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Sensors and Actuators B 160 (2011) 549– 554

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

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

olycrystalline tungsten oxide nanofibers for gas-sensing applications

uan-Anh Nguyena, Sungyeol Parka, Jun Beom Kima, Tae Kyu Kimb, Gi Hun Seongc,aebum Chooc, Yong Shin Kima,∗

Graduate School of Bionano Technology, Hanyang University, Ansan 426-791, South KoreaDepartment of Chemistry and Chemistry Institute of Functional Materials, Pusan National University, Busan 609-735, South KoreaDepartment of Bionano Engineering, Hanyang University, Ansan 426-791, South Korea

r t i c l e i n f o

rticle history:eceived 16 May 2011eceived in revised form 11 August 2011ccepted 13 August 2011vailable online 22 August 2011

eywords:

a b s t r a c t

Tungsten oxide nanofibers were successfully prepared via thermal treatment of electrospun compositenanofibers consisting of polyvinylpyrrolidone (PVP) and tungstic acid at 500 ◦C in air. The morphology,crystal structure, and chemical composition of the nanofibers were characterized by SEM, EDX, TEM,XRD, and FT-IR before and after the thermal treatment. It was confirmed that the calcination processwas responsible for the removal of PVP component and the growth of crystalline WO3. The resultingtungsten oxide nanofibers, which had a rough surface morphology and an average diameter of around

ungsten oxideanofiberlectrospinningas sensor

40 nm, were found to be formed by the axial agglomeration of prolate spheroid-like WO3 nanoparticleswith monoclinic crystalline phases. Gas-sensing measurements of the polycrystalline WO3 nanofibermats were performed upon exposure to ammonia gas. They demonstrated n-type sensing response andsensitive NH3 detection up to 10 ppm with a well-defined relationship between the concentration anddetection response at an operating temperature of 300 ◦C. These results were interpreted by applying thespace-charge layer model used in the semiconducting metal-oxide sensor systems.

. Introduction

Metal oxide semiconductors have been widely used as sensinglements in gas sensor devices [1,2]. One of the most promis-ng metal oxide materials is tungsten oxide. Several studies haveemonstrated that tungsten oxide sensors can be used for theetection of nitrogen oxide (NO and NO2), ammonia, hydrogenulfide, and ethanol [3–9]. These devices are typically composedf a thin continuous film of tungsten oxides formed on pat-erned metal electrodes, and their electrical resistance is modifiedhen oxidizing or reducing gas molecules are adsorbed on thelm surface. Since nanostructured metal oxide semiconductorsre promising candidates as a sensing layer due to their highurface-to-volume ratio and small grain size, various tungstenxide nanomaterials such as hollow-sphere nanoparticles [10],anowires or nanorods [11–13], nanofibers [14–16], nanoplates17], and three-dimensional hierarchical nanostructures [18] haveeen extensively investigated to develop gas sensors of high per-ormance. These materials have demonstrated excellent sensing

roperties (high sensitivity, fast response time, and low opera-ion temperature) which are unattainable using the conventionalungsten oxide sensors with submicrometer-sized grains.

∗ Corresponding author. Tel.: +82 31 400 5507; fax: +82 31 400 5457.E-mail address: yongshin@hanyang.ac.kr (Y.S. Kim).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.08.028

© 2011 Elsevier B.V. All rights reserved.

An electrospinning technique was proved to be a simple and ver-satile method for the production of nano-sized fibers of polymers[19], inorganic materials [20], and organic/inorganic composites[21]. When a high voltage is applied to the needle of a syringe con-taining a precursor solution, the solution is ejected from the needleand deposited as a nonwoven fibrous mat on a substrate placedon the ground collector. Recently, tungsten oxide nanofibers havebeen synthesized from the tungsten precursors of WCl6 [14] andW(OiPr)6 [15] by applying the electrospinning and sol–gel chem-istry. However, since these tungsten oxide precursors are expensiveand highly reactive towards hydrolysis, they must be handled in ananhydrous environment such as in a glove box. They can also reactwith moisture during the electrospinning process, which leads touncontrolled precipitation especially around the needle tip. Thisprecipitation greatly affects the ejection dynamics of solutions,making it difficult to obtain narrow nanofibers with a uniformdiameter.

