A Noncontact Dibutyl Phthalate Sensor Based on a

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Jul 21, 2017 - homemade wireless-electrodeless quartz crystal microbalance with dissipation (QCM-D) coated with nano-structured nickel hydroxide is ...
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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

2

*

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.) 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 (1269–7104 ng/m3 ), indoor dust (4.4–2300 mg/kg) and ambient air (PM2.5 associated 8.72 µg/m3 , PM10 -associated 12.90 µg/m3 ) [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

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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

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 electric field Sensors 2017, 17,was 1681 generated and the quartz oscillator was excited because of the converse piezoelectric 3 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 was homemade and The the impedance matching network was a HF automatic antenna tuner (mAT-125E). piezoelectric effect. received signals entered a narrowband amplifier then to the oscilloscope so Figure 1a shows the homebuilt electrodeless QCM-D gas chamber. The AT-cut 6.0 MHz quartz plate that the damping vibration signal was recorded. Then, the PC was used to real-time analyze the was placed and at thedissipation bottom of the chamber where 9 mm diameter holefrequency was machined to quartz fix the frequency value. During the aexperiment, the blind resonant of the location of the quartz plate. Two spiral coils were placed below the chamber and right below the oscillator will change, but we did not change the frequency of the RF signal after the resonant quartz plate. The distance between the coils and the quartz plate was 3 mm. The volume capacity of frequency of the quartz crystal reached the base line in case of an effect on the vibration state of the the chamber was 1 L. At the top of the chamber, a small plastic airbag was used to keep the pressure as quartz oscillator. 1 atm.

(a)

(b) Figure 1. 1. (a) (QCM) gas gas chamber. Two spiral spiral Figure (a) The The homebuilt homebuilt electrodeless electrodeless quartz quartz crystal crystal microbalance microbalance (QCM) chamber. Two coils are placed outside the chamber and right below the quartz plate. They are used to radiate the coils are placed outside the chamber and right below the quartz plate. They are used to radiate the electric field, field, which which excites excites the the quartz quartz oscillator oscillator and and receives receives the the vibrational vibrational signals signals of of the the quartz quartz electric oscillator. The quartz oscillator is placed at the bottom of the chamber and above the middle of two two oscillator. The quartz oscillator is placed at the bottom of the chamber and above the middle of coils. (b) The whole structure of the wireless-electrodeless quartz crystal microbalance with coils. (b) The whole structure of the wireless-electrodeless quartz crystal microbalance with dissipation dissipation (QCM-D) The signal generator burst radio frequency signal and (QCM-D) system. The system. signal generator generates the generates burst radiothe frequency signal and finally sends it finally sends it to coil. the transmitting The the other coil and receives the signal andnarrowband then sendsamplifier it to the to the transmitting The other coilcoil. receives signal then sends it to the narrowband amplifier then the oscilloscope finally to the PC for analyzing. then to the oscilloscope and to finally to the PC forand 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 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 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

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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. Vx =

V×C×M 273 + TR × 10−9 × 22.4 × d × P 273 + TB

(1)

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 solution (g/cm3 ), 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 Figure 1a). 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 Figure 2a,b, the Ni(OH)2 sample presented a sheet-like morphology.

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(a) (a)

(b) (b)

Figure 2. 2. SEM SEM morphology morphology of of the the nano–Ni(OH) nano–Ni(OH)22 sample sample at at different different scales. scales. (a) (a) ×20,000, ×20,000, (b) (b) ×50,000. ×50,000. Figure Figure 2. SEM morphology of the nano–Ni(OH) 2 sample at different scales. (a) ×20,000, (b) ×50,000.

