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Sensors and Actuators B 157 (2011) 361– 367

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

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

sing a TiO2/ZnO double-layer film for improving the sensing performance ofnO based NO gas sensor

hia-Yu Lina, Jiang-Ging Chena, Wei-Yi Fengb, Chii-Wann Linb,c, Ju-Wen Huanga,ames J. Tunneyd, Kuo-Chuan Hoa,e,∗

Department of Chemical Engineering, National Taiwan University, Taipei 10617, TaiwanInstitute of Biomedical Engineering, National Taiwan University, Taipei 10617, TaiwanInstitute of Electrical Engineering, National Taiwan University, Taipei 10617, TaiwanInstitute for Chemical Process and Environmental Technology, National Research Council of Canada, Ottawa K1A 0R6, CanadaInstitute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

r t i c l e i n f o

rticle history:eceived 19 December 2010eceived in revised form 18 April 2011ccepted 19 April 2011vailable online 27 April 2011

eywords:as sensor

a b s t r a c t

NO gas sensors, based on ZnO thin film (ZnOfilm), TiO2 nanoparticulate film (TiO2NP), and TiO2NP/ZnOfilm

double-layer film, were fabricated, and their sensing characteristics towards NO gas were investigated inthis study. The maximal response of a ZnOfilm deposited onto a rougher Al2O3 substrate, towards NO gas,was higher than that of a ZnOfilm deposited on a smoother glass substrate. Although the sensing responseof the TiO2NPs itself towards NO gas was minute, the TiO2NP/ZnOfilm double-layer film showed enhancedresponse as compared with TiO2NP or ZnOfilm single-layer film. In addition, the sensor response of theTiO2NP/ZnOfilm double-layer film was strongly influenced by the annealing time for the film prepara-

itric oxideitanium oxideinc oxide

tion; the maximum response to NO was enhanced about 6.2 times as the annealing time was increasedfrom 30 min to 2 h. Based on the XPS results, the increase in the transition zone between TiO2NP andZnOfilm along with the appearance of Ti3+ state was noticed when the annealing time was increased.With the high sensitive TiO2NP/ZnOfilm/Al2O3 electrode, the limit of detection (S/N = 3) can be achievedat 8.8 ppb. The double-layer TiO2NP/ZnOfilm also showed improved selectivities with respect to NO2

and CO.© 2011 Elsevier B.V. All rights reserved.

. Introduction

Nitric oxide (NO), generated by the combustion processes, hasetrimental impacts on human health and environment. It is notnly a precursor of the acid rain, but also the cause for the deple-ion of ozone [1]. In the presence of excess oxygen, NO is easilyxidized to nitrogen dioxide. Frequent exposure to NO and NO2as would cause pulmonary edema and fatality [2]. As a result,eliable sensors for detecting low concentration of NO are highlyemanded. Chemi-resistive gas sensors, based on thin film’s con-uctivity changes upon interaction with target gas molecules, haveeen widely used as the sensing platform for this purpose due to

eal-time measurement in gas phase, lower cost and better longerm stability. Among the metal oxides, zinc oxide (ZnO) has beenroposed as a promising gas sensing material since it is highly sen-

∗ Corresponding author at: Department of Chemical Engineering, National Taiwanniversity, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan. Tel.: +886 2 2366 0739;

ax: +886 2 2362 3040.E-mail address: kcho@ntu.edu.tw (K.-C. Ho).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.04.056

sitive to some pollutant gases, such as CH4 [3], NO2 [4–6], CO [7],NH3 [8,9], ethanol [10] and NO [11]. However, the lack of selec-tivity of ZnO, due to the unspecific sensing mechanism, hindersits practical use. Recently, the use of the double-layer structurehas been proven an effective way to improve the sensing perfor-mance of metal-oxide based gas sensors, though the mechanismbehind is still debated. Vaezi [12] reported that the selectivity andstability of the SnO2/ZnO double-layer film showed higher selectiv-ity and stability than that of either SnO2 or ZnO single-layer film.It is also reported that as a TiO2 based layer was deposited ontothe In2O3–MgO film, the resultant double-layer film showed muchhigher sensor response to ammonia gas [13].

