title: Heavy Metal Ion Extraction and Electrochemical Detection with a Thermomorphic Ionic Liquid
Creators: name: Schmitt, Fabian
Publisher: name: UNSPECIFIED
Year: 2023

description: Heavy metals comprise a large proportion of the elements in the periodic table and can be found anywhere from the earth's crust to its core. Depending on their concentration, some perform important functions in living nature, but can also pose a risk to health, life, and the environment. Unlike organic materials, they are not biodegradable and can therefore accumulate. Human activities additionally bring heavy metals to the earth's surface. Corrosion produces ionic, water-soluble species. As these pose a risk even in low concentrations, the World Health Organization (WHO) proposes upper limits, such as 5 μg L⁻¹ for lead (Pb) in tap water. The concentrations are usually monitored using established, highly sensitive laboratory methods such as atomic absorption spectrometry (AAS) or inductively coupled plasma optical emission spectrometry (ICP-OES). However, these are stationary, investment-intensive methods and require laboratory infrastructures and trained personnel. In order to meet point-of-care (PoC) requirements, mobile devices for electrochemical detection methods have been developed and their potential demonstrated in scientific work. So far, however, these applications have not been able to establish, while electrochemical sensors continue to be used in other areas, e.g., for pH and blood sugar measurements.
To increase the sensitivity and selectivity of electrochemical heavy metal detection, the electrodes used can be chemically modified. Modified electrodes usually consist of various carbon materials such as glassy carbon, graphite, or nanotubes to which bismuth, which has replaced mercury in this function for environmental reasons, and Nafion®, a perfluorinated polymer with sulfonic acid functionality, are applied. In addition, ion selectivity can be increased by modification with metal ion-complexing molecules. Electrode materials with limits of detection (LOD) far below the WHO specifications are described in the scientific literature.
In addition to (waste) water treatment, the extraction of heavy metals is also motivated by the recovery of metals due to their economic value. In the field of hydrometallurgy, research is being conducted into extracting agents that are selective and enable high extraction coefficients. Ionic liquids (ILs) can be used for this purpose.
ILs are salts with melting points below 100 °C or room temperature (RTIL) and form a class of substances with negligible vapor pressure and electrical conductivity. Due to the large number of possible combinations of anions and cations and the modification of the chemical constitution, particularly in the case of organic ions, and the associated adjustable physico-chemical properties, ILs are intensively studied, and electrochemical applications are being investigated. In electrochemical heavy metal detection, ILs have so far been used almost exclusively as binders in electrode preparation. The IL betaine bis(trifluoromethylsulfonyl)imide [HBet][NTf₂], whose selective extraction properties for metal ions from aqueous solution were investigated, is able to dissolve various metal oxides, and the subsequent electrochemical detection was documented. The mixture of [HBet][NTf₂] and water exhibits thermomorphic phase behaviour. Above the upper critical solution temperature (UCST), a single-phase system is present for all compositions. When the mixture cools down, the IL-rich and water-rich phases separate. This behaviour can be exploited for extraction, whereby stirring can be avoided to increase the phase exchange surface. This could have advantages in a mobile application with small volumes and highly viscous ILs.
Another discipline within PoC is the development of integrated, microfluidic sensors. For single use, these should be small and consist of inexpensive, available, and non-critical materials. Paper-based materials fulfil these requirements as a fluid channel and substrate and are already a prominent representative in this function.
In order to avoid disturbing interferences during detection with other metal ions in a sample and at the same time to enable the use of inexpensive working electrodes, electrochemical methods with upstream extraction of the analyte (here Pb²⁺) are to be investigated in this work. An IL shall be employed as both the extraction agent and the electrolyte. Finally, this concept is to be transferred to the application in paper-based, microfluidic sensors. The IL must therefore combine the requirements of an extracting agent, an electrolyte, and a fluid in a paper channel.
Despite comparatively high water uptake, the IL [HBet][NTf₂] is suitable due to its extraction properties provided by the carboxy function in the cation and was chosen for this work. Additionally, it features thermomorphic properties. Water-saturated [HBet][NTf₂] could be characterized using the Walden plot. The extraction of Pb²⁺ from aqueous medium after heating above the UCST was confirmed by ICP-OES. The analyte concentration could be increased both by increasing the initial water-to-IL ratio and by adding zwitterionic betaine to the mixture. However, increasing the amount of water ultimately led to complete dissolution of the IL-rich phase.
Pb²⁺ could be detected in the IL after extraction using square wave voltammetry (SWV), whereby method parameters can be optimized for stronger signals starting from parameters for aqueous systems. Measurement at elevated temperatures (42 °C) leads to further signal amplification, which can be explained by the lower viscosity and higher electrical conductivity. The addition of zwitterionic betaine can amplify the measured signals, but the conductivity decreases with increasing mass ratio of the zwitterionic betaine, and the effect is no longer detectable. The strong increase in the concentration during extraction therefore only leads to larger signals in electrochemical detection to a limited extent.
The signal intensity of the SWV could be further increased by modifying the working electrodes with Nafion® and bismuth. The strongest effect was seen when bismuth was dissolved during electrode preparation. The addition of carbon materials (carbon black or graphite powder) did not lead to an increase but revealed the need for further parameter adjustments when changing the electrode material.
A combination of increased initial water-to-IL ratio and betaine addition during extraction with temperature increase and electrode modification during detection led to a limit of detection (LOD) of 30 ppb. This is above the WHO guidelines but demonstrates the potential of the combination of extraction and detection.
Finally, the detection of Pb²⁺ in [HBet][NTf₂] after extraction was carried out in a paper-based cell. Due to the flow rate of IL in paper, material containing glass fibres was used for this purpose, although detection was conducted without flux. Despite shorter times for enrichment of the analyte and with non-optimized extractions and electrodes, signal intensities were measured that were ten times higher and resistances that were a hundred times lower compared to bulk cell experiments. In addition, the small dimensions of the paper-based cell with the small electrode spacing allow the use of small amounts of electrolyte.
In summary, this work demonstrated the combination of extraction with subsequent electrochemical detection using the thermomorphic IL [HBet][NTf₂]. Possibilities for analyte enrichment in the electrolyte and increasing the signal intensities were identified and utilized. The concept could be transferred to paper-based cells. The small electrode spacing reduces the cell resistance and higher signal intensities could be measured.