Localized fluorescence response

Figure 8 Localized fluorescence response to a 10 V step DC field. The central tree-like pattern is a fluorescein-stained water pathway extending from the top metallization line toward a bottom line not shown in the photograph.

Fluorescence in Sinusoidal Electric Fields. Fluorescein is an indicator whose fluorescence intensity changes with pH. Consequently, it is reasonable to interpret fluorescence response to applied AC voltage in terms of electrochemical reactions that alter the local pH. This AC-response of local fluorescence intensity exhibits classic relaxation behavior. At high frequencies, electrochemical reactions do not proceed long enough during each half-cycle to produce any appreciable pH change consequently, specimens exhibit time-... 

The parameters of saturation curves depend on photophysical characteristics of molecules fluorophores, so that such characteristics can be extracted from these curves after resolving an inverse problem (Fadeev et al., 1999). This is a basement of nonlinear laser fluoiimetiy as a method for investigation of photophysical properties of CCXI. To solve the inverse problem, we should first calculate (either analytically or numerically) the theoretical saturation curves by using the fluorescence response formation model of an ensemble of fluorescent molecules under their excitation by laser radiation. In present work two models have been used the conventional model of fluorescence response formation and the model of localized donor-acceptor (LDA) pairs. 

Skin Photodynamic therapy causes selective destruction of abnormal cells by activating a photosensitizer in the presence of oxygen. Local phototoxic reactions and pain are the most common limiting adverse reactions. In a randomized comparison in healthy volimteers of the local phototoxic response to photodynamic therapy with 5-aminolevulinic acid 20% or methylaminolevulinate 160 mg/g cream, pain, detection of substance P, change in fluorescence intensity from before to 5 hours after cream application, and adverse reactions not related to local phototoxicity were studied [57 ]. Aminolevulinic acid and methylaminolevulinate caused equivalent frequencies of local adverse reactions, except for a higher frequency and extent of hyperpigmentation after exposure to aminolevulinic acid for 28 days. 

The dye is excited by light suppHed through the optical fiber (see Fiber optics), and its fluorescence monitored, also via the optical fiber. Because molecular oxygen, O2, quenches the fluorescence of the dyes employed, the iatensity of the fluorescence is related to the concentration of O2 at the surface of the optical fiber. Any glucose present ia the test solution reduces the local O2 concentration because of the immobilized enzyme resulting ia an iacrease ia fluorescence iatensity. This biosensor has a detection limit for glucose of approximately 100 ]lM , response times are on the order of a miaute. 

It is possible, however, that the electrochromic response of some styrylpyridi-nium probes, for example, RH421 (see Fig. 2), is enhanced by a reorientation of the dye molecule as a whole within the membrane. There is a steep gradient in polarity on going from the aqueous environment across the lipid headgroup region and into the hydrocarbon interior of a lipid membrane. Therefore, any small reorientation of a probe within the membrane is likely to lead to a change in its local polarity and hence a solvatochromic shift of its fluorescence excitation spectrum. Such a... 

Fig. 11 (a) Chemical structure left, 9 90°) and cation response right) of virtually decoupled probe 30 for Hg2+ and Ag+. Absorption and emission spectra of 30 in the absence (black, dotted line = fit of the CT emission LE = fluorophore-localized emission band) and presence (at full complexation) of Hg2+ red) and Ag+ blue) in MeCN fluorometric titrations of 1 with Hg2+ and Ag+ shown in the inset FEF (LE) determined from the integrated fluorescence intensity of the LE band, (b) Chemical structures of other virtually decoupled probes for Na+ (31), Pb2+ (32), and Ni2+ (33). For color code, see Fig. 3. (Adapted in part from [115], Copyright 2000 American Chemical Society)...

The success of fluorescent sensors can be explained by the distinct advantages offered by fluorescence detection in terms of sensitivity, selectivity, response time, local observation (e.g. by fluorescence imaging spectroscopy). Moreover, remote sensing is possible by using optical fibers. The great improvement in the sensitivity... 

Fluorescence is also a powerful tool for investigating the structure and dynamics of matter or living systems at a molecular or supramolecular level. Polymers, solutions of surfactants, solid surfaces, biological membranes, proteins, nucleic acids and living cells are well-known examples of systems in which estimates of local parameters such as polarity, fluidity, order, molecular mobility and electrical potential is possible by means of fluorescent molecules playing the role of probes. The latter can be intrinsic or introduced on purpose. The high sensitivity of fluo-rimetric methods in conjunction with the specificity of the response of probes to their microenvironment contribute towards the success of this approach. Another factor is the ability of probes to provide information on dynamics of fast phenomena and/or the structural parameters of the system under study. 

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