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

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)/3,3,3trifluoropropyltrimethoxysilane (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 oxygen in gas or liquid phases. Optical oxygen sensors based on the quenching of fluorescence or phosphorescence by molecular oxygen overcome the limitations of the conventional Clark electrode and are extensively applied in the chemical [1–4], clinical [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).

0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.12.010

light sources. Furthermore, the phosphorescence wavelengths of Pt complexes are well separated from the excitation LED wavelength, and hence the influence of the excitation light source can be easily eliminated. In general, the support matrix of an optical 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 luminescence 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 fluorescence intensity and fluorescence decay time are influenced

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by the ambient temperature [12,13]. Various researchers have employed multi-sided models to explore this temperature effect [14–16]. Gouin and Gouterman proposed the use of annealing [17] or the addition of various polymer materials and pigments [18] to reduce the temperature-dependence of the luminescence of pressure-sensitive paints. Several researchers have proposed temperature compensation schemes for oxygen optrodes. For example, Lam et al. [19] presented a phosphorescence-based oxygen sensor based on erythrosine B doped sol–gel silica. The proposed sensor exploited the fact that phosphorescence and delayed fluorescence have contrasting temperature dependencies to carry out the temperature compensation of the sensor measurements. To account for the effects of temperature, many oxygen optrodes are designed to measure not only the oxygen concentration, but also the sensing temperature. In such sensors, an accurate indication of the oxygen concentration is then obtained by applying a suitable temperature calibration factor to the measured oxygen concentration value. Borisov and Wolfbeis [20] presented a temperature-sensitive probe in which a palladium dye was employed as a luminescent indicator for oxygen, and europium(III) complexes were used as a temperature indicator. The authors showed that both indicators could be excited by a 405 nm LED. Significantly, the two indicators yielded well-separated bright luminescence, and therefore the two signals could be processed individually to provide independent values of the oxygen and the temperature, respectively. Chu and Lo [21] presented a plastic optical fiber for the dual sensing of temperature and oxygen. The sensor features commercially available epoxy glue coated on the side-polished fiber surface for temperature sensing and a fluorinated xerogel doped with platinum tetrakis pentafluorophenyl porphine (PtTFPP) coated on the fiber end for oxygen sensing. Stehning and Holst [22] presented a hybrid temperature-oxygen fiber-optic sensor in which all of the signal generation and processing was performed using a digital signal processor (DSP). In principle, this device provided the means to resolve even more contributing lifetimes of the luminescence signals. However, difficulties occurred in practice due to the closely spaced lifetimes of the luminophores and the low signal-to-noise ratio of the optical setup [22]. Recently, we have described the oxygen sensor based on Pt(II) complexes embedded in TEOS/Octyl-triEOS [23] or n-propyl-TriMOS/TFP-TriMOS [24] to produce better performance fiber-optic oxygen sensors. To the best of our study, PtTFPP is more stable and suitable than PtOEP for the fiberoptic oxygen sensor’s study. Based on the reason, this current study fabricates fiber-optic oxygen sensors based on teraethylorthosilane (TEOS)/n-octyltriethoxysilane (Octyl-triEOS) or n-propyltrimethoxysilane (n-propyl-TriMOS)/3,3,3trifluoropropyltrimethoxysilane (TFP-TriMOS) composite xerogels doped with platinum tetra(pentafluorophenyl)porphine (PtTFPP), as illustrated in Fig. 1. However, the use of luminescence intensity as oxygen monitor presents several measurement problems, e.g. variations in the amplitude of the light signal due to photodegradation of the lumiphore, shifts in the instrument or optical interferences [20]. Also, the temperature effect in luminescence of dye is a key issue in the intensity-based sensors. Therefore, a modified Stern–Volmer model is then

Fig. 1. Chemical structure of PtTFPP.

