Ratiometric optical sensor for dual sensing of temperature and oxygen

Sensors and Actuators B 210 (2015) 302–309

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Ratiometric optical sensor for dual sensing of temperature and oxygen Cheng-Shane Chu ∗ , Ting-Hsun Lin Department of Mechanical Engineering, Ming Chi University of Technology, Taishan Dist., New Taipei City, Taiwan

a r t i c l e

i n f o

Article history: Received 17 November 2014 Received in revised form 22 December 2014 Accepted 23 December 2014 Available online 8 January 2015 Keywords: Ratiometric Optical dual sensor Temperature Oxygen Sol–gel

a b s t r a c t This paper proposes a ratiometric optical temperature and oxygen sensor that incorporates a sol–gel matrix doped with platinum tetrakis pentafluorophenyl porphine (PtTFPP) as the oxygen-sensitive material, 7-amino-4-trifluoromethyl coumarin (AFC) as the temperature-sensing material, and rhodamine 110 (Rh110) as the temperature/oxygen-independent fluorescent dye. The feasibility of coating a color sensor with the sensing film for fabricating a ratiometric optical dual sensing device is investigated. Using an LED with a central wavelength of 405 nm as the excitation source, it is shown that the emission wavelengths of the temperature-sensitive, oxygen-sensitive, and reference dyes have no spectral overlap and therefore permit the temperature and oxygen concentration to be measured using a ratiometric-based method. The ratiometric optical dual sensor was tested with regard to monitoring various temperatures and oxygen concentrations. The results show that the luminescence properties of the temperature sensor are independent of the presence of the oxygen sensor, and have a uniquely good linear response in the 25–65 ◦ C range. The oxygen sensing scheme presented in this work is intended for use in temperature compensation. The proposed ratiometric sensing approach suppresses the effects of spurious fluctuations in the intensity of the excitation source. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The determination of the properties of multiple analytes simultaneously can be achieved using optical sensors [1]. An example of multianalyte sensing is that of temperature and oxygen. It is important to know the temperature in the luminescent sensing of oxygen since quenching by oxygen is always highly temperaturedependent [2]. There is thus a need for optical sensors that measure both temperature and oxygen. Optical oxygen sensors determine the oxygen concentration based on the reduction in luminescence intensity caused by the oxygen quenching of the emitting state. Optical oxygen sensors consist of an oxygen-sensitive dye entrapped in a matrix with a high permeability to oxygen. Many researchers have reported that sol–gel-derived glass is a suitable matrix material for oxygen since it has high oxygen permeability, good mechanical and chemical stability, and superior optical clarity [3–5]. Furthermore, it has been shown that the use of organically modified silicate (ORMOSIL) matrices improves the response and sensitivity of ruthenium-based oxygen sensors [6–8]. ORMOSILs can accommodate and disperse analyte-sensitive dyes and have a porous structure, which is essential for improving the

∗ Corresponding author. Tel.: +886 2 290898999x4535; fax: +886 2 29063269. E-mail address: [email protected] (C.-S. Chu). http://dx.doi.org/10.1016/j.snb.2014.12.133 0925-4005/© 2015 Elsevier B.V. All rights reserved.

sensor response. Long-lived, fluorescent oxygen-sensitive dyes are needed to improve the sensitivity of oxygen sensors. Phosphorescent porphyrins of platinum [9–11], such as platinum tetrakis pentrafluoropheny porphine (PtTFPP), have long lifetimes, convenient excitation and emission wavelengths with large Stokes shifts (100–170 nm), and a reasonable luminescence quantum yield. Recently, our lab developed fiber-optic oxygen sensors based on 3,3,3-trifluoropropyltrimethoxysliane (TFP-TriMOS) or npropyltrimethoxysilane (n-propyl-TriMOS)/tetraethylorthosilane (TEOS)/n-octyltriethoxysilane (octyl-triEOS) composite xerogel doped with platinum complexes. The resulting fiber-optic oxygen sensors have linear Stern–Volmer plots and show better performance than that of existing oxygen sensors based on ruthenium dye immobilized in various sol–gel matrices [12]. Fluorescent temperature sensing is a common approach for the optical determination of temperature. Another optical spectroscopic technology that can be used for this task is luminescence quenching [13–15]. In the latter approach, an excited fluorophore molecule can drop back to its ground state through radiation emission (luminescence) or through collision with another molecule. The luminescence emission and collision processes are in equilibrium for a system at a certain temperature. A change in temperature will change the frequency of collisions and thus shift the equilibrium of the two processes, resulting in changes in the luminescence intensity and lifetime. Optical temperature sensors have been developed based this characteristic [13–15].

