Selective and visual Ca(2+) ion recognition in solution and in a self-assembly organogel of the terpyridine-based derivative triggered by ultrasound.

A new kind of terpyridine-based Ca(2+) sensor TS was designed and studied based on the internal charge transfer (ICT). In the diluted solution state, TS sensed Ca(2+) and Mg(2+) ions among test ions via an "off-on" approach as seen from fluorescence spectra of test ions. Moreover, TS was able to form stable fluorescent gels in organic solvents accelerated by ultrasound, indicating the ultrasound responsive properties of TS molecules. The S-gel of TS could be successfully used to selectively recognize Ca(2+) through fluorescent emission color and morphological changes, which was different from that of the solution state. It was predicated that the competition between the self-assembly of TS molecules and the host–guest interaction of TS with Ca(2+) or Mg(2+) was responsible for the sensing properties. To the best of our knowledge, this is the first example that organogels could selectively sense Ca2+ ions.