Potentiometric sensors based on fluorous membranes doped with highly selective ionophores for carbonate

Abstract

Manganese(III) complexes of three fluorophilic salen derivatives were used to prepare ion-selective electrodes (ISEs) with ionophore-doped fluorous sensing membranes. Because of their extremely low polarity and polarizability, fluorous media are not only chemically very inert but also solvate potentially interfering ions poorly, resulting in a much improved discrimination of such ions. Indeed, the new ISEs exhibited selectivities for CO(3)(2-) that exceed those of previously reported ISEs based on nonfluorous membranes by several orders of magnitude. In particular, the interference from chloride and salicylate was reduced by 2 and 6 orders of magnitude, respectively. To achieve this, the selectivities of these ISEs were fine-tuned by addition of noncoordinating hydrophobic ions (i.e., ionic sites) into the sensing membranes. Stability constants of the anion-ionophore complexes were determined from the dependence of the potentiometric selectivities on the charge sign of the ionic sites and the molar ratio of ionic sites and the ionophore. For this purpose, a previously introduced fluorophilic tetraphenylborate and a novel fluorophilic cation with a bis(triphenylphosphoranylidene)ammonium group, (R(f6)(CH(2))(3))(3)PN(+)P(R(f6)(CH(2))(3))(3), were utilized (where R(f6) is C(6)F(13)). The optimum CO(3)(2-) selectivities were found for sensing membranes composed of anionic sites and ionophore in a 1:4 molar ratio, which results in the formation of 2:1 complexes with CO(3)(2-) with stability constants up to 4.1 × 10(15). As predicted by established theory, the site-to-ionophore ratios that provide optimum potentiometric selectivity depend on the stoichiometries of the complexes of both the primary and the interfering ions. However, the ionophores used in this study give examples of charges and stoichiometries previously neither explicitly predicted by theory nor shown by experiment. The exceptional selectivity of fluorous membranes doped with these carbonate ionophores suggests their use not only for potentiometric sensing but also for other types of sensors, such as the selective separation of carbonate from other anions and the sequestration of carbon dioxide.

Figure 1

Potentiometric CO₃²⁻ response of an ISE based on a liquid membrane with perfluoroperhydrophenanthrene doped with (1) 1.0 mM cationic sites, 1, and 1.5 mM ionophore Mn-1, (2) 2.0 mM cationic sites, 1, and 1.5 mM ionophore Mn-1, and (3) 1.0 mM anionic sites, 2, and 4.0 mM ionophore Mn-1. Sample solutions contained sodium bicarbonate buffered with 10 mM Tris-H₂SO₄ to pH=8.75. Response curves were shifted vertically relative to one another for enhanced clarity.

Figure 2

Logarithmic representation of selectivities for CO₃²⁻ of a conventional ISE based on the ionophore heptyl 4-trifluoroacetylbenzoate and fluorous ISEs with 1.0 mM cationic sites 1 and 1.5 mM fluorophilic ionophore Mn-1 (center, No. 1 in Table 1), and 1.0 mM anionic sites, 2, and 4.0 mM fluorophilic ionophore Mn-2 (right, No. 6 in Table 1).

Figure 3

Logarithmic representation of selectivities with respect to BPh₄⁻ of (from left to right) fluorous ion-exchanger ISEs based on 2.0 mM cationic sites, 1, and fluorous ionophore-based ISEs doped with 1 (1.0 mM) and ionophore Mn-1 (1.5 mM); 1 (2.0 mM) and Mn-1 (1.5 mM); 1 (1.0 mM) and Mn-1 (4.0 mM); 1 (1.0 mM) and ionophore Mn-2 (1.5 mM); 1 (2.0 mM) and Mn-2 (1.5 mM); 1 (1.0 mM) and Mn-2 (4.0 mM); anionic sites, 2 (1.0 mM) and Mn-3 (4.0 mM).

Figure 4

Schematic representation of the ratios of ionophore and ionophore complexes in fluorous sensing membranes with different ionic site-to-ionophore ratios for the primary ion CO₃²⁻ (A) and the interfering ions SCN⁻ (B) and HO⁻ (C). For each membrane composition, the anion stabilized the least in the sensing membrane (i.e., the most readily exchangeable anion) is circled.

Chart 1

Structure Formulae of Ionophores and Other Membrane Components