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Sensors and Actuators B 159 (2011) 301– 306

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h o mepage: www.elsev ier .com/ locate /snb

ighly stable and sensitive humidity sensors based on quartz crystalicrobalance coated with bacterial cellulose membrane

eili Hua, Shiyan Chena, Bihui Zhoua, Luting Liua, Bin Dinga,b,∗, Huaping Wanga,∗

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, The Key Laboratory of High-Performance Fiber and Product,inistry of Education, College of Materials Science and Engineering, Donghua University, Shanghai 201620, ChinaNanomaterials Research Center, Modern Textile Institute, Donghua University, Shanghai 200051, China

r t i c l e i n f o

rticle history:eceived 28 April 2011eceived in revised form 2 July 2011ccepted 7 July 2011vailable online 19 July 2011

a b s t r a c t

A novel highly stable and sensitive humidity sensor based on bacterial cellulose (BC) coated quartz crys-tal microbalance (QCM) has been successfully fabricated. The results showed that the sensors possessedgood sensing characteristics by increasing more than two orders of magnitude with increasing relativehumidity (RH) from 5 to 97%, and the Log(�f) showed good linearity (20–97% RH). The sensitivity of

eywords:acterial celluloseanofibrous membraneuartz crystal microbalanceumidity sensors

sensors coated with BC membranes was four times higher than that of the corresponding cellulose mem-branes at 97% RH. In addition, the sensor sensitivity is greatly enhanced by increasing the coating load ofthe BC membranes with more absorption sites in the sensing membranes. Moreover, the experimentalresults prove that the resultant sensors exhibited a good reversible behavior and good long term stability.Herein, not only a novel and low-cost humidity sensor material was exploited, but also a new applicationarea for BC nanofibrous membranes was opened up.

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

. Introduction

Sensor technology has been improved significantly in recentears. Compared to existing sensor products and technologies, theain development focuses on improving sensitivity, reliability and

epeatability [1]. Humidity sensors have gained increasing appli-ations in industrial processing and environmental control, suchs weather forecast, medical or domestic applications for humanomfort, industrial uses, agriculture, automobiles and so on [2,3].n recent years, many efforts have been taken in the investigation ofumidity sensors with high sensitivity, rapid response, fast recov-ry and small hysteresis [4–8]. Thus, the requirement for cheap,eliable and highly sensitive humidity sensors is urgent. Accord-ng to this need, recent advances in devices and materials haveffered new technologies for the detection of relative humidityRH). For example, the mechanical methods are based on surface

coustic wave (SAW), bulk acoustic wave (BAW) or quartz crys-al microbalance (QCM), which can detect mass changes due tohe adsorption of water vapor [9–13]. Among them, the QCM is

∗ Corresponding authors at: State Key Laboratory for Modification of Chemicalibers and Polymer Materials, The Key Laboratory of High-performance Fiber androduct, Ministry of Education, College of Materials Science and Engineering, Nano-aterials Research Center, Modern Textile Institute, Donghua University, Shanghai

01620, China. Tel.: +86 21 67792958; fax: +86 21 67792726.E-mail addresses: binding@dhu.edu.cn (B. Ding), wanghp@dhu.edu.cn (H. Wang).

925-4005/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rioi:10.1016/j.snb.2011.07.014

a very stable and sensitive device which is highly sensitive to masschanges on the nanogram scale (1 ng/cm2) [14]. As a sensor, QCMhas been widely used to monitor the change in mass loading bymeasuring the shift of its resonant frequency [15,16]. The perfor-mance of the QCM-based sensor greatly depends on the chemicalnature and physical properties of the coating material. At present,different sensing membrane materials coated on the electrode ofQCM to detect vapor and humidity have been reported. Thesematerials included polymer, metal and carbon absorption materi-als [17–22]. Nanomaterials were also used for sensing membranes,such as ZnO nanostructure [23], carbon nanotube/Nafion compos-ite material [24], multi-walled carbon nanotube/Nafion compositematerial [17] and electrospinning fibrous PAA/PVA membrane [25].However, limit to our knowledge, no reports on bacterial cellulose(BC) humidity sensors combined with QCM have been seen.

