A Plastic Optical Fiber Sensor for the Dual Sensing of Temperature and Oxygen

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1 Proceedings of the XIth International Congress and Exposition June 2-5, 28 Orlando, Florida USA 28 Society for Experimental Mechanics Inc. A Plastic Optical Fiber Sensor for the Dual Sensing of Temperature and Oxygen Chen-Shane Chu and *Yu-Lung Lo Department of Mechanical Engineering, National Cheng Kung University, No.1, Ta-Hsueh Road, Tainan 71, Taiwan *Correspond author: loyl@mail.ncku.edu.tw ABSTRACT This study presents a low-cost plastic optical fiber sensor for the dual sensing of temperature and oxygen. The sensor features a commercially available epoxy glue coated on the side-polished fiber surface for temperature sensing and a fluorinated xerogel doped with platinum tetrakis pentrafluoropheny porphine (PtTFPP) coated on the fiber end for oxygen sensing. The temperature and oxygen indicators are both excited using a UV LED light source with a wavelength of 38 nm. The luminescence emission spectra of the two indicators are well resolved and exhibit no cross-talk effects. Our studies show that the temperature response of the sensor is independent of the oxygen concentration. Overall, the results indicate that the dual sensor presented in this study provides an ideal solution for the non-contact, simultaneous sensing of temperature and oxygen in general biological and medical applications. 1. Introduction In general, the ambient temperature has a significant effect upon the performance of opto-chemical sensors [1]. For example, the luminescence quenching effect of oxygen is highly sensitive to the temperature, and hence a knowledge of the ambient temperature is essential if the quenching results are to be reliably interpreted [2]. 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 [2-4] or two different wavelengths [5] for excitation purposes. Dual sensing techniques are commonly based on measuring the lifetime of the luminescence signal since this property is independent of drifts in the optical path, or variations in the gain of the detection system. However, the electronic instrumentation required in such techniques tends to be complex and somewhat expensive. Besides, the sensing layer embedded with multiple indicators occasionally display increased photodecomposition and signal drifts compared to single sensors, particularly when oxygen sensors (which generate singlet oxygen) are present [6]. Accordingly, the objective of the current study is to develop a simple, low-cost dual temperature and oxygen sensor based on a plastic optical fiber coated with suitable sensing indicators. The current study presents a simple, low-cost plastic optical fiber sensor for the dual sensing of temperature and oxygen. In the proposed device, a commercial epoxy glue is used as the temperature indicator and PtTFPP [7-8] embedded in an 3,3,3-trifluoropropyltrimethoxysliane (TFP-TriMOS)/ n-propyltrimethoxysilane (n-propyl-trimos) matrix is used as the oxygen indicator. It is found that the temperature response of the sensor is independent of the oxygen concentration. Thus, the sensed temperature provides a convenient means of calibrating the derived values of the oxygen concentration.

2 2. Basic theory 2.1 Optical temperature sensor A linear calibration curve for measuring temperature using the fiber optic sensor of this work can be obtained by using the attenuation of the luminescence signal in db as a sensing signal. The db signal is defined as [9]: 22 C I F db = 1 log( ) I F (1) 22 C I F where is the luminescence intensity of the sensor at 22 and I F is the luminescence intensity at a measured temperature. 2.2 Optical oxygen sensor The luminophore quenching effect depends on several factors. However, in the simplest scenario of a luminophore in a homogeneous microenvironment, quenching takes place in accordance with the Stern-Volmer equation [1], i.e. I /I = 1 + K sv [O 2 ] = 1 + kqτ [O 2 ] (2) where I and I represent the steady-state luminescence intensities in the absence and presence of O 2, respectively; K sv is the Stern-Volmer quenching constant; [O 2 ] is the O 2 concentration; τ is the excited state luminescence lifetime in the absence of a quencher, and kq is the bimolecular rate constant describing the efficiency of the collisional encounters between the luminophore and the quencher. For this ideal case, a plot of I /I vs. [O 2 ] will be linear with a slope equal to K sv and an intercept of unity. 3. Experiment setup and results Fig. 1 illustrates the experimental setup used to characterize the performance of the plastic optical fiber sensor for dual sensing of temperature and oxygen. The dual sensor was excited by a UV LED light source driven by a 4 MHz Arbitrary Waveform Generator. The luminescence intensity was measured at a pressure of 11.3 kpa using a USB 2-FLG spectrofluorometer. Six different oxygen concentrations were obtained by mixing oxygen and nitrogen and controlled by gas flowmeters. The mixed gas was heated to temperatures ranging from 22 to 73 in a Hot Circulator Standard Oven. The mixed gas temperature was measured by thermometer. Figure 1 Experimental setup for optical fiber dual sensor system.

