Change in Fluorescence

When a liquid crystal is reoriented in an external field the observed intensity of the fluorescence of impurity molecules dissolved in the liquid crystal is changed. For example, if the dye brilliant phosphine is dissolved in the nematic phase of p-n butoxybenzoic acid the intensity of its fluorescence increases by a factor of 3 when a field is applied [178]. The use of a fluorescent probe (stilbene dye) for investigating the kinetics of the Prederiks effect is described in [56]. Fluorescence polarization measurements allow us to obtain information on liquid crystal electronic spectra [179] and order parameter [180]. 

References [181, 182] demonstrated fluorescent displays with the possibility of switching between two colors [181] and optical shutters, switched by short electrical pulses [182] (less than 1 ms). The color of fluorescent displays is readable in the dark at a low level of ambient light, the displays possess excellent viewing characteristics at oblique viewing and are insensitive to any irregularity in cell thickness. Fluorescent displays can compete with the usual guest-host devices in some indoor applications. 

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Fig. 17. Changes in fluorescent background on changing the excitation wavelength. Raman spectra of o-xylene using different exciting lines (a) Ar+ 488 nm (b) Kr+ 647.1 nm (c) Ar+ 514.5 nm (d) Kr+ 568.2 nm. Fluorescent background was substantially reduced in spectrum (b). (Courtesy Spex Industries, Inc.)...

Connection between the reversible target binding and the change in fluorescence intensity can be easily established based on the mass action law. In the simplest case... 

Fig. 2 The changes in fluorescence decay kinetics on binding the analyte, (a) The analyte is the dynamic quencher. The decay becomes shorter gradually as a function of its concentration, (b) The analyte binding changes the lifetime. Superposition of decay kinetics of bound and unbound forms is observed...

The basis for the different response times of these probes is their response mechanism. In order to produce a change in fluorescence, a change in electric field must induce some movement either of the dye molecule as a whole or of its electrons. The degree of movement determines the speed of the fluorescence response. 

Zhang and Seitz somewhat later described a sensor for carbon dioxide that is based on measurement of fluorescence60. It was prepared by covering a pH sensor based on fluorescence with a C02-permeable membrane and contacting the pH-sensitive membrane with a reservoir of hydrogen carbonate. As CO2 diffuses across the membrane it causes a change in pH which is measured via the change in fluorescence from the base form of the... 

Figure 4. Synthesis of an indicator dye for amines which exhibits methacrylate groups for preparation of copolymers. The dye shows a reversible change in fluorescence from green to blue upon interaction with amphetamine.

In phase-fluorimetric oxygen sensors, active elements are excited with periodically modulated light, and changes in fluorescence phase characteristics are measured. The delay or emission (phase shift, ( ), measured in degrees angle) relates to the lifetime of the dye (x) and oxygen concentration as follows ... 

Measurements of binding curves without influencing the equilibria can be performed if the readout for complex formation is correlated with a change in a macroscopic signal. This can be either a change in fluorescence intensity, fluorescence polarization, optical absorption, or heat of association (see next chapter). Assume an equilibrium... 

Cornea, A. and Michael Conn, P. (2002) Measurement of changes in fluorescence resonance energy transfer between gonadotropin-releasing hormone receptors in response to agonists. Methods 27, 333. 

Cross-reactive sensing arrays were developed to detect odors and vapors in an artificial nose manner. Solvatochromic dyes such as Nile Red are adsorbed on the surface or embedded into various polymeric or porous silica beads. The beads respond to analyte vapor by a change in fluorescence maxima or/and intensity due to changes of polarity inside the bead. A portable instrument and preliminary field test for the detection of petroleum products was recently described [106]. 

Figure 10.14 shows examples of chelating PET sensors. PET-12 to PET-14 are selective for calcium. In PET sensors, the changes in fluorescence quantum yield are accompanied by proportional changes in excited-state lifetime. Therefore, compounds PET-12 to PET-14 were found to be suitable for fluorescence lifetime imaging of calcium. 

In calixarene-based compound M-8 (Figure 10.28), bearing four anthracene moieties on the lower rim, some changes in fluorescence intensity were observed on binding of alkali metal ions but no excimer emission was detected. Quenching of the fluorescence by Na+ may arise from interaction of four anthracene residues brought in closer proximity to one another enhancement of fluorescence by K+ is difficult to explain. 

A distinct advantage of PET sensors is the very large change in fluorescence intensity usually observed upon cation binding, so that the expressions off-on and on-off fluorescent sensors are often used. Another characteristic is the absence of shift of the fluorescence or excitation spectra, which precludes the possibility of intensity-ratio measurements at two wavelengths. Furthermore, PET often arises from a tertiary amine whose pH sensitivity may affect the response to cations. 

Fluctuations in fluorescence intensity in a small open region (in general created by a focused laser beam) arise from the motion of fluorescent species in and out of this region via translational diffusion or flow. Fluctuations can also arise from chemical reactions accompanied by a change in fluorescence intensity association and dissociation of a complex, conformational transitions, photochemical reactions (Figure 11.10) (Thompson, 1991). 

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