BC has gained more and more attention in these years for thefavorable properties it possesses, such as its remarkable mechanicalproperties in both dry and wet states, porosity, water absorbency,moldability, biodegradability and excellent biological affinity [26].In addition, the BC materials are environmentally friendly, low-cost and in particular are commercially available in the market.The high surface area and a large numbers of nanopores and sur-face hydroxyl groups act as convenient anchoring points for organic

functionalization. Moreover, the nanoscale pore space makes thenanoporous materials be attractive media for applications such asnanocomposite preparation [27,28]. BC also can be considered asprime material or substrate material for the moisture adsorption

ghts reserved.

3 ctuators B 159 (2011) 301– 306

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ue to their large specific surface areas, a large number of surfaceydroxyl groups, good water absorbance and water holding capac-

ty. So, QCM coating with BC nanofibrous membrane can be a goodonsideration for constructing highly sensitive humidity sensors.n this work we planned to develop the humidity sensor of QCM

ith BC nanoporous membranes as sensitive coatings and the sens-ng characteristics of resultant sensors such as response, linearity,eversibility and stability were investigated. The effect of the coat-ng load of BC nanofibrous membranes on sensor sensitivity haslso been investigated.

. Experimental

.1. Materials

BC membrane was kindly provided by Hainan Yeguo Foods Co.td. The bacterial strain Acetobacter xylinum was incubated for 10ays in a static culture containing 5% (v/v) coconut milk (nitro-en content, 0.8%; lipid, 30%) and 8% (w/v) sucrose, adjusted toH 3.0 by acetic acid. The content of BC nanofiber in the pelliclesas approximately 1% (v/v). BC membrane was kindly provided

y Hainan Yeguo Foods Co. Ltd. The membrane was washed withistilled water and treated with 1% sodium hydroxide at 80 ◦C for

h, followed by rinsing with water. The other chemicals were pur-hased from Shanghai Chemical Company. All chemicals were useds received without any further purification.

.2. Fabrication of nanofibrous membranes on QCM

A typical procedure to fabricate QCM sensors was as follows.n AT-cut 5.0 MHz crystal was rinsed repeatedly into ethanol andeionized water and dried in air at room temperature. Severalieces of water-wet sample containing about 200 mg dry celluloseas immersed in a breaker containing 200 mL distilled water and

he mixture was pulped with the rotation speed of 12,000 r/miny a IKA T25 digital beating machine for 10 min at 25 ◦C. Then0 �l of the treated mixture was dispensed onto the surface of thelectrode of the QCM using a micropipette. The membrane coatedCM was dried at 60 ◦C for 1 h in vacuum. To study the sensitiv-

ty of the QCM coated with BC membranes with different coatingoads, three samples were prepared, i.e., the frequency shift of theCM was different from each other. Table 1 gives the informationf the samples about the fundamental frequency, the frequencyhift at the same environment before and after being coated withC membranes and the mass loads of BC membranes on QCM.he QCMs were labeled as S1, S2 and S3, respectively. In ordero compare the sensitivity of the BC membrane and natural cel-ulose membrane coated QCM sensors, a cellulose membrane wasrepared by the same method on the electrode of the QCM. Theample was labeled as S4 and its detailed information also coulde seen in Table 1. The resonance frequencies were measured by

QCM digital controller (QCM200, Stanford Research Systems). All◦

he membranes coated QCMs were dried at 25 C in vacuum for

0 min prior to the subsequent characterizations. The morphologyf nanofibrous membranes on the QCM was examined with fieldmission scanning electron microscopy (FE-SEM) (S-4800, Hitachi

able 1ensors information.

Samplenumber

Fundamentalfrequency (Hz)

Frequencyshift (Hz)

Mass loadsof BC (ng)

S1 5009982.8 −2522 42041.7S2 5005150.0 −3508 58478.4S3 5008450.2 −4510 75181.7S4 5008504.9 −4572 76215.2

Fig. 1. Schematic view of the experimental setup for humidity detection.

Ltd., Japan). The diameters of fibers were measured using an imageanalyzer (Adobe Photoshop CS2 9.0). The Brunauer–Emmett–Teller(BET) surface area, pore volume, and pore width of the fibrous sam-ples were characterized by nitrogen gas adsorption (Micromeritics,ASAP 2020 analyzer).

2.3. Apparatus for humidity sensing

The sensing properties of the resultant humidity sensors can bemeasured by the experimental setup shown in Fig. 1. The sensorwas installed in the testing chamber with constant temperature26 ◦C inside. Through injecting water into the chamber, the con-trolled humidity environments can be achieved, which yieldedhumidity ranging from 5 to 97% (nitrogen was used to dry the airin the chamber for humidity testing down to about 5% RH afterchecking with a commercial humidity sensor) as required. Evapo-ration of the water droplet hanging at the very tip of the needle wasaccelerated using a fan. Furthermore, a Thermo-hygrometer wasapplied in the system to monitor the RH in the chamber and sen-sors’ operating temperature. The sensing properties of the sensorsto humidity were examined by measuring the resonance frequencyshifts of QCM because of the additional mass loading. The data fromthe sensors were recorded by a personal computer.

