Dual off–on and on–off fluorescent detection of Zn2+/Cd2+ ions based on carbazolone substituted 2-aminobenzamides

Abstract

Two new 2-aminobenzamide structural isomers, 4-isoACOBA and 5-isoACOBA, as fluorescent probes for Cd²⁺ and Zn²⁺ were fabricated with carbazolone as fluorophore and N,N-bis(2-pyridylmethyl)ethylenediamine (BPEA) as chelator. With Cd²⁺/Zn²⁺ as input, the two probes are characteristic of the transformation from “off–on” to “on–off” molecular switch by interchanging the substitution position of the fluorophore from C-4 to C-5 at the benzene ring. 4-IsoACOBA is a Cd²⁺-specific turn-on fluorescent probe exhibiting good discrimination between Cd²⁺ and Zn²⁺ with F_Cd2+/F_Zn2+ = 2.48, while 5-isoACOBA is a Zn²⁺-specific turn-off probe with F_Cd2+/F_Zn2+ = 4.50. The binding behaviours of 4-isoACOBA–Cd(II) and 5-isoACOBA–Zn(II) were deeply investigated by UV and fluorescence titration, ESI-MS analysis, and DFT study. The results indicate that both the electron donating/withdrawing ability and the substituted position of the fluorophore have remarkable influences on the probe sensing properties and selectivity.