In this study, a TiO2NP/ZnOfilm double-layer film was preparedby depositing a TiO2NP layer using doctor-blade method, onto theZnO thin film, which was prepared by RF sputtering. The experi-ment results on sensing characteristics of the single-layer films ofTiO2NP and ZnOfilm, and the double-layer film of TiO2NP/ZnOfilm

are presented. The results showed that the TiO2NP/ZnOfilm double-layer film exhibited much higher sensing response to NO gas eitherthan TiO2NP or ZnOfilm single-layer one. The sensing mechanismwas also proposed and discussed.

3 Actuators B 157 (2011) 361– 367

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20

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40

50

60

Res

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Temperature oC

ZnOfilm/Al2O3

ZnOfilm/glass

62 C.-Y. Lin et al. / Sensors and

. Experimental

.1. Preparation of ZnO, TiO2NP, and TiO2NP/ZnOfilm double-layerlms

Gold interdigitated electrodes (AuIDEs) were coated on bothl2O3 and glass substrates and the AuIDEs coated substratesere used for the deposition of ZnO thin films. The AuIDEs wereeposited on Al2O3 and glass substrates by screen-printing tech-ology and traditional photolithography, respectively. ZnO thinlms, with a thickness around 150 nm, were RF sputtered ontohe AuIDEs coated substrates under a working pressure of 3 mtorrnder Ar/O2 atmosphere at a power of 150 W for 60 min. ThenOfilm coated sensing elements are designated as ZnOfilm/glassnd ZnOfilm/Al2O3, respectively.

The paste of TiO2 nanoparticles (∼20 nm) was prepared accord-ng to the previous literature [14], and then the TiO2 paste

as deposited using the doctor-blade method on an Al2O3 andnOfilm/Al2O3 substrates. The resultant sensing elements are des-gnated as TiO2NP/Al2O3 and TiO2NP/ZnOfilm/Al2O3 electrodes,espectively. Thereafter, the TiO2NP-coated electrodes were driedn air at room temperature for 30 min, followed by heating the filmo 500 ◦C at a rate of 20 ◦C/min, and maintained for 30 min, 1 h, and

h before cooling to room temperature.

.2. Gas sensing experiment

The set-up for the gas sensing experiment is shown in Scheme 1.he gas sensing measurements were carried out in a closed glasshamber situated in a heater. The sensing element, facing againsthe gas inlet at a flow rate of 250 ml/min, was connected by goldires which were attached to the interdigitated gold electrode with

ilver paste. It is well known that some general gas molecules, suchs O2, H2O, CO2, etc., could adsorb onto the electrode. The resis-ance of the electrode would change when these gas moleculesesorb from the electrode surface. To minimize the baseline drifturing the desorption process, the sensing electrodes were exposedo ultrapure N2 at 300 ◦C for several hours before data collec-ion. The transient current responses of the sensing electrodes to0 ppm NO were obtained at different temperatures by applying

10 V DC across both terminals of the electrodes. The concen-rations of NO gas were tuned by mixing 100 ppm NO with anltra-pure N2 gas stream from a tank (ShenYi Gas Co., Taiwan,9.9995%) through a flow meter. The controlled humidity envi-onments were achieved using saturated aqueous solutions ofifferent salts: MgCl2, Mg(NO3)2, and KNO3 in a closed glass vesselt ambient temperature, which yielded 33%, 54%, and 95% relativeumidity, respectively [15].

The current responses were recorded by a potentiostat (CHI 440,H Instruments, Inc., USA). Since the current–voltage curve of theensing element is ohmic (see Scheme 1(c)), the current was con-erted into resistance according to the ohmic law. The responses,, measured under ultrapure nitrogen and zero-grade synthetic airtmospheres, are defined by Eqs. (1) and (2), respectively:

= Rg − RN2

RN2

(1)

= Rg − Rair

Rair(2)

here RN2 , Rair, and Rg are the resistances of the film in ultrapure

2 gas, zero-grade synthetic air, and in testing gas, respectively. Inddition, the response time (t95) is defined as the time takes to reachhe 95% of the stable response to NO gas, and the recovery time (t95)s defined as the time takes to reach the 95% of the baseline.

Fig. 1. Temperature dependence of the responses of ZnOfilm/Al2O3 and ZnOfilm/glasssensing elements. The concentration of NO gas used is 10 ppm.