In this work, a nonwoven mat of tungsten oxide nanofibers wassuccessfully prepared by electrospinning a solution prepared froman inexpensive tungstic acid (H2WO4) precursor. Since the H2WO4precursor was dissolved in aqueous ammonia, the electrospiningprocess became free of the unpleasant precipitation problem. To

the best of our knowledge, this is the first successful attempt to pre-pare tungsten oxide nanofibers using tungstic acid as a precursor.The average diameter of WO3 nanofibers was around 40 nm, whichis more than two times less than those of WO3 nanofibers prepared

550 T.-A. Nguyen et al. / Sensors and Act

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ig. 1. Schematic illustrations of (a) the electrospinning system used for the forma-ion of composite nanofibers and (b) the thermal treatment process. A fabricated

O3 sensor is displayed in (c).

rom other precursors such as W(OiPr)6, WCl6, and metallic tung-ten [14–16]. Since metal oxide nanofibers with a small diameterenerally have advantages from the viewpoint of sensitive and fastetection due to a high surface-to-volume ratio in the metal oxideensor devices, we evaluated the NH3-sensing properties of ourarrow WO3 nanofibers prepared from the H2WO4 precursor. Theensors demonstrated the good ability to detect 10 ppm NH3 with

signal-to-noise ratio of about 29 at an operating temperature of00 ◦C.

. Experiment

A primary solution was prepared by dissolving 1.12 g of tungsticcid powder in 10 mL of NH4OH (35%), which was then mixedith a 10 mL ethanol solution containing 1.0 g of polyvinylpyrroli-one (PVP, Mw = 1,300,000). The resulting mixture was stirred for

days at room temperature to produce a homogenous solution forlectrospinning. Fig. 1a shows a schematic diagram of the electro-pinning setup used to produce nanofibers. The composite solutionas loaded into a plastic syringe equipped with a 22 gauge metaleedle. The needle was connected to a high-voltage power sup-ly (ES-30P, Gamma High Voltage Research) capable of generatingC voltages up to 30 kV. Typically, a voltage of 20 kV was applied

o the needle and the mixture solution was delivered by a syringe

ump at a flow rate of 0.7 mL/h. A cylindrical ring collector waslaced 15 cm away from the needle tip. The collector, being coveredith an aluminum foil for the easy maintenance, was grounded

nd rotated at a speed of around 150 rpm to achieve a homogenous

uators B 160 (2011) 549– 554

electrical field during the electrospinning process. The compositenanofibers were collected on both pieces of Si wafer and sensorsubstrates that were stuck to the Al foil. Tungsten oxide nanofiberswere prepared after the thermal treatment of the as-electrospuncomposite fibers at 500 ◦C for 3 h in air (see Fig. 1b). A slow heatingrate of 2 ◦C/min was used to ensure the removal of organic moietieswithout destroying the fibrous nanostructures. Fig. 1c shows a pic-ture of the WO3 sensor fabricated on the sensor substrate. A pairof comb-shaped Au electrodes was formed on a SiO2/Si substratevia consecutive e-beam depositions of 5 nm Cr and 100 nm Au lay-ers through a shadow mask. This interdigitated electrodes have asensing area of 3 × 10 mm2 and an electrode gap of 300 �m.

The microstructures of the tungsten oxide nanofibers wereobserved by scanning electron microscopy (SEM; S-4800, Hitachi)and transmission electron microscopy (TEM; JEM-2100F, JEOL).The chemical compositions were evaluated using energy dispersiveX-ray spectroscopy (EDS) and Fourier transform infrared spec-troscopy (FT-IR; Scimitar 1000, Varian). X-ray diffraction (XRD)patterns were also obtained by an X-ray diffractometer (D/Max-2500, Rigaku) with Cu K� radiation (� = 1.5418 A). The nanofibersamples prepared on bare Si substrates were directly used for SEM,EDX, and XRD analyses. The nanofiber fragments detached fromthe Si sample were mixed with KBr powder and suspended in anethanol solution to prepare specimens for FT-IR and TEM analyses,respectively.

