## 3 Results and Discussion
### Size and Surface Chemical Composition  
The size distribution obtained from TEM (see Figure 1 for a representative micrograph) shows all three types of NPs have particle sizes of few tens of nm with an average diameter of 11 nm and a standard deviation of 5 nm. These values are in very good agreement with the sizing obtained from the analysis of the absorbance at the wavelength of the surface plasmon (\(9\pm 2\)) and (\(8\pm 2\)) nm for gold and silver, respectively. In the platinum case, the surface plasmon resonance falls in the far UV region with a position that does not depend on the particle size. In that case, the only possible cross-validation is checking the size distribution obtained from the autocorrelation function (ACF) of the DLS, taking into account the strong dependence of the light scattering power on the particle size. This (also shown in Figure 2C) is centered at a slightly larger diameter, 14 nm. The discrepancy between TEM, DLS, and SPR is negligible if one takes into account that all such techniques, in the present polydisperse system, have an associated error of at least 2 nm. This is an important achievement because, in the case of platinum, as discussed above, the position of the SPR peak cannot be relied upon for the sizing. The size distributions obtained from the ACF of the DLS for silver and gold are centered to a slightly lower diameter (13 and 10 nm, respectively) and thus are in very good agreement with the TEM results. The surface chemical composition was investigated by XPS. A representative XP spectrum of the Au4f region is shown in Figure 3A. The doublet shows one main contribution associated with Au(0), in which the Au4f\({}_{7/2}\) component falls at BE = 83.4 \(\pm\) 0.2 eV. The low BE value found in this work as compared to 84 eV typical of bulk gold is not unexpected [24]. This trend is ascribed to both negative charge density given by chloride on the NP's surface and the effect of the substrate [24]. An additional couple of peaks were included to fit this signal: Au4f\({}_{7/2}\) associated with the minor doublet which falls at 85.4 \(\pm\) 0.2 eV, being then compatible with Au(I). However, it should be pointed out that its relative abundance is below 3%, hence within the fitting error. The Ag3d XP region relevant to silver nano-colloids is presented in Figure 3B. Three peaks are clearly visible: the Ag3d doublet with components at BE = 367.0 \(\pm\) 0.2 eV (Ag3d\({}_{5/2}\)) and 373.0 \(\pm\) 0.2 eV (Ag3d\({}_{3/2}\)), and the K2s peak at about 377 eV (from the KCl). Interestingly, Ag3d\({}_{5/2}\) falls at very low BE as compared to the typical value for Ag(0) (about 368 eV). Such findings may be explained in terms of chloride adsorption on the surface of silver nanoparticles, which generates a high negative charge density on the surface thus shifting downwards Ag3d position. Nevertheless, the chemical speciation of AgNPs returns only Ag(0), as estimated by the calculation of the modified Auger parameter (\(\alpha^{\prime}\) = BE(Ag3d\({}_{5/2}\)) and KE(Auger)). In fact, the Ag\({}_{4}\)N\({}_{45}\)N\({}_{45}\) Auger signal falls at a kinetic energy (KE) of 358.8 \(\pm\) 0.2 eV for such samples, thus resulting in \(\alpha^{\prime}\) = 725.8 \(\pm\) 0.3 eV, which is in agreement with Ag(0) state [23; 25]. Additional components ascribed to Ag(I) were not detected, thus suggesting that surface oxidation did not occur. XPS analysis of platinum nanoparticles (Figure 3C) identifies three chemical \begin{table}
\begin{tabular}{c c c c} \hline
**Element\%** & **AuNPs** & **AgNPs** & **PtNPs** \\ \hline C\% & 22.2 & 30.8 & 31.9 \\ O\% & 28.2 & 28.3 & 29.3 \\ Si\% & 33.0 & 25.5 & 20.4 \\ K\% & 2.4 & 2.2 & 4.5 \\ Cl\% & 2.8 & 1.6 & 4.3 \\ Au\% & 11.4 & / & / \\ Ag\% & / & 11.6 & / \\ Pt\% & / & / & 9.6 \\ \hline \end{tabular}
\end{table} 
Table 3: XPS surface chemical composition of metal nanoparticles prepared by pulsed laser ablation in 100 \(\mu\)M KCl aqueous solution.