A fast-response and highly linear humidity sensor based on quartz crystal microbalance

A fast-response and highly linear humidity sensor based on quartz crystal microbalance

Accepted Manuscript Title: A fast-response and highly linear humidity sensor based on quartz crystal microbalance Authors: Xinyu Zheng, Rongrong Fan, ...

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Accepted Manuscript Title: A fast-response and highly linear humidity sensor based on quartz crystal microbalance Authors: Xinyu Zheng, Rongrong Fan, Chunru Li, Xingyue Yang, Huizi Li, Jiandi Lin, Xuechou Zhou, Rixin Lv PII: DOI: Reference:

S0925-4005(18)32201-9 https://doi.org/10.1016/j.snb.2018.12.081 SNB 25842

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

10 August 2018 30 November 2018 16 December 2018

Please cite this article as: Zheng X, Fan R, Li C, Yang X, Li H, Lin J, Zhou X, Lv R, A fast-response and highly linear humidity sensor based on quartz crystal microbalance, Sensors and amp; Actuators: B. Chemical (2018), https://doi.org/10.1016/j.snb.2018.12.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A fast-response and highly linear humidity sensor based on quartz crystal microbalance Xinyu Zheng*, Rongrong Fan, Chunru Li, Xingyue Yang, Huizi Li, Jiandi Lin, Xuechou Zhou, RixinLv*

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School of Life Sciences, Fujian Agriculture and Forestry University, Fuzhou, 350002, China.

*Corresponding

author.

E-mail address: [email protected]

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Graphical Abstract:

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Fig. 1. Response and recovery times of the humidity sensor.



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Research Highlights:

Polyvinyl alcohol and nano-silica composites are used as the sensing film

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of humidity sensor.

The preparation of sensitive film is simple.



The prepared humidity sensor has short response time (5 s) and short

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recovery time (21 s). The frequency shifts linearly respond to the relative humidity (R2=0.9990).

Abstract: A fast-response and highly linear humidity sensor based on quartz crystal microbalance (QCM) was prepared by using polyvinyl alcohol (PVA) and nano-silica composites as sensing film. The mass ratio of PVA to nano-silica and the total mass of sensing film which can significantly influence the response time and linearity of humidity sensor were well discussed. The results show that the optimal mass ratio of

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PVA to nano-silica is 1:1.5 and the total mass of the sensing film coated on the surface

of QCM is 1.0 μg. Under the optimum conditions, the frequency shifts linearly

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respond to the relative humidity. The prepared humidity sensor has short response time (5 s), short recovery time (21 s), good stability and reproducibility. The developed humidity sensor is expected to be used for the detection of humidity in

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practice.

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Key words: Fast-response; Highly linear; Quartz crystal microbalance; Humidity

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sensor; Polyvinyl alcohol; Nano-silica

1 Introduction Humidity sensor is a kind of sensor which can measure the content of water vapor in the air. This technique has been widely used in the fields such as the management of archives and books, weather forecasting, protection and use of precision instruments, storage of foods, online monitoring of industrial production,

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cultivation of agricultural greenhouses and health care [1-3]. At present, several kinds of the sensors used for humidity detection mainly include the types of impedance, capacitance and piezoelectricity [4-6]. Piezoelectric

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sensor includes surface acoustic waves (SAW) sensor [7] and quartz crystal

microbalance sensors (QCM) [8]. Quartz crystal microbalance (QCM) sensors are used to measure the contents of adsorbed substances by detecting the frequency shifts

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of crystal oscillator. According to Sauerbrey's equation [9], the frequency shifts of

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QCM are proportion to the adsorbed-mass loading on the surface of the electrodes.

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Due to several advantages of QCM sensors such as high sensitivity, high resolution

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and low cost, they have been widely used in the fields of chemistry, biology, physics and medicine [10], especially in the analysis of gas or liquid composition. The

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performance of QCM humidity sensors is mainly determined by the sensing films coated on the surface of QCM. The sensing films of humidity sensors can be divided

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into several categories as follows: ①organic molecules and organic polymers [11,12]. These sensors used in low humidity detection have been proven to be effective with

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high sensitivity. However, the sensing films may be dissolved in the high humidity environment. ② inorganic nano-materials [13,14]. These sensors have good reproducibility and high sensitivity, but the sensing films are difficult to be

