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Sensors and Actuators B 150 (2010) 154–159

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

hin film-coated plastic optical fiber probe for aerosol chemicalensing applications

tul Kulkarnia,1, Jun-Ho Leeb, Jae-Do Namb, Taesung Kima,c,∗

School of Mechanical Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, South KoreaGyeonggi Regional Research Center (GRRC), Department of Polymer Science and Engineering Sungkyunkwan University,00 Chunchun-dong, Jangan-gu, Suwon 440-746, South KoreaSKKU Advanced Institute of nanotechnology (SAINT), Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, South Korea

r t i c l e i n f o

rticle history:eceived 7 January 2010eceived in revised form 9 July 2010ccepted 19 July 2010

a b s t r a c t

The increasing importance of aerosols in understanding environmental processes as well as the generaluse of aerosol technologies has led to great interest in aerosol characterization. Thus, we present thedevelopment of a thin film-coated plastic optical fiber probe for aerosol chemical composition sensing.To prepare the sensor probe, a small length of cladding was removed from the middle portion of the

vailable online 16 August 2010

eywords:erosolhemical compositionlastic optical fiber probe

plastic optical fiber. A thin film of tetraethylorthosilane (TEOS), prepared using the sol–gel technique,was deposited on the surface of the unclad portion of the fiber via dip coating. Sensor responses arereported for various aerosols and are found to be encouraging. The thin film-coated plastic optical fiberis found to increase the sensitivity of the sensor. A plastic fiber optic aerosol chemical sensor based onevanescent wave absorption is presented here, to the best of our knowledge, for the first time.

hin filmhemical sensor

. Introduction

Tropospheric aerosols are of great importance because of theirmpact on human health, general visibility, continental and mar-time ecosystems, and the Earth’s climate. Atmospheric aerosolsn the troposphere are mainly composed of solid and liquid parti-les of varying compositions and phases, consisting of preliminaryarbonaceous species including elemental carbon and organic car-on as well as inorganic species such as metals, sulfates, nitrates,nd ammonium [1]. Recent atmospheric chemistry studies haveocused on developing new techniques for monitoring aerosolhemical compositions [2–10]. Almost all measuring methodsepend on mass spectroscopic techniques that are mainly basedn offsite laboratory analyses and are costly and time-consuming.n addition, there are limitations in sampling and analytical tech-iques. For this reason, there is a need for accurate, inexpensive,

ontinuous, and long-term monitoring of environmental contami-ants using sensors that can be operated on site.

A number of chemical sensors have been developed for envi-onmental monitoring applications [11]. These can be classified

∗ Corresponding author at: School of Mechanical Engineering, Sungkyunkwanniversity, 300 Chunchun-dong, Jangan-gu, Suwon, South Korea.el.: +82 31 290 7466.

E-mail addresses: atulkin@gmail.com (A. Kulkarni), tkim@skku.edu (T. Kim).1 Tel.: +82 31 299 4751.

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.07.026

© 2010 Elsevier B.V. All rights reserved.

based on their principal physical and operating mechanisms.Many devices based on optical methods have been used inchemical sensors and biosensors, including ellipsometry, spec-troscopy (luminescence, phosphorescence, fluorescence, Raman),interferometry (white light interferometry, modal interferometryin optical waveguide structures), spectroscopy of guided modesin optical waveguide structures (grating coupler, resonant mirror),and surface plasmon resonance [12–17].

Fiber optic and integrated optic technologies were primarilydeveloped for telecommunication applications. However, advancesin the development of high quality and competitively pricedoptoelectronic components and fibers have contributed to theexpansion of guided wave technology for sensing [18]. Fiberoptic chemical sensor principles are based on the monitoring ofabsorbance, reflectance, luminescence, reflective index change, andlight scattering as well as on measuring oxygen, pH, carbon diox-ide, ammonia, detergents, biochemical oxygen demand, pesticides,and humidity [19]. The utilization of the sol–gel process to pro-duce thin film membrane materials for optical chemical sensorsand biosensors is attracting considerable research interest. Herein,the sensor response mainly depends on the refractive indices ofthe fiber core, thin film, and the surrounding medium. The sensing

mechanism is based on the monitoring of the absorption or fluo-rescence characteristics of indicator dyes/sensing films entrappedwithin a thin film deposited onto an optical fiber as substitutecladding. The targeted aerosols interact with the immobilized indi-cator, resulting in changes in absorbance or emission spectra, which

