A Capacitance-to-Frequency Converter with On-Chip Passivated Microelectrodes for Bacteria Detection in Saline Buffers up to 575 MHz

To enable impedimetric sensing of whole-cell bacteria in saline buffer with passivated microelectrodes, we designed a very high frequency (up to 575 MHz) CMOS oscillator connected to on-chip Al/Al2O3 interdigitated microelectrodes. The output frequency was theoretically and experimentally shown to be inversely proportional to the permittivity of the solution with maximal error of 2 %, up to an electrical conductivity of 1 S/m. As a biosensing proof-of-concept, successful experimental detection of whole-cell Staphylococcus epidermidis was performed in physiological buffers, showing a detection limit of 10^7 CFU/ml after 20 min of bacterial incubation. The 0.25 μm CMOS circuit, combining sensing/readout parts, has a 0.05 mm2 area, consumes 29 mW at 250 kS/s and can be connected to microcontrollers, making the system suitable for point-of-care diagnostics