Fiber-optic sensors, use

Blum L.J., Gautier S.M., Coulet P.R., Highly stable bioluminescence-based fiber-optic sensor using immobilized enzymes from Vibrio harveyi, Anal. Lett. 1989 22(10) 2211-2222. 

Most present-day fiber-optic sensors use linear diode arrays combined with optical gratings and measure the absorption, transmission, fluorescence, and reflection in UV, visible, and NIR regions (see Table 3.1). Light travels to the sampling probe via one fiber-optic cable and returns to the instrument via a second. Laser excitation permits long-distance transmission of excitation radiation to get a useful signal from the sample. 

Capacitance or conductance measurement This method is applied where the working fluid acts as a capacitive or conductive element in a circuit (Jones et al., 1981). Use of fiber optics sensors has been developed recently (Moujaes and Dougall, 1987, 1990). These methods are used to measure film thickness in annular flow. Further discussion appears in Section 3.3.4.4. For other regimes, the use of the electrical impedience imaging method has also been introduced (Lin et al., 1991). 

Kopelman et al.73 have prepared fiber optic sensors that are selective for nitric oxide and do not respond to most potential interferents. Both micro-and nanosensors have been prepared, and their response is fast (<1 s), reversible, and linear up to 1 mM concentrations of nitric oxide. The respective "chemistry" at the fiber tip was contacted with the sample, light was guided to the sample through the microfiber, and emitted light was collected by a microscope (without the use of fibers, however). 

Petersons pH probe also was modified in order to give a miniature fiber optic sensor potentially suitable for glucose measurements90. Kopelman et al.91 developed a fiber-optic pH nanosensor for physiological measurements using a dual-emission sensitive dye. The performance of a pH sensor was reported92. An unclad fiber was dip-coated with a thin layer of porous cladding within which a pH-sensitive dye was entrapped. The fundamental... 

A novel fiber optic sensor concept using antibody-antigen reactions at a glass-liquid interface was reported by Daehne146. The reaction of antibodies immobilized onto the surface of fused silica fiber optic or planar waveguides with antigens in solution was detected by interaction with the evanescent wave. By detecting in-line fluorescence, the measurement of human IgG is described. 

Figure 4. Luminescence decay profile of an oxygen indicator dye excited by a short flash of light, in (a) solution and (b) embedded into a gas-permeable film used to fabricate fiber-optic sensors for such species. The logarithmic scale of the Y-axis allows to compare the exponential emission decay in homogeneous solution and the strongly non-exponential profile of the photoexcited dye after immobilization in a polymer matrix.

Identical olfactory neurons are located in different places in the cavity, and therefore occupy different positions in the flow path. By using a nasal cavity model, we investigated the influence of the dynamic flow on the sensors response14. The responses from identical fiber optic sensors located... 

Fig. 20a. 1. Schematic of an optical sensor using optical fibers as waveguide.

Newer techniques for measuring the refractive index allow for instantaneous, real-time measurement in process streams, or alternatively, a special continuous-flow sample well can be installed on bench top instruments. Small, pocket-sized refractometers also make held measurement very simple and reliable. Fiber optic sensors find uses in biomedical applications. 

A fiber optic sensor for the determination of sodium was reported by Burgess.<52) A bifurcated fiber with a reference fiber 5 mm apart from the tip was used to observe the changes of bromothymol blue (Amax = 620 nm) attached to Nafion in the presence of sodium ions. As the tip was saturated, the probe was renewed with fresh reagent. However, the epoxy holding the fibers was prone to damage from high sodium concentrations of around 2.5 M and the sensitivity of analysis was low. 

Figure 9.16. Performance of alexandrite based real time optical temperature sensors versus standard (Neslab RTE-J l IM) Equation 9.107 is used to obtain a working relation between Temperature and r. The fiber optic sensor monitored the bath temperature (—) in equilibrium with the standard (-).

It is already over two decades since the first concepts of the use of fiber optic techniques for sensor purposes were discussed. The initial drive for the development of fiber optic sensors came from their potential use in military and aerospace applications where the cost factors of the introduction of new technology were less rigid and the working environment more hostile than is experienced with other areas of application. 

The primary reason for interest in fiber optic sensors, in most cases, stems from the fundamental differences between the use of optical fiber and a metal wire for signal transmission.(2) These differences give fiber optic sensors the following advantageous characteristics. 

Perhaps the first detailed discussion of such a technique in fluorescent thermometry (shown in Figure 11.10) was given by Zhang et al. in their work(36) based on both mathematical analysis and experimental simulation. Examples of the electronic design of the corresponding system and the application of the technique in a ruby fluorescence-based fiber-optic sensor system are also listed. This shows that there is no difference in the measurement sensitivity between a system using square-wave modulation and one using sinusoidal modulation. However, the former performs a little better in terms of the measurement resolution. 

Seo et al. (1999) used a planar optic biosensor that measures the phase shift variation in refractive index due to antigen binding to antibody. In this method, they were able to detect S. enterica serovar T) himurium with a detection limit of 1 x 10 cfu/ml. When chicken carcass fluid was inoculated with 20 cfu/ml, the sensor was able to detect this pathogen after 12 h of nonselective enrichment. A compact fiber optic sensor was also used for detection of S. T) himurium at a detection limit of 1 X 10" cfu/ml (Zhou et al., 1997, 1998) however, its efficacy with food samples is unproven. Later, Kramer and Lim (2004) used the fiber optic sensor, RAPTOR , to detect this pathogen from spent irrigation water for alfalfa sprouts. They showed that the system can be used to detect Salmonella spiked at 50 cfu/g seeds. An evanescent wave-based multianalyte array biosensor (MAAB) was also employed for successful testing of chicken excreta and various food samples (sausage, cantaloupe, egg, sprout, and chicken carcass) for S. T) himurium (Taitt et ah, 2004). While some samples exhibited interference with the assay, overall, the detection limit for this system was reported to be 8 x 10 cfu/g. 

Sampling with fiber optic sensors can be continuous if needed otherwise they can be operated discontinuously, with a lower duty cycle. These sensors could be used for laboratory-based or in situ applications. The cost of instrumentation for fiber optic systems should be 25,000 to 50,000. Sensors would need to be replaced periodically (several weeks to many months), depending upon their design. Sensors using fiber optic probes will be available within 5 years for some applications and within 10 years for some others. Sensors for pH, C02, and 02 are in development now new sensors should be capable of measuring from high concentrations down to 1 part per million for ions and organic materials. Basic research is still required for specific applications. 

Fiber optic sensors are an alternative to thermocouples as embedded temperature distribution mapping sensors. As described in Section 2.2.7, McIntyre et al.104 developed two distinct fiber optic temperature probe technologies for fuel cell applications (free space probes and optical fiber probes). Both sensor technologies showed similar trends in fuel cell temperature and were also used to study transient conditions. 

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