Chiral octahedral cobalt(III) complex immobilized on Carboblack C as a novel robust and readily available enantioselective voltammetric sensor for the recognition of tryptophan enantiomers in real samples

Abstract

Enantioselective voltammetric sensors (EVSs) have significant interest in analytical and medicinal chemistry, as they allow for the express analysis and recognition of pharmacologically active components in drugs, as well as the establishment of the enantiopurity of compounds. The most common methods of modifying the electrode surface are through molecular imprinting techniques and the addition of a non-electroactive chiral modifier to the electrode composition. By varying the structure and properties of the chiral modifier, the sensor can be designed to have the highest enantioselectivity for a chosen chiral analyte. In this study, we describe the development of a new type of EVS based on a Carboblack C paste electrode modified with a chiral octahedral cobalt(III) complex for use as a selective enantioselector. It was found that the EVS, prepared from enantiopure Co(III) complexes, can selectively recognize L- and D-tryptophan (Trp) in phosphate buffer solution (PBS) with pH = 6.86. The peak current ratio (iD/iL) between the two enantiomers reached 1.36, demonstrating the effectiveness of the sensor. The experimental results indicate a linear correlation between the peak currents of Trp enantiomers and their concentration in the range from 1.00·10−6 to 5.00·10−4 М, leading to a detection limit of 2.42 µM and 3.30 µM for D- and L-Trp, respectively, when using chiral octahedral Co(III) complex with chloride counter-anion. Next, the EVSs developed demonstrated practical applicability in measuring the concentration of Trp in pharmaceutical samples and human urine and blood plasma. An important advantage of these sensors is the difference in the oxidation potentials between Trp enantiomers (ΔEp=35 mV), which makes it possible to determine Trp enantiomers in their mixtures. Quantum chemical calculations and experimental data show that the octahedral Co(III) complex binds to one of the Trp enantiomers with a stronger affinity, thus increasing its concentration at the electrode surface and promoting its oxidation. The easy preparation of the chiral Co(III) complex from inexpensive commercial materials, its high stability and insolubility in water make it a promising selector for the creation of efficient multifunctional EVSs for the analysis of various aqueous solutions.