developed to compensate for the temperature effect on the oxygen concentration measurements obtained using these sensors over a working temperature range of 25–70 ◦ C. If a variable attenuator according to the temperature compensation coefficient is designed to compensate for fluorescence intensity change, the fiber-optic oxygen sensor could work in a broad temperature range without using a temperature sensor. 2. Modified Stern–Volmer model for temperature compensation If the dynamic quenching of luminescence by oxygen is used as a sensing principle, the relation between the oxygen concentration [%O2 ] and the measurable luminescence intensity I of a given fluorophore can best be described by the two-site model of the Stern–Volmer equation [4]:   I f1 f2 = + (1) I0 1 + Ksv1 [%O2 ] 1 + Ksv2 [%O2 ] where I0 is the luminescence intensity in the absence of oxygen, f1 and f2 represent the fraction of each of the two sites contributing to the unquenched intensity, and f1 + f2 = 1. KSV1 , KSV2 are the quenching coefficients that describe the oxygen sensitivity of each component. For practical applications, it can be assumed that one of the components cannot be quenched by oxygen (KSV2 = 0) [25], and this yields an experimentally modified equation [10,25,26]:   I0 1 1 = + I0 − I f1 Ksv1 [%O2 ] 1 − f2   1 1 = (2) + f1 Ksv1 [%O2 ] f1 Eqs. (1) and (2) disregard the temperature effect on the luminescence intensity. Although the temperature-dependence of quenching-based oxygen sensors has been discussed in [14,15], these studies did not develop temperature models to compensate for the temperature-induced variation in the fluorescence intensity. Accordingly, the current study develops a simple

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mathematical approach for the temperature compensation of fiber-optic oxygen sensors. The basic concept of the proposed approach is to correct the Stern–Volmer constants to the reference Stern–Volmer constant at different temperatures. Therefore, the modified Stern–Volmer model has the form as   I0 (Tref ) 1 1 = 1+ (3) I0 (Tref ) − Im (T ) f1 (T ) Ksv1 (Tref )[%O2 (T )] and Im (T ) = I(T )C(T )

(4)

where I0 (Tref ) is the luminescence intensity in the absence of oxygen at the reference temperature (generally specified as room temperature), Im (T) is the modified luminescence intensity at the given temperature of the measurement environment, and I(T) is the steady-state luminescence intensity in the presence of O2 at the given different temperatures of the measurement environment. KSV1 (Tref ) is the Stern–Volmer constant at the reference temperature, f1 (T) is the fractional intensity of the component contributing to the total luminescence at the given temperature of the measurement, and C(T) is the value of the temperature compensation coefficient at the given temperature of the measurement environment. As discussed later in Section 4, the correlation between the oxygen concentration and the fluorescence intensity is evaluated by using the fiber-optic sensors to measure six known oxygen concentrations at six different temperatures. The experimental data are then fitted to the modified Stern–Volmer model given in Eq. (3) in order to compute a calibration curve plotting, therefore the temperature compensation coefficient regarding to the working temperature can be obtained as C(T ) =

I0 (Tref ) I0 (Tref )f1 Ksv1 (Tref )[%O2 (T )] − I(T ){1 + Ksv1 (Tref )[%O2 (T )]} I(T )

(5)

Therefore, the temperature-compensated value of the oxygen concentration at any temperature T can then be derived from the measured fluorescence intensity I(T) by applying the following rearranged form of Eq. (3) with the appropriate value of C(T) and 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)

3. Preparation of fiber-optic oxygen sensors In this study, the fiber-optic oxygen sensors were fabricated by doping TEOS/Octyl-triEOS or n-propyl-TriMOS/TFPTriMOS composite sols with a PtTFPP complex, as described below.

