and the medium term (in temperature (20 controlled by an incubator. To improve the frequency stability, the oscillation insertion loss by a strategically phase modulation [15]. The measured short-term (in seconds) conducted the frequency point corresponding to the lowest insertion loss by strategically hours)was frequency is better than ±25/h the equilibrium status (Figure 4). of theterm reasons frequency stabilitystability ofat the oscillator without a at sensor coating is ±1.5 Hz/s, and theaOne medium (in phase modulation [15]. The measured short-term (in seconds) frequency stability of the oscillator for thefrequency frequencystability fluctuations is the temperature which can 4). only beofstabilized to hours) is better thanenvironmental ±25/h at the equilibrium status (Figure One the reasons without a sensor coating is ˘1.5 Hz/s, and the medium term (in hours) frequency stability is better within °C. for the ±0.5 frequency fluctuations is the environmental temperature which can only be stabilized to than ˘25/h at the equilibrium status (Figure 4). One of the reasons for the frequency fluctuations is withinthe ±0.5 °C. environmental temperature which can only be stabilized to within ˘0.5 ˝ C.

Figure 3. The oscillation PCB for methane gas sensor. Figure 3. The oscillation PCB PCB for gasgas sensor. Figure 3. The oscillation formethane methane sensor.

Oscillation frequency (Hz)(Hz) Oscillation frequency

375975 376050 375975 376050 375950 376000

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375950 376000 375925 375950 375925 375950 375900 375900 375900 375900 375875 375850 375875 375850 375850 375800 375850 375800 375825 375750 375825 375750

100 100

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Time1000 (sec)

10000 10000

Time (sec) Figure 4. The measuredfrequency frequency stabiltiy thethe SAW oscillator. Figure 4. The measured stabiltiyofof SAW oscillator.

Figure 4. The measured frequency stabiltiy of the SAW oscillator.

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2.3. Synthesis and Characterization of CrypA Sensors 2016, 16, 73

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2.3. Synthesis and Characterization of CrypAin Scheme 1, which was followed from the well-known twoThe synthesis of CrypA is depicted 1H-NMR (Figure 5), recorded 2.3. Synthesis Characterization of CrypA step method [16]. Then, the synthesized CrypA was characterized byfollowed The synthesis ofandCrypA is depicted in Scheme 1, which was from the well-known 1 H-NMR in CDCl 3 on The a Bruker 400 MHz spectrometer and referenced to the residual solvent peak.twoThe NMR synthesis of CrypA is depicted inCrypA Scheme was 1, which was followedby from the well-known two-step method [16]. Then, the synthesized characterized (Figure 5), recorded 1H-NMR (Figure 5), recorded step method [16]. Then, the synthesized CrypA was characterized by δ = 6.77 400 (s, 6H), 6.68 (s, 6H), 4.63 (d, 13.7 Hz, 6H), 4.17residual (m, 12H), 3.81 (s,peak. 18H), The 3.44 NMR (d, in results CDCl3were: on a Bruker MHz spectrometer andJ =referenced to the solvent in CDCl on a Bruker 400 MHz spectrometer and referenced to the residual solvent peak. The NMR J = 13.8 Hz, 6H)3 ppm. results were: δ = 6.77 (s, 6H), 6.68 (s, 6H), 4.63 (d, J = 13.7 Hz, 6H), 4.17 (m, 12H), 3.81 (s, 18H), 3.44 (d, results were: δ = 6.77 (s, 6H), 6.68 (s, 6H), 4.63 (d, J = 13.7 Hz, 6H), 4.17 (m, 12H), 3.81 (s, 18H), 3.44 (d,

J = 13.8 Hz, J =6H) 13.8 ppm. Hz, 6H) ppm.

Scheme 1.Synthesis Synthesis ofofCrypA. Scheme CrypA. Scheme 1. 1. Synthesis of CrypA.

Figure 5. 1H-NMR spectrum of the synthesized CrypA.