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synthesized and immobilized on the surface of electrodes. ③biomass material [15]. These sensors have strong adsorption capacity for water molecular and have high sensitivity due to the possession of huge amounts of hydrophilic groups, but the reproducibility of the sensors is poor.④porous materials [16,17]. The porous material can be easy to be fixed on the surface of electrodes. These sensors have the strong ability of water-absorption due to the porous structure, however, the response

time also become longer.⑤inorganic nanoparticles and organic polymer composite film [18,19]. These sensors have the merits of rapid response, high sensitivity, low hysteresis and wide linear range. Polyvinyl alcohol (PVA) is a kind of water-soluble polymer. Each repeat unit in the backbone chain of PVA is bonded with a hydroxyl group. In addition, PVA has the film forming property. It can absorb and desorb water quickly in air [20, 21]. The

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surface of nano-silica has lots of hydroxyl groups. similarly, nano-silica can also absorb and desorb water in air. The two materials can be used as the sensitive

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materials of QCM humidity sensors. However, absolute value for the slop of calibration curve increase with the increase of relative humidity when PVA is used solely as the sensitive material for the determination of humidity leading to poor

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linearity [20, 22]. Unlike the sensing film of PVA, when nano-silica is used alone as

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sensing materials of QCM, the absolute value for the slop of calibration curve is

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rather large in low humidity and decrease with the increase of relative humidity [19, 23]. Moreover, nano-silica can hardly to be immobilized on the surface of electrodes.

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Our previous research has reported a humidity sensor based on QCM coated with

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urea formaldehyde resin / nano silica composite films, the response and recovery times of the humidity sensor are 12 and 25 s. Because PVA has the good affinity to water, the ease of coating it on the surface of electrode [20], and the rapid absorption /

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desorption of water, it was considered for replacing the urea formaldehyde resin (UFR). So PVA and nano-silica composites were selected as sensitive materials of

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QCM for constructing a faster response humidity sensor in this study. The mass ratio of PVA to nano-silica and total mass of the sensing film loaded on the QCM were well investigated to obtain a rapid response and highly linear humidity sensor.

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2 Experimental 2.1 Material

PVA and nano-silica (particle size 7~40 nm, specific surface area 150 m2·g-1, purity 99.8%) were purchased from Aladdin Chemistry Co. Ltd. LiCl, MgCl 2, Mg(NO3)2, NaCl and KCl were of analytical grade and bought from local commercial sources. 20 MHz quartz crystals (consisting of two silver electrodes with a diameter

of 3.0 mm) were purchased from Yangxing technology Co.,LTD. Double distilled water was used for preparation of all solutions and for washing. 2.2 Preparation of solution 0.1 g of PVA and 0.1 g of nano-silica were added to two 100 mL volumetric flasks, respectively, and then diluted with double distilled water to volume. Two

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solutions were treated by ultrasonic oscillation for 10 minutes to prepare the homogeneous solutions. The PVA solution and the nano-silica solution were mixed with the volume proportion of 1:0, 1:0.5, 1:1, 1:1.5, and 1:2. The total mass (PVA and

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nano-silica) concentration of the mixed solution is 1.0 ×10-3 g·mL-1. 10 mL of the mixed solution was pipetted into 100 mL volumetric flask and brought to volume by double distilled water. The solution was defined as solution (A).

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2.3 Driver circuit

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The electronic components were assembled as shown in Fig. 1. The oscillation

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circuit of sensor is composed of transistors Q1, Q2 and their surrounding components.

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The oscillation circuit of reference crystal is formed by Q3, Q4 and their surrounding components. Differential frequency circuit between sensor and reference crystal is

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made up of Q5, Q6, Q7 and their surrounding components. In oscillation circuit of the sensor, the signal which output from Q2 emitter is fed back positively to Q1 base pole

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by sensor Y1. After amplification by Q1 and Q2, the signal is coupled to Q5 base pole via C5. The frequency of this signal is the frequency of the sensor. Similarly, the

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signal of the reference crystal is coupled to Q5 base via C9. There is a phase difference between two high-frequency signals of different frequencies. A high-frequency signal whose pulsation frequency is the difference frequency of two

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high-frequency signals will inevitably be generated on the Q5 base pole due to the linear superposition of two high-frequency signals. The high-frequency signal is amplified by Q5 and filtered by the π filter which is made up of C11, L7 and C12 for getting the difference frequency signal between the sensor and reference crystal. The signal is coupled to Q6 base pole via C14, amplified by Q6, and then coupled by C19 to Q7. After shaping, the square wave signal whose frequency is the same as the

difference frequency signal can be obtained. The square wave signal is sent to the frequency meter by R16 for measurement. 2.4 Experimental apparatus The detection system including a humidity generating system and driver circuit is designed by our group [19]. 2.5 Experimental methods

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2.5.1 Preparation of sensing film

The silver electrode of QCM was placed in acetoneby ultrasonic cleaning. After

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drying, a certain volume of solution (A) was dispersed on the front and

back surfaces of the electrode. The electrode was dried in vacuum drier at 60℃ for 5 min.