2.3. Film characterization

The microstructure of ZnO was determined by field emissionscanning electron microscopy (FESEM, JEOL JSM-6700F, Japan).The oxidation states of surfaces were determined with an X-rayrecorded on a PHI 5000 VersaProbe (ULVAC-PHI, Chigasaki, Japan)system using a microfocused (100 �m, 25 W) Al X-ray beam. AWienfiltered C+

60 ion source (IOG C60-10, Ionoptika, Chandler’sFord, UK) was operated at 10 nA and 10 kV. The angle between theAr+ and C+

60 ion beam was 33◦. The ion-beam current was mea-sured with the target current of an Au foil. The base pressure ofthe main chamber (<1 × 10−7 Pa) was achieved by evacuation usingturbomolecular and ion-getter pumps.

3. Results and discussions

3.1. Effects of the substrates

NO behaves as an oxidizing gas to ZnO film, and therefore, theresistance of the n-type ZnO film increases as NO gas chemisorbs onthe surface of the ZnO film. The increase in the resistance of the ZnOthin film can be attributed to NO adsorption in the charged form(NOads

−), or to the reduction of NO to form N2O [16], as describedby Eq. (3) or Eq. (4), respectively.

NO + e− → NOads− (3)

2NO + e− → N2O + Oads− (4)

Fig. 1 shows the temperature dependence of the responses, to10 ppm NO gas, against ZnOfilm/Al2O3 and ZnOfilm/glass electrodes.A quasi-stable response could be obtained after the sensing filmswere exposed to 10 ppm NO for 15 min. It was found that the opti-mal sensing temperature for ZnOfilm shifted from 260 to 320 ◦C asthe substrate was changed from glass to Al2O3. The reason is notclear yet, but the larger maximal response for the ZnOfilm/Al2O3can be explained by a rougher Al2O3 as compared to that of glass,as shown in Fig. 2. Since a higher response is preferred, the Al2O3substrate was selected for this study.

3.2. Effect of TiO2NP layer

Fig. 3 shows the temperature dependence of the responses, to

10 ppm NO gas, against TiO2NP thick film (∼6 �m) deposited onAl2O3 substrate. Due to the low conductivity of the TiO2NP filmand further decrease in conductivity upon exposure to NO gas, thebackground noise becomes a serious issue. In addition, the sensing

C.-Y. Lin et al. / Sensors and Actuators B 157 (2011) 361– 367 363

S ambeu

rth

Tcelris

cheme 1. (a) Global setup for gas sensing experiments; (b) illustration of sensing chnder ultrapure N2 atmosphere.

esponse of the TiO2NP film is much lower as being compared tohat of the ZnOfilm. This low response of the TiO2NP to NO gas atigh temperature was also observed in the literature [17].

However, the situation was improved by depositing a layer ofiO2NP film on top of the ZnOfilm/Al2O3 substrate. Fig. 4 shows theross-sectional view of SEM image for the TiO2NP/ZnOfilm/Al2O3lectrode. It can be seen that the grain sizes of TiO2NP and ZnOfilm

ayers are about 20 and 50 nm, respectively. In addition, the sensingesponse of the TiO2NP/ZnOfilm double-layer film, with an anneal-ng time of 30 min, is higher than either TiO2NP or ZnOfilm alone (aseen in Figs. 3 and 5). Furthermore, the enhancement in maximal

r; (c) current–voltage curve measured using TiO2NP/ZnOfilm/Al2O3 sensing element

sensor response along with change in optimal sensing tempera-ture was also observed as the annealing time was increased from30 min to 2 h (see Fig. 5), presumably has to do with the change inthe sensing mechanism. It should be noted from Fig. 5 that the sens-ing temperature has to be increased when the annealing time wasincreased. However, the instability of the sensing chamber at highersensing temperature prevents further experiments. As shown in

Fig. 6(a)–(c), binding energies for Ti 2p3/2 and Zn 2p3/2 slightlyshift during the annealing process, which is indicative of stronginteraction of Ti and Zn. In addition, as revealed in Fig. 6(d)–(f), anextension of a transition zone and a decrease in the binding energies

364 C.-Y. Lin et al. / Sensors and Actuators B 157 (2011) 361– 367

Fig. 2. The top-view of SEM images for the ZnO thin film deposited on (A) glass substrate and (B) Al2O3 substrate.

