Extruded filament electrodes for lactate biosensing in continuous-injection paper-based microfluidic devices.

Versatile electrodes were templated into poly (methyl methacrylate) (PMMA) molds with a 3D printing pen and commercial carbon black filament. The working electrodes (WE) were modified with Prussian Blue (PB), lactate oxidase (LOx)/chitosan solution, and Nafion. Under optimized modification conditions, a 3-electrode thermoplastic chip was integrated with paper-based analytical devices (μPADs). This integration was conducted by attaching a circular piece of paper to the 3-electrode thermoplastic chip, enclosing the system with a PMMA cover and clamps. The injections of the analyte solution were performed at the center of the μPADs, where the WE is positioned. The injections generated a radial flow during the analysis, eliminating the need for channels containing hydrophobic barriers to constrain the solution. This system also allows sequential/multiple injections of analyte solution, providing rapid responses in peak format. The μPADs were initially characterized with a food dye solution and ferricyanide as a redox probe. Increasing the pore size of the paper substrates increased the flow rate of the μPADs, providing sharper and more intense transient signals. In addition, increasing the injection volume produced broader peaks, also limiting the number of injections. The proposed system provided a linear range from 0.5 to 4 mmol L lactate. Requiring only 2 μL of the sample, the analytical applicability of the μPADs was further demonstrated for lactate determination through discrete sampling of real sweat. Therefore, this work brings a straightforward approach to fabricating μPADs for lactate quantification, opening possibilities for new sensing applications demanding sample volumes as small as a few μL.