# Theranostic Nanoplatforms of Thiolated Reduced Graphene Oxide Nanosheets and Gold Nanoparticles
## 1 Introduction  
Theranostics has a high potential to develop personalized medicine, since it combines diagnostic and therapeutic capabilities into a multicomponent platform, thus offering enhanced specific targeting, therapeutic efficacy, and real-time imaging of the therapeutic process [1]. Several classes ofnanomaterials and hybrid nanocomposites, owing to their unique physicochemical properties, are currently exploited as theranostic nanoformulation, especially for cancer diagnosis and treatment [2; 3]. Graphene, made up of sp\({}^{2}\)-hybridized carbon atoms monolayers arranged into hexagonal crystal lattice [4], has shown intriguing properties such as ultra-high specific area [5], high thermal [6] and electrical [7] conductivity, low charge-transfer resistance [8], high tensile strength [9], and elasticity [10]. Despite such promising outlook, so far, a wide use of graphene in nanomedicine has been limited by its superhydrophobic character, i.e., the poor solubility in aqueous solvent. The development of chemically or physically modified graphene-based materials, including graphene oxide (GO) and reduced graphene oxide (rGO), allows for a more versatile use of graphene with tailored properties for applications in biosensing, imaging, therapeutic, and drug delivery [11; 12; 13; 14; 15]. The main oxygen-containing functional groups on GO nanosheets are epoxides (C-O-C), phenolic hydroxyl (-OH), carboxylic (-COOH), and carbonyl groups (C=O), which provide polar properties to the graphene-based material. Indeed, due to these functionalities, GO nanosheets are hydrophilic and stable in aqueous media. It is commonly accepted that in GO the -COOH groups are mainly located at the edges, while -OH and C-O-C groups are located at the basal plane of the nanosheets [16]. The introduction of heteroatoms on GO, including nitrogen, sulfur, and boron moieties [17; 18], as well as polymers [19], enzymes [20], semiconductor quantum-dots [21], and magnetic and metal nanoparticles [22; 23; 24] has been successfully demonstrated either via covalent or noncovalent interactions [25]. To note, the reduction of GO after its chemical modification provides the possibility to improve the electrical, physical, chemical, and biological properties of the nanosheets [26; 27; 28; 29; 30] and, at the same time, to modulate their stability in aqueous dispersion by preventing aggregation caused by Van der Waals and \(\pi\)-\(\pi\) stacking interactions [31]. In particular, the functionalization of GO and rGO with sulfur-containing moieties, either directly linked to the graphitic sp\({}^{2}\) carbon lattice or via sulfur-containing organic molecules, e.g., cysteine, has been exploited extensively for application in catalysis, electrochemistry, imaging, and biosensing and for the implementation of hydrophobic drug nanocarriers [32; 33; 34; 35; 36]. To this respect, great attention has been given to the high stability of the covalent bond between gold and sulfur, usually mediated by thiol (-SH) groups [37]. Along with the applications of GO in nanomedicine and theranostics devices, contrasting results were obtained from different authors for its cytotoxicity. Indeed, the different cytotoxic effects of GO in several cellular lines (both in vivo and in vitro) [38; 39; 40; 41] are generally attributed to the internalization of the nanosheets inside the cells' matrix with the subsequent induction of reactive oxygen species' (ROS) production. Moreover, it was demonstrated that either GO or rGO cause loss of cell adhesion, can induce apoptosis and subcellular organelles morphology modification, destruction of the membranes' integrity, and DNA mutation [42; 43]. To this regard, the cytotoxic effects of graphene oxide strongly depend on the physical-chemical characteristics of the nanosheets surface. Interestingly, rGO were found to be less toxic then bare GO, as tested with in vitro models [44] as well as with in vivo experiments, which did not induce thrombus formation caused by blood coagulation [45].