Multifolding\nVertical-Flow Electrochemical Paper-Based\nDevices with Tunable Dual Preconcentration for Enhanced Multiplexed\nAssays of Heavy Metals

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