Label-Free Electrical Sensing of Single Cells Translocating through Micropores at Gigahertz Frequencies

Abstract

The first part of the thesis is dedicated to the resistive pulse method operating with direct currents. Borosilicate glass micropores, used for the experiments, are manufactured by direct laser ablation with an ArF-based excimer laser. The electric current measurements across the micropore are discussed numerically and experimentally. It is shown how time domain direct current measurements across the micropores, which are filled with a conductive electrolyte solution, deliver information about the sizes of individual translocating particles. Noise measurements are performed to illustrate that the resistive pulse method is not capable to perform high speed measurements which are needed for the application of the method for e.g. DNA sequencing. It is explained in the second part of the thesis how a micropore is embedded between two in-plane metal electrodes. These electrodes are used to bring the electrical field of a radio frequency wave into the vicinity or the pore. The interaction of the electrical field with Jurkat T cells is used to characterize the translocating cells and to extract information about the cells in addition to their size. In detail, it is shown that these measurements enable the experimentalist to probe specific properties of the cell such as its electrical conductivity and dielectric permittivity. The preparation of the measurement chip is discussed in detail. It is shown how the micropore is aligned between the two electrodes and how this microscopic sensing region is interfaced with the measurement equipment. Coplanar waveguides are introduced which lead the radio frequency wave into the sensing region and reflection measurements are performed to measure the particle of interest. The chip design is tested by time domain measurements with a vector network analyzer, which is capable of directly measuring the chip reflection versus time. A heterodyne measurement setup is introduced which enables to resolve the reflection using lock-in amplifiers with increased sampling speed compared to the vector network analyzer. The setup is used to observe translocating polystyrene beads and Jurkat T cells in different electrolyte solutions. The findings are discussed using time resolved optical microscopy and patch clamp experiments and it is shown that cells which undergo apoptosis can be identified with this method.