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CHAPTER 6. OPTICAL AND FIBER OPTIC CHEMI... > 7. LUMINESCENCE (FLUORESCENCE)-BASED... - Pg. 343

OPTICAL AND FIBER OPTIC CHEMICAL SENSORS 343 7. LUMINESCENCE (FLUORESCENCE)-BASED SENSORS Luminescence (fluorescent) chemical sensors occupy a prominent place among the optical devices be- cause of their superb sensitivity (just a single photon is sometimes suffi cient for quantifying lumines- cence, compared to detecting the intensity difference between two beams of light as in absorption techniques) (Orellana 2006). Fluorescence spectra are composed of both excitation and emission com- ponents, which should in principle give this spectroscopic approach a great potential for selective analy- sis. For a detailed review of fluorescence spectroscopy, the reader is referred to Lakowicz (1982). However, as noted by Steinberg et al. (1994), because both excitation and emission bands are nor- mally broad with very little fine structure, it is diffi cult to use these bands to develop both sensitive and selective analytical methods. Therefore, most fluorescence-based methods require the additional use of separation and chromatography to obtain analytically useful selectivity. Thus, in many applications, lu- minescence analysis is at best a semiquantitative method. However, luminescence has a multidimensional character which can be manipulated to produce additional selectivity. This multidimensional character of luminescence measurements was reviewed by Warner et al. (1985), who discussed at least nine parameters that can be modulated to achieve additional selectivity with luminescence measurements. In addition to specific excitation and emission measurements, selectivity can be enhanced for fluorescence by utilizing matrix effects (micelles, surfaces, heavy atoms), temperature effects (Shpol'skii effect), fluorescence and phosphorescence lifetime filtering, and synchronous scanning. Selectivity can also be obtained by cou- pling fluorescence measurements to techniques such as column or thin-layer chromatography (TLC). Luminescence (fluorescent) chemical sensors may be based either on measurement of the intrinsic fluorescence of the target analyte or on the variation of the fluorescence of an indicator dye with the concentration of the determinand. In the latter case, the probe molecule can be immobilized onto a (thin) polymer support (sometimes even the waveguide surface itself ) and placed at the distal end or in the evanescent field of an optical fiber or integrated optic sensor. One of the variants of such sensors is shown in Figure 6.23. The mechanisms of luminescence (fluorescent) chemical sensors may also be different (Orellana 2006; Gansert et al. 2006). Quenching of the indicator dye fluorescence by the analyte itself or by an analyte-sensitive third party is one of the most widespread methods adopted to develop chemical optosensors. It is also possible to fabricate fluorescent sensors based on excited-state energy-transfer quenching. Effi cient deactivation of both singlet and triplet electronic excited states by molecular oxygen was the basis of the first fiber optic fluorosensors. Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g., the sample acidity, chloride, cations, anions, etc.) leads to a variation of the absorption band so that the effi ciency of energy transfers from the fluorophore changes. In this way, both emission intensity- and lifetime-based sensors may be fabricated. Photoinduced electron-transfer (PET) quenching processes can also be employed to develop fluorescence optosensors. Due to the electron promotion upon absorption of light, every electronic excited state is both a better oxidant and a better reducing agent than its corresponding ground state. Therefore, the analyte will undergo a photoredox reaction with the appropriate (luminescent) indicator dye (intermolecular quenching) or will interfere with an existing intramolecular PET quenching.