(96b) Real-Time PCR Quantification by Asymmetric Electrophoresis and Warburg Impedance Across Nanoslots

The development of portable, inexpensive, label-free and accurate platform for the quantification of DNA concentration is highly desirable, especially for real-time monitoring of the number of copies of DNA amplified in PCR. A novel nano-slot sensor design is proposed that employs an electrochemical impedance meter and a small AC field to quantify DNA concentration with pico-molar sensitivity. The DNA solution is introduced into two micro-reservoirs connected by a nano-slot across which a low-frequency AC field is applied. The cross-channel impedance spectrum of the system varies with time, as the co-ion DNA molecules are driven to the nano-slot entrance by the AC field. This trapping mechanism is shown to be a result of asymmetric electrophoresis due to enrichment/depletion symmetry breaking across the nanoslot. The larger voltage drop at the depletion region reduces its bulk field, resulting in a smaller bulk electrophoretic displacement during the depletion half cycle. The high field and ion depletion force in the depletion region also combines with the dominant dielectrophoretic force to endow a size-sensitive concentration rate. To validate this theory, microscopy experiments are performed using fluorescently labeled DNA to visualize the asymptotic electrophoresis of DNA at the nano slot. Due to the high charge of the DNA macro-ions and its large associated counter-ions, their presence in the depletion layer introduces a surfeit of counter-ions that significantly increases the conductivity of the depletion layer. Since the depletion layer is the region with the lowest conductivity in the entire device, the presence of DNA molecules significantly changes the overall conductance of the nano-slot system. This conductance change appears as a shift in the low-frequency impedance spectrum. The magnitude of this shift is directly correlated with the concentration of DNA present in the system. This allows for a highly sensitive, rapid, label-free and inexpensive real-time DNA quantification device for point-of-care application.

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