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Laser Ablation Synthesis in Solution and Characterization of Magnetic-plasmonic alloy nanoparticles
Full text linkIn the history of nanoparticle synthesis, combining the properties of different elements in a single nanostructure has always been an important objective. In particular, alloy nanoparticles (NPs) are attracting a great research interest from the scientific community, since alloying is a way for combining different properties in a single nano-object, and obtaining peculiar structures such as NPs with magnetic and plasmonic response. Besides, by acting on alloy composition, it becomes possible to finely tune a given physical or chemical property, such as intensity and position of the surface plasmon resonance in Ag-Au NPs, or the efficiency of hydrogenation of nitro-substituted aromatics with Ni-Pd nanoalloys. The most frequent approach for the synthesis of metal alloy NPs relies on wet-chemistry methods. There are many available reactions, such as the one used to obtain in one step Ag-Au alloy NPs by co-reduction of HAuCl4 and AgNO3, and they depend on the type of
metal precursors and solvents employed. Other chemical methods have also been used, such as radiolysis, electrochemistry, sonochemistry and biosynthesis. All these methods run in thermodynamic or near-to-thermodynamic equilibrium conditions, thus limiting the type and composition of achievable nanoalloys.
Physical methods such as ion implantation and molecular beams can also produce metastable nanoalloys, but are less frequently employed because of the limitations in the scalability and use of final products. With these methods, NPs are produced inside a solid matrix or on a substrate where they often undergo irreversible agglomeration.
A different and more promising physical method for the production of both thermodynamically stable and metastable alloy NPs consists in Laser Ablation Synthesis in Solution (LASiS). LASiS employs a pulsed laser focused on the surface of a bulk metal target for the generation of a colloidal solution of NPs. Following laser absorption by the bulk target, the ablation process takes place through the generation of a plasma plume and the formation of a cavitation bubble, where NPs form. The structure and composition of the NPs are determined by a number of ablation parameters, like laser power, pulse duration,
solvent, target, and molecules available in the solvent. In recent years, LASiS has been employed to create metal alloy NPs such as Au-Ag, Pt-Au, Pt-Ir.
In this PhD work, the synthesis of alloy nanoparticles by LASiS was investigated, considering in particular the Au-Fe and Ag-Fe systems. Au-Fe alloy nanoparticles were obtained by laser ablation of a bulk Au73Fe27 target. Different solvents were used to obtain more insight about the influence of the ablation liquid
environment on the structure and composition of the nanoparticles. The plasmonic and magnetic properties of the nanoparticles were also studied in detail and, taking advantage from their coexistence, these nanoalloys were optimized as multimodal contrast agents for Magnetic Resonance Imaging, x-ray absorption computerized tomography, and surface enhanced Raman scattering (SERS) imaging. In-vitro and in-vivo tests of their biocompatibility and functionality as multimodal contrast agents were also carried out.
Ag-Fe nanoparticles were synthesized in water and ethanol from targets with different composition. The nanoparticles were studied for their plasmonic and magnetic properties, and in-depth understanding of their complex structure required the use of various complementary techniques of analysis. Due to their magnetic properties, these nanoparticles were used to create customizable arrays for SERS analysis. Various nanoparticles parameters were studied to optimize the SERS efficiency of these arrays, such as surface coating with thiolated ligands or nanoparticles concentration. As a last step, the possibility of reusing these arrays was investigated.
In summary, the application of LASiS to the synthesis of magnetic-plasmonic alloy NPs, which has been attempted in this thesis for the first time, opens several fascinating opportunities for the development of new multifunctional tools in various fields ranging from nanophotonics to nanomedicine
Magnetic amplification of surface enhanced raman scattering signals in plasmonic alloy nanoparticles
No full textMetastable alloy nanoparticles, metal-oxide nanocrescents and nanoshells generated by laser ablation in liquid solution: influence of the chemical environment on structure and composition
No full textAlloy nanoparticles are characterized by the combination of multiple interesting properties, which are attractive for technological and scientific purposes. A frontier topic of this field is nanoalloys with compositions not thermodynamically allowed at ordinary temperature and pressure (i.e. metastable), because they require out-of-equilibrium synthetic approaches. Recently, laser ablation synthesis in solution (LASiS) was successfully applied for the realization of metastable nanoalloys because of the fast kinetics of nanoparticle formation. However, the role played by the chemical environment on the final composition and structure of laser generated nanoalloys still has to be fully elucidated. Here, we investigated the influence of different synthetic conditions on the LASiS of metastable nanoalloys composed of Au and Fe, such as the use of water instead of ethanol, the bubbling of inert gases and the addition of a few vol% of H2O2 and H2O. The two elements showed different reactivity when LASiS was performed in water instead of ethanol, while minor effects were observed from bubbling pure gases such as N2, Ar and CO2 in the liquid solution. Moreover, the plasmonic response and the structure of the nanoalloys were sensibly modified by adding H2O2 to water. We also found that nanoparticle production is dramatically influenced just by adding 0.2% of H2O in ethanol. These results suggest that the formation of a cavitation bubble with long lifetime and large size during LASiS is useful for the preservation of the metastable alloy composition, whereas an oxidative environment hampers the formation of metastable alloy nanoparticles. Overall, by acting on the type of solvent and solutes, we were able to switch from a traditional synthetic approach for the composition of Au–Fe nanoalloys to one using a reactive environment, which gives unconventional structures such as metal@iron-oxide nanoshells and nanocrescents of oxide supported on metal nanospheres. These results expand the knowledge about the mechanism of the formation of nanoalloys using LASiS and show how to obtain multielement nanoparticles of enormous interest for nanomedicine, plasmonics, magneto-plasmonics and catalysis
Gold-Iron bimetallic nanoparticles as multimodal contrast agents
