A Noncontact Dibutyl Phthalate Sensor Based on a Wireless-Electrodeless QCM-D Modiﬁed with Nano-Structured Nickel Hydroxide

sensors

Article A Noncontact Dibutyl Phthalate Sensor Based on a Wireless-Electrodeless QCM-D Modiﬁed with Nano-Structured Nickel Hydroxide

Daqi Chen 1 ID , Xiyang Sun 1, Kaihuan Zhang 1, Guokang Fan 2, You Wang 1, Guang Li 1 and Ruifen Hu 1,*

1 State Key Laboratory of Industrial Control Technology, Institute of Cyber Systems and Control, Zhejiang University, Hangzhou 310027, China; [email protected] (D.C.); [email protected] (X.S.); [email protected] (K.Z.); [email protected] (Y.W.); [email protected] (G.L.) 2 School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China; [email protected] * Correspondence: [email protected]; Tel./Fax: +86-571-8795-2268 (ext. 8232)

Received: 23 June 2017; Accepted: 19 July 2017; Published: 21 July 2017

Abstract: Dibutyl phthalate (DBP) is a widely used plasticizer which has been found to be a reproductive and developmental toxicant and ubiquitously existing in the air. A highly sensitive method for DBP monitoring in the environment is urgently needed. A DBP sensor based on a homemade wireless-electrodeless quartz crystal microbalance with dissipation (QCM-D) coated with nano-structured nickel hydroxide is presented. With the noncontact conﬁguration, the sensing system could work at a higher resonance frequency (the 3rd overtone) and the response of the system was even more stable compared with a conventional quartz crystal microbalance (QCM). The sensor achieved a sensitivity of 7.3 Hz/ppb to DBP in a concentration range of 0.4–40 ppb and an ultra-low detection limit of 0.4 ppb of DBP has also been achieved.

Keywords: wireless-electrodeless QCM with dissipation; dibutyl phthalate; overtone

1. Introduction Dibutyl phthalate (DBP) is a kind of phthalate plasticizers which has been used as an additive in lots of applications, including nail lacquers, food packaging ﬁlms, adhesives, pharmaceuticals and even toys for children [1–3] in the last few decades. As a result, DBP vapor has become ubiquitous in the environment. However, being exposed to DBP vapor can be harmful to human health. Recent research has found that DBP is a reproductive and developmental toxin to males and DBP has been considered as an endocrine disrupting compound (EDC) [4,5]. As air is an important medium for DBP to enter human bodies, a lot of research has assessed the concentration of DBP in indoor air 3 3 (1269–7104 ng/m ), indoor dust (4.4–2300 mg/kg) and ambient air (PM2.5 associated 8.72 µg/m , 3 PM10-associated 12.90 µg/m )[6–8]. In the Proposition 65, the maximum allowable dose levels (MADL) for DBP is 8.7 µg/day [9], which means if breathing contains 10% of the DBP intake, the concentration of the DBP vapor in the air should be lower than 0.087 µg/m3 (6.8 ppb) to ensure safety. Considering the facts above, the need for a DBP sensor that can monitor the concentration of DBP vapor in air is very urgent to avoid adverse effects, especially for people who work in a plastic production workshop. Such people could easily suffer from staying for a long time in an environment that contains relatively high concentrations of DBP vapor. The common methods used to determine the DBP concentration in air are gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). Both the methods can measure extremely low concentration (respectively 1218–2453 ng/m3

