## 3 Results and Discussion
### Effect of Added Metal Cations
On the other hand, the experimental ccc values for Cu\({}^{2+}\) (ccc = 6 \(\upmu\)M) and Pb\({}^{2+}\) (ccc = 13 \(\upmu\)M) are smaller than the DLVO prediction by two orders of magnitude, which is too much to be justified by ion-specific effects (that holds at high ion concentration). The inspection of these results suggests that in the case of Cu\({}^{2+}\) and Pb\({}^{2+}\), the mechanism of destabilization is different from the simple charge screening. Interestingly, these are the two cations having a high standard reduction potential. This suggests that the cation-induced decrease in the absolute value of NP \(\zeta\)-potential, which leads to the coagulation, is associated with the NP oxidation by the cations. However, a mismatch of a factor of a hundred between theory and experiments is not enough to be proof. For this reason, the attention was focused on the silver cation, which has a reduction potential of 0.8 V. Besides, it is a monovalent ion for which one can exclude the role of bidentate ligand bridging two NPs. To test the impact of Ag\({}^{+}\) on the colloidal stability any trace of halides must be avoided. Therefore, the AuNPs were ablated in 0.1 mM KNO\({}_{3}\) (in order to leave the ionic strength steady at the level of the previous experiments). The resulting AuNPs have the same size (R = 5 nm) and \(\zeta\)-potential (\(-\)59 mV) of those ablated in KCl 0.1 mM. The AuNP response at increasing concentrations of the two monovalent nitrates was observed: AgNO\({}_{3}\) and KNO\({}_{3}\). Pictures of such ionic strength scans are shown in Figure 5. Immediately after the salt addition, the AuNP solutions exposed at concentrations of KNO\({}_{3}\) larger than 5 mM turn blue (upper picture in Figure 5D) and completely precipitate in few hours (lower picture in Figure S1). In Figure 5B the absorbance spectra of the AuNPs solution produced in KNO\({}_{3}\) have been acquired immediately after the loading of different amount of KNO\({}_{3}\) in order to evidence the size variation of AuNPs as a function of salt concentration. This allows the estimation of the K\({}^{+}\)ccc at 8 mM, in agreement with the order of magnitude of the DLVO prediction. The situation in the case of the addition of AgNO\({}_{3}\) is completely different. The solutions with an Ag\({}^{+}\) concentration larger than 1 \(\upmu\)M change color after the salt addition (lower picture in Figure 5D). In the case of [AgNO\({}_{3}\)] = 5 \(\upmu\)M, the kinetics of color change are slightly slower, but the observation of the samples after few hours (Figure S2) also confirms that such a sample is well above the ccc. In Figure 5A, the absorbance spectra of the AuNPs solution produced in KNO\({}_{3}\) have been acquired immediately after the loading of different amounts of AgNO\({}_{3}\) in order to put in evidence the size variation of AuNPs as a function of salt concentration. The LDE-PALS measurements, taken immediately after the salt addition (before macroscopic precipitation), are shown in the main panel of Figure 5C and confirm the \(\zeta\)-potential of the particles becomes less negative upon loading with AgNO\({}_{3}\) and vanishes at a concentration (0.1 mM)at which the KCl does not affect it. The above-described experiments demonstrate that keeping the cation valency constant, there is a decrease in the ccc of four orders of magnitude passing from the low reduction potential metal potassium to the high reduction potential metal silver. This is very difficult to explain without assuming that the mechanism by which the cations decrease the nanoparticle's negative charge is the oxidation of the NPs surface. However, the XPS analysis described in Section 3.1 has assessed that on the surface of the gold nanoparticles, there is only elemental gold, that can be hardly oxidized by silver ions. A case that could reconcile these two pieces of evidence is the presence of an excess of electrons on the metal particle that can reduce the Ag\({}^{+}\) ions without oxidizing the elemental Au\({}^{0}\).