Fibre optic oxygen sensor based on fluorescence quenching of evanescent-wave excited ruthenium complexes in sol–gel derived porous coatings

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Abstract
A simple, low-cost technique for the fabrication of optical sensors for oxygen is described and preliminary results obtained using these sensors are reported. The technique is based on coating a declad portion of an optical fibre with a microporous glass film prepared by the sol–gel process. A ruthenium complex [RuII–tris-(2,2′-bipyridine) or RuII–tris(4,7-diphenyl-1,10-phenanthroline)] is trapped in the nanometre-scale cage-like structure of the porous film. In this sensor configuration the complex is excited by the evanescent field of the 488 nm radiation guided by the optical fibre. The luminescence from such complexes is known to be quenched by oxygen and the sensors exhibit repeatable quenching behaviour when exposed to various concentrations of oxygen. The ratio R=I0/I100 where I0 and I100 represent the detected signals from a sensor exposed to 100% nitrogen and 100% oxygen, respectively, is used as a measure of the sensitivity of the sensor. Sensors based on the diphenylphenanthroline complex exhibit greater sensitivity than those based on the bipyridine complex, in accordance with theoretical predictions. More importantly, however, the design potential of the sol–gel process for sensor fabrication is demonstrated by the achievement of a substantial increase in R when the process parameters are adjusted to increase the pore volume.