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the sol solution to catalyze the organically modified silicate (ORMOSIL) reaction. The solution was then capped and stirred magnetically for 1 h at room temperature. In this stage, Triton-X-100 (0.1 mL) was added to the solution to improve the homogeneity of the silica sol, resulting in a crack-free monolith. The sensor dye solution was prepared by dissolving 2 mg of PtTFPP in 10 mL of tetrahydrofuran (THF). PtTFPP dissolves thoroughly in THF, resulting in a highly homogeneous dye solution. The uniform distribution of the sensing dye molecules ensures that the dye solution is highly sensitive to oxygen. The luminophore-doped sol solution was prepared by mixing the PtTFPP/THF solution into the sol solution. The sol mixtures were then capped and stirred mechanically for 10 min. PtTFPPdoped TEOS/Octyl-triEOS composite xerogels were prepared using a sol–gel process performed under room temperature conditions. Prior to the dip-coating process, a multimode optical fiber was cleaned by soaking in a NaOH solution for 24 h, rinsing with copious amounts of de-ionized water and EtOH, and then drying at room temperature for 1 h. Finally, a xerogel film was formed on the end of the multimode optical fiber using a dipcoating process conducted with a dipping velocity of 0.25 mm/s. The related characteristic of the oxygen sensor based on Pt(II) complex embedded in TEOS/Octyl-triEOS at room temperature could be found in [23]. Besides, the fiber-optic oxygen sensors also could be prepared by the electropolymerization of porphyrin molecules but it is more difficult than doping method to carry out on the optical fiber. 3.2. n-Propyl-TriMOS/TFP-TriMOS doped with PtTFPP The n-propyltrimethoxysilane (n-propyl-TriMOS)/3,3,3trifluoropropyltrimethoxysilane (TFP-TriMOS) composite prepared by mixing n-propyl-TriMOS (0.69 mL) and TFPTriMOS (1.5 mL) to form precursor solutions [24]. EtOH (1.5 mL), de-ionized water (0.635 mL) and HCl (0.08 mL of 0.1 M HCl) were added to the sol solution to catalyze the ORMOSIL reaction. The solution was then capped and stirred magnetically for 1 h at room temperature. In this stage, Triton-X-100 (0.1 mL) was added to improve the homogeneity of the silica sol. The sensor dye solution was prepared by dissolving 2 mg of PtTFPP in 10 mL of tetrahydrofuran (THF). The luminophoredoped sol solution was prepared by mixing the PtTFPP/THF solution into the sol solution. The sol mixtures were then capped and stirred magnetically for 10 min prior. PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS composite xerogels were prepared using a sol–gel process performed at room temperature. The related characteristic of the oxygen sensor based on Pt(II) complex embedded in n-propyl-TriMOS/TFP-TriMOS at different temperatures can be found in [24].

3.1. TEOS/Octyl-triEOS doped with PtTFPP 4. Experimental setup The composite sols were prepared by mixing teraethylorthosilane (TEOS) (4 mL) and n-octyltriethoxysilane (Octyl-triEOS) (0.2 mL) to form precursor solutions. Adopting a similar 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

Fig. 2 presents a schematic illustration of the current experimental setup. The dye molecules in the fiber-optic sensors were excited by a UV LED light source (Ocean Optics, Model LS-450, 395 nm wavelength) driven by a waveform generator (Thurlby

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

Thandar Instruments Ltd., Model TGA1240) in 10 kHz pulse signal. The fiber-optic oxygen sensors comprised a multimode silica glass fiber (600/630 ␮m) and a bifurcated optical fiber (Ocean Optics, BIF-600-UV-VIS). The emission measurements were performed in a gas cell and the pressures dependent upon mixed gas temperature were adjusted by pressure gauge to be 1 atm. The resulting photoluminescence intensity was measured using a USB 2000-FLG spectrofluorometer. The six different oxygen concentrations was obtained by mixing oxygen and nitrogen and controlled by the gas flow meter. The mixed gas was heated to temperatures ranging from 25 to 70 ◦ C in a hot circulator standard oven (RISEN Co. Ltd., D9LR-RHD452). The mixed gas temperature was measured by thermometer (Centenary Materials Co. Ltd., Model TM-905). 5. Experimental results and discussion 5.1. Optical properties of PtTFPP Fig. 3 presents the absorption spectrum of the PtTFPP complex immobilized in a sol–gel matrix [23]. It is observed that

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

Fig. 4. Emission spectra of PtTFPP-doped TEOS/Octyl-triEOS sensor in nitrogen-only environment at different temperatures.