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A. Kulkarni et al. / Sensors an

re monitored using a proper detector module via an optical fiber.vanescent wave absorption is an effective technique for perform-ng such analysis [20]. When a beam of light propagates along anptical fiber, the electromagnetic field does not abruptly fall to zerot the core/cladding interface. Instead, the overlap of the incom-ng beam and the internally reflected beam leads to a field thatenetrates into the medium next to the core. This electromagneticeld, which tails but does not propagate into the second medium,

s called the evanescent field. Its intensity Iz decays exponentiallyith the perpendicular distance to the interface z as follows:

z = I0 exp

(− z

dp

)(1)

here I0 is the intensity of the incident radiation. The depth of pen-tration dp of the evanescent field is related to the angle of incidenceat the interface, refractive indexes of core n1 and cladding n2, and

he wavelength of the radiation � as follows [21–23];

p = �

2�

√n2

1 sin2 � − n22

(2)

The evanescent field is able to interact with dye materials con-ained within the coating. This mechanism can be employed inather ingenious ways to develop fiber optic evanescent aerosolensors. In these kinds of sensors, the parameter measured is theight intensity carried by the fiber that is modulated by the changen the absorption of the aerosol-sensitive chromophore.

In the present work, two types of sols were utilized to formsensitive cladding for the plastic optical fiber sensor probe: therst consisted of pure TEOS and the second was functionalized byhe inclusion of thymol blue during synthesis. These prepared solsere coated on a plastic optical fiber core via dip coating, and an

nvestigation of the interaction of aerosols such as black (elemen-al) carbon (BC) and ammonium sulfate ((NH4)2SO4) with the thinlms was conducted. The utilized aerosols were characterized prioro the evaluation via standard aerosol characterization equipmentuch as a differential mobility analyzer (DMA) and a condensationsarticle counter (CPC).

. Experimental

.1. Preparation of the sols

The sol–gel process is a relatively straightforward method forroducing high quality glasses and ceramics [22]. In the context ofptical sensors, the process is generally employed in the produc-ion of silica or titania glass, or mixtures of these two. Two types

Fig. 1. (a) Plastic optical fiber sensor element with enlarged view of the sol–gel film co

ators B 150 (2010) 154–159 155

of sols: tetraethylorthosilane (TEOS), and TEOS + thymol blue (TB)(Sigma–Aldrich, Inc., USA) were used for the preparation of the sil-ica cladding. For the first sol–gel, 5 ml of TEOS was dissolved in 6 mlof ethanol (EtOH), and the resulting solution was added to 6 ml ofa stoichiometric excess of water (6 ml acidified with concentratedHCl to pH 2). The mixture was stirred vigorously and heated uncov-ered to 60 ◦C at 300 RPM. After 3 h of mixing, a sol was obtained atpH 2.46. This solution is hereafter referred to as TEOS. For the sec-ond sol–gel, 4 ml of TEOS was dissolved in 8 ml of EtOH and 8 ml ofwater acidified with HCl (pH 2). The mixture was stirred for 1 h toobtain a clear silica sol. 1 mg of TB dye was mixed into the sol, andthe mixture was stirred for 2 h. This solution is hereafter referredto as thymol blue.

2.2. Coating the optical fibers

The plastic optical fiber (POF) had a 980 �m core and a 20 �mcladding layer. The overall diameter with the protective jacket was2.2 mm. The plastic optical fiber’s protective cover was mechani-cally removed by means of a surgical blade on a 1 cm-long segmentin the central part of the fiber as depicted in Fig. 1(a). Cladding wasremoved with a razor blade while paying great attention such thatno scratches were inflicted on the surface of the fiber core; the areawas then treated with acetone, washed with a large quantity ofwater, and dried under a nitrogen flow. The unclad fiber was thendipped into the prepared sols and withdrawn with a velocity of10 cm mn−1 via a micro-dip coater (E-flex, EF-3100). Three coatingprocesses were performed. After each process, the fiber was curedat 90 ◦C for 1 h [24]; the last curing time was 20 h to produce a hardporous film. Although the plastic optical fiber is polymer based, nodamage was observed for the curing at 90 ◦C. The SEM image of thethin film-coated fiber is shown in Fig. 1(a). Each dip coating filmthickness on the optical fiber core was about 6–7 nm. Hence, afterthree dip coatings we achieved a film thickness of about 20–22 nm.The sol–gel film transmittance was also measured in the visiblerange and the observation is depicted in the Fig. 1(b). We observedthat this long deposition process and the high temperature treat-ment improved the adhesion of the sols to the plastic optical fiber.10 sol–gel-coated POF probes obtained by this method were usedfor the experimental evaluation.

