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

Abstract

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/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> 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 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> CFU/ml after 20 min of bacterial incubation. The 0.25-μm CMOS circuit, combining sensing/readout parts, has a 0.05 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area, consumes 29 mW at 250 kS/s, and can be connected to microcontrollers, making the system suitable for point-of-care diagnostics.