## 3 Results and Discussion
### HR-XPS of AgNPs Dye  
To investigate the chemical structure of AgNPs-FITC and AgNPs-RITC, SR-XPS studies were carried out. SR-XPS measurements were performed on samples with a 1/1 metal/ dye molar ratio to obtain better insight into the dyes' molecular structure stability and to probe the macromolecule-NP interaction at the dye/metal interface. Measurements were carried out at C1s, S2p, N1s, O1s, and Ag3d core levels; all data analysis results (binding energy (BE), full width half-maximum (FWHM), atomic ratio values, and proposed assignments) are collected in Table S2 in the Supplementary Materials. The measured C 1s core level spectra confirmed the stability of the dyes' molecular structure, showing all the expected spectral components (see Figure 4a) in atomic ratios compatible with the theoretical values calculated by the molecular structure, as reported in the following Table 1.  
N1s data also confirmed the chemical stability of both dyes; as expected, the N1s spectrum of AgNPs-FITC (Figure 4b, top) has a single component at 398.40 eV BE, as expected for N atoms in -N=C=S moieties; the N1s signal measured on AgNPs-FITC samples is asymmetric, with a main component ascribed to -NCS, tertiary amines at 398.48 eV, and a small feature of lower intensity at higher BE values (400.48 eV), as expected for positively charged N atoms. Focusing on the dye/AgNPs interaction, Ag3d and S2p signals allowed a probing of the electronic structure at the organic molecule/metallic cluster interface. The S2p and Ag3d spectra of AgNPs-FITC are, respectively, shown in Figure 5a,b, and are representative also for the RITC-containing sample. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **BE (eV)** & **FWHM (eV)** & **Exp. Atomic Ratio** & **\textasciitifector* & **Theor. Atomic Ratio** & **Assignment** \\ \hline \multirow{4}{*}{AgNPs–FITC} & 284.7 & 1.11 & 7.5 & 7.5 & C–C \\  & 285.84 & 1.11 & 1 & 1 & C–N, SCN \\  & 286.43 & 1.11 & 1.2 & 1 & C–O \\  & 287.66 & 1.11 & 0.5 & 1.5 & C=O \\  & 288.84 & 1.11 & - & - & (impurities) \\ \hline \multirow{4}{*}{AgNPs–FITC} & 284.7 & 1.31 & 20.9 & 18 & C–C \\  & 285.8 & 1.31 & 8.1 & 7 & C–N, SCN \\ \cline{1-1}  & 286.78 & 1.31 & 3.4 & 2 & C–O \\ \cline{1-1}  & 287.85 & 1.31 & 2 & 2 & C=O \\ \cline{1-1}  & 288.92 & 1.31 & 1 & 1 & COOH \\ \hline \hline \end{tabular}
* Intensity ratios: for AgNPs–FITC, calculated as signal intensity/C–N, SCN signal intensity; for AgNPs–FITC, calculated as signal intensity/COOH signal intensity.  
\end{table} 
Table 1: Synchrotron radiation (SR)-XPS C1s core level data fit results: BE, binding energy; FWHM, full width half-maximum; atomic ratios; and proposed assignments (the statistical incertitude in semiquantitative eValuation by XPS was estimated as 5% of the calculated value [66]). Figure 4: (a) C1s SR-XPS spectra and (b) N1s SR-XPS spectra collected on AgNPs–FITC (**top**) and AgNPs-RITC (**bottom**). Both spectra appear structured, and by following a peak-fitting procedure, two pairs of spin-orbit components can be individuated for the two signals. The first feature in the Ag3d spectrum (Ag3d5/2 BE = 367.84 eV) is associated with metallic silver and arises by the Ag atoms at the nanoparticles' core. The spin-orbit pair of very low intensity at higher BE values (Ag3d5/2 BE = 368.79 eV) is indicative for positively charged metal atoms, as expected for surface atoms interacting with the organic macromolecule [67]. The S2p spectra provide complementary information about the envisaged chemical interaction between the NCS functional groups of both dyes and the surface atoms of the silver clusters, as extensively discussed previously [57] for gold nanoparticles functionalized with RITC.