Analytical fluorescent sensors

It has been demonstrated that dendrimers can be used also as fluorescent sensors for metal ions. Poly(propylene amine) dendrimers functionalized with dansyl units at the periphery like 34 can coordinate metal ions by the aliphatic amine units contained in the interior of the dendrimer [80]. The advantage of a dendrimer for this kind of application is related to the fact that a single analyte can interact with a great number of fluorescent units, which results in signal amplification. For example, when a Co ion enters dendrimer 34, the fluorescence of all the 32 dansyl units is quenched with a 32-fold increase in sensitivity with respect to a normal dansyl sensor. This concept is illustrated in Fig. 3. 

Zourob, M. Mohr, S. Fielden, P. R. McDonnell, M. B. Goddard, N. J., Small volume refractive index and fluorescence sensor for micro total analytical system (p TAS) applica tions, Sens. Actuators B 2003, 94, 304 312... 

The design of fluorescent sensors is of major importance because of the high demand in analytical chemistry, clinical biochemistry, medicine, the environment, etc. Numerous chemical and biochemical analytes can be detected by fluorescence methods cations (H+, Li+, Na+, K+, Ca2+, Mg2+, Zn2+, Pb2+, Al3+, Cd2+, etc.), anions (halide ions, citrates, carboxylates, phosphates, ATP, etc.), neutral molecules (sugars, e.g. glucose, etc.) and gases (O2, CO2, NO, etc.). There is already a wide choice of fluorescent molecular sensors for particular applications and many of them are commercially available. However, there is still a need for sensors with improved selectivity and minimum perturbation of the microenvironment to be probed. Moreover, there is the potential for progress in the development of fluorescent sensors for biochemical analytes (amino acids, coenzymes, carbohydrates, nucleosides, nucleotides, etc.). 

At present, most fluorescence sensors or assays are based on intensity measurements, i.e., intensity-based sensing, in which the intensity of the probe molecules change in response to the analyte of interest. Intensity-based methods are initially the easiest to implement because many fluorescent probes change intensity in response to analytes. These intensity changes can be due to changes in extinction coefficient due to probe ionization, changes in quantum yield of the probe on analyte binding, or due... 

The opportunities for near-IR fluorescence sensors are of course not only limited to analytical chemistry. Physical parameters such as temperature can also be measured. For example, Grattan and Palmer have used the fluorescence lifetime quenching of neodymium glass fluorescence at 1054 nm, excited at 810 nm with a gallium-alumi-... 

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. 

The quantitation of undesired enantiomers in drug raw materials is one of the objectives of the pharmaceutical industry. Several calixarene derivatives were investigated as fluorescence sensors for chiral pharmaceutical compounds. The mechanism of these examples is based on different fluorescence quenching of the calixarenes by the two enantiomeric forms of a specific analyte. 

Figure 16.25 Semiconductor nanoparticle-based fluorescent sensors (a) Forster resonant energy transfer (FRET) between two nanoparticles induced by analyte, (b) crown ether receptor for potassium ions, and (c) operation principle of maltose fluorescent sensor. (Adapted from Chen et at. [144] and Medintz et at. [146])...

Both empirical and rational methods have been successful in developing novel fluorescent sensors. However, on the one hand, empirical design and synthesis may require considerable trial and error. On the other hand, the rational design approach described above is limited to analytes that can sufficiently change the oxidation or reduction potential. Further, even in the case of theoretically designed molecules, the fluorescence properties may be unexpectedly influenced by environmental factors. The construction of libraries of fluorescent molecules is one way to overcome some of these problems in the development of novel fluorescent sensors. 

Hale Z M and Payne F P 1994 Fluorescent sensors based on tapered single-mode optical fibres Sensors Actuators 17 233 0 Luo S and Walt D R 1989 Avidin-biotin coupling as a general method for preparing enzyme-based fiber-optic sensors Anal. Chem. 61 1069-72 Li L and Walt D R 1995 Dual-analyte fiber-optic sensor for the simultaneous and continuous measurement of glucose and oxygen Anal. Chem. 67 3746-52 Tan W, Shi Z-Y, Smith S, Bimbaum D and Kopelman R 1992 Submicrometer intracellular chemical optical fiber sensors Science 258 778-81 Tan W, Shi Z-Y and Kopelman R 1992 Development of submicron chemical fiber optic sensors Anal. Chem. 64 2985-90... 

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