Cyclodextrin supramolecular complex as a water-soluble ratiometric sensor for ferric ion sensing.

Heightened concern for human health and environmental protection has stimulated active research on the potential impact of transition-metal ions and their toxic effects, thus it is very demanding to design transition-metal ion detection methods that are cost-effective, rapid, facile, and applicable to the environmental and biological milieus. In this study, we demonstrated an alternative strategy for constructing a water-soluble FRET-based ratiometric sensor for ferric ion detection by forming a supramolecular beta-cyclodextrin/dye complex. This water-soluble FRET system consists of a dansyl-linked beta-cyclodextrin (beta-CD-DNS) and a spirolactam rhodamine-linked adamantane (AD-SRhB). The dansyl moiety serves as the donor, and the spirolactam-rhodamine B derivative (SRhB) was chosen as a sensitive, selective chemosensor for Fe(III) ions and a very efficient ring-opening reaction induced by Fe(III) generates the long-wavelength rhodamine B fluorophore that can act as the energy acceptor. Moreover, the adamantyl (AD) group, which is known for its capability to form stable host-guest inclusion complexes with beta-CD derivatives, was covalently linked to the spirolactam rhodamine, thus the adamantyl moiety of the ion-recognition element can be anchored inside the CD cavity. In this way, the donor-acceptor separation can be kept within the critical Forster distance; accordingly, energy transfer can take place from the donor (dansyl) to the acceptor (rhodamine derivative/Fe(III) complex), and thus ratiometric detection for Fe(III) in an aqueous medium can be fulfilled. This FRET-based supramolecular sensor can be readily formed via an inclusion process using the donor part and the acceptor part, hence this strategy could afford a robust approach for constructing a wide range of FRET-based water-soluble sensing systems simply by assembling a specifically predesigned donor-linked CD and acceptor-linked adamantane.