In this paper the plasmon enhanced ablation for elemental analysis is investigated with several experiments in order to point out the crucial questions concerning the laser matter interaction under the effect of plasmonic coupling between the nanoparticle (NP) system and the laser ablation pulse. The correlation between the electromagnetic field enhancement and the signal enhancement during NP enhanced laser induced breakdown spectroscopy (NELIBS), as well as the laser matter interaction at the nanoscale, is discussed in the case of noble metal NPs deposited on metal samples. The results suggest that, while laser pulse energy is concentrated in the space between the NPs, the NP system is shielded by the field enhancement distribution after the laser pulse interacts with the plasmons of the NP system. Finally the comparison of the laser energy transfer to the sample between NELIBS and conventional LIBS is discussed. ? 2020 Elsevier B.V.

Metal nanoparticles (NPs) made of gold, silver, and platinum have been synthesized by means of pulsed laser ablation in liquid aqueous solution. Independently from the metal nature, all NPs have an average diameter of 10 ? 5 nm. The ?-potential values are:-62 ? 7 mV for gold,-44 ? 2 mV for silver and-58 ? 3 for platinum. XPS analysis demonstrates the absence of metal oxides in the case of gold and silver NPs. In the case of platinum NPs, 22% of the particle surface is ascribed to platinum oxidized species. This points to a marginal role of the metal oxides in building the negative charge that stabilizes these colloidal suspensions. The investigation of the colloidal stability of gold NPs in the presence of metal cations shows these NPs can be destabilized by trace amounts of selected metal ions. The case of Ag+ is paradigmatic since it is able to reduce the NP ?-potential and to induce coagulation at concentrations as low as 3 ?M, while in the case of K+ the critical coagulation concentration is around 8 mM. It is proposed that such a huge difference in destabilization power between monovalent cations can be accounted for by the difference in the reduction potential. ? 2020 by the authors.

aser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) is a powerful and well-established analytical technique, withhigh sensitivity and fast response, extensively applied to investigateinorganic elements in solid specimen [1-3]. Little or no sample pre-paration is required and analyses can be performed on a large range ofmaterials: conducting, non-conducting, opaque and transparent. Thesample mass size required is in the order of sub-micrograms, whichessentially keeps the item aspect unaltered. It can be applied to dif-ferent analysis approaches: from bulk analysis and depth profiling toelemental/isotope mapping.All these advantages make the technique both extremely versatileandflexible, so it is currently applied in various scientificfields, such asbiology, metallurgy, archeology, material science, geology, etc.Nevertheless, LA-ICPMS cannot be considered as a direct all-purposetechnique to be applied any solid item due to some issues: the aerosolsample composition is not always perfectly representative, transportefficiency is problematic and there may be a possible incomplete de-composition of particles that reach the ICP.Studies to improve LA-ICPMS instrumentation and quantificationstrategies are still ongoing. The biggest challenge regards elementalfractionation, which is related to abundances of detected ions, fre-quently not stoichiometrically corresponding to the composition of thepristine sample [1-5]. In the aerosol, differing sizes and geometry ofparticles ablated from different matrices represent another importantissue to be investigated and is related to the laser-sample interactionwhich affects sample transport efficiency from the ablation cell to theplasma and the subsequent atomization of particles in the ICP.Various efforts have been carried out to control these issues[1, 6-20]. Most studies have investigated the effect of instrumentalparameters on aerosol formation. In particular, the influence of bothlaser wavelength and pulse duration on the formation of a homo-geneous aerosol have been extensively monitored.Recently, the use of metallic nanoparticles (NPs) has been proposedto improve the efficiency of energy transfer from the ns-laser pulse tothe sample in Laser Induced Breakdown Spectroscopy (LIBS), namelyNELIBS [21]. In these works, an improvement of the LOD up to 2 ordersof magnitude has been found. LIBS and LA-ICPMS are clearly two dif-ferent analytical approaches, as thefirst is based on the direct ob-servation of the laser induced plasma, while the latter also involves thetransportation of the aerosol to the ICP torch, its atomization and io-nization. In any case, both techniques are based on the same sampleoperation, i.e. the laser ablation, so the use of NPs deposited on thesurface can improve, although in a different extent, LA-ICPMS too.Recently, the feasibility of enhancing the LA-ICPMS signal of major andminor elements in Al alloy and brass by Ag and Au NPs was proven [22]but many questions remain unanswered. In this paper a detailed char-acterization of the processes occurring during NELA-ICPMS (Nano-particle-Enhanced LA-ICPMS) is proposed, in order to improve itsgeneral performance. Inside this framework, the effects of NP size andtype(i.e. Au, Ag, Pt), with specific SPR (Surface Plasmon Resonance),on the signal enhancement of a large variety of trace elements, both inconductive and dielectric matrices, were tested. The obtained resultsopen the way to several fundamental issues concerning both NPs' en-hanced photoablation and the consequent effect of NPs on particleformation and the subsequent stages (transport, ionization, atomiza-tion).Although further studies are still required to obtain a completeunderstanding of the effect of the use of NPs in LA-ICPMS, this paperaims to provide a general idea of the perspectives of NELA-ICPMS in-vestigating the effect of NPs during ablation and subsequent transportin the ICP torch, in order to extend knowledge on the causes of theenhancement of the signal of some trace elements in a standard sample

