Ion-selective electrodes (ISEs) are routinely used for determination of electrolytes and pH in biological samples, such as blood serum and urine. However, ISEs are sensitive to fouling by substances present in the samples. Therefore, a reliable determination of ions in e.g. blood serum requires strict measurement protocols including regular calibration, washing and conditioning of ISEs. The need for such maintenance may be one reason why ISEs are not yet widely employed in other sectors, such as process- and environmental analysis. In order to address these challenges related to sampling and maintenance of ISEs, a novel type of measurement protocol for ISEs was developed and introduced by Prof. Johan Bobacka, Dr. Grzegorz Lisak and Ms. Jingwen Cui at the Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Åbo-Turku, Finland. The new measurement protocol includes paper-based sampling that simplifies the handling and delivery of the sample to the ISEs. At the same time, paper-based sampling protects ISEs from fouling by solid impurities.
Paper is a promising medium for sampling of electrolytes in various sample types, especially when the sample contains only a small amount of liquid, which may be the case in samples from e.g. foodstuff and soil. In this approach, the ISEs and the reference electrode are simply pressed against the paper substrate to detect the concentration or activity of ions that are transported by wicking action (capillary forces) into the paper substrate. This eliminates liquid handling in the traditional way as the electrochemical sensors themselves are not immersed into the original sample during measurements. Here, only the paper is disposable while the ISEs are reused.
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. The paper substrate was also found to influence the determination of heavy metal ions (Pb2+) at low concentrations, indicating complexation of Pb2+ by chemically active groups present in paper.
In conclusion, this work shows that paper-based sampling is feasible when coupled with solid-state ion-selective electrodes. Paper-based microfluidics offer new possibilities for rapid measurement of ions and pH in problematic samples. Some limitations of paper-based sampling were identified, which highlights the need for paper substrates that are chemically more inert.
Grzegorz Lisak, Jingwen Cui, Johan Bobacka. Sensors and Actuators B: Chemical, Volume 207, Part B, 2015, Pages 933-939.
Åbo Akademi University, Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Biskopsgatan 8, FIN-20500 Åbo-Turku, Finland.
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 food stuff.