Electrochemical Tape-and-Paper-Based Sensor for the Quantification of Potassium

Abstract

Potassium levels in serum are used in the diagnosis of diseases involving cardiac arrhythmias, neuromuscular weakness, and chronic kidney diseases. These illnesses are becoming more prevalent, therefore, developing new potassium quantification methods would aid in advancing preventative care. Current methods of quantifying potassium mainly rely on the use of glass ion-selective electrodes which are costly, fragile, and requires frequent maintenance and recalibration. For faster and more accessible quantification of potassium, we are developing low cost, portable, and easy to fabricate electrochemical tape-and-paper-based devices. Our sensor bypasses the inconveniences of ion-selective electrodes and could ultimately serve as a point-of-care device to allow for regular monitoring or even home-use. Our sensing method relies on Prussian blue immobilized on the surface of electrodes as a potassium recognition element. Potassium ions intercalate into the Prussian blue lattice and subsequently changes the electrochemical characteristics of Prussian blue such as the redox peak potentials. These devices are highly robust, feature a limit of detection of 1.3 mM potassium and the response is linear to at least 100 mM, which contains the clinically relevant ranges required for diagnostics. Quantification was developed using cyclic voltammetry, demonstrated in Chapter 3. We observed changes in Prussian blue redox peak potentials at different concentrations of potassium and followed the expected Nernstian response. We investigated multiple methods of immobilizing Prussian blue onto the electrode surfaces to investigate stability and reproducibility in Chapter 4. Adsorption, in-situ synthesis, and carbon paste incorporation of Prussian blue was tested. Prussian blue-carbon paste devices had reproducibility issues and featured broad reduction peaks. In-situ synthesis of Prussian blue directly onto the surface of the electrodes also featured broad reduction peaks but the Prussian blue response was reproducible. The issue with in-situ synthesis was the stability of the Prussian blue layer, which was susceptible to degradation after repeated use of the device, which is required for evaluating the performance of the device. Although adsorption using Prussian blue in water had some reproducibility issues as well, this method led to the most stable Prussian blue layer, had distinct reduction peaks, and was simple to perform. Various solvents were used to dissolve Prussian blue in Chapter 5 to investigate methods of increasing device reproducibility when using adsorption. A few organic solvents were able to dissolve Prussian blue to form a stable solution with the goal of forming a more uniform Prussian blue layer and potentially improving consistency of the layer immobilization. While these alternative solvents were able to dissolve Prussian blue, they also damaged the graphite electrodes on the devices, which altered the electrochemical responses of the devices to the point where potassium quantification was no longer possible. Due to incompatibility between these alternative solvents and the devices, adsorption of Prussian blue in water continued to be used. Different modes of adsorption were explored and was optimized in Chapter 6. By altering the adsorption setup and allowing the Prussian blue particles to settle evenly onto a level electrode surface, device reproductivity increased substantially. To understand the applicability of the devices in real samples, interferent studies were performed in Chapter 7. Other cations such as Na+, Li+, Ca2+, Mg2+, and Ba2+ were not observed to enter the Prussian blue lattice in the cyclic voltammograms. Monovalent cations that share the same charge as K+ but have smaller ionic radius, Na+ and Li+, were able to decrease K+ sensitivity. Divalent cations that had a smaller ionic radius than K+ did not alter sensitivity. The exception was Ba2+, which also decreased K+ sensitivity. These results suggested that both ionic radius and charge of a species were important factors in impacting K+ intercalation into the Prussian blue lattice. Other interferents such as sulfates, phosphates, carbonates, urea, and lactic found in serum and sweat samples were tested. The presence of these interferents decreased the current intensity of the reduction peak of Prussian blue, which resulted in less definition in the peaks. For the future of this project, the effects of interferents found in serum and sweat must be investigated further. Additionally, reproducibility of the devices could be improved further if less harsh organic solvents are tested for adsorption, square wave voltammetry could be used for quantification to evaluate the viability of alternative voltametric techniques, and Prussian blue analogues could be implemented into the devices for quantification of other cations.