The laser induced plasma during the nanosecond Pulsed Laser Ablation in Liquid (PLAL) plays a crucial role in the nanoparticles (NPs) formation and charging. It was demonstrated that during the plasma phase evolution, once the NPs are formed, they are charged with the excess of plasma electrons. Immediately after the plasma phase extinguishes, the NPs will be released in the induced vapor bubble, generated by the fast energy exchanges between the plasma and the liquid. The excess of charge in the NPs preserves them from the agglomeration during the bubble evolution and can induces an electrostatic pressure able to eject the particles outside the cavitation bubble.

In this paper experimental temperature and density maps of the laser induced plasma in water during pulsed laser ablation in liquid (PLAL) for the production of metallic nanoparticles (NPs) has been determined. A detection system based on the simultaneous acquisition of two emission images at 515 and 410 nm has been constructed and the obtained images have been processed simultaneously by imaging software. The results of the data analysis show a variation of the temperature between 4000 and 7000 K over the plasma volume. Moreover, by the study of the temperature distribution and of the number densities along the plasma expansion axis it is possible to observe the condensation zone of the plasma where NPs can be formed. Finally, the time associated with the electron processes is estimated and the plasma charging effect on NPs is demonstrated. The set of observations retrieved from these experiments suggests the importance of the plasma phase for the growth of NPs and the necessity of considering the spatial distribution of plasma parameters for the understanding of one of the most important issues of the PLAL process, that is the source of solid material in the plasma phase. ? 2019 IOP Publishing Ltd.

In this paper, the Variational Method based on the H?ckel Theory is applied to NPs chain and aggregate systems in order to estimate the energy of the plasmon and, in turn, the resonance wavelength shift, which is caused by the interaction of adjacent NPs. This method is based on the analogies of NPs dipole interactions and the ?-system in molecules. Differently from the Hartree-Fock method that is a self-consistent model, in this approach, the input data that this method requires is the dimer energy shift with respect to single NPs. This enables us to acquire a simultaneous estimation of the wavefunctions of the NPs system as well as the expectation energy value of every kind of NPs system. The main advantage of this approach is the rapid response and ease of application to every kind of geometries and spacing from the linear chain to clusters, without the necessity of a time-consuming calculation. The results obtained with this model are closely aligned to related literature and open the way to further development of this methodology for investigating other properties of NPs systems. ? 2019 by the authors. Licensee MDPI, Basel, Switzerland.

In this work Nanoparticle Enhanced Laser Induced Breakdown Spectroscopy (NELIBS) has been employed for quantitative metal detection using amyloid fibrils coated with gold nanoparticles as enhancers. Amyloid fibrils represent, from one hand, an extremely interesting system for novel technologies, ranging from water purification to medical applications, and on the other hand, an ideal system for investigating the performance of laser-matter interaction in biological systems. The results obtained in this work show the potentiality of NELIBS for the quantification at sub-ppm (mg/kg) level of metallic elements (Cr, Pb, Tl and Cd) even in the protein and/or biological environment, employing amyloid fibrils with gold nanoparticles. Moreover, the single-shot measurements reveal the promising use of this technique in applications where high sensitivity and/or limitation in the sample amount are demanded. ? 2019 Elsevier B.V.

In this paper, the promising variant of the Laser Induced Breakdown Spectroscopy (LIBS) technique, namely Nanoparticle - Enhanced LIBS (NELIBS) is described. The underlying mechanisms responsible for NELIBS are described. This is done by presenting both the properties of metallic particles interacting with high-energy laser radiation and the mechanisms of laser ablation enabled by the presence of nanoparticles. Clarifications are also made about the sample preparation for NELIBS in particular about how to reach the optimal surface concentration of nanoparticles. NELIBS applications on different kinds of samples are also described such as metals, transparent samples, fresh samples and liquids including biological fluids. ? 2018 Elsevier B.V.

