Fluorescent chemical sensors

Fluorescent chemical sensors occupy nowadays a prominent place among the optical devices due to its superb sensitivity (just a single photon sometimes suffices for quantifying luminescence compared to detecting the intensity difference between two beams of light in absorption techniques), combined with the required selectivity that photo- or chemi-luminescence impart to the electronic excitation. This is due to the fact that the excitation and emission wavelengths can be selected from those of the absorption and luminescence bands of the luminophore molecule in addition, the emission kinetics and anisotropy features of the latter add specificity to luminescent measurements8 10. 

In conclusion, the advantages of microfluidic devices, parallel synthesis, and combinatorial approaches can be merged to integrate a fluorescent chemical sensor array in a microfluidic chip. Fluorescent microchannel array can be produced by parallel synthesis of fluorescent monolayers covalent attached to the walls of glass microchannels. 

It was shown that polymeric matrices have their sensing response stability over several years.911 However, in fluorescent chemical sensors, photobleaching of matrix-immobilized organic dyes significantly hinders the applicability of such... 

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

Fig. 5 shows the instrumental arrangement of the commercially most successful optical chemical sensor between 1984 and 2000. It is used in about 70% of all critical care operations in the US to monitor pH, pC02 and p02 in the cardiopulmonary bypass operations35. It contains 3 fluorescent spots, each sensitive for one parameter, in contact with blood. Fluorescence intensity is measured at two wavelengths and the signals are then submitted to internal referencing and data processing. 

The Stem-Volmer equations discussed so far apply to solutions of the luminophore and the quencher, where both species are homogeneously distributed and Fick diffusion laws in a 3-D space apply. Nevertheless, this is a quite unusual situation in fluorescent dye-based chemical sensors where a number of factors provoke strong departure from the linearity given by equation 2. A detailed discussion of such situations is beyond the scope of this chapter however, the optosensor researcher must take into account the following effects (where applicable) ... 

Lubbers D.W., Fluorescence Based Chemical Sensors, in Advances in Biosensors, Vol. 

Munkholm C., Walt D.R., Milanovich F.P., Klainer S.M., Polymer modification of fiber optic chemical sensors as a method of enhancing fluorescence signal for pEl measurement, Analytical Chemistry 1986 58 1427-1430. 

As opposed to conventional analytical techniques, optical sensors and biosensors, particularly those employing absorption and fluorescence-based sensing materials potentially allow for measurement through transparent or semi-transparent materials in a non-destructive fashion4, 5> 9 10. Chemical sensor technology has developed rapidly over the past years and a number of systems for food applications have been introduced and evaluated with foods. 

L. Chen, D.W. McBranch, H.-L. Wang, R. Helgeson, F. Wudl, and D.G. Whitten, Highly sensitive biological and chemical sensors based on reversible fluorescence quenching in a conjugated polymer, Proc. Natl. Acad. Sci. USA, 96 12287-12292, 1999. 

Thomas SW III, Joly GD, Swager TM (2007) Chemical sensors based on amplifying fluorescent conjugated polymers. Chem Rev 107 1339-1386... 

Wolfbeis OS (2005) Materials for fluorescence-based optical chemical sensors. J Mater Chem 15 2657-2669... 

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.). 

Another distinction should be made (independently of the fluorescence aspects) between chemical sensors (also called chemosensors) and biosensors. In the former, the analyte-responsive moiety is of abiotic origin, whereas it is a biological macromolecule (e.g. protein) in the latter. 

Fluorescent pH indicators offer much better sensitivity than the classical dyes such as phenolphthalein, thymol blue, etc., based on color change. They are thus widely used in analytical chemistry, bioanalytical chemistry, cellular biology (for measuring intracellular pH), medicine (for monitoring pH and pCC>2 in blood pCC>2 is determined via the bicarbonate couple). Fluorescence microscopy can provide spatial information on pH. Moreover, remote sensing of pH is possible by means of fiber optic chemical sensors. 

From a general point of view, a chemical sensor is a device capable of continuously monitoring the concentration of an analyte. The two main classes are electrochemical sensors and optical chemical sensors. The latter are based on the measurement of changes in an optical quantity refractive index, light scattering, reflectance, absorbance, fluorescence, chemiluminescence, etc. For remote sensing, an optical fiber is used, and the optical sensor is then called an optode because of... 

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