Highly selective methylammonium-selective membrane electrode made by the modification of the upper rim of calix[6]arene derivatives

The upper rim of calix[6]arene-hexaacetic acid hexaethyl esters was modified with various alkyl substituents to develop ionophores for constructing organic ammonium ion-selective membrane electrodes. We found that the substitution of bulkier alkyl groups enabled a more effective discrimination of simple aliphatic ammonium ions such as methylammonium and ethylammonium. In particular, an electrode based on p-1,1,3,3-tetramethylbutylcalix[6]arene-hexaacetic acid hexaethyl ester showed the highest response to methylammonium among various organic ammonium ions and inorganic cations, giving the following selectivity coefficients; logkCH3NH3+,XPot: C2H5NH3+, −0.2; C3H7NH3+, −1.4; C4H9NH3+, −1.9; C6H13NH3+, −1.6; phenethylammonium, −1.9; Na+, −2.8; K+, −1.8; Rb+, −1.6; Cs+, −0.2; NH4+, −2.8. The electrode exhibited a near Nernstian response to methylammonium in the concentration range of 1 × 10−5 to 1 × 10−2 M with a slope of 58.5 mV per concentration decade in 0.1 M MgCl2. The limit of detection was 3 × 10−6 M. The electrode could be used over a pH range of 2-10. Formation of a 1:1 complex between p-1,1,3,3-tetramethylbutylcalix[6]arene-hexaacetic acid hexaethyl ester and methylammonium was confirmed by electrospray ionization mass spectrometry. The response characteristics of the methylammonium-selective electrode were compared with those of NH4+-selective electrodes so far developed.