3.2. Optimization Optimization of the the Thickness Thickness of of the the Sensing Sensing FILM FILM 3.2. 3.2. Optimization of of the Thickness of the Sensing FILM The thickness thickness of of the the sensing sensing film film on on the the quartz quartz plate plate was was aa key key factor factor to to the the sensitivity. sensitivity. Thicker Thicker The The thickness of the sensing film on the quartz plate was a key factor to the sensitivity. Thicker sensing film film could could provide provide more more adsorption adsorption sites and and thus thus more more analyte molecules molecules would be be sensing sensing film could provide more adsorption sites sites and thus more analyte analyte molecules would bewould adsorbed adsorbed in the same concentration. However, when there were too many layers of the sensing adsorbed in concentration. the same concentration. However, werelayers too many of material the sensing in the same However, when therewhen were there too many of thelayers sensing on material on on the the film, film, the the analyte analyte molecules molecules could could not not attach attach to to the the inner inner sites sites and and the the adsorb adsorb mass mass material the film, the analyte molecules could not attach to the inner sites and the adsorb mass would reach would reach reach saturation. saturation. Worse, Worse, ifif the the sensing film film was was too too thick, thick, itit would would deteriorate deteriorate the the sensing sensing would saturation. Worse, if the sensing film wassensing too thick, it would deteriorate the sensing ability of the QCM ability of the QCM sensors because the sensing film is also the adsorb mass of the QCM. Based on ability ofbecause the QCM the the sensing film is also theQCM. adsorb masson ofthis, the QCM. Based on sensors the sensors sensingbecause film is also adsorb mass of the Based 10 sensors with this, 10 10 sensors sensors with with different different thickness thickness sensing sensing films films were were made made by by controlling controlling the the volume volume of of the the this, 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 nano-Ni(OH) solution coated onthe thefilm QCM to optimize the film thickness. Thesensors responses all these coated on the2QCM to optimize thickness. The responses of all these to 24ofppb DBP sensors to to 24 24 ppb ppb DBP DBP vapor vapor were were measured measured and and are are shown shown in in Figure Figure 3. 3. The The response response of of the the sensor sensor sensors vapor were measured and are shown in Figure 3. The response of the sensor is proportional to the2 is proportional proportional to to the the thickness thickness of of the the nano-Ni(OH) nano-Ni(OH)22 film film when when the the loaded loaded mass mass is is2 below below 3.14 3.14 μg/mm μg/mm2,, is thickness of the nano-Ni(OH) 2 film when the loaded mass is below 3.14 µg/mm , and then reaches a and then reaches a saturation level after that. In order to achieve a fast response and avoid error and then reaches a saturation leveltoafter that.aIn order to achieve a fasterror response and error saturation level after that. In order achieve fast response and avoid caused by avoid the coating caused by the coating process, the optimal thickness of nano-Ni(OH) 2 film was chosen as 3.46 caused by coating process, the optimal thickness of nano-Ni(OH)2 film 2was chosen as 3.46 process, thethe optimal thickness of nano-Ni(OH) 2 film was chosen as 3.46 µg/mm , a little thicker than 2 μg/mm2,, aa little little thicker thicker than than the minimum stable point. μg/mm the minimum stable point. the minimum stable point. 70 70 60 60

Response(Hz) Response(Hz)

50 50 40 40 30 30 20 20 10 10 00 0.5 0.5

11

1.5 1.5

22

2.5 2.5

33

3.5 3.5

44

4.5 4.5

55

Loaded mass mass per per unit unit area(μg·mm area(μg·mm-2-2)) Loaded Figure 3. Response curve of the nano-Ni(OH)2 QCM sensors to 24 ppb dibutyl phthalate (DBP) with Figure 3. 3.loaded Response curve of the the nano-Ni(OH) nano-Ni(OH) QCM sensors sensors to to 24 24 ppb ppb dibutyl dibutyl phthalate phthalate (DBP) (DBP) with with Figure Response curve of 22 QCM different mass of sensing material. different loaded loaded mass mass of of sensing sensing material. material. different