PtTFPP has several bands, including a Soret band at 392 nm and two Q bands between 508 and 541 nm. The absorption spectrum indicates that a UV LED or a green LED is suitable for use as an excitation light source for the current fiber-optic oxygen sensors. 5.2. Oxygen sensing properties of PtTFPP in TEOS/Octyl-triEOS or n-propyl-TriMOS/TFP-TriMOS Figs. 4 and 5 present the photoluminescence spectra of PtTFPP-doped TEOS/Octyl-triEOS or n-propyl-TriMOS/TFPTriMOS xerogels at various temperatures in a nitrogen only environment. Note that in obtaining these results, the integration time of the CCD spectrometer was set at 100 ms. As a result, the sensor probes yield strong phosphorescent emissions at 650 nm. Fig. 4 shows that the fluorescence intensity of the PtTFPPdoped TEOS/Octyl-triEOS sensor reduces by approximately 56.33% as the temperature increases from 24.8 to 70.3 ◦ C. Similarly, Fig. 5 indicates that the fluorescence intensity of the PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor reduces by approximately 45.44% as the temperature increases from 25.1 to 69 ◦ C.

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

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

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

Figs. 6 and 7 illustrate the variation of the fluorescence intensity decay with the ambient temperature as a function of the oxygen concentration for the PtTFPP-doped TEOS/Octyl-triEOS sensor and the PtTFPP-doped n-propylTriMOS/TFP-TriMOS sensor, respectively. These results will enable the oxygen measurements to be corrected at different temperatures. According to the data from Figs. 6 and 7, the Stern–Volmer plots for the PtTFPP-doped TEOS/Octyl-triEOS sensor and the PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor using Eq. (1) can be shown in Figs. 8 and 9, respectively. As shown, the ratio I0 /I (where I0 and I are the luminescence intensities in the absence of oxygen and in the presence of oxygen, respectively) is plotted for six different oxygen concentrations ranging from 0 [vol%] to 100 [vol%] and six different temperatures ranging from 25 to 70 ◦ C. It is observed that the sensitivities of both oxygen sensors increase as the ambient temperature increases. However, the enhanced sensitivity will have poorer S/N values and it is due to the effect of temperature on the fluorescence of the molecule. Furthermore, it is noted that the PtTFPP-doped npropyl-TriMOS/TFP-TriMOS sensor is more sensitive than the PtTFPP-doped TEOS/Octyl-triEOS/TEOS sensor.

Figs. 10 and 11 present the modified Stern–Volmer plots for the PtTFPP-doped TEOS/Octyl-triEOS sensor and the PtTFPPdoped n-propyl-TriMOS/TFP-TriMOS sensor using Eq. (2), respectively. I0 /(I0 − I) against 1/[O2 ] was plotted with the same data from Figs. 8 and 9. As shown in Figs. 10 and 11, the ratio I0 /(I0 − I) against 1/[O2 ] showed good linearity, which highly enhanced compared with I0 /I against [O2 ]. Regression values, the R2 were >0.998. The accuracy of the uncompensated oxygen concentration measurement results obtained by the PtTFPP-doped TEOS/Octyl-triEOS sensor and the PtTFPP-doped n-propylTriMOS/TFP-TriMOS sensor (see Figs. 10 and 11) is evaluated in Tables 1 and 2, respectively. In both tables, the header row O2 Set denotes the oxygen concentration (vol%) set by the gas flow controller, the column header O2 M Diff denotes the difference of the oxygen concentration measured without the temperature compensation and the actual oxygen concentration, and R2 denotes the linearity of the corresponding Stern–Volmer plot. The results presented in these tables indicate that both sensors overstate the oxygen concentration when the temperature effect is neglected.

Fig. 7. Variation of intensity decay of PtTFPP-doped n-propyl-TriMOS/TFPTriMOS sensor with ambient temperature as function of oxygen concentration [24].

