A Smartphone‐Assisted Sensitive, Selective and Reversible Recognition of Copper Ions in an Aqueous Medium

A photochromic substituted spiropyran (8‐methoxy‐1,3’,3’‐trimethylspiro[chromene‐2,2’indoline]) was successfully synthesized and its structural integrity was ascertained using spectroscopic methods. Owing to the uniquely positioned methoxy and phenolic groups for potential coordination with a metal ion, it was investigated for the recognition of toxic metal ions. The spiropyran derivative responded to the presence of Cu 2+ ions in an aqueous solution by displaying a color change visible to the naked eye (colorless to pink). The color change was witnessed due to the Cu 2+ ion‐induced transformation of the closed‐form ( spiro ) of the substituted spiropyran derivative into an open merocyanine (MC) form, which complexes the Cu 2+ ion. The color change was further used for the quantification of Cu 2+ ion concentration in water using a smartphone captured digital images via pixel intensity analysis. The spiropyran derivative displayed 0.24±0.01 μM, 0.65±0.06 μM (0.61±0.06 μM using paper strips) as the LOD for Cu 2+ ions using UV‐Visible spectroscopy and digital colorimetry, respectively. The density functional theory (DFT) calculations and Job's plot supported the formation of a 2 : 1 (H: G) complex between the spiropyran derivative and copper ions. The time‐dependent DFT (TD‐DFT) investigations were also used to understand the color change during the complex formation, which indicated a good correlation between the experimental and theoretical results at the molecular level.