Cationic or anionic sites? Selectivity optimization of ion-selective electrodes based on charged ionophores

Area of Science:

• Electrochemistry
• Analytical Chemistry
• Materials Science

Background:

• Ion-selective electrodes (ISEs) are crucial for ion detection.
• The role of ionic sites in ISE selectivity has been studied, but a generalized model was lacking.
• Previous understanding suggested only ionic sites matching the primary ion's charge improve selectivity in charged ionophore ISEs.

Purpose of the Study:

• To present a generalized phase boundary potential model for ISE selectivity.
• To investigate the influence of ionic sites on ISE selectivity for various ion charges and ionophore types.
• To challenge the existing paradigm regarding the charge sign of ionic sites for optimal selectivity.

Main Methods:

• Development of a generalized phase boundary potential model.
• Experimental validation using three ISEs with different charged ionophores (lasalocid, ETH 5639, and a molybdenum-porphyrin complex).
• Analysis of potentiometric selectivities and ion-ionophore complex stabilities.

Main Results:

• The model applies to primary and interfering ions of any charge and to charged or neutral ionophores.
• Optimal ionic site charge sign depends on primary/interfering ion charges and complex stoichiometry.
• Two novel ISEs demonstrated maximum selectivity with ionic sites opposite in charge to the primary ion.
• An F- ISE showed improved selectivity with increasing anionic sites.

Conclusions:

• The charge sign of ionic sites is not the sole determinant of ISE selectivity; ion charge and complex stoichiometry are critical.
• The developed model provides a more comprehensive understanding of ISE selectivity.
• Experimental results validate the model and reveal new strategies for designing high-performance ISEs.