10
Available online at www.sciencedirect.com Sensors and Actuators B 131 (2008) 479–488 Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model Yu-Lung Lo a,, Chen-Shane Chu a , Jiahn-Piring Yur b , Yuan-Che Chang a a Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan b Department of Mechanical Engineering, Kun Shan University, Tainan, Taiwan Received 25 January 2007; received in revised form 15 November 2007; accepted 5 December 2007 Available online 14 December 2007 Abstract In practical sensing applications, temperature effects are of particular concern, and hence it is necessary to develop the means to correct the fluorescence intensity measurement in accordance with the working temperature. Accordingly, this study develops a modified Stern–Volmer model to compensate for the temperature drift of oxygen concentration measurements obtained using fiber-optic sensors. The oxygen sensors considered in this study are based on teraethylorthosilane (TEOS)/n-octyltriethoxysilane (Octyl-triEOS) or n-propyltrimethoxysilane (n-propyl-TriMOS)/3,3,3- trifluoropropyltrimethoxysilane (TFP-TriMOS) composite xerogels doped with platinum meso-tetrakis(pentafluorophenyl)porphine (PtTFPP). The experimental results are fitted to the modified Stern–Volmer model in order to compute suitable values for a temperature compensation coefficient at different working temperatures. It is found that the proposed temperature compensation method reduces the difference in the oxygen concentration measurement for working temperatures in the range of 25–70 C as compared to data without compensation. The linearity and sensitivity of PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor are better than PtTFPP-doped TEOS/Octyl-triEOS sensor for working temperatures in the range of 25–70 C. The proposed approach could provide a straightforward and effective means of improving the accuracy of fiber-optic oxygen sensors if a variable attenuator is designed according to the temperature compensation coefficient. Thus, the fiber-optic oxygen sensor with a variable attenuator could work in a broad temperature range without using a temperature sensor. © 2007 Elsevier B.V. All rights reserved. Keywords: Temperature effect; Modified Stern–Volmer model; Fiber-optic oxygen sensor 1. Introduction Various techniques have been developed for detecting oxy- gen in gas or liquid phases. Optical oxygen sensors based on the quenching of fluorescence or phosphorescence by molecu- lar oxygen overcome the limitations of the conventional Clark electrode and are extensively applied in the chemical [1–4], clin- ical [5,6] and environmental [7] fields. Generally, the membrane of such sensors consists of an analyte-sensitive dye and a support matrix in which the dye is dispersed or dissolved. Although many different oxygen-sensitive dyes can be used in optical oxygen sensors, organic dyes, Ru complexes and Pt complexes [8–11] are among those most commonly employed. Of these dyes, Pt complexes are easily excited using compact and low-cost LED Corresponding author. E-mail address: [email protected] (Y.-L. Lo). light sources. Furthermore, the phosphorescence wavelengths of Pt complexes are well separated from the excitation LED wave- length, and hence the influence of the excitation light source can be easily eliminated. In general, the support matrix of an opti- cal sensor not only immobilizes the dye, but also supplies for oxygen to penetrate into the thin film to react with the sensitive dye. Different matrixes yield different oxygen diffusion rates, and hence have a direct influence on the quenching efficiency of the indicator by the oxygen. Furthermore, the oxygen diffusion rate decreases/increases as the ambient temperature increases. To compensate for temperature-induced variations in the lumi- nescence intensity, it is necessary to determine the temperature at the sensor tip when measuring the oxygen concentration and to apply an appropriate calibration factor. Ideally, oxygen sensors should be temperature-independent such that they can be used in various environments. However, in optical sensors based on fluorescence quenching, both the flu- orescence intensity and fluorescence decay time are influenced 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.12.010

Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model

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Page 1: Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model

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Available online at www.sciencedirect.com

Sensors and Actuators B 131 (2008) 479–488

Temperature compensation of fluorescence intensity-based fiber-opticoxygen sensors using modified Stern–Volmer model

Yu-Lung Lo a,∗, Chen-Shane Chu a, Jiahn-Piring Yur b, Yuan-Che Chang a

a Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwanb Department of Mechanical Engineering, Kun Shan University, Tainan, Taiwan

Received 25 January 2007; received in revised form 15 November 2007; accepted 5 December 2007Available online 14 December 2007

bstract

In practical sensing applications, temperature effects are of particular concern, and hence it is necessary to develop the means to correct theuorescence intensity measurement in accordance with the working temperature. Accordingly, this study develops a modified Stern–Volmer model

o compensate for the temperature drift of oxygen concentration measurements obtained using fiber-optic sensors. The oxygen sensors considered inhis study are based on teraethylorthosilane (TEOS)/n-octyltriethoxysilane (Octyl-triEOS) or n-propyltrimethoxysilane (n-propyl-TriMOS)/3,3,3-rifluoropropyltrimethoxysilane (TFP-TriMOS) composite xerogels doped with platinum meso-tetrakis(pentafluorophenyl)porphine (PtTFPP).

