Sensors and Actuators A 158 (2010) 328–334

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Sensors and Actuators A: Physical

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A breath sensor using carbon nanotubes operated by field effectsof polarization and ionization

Xiaohang Chen, Yanyan Wang, Yuhua Wang, Zhongyu Hou, Dong Xu, Zhi Yang, Yafei Zhang ∗

National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education,Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China

a r t i c l e i n f o

Article history:Received 5 July 2009Received in revised form19 November 2009Accepted 11 January 2010Available online 28 January 2010

Keywords:Microfabrication breath sensor

a b s t r a c t

A novel microfabricated breath sensor (MBS) based on multi-walled carbon nanotubes (MWNTs) hasbeen presented and tested. It has a simple structure of two nickel beams incorporating with MWNTs. Theresponses of the MBS to the behavior of the breath dynamic characteristics are consistent with the exha-lation pulse of the human-volunteers, e.g. the exhale flow strength and frequency. It has a rapid responseand high sensitivity in detecting feeble breath, and no recovery issues such as the adsorption-type sensorsare detected. Furthermore, strong anti-interference ability to external air flow and temperature shift isobserved. These unique results ensure this MBS can be successfully employed to real-time monitoring ofhuman breath.

Multi-walled carbon nanotubesExhaled breathRS

© 2010 Elsevier B.V. All rights reserved.

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esponseensitivity

. Introduction

The breath of human, a basic characteristic of life carrying var-ous information, is significant to the comprehensive analysis ofung function [1,2], asthma detection [3], diabetes mellitus diag-osis [4], etc. Meanwhile, health condition can also be reflected byhe physical characteristics of the exhaled breath, such as breathrequency and tidal volume. The breath measurement has beenccomplished by different approaches, such as the pressure sensors5], which examine the pressure change induced by the exhaled gasow. A tube connecting mouth or nose and the windtight device areecessary while operation of such sensor; otherwise, its pressure-ensitive element may be disturbed by the ambient flow. The shapeodification of chest or abdomen will also reflect the breath con-

ition indirectly via pressure sensor; however, it will cause theiscomfort for tying on the body. Another type of breath sensorakes advantage of the temperature shift near the nose duringhe breath. However, the stochastic nature of ambient tempera-ure may deteriorate its signal quality significantly. Besides these

ommercial breath sensors, short time response humidity sensors6–10] focus an increasing interest in breath diagnosis applications.owever, response cycle of subsecond is difficult to accomplish.here is another breath sensor [11] based on the fact that the

∗ Corresponding author. Tel.: +86 21 3420 5665; fax: +86 21 3420 5665.E-mail address: yfzhang@sjtu.edu.cn (Y. Zhang).

924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2010.01.021

velocity of sound is directly modulated by air flow. However, theanti-interference ability to ambient noise is a weakness.

Due to the unique physical properties [12,13] and poten-tial in diverse range of applications [14–18], carbon nanotubeshave attracted considerable attentions. Its high aspect ratio, smalltip ratios and high electrical conductivity result in strong fieldenhancement effect [19,20]. There is another microfabricatedbreath sensor (MBS) based on multi-walled carbon nanotubes(MWNTs) reported by our group, which takes advantage of the tipeffect of MWNTs [21]. In this paper herein; however, microfabri-cation processes are improved to deal with its uncontrollability ofMWNTs distribution and unsatisfied bonding of MWNTs film onmicroelectrodes caused by screen printing. The focus of the MBSis on the performance in monitoring human breath under alter-native current (AC). The information that MBS can provide is thefrequency and strength of the exhaled breath. It has a consider-able high quality of response, sensitivity and recovery performance,with excellent ability of anti-interference to ambient air flow andtemperature shift.

