Development of a paper-based ultra-microfluidic analytical device based on low-cost materials for electrochemical sensing of folic acid

Abstract

The evolution of miniaturization has given rise to microfluidics, marking a significant advancement in chemical analysis. The pioneering work of Andreas Manz and his team in developing a chip-like platform for capillary electrophoresis in the early 1990s, characterized by its simplicity, costeffectiveness, rapid in-situ analysis capability, ultra-sensitivity, and lack of sample pre-treatment steps, laid the groundwork for the invention of the first paper-based analytical device by Whitesides and coworkers in 2007. Since then, there has been a growing interest in developing microfluidic paper-based analytical devices (µPADs). Among various detection techniques, electrochemical sensing has been widely adopted in paper-based microfluidics due to the ease of fabricating electrodes, the availability of portable potentiostats, and the ability to integrate established methods. In this study, a simple μPAD was developed for the detection of folic acid (FA). It was chosen as the target analyte due to the increasing need for improved FA quantification driven by its physiological importance, as suboptimal intake can lead to adverse health effects such as severe birth defects of the brain and spinal cord in developing embryos, carcinogenesis, anemia, cardiovascular disease, and psychiatric disorders. Cyclic voltammetry (CV) was performed in the presence of phosphate-buffered saline (PBS) to maintain a pH of around 7 and also to serve as a supporting electrolyte to decrease the solution resistance. Varnish was used as the binding material to fabricate hydrophobic barriers due to its superior hydrophobic properties, higher viscosity, flow control, adhesiveness, slower drying time, and enhanced sealing capabilities compared to super glue and nail polish. Among various screen-printed electrodes, carbon paste electrodes (CPEs) effectively overcome limitations such as fracturing during polishing and irregular surface formation. In this study, the CPE exhibited better electrochemical properties due to its chemical inertness, excellent electron transfer, and ease of fabrication, as well as uniform consistency when a 1:1 mass ratio of graphite powder to varnish was employed. The incorporation of sodium alpha-olefin sulfonate (SAOS) into the CPE was found to be effective in improving the detection limits. To assess sensitivity, the limit of detection (LOD) and limit of quantification (LOQ) were determined using LOD = 3.3 (Sy/S) and LOQ = 10 (Sy/S), based on the standard deviation (Sy) and the calibration slope (S). The modified CPE exhibited superior analytical performance by lowering the overpotential and detection limits, with a satisfactory linear relationship (R² = 0.9826) observed over the range of 0.795- 7.95 mM. The LOD and LOQ values of 0.50 mmol/L and 1.53 mmol/L, respectively, demonstrated the high sensitivity and promising potential of the developed µPAD. Furthermore, this method is wellsuited for detecting similar redox-active analytes, which can be identified through reduction within the water window and oxidation at potentials above the water window.