Analytical probes, fiber optic fluorescent

The sample cells for molecular fluorescence are similar to those for optical molecular absorption. Remote sensing with fiber-optic probes (see Figure 10.30) also can be adapted for use with either a fluorometer or spectrofluorometer. An analyte that is fluorescent can be monitored directly. For analytes that are not fluorescent, a suitable fluorescent probe molecule can be incorporated into the tip of the fiber-optic probe. The analyte s reaction with the probe molecule leads to an increase or decrease in fluorescence. 

In particular optical sensing systems belong to this group, which are described in detail in Chapter 27. For example, fluorescent reporter groups are incorporated into the MIP, the properties of which are altered upon analyte binding [14-16]. A very sensitive sensor for a hydrolysis product of the chemical warfare agent Soman has been described based on a polymer-coated fiber optic probe and a luminescent... 

The wetted fiber types listed in Table 3 have been based on multimode fibers or fiber array bundles. In Fig. 4, the large diameter (1,000 pm) multimode cores are acid etched or stretched to create a sharp probe tip. In either case, the fiber surface is treated for binding proteins or other molecular probes. Walt et al. [7] (Table 3,5th row) developed a chip using a wetted fiber-optic array of thousands of individual fibers in a coherent bundle. By etching the core at the fiber bundle s tip, an array of micro wells was created and loaded with microspheres. The microspheres are coated with various sensing materials such as antibodies. Resulting changes in fluorescence intensity correspond to analyte concentration. 

Fig. 5 - Displacement of the fluorescent probe FHMI by the imidazolinone and non-imidazolinone compounds, (left) Chemical structures of three imidazolinone class compounds and three non-imidazoline agrochemicals used to demonstrate the specificity of the fiber optic sensor, (right) Reduction of steady-state fluorescence by the various analytes. A 25 nM FHMI in casein/PBS solution was perfused to reach a steady state before a solution of 1 jlM compound was added to the FHMI, casein/PBS. Plot 1, imzamethabenz methyl 2, imazapyr 3 imzaaquin 4, chlorimuron ethyl 5, primisulfuron 6, sethoxydim. Reproduced with permission from reference 2, Copyright 1993 American Chemical Society.

Biosensor Probes. For the fiber optic biosensor used here, a portion of protective cladding on the exterior of the optical fiber is removed from the distal 10 cm of the fiber to expose a core of fused silica. This exposed region becomes the probe. Antibodies are covalently attached to the exposed core. When the probe is in contact with a sample containing an analyte, the immobilized antibody specifically binds the analyte from the bulk solution and concentrates it on the surface of the fiber within the evanescent zone. Any fluorophore associated with the analyte is also immobilized within the evanescent wave. Excitation of the fluorophore by light in the evanescent wave leads to fluorescent emission which generates a detectable signal. Two different methods of associating a fluorophore with the analyte are described below. 

Fiber-optic biosensor can be potentially used for clinical sample analysis. As described above, by using enzymes and antibodies immobilized on the fiber tip or around the fiber core (evanescent field), different fiber-optic biosensors have been developed. In most cases, the measurement is based on changes in fluorescence intensity. The target analytes include glucose, cholesterol, enzymes, antibodies, bacteria, and viruses. Measurements are usually fast and simple, and in many devices, the probe is disposable however, the instability of the biological recognition materials reduces the sensor lifetime. 

Fang, XH and Tan, WH, Imaging single fluorescent molecules at the interface of an optical fiber probe by evanescent wave excitation. Analytical Chemistry 71 (1999) 3101-3105. 

---