280 300 320 340 3600

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70

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pons

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Temperature oC

ZnOfilm/Al2O3

TiO2NP/Al2O3

Fig. 3. Temperature dependence of the responses of ZnOfilm/Al2O3 andT

otdiioeu

280 300 320 340 360 380 4000

100

200

300

400

500ZnOfilm/Al O2 3

TiO NP/ZnO2 film /Al O2 3annealed for 0.5 hTiO NP/ZnO2 film /Al O2 3annealed for 1 hTiO NP/ZnO2 film /Al O2 3annealed for 2 h

Res

pons

e

Temperature (ºC)

Fig. 5. Temperature dependence of the responses of ZnOfilm/Al2O3 and

iO2NP/Al2O3 sensing elements. The concentration of NO gas used is 10 ppm.

f Ti 2p3/2 and Zn 2p3/2 were noticed when increasing the annealingime. This implies that Ti and Zn species do interact with each otheruring the annealing process. Furthermore, it was found that the

ncrease in the annealing time caused not only a reduction in thentensity of the main Ti 2p peak at 459.2 eV, but also the growth

3/2f a shoulder at around 458.3 eV, which has been assigned for thevolution of T3+ cations [18]. As zinc–titanium oxide was annealednder air atmosphere, the adsorbed O2 molecules react with two

Fig. 4. The cross-sectional view of SEM images for the TiO2NP/ZnOfilm/

TiO2NP/ZnOfilm/Al2O3 with annealing time of 0.5, 1, and 2 h. The concentration ofNO gas used is 10 ppm.

electrons trapped at an oxygen vacancy site and transformed intotwo adsorbed oxygen ions (Oads

−), reducing adjacent or neighbor-ing Ti4+ cations to generate O–Ti3+ bond. As a result, the amountof Ti3+ site increased as the annealing time was increased. Theenhancement in the sensor response could therefore be attributed

to the evolution of the new phase (zinc titanate) or the change in thesurface defect during the annealing process, i.e., Ti3+ cation. One ofthe possible mechanisms will be proposed in the following section.

Al2O3 substrates with magnifications of (A) 5000 and (B) 30,000.

C.-Y. Lin et al. / Sensors and Actuators B 157 (2011) 361– 367 365

458 460 462

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nsity

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Ti 2P3/2

Ti3+

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(f) Annealed for 2 h (a) Ti 2p3/2

(b) O 1s (c) Zn 2p3/2

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528 530 532 534

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1018 1020 1022 1024 1026

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0.5 h 1 h 2 h

Zn 2P3/2

(c)

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(a) Ti2p3/2

(b) O 1s (c) Zn 2p3/2

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(a)

(d) Annealed for 0.5 h

transition zone

100 20 30 40 50 60 700

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transition zone

(e) Annealed for 1 h (a) Ti 2p 3/2 (b) O 1s (c) Zn 2p3/2

Con

cent

ratio

n (a

tom

ic %

)

Sputtering time (min)

(b) (c)

(a)

F r filmp

3

sqNttwaow

F#

ig. 6. XPS (a) Ti 2p3/2, (b) O 1s, (c) Zn 2p3/2 spectra of TiO2NP/ZnOfilm double-layerofiles obtained on the samples annealed for (d) 30 min, (e) 1 h, and (f) 2 h.

.3. Gas characteristic of TiO2NP/ZnOfilm double-layer film

Fig. 7 shows the transit response of the TiO2NP/ZnOfilm/Al2O3ensing element to various concentrations of NO gas at 360 ◦C. Auasi-stable response was obtained after the film was exposed toO gas for 15 min and then N2 gas was exposed for 30 min to allow

he sensor response recovered. The response (t90) and recovery (t90)imes of the sensor to 10 ppm NO gas under N2 atmosphere at 360 ◦Cere estimated to be 8.2 and 5.6 min, respectively. It was found that

ll the responses are linear with the increase in the concentrationf NO. The limit of detection, based on signal-to-noise ratio of 3,

as found to be 8.8 ppb.

The interfering effects of CO and NO2 were also examined.ig. 8 shows the responses of the ZnO/filmAl2O3 (configuration1) and the TiO2NP/ZnOfilm/Al2O3 (configuration #2) electrodes

after calcinations at 500 ◦C for different times ranging from 0.5 to 2 h. XPS depth

to 10 ppm of NO, CO, and NO2 gases at 360 ◦C. The times toacquire quasi-stable responses for CO, NO and NO2 are 10, 15,and 25 min, respectively. It was found that the ZnOfilm/Al2O3sensing element shows a much higher response to NO2 eitherthan CO or NO before the addition of the TiO2NP layer. However,after the introduction of the TiO2 nanoparticles upper layer,the response of the resultant double-layer film to NO gas wasgreatly improved while the responses to CO and NO2 remainedessentially the same, which means the selectivity with respect toNO2 and CO can be improved by the introduction of the TiO2NPupper layer. The improvement in selectivity with respect to

