Redox potential, fluorescent sensors

The Bronsted acid/base pair pyridinium/pyridine is unusual because redox potentials are available for both partners in an accessible range [35]. This allows detailed anticipation of the sensory behaviour of systems containing pyridine receptors. (49), whose fluorescence properties have received some attention before [113], is a member of a family of PET sensors for protons whose excitation and emission wavelengths spread over a substantial fraction of the accessible optical spectrum without compromising sensory action [114]. 

Figure 30 A two-component fluorescent sensor of the redox potential. The Ni" derivative quenches the emission of the nearby dansyl subunit via a fluorophore-to-metal eT process, the Ni" derivative does not. The Ni /Ni redox couple potential is 0.08 V vs. Fc /Fc. When containing a redox agent with a potential lower than 0.08 V, the solution will be fluorescent If an oxidizing agent with a potential higher than 0.08 V is present, the solution will not emit any more.

Fluorescent proteins can be used as mono-chromophore sensors by exploiting the pH and halide-sensitivity of these proteins [70,96-99]. In general, in FPs moderate acidification leads to reversible quenching of fluorescence. This pH sensitivity varies between different mutants and can be used to measure the ambient pH. Successful engineering of fluorescent proteins to render them sensitive to other parameters such and Zn2+, redox potentials or Ca2+ has also been reported [76,100]. 

It was found, however, that metallo-complexes that contain transition metal centers are especially convenient for fluorescence sensors as they undergo fast and kinetically uncomplicated one-electron redox changes (Bergonzi et al. 1998). Moreover, transition metals tend to participate in luminescence quenching of both the electron transfer, eT, and energy transfer, ET, varieties (see Fig. 10.5). Finally, the potential of the metal-centered redox couple can be modulated by varying the nature of the hosting... 

Nitrite Nitrite is an important indicator of fecal pollution in natural waters as well as a potential precursor of carcinogenic species. A rush of flow and sequential injection spectrophotometric method based on Griess-type reactions has been proposed, also coupled to online sorbent enrichment schemes. The catalytic effect of nitrite on the oxidation of various organic species constitutes the basis of fairly sensitive spectrophotometric methods. Fluorometric methods based on the formation of aromatic azoic acid salts, quenching of Rhodamine 6G fluorescence, and direct reaction with substituted tetramine or naphthalene species have been also reported. Indirect CL methods usually involve conversion into nitric oxide and gas-phase detection as mentioned in the foregoing section. The redox reaction between nitrite and iodide in acidic media is the fundamental of a plethora of flow injection methodologies with spectrophotometric, CL, or biamperometric detection. New electrochemical sensors with chemically modified carbon paste electrodes containing ruthenium sites, or platinum electrodes with cellulose or naphthalene films, have recently attracted special attention for amperometric detection. 

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