Other Fluorescent Sensors

Hayashita and Teramae have prepared an interesting fluorescent ensemble that consists of compound 38 and p-cyclodextrin [98], The system displays fluorescence enhancement on saccharide binding and, as expected for a monoboronic acid, the highest binding was observed with D-fructose. Observed fC pp for 38 were 2515 for D-fructose and 79 M for D-glucose in 2% (v/v) DMSO-water at pH 7.5. 

Lakowicz and Geddes have explored the use of several quaternized quinolium boronic acids (39a-f) for D-glucose monitoring within contact lenses [99]. 

Diboronic acids can bind monosaccharides selectively, where the 1 1 binding creates a rigid molecular complex [100-105]. This rigidification effect can also be utilized in designing fluorescent sensors for disaccharides. Sandanayake has investigated the binding of diboronic acid 40 (F igure 12.10) with disaccharides in basic aqueous media [106]. Excited stilbene is quenched by radiationless decay via rotation of flie ethylene double bond. Obstruction of this rotation leads to increased fluorescence 

L-ffuctose, 2100 for D-glucose and 1900 for L-glucose in 1% (v/v) methanol-water at pH 10.8 (carbonate buffer). 

Kijima et al. have developed a D-lactulose selective system (43) based on a diboron-ic acid porphyrin [110]. The spatial disposition of the two boronic acids in 43 produces the perfect cleft for the disaccharide D-lactulose. found for 43 was 560 for 

109 X=0CH3, meta 110 X=0CH3, ortho 111 X=0CH3, para 112 X=CH3, meta 113 X=CH3, meta 114 X=CH3, para 

Diboronic acids can bind monosaccharides selectively, where the 1 1 binding creates a rigid molecular complex. This rigidification effect can also be 

Takeuchi has used molecular rigidification of cyanine diboronic acid 116 to generate a fluorescence increase with added saccharides. The observed stability constants (ilobs) for 116 were 130,000 M for D-fructose and 1400 M for D-glucose in 1 1 (v/v) methanol/water at pH 10.0 (carbonate buffer). 

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Fluorophores are relative small molecules that, with some exceptions, are not naturally occurring and have to be synthesized chemically. There has been a large development in the synthesis of fluorescent molecules and nowadays there is a vast range of alternatives including dyes with improved photochemical properties, solubility or modified reactivity that allow for conjugation to other molecules of interest and the synthesis and application of fluorescent sensors [10, 13], Although a lot is known about the physics of fluorescence and a lot of information is available about the properties of dyes, their prediction from the chemical structures cannot be accurately done. For this reason, there has been a... 

The fourth type of mediator-based cation optical sensing is using potential sensitive dye and a cation selective ionophore doped in polymer membrane. Strong fluorophores, e.g. Rhodamine-B C-18 ester exhibits differences in fluorescence intensity because of the concentration redistribution in membranes. PVC membranes doped with a potassium ionophore, can selectively extract potassium into the membrane, and therefore produce a potential at the membrane/solu-tion interface. This potential will cause the fluorescent dye to redistribute within the membrane and therefore changes its fluorescence intensity. Here, the ionophore and the fluorescence have no interaction, therefore it can be applied to develop other cation sensors with a selective neutral ionophore. 

Most PET fluorescent sensors for cations are based on the principle displayed in Figure 10.7, but other photoinduced electron transfer mechanisms can take place with transition metal ions (Fabbrizzi et al., 1996 Bergonzi et al., 1998). In fact, 3d metals exhibit redox activity and electron transfer can occur from the fluorophore... 

System 22 is an earlier example which incorporates Tsien s selective calcium receptor 23. ° System 23 has also been employed for the construction of ratiometric fluorescent sensors involving wavelength shifts. System 22 and other related PET sensors provide some of the most visually dramatic fluorescence off-on switching induced by biologically relevant levels of calcium ions in addition to their consistent predictability of most sensor parameters. 

Fluorescence sensors have been used since 1957 to measure cell internal NAD(P)H at 450 nm. Later on they were applied for in situ determination of the cell concentration. However, the culture fluorescence intensity is not only influenced by the cell concentration, but also by the physiological state of the cells [56] and, in addition to that, there are several other compounds that participate in the fluorescence emission besides NAD(P)H. To identify the fluorophores in the cells and cultivation medium, the excitation and the emission wave lengths are varied in a broad range [57,58]. Two instruments were applied for the 2D-fluorescence spectroscopy Model F-4500 (Hitachi) and the BioView Sensor (Delta light Optics). Each of them uses an excitation range of 250-560 nm,an emission range of 260/300-600 nm and the measuring time of 1 min [59,60]. The application of this technique for CPC production was performed by Lindemann [61]. 

The last group of fluorescent sensors is based on neither photoinduced proton transfer nor photoinduced electron transfer. The best-known example of this kind of molecular device is fluorescein (Figure 16.2e). The evolution of the fluorescence spectrum versus pH should be similar to that of the absorption spectrum. In other words, when increasing the pH, the absorption and emission bands of the acidic form should decrease with a concomitant increase in the absorption and emission bands of the basic form [1],... 

Coordination of NO to the iron results in labilization of the quinoline moiety and fluorescence decrease [94], Other NO sensors are based on Fe [102], Mn [95], and Co [103-105] tropocoronand complexes. One example is shown in Figure 16.23. The tropocoronand ligands incorporate substituted dimethylaminonaphthalene sulphonate fluorophores. Free ligands exhibit strong fluorescence, which is largely... 

Owing to these imique properties, lanthanides have several advantages over traditional organic fluorophores, quantum dots, or other fluorescent species commonly used as sensors. These... 

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