Study of the Effect of Water Pressure on Plasma and Cavitation Bubble Induced by Pulsed Laser Ablation in Liquid of Silver and Missed Variations of Observable Nanoparticle Features.

In this work the effects of the pressure between 1-150 Bar on pulsed laser ablation in liquids (PLAL) during the production of silver nanoparticles (AgNPs) in water was investigated. The produced NPs are the results of two different well-known stages which are the plasma and the bubble evolution occurring until the generated material is released into the solution. The main aim of this work is to show which roles is played by the variation of water pressure on the laser induced plasma and the cavitation bubble dynamics during the NPs formation. Their implication on the comprehension of the as-produced NPs formation mechanisms is treated. The typical timescales of the different stages occurring in water at different pressures have been studied by optical emission spectroscopy (OES), imaging and shadowgraph experiments. Finally surface plasmon resonance (SPR) spectroscopy, transmission electron microscopy (TEM), dynamic light scattering (DLS) and scanning electron microscopy (SEM) for characterization of the material released in solution, have been used.

On the stability of gold nanoparticles synthesized by laser ablation in liquids.

"Naked" gold nanoparticles (AuNPs), synthesized in the absence of any capping agents, prepared by pulsed laser ablation in liquid (PLAL) are stabilized by negative charges. Common explanations for this phenomenon involve the presence of gold oxides and/or the anion adsorption. We have found that AuNP ablated in solutions of acids with very different oxidation power, viz. HCl, H2SO4, HNO3 share the same size and ζ-potential. Although, gold oxides have pKas≈4, the ζ-potential of AuNPs ablated in solutions with pH⩽4 is always negative. These evidences suggest that the gold oxidation and anion adsorptions have only a minor role on building the negative surface potential and we hypothesize, for the first time, that excess electrons formed within the plasma phase could charge the metallic particles. In our model, a crucial point is that the colloidal size of the NP maintains the energy of the electrons small enough to preclude chemical reactions but with a surface potential yet large enough to stabilize the AuNPs with respect to aggregation. A confirmation of the hypothesis of "electron-stabilized nanoparticles" is that either the addition of macroscopic metallic objects either the contact with a "grounded" copper wire induce the loss of charge and AuNPs aggregation.

Nanoparticle Enhanced Laser-Induced Breakdown Spectroscopy for Microdrop Analysis at subppm Level.

In this paper, nanoparticle enhanced laser-induced breakdown spectroscopy (NELIBS) was applied to the elemental chemical analysis of microdrops of solutions with analyte concentration at subppm level. The effect on laser ablation of the strong local enhancement of the electromagnetic field allows enhancing the optical emission signal up to more than 1 order of magnitude, enabling LIBS to quantify ppb concentration and notably decreasing the limit of detection (LOD) of the technique. At optimized conditions, it was demonstrated that NELIBS can reach an absolute LOD of few picograms for Pb and 0.2 pg for Ag. The effect of field enhancement in NELIBS was tested on biological solutions such as protein solutions and human serum, in order to improve the sensitivity of LIBS with samples where the formation and excitation of the plasma are not as efficient as with metals. Even in these difficult cases, a significant improvement with respect to conventional LIBS was observed.