Evaluating Nonlinear Impedance Excitation as Detection Method for Biosensors

Abstract

Chemical and microbial contamination of natural water supplies is one of the biggest challenges faced by developing nations, especially communities in rural areas. An estimated 1.6 million children die each year due to water-related deaths. Low-cost, portable, and point-of-use biosensors are required to monitor water quality and improve living standards. This paper investigates the possibility of developing a biosensor using a nonlinear impedance spectroscope (NLIS) and low-cost patterned platinum and titanium electrode chips. Divalent (Mg $^{2+}$ ) and monovalent (Cl $^-$ , Na $^+$ , and K $^+$ ) ions, and Escherichia coli strain B44 served as targets. The NLIS was set at a high excitation voltage and a fixed frequency. The spectra were imaged using a low-cost desktop audio interface and a computer. Electrical characteristics were optimized for the sensor, and the detection setup carefully defined. As little as 0.1 mg/ml NaCl and as few as $10^{11}$ cells of E. coli per milliliter were detected in less than 4 min. The sensitivity of the method was demonstrated by recording different electrochemical patterns for cells of E. coli treated with ethanol and for cells killed by heat. These findings form the basis of developing a cost-effective biosensor to detect changes in ion levels and bacterial cell numbers, and record the electrical activity (metabolic state) of bacterial cells. Apart from testing water quality, the biosensor may also be used to determine the microbial load in food and beverages.