Selective Binding of NH4+ by Redox-Active Crown Ethers: Application to a NH4+ Sensor
journal contributionposted on 01.07.2004, 00:00 by Md. Aminur Rahmen, Nak-Hyun Kwon, Mi-Sook Won, Myung-Ho Hyun, Yoon-Bo Shim
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.