Supramolecular function of fluorescent probe/cyclodextrin complex sensors in water

This paper reviewed the supramolecular function of fluorescent probe/cyclod extrin (CyD) complexes for ion and molecule recognition in water. Benzo-15- crown-5 fluoroionophores, Cn-15C5 (n = 1, 3, 5), with different alkyl space r lengths were first examined to develop supramolecular Cn-15C5/gamma -CyD complex sensors for alkali metal-ion recognition in water. In organic solut ions, C3-15C5 shows moderate Na+ selectivity based on 1 : 1 complex formati on. However, the C3-15C5/gamma -CyD complex is found to selectively respond to K+ ion in water and to exhibit pyrene dimer emission. An equilibrium an alysis of the gamma -CyD inclusion complexes in water reveals that the majo r component for the dimer emission is a 2 : 1 : 1 complex of C3-15C5 with K + and gamma -CyD. Although the K+ sensitivity of the C5-15C5/gamma -CyD com plex is comparable to that of the C3-15C5/gamma -CyD complex, it also respo nds to Na+. The fluoroionophore C1-15C5, which has the shortest methylene s pacer, exhibits no response to alkali metal cations in the presence of gamm a -CyD. Thus, the response function is strongly affected by the alkyl space r length of Cn-15C5, and the highest K+ selectivity in water is obtained fo r the C3-15C5/gamma -CyD complex. The boronic acid fluoroionophore C4-PB/be ta -CyD complex binds sugars and produces increase fluorescence emission in water. A pH-fluorescence profile for the C4-PB/beta -CyD complex reveals t hat the fluorescence intensity increases upon the formation of the boronate conjugate base. Upon the addition of fructose, the apparent pK(a) decrease s to a lower pH, resulting in increased fluorescence at neutral pH. The flu orescence emission response of the C4-PB/beta -CyD complex upon sugar bindi ng appears to be due to suppression of the photoinduced electron transfer ( PET) from the pyrene donor to the trigonal arylboronic acid acceptor.