## 4 Conclusions  
Metal nanoparticles made of gold, silver, and platinum have been synthesized by means of pulsed laser ablation in liquid. The particles share the same size and have the same \(\zeta\)-potential corresponding to 19 negative charges per nanoparticle. With respect to the chemical composition of gold and silver nanoparticles, XPS analysis has demonstrated the absence of a measurable surface concentration of metal oxides. On the contrary, in the case of nanoparticles made by platinum, only 77.6% of the particle's surface is made of elemental platinum. The remaining composition is attributed to platinum hydroxide (Pt(II) 10.8%) and oxide (Pt(IV) 11.6%). Despite such a large difference in chemical composition, the fact that the nanoparticles have the very same \(\zeta\)-potential points to a marginal role of the metal oxides in building the negative charge that stabilizes these colloidal suspensions. These metal colloids can be destabilized by trace amounts of selected metal ions (Ag\({}^{+}\), Cu\({}^{2+}\), and Pb\({}^{2+}\)). The case of Ag\({}^{+}\) is paradigmatic, since it is able to reduce the NP \(\zeta\)-potential and to induce the coagulation at concentrations as low as 3 \(\upmu\)M, while in the case of K\({}^{+}\), the critical coagulation concentration is around  
Figure 5: Effect of AgNO\({}_{3}\) and KNO\({}_{3}\) on AuNPs ablated in KNO\({}_{3}\) 100 \(\upmu\)M. The NP concentration is 1.5 nM. (**A,B**) absorbance spectra of the AuNPs solutions as a function of salt concentration, AgNO\({}_{3}\) and KNO\({}_{3}\), respectively. (**C**) \(\zeta\)-potential of the solutions shown in A and B as a function of salt concentration. (**D**) Visual appearance of the AuNPs solution immediately after the loading with different amount of KNO\({}_{3}\) (upper) and AgNO\({}_{3}\) (lower).  
8000 uM. It is proposed that such a huge difference in destabilization power between monovalent cations can be accounted for by the differences in the reduction potential (being Ag\({}^{+}\) an oxidant stronger than K\({}^{+}\)). According to such an interpretation, while K\({}^{+}\) acts merely as a counterion, screening the negative charge on the nanoparticle only at a high salt concentration, Ag\({}^{+}\) reduces itself by subtracting electrons from (oxidizing) the nanoparticles that lose their negative charge.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/12/4169/s1](http://www.mdpi.com/2076-3417/10/12/4169/s1)]([http://www.mdpi.com/2076-3417/10/12/4169/s1](http://www.mdpi.com/2076-3417/10/12/4169/s1)), Figure S1: Time evolution upon addition of KNO\({}_{3}\), Figure S2: Time evolution upon the addition of AgNO\({}_{3}\).  
Conceptualization, G.P., N.C. and A.D.G.; methodology, H.M., A.M., M.D.; formal analysis, M.D., H.M. and R.A.P.; investigation, H.M., M.D., R.A.P.; data curation, H.M., A.M. and R.A.P.; writing--original draft preparation, H.M. and G.P.; writing--review and editing, M.D., A.M., H.M. and G.P; supervision, G.P. All authors have read and agreed to the published version of the manuscript.  
Partial financial support by the Center for Colloid and Surface Science (CSGI) is acknowledged.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.