3.3. Selectivity 3.3. Selectivity Selectivity 3.3. To investigate the selectivity of the sensing film, diverse VOCs interferences, including two other To investigate investigate the selectivity selectivity ofand theseveral sensingconventional film, diverse diverse VOCs VOCs interferences, including two other other To the the sensing film, interferences, including two plasticizer vapors, DEP and DMP,of solvents like ethanol, chloroform, ethyl plasticizer vapors, DEP and DMP, and several conventional solvents like ethanol, chloroform, ethyl plasticizer vapors, DEP and DMP, and several conventional solvents like ethanol, chloroform, ethyl acetate, acetic acetic acid, acid, acetaldehyde acetaldehyde and and benzene, benzene, were were tested. tested. Among Among them, them, DEP DEP and and DMP DMP were were tested tested acetate,

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Sensors acetic 2017, 17,acid, 1681 acetaldehyde and benzene, were tested. Among them, DEP and DMP were 6 of 11 acetate, tested at 40 ppb and the other conventional solvents were tested at 20 ppm. Figure 4 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 other conventional inferences andand suggested thatthat thethe sensor cancan detect plasticizer vapors at ppb levels. other conventional inferences suggested sensor detect plasticizer vapors at ppb The reason possibly that DBP, and DMP moremore hydrogen bond acceptors (4) (4) than levels. Thewas reason was possibly thatDEP DBP, DEP and contain DMP contain hydrogen bond acceptors thethan other interferences ( ≤ 2), which means they are more easily able to interact with the − OH groups the other interferences (≤2), which means they are more easily able to interact with the −OH ongroups the surface of surface the nano-structured nano-Ni(OH) hydrogen bonding so that so thethat physical 2 through on the of the nano-structured nano-Ni(OH) 2 through hydrogen bonding the adsorption was facilitated. Additionally, the benzene and ring the symmetrical molecular structure physical adsorption was facilitated. Additionally, the ring benzene and the symmetrical molecular of structure DBP, DEPofand DMP lead to DMP their good hydrophobicity, which contributes their affinity. DBP, DEP and lead to their good hydrophobicity, whichtocontributes to Finally, their among DBP, DEP and DMP, theDEP response amplitudes decreased in turn,decreased the length the the alkyl chains affinity. Finally, among DBP, and DMP, the response amplitudes inof turn, length of the alkylanalytes chains inmay the target analytes account for this [13]. in the target account for thismay [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 After the thickness of the2sensing film in and testing the selectivity, the QCMvapor sensor with with theoptimizing film thickness of 3.46 μg/mm was tested different concentrations of DBP that 2 was tested in different concentrations of DBP vapor that ranged theranged film thickness of 3.46 µg/mm from 0.4 ppb to 40 ppb. For each concentration, three cycles were carried out to test the from 0.4 ppb toand 40 ppb. For each three cyclesthe were carried out to test theofrepeatability repeatability the stability of concentration, the sensor. Figure 5 shows dynamic response curves the QCM and the stability of the sensor.concentration Figure 5 shows dynamic response curves of the QCM when sensor when three different DBPthe vapor were injected into the chamber. Fromsensor the figure three DBP time vapor injected thethe chamber. From the figure we know we different can knowconcentration that the response is were less than 300 sinto when concentration is higher thancan 8 ppb that thethe response time is is less than600 300ssfor when the concentration is higher thanthe 8 ppb the recovery and recovery time nearly all concentration, which can satisfy fast and monitor of the concentration of sthe vapor. The response longer the concentration was lower, of time is nearly 600 forDBP all concentration, whichtime can was satisfy the when fast monitor of the concentration because smaller concentration gradient difference that slow down the absorption process. theprobably DBP vapor. Theofresponse time was longer when the concentration was lower, probably because Moreover, the response curves were extremely stable with the error below 1 Hz, showing goodthe of smaller concentration gradient difference that slow down the absorption process. Moreover, stability and accuracy of the new system. The noncontact configuration rather than using a long wire response curves were extremely stable with the error below 1 Hz, showing good stability and accuracy to connect the oscillator to the circuit was the key reason. Thanks to this, good SNR (>50) of the of the new system. The noncontact configuration rather than using a long wire to connect the oscillator receiver signal achieved. High spikes in(>50) the frequency response shortly N2 to the circuit waswas the key reason. Thanks towere this, observed good SNR of the receiver signal wasafter achieved. 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. Figure 6 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)2 -coated QCM sensor to different DBP concentrations. The mass linearly when the concentration was between 0.4 ppb and 16 ppb. Then, with the concentration