The experimental results are fitted to the modified Stern–Volmer model in order to compute suitable values for a temperature compensationoefficient at different working temperatures. It is found that the proposed temperature compensation method reduces the difference in thexygen concentration measurement for working temperatures in the range of 25–70 ◦C as compared to data without compensation. The linearitynd sensitivity of PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor are better than PtTFPP-doped TEOS/Octyl-triEOS sensor for working

emperatures in the range of 25–70 C.The proposed approach could provide a straightforward and effective means of improving the accuracy of fiber-optic oxygen sensors if a variable

ttenuator is designed according to the temperature compensation coefficient. Thus, the fiber-optic oxygen sensor with a variable attenuator couldork in a broad temperature range without using a temperature sensor.2007 Elsevier B.V. All rights reserved.

gen se

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eywords: Temperature effect; Modified Stern–Volmer model; Fiber-optic oxy

. Introduction

Various techniques have been developed for detecting oxy-en in gas or liquid phases. Optical oxygen sensors based onhe quenching of fluorescence or phosphorescence by molecu-ar oxygen overcome the limitations of the conventional Clarklectrode and are extensively applied in the chemical [1–4], clin-cal [5,6] and environmental [7] fields. Generally, the membranef such sensors consists of an analyte-sensitive dye and a supportatrix in which the dye is dispersed or dissolved. Although many

ifferent oxygen-sensitive dyes can be used in optical oxygen

ensors, organic dyes, Ru complexes and Pt complexes [8–11]re among those most commonly employed. Of these dyes, Ptomplexes are easily excited using compact and low-cost LED

∗ Corresponding author.E-mail address: [email protected] (Y.-L. Lo).

nat

soo

925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2007.12.010

nsor

ight sources. Furthermore, the phosphorescence wavelengths oft complexes are well separated from the excitation LED wave-

ength, and hence the influence of the excitation light source cane easily eliminated. In general, the support matrix of an opti-al sensor not only immobilizes the dye, but also supplies forxygen to penetrate into the thin film to react with the sensitiveye. Different matrixes yield different oxygen diffusion rates,nd hence have a direct influence on the quenching efficiency ofhe indicator by the oxygen. Furthermore, the oxygen diffusionate decreases/increases as the ambient temperature increases.o compensate for temperature-induced variations in the lumi-escence intensity, it is necessary to determine the temperaturet the sensor tip when measuring the oxygen concentration ando apply an appropriate calibration factor.

Ideally, oxygen sensors should be temperature-independentuch that they can be used in various environments. However, inptical sensors based on fluorescence quenching, both the flu-rescence intensity and fluorescence decay time are influenced

Page 2: Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model

4 Actuators B 131 (2008) 479–488

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80 Y.-L. Lo et al. / Sensors and

y the ambient temperature [12,13]. Various researchers havemployed multi-sided models to explore this temperature effect14–16]. Gouin and Gouterman proposed the use of annealing17] or the addition of various polymer materials and pigments18] to reduce the temperature-dependence of the luminescencef pressure-sensitive paints. Several researchers have proposedemperature compensation schemes for oxygen optrodes. Forxample, Lam et al. [19] presented a phosphorescence-basedxygen sensor based on erythrosine B doped sol–gel silica. Theroposed sensor exploited the fact that phosphorescence andelayed fluorescence have contrasting temperature dependen-ies to carry out the temperature compensation of the sensoreasurements. To account for the effects of temperature, many

xygen optrodes are designed to measure not only the oxygenoncentration, but also the sensing temperature. In such sen-ors, an accurate indication of the oxygen concentration is thenbtained by applying a suitable temperature calibration factoro the measured oxygen concentration value. Borisov and Wolf-eis [20] presented a temperature-sensitive probe in which aalladium dye was employed as a luminescent indicator forxygen, and europium(III) complexes were used as a temper-ture indicator. The authors showed that both indicators coulde excited by a 405 nm LED. Significantly, the two indicatorsielded well-separated bright luminescence, and therefore thewo signals could be processed individually to provide indepen-ent values of the oxygen and the temperature, respectively. Chund Lo [21] presented a plastic optical fiber for the dual sensingf temperature and oxygen. The sensor features commerciallyvailable epoxy glue coated on the side-polished fiber surfaceor temperature sensing and a fluorinated xerogel doped withlatinum tetrakis pentafluorophenyl porphine (PtTFPP) coatedn the fiber end for oxygen sensing. Stehning and Holst [22] pre-ented a hybrid temperature-oxygen fiber-optic sensor in whichll of the signal generation and processing was performed usingdigital signal processor (DSP). In principle, this device pro-

ided the means to resolve even more contributing lifetimes ofhe luminescence signals. However, difficulties occurred in prac-ice due to the closely spaced lifetimes of the luminophores andhe low signal-to-noise ratio of the optical setup [22].

