## 1 Introduction  
Graphene-based nanomaterials have garnered much attention when it comes to sensors as they require enhanced measuring sensitivity and functionality due to their unique electrochemical, mechanical, and optical properties [1]. Using electrochemical measurements, graphene oxide (GO)-deposited electrode sensors have been used in the ultrasensitive detection of proteins and living microorganisms [2; 3; 4]. Graphene-nanoparticle-based hybrids have been developed to amplify the sensitivity and selectivity of the sandwich-type electrochemical immunosensor [5], the cell signaling-protein sensor [6], or apoptotic cell sensor [7]. The hybrid material of graphene anchored with nanoparticles gives rise to synergetic effects such as encouraging biocompatible conditions [8] or increasing the electrochemical sensing area of its tangled construction [9]. Moreover, the electrochemical measurement sensitivity of biomolecules was also shown to be reinforced by employing nanoparticles, which were deposited or patterned on the electrode surface by using electrodeposition or thermal deposition with evaporation masks [10; 11; 12]. Additionally, graphene-nanoparticle composites along with electrochemical measurement showed great potential, feasibility, and increased sensitivity for label-free and real-time biomolecular and protein detection due to its improved ability to capture analytes [13; 14; 15].  
This improved binding ability of graphene nanocomposites to proteins can be utilized to develop new cytosensors for detecting target cells or cell-based assays for monitoring cell behavior. The cytosensor employing the graphene-nanomaterial composites showed high sensitivity and selectivity for sensing Du-145 cancer cells (limit of detection: 20 cells, linear range: \(10^{2}\) to \(10^{6}\) cells per mL) [16] or HL-60 cells exposed to apoptotic inducer (limit of detection: 48 cells, measurement range: \(10^{3}\) to \(10^{7}\) cells per mL) [7]. Cell-based assays using electrochemical impedance measurement have already been applied to non-destructively detect real-time cellular responses to drug candidates and disease risk factors [17]. Electrochemical impedance monitoring of cells on biocompatible gold or platinum electrodes has also been used in a wide range of applications, including wound healing assays, clinical diagnoses, cytotoxic assays, and drug development [18; 19]. The transparent indium tin oxide (ITO) electrode made it possible to characterize the impedance of cells with better optical transmittance when compared to typical gold or platinum electrodes [20; 21]. The surface of the ITO electrode could be modified via protein, peptide, or organic polymer coatings to achieve the biocompatibility [22; 23; 24; 25] required for the impedance monitoring of cells. However, organic materials are easily affected by environmental factors including temperature, humidity, pH, and solvent ions, and can be denatured such that substrate conditions cannot guarantee cell cultivation. Additionally, cell behavior is dependent on protein environments and the extracellular matrix.  
In this study, we investigated the feasibility of a graphene nanoparticle (alternative to denaturable organic materials)-coated ITO electrode for the impedance characterization of cell growth. For this, an ITO microelectrode, deposited with reduced graphene oxide (rGO) and gold nanoparticles (AuNP), was prepared and characterized using electrochemical and optical analysis. The modified ITO electrode was then used for impedance measurements of HEK293 cells during cell growth to evaluate whether the rGO-AuNP could be used as biocompatible substrates for the impedance monitoring of cells. Furthermore, the measurement range of the cellular impedance measurement (the difference between the largest and smallest measurable value) using the highly-conductive rGO-AuNP-modified ITO electrode was compared to a bare ITO electrode to evaluate the enhancement of the hybrid sensor's performance.