Development of the Smartphone-Assisted Colorimetric Detection of Thorium by Using New Schiff's Base and Its...

In this paper, a new Th⁴⁺ ion-selective chromogenic sensor (L) was developed by reacting 1,10-phenanthroline-2,9-dicarbohydrazide with 2-hydroxy naphthaldehyde. The sensing ability of L toward Th⁴⁺ was investigated in solution and paper strips loaded with L using spectrophotometric and colorimetric methods. The selective interaction of L was examined with various f-metal ions and other selected metal ions from s-block and d-block elements. Results show that by the colorimetric method in solution-phase dimethyl sulfoxide/H₂O (7:3, v/v) and paper strip methods, the naked-eye detectable color change of L occurred from colorless solution to yellow-orange and pale yellow colour upon interacting with Th⁴⁺ and Al³⁺, respectively, whereas other metal ions did not interfere. The ligand L exhibits two absorbance bands at 320 and 375 nm because of ligand-to-ligand charge transfer. Upon interaction with Th⁴⁺, L undergoes red shift of both absorption bands and the formation of a new UV-vis band at 335 and 440 nm. The UV-visible spectral studies indicate the formation of a 1:1 host-guest complex between L and Th⁴⁺ with an association constant of 4.7 × 10³ M^-1. The limit of quantification and limit of detection of L for the analysis of Th⁴⁺ are found to be 167 and 50 nM, respectively. The visually detectable color change of L has been well integrated with a smartphone RGB color value to make it an analytical signal for real-time analysis of Th⁴⁺ with the detection limit down to 116 nM. Besides, L was applied for the analysis of Th⁴⁺ content present in various real water samples, monazite, and lantern mantle samples by spectrophotometry and RGB color values. The binding mode of L with Th⁴⁺ is investigated by ¹H NMR, electrospray ionization-mass, and theoretical studies.