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2.5.2 Testing process

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The as-prepared sensor was connected to the driver circuit of the instrument and

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placed in the testing rooms with different humidity. The frequency of the sensor was

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detected. The standard humidity involved in the experiment was the different humidity produced by the saturated salt solutions (Table 1). After completing each

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measurement of humidity, the sensor was switched to the next humidity testing room immediately. The switching time is less than 2 s. At the same time, the saturated salt

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solution was stirred evenly with a magnetic stirrer for measuring the frequency variation under different relative humidity conditions.

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3. Results and discussion

3.1The morphology of sensitive films SEM pictures of different sensitive films are shown in Fig. 2. It can be seen from

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Fig. 2a that large amounts of PVA (gray villous bulges) are evenly distributed on the surface of the sensor. Accordingly, nano-silica whose morphology is spherical particle (small white dots) can be observed obviously on the surface of the senor (Fig. 2b). As displayed in Fig. 2c, the nano-silica particles which are partially wrapped with gray villous bulges are PVA and nano-silica composite particles. The size of the composite particles appearing on the surface of the sensor is slightly larger than that of

nano-silica, indicating that nano-silica in the composite films are coated with PVA. 3.2The mass ratio of PVA to nano-silica When the total mass of the sensitive film was fixed as 1.0 μg, the effects of different mass ratios of PVA to nano-silica composite films on linearity of the calibration curve was carefully studied at 20℃. As shown in Fig.3, once PVA is used

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alone, the absolute value for the slop of calibration curve increase with the increase of relative humidity (curve a). This phenomenon indicates that the adsorption property of PVA to water molecules is multi-layer adsorption. While the absolute value for the

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slop of calibration curve reduce to zero with the increase of relative humidity for a

sole use of nano-silica, suggesting that the adsorption property of nano-silica water molecules is a monolayer adsorption (curve f). It can be seen that the absolute value

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for the slope of the curve increase gradually with the increase of the mass ratio of

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PVA to nano-silica at lower relative humidity, and decrease conversely at higher

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relative humidity (from curve a to curve e and Table 2). When the mass ratio of PVA

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to nano-silica is 1:1.5, the best linearity of calibration curve can be obtained (curve d, R2=0.9990). The corresponding frequency shifts were taken to calculate the standard

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deviation (the value of the noise) when the each relative humidity reached a relative constant. The value of the noise is less than 0.88 Hz (0.037% RH). The reason mainly

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attribute to the synergy effect of the two sensitive materials. 3.3 Effect of the thickness of sensitive film

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The mass ratio of two sensitive materials (PVA and nano-silica) was set as 1:1.5. The relationship between frequency shifts and relative humidity under different thickness of the sensitive material conditions was well investigated at 20℃. As

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displayed in Fig. 4 and table 3, the absolute value for the slop of calibration curve tend to be larger at thicker sensitive films. The probably reason may be explained that the hydroxyl groups absorbing water molecules on the sensor increase with the increasing thickness of the films. Good linearity of the calibration curves with a slight change for different thickness of the films can be obtained within a certain total mass of sensitive films. However, the more PVA is exposed to the surface of the sensor with

the increasing total mass of sensitive films (Fig.2d), so the water absorption characteristics of composite films is chiefly determined by PVA, resulting the decrease of linear correlation coefficient (Fig.4, curve e). Thus, 1.0 μg was selected as the optimal total mass of the sensitive film. 3.4 Response and recovery time The response and recovery time was taken to study the response characteristics

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of the sensor. The sensor was placed in the testing room with a relative humidity of

11.31%. After 10 minutes, the sensor was switched to another room with a relative

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humidity of 85.11% immediately. The graph of the frequency variation was exhibited

in the Fig. 5. The results suggest that the response time is about 5 s and the recovery time is about 21 s, indicating that the sensor has extremely fast-response speed. The

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experimental results also confirm that the developed humidity sensor has shorter

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response and recovery time than the humidity sensor based on QCM coated with urea

3.5 The reproducibility of sensor

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For validating the precision of the method, the reproducibility of sensor was

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evaluated. The sensor was placed periodically in high-humidity (85.11%) and low-humidity (11.31%) environments (Fig. 6). There is a little change in frequency shift with the relative humidity within 18 cycles of testing. The standard deviation of

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the frequency variation is less than 1.5 Hz. The results manifest that the adsorption and desorption process of water molecular on the composite film is a physical process.

