IEEE SENSORS JOURNAL, VOL. 12, NO. 1, JANUARY 2012 71 A Cylindrical-Core Fiber-Optic Oxygen Sensor Based on Fluorescence Quenching of a Platinum Complex Immobilized in a Polymer Matrix Rongsheng Chen, Member, IEEE, Andrew D. Farmery, Andy Obeid, and Clive E. W. Hahn Abstract—A miniature (200 in diameter) cylindrical-core fiber-optic oxygen sensor has been developed for measuring rapid change in oxygen partial pressure . The fiber-optic sensing element is based on a cylindrical-core waveguide structure formed by coating a thin medical grade polymer sensing film that con- tains immobilized Pt(II) complexes on silica optical fiber. The per- formance such as sensitivity and time response of the fiber-optic oxygen sensors were evaluated using luminescence intensity mea- surement. To determine accurately the response time of the fiber- optic oxygen sensors, a test chamber was used to provide rapid changes in the partial pressure of oxygen. The result showed that the time response (time-constant, ) of this cylindrical-core fiber- optic oxygen sensor is less than 50 ms. To our knowledge, this is the fastest such sensor of this size covering the full dynamic range of pO2 from 0 to 100 kPa. Index Terms—Cylindrical-core optical fiber, fiber-optic oxygen sensor, fluorescence quenching. I. INTRODUCTION A FAST, reliable, and accurate oxygen sensor is important for a wide range of industrial, medical, and environmental applications. Examples of medical applications include mea- surement of the rate of oxygen consumption by patients and measurement of oxygen partial pressure in the inspired and ex- pired gas of patients undergoing anaesthesia or in the critical care setting. More recently, the value and utility of measuring rapid oscillations in arterial blood, which may occur on a breath to breath basis in patients with acute lung injury, is being understood [1]. In the diseased lung, alveolar units may begin to collapse in expiration and reopen in inspiration. This process, known as cyclical atelectasis, causes the oxygen partial pressure in the arterial blood to oscillate widely on a breath-by-breath basis. Manuscript received February 12, 2011; accepted March 10, 2011. Date of publication March 28, 2011; date of current version November 29, 2011. This work was supported in part by the Engineering and Physical Sciences Research Council (EPSRC), U.K., under Grant EP/F02987X/1. The technical assistance given by colleagues from Oxford Optronix is duly acknowledged. The associate editor coordinating the review of this paper and approving it for publication was Prof. Jose Lopez-Higuera. R. Chen, A. D. Farmery, and C. E. W. Hahn are with the Nuffield Depart- ment of Clinical Neurosciences, University of Oxford, Oxford OX3 9DU, U.K. (e-mail: rongsheng.chen@nda.ox.ac.uk; andrew.farmery@nda.ox.ac.uk; clive.hahn@nda.ox.ac.uk). A. Obeid is with Oxford Optronix Ltd., Milton Park, Oxford OX14 4SA, U.K. (e-mail: andy@oxford-optronicx.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2011.2132758 These oscillations in arterial blood can be used to detect the onset of cyclical atelectasis in the lung and to direct the clinician to adjust the ventilator settings to reduce the amplitude of the os- cillations and thus moderate the atelectasis process itself. There is therefore a clinical need to measure these intra-breath oscillations online and in real time with a device small enough to fit into a human artery. The pO2 measurement range needs to be from 5% to 60% in this application. Electrochemical oxygen sensors such as the Clark electrode are widely used but a number of disadvantages still remain un- resolved, particularly for clinical applications, where they can only provide a “static” reading. In addition, optical oxygen sen- sors based on luminescence quenching can offer some advan- tages over the traditional Clark electrode, such as their small size and suitability for remote sensing and multiplex sensor net- working. They do not consume oxygen and can be used to mea- sure oxygen in both gas and liquid phases. In many applications, the critical advantage is potential rapid response time making them suitable for continuous dynamic pO2 monitoring. The principle of the optical oxygen sensor is based on the oxygen quenching effect on fluorescence of luminescent molecules (luminophores) that are immobilized in a matrix. Many materials have been used as the matrix including silicone rubbers [2], [3], silica gels [4], sol-gels [5], [6], and polymers [7]–[9]. Most of these materials were chosen because they have a high oxygen permeability, good mechanical and chemical stability, and superior optical clarity. In many previous studies, ruthenium complexes have been used as the fluorophore since this type dye has a long excitation lifetime (5.3 ). How- ever, phosphorescent porphyrins of platinum such as platinum tetrakis pentrafluoropheny porphine (PtTFPP) and platinum octaethylporphine (PtOEP) have more desirable features such as longer lifetimes (60 ), larger Stokes shifts (145 nm for Pt complexes compared to 100 nm for ruthemium). These plat- inum complexes have been extensively used in optical oxygen sensors [10]–[13]. The fiber-optic oxygen sensors based on the evanescent wave has been reported [14]–[16]. In these sensors, the evanescent wave (which comprises only a small part of the excitation light) penetrates the surrounding sensing matrix. The depth of the evanescent field is approximately 0.5 , it being approxi- mately equal to the wavelength of the excitation light (505 nm). The fluorophores, immobilized in the matrix on the surface of the decladded fiber, are excited by the evanescent wave and the fluorescence produced from the fluorophore is emitted into the fiber core for transmission to an optical detection system. In this 1530-437X/$26.00 © 2011 IEEE