2. Experiment

2.1. Sensor fabrication process

Traditional “lift-off” and novel electrophoresis processes wereemployed to generate the microstructure. Firstly, the LOR 3A (notphotosensitive) and AZ3612 (photosensitive) were spun on the

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ig. 1. Schematics of the fabrication processes: (a) spinning of photoresist;b)lithograph and etching of photoresist; (c) deposition of Cr/Cu seed layer; (d) “lift-ff” of resist; (e) electroplate of Ni beams; (f) electrophoresis of MWNTs film andlectroplate of Ni coat; (g) schematic of MBS.

afer in order. After spinning and baking, the AZ3612 was exposednd the developer (AZ400) was used to clear the exposed AZ3612reas, but also etch away the LOR 3A. Since the LOR 3A had a higherissolution rate in developer than the AZ3612, the resulting pro-le would have an overhang of photoresist, which prevented the

idewall deposition of film, as illustrated in Fig. 1a. Then, Cr/Cueed layer (10 nm/140 nm) was deposited on the patterned resist,s shown in Fig. 1b. After deposition of Cr/Cu film, the resist wasissolved in dimethylsulfoxide and the patterns of microelectrodesere obtained, as shown in Fig. 1c. Subsequently, nickel beams

ig. 2. The test platform: (a) the practical experimental configuration; (b) diagram of MBection of MFM and MBS under test is ∼10 cm).

ors A 158 (2010) 328–334 329

were electroplated on the Cr/Cu seed layer, which thickened theheight of the electrodes to be 4–6 �m, in order to increase the effec-tive current generation areas of the electrodes, as shown in Fig. 1d.Finally, The Cr/Cu/Ni electrodes were coated by a film of MWNTs(>95% purity) using an electrophoresis process. It was accomplishedin acetone with a MWNTs content of 5 mg/L, at a constant elec-tric field of 10 V/cm, for 5 min. Mg(NO3)2 was used as an additive(6 mg/L) and the MWNTs were homogeneously dispersed in ace-tone using ultrasonic machine at power of 50 W for 30 min beforeelectrophoresis. Additionally, another much thinner nickel film waselectroplated on the MWNTs for better bonding and consistence ofMWNT–film on the nickel surface, as shown in Fig. 1e. After wash-ing with deionized water and drying, the device was completed, asschematically illustrated in Fig. 1f.

2.2. Measurement set-up

A platform of detecting MBS performance in monitoring humanbreath was presented, As shown in Fig. 2. One electrode of sensorwas connected to the signal generator (NF 1946B Function Gener-ator), which supplied an AC signal source. Another was linked to alock-in amplifier, for signal amplification and amplitude demodu-lation. The MBS was fixed at a controlled distance (2 cm) to the exitof a microbridge mass flowmeter (MFM, AWM 5000, Honeywell)which was widely applied in the field of life-support machine. ThisMFM was introduced here to indicate the flow rate of various gasesunder test. During the test, the operating voltage was approximate

(∼) 1 V and the frequency was chosen ranging from 1 to 10 kHz.All signals could be translated into voltage signals and recordedby a digital oscilloscope. The mathematical conversion equation ofthe voltage (Vo) recorded by the oscilloscope and the response cur-rent (Io) of MBS was: Vo = 4Io (V/�A). The mathematical conversion

S and MFM; (c) the schematic of the platform (the distance between the sensitive

330 X. Chen et al. / Sensors and Actuators A 158 (2010) 328–334

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demonstrated in Fig. 4. There are few detectable responses of themetallic device without MWNTs. A signal delay of the MBS follow-ing that of MFM is observed (∼0.17 s for beginning of expirationand ∼1 s for end), which may be ascribed to the distance betweenMBS and the sensitive section of MFM.

ig. 3. Surface characteristics of MBS: (a) the whole picture of the MBS; (b) the tochematic of MWNT-tips on the sidewall.

quation of the MFM output voltage (VMFM) and its indicated flowate (WD) was: WD = 2.5(VMFM − 1) (slpm/V).

