Development of a fast and simple electrochemical sensor for trace determination of sulfite using copper(II) triazole complexes

A glassy carbon electrode (GCE) was modified with copper triazole complexes (Cu(L)₂) containing three different substituents (OMe, Br, and NO₂), and the fabricated electrochemical sensors were used to detect sulfite. The materials were characterized by cyclic voltammetry and scanning electron microscopy (SEM/EDX). The copper(II) 5-methyl-1-(4-nitrophenyl)-1H-1,2,3-triazole-4-carboxylate (Cu(L^NO2)₂) complex exhibited the best electrochemical performance toward the sulfite electron transfer process regarding onset potential and peak current. Best results were obtained at pH 4, and the following analytical parameters for sulfite detection were determined by amperometry at 0.90 V, as follows: reproducibility (1.9 %), repeatability (1.6 %), limit of detection (0.75 µmol/L), limit of quantification (2.27 µmol/L), dynamic concentration range (10 µmol/L to 5.00 mmol/L, R² = 0.995), and sensitivity (7.43x10^-3 µA µmol⁻¹ L). Precision was evaluated in terms of repeatability, and a relative standard deviation < 15 % was found at three concentration levels. The method́s accuracy was assessed by determining sulfite in commercial wine samples, and acceptable errors were obtained compared to values reported by an external analytical laboratory. The selectivity towards interfering species (Ca²⁺, Zn²⁺, Mg²⁺, NO₃⁻, glucose, sucrose, NaCl, and KCl) and the stability was also evaluated. The practical application of the Cu(L^NO2)₂/GCE sensor was examined by analyzing the sulfite content in white and red commercial wines.