A series of ionophores for ammonium ion-selective electrodes was designed and synthesized, and their characteristics were examined. The design of the ionophores is based on two different strategies: (1) introduction of bulky blocking subunits (decalino groups) in 20- or 21-membered crown ethers (TD20C6 and TD21C6), the ring size of which is expected to be suitable for selective NH4+ recognition, as compared to the slightly smaller K+; and (2) preorganized tripodal ionophores based on a 6-fold substituted benzene ring in order to complementarily recognize the tetrahedral NH4+, in contrast to the spherical K+. Compared to nonactin, a natural product that is used as a representative NH4+ ionophore, the newly developed TD20C6 showed higher NH4+ selectivity over K+ while retaining the selectivity over Na+(log KNH4+,K+pot≡ -1.5 and log KNH4+,Na+pot = -2.5). On the other hand, a tripodal ionophore with pyrazole nitrogen atoms as NH4+ binding sites showed high NH4+/K+. selectivity but suffered from increased Ca2+ interference (log KNH4+,K+pot = -2.1 and log KNH4+,Ca2+pot = -1.6). As an overall conclusion, the cyclic ionophores TD19C6 and TD20C6 are the best ammonium-selective ionophores developed to date.
ASJC Scopus subject areas
- Analytical Chemistry