The gas-sensing characteristics of the nanofibers were evaluatedusing a home-made measurement system consisting of analyte-diluted gas delivery lines and a small tube furnace (see Fig. 2).Measurements were carried out by placing a sensor in a quartztube of the furnace with electrical feedthroughs, and by blowingammonia vapors over it at a flow rate of 1.000 mL/min. Ammo-nia concentrations in the range of 10–200 ppm were introducedby adjusting the relative ratio of the NH3 flow rate to the dilu-tion air flow rate. The chemoresistive responses of the sensor weremeasured by a digital multimeter (34410A, Agilent) at operationtemperatures of 250–350 ◦C. The sensor temperature was con-trolled by a heater of the tube furnace.

3. Results and discussion

3.1. Material properties of WO3 nanofibers

A surface SEM image of as-electrospun PVP/tungsten oxidecomposites is shown in Fig. 3a, demonstrating randomly orientednanofibers with a smooth surface. The diameters of the compos-ite nanofibers were distributed in the range of 50–100 nm with anaverage diameter of 71.5 nm, as shown in the inset. This exhibitsthat composite nanofibers with an average diameter of around70 nm were readily formed from the mixture solution of tungsticacid and PVP via electrospinning. A comparable surface SEM imagewas obtained for the sample thermally-treated at 500 ◦C in air (seeFig. 3b). There were distinct differences in surface morphologyas the annealed nanofibers had small diameters and rough sur-faces. The average diameter of the annealed fibers was 37.8 nm,which corresponds to a shrinkage of 47%. The diameter decreasemay be attributed to the calcination of organic PVP from the as-prepared composite nanofibers. In addition, amorphous tungstenoxide materials might aggregate to produce crystalline nanoparti-cles located along the 1-D direction to form a nanofiber with a roughsurface during the calcination process. Based on the SEM observa-

tion that the composite nanofibers collected on a background Al foilalso had a similar fiber diameter and a surface morphology, the kindof a substrate seemed to have a little effect on nanofiber microstruc-tures. The morphology of WO3 nanofibers might mainly depend on

T.-A. Nguyen et al. / Sensors and Actuators B 160 (2011) 549– 554 551

easure

bs

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Fig. 2. Schematic diagram of the sensor m

oth the electric filed-induced ejection dynamics of a compositeolution and the conditions used in the calcination process.

FT-IR analysis was performed to confirm the removal ofVP from the composite nanofibers after the thermal treat-

ig. 3. SEM images of (a) as-electrospun composite nanofibers and (b) tungstenxide nanofibers calcined at 500 ◦C for 3 h. The insets display a fiber diameter dis-ribution obtained by measuring a width of nanofibers in the SEM image. The whitecale bars at the right-bottom corner correspond to 1 �m.

ment system (MFC, mass flow controller).

ment. Fig. 4 shows the FT-IR spectra of the as-electrospunand thermally-treated samples. The upper spectrum obtainedfrom the composite nanofibers exhibits several strong bandstructures in the region of 1150–1750 cm−1 and a broad fea-ture in the range of 500–1000 cm−1. These characteristic bandsobserved in the high and low energy regions suggest thepresence of PVP and tungsten oxide within the compositenanofibers, respectively, based on the previous band assign-ments of �(C O) = 1650–1700 cm−1, �(C O) = 1230–1330 cm−1,�(CH3 or CH2) = 1350–1500 cm−1, �(C–N) = 1020–1250 cm−1, and�(O–W–O) = 600–900 cm−1 [22–24], where � and ı symbols presentinternal motions of stretching and deformation in a polyatomicmolecule, respectively. On the other hand, the calcined sample (thelower spectrum) displays a very intense band with three peaks atthe wavenumbers of 873, 817 and 781 cm−1 in the low energyregion. Based on previously reported values (870, 815, 755 and665 cm−1) these peak positions could be assigned to the O–W–Ostretching modes of monochromic tungsten oxide [23]. Meanwhile,the weak bands at 1635 and 1395 cm−1 seem to result from weakly-bound water contaminants due to exposure of the sample to theambient atmosphere. These observations imply that thermal treat-ment induces the removal of the PVP constituent and the growth

of crystalline WO3. These results are very consistent with the PVPthermo-gravimetrogram, which show an abrupt mass change in thetemperature region of 350–500 ◦C [25,26]. Furthermore, no other

Fig. 4. FT-IR spectra of the as-electrospun composite and thermally-treatednanofibers.

