Optical fibers covered with membranes

The membrane method (Sec. 6.5.2) has been described by Opitz et al. (1983) and in other publications (Wyzgol et al., 1989 Heinrich et al., 1990). Already in the first patent 

Such fibers should have the same properties as good IRE, e.g. sufficient transmission for IR radiation, insolubility in solvents, and chemical inertness. Further, they should have good mechanical properties (e.g. flexibility), they must be step index fibers, and it must be possible to remove the original cladding and to apply the sensitive one without any damage to the fiber. 

There are at this time no IR-fibers which fulfill all of these demands. The mainly used fibers are made of poly-crystalline silver halide (refractive index 2.2 at 10.6 pm) with very good transmission over the whole MIR (2 to 16 pm, see Fig.6.5-5), and good mechanical properties. The disadvantages are that they react with many commonly used metals and that they are sensitive to visible radiation, especially to UV radiation (Ceram optec, 1991). 

Other possible types of fibers are the chalcogenide glass fibers (e.g. Ge-As-Se) or heavy-metal fluoride fibers. Their transmission region is limited (2 to 10 pm. Fig. 6.5-5) and they are very fragile if the original cladding is removed. 

The covering of the fibers with an enriching membrane is more complicated but also possible with the same procedures as used for normal IRE. 

---

The actual temperature increase is more moderate because the BP filter head is covered with the collection fiber, another metal tube, and the protection coat. We also tried to deposit a dielectric coating directly on the distal end of the optical fiber. Several dozen fibers were held separately in a holder that was inserted into the deposition chamber and the filter was deposited with the usual process. In this experiment, the filter was successfully deposited on the end face of the fibers, but its properties were not constant. This seems to be ascribable to a localized thermal irregularity in the deposition chamber, which affected the temperature of the end face of the individual fibers. The low quality of the deposited filter at the circumference of the fiber end was also a problem. It seems that the layer structure of the filter membrane at the edge of the fiber end face was in disorder and this portion got into the core part. The thickness of the cladding was only 5.5 pm in our fiber. It may be possible to avoid this problem by employing a thicker fiber. 

The Fiber optic immnnosensors are the second major generation of biosensors. Many of the biosensors are a mini version of the classical spectrophotometry. The most freqnently nsed is the fiber optic chain or membrane, covered with a biological element. The optical biosensors are only optical sensors where the element to be detected is of biological origin. This provides greater specificity for the analyte. 

The principle of the fiber-optic pH sensor led Vurek et al. [133] to devise a CO2 sensor. Instead of coupling the pH indicator dye to an insoluble polymer, a simple isotonic solution of salt, hydrogencarbonate, and dye was used, which was covered with a C02-permeable silicone-rubber membrane. The sensor s performance was demonstrated in vivo. Similarly, a fluorescein-based C02-sensitive system was reported by Hirschfeld et al. [134]. 

---