Analysis of Electrochemical Impedance Spectroscopy and Its Application in Detection of Toxic Chemicals in Beverage

Abstract

Food safety is a major concern in today’s world causing serious health and financial consequences for food industry and health care services. The presence of toxic chemicals in food materials is one of the problems that food packaging industry should overcome to ensure public health. For example, the addition of plasticizers such as phthalate family (like di(2-ethylhexyl) phthalate, DEHP) to plastic bottles may result in diffusion of these materials in bottled soft drinks, water and juice. The detection of these toxins would help the retail stores and consumers to determine whether it is safe to drink bottled beverages. One way to determine the chemical ingredients of a beverage is to use electrochemistry impedance spectroscopy (EIS), where samples of contaminated and safe beverage respond differently to the electrical signal.

This work presents a thorough analysis of electrochemical phenomena and EIS responses of aqueous samples with chemical contamination. MEMS (microelectromechanical system) biochemical sensors are used to study the response of an aqueous solution to frequency spectrum using equivalent electrical circuits that can simulate the electrochemical reactions in the system. Electrochemical phenomena such as double-layer formation, charge transport in the bulk solution, and charge diffusion at the sensor-solution interface are studied. Different equivalent circuits are designed using a variety of combinations of capacitive and resistive components such that circuit can properly simulate the behavior of the sample. A comprehensive study is conducted to compare different circuit models and compare their simulation results. The experimental/simulation analysis show that it is possible to use equivalent circuits with different components, configurations and level of complexity, and still accurately predict the behavior of the system by each of them, as long as the circuit and its elements provide physical significance. Moreover, the effect of sensor coating/contamination on the response of the system is studied, and qualitative and quantitative analyses are performed using two geometrically identical sensors with different surface coatings. The results of this analysis verify the selection of electrical components and electrical circuit in determining the changes in electrochemical phenomena as the sensor or the solution change.

After comprehensive EIS analysis, the MEMS sensors are used to detect toxin chemicals in bottled beverages. Three beverages, water, orange juice and unsweetened tea, are used and different concentrations of DEHP (as the toxic contaminant) are tested. The experimental data shows that the sensors capture low concentrations of DEHP in different beverages. However, each sample reacts differently to the presence of DEHP molecules. For water, the sensor can capture small changes in DEHP concertation and provide distinct EIS responses with high sensitivity. For samples with ionic or polarized molecules, such as orange juice (citric acid) and unsweetened tea, the addition of another polar species (DEHP molecules) results in a smaller change in EIS response of the sample, as the concentration of DEHP changes. This makes the sensor less sensitive to the change of DEHP concentration. Nonetheless, the sensors and test setup used in this work successfully detect the added chemicals to a beverage of interest. This work has a great significance in low cost, rapid and effective detection of toxins in food materials, and the results of this work may be extended to detection of cells and biomarkers as the future work in the research team at SIUE.