The basic sensing responses to spontaneous breath had beeneasured through output voltage in atmospheric air at room tem-

erature and 45–90% relative humidity. The experiments werearried out in the decontamination chamber in which the temper-ture and humidity could be controlled conveniently and exactly.t had been examined by more than 10 volunteers for the reliabil-ty. Unusual human exhaled breath was tested in order to observeerformance of the sensor under the condition of varying breathrequency and strength. Buffered compressed air had been intro-uced through a tube to check the influence of ambient air flow.he device was placed in an oven to examine its performance underifferent temperatures ranging from 30 to 60 ◦C. Volunteers’ breathow rates were recorded by MFM consistently during all tests.

. Result and discussion

.1. Structure characteristics

Fig. 3a–c shows a SEM image of the MBS, the top view images ofhe gap (∼8 �m) between electrodes and morphology of MWNTsayer on the sidewall respectively. As shown in the images, nickellectrodes are coated by a film of MWNTs with diameters of about0–50 nm, and tilted tips of MWNTs can be observed, especially athe sidewall of electrodes.

As schematically shown in Fig. 3d, the equivalent area of thisapacitor structure can be calculated as follows:

�S

S= �d × L × N

h × W(1)

here �S and S denote the increment of relative area and initial

elative area without MWNTs respectively. And d, L, W, N, h is theean diameter of MWNTs, the mean length of titled tips, the length

f electrode, the number of titled MWNTs in electrode length of W,nd the thickness of electrodes, respectively. More than 50 imagesere captured by SEM in order to estimate the increment propor-

image of the electrode gap; (c) morphology of MWNTs layer on the sidewall; (c)

tion of sidewall area. From the SEM images, L, W and N can bestatistically calculated. As d is kept as a constant, the incrementproportion of sidewalls area can be estimated as ∼40%.

3.2. Basic performance

The current output modification of this MBS reflecting thespontaneous breath was collected by the lock-in amplifier. Afterprocessing, the signals were plotted by an oscilloscope to indicatethe characteristics of the breath, including frequency, duration, andamplitude. Typical responses of the MFM and the MBS as well asthe bald metal electrode structure (with the same size of MBS) are

Fig. 4. The curves indicating responses of three devices to spontaneous breath: MBS(baseline 0 V); MFM (baseline 1 V); and metal electrodes device (baseline arbitrary).

X. Chen et al. / Sensors and Actuators A 158 (2010) 328–334 331

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3.4. Modeling and theoretical analysis

The differences in the performances of MBS and the bald metalelectrode structure are inconsistent with the absorption or otherrelated mechanism [22–24]. Furthermore, the increment propor-

ig. 5. The curves indicating MBS responses to unnatural breaths of different and ch

To investigate the ability of monitoring breath of high frequencynd low strength, unnatural breath was tested, as indicated inig. 5a–d. The former two volunteers breathed with a constantrequency and strength, and the latter two volunteers breathedith an abruptly changing frequency and strength. The curves

ndicate stable operation of MBS in monitoring unnatural breath.his MBS can distinguish exhaled breath of a high frequency (100imes per minute), which is much higher than normal breath fre-uency (16–20 times per minute). Further, the delay time (�t) ofhe exhaled gas acts on MBS and MFM can be denoted as Eq. (2):

t = D × A

F(2)

here D stands for the distance between the sensitive section ofFM and MBS, A for the cross-sectional area of MFM flow path,

nd F for the flow rate recorded by the MFM. When D is equal to0 cm, the delay time (�t) of the exhaled breath act on MBS andFM can be calculated as ∼0.5 s. As indicated in the curves in Fig. 5,

he response delay time (�T) of MBS to MFM is averagely ∼0.15 s�T < �t), indicating higher response speed of MBS than MFM.

Meanwhile, exhalant air flow of 0.5 slpm (1.2 V voltage of MFM)s clearly reflected in Fig. 5, exhibiting unique sensitivity in detect-ng feeble breath.

