A Facile Method to Fabricate an Enclosed Paper-Based Analytical Device via Double-Sided Patterning for Ionic...

Microfluidic paper-based analytical devices (μPADs) have been developed for use in a variety of diagnosis and analysis fields. However, conventional μPADs with an open-channel system have limitations for application as analytical platforms mainly because of the evaporation and contamination of the sample solution. This study demonstrates the design and fabrication of an enclosed three-dimensional(3D)-μPAD and its application as a primary early analysis platform for ionic contaminants. To generate the hydrophobic PDMS barrier, double-sided patterning is carried out using a PDMS blade-coated stamp mold that is fabricated using 3D printing. The selective PDMS patterning can be achieved with controlled PDMS permeation of the cellulose substrate using 3D-designed stamp molds. We find the optimal conditions enabling the formation of enclosed channels, including round shape pattern and inter-pattern distance of 10 mm of stamp design, contact time of 0.5 min, and spacer height of 300 µm of double-sided patterning procedure. As a proof of concept, this enclosed 3D-μPAD is used for the simultaneous colorimetric detection of heavy metal ions in a concentration range of 0.1-2000 ppm, including nickel (Ni²⁺), copper (Cu²⁺), mercury (Hg²⁺), and radioactive isotope cesium-137 ions (Cs⁺). We confirm that qualitative analysis and image-based quantitative analysis with high reliability are possible through rapid color changes within 3 min. The limits of detection (LOD) for 0.55 ppm of Ni²⁺, 5.05 ppm of Cu²⁺, 0.188 ppm of Hg²⁺, and 0.016 ppm of Cs+ are observed, respectively. In addition, we confirm that the analysis is highly reliable in a wide range of ion concentrations with CV values below 3% for Ni²⁺ (0.56%), Cu²⁺ (0.45%), Hg²⁺ (1.35%), and Cs⁺ (2.18%). This method could be a promising technique to develop a 3D-μPAD with various applications as a primary early analysis device in the environmental and biological industries.