Fiber-optic bundle sensors

Many fluorescence sensors are based, not on direct fluorescence, but on the quenching of fluorescence. Molecular oxygen, for example, is one of the best col-lisional quenchers. Oxygen can quench the fluorescence from polycyclic aromatic hydrocarbons complexes of ruthenium, osmium, iridium, and platinum and a number of surface-adsorbed heterocyclic molecules. An oxygen sensor can be made by immobilizing the fluorophore in a thin layer of silicone on the end of a fiber-optic bundle. Sensors for SO-, haliilcs, H-O-, and several other molecules have been ba.sed on fluorescence quenching. 

Figure 3. First fiber optic chemical sensor (from ref. [17] used for sensing oxygen). Alos shown is a cross-sedction of the fiber bundle used. 6 light source 9 photodectectors 16 chemically sensitive layer.

Fuh et al. (1988) devised an enzyme optrode for penicillin. (5-Lactamase was immobilized on a fluorescein isothiocyanate-labeled porous glass particle which was glued to the tip of a fiber optic bundle. Excitation was carried out by an argon laser. pH changes resulting from the enzyme reaction led to changes of the fluorescence intensity. The response time of the sensor was 20-45 s, the detection limit being 0.1 mmol/1 penicillin. 

The first step in preparing a sensor is to dissolve 100 mg of PVOH/indicator conjugate in 2.0 mL of water. Five minutes in a water bath at 30°C is required to get the PVOH to dissolve. After cooling to room temperature, 0.5 mL of PVOH/indicator solution is combined with 0.050 mLs each of 2% aqueous glutaraldehyde and 4 M HCl. This mixture can be manipulated as a liquid for about five, minutes until sufficient crnsslinking takes place to cause the solution to gel. During this interval a micropipet is used to precisely transfer 3 microliters to the common end of the bifurcated fiber optic bundle so that the gel forms in situ. 

This reaction produces p-nitrophenoxide which strongly absorbs 404 nm radiation. The sensor tip is constructed with alkaline phosphatase covalently immobilized on a nylon membrane. This membrane is positioned at the common end of a bifurcated fiber-optic bundle. One arm of this bundle is connected to the source optics and the other is connected to the detector optics. Incident radiation is transported from a 100 watt tungsten-halogen lamp source to the sensor tip. A fraction of this incident radiation is back scattered off the nylon mesh and a fraction of this back scattered radiation is collected by the fiber-optic bundle and directed to a 404.7 nm interference filter and then to a photomultiplier tube detector. 

Another interesting approach to fiber-optic sensor development has been reported by Walt and co-workers [114,115], who have created multiplexed extrinsicmode fiber optic microarray biosensor. The sensor employed fiber optic bundles (2-3 feet in length) composed of many optical fibers each 200-350 Lun in diameter. The microarray was fabricated by immobilizing a different oligonucleotide probe sequence onto the distal end of each fiber. Individual fibers within the bundle were made reactive for oligonucleotide immobilization by immersing the bundle in a solution of... 

FIGURE 5 Design principle of fiber-optic chemical sensors, (a) Two single fibers, (b) Two fiber bundles, (c) Bifurcated fiber. 

Figure 20-19 Fiber-optic sensor lor detection of specific DNA sequences. Upper half shows circular wells in etched tip of bundle. Fluorescence image in lower half identifies wells to which fluorescent target DNA has bound. [From J R. Epstein. M. Lee. and D. R. Waft High-Density Fiber-Optic Genosensor Microsphere Array Capable of Zofriomoto Detection limits, ...

Colorimetric and fluorimetric NH3 sensors contain mixtures of pH indicators having suitable dissociation constants at the tip of the fiber bundle. The measuring solution is separated from this indicator layer by an NH3 gas-permeable membrane covered by an immobilized de-aminating enzyme, e.g. urease (Wolfbeis, 1987 Arnold, 1987). The fluorimetric indication of NADH has been used in optical biosensors for lactate, pyruvate, and ethanol, where the respective dehydrogenase is immobilized at the tip of an optical NADH sensor (Arnold, 1987 Wangsa and Arnold, 1988). 

Fiber-optic sensors may also be combined to form a bundle of fiber applicable to the simultaneous sensing of, eg, physiological pH, oxygen, electrolytes, anesthetics, glucose, creatinine, temperature, and flow rate. This is possible because of the minute size of the fibers. There is considerable industrial activity in this direction. 

Another type of photoelectric sensor is the fiber optics sensor, which uses a bundle of transparent fibers of glass or plastic to conduct and guide light energy using the principle of total internal reflection, as depicted in Fig. 6. Fiber optics serve as light pipes to transmit the light from the source to the detector. With a small diameter and flexibility of the fibers to be... 

An optical-fiber CL sensor is reported for trichlorethylene assay [87], The sensor consists of a glass fiber bundle and a transducer consisting of three components (i) a gas-permeable membrane to separate trichlorethylene from water, (ii) H2S04-NaN03 mixture as oxidizing agent, and (iii) a luminol solution. The assay of trichloroethylene can be done in the 0.05-0.6- J,g/mL concentration range with a detection limit of 0.03 J.g/mL. 

Since the optical fibers are placed info locations fhat are difficult to access, the coatings are expected to last over 20 years. Tests indicated that UV cured protective coatings would be mechanically stable up to 100 years under ambient conditions. Radiation curable adhesives can also be used for end-to-end splicing, termination of bundles, construction of optical sensors, and other areas in the optical field. ... 

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