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

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

Fig. 11. Modified Stern–Volmer plot for PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor at different temperatures (no temperature compensation).

5.3. Temperature-compensated measurement of oxygen concentration Using the experimental results presented above and the modified Stern–Volmer model introduced in Section 2, a cal-

ibration curve can be developed to compensate for the effect of temperature drift on the measurement performance of the PtTFPP-doped TEOS/Octyl-triEOS or n-propyl-TriMOS/TFPTriMOS sensors. By calibrating the fiber-optic oxygen sensors at different temperatures, the temperature compensation coef-

Table 1 Oxygen concentration results obtained using PtTFPP-doped TEOS/Octyl-triEOS sensor with no temperature compensation Temperature (◦ C)

O2 Set (%)

0

20

40

24.8

O2 M Diff (%) R2

0

−0.32

−0.72

−2.67 0.9987

−2.09

3.7

34.9

O2 M Diff (%) R2

0.83

18.44

36.92

54.26 0.9997

73.33

101.05

45.3

O2 M Diff (%) R2

1.71

43.39

86.49

134.08 0.9984

194.91

258.84

55.3

O2 M Diff (%) R2

2.8

78.46

167.98

271.12 0.9995

407.39

606.35

64.8

O2 M Diff (%) R2

4.23

122.57

267.72

480.41 0.9996

751.64

1956.94

70.3

O2 M Diff (%) R2

5.59

194.28

573.14

1621.19 0.9997

13039.79

−4227.06

60

80

100

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485

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

O2 Set (%)

0

25.1

O2 M Diff (%) R2

0

O2 M Diff (%) R2

0.3

O2 M Diff (%) R2

0.43

O2 M Diff (%) R2

0.69

60

O2 M Diff (%) R2

0.97

32.74

69

O2 M Diff (%) R2

1.21

43.68

36.6 42.1 51.5

20 0.03

40

60

80

−2.36

−2.26

9.15

30.66

46.76

61.46

33.53

87.38

121.13

81.28

129.79

179.21

71.95

109.65 0.9996

170.75

247.83

100.78

164.26 0.9997

248.48

379.55

0.25

100

0.9987 10.16

19.93 0.9997

17.56

33.39 0.9984

25.25

52.64 0.9995

six known oxygen concentrations at six known temperatures. In computing the oxygen concentrations at each temperature, the intensity measurements were scaled using the appropriate values of the two parameters (C(T), f1 (T)) taken from Figs. 12 and 13, respectively. The temperature-compensated measurement results obtained by the two PtTFPP-doped oxygen sensors are plotted graphically in Figs. 14 and 15 and are evaluated in Tables 3 and 4.

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

ficients C(T) can be obtained as a function of temperature by 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 compensation coefficient and f1 with the working temperature for the two sensors, respectively. For both sensors, they are observed that linear relationships exist between the temperature compensation coefficient and the temperature. Having completed the calibration process, the two sensors were employed to measure

Fig. 14. Temperature-compensated oxygen concentration measurements obtained using PtTFPP-doped TEOS/Octyl-triEOS sensor.

Fig. 13. Variation of f1 with ambient temperature for (a) PtTFPP-doped TEOS/Octyl-triEOS sensor and (b) PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor.

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

O2 Set (%)

24.8

O2 M Diff (%) R2

34.9 45.3 55.3 64.8 70.3

0 0

20

40

−0.32

−0.73

60

80

100

−2.67

−2.09

3.7

0.39

−0.04

2.83

1.33

2.34

1.58

2.49

1.18

0.6

1.85

−2.37

7.27

6.06

2.34

−3.22

0.9995

O2 M Diff (%) R2

−1.23

O2 M Diff (%) R2

−1.82

O2 M Diff (%) R2

−2.12

O2 M Diff (%) R2

−2.3

O2 M Diff (%) R2

−2.27

0.04

0.93 0.9998

0.83

1.52 0.9973

1.87

3.45 0.9832

2.25

2.55 0.9758

4.52

6.96 0.8944

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

O2 Set (%)

