# Theranostic Nanoplatforms of Thiolated Reduced Graphene Oxide Nanosheets and Gold Nanoparticles
## 3 Results and Discussions  
Physicochemical Characterisation UV-Visible Characterization of GO-Based Nanosheet Dispersions and Plasmonic Nanoplatforms  
From all UV-VIS spectroscopic studies, carried out in the comparison of thiolated reduced graphene oxide with graphene oxide, it can be inferred that the optical absorption of GO is dominated by the \(\pi\)\(\rightarrow\)\(\pi^{\ast}\) plasmon peak near 230 nm (Figure 1). Figure 1a displays the experimental spectra of as-received pristine GO (arGO) as well as of GO after the base-wash treatment (bwGO) and the functionalization with thiol groups (rGOSH). The as-received GO exhibits an absorption peak centered at 232 nm, attributed to \(\pi\)\(\rightarrow\)\(\pi^{\ast}\) transition of aromatic C=C bonds and a shoulder at 300 nm, attributed to n\(\rightarrow\)\(\pi^{\ast}\) transition of carboxyl and other carbonyl and hydroxyl auxochromes [84]. The base-wash treatment on GO was expected to increase the hydrophobic character of the GO by the desorption of OD from the nanosheets' surface [71], which decreased the number of oxygen-containing groups and produced a partial restoration of the \(\pi\) conjugated systems. Accordingly, the spectrum of bwGO displayed a redshift to 236 nm of the \(\pi\)\(\rightarrow\)\(\pi^{\ast}\) transition band accompanied by an increased broadening. No significant differences were observed in the n\(\rightarrow\)\(\pi^{\ast}\) absorption shoulder, except in the relative intensity increase (from about 50% to about 70%) compared to the peak of the \(\pi\)\(\rightarrow\)\(\pi^{\ast}\) transition. Since the latter depends on two kinds of conjugative effects, related, respectively, to nanometer-scale sp\({}^{2}\) clusters and linking chromophore units such as C=C, C=O and C-O bonds [85], it was inferred that the change of the UV-VIS absorption relative intensity of n\(\rightarrow\)\(\pi^{\ast}\) to \(\pi\)\(\rightarrow\)\(\pi^{\ast}\) transition in bwGO in comparison with arGO was caused by the decreased conjugative effect of chromophore aggregation, which mostly influenced the \(\pi\)\(\rightarrow\)\(\pi^{\ast}\) plasmon peak [17]. Finally, the UV-VIS spectrum of rGOSH sample showed the disappearance of the shoulder at around 300 nm and the absorption peak associated with the aromatic bonds significantly redshifted to 282 nm. The bathochromic effect, which is characteristic of the electrical continuity of the nanosheets and enhanced restoration of \(\pi\)-aromatic bonds [86], was a result of the brominating treatment at  
Figure 1: (a) Experimental UV-VIS spectra normalized to the \(\pi\)\(\rightarrow\)\(\pi^{\ast}\) absorbance peak of as-received graphene oxide (arGO, in water, solid line), base-washed GO (bwGO, in water, dashed line), and thiolated reduced GO (rGOSH, in dimethyl sulfoxide DMSO, dotted line). (b) Theoretical UV-VIS spectra of GO and rGOSH from semi-empirical computational calculations. elevated temperature, which provided the reduction of the bwGO [87]. Indeed, it was demonstrated that the thiodiation reaction produced hydrosulfide byproduct (H\({}_{2}\)S), which, in turn, reduced GO partially to rGO, inducing a redshift in the UV-VIS absorption [32]. Noteworthy, the lower solubility of rGOSH than hydrophilic bwGO, and the tendency of the dispersion to aggregate in water (which required the samples to be dispersed in DMSO), ruled out the increased hydrophobicity of the nanosheets as further mark of the graphene oxide reduction. Within the limits determined by the semi-empirical approach, the calculated UV-visible spectra, reported in Figure 1b, were in good agreement with experimental data. In particular, the theoretical UV-VIS spectrum of GO showed the peak related to the \(\pi\)\(\rightarrow\)\(\pi^{\bullet}\) transitions at 240 nm, which was slightly redshifted than the experimental finding (at 236 nm), while the expected n\(\rightarrow\)\(\pi^{\bullet}\) absorption near 300 nm was calculated at 370 nm. Additionally, theoretical data showed a \(\varepsilon_{\text{shoulder}}/\varepsilon_{\text{peak}}\) ratio of about 0.6.