New 1,8-naphthyridine-based probes for the selective fluorescence signalling of toxic cadmium: synthesis, photophysical studies and molecular modelling

Newly synthesised 1,8-naphthyridine-based molecular probes, NAP-1 and NAP-2, exhibit highly selective fluorescence responses towards the toxic cadmium over coordinatively competing Zn2+ and several other metal ions examined. On the one hand, NAP-1 (MeOH:H2O, v/v 80:20, pH 7.4) exhibits ca. 1.5 order of magnitude higher stability constant for Cd2+ over Zn2+; on the other hand, NAP-2 in MeOH offers unique selectivity only towards Cd2+, exhibiting both absorbance and emission red shifts as well as fluorescence enhancement. By 1H NMR analysis, the tetra-coordinated binding is indicated at least for the NAP-1+Cd2+ complex and theoretical calculations reveal relatively stronger binding of Cd2+ over Zn2+.