Selective binding of Ni²⁺ and Cu²⁺ metal ions with naphthazarin esters isolated from Arnebia euchroma

Naphthazarin esters (C1-C4) isolated from the roots of Arnebia euchroma are found as skilled dual chemosensors for Ni²⁺ and Cu²⁺ among Pb²⁺ , Na²⁺ , K²⁺ , Hg²⁺ , Mg²⁺ , and Ca²⁺ metal ions. C1-C4 esters exhibited a red shift of 54 nm with Ni²⁺ and 30 nm with Cu²⁺ metal ions in absorption. There is a formation of red-shifted bands between 517 and 613 nm in the absorption spectrum of C1-C4 sensors on binding with Ni²⁺ and Cu²⁺ ions. The addition of Ni²⁺ and Cu²⁺ ions to sensors C1-C4 stimulates a remarkable color change from reddish pink to purple and light blue, respectively. These color changes can be identified with the naked eye. The significant downfield shifts of CO and OH peaks in nuclear magnetic resonance (NMR) spectrum confirm the chelation as binding mechanism. With ultraviolet-visble and NMR studies, it is found that C1-C4 esters possessed notable selectivity and sensitivity toward Ni²⁺ and Cu²⁺ over other metal ions.