A functional Fe^III-based NH₂-MOF as an electrochemical sensor for Cu²⁺ detection in ethanol fuel

The presence of Cu²⁺ in ethanol fuel represents a problem in terms of quality, leading to issues in the engines of automotive vehicles. Thus, this study brings an innovative Fe^III-organic framework to develop an electrochemical sensor to detect Cu²⁺ in ethanolic media. The synthesized metal–organic framework (MOF) was MIL-88b-NH₂ and MIL-82, and these materials were characterized using powder X-ray diffraction and infrared spectroscopy. The results showed the successful synthesis of both MOFs with minor synthetic modifications. Subsequently, the modified carbon paste selected to analyze Cu²⁺ in ethanol fuel was MIL-88b-NH₂ based on the results obtained from cyclic voltammetry and electrochemical impedance spectroscopy. Subsequently, square wave anodic stripping voltammetry parameters were optimized using a design of experiments approach. The analytical signal of the electrode exhibited good stability (relative standard deviation = 5.2%). Besides, the correlation coefficient (r) and coefficient of determination (R²) calculated in the calibration curve were 0.9993 and 99.85%, respectively, indicating a good fit of the linear model to the experimental data. The limit of detection obtained from the linear equation was 0.020 µmol L⁻¹. Finally, in the recovery test using spiked samples of ethanol fuel, the obtained values were 83 and 96%, indicating the absence of matrix effects. Our findings demonstrate that the MIL-88b-NH₂ modified carbon paste electrode is a suitable sensor for assessing Cu²⁺ contamination in ethanol fuel under and above the limit permitted by Brazilian legislation.