In the recent years, there has been a merger of microelectronics and biological sciences to develop what are called “Micro Total Analysis System ($μ-TAS$)”. To enhance the portability and disposability of microfabricated analysis systems, much work has been directed increasingly towards the development of on-chip detection for the direct interrogation of the electrical properties and behavior of biological species. In this thesis, electrochemical impedance sensors have demonstrated in terms of the feasibility of label-free deoxyribonucleic acid (DNA) detection and the frequency characteristics and design rule of coplanar impedance sensors. Glass-based microchannel chips were fabricated using micromachining technology, and Pt thin-film microelectrodes, as coplanar impedance sensors, were integrated on them. In chapter 4, DNA detection method based on dielectric properties was described. From capacitance-frequency measurements at various interelectrode distances and ionic concentrations, a significant difference between the deionized (DI) water and the TE buffer (tris-HCl + ethylenediaminetetraacetic acid (EDTA)) was observed. This was discussed from the viewpoint of interfacial phenomena, such as the electrical double layer and Faradaic reactions, the dielectric constant related to the conductivity, and the capacitance inversely proportional to the interelectrode distance. From the nondependence of capacitance on interelectrode distance in the low-frequency region of the TE buffer, we focused on the electrode-electrolyte interface and explained the behavior observed using the double layer model and Fradaic reactions. The addition of ions and molecules increased capacitance due to the diffusive mobile layer and the Faradaic reaction at the electrode-electrolyte interface. The addition of DNA molecules (10ng/㎕) resulted in an increase in capacitance and dielectric loss in the TE buffer at low frequency. It is feasible to use dielectric properties for rapid and direct detection of biomolecules, particularly DNA molecules, without the addition of labels or indicators. However, further investigations are required to obtain reliable dielectric sensors for specific application to DNA detection. In chapter 5, we discuss the alternating current (AC) frequency characteristics of coplanar impedance sensors as design parameters. We suggest the guidelines of dominant components, such as interfacial capacitance ($C_{DL}$), solution resistance ($R_{Sol}$) and cell capacitance ($C_{Cell}$), for sensing as design parameters. Longitudinal design parameters, such as interelectrode spacing and electrode width, of coplanar impedance sensors were changed to determine AC frequency characteristics as design parameters. Through developing total impedance equations and modeling equivalent circuits, the dominant components in each frequency region were illustrated for coplanar impedance sensors and measured results were compared with fitted values. As the ionic concentration increased, the value of frequency-independent region decreased and cut-off frequencies increased. As the interelectrode spacing increased, cut-off frequencies decreased and total impedance increased. However, the width of each frequency-independent region was similar. As the electrode area increased, low cut-off frequency ($f_{low}$) decreased but high cut-off frequency ($f_{high}$) was fixed. We think that the decrease in $R_{Sol}$ dominated over the influence of other components, which resulted in heightening $f_{low}$ and $f_{high}$. The interelectrode spacing is more significant parameter than the electrode area in frequency characteristics of coplanar sensors. The cell constant dependent on the geometry of the sensor was developed, and we compared the theoretically predicted values with the experimentally obtained results. The deviation of experimentally obtained results from theoretically predicted values may result form the fringing effect of coplanar electrode structure and parasitic capacitance due to dielectric substrates.
