## 3 Results and Discussion
The relative concentrations of GO and AuNPs in the hybrid nanocomposites, estimated by considering the molar extinction coefficients respectively of \(\varepsilon\) = 37.81 \(\pm\) 0.06 mL\(\cdot\)mg\({}^{-1}\)cm\({}^{-1}\) for GO and \(\varepsilon\) = 1.89\(\cdot\)10\({}^{8}\) M\({}^{-1}\)cm\({}^{-1}\) for AuNP, are shown in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **\(\mathbf{\lambda}\) (nm)** & **FWHM (nm)** \\ \hline GO-AuNP(0) & 525 & 50 \\ GO-AuNP(1) & 525 & 53 \\ GO-AuNP(2) & 525 & 53 \\ AuNP(0) & 522 & 54 \\ AuNP(1) & 523 & 56 \\ AuNP(2) & 523 & 56 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optical parameters from UV-vis spectra of hybrid GO-AuNP composites and AuNPs before (’0’) and after the centrifugation for pellet 1 (’1’) and pellet 2 (’2’). Figure 2: UV-visible experimental spectra of AuNP, GO, GO-AuNP (**a**: pellet 1; **b**: pellet 2) and GO calculated spectrum by the difference (GO-AuNP)-(AuNP). NMR spectrometry is often used to study the structure of organic compounds, as well as ligand molecules on the nanoparticles surface. NMR shifts are affected by the perturbation of the magnetic field at the nuclei from the surrounding environment and NMR studies on nanoparticle systems are focalized on the effect of the particles size on the NMR signals [48; 49]. In view of that, to characterize and confirm the interaction between gold nanoparticles and GO sheets into the hybrid systems, \({}^{1}\)H-NMR measurements were performed. Figure 3 shows the acquired NMR spectra for: (a) hybrid GO-AuNP, (b) citrate-stabilized AuNPs at the concentration of 3.5 \(\times\) 10\({}^{-8}\) M; (c) GO (0.025 mg/mL in a 3.09 \(\times\) 10\({}^{-3}\) M sodium citrate solution in D\({}_{2}\)O); (d) 3.09 \(\times\) 10\({}^{-3}\) M sodium citrate solution in D\({}_{2}\)O. According to literature data, the AB quartet (J = 15 Hz) peaks, centred at 2.52 ppm, correspond to the citrate-CH\({}_{2}\) signal [50]. The intensity of the signal considerably decreases for the hybrid GO-AuNP and the peaks are also broadened and unresolved. The line broadening in the NMR spectra of metal nanoparticles can be related to several factors, including the change in the binding modes of small molecules, dipole-dipole coupling, the increasing size of the NP core, the heterogeneity of the dispersion [51]. In our case, since the stability and homogeneity of the dispersions were tested by UV-visible spectroscopic analyses during aging times up to two weeks, the most likely picture that can be figured out is that the line broadening is due to hindered mobility of the citrate molecules interacting simultaneously with both the AuNPs and the GO nanosheets. Conversely, the intensity and the peak-shape of the citrate signal, in the NMR spectrum of the bare AuNPs, is comparable to the signal obtained for the controls (i.e., GO alone and sodium citrate solution). Together, these experimental findings suggest that most of the citrate molecules on the gold surface are replaced by GO sheets, thus confirming the effective strong interaction in the assembling of GO and AuNPs. The atomic force microscopy height images of AuNP, GO, and GO-AuNP samples irreversibly adsorbed on mica (see experimental) confirm the actual formation of the hybrid nanocomposites (Figure 4). In fact, similar spherical AuNPs, about 20 nm in size, are found strongly associated to the GO single layers, which have dimensions of about 1 nm of thickness and roughly 300-400 nm of lateral size. Noteworthy, the preferential reaction of AuNPs with the GO in-plane defects [4] can be deduced by the allocation of the AuNPs, which are mostly at the basal plane of the GO sheets. Figure 3: NMR spectra of (**a**) hybrid nano-systems GO-AuNP pellet 2, 3.5 \(\times\) 10\({}^{-8}\) M; (**b**) AuNPs pellet 2, 3.5 \(\times\) 10\({}^{-8}\) M; (**c**) GO 0.025 mg/mL in 3.09 \(\times\) 10\({}^{-3}\) M sodium citrate solution (**d**) 3.09 \(\times\) 10\({}^{-3}\) M sodium citrate solution.