Available online at www.sciencedirect.com Talanta 74 (2008) 1621–1629 Electrical impedance spectroscopy for detection of bacterial cells in suspensions using interdigitated microelectrodes Liju Yang ∗ Biomanufacturing Research Institute & Technology Enterprise (BRITE), and Department of Pharmaceutical Sciences, North Carolina Central University, Durham, NC 27707, United States Received 8 August 2007; received in revised form 6 October 2007; accepted 8 October 2007 Available online 18 October 2007 Abstract In this study, we present a new, simple and rapid impedance method to detect bacterial cells by making use of the impedance properties of bacterial cell suspensions using interdigitated microelectrodes. It was found that bacterial cell suspensions in deionized (DI) water with different cell concentrations could generate different electrical impedance spectral responses, whereas cell suspensions in phosphate buffered saline (PBS) solution could not produce any significant differences in impedance spectra in response to different cell concentrations. In DI water suspensions, impedance at 1 kHz decreased with the increasing cell concentrations in the suspensions. The impedance of cell suspensions in DI water was discussed and found that it was resulted from the cell wall charges and the release of ions or other osmolytes from the cells. A linear relationship between the impedance and the logarithmic value of the cell concentration was found in the cell concentration range from 10 6 to 10 10 cfu/ml, which can be expressed by a regression equation of Z (k)= -2.06 log C (cells/20 l) + 5.23 with R 2 = 0.98. The detection limit was calculated to be 3.45 × 10 6 cfu/ml, which is comparable with many label-free immunosensors for detection of pathogenic bacteria reported in the literature. To achieve the selectivity of this method, we also demonstrated the feasibility of integrating magnetic separation to this impedance method. This study has demonstrated that bacterial cell concentration can be inferred by measuring the impedance of cell suspensions in DI water. This new detection mechanism could be an alternative to current impedance methods that have been reported for the detection of bacterial cells, e.g. impedance microbiology and electrical/electrochemical impedance biosensors. Published by Elsevier B.V. Keywords: Impedance spectroscopy; Bacterial detection; Bacterial suspension; Salmonella 1. Introduction Impedance technique, as one of the principle electri- cal/electrochemical transductions, is becoming a fertile area for developing interdisciplinary methods for applications in a wide range of biological and biomedical detection. This is due to a number of facts including: (i) the electrical properties of the biological entities and/or biological reactions motivates attention to impedance techniques; (ii) impedance is one of the most promising techniques for developing label-free, real- time, and non-invasive methods for biological detection; (iii) impedance as an electronic detecting mechanism can easily interface with miniaturized devices, such as biosensors and biochips, to meet the growing need to offer an analytical ∗ Tel.: +1 919 530 6704; fax: +1 919 530 6600. E-mail address: lyang@nccu.edu. foot print considerably smaller than laboratory-based instru- ments. Detection and quantification of foodborne pathogenic bac- teria remains a challenge and an important issue for ensuring food safety and security for our society. It is reported that foodborne disease causes approximately 76 million illnesses, accounting for 325,000 hospitalizations and more than 5000 deaths each year in the United States [1]. Salmonella is one of the major causes of worldwide foodborne disease. The US Centers for Disease Control and Prevention (CDC) estimates that 1.4 million people are infected and 1000 people die each year with salmonellosis in the US. These infections account for 1 billion dollars yearly in direct and indirect medical costs [1]. Salmonella Typhimurium is recognized as the second most com- mon serotype (after Salmonella enteritidis) of Salmonella found in humans. The impedance technique has been applied in the field of microbiology as a means to detect and/or quantify foodborne 0039-9140/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.talanta.2007.10.018