No full textIn a one-step approach to obtain a multimodal bimetallic nanoparticle, different from standard chemical methods process, using laser ablation synthesis in solution (LASiS) of a gold-iron alloy target (73% Au- 27% Fe) surface of the particles, and their optical and magnetic characteristics were analysed using XPS, UV-Visible and SERS instruments. In a second series of experiments, bimetallic nanoparticles were tested for CT, SERS and MRI analysis, yielding good results in all the three fields. In vivo analysis on mice was also carried out, showing a good retention in tumour sites. The bioocompatibility and MRI performance of the nanoparticles were compared to those of a commercially available MRI contrast agent, showing equivalent biocompatibility and comparable results. With the added benefit of the CT functionality, these nanoparticles show great promise as multimodal agent
Formation of alloy nanoparticles by laser ablation of Au/Fe multilayer films in liquid environment
No full textLaser ablation in liquids (LAL) emerged as a powerful technique for the synthesis of multielement nanoparticles (NPs) such as metal alloys with thermodynamically forbidden composition. Consequently, there is a great interest in expanding the current knowledge about NPs formation during LAL, in order to improve the control on product structure and to extend the range of compositions accessible by this technique. Here we performed a systematic investigation on alloy NPs formation by nanosecond LAL of Au/Fe/glass multilayers with different thickness and order of deposition. The experiments were carried out in ethanol and water, which have, respectively, favourable and unfavourable effects on alloy formation. Results were analyzed with optical absorption spectroscopy, transmission electron microscopy and Mie theory for simple and core-shell spheres. Since alloy NPs were obtained in all cases, our findings provide the evidence that the two metals are mixed during particles formation. Besides, our results suggest that the probability of interaction between ablated matter and solution species is higher for the topmost layer of the target, i.e. the one closer to the solid/liquid interface. This provides useful insight for the synthesis of nanoalloys with new compositions, that are of interest in several fields, from catalysis to photonics and nanomedicine
Magnetic-plasmonic nanoalloys: a new class of multimodal theranostic tool
No full textThe integration of multiple functionalities in a single object with
nanometric size is crucial for the development of efficient tools for nanomedicine
applications. Here we show how a synthetic approach based on laser ablation of a
solid target in a liquid solution gives access to a library of multifunctional
nanomaterials. In particular, we discuss the example of a binary metal
alloy in which one of the two elements is a noble metal with plasmonic properties
(gold), and the other element is a transition metal with magnetic properties
(iron). The structure and surface conjugation of the magneto-plasmonic
nanoalloy was engineered in order to gain the functions of a multimodal contrast
agent for magnetic resonance imaging, x-ray computed thomography, photoacustic
imaging and surface enhanced Raman imaging. The nanoalloy also shows
promising performances as near infrared photothermal agent, thus it can be
considered as a multimodal theranostic tool
Synthesis of gold nanoparticles in liquid environment by laser ablation with geometrically confined configurations: Insights to improve size control and productivity
No full textLaser ablation of solid targets in liquid environment allows the
generation of nanoparticles (NPs) with several useful properties such as high
purity, easily functionalizable surface, metastable composition, or complex
structure, including doped nanocrystals, core−shells, hollow microspheres,
nanotruffles, or nanocrescents. However, the mechanisms of NPs formation is
still not well understood, and challenges remain in size control and productivity.
Here, we investigate how the asymmetry intrinsic of laser-matter interaction can
influence the structure and yield of gold NPs produced with nanosecond pulses.
In particular, we confined the geometry of the laser ablation configuration in
three ways: by reducing the thickness of the solid target from bulk size to a few
tens of nanometers, by reducing the size of the laser spot on the solid target, and finally, by reducing the lateral size of the bulk
target. The interpretation of results was supported with numerical simulations of heat distribution inside the metal target in the
three configurations. Surprisingly, we found that only the average size of NPs is affected by target thickness, whereas NPs
polydispersity is reduced by confining the ablation geometry in transversal direction to the light propagation axis, that is, by
decreasing transversal target size or laser spot size. In addition, we observed a strong dependence of yield versus target thickness,
suggesting that targets below ∼0.1 mm should be avoided for optimal ablation rate. Taken together, these findings indicate that
NPs formation mechanism changes with the depth of the ablated layer inside the bulk target and with the spatiotemporal
temperature gradient in the material. By adding another piece to the puzzle of laser ablation synthesis in liquid solution, this
study provides useful indications to improve the size distribution and productivity of laser-generated NPs
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Magnetic tuning of SERS hot spots in polymer-coated magnetic–plasmonic iron–silver nanoparticles
No full textPlasmonic nanostructures are intensively studied for their ability to create electromagnetic hot spots, where
a great variety of optical and spectroscopic processes can be amplified. Understanding how to control the
formation of hot spots in a dynamic and reversible way is crucial to further expand the panorama of plasmon
enhanced phenomena. In this work, we investigate the ability to modulate the hot spots in magnetic–
plasmonic iron-doped silver nanoparticles dispersed in aqueous solution, by applying an external
magnetic field. Evidence of magnetic field induction of hot spots was achieved by measuring the
amplification of surface enhanced Raman scattering (SERS) from analytes dispersed in the solution
containing Ag–Fe NPs. A polymeric shell was introduced around Ag–Fe NPs to confer colloidal stability,
and it was found that the length and density of the polymer chains have a significant influence on SERS
performance, and therefore on the formation of electromagnetic hot spots, under the action of the
external magnetic field. These findings are expected to provide an important contribution to
understanding the growing field of tuneable electromagnetic enhancement by external stimuli, such as
magnetic fields applied to magnetic–plasmonic nanoparticles
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