2.4. CrypA Deposition Usually, the CrypA molecules are easily gathered and crystallized on the piezoelectric substrate Figure5.5.11H-NMR H-NMR spectrum of the synthesized Figure synthesizedCrypA. CrypA. surface, so the CrypA molecules are firstspectrum dissolved of in the tetrahydrofuran (THF) prior to deposition on the sensing SAW device surface. The detailed composition of the CrypA-solution is that 3.0 mg 2.4. CrypA Deposition 0.3 mg polyvinyl chloride (PVC) and 0.6 mg dioctyl sebacate were dissolved in 2 mL THF. 2.4. CrypACrypA, Deposition The polyvinyl chloride was usedare to improve the adhesion of crystallized CrypA. To determine more effective Usually, the CrypA molecules easily gathered and on the the piezoelectric substrate waythe for CrypA formation, drop-coating andgathered spin-coating were tested. Beforeon thethe CrypA deposition, substrate Usually, CrypA molecules are easily and crystallized piezoelectric surface, so the CrypA molecules are first dissolved in tetrahydrofuran (THF) prior to deposition on SiOCrypA 2 surface of the sensing SAW device was cleaned of any contaminants by a routine cleaning surface, sothethe molecules are dissolved in tetrahydrofuran (THF) prior that to deposition the sensing SAWinvolving device surface. Thefirst detailed 3.0 mg procedure rinsing in piranha solution composition (H2SO4 - H2O2 = of 3:1 the v/v),CrypA-solution a DI water rinse andisdrying onCrypA, the sensing SAW device surface. The detailed composition of the CrypA-solution is thatTHF. 3.0 mg 0.3 mg polyvinyl chloride (PVC) and 0.6 mg dioctyl sebacate were dissolved in 2 mL by N2. For drop-coating, 0.3 μL CrypA-solution was dropped on the cleaned SiO2 film surface CrypA, 0.3between mg polyvinyl and 0.6 mgcured dioctyl were dissolved in effective 2 mL THF. the IDTswas ofchloride theused sensing resonator, and then at CrypA. 80 sebacate °C for To 40 min in an oven. spinThe polyvinyl chloride to(PVC) improve the adhesion of determine theFor more coating, 100 μL solutions were spin at 2000 rpm for 30 s on a ST-quartz wafer with SAW resonator The polyvinyl chloride was used to improve the adhesion of CrypA. To determine the more effective way for CrypA formation, drop-coating and spin-coating were tested. Before the CrypA deposition, patterns. The wafer was also cured at 80 °C for 40 min and then dicedtested. into individual SAW resonators. way for CrypA formation, drop-coating and spin-coating were Before the CrypA deposition, the SiO2 surface of the sensing SAW device was cleaned of any contaminants by a routine cleaning Then, the surface topography of CrypA-coated sensor chips utilizing different CrypA coating theprocedure SiO2 surface of therinsing sensing device was(H cleaned of contaminants by rinse a routine cleaning involving in SAW piranha solution 2SO4 - H 2Oany 2 = 3:1 v/v), a DI water and drying methods was characterized by the atomic force microscope (AFM), as shown in Figure 6. As a rough procedure involving rinsing in Piranha solution (V(H SO ):V(H O ) = 3:1), a DI water rinse and drying by N2. For drop-coating, 0.3 μL CrypA-solution was dropped on the cleaned SiO 2 film surface 2 4by spin-coating 2 2 estimate, the thicknesses of the CrypA coating deposited and drop-coating are ~600

the IDTs of the0.3 sensing resonator, andwas thendropped cured aton 80the °C cleaned for 40 min an oven. Forbetween spinbybetween N2 . For drop-coating, µL CrypA-solution SiOin2 film surface ˝ solutions were spin at then 2000 cured rpm forat30 a ST-quartz wafer with SAW resonator thecoating, IDTs of100 theμL sensing resonator, and 80s on C for 40 min in an oven. For spin-coating, The wafer was also 80 °C minaand then diced intowith individual SAW resonators. 100patterns. µL solutions were spin atcured 2000 at rpm forfor 3040 s on ST-quartz wafer SAW resonator patterns. ˝ Then, the surface topography of CrypA-coated sensor chips utilizing different CrypA coating The wafer was also cured at 80 C for 40 min and then diced into individual SAW resonators. methods characterized by the atomic force microscope shown in Figure 6. As a rough Then, was the surface topography of CrypA-coated sensor(AFM), chips as utilizing different CrypA coating estimate, the thicknesses of the CrypA coating deposited by spin-coating and drop-coating area~600 methods was characterized by the atomic force microscope (AFM), as shown in Figure 6. As rough estimate, the thicknesses of the CrypA coating deposited by spin-coating and drop-coating are ~600 nm

