Interdigitated array microelectrodes based impedance biosensors for detection of bacterial cells

Abstract

Impedance spectroscopy is a sensitive technique to characterize the chemical and physical properties of solid, liquid, and gas phase materials. In recent years this technique has gained widespread use in developing biosensors for monitoring the catalyzed reaction of enzymes; the bio-molecular recognition events of specific proteins, nucleic acids, whole cells, antibodies or antibody-related substances; growth of bacterial cells; or the presence of bacterial cells in the aqueous medium. Interdigitated array microelectrodes (IDAM) have been integrated with impedance detection in order to miniaturize the conventional electrodes, enhance the sensitivity, and use the flexibility of electrode fabrication to suit the conventional electrochemical cell format or microfluidic devices for variety of applications in chemistry and life sciences. This article limits its discussion to IDAM based impedance biosensors for their applications in the detection of bacterial cells. It elaborates on different IDAM geometries their fabrication materials and design parameters, and types of detection techniques. Additionally, the shortcomings of the current techniques and some upcoming trends in this area are also mentioned.

Introduction

Electrochemical biosensors, often referred to as amperometric, potentiometric, conductimetric, or impedimetric, are advantageous as they are highly sensitive, rapid, inexpensive, and are suitable for designing integrated microsystems (Radke and Alocilja, 2005a, Bakkar and Qin, 2006). Electrochemical impedance combines the analysis of the resistive and capacitive (or inductive) properties of materials in response to the small amplitude sinusoidal excitation signal (Bott, 2001, Guam et al., 2004). Impedance detection works by measuring the impedance change caused by binding of target molecules to receptors (antibodies, DNA, proteins, and other bio-recognition elements) immobilized on the surface of the electrodes (Yang et al., 2004a, Radke and Alocilja, 2004, Radke and Alocilja, 2005a, Radke and Alocilja, 2005b, Yang and Li, 2005), change in the conductivity of the medium caused by the growth of bacteria (Gomez et al., 2001, Gomez et al., 2002, Gomez-Sjöberg et al., 2005, Yang et al., 2004b, Yang and Li, 2006), change in conductivity of the medium due to suspension of target molecule in the aqueous medium (Varshney and Li, 2007a, Varshney et al., 2007b), capturing bacterial cells on the surface of electrodes using dielectrophoresis (DEP) (Li and Bashir, 2002, Suehiro et al., 1999, Suehiro et al., 2001, Suehiro et al., 2003a, Suehiro et al., 2003b, Aldaeus et al., 2005), and change in the ionic concentration of the medium caused by the activity of enzyme used as labels for the signal amplification (Laureyn et al., 2000, Ruan et al., 2002, Kim et al., 2004, Thomas et al., 2004). Generally impedance measurement is divided into two categories: faradic and non-faradic (Yang et al., 2004a). Faradic requires a redox probe for impedance measurement, while non-faradic measurement can be performed in the absence of a redox probe. Traditionally, macro-sized metal rods or wires were used as electrodes immersed in the medium to measure impedance (Towe and Pizziconi, 1997, Berggren et al., 1998, Mirsky et al., 1998). In an attempt to miniaturize the sensor and improve the sensitivity, microelectrodes have been considered as potential candidate to combine with traditional detection systems. Microelectrodes favor a greater rate of reactant supply (while macroelectrodes cause greater depletion of reactants) and require lower concentrations of electro-active ions to form double layer as compared to macroelectrodes (Ciszkowska and Stojek, 1999, Min and Baeumner, 2004). As a result, microelectrodes can perform impedance measurement even in low conductivity solution, where macroelectrodes may not be sensitive.

Among microelectrodes, interdigitated array microelectrodes (IDAM) present promising advantages in terms of low ohmic drop, fast establishment of steady-state, rapid reaction kinetics, and increased signal-to-noise ratio (Amatore et al., 1983, Ciszkowska and Stojek, 1999, Mauyama et al., 2006). IDAM consist of a series of parallel microband electrodes in which alternating microbands are connected together, forming a set of interdigitating electrode fingers (Fig. 1). Due to proximity of cathodic and anodic electrodes, minute amounts of ionic species can be efficiently cycled between the electrodes resulting in very large (>0.98) collection efficiencies, giving the IDAM an advantage in detecting small amounts of generated electrode products (Postlethwaite et al., 1996, Thomas et al., 2004). Additionally, IDAM eliminates the need for a reference electrode and provides simple means for obtaining a steady-state current response, which is comparatively simpler to detect as compared to three or four electrode systems (Liu et al., 2004, Nebling et al., 2004). Their low response time also favors rapid detection. The typical dimensions of an individual microband “finger” are 0.1–0.2 μm in height, 1–20 μm in width; 2–10 mm in length, with a gap of 1–20 μm between the fingers.