Sponge and Paper Substrates Coupled with Ion Sensors Utilized in Microfluidic Sampling of High Solid-to-Liquid Ratio Samples
Authors: Ding Ruiyu , Lisak Grzegorz 
Published: May 2020

The on-site analysis of any ion concentrations in environmental samples faces many challenges, such as need of sample manipulations and use of a non-portable analytical instrumentation that in turn results in higher uncertainties of the determination and the same time the delay between sampling and the actual measurement. On the other hand, wearable sensors gain more and more attention for instant monitoring of clinically relevant analytes. The portable and relatively low-cost ion-selective electrodes (ISEs) have been extensively studied in context of on-site analysis of ions. However, their potentiometric response majorly depends on the physico-chemical state of an ion-selective membrane (ISM) and the protocol of the analysis. In the situation of nature samples containing high solid-to-liquid ratio, paper substrates and textile were investigated before to integrated with the ISEs to protect the ISM from mechanical damage and the same time deliver micro volumes of liquid to the electrode [1, 2]. Nevertheless, measurements of pH and heavy metals were found to be affected by side reactions and nonspecific adsorption of metal ions to the sampling substrates.
In this study, method of pre-modification of paper substrates was investigated to eliminate the detrimental super-Nernstian response otherwise observed [3]. Potentiometric determination of heavy metals in environmental samples (Cd2+ and Pb2+) utilizing paper based microfluidic sampling was studied with a commercial crystalline membrane solid-state Cd2+-and a solid-contact Pb2+-ion selective electrodes. Pre-treatment of paper substrates in the solutions containing the salt with cation the same as the ISEs primary cation, successfully eliminated super Nernstian response. Moreover, the concentration dependent sorption-desorption of metal ion to/from paper substrates was found as possible interferences of using microfluidic paper-based sampling in heavy metal analysis. Then, the amount of heavy metal ions binding onto different substrates was detected.Owning to the lowest heavy metal sorption capacity of PU sponge, microfluidic sponge-based sampling was developed and studied as novel sampling and sample handling substrate to serve as alternative for solution sampling for ion analysis. Possibility of direct measurements of ions in such high solid-to-liquid ratio samples as mud, sludge, wet soil were explored [4].

References:

[1] J. Cui, G. Lisak, S. Strzalkowska, J. Bobacka, Potentiometric sensing utilizing paper-based microfluidic sampling, Analyst. (2014). doi:10.1039/c3an02157b.
[2] G. Lisak, T. Arnebrant, T. Ruzgas, J. Bobacka, Textile-based sampling for potentiometric determination of ions, Anal. Chim. Acta. (2015). doi:10.1016/j.aca.2015.03.045.
[3] R. Ding, V. Krikstolaityte, G. Lisak, Inorganic salt modified paper substrates utilized in paper-based microfluidic sampling for potentiometric determination of heavy metals, Sensors Actuators, B Chem. (2019). doi:10.1016/j.snb.2019.03.079.
[4] R. Ding, G. Lisak. Sponge-based microfluidic sampling for potentiometric ion sensing. (2019) Analytica Chimica Acta.

Figure 1