3 3.1 Fabrication of plastic optical fiber dual sensor The plastic optical fiber dual sensor developed in this study uses epoxy glue coated on the side-polished fiber surface as a temperature sensor and PtTFPP embedded in a fluorinated xerogel on the end of the fiber as an oxygen indicator (Note that full details of the oxygen indicator preparation process are presented by the current authors in [1]). Having dip-coated the fiber with the two sensing media, it was dried at a temperature of 6 overnight and then left to stabilize under ambient conditions for one week prior to use. 3.2 Temperature sensing properties Fig. 2 presents the emission spectra of the optical fiber dual sensor as a function of temperature at oxygen concentrations ranging from % to 1%. Note that in recording the luminescence intensity, the integration time of the CCD spectrometer was set to 1 ms. Regression analysis can then be applied to determine a logarithmic relationship between the luminescence intensity and temperature. Applying Eq. (1), a linear calibration scale based on the attenuation of luminescence signal in db, as shown in Fig. 3. Relative Luminescence Intensity (a.u.) nm LED Light Source 65 nm Wavelength (nm) Temperature Figure 2 Emission spectra of the optical fiber dual sensor as function of temperature, for the temperature sensor and 65 nm for oxygen sensor both at % oxygen concentration. db R 2 = Temperature ( ) Figure 3 Calibration curves of optical fiber dual sensor for temperature measurement: calibration curve using attenuation of luminescence signal in db as sensing signal.

4 3.2 Oxygen sensing properties Fig. 4 presents the luminescence intensity spectra of plastic optical fiber dual sensor under six different temperature and oxygen concentrations. The optical fiber dual sensor is excited by a UV LED and two emissions of temperature sensor and oxygen sensor has no spectral overlap, so that the temperatures and oxygen concentrations can be independently monitored. Relative Luminescence Intensity (a.u.) nm LED Light Source 65 nm Wavelength (nm) Oxygen % 2% 4% 6% 8% 1% Figure 4 Emission spectra of temperature- and oxygen-sensing membranes as function of oxygen concentration at temperatures of: 22.2 Fig. 5 illustrates the Stern-Volmer plots of the optical fiber dual sensor derived from the experimental data presented in Fig. 4. It is observed that the plots corresponding to the oxygen-sensing membrane are linear for temperature less or equal to 38.1, which implies that the magnitude of luminescence quenching effect is directly related to the oxygen concentration. In terms of temperature increasing from 52.4 to around 73.2, the Stern-Volmer plot becomes more and more nonlinear. The results of Fig. 5 confirm that the temperature response of the epoxy glue membrane is insensitive to the oxygen concentration nm R 2 = Io / I Temperature R 2 =.9954 R 2 =.9958 R 2 = Oxygen Concentration (%) Figure 5 Stern-Volmer plots of optical fiber dual sensor as function of oxygen concentration and temperature.

5 4. Conclusions This study has presented a simple, low-cost plastic optical fiber sensor for the dual sensing of temperature and oxygen. The sensor incorporates a platinum complex (PtTFPP) embedded in a fluorinated xerogel as the oxygen sensor and epoxy glue as the temperature indicator. Both indicators are excited using a single UV LED light source with a wavelength of 38 nm. The emission spectra of the two sensing materials are well resolved and exhibit no cross-talk effect. As a result, the oxygen concentration and temperature can be independently monitored using the single sensor. In conclusion, the plastic optical fiber sensor developed in this study enables the simultaneous, non-contact sensing of temperature and oxygen and therefore represents a suitable probe for the temperature-compensated sensing of oxygen, high-resolution oxygen profiling, and a variety of biological and medical applications. References [1] V. I. Ogurtosov and D. B. Papkovsky, Modelling of phase-fluorometric oxygen sensors:consideration of temperature effects and operational requirements, Sens. Actuators B Chem. vol. 113, pp , 26. [2] L. M. Coyle and M. Gouterman, Correcting lifetime measurements for temperature, Sens. Actuators B Chem. vol. 61, pp , [3] M. E. Koese, B. F. Carroll, K. S. Schanze, Preparation and Spectroscopic Properties of Multiluminophore Luminescent Oxygen and Films, Langmuir vol. 39, pp , 21. [4] S. M. Borisov and O. S. Wolfbeis, Temperature-sensitive europium (III) probes and their use for simultaneous luminescent sensing of temperature and oxygen, Anal. Chem. vol. 78, pp , 26. [5] S. M. Borisov, A. S. Vasylevska, C. Krause, O. S. Wolfbeis, Composite luminescent material for dual sensing of oxygen and temperature, Adv. Mater. vol. 16, pp , 26. [6] S. Nagl and O. S. Wolfbeis, Optical multiple chemical sensing: status and current challenges, Analyst vol. 132, pp , 27. [7] T. S. Yeh, C. S. Chu, Y. L. Lo, Highly sensitive optical fiber oxygen sensor using Pt(II) complex embedded in sol gel matrices, Sens. Actuators B Chem. vol. 119, pp , 26. [8] C. S. Chu and Y. L. Lo, High-performance fiber-optic oxygen sensors based on fluorinated xerogels doped with Pt(II) complexes, Sens. Actuators B Chem. vol. 124, pp , 27. [9] S. Q. Tao and A. Jayaprakash, A fiber optic temperature sensor with an epoxy-glue membrane as a temperature indicator, Sens. Actuators B Chem. vol. 119, pp , 26. [1] J. R. Lakowicz, Principles of Luminescence Spectroscopy, 2nd ed; Kluwer Academic/ Plenum Press, New York, 1999.