In this work, LIBS (Laser Induced Breakdown Spectroscopy) capability to operate in stand-off configuration, without the need of a direct contact with the sample, has been coupled with the calibration-free (CF) approach to LIBS data analysis. The latter does not require the use of standard calibration. The feasibility of the calibration free method on stand-off LIBS spectra has been thereby tested. The quantitative analysis was performed on samples of two well-known meteorites, Toluca (iron meteorite) and Sahara 98222 (L6 chondrite) by using a stand-off configuration at a distance of 5 m. The plasma temperature and the electron number densities were determined for each sample and for each laser shot in order to apply the CF method. For the Toluca meteorite sample Fe, Ni and Co content was quantified during the depth profile. For the Sahara 98222 major and minor elements (Fe, Mg, Si, Na, Ti, Al, Cr, Mn, Ca, Ni, Co) were analysed by averaging different meteorite zones because of its strong inhomogeneity. Results demonstrate the possibility of remote analysis of minor bodies and space debris. ? 2018 Elsevier B.V.

In this paper, Nanoparticle-Enhanced Laser Induced Breakdown Spectroscopy is applied to transparent samples and gemstones with the aim to overcome the laser induced damage on the sample. We propose to deposit a layer of AuNPs on the sample surface by drying a colloidal solution before ablating the sample with a 532 nm pulsed laser beam. This procedure ensures that the most significant fraction of the beam, being in resonance with the AuNP surface plasmon, is mainly absorbed by the NP layer, which in turn results the breakdown to be induced on NPs rather than on the sample itself. The fast explosion of the NPs and the plasma induction allow the ablation and the transfer in the plasma phase of the portion of sample surface where the NPs were placed. The employed AuNPs are prepared in milliQ water without the use of any chemical stabilizers by Pulsed Laser Ablation in Liquids (PLAL), in order to obtain a strict control of composition and impurities, and to limit possible spectral interferences (except from Au emission lines). Therefore with this technique it is possible to obtain, together with the emission signal of Au (coming from atomized NPs), the emission spectrum of the sample, by limiting or avoiding the direct interaction of the laser pulse with the sample itself. This approach is extremely useful for the elemental analysis by laser ablation of high refractive index samples, where the laser pulse on an untreated surface can otherwise penetrate inside the sample, generate breakdown events below the superficial layer, and consequently cause cracks and other damage. The results obtained with NELIBS on high refractive index samples like glasses, tourmaline, aquamarine and ruby are very promising, and demonstrate the potentiality of this approach for precious gemstones analysis. ? 2018 Elsevier B.V.

"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 ?-potential. Although, gold oxides have pKas ? 4, the ?-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. ? 2016 Elsevier Inc.

In this paper, emission spectroscopy and fast imaging surveys during pulsed laser ablation in liquid (PLAL) for nanoparticles (NPs) production have been used, in order to provide further details about the process involved and the potentialities offered by a wire-shaped sample ablated in a flowing water jet. This kind of set-up has been explored because the laser ablation efficiency in water increases when a thin water layer and a wire-shaped target are used. In order to understand the physical processes causing the increasing ablation efficiency, both the laser-induced plasma and bubble dynamics generated in a flowing liquid jet have been analysed. The plasma parameters and the bubble behaviour in such a system have been compared with those observed in conventional PLAL experiments, where either a bulk or a wire-shaped target is immersed in bulk water. From the data presented here it is evidenced that the plasma and shockwave induced during the breakdown process can play a direct role in the ablation efficiency variation observed. With regard to the cavitation bubbles evolving near a free surface (the interface between water and air) it should be noted that these have to be treated with caution as a consequence of the strong influence played in these circumstances by the boundary of the water jet during its expansion dynamics. The effects due to the size of the liquid layer, the presence of the water/air interface, the liquid characteristics, the target shape, the plasma evolution and the bubble dynamics together with their outcomes on the NPs' production, are presented and discussed.

There is a growing interest in the development of technologies of augmented information of use in exploration of the deep ocean. Of special interest are techniques for the chemical analysis of submerged solids, which show promise for subsea mining applications where a rapid sorting of materials found in the sea bottom is required. Laser-induced breakdown spectroscopy has demonstrated potential for this application by its unique capability of providing the atomic composition of underwater solids. Here we present a study on the parameters that configure the spectral response of metallic targets in an oceanic pressure environment. Following laser excitation of the solid, the plasma persistence and the cavitation bubble size are considerably reduced as the hydrostatic pressure increases. These effects are of particular concern in dual pulse excitation as reported here, where a careful choice of the interpulse timing is required. Shadowgraphic images of the plasmas demonstrate that cavitation bubbles are formed early after the plasma onset and that the effect of hydrostatic pressure is negligible during the early stage of plasma expansion. Contrarily to the effect observed at atmospheric pressure, emission spectra observed at high-pressures are characterized by self-absorbed atomic lines on continuum radiation resulting from strong radiative recombination in the electron-rich confined environment. This effect is much less evident in ionic lines due to the much larger energy of the levels involved and to the lower extent of absorption effects occurring in the inner part of the plasma, where the ionized states are more abundant. As a result of the smaller shorter-lived cavitation bubble, the LIBS intensity enhancement factor resulting from dual pulse excitation is decreased by increasing the applied pressure.

