Fluorescence sensors fluorescein-based

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],... 

The intrinsic sensors are based on the direct recognition of the chemicals by its intrinsic optical activity, such as absorption or fluorescence in the UV/Vis/IR region. In these cases, no extra chemical is needed to generate the analytical signal. The detection can be a traditional spectrometer or coupled with fiber optics in those regions. Sensors have been developed for the detection of CO, C02 NOx, S02, H2S, NH3, non-saturated hydrocarbons, as well as solvent vapors in air using IR or NIR absorptions, or for the detection of indicator concentrations in the UV/ Vis region and fluorophores such as quinine, fluorescein, etc. 

The structure of the fluorescent sensor Fluo-3 consists of BAPTA attached to a xanthene derivative as the fluorophore (Fig. 18.2). Many xanthene-based (sometimes referred to fluorescein-based) fluorescent sensors such as Fluo-3 have been reported. These compounds have an aromatic moiety that recognizes or reacts with the target at the 9-position of the xanthene ring. 

Fluorescent sensors have been widely investigated for the detection of many types of compounds and a particularly fruitful field is the selective detection of metal ions for example, a series (Zinpyr) of zinc sensors, based on fluorescein-containing attached chelating ligands, which can be used for quantitative determination of Zn or imaging of zinc-containing biological structures. ... 

Figure 2 shows emission intensity at 520 nm vs pH for excitation at 420 and 480 nm. The band excited at 480 nm increases in intensity as the pH goes from about 3.5 to 7.0. This is similar to that observed for previously reported pH sensors based on immobilized fluoresceinamine and is due to the formation of the highly fluorescent base form of fluorescein. 

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