An Environmentally Friendly Three-Dimensional Printed Graphene-Integrated Polylactic Acid Electrochemical Sensor for the Quality Control of Biofuels

Additively manufactured sensors have shown promising features regarding sustainability, because of the reduction of waste, feasible use of biopolymers, the fabrication process, and large-scale production. Graphene-integrated polylactic acid (G/PLA) is an excellent material to three-dimensional (3D)-print electrochemical sensors with outstanding stability, robustness, and sensitivity. Herein, we propose 3D-printed G/PLA electrochemical sensors to the monitoring of quality control of biodiesel and/or biokerosene, which have been widely demanded worldwide to replace fossil fuels. The main drawback of such biofuels is their oxidation instability, which can be controlled by the monitoring of additive antioxidants added to retard oxidation processes. This is the first application of a 3D-printed sensor for such biofuels, and as a proof-of-concept, we demonstrate that these sensors can determine tert-butylhydroquinone (TBHQ), the most used antioxidant in biofuels, in soybean biodiesel and coconut biokerosene. TBHQ is electrochemically oxidized at +0.3 V (vs Ag|AgCl|KCl_(sat.)), and a microliter aliquot of the samples was diluted in 0.12 mol L^–1 Britton–Robinson solution (pH 4.05) containing sodium dodecyl sulfate to stabilize the emulsion before the voltammetric measurement. An STL 3D printer and a 3D pen were used to fabricate the sensors in a cylindrical shape (3.5 cm length × 0.4 cm diameter). After selecting optimal parameters, the sensor presented a linear range up to 400 μmol L^–1 (r > 0.99) and a detection limit estimated as 0.1 μmol L^–1. High precision was attested by relative standard deviation values lesser than 5% (interday and interelectrode measures). The metallic species Fe³⁺, Pb²⁺, Cu²⁺, Mn²⁺, and Cr²⁺, which can be found because of corrosion of metallic parts by the action of storing and transporting biofuels, did not statistically interfere (t_tab = 1.833 > t_cal = 0.114) in the determination of TBHQ. The analysis of samples spiked with TBHQ resulted in recovery values that ranged from 86 to 105%. A commercial biodiesel sample was analyzed by both the proposed and high-performance liquid chromatography methods, and the results were statistically similar. Importantly, the sensor was compatible with biofuels, and its surface can be renewed after polishing for its continuous use for routine applications.