Recently, we have described the oxygen sensor based ont(II) complexes embedded in TEOS/Octyl-triEOS [23] or-propyl-TriMOS/TFP-TriMOS [24] to produce better perfor-ance fiber-optic oxygen sensors. To the best of our study,tTFPP is more stable and suitable than PtOEP for the fiber-ptic oxygen sensor’s study. Based on the reason, this currenttudy fabricates fiber-optic oxygen sensors based on teraethy-orthosilane (TEOS)/n-octyltriethoxysilane (Octyl-triEOS)r n-propyltrimethoxysilane (n-propyl-TriMOS)/3,3,3-rifluoropropyltrimethoxysilane (TFP-TriMOS) compositeerogels doped with platinum tetra(pentafluorophenyl)porphinePtTFPP), as illustrated in Fig. 1. However, the use of lumines-ence intensity as oxygen monitor presents several measurementroblems, e.g. variations in the amplitude of the light signal due

o photodegradation of the lumiphore, shifts in the instrumentr optical interferences [20]. Also, the temperature effectn luminescence of dye is a key issue in the intensity-basedensors. Therefore, a modified Stern–Volmer model is then

qtsi

Fig. 1. Chemical structure of PtTFPP.

eveloped to compensate for the temperature effect on thexygen concentration measurements obtained using theseensors over a working temperature range of 25–70 ◦C. If aariable attenuator according to the temperature compensationoefficient is designed to compensate for fluorescence intensityhange, the fiber-optic oxygen sensor could work in a broademperature range without using a temperature sensor.

. Modified Stern–Volmer model for temperatureompensation

If the dynamic quenching of luminescence by oxygen is useds a sensing principle, the relation between the oxygen concen-ration [%O2] and the measurable luminescence intensity I of aiven fluorophore can best be described by the two-site modelf the Stern–Volmer equation [4]:

I

I0=

(f1

1 + Ksv1[%O2]+ f2

1 + Ksv2[%O2]

)(1)

here I0 is the luminescence intensity in the absence of oxy-en, f1 and f2 represent the fraction of each of the two sitesontributing to the unquenched intensity, and f1 + f2 = 1. KSV1,SV2 are the quenching coefficients that describe the oxygen

ensitivity of each component. For practical applications, it cane assumed that one of the components cannot be quenchedy oxygen (KSV2 = 0) [25], and this yields an experimentallyodified equation [10,25,26]:

I0

I0 − I=

(1

f1Ksv1[%O2]+ 1

1 − f2

)

=(

1

f1Ksv1[%O2]+ 1

f1

)(2)

qs. (1) and (2) disregard the temperature effect on the lumi-escence intensity. Although the temperature-dependence of

uenching-based oxygen sensors has been discussed in [14,15],hese studies did not develop temperature models to compen-ate for the temperature-induced variation in the fluorescencentensity. Accordingly, the current study develops a simple
Page 3: Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model

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Y.-L. Lo et al. / Sensors and

athematical approach for the temperature compensation ofber-optic oxygen sensors. The basic concept of the pro-osed approach is to correct the Stern–Volmer constants tohe reference Stern–Volmer constant at different temperatures.herefore, the modified Stern–Volmer model has the form as

I0(Tref)

I0(Tref) − Im(T )= 1

f1(T )

(1 + 1

Ksv1(Tref)[%O2(T )]

)(3)

nd

m(T ) = I(T )C(T ) (4)

here I0(Tref) is the luminescence intensity in the absence ofxygen at the reference temperature (generally specified as roomemperature), Im(T) is the modified luminescence intensity at theiven temperature of the measurement environment, and I(T) ishe steady-state luminescence intensity in the presence of O2 athe given different temperatures of the measurement environ-

ent. KSV1(Tref) is the Stern–Volmer constant at the referenceemperature, f1(T) is the fractional intensity of the componentontributing to the total luminescence at the given temperaturef the measurement, and C(T) is the value of the temperatureompensation coefficient at the given temperature of the mea-urement environment.

As discussed later in Section 4, the correlation between thexygen concentration and the fluorescence intensity is evaluatedy using the fiber-optic sensors to measure six known oxygenoncentrations at six different temperatures. The experimentalata are then fitted to the modified Stern–Volmer model given inq. (3) in order to compute a calibration curve plotting, there-

ore the temperature compensation coefficient regarding to theorking temperature can be obtained as

(T ) = I0(Tref)

I(T )− I0(Tref)f1Ksv1(Tref)[%O2(T )]

I(T ){1 + Ksv1(Tref)[%O2(T )]} (5)

herefore, the temperature-compensated value of the oxygenoncentration at any temperature T can then be derived from theeasured fluorescence intensity I(T) by applying the following

earranged form of Eq. (3) with the appropriate value of C(T)nd f1(T) taken from the calibration curve as

%O2(T )] = I0(Tref) − I(T )C(T )

{I0(Tref)[f1(T ) − 1] + I(T )C(T )}Ksv1(Tref)(6)

. Preparation of fiber-optic oxygen sensors

In this study, the fiber-optic oxygen sensors were fabricatedy doping TEOS/Octyl-triEOS or n-propyl-TriMOS/TFP-riMOS composite sols with a PtTFPP complex, as describedelow.

