## 3 Results and Discussion  
Plasmonic Sensing Capabilities and Physicochemical Characterisation of the Hybrid GO-AuNP Nanocomposites  
The plasmon resonance properties of gold nanoparticles are very sensitive to the local environment of the nanostructure, owing to the role of the dielectric constant of the surrounding medium as well as to charge transfer effects in core-shell metal nanostructures, which results in a redistribution of electron populations in the gold nanoparticles [40]. For instance, an increase in the refractive index of the embedding medium or in the thickness of a shell leads to a red shift in the absorption band [41]. Such a shift, measured as the change in the peak wavelength at the maximum absorbance in the UV-visible spectrum, gives quantitative information on the molecular layer adsorbed on the nanoparticle surface [42]. In conjunction with the shape sensitivity, such features of the plasmonic nanopalforms offer multiple modalities for surface-based applications such as sensing, spectroscopy and imaging [43]. The optical properties of GO and AuNPs result in characteristic absorbance features, as displayed by UV-vis spectra (Figure 1). GO exhibits a maximum absorption peak centred at 230 nm, attributed to \(\pi\)-\(\pi^{\star}\) transitions of aromatic C=C bonds, and a shoulder peak at about 300 nm, attributed to n-\(\pi^{\star}\) transitions of C=O bonds, respectively [44]. AuNPs present a plasmon peak centred at around 520 nm of wavelength, distinctive of spherical gold nanoparticles with a diameter of around 12 nm [24; 45]. The spectra of the hybrid GO-AuNPs show evidence of hyperchromic effect as well as a red-shift in the plasmonic band of about 3 nm, which is indicative of spatial distribution changes for the electric near field of the gold nanoparticles [24; 46]. Such a finding points to an effective interaction between the gold nanoparticles and the GO nanosheets. Accordingly, an increase of the absorption related to \(\pi\)-\(\pi^{\star}\) transitions is visible around 230 nm. Figure 1 displays three spectra for each sample, corresponding to the as prepared as well as to the re-suspended pellets after the two rinsing steps by centrifugation, in order to remove the excess of reactants as well as to concentrate the nanomaterials in the aqueous dispersion. Figure 1: UV-visible spectra of graphene oxide gold nanoparticles (GO-AuNP) (blue) and AuNP (wine) samples before (’0’) and after the two rinsing steps in Milli-Q water (centrifugation at 8000 r.p.m, 15 min, RT) for the re-suspended pellets 1 (’1’) and 2 (’2’), respectively. Spectra of GO (black) before (’0’) and after two rinsing steps in Milli-Q water (centrifugation at 13,000 r.p.m, 20 min, RT) for the re-suspended pellets 1 (’1’) and 2 (’2’) are shown for reference. The characteristic values of wavelength at the absorbance maximum and of full width at half maximum (FWHM) in Table 1 show that the washing steps do not cause significant aggregation of the nanosystems, since minimal red shift (only 1 nm) and band broadening (2-3 nm) are found. The analysis of the GO-AuNP spectra subtracted by those of AuNP (Figure 2) emphasizes the occurrence of charge transfer processes at the interface between the GO nanosheets and the metal. In fact, for both pellet 1 (Figure 2a) and pellet 2 (Figure 2b) samples, the difference spectra show an increase of the shoulder at 300 nm, which can be indicative of the near contact between Au and the GO, which in turn affect the electronic conjugation between the GO sheets [6]. This finding is advantageous for the use of these hybrid nanocomposites as selective energy transfer probes in sensing applications at the biointerface. Indeed, the preferential binding of biomolecules to the gold [47; 24] or the GO is expected to induce different changes in the optical response.