A metal-organic framework-based amperometric sensor for the sensitive determination of sulfite ions in the presence of ascorbic acid

A carbon paste electrode modified with the MIL-101(Cr) metal–organic framework (MIL-101(Cr)-CPE) was prepared, characterized and assayed to the voltammetric detection of sulfite ions. Electrochemical impedance spectroscopy and cyclic voltammetry indicated a significant increase in charge transfer ability and real surface area (Rct = 1.1 kΩ and S = 0.220 cm2) of the MIL-101(Cr)-CPE in comparison with the bare CPE (Rct = 7.8 kΩ and S = 0.031 cm2). It was demonstrated by cyclic voltammetry that the oxidation of sulfite ions at the MIL-101(Cr)-CPE follows an irreversible and diffusion-controlled mechanism. Also, the peak current of sulfite ions was 1.5-fold more intense on MIL-101(Cr)-CPE compared to the unmodified CPE. An analytical method was therefore developed, based on square wave voltammetry: under optimized experimental conditions, the MIL-101(Cr)-CPE electrode displayed a linear relationship between the oxidation peak current and the concentration of sulfite in the concentration range of 2 µM–70 µM, with a detection limit of 0.58 µM (S/N = 3). The sensor was furthermore exploited for the simultaneous determination of sulfite and ascorbic acid, which gave well separated anodic peak at 0.35 V and 0.90 V (vs. Ag/AgCl, 3 M KCl), respectively. The proposed method was further used to determine both analytes in commercial wines, thus establishing its applicability as inexpensive analytical tool for quality control.