Sensors 2017, 17, 1681; doi:10.3390/s17071681 www.mdpi.com/journal/sensors Sensors 2017, 17, 1681 2 of 11 and 52–1100 ng /m3)[10,11], but they both take considerable time and are expensive, which disagrees with the demand for fast and cheap monitoring in daily life. For the fast and cheap detection of DBP in air, the quartz crystal microbalance (QCM) sensor is a popular choice because of their high sensitivity and room temperature working conditions. Different sensing material have been developed to modify the QCM sensor in order to monitor DBP in the air. Wang [12] deposited the polyaniline nanofibers on the electrode of a quartz crystal oscillator as the sensitive film and detected the DBP in the range of 20–1000 ppb with a detection limit of 20 ppb. Hu and Zhang [13,14] respectively used nano-structured nickel hydroxide and Au-decorated ZnO to coat QCM sensors and achieved a detection range of 5–40 ppb and 2–30 ppb with detection limits of 5 ppb and 2 ppb. However, conventional QCM sensors have some drawbacks which limit their application: (1) electrodes on both sides of the quartz disc deteriorates the sensitivity of the sensor and limits the available frequency because they are also mass adsorbed on the oscillator’s surface [15]; (2) some coating materials for the oscillators were restricted. For example, bioactive materials were hard to coat on the QCMs for a long time because of the biotoxicity of the metal electrodes; (3) this configuration is only applicable in the case when the sensing film is firmly attached to the quartz crystal surface, oscillating rigidly together with the crystal throughout the experiment [16]. When there are visco-elastic changes of the deposited film, the mass would not be a linear relationship with the mass loaded. To solve all the problems mentioned above, we developed a wireless-electrodeless QCM with a dissipation system. This system could simultaneously monitor the frequency and the dissipation value in a noncontact way, which means that it combines both the advantages of the quartz crystal microbalance with dissipation (QCM-D) system [17] and the wireless-electrodeless QCM system [18–24]. Based on this system, a DBP sensor was constructed and nano-structured nickel hydroxide [14] was deposited as the sensing film. The depositing load of the sensing film was optimized and the sensor performance including sensitivity, selectivity and limit of detection was evaluated. Then the operating frequency was raised to 3rd harmonics, and the same experiment was carried out. With the homemade sensor applied in DBP detection, drilling in the chamber for wires was avoided, and thus reduced the risk of leaking of the test vapor. Meanwhile, the raised operating frequency improved the sensitivity of the sensor (7.3 Hz/ppb to DBP in a concentration range of 0.4–40 ppb) and achieved lower limit of detection (0.4 ppb). Moreover, the signal-to-noise ratio (SNR) of the signal was improved and the system was thus more stable. Although influence on performance of the sensor by fluctuation of temperature were discussed in some research [25–27], we did not discuss it in this thesis because the temperature in production workshops is quite stable in order to ensure the stability of product quality. In conclusion, the wireless-electrodeless QCM-D system is a good and new choice to achieve fast and highly sensitive detection of volatile organic compounds (VOCs).

2. Experimental Section

2.1. Materials All the chemicals and reagents were analytical grade. Nickel dichloride, ammonia, dibutyl phthalate (DBP), diethyl phthalate (DEP), dimethyl phthalate (DMP), ethanol, chloroform, ethyl acetate, acetic acid, acetaldehyde (40% v/v) and benzene were purchased from Sigma–Aldrich (Shanghai, China). The AT–cut 6.0 MHz quartz crystal discs were purchased from Kesheng Electronics (Yantai) Ltd., Yantai, China.

2.2. The Wireless-Electrodeless QCM-D System Base on the traditional QCM-D system and the wireless-electrodeless QCM systems, a homemade QCM-D system was built combining their advantages. Several instruments was used to form this system, the function generator was Tektronix AFG3102C (Tektronix, Beaverton, OR, USA), the oscilloscope was Tektronix TDS5054B (Tektronix, Beaverton, OR, USA), the narrow band ampliﬁer Sensors 2017, 17, 1681 3 of 11 machined to fix the location of the quartz plate. Two spiral coils were placed below the chamber and right below the quartz plate. The distance between the coils and the quartz plate was 3 mm. The volume capacity of the chamber was 1 L. At the top of the chamber, a small plastic airbag was used to keep the pressure as 1 atm. In order to make the whole system to work, circuits and signal processing units were needed to form a whole system. In Figure 1b, to excite and detect the vibrations of the quartz oscillator, a radio frequency (RF) signal whose frequency was very close to the resonant frequency of the quartz crystal was generated by the function generator and sent to the transmitting coil. After that, an alternating Sensorselectric2017 field, 17 ,was 1681 generated and the quartz oscillator was excited because of the converse piezoelectric3 of 11 effect. After the quartz plate was steadily vibrating, the RF signal was removed and the receiving coil received the exponentially decaying mechanical vibration signals of the quartz plate through the waspiezoelectric homemade effect. and The the impedancereceived signals matching entered network a narrowband was a HF automaticamplifier then antenna to the tuner oscilloscope (mAT-125E). so Figurethat the1a damping shows the vibration homebuilt signal electrodeless was recorded. QCM-D Then, gas chamber.the PC was The used AT-cut to 6.0real-time MHz quartz analyze plate the wasfrequency placed and at the dissipation bottom of thevalue. chamber During where the aexpe 9 mmriment, diameter the blindresonant hole frequency was machined of the to quartz ﬁx the locationoscillator of will the change, quartz plate.but we Two did spiral not change coils were the placedfrequency below of thethe chamberRF signal and after right the below resonant the quartzfrequency plate. of Thethe quartz distance crystal between reached the coils the base and theline quartz in case plate of an was effect 3 mm. on the The vibration volume capacitystate of the of thequartz chamber oscillator. was 1 L. At the top of the chamber, a small plastic airbag was used to keep the pressure as 1 atm.

