Paper-based microfluidic sampling and separation of analytes for potentiometric ion sensing

This work demonstrates a paper-based microfluidic sampling and separation platform that allows potentiometric sensing of chloride ions in presence of strongly interfering salicylate ions using a solid-contact ion-selective electrode as a detector. The device was composed of two pieces of paper with different shapes and pore sizes. A “T” shaped filter paper with a pore size of 12–25 μm was used as the detection zone. A filter paper with a pore size of 2.0 μm was modified with a complexing agent (Fe³⁺) and served as the separation zone. The two pieces of the paper were joined together just like a jigsaw. A solid-contact Cl⁻ −selective electrode and a reference electrode were gently pressed onto the detection zone to create a direct contact between the electrodes and the solution absorbed in the paper. Utilizing the possibility to form stable complexes between Fe³⁺ and salicylate, the proposed platform enables the separation of salicylate and detection of chloride. This system offers a convenient platform for both sampling and separation of ions, in which sample pretreatment procedures can be simplified or avoided.