Chiral Selectors in Voltammetric Sensors Based on Mixed Phenylalanine/Alanine Cu(II) and Zn(II) Complexes

A practical application composite based on mixed chelate complexes [M(S-Ala)2(H2O)n]–[M(S-Phe)2(H2O)n] (M = Cu(II), Zn(II); n = 0–1) as chiral selectors in enantioselective voltammetric sensors was suggested. The structures of the resulting complexes were studied by XRD, ESI-MS, and IR- and NMR-spectroscopy methods. It was determined that enantioselectivity depends on the metal nature and on the structure of the mixed complex. The mixed complexes, which were suggested to be chiral selectors, were stable under the experimental conditions and provided greater enantioselectivity in the determination of chiral analytes, such as naproxen and propranolol, in comparison with the amino acids they comprise. The best results shown by the mixed copper complex [Cu(S-Ala)2]–[Cu(S-Phe)2] were: ipS/ipR = 1.27 and ΔEp = 30 mV for Nap; and ipS/ipR = 1.37 and ΔEp = 20 mV for Prp. The electrochemical and analytical characteristics of the sensors and conditions of voltammogram recordings were studied by differential pulse voltammetry. Linear relationships between the anodic current and the concentrations of Nap and Prp enantiomers were achieved in the range of 2.5 × 10−5 to 1.0 × 10−3 mol L−1 for GCE/PEC-[Cu(S-Ala)2]–[Cu(S-Phe)2] and 5.0 × 10−5 to 1.0 × 10−3 for GCE/PEC–[Zn(S-Ala)2(H2O)]–[Zn(S-Phe)2(H2O)], with detection limits (3 s/m) of 0.30–1.24 μM. The suggested sensor was used to analyze Nap and Prp enantiomers in urine and plasma samples.