552 T.-A. Nguyen et al. / Sensors and Act

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ig. 5. A XRD diffraction pattern of the thermally-treated nanofibers. The bottom barlot corresponds to reference data obtained for the monoclinic WO3 crystal (JCPDSard No. 83-0950).

lements except for tungsten and oxygen atoms were detected inhe EDS spectrum of the calcined nanofibers, and the atomic ratiof W and O was found to be close to that of stoichimetric WO3. Thisrovides further evidence of the PVP removal from the compositeanofibers during the calcination process.

Fig. 5 shows a typical XRD pattern of the tungsten oxideanofibers. There are many diffraction peaks containing the fourtrongest peaks observed at 2� positions of 23.1, 23.6, 24.4,nd 34.2◦. These positions were well assigned to the crystallanes of the monoclinic WO3 phase with cell parameters of

= 7.300 A, b = 7.538 A, c = 7.689 A and = 90.892◦ (JCPDS Card No.3-0950), indicating the successful formation of polycrystallineO3 nanofibers. The average crystal size D can be calculated using

he Debye–Scherrer equation:

= K�

B cos �(1)

ig. 6. (a) Low-resolution and (b) high-resolution TEM images of polycrystalline WO3 naegions of the dotted ellipses. A typical SAED pattern obtained from the sample is shown

uators B 160 (2011) 549– 554

where K is a constant dependent on the crystallite shape, � is thewavelength of the X-ray radiation, and B represents the full widthat half maximum of a peak. The average crystal size from the mostintensive (2 0 0) peak was calculated to be 32 nm, while the threeother strong peaks of (0 0 2), (0 2 0), and (2 2 0) yielded crystal sizesin the range of 20–30 nm.

Detailed microstructures of the polycrystalline WO3 were fur-ther investigated by TEM. Fig. 6a shows a typical low-resolutionTEM image of a sample, exhibiting axial nanofibers with diametersof around 50 nm. The nanofibers were formed by agglomeratingprolate spheroids in which the length of the polar axis and the equa-torial diameter were 20–40 nm and 10–30 nm, respectively. Thesegrain sizes are very consistent with the values calculated by theDebye–Scherrer equation. The lattice structures of WO3 nanopar-ticles were further analyzed in the high-resolution TEM image (seeFig. 6b). The inside regions indicated by the dotted ellipse sym-bols clearly exhibits highly crystalline characteristics of WO3 grainswith the interplanar distances of 0.37 nm and 0.26 nm for the Aand B areas, respectively. These distances agree well with the mostintensive two deflection planes of (2 0 0) and (2 2 0)/(2 0 2) in theXRD pattern of Fig. 5. The axially agglomerated microstructure ofpolycrystalline nanoparticles was previously reported in similarelectrospun metal oxide nanofiber systems such as TiO2, SnO2, andWO3 [15,27,28]. Furthermore, the selected area electron diffraction(SAED) pattern clearly displays partially orientated polycrystallineWO3 features of the spheroids (see Fig. 6c).

3.2. Gas-sensing properties of WO3 nanofibers

The gas-sensing performance of the WO3 nanofibers was evalu-ated using ammonia analyte. Fig. 7 shows a typical time-profiledresponse curve of the WO3 sensor at an operating temperature

of 300 ◦C when ammonia gas was injected for 10 min followedby a 10-min recovery period. The measurements were carried outtwo times at the same NH3 concentration. The concentration wasincreased successively from 10 to 200 ppm, as designated by the

nofibers. Well-resolved lattice structures of the WO3 crystals are shown in inside in (c).

T.-A. Nguyen et al. / Sensors and Act

Fig. 7. The response curve of the WO3 nanofiber sensor operated at 300 ◦C whenacu

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mmonia analytes were supplied in the pulsed mode with gradually increasing aoncentration, as shown in the bottom. The numbers indicate the NH3 concentrationsed in the measurements.

umerical values in Fig. 7. The response curve exhibits a decreasen resistance upon exposure to ammonia, corresponding to n-typeesponse in the semiconducting metal oxide sensor system. Forxample, the sensor resistance decreased from 3.95 to 2.93 M� for0 ppm NH3. This result demonstrates that our electrospun WO3anofiber sensor can successfully detect NH3 even at a low con-entration of 10 ppm. The background noise level was found to be5 k� for the 10-min baseline resistances just before the NH3 injec-ion. The signal-to-noise ratio defined by the relative ratio of theesistance change to the background noise level was calculated toe 29.

