## 3 Results and Discussion
### Impedimetric Measurement of Cell Growth on the rGO-AuNP/ITO Electrode  
The measured electrochemical impedances of the bare ITO, AuNP/ITO, rGO/ITO, and rGO-AuNP/ITO in are shown in the Nyquist plot in Figure 4. The measured impedance data of the electrode (\(Z_{EL}\)) are presented as symbols and analyzed via non-linear curve fitting using the following equation:  
\[Z_{EL}=(Z_{CPE}\cdot(R_{CT}+Z_{W}))/(Z_{CPE}+R_{CT}+Z_{W})+R_{S} \tag{3}\]  
where \(Z_{CPE}\) is the constant phase element for the non-ideal capacitive characteristic of the electrode-electrolyte (=\(T/(j\omega)^{P}\); \(j\) is the imaginary unit, and \(\omega\) is the angular frequency), \(R_{CT}\) is the charge transfer resistance, \(Z_{W}\) is the Warburg impedance, and \(R_{S}\) is the solution resistance. The results showed that the measured impedance data of the electrode agreed with the fitted lines using Equation (3), and that the electrode interfacial impedance determining the faradaic and non-faradaic current was decreased by the electrochemical deposition of the electrode surface. In the parameters of Equation (3) extrapolated from the fitting analysis, \(Z_{CPE}\) and \(R_{CT}\) were selected to characterize the surface modification of the ITO electrode. The \(Z_{CPE}\) values of the bare, AuNP-, rGO-, and rGO-AuNP-deposited ITO electrodes were found to be \(90.05/(j\omega)^{0.94}\), \(14.4/(j\omega)^{0.88}\), \(4.71/(j\omega)^{0.91}\), and \(1.7/(j\omega)^{0.88}\) M\(\Omega\), respectively. The \(R_{CT}\) values of the bare, AuNP-, rGO-, and rGO-AuNP-deposited ITO electrodes were found to be 118.07, 77.59, 34.67, and 23.4 k\(\Omega\), respectively. The decrease in the \(P\) value of \(Z_{CPE}\) of the AuNP- and rGO-AuNP-deposited electrodes can be explained by an increase in the surface roughness of the electrode caused by the electrochemical deposition process. The significant decreases in the \(T\) value of \(Z_{CPE}\) and \(R_{CT}\) of the rGO-AuNP-deposited ITO electrode can be induced by an increase of the sensing area and charge transfer conductance of the modified electrode. Figure 5a-c shows the fluorescent microscopic images of HEK293/GFP cells cultured for 40 h on bare, AuNP-, and rGO-AuNP-deposited ITO electrodes, when the cells were exposed to the blue light  
Figure 4: Impedance Nyquist plot of the bare, rGO-, AuNP-, and rGO–AuNP-deposited ITO electrodes measured with an electrolyte containing 10 mM K\({}_{3}\)Fe(CN)\({}_{6}\) in 0.1 M KCl at a frequency of 0.1 Hz to 100 kHz (measure) and curves (fit) fitted to the measured spectra via non-linear curve fitting using Equation (3). (400-490 nm). The average and standard deviation of the fluorescent intensity of the cells cultured on each electrode analyzed by ImageJ is shown in Figure 5d. The microscopic images proved that the rGO-AuNP deposited ITO electrode is biocompatible and that the optical density of the cells was observed to be higher on the rGO-AuNP deposited ITO electrode when compared to the bare, AuNP, or rGO-deposited ITO. The impedance magnitudes without or with HEK293/GFP cells cultured for 48 h on the bare or rGO-AuNP-deposited ITO electrode measured at a frequency of 10 Hz to 100 kHz is shown in Figure 6a. The impedance magnitude of the electrode without cells showed the capacitive impedance of the electrode interface at low frequencies and the resistance of the culture media at high frequencies. As the cells adhered and proliferated on the electrodes during cell growth, an increase in the impedance magnitude at frequencies above 4.64 kHz for the bare ITO or 215.4 Hz for rGO-AuNP-deposited ITO electrode was observed. Figure 6b demonstrates the normalized impedance magnitude at 21.5 kHz recorded during cell culture on the bare, rGO-, AuNP-, and rGO-AuNP-deposited ITO electrodes. The normalized impedance magnitude (\(\left|\right.Z\left|\right. {}_{norm}\)) of the HEK293/GFP cells was calculated as follows:  
\[\left|Z\right|_{norm}=\left|Z\right|_{Cells}/\left|Z\right|_{No\ Cells} \tag{4}\]  
where \(\left|\right.Z\left|\right.