Selective Binding of NH₄⁺ by Redox-Active Crown Ethers:  Application to a NH₄⁺ Sensor

Two redox-active crown ethers, (1,1‘- bi-2-naphthyl)-23-crown-6 incorporating 9,10-anthraquinone (BNAQ) and 1,4-benzoquinone (BNBQ), were synthesized and employed in the selective binding of NH₄⁺ over K⁺. Their applications to NH₄⁺ detection were studied by cyclic voltammetry and amperometry in aqueous media. The results showed that the magnitude of the quinone redox peak decreased linearly as the concentration of NH₄⁺ increased, indicating the formation of BNAQ−NH₄⁺ and BNBQ−NH₄⁺ complexes. Formation constants of BNAQ−NH₄⁺ and BNBQ−NH₄⁺ complexes were determined to be 4.3 × 10³ and 4.0 × 10³ M^-1, respectively, which were 2 orders of the magnitude greater than those of BNAQ−K⁺ and BNBQ−K⁺ complexes. The ¹H NMR titration method carried out in DMSO-d₆ showed that both complexes possessed 1:1 stoichiometry, and association constants were determined to be 648 ± 35 and 600 ± 47 M^-1 for BNAQ−NH₄⁺ and BNBQ−NH₄⁺, respectively. Interference effects from other alkali and alkaline earth metal ions in the analysis of NH₄⁺ were also investigated. The BNAQ-modified sensor showed a linear response from 1.0 μM to 1.0 mM for NH₄⁺, and the detection limit was determined to be 0.9 ± 0.03 μM.