3. Results and discussion

The morphology of original BC nanofibers, the BC regeneratedmembranes after beating treatment was studied using FE-SEM.Fig. 2a and b shows the FE-SEM images of original freeze-dried andvacuum-dried BC membranes. The images show the microfibrillarstructure of cellulose and aggregates of semicrystalline extended-cellulose chains in an ultrafine 3D nanoporous network structure.It consists of continuous nanofibers with a diameter in the rangefrom 30 to 70 nm. From Fig. 2b, it can be seen that the microfibrilsof vacuum-dried BC are densely coagulated with the impercepti-ble interstitial cavities due to the hydrophilicity of cellulose andthe strong surface tension of water. In contrast to this, the beatingtreated BC nanofibers are separated from each other which are sim-ilar to the freeze-dried BC membranes. Additionally, the nanofibershas a sticky phenomenon due to the formation of the strong hydro-gen bond between the oxygen of the water molecule and the largeamount of hydroxyl groups of BC [29]. The hydroxyl groups overthe surface of nanofibers to establish hydrogen bond in a certainhumidity environment can be shown in Fig. 2c. The formation offiber joints was preferred to prevent the separation between fibersand the electrode of QCM.

In principle, high surface area is beneficial to enhancing the

sensor sensitivity. The results of the BET analysis of the freeze-dried BC, vacuum-dried BC and beating treated BC membranes arelisted in Table 2. As shown in Table 2, the specific surface areaof freeze-dried BC membranes is 55.37 m2/g, which is 16 times

W. Hu et al. / Sensors and Actuators B 159 (2011) 301– 306 303

cuum-dried BC membranes and (c) beating treated BC membranes.

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Fig. 2. FE-SEM images of (a) freeze-dried BC membranes (b) va

ore than that of electrospun PAA membranes [25], indicatinghat the BC nanofibrous network structure with increased spe-ific surface area is more beneficial to the application of humidityensors. The vacuum-dried BC membranes have much smaller sur-ace areas and pore volumes than the freeze-dried BC membranes,hich may be attributed to the densely coagulated phenomena

mong fibers (Fig. 2b). As shown in Table 2, the specific surfacerea, pore volume and pore diameter of beating treated BC mem-ranes falls in between forementioned two kinds of membranesnd we have found that it also has the best adhesion with QCM.ince BC membrane has large surface area and many hydroxylroups over the surface of nanofibers, the BC membrane can estab-ish an adsorption–desorption equilibrium under the actions of vaner Waals force and hydrogen bond in a certain humidity environ-ent. When the humidity environment changes, the coating will

bsorb or desorb the water molecule, then it will definitely changehe mass on the top of the crystal and consequently the resonantrequency of the crystal will change. By measuring the change of fre-uency, the humidity can be known. High response is contributed to

arge surface area and rich hydroxyl groups of nanoporous BC mem-rane allowing water molecules to reach more active sites from theurfaces and the pores.

The frequency shift (�f) of four QCM humidity sensors coatedith cellulose membranes and different coating loads of BC mem-

ranes as a function of the RH within the same time interval of 150 ss shown in Fig. 3. Here, we define the frequency changing rate Sfs frequency sensitivity, Sf = |�f/�t|, where �f is the response fre-uency shift within the time interval of �t [30]. The larger Sf valueeans the bigger frequency shift within the time unit and thus the

igher sensitivity. The results show that the resonant frequencyhift of sensors to RH increased with increasing RH for all sensors.

t can be also seen from the figures that the QCM sensor coated

ith cellulose membrane did not show an obvious change of fre-uency until the humidity reached as high as 48% RH. At 30% RH, theensor coated with BC nanofibrous membranes began to present a

able 2ET surface area analyses of the freeze-dried BC, vacuum-dried BC and beating treated BC

Samples Average fiber diameter (nm) BET surface

Freeze-dried BC 45.2 55.37

Vacuum-dried BC 46.5 20.70

Beating treated BC 45.9 35.56

Fig. 3. Frequency shifts of QCM humidity sensors of (a) S4, (b) S1, (c) S2 and (d) S3.

relatively obvious decrease of frequency shift. Moreover, the resultsindicated that the QCM sensors coated with treated BC membraneshave a larger frequency shift and sensitivity at same RH comparewith that coated with cellulose membranes, and the sensor sen-sitivity of nanofibrous BC membranes was four times higher thanthat of cellulose membranes at 97% RH which is mainly due to thehigher specific surface area and richer hydroxyl groups of the BCmembranes absorbing moisture easily and uniformly [31,32].