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In recent years, researchers have proposed various materials for the dual sensing of temperature and oxygen. These sensing materials require the use of either a single wavelength [16–26] or two different wavelengths [27,28] for excitation. Luminescence-based optical sensors have been demonstrated to be sensitive, highly reproducible, ready for development into high throughput formats, and feasible for measuring two or more parameters simultaneously. However, excited-state lifetime measurements are preferred in terms of accuracy since they are not prone to errors derived from drifts in the opto-electronical setup, leaching or photobleaching of the sensing material, or transmission changes in the optics [29]. The resulting complexity of the equipment required for these lifetime measurements (based on pulsed or phase modulation techniques) is a drawback. As an alternative, ratiometric methods based on the use of an internal reference have been developed. Lifetime and ratiometric approaches usually result in more robust sensing systems that are less affected by problems related to non-analyteinduced intensity changes [30–32]. The present study proposes a ratiometric optical dual sensor system with three emission colors. Intensity-based ratiometric measurements are used for temperature and oxygen concentration measurements. The ratiometric optical dual sensor uses PtTFPP as the oxygen-sensitive dye with red emission, AFC as the temperature-sensitive dye with blue emission, and Rh110 as the temperature/oxygen-insensitive reference dye with green emission immobilized in a sol–gel matrix. The preparation of the tri-color ratiometric optical temperature and oxygen sensing film, the characterization of sensing response, and the application of the sensing film for evaluating the temperature and oxygen concentration are presented. 2. Basic theory Luminophore quenching depends on several factors. In the simplest scenario of a luminophore in a homogeneous microenvironment, quenching follows the Stern–Volmer equation [33]: I0 = 1 + KSV [O2 ] I

(1)

where I0 and I represent the steady-state luminescence intensities in the absence and presence of O2 , respectively; KSV is the Stern–Volmer quenching constant; [O2 ] is the O2 concentration. In the ideal case, a plot of I0 /I versus [O2 ] has a linear form with a slope equal to KSV and an intercept of unity, allowing a simple single-point sensor calibration. Eq. (1) describes the idealized behavior of a luminophore with a single excited-state lifetime in a homogeneous environment undergoing dynamic quenching. However, the Stern–Volmer plot of fluorescence quenching of the sensors is nonlinear due to the presence of both static and dynamic quenching. Demas et al. reported a multisite model for a sensing film with various oxygenaccessible sites [34]. In this model, the oxygen molecule can exist at various sites, each with its own characteristic quenching constant. Since the observed fluorescence intensity is the sum of emissions from all oxygen accessible sites, the Stern–Volmer relationship is modified as: I0 = I



n   1

fn (1 + KSVn [O2 ])



−1

303

maximum luminescence intensity of the PtTFPP dye to that of the oxygen/temperature-insensitive Rh110 dye: R=

IPt

(650nm)

IRh110

(3)

(540nm)

where IPt(650 nm) and IRh110(540 nm) are the steady-state luminescence intensities of the PtTFPP-doped oxygen sensor and the reference dye in the presence of O2 , respectively. The response of the ratiometric sensor can be evaluated by replacing I0 and I in the Stern–Volmer equation by R0 and R, respectively: R0 = R



n   1

fn



−1

(1 + KSVn [O2 ])