.1. TEOS/Octyl-triEOS doped with PtTFPP

The composite sols were prepared by mixing teraethy-

orthosilane (TEOS) (4 mL) and n-octyltriethoxysilaneOctyl-triEOS) (0.2 mL) to form precursor solutions. Adoptingsimilar approach to that employed by Yeh et al. [23], EtOH

1.25 mL) and HCl (0.4 mL of 0.1 M HCl) were added to

me3

tors B 131 (2008) 479–488 481

he sol solution to catalyze the organically modified silicateORMOSIL) reaction. The solution was then capped andtirred magnetically for 1 h at room temperature. In this stage,riton-X-100 (0.1 mL) was added to the solution to improve theomogeneity of the silica sol, resulting in a crack-free monolith.

The sensor dye solution was prepared by dissolving 2 mg oftTFPP in 10 mL of tetrahydrofuran (THF). PtTFPP dissolves

horoughly in THF, resulting in a highly homogeneous dye solu-ion. The uniform distribution of the sensing dye moleculesnsures that the dye solution is highly sensitive to oxygen.

The luminophore-doped sol solution was prepared by mixinghe PtTFPP/THF solution into the sol solution. The sol mixturesere then capped and stirred mechanically for 10 min. PtTFPP-oped TEOS/Octyl-triEOS composite xerogels were preparedsing a sol–gel process performed under room temperature con-itions. Prior to the dip-coating process, a multimode opticalber was cleaned by soaking in a NaOH solution for 24 h, rins-

ng with copious amounts of de-ionized water and EtOH, andhen drying at room temperature for 1 h. Finally, a xerogel filmas formed on the end of the multimode optical fiber using a dip-

oating process conducted with a dipping velocity of 0.25 mm/s.he related characteristic of the oxygen sensor based on Pt(II)omplex embedded in TEOS/Octyl-triEOS at room temperatureould be found in [23]. Besides, the fiber-optic oxygen sensorslso could be prepared by the electropolymerization of porphyrinolecules but it is more difficult than doping method to carry

ut on the optical fiber.

.2. n-Propyl-TriMOS/TFP-TriMOS doped with PtTFPP

The n-propyltrimethoxysilane (n-propyl-TriMOS)/3,3,3-rifluoropropyltrimethoxysilane (TFP-TriMOS) compositerepared by mixing n-propyl-TriMOS (0.69 mL) and TFP-riMOS (1.5 mL) to form precursor solutions [24]. EtOH1.5 mL), de-ionized water (0.635 mL) and HCl (0.08 mLf 0.1 M HCl) were added to the sol solution to catalyzehe ORMOSIL reaction. The solution was then capped andtirred magnetically for 1 h at room temperature. In this stage,riton-X-100 (0.1 mL) was added to improve the homogeneityf the silica sol.

The sensor dye solution was prepared by dissolving 2 mg oftTFPP in 10 mL of tetrahydrofuran (THF). The luminophore-oped sol solution was prepared by mixing the PtTFPP/THFolution into the sol solution. The sol mixtures were thenapped and stirred magnetically for 10 min prior. PtTFPP-doped-propyl-TriMOS/TFP-TriMOS composite xerogels were pre-ared using a sol–gel process performed at room temperature.he related characteristic of the oxygen sensor based on Pt(II)omplex embedded in n-propyl-TriMOS/TFP-TriMOS at differ-nt temperatures can be found in [24].

. Experimental setup

Fig. 2 presents a schematic illustration of the current experi-ental setup. The dye molecules in the fiber-optic sensors were

xcited by a UV LED light source (Ocean Optics, Model LS-450,95 nm wavelength) driven by a waveform generator (Thurlby

Page 4: Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model

482 Y.-L. Lo et al. / Sensors and Actuators B 131 (2008) 479–488

Tss(wm1uonwcmn

5

5

p

Fn

Ptie

5T

PTetsFd

Fig. 2. Schematic diagram of experimental setup [24].

handar Instruments Ltd., Model TGA1240) in 10 kHz pulseignal. The fiber-optic oxygen sensors comprised a multimodeilica glass fiber (600/630 �m) and a bifurcated optical fiberOcean Optics, BIF-600-UV-VIS). The emission measurementsere performed in a gas cell and the pressures dependent uponixed gas temperature were adjusted by pressure gauge to beatm. The resulting photoluminescence intensity was measuredsing a USB 2000-FLG spectrofluorometer. The six differentxygen concentrations was obtained by mixing oxygen anditrogen and controlled by the gas flow meter. The mixed gasas heated to temperatures ranging from 25 to 70 ◦C in a hot cir-

ulator standard oven (RISEN Co. Ltd., D9LR-RHD452). Theixed gas temperature was measured by thermometer (Cente-

ary Materials Co. Ltd., Model TM-905).