The sensing properties of nanofibrous BC membranes with dif-ferent coating loads of BC were investigated shown in Fig. 3b–din order to know the effect of the coating loads of BC nanofibrous

membranes on the sensor sensitivity. With increasing coating loadsof the BC membranes, the frequency shift tended to increase underthe same humidity condition, indicating that the sensitivity was

membranes.

area (m2 /g) Pore volume (cm3/g) Pore diameter (nm)

0.258 18.60.068 6.10.122 10.2

304 W. Hu et al. / Sensors and Actuators B 159 (2011) 301– 306

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Fig. 5. Response and recovery curve of QCM sensors of (a) S4, (b) S1, (c) S2 and (d)S3.

A simple, highly stable and sensitive, and low-cost humidity

ig. 4. The dependence of the frequency shifts of sensors of (a) S3, (b) S2 and (c) S1n RH (20–97%) and the calibration curves obtained from Fig. 3.

nhanced. At 97% RH, the maximum frequency shift was 2043 Hzith a coating load of 4500 Hz, which was nearly 1.5 times higher

han that (1306 Hz) of the sensor with a coating load of 2500 Hz. TheCM sensors with a larger coating load exhibited a larger frequency

esponse for the same RH since the absorption sites increased withncreasing coating load [33].

It can be observed in Fig. 3 that the frequency shifts as a func-ion of the RH did not show a linear tendency with increasing RH.he phenomenon was in accordance with humidity sensor systemased on QCM coated with nanostructured ZnO as sensing element34]. The BET model adsorption theory which indicates the adsorp-ion of the water vapor to the ZnO nanostructured surface is the

ulti-molecular-layer adsorption and the amount of the adsorbedater vapor in different RHs is not linear was introduced to explain

he nonlinear phenomenon. The humidity sensors based on nanofi-rous PAA membranes as sensitive coatings on QCM has also been

nvestigated and the result presented a nonlinear tendency withncreasing RH for the frequency shifts as a function of the RH [25].owever, the frequency shifts followed a logarithmic increase withood linear Log(�f) towards RH in the range of 20–95%. Similarly,e can see in Fig. 4 that all the QCM sensors coated with nanofi-

rous BC membranes exhibited good linear Log(�f) towards RHn the range of 20–97% and the coefficients of determination, R2,

ere 0.9980, 0.9956, and 0.9963 for BC membranes with the coatingoads of 2500, 3500 and 4500 Hz, respectively.

It is well known that response and recovery behavior is anmportant characteristic for evaluating the performance of theensing materials. The humidity response and recovery character-stic curves of sensors coated with three BC membranes with the RHhanging from 60 to 97% are given in Fig. 5. After reaching the equi-ibrium at 97% RH, the sensors were exposed to ambient air with RHf 60% until the full desorption were achieved. The time taken byhe sensor to achieve 90% of the total frequency change is defineds the response time in the case of adsorption or the recovery timen the case of desorption [35]. When the RH was increased from 60o 97%, the response time gradually increased. On the other hand,hen the RH was increased from 60 to 70%, the response timesere 119, 102, 89 and 141 s for S1, S2, S3, and S4, respectively. The

btained values of response times are shown only for comparison.t could be observed at the same time that the frequencies of the

ensors were backshifted to their initial values by exposure to ambi-nt air with RH of 60%, and the recovery times were 53, 57, 62 s for1, S2, and S3, respectively. Moreover, the QCM sensor coated with

Fig. 6. Stability of the nanofibrous BC membrane coated QCM sensor S3 after expo-sure in air for 6 weeks.

nanofibrous BC membranes showed a larger frequency shifts andfaster response compared to S4, and the increased coating load isbeneficial to the sensibility which is in accordance with the resultsshown in Fig. 3(b–d).