(4)

where R0 is the luminescence signal ratio of the sensor in the absence of oxygen. 3. Preparation of ratiometric temperature and oxygen sensing material TEOS/Octyl-triEOS composite sols used as the matrix material in the proposed ratiometric optical dual sensor were prepared by mixing Octyl-triEOS (0.2 mL) and TEOS (4 mL) to form a precursor solution. Adopting an approach similar to that employed by Yeh et al. [9], EtOH (1.25 mL) and HCl (0.4 mL of 0.1 M HCl) were then added to the sol solution to catalyze the ORMOSIL reaction. The resulting solution was then capped and stirred magnetically for 1 h at room temperature. During the mixing process, 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 (designated as solution A). The oxygen sensor dye solution was prepared by dissolving 2 mg of PtTFPP into 10 mL of tetrahydrofuran (THF) (designated as solution B). The temperature-sensitive dye solution was prepared by dissolving 2 mg of AFC into 10 mL of EtOH and 3(triethoxysilyl)propylisocyanate (TEPIC) (designated as solution C). The temperature/oxygen-independent fluorescent dye solution was prepared by dissolving 2 mg of Rh110 into 10 mL of EtOH (designated as solution D). The luminophore-doped sol solutions were then prepared by mixing the solution B (1 mL), solution C (0.2 mL), and solution D (1 mL) into the solution A (3 mL). The sol mixtures were then capped and stirred mechanically for 10 min. Finally, PtTFPP/THF, Rh110/EtOH, and AFC/TESPIC/EtOH-doped TEOS/Octyl-triEOS composite xerogels were prepared using a sol–gel process performed at room temperature. A flow chart showing the basic synthesis procedure of fabricating composite xerogels with embedded PtTFPP/THF, Rh110/EtOH, and AFC/TESPIC/EtOH is illustrated in Fig. 1. The sensing film was prepared by drop-coating approximately 2 ␮L of PtTFPP/THF, Rh110/EtOH, and AFC/TESPIC/EtOH-doped composite xerogels onto the active area surface (4 × 4.8 mm2 ) of color sensor. The coated substrates were dried at room temperature in the darkness. Fig. 2 shows a photograph of a coated color sensor. All of the prepared color sensors were stored in darkness until use to avoid photodegradation. 4. Experimental setup and results

(2)

where fn denotes the fractional contribution to the total emission from domain/site n and KSVn is the Stern–Volmer quenching constant associated with domain/site n. For the ratiometric optical dual sensor developed in this study, the oxygen concentration is derived from the ratio of the

4.1. Instrumentation Fig. 3 shows a schematic illustration of the experimental arrangement used to characterize the performance of the ratiometric optical dual sensor. In the sensing experiments, the luminescence excitation was provided by an LED (LED405E, Thorlabs, Inc.) with a central wavelength of 405 nm. The ratiometric

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Fig. 1. Flow chart showing the basic synthesis procedure of fabricating composite xerogels.

4.2. Circuits and program

Fig. 2. Photograph of a coated color sensor.

optical temperature and oxygen sensing system consisted of a coated color sensor (Hamamatsu Photonics, S9032-02) and an electronic circuit. Nine oxygen concentrations were obtained by mixing oxygen and nitrogen under control of gas flowmeters. The mixed gas was heated to temperatures ranging from 25 to 65 ◦ C in a hot circulator standard oven (RISEN Co., Ltd, D9LR-RHD452). The mixed gas temperature was measured with a thermometer (Lutron Electronic Co., Ltd., Model TM-925).

The coated color sensor described above was held inside a developed electronic instrument with a low-cost microcontroller as the control unit (HT32, Holtek Semiconductor Inc.). This microcontroller controls the excitation and detection of the optical luminescence signals. The relative luminescence intensities of the temperature and oxygen sensors are directly read from the liquid crystal display (LCD). The instrument has electronic circuitry for power management from a standard AC/DC adapter. Fig. 4(a) and (b) shows the functional block diagram and electronic circuits of the ratiometric optical dual sensor system, respectively, which comprises a color sensor module (LED and color sensor), a control unit (microcontroller), and power management circuits. In the color sensor module, with three optical channels, the temperature, oxygen, and reference signal sensing films are optically excited by an ultraviolet (UV) LED with a central wavelength of 405 nm driven by an electronic circuit with a 10-kHz square wave. The color sensor is connected to the operational amplifiers and converts the photoelectric current into proportional RGB voltage output. The RGB voltage output is connected to the microcontroller to convert these analog voltages to three 16-bit digital data. This 48-bit RGB data is linked to the computer through the serial port that is interfaced with the microcontroller. The data thus obtained in the computer can be processed as per the color measurement technique to display the temperature and oxygen concentration.