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and nm, nm, respectively. It is Italso obviously thatthat the the CrypA filmfilm formed by by drop-coating hashas an nm ~200 and ~200 respectively. is also obviously CrypA formed drop-coating obviously larger rougher surface with many fluctuations and bubbles, whereas the film surface coated an obviously larger rougher surface with many fluctuations and bubbles, whereas the film surface with spin-coating is much issmoother. coated with spin-coating much smoother.

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nm and ~200 nm, respectively. It is also obviously that the CrypA film formed by drop-coating has an obviously larger rougher surface with many fluctuations and bubbles, whereas the film surface coated with spin-coating is much smoother.

(a)

(b)

Figure 6. AFM characterization of CrypA coated by (a) Spin-coating; (b) Drop-coating. Figure 6. AFM characterization of CrypA coated by (a) Spin-coating; (b) Drop-coating.

3. Sensor Experiments 3. Sensor Experiments 3.1. Gas Sensor (a) Experimental Setup (b) 3.1. Gas Sensor Experimental Setup Figure 6. AFM characterization of CrypA coated by (a) Spin-coating; Drop-coating. The developed CrypA-coated sensing chip and(b) reference chip were placed in a surface nickelThe developed CrypA-coated sensing chip and reference chip were placed in a surface plated Aluminum gas chamber with volume of 500 mL(Figure 7a), and connected to corresponding 3. Sensor Experiments Aluminum gas chamber with volume of 500 mL(Figure 7a), and connected to nickel-plated oscillation circuit, respectively. The experimental set up in Figure 7b was utilized to characterize the corresponding oscillation circuit, respectively. The experimental set up in Figure 7b was utilized 3.1. Gas Sensor Experimental Setup sensor responses towards methane gas at room temperature. The SAW sensor was exposed to N2 and to characterize the sensor responses towards methane gas at room temperature. The SAW sensor was 4 gas alternately the gas as shown Figure study the humidity effects, air was The CH developed CrypA-coatedvia sensing chip path, and reference chip in were placed7b. in aTo surface nickelexposed to N2 and CH gas alternately via the7a), gas path, as shown in Figure 7b. To study the humidity 4 volume plated Aluminum gas chamber with of 500 mL(Figure and was connected to corresponding used as the diluents, and the relative humidity (RH) controlled by a streaming standard humidity effects, was used asexperimental the diluents, and the relative humidity (RH) was oscillation circuit,air respectively. The set up in Figure 7b was utilized to characterize the controlled by a streaming generator (RST-GX-2, Beijing Naisisa New Technology Development Corp, Beijing, China.). The sensor responses methane gas at room temperature. The SAW sensor New was exposed to N2 andDevelopment Corp, Beijing, standardtowards humidity generator (RST-GX-2, Beijing Naisisa Technology sensor signal were collected at 60 points To per minute by theeffects, FSAM, and plotted by the personal CH4 gasChina.). alternatelyThe via the gas path, as shown Figure 7b.at study the humidity airthe wasFSAM, and plotted by the sensor signal wereincollected 60 points per minute by computer in the realrelative time.humidity (RH) was controlled by a streaming standard humidity used as the diluents, and personal computer in real time. Sensors 2016, 16, 73 generator (RST-GX-2, Beijing Naisisa New Technology Development Corp, Beijing, China.). The

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sensor signal were collected at 60 points per minute by the FSAM, and plotted by the personal computer in real time.

(a)

(a)

(b)

Figure 7. The gas chamber (a) and experimental set up (b) of the sensor system.

