Paper-based microfluidic sampling for potentiometric determination of ions

In this work, paper-based microfluidic sampling was used together with potentiometric determination of ions (Cd2+, Cl-, Pb2+) and pH. Calibration solutions and samples were absorbed into paper substrates and potentiometric detection was performed by placing solid-state ion-selective electrodes (ISEs) and a reference electrode in direct contact with the paper substrate. The paper substrate was found to influence the determination of Pb2+ at low concentrations, indicating complexation of Pb2+ by chemically active groups present in paper. Paper-based microfluidic sampling was successfully applied in measurements of pH in various foodstuffs and environmental samples by employing a conventional pH glass electrode. However, some influence of the paper substrate on the measured pH was observed for samples with pH close to neutral. Despite some limitations, paper-based microfluidic sampling with potentiometric sensing opens new possibilities for direct chemical analysis of specific sample types. The method described in this work is particularly interesting for analysis of small sample volumes with a high content of solid impurities and for analysis of e.g. liquid or moisture from polluted surfaces and foodstuff.