.3. Environmental influence

In order to measure the ambient influence to MBS when mon-toring breath, ambient air flow (buffered compressed air) wasnduced to MBS and the responses were recorded. As typically illus-rated in Fig. 6, the responses of MBS are much less detectable thanhat of exhaled gas even the flow rate is much higher than sponta-

eous breath. It indicates its favorable anti-interference ability ofurrounding flow.

To examine the impact of temperature, tests of MBS at differ-nt temperatures are carried out and the outputs are collected.he ambient temperature was controlled by the oven, which varied

frequency and strength. Experimental results of four volunteers are demonstrated.

from 35 to 60 ◦C gradually. It is shown in Fig. 7a that the responsesignal amplitudes of the sensor to the breath in various temper-atures are distributed in a very narrow range (3.3–4.3 V), whichshows its thermal stability. Additionally, the higher temperaturesenhance the removal of the exhaled flow and then shorten therecovery duration, indicating better performance in recovery pro-cess, as shown in Fig. 7b. In the tests, we also found that it couldwork steadily with various sampling distance of the spontaneousbreath, ranging from 10 to 20 cm.

Fig. 6. Response spectra of MBS and MFM to the air flow pulse: MBS (baseline 0 V)and MFM (baseline 0 V).

332 X. Chen et al. / Sensors and Actuators A 158 (2010) 328–334

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ig. 7. The curve of MBS responses under different temperatures: (a) the responsehe MBS response cycle under 30, 45 and 60 ◦C.

ion of sidewalls areas between the MBS and bald nickel beams cane estimated as ∼40% (Fig. 3), much smaller than the signal incre-ent proportion (more than 10 as shown in Fig. 4). The remarkable

erformance of the MBS cannot be attributed absolutely to thencrease of capacitance.

A new model is proposed by us herein to explain the perfor-ance of MBS, which is detailed in [25]. As shown in Fig. 8a, the

esponse process of MBS to exhaled breath can be divided into fourtages.

In stage I, the current is resulted from ionization current of initialnvironment (Ib), which is composed of ambient atmosphere.

In stage II, as exhaled gas is introduced to MBS, the backgroundir is chemically changed because of numerous atomized water

∼6.2% fractional concentration) [26,27] and charged particles con-ained in breath gas [28,29]. Charges induced by nonuniformolarization of droplets result in polarization current (Jpd) whileroplets drifting across the gaps. Jpd is much greater than base cur-

ig. 8. Schematic of proposed mechanism: (a) the response curve of MBS to exhaledreath according to the proposed model; (b) three stages of the actual response andecovery circle. The response voltages of MBS (baseline 0 V) and MFM (baseline 0 V)re denoted by left and right vertical axes, respectively; (c) schematic of MBS underuman breath.

oltage distribution under temperature ranging from 30 to 60 ◦C; (b) comparison of

rent (Ib) in atmosphere. Here, the sensitivity of MBS can be definedas:

∂Jpd

∂Wd= − q

VE

∫ ∫ ∫VE

(�E + ∂�E

∂WdWd

)ε0∇ �E(x, y, z)dV (3)

where Wd is the flow rate of exhaled breath, q is the electrical quan-tity of elementary charge, VE is the volume of current generationspace, �E is the averaged electric susceptibility of the medium inVE, ε0 is the dielectric constant in vacuum, E is the electric fieldvector, and dV is the differential unit of VE. Especially, �E is relatedby inverse proportion of background air and the liquid droplets.Under the external AC source, electric field between the electrodesis greatly enhanced by the strong field convergence of the MWNTstips [19]. As a result, the sensitivity of MBS to exhaled breath isgreatly improved.