Fig. 3. Schematic diagram of experimental arrangement used for ratiometric temperature and oxygen sensing.

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305

Fig. 4. (a) Functional block diagram and (b) electronic circuits of ratiometric optical dual sensor.

Moreover, neither a lens nor optical filters are required, resulting in a compact and robust sensing module. 4.3. Optical properties of ratiometric optical dual sensor Fig. 5(a) and (b) depicts the absorption and emission spectra of the materials (PtTFPP, AFC and Rh110), respectively. As shown, the AFC as well as the oxygen indicator PtTFPP are both efficiently excited a 405-nm LED and Rh110 is excitable only to a minor extent.

Notably, the absorption maximum of Rh110 is located at ∼500 nm so that the dye is also excitable with the blue emission of AFC. Therefore, the three optical probes have well separated spectral windows and can be excited using the same excitation wavelength, 405 nm. Fig. 6 shows the room-temperature luminescence spectra of these dyes (PtTFPP, AFC and Rh110). When excited with a 405-nm LED, these materials exhibit strong phosphorescent emissions at 650, 480, and 540 nm, respectively. Their bright emissions are well

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6

1.2

PtTFPP Absorption

Normalized Emission Intensity (a.u.)

Normalized Absorbance (a.u.)

1.4 Rh110 Absorption

AFC Absorption

1 0.8 0.6 0.4 0.2

5

405 nm LED

4 3 2 AFC

1

Rh110

PtTFPP

0

0 370

390

410

430

450 470 490 510 Wavelength (nm)

530

550

570

(a)

380

430

480

530 580 Wavelength (nm)

630

680

(b)

Fig. 5. Spectral properties of the materials used in the ratiometric optical dual sensor: (a) absorption spectra (b) emission spectra.

resolved and thus the temperatures and oxygen concentrations can be independently monitored. S9032-2 is a color sensor molded into a plastic package and features a 3-channel (RGB) photodiode sensitive to the blue (p = 460 nm; photosensitivity: 0.18 A/W), green (p = 540 nm; photosensitivity: 0.23 A/W), and red (p = 620 nm; photosensitivity: 0.168 A/W) regions of the spectrum. Based on the sensing membrane coated on the active area of the color sensor, the temperature and oxygen concentration can be independently monitored and the effects of spurious fluctuations in the intensity of the excitation source are suppressed. 4.4. Temperature sensing properties of ratiometric optical dual sensor AFC was used as the temperature indicator in this work [26]. It is sensitive to temperatures in the tested range of 25 to around 65 ◦ C, as shown in Fig. 7. As the temperature increased, the luminescence intensity at the peak wavelength (480 nm) of AFC decreased. The resulting calibration curves, shown in Fig. 9, can be used for temperature measurement in the tested range. A linear calibration curve for measuring temperature using the ratiometric optical dual sensor developed in this work can be obtained using the attenuation of the relative luminescence signal as a sensing signal. Fig. 8 shows the linear calibration curve obtained using the data from Fig. 7. The relationship between the temperature and the luminescence intensity at the peak wavelength (540 nm) of Rh110 was investigated. The ratiometric optical dual sensor was heated from

Fig. 7. Variation of relative luminescence intensity of ratiometric optical dual sensor at 480 nm with oxygen concentration ranging from 0 to 100% and temperature of 25 to 65 ◦ C.

25 to 65 ◦ C and the luminescence intensity at the peak wavelength (540 nm) of Rh110 was simultaneously measured. The results are shown in Fig. 8. The results show good repeatability and minimal hysteresis, ensuring that there are no unwanted temperature effect

Relative Luminescence Intensity (a.u.)