CO and NO2 could also be attributed to the formation of Ti3+

cations. The formation of Ti3+ cations could provide additionalsites for NO adsorption and reduction, as described by Eq. (5).However, it appeared that neither NO2 nor CO interacted with

366 C.-Y. Lin et al. / Sensors and Actuators B 157 (2011) 361– 367

0.0 0.5 1. 0 1.5 2.00

1020304050607080

Y=32.88*XR2=0.996

Res

pons

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NO concentration (ppm)

6420 8

0

50

100

150

200

0.25 ppm0.5 ppm

1 ppm

Res

pons

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Time (hr)

2 ppm

Fig. 7. The responses of the TiO2NP/ZnOfilm/Al2O3 sensing element, annealed at500 ◦C for 2 h, to various NO concentrations ranging from 250 ppb to 2 ppm. Theinset is the calibration curve.

210

100

200

300

400

Res

pons

e

Type of the configuration

10 ppm NO10 ppm NO 2

10 ppm CO

Fig. 8. The responses of the ZnOfilm/Al2O3 (configuration #1) andTiO2NP/ZnOfilm/Al2O3 (configuration #2), annealed at 500 ◦C for 2 h, to 10 ppm ofN

200 40 60 80 1000

100

200

300

400

500

10 ppm NO at 360 oC

Res

pons

e

Relative humidity (%)

addition, as the relative humidity was further increased (>33%),

Fs

O, NO2 and CO at 360 ◦C under N2 atmosphere.

300 60 90 120 150

20

40

60

80

100

120

140 Under zero-grade synthetic air under nitrogen

Time (min)

Res

ista

nce

M

(a)

0

1

2

3

4

5

Res

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nce

M

ig. 9. (a) The transit responses and (b) the overall responses of the TiO2NP/ZnOfilm/Alynthetic air atmospheres.

Fig. 10. The responses of the TiO2NP/ZnOfilm/Al2O3 to 10 ppm NO in various relativehumidities at 360 ◦C under ultrapure N2 atmosphere.

these sites.

2NO + e− + Ti3+ → N2 + Ti4+ + 2Oads− (5)

Fig. 9 compares the NO sensing characteristics of TiO2NP/ZnOfilmdouble-layer film under ultrapure N2 atmosphere and zero-gradesynthetic air. The resistances of the sensing film under the nitro-gen (RN2 ) and synthetic air (Rair) atmospheres are about 11 k� and1.6 M�, respectively. According to Eqs. (1) and (2), when exposingto NO gas, a relatively small resistance change under N2 atmospherecould result in a much higher response, as compared to that underzero-grade synthetic air atmosphere. This can be explained by thepartial oxidation of NO to form NO2 in air and the low sensitiv-ity fact of TiO2NP/ZnOfilm to NO2. Therefore, we did not furtherinvestigate the interference effect from other pollutant gases underzero-grade synthetic air atmosphere. The humidity effect on thesensor response was also examined at 360 ◦C under N2 atmosphereand the results are shown in Fig. 10. The results revealed that thesensor response was greatly influenced by the relative humidity,which was varied from about 0% to 95%. Due to the co-existence ofH2O and NO, some NO dissolve in H2O to form HNO2, and smallerquantity of NO resulted in the decrease of the sensor response. In

more water molecules chemisorb onto the sensing film in the formof OH− accompanying with an electron transfer from the sens-

0

100

200

300

400

500

600

Res

pons

e

Type of background atmosphere

10 ppm NO under N2 atmospher e 10 ppm NO under zero- grade synthetic ai r

(b)

2O3 sensing element to 10 ppm NO at 360 ◦C under ultrapure N2 and zero-grade

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C.-Y. Lin et al. / Sensors and

ng film (see Eq. (6)) [19], resulting in the increase of the sensoresponse.