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of DBP molecular adsorbed to the nano-Ni(OH)2 film increased linearly when the concentration was Sensors 2017, 17, 1681 7 of 11 between 0.4 17, ppb and 16 ppb. Then, with the concentration increased, the increments of the Sensors 2017, 1681 7 ofmass 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 work [14], showing that method of data acquisition would not affect the adsorption increased, the increments ofwireless the mass adsorbed become fewer. The sensitivity was achieved as 2.4 Hz/ppb, which is thethe same as the previous work [14], showing that the wireless method of data 2.4 Hz/ppb, which is the same as the previous work [14], showing that the wireless method of data process itself. Finally, the limit of detection of the sensor was achieved as 1.2 ppb (calculated as three acquisition would not affect the adsorption process itself. Finally, the limit of detection of the sensor acquisition would affect the adsorption itself. Finally, the ratio). limit of detection of the sensor times the signal-to-noise ratio). was achieved as 1.2not ppb (calculated as threeprocess times the signal-to-noise was achieved as 1.2 ppb (calculated as three times the signal-to-noise ratio).

Figure Responseofofthe thenano-Ni(OH) nano-Ni(OH)2-coated -coated QCM ppb (from bottom to top) Figure 5. 5. Response QCMsensor sensortoto8,8,24, 24,and and4040 ppb (from bottom to top) Figure 5. Response of the nano-Ni(OH)22-coated QCM sensor to 8, 24, and 40 ppb (from bottom to top) of DBP. of 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 the calibration nano-Ni(OH) 2-coated QCM sensor to different concentrations of DBP vapor. The insetcurve indicates curve of the linear range. Figure 6. Calibration of the nano-Ni(OH) 2 -coated QCM sensor 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 Withthe theperformance noncontact configuration, operating at higher harmonics becomesofa the realistic way to to improve of the DBP sensor. We simply changed the frequency RF signal improve the performance of the DBP sensor. We simply changed the frequency of the RF signal about 18 MHz (the 3rd harmonic of the quartz plate), and carried out the same experiment. Figureto 7 about 18 MHz (the 3rdofharmonic of thewhen quartz plate), and out the same experiment. shows the responses 24 ppb DBP operating at carried the fundamental frequency and Figure the 3rd7 shows the responses of 24 DBP when at times the fundamental the was 3rd harmonics, respectively. Theppb frequency shiftsoperating were three larger whenfrequency the QCMand sensor harmonics, The frequency shifts were three times when the QCM sensor operated at respectively. the 3rd harmonics than when being operated at the larger fundamental frequency, whilewas the operated at the 3rd harmonics than when being operated at the fundamental frequency, while the

