Optical sensors based on fluorescence

Ruorescence is better adapted to analysis by optical sensors than absorption. Only a single fiber is used because the wavelength of emission is different from that of excitation. Furthermore, fluorescence is a sensitive technique that can detect voy low concentrations. It has the furtlKT advantage of having a signal that is linear as a function of low concentrations. 

A pH optical sensor can be constructed to measure the fluorescence emitted by fluoresceinamine immobilized on the tip of an opfical fiber [10]. An argon laser excites the sample at 488 nm with a beam intensity of 1.5 W at the tip of the fiber. The response to pH is 

Other fluorescent colorants can be used to produce pH, pNHs, and pCX)2 optical sensors. One example is the trisodium salt of 8-hydroxy 1,3,6-pyrenetrisulfonic acid (called HOPS A or HPTS) whose basic form can be excited at 470 nm to emit fluorescence at 510 nm [203, 204]. The CX)2 optical sensor requires a silicone membrane to contain a bicarbonate solution, just like potentiometric pC02 electrodes. A further similarity with potentiometric electrodes is that the pH, pNHs and pC02 optical sensors serve as the bases for enzyme sensors. The appropriate enzymatic membrane is simply attached. The optical sensor for penicillin uses penicillinase to catalyse the transformation of penicillin into penicilloic acid. The acid is detectable with a pH optical sensor that uses fluorescein isothiocyanate (FITC) as a colored indicator [205]. 

The ammonia optical sensor also uses an indicamr with a fluorescent basic form. An optical biosensor for glutamate [206] is produced by immobilizing glutamate oxidase on this ammonia optical sensor. The enzyme catalyses the following reaction  

The determination of the ammonia produced gives a measure of the glutamate concentration present, which is possible between concentrations of 1 and 150 jM. 

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V and therefore the cell reaction occurs spontaneously. These cells are sometimes referred to as Mackereth sensors . Such galvanic sensors are also often termed fuel cell sensors and it is possible to measure either the resulting current or the cell voltage. In the former case, the term amperometric sensor in its widest definition is still correct, although the cell is fundamentally different from the usual Faradaic amperometric systems. Note that for the determination of atmospheric oxygen an alternative has become available in recent years in the form of optical sensors based on fluorescence. These sensors are very robust as they do not contain electrodes or a liquid phase and show very fast response times. 

Noire, M. H. Dureault, B. A ferrous ion optical sensor based on fluorescence quenching. Sens. Actuators B 1995, B29, 386-391. 

MacCraith, B.D., Mcdonagh, C.M., Okeefe, G., Keyes, T.E., Vos, J.G., Okelly, B., and Megilp, J.F. (1993) Fiber optic sensor based on fluorescence quenching of evanescent-wave excited ruthenium complexes in sol-gel derived porous coatings. Analyst, 118, 385-388. 

Huber C., Krause C., Werner T., Wolfbeis O.S., Serum chloride optical sensors based on dynamic quenching of the fluorescence of photo-immobilized lucigenin, Microchimica Acta 2003 142 245-253. 

Whitten D, Jones R, Bergstedt T, McBranch D, Chen L, Heeger P (2001) From superquenching to biodetection building sensors based on fluorescent polyelectrolytes. In Ramamurthy V, Schanze KS (eds) Optical sensors and switches. Marcel Dekker Inc, New York... 

K. S. Litwiler, P. M. Kluczynski, and F. V. Bright, Determination of the transduction mechanism for optical sensors based on rhodamine 6G impregnated perfluorosulfonate films using steady-state and frequency-domain fluorescence, Anal Chem. 63,797-802(1991). 

An ideal sensor recognizes analytes in a sensitive, selective, and reversible manner. This recognition is in turn reported as a clear response. In recent years, conducting polymers have emerged as practical and viable transducers for translating analyte-receptor and nonspecific interactions into observable signals. Transduction schemes include electronic sensors using conductometric and potentiometric methods and optical sensors based on colorimetric and fluorescence methods [1]. 

The fluorescence response of an optical sensor based on a PVC matrix liquid membrane containing receptor 22 and ionic sites [bis(2-ethylhexyl) sebacate... 

The reflecting surface can be a mirror or a membrane with a light-scattering surface. In any case, the sensor has the appearance of a monolithic probe (i.e., a dip-stick probe). Optical sensors based on absorption, fluorescence, phosphorescence, and luminescence can employ such a configuration. Various highly optimized fiberoptic probes for UV-Vis, NIR, and IR ranges are now commercially available, and their designs are shown in Fig. 9.23. 