(a)

(b)

Figure 1. (a) The homebuilt electrodeless quartz crystal microbalance (QCM) gas chamber. Two spiralspiral coilscoils areare placedplaced outsideoutside thethe chamberchamber andand rightright belowbelow thethe quartzquartz plate.plate. They are usedused toto radiateradiate thethe electricelectric field,field, whichwhich excitesexcites thethe quartzquartz oscillatoroscillator and receivesreceives thethe vibrationalvibrational signalssignals ofof thethe quartzquartz oscillator. The quartz oscillator is placed at the bottombottom of thethe chamberchamber andand aboveabove thethe middlemiddle ofof twotwo coils.coils. (b(b)) The The whole whole structure structure of the of wireless-electrodeless the wireless-electrodeless quartz crystalquartz microbalance crystal microbalance with dissipation with (QCM-D)dissipation system. (QCM-D) The system. signal generator The signal generates generator the generates burst radio the frequency burst radio signal frequency and finally signal sends and it tofinally the transmitting sends it to coil.the transmitting The other coil coil. receives The theother signal coil andreceives then sendsthe signal it to theand narrowband then sends amplifierit to the thennarrowband to the oscilloscope amplifier then and to finally the oscilloscope to the PC for and analyzing. finally to the PC for analyzing.

2.3. Fabrication of a QCM Gas Sensor In order to make the whole system to work, circuits and signal processing units were needed to form a whole system. In Figure1b, to excite and detect the vibrations of the quartz oscillator, a radio frequency (RF) signal whose frequency was very close to the resonant frequency of the quartz crystal was generated by the function generator and sent to the transmitting coil. After that, an alternating electric ﬁeld was generated and the quartz oscillator was excited because of the converse piezoelectric effect. After the quartz plate was steadily vibrating, the RF signal was removed and the receiving coil received the exponentially decaying mechanical vibration signals of the quartz plate through the piezoelectric effect. The received signals entered a narrowband ampliﬁer then to the oscilloscope so that the damping vibration signal was recorded. Then, the PC was used to real-time analyze the frequency and dissipation value. During the experiment, the resonant frequency of the quartz oscillator Sensors 2017, 17, 1681 4 of 11 will change, but we did not change the frequency of the RF signal after the resonant frequency of the quartz crystal reached the base line in case of an effect on the vibration state of the quartz oscillator.

2.3. Fabrication of a QCM Gas Sensor

The preparation of nano-Ni(OH)2 was carried out in the same way as the previous work [14]. Before the experiment, the quartz plates should be coated with nano-Ni(OH)2 as the sensing ﬁlm. The AT–cut 6.0 MHz quartz plates were washed with deionized water and anhydrous alcohol for 15 min respectively. Then they were dried by high–purity N2 at room temperature. Then 10 mg nano-Ni(OH)2 was added to 1 mL deionized water in a burette and dispersed by ultrasonication to form a 10 mg/mL Ni(OH)2 solution. Finally, the nano-Ni(OH)2 solution sample was evenly smeared onto the surface of the quartz plates and the quartz plates were dried for 24 h at room temperature. The QCM sensors coated with nano-structured Ni(OH)2 sensing ﬁlm were obtained.

2.4. Preparation of Measured Vapors The vapors to be measured in the experiment were DBP, DEP, DMP, ethanol, chloroform, ethyl acetate, acetic acid, acetaldehyde (40% v/v) and benzene. All of them are liquid state at room temperature. To form a mixed gas with a known concentration, small amounts of the analyte solutions were injected into 2 L airbag full of high–purity N2. According to the Equation (1), the volume of every kind of the analyte solutions injected were calculated except for DBP, DEP and DMP.

V × C × M −9 273 + TR Vx = × 10 × (1) 22.4 × d × P 273 + TB where Vx is the analyte solution (mL), V is the volume of the airbags (mL), C is the target concentration of the measured vapors (ppm), M is the molecular weight of the analyte, d is the density of the analyte 3 solution (g/cm ), P is the purity of the analyte solution (%), TR is the room temperature and TB is the airbag temperature. Through this method, vapors whose concentration were 1 ppm of ethanol, chloroform, ethyl acetate, acetic acid, acetaldehyde and benzene were obtained. Last, in order to form a known concentration mixed gas of DBP, DEP and DMP, an excess amount of them was respectively injected into the 2 L airbag to get saturated vapors. The concentration of the saturated vapors were 0.84 ppm (DBP), 2.76 ppm (DEP) and 4.05 ppm (DMP) at 25 ◦C.