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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. Figure 7 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 3rd harmonics thanthe when being time operated the fundamental frequency, the response time and response time and recovery wereatexactly the same. This means while that changing the operating the recovery time were exactly the same. This means that changing the operating frequency would frequency would not affect either the vapor adsorption process or mechanism, but only made not the affect either the vapor adsorption process or mechanism, but only made the response of frequency response of frequency increase three times and thus increased the sensitivity of the sensor. It is increase three and thus increased the operated sensitivity the sensor. Itwas is interesting thatthree the height interesting thattimes the height of the spike when atof 3rd harmonics far more than times of the spike when operated at 3rd harmonics was far more than three times of its value when being of its value when being operated at the fundamental frequency. This confirms that the spike in operated at the fundamental frequency. This confirms that the spike in frequency when N started to 2 frequency when N2 started to inject to desorb the sensor was not caused by a mass increase, but by inject desorb change the sensor was not caused by a mass increase, but byTherefore, an air pressure change because an airto pressure because of their different growth multiples. we can speculate that of their different growth multiples. Therefore, we can speculate that different growth multiples in different growth multiples in frequency shifts when operated in 3rd harmonics may help to research frequency when operated in 3rdand harmonics may help to in theand kinetics of the more in theshifts kinetics of the absorption desorption process of research the vapormore molecule the sensing absorption and desorption process of the vapor molecule and the sensing film, and more research film, and more research needs to be done in the future. Figure 8 shows the calibration curves of the needs to be done in the future. Figure 8 shows the calibration curves of the nano-Ni(OH)2 -coated QCM nano-Ni(OH) 2-coated QCM sensors to DBP vapor when operated at different frequencies (the sensors to DBPfrequency vapor when different frequencies frequency and the 3rd fundamental andoperated the 3rd at harmonics). It implies (the that fundamental the sensitivity of the sensor being harmonics). It implies that the sensitivity of the sensor being operated at the 3rd harmonics is three operated at the 3rd harmonics is three times of that when being operated at the fundamental times of thatThe when being operatedworking at the fundamental frequency. The limitthe of value detection at the frequency. limit of detection at the fundamental frequency wasworking 1.2 ppb, while fundamental frequency the value was 1.2 ppb, while it was about 0.4 ppb when working at the 3rd it was about 0.4 ppb when working at the 3rd harmonics (calculated as three times the signal-to-noise harmonics (calculated as three times the signal-to-noise ratio). From all the data above and compared ratio). From all the data above and compared to the previous work, the use of the to the previous work, the use ofsensor the wireless-electrodeless QCM-D withofsimply the wireless-electrodeless QCM-D with simply increasing the sensor frequency the RFincreasing signal could frequency of the RF signal could achieve better sensitivity and a lower limit of detection by using achieve better sensitivity and a lower limit of detection by using exactly the same sensing material, exactly the same sensing material, means better film an was not foundtoyet, we could which means if better sensing film which was not foundif yet, wesensing could have alternative improve the have an alternative to improve the sensing performance. sensing performance.

Figure 7. sensor to 24 DBP.DBP. The red the is response when 7. Responses Responsesofofthe thenano-Ni(OH) nano-Ni(OH) 2 QCM sensor to ppb 24 ppb Theline redisline the response 2 QCM the quartz oscillator works at the fundamental frequency, while the blue one works at 3rd harmonics. when the quartz oscillator works at the fundamental frequency, while the blue one works at 3rd

harmonics.

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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 Thered red is response the response the quartz oscillator at the fundamental DBP lineline is the whenwhen the quartz oscillator works atworks the fundamental frequency, frequency, while blueatone at 3rdThe harmonic. The inset thecurve calibration curve range. of the while the blue onethe works 3rdworks harmonic. inset indicates theindicates calibration of the linear 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) Acknowledgments: work Project is supported by the Natural Science Foundation of China (Grant No. 61403339) and the AutonomousThe Research of the State Key Laboratory of Industrial Control Technology, China (Grant No. and ICT1601). the Autonomous Research Project of the State Key Laboratory of Industrial Control Technology, China (Grant No. ICT1601). Daqi Chen, You Wang and Huifen Hu conceived and designed the experiments; Chen Daqi Author Contributions: 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, Daqi built platform and device; Guokang Fan prepared the used sensing materials; Daqi Chen, Ruifen Hu the andexperimental Guang Li wrote the paper. 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.

Conflicts of Interest: The authors declare no conflicts of interest. References 1. Al-Natsheh, M.; Alawi, M.; Fayyad, M.; Tarawneh, I. Simultaneous GC-MS determination of eight phthalates References in total and migrated portions of plasticized polymeric toys and childcare articles. J. Chromatogr. B 2015, 985, 1. Al-Natsheh, M.; Alawi, M.; Fayyad, M.; Tarawneh, I. Simultaneous GC-MS determination of eight 103–109. [CrossRef] [PubMed] phthalates in total and migrated portions plasticizedofpolymeric and childcare J. Chromatogr. 2. Wang, Z.-W.; Gao, S.; Hu, C.-Y.; Wu, Y.-M.ofModelling migrationtoys from printing inksarticles. on paper packaging. B 2015, Technol. 985, 103–109. Packag. Sci. 2015, 28, 357–366. [CrossRef] 2. Wang, Z.-W.; Gao, S.; Hu, C.-Y.; Wu, Y.-M. Modelling of migration from printing inks on paper packaging. Packag. Technol. Sci. 2015, 28, 357–366.