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.

To scale-up pulsed laser ablation in liquids for nanoparticle synthesis, we combine two promising approaches, a wire-shaped target and a small liquid layer, in one setup. Using thin liquid layers a significant increase in nanoparticle productivity (up to 5 times) is obtained. This increase is attributed to the dynamics, shape of the cavitation bubble and the spring-board like behavior of the wires in the small liquid filament. It is found that despite the increase in productivity, the particle size is independent of the productivity-related ablation parameters such as repetition rate, liquid layer thickness and wire diameter. In addition to the cavitation bubble, further shielding effects have been related to both, the laser ablated material and the presence of generated small vapor bubbles. The obtained results show that this setup can provide a good strategy to realize a continuous and process-stable (particle size and quality) ablation process without the need of target replacement.

Comment on: Comment on "Nanoparticle Enhanced Laser-Induced Breakdown Spectroscopy for Microdrop Analysis at subppm Level": Several Issues to Consider When Quantitatively Measuring Fluids Using Nanoparticle-Enhanced Laser-Induced Breakdown Spectroscopy. Zhao C, Dong D.

In this paper, the new approach for Laser Induced Breakdown Spectroscopy (LIBS) based on nanoparticle deposition on the sample surface is reviewed from both fundamental and application points of view. The case of Nanoparticle-Enhanced LIBS (NELIBS) of metal samples is used for describing and discussing the main causes of the emission signal enhancement. A set of test cases is presented, which shows enhancements up to 1-2 orders of magnitude obtained using NELIBS with respect to LIBS. The feasibility and potential of NELIBS are also discussed for several analytical applications, including analysis of metallic samples, transparent samples and aqueous solutions. ? 2016 The Royal Society of Chemistry.

The interaction of nanoparticles (NPs) with proteins is widely investigated since it can be a key issue in addressing the problem of nanotoxicity, particularly in the case of biological and medical applications. In this work, silver and gold nanoparticles (AgNPs and AuNPs) were produced in water by Pulsed Laser Ablation in Liquid (PLAL) and allowed to react with Ubiquitin (Ub) (a small human protein essential for degradative processes in cells). NPs produced by PLAL are completely free of undesired contaminants and do not require the use of stabilizers. We found that the NPs + Ub system behaves differently if the NPs are or are not treated with a stabilizer before performing the interaction with Ub, since the presence of capping agents modifies the surface reactivity of the metal-NPs. The surface plasmon resonance (SPR) absorption spectroscopy was employed to monitor the fast changes occurring in the NP colloidal solutions upon interaction with Ub. The results obtained by SPR were confirmed by TEM analysis. Therefore, when Ub interacts with bare NPs a rapid aggregation occurs and, at the same time, Ub undergoes an amyloid transition. Notably, the aggregation of AuNPs occurs at a much greater rate than that of analogous AgNPs and the Ub fibrils that are formed can be imaged by thioflavin T fluorescence. ? 2015 Elsevier B.V. All rights reserved.

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. ? 2016 American Chemical Society.

In the last decade Pulsed Laser Ablation in Liquids (PLAL) has been widely investigated from the fundamental point of view, and various theories have been proposed. Although many important achievements have been obtained by the scientific community, many aspects still need to be clarified and many contradictions arise when comparing the interpretation of similar experiments carried out by different authors. In this paper we have reconsidered previous works focused on specific processes and stages of the PLAL, in order to outline a modern and comprehensive point of view of the overall physical aspects of PLAL. With this aim, several simultaneous diagnostic methods have been applied during the production of metallic nanoparticles (NPs), i.e. optical emission spectroscopy and fast imaging for the investigation of the laser-induced plasma, shadowgraph for the study of the cavitation bubble, and Double Pulse Laser Ablation in Liquid (DP-LAL) and laser scattering for the investigation of NPs location and mechanisms of release in solution. The connection between the various stages of the DP-LAL allows understanding the main characteristics of the produced NPs and the typical timescales of the basic mechanisms involved in PLAL.