# Theranostic Nanoplatforms of Thiolated Reduced Graphene Oxide Nanosheets and Gold Nanoparticles
## 3 Results and Discussions
The hybrid graphene-plasmonic gold nanoparticle nanocomposites were fabricated by a one-pot synthetic process through the reduction of the precursor metal salt in the aqueous dispersions of base-washed GO or thiolated reduced GO nanosheets. Figure 4 shows the UV-VIS spectra of graphene-based hybrids with gold nanosphere (AuNS, Figure 4a) and nanorods (AuNR, Figure 4b), in the comparison with the spectra of the corresponding nanoparticles alone. To note, for all hybrid samples, the optical absorption of GO was buried by the spectral features of the gold nanoparticles, which instead maintained or even enhanced their plasmonic-sensing capability in the AuNS@GO and AuNR@GO nanocomposites. The absorption spectrum of metal nanoparticles with diameters between 3-100 nm is characterized by a strong band caused by collective excitation of free electrons, which is usually ascribed to the surface plasmon resonance phenomenon. This absorption band is very sensitive to the structure of the particle and the nature of the surrounding environment, namely the size and shape of the nanosystem, and the presence of the ligands as well as the solvent in which the particles are dispersed [93]. Spherical gold nanoparticles having a diameter between 12 and 40 nm show an absorption peak in the visible region, at 520-530 nm, which gradually redshifts as the size of the nanoparticle increases [74]. Accordingly, Figure 4a shows that citrate capped AuNS exhibited a plasmon peak centered at 525 nm, which corresponds to spherical nanoparticles with an optical diameter of about 27 nm (Table 3). As to the NPs grown in the presence of bwGO or rGOSH, in both cases the SPR band of the hybrid was slightly broader and redshifted by about 5 nm, suggesting an effective interaction of the AuNS with the GO sheets, which provided new relaxation pathways to the free electrons on the metal nanosystems [67]. Figure 4: UV-VIS spectra for as-prepared samples of: (**a**) citrate-capped gold nanospheres (AuNS, wine line) and hybrids with base-washed GO (bwGO@AuNS, red line) or thiolated reduced GO (rGOSH@AuNS, orange line); (**b**) cetyl trimethylammonium bromide-capped gold nanorods (AuNR, cyan line) and bwGO@AuNR (light blue line) or rGOSH@AuNR (navy line) hybrids. When the shape of AuNPs changed from spheres to rods (Figure 4b), the SP signal was split into two bands: A strong peak in the near-infrared (NIR) region (716 nm), corresponding to electron oscillations along the long axis, referred to as longitudinal band, and a weak band in the visible region at a wavelength similar to that of spherical gold (512 nm) due to the transverse electronic oscillation [94]. Interestingly, a blueshift of about 28 nm and 29 nm for the longitudinal plasmon band, respectively, of bwGO@AuNR and rGOSH@AuNR, indicated that the aspect ratio of AuNRs was significantly reduced when the seed-mediated growth process occurred in the presence of GO nanosheets. To note, for the rGOSH@AuNR hybrid the SP band at ~512 nm almost completely disappeared and the longitudinal plasmon band decreased in intensity, thus evidencing electronic and optical interference of GO on the plasmonic nanostructures [67]. In order to remove the excess of reactant, especially the CTAB used for the growth of nanorods, the nanocomposite systems were purified by centrifugation and water-washing steps (see Materials and Methods section) and characterized by UV-VIS spectroscopy (Figure 5). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \(\mathbf{\lambda_{max}}\) **(nm)** & **FWHM (nm)** & **D (nm)/** & **R** \\ \hline
**AuNS** & 525 & 53 & 27 & \\
**bwGO@AuNS** & 530 & 63 & 41 & \\
**rGOSH@AuNS** & 530 & 62 & 41 & \\
**AuNR** & 716 & 111 & & 3.12 \\
**bwGO@AuNR** & 689 & 86 & & 2.83 \\
**rGOSH@AuNR** & 687 & - & & 2.81 \\ \hline \hline \end{tabular}
\end{table}