10000 9000 O2/T Reference Signal

8000 7000 6000

LED Signal

5000 T Sensing Signal

4000

O2 Sensing Signal

3000 2000

538 nm Q band of PtTFPP

1000 0 380

430

480 530 580 Wavelength (nm)

630

Fig. 6. Excitation and emission spectra of PtTFPP, AFC, and Rh110.

680 Fig. 8. Variation of relative luminescence intensity of ratiometric optical dual sensor at 540 nm with oxygen concentration ranging from 0 to 100% and temperature of 25 to 65 ◦ C.

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307

Fig. 9. Sensor response to temperature changes: (a) AFC and Rh110 measurements and (b) ratiometric measurement.

3.5 25 °C 3

Increasing Temperature

35 °C

R0/R

2.5 2

45 °C

1.5 55 °C

1 0.5

65 °C

0 0

Fig. 10. Variation of relative luminescence intensity of ratiometric optical dual sensor at 650 nm with oxygen concentration ranging from 0 to 100% and temperature of 25 to 65 ◦ C.

presents in the ratiometric optical dual sensor. In contrast to the strong oxygen dependence of the luminescence intensity of PtTFPP, the luminescence intensity of the AFC temperature sensor is clearly not oxygen-dependent and the luminescence intensity of Rh110 is oxygen/temperature-independent at nine oxygen concentrations (0–100%) and five temperatures (ranging from 25 to 65 ◦ C). 3.5

35 °C

I0/I

2

45 °C

1.5 55 °C

1 0.5

65 °C

0

100

Fig. 9(a) shows the luminescence intensities of AFC and Rh110. It can be seen that the temperature dependence of AFC is much stronger than that of Rh110. AFC and Rh110 show 0.82% and 0.09% decreases in voltage per degree increase in temperature, respectively. The temperature response of AFC is thus an order of magnitude greater than that of Rh110. Fig. 9(b) shows a plot of the VAFC /VRh110 ratio, where VAFC and VRh110 are the luminescence intensities of AFC and Rh110, respectively. 3500 Relative Luminescence Intensity (mV)

Increasing Temperature

2.5

40 60 80 Oxygen Concentration (%)

Fig. 12. Stern–Volmer plots for ratiometric optical dual sensor with oxygen concentration ranging from 0 to 100% and temperature of 25 to 65 ◦ C.

25 °C

3

20

Oxygen Sensing Signal (PtTFPP)

100% N2

3000 2500 2000 100% O2

1500 1000

Temperature Sensing Signal (AFC)

500 Reference Signal (Rh110) 0

0

20

40 60 80 Oxygen Concentration (%)

100

Fig. 11. Stern–Volmer plots for optical dual sensor with oxygen concentration ranging from 0 to 100% and temperature of 25 to 65 ◦ C.

0

500

1000

1500 2000 Time (s)

2500

3000

3500

Fig. 13. Response characteristics of ratiometric optical dual sensor when switching alternately between 100% nitrogen and 100% oxygen.

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Fig. 14. Relative luminescence intensities of ratiometric optical dual sensor in the (a) dry (b) water vapor O2 gas atmosphere.

4.5. Oxygen sensing properties of ratiometric optical dual sensor The dependence of the luminescence intensity of PtTFPP on the ambient temperature as a function of oxygen concentration for the PtTFPP/AFC/Rh110-doped TEOS/Octyl-triEOS sensor is shown in Fig. 10. It can be seen that the relative luminescence intensity of the PtTFPP-doped oxygen sensor decreases with increasing temperature and oxygen concentration. This relationship between the temperature and the luminescence intensity decay enables the oxygen measurements to be corrected at different temperatures. Fig. 11 presents the Stern–Volmer plots for the oxygen response in the ratiometric optical dual sensor system obtained by determining the relative luminescence intensity at nine oxygen concentrations (0–100%) and five temperatures (ranging from 25 to 65 ◦ C). For the developed sensor, the relative luminescence intensity is dependent on both oxygen and temperature. The Stern–Volmer plots in this study provide an indication of the relative response of the sensor to O2 quenching. Fig. 11 plots the ratio I0 /I for nine oxygen concentrations, ranging from 0 vol% to 100 vol%, and five temperatures, ranging from 25 to around 65 ◦ C. The luminescence quenching leads to a reduction in the response toward oxygen at elevated temperatures because the oxygen molecules have less time to interact with the oxygen-sensitive dye. The overall response of the ratiometric optical dual sensor can be evaluated by computing the value of the ratio R0 /R at discrete values of the oxygen concentration (see Eq. (3)) to obtain the calibration curves shown in Fig. 12.

