Acidified paper substrates for microfluidic solution sampling integrated with potentiometric sensors for determination of heavy metals

To address analysis of heavy metals in samples containing low volumes and/or high solid-to-liquid contents, microfluidic paper-based solution sampling coupled with Pb2+-ISEs was investigated. The acidification of paper substrates prior to their use was found effective at eliminating the otherwise observed super-Nernstian response of Pb2+-ISEs. Acidified paper substrates (at different pH) were evaluated and through 3D approximate (concentration-EMF-pH) analyses of the Pb2+-ISEs responses coupled with acidified paper substrates, the acceptable operational pH range (for paper substrate containing sample solution) of the potentiometric sensors was found to be between 3-4. Furthermore, to simplify the analytical protocol of heavy metal sensing using Pb2+-ISEs coupled with microfluidic paper-based solution sampling, a colour-coded matching graph was proposed to match the appropriate acidified paper substrate to the pH of the unbuffered sample solution. Such an approach was used for determination of Pb2+ in various complex environmental samples with small volumes of solution and/or high solid-to-liquid ratios, such as wet soil, street runoff, chemical spillage and plant dew. Although the samples were characterized with high ionic complexity, determinations by potentiometry and ICP-OES were found to be comparable, which validates the use of acidified paper substrates in reliable determination of lead(II) in complex sample matrices.

Automated summary

The combination of acidified paper substrates and Pb2+-ISEs proved to be effective in simplifying the analysis of heavy metals in complex samples with low volumes and/or high solid-to-liquid ratios. By evaluating acidified paper substrates at different pH levels and analyzing the responses of Pb2+-ISEs, an acceptable operational pH range for potentiometric sensors was determined to be between 3-4. The proposed colour-coded matching graph can be used to match appropriate acidified paper substrates to the pH of unbuffered sample solutions, allowing for reliable determination of lead(II) in various complex environmental samples.