Study on electrical impedance spectroscopy for real-time monitoring of portable PCR

Abstract

Polymerase chain reaction (PCR) has been widely used for major definite diagnostic tool, but very limited to indoor use such as hospital or diagnosis lab. For the rapid on-site detection of virus in an outdoor environment, a portable PCR thermal cycler is crucial module. In this study, we fabricated a portable thermal cycler with electrochemical impedance spectroscopy (EIS) for real-time monitoring of PCR process. The heating in the thermal cycler used a Pt resistive heater with a small thermal mass and high heat efficiency. A resistance temperature detector (RTD) made of the same material as the heater was used as a temperature sensor capable of measuring absolute temperature. The cooling in the thermal cycler used dual blower fans with lower power consumption than the Peltier element. The PCR components were electrically connected to a control module that could be operated with a Li-ion battery (12 V). The accumulated power consumption for 30 PCR cycles was 7.3 Wh. Escherichia coli genomic DNA (510 bp, 80 ng/μl) was amplified using the proposed PCR thermal cycler and the disposable PCR chip. A similar DNA amplification capability was confirmed using the proposed portable PCR thermal cycler compared with a conventional thermal cycler. Meanwhile, conventional real-time PCR system, which uses a fluorescence detection (FD), requires expensive optical detection including fluorescence labeling. To simplify the real-time PCR monitoring, we proposed an EIS using interdigitated electrodes (IDEs). The proposed EIS electrode makes a direct contact with the PCR sample requiring no labeling or surface pretreatment. The lowest noise in the EIS measurement was witnessed at the input AC voltage of 100 mVrms. Electrical impedances in a frequency domain were measured during 30 cycles in the PCR of Escherichia coli genomic DNA (180 bp, 10 ng/μl). From the analysis of the real and imaginary values, electrical impedance was monotonously increased with PCR cycle at six frequencies (24.9 Hz, 100.4 Hz, 117.2 Hz, 135.2 Hz, 158.4 Hz, and 186.0 Hz). The damage of the sample monitored by EIS measurement was confirmed to be negligible through electrophoresis.