25.1

O2 M Diff (%) R2

36.6 42.1 51.5 60 69

0 0

20

40

0.03

0.25

60

80

100

−2.36

−2.26

9.15

−1.02

0.47

0.11

−0.02

3.71

4.88

−0.92

1.28

0.56

−1.57

−1.14

−1.05

0.39

−1.78

−2.34

0.9987

O2 M Diff (%) R2

−0.21

O2 M Diff (%) R2

−0.3

O2 M Diff (%) R2

−0.35

O2 M Diff (%) R2

−0.37

O2 M Diff (%) R2

−0.41

0.26

−0.24 0.9992

1.3

0.38 0.992

0.96

0.94 0.9945

0.67

1.24 0.9959

As in Tables 1 and 2, the header row O2 Set denotes the oxygen concentration (vol%) set using the gas flow controller, O2 M Diff denotes the difference of the oxygen concentration measured using the temperature compensation and the actual oxygen concentration, and R2 denotes the linearity of the corresponding modified Stern–Volmer plot. It is observed

1.07

2.24 0.9915

that the temperature-compensated measurement results obtained from the two sensors are slightly over-stated compared to the actual oxygen concentrations. However, comparing the results presented in Tables 3 and 4 with those given in Tables 1 and 2, respectively, it is found that the temperature compensation method reduces the difference in the oxygen concentration measurement for working temperatures in the range of 25–70 ◦ C. Again comparing the results presented in Tables 3 and 4, the linearity and correctness of PtTFPPdoped 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. On the other hand, the modified Stern–Volmer plots are nonlinear at higher temperature (50–70 ◦ C) and PtTFPP-doped TEOS/Octyl-triEOS sensor is worse than PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor. 5.4. Fiber-optic variable attenuator design

Fig. 15. Temperature-compensated oxygen concentration measurements obtained using PtTFPP-doped n-propyl-TriMOS/TFP-TriMOS sensor.

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

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Fig. 16. Fiber-optic variable attenuator design.

would like to introduce a simple fiber-optic variable attenuator design which use negative thermal expansion material to induce the light intensity change as temperature fluctuations. The proposed fiber-optic variable attenuator design for oxygen sensor temperature compensation is shown in Fig. 16. Based on the linear relationships exist between two parameters (C(T), f1 (T)) and temperature, a suitable negative thermal expansion material can be applied to design the fiber-optic variable attenuator. As temperature increases, the distance between optical fiber and sensing probe could be reduced and, therefore the luminescence intensity increases. Hence, luminescence intensity decay with the ambient temperature could be compensated by this fiber-optic variable attenuator design. In this study, authors presented a straightforward and effective means of improving the accuracy of fiber-optic oxygen sensors at different temperatures. Based on this modified Stern–Volmer model, a fiber-optic variable attenuator could be designed for oxygen sensor temperature compensation. Thus, the fiber-optic oxygen sensor with a variable attenuator could work in a broad temperature range without using a temperature sensor. 5.5. Intensity drifts from light source and/or lead-in/out fibers Ratiometric fluorescence method has been used to solve the problem of intensity drift from light source and/or lead-in/out fibers [27–29]. The ratiometric method employing two different luminescent indicators immobilized in the same matrix is presented by Park et al. [27]. One of the luminescent indicators is designed for sensing, and the other for reference. Alternative is based on using a light source to excite two emission wavelengths from a single luminescent indicator [28,29]. One of emission wavelengths is designed for sensing, and the other for reference. The general schematic view of the fiber-optic oxygen sensor device according to ratiometric fluorescence method and variable attenuator design is presented in Fig. 17. The illustrated system includes a light source for directing excitation light into the device, as well as the light detectors for detecting the emission lights from the sensing probe in Dye 1 (oxygen-sensitive dye) and reference probe in Dye 2 (oxygen and temperature-insensitive dye). Therefore, the intensity-based

Fig. 17. Schematic diagram of ratiometric fiber-optic sensor for a thermal oxygen measurement.

