MF-4SC Membranes Modified with Carboxylated Carbon Nanotubes for Potentiometric Determination of Alanine, Valine, and Phenylalanine in Alkaline Solutions

Perfluorosulfonic acid membranes MF-4SC containing 0.5–1.0 wt % carbon nanotubes with carboxyl groups on the surface have been manufactured. On their basis, multisensory systems have been developed for the simultaneous determination of amino acids (the total concentration of their anionic and zwitterionic forms) and K⁺ cations in aqueous solutions at pH 8–10 in the concentration range from 1.0 × 10^–4 to 5.0 × 10^–2 M. The relative error in the determination of alanine (5–15%), valine (0.3–10%), and phenylalanine (0.7–5%) is comparable with that for K⁺ cations (1.4–11%). Differences in the cross sensitivity of sensors based on the pristine and modified membranes, necessary for their use in multisensory systems, are achieved due to changes in the microstructure of membranes and the appearance of new reaction centers that affect the conditions of non-exchange sorption of aliphatic and aromatic amino acids in different ways.