Sensors 2017, 17, 1681

3. 4.

5. 6. 7.

8.

9.

10.

11.

12. 13.

14.

15. 16. 17. 18.

19.

20.

21. 22. 23.

10 of 11

Bonini, M.; Errani, E.; Zerbinati, G.; Ferri, E.; Girotti, S. Extraction and gas chromatographic evaluation of plasticizers content in food packaging films. Microchem. J. 2008, 90, 31–36. [CrossRef] Mylchreest, E.; Sar, M.; Cattley, R.C.; Foster, P.M.D. Disruption of androgen-regulated male reproductive development by Di (n-butyl) phthalate during late gestation in rats is different from flutamide. Toxicol. Appl. Pharmacol. 1999, 156, 81–95. [CrossRef] [PubMed] Naarala, J.; Korpi, A. Cell death and production of reactive oxygen species by murine macrophages after short term exposure to phthalates. Toxicol. Lett. 2009, 188, 157–160. [CrossRef] [PubMed] Wang, X.; Song, M.; Guo, M.; Chi, C.; Mo, F.; Shen, X. Pollution levels and characteristics of phthalate esters in indoor air in hospitals. J. Environ. Sci. 2015, 37, 67–74. [CrossRef] [PubMed] Bi, X.; Yuan, S.; Pan, X.; Winstead, C.; Wang, Q. Comparison, association, and risk assessment of phthalates in floor dust at different indoor environments in Delaware, USA. J. Environ. Sci. Health Part A Toxic/Hazard. Subst. Environ. Eng. 2015, 50, 1428–1439. [CrossRef] [PubMed] Kong, S.; Ji, Y.; Liu, L.; Chen, L.; Zhao, X.; Wang, J.; Bai, Z.; Sun, Z. Spatial and temporal variation of phthalic acid esters (PAEs) in atmospheric PM10 and PM2.5 and the influence of ambient temperature in Tianjin, China. Atmos. Environ. 2013, 74, 199–208. [CrossRef] Kopelovich, L.; Perez, A.L.; Jacobs, N.; Mendelsohn, E.; Keenan, J.J. Screening-level human health risk assessment of toluene and dibutyl phthalate in nail lacquers. Food Chem. Toxicol. 2015, 81, 46–53. [CrossRef] [PubMed] Fromme, H.; Lahrz, T.; Piloty, M.; Gebhart, H.; Oddoy, A.; Rüden, H. Occurrence of phthalates and musk fragrances in indoor air and dust from apartments and kindergartens in Berlin (Germany). Indoor Air 2004, 14, 188–195. [CrossRef] [PubMed] Rudel, R.A.; Camann, D.E.; Spengler, J.D.; Korn, L.R.; Brody, J.G. Phthalates, alkylphenols, pesticides, polybrominated diphenyl ethers, and other endocrine-disrupting compounds in indoor air and dust. Environ. Sci. Technol. 2003, 37, 4543–4553. [CrossRef] [PubMed] Wang, Y.; Ding, P.; Hu, R.; Zhang, J.; Ma, X.; Luo, Z.; Li, G. A dibutyl phthalate sensor based on a nanofiber polyaniline coated quartz crystal monitor. Sensors 2013, 13, 3765–3775. [CrossRef] [PubMed] Zhang, K.; Fan, G.; Hu, R.; Li, G. Enhanced dibutyl phthalate sensing performance of a quartz crystal microbalance coated with au-decorated ZnO porous microspheres. Sensors 2015, 15, 21153–21168. [CrossRef] [PubMed] Hu, R.; Zhang, K.; Fan, G.; Luo, Z.; Li, G. Development of a high-sensitivity plasticizer sensor based on a quartz crystal microbalance modified with a nanostructured nickel hydroxide film. Meas. Sci. Technol. 