Sensors and Actuators B 137 (2009) 357–362 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Solid-medium-integrated impedimetric biosensor for real-time monitoring of microorganisms Ahmi Choi a , Jae-Sung Park b , Hyo-Il Jung a,b,∗ a National Core Research Center for Nanomedical Technology, Yonsei University, Seoul, South Korea b School of Mechanical Engineering, Yonsei University, Seoul, South Korea article info Article history: Received 27 September 2008 Received in revised form 20 December 2008 Accepted 30 December 2008 Available online 9 January 2009 Keywords: Solid medium Impedance microbiology Biosensor Real-time monitoring abstract Rapid, real-time detection of pathogenic microorganisms is an emerging and quickly evolving field of research, especially with regard to microorganisms that pose a major threat to public health. Herein, a new method that uses bioimpedance and solid culture medium for the real-time monitoring of bacterial growth is introduced. We fabricated a new impedimetric biosensor by integrating solid medium and two plane electrodes attached on two facing sides of an acryl well. During bioelectrical impedance analysis, the solid medium showed the characteristics of a homogenous conductive material. In a real-time impedance measurement, our solid medium biosensor could monitor bacterial growth in situ with a detection time of ∼4h. We applied different culture media (e.g. Tryptic Soy Agar, Luria Agar, and Nutrient Agar) to our biosensor and found that a low conductive medium was optimal for the monitoring of the bacterial growth due to low threshold in the variation of the impedance signal. Our data indicate that the solid medium biosensor is useful for detection of pathogenic microorganisms, thereby providing a new analytical tool for impedance microbiology. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The real-time detection of pathogenic microorganisms is essential for public health and welfare. Generally, pathogenic microorganisms that exist in water, food, and the atmosphere can cause severe diseases such as food poisoning and respiratory infec- tions [1]. Moreover, airborne biological particles responsible for diseases such as measles, anthrax, Legionella, influenza, smallpox, and rhinovirus can be used as bioterrorism agents [2,3]. Conven- tional bacteria detection is achieved by colony counting, which cannot detect bacteria until the bacterial colony is formed, and requires a long incubation time of over 24 h. For rapid and effective detection, several methods, including immunoassays, molecular biological tests, optical, and electrical methods, have been stud- ied for multiple decades [4–6]. Others have demonstrated that an ATP-bioluminescence transducer can be used to detect airborne bacteria in real-time [7]. Electrical methods also have been devel- oped to detect microorganisms using dielectrical properties like impedance, conductance, and capacitance [8–11]. To date, the impedance method in microbiology has been applied primarily to liquid medium. Bacterial growth has been monitored by measuring the impedance change of the medium ∗ Corresponding author at: School of Mechanical Engineering, Yonsei University, Seoul, South Korea. Tel.: +82 2 2123 5814; fax: +82 2 312 2159. E-mail address: uridle7@yonsei.ac.kr (H.-I. Jung). based on the following two factors. The first factor is the release of gases and ionic molecules such as carbon dioxide, bicarbonate, and hydrogen ions, which are produced during bacterial metabolism. The ionic materials diffuse into the whole medium and induce changes in the dielectric properties of the medium [12]. The sec- ond factor is the attachment of bacterial cells directly to the electrode, which causes a change of the Debye length and sub- sequent change of electrode–solution interface properties [13]. In recent researches of the liquid-medium based impedimetric biosensor, the interdigitated microelectrodes (IDMs) and the equiv- alent circuit analysis were employed to improve the monitoring modalities [14,15]. Particularly, an impedimetric immunosensor using magnetic nanoparticle–antibody conjugates could separate and concentrate pathogens from both pure culture and real sample and then lower the detection limit [16,17]. We propose a solid-medium-integrated impedimetric biosen- sor for real-time monitoring of bacterial growth by measuring the medium impedance. Solid medium has advantages in that it is easy to handle and portable compared with liquid medium. Despite these advantages, it has been used only for the cultivation of bacte- ria and rarely employed as a material in a biosensor for bacterial growth monitoring. To our knowledge, this work represents the first attempt to use solid medium in a biosensor, allowing for the investigation of its bioelectrical impedance. In this paper, the bio- electrical characteristics of the solid medium were investigated extensively with respect to the volume of medium and the gap between electrodes. We also observed that the real-time change 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.12.062