Barrier-free patterned paper sensors for multiplexed heavy metal detection

Patterned paper sensors such as letter- and barcode-shaped sensors have become a subject of growing interest due to the potential in information embedding and data interpretation, which brings a requirement on easily fabricating these paper-based analytical devices. The answer, in part, may lie in the influence of paper structure on the performance of paper-based analytical devices. After investigating the effect of physical properties of paper on precipitation and non-precipitation assays for detecting Fe(II), here we propose a simple and promising approach for barrier-free paper patterning. Without building hydrophobic boundaries, the precipitates of sensing reactions on low-bulk and medium-thick paper substrates allow patterned signal readout directly. As a proof of concept, barrier-free patterned paper sensors for detecting heavy metals were fabricated, with detection limits of 0.25 ppm for Fe(II), 0.4 ppm for Ni(II), and 0.5 ppm for Cu(II). Our work provides a valuable perspective on fabrication of patterned paper sensors.