# Perspective of Using Vertically Oriented Graphene as an Electrochemical Biosensing Platform
## 3 Results and Discussions
### EIS Analysis
The \(\beta\) region, defined between 100 kHz and 1 MHz, is represented by the results from the build-up of charge at cell membranes due to the Maxwell-Wagner effect. In the Figures 6 and 7 are presented the variation of complex electrical permittivity for SW403 and HT 29 at the two concentrations: \(10^{4}\) and \(10^{6}\) cells/mL. Figure 5: Nyquist diagram: (**a**) Concentration of \(10^{4}\) cells/mL and \(10^{6}\) cells/mL for HT29 cells and (**b**) Concentration of \(10^{4}\) cells/mL and \(10^{6}\) cells/mL for SW403 cells. Complex permittivity measurements can help characterize and monitor tumor cells. The applied electric field will interact with the ions in the conductive medium, determining the alignment of the ions around the cell and causing interfacial polarization. Applications of high potential can permeabilize the cell membrane. That is why low potentials of the order of tens of mV are applied to measure cell impedance. In Table 1, we can see the extracted impedance parameters found by fitting the impedance measured experimentally. The sample of SW 403 at a concentration of 10\({}^{4}\) cells/mL has the highest charge transfer resistance, Rct = 3250 \(\Omega\), and the conductivity is lower, Cdl = 6.38 \(\mu\)F, than the sample of SW 403 at a concentration of 10\({}^{6}\) cells/mL (Cdl = 8.92 \(\mu\)F) but higher than HT 29 (Cdl = 5.66 \(\mu\)F and 4.85 \(\mu\)F, respectively) and blank VG (Cdl = 5.98 \(\mu\)F). The explanation for these parameters is that SW 403 forms large cell conglomerates that impede the current's passage. When tumor cells were captured on the electrode, the electron transfer at the electrode-molecule interface was obstructed, allowing for the measurement of tumor cells through \begin{table}
\begin{tabular}{c c c c c c} \hline
**Sample** & **R\({}_{\text{e}}\) (\(\Omega\))** & **R\({}_{\text{p}}\) (\(\Omega\))** & **Rct (\(\Omega\))** & **Z\({}^{\prime\prime}\)\({}_{\text{max}}\) (\(\Omega\))** & **Cdl (\(\mu\)F)** \\ \hline VG & 53.8 & 2630 & 1474 & 1237 & 5.98 \\ \hline SW 403—10\({}^{4}\) cells/mL & 32.8 & 6071 & 3250 & 2921 & 6.38 \\ \hline HT 29—10\({}^{4}\) cells/mL & 49.9 & 4071 & 1190 & 905 & 5.66 \\ \hline SW 403—10\({}^{6}\) cells/mL & 29.9 & 5236 & 2787 & 1825 & 8.92 \\ \hline HT 29—10\({}^{6}\) cells/mL & 39.4 & 5380 & 2409 & 1999 & 4.85 \\ \hline \end{tabular}
\end{table} 
Table 1: Impedance parameters. Figure 6: Variation in complex electrical permittivity for SW403 and HT29 at 10\({}^{4}\) cells/mL concentration: (**a**) imaginary permittivity and (**b**) real permittivity. Figure 7: Variation in complex electrical permittivity for SW403 and HT29 at 10\({}^{6}\) cells/mL concentration: (**a**) imaginary permittivity and (**b**) real permittivity. changes in impedance. This was evaluated using electrochemical impedance spectroscopy and can be observed from the Nyquist plot. The Faraday impedance in the electrical circuit consists of two components for charge transfer control and diffusion control: the Warburg impedance (W) and the electrode polarization resistance (Rp). The solution resistance (Rs) and the double-layer capacitance (Cdl) are ideal circuit elements. Rp and W are not exemplary components; they have a particular relationship with the measurement frequency. The interaction of cells with the local electric field can cause the separation of charges and the creation of new dipoles, known as induced polarization. In biological systems, there are two important examples of induced polarization. One is the movement of ions on the surface of macromolecules due to the electric field, and the other is the interfacial polarization known as Maxwell-Wagner polarization on non-conducting surfaces. Biomaterials are made up of molecules with varying permittivity and conductivity. When an electric field is applied to these materials, the mobility of charge carriers, such as ions, is higher in certain areas (e.g., aqueous phases) than in others (e.g., lipid phases). This leads to a non-uniform distribution of charges on surfaces that are non-conductive.