## 1 Introduction  
It is difficult to overestimate the importance of nanoparticles (NP) in the scientific research scene of the latest 20 years. They have been the focus of much research and have found applications in many fields, such as sensing and biosensing, nanomedicine, and chemical catalysis, just to cite a few [1; 2; 3; 4; 5; 6; 7]. Chemical synthetic strategies to produce metal nanoparticles of controlled sizes require the use of capping agents to prevent uncontrolled growth [8], but also to avoid their coagulation [9]. There are, however, applications where the presence of chemicals adsorbed on the surface of the NP is detrimental, which can also be true from an environmental point of view. In this respect, pulsed laser ablation in liquid (PLAL) represents a promising approach to the synthesis of nanostructured materials. In recent times, PLAL has been employed for the preparation of several metals and semiconductor materials in different liquid media [10]. During PLAL, a small amount of material is removed from a solid target submerged in a liquid by irradiating it with a laser beam. The interaction of the radiation with matter induces the breakdown of the submerged target that is characterized by visible plasma emission and the production of shock waves and cavitation bubbles. The atomized metal nucleates and grows in the plasma environment during the first few hundreds of nanoseconds. In the following microseconds, the NPs will rearrange in the cavitation bubble and eventually, be released in the surrounding liquid medium [11]. The advantages of PLAL as an alternative method of preparation of NPs generation has prompted a large body of fundamental studies on the nanoparticle growth within the laser-induced plasma. At variance, several aspects of physics from the resulting NPs solution are often overlooked in the literature. In particular, the origin of the high colloidal stability of NPs prepared by PLAL is puzzling. Noble metal colloids (Au, Ag, Pt), especially, have high Hamaker constant values and, as a consequence, are subject of strong attractive inter-particle forces. Being free from capping agents or surfactants, NPs synthesized by PLAL cannot rely on repulsive steric interactions to prevent coagulation. Experimentally, noble metal NPs ablated in water are characterized by a large negative \(\zeta\)-potential and, therefore, their stability should be electrostatic in nature. Indeed, they can easily be made unstable and coagulated by increasing the solution's ionic strength. However, the origin of the negative \(\zeta\)-potential is less obvious. The case of the gold NPs is archetypal. Metallic gold (Au\({}^{0}\)) is a noble metal and, therefore, inert under most conditions. Nonetheless, gold NPs (AuNPs) ablated in water have a large negative \(\zeta\)-potential. Several authors attribute the negative \(\zeta\)-potential observed in water to partially oxidized gold species (AuO\({}^{-}\) and AuCO\({}_{3}\)\({}^{-}\)) onto the surface of the \"naked\" particles [10; 12]. On the other hand, there are many examples where the presence of gold surface oxides is not detected (a comprehensive survey of the results in the literature is discussed in the recent paper by [PERSON] and co-workers [13]). Currently, the origin of those negative charges is yet a matter of debate [14; 15; 16]. Recently, based on several experimental pieces of evidence, our group has proposed that a significant portion of the negative charge sitting on the gold nanoparticle should be due to the presence of excess electrons formed within the plasma phase and \"trapped\" by the nanoparticle [16]. As a matter of fact, the nature of particles electron charging in the plasma phase, as well as the role of the excess electrons on the NPs surface in the cavitation bubble have been recently investigated both with experiments and models [17; 18].