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The development of sensitive and selective analytical devices for monitoring glucose levels (GLU) in biological fluids is extremely important for clinical diagnostics. In this work, we produced a new composite based on NiFe2O4 and multi-walled carbon nanotubes (MWCNT), called NiFe2O4@MWCNT, to be applied as a non-enzymatic amperometric sensor for GLU. Both NiFe2O4 and NiFe2O4@MWCNT composites were properly characterized by XRD, SEM, FTIR, and Raman spectroscopy, which confirmed that the composite was successfully prepared. A glassy-carbon electrode (GCE) modified with NiFe2O4@MWCNT was investigated by cyclic voltammetry and applied for the amperometric GLU detection using batch-injection analysis (BIA). A linear working range between 50 and 600 µmol L−1 GLU with a significant increase in sensitivity (3-fold) in comparison with MWCNT/GCE was verified, with a detection limit of 36 µmol L−1. Inter-electrode measurements (n = 4, RSD = 10%) indicated that the sensor fabrication is reproducible. Furthermore, the proposed non-enzymatic sensor was selective even in the presence of other biomarkers found in urine. When applied to synthetic urine samples, recovery levels between 84 and 95% confirmed analytical accuracy and the absence of sample matrix effect. Importantly, the developed approach is simple (free of biological modifiers), fast (77 injections per hour), and practical (high-performance tool), which are suitable features for routine analyses. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) printing is an emerging technology to develop devices on a large scale with potential application for electroanalysis. However, 3D-printed electrodes, in their native form, provide poor electrochemical response due to the presence of a high percentage of thermoplastic polymer in the conductive filaments. Therefore, surface treatments are usually required to remove the nonconductive material from the 3D-printed electrode surfaces, providing a dramatic improvement in the electroanalytical performance. However, these procedures are time-consuming, require bulky equipment, or even involve non-eco-friendly protocols. Herein, we demonstrated that portable and low-cost atmospheric air plasma jet pens can be used to activate electrodes additively manufactured using a commercial poly(lactic acid) filament containing carbon black as conductive filler, improving the electrochemical activity. Remarkable electrochemical results were obtained (voltammetric profile) using [Fe(CN) 6 ] 3-/4- , dopamine and [Ru(NH 3 ) 6 ] 2+/3+ as redox probes. Microscopic, spectroscopic, and electrochemical techniques revealed that the air-plasma jet pen removes the excess PLA on the 3D-printed electrode surface, exposing the conductive carbon black particles and increasing the surface area. The performance of the treated electrode was evaluated by the quantification of capsaicin in pepper sauce samples, with a limit of detection of 3 nM, suitable for analysis of food samples. Recovery values from 94% to 101% were obtained for the analysis of spiked samples. The new treatment generated by a plasma jet pen is an alternative approach to improve the electrochemical activity of 3D-printed electrodes that present sluggish kinetics with great advantages over previous protocols, including low-cost, short time of treatment (2 min), environmentally friendly protocol (reagentless), and portability (hand-held pen).