2O + NO− + e− → 1/2 N2 + 2OH− (6)

. Conclusion

The TiO2NP, ZnOfilm and TiO2NP/ZnOfilm double-layer film haveeen prepared for NO gas sensing application. As compared toither TiO2NP or ZnOfilm single-layer film, the TiO2NP/ZnOfilmouble-layer film showed much higher sensing response to NOas. In addition, the enhancement in the sensor response istrongly affected by the annealing time during the film preparation,hich can be attributed to the evolution of Ti3+ cations. Further-ore, the double-layer also showed improved selectivities with

espect to NO2 and CO. The results of this work suggest that theiO2NP/ZnOfilm double-layer film can be a potential sensing layeror environmental monitoring application.

cknowledgements

This work was sponsored by the National Research Council ofaiwan under grant numbers NSC 96-2120-M-002-004 and NSC8-EC-17-A-02-S2-0125.

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Biographies

Chia-Yu Lin received his PhD. degree in Chemical Engineering from the NationalTaiwan University, Taipei, Taiwan in 2010. Currently, he is doing his compulsorymilitary service. His research interest mainly surrounds synthesis of nanomate-rials for the applications of electrochemical sensors, chemical gas sensors, anddye-sensitized solar cells.

Jian-Ging Chen completed his B.S. and M.S. degrees in Chemical Engineering atTunghai University and National Central University, Taiwan, respectively. In 2003he joined Electro-optical Materials Lab at National Taiwan University as a PhDstudent and devoted to the field of dye-sensitized solar cells, emphasizing theirelectrochemical characterization.

Wei-Yi Feng received the B.S. degree in electrical engineering from Yuan-Ze Uni-versity, Taoyuan, Taiwan, in 2006, and the M.S. degree in the Institute of BiomedicalEngineering from the National Taiwan University, Taipei, Taiwan, in July 2008. Hiscurrent research interests include nano/micro electro-mechanical systems process-ing, applications of metal oxide semiconductors, biochemical sensors, gas sensors,as well as SPR sensors.

Chii-Wann Lin received his B.S. from the Department of Electrical Engineering,National Cheng Kung University in 1984. He then started his career in biomedicalengineering with a M.S. degree from the Graduate Institute of Biomedical Engi-neering, National Yang Ming University in 1986. After two years of military service,he attended the Case Western Reserve University and received his Ph.D. degreein Biomedical Engineering in January 1993. Before his return to Taiwan, he hadbeen a research associate in the Neurology Department, CWRU from January 1993to August 1993. He worked in the Center for Biomedical Engineering, College ofMedicine, National Taiwan University from September 1993 to August 1998. Heis now a Professor in the Graduate Institute of Biomedical Engineering and holdsjoint appointments in both the Department of Electrical Engineering and the Insti-tute of Applied Mechanics, National Taiwan University. He is also a member ofIEEE EMBS, IFMBE, and Chinese BMES. He held the position of President of theTaiwan Association of Chemical Sensors from 2008 to 2010. His research inter-ests include biomedical micro sensors, optical biochip, surface plasmon resonance,bio-plasmonics, nano-medicine, and personal e-health system.

Ju-Wen Huang received the B.S. degree in Chemical Engineering from the NationalCheng Kung University, Tainan, Taiwan, in 2006. His current research interestsinclude the fabrication of nanosized titanium oxide for dye-sensitized solar cellapplication.

James J. Tunney obtained his Ph.D. in Chemistry in 1995 from the University ofOttawa, Canada. He joined the National Research Council of Canada in 1996 first as aPost-Doctoral Fellow, and later as a Research Officer. Since 2006, he has also servedas a Competency Leader for Organic Materials at NRC-ICPET. His research interestsinclude the use of thin and thick film technology applied to chemical sensing.

Kuo-Chuan Ho received B.S. and M.S. degrees in Chemical Engineering from theNational Cheng Kung University, Tainan, Taiwan, in 1978 and 1980, respectively.In 1986, he received his Ph.D. degree in Chemical Engineering at the Univer-sity of Rochester. The same year he joined PPG Industries, Inc., first as a SeniorResearch Engineer and then, from 1990 until 1993, as a Research Project Engineer.He has worked on the electrochemical properties of various electrode materials,with emphasis on improving the performances of sensor devices. Following a six-year industrial career at PPG Industries, Inc., he joined his alma mater at National

Cheng Kung University in 1993 as an Associate Professor in the Chemical Engineer-ing Department. In 1994, he moved to the Department of Chemical Engineeringat the National Taiwan University. Currently, he is a Professor jointly appointedby the Department of Chemical Engineering and Institute of Polymer Science andEngineering at the National Taiwan University.