Resonant dielectrophoresis and electrohydrodynamics for high-sensitivity impedance detection of whole-cell bacteria†
We present the co-integration of CMOS-compatible Al/Al2O3 interdigitated microelectrodes (IDEs) with an electrokinetic-driven macroelectrode for sensitive detection of whole-cell bacteria in a microfluidic channel. Two frequency ranges applied to the macroelectrode were identified to notably increase the bacterial coverage of the impedimetric sensor per unit time. Around 10 kHz, the bacterial cells were directed towards the IDE center thanks to AC electroosmosis (AC-EO) and the sensor capacitance linearly increased, achieving a limit of detection (LoD) of 3.5 × 105 CFU mL−1 after an incubation time of 20 min with Staphylococcus epidermidis. At 63 MHz precisely, a resonance effect due to the device was found to dramatically increase the trapping of S. epidermidis on the sensor periphery, due to the combined actions of short-range contactless dielectrophoresis (cDEP) and long-range Joule heating electrothermal (J-ET) flow. Thanks to a flow-based method, the bacterial cells were redirected towards the sensor center and an LoD of 105 CFU mL−1 was achieved within 20 min of incubation, which is almost two orders of magnitude better than the impedimetric sensor alone. Analytical models and 2D simulations using the Maxwell stress tensor (MST) provide a comprehensive analysis of the experimental results, especially about the spectral balance between cDEP, AC-EO and J-ET accounting for the 33-nm thick insulating layer atop the electrodes. Electrode CMOS compatibility confers portability, miniaturization and affordability capabilities for building point-of-care (PoC) diagnostic tests in a lab-on-a-chip (LoC).