Low-cost capacitive CMOS capacitive E. Coli biosensor for point-of-need water quality monitoring

Abstract

Exposure to pathogenic Escherichia coli (E. coli) bacteria through contaminated water can cause potentially life-threatening diarrhea and vomiting, and it is a useful water quality indicator. Simulations and experiments are conducted to give guidelines on low-cost capacitive biosensing devices aimed at bacterial sensing, and a custom integrated circuit that can be used in a low-cost capacitive biosensing device is delivered. Finite element modelling was conducted to compare the electric fields (E-fields) and capacitance across different electrode geometries and materials. The size of electrode features had the biggest impact on electric field strength and relative capacitance change in the presence of cell-like structures. The simulation results are used to clarify assumptions on how simulations for the design of capacitive sensing electrodes need to be conducted. Capacitance measurement of low-cost and commercial electrodes was conducted using a benchtop LCR meter and 3 μm microbeads as substitutes for E. coli cells. It was found that the measured capacitance increases as the concentration of microbeads increases, and the low-cost electrodes seem to show a higher-than-expected sensitivity when compared to commercial electrodes with smaller feature sizes. Electric impedance spectroscopy experiments conducted on E. coli cells showed a similar performance as characterisation experiments using microbeads. These insights inform the development of guidelines that may be used to design low-cost electrodes for similar applications. A custom integrated circuit (IC) featuring a capacitive sensing array with sub-surface electrodes was designed and delivered. This IC also includes the custom operational amplifier used in the sensing array, used in a low-cost capacitive biosensor prototype for point-of-need use. The low-cost device was characterised with 3 μm microbeads using a subset of the electrodes used in the LCR experiments, with comparable results achieved using the low-cost device. Lessons learned in the design of the low-cost system guide the development of a design flow for the design of point-of-need water quality monitoring devices and gives guidance on the required hardware to build such devices.