Sensors and Actuators B 74 (2001) 124±130
Phase ¯uorometric dissolved oxygen sensor C. McDonagha,*, C. Kolleb, A.K. McEvoya, D.L. Dowlinga, A.A. Cafollaa, S.J. Cullena, B.D. MacCraitha a
Optical Sensors Laboratory, National Centre for Sensor Research, Dublin City University, Glasnevin, Dublin 9, Ireland b In®neon Technologies MDCA Villach, Siemen sstr. 2, 29500-Villach, Austria
Abstract The design and performance of a ruggedised dissolved oxygen (DO) probe, which is based on phase ¯uorometric detection of the quenched ¯uorescence of an oxygen-sensitive ruthenium complex, is reported. The complex is entrapped in a porous hydrophobic sol±gel matrix that has been optimised for this application. The LED excitation and photodiode detection are employed in a dipstick probe con®guration, with the oxygen-sensitive ®lm coated on a disposable PMMA disc, which in turn is designed to guide light into the ®lm by total internal re¯ection. A key element of the design is the common mode rejection of phase between the signal and reference channels, requiring careful selection of the relevant optoelectronic components. The advantages of the phase ¯uorometric approach over intensity measurement are highlighted. The phase detection electronics exhibits excellent long-term stability with a temperature coef®cient of 0.00087. The sensor response exhibits excellent signal-to-noise ratio (SNR) and reversibility and has been corrected for both temperature and pressure. The limit of detection (LOD) is typically 0.15 hPa or 6 ppb. The sensor has many potential applications but it has been designed primarily for application in waste-water monitoring. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Optical; Oxygen sensing; Phase ¯uorometry; Sol±gel
1. Introduction The determination of oxygen concentration is important in many areas of industry, medicine and the environment. The amount of oxygen dissolved in water is an indication of the quality of the water and careful control of oxygen levels is important in waste-water management and in fermentation processes. Optical dissolved oxygen (DO) sensors [1] are more attractive than conventional amperometric devices, because, in general, they have a fast response time, do not consume oxygen and are not easily poisoned. Sensor operation is usually based on the quenching of ¯uorescence in the presence of oxygen. In this work, the ¯uorescent ruthenium complex, [RuII-Tris(4,7-diphenyl-1,10-phenanthroline)]2, (Ru-(Ph2phen)32) was chosen because of its highly emissive metal-to-ligand charge-transfer state, long lifetime, and strong absorption in the blue-green region of the spectrum, which is compatible with high-brightness blue light-emitting diodes. The dye is entrapped in a porous, hydrophobic sol± gel ®lm. The oxygen quenching process is described by the
* Corresponding author. Tel: 353-1-704-5301; fax: 353-1-700-5384. E-mail address:
[email protected] (C. McDonagh).
Stern±Volmer equations: I0 1 KSV pO2 I t0 1 KSV pO2 t KSV kt0
(1) (2) (3)
where I and t are, respectively, the fluorescence intensity and excited state lifetime of the fluorophore, the subscript 0 denotes the absence of oxygen, KSV the Stern±Volmer constant, k the diffusion-dependent bimolecular quenching constant, and pO2 the oxygen partial pressure. Much has been published [2±4] on optical oxygen sensors which are based on intensity quenching of the ¯uorescence Eq. (1). It has now been established that these sensors have many drawbacks. These include susceptibility to light source and detector drift, to changes in optical path, and drift due to degradation or leaching of the dye. These effects can be minimised by operating the sensor in the time domain instead of the intensity domain [5,6]. The lifetime, t, is an intrinsic property of the ¯uorophore which, unlike intensity, is virtually independent of external perturbations. The lifetime is quenched (reduced) in the presence of oxygen and this quenching is described in Eq. (2). In the DO sensor
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 0 ) 0 0 7 2 1 - 8
C. McDonagh et al. / Sensors and Actuators B 74 (2001) 124±130
reported here, the lifetime is monitored as a function of oxygen concentration using the phase ¯uorometry technique [7,8], where an oxygen-sensitive phase difference is measured between the modulated ¯uorescence signal and a modulated reference signal. The objective of this work was to build a probe-based optical DO sensor based on the phase ¯uorometric principle. In this work, the main application for the sensor is in wastewater treatment. Speci®cations include a measurement range of 0±15 ppm, stability of 0.1 ppm per week, a limit of detection (LOD) of