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The adsorbent and desorbent velocities are fast. As is shown in Fig. 7, it can be found that the adsorption curve and the desorption curve are symmetric, suggesting that the process of adsorption and desorption is reversible. In addition, pointy projections can

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also be observed when the humidity of the testing room is significantly different from the ambient humidity. However, this phenomenon has a very short duration. It could be the influence of environmental humidity on the sensor when the sensor was switched to another testing room. The maximum hysteresis of humidity sensor is calculated to be 1.3%. These experimental results demonstrate that the sensor has good reproducibility.

3.6 The stability of the sensor The stability of sensor was also investigated carefully. In order to obtain the response curve of the frequency variation, the sensor was exposed to different relative humidity for a long time. The results from Fig. 8 illustrate that the frequency variation of the sensor coated with PVA and nano-silica composite film is fairly stable under the different relative humidity. The standard deviation of the frequency shift and the

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relative humidity were 10~20 Hz and 0.32%~0.64%, respectively. High stability of

the sensor may be due to the good adhesion performance of PVA. The PVA is

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organic conjugated polymers, which can attach itself and nano-silica to the quartz crystal well.

Two different environmental humidity were also utilized to investigate the

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influences of environmental humidity on the humidity sensor. The mass ratio of PVA

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to nano-silica and the total mass of the sensing film are 1:1.5 and 1.0 μg, respectively.

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Firstly, the temperature of laboratory was adjusted to 20℃ by air-condition (the

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outdoor temperature was 15℃), the measured relative humidity was 48.12% at this moment. Secondly, the ultrasonic humidifier was employed to adjust the laboratory

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humidity to 81.56%, the humidity calibration curves of the sensor were measured under two different environmental humidity conditions. As can be seen from Fig. 9,

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the regression equations of the sensor for relative humidity 48.12% and 81.56% were y = -2280.5x + 201.77 (R² = 0.9943) and

y = -2311.1x + 217.14 (R² = 0.9968),

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respectively. The experimental results suggest that the environmental humidity has little influence on the measurements of the calibration curves. Thus, the sensor possesses the good performance of stability and anti-aging.

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4. Conclusion

A fast-response and highly linear humidity sensor based on quartz crystal

microbalance (QCM) was prepared in this work. PVA and nano-silica composites film was used as the sensitive material. After a series of experiments, it was demonstrated that the prepared humidity sensor has the excellent characteristics of fast response speed, high linearity, good reproducibility, satisfactory stability and nice anti-aging,

especially in terms of the response speed and the linearity of calibration curve comparing the reported literatures (Table 4) [15, 17, 19, 24-26]. Undoubtedly, the sensor has promising potentiality in application of daily life, industrial and agricultural production. Acknowledgement The authors acknowledge the support from the Opening Foundation of National

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Engineering Research Center for Sugarcane (PTJH1500114), China Postdoctoral

Science Foundation (2017M612106), Scientific Research Starting Foundation for the

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Returned Overseas Chinese Scholars, Ministry of Education of China (for Dr. Jian-Di Lin), and the Open Fund of State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences

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(20160025).

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Author Biographies Xinyu Zheng received his PhD from Fujian Agriculture and Forestry University, Fujian, China, in July, 2014. His research interests include polymer materials, organic/inorganic composites for chemical sensors and quartz crystal microbalance. Rongrong Fan is a postgraduate of chemical ecology in School of Life Sciences,

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Fujian Agriculture and Forestry University, Fujian, China. The humidity sensor is one of her current research interests.

Agriculture and Forestry University, Fujian, China.

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Chunru Li is an undergraduate course student in School of Life Sciences, Fujian

Xingyue Yang is an undergraduate course student in School of Life Sciences,

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Fujian Agriculture and Forestry University, Fujian, China.

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Huizi Li is an undergraduate course student in School of Life Sciences, Fujian

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Agriculture and Forestry University, Fujian, China.

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Jiandi Lin received his PhD in Physical chemistry from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou,Fujian,

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China, in July, 2010. His research interests include functional polymers with optical and electrical characteristics.

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Xuechou Zhou received his PhD in Physical chemistry from Fuzhou University, Fujian, China, in July, 2009. He has been longly engaged in the research fields of

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chemical sensors.