To test the long-term stability of the sensor, the frequencyshift curves with time at various RH levels for the coating load of4500 Hz BC membrane coated QCM sensors are shown in Fig. 6. Themeasurements were repeated at 26 ◦C every 7 days for 6 weeks, fol-lowed by measuring frequency between 5% and 97% RH. As shownin Fig. 6, slight variation in frequency shift is observed at eachhumidity region over time. So there is acceptable change in the fre-quencies, providing a good long-term stable characteristic of thesensors. In a word, the QCM sensors coated with the environmen-tally friendly, low-cost and commercially available BC materialsexhibit good performances in comparison with commercial humid-ity sensors [36] and it would have great potential in the sensingapplication in medical, domestic or industrial fields.

4. Conclusions

sensor based on BC nanofibrous coated QCM has been fabricated.The results show that the resultant sensors have good sensingcharacteristics by increasing more than two orders of magnitude

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in 2002, and PhD from Institute of Chemistry, ChineseAcademy of Sciences in 2006. Then she joined in DonghuaUniversity. She focuses on molecular electronics includingpreparation and characterization of organic and polymermaterials and optoelectronic devices.

W. Hu et al. / Sensors and A

ith increasing RH from 5 to 97% and the Log(�f) exhibited aood linearity in the range of 20–97% RH. The sensitivity of sen-ors coated with BC membranes was 4 times higher than thatf the corresponding cellulose membranes at 97% RH. Comparedith cellulose membranes, the nanofibrous BC membranes exhib-

ted remarkably enhanced humidity sensitivity due to their largerpecific surface area caused by the ultrafine three-dimensionalanofibrous network structure. Moreover, the sensor sensitivity isreatly enhanced by increasing the coating load of the membranesith more absorption sites in the sensing membranes. Addition-

lly, the resultant sensors exhibited a good reversible behavior andood long term stability. Herein, BC nanofibrous membranes coulde a promising candidate for the construction of high performance

ow-cost humidity sensors and the novel highly stable and sen-itive humidity reveals a new application area for BC membraneaterials.

cknowledgements

The authors thank Hainan Yeguo Foods Co. Ltd. for supplyingC samples. This work was financially supported by Doctoral Fundf Ministry of Education of China (20090075120011), Program ofntroducing Talents of Discipline to Universities (B07024), Shanghaieading Academic Discipline Project (B603), The National Naturalcience Foundation of China (51003012), Project of the Action oncientists and Engineers to Serve Enterprises (2009GJE20016) andhe Innovation Funds for Ph.D. Students (Weili Hu) of Donghuaniversity.

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Biographies

Weili Hu received her BS degree from the College ofMaterials Science and Engineering, Donghua Universityin 2008. From 2008 to 2010 she took part in a continuousacademic project that involved postgraduate and doctoralstudy in the same college of Donghua University. Now sheis a visiting scholar at University of California, Los Ange-les. Her primary research interests include fabrication offunctional nanocomposites based on bacterial cellulose,gas sensors and flexible electronics.

Shiyan Chen is an associate professor of Donghua Univer-sity. She received her MS degree in Shandong University

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06 W. Hu et al. / Sensors and A

Bihui Zhou received her bachelor degree from the Collegeof Materials Science and Engineering, Donghua Universityin 2009. She is currently pursuing a MS degree in the samecollege, majoring in the polymer materials. Her researchfocused on the functional nanomaterials based on bacte-rial cellulose.

Luting Liu studied for her BS degree at the College of Mate-rials Science and Engineering, Donghua University since2008. Now she is a junior student studying on the polymermaterials. Her primary research interests include nanobacterial cellulose, protective material and gas sensors.

ors B 159 (2011) 301– 306

Bin Ding received his BS degree from the Departmentof Applied Chemistry of Northeast Normal University(China) in 1998. From 1998 to 2000 he was appointed anInstructor at the same university. In 2003, he received hisME degree in Polymer Engineering from Chonbuk NationalUniversity in South Korea. In 2005, he received his PhDdegree from Keio University in Japan. He was a visitingscientist at the Keio University (2005–2007) and the Uni-versity of California at Davis (2007–2008). He is now aprofessor at Donghua University. His primary researchinterests include fabrication of nanostructured materials,gas sensors, bio-sensors, catalysts, solar cells, and filters.

Huaping Wang is a professor of Donghua University.He received his MS degree and PhD in Donghua Univer-sity in 1989 and 2002, respectively. His research focuses

on nano-composites and energy materials, fundamentaland technology of novel spinning (including the develop-ment and industrialization of new products and cleanerproduction), clean preparation of natural fibres and high-performance fibres.