Potentiometric heavy metal sensing integrated with paper-based microfluidic solution sampling

• Main Author: Silva Mahamalage Rochelle Udani
• Other Authors: Grzegorz Lisak
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Environmental engineering; Engineering::Materials
• Online Access: https://hdl.handle.net/10356/170231
• Institution: Nanyang Technological University

Paper-based microfluidic solution sampling is a viable option for potentiometric sensors to be used for determination of analytes in samples with high solid-to-liquid ratios. Unfortunately, potentiometric heavy metal sensitive electrodes cannot be easily integrated with paper-based solution sampling as heavy metals have strong affinity towards the paper matrix. This work addresses development of paper modifications to diminish, control or eliminate the heavy metals-paper matrix interactions in order to reliably measure ion activities in microvolumes of analytes when ion sensors are coupled with paper-based solution sampling. In the first study, 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. 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. Characterization of paper substrates showed that the favourable quality of paper substrates being able to absorb the analyte was preserved while the thin metallic layers blocked much of the negatively charged sites on the paper substrates. Durability studies indicated that the lifetime of Pb2+-ISEs could be doubled when ISEs are used with paper-based substrates in complex environmental samples with high solid-to-liquid content compared to when ISEs are used without a paper-based substrate. 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 as compared to sensors without the use of paper substrates. In the second study, paper substrates were modified with ion-selective membrane (ISM) cocktail (used for Pb2+-ion-selective electrodes (ISEs) preparation) and coupled with model heavy metal Pb2+-ISEs. Upon investigations with various compositions of ISM cocktail, it was found that the super Nernstian response of Pb2+-ISEs was eliminated when 10 to 50 mg ml-1 ISM cocktail was used for modification. The modification of the paper substrates by Pb2+-ISM allowed elimination of adsorption sites. Also, it resulted in improvement of sensor performance in terms of their detection limits to be similar of those for conditioned electrodes in standard beaker-based measurements. It is believed that the elimination of super-Nernstian response of the electrodes, improving the potentiometric responses and detection limits of ISEs was attributed to compatibility improvement of the paper substrates and Pb2+-ISEs. In the third study, the selectivity of Pb2+-ISEs coupled with newly introduced paper-based sampling strategies was studied using the separate solutions method (SSM).The results confirmed that the modifications of paper-based substrates to improve the potentiometric performance with heavy metals influence selectivity of the Pb2+-ISEs, however does not adversely impact overall selectivity pattern and ability of the Pb2+-ISEs to discriminate interfering ions in wide concentration range. The selectivity coefficients were also compared with those obtained with solution-based sampling, unmodified paper-based sampling and a previously introduced paper-based substrate modification in the literature.