Rixin Lv is currently a lecturer of Fujian Agriculture and Forestry University,

Fujian, China. His research is mainly focused on the sensor technology and its

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Captions: Fig. 1. Driving circuit of a quartz crystal microbalance. Q1, Q2, Q3, Q4, Q5, Q6: 9018; C1, C2, C3, C4, C5, C6, C7, C8, C9, C11, C12, C13: 0.01 μF; C10: 56 pF; C14, C15, C16, C17, C18, C19: 100 μF; L1, L2, L3, L4, L5, L6, L7: 100 μH; R1, R2, R4, R5, R7, R8, R10, R11, R14, R21: 10 KΩ; R3, R6, R9, R12, R16, R22: 560 Ω; R13, R19: 30 KΩ; R15, R20, R24: 2 KΩ; R17: 200 Ω; R18: 100 Ω; R23, R26: 1 KΩ,R25:100Ω;Y1: the sensor; Y2: the reference crystal oscillator.

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Fig. 2. SEM images of the sensitive films. The total mass of sensitive film: (a) 1.0 μg polyvinyl alcohol, (b)1.0 μg nano-silica (c) 1.0 μg composite film and (d) 4.0 μg composite film. Fig. 3. The relationships between the calibration curve linearity and the mass ratio of polyvinyl alcohol to nano-silica. The ratio of polyvinyl alcohol to nano-silica: (a) 1.0:0, (b) 1.0:0.5, (c) 1.0:1.0, (d) 1.0:1.5, (e) 1.0:2.0, (f) 0:1.0. Fig. 4. The relationships between the calibration curve linearity and the total mass of sensitive film. The total mass of sensitive film: (a) 0.5 μg, (b) 1.0 μg, (c) 1.5 μg, (d) 2.0 μg, (e) 4.0 μg. Fig. 5. Response and recovery times of the sensor. Fig. 6. The response and recovery curves of the sensor for different relative humidity. Fig. 7. The comparation between response and recovery calibration curves. Fig. 8. The stability of the humidity sensor. Fig. 9. The influence of environmental humidity on the response of the humidity sensor. a-relative humidity 48.12%, b- relative humidity 81.56%

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M

Fig. 9

Table 1 The relative humidity of different saturated salts solutions (20℃) Relative humidity (RH%)

LiCl

11.31

MgCl2

33.07

Mg(NO3)2

55.87

NaCl

75.47

KCl

85.11

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Saturated salt solution

Table 2 Regression equations of the sensors for different mass ratios of the composite films a The correlation coefficient (R²)

a

Regression equation y = -1944.9x + 346.20

b

y = -2926.3x + 519.41

0.9467

c

y = -2652.2x + 335.78

0.9918

d

y = -2385.4x + 247.16

0.9990

e

y = -4092.4x – 274.34

0.7926

f

y = -4551.3x– 494.39

0.7139

0.9300

a

IP T

Samples

A

CC E

PT

ED

M

A

N

U

SC R

The ratio of polyvinyl alcohol to nano-silica: (a) 1.0:0, (b) 1.0:0.5, (c) 1.0:1.0, (d) 1.0:1.5, (e) 1.0:2.0, (f) 0:1.0.

IP T SC R U N A M ED PT

A

CC E

Table 3 Regression equations of the sensors for different total mass of the composite

a

films a

Samples

Regression equation

The correlation coefficient (R²)

a

y = -1271.0x + 168.17

0.9969

b

y = -2385.4x + 247.16

0.9990

c

y = -6131.8x + 733.89

0.9979

d

y = -8405.6x + 1074.1

0.9961

e

y = -14752x + 2788.1

0.9032

The total mass of sensitive film: (a) 0.5 μg, (b) 1.0 μg, (c) 1.5 μg, (d) 2.0 μg, (e) 4.0 μg.

A ED

PT

CC E

IP T

SC R

U

N

A

M

IP T SC R

Response

coefficient

time (s)

[15]

14

16

2% RH

[17]

12

25

1.3% RH

[19]

– – 0.9860

60 20 4

180 50 20

3% RH – 2% RH

0.9990

5

21

1.3% RH

[24] [25] [26] This work

M



PT

ED

0.9998

a

bacterial cellulose.

b

Li-doped mesoporous silica A-SBA-15

c

Fe/SBA-15- Fe-doped mesoporous silica SBA-15.

CC E

time (s) –

Mesoporous SnO2–SiO2

A

Ref.

53

0.9980

d poly(dimethylaminoethyl

Hysteresis

119

BCa

Urea formaldehyde resin/nano silica Li/SBA-15b Fe/SBA-15c PDMAEM/PGMAd Polyvinyl alcohol/nano-silica

Recovery

N

linear correlation

A

Sensitive materials

U

Table 4 A comparison of humidity sensors with different sensitive materials based on QCM

methacrylate) and poly(glycidyl methacrylate).