# Perspective of Using Vertically Oriented Graphene as an Electrochemical Biosensing Platform
## 3 Results and Discussions
### EIS Analysis  
The VG-based sensor [19], integrated into a microfluidic system and developed within the Research and Development Institute for Microtechnologies--IMT Bucharest, is made on a monocrystalline silicon wafer and consists of 2 interdigitated working microelectrodes on the surface of which vertical graphene is deposited at a temperature of 700 \({}^{\circ}\)C, each with 64 digits, a reference electrode, and an auxiliary electrode. The microfluidic system is made half on the chip with the sensor and the other half on a Corning 7740 glass chip, which is also the device's cover. Applying a stimulus to the cells leads to a change in the permeability of the cell membrane--the result in impedance models the conductance of the membrane in the area of the microelectrode array. The characterization of the cells using the device [19] was carried out by electrochemical impedance spectroscopy. When applying an electric voltage between the interdigitated working electrode based on vertical graphene and the auxiliary electrode of several tens of mV, an oxidation reaction occurs with the release of electrons on the digits of one of the electrodes, the anode. On the digits of the other, a reduction reaction with electron captures and the solid-liquid interface behaves as a capacitor. The larger the interface, with the capacity to grow, the more a flattening of the initial drop, respectively, decreases the contact angle. Figure 4: FTIR spectra for VG grown on the working electrode before and after the electrochemical processes. We conducted the EIS analysis to illustrate the electrical characteristics of adenocarcinoma cells, including the processes occurring at the VG electrode and the associated characteristic parameters. It can be seen from the Nyquist diagram that HT29 cells, compared to SW403, have lower electrical charge transfer resistance and higher electrical permittivity and conductivity. HT29 tumor cells have a highly folded membrane surface due to microvilli and, therefore, have a much larger cell surface than cells in the SW403 line. HT29 has a higher membrane capacity and a lower capacitive reactance. The higher membrane capacitance of HT29 cells results in storing more electrical charges, leading to higher conductivity and lower charge transfer resistance. At the microvilli level, lipid bridges have a very high concentration, especially cholesterol-ol-glycosophilipids. If the surface area of HT29 cells is much larger compared to SW403, it means that the concentration of lipid bridges, that is, cholesterol-ol-glycosophilipids, is also much higher. Therefore, tumor cells from the SW403 line adhere and form large cell conglomerates. The variation curves in Figure 5a,b for SW403 (b) look different from those for HT29 (a) because the SW403 cells agglomerate to form large conglomerates. The Nyquist diagram (Figure 5) shows the capacitive reactance (imaginary impedance) as a function of the real impedance for two different concentrations (\(10^{4}\) cells/mL and \(10^{6}\) cells/mL) for, respectively, HT29 and SW 403 cells. From the semicircular curve corresponding to high frequencies, the resistance can be determined using charge transfer (R\({}_{\text{ct}}\)) and the capacity of the electric double layer (C\({}_{\text{dl}}\)) that appears on the surface of the membrane. Still, it is also possible to determine the impedance modulus corresponding to the impedance resonance, the maximum of the semicircular curve. The linear variation in the imaginary impedance as a function of the real impedance related to low frequencies is due to Warburg diffusion. Electrical permittivity is inversely proportional to impedance and describes the ability of cells to resist an electric field. It decreases with increasing frequency when conductivity rises. The permittivity of polarized cells in the frequency range of 100 mHz-1 MHz is divided into two regions. The \(\alpha\) dispersion region is defined below 100 kHz and represents the polarization of ions in a conductive medium.