Terpyridine-4-amino-1,8-naphthalimide chemosensor for discriminative fluorescent sensing of divalent metal cations at ppb level of sensitivity

Abstract

A new terpyridine-4-amino-1,8-naphthalimide fluorescence sensor, TERPY-Nap, has been designed and synthesized in the analytically pure form via a one-step imidation reaction between 4-amino-1,8-naphthalic anhydride and N-[2,2′;6′,2″]Terpyridin-4′-yl-propane-1,3-diamine. The internal-charge-transfer (ICT) transition makes TERPY-Nap to be highly emissive and exhibits solvent polarity-dependent emission characteristics in different organic solvents. Inspired by the strong fluorescence, TERPY-Nap was used as an efficient fluorescence sensor for discriminative fluorescence sensing of divalent metal cations such as Cu(II), Co(II), and Ni(II) in ethanol. The initial fluorescence emission intensity of TERPY-Nap was quenched drastically upon the addition of Cu(II), Ni(II), and Co(II) ions as their nitrate salt, and the fluorescence quenching constants (KSV) were calculated to be in the order of 104–105 M−1. Furthermore, it was observed that TERPY-Nap can sense Cu(II), Ni(II), and Co(II) as low as ppb level of sensitivity and the quenched fluorescence emission of complexes can be reversed by the addition of well-known metal scavenger EDTA in ethanol. Interestingly, similar fluorescence titration studies using other metal ions like Zn(II), Cd(II), Mn(II), Ba(II), Al(III), Mg(II), and Ce(III) showed almost negligible changes in emission intensity due to their weak binding at the terpyridine receptor sites. Therefore, TERPY-Nap can be used as a potential ‘turn-off’ fluorescence chemosensor for reversible detection and trace analysis of biologically relevant and environmentally pertaining divalent metal cations.