The variation of the response magnitudes observed in Fig. 7,.e., the decrease tendency in sensing resistance with the increasef an NH3 concentration, was quantitatively evaluated in terms ofhe detection response. The detection response was defined as theelative ratio between the initial resistance in air, Rair, and the NH3-ensing resistance, RNH3 , Response = Rair/RNH3 . Fig. 8 displays theariation of the response as a function of the NH3 concentration,NH3]. These data can be approximated by an equation with theorm of Response = 1 + a [NH3]b, where a and b are prefactor andxponential parameters, respectively. The solid line correspondso the best fit result with a = 0.11 and b = 0.43. This power depen-ence of the sensor responses was previously reported in metal

xide semiconductor sensor systems [1,29], which provides a rela-ionship between the sensor response and the gas concentration.

The sensing mechanism of the WO3 nanofiber sensor can bexplained by the space-charge layer model [17]. Tungsten oxide

ig. 8. The detection response variation of the WO3 nanofiber sensor as a functionf the NH3 concentration.

uators B 160 (2011) 549– 554 553

is well known as an n-type semiconductor. If the sensor is keptin air at 300 ◦C, oxygen molecules will adsorb on the surfaces ofWO3 nanofibers, and then transform to negatively-charged oxygenspecies (e.g., O2

−, O−, or O2−) by capturing free electrons from theconduction band of WO3. This electron-capture process conductsto the formation of a negative surface potential and a depletionregion around the grain boundary of WO3 crystals, which results ina decrease in conductivity of electron carriers along the nanofiber.Among the possible negative-charged oxygen species, the atomicoxygen ion adsorbate, O− (ad), had been reported to exist predomi-nantly at the temperature above 200 ◦C in SnO2 sensor systems [1].When the nanofiber sensor is exposed to reducing ammonia gas,NH3 molecules can chemisorb on both Lewis and Brønsted acidicsurface sites of WO3. If not simply desorbed, NH3 can undergo sev-eral competitive oxidation reactions with the oxygen adsorbate onthe nanofiber surface. Three main reactions have been proposedfor NH3 oxidation, being accompanied with the generation of a freeelectron, e− [30,31]:

2NH3 + 3O− (ad) → N2 + 3H2O + 3e− (2)

2NH3 + 5O− (ad) → 2NO + 3H2O + 5e− (3)

2NH3 + 4O− (ad) → N2O + 3H2O + 4e− (4)

These surface reactions at the solid–gas interface lead to anincrease of free electrons in the conduction band of n-type WO3and a decrease in inter-grain barrier potential. The barrier energydecrease, �Ebarr, upon exposure to the reducing NH3 can explainthe observed n-type response, i.e., a conductivity increase in theWO3 nanofiber sensor system.

Similar NH3-sensing measurements were performed at differ-ent operating temperatures in the range of 250–350 ◦C. The initialsensor resistance was decreased from 4.3 to 1.6 M� as the tem-perature goes higher in the temperature region. This variationsuggests the semiconducting nature of the WO3 nanofibers. Theresponse curves measured at 350 ◦C exhibited an inferior detectionresponse compared with that at 300 ◦C. For example, upon expo-sure to 100 ppm NH3, the detection responses were 1.1 at 350 ◦Cand 1.7 at 300 ◦C. If the sensor conductivity is predominantly con-trolled by the probability of a free electron for surmounting theinter-grain potential barrier, the detection response can be roughlyexpressed by the relationship, Response ∝ exp(�Ebarr/RT), where Ris a gas constant and T is an operation temperature in Kelvin scale.The decrease in response could be understood with the tempera-ture insensitivity of the barrier energy change. On the other hand,the detection response at the operating temperature of 250 ◦C wascomparable to that at 300 ◦C. However, the response and recoverytimes were observed to increase by more than a factor of 2 as thetemperature was decreased from 300 to 250 ◦C. Judging from theobservation of requiring a long time more than 1 h at 250 ◦C fora stabilized initial sensor resistance, the process of forming sur-face oxygen ions expected great endothermicity and slow kinetics.These features could be responsible for the slow response recoveryobserved at 250 ◦C. The long response time may be attributed toslower rate constants of the NH3 oxidation surface reactions. Con-sequently, the optimum operating temperature to achieve a greatresponse and fast detection times was around 300 ◦C.