Figure 7. The gas chamber (a) and experimental set up (b) of the sensor system. 3.2. Sensor Performance Evaluation

First, the repeatability of the developed CrypA-coated SAW sensor was evaluated. Figure 8 showed a response profile obtained from three consecutive 18 min on-off exposures to 5% of CH4 in pure N2 at 20 °C using the developed sensors with CrypA deposited by different coating approaches. ~1 kHz of frequency response was achieved from the sensor with CrypA coated by drop-coating method (Figure 8a). It can also be noted that three gas exposures are in good reproducible run. The gathered frequency signal showed a rapid rise upon exposure to CH4 and reaches approximately the

(b)

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Figure 7. The gas chamber (a) and experimental set up (b) of the sensor system.

3.2. Sensor Performance Evaluation 3.2. Sensor Performance Evaluation First, the repeatability of the developed CrypA-coated SAW sensor was evaluated. Figure 8 First, the repeatability of the developed CrypA-coated SAW sensor was evaluated. Figure 8 showed a response profile obtained from three consecutive 18 min on-off exposures to 5% of CH4 in showed a response profile obtained from three consecutive 18 min on-off exposures to 5% of CH4 in pure N2 at 20 ˝ C using the developed sensors with CrypA deposited by different coating approaches. pure N2 at 20 °C using the developed sensors with CrypA deposited by different coating approaches. ~1 kHz of frequency response was achieved from the sensor with CrypA coated by drop-coating ~1 kHz of frequency response was achieved from the sensor with CrypA coated by drop-coating method (Figure 8a). It can also be noted that three gas exposures are in good reproducible run. The method (Figure 8a). It can also be noted that three gas exposures are in good reproducible run. The gathered frequency signal showed a rapid rise upon exposure to CH4 and reaches approximately the gathered frequency signal showed a rapid rise upon exposure to CH4 and reaches approximately the equilibrium (saturation) value in 12 s. When the gas was removed by N2 injection, the sensor response equilibrium (saturation) value in 12 s. When the gas was removed by N2 injection, the sensor response returned to its initial baseline within 20 s. It means the 90% response time of ~12 s and recovery time of returned to its initial baseline within 20 s. It means the 90% response time of ~12 s and recovery time ~20 s with good repeatability were obtained at room temperature. These promising results indicated of ~20 s with good repeatability were obtained at room temperature. These promising results that this sensor exhibits fast response and excellent repeatability in response to CH4 . The sensor indicated that this sensor exhibits fast response and excellent repeatability in response to CH4. The response towards 5% CH4 was also conducted from the sensor with CrypA coated by spinning-coating, sensor response towards 5% CH4 was also conducted from the sensor with CrypA coated by as shown in Figure 8b, only ~100 Hz of frequency response was observed, much smaller than that spinning-coating, as shown in Figure 8b, only ~100 Hz of frequency response was observed, much of drop-coating CrypA sensor. Considering the CrypA film topography characteristics mentioned smaller than that of drop-coating CrypA sensor. Considering the CrypA film topography above, the experimental results indicate that the surface roughness of CrypA coating influences greatly characteristics mentioned above, the experimental results indicate that the surface roughness of on the sensor response. Sensitive film with larger rougher surface provides larger surface-to-volume CrypA coating influences greatly on the sensor response. Sensitive film with larger rougher surface ratio, thus more CrypA molecules are able to contact and absorb methane gas, resulting in higher provides larger surface-to-volume ratio, thus more CrypA molecules are able to contact and absorb sensor response. methane gas, resulting in higher sensor response. 377200

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(b) Figure 8. Sensor Sensorresponse response and repeatability testing towards 5% from CH4 the from thesensor SAWwith sensor with Figure 8. and repeatability testing towards 5% CH SAW different 4 different CrypA coating method. (a) Dropping-method; (b) Spinning-coating. CrypA coating method. (a) Dropping-method; (b) Spinning-coating.

