Aisling K. McEvoy

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.

Sol–gel based optical carbon dioxide sensor employing dual luminophore referencing for application in food packaging technology

An optical sensor for the measurement of carbon dioxide in Modified Atmosphere Packaging (MAP) applications has been developed. It is based on the fluorescent pH indicator 1-hydroxypyrene-3,6,8-trisulfonate (HPTS) immobilised in a hydrophobic organically modified silica (ormosil) matrix. Cetyltrimethylammonium hydroxide was used as an internal buffer system. Fluorescence is measured in the phase domain by means of the Dual Luminophore Referencing (DLR) sensing scheme which provides many of the advantages of lifetime-based fluorometric sensors and makes it compatible with established optical oxygen sensor technology. The long-term stability of the sensor membranes has been investigated. The sensor displays 13.5 degrees phase shift between 0 and 100% CO2 with a resolution of better than 1% and a limit of detection of 0.08%. Oxygen cross-sensitivity is minimised (0.6% quenching in air) by immobilising the reference luminophore in polymer nano-beads. Cross-sensitivity towards chloride and pH was found to be negligible. Temperature effects were studied, and a linear Arrhenius correlation between ln k and 1/T was found. The sensor is stable over a period of at least seven months and its output is in excellent agreement with a standard reference method for carbon dioxide analysis.

Dissolved oxygen sensor based on fluorescence quenching of oxygen-sensitive ruthenium complexes immobilized in sol–gel-derived porous silica coatings

A dissolved oxygen sensor based on the quenching of fluorescence from a ruthenium dye complex entrapped in a porous sol–gel film is reported. Sol–gel-derived silica films were fabricated by dip-coating on to planar and optical fibre substrates. The films were pre-doped with the oxygen-sensitive ruthenium complex [RII–tris(4,7-diphenyl-1,10-phenanthroline)], the fluorescence of which is quenched in the presence of oxygen. The structure and behaviour of sol–gel films are dependent on the fabrication parameters. In particular, enhancement of the surface hydrophobicity increases the quenching response in water. This is achieved by using suitable proportions of modified precursors of silica of the form R(OEt)3Si, where R is an alkyl group, in the standard fabrication procedure. It is shown that by increasing the ratio of modified precursor, the quenching response in the aqueous phase increases. A very low limit of detection, 6 ppb, was determined for the modified films. Using a high-brightness blue LED, combined with a miniature photodiode detection system, these results indicate the potential for a low-cost, high-performance, portable dissolved oxygen sensor for use in many varied situations from aeration control to on-line river pollution monitoring.

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.

Lifetime-based optical sensor for high-level pCO2 detection employing fluorescence resonance energy transfer

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. Fluorescence 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% CO2 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.

OPTICAL SOL-GEL-BASED DISSOLVED OXYGEN SENSOR : PROGRESS TOWARDS A COMMERCIAL INSTRUMENT

A dissolved oxygen sensor based on fluorescence quenching of the oxygen-sensitive ruthenium complex, [Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline]2+, which has been immobilized in a porous silica sol-gel-derived film, is reported. Ormosil sensing films were fabricated using modified silica precursors such as methyltriethoxysilane (MTEOS) and ethyltriethoxysilane (ETEOS) and were dip-coated onto planar glass substrates. Tailoring of the films for dissolved oxygen (DO) sensing is described whereby sensor response is optimized by maximizing film hydrophobicity by the use of the modified precursors. Sensor performance parameters such as limit of detection and sensor resolution are reported. Issues such as dye leaching and photobleaching are discussed. Progress towards a commercial instrument is reported.

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.

Optimisation of sol-gel-derived silica films for optical oxygen sensing

Sol-gel-derived silica films were fabricated by dip-coating onto planar and optical fibre substrates. The films were pre-doped with the oxygen-sensitive ruthenium complex [Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline)], whose fluorescence is quenched in the presence of oxygen. The structure and behaviour of sol-gel films is related to the fabrication parameters. In order to optimise the films for oxygen sensing in gaseous and in aqueous media, the quenching behaviour was monitored as a function of dip-speed and water: precursor ratio. By adjusting the above parameters, film properties can be tailored to optimise oxygen quenching in particular concentration ranges and environments.

Breath-by-breath measurement of oxygen using a compact optical sensor

We report on the development of a novel optical oxygen sensor for breath monitoring applications using the technique of phase fluorometry. The principal design criteria are that the system be compact, lightweight, and employ a disposable sensing element (while performing competitively with current commercial analyzers). The oxygen-sensitive, luminescent ruthenium complex Ru[dpp](3)(2+) is encapsulated in a sol-gel matrix and deposited onto a custom-designed, polymer sensor chip that provides significantly improved luminescence capture efficiency. The performance of the sensor module is characterized using a commercially available lung simulator. A resolution of 0.03% O(2) is achieved, which compares well with commercial breath monitoring systems and, when combined with its immunity to humidity and ability to respond effectively across a broad range of breathing rates, makes this device an extremely promising candidate for the development of a practical, low-cost biodiagnostic tool.

Optical sensors for application in intelligent food-packaging technology

Modified Atmosphere Packaged (MAP) food employs a protective gas mixture, which normally contains selected amounts of carbon dioxide (CO2) and oxygen (O2), in order to extend the shelf life of food. Conventional MAP analysis of package integrity involves destructive sampling of packages followed by carbon dioxide and oxygen detection. For quality control reasons, as well as to enhance food safety, the concept of optical on-pack sensors for monitoring the gas composition of the MAP package at different stages of the distribution process is very attractive. The objective of this work was to develop printable formulations of oxygen and carbon dioxide sensors for use in food packaging. Oxygen sensing is achieved by detecting the degree of quenching of a fluorescent ruthenium complex entrapped in a sol-gel matrix. In particular, a measurement technique based on the quenching of the fluorescence decay time, phase fluorometric detection, is employed. A scheme for detecting CO2 has been developed which is compatible with the oxygen detection scheme. It is fluorescence-based and uses the pH-sensitive 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) indicator dye encapsulated in an organically modified silica (ORMOSIL) glass matrix. Dual Luminophore Referencing (DLR) has been employed as an internal referencing scheme, which provides many of the advantages of lifetime-based fluorometric methods. Oxygen cross-sensitivity was minimised by encapsulating the reference luminophore in dense sol-gel microspheres. The sensor performance compared well with standard methods for both oxygen and carbon dioxide detection. The results of preliminary on-pack print trials are presented and a preliminary design of an integrated dual gas optical read-out device is discussed.