4. Conclusion

We have fabricated tungsten oxide nanofiber gas sensors using asimple electrospinning method and sol–gel reactions of humidity-

durable tungstic acid. Pure polycrystalline WO3 nanofibers, havingan average diameter of 37.8 nm and rough surface morphology,were synthesized through the calcination of PVP-WO3 compos-ite nanofibers with an average diameter of 71.5 nm, prepared

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54 T.-A. Nguyen et al. / Sensors a

y electrospinning of a solution of PVP and H2WO4. The NH3-ensing characteristics of the sensors were evaluated underifferent concentrations (10–200 ppm) and operating tempera-ures (250–350 ◦C). The measurements have demonstrated thatmmonia analyte could be detected with a sufficient detectionesponse even at the low concentration of 10 ppm, which isrobably due to the high surface-to-volume ratio and effectiveodulation in the electron transportation, resulting from the

xially aggregated nanofiber structure of the nano-sized mon-clinic WO3 particles. In addition, the detection response andesponse times were dependent on an operating temperature.he optimal operating temperature was found to be 300 ◦C. Theseesults suggest potential applications of electrospun tungsten oxideanofibers as an active sensing material for NH3 detection.

cknowledgement

This research was supported by the National Research Founda-ion of Korea (NRF) funded by the Ministry of Education, Sciencend Technology (No. 2010-0020638).

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Biographies

Tuan-Anh Nguyen received his BS degree from the Hanoi University of Technol-ogy, Vietnam in 2006. Now, he is a graduate student in the department of BionanoEngineering, Hanyang University, South Korea. His main research interests includethe development of novel sensing materials such as nanostructured metal oxidesemiconductors.

Sungyeol Park received his BS degree in the department of Applied Chemistry,Hanyang University, South Korea in 2010. He is a graduate student in the depart-ment of Bionano Engineering, Hanyang University at present. His research interest ismainly centered on the development of highly sensitive electrochemiluminescence(ECL) Ru(bpy)3

2+ sensors.

Jun Beom Kim received his BS degree in the department of Applied Chemistry,Hanyang University, South Korea in 2011. After the graduation, he becomes a grad-uate student in the department of Bionano Engineering, Hanyang University. Hisresearch activities are focused on the development of a solid-state electrochemilu-minescence (ECL) sensor based on Tris(2,2′-bipyridyl) ruthenium.

Tae Kyu Kim received a PhD degree in chemistry from Korea Advanced Instituteof Science and Technology (KAIST) in 2004. He has worked for Pusan National Uni-versity since March 2007. His current research focuses on the reaction dynamicsof simple chemical reactions in gaseous and liquid phase by means of X-ray spec-troscopy.

Gi Hun Seong received the BS and MS degrees in chemical engineering fromKorea Advanced Institute of Science and Technology (KAIST) in 1993 and 1995,respectively, and his PhD degree in bioscience and biotechnology from TokyoInstitute of Technology, Japan in 2001. He joined the chemistry department,Texas A&M University as a postdoctoral research fellow from 2001 to 2003. Heis presently an associate professor in the department of Bionano engineering,Hanyang University, South Korea. His research interests are carbon nanotubes-based (bio)sensors, microbiochips, and nanoparticles-based solar cell and fuelcell.

Jaebum Choo is a Professor in the Department of Bionano Engineering at HanyangUniversity. He is also a Director of Integrated Human Sensing System (ERC)Research Center. He received his PhD from Texas A&M University in1994. After1-year post-doctorial experience, he joined as a faculty member of Hanyang Uni-versity. He has published more than 120 papers in peer reviewed journals andhas given over 90 invited lectures. His research interest is mainly centered onthe development of highly sensitive optical detection technologies using metalnanoparticles.

Yong Shin Kim received a PhD in chemistry from Korea Advanced Institute ofScience and Technology (KAIST) in 1997. After his degree, he worked as a seniorresearch member at Electronics and Telecommunications Research Institute (ETRI)

in South Korea for developing flat panel display devices and miniaturized elec-tronic nose systems. Since 2007, he has been employed at Hanyang University inSouth Korea as an associate professor. Now his research activities are focused onthe development of smart chemical sensors and novel nanomaterials for sensorapplications.