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Ruthenium(II)-bipyridine-substituted calix[4]arene sensor 20 is a turn-off probe for CN−, which causes fluorescence quenching and a blue-shift of the emission band at 624 nm [93]. The simple, polyanionic calix[4]arene-based sensor 21 is an ‘on-off’ probe for spermine 22 [94], while a QD-appended calix[8]arene receptor forms the complex 23 with C60 fullerene, which quenches the QD fluorescence output [95].