. Experimental results and discussion

.1. Optical properties of PtTFPP

Fig. 3 presents the absorption spectrum of the PtTFPP com-lex immobilized in a sol–gel matrix [23]. It is observed that

Fig. 3. Absorption spectrum of PtTFPP doped in sol–gel matrix [24].

5iPbt

Fs

ig. 4. Emission spectra of PtTFPP-doped TEOS/Octyl-triEOS sensor initrogen-only environment at different temperatures.

tTFPP has several bands, including a Soret band at 392 nm andwo Q bands between 508 and 541 nm. The absorption spectrumndicates that a UV LED or a green LED is suitable for use as anxcitation light source for the current fiber-optic oxygen sensors.

.2. Oxygen sensing properties of PtTFPP inEOS/Octyl-triEOS or n-propyl-TriMOS/TFP-TriMOS

Figs. 4 and 5 present the photoluminescence spectra oftTFPP-doped TEOS/Octyl-triEOS or n-propyl-TriMOS/TFP-riMOS xerogels at various temperatures in a nitrogen onlynvironment. Note that in obtaining these results, the integrationime of the CCD spectrometer was set at 100 ms. As a result, theensor probes yield strong phosphorescent emissions at 650 nm.ig. 4 shows that the fluorescence intensity of the PtTFPP-oped TEOS/Octyl-triEOS sensor reduces by approximately6.33% as the temperature increases from 24.8 to 70.3 ◦C. Sim-larly, Fig. 5 indicates that the fluorescence intensity of the

tTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor reducesy approximately 45.44% as the temperature increases from 25.1o 69 ◦C.

ig. 5. Emission spectra of PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sen-or in nitrogen-only environment at different temperatures [24].

Page 5: Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model

Y.-L. Lo et al. / Sensors and Actuators B 131 (2008) 479–488 483

Fs

cfTTet

pPErai0foHatpP

FT[

Ff

tdrdIet

tTTiOgdt

ig. 6. Variation of intensity decay of PtTFPP-doped TEOS/Octyl-triEOS sen-or with ambient temperature as function of oxygen concentration.

Figs. 6 and 7 illustrate the variation of the fluores-ence intensity decay with the ambient temperature as aunction of the oxygen concentration for the PtTFPP-dopedEOS/Octyl-triEOS sensor and the PtTFPP-doped n-propyl-riMOS/TFP-TriMOS sensor, respectively. These results willnable the oxygen measurements to be corrected at differentemperatures.

According to the data from Figs. 6 and 7, the Stern–Volmerlots for the PtTFPP-doped TEOS/Octyl-triEOS sensor and thetTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor usingq. (1) can be shown in Figs. 8 and 9, respectively. As shown, the

atio I0/I (where I0 and I are the luminescence intensities in thebsence of oxygen and in the presence of oxygen, respectively)s plotted for six different oxygen concentrations ranging from

[vol%] to 100 [vol%] and six different temperatures rangingrom 25 to 70 ◦C. It is observed that the sensitivities of bothxygen sensors increase as the ambient temperature increases.owever, the enhanced sensitivity will have poorer S/N values

nd it is due to the effect of temperature on the fluorescence ofhe molecule. Furthermore, it is noted that the PtTFPP-doped n-ropyl-TriMOS/TFP-TriMOS sensor is more sensitive than thetTFPP-doped TEOS/Octyl-triEOS/TEOS sensor.

ig. 7. Variation of intensity decay of PtTFPP-doped n-propyl-TriMOS/TFP-riMOS sensor with ambient temperature as function of oxygen concentration24].

apse

Fs

ig. 8. Stern–Volmer plot for PtTFPP-doped TEOS/Octyl-triEOS sensor at dif-erent temperatures (no temperature compensation).

Figs. 10 and 11 present the modified Stern–Volmer plots forhe PtTFPP-doped TEOS/Octyl-triEOS sensor and the PtTFPP-oped n-propyl-TriMOS/TFP-TriMOS sensor using Eq. (2),espectively. I0/(I0 − I) against 1/[O2] was plotted with the sameata from Figs. 8 and 9. As shown in Figs. 10 and 11, the ratio0/(I0 − I) against 1/[O2] showed good linearity, which highlynhanced compared with I0/I against [O2]. Regression values,he R2 were >0.998.