2.5. Gas Sensing Experiments The quartz plate with sensing film was first put in the blind hole of the chamber. Before the experiment, the frequency of the burst RF signal was set to a suitable value to guarantee the oscillation of the quartz plate. Then, high-purity N2 was continuously injected to purge the chamber and to desorb the sensors at the velocity of 120 L/min until the resonant frequency of the sensor reached a stable baseline. After this, the frequency of the burst RF signal was tuned to approach the baseline frequency and held. In every experiment, N2 was first injected to desorb the sensor until its resonant frequency returned to the baseline. Then precise volumes of mixed gas of the analyte were injected into the chamber and left to stand for 5 min to obtain appropriate concentrations of the vapors in the chamber. At the same time, the frequency was continuously monitored and the frequency differences were the responses. To maintain the balanced pressure in the gas chamber when injecting the gas samples, a small elastic bag was connected to the chamber through a thin catheter (see Figure1a).

3. Results and Discussion

3.1. SEM Morphology The scanning electron microscopy (SEM) technique was used to investigate the morphology and nanostructure of the Ni(OH)2 ﬁlm. In Figure2a,b, the Ni(OH) 2 sample presented a sheet-like morphology. Sensors 2017, 17, 1681 5 of 11 SensorsSensors 20172017,, 1717,, 16811681 55 ofof 1111

((aa)) ((bb))

FigureFigure 2.2. SEMSEM morphologymorphology ofof thethe nano–Ni(OH)nano–Ni(OH)22 samplesample atat differentdifferent scales.scales. ((aa)) ×20,000,×20,000, ((bb)) ×50,000.×50,000. Figure 2. SEM morphology of the nano–Ni(OH)2 sample at different scales. (a) ×20,000, (b) ×50,000. 3.2. Optimization of the Thickness of the Sensing FILM 3.2. Optimization of the ThicknessThickness of the Sensing FILM The thickness of the sensing film on the quartz plate was a key factor to the sensitivity. Thicker The thickness of the sensing filmfilm on the quartz plateplate was a key factor to the sensitivity. Thicker sensing film could provide more adsorption sites and thus more analyte molecules would be sensing filmfilm could could provide provide more more adsorption adsorption sites sites and thusand morethus analytemore analyte molecules molecules would be would adsorbed be adsorbed in the same concentration. However, when there were too many layers of the sensing inadsorbed the same in concentration.the same concentration. However, whenHowever, there when were there too many were layerstoo many of the layers sensing of materialthe sensing on material on the film, the analyte molecules could not attach to the inner sites and the adsorb mass thematerial film, theon the analyte film, moleculesthe analyte could molecules not attach could to no thet attach inner sitesto the and inner the si adsorbtes and mass the wouldadsorb reachmass would reach saturation. Worse, if the sensing film was too thick, it would deteriorate the sensing saturation.would reach Worse, saturation. if the sensing Worse, film if the was sensing too thick, film it wouldwas too deteriorate thick, it would the sensing deteriorate ability ofthe the sensing QCM ability of the QCM sensors because the sensing film is also the adsorb mass of the QCM. Based on sensorsability of because the QCM the sensors sensing because film is also the thesensing adsorb film mass is also of the the QCM. adsorb Based mass on of this,the QCM. 10 sensors Based with on this,this, 1010 sensorssensors withwith differentdifferent thicknessthickness sensingsensing filmsfilms werewere mademade byby controllingcontrolling thethe volumevolume ofof thethe different thickness sensing films were made by controlling the volume of the nano-Ni(OH)2 solution nano-Ni(OH)2 solution coated on the QCM to optimize the film thickness. The responses of all these coatednano-Ni(OH) on the2QCM solution to optimize coated on the the film QCM thickness. to optimize The responsesthe film thickness. of all these The sensors responses to 24 of ppb all these DBP sensors to 24 ppb DBP vapor were measured and are shown in Figure 3. The response of the sensor vaporsensors were to 24 measured ppb DBP andvapor are were shown measured in Figure and3. Theare shown response in Figure of the sensor3. The response is proportional of the tosensor the is proportional to the thickness of the nano-Ni(OH)2 film when the loaded mass is below 3.14 μg/mm22, thicknessis proportional of the to nano-Ni(OH) the thickness filmof the when nano-Ni(OH) the loaded2 film mass when is below the loaded 3.14 µ g/mmmass is2 below, and then 3.14 reachesμg/mm a, and then reaches a saturation2 level after that. In order to achieve a fast response and avoid error saturationand then reaches level after a saturation that. In order level to after achieve that. a In fast order response to achieve and avoid a fast error response caused and by avoid the coating error causedcaused byby thethe coatingcoating process,process, ththee optimaloptimal thicknessthickness ofof nano-Ni(OH)nano-Ni(OH)22 filmfilm 2 waswas chosenchosen asas 3.463.46 process, the optimal thickness of nano-Ni(OH)2 film was chosen as 3.46 µg/mm , a little thicker than μg/mm22, a little thicker than the minimum stable point. theμg/mm minimum, a little stable thicker point. than the minimum stable point.