When Wd is kept as a constant, the Jpd is only related to theexhaled flow composition, indicating stable response for every cer-tain volunteer, as shown in Fig. 8a. When the exhaled breath isintroduced with a fixed distance between exhaled breath and MBS,the curve of response current to time is analogical to the breathflow rate. Furthermore, as shown in Fig. 4, the third peak of MBSis huger than the former ones, which is caused by the reduction ofdistance between MBS and exhaled breath source. While the dis-tance is shortened and the exhaled breath flow rate (WD) is kept asa constant, the output voltage of MFM maintains unchanged, butless liquid droplets in breath gas will be lose during the diffusionprocess from exhaled breath source to MBS, leading to a higher pro-portion of liquid droplets in VE. As a result, the sensitivity of MBS isincreased (as shown in Fig. 4).

In this proposed model, the proportion of the liquid dropletscontained in the exhaled gas is the key factor which affects thesensitivity of MBS according to the Eq. (3). When exhaled breathis introduced to MBS, the value of �E is greatly increased as thebackground air is chemically changed by the exhaled gas. Whenambient air flow pulse is introduced to MBS, the backgroundgas remains unchanged, leading to low sensitivity of MBS. Thisis the reason why MBS is relatively insensitive to the ambientair.

In stage III, the residual atomized droplets will be vaporizedwhen the exhalation stops, which leads to sustaining decrease of

response current. In this stage, current is composed of ionizationcurrent (Ii) and another polarization current (Idp) caused by thedielectrophoresis force to droplets. Taking Dl to denote the meanparticle of liquid droplets in VE, Nl to denote the equivalent numberof the droplets of Dl in diameter, and Wdp to denote the droplet drift

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elocity due to dielectrophoresis forces, Jdp can be defined as:

dp(t) = K · Nl ·∫ ∫ ∫

VE

D3l

· Wdp · ∇ · �EdV

V2E

(4)

here K is only related to the electric susceptibility of liquid atom-zed droplets in breath gas. For the same volunteer, K and Dl areonsidered as a constant. The Idp is in direct proportion to Nl underteady electric field and decreases with vaporization of residualroplets. When the temperature increases, the evaporation rate ofhe liquid droplets in VE increases as a result of the more strenuousiffusion motion, leading to faster decrease of the equivalent num-er of the liquid droplets (Nl). As demonstrated in Fig. 7b, higheremperature indicates higher speed decrease of response current intage III and exhibits faster recovery. The elapsed time of vaporiza-ion is much longer than that of exhaled breath introduction, whichs indicating that the recovery process is slower than the responserocess.

In stage IV, when the environmental gas is restored, the currentagnitude will return back to the initial value as shown in Fig. 8a.The curve of MBS response to exhaled breath is shown in Fig. 8a,

ccording to this proposed model. In stage II, the response currents a constant while the exhaled flow rate (Wd) is assumed as a con-tant in the model. Actually, Wd is always varying and there is a peakalue during one breath, leading to a corresponding peak value ofesponse current of MBS, as shown in Fig. 8b. After careful verifica-ion, this proposed model is validated to match the experimentalesults well.

. Conclusion

A novel MBS incorporating MWNTs has been fabricated andested. The sensor has a simple capacitor structure, including

pair of metal electrodes, coated by a layer of MWNTs andollowed by a nickel protection layer. The MWNT–film greatlymproves the performance of the sensor in monitoring the exhaledreath. Meanwhile, this MBS can operate steadily with ambi-nt interference of ambient air flow and temperature. Finally,he performances of response and recovery time are plausible,mplying a congruous response to the characteristics of breathynamics. The experimental results in this paper ensure that thisBS can be successfully employed to monitor human breath.

his attractive feature will be exclusively examined in our futureesearch.

cknowledgements

This work is supported by Hi-Tech Research and Developmentrogram of China No. 2007AA03Z328, Shanghai Natural Scienceoundation (No. 09ZR1415000), National Natural Science Founda-ion of China (Nos. 60871032, 60906053 and 50902092).

eferences

[1] D.T.V. Anh, W. Olthuis, P. Bergveld, A hydrogen peroxide sensor forexhaled breath measurement, Sens. Actuators B: Chem. 111–112 (2005)494–499.

