Journal of Sol-Gel Science and Technology, 2, 661-665 (1994) © 1994 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands. Fibre Optic Chemical Sensors Based on Evanescent Wave Interactions in Sol-Gel-Derived Porous Coatings Code: F7 B.D. MAC CRAITH, C. MCDONAGH, G. O'KEEFFE AND T. BUTLER School of Physical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland B. O'KELLY AND J.F. MCGILP Dept. of Pure and Applied Physics, Trinity College, Dublin 2, Ireland Abstract. A simple, low-cost technique for fabricating reagent-mediated fibre-optic chemical sensors (optrodes) is described and the performance of a range of such sensors is reported. The technique is based on coating an unclad portion of an optical fibre with a microporous glass film prepared by the sol-gel process. Although tip- and side-coating are both possible with this technique, the latter, which employs evanescent wave interactions, offers particular advantages in terms of sensor performance, control of sensitivity and quality of coating. The sol-gel- derived film is used to provide a robust support matrix in which analyte-sensitive dyes are entrapped and into which smaller analyte molecules may diffuse. The benefits of this sol-gel approach to sensor fabrication are illustrated by results from a range of sensors for pH, ammonia and oxygen based on both evanescent wave absorption and evanescent wave excitation of fluorescence. Keywords: fibre optic sensor, evanescent wave, microporous coating, dye entrapment 1. Introduction There is considerable research interest in the area of fi- bre optic chemical sensors with a view mainly to their application in industrial chemical analysis, medical di- agnostics and environmental monitoring [ 1, 2]. Essen- tially the technology involves probing matter in any of its phases with photons guided along an optical fibre or waveguide. The motivation for using waveguided photons for chemical sensing derives from a number of attractive features. Principally, the technology can combine the advantages of optical fibre lightguiding with a multiplicity of optical techniques developed for routine chemical analysis. These techniques are usu- ally highly specific and very sensitive. The small size and flexibility of optical fibres confers geometric versa- tility and their low attenuation enables remote in-situ sensing in difficult or hazardous environments. Fur- thermore, fibre optic chemical sensors can capitalise on the availability of components developed for the wider optoelectronics industry. In general terms two categories of fibre optic chem- ical sensors may be distinguished. In direct spectro- scopic sensors the optical radiation interacts directly with the analyse and the fibre or waveguide acts as a simple light guide which separates the sensing loca- tion from the monitoring instrumentation. The optical fibre enables direct spectral analysis (fluorescence, ab- sorption etc.) to be performed remotely on a sample. Alternatively, reagent-mediated sensors (or optrodes) employ an intermediate reagent which responds opti- cally (e.g. by fluorescence or absorption change) to the analyte. Reagents are chosen to react sensitively and specifically to a particular analyse and the resultant op- tical response is used to measure analyte concentration. In this paper we describe a generic fabrication pro- cess for reagent-mediated fibre optic chemical sensors. The process utilises microporous coatings produced by the sol-gel process and has particular potential for low- cost disposable devices. We have also shown that sol- gel-derived coatings may also be used to advantage in direct spectroscopic sensors but these will not be dis- cussed here [3].