The accuracy of the uncompensated oxygen concentra-ion measurement results obtained by the PtTFPP-dopedEOS/Octyl-triEOS sensor and the PtTFPP-doped n-propyl-riMOS/TFP-TriMOS sensor (see Figs. 10 and 11) is evaluated

n Tables 1 and 2, respectively. In both tables, the header row2 Set denotes the oxygen concentration (vol%) set by theas flow controller, the column header O2 M Diff denotes theifference of the oxygen concentration measured without theemperature compensation and the actual oxygen concentration,nd R2 denotes the linearity of the corresponding Stern–Volmerlot. The results presented in these tables indicate that both sen-

ors overstate the oxygen concentration when the temperatureffect is neglected.

ig. 9. Stern–Volmer plot for PtTFPP-doped n-propyl-TriMOS/TFP-TriMOSensor at different temperatures [24] (no temperature compensation).

Page 6: Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model

484 Y.-L. Lo et al. / Sensors and Actuators B 131 (2008) 479–488

Fig. 10. Modified Stern–Volmer plot for PtTFPP-doped TEOS/Octyl-triEOS sensor at different temperatures (no temperature compensation).

/TFP-

5c

m

i

TO

T

2

3

4

5

6

7

Fig. 11. Modified Stern–Volmer plot for PtTFPP-doped n-propyl-TriMOS

.3. Temperature-compensated measurement of oxygen

oncentration

Using the experimental results presented above and theodified Stern–Volmer model introduced in Section 2, a cal-

oPTa

able 1xygen concentration results obtained using PtTFPP-doped TEOS/Octyl-triEOS sen

emperature (◦C) O2 Set (%) 0 20

4.8 O2 M Diff (%) 0 −0.32R2

4.9 O2 M Diff (%) 0.83 18.44R2

5.3 O2 M Diff (%) 1.71 43.39R2

5.3 O2 M Diff (%) 2.8 78.46R2

4.8 O2 M Diff (%) 4.23 122.57R2

0.3 O2 M Diff (%) 5.59 194.28R2

TriMOS sensor at different temperatures (no temperature compensation).

bration curve can be developed to compensate for the effect

f temperature drift on the measurement performance of thetTFPP-doped TEOS/Octyl-triEOS or n-propyl-TriMOS/TFP-riMOS sensors. By calibrating the fiber-optic oxygen sensorst different temperatures, the temperature compensation coef-

sor with no temperature compensation

40 60 80 100

−0.72 −2.67 −2.09 3.70.9987

36.92 54.26 73.33 101.050.9997

86.49 134.08 194.91 258.840.9984

167.98 271.12 407.39 606.350.9995

267.72 480.41 751.64 1956.940.9996

573.14 1621.19 13039.79 −4227.060.9997

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Y.-L. Lo et al. / Sensors and Actuators B 131 (2008) 479–488 485

Table 2Oxygen concentration results obtained using PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor with no temperature compensation

Temperature (◦C) O2 Set (%) 0 20 40 60 80 100

25.1 O2 M Diff (%) 0 0.03 0.25 −2.36 −2.26 9.15R2 0.9987

36.6 O2 M Diff (%) 0.3 10.16 19.93 30.66 46.76 61.46R2 0.9997

42.1 O2 M Diff (%) 0.43 17.56 33.39 33.53 87.38 121.13R2 0.9984

51.5 O2 M Diff (%) 0.69 25.25 52.64 81.28 129.79 179.21R2 0.9995

60 O2 M Diff (%) 0.97 32.74 71.95 109.65 170.75 247.83R2 0.9996

69 O2 M Diff (%) 1.21 43.68 100.78 164.26 248.48 379.55R2

Fpn

fib[

sttsc

sItaFmgen sensors are plotted graphically in Figs. 14 and 15 and areevaluated in Tables 3 and 4.

F

ig. 12. Variation of temperature compensation coefficient with ambient tem-erature for PtTFPP-doped TEOS/Octyl-triEOS sensor (�) and PtTFPP-doped-propyl-TriMOS/TFP-TriMOS sensor (�).

cients C(T) can be obtained as a function of temperaturey substituting the calibration datum (I0(Tref), I(T), KSV(Tref),O2(T)] and f1(T)) into Eq. (5).

Figs. 12 and 13 plot the variation of the temperature compen-ation coefficient and f1 with the working temperature for the

wo sensors, respectively. For both sensors, they are observedhat linear relationships exist between the temperature compen-ation coefficient and the temperature. Having completed thealibration process, the two sensors were employed to measure

Fo

ig. 13. Variation of f1 with ambient temperature for (a) PtTFPP-doped TEOS/Octyl

0.9997

ix known oxygen concentrations at six known temperatures.n computing the oxygen concentrations at each temperature,he intensity measurements were scaled using the appropri-te values of the two parameters (C(T), f1(T)) taken fromigs. 12 and 13, respectively. The temperature-compensatedeasurement results obtained by the two PtTFPP-doped oxy-

ig. 14. Temperature-compensated oxygen concentration measurementsbtained using PtTFPP-doped TEOS/Octyl-triEOS sensor.

