Development of a smartphone-based colorimetric chemosensor for on-site detection of indium ions in environmental water samples: solution and solid-state analysis.

A novel diaminomaleonitrile-based colorimetric probe, 2-amino-3-[(8-hydroxy-2-quinolinyl)methylene]amino]-2-butenedinitrile (HQ-DAMN), was successfully synthesized and extensively characterized using a range of spectroscopic techniques. The sensing ability of HQ-DAMN towards multiple cations was investigated using colorimetric analysis, paper strips, an RGB model, alginate beads, and a UV-Vis spectrophotometer method in ACN: H₂O (4 : 6, v/v, pH 6.5). Upon interaction with the In³⁺ ion, the colorless solution of HQ-DAMN immediately transforms into orange, while no visible color change was observed with other metal ions. UV-Vis spectrophotometric studies revealed that HQ-DAMN exhibits a strong absorption peak at 374 nm and a weak absorption peak at 266 nm, attributed to intramolecular charge transfer (ICT) and ligand-to-ligand charge transfer (LLCT), respectively. Upon binding with the In³⁺ ion, new peaks emerge at 472 nm, indicative of ligand-to-metal charge transfer (LMCT). The HQ-DAMN-In³⁺ complex exhibited a 2 : 1 binding stoichiometry, with a binding constant of 7.70 × 10³ M^-2, as determined using Job's plot and Benesi-Hildebrand (B-H) plot analysis. HQ-DAMN was found to be stable in a pH range of 4.0-7.0, retaining its high selectivity in the presence of competing ions. Moreover, it achieved excellent sensitivity, with detection limits as low as 66.7 nM using spectrophotometric methods and 0.68 μM with the RGB model. The interaction between HQ-DAMN and In³⁺ ions was thoroughly investigated using ¹H NMR spectra, ESI mass spectrometry, and FT-IR spectroscopy, supported by theoretical computational studies. These findings affirm the potential of HQ-DAMN as a colorimetric and smartphone RGB sensor for In³⁺ ion detection.