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Electrochemical Strategies for Enabling the In-field Detection and Quantification of Per- and Polyfluoroalkylsubstances (PFAS)
Per- and polyfluoroalkyl substances (PFAS), once considered to be emerging micro-pollutants, are now a very present class of pervasive and persistent micropollutant. Frequently referred to as “forever chemicals”, once they’re in the environment, they do not break down owing to the strength of their network of carbon-fluorine bonds. Their persistence is of particular concern, as they have been shown to have a plethora of negative health effects on living things including low infant birth weights, dyslipidemia, and cancer, to name a few. Due to both their persistence and negative health effects, the ability to rapidly test waters (i.e., drinking water, river water, lake water, etc.) is of critical importance. The current “gold-standard” method for testing waters is the collection and transport of a sample to a centralized facility where chromatography and mass spectrometric methods can be performed for the separation, identification, and quantification of PFAS; however, this method is not able to be used for real-time analyses and is not sufficient for efficiently informing consumers or remediation efforts. An in-field detection method that is capable of providing real-time analyses is needed.
Electrochemistry stands well-poised to offer a suite of techniques that can be used for in-field detection. Electrochemistry is cost-effective, easy to perform and analyze, and readily portable; however, it lacks specificity and typically requires an electroactive analyte. These limitations can be overcome through the use of a surface functionalization strategy which adds specificity through the imprinting of the analyte of interest and monitors the change in signal from an alternate mediator molecule. Molecularly imprinted polymers (MIPs) are the chosen surface functionalization strategy that will be used and discussed in this work. While MIPs overcome the specificity and requirement of an electroactive analyte limitations and have been previously demonstrated for the detection of perfluorooctane sulfonate (PFOS), they traditionally require the use of added buffers and one electron mediators, which are not found in natural waters. Thus, to expand MIP-based electrochemical detection to in-field use strategies must be developed and employed to mitigate these concerns.
This work provides significant strides forward in enabling in-field, MIP-based electro-chemical sensing. We take advantage of ambient dioxygen present in river water to quantify one of the more harmful PFAS molecules, perfluorooctane sulfonate (PFOS), from 0 to 0.5 nM on a MIP-modified carbon substrate. Differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) generated calibration curves for PFOS in river water using oxygen as the mediator. Importantly, we show that electrochemical impedance spectroscopy is superior to voltammetric techniques: like ultramicroelectrodes, this technique can be used in low-conductivity matrices like river water with high reproducibility. Further, impedance provides a PFOS limit of detection of 3.4 pM. We also demonstrate that the common interferents humic acid and chloride do not affect the sensor signal. The use of dioxygen is predicated on the assumption that there will be consistent ambient dioxygen levels in natural waters. This is not always the case in hypoxic groundwater and at high altitudes. To overcome this challenge, and further advance the strategies that will enable in-field electroanalysis of PFAS, we demonstrate that dioxygen can be generated in solution through the hydrolysis of water. The electrogenerated dioxygen can then be used as a mediator for molecularly imprinted polymer (MIP)-based electroanalysis. We demonstrate that calibration curves can be constructed with high precision and sensitivity (LOD > 1 ppt). We also demonstrate the development and use of a universal multiplexer and electrode array, which can enable high throughput, in-field electroanalysis for a wide variety of compounds. In this work, we demonstrate it specifically for detecting PFOS from 0.05 to 0.05 nM and lead at a concentration of 1 nM.
Additionally, in this work, we lay the groundwork for the future direction of developing a more fundamental understanding of MIPs to be able to fine-tune their selectivity and performance. Preliminary data and experimental approaches are shown for using nanoparticle deposition and visualization, with scanning electrochemical microscopy, to characterize surface reactivity and binding site distribution, functional group studies to better understand what groups and molecular interactions affect the binding of the analyte to the MIP the most, and using cyclic voltammetry to determine the capacitance and resistance of the polymer. Further approaches are outlined to relate the conditions under which the polymer was created to the polymer’s characteristics and then the polymer’s performance. Future improvements to make the in-field use of the multiplexer more efficient are also shown. In total, this work shows the feasibility and nearness of in-field, MIP-based electrochemical detection for PFAS by advancing the strategies and hardware necessary to do so.
- Doctor of Philosophy
- West Lafayette