## Materials and Methods  
### NP Synthesis and Preliminary Characterization  
Colloidal particles in aqueous solution unavoidably acquire electrical charge. On materials that are not intrinsically charged, these charges can be generated by (i) adsorption of charged species in solution; (ii) titratable groups that are present on the surface, making their charge pH-dependent.  
The experiments here described have been designed to test the existence of titratable oxidized metal species (mechanism ii above). In order to keep the ion adsorption constant of mechanism (i), the ionic strength was fixed to a low and constant value. For this reason, the metal NPs in this work were synthesized by laser ablation in aqueous solutions of KCl at a fixed concentration of \(10^{-4}\) M.  
To produce metal NPs by LAL, an ns NdYAG (Quanta System PILS-GIANT) laser was focused on the metal target located on the bottom of a cuvette filled with 3 mL of \(10^{-4}\) M KCl aqueous solution for all the metal targets. In the case of the experiments described in Section 3.3, to prevent the precipitation of silver ions, the gold target was ablated in a solution of \(10^{-4}\) M KNO\({}_{3}\) aqueous solution. The laser, with a nominal pulse duration of 8 ns, operates with a 1064 nm laser wavelength, with a fluence of 64 J/cm\({}^{2}\) and a laser frequency of 10 Hz. The laser beam was focused by using a 5 cm focal lens and the focal plane of the laser beam was put inside the target, in order to obtain a laser crater on the target surface with a diameter of \(1\pm 0.2\) mm. The laser ablation time was 1 min for each solution. Immediately after the production, the NPs solutions were spun for 1 min on a benchtop centrifuge to eliminate some debris of the ablation. The concentrations of NP solutions (of the order of 3-8 nM) are listed in Table 1.  
The concentration of AuNPs and AgNPs has been evaluated from the visible spectrum of the surface plasmon resonance (SPR) as described in [20]. The concentration of PtNP has been calculated from the total amount of Pt (determined by Flame Atomic Absorption) and the average size determined by TEM.  
The metal targets were purchased from Kurt J. Lesker Company. Gold and platinum targets were cleaned using aluminium oxide 150 mesh. The cleaning was repeated three times, diluting the alumina each time with increasing amounts of water. After each cleaning step (5 min) with alumina, the target was washed 15 min in an ultrasound bath in Milli-Q water. Finally, the target was exposed to the flame of a Bunsen until it reached a white color.  
After each cleaning step, 5 min with alumina, the silver target was repeatedly cleaned three times with sodium bicarbonate, followed by a 15-min ultrasound bath in Mili-Q water. This target was not exposed to any flame.  
The visible spectrum of the SPR of AuNPs and AgNPs was registered using an Agilent 8453 UV-Visible spectrophotometer. Using the absorbance values and the calibration described elsewhere [20] it is possible to estimate the NP diameter and the NP concentration. Eventually, the solutions of ablated NPs were diluted to a NP concentration of 1.5 nM with KCl \(10^{-4}\) M for characterization by dynamic light  
\begin{table}
\begin{tabular}{l c c c} \hline \multicolumn{3}{c}{**Size (Averaged TEM Diameter)**} & \multicolumn{2}{c}{**NPs Concentration**} \\ \hline  & **nm** & **nmol-NPs/L (nM)** & **n\({}^{\circ}\) NPs/L** \\ \hline AuNPs & \(11\pm 5\) & 8.8 & \(5.3\times 10^{15}\) \\ AgNPs & \(11\pm 5\) & 3.5 & \(2.2\times 10^{15}\) \\ PtNPs & \(11\pm 5\) & 6.6 & \(4.0\times 10^{15}\) \\ \hline \end{tabular}
\end{table}
Table 1: Sizes (diameter), NPs concentration in KCl \(10^{-4}\) M, after centrifugation.  
scattering (DLS) and by Laser Doppler Electrophoresis (LDE) measurements using a Zetasizer-Nano ZS from Malvern Instruments. Instrumental conditions are as in [21] (DLS) and [22] (LDE); the \(\zeta\)-potential was subsequently evaluated from the electrophoretic mobility measured by LDE according to the Huckel approximation.