Multifolding Vertical-Flow Electrochemical Paper-Based Devices with Tunable Dual Preconcentration for Enhanced Multiplexed Assays of Heavy Metals.

This work describes fully integrated multifolding electrochemical paper-based devices (ePADs) for enhanced multiplexed voltammetric determination of heavy metals (Zn(II), Cd(II), and Pb(II)) using tunable passive preconcentration. The paper devices integrate five circular sample preconcentration layers and a 3-electrode electrochemical cell. The hydrophobic barriers of the devices are drawn by pen-plotting with hydrophobic ink, while the electrodes are deposited by screen-printing. The devices exploit the wicking ability of cellulose paper to perform passive preconcentration of the target analytes, resulting in a ∼6-fold signal enhancement. For this purpose, drops of the sample are placed at the five sample pads of the preconcentration layers, the device is folded, and the target metals are eluted in a vertical-flow mode to the electrochemical cell, where they are measured directly by anodic stripping voltammetry (ASV). The working electrode of the ePADs is bulk-modified with bismuth citrate; during the ASV measurements, the bismuth precursor is converted to nanodomains of metallic bismuth at the surface of the working electrode. By combining the triplex signal amplification through passive preconcentration, electrochemical preconcentration, and judicious working electrode modification with generated bismuth nanoparticles, ultrasensitive and multiplexed heavy metal assays can be achieved. Due to their high degree of integration, low cost, easy and fast fabrication, and sensitivity, the multifolding ePADs are particularly suitable for on-site heavy metals' monitoring applications.