7070

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5050

4040

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2020 Response(Hz) Response(Hz) 1010

00 0.511.522.533.544.550.511.522.533.544.55 LoadedLoaded massmass perper unitunit area(μg·mmarea(μg·mm-2-2))

Figure 3. Response curve of the nano-Ni(OH)2 QCM sensors to 24 ppb dibutyl phthalate (DBP) with differentFigureFigure 3.3. loaded ResponseResponse mass curvecurve of sensing ofof thethe nano-Ni(OH)nano-Ni(OH) material. 22 QCMQCM sensorssensors toto 2424 ppbppb didibutylbutyl phthalatephthalate (DBP)(DBP) withwith differentdifferent loadedloaded massmass ofof sensingsensing material.material. 3.3. Selectivity 3.3.3.3. SelectivitySelectivity To investigate the selectivity of the sensing ﬁlm, diverse VOCs interferences, including two other plasticizerToTo investigateinvestigate vapors, DEP thethe selectivityselectivity and DMP, ofof and thethe several sensingsensing conventional film,film, diversediverse VOCsVOCs solvents interferences,interferences, like ethanol, includingincluding chloroform, twotwo otherethylother plasticizerplasticizer vapors,vapors, DEPDEP andand DMP,DMP, andand severalseveral conventionalconventional solventssolvents likelike ethanol,ethanol, chloroform,chloroform, ethylethyl acetate,acetate, aceticacetic acid,acid, acetaldehydeacetaldehyde andand benzene,benzene, werewere tested.tested. AmongAmong them,them, DEPDEP andand DMPDMP werewere testedtested

Sensors 2017, 17, 1681 6 of 11 acetate,Sensors acetic2017, 17 acid,, 1681 acetaldehyde and benzene, were tested. Among them, DEP and DMP were6 of tested 11 at 40 ppb and the other conventional solvents were tested at 20 ppm. Figure4 indicates that the at 40 ppb and the other conventional solvents were tested at 20 ppm. Figure 4 indicates that the response of the nano-Ni(OH)2 QCM sensor to the DBP vapor is greater than that of the other two response of the nano-Ni(OH)2 QCM sensor to the DBP vapor is greater than that of the other two plasticizer vapors under the same concentration. Moreover, although other conventional inferences plasticizer vapors under the same concentration. Moreover, although other conventional inferences were 500 times the concentration of DBP vapor, the responses were still far smaller than that of DBP. were 500 times the concentration of DBP vapor, the responses were still far smaller than that of DBP. These results indicated that the afﬁnity of the sensor to plasticizer vapors were much better than These results indicated that the affinity of the sensor to plasticizer vapors were much better than otherother conventional conventional inferences inferences and and suggested suggested that that the the sensor sensor can can detect detect plasticizer plasticizer vapors vapors at ppb at ppb levels. Thelevels. reason The was reason possibly was possibly that DBP, that DEP DBP, and DEP DMP and containDMP contain more more hydrogen hydrogen bond bond acceptors acceptors (4) (4) than thethan other the interferences other interferences (≤2), which (≤2), which means means they are they more are easilymore easily able to able interact to interact with thewith− OHthe groups−OH ongroups the surface on the of surface the nano-structured of the nano-structured nano-Ni(OH) nano-Ni(OH)2 through2 through hydrogen hydrogen bonding bonding so that so the that physical the adsorptionphysical adsorption was facilitated. was facilitated. Additionally, Additionally the benzene, the ringbenzene and ring the symmetricaland the symmetrical molecular molecular structure ofstructure DBP, DEP of and DBP, DMP DEP lead and to DMP their goodlead to hydrophobicity, their good hydrophobicity, which contributes which to contributes their afﬁnity. to Finally,their amongaffinity. DBP, Finally, DEP andamong DMP, DBP, the DEP response and DMP, amplitudes the response decreased amplitudes in turn, decreased the length in ofturn, the the alkyl length chains in theof the target alkyl analytes chains in may the target account analytes for this may [13 ].account for this [13].

