# Theranostic Nanoplatforms of Thiolated Reduced Graphene Oxide Nanosheets and Gold Nanoparticles
## 1 Introduction
It has been reported that GO cytotoxicity is strictly dependent on the solution concentration, the ionic strength of the solvent, the absorption of plasma corona proteins, the nanoplates' lateral size, and, most importantly, the number of oxygenated groups present on its surface [46; 47; 48]. Noteworthy, a direct correlation between the reduction degree of rGO and the cells' viability was highlighted as result of lower interferences with the electron transport chain with oxygen surface groups of GO [49], which in turn prevents the donation of electrons to site I/II of the electron transport chain and thereby slows down the generation of ROS [50]. To note, during oxidation of the graphene layers of graphite to obtain GO, defects are generated, and oxidation occurs at these defect sites. The main possible oxygen-containing functionalities at defect sites are mainly ketone- and quinone-type groups; however, depending on the size of the defects, other functional groups can also be formed. The oxidation process also activates a side product formation during GO synthesis, referred to as oxidative debris (OD), which acts as a natural surfactant of the GO nanosheets in dispersions and affects agglomeration of graphene oxide layers, thus influencing significantly the stability of GO nanosheets in the suspension [51]. Plasmonic properties of gold nanoparticles (AuNPs) are commonly exploited in biomedical imaging and biosensing [24; 52], in biochemistry [53], photothermal therapy [54] and anti-angiogenic applications [55; 56; 57; 58] as a result of the easily tunability of the surface plasmon (SP) band by varying size, shape, or by gold surface modification [59; 60; 61]. Specifically, gold nanospheres (AuNS) and nanorods (AuNR) have attracted a great interest in nano-biotechnology because of their easily adjustable cytotoxicity by size control, dose, or the capping agent and stabilizer used, facile way of synthesis, and high sensitivity to the microenvironment change [62; 63; 64]. The integration of GO layers with gold(0) nanostructures, to obtain hybrids GO@Au nanocomposite biosensors, exhibits the dual advantage of increasing the sensing performance, due to the localized surface plasmon resonance (LSPR) property [65], and owning the intrinsic biological activities of nanogold [24; 66]. Previous studies have shown that the presence of GO substrates negatively affects the stability of gold nanospheres and nanorods, the main mechanism proposed for the nanoparticle destabilization involving the disruption and stripping of the protective capping ligands by the GO through strong electrostatic interactions [67]. In this study, we proposed a new theranostic platform based on thiolated reduced graphene oxide and integrated with plasmonic spherical and rod-like gold nanoparticles. To the best of our knowledge, although several examples exist in literature on AuNP-decorated GO nanocomposites obtained by in situ growth methods, including green approaches [24; 68; 69], only few studies have focused on the functionalization of thiolated GO with gold nanoparticles [70]. To avoid uncontrolled reactions and to modulate the hydrophobic character, the OD was removed from the as-received pristine GO by a treatment referred to as base-washing of GO (bwGO) [71]. Reduced monothiolated GO (rGOSH) was synthetized by a selective substitution of epoxydic and hydroxylic groups with thiourea at high temperature (70 \({}^{\circ}\)C), after a brominating treatment. Meanwhile, the surface decoration with AuNS and AuNR was performed by in situ reduction of the metal salt precursor and seed-mediated growth processes, by using the GO dispersion as solvent, either bwGO or rGOSH. The plasmonic-sensing capability of the nanohybrids, in comparison to bare AuNP and GO, were investigated by UV-visible (UV-VIS) spectroscopy. The physicochemical characterization of the nanocomposites was performed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), thermogravimetric analyses (TGA) in nitrogen environment, and atomic force microscopy (AFM).