Gerard O'Keeffe

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

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.

Sol-gel coatings for optical chemical sensors and biosensors☆

In this paper an overview is presented of the state-of-the-art of optical sensors employing sol-gel-derived coatings. The technique is particularly suited to the side-coating of optical fibres or waveguides in evanescent-wave sensors because precise control of sensitivity-determining parameters, such as the coating thickness and length, is achievable. Sensors based on entrapped organic and inorganic dyes, enzymes and other biomolecules have been reported. The main features of the process are illustrated by examples of chemical sensors and biosensors from the literature. In particular, the development of an oxygen sensor based on the quenching of fluorescence from a sol-gel-entrapped ruthenium complex and an absorption-based pH sensor are described. Issues that require further investigation before this technology can proceed to the stage of industrial development are also highlighted.

Development of a LED-based phase fluorimetric oxygen sensor using evanescent wave excitation of a sol-gel immobilized dye

Abstract In this paper we report preliminary results from an intrinsic fibre-optic oxygen based on phase fluorimetry. Phase fluorimetry is a method of measuring the luminescence lifetime of a fluorophore and, where suitable, has many advantages over other reported optical sensing techniques such as absorption and fluoresecnece intensity monitoring. Lifetime measurements are absolute quantities, which offer the possibility of inherent referencing, and are usually independent of indicator concentration, photobleaching the excitation source intensity. The long excited-state lifetime of ruthenium polypyridyl complexes are efficiently quenched by oxygen. Sol-gel immobilization of the ruthenium complex (Ru II -tri(4,7-diphenyl-1,10-phenanthroline)) on a multimode optical fibre yields highly durable, inert, microporous claddings which exhibit almost complete quenching under evanescent-wave excitation. The fluorophore decay curve exhibits a double exponential behaviour consisting of a fast and a slow component, both of which undergo quenching on exposure to oxygen. These data are used to model and expected behaviour of the sensor in phase fluorimetric mode. The predictions are in close agreement with experimental measurements performed with the coated fibre under blue light-emitting diode (LED) excitation. An optimum modulation frequency is identified and the performance of the intrinsic oxygen sensor in real-time measurement mode is reported. The use of an inexpensive light source, combined with a simple fabrication technique and the advantages of the phase fluorimetric method, facilitates the production of low cost, high performance optical oxygen sensors.

Fibre optic chemical sensors based on evanescent wave interactions in sol-gel-derived porous coatings: Code: F7

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.

Light-emitting-diode-based oxygen sensing using evanescent wave excitation of a dye-doped sol-gel coating

Two evanescent wave fiber optic sensors for oxygen are reported: one intensity based and the other based on phase fluorimetry. Both sensors employ the quenching by oxygen of the fluorescence from a ruthenium complex trapped in the cagelike structure of a sol-gel-derived porous film on a declad section of multimode optical fiber. The sensors exhibit excellent performance using excitation from new high brightness blue LEDs and establish the viability of low-cost portable sensor devices based on the 501-gel process.

Optical immunosensing of lactate dehydrogenase (LDH)

Abstract Detection of the lactate dehydrogenase (LDH) enzyme is important in many areas of clinical diagnosis. A fibre-optic evanescent wave immunosensor (EWIS) was developed for the determination of this enzyme. A polyclonal antibody preparation was used in both one-step and two-step assays, developed for the analysis of LDH. Similar assays were performed on the commercially available biosensor based on surface plasmon resonance, the BlAcore ™ , and the results compared to those obtained with the EWIS. It was established that although the same polyclonal antibody preparation can be used for one-step immunoassays, if the antigen is large, the sensitivity of the sensor system is reduced when compared to two-step assays. The working ranges for both systems were determined.

Assessment of the Effect of Increased Fluorophore Labelling on the Binding Ability of an Antibody

Abstract The effect of the fluorophore-to-protein labelling ratio on the binding ability or ‘affinity’ of an antibody preparation, using two immunosensors, the BIAcoreTM (Pharmacia Biosensor AB) and a fibre optical fluoroimmunosensor based on the evanescent wave technique (EWIS), constructed in our laboratory, was assessed. The BIAcoreTM was used to determine the effect of increased labelling on the ‘affinityy’ of the FITC (fluorescein isothiocyanate)-labelled anti-LDH antibodies, while the EWIS was used to assess how the fluorescence intensity of the conjugates may compensate for loss of binding ability in, for example, fluoroimmunoassay applications. The optimal labelling ratio was determined to be 2-5 FITC molecules per antibody protein molecule.

Development of an LED-based fiber optic oxygen sensor using a sol-gel-derived coating

Two evanescent wave fiber optic sensors for oxygen are reported: (i) intensity-based, and (ii) based on phase fluorimetry. Both sensors employ the quenching by oxygen of the fluorescence form a ruthenium complex trapped in the cage-line structure of a sol-gel-derived porous film on a declad section of multimode optical fiber. The sensors exhibit excellent performance using excitation from new high brightness blue LEDs and establish the viability of low-cost portable sensor devices based on the sol-gel process. The data presented for the intensity-based sensor were obtained using an all solid state excitation and detection system. Preliminary results obtained by application of the intensity- based sensor to dissolved oxygen measurement are also reported.

Sol-gel coatings for optical chemical sensors and biosensors

In this paper an overview is presented of the state-of-the-art of optical sensors which employ sol-gel-derived coatings. The technique is particularly suited to the side-coating of optical fibers or waveguides in evanescent-wave sensors because precise control of sensitivity- determining parameters, such as the coating thickness and length, is achievable. Sensors based on entrapped organic and inorganic dyes, enzymes and other biomolecules have been reported. The main features of the process are illustrated by examples of chemical sensors and biosensors from the literature. In particular, the development of an oxygen sensor based on the quenching of fluorescence from a sol-gel-entrappd ruthenium complex is described. This sensor may be operated in intensity or decay-time mode. The latter offers many advantages over intensity-based sensing and may also be used to provide useful diagnostic information concerning the distribution/accessibility of the sensor fluorophore. Issues which require further investigation before this technology can proceed to the stage of industrial development are also highlighted.

Development of an intrinsic phase fluorimetric oxygen sensor using a high-intensity blue LED

The ability to determine oxygen concentration is of great importance for many industhal, medical and environmental applications. Optical oxygen sensors are attractive because of their many advantages over other sensor types, in that they offer the possibility of miniaturization, have fast response times, do not consume oxygen, and are not easily poisoned.