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 a saline buffer with passivated microelectrodes, we designed a very high frequency (up to 575 MHz) complementary meta-oxide-semiconductor (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 a 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 10⁷ 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.