Highly selective fluorescent chemosensors for cadmium in water

The design, synthesis and photophysical evaluation of two new chemosensors and is described for the selective detection of Cd(II) in water at pH 7.4. Both are based on the use of aromatic iminodiacetate receptors that connected to an anthracene fluorophore by covalent methyl spacers. These are highly water-soluble sensors where the fluorescence is ‘switched off’ between pH 3–11, due to photoinduced electron transfer (PET) quenching of the anthracene excited state by the receptor. Upon protonation of the receptor, the emission was however, ‘switched on’. From these changes pKas of 1.8 and 2.5 were determined for and respectively. Both showed good selectivity for Cd(II) over competitive ions such as group II and Zn(II), Cu(II), Co(II). For , having a single receptor, only a weak monomer anthracene emission was observed for the free sensor at pH 7.4 (HEPES buffer, 135 mM NaCl). Upon Zn(II) titration, a broad red shifted emission occurred, centred at 468 nm. In the presence of Cd(II), a similar red shifted emission was also observed, however, this time centred at 506 nm. In contrast to these results, the fluorescence of in the presence of Zn(II) gave rise to typical monomeric anthracene emission, due to suppression of PET, that is, the anthracene emission was ‘switched on’. Nevertheless, in the presence of Cd(II) a broad emission centred at 500 nm was observed, similar to that seen for . These ion induced long wavelength emission bands were assigned to the formation of charge-transfer complexes (exciplexes) between the anthracene moieties and the ion-receptor complexes. Importantly, for both and , a selective detection of Cd(II) was possible, even in the presence of Zn(II).