Squaramide-based tripodal ionophores for potentiometric sulfate-selective sensors with high selectivity

A class of squaramide-based tripodal molecules was employed as new ionophores for highly sensitive and selective sulfate-selective sensors. Compared with the reported tripodal ionophore with urea, squaramide groups as superior hydrogen bonding donor were introduced into the tripodal structure to obtain new ionophores leading to better electrode performance. Three derivatives with unsubstituted (I), p-carbon trifluoride (II), and p-nitro (III) phenyl groups were attached to squaramide groups for the optimization of ionophore based sensors. Electron withdrawing p-nitrophenyl groups gave a greater enhancement of the hydrogen bond donor ability of squaramide so that Ionophore III was chosen as the best candidate for sulfate ion recognition. Such a membrane with 30 mol% TDMACl exhibited a Nernstian slope of -30.2 mV per decade to sulfate ions with a linear range from 1 μM to 100 mM in potentiometric measurement. The selectivity coefficients of the proposed sensor over H2PO4(-), Cl(-), Br(-), NO3(-), SCN(-), I(-) and ClO4(-) were -4.3, -3.4, -2.5, -0.6, +3.1, +3.4 and +5.9, respectively, which were much better than the existing sulfate-selective sensors. The new sensors with high selectivity were successfully applied for the quantification of sulfate in cell lysates and drinking water with good recoveries.