2.3. Determination of the physical properties of the Aerosols

Black carbon (BC) and sulfate (NH4)2SO4, were selected for thestudy due to their important role in human life. The refractiveindexes, which were the main concern in our evaluation, were1.54 + 0i and 1.96 + 0.66i for sulfate and BC, respectively. The setup

ating SEM image. (b) Transmittance of the prepared sol–gel in the visible range.

156 A. Kulkarni et al. / Sensors and Actu

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ber concentrations were found for these aerosol particles. The

ig. 2. Schematic presentation of Aerosol characterization setup for mass concen-ration measurements.

epicted in Fig. 2 was used to characterize aerosol physical prop-rties, such as the number of particles and size distribution. Theetup utilized a differential mobility analyzer (DMA) (Model 3081,SI Inc.) and a condensation particle counter (CPC) (Model 3025A,

SI Inc.). The solution concentrations of BC and sulfate were pre-ared such that the final concentrations that emerge from the DMAere the same. Hence the same concentration of the aerosols could

e used to evaluate sensor performance.

Fig. 3. Schematic presentation of fiber optic sensor evaluati

ators B 150 (2010) 154–159

2.4. Investigation of the interaction of porous layers with theAerosols

The aerosol characterization setup is further modified to evalu-ate the sensor performance is as illustrated in Fig. 3.

Compressed air from the air supply system was passed throughan air filter to remove contaminant particles that might have beenpresent in the compressed air. Then, the clean air was injected intothe atomizer in which the aerosols were generated. After passingthrough the diffusion dryer, the aerosol sample was delivered tothe sensor chamber. In this experiment, the following light sourceswere used: UV (� = 340 nm), blue (� = 440 nm), red (� = 660 nm),and infrared (� = 940 nm). The Si photodiode detector (Thorlab,PDA36A) was used. The detector output was recorded using a per-sonal computer (PC) for further data processing via a standarddigital multimeter (Keithley digital multimeter 2700). A thin film-coated plastic optical fiber, a light source, and a detector wereplaced in the sensor chamber. All experiments were carried out ina dark room at an ambient temperature under normal atmosphericpressure conditions.

3. Results and discussion

For an accurate evaluation of aerosol chemical composition,the aerosol particles need to have the same size distribution. Toachieve homogeneity of the particle total number concentrations,the amount of the dispersed aerosols was altered accordingly inde-ionized (DI) water during the preparation of the solution. Theselected aerosol particle size distributions of sulfate and BC wereobtained using a differential mobility analyzer (DMA) and conden-sation particle counter (CPC), and are depicted in Fig. 4.

No significant differences in the diameters and total num-

mean diameter size of sulfate was approximately 37.5 nm, and themean diameter size of BC was approximately 37 nm. The modediameter—the most frequent size of sulfate and BC—was 35.5 nmand 32.4 nm, respectively. The total number concentration was 3.28

on setup for aerosol chemical composition detection.

A. Kulkarni et al. / Sensors and Actu

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afttcr

respectively. However, for BC the percentage changes are 0.88%,

Fig. 4. Aerosol concentration classified using DMA and CPC.

+ 06 (#/cm3) for sulfate and 3.17 E + 06 (#/cm3) for BC. Hence, theize distributions of the aerosols, because they are similar, shouldot affect the output data of the detector.

The thin film-coated optical fiber sensor response was evalu-ted using the light sources mentioned in the experimental sectionor BC and (NH4)2SO4. When the aerosols were introduced into

he chamber, the aerosol molecules diffused into the inner layer ofhe sensing film where the dye complex could absorb the evanes-ent field. For ready reference, the plastic optical fiber core had aefractive index of 1.48, and the thin film sol–gel-coated cladding

Fig. 5. TEOS thin film-coated fiber optic sensor response fo

Fig. 6. Thymol blue film-coated fiber optic sensor response f

ators B 150 (2010) 154–159 157

of TEOS and Thymol blue had refractive indices of 1.5 and 1.52,respectively. The surrounding aerosols, sulfate and BC, had refrac-tive indices of 1.54 and 1.96, respectively. The sensor response forthe change in the intensity with and without analytes for the TEOSthin film-coated fiber is depicted in Fig. 5. Here, both BC and sulfateexhibit maximum sensitivity for the blue light source, whereas thesensitivity is lower for the red light source. This implies that, forthe TEOS film-coated fiber, the absorption by the both analytes isat its maximum for the red light source; hence the output intensityis low.