Then, we exposed the CrypA-coated sensors by using the drop-coating to CH4 with different concentrations to characterize their sensitivity. Figure 9 shows the real-time response of the CrypA sensor to low (Figure 9a) and high (Figure 9b) CH4 concentrations at room temperature (20 °C). And the maximum frequency response was plotted vs. the corresponding CH4 concentrations as shown in Figure 10. It is obviously that as the gas concentration increased, the sensor frequency signal also increased with approximately linearity. The sensitivity in CH4 concentration range of 0.2%~5% was

(b) Figure 8. Sensor response and repeatability testing towards 5% CH4 from the SAW sensor with 8 of 10 different CrypA coating method. (a) Dropping-method; (b) Spinning-coating.

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Then, we we exposed exposed the the CrypA-coated CrypA-coated sensors sensors by by using using the the drop-coating drop-coating to to CH CH4 with with different different Then, 4 concentrations to characterize their sensitivity. Figure 9 shows the real-time response of the CrypA concentrations to characterize their sensitivity. Figure 9 shows the real-time response of the CrypA sensor to to low low (Figure (Figure 9a) 9a) and and high high (Figure (Figure 9b) 9b) CH CH4 concentrations concentrations at at room room temperature temperature (20 (20 ˝°C). And sensor C). And 4 the maximum frequency response was plotted vs. the corresponding CH 4 concentrations as shown in the maximum frequency response was plotted vs. the corresponding CH4 concentrations as shown Figure 10.10. It It is isobviously signal also also in Figure obviouslythat thatasasthe thegas gasconcentration concentrationincreased, increased, the the sensor sensor frequency frequency signal increased with approximately linearity. The sensitivity in CH 4 concentration range of 0.2%~5% was increased with approximately linearity. The sensitivity in CH4 concentration range of 0.2%~5% was evaluated as as ~204 ~204 Hz/% Hz/% with 9b, the the sensor sensor response response of of evaluated with well well linearity linearity of of 0.99827. 0.99827. Also, Also, from from Figure Figure 9b, ~50 Hz Hz occurs occurs at at CH CH4 concentration concentration of of 0.2%. 0.2%. It It means means lower lower detection detection limit limit will will be be expected expected because because ~50 4 the present oscillator exhibits excellent short-term frequency stability of ±1.5 Hz/s. Hence, based on the present oscillator exhibits excellent short-term frequency stability of ˘1.5 Hz/s. Hence, based on the International Union of Pure and Applied Chemistry (IUPAC) (Zurich, Switzerland), the detection the International Union of Pure and Applied Chemistry (IUPAC) (Zurich, Switzerland), the detection limit of of the the developed developed sensor sensor can can be be estimated estimated to to less less than than 0.05%, 0.05%, the the same same rank rank to to the the reported reported limit CrypA-coated QCM sensor but the dynamic range is larger more [9]. The measure results indicate CrypA-coated QCM sensor but the dynamic range is larger more [9]. The measure results indicate that the presented CrypA-coated SAW sensor was very promising for CH 4 detection and monitor in that the presented CrypA-coated SAW sensor was very promising for CH4 detection and monitor in homes, industries industries and and mines. mines. homes,

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Time (min) (b) Figure 9. 9. Responses Responses of of CrypA CrypA based based SAW SAW sensor sensor to to (a) (a)High HighCH CH44 concentrations concentrations and and (b) (b) Low Low CH CH44 Figure concentration at room temperature. concentration at room temperature.

3.3. Humidity Effect Evaluation Evaluation 3.3. Humidity Effect Usually, the the gas gas sensors sensors are are usually usually deployed deployed in in the the ambient ambient dynamic dynamic environment environment that that may may Usually, affect significantly the sensor performance [17]. Therefore, it is necessary to study the humidity effect affect significantly the sensor performance [17]. Therefore, it is necessary to study the humidity effect on these sensors. Figure 11 illustrated the crossed humidity sensitivity of the developed SAW sensor, it clearly indicates that the sensor signal increased as the RH increased. The response to a constant methane concentration of 5% but different humidity levels of 0%, 20%, and 30% was 1015 Hz, 1258 Hz, and 1381 Hz, respectively. Obviously, the humidity affected the sensor performance significantly. The reason for the humidity effect is the adsorption of water molecules in CrypA while methane

Figure 9. Responses of CrypA based SAW sensor to (a) High CH4 concentrations and (b) Low CH4 concentration at room temperature.