So far, the selection of polymers for chemical sensors based on fluorescent dyes in polymer matrices has been largely empirical, based on the accomplishment by the polymer of a number of requisites that are desirable for any support aimed to be used in optical sensing. These features are the following ... 

This chapter deals with the different separation mechanisms of chiral discrimination which are applied for optical sensors. Several types of optical sensors based on enrichment of analyte molecules in thin polymer films and fluorescence sensors were introduced for sensing of enantiomers in gaseous and aqueous media. 

Fiber-optic sensors based on controlled-release polymers provide sustained release of indicating reagents over long periods. This technique allows irreversible chemistries to be used in the design of sensors for continuous measurements. The sensor reported in this paper is based on a fluorescence energy transfer immunoassay. The sensor was cycled through different concentrations of antigen continuously for 30 hours. 

One of the most important biomolecules for which fluorescent sensing [94-96] is of great importance is nitric oxide [97-101]. Nitric oxide can react with several organic dyes, switching on their fluorescence as a result of a triazole ring closure reaction [94], There are also useful and selective NO optical sensors based on transition metal complexes (Figures 16.22 and 16.23) [94-96],... 

Baleizao C, Nagl S, SchaferUng M, Berberan-Santos MN, Wolfbeis OS (2008) Dual fluorescence sensor for trace oxygen and temperature with unmatched range and sensitivity. Anal Chem 80 6449-6457 Barone PW, Baik S, HeUer DA, Strano MS (2005) Near-infrared optical sensors based on single-walled carbon nanotubes. Nat Mater 4(l) 86-92... 

Optical sensors based on UV-visible and fluorescence spectrophotometry and a visual color change in a material are other directions for mesoporous silicates application (Melde et al. 2008). Several examples of such sensors are presented in Table 8.2. Usually optical detection of gases in mesoporous silica-based sensors takes place through the use of an incorporated dye. In particular, oxygen sensing... 

Posch H, Wolfbeis OS (1988) Optical sensors. Part 13 fiber-optic humidity sensor based on fluorescence quenching. Sens Actuators 15 77-83... 

A fiber-optic pH sensor based on fluorescence energy transfer can be construeted by coimmobilizing a pH-sensitive fluorophore and apH-sensitive absorber. For example, eosin (donor) and phenol red (acceptor) were coimmobilized in a polymer on the distal end of a silanized single-core optical fiber. Eosin s emission spectrum overlaps with the absorption of the basic form of phenol red. The concentration of the basic form of phenol red increases with an increase in pH. As a result, energy transfer from eosin to phenol red increases and the fluorescence intensity of eosin decreases. Thus, the pH-dependent absorption change of phenol red can be detected as changes in the fluorescence signal of eosin. 

Optical biosensors based on fluorescence detection often use the combination of a fluorescent bioreceptor associated with an optical transducer. Fluorescent biosensors may also be obtained by immobilizing whole cells on the surface of a sensor layer. This bioactive layer is usually placed in front of the tip of an optical fibers bundle to generate a fluorescent signal. The optical fibers are required to send the excitation radiation to the fluorescent bioelement and convey the fluorescence radiation up to a fluo-rimeter. In order to improve the simpHcity and reliability of fluorescence-based biosensors, optically translucent supports are used because their optical properties enable detection of fluorescence emitted by the algal cells. 

Optical sensors based on sol-gel coatings have been developed since the early 1990s [157], The working principle of the optical sensors is based on changes in absorbance or fluorescence [158] of the coating in the presence of the analyte. The specific features of sol-gel glasses in terms of porosity, permeability, and optical quality make this type of materials especially appropriate for the development of optical sensors [159]. 

Sol-gel optical sensors were developed for many different applications the determination of pH [160], measuring the absorbance of common pH indicators immobilized in thin-film silica matrices [161,162] or as a coating in fiber-optics, where the evanescent wave absorption is measured [163,164], and sensors based on fluorescent pH indicators in hybrid matrices [165,166] were also developed, showing excellent reproducibility and reversibility. Zirconia in hybrid films doped with methyl-red showed reversible response to acid and base vapors [167]. Silica-titania thin films with entrapped sensing molecules have been developed as pH sensors [168]. 

Figure 4.35 Glucose-sensitive optical sensor based on the competition between glucose ( ) and a fluorescent ligand (Hi) for concanavalin A attached to the detector walls.

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