## 1 Introduction  
Graphene, made up of sp\({}^{2}\) carbon monoatomic layers hexagonally arranged into honeycomb lattice structure nanosheets [1], has emerged as an ideal material for use in multifunctional fast sensors, thanks to its electrical and thermal conductivity, the high tensile strength, the low charge transfer resistance, and the broad electrochemical window in 0.1 M phosphate saline buffer [2]. Recently, a great interest for biosensing applications has been focused onto its analogues, graphene oxide (GO), and reduced-graphene oxide (rGO), due to the presence of defective oxygen-bound sp\({}^{3}\) carbon atoms, which lead to a strong hydrophilicity and contribute to the formation of highly stable colloidal dispersions in aqueous solvents [3]. Such hydrophilic groups on the GO surface include carboxylic (-COOH) and hydroxyl (C-OH) groups, typically located at the edges of the sheets, while carbonyl (C=O) and epoxide (C-O-C) groups evolve at the basal plane of the graphene sheets creating the in-plane defects and disorder [4]. Graphene oxide can be functionalized through the addition of functional groups which affect its reactivity and converts the nanosheets into versatile nanoplatforms both for sensing, e.g., GO probes for cell imaging [5], and for drug delivery [6], thus opening new horizons in the fields of nanomedicine [7]. The hydrophilic groups onto the GO offer high versatility for the nanosheets derivatisation [8, 9], to immobilise enzymes for bioelectrodes and biofuel cells [10], drugs for anticancer therapies [6, 11],but also polymers such as chitosan, polyethylene glycol, poly-l-lysine, polyacrylate, to enhance the biocompatibility, the biodegradability, the solubility in aqueous medium, the responsiveness to physical (temperature) and chemical (pH) stimuli, and the non-toxicity of the graphene [12; 13; 14; 15]. Noteworthy, the chemical functionalization of GO can prevent concomitantly the uncontrolled aggregation of the nanosheets caused by Van der Waals and \(\pi\)-\(\pi\) stacking interactions [16]. Graphene can also be used in conjunction with metallic nanostructures to enhance biosensor performance with localised surface plasmon properties as well as with a larger sensitivity [17; 18]. To this respect, a promising approach is integrating graphene layers with conventional plasmonic nanostructures, such as gold nanoparticles (AuNPs) [19; 20], to obtain hybrid GO-AuNP composites exhibiting the dual advantage of increased interlayer distance between the single nanosheets (which minimizes the aggregation process) and additional intrinsic optical and biological properties of nanogold [21]. Indeed, AuNPs plasmonic properties allow for their extensive use in biomedical imaging and biodetection [22] in cytobiology and biochemistry [23], as well as photothermal and anti-angiogenic applications [24; 25]. Along with the applications of hybrid GO-AuNP nanocomposites in theranostics (i.e., sensing/imaging + therapy), while AuNPs are considered to be relatively safe, and their cytotoxicity can be easily controlled by the dose, the capping and stabilizer agents used [24; 26], contrasting results were obtained from different authors for GO cytotoxicity. For example, GO were found non-toxic in human breast adenocarcinoma cell line (MCF-7 line) and kidney epithelial (Vero line) cells [27] but in normal human lung (BEAS-2B line) and in human liver cancer (HepG2 line) cells, either GO or rGO reduced cell adhesion, induced cell apoptosis, and entered lysosomes, mitochondria, cell nuclei and endoplasm, thus inducing cell morphology modification, destruction of the membrane integrity and induction of DNA damage [28; 29]. Mitochondria are organelles crucial to maintain cellular energy balance, indeed are responsible for ATP synthesis and are implicated into the calcium signaling modulation, being essential for the cell life [30].