Analytica Chimica Acta 480 (2003) 275–283 Lifetime-based optical sensor for high-level pCO 2 detection employing fluorescence resonance energy transfer Christoph von Bültzingslöwen, Aisling K. McEvoy, Colette McDonagh, Brian D. MacCraith ∗ School of Physical Sciences, National Centre for Sensor Research, Dublin City University, Glasnevin, Dublin 9, Ireland Received 1 July 2002; received in revised form 10 December 2002; accepted 20 December 2002 Abstract An optical sensor for the measurement of high levels of carbon dioxide in gas phase has been developed. It is based on fluorescence resonance energy transfer (FRET) between a long-lifetime ruthenium polypyridyl complex and the pH-active disazo dye Sudan III. The donor luminophore and the acceptor dye are both immobilised in a hydrophobic silica sol–gel/ethyl cellulose hybrid matrix material. Tetraoctylammonium hydroxide (TOA-OH) is used as an internal buffering system. Fluo- rescence lifetime is measured in the frequency domain, using low-cost phase modulation measurement technology. The use of Sudan III as an acceptor dye has enabled the sensor to have a dynamic range up to 100% carbon dioxide. The sensor displays 11.2 ◦ phase shift between the limit of detection (LOD) of 0.06 and 100% CO 2 with a resolution of better than 2%. The encapsulation in the silica/polymer hybrid material has provided the sensor with good mechanical and chemical stability. The effect of molecular oxygen, humidity and temperature on the sensor performance was studied in detail. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Carbon dioxide; Sensor; Fluorescence; Lifetime; Sol–gel 1. Introduction Carbon dioxide is an important parameter in many clinical, biochemical and industrial applications. Great progress has been made in the last 25 years in the development of optical chemical sensor ma- terials, that are capable of reliable carbon dioxide detection [1]. These sensors are normally based on the interrogation of colourimetric or fluorometric pH indicators. An important step in establishing pCO 2 sensor materials was the introduction of solid sensor membranes by Mills [1], using phase transfer agents ∗ Corresponding author. Tel.: +353-1-7005299; fax: +353-1-7005384. E-mail address: brian.maccraith@dcu.ie (B.D. MacCraith). to solubilise the pH-indicator dye into a hydrophobic polymer matrix and to provide the sensing chemistry with the necessary water molecules [2]. Sensors based on luminescence detection usually result in higher sensitivity than those based on ab- sorption or reflectance [3]. The detection of lumines- cence lifetime, rather than intensity, can overcome many of the problems associated with intensity-based systems such as drift in optoelectronic components [4]. An approach, which enables the use of colouri- metric pH indicators in lifetime-based sensors, is fluorescence resonance energy transfer (FRET). In this technique the colour change of a colourimetric pH indicator is converted into lifetime information [5]. An inert fluorescent dye, whose emission band overlaps the absorption band of the pH indicator, is 0003-2670/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0003-2670(02)01653-7