## 3 Results and Discussion
### Effect of Added Metal Cations  
In a previous work [16], our group observed that grounding with copper wire solutions of AuNPs synthesized by PLAL induces a rapid discharge of the \(\zeta\)-potential and eventually the coagulation of the NPs. However, this effect is not seen in AuNPs of the same size and concentration synthesized by citrate reduction of HAuCl\({}_{4}\)[16]. The interpretation of such behavior was associated with an electric discharging of the NPs and, therefore, as a proof of the presence of trapped electrons. However, the present investigation has shown that the very same behavior can be reproduced by dipping a small piece of metal copper, or by doping the solution with traces amount of copper salt. The unresponsivity of chemically synthesized AuNPs is likely to be ascribed to the presence of citrate that can complex the Cu\({}^{2+}\) ions. In the case of AuNPs, nanoparticle aggregation is easily probed by naked eye because it is preceded by a color change from pink to blue or grey (depending on the extent of the aggregation). Such color change reflects the shift of the gold surface plasmon resonance due to the formation of clusters [28]. Such an effect is less marked in the case AgNPs for which the well-known reactivity of silver cation towards halides complicates the understanding of the metal halides induced destabilization. In the case of PtNPs, the absence of a visible surface plasmon makes them unsuitable for studying the impact of oxidant cations. For these reasons, the study of the effect of oxidant cations has been performed on AuNPs. Exploiting the change of the surface plasmon, the minimum amount of Cu\({}^{2+}\) that results in an immediate shift of the plasmon maximum from \(\sim\)520 nm to \(\sim\)800 nm, (i.e., the critical coagulation concentration or ccc) has been determined. In the case of Cu\({}^{2+}\) the value is ccc = \(6\pm 2\) uM. Of course, an increase in the ionic strength of a colloidal dispersion screens the charges on the particles  
Figure 4: \(\zeta\)-potential distribution of the different metal nanoparticles in 100 μM KCl. (whatever the origin), and, for large enough ionic strengths, it eventually destabilizes the dispersion. The observed ccc for CuCl\({}_{2}\) is, however, quite low. Lower than one would expect for customary electrostatic stabilized colloids. On the other hand, the world of colloids is rich with ion-specific effects that scatter the response of the systems to ionic strength around the theoretical value. For this reason, the ccc values of AuNP synthesized by PLAL have been determined, for other bivalent (PbCl\({}_{2}\), MgCl\({}_{2}\), CaCl\({}_{2}\)) and monovalent (NaCl) chloride salts. The ccc values are listed in Table 4. The experimental values can be profitably compared with the expectation of the DLVO theory for mono- and divalent electrolytes. According to the DLVO, an estimate of the ccc is given by the approximate expression below [27]:  
\[\text{ccc (M)}=\frac{87.4\cdot 10^{-40}}{z^{6}H^{2}}=\frac{55\cdot 10^{-3}}{z^{6}} \tag{3}\]  
where \(z\) is the valency of the counterion, in the present case the cation, and the Hamaker constant \(H\), is in joules. The very strong dependence of the critical coagulation concentration on the valency was at the basis of the empirical Schultze-Hardy rule. The last equality is obtained considering the Hamaker constant of gold \(H\sim\)100 \(k_{\text{b}}\)T [29]. According to Equation (3) the expected ccc for NaCl is \(\sim\)55 mM which order of magnitude is in agreement with the experimental value of 35 mM, as reported in Table 4. Passing to divalent cations, the ccc is reduced by a factor of 64, corresponding to 800 \(\upmu\)M. In the case of divalent cations, the experimental values have an order of magnitude that are in agreement only in the cases of Mg\({}^{2+}\) and Ca\({}^{2+}\) (ccc = 150 \(\upmu\)M for both).