Figure 4. Responses of the nano-Ni(OH)2 QCM sensor to various organic vapors. The concentration Figure 4. Responses of the nano-Ni(OH)2 QCM sensor to various organic vapors. The concentration of DBP, diethyl phthalate (DEP) and dimethyl phthalate (DMP) was 40 ppb, while other vapors were of DBP, diethyl phthalate (DEP) and dimethyl phthalate (DMP) was 40 ppb, while other vapors were 20 ppm. The response of DBP was far bigger than the inferences. 20 ppm. The response of DBP was far bigger than the inferences. 3.4. Sensitivity at the Fundamental Frequency 3.4. Sensitivity at the Fundamental Frequency After optimizing the thickness of the sensing film and testing the selectivity, the QCM sensor withAfter the optimizing film thickness the thicknessof 3.46 μg/mm of the2 sensingwas tested film in and different testing theconc selectivity,entrations theof DBP QCM vapor sensor that with 2 theranged film thickness from 0.4 ofppb 3.46 to µ40g/mm ppb. Forwas each tested concentratio in differentn, three concentrations cycles were of ca DBPrried vapor out to that test ranged the fromrepeatability 0.4 ppb to and 40 ppb.the stability For each of concentration,the sensor. Figure three 5 shows cycles the were dynamic carried response out to testcurves the of repeatability the QCM andsensor the stability when three of the different sensor. concentration Figure5 shows DBP the vapor dynamic were injected response into curves the chamber. of the QCM From sensor the figure when threewe differentcan know concentration that the response DBP time vapor is wereless than injected 300 s into when the the chamber. concentration From the is higher figure wethan can 8 ppb know thatand the the response recovery time time is is less nearly than 600 300 s sfor when all concentration, the concentration which is highercan satisfy than the 8 ppbfast andmonitor the recoveryof the timeconcentration is nearly 600 of sthe for DBP all concentration,vapor. The response which time can was satisfy longer the when fast monitor the concentration of the concentration was lower, of theprobably DBP vapor. because The of response smaller concentration time was longer gradient when di thefference concentration that slow down was lower,the absorption probably process. because ofMoreover, smaller concentration the response gradient curves were difference extremely that stable slow downwith the the error absorption below process.1 Hz, showing Moreover, good the responsestability curves and accuracy were extremely of the new stable system. with The the noncontact error below configuration 1 Hz, showing rather good than stability using a and long accuracy wire of theto connect new system. the oscillator The noncontact to the circuit configuration was the ratherkey reason. than usingThanks a longto this, wire good to connect SNR (>50) the oscillatorof the to thereceiver circuit signal was was the keyachieved. reason. High Thanks spikes to were this, ob goodserved SNR in (>50)the frequency of the receiver response signal shortly was after achieved. N2 was injected to desorb the sensor, which was caused by instantly increased air pressure in the High spikes were observed in the frequency response shortly after N2 was injected to desorb the chamber. Figure 6 shows the calibration curve of the nano-Ni(OH)2-coated QCM sensor to different sensor, which was caused by instantly increased air pressure in the chamber. Figure6 shows the DBP concentrations. The mass of DBP molecular adsorbed to the nano-Ni(OH)2 film increased calibration curve of the nano-Ni(OH) -coated QCM sensor to different DBP concentrations. The mass linearly when the concentration was2 between 0.4 ppb and 16 ppb. Then, with the concentration

Sensors 2017, 17, 1681 7 of 11

of DBP molecular adsorbed to the nano-Ni(OH)2 ﬁlm increased linearly when the concentration was Sensors 2017, 17, 1681 7 of 11 betweenSensors 2017 0.4, 17 ppb, 1681 and 16 ppb. Then, with the concentration increased, the increments of the7 of mass 11 adsorbed become fewer. The sensitivity was achieved as 2.4 Hz/ppb, which is the same as the previous increased, the increments of the mass adsorbed become fewer. The sensitivity was achieved as work2.4increased, Hz/ppb, [14], showing the which increments is that the the same of wireless the as themass previous method adsorbed ofwork databecome [14], acquisition showing fewer. Thethat would sensitivitythe wireless not affect was method theachieved adsorption of data as processacquisition2.4 Hz/ppb, itself. would Finally,which not is the the affect limit same the of as adso detection therption previous of process the work sensor itself. [14], was Finally,showing achieved the that limit asthe 1.2of wireless detection ppb (calculated method of the ofsensor as data three timeswasacquisition theachieved signal-to-noise would as 1.2 not ppb affect ratio). (calculated the adso asrption three processtimes the itself. sign Finally,al-to-noise the ratio).limit of detection of the sensor was achieved as 1.2 ppb (calculated as three times the signal-to-noise ratio).

Figure 5. Response of the nano-Ni(OH)2-coated QCM sensor to 8, 24, and 40 ppb (from bottom to top) Figure 5. Response of the nano-Ni(OH)2-coated QCM sensor to 8, 24, and 40 ppb (from bottom to top) Figure 5. Response of the nano-Ni(OH)2-coated QCM sensor to 8, 24, and 40 ppb (from bottom to top) ofof DBP. DBP. of DBP.

