Carbon nanofibers modified screen-printed electrode for copper analysis in bioethanol by anodic stripping voltammetry

Bioethanol is a renewable, clean and self-sustaining biofuel. Their use helps reduce emissions of gaseous pollutants into the atmosphere compared to fossil fuels. Current public policies require this biofuel to have a high degree of purity. Moreover, the presence of metals in high concentrations in their composition is detrimental to the engines, reducing their efficiency, in addition to being able to form undesirable residues.1 Considering these assumptions into account and the need for metals quantification in bioethanol, carbon nanofibers (CNF) were selected as a modifier of carbon screen-printed electrode (SPE) due to their mechanical resistance and high sensitivity, which allows the development of resistant and efficient sensors. In this work was developed a carbon screen-printed electrode (SPE) modified with carbon nanofibers (CNF) for copper determination in bioethanol sample. The SPE was activated in a phosphate buffer (pH 7) solution containing KCl applying potentials from 0.0 to 2.0 V at 50 mV s-1 by 10 voltammetric cycles. The nanofibers were immobilized on electrode surface through a suspension containing CNF, Nafion and ethanol. From the scanning electron microscopy images, it is possible to note a uniform distribution of the CNF and mean diameter of 93 ± 23 nm. Energy dispersive x-ray spectroscopy spectrum showed carbon, oxygen and fluorine peaks, indicating that the modification was successful. In the characterization by electrochemical impedance spectroscopy (frequency of 0.10 Hz to 100 kHz and potential 0.22 V) with 10 mM Fe(CN)64-/3- in 1.0 M KCl solution showed that the electron transfers resistance (Rct) of the CNF/SPE (Rct: 0 Ω) decreased in comparison with unmodified SPE (Rct: 704 Ω).The Raman spectrum presented two intense bands at ~ 1328 and ~ 1600 cm-1, associated with band D and G, respectively, for the modified and unmodified surfaces. For analytical method develepment was investigated the copper eletrooxidation with linear sweep (LSASV), square wave (SWASV) and differential pulse anodic stripping voltammetry (DPASV). The LSASV (3,4×10-4 A) showed the best sensitivity compared to SWASV (8,3×10-5 A) and DPASV (6,5×10-5 A). Using the LSASV under optimized conditions, the analytical method was developed. Copper solutions were prepared in concentration range from 1.0×10-8 to 1.0×10-3 M.The analytical curve presented a linear range of 1.0×10-7 to 1.0×10-5 (y = -3.6x + 5.5×10-6) with detection limit of 5.0×10-8 M, quantification limit of 1.5×10-7 M, sensitivity of 3.6 A mol-1 L and correlation coefficient of 0.996. The recovery study showed that the method has good accuracy and repeatability, with recovery of 101 ± 2 %. The copper concentration found in bioethanol sample by standard addition method was 1.1×10-7 ± 2.5×10-9 mol L-1. In addition, the stability and reproducibility of the prepared modified electrode were investigated. The excellent long-term stability and reproducibility of the prepared CNF/SPE make them attractive as sensor. The results indicated that the analytical method proposed employing the modified electrode and LSASV presented better analytical performance. Moreover, the sensor has shown to be an excellent alternative for the copper determination in bioethanol samples.