Zn2+/Cd2+ optical discrimination by fluorescent acridine-based bis-macrocylic receptors

A new acridine-based fluorescent chemosensor, L2, featuring two sulfurated macrocyclic units has been synthesised. Proton and metal binding properties of L2 have been studied in H2O/CH3CN (1:1 v/v) by potentiometric and spectrophotometric measurements and compared with those of the analogous and previously published bis-macrocyclic receptor featuring two [9]aneN3 units (L1). Despite a thermodynamic selectivity observed for Zn2+ with both L1 and L2, L2 shows a marked sensing selectivity towards Cd2+ via a chelation enhancement of fluorescence (CHEF) effect both in H2O/CH3CN (1:1 v/v) and in H2O using silica core-PEG shell nanoparticles as vehiculant. On the other hand, L1 shows a selective optical sensing towards Zn2+ in agreement with the thermodynamic selectivity observed for the same metal ion.