Figure 6. Calibration curve of the nano-Ni(OH)2-coated QCM sensor to different concentrations of Figure 6. Calibration curve of the nano-Ni(OH)2-coated QCM sensor to different concentrations of FigureDBP vapor. 6. Calibration The inset curve indicates of the the nano-Ni(OH) calibration curve2-coated of the QCMlinear sensorrange. to different concentrations of DBP vapor. The inset indicates the calibration curve of the linear range. DBP vapor. The inset indicates the calibration curve of the linear range. 3.5. Experiment at 3rd Harmonics 3.5. Experiment at 3rd Harmonics With the noncontact configuration, operating at higher harmonics becomes a realistic way to improveWith the the performance noncontact configuration,of the DBP sensor. operating We simp at higherly changed harmonics the frequency becomes of a therealistic RF signal way to aboutimprove 18 MHzthe performance (the 3rd harmonic of the DBPof the sensor. quartz We plate), simp andly changedcarried out the the frequency same experiment. of the RF signalFigure to 7 showsabout 18the MHz responses (the 3rd of harmonic 24 ppb DBPof the when quartz operating plate), and at carriedthe fundamental out the same frequency experiment. and Figurethe 3rd 7 harmonics,shows the responsesrespectively. of 24The ppb frequency DBP when shifts operating were three at timesthe fundamental larger when frequency the QCM and sensor the was3rd operatedharmonics, at respectively.the 3rd harmonics The frequency than when shifts being we opreerated three timesat the largerfundamental when thefrequency, QCM sensor while wasthe operated at the 3rd harmonics than when being operated at the fundamental frequency, while the

Sensors 2017, 17, 1681 8 of 11

3.5. Experiment at 3rd Harmonics With the noncontact configuration, operating at higher harmonics becomes a realistic way to improve the performance of the DBP sensor. We simply changed the frequency of the RF signal to about 18 MHz (the 3rd harmonic of the quartz plate), and carried out the same experiment. Figure7 shows the responses of 24 ppb DBP when operating at the fundamental frequency and the 3rd harmonics, Sensors 2017, 17, 1681 8 of 11 respectively. The frequency shifts were three times larger when the QCM sensor was operated at the 3rdresponse harmonics time and than the when recovery being time operated were at exactly the fundamental the same. This frequency, means whilethat changing the response the operating time and thefrequency recovery would time werenot affect exactly either the same. the vapor This meansadsorption that changing process or the mechanism, operating frequency but only wouldmade notthe affectresponse either of thefrequency vapor adsorptionincrease three process times or and mechanism, thus increased but only the made sensitivity the response of the ofsensor. frequency It is increaseinteresting three that times the height and thus of the increased spike when the operat sensitivityed at of3rd the harmonics sensor. It was is interesting far more than that three the height times of theits spikevalue whenwhen operatedbeing operated at 3rd harmonics at the fundamen was fartal more frequency. than three This times confirms of its valuethat the when spike being in operated at the fundamental frequency. This confirms that the spike in frequency when N started to frequency when N2 started to inject to desorb the sensor was not caused by a mass increase,2 but by injectan air topressure desorb change the sensor because was notof their caused different by a mass growth increase, multiples. but by Therefore, an air pressure we can change speculate because that ofdifferent their different growth multiples growth multiples. in frequency Therefore, shifts when we can operated speculate in 3rd that harmonics different may growth help multiples to research in frequencymore in the shifts kinetics when of the operated absorption in 3rd and harmonics desorption may process help toof researchthe vapor more molecule in the and kinetics the sensing of the absorptionfilm, and more and research desorption needs process to be ofdone the in vapor the fu moleculeture. Figure and 8 the shows sensing the film,calibration and more curves research of the needs to be done in the future. Figure8 shows the calibration curves of the nano-Ni(OH) -coated QCM nano-Ni(OH)2-coated QCM sensors to DBP vapor when operated at different frequencies2 (the sensorsfundamental to DBP frequency vapor when and operated the 3rd atharmonics). different frequencies It implies (thethat fundamentalthe sensitivity frequency of the sensor and the being 3rd harmonics).operated at Itthe implies 3rd harmonics that the sensitivity is three oftimes the sensorof thatbeing when operated being operated at the 3rd at harmonics the fundamental is three timesfrequency. of that The when limit being of detection operated working at the fundamental at the fundamental frequency. frequency The limit the of value detection was working1.2 ppb, while at the fundamentalit was about 0.4 frequency ppb when the working value was at the 1.2 3rd ppb, harmon whileics it was(calculated about 0.4 as ppbthree when times working the signal-to-noise at the 3rd harmonicsratio). From (calculated all theas data three timesabove theand signal-to-noise compared ratio).to the From previous all the datawork, above the and use compared of the towireless-electrodeless the previous work, theQCM-D use of sensor the wireless-electrodeless with simply increasing QCM-D the sensorfrequency with of simply the RF increasing signal could the frequencyachieve better of the sensitivity RF signal and could a lower achieve limit better of detection sensitivity by using and a exactly lower limitthe same of detection sensing bymaterial, using exactlywhich means the same if better sensing sensing material, film whichwas not means found if ye bettert, we sensing could have film an was alternative not found to yet, improve we could the havesensing an performance. alternative to improve the sensing performance.

