Abstract
This paper reviewed the supramolecular function of fluorescent probe/cyclodextrin (CyD) complexes for ion and molecule recognition in water. Benzo-15-crown-5 fluoroionophores, Cn-15C5 (n=1, 3, 5), with different alkyl spacer lengths were first examined to develop supramolecular Cn-15C5/γ-CyD complex sensors for alkali metal-ion recognition in water. In organic solutions, C3-15C5 shows moderate Na+ selectivity based on 1 : 1 complex formation. However, the C3-15C5/γ-CyD complex is found to selectively respond to K+ ion in water and to exhibit pyrene dimer emission. An equilibrium analysis of the γ-CyD inclusion complexes in water reveals that the major component for the dimer emission is a 2 : 1 : 1 complex of C3-15C5 with K+ and γ-CyD. Although the K+ sensitivity of the C5-15C5/γ-CyD complex is comparable to that of the C3-15C5/γ-CyD complex, it also responds to Na+. The fluoroionophore C1-15C5, which has the shortest methylene spacer, exhibits no response to alkali metal cations in the presence of γ-CyD. Thus, the response function is strongly affected by the alkyl spacer length of Cn-15C5, and the highest K+ selectivity in water is obtained for the C3-15C5/γ-CyD complex. The boronic acid fluoroionophore C4-PB/β-CyD complex binds sugars and produces increased fluorescence emission in water. A pH-fluorescence profile for the C4-PB/β-CyD complex reveals that the fluorescence intensity increases upon the formation of the boronate conjugate base. Upon the addition of fructose, the apparent pKa decreases to a lower pH, resulting in increased fluorescence at neutral pH. The fluorescence emission response of the C4-PB/β-CyD complex upon sugar binding appears to be due to suppression of the photoinduced electron transfer (PET) from the pyrene donor to the trigonal arylboronic acid acceptor.
