Electrical/ Electrochemical Impedance Measurement for Biological Applications
Foodborne diseases caused by foodborne pathogens have been a serious threat to public health for decades and remain one of the major concerns of our society. The development of rapid, sensitive, and specific methods is central to implementing effective practice to ensure food safety and security. We have been using impedance technique as a means to detect and/or quantify foodborne pathogenic bacteria. This paper will particular introduce the recent significant development in this field, including the use of microfabricated microelectrodes, microfluidic chip-based devices, nanoparticles and integration with other techniques such as dielectrophoresis. Based on the fundamental impedance microbiology, which is a technique based on the measurement of changes in electrical impedance of a culture medium or a reaction solution resulting from the bacterial growth, we have added some new aspects to this subject that have made the impedance technique a more valuable technique for detection of foodborne bacterial pathogens. These aspects include the use of different electrode systems and the analysis of impedance components using equivalent circuits for better improvement on the detection systems. The advances in microfabrication technologies have launched the use of microfabricated microarray electrodes in impedance detection and the miniaturization of impedance microbiology into a chip format, which has shown great promising in rapid detection of bacterial growth. Further, the integration of impedance technique with biosensor technology has led to the development of impedance biosensors that is expending rapidly for bacteria detection in recent years. Impedance biosensors for bacteria detection are based on the analysis of electrical properties of bacterial cells when they are attached to or associated with the electrodes. Such impedance biosensor methods have substantially reduced the assay time down to 30 min~2 h compared with growth-based impedance methods. The dimensional compatibility of those microfabricated biosensors with the target bacterial cells has enabled them to detect the binding of bacterial cells on its surface without any amplification step.
On the other hand, electrical impedance spectroscopy (EIS) enables direct sensing of cellular activities occurring on an electrode surface by measuring the induced capacitance and/or resistance changes by cells attached on the electrode. Particular, EIS is capable of distinguishing different types of cells based on the cellular activity-induced electrical signals. We have demonstrated the use of real-time impedance technique to study the cellular activities of oral cancer cells in a label-free manner, including cell proliferation, adhesion, spreading, and apoptosis induced by drug treatments. Furthermore, it is known that during transformation from non-cancer cells to caner cells, the cellular morphology, structure, and function are altered, which may includes, but are not limited to changes in the nuclei, composition of the cytoplasm, structure of the cell membrane, changes in the nuclei, composition of the cytoplasm, structure of the cell membrane, and expression levels of ion channels, etc. These changes may result in alterations in cancer cell properties that could be detected by the EIS well before these morphological features that normally used in detection can be seen or biomarkers are developed. We have explored the use of real-time impedance measurements to distinguish oral cancer cells from normal oral epithelial cells in label-free manner by studying the kinetics of cell spreading and attachment of the two cell types.