O2 sensors insensitive to thermal effects and intensity drifts from light source and/or lead-in/out fibers could be possibly achieved. 6. Conclusions This study has developed a modified Stern–Volmer model to compensate for the effects of temperature drift under ambient temperatures in the range of 25 to around 70 ◦ C for PtTFPP-doped TEOS/Octyl-triEOS and n-propylTriMOS/TFP-TriMOS fiber-optic oxygen sensors. It is found that the temperature compensation coefficient from both sensors has a good linearity with respect to temperature variations, and this results in an easier signal process. Therefore, a variable attenuator according to the temperature compensation coefficient could be designed due to its linear characterization. In the future, authors will design a variable attenuator, thus the fiberoptic oxygen sensor with a variable attenuator could work in a broad temperature range without using a temperature sensor. Acknowledgements The funding received from the Advanced Optoelectronic Technology Center, National Cheng Kung University under projects from the Ministry of Education and the National Science Council of Taiwan (Grant No. NSC 95-219-M-009-008) is gratefully acknowledged. References [1] D.B. Papkovsky, Luminescent porphyrins as probes for optical (bio) sensors, Sens. Actuator B: Chem. 11 (1993) 293–300. [2] J.N. Demas, B.A. Degraff, P.B. Coleman, Oxygen sensors based on luminescence quenching, Anal. Chem. 71 (1999) 793A–800A. [3] C. Preininger, I. Klimant, O.S. Wolfbeis, Optical fiber sensor for biological oxygen demand, Anal. Chem. 66 (1994) 1841–1846. [4] E.R. Carraway, J.N. Demas, B.A. DeGraff, J.R. Bacon, Photophysics and photochemistry of oxygen sensors based on luminescent transition-metal complexes, Anal. Chem. 63 (1991) 337–342.

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Biographies Yu-Lung Lo received his BS degree from National Cheng Kung University, Tainan, Taiwan, in 1985, and his MS and PhD degrees in mechanical engineering from the Smart Materials and Structures Research Center, University of Maryland, College Park, USA, in 1992 and 1995, respectively. After graduation, he joined the Opto-Electronics and Systems Laboratories of the Industrial Technology Research Institute (ITRI), working on fiber-optic smart structures and fiber communications. He has been a member of the Mechanical Engineering Department, National Cheng Kung University, since 1996, where he is now a full professor. Also, he is an affiliate professor in Institute of Nanotechnology and Microsystem Engineering. His research interests lie in the areas of fiberoptic sensors, passive components in optical fiber communications, experimental mechanics, optical techniques in precision measurements on LCD panels, and MOEMS. Chen-Shane Chu received his BS and MS degrees from the Mechanical Engineering Department, National Cheng Kung University, Taiwan ROC, in 2002 and 2004, respectively. He is currently pursuing his PhD degree at the Mechanical Engineering Department, National Cheng Kung University. Jiahn-Piring Yur received his BS, MS, and PhD degrees from the Mechanical Engineering Department, National Cheng Kung University, Tainan, Taiwan ROC, in 1985, 1990, and 2002, respectively. He worked as a teaching assistant at NCKU from 1987 to 1988 and then joined the Mechanical Engineering Department at Kun Shan University (KSU), Tainan, Taiwan ROC, in 1990. As part of his responsibilities at KSU, he established the MEMS Center, the Microsensor Technical R&D Center, and the Nano-technology R&D Center in 1999, 2001, and 2002, respectively. His research interests lie in the fields of precision manufacturing, material science, microstructure detecting techniques in scanning electron microscopes (SEMs), fiber-optic sensors, optical techniques in the backlight units (BLUs) of liquid crystal displays (LCDs), MEMS, and nano-technology. Yuan-Che Chang received his BS degree from the Mechanical Engineering Department, Tamkang University, Taiwan ROC, in 2004, and his MS degree in mechanical engineering from National Cheng Kung University in 2006.