However, in case of Thymol blue, the coated fiber exhibited ahigher sensitivity than the TEOS film-coated fiber for all utilizedlight sources, as seen in the sensor response Fig. 6. Changing therefractive indices of the fiber core, thin film clad, and surroundingaerosols resulted in a change in the output intensity, and hencethe detector output. For the TEOS film-coated fiber, the percentagechange in the detector output, and hence intensity, between BC andsulfate was 0.28% for the red light source followed by the blue lightsource at 0.12%, whereas both UV and IR light sources exhibited achange in detector output of 0.04%. Whereas for the Thymol bluefilm-coated fiber, UV and red light sources showed the same (i.e.0.2%) change followed by blue (0.18%) and IR (0.16%) light sources.On the other hand, if we compare the effect of thin films for sulfateonly, it can be seen that the percentage changes in the detectoroutput are 1.04%, 0.36%, 2.22%, and 0.48% for UV, red, blue and IR

0.28%, 2.16%, and 0.68%.The effect of the sensing mechanism is such that it slows down

the response of the sensor. However, in this research, we found thatthis limitation can be somewhat minimized through either opti-

r (a) sulfate and (b) BC for UV, Red, Blue, and IR light.

or (a) sulfate and (b) BC for UV, Red, Blue, and IR light.

158 A. Kulkarni et al. / Sensors and Actu

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ig. 7. TEOS thin film-coated fiber response for output stability for a fixed concen-ration of sulfate in the chamber.

ization of the thickness of substitutional cladding or via controlf the ambient temperature. When the cladding thickness is closeo the depth of penetration dp of the evanescent field expressed inq. (2), the sensor exhibits an optimized response. However, whenhe cladding thickness decreases further, the system sensitivity willecrease due to less absorption of the evanescent field by the thinlm (results are not shown here).

To observe sensor stability with respect to time, we blocked theow of aerosol into the chamber with the chamber outlet in itslosed condition. The response of the sensor is plotted in Fig. 7. Webserved that after stopping the flow of aerosol, there was onlymarginal change in the detector output. This signifies that, forfixed concentration of aerosols inside the sensing chamber, the

ensor response is stable.

. Conclusion

Preliminary results of a test on a TEOS and Thymol blue thinlm-coated plastic optical fiber for an aerosol chemical sensingechnique were reported. This technique has many advantagesver conventional aerosol chemical composition techniques andan achieve better performance by optimizing the thin film prop-rties. We believe that optimization of the sensor enables it to beailored to specific applications through consideration of the sen-itivity and response time of the sensor. The sensor evaluationsor the long-term stability of the sensing area of the plastic opti-al fiber core with respect to continuous exposure to the variouserosols and for various aerosol concentrations are underway. Inonclusion, the reported sensing technique for aerosol chemicalomposition is highly encouraging.

cknowledgement

This work was supported by the GRRC Program of Gyeonggirovince and through instrumental support.

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Biographies

Atul Kulkarni received his Ph.D. degree from the University of Pune, India in2005. His research topic involved multidisciplinary sensors. Since 2006, he hasbeen a Research Professor at Sungkyunkwan University, Nanoparticle TechnologyLab, School of Mechanical Engineering, South Korea. His research interests includedeveloping sensors using optical, nano, and biotechnologies.

Jun Ho Lee received his B.S., M.S., and Ph.D. degrees from the Department of Poly-mer Science and Engineering at Sungkyunkwan University, Korea, in 1996, 2000, and2008, respectively. He worked for the Dongjin Semichem Company as an Assistant

University, Korea. He is actively working in the areas of polymer sensors andactuators, photoresist for flat panel displays, and biodegradable polymers andcomposites.

Jae-Do Nam received his B.S. and M.S. degrees in Chemical Engineering from SeoulNational University in 1984 and 1986, respectively. In 1991, he received the Ph.D.

d Actu

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A. Kulkarni et al. / Sensors an

egree in Chemical Engineering from the University of Washington, Seattle, WA,

SA. He was a Research Associate at the Polymeric Composites Laboratory, Univer-

ity of Washington, from 1991 to 1993. Returning to Korea, he joined the Samsungroup in 1993–1994 and moved to Sungkyunkwan University, Suwon, Korea in 1994here he is a Professor in the Department of Polymer Science and Engineering. He

s actively working in the areas of polymer sensors and actuators, organic/inorganicybrid nanocomposites, biodegradable polymers, and energy storage.

ators B 150 (2010) 154–159 159

Taesung Kim received his Ph.D. degree in 2003 from the Department of Mechan-

ical Engineering, University of Minnesota, Twin Cities Campus, USA. He iscurrently a Professor in the School of Mechanical Engineering, SungkyunkwanUniversity, South Korea. His research interests are in nanoparticle instru-mentation and manufacturing, nano/micro contamination and atmosphericaerosol control, nanogrinding/CMP, nanostructured material research, andsensors.