A light activated CMP conjugated 8-aminoquinoline turn-on fluorescent optode for selective determination of Th⁴⁺ in...

A new dibutyl(2-oxo-2-(quinolin-8-ylamino)ethyl)phosphinate (L) was designed, synthesised and developed as a light activated optode for Th⁴⁺ determination. The sensing performance of L was studied in solution and in the polymeric membrane phase using absorbance and emission techniques. The neutral ion carrier L exhibits weak fluorescence at 400 nm in CH₃CN : H₂O (9 : 1, v/v). Upon complexation of L with Th⁴⁺, the emission intensity at 486 nm was increased 100-fold due to chelation induced enhanced fluorescence. The L-Th⁴⁺ binding interaction was studied using both Hill's and Job's plots. They indicate the formation of complex species in a 2 : 1 ratio with an estimated binding constant of 2.66 × 10⁴ M^-2. Complex formation between L and Th⁴⁺ was studied by NMR (¹H, ³¹P), FTIR and LC-MS analyses. In order to synthesize a probe for the sensing of Th⁴⁺ in 100% aqueous medium, the optode was prepared by immobilizing L as a neutral ion carrier in a PVC support using dioctyl phthalate (DOP) as a membrane solvent. The best performance of the optode was observed with a membrane composition of PVC : L : NaTPB : DOP in proportions of 35 : 5 : 3 : 57 (%, w/w) in the pH range of 4.0-8.0. The optode can detect Th⁴⁺ concentrations down to 1.1 nM with a fast response time of 15 s and the optical response remains unaltered even after 3 months; the optode sensor can be regenerated using Na₂EDTA solution. Finally, the optode was used for the quantification of Th⁴⁺ in various water samples, monazite sand, and gas mantle samples.