Corrective protocol to predict interference free sensor response for paper-based solution sampling coupled with heavy metal sensitive ion-selective electrodes.

Paper-based microfluidics combined with potentiometric measurement has emerged as an attractive approach for detecting various chemical ionic moieties. Detection of heavy metal ions, using paper substrates as solution sampling and delivery systems remains challenging despite efforts to introduce several physico-chemical paper substrate modifications to stop adsorption of ions onto the paper substrates. This study quantitatively investigates the adsorption of heavy metal ions on the paper substrates during paper-based potentiometric measurements and explains the super-Nernstian response of potentiometric sensors through local depletion of heavy metal ions from the solution. Consequently, based on the investigated ion adsorption, a corrective potential protocol was established for the electrodes coupled with paper-based solution sampling by predicting interference free sensor response from paper-based measurement. Furthermore, the ion adsorption was also recorded for mixed metal ion solutions to understand competitive primary/interfering ions adsorption onto the paper substrates and establish corrective measures to predict interference free sensor response. In this method, no modifications of the paper substrates are necessary before actual potentiometric measurements. The proposed corrective protocol allows prediction of sensor response based on the paper-based solution sampling potentiometric measurement, providing a simple methodological approach based on correction of potential readout of the potentiometric sensor, thus completely resigning from the need of modifying paper substrate for measurements of heavy metal ions.

Ion-selective membrane modified microfluidic paper-based solution sampling substrates for potentiometric heavy metal detection.

Paper-based microfluidic solution sampling is a viable option for potentiometric sensors to be used for the determination of analytes in samples with high solid-to-liquid ratios. Unfortunately, heavy metal sensitive electrodes cannot be easily integrated with paper-based solution sampling as heavy metals have strong physicochemical adsorption affinity towards paper substrates. In this work, paper substrates were modified with an ion-selective membrane (ISM) cocktail (used for the preparation of Pb2+-ion-selective electrodes (ISEs)) and coupled with model heavy metal Pb2+-ISEs. It was found that the super-Nernstian response of Pb2+-ISEs was eliminated when 10 to 50 mg ml-1 of the ISM cocktail was used for the modification of paper substrates. The modification of the paper substrates by Pb2+-ISM allowed the elimination of adsorption sites. In addition, it resulted in an improvement of sensor performance in terms of their detection limits to be similar to those for conditioned electrodes in standard beaker-based measurements. It is believed that the elimination of super-Nernstian response of the electrodes and improving the potentiometric responses and detection limits of ISEs were attributed to the compatibility improvement of the paper substrates and Pb2+-ISEs to the same type of ISM.

Non-equilibrium potentiometric sensors integrated with metal modified paper-based microfluidic solution sampling substrates for determination of heavy metals in complex environmental samples.

Metal modified paper-based substrates were utilized for microfluidic paper-based solution sampling coupled with Pb2+-ion selective electrodes (ISEs) with the aim of controlling the super-Nernstian response which usually occurs when using unmodified paper substrates. Potentiometric responses of Pb2+-ISEs coupled with gold, platinum and palladium coated paper substrates were investigated. Potentiometric response time was found to be predominantly dependent on the thickness of metallic layer deposited at the paper substrates. Paper-based substrates coated on both sides with 38 nm gold layers were found to be the most advantageous in controlling the super-Nernstian response of ISEs at non-equilibrium conditions. Durability studies indicated that the lifetime of Pb2+-ISEs could be doubled when used with paper-based substrates in complex environmental samples with high solid-to-liquid content. Determination of lead in real samples using metal modified paper substrates coupled with Pb2+-ISEs was validated by inductively coupled plasma optical emission spectrometry (ICP-OES). Detailed life cycle assessments were performed for model screen printed potentiometric sensors with and without metal modified paper-based solution sampling substrates. The results confirmed that the use of modified paper substrates demonstrated lower environmental impact per potentiometric measurement of Pb2+-ISE in prepared simulated environmental sample as compared to sensors without the use of paper substrates.