3.3. Humidity Effect Evaluation SensorsUsually, 2016, 16, 73the

gas sensors are usually deployed in the ambient dynamic environment that9 of may 10 affect significantly the sensor performance [17]. Therefore, it is necessary to study the humidity effect on these sensors. Figure 11 illustrated the crossed humidity sensitivity of the developed SAW sensor, on these sensors. Figure 11 illustrated the increased crossed humidity sensitivity of the SAW sensor, it clearly indicates that the sensor signal as the RH increased. Thedeveloped response to a constant itmethane clearly indicates that the sensor signal increased as the RH increased. The response to a constant concentration of 5% but different humidity levels of 0%, 20%, and 30% was 1015 Hz, 1258 methane concentration of 5% but different humidity levels of 0%, the 20%, and 30% was 1015 significantly. Hz, 1258 Hz, Hz, and 1381 Hz, respectively. Obviously, the humidity affected sensor performance and 1381 Hz, respectively. Obviously, the humidity affected the sensor performance significantly. The reason for the humidity effect is the adsorption of water molecules in CrypA while methane The reason the humidity effect is the adsorption of water molecules in CrypA while sensing. sensing. Asfor described in Reference [10], the humidity effect on sensor response canmethane be alleviated in As described in Reference [10], the humidity effect on sensor response can be alleviated in different different ways like calibration by creating a special database relating to different humidity level, or ways like acalibration by creating a special database relating to different humidityeffect level, by or utilizing a utilizing polymer coating on the reference device to estimate the humidity difference polymer method. coating on the reference device to estimate the humidity effect by difference method. 1200

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Figure 10. Frequency shifts of CrypA-coated SAW sensor vs. different concentrations of CH4 . Figure 10. Frequency shifts of CrypA-coated SAW sensor vs. different concentrations of CH4.

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4. Conclusions 4. Conclusions A A room room temperature temperature SAW SAW sensor sensor incorporating incorporating aa CrypA CrypA coating coating was was developed developed for for sensing sensing methane gas. The two-port SAW resonsators with low insertion loss, high quality factor, and methane gas. The two-port SAW resonsators with low insertion loss, high quality factor, and single single resonation fabricated on aon temperature-compensated ST-X quartz as the feedback resonation mode modewere were fabricated a temperature-compensated ST-X substrate quartz substrate as the element oscillation path. Thepath. synthesized Supramolecular CrypA was deposited onto the onto sensing feedbackinelement in oscillation The synthesized Supramolecular CrypA was deposited the SAW device as the sensitive material for sensing methane gas. Different CrypA film deposition sensing SAW device as the sensitive material for sensing methane gas. Different CrypA film approaches were conducted achieve higher sensorhigher response. Theresponse. sensor responses to methane were deposition approaches weretoconducted to achieve sensor The sensor responses to

methane were evaluated experimentally. Fast response and excellent repeatability were observed, and the estimated detection limit and sensitivity are ~0.05% and ~204 Hz/%, respectively. Acknowledgments: This work was partly supported by National Natural Science Foundation of China (No.11274340, 11374254).

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evaluated experimentally. Fast response and excellent repeatability were observed, and the estimated detection limit and sensitivity are ~0.05% and ~204 Hz/%, respectively. Acknowledgments: This work was partly supported by National Natural Science Foundation of China (No. 11274340, 11374254). Author Contributions: All authors participated in the work presented here. Wen Wang and Shitang He defined the research topic. Haoliang Hu and Xinlu Liu carried out most of the experiments. Yong Pan contributed the CrypA deposition. Caihong Zhang and Chuan Dong contributed the synthesis of the CrypA material. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3.

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Development of a Room Temperature SAW Methane Gas Sensor Incorporating a Supramolecular Cryptophane A Coating.

A new room temperature supra-molecular cryptophane A (CrypA)-coated surface acoustic wave (SAW) sensor for sensing methane gas is presented. The senso...
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