Characterization of lapachol in artificial organic-film membrane: Application for the trans-membrane transport of Mg2+

Species transfer across a liquid|liquid interface is studied by means of a thin film-modified electrode using cyclic voltammetry and square-wave voltammetry. The thin film-modified electrode consists of an edge plane pyrolytic graphite electrode (EPG) covered with a thin film of a water-immiscible electro-inactive organic solvent (nitrobenzene) containing a neutral redox probe and/or a suitable electrolyte. For this study we used, as redox probe in the organic phase, 2-hydroxy-3-isopropenyl-1,4-naphthoquinone also known as lapachol (Q) and an appropriate electrolyte. The redox transformations of Q at the graphite electrode/organic (EG|NB) interface was coupled to an ion-transfer reaction from aqueous to organic phase. The proton transfer at the nitrobenzene/water (NB|W) interface is essential for the electrochemical conversion of Q within the membrane. The voltammograms obtained are influenced by the pH of the aqueous phase. Q has two reduction systems due to the redox transformation of its two tautomeric forms resulting from the migration of a proton between the hydroxyl group in position 2 and the carbonyl group in position 4. The electrochemical mechanism consist of 2e−/2H+ exchange to form the separate redox compounds H2Q. The experiments conducted reveal the ability of both tautomers to form 1:1 complexes with Mg2+ when this cation is present in the aqueous phase.