4.6. Oxygen response and recovery time of ratiometric optical dual sensor Fig. 13 shows the typical oxygen responses of the ratiometric optical dual sensor when switching between fully oxygenated and fully deoxygenated environments. It can be seen that the optical sensor provided stable and reproducible signals as the environment alternated between the fully oxygenated and fully deoxygenated conditions. From inspection, the response time (t95 : time required for 95% of the total intensity change to take place) of the PdTFPPdoped oxygen sensor was found to be 155 s when switching from nitrogen to oxygen and 279 s when switching from oxygen to nitrogen. The sensor is thus suitable for practical measurement applications when a reliable indication of the oxygen concentration is required.

4.7. Water vapor interferences of the ratiometric optical dual sensor The effects of humidity were tested by passing the gases (N2 and O2 ) through a wash bottle before entering the test chamber. Fig. 14 shows the results obtained by first measuring with dry gases and then after bubbling the humidified gases through the cell. Humidity is known to affect the permeability coefficient of some hydrophilic films. The membrane examined showed an increase in the fluorescence intensities for water vapor, indicating that perhaps the humidity was having some interference effect. 5. Conclusions This paper presented a ratiometric optical device for temperature and oxygen concentration dual sensing applications. The ratiometric optical dual sensor comprises a color sensor with a composite sol–gel membrane doped with a oxygen-sensitive material (PtTFPP), a temperature-sensitive fluorescent dye (AFC), and a temperature/oxygen-insensitive dye (Rh110). Utilizing an LED with a central wavelength of 405 nm, it was found that the three dopants yield discrete emission spectra with no spectral overlap. As a result, the oxygen concentration can be extracted by computing the ratio of the peak luminescence intensities of the two spectra. In addition, the results show that the luminescence properties of the temperature sensor are independent of the presence of the oxygen sensor, and have a uniquely good linear response in the 25–65 ◦ C range. The ratiometric optical dual sensor presented in this study is insensitive to fluctuations in excitation light intensity. Consequently, it is a candidate for temperature and oxygen sensing applications in the medical, industrial, and environmental monitoring domains. Acknowledgements The authors gratefully acknowledge the financial support provided to this study by the Ministry of Science and Technology of Taiwan under grant no. MOST 103-2221-E-131-014. References [1] O.S. Wolfbeis, Materials for fluorescence-based optical chemical sensors, J. Mater. Chem. 15 (2005) 2657–2669. [2] K.F. Mongey, J.G. Vos, B.D. MacCraith, C.M. McDonagh, C. Coates, J.J. McGarvey, Photophysics of mixed-ligand polypyridyl ruthenium(II) complexes immobilised in silica sol–gel monoliths, J. Mater. Chem. 7 (1997) 1473–1479.

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Biographies Cheng-Shane Chu received his M.S. and Ph.D. degrees from the Mechanical Engineering Department at National Cheng Kung University, Taiwan, in 2004 and 2009, respectively. He has been a member of the Mechanical Engineering Department, Ming Chi University of Technology, since 2010, where he is now an assistant professor. His research interests include the optical techniques in precision measurements, development, optimization, and characterization of novel optical sensors for industrial, biological, and environmental applications. Ting-Hsun Lin received his BS degree in Mechanical Engineering from Ming Chi University of Technology, Taiwan, in 2012. He is currently working toward the M.S. degree in mechanical engineering from Ming Chi University of Technology, Taipei, Taiwan. His research interests in the areas of optical fiber sensors and optical measurement.