Figure 7.7. ResponsesResponsesof of the the nano-Ni(OH) nano-Ni(OH)2 QCM2 QCM sensor sensor to 24to ppb24 ppb DBP. DBP. The redThe linered isline the is response the response when thewhen quartz the quartz oscillator oscillator works atworks the fundamental at the fundamen frequency,tal frequency, while the while blue one the works blue one at 3rd works harmonics. at 3rd harmonics.

Sensors 2017, 17, 1681 9 of 11 Sensors 2017, 17, 1681 9 of 11

Figure 8. Calibration curve of the nano-Ni(OH)2-coated QCM sensor to different concentrations of Figure 8. Calibration curve of the nano-Ni(OH)2-coated QCM sensor to different concentrations of DBP vapor.vapor. The The red red line line is theis responsethe response when when the quartz the quartz oscillator oscillator works at wo therks fundamental at the fundamental frequency, whilefrequency, the blue while one the works blueat one 3rd works harmonic. at 3rd The harmonic inset indicates. The inset the indicates calibration the curve calibration of the linearcurve range.of the linear range. 4. Conclusions 4. Conclusions In this paper, a DBP gas sensor based on a homemade wireless-electrodeless QCM-D is present. In this paper, a DBP gas sensor based on a homemade wireless-electrodeless QCM-D is present. With the noncontact configuration, wires or electrodes were removed so that damage to the chamber With the noncontact configuration, wires or electrodes were removed so that damage to the chamber for wire was avoided and the operated frequency was raised to 3rd harmonics. Nano-structured nickel for wire was avoided and the operated frequency was raised to 3rd harmonics. Nano-structured hydroxide [14] was used as the sensing film. With the 3rd harmonics operating frequency (18 MHz), nickel hydroxide [14] was used as the sensing film. With the 3rd harmonics operating frequency a higher sensitivity of 7.3 Hz/ppb was achieved with half the usage of the sensing material. A low (18 MHz), a higher sensitivity of 7.3 Hz/ppb was achieved with half the usage of the sensing material. limit of detection 0.4 ppb was also achieved, which was below the safety concentration level and A low limit of detection 0.4 ppb was also achieved, which was below the safety concentration level satisfied the detecting demand. Additionally, the system showed more stable than the traditional QCM and satisfied the detecting demand. Additionally, the system showed more stable than the traditional system owing to the fitting process when calculating the frequency. QCM system owing to the fitting process when calculating the frequency. In the future, more work needs to be done to improve the performance of this new kind of In the future, more work needs to be done to improve the performance of this new kind of wireless-electrodeless QCM sensor. Using a thinner quartz plate and operating in higher harmonics wireless-electrodeless QCM sensor. Using a thinner quartz plate and operating in higher harmonics will be tried to achieve a better performance of the gas sensor. Moreover, the potential for obtaining will be tried to achieve a better performance of the gas sensor. Moreover, the potential for obtaining more information about the experiment subject through simultaneously detecting the frequency and more information about the experiment subject through simultaneously detecting the frequency and the dissipation value will be researched. the dissipation value will be researched. Acknowledgments: The work is supported by the Natural Science Foundation of China (Grant No. 61403339) andAcknowledgments: the Autonomous The Research work Projectis supported of the Stateby the Key Natural Laboratory Science of IndustrialFoundation Control of China Technology, (Grant No. China 61403339) (Grant No.and ICT1601).the Autonomous Research Project of the State Key Laboratory of Industrial Control Technology, China Author(Grant No. Contributions: ICT1601). Daqi Chen, You Wang and Huifen Hu conceived and designed the experiments; Chen Daqi built the experimental platform and device; Guokang Fan prepared the used sensing materials; Daqi Chen, Author Contributions: Daqi Chen, You Wang and Huifen Hu conceived and designed the experiments; Chen Xiyang Sun performed the experiments; Daqi Chen and Xiyang Sun analyzed the experimental ressults; Daqi Chen, RuifenDaqi built Hu the and experimental Guang Li wrote platform the paper. and device; Guokang Fan prepared the used sensing materials; Daqi Chen, Xiyang Sun performed the experiments; Daqi Chen and Xiyang Sun analyzed the experimental ressults; Daqi Conflicts of Interest: The authors declare no conflicts of interest. Chen, Ruifen Hu and Guang Li wrote the paper.

ReferencesConflicts of Interest: The authors declare no conflicts of interest.

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