2015, 26, 055102. [CrossRef] Ogi, H.; Motoshisa, K.; Matsumoto, T.; Hatanaka, K.; Hirao, M. Isolated electrodeless high-frequency quartz crystal microbalance for immunosensors. Anal. Chem. 2006, 78, 6903–6909. [CrossRef] [PubMed] Erol, A.; Okur, S.; Yagmurcukarde¸ ˘ s, N.; Arıkan, M.Ç. Humidity-sensing properties of a ZnO nanowire film as measured with a QCM. Sens. Actuators B Chem. 2011, 152, 115–120. [CrossRef] Rodahl, M.; Kasemo, B. A simple setup to simultaneously measure the resonant frequency and the absolute dissipation factor of a quartz crystal microbalance. Rev. Sci. Instrum. 1996, 67, 3238. [CrossRef] Ogi, H.; Motohisa, K.; Hatanaka, K.; Ohmori, T.; Hirao, M.; Nishiyama, M. Concentration dependence of IgG-protein A affinity studied by wireless-electrodeless QCM. Biosens. Bioelectron. 2007, 22, 3238–3242. [CrossRef] [PubMed] Ogi, H.; Fukunishi, Y.; Nagai, H.; Okamoto, K.; Hirao, M.; Nishiyama, M. Nonspecific-adsorption behavior of polyethylenglycol and bovine serum albumin studied by 55-MHz wireless-electrodeless quartz crystal microbalance. Biosens. Bioelectron. 2009, 24, 3148–3152. [CrossRef] [PubMed] Ogi, H.; Yanagida, T.; Hirao, M.; Nishiyama, M. Replacement-free mass-amplified sandwich assay with 180-MHz electrodeless quartz-crystal microbalance biosensor. Biosens. Bioelectron. 2011, 26, 4819–4822. [CrossRef] [PubMed] Stevenson, A.C.; Lowe, C.R. Magnetic–acoustic–resonator sensors (MARS)—A new sensing methodology. Sens. Actuators A Phys. 1999, 72, 32–37. [CrossRef] Stevenson, A.C.; Araya-Kleinsteuber, B.; Sethi, R.S.; Mehta, H.M.; Lowe, C.R. Hypersonic evanescent waves generated with a planar spiral coil. Analyst 2003, 128, 1175. [CrossRef] [PubMed] Stevenson, A.C.; Araya-Kleinsteuber, B.; Sethi, R.S.; Metha, H.M.; Lowe, C.R. Planar coil excitation of multifrequency shear wave transducers. Biosens. Bioelectron. 2005, 20, 1298–1304. [CrossRef] [PubMed]

Sensors 2017, 17, 1681

24. 25. 26. 27.

11 of 11

Hu, R.; Stevenson, A.C.; Lowe, C.R. An acoustic glucose sensor. Biosens. Bioelectron. 2012, 35, 425–428. [CrossRef] [PubMed] Selyanchyn, R.; Wakamatsu, S.; Hayashi, K.; Lee, S.W. A nano-thin film-based prototype QCM sensor array for monitoring human breath and respiratory patterns. Sensors 2015, 15, 18834–18850. [CrossRef] [PubMed] Procek, M.; Stolarczyk, A.; Pustelny, T.; Maciak, E. A study of a QCM sensor based on TiO2 anostructures for the etection of NO2 and explosives vapours in air. Sensors 2015, 15, 9563–9581. [CrossRef] [PubMed] Sabri, Y.M.; Kandjani, A.E.; Ippolito, S.J.; Bhargava, S.K. Nanosphere monolayer on a transducer for enhanced detection of gaseous heavy metal. ACS Appl. Mater. Interfaces 2015, 7, 1491–1499. [CrossRef] [PubMed] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).