CDs in luminescence spectroscopy

Molecular luminescence spectrometry, especially molecular fluorescence spectrometry, has become estabhshed as a routine technique in many analytical applications. In many cases, molecular luminescence spectrometry can yield a lower detection limit and greater selectivity than molecular absorption spectrometry. However, although most compounds exhibit strong fluorescence or 

Inclusion complex formation with CDs usually results in a higher fluorescence quantum yield or the lifetimes of the excited states. It has been found that the 

The fluorescence intensity of naphthalene in aqueous solution decreases upon aeration. However, the quenching of naphthalene by aeration is totally suppressed in the presence of a water-soluble sulfopropylated fi-CD [70]. Similarly, the quenching of halonaphthalene phosphorescence in water by NaN02 can be substantially inhibited by j8-CD. The rate of inhibition depends on the bond-tightness between the analyte and the CD. Retinal, which is normally insoluble in water and is not fluorescent in solution at room temperature, emits luminescence in the region of 450 nm and permits fluorescence detection when incorporated by j8- or y-CD even in air-saturated aqueous solution [71]. In the luminescence detection of volatile compounds, CDs can be used as solid matrices which efficiently trap the volatile compounds for obtaining room temperature 

The CDs also can be used to enhance chemiluminescence of the luminol related compounds. It was foimd that CDs were capable of increasing the light output by factors up to 300 in aqueous peroxyoxalate solution. The enhancement could be attributed to increases in reaction rate, excitation efficiency and fluorescence efficiency of the emitting species. 

In most cases, the present of CDs will enhance the luminescence. However, CDs can also selectively quench the luminescence of some compoimds. A study of the effect of /3-CD on the fluorescence of xanthene dyes, coumarins and pyromethene-difluoroboron complexes in aqueous solution shows that fi-CD enhances the fluorescence of 7-hydroxycoumarin and coumarins, but quenches the fluorescence of the 7-hydroxy-4-methylcoumarins [73]. This behavior of CDs provides a new approach to multicomponent fluorometric analysis and indicates that CDs can be used for differentiating the structures of similar compounds such as the positional isomers by the selective incorporation of the analyte. 

---