[2] C. Di Natale, A. Macagnano, E. Martinelli, R. Paolesse, G. D’Arcangelo, C. Roscioni,A. Finazzi Agrò, A. D’Amico, Lung cancer identification by the analysis of breathby means of an array of non-selective gas sensors, Biosens. Bioelectron. 18(2003) 1209–1218.

[3] M. Fleischer, E. Simon, E. Rumpel, H. Ulmer, M. Harbeck, M. Wandel, C. Fiet-

zek, U. Weimar, H. Meixner, Detection of volatile compounds correlated tohuman diseases through breath analysis with chemical sensors, Sens. ActuatorsB: Chem. 83 (2002) 245–249.

[4] J.B. Yu, H.G. Byun, M.S. So, J.S. Huh, Analysis of diabetic patient’s breathwith conducting polymer sensor array, Sens. Actuators B: Chem. 108 (2005)306–308.

ors A 158 (2010) 328–334 333

[5] S. Brady, D. Diamond, Inherently conducting polymer modified polyurethanesmart foam for pressure sensing, Sens. Actuators A: Phys. 119 (2005)398–404.

[6] Y. Ma, S. Ma, T. Wang, W. Fang, Air-flow sensor and humidity sensor applicationto neonatal infant respiration monitoring, Sens. Actuators A: Phys. 49 (1995)47–50.

[7] A. Tetelin, V. Pouget, J.L. Lachaud, C. Pellet, Dynamic behavior of a chemicalsensor for humidity level measurement in human breath, IEEE Trans. Instrum.Meas. 53 (2004) 1262–1267.

[8] A. Tetelin, C. Pellet, C. Laville, G. N’Kaoua, Fast response humidity sensors for amedical microsystem, Sens. Actuators B: Chem. 91 (2003) 211–218.

[9] C. Laville, C. Pellet, Comparison of three humidity sensors for a pulmonaryfunction diagnosis microsystem, IEEE Sens. J. 2 (2002) 96–101.

10] A.K. Kalkan, H. Li, C.J. O’Brien, S.J. Fonash, A rapid-response, high-sensitivitynanophase humidity sensor for respiratory monitoring, IEEE Electron DeviceLett. 25 (2004) 526.

11] B. Hok, A. Bluckert, G. Sandberg, A non-contacting sensor system for respiratoryair flow detection, Sens. Actuators A: Phys. 52 (1996) 81–85.

12] Y.B. Zhu, W.L. Wang, The effects of pores of template on the field emissionproperties of template-synthesized array of carbon nanotubes, J. Wide BandgapMater. 10 (2002) 29–41.

13] N. Hamada, S. Sawada, A. Oshiyama, New one-dimensionalconductors—graphitic microtubules, Phys. Rev. Lett. 68 (1992) 1579–1581.

14] G. Ruffini, S. Dunne, L. Fuentemilla, C. Grau, E. Farres, J. Marco-Pallares, Firsthuman trials of a dry electrophysiology sensor using a carbon nanotube arrayinterface, Sens. Actuators A: Phys. 144 (2008) 275–279.

15] K.A. Sablon, Polymer single-nanowire optical sensor, Nanoscale Res. Lett. 4(2009) 94–95.

16] L. Zhu, J. Xu, Y. Xiu, Y. Sun, D.W. Hess, C.P. Wong, Growth and electrical char-acterization of high-aspect-ratio carbon nanotube arrays, Carbon 44 (2006)253–258.

17] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56.18] O.K. Varghese, P.D. Kichambre, D. Gong, K.G. Ong, E.C. Dickey, C.A. Grimes,

Gas sensing characteristics of multi-wall carbon nanotubes, Sens. ActuatorsB: Chem. 81 (2001) 32–41.

