A challenging problem in alternating current (AC) impedance sensing of particles (e.g., blood cells in plasma) with micro electrodes is that with the shrinking of electrode surface area the electrode double layer capacitance decreases. This double-layer capacitor dominates the system impedance in low frequency range, while the parallel stray capacitor dominates the system impedance in high frequency range. Hence the sensitivity for particle sensing for micro impedance sensors decreases over a wide frequency range. In this paper, we propose an approach to solve the problem. The idea is to use resonant sensing by connecting an external parallel inductor to the system. At the resonant frequency, the capacitive components in the system are nullified by the inductor, leaving the channel impedance (including the particle impedance) to be a major component in the system impedance. We then successfully demonstrate this idea by sensing 5 μm polystyrene beads. More important, this technique is extended to sensing blood cells in diluted human whole blood and leukocyte-rich plasma. The sensitivity can be improved by two orders of magnitude over more than three decades in frequency domain. The measured signal peak height histogram at low frequency matches well with known volume distribution of erythrocytes and leukocytes.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Surfaces, Coatings and Films
- Metals and Alloys
- Electrical and Electronic Engineering