-triEOS sensor and (b) PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor.

Page 8: Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model

486 Y.-L. Lo et al. / Sensors and Actuators B 131 (2008) 479–488

Table 3Results of the oxygen measurement with PtTFPP doped in TEOS/Octyl-triEOS (temperature compensated)

Temperature (◦C) O2 Set (%) 0 20 40 60 80 100

24.8 O2 M Diff (%) 0 −0.32 −0.73 −2.67 −2.09 3.7R2 0.9995

34.9 O2 M Diff (%) −1.23 0.04 0.93 0.39 −0.04 2.83R2 0.9998

45.3 O2 M Diff (%) −1.82 0.83 1.52 1.33 2.34 1.58R2 0.9973

55.3 O2 M Diff (%) −2.12 1.87 3.45 2.49 1.18 0.6R2 0.9832

64.8 O2 M Diff (%) −2.3 2.25 2.55 1.85 −2.37 7.27R2 0.9758

70.3 O2 M Diff (%) −2.27 4.52 6.96 6.06 2.34 −3.22R2 0.8944

Table 4Results of the oxygen measurement with PtTFPP doped in n-propyl-TriMOS/TFP-TriMOS (temperature compensated)

Temperature (◦C) O2 Set (%) 0 20 40 60 80 100

25.1 O2 M Diff (%) 0 0.03 0.25 −2.36 −2.26 9.15R2 0.9987

36.6 O2 M Diff (%) −0.21 0.26 −0.24 −1.02 0.47 0.11R2 0.9992

42.1 O2 M Diff (%) −0.3 1.3 0.38 −0.02 3.71 4.88R2 0.992

51.5 O2 M Diff (%) −0.35 0.96 0.94 −0.92 1.28 0.56R2 0.9945

60 O2 M Diff (%) −0.37 0.67 1.24 −1.57 −1.14 −1.05R2 0.9959

6 1.07

oOtat

Fo

tf

9 O2 M Diff (%) −0.41R2

As in Tables 1 and 2, the header row O2 Set denotes thexygen concentration (vol%) set using the gas flow controller,

2 M Diff denotes the difference of the oxygen concentra-

ion measured using the temperature compensation and thectual oxygen concentration, and R2 denotes the linearity ofhe corresponding modified Stern–Volmer plot. It is observed

ig. 15. Temperature-compensated oxygen concentration measurementsbtained using PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor.

trTccridPpmtis

5

pm

2.24 0.39 −1.78 −2.340.9915

hat the temperature-compensated measurement results obtainedrom the two sensors are slightly over-stated compared tohe actual oxygen concentrations. However, comparing theesults presented in Tables 3 and 4 with those given inables 1 and 2, respectively, it is found that the temperatureompensation method reduces the difference in the oxygenoncentration measurement for working temperatures in theange of 25–70 ◦C. Again comparing the results presentedn Tables 3 and 4, the linearity and correctness of PtTFPP-oped n-propyl-TriMOS/TFP-TriMOS sensor are better thantTFPP-doped TEOS/Octyl-triEOS sensor for working tem-eratures in the range of 25–70 ◦C. On the other hand, theodified Stern–Volmer plots are nonlinear at higher tempera-

ure (50–70 ◦C) and PtTFPP-doped TEOS/Octyl-triEOS sensors worse than PtTFPP-doped n-propyl-TriMOS/TFP-TriMOSensor.

.4. Fiber-optic variable attenuator design

The purpose of this report is to describe a temperature com-ensation method based on the simple modified Stern–Volmerodel. According to this modified Stern–Volmer model, authors

Page 9: Temperature compensation of fluorescence intensity-based fiber-optic oxygen sensors using modified Stern–Volmer model

Y.-L. Lo et al. / Sensors and Actuators B 131 (2008) 479–488 487

wdtpt

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tsSdtws

5fi

pfiepitwof

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Fig. 16. Fiber-optic variable attenuator design.

ould like to introduce a simple fiber-optic variable attenuatoresign which use negative thermal expansion material to inducehe light intensity change as temperature fluctuations. The pro-osed fiber-optic variable attenuator design for oxygen sensoremperature compensation is shown in Fig. 16.

Based on the linear relationships exist between two param-ters (C(T), f1(T)) and temperature, a suitable negative thermalxpansion material can be applied to design the fiber-optic vari-ble attenuator. As temperature increases, the distance betweenptical fiber and sensing probe could be reduced and, there-ore the luminescence intensity increases. Hence, luminescencentensity decay with the ambient temperature could be compen-ated by this fiber-optic variable attenuator design.