19] W.A. de Heer, A. Chatelain, D. Ugarte, Carbon nanotube field-emission electronsource, Science 270 (1995) 1179–1180.

20] R.B. Rakhi, K. Sethupathi, S. Ramaprabhu, Effect of purity and substrate on fieldemission properties of multi-walled carbon nanotubes, Nanoscale Res. Lett. 2(2007) 331–336.

21] Z. Hou, B. Cai, D. Xu, Application of carbon nanotubes to human breath dynamicscharacterization, Appl. Phys. Lett. 89 (2006) 053105.

22] T.S. Poet, R.A. Corley, K.D. Thrall, J.A. Edwards, H. Tanojo, K.K. Weitz, X. Hui,H.I. Maibach, R.C. Wester, Assessment of the percutaneous absorption oftrichloroethylene in rates and humans using MS/MS real-time breath analysisand physiologically based pharmacokinetic modeling, Toxicol. Sci. 56 (2000)61–72.

23] Y.Y. Xu, X.J. Li, J.T. He, X. Hu, H.Y. Wang, Capacitive humidity sensing propertiesof hydrothermally-etched silicon nano-porous pillar array, Sens. Actuators B:Chem. 105 (2) (2005) 219–222.

24] J.T.W. Yeow, J.P.M. She, Carbon nanotube-enhanced capillary condensation fora capacitive humidity sensor, Nanotechnology 17 (2006) 5441–5448.

25] Z. Hou, B. Cai, H. Liu, Sensing of atomized liquids through field effects of polar-ization and ionization induced by nanostructures, Appl. Phys. Lett. 94 (2009)223503.

26] B.L. Mao, Clinical Blood Gas Analysis, vol. 1, People’s Military Medical Press,Beijing, 1985, p. 2.

27] M. Phillips, Method for the collection and assay of volatile organic compoundsin breath, Anal. Biochem. 247 (1997) 272–278.

28] W.Q. Cao, Y.X. Duan, Breath analysis: potential for clinical diagnosis and expo-sure assessment, Clin. Chem. 52 (2006) 800–811.

29] R.M. Effros, K.W. Hoagland, M. Bosbous, D. Castillo, B. Foss, M. Dunning, M.Gare, W. Lin, F. Sun, Dilution of respiratory solutes in exhaled condensates,Am. J. Respir. Crit. Care Med. 165 (2002) 663–669.

Biographies

Xiaohang Chen received the B.S. degree in electronic science and technology fromShaanxi University of Science and Technology, China, in 2007. He is currentlyworking toward the M.S. degree of microelectronics and solid-state electronics atShanghai Jiaotong University, China. His research interests include digital integratedcircuits, gas sensors, and humidity sensors.

Yanyan Wang received the M.S. degree in condensed matter physics from DalianUniversity of Technology, China, in 2007. She is currently working toward thePh.D. degree of microelectronics and solid-state electronics at Shanghai JiaotongUniversity, China. Her research interests include MEMS-based electronic devices,

particularly micro-gas sensors.

Yuhua Wang received the B.S. degree in Huazhong University of Scienceand Technology, China, in 2008. She is currently working toward the M.S.degree of microelectronics and solid-state electronics at Shanghai Jiaotong Uni-versity, China. His research interests include gas sensors, and DBD structuresensors.

3 Actua

ZCo

Di

34 X. Chen et al. / Sensors and

hongyu Hou is currently an assistant professor at Shanghai Jiaotong University,hina. His research interests include plasma science and technology in the contextf low dimensional systems.

ong Xu is currently a professor at Shanghai Jiaotong University, China. Her researchnterests include micro/nano-electromechanical systems.

tors A 158 (2010) 328–334

Zhi Yang is currently an assistant professor at Shanghai Jiaotong University, China.His research interests include nanoscale science and technology.

Yafei Zhang is currently a professor at Shanghai Jiaotong University, China. Hisresearch interests include nanoscale science and technology.