Development of a highly efficient gadolinium(III) potentiometric sensor using two aza-crown ether-based multiple diglycolamides (DGAs)

The present investigation deals with the development of two polyvinyl chloride- (PVC) based potentiometric sensors for gadolinium(III) ion with two multiple diglycolamide (DGA) ligands with the DGA moieties tethered onto aza-crown ethers as ionophores, 2-nitrophenyl octyl ether (NPOE) as the plasticizer and sodium tetraphenylborate (NaTPB) as ionic additive. The ionophores are a tripodal DGA ligand containing a triaza-9-crown-3 core (L_I) and a tetrapodal DGA ligand with a tetraaza-12-crown-4 as the scaffold (L_II). The polymer inclusion membranes (PIMs) were tested for gadolinium ion detection in a mildly acidic solution (pH 3.0 ± 0.1). The PIM containing an optimized composition of 7.8% L_I, 29.6% PVC, 60.5% NPOE, 2.1% NaTPB exhibited a linear response behaviour from 7.2 × 10⁻⁷ to 4.9 × 10⁻³ M Gd³⁺ with a slope of 19.8 ± 0.4 mV/decade and a limit of detection (LOD) of 6.0 × 10⁻⁷ M. The sensor showed fast response time (<20 s) and a lifetime of at least two months. It exhibited good tolerance to monovalent/divalent interfering ions and moderate to poor tolerance to trivalent ions. The effect of temperature on the selectivity coefficient of Gd³⁺ over La³⁺ was determined which indicated a decreasing trend with temperature. The concentration of Gd³⁺ in the inner filling solution (IFS) had a significant effect on the sensor sensitivity and on the detection limit. The other sensor containing L_II performs poorly as a potentiometric sensor. The physical characterization of the membranes was done using thermogravimetry (TGA), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and photoluminesce (PL) spectroscopy. A PL study was performed using Eu³⁺ as a surrogate probe for Gd³⁺ as the latter ion does not show luminescence properties. The PL spectra suggest the presence of a single analyte ion species in the membrane. The sensor containing L_I can be used as an indicator electrode for the potentiometric titration of Gd³⁺ which indicated a recovery of 94.6 ± 4.3% of Gd³⁺ for triplicate measurements and the results match well with the complexometric titration result using xylenol orange as a colorimetric indicator for the endpoint detection.