In this study, authors presented a straightforward and effec-ive means of improving the accuracy of fiber-optic oxygenensors at different temperatures. Based on this modifiedtern–Volmer model, a fiber-optic variable attenuator could beesigned for oxygen sensor temperature compensation. Thus,he fiber-optic oxygen sensor with a variable attenuator couldork in a broad temperature range without using a temperature

ensor.

.5. Intensity drifts from light source and/or lead-in/outbers

Ratiometric fluorescence method has been used to solve theroblem of intensity drift from light source and/or lead-in/outbers [27–29]. The ratiometric method employing two differ-nt luminescent indicators immobilized in the same matrix isresented by Park et al. [27]. One of the luminescent indicatorss designed for sensing, and the other for reference. Alterna-ive is based on using a light source to excite two emissionavelengths from a single luminescent indicator [28,29]. Onef emission wavelengths is designed for sensing, and the otheror reference.

The general schematic view of the fiber-optic oxygen sen-or device according to ratiometric fluorescence method andariable attenuator design is presented in Fig. 17. The illus-rated system includes a light source for directing excitation

ight into the device, as well as the light detectors for detect-ng the emission lights from the sensing probe in Dye 1oxygen-sensitive dye) and reference probe in Dye 2 (oxygennd temperature-insensitive dye). Therefore, the intensity-based

ig. 17. Schematic diagram of ratiometric fiber-optic sensor for a thermal oxy-en measurement.

2 sensors insensitive to thermal effects and intensity driftsrom light source and/or lead-in/out fibers could be possiblychieved.

. Conclusions

This study has developed a modified Stern–Volmerodel to compensate for the effects of temperature drift

nder ambient temperatures in the range of 25 to around0 ◦C for PtTFPP-doped TEOS/Octyl-triEOS and n-propyl-riMOS/TFP-TriMOS fiber-optic oxygen sensors. It is found

hat the temperature compensation coefficient from both sen-ors has a good linearity with respect to temperature variations,nd this results in an easier signal process. Therefore, a variablettenuator according to the temperature compensation coeffi-ient could be designed due to its linear characterization. In theuture, authors will design a variable attenuator, thus the fiber-ptic oxygen sensor with a variable attenuator could work in aroad temperature range without using a temperature sensor.

cknowledgements

The funding received from the Advanced Optoelectronicechnology Center, National Cheng Kung University underrojects from the Ministry of Education and the National Sci-nce Council of Taiwan (Grant No. NSC 95-219-M-009-008) isratefully acknowledged.

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iographies

u-Lung Lo received his BS degree from National Cheng Kung University,ainan, Taiwan, in 1985, and his MS and PhD degrees in mechanical engi-eering from the Smart Materials and Structures Research Center, University ofaryland, College Park, USA, in 1992 and 1995, respectively. After graduation,

e joined the Opto-Electronics and Systems Laboratories of the Industrial Tech-ology Research Institute (ITRI), working on fiber-optic smart structures andber communications. He has been a member of the Mechanical Engineeringepartment, National Cheng Kung University, since 1996, where he is now a

ull professor. Also, he is an affiliate professor in Institute of Nanotechnologynd Microsystem Engineering. His research interests lie in the areas of fiber-ptic sensors, passive components in optical fiber communications, experimentalechanics, optical techniques in precision measurements on LCD panels, andOEMS.

hen-Shane Chu received his BS and MS degrees from the Mechanical Engi-eering Department, National Cheng Kung University, Taiwan ROC, in 2002 and004, respectively. He is currently pursuing his PhD degree at the Mechanicalngineering Department, National Cheng Kung University.

iahn-Piring Yur received his BS, MS, and PhD degrees from the Mechani-al Engineering Department, National Cheng Kung University, Tainan, TaiwanOC, in 1985, 1990, and 2002, respectively. He worked as a teaching assis-

ant at NCKU from 1987 to 1988 and then joined the Mechanical Engineeringepartment at Kun Shan University (KSU), Tainan, Taiwan ROC, in 1990. Asart of his responsibilities at KSU, he established the MEMS Center, the Micro-ensor Technical R&D Center, and the Nano-technology R&D Center in 1999,001, and 2002, respectively. His research interests lie in the fields of preci-ion manufacturing, material science, microstructure detecting techniques incanning electron microscopes (SEMs), fiber-optic sensors, optical techniquesn the backlight units (BLUs) of liquid crystal displays (LCDs), MEMS, and

ano-technology.

uan-Che Chang received his BS degree from the Mechanical Engineeringepartment, Tamkang University, Taiwan ROC, in 2004, and his MS degree inechanical engineering from National Cheng Kung University in 2006.