Laser ablation synthesis in liquid solution (LASiS) is a "green" technique that gives access to the preparation of a library of nanomaterials. Bare noble metal spherical particles, multiphase ...core-shell oxides, metal-semiconductor heterostructures, layered organometallic compounds and other complex nanostructures can be obtained with the same experimental set up, just by varying a few synthetic parameters. How to govern such versatility is one of the current challenges of LASiS and requires a thorough understanding of the physical and chemical processes involved in the synthesis. In this perspective, the fundamental mechanisms of laser ablation in liquids are summarized, organized according to their temporal sequence and correlated with relevant examples taken from the library of nanomaterials disclosed by LASiS, in order to show how synthesis parameters influence the composition and the structure of products. The resulting framework suggests that, to date, much attention has been devoted to the physical aspects of laser-matter interaction and to the characterization of the final products of the synthesis. Conversely, the clarification of chemical processes active during LASiS deserves more research efforts and requires the synergy among multiple investigation techniques.
In the past years, laser ablation synthesis in solution (LASiS) emerged as a reliable alternative to traditional chemical reduction methods for obtaining noble metal nanoparticles (NMNp). LASiS is a ..."green" technique for the synthesis of stable NMNp in water or in organic solvents, which does not need stabilizing molecules or other chemicals. The so obtained NMNp are highly available for further functionalization or can be used wherever unprotected metal nanoparticles are desired. Surface functionalization of NMNp can be monitored in real time by UV-visible spectroscopy of the plasmon resonance. However LASiS has some limitations in the size control of NMNp, which can be overcome by "chemical free" laser treatments of NMNp. In this paper we provide an overview of LASiS, size manipulation by laser irradiation and functionalization of NMNp, with special care in pointing out some of the main issues about this research area.
We present a method for the evaluation of the average size of gold nanoparticles based on the fitting of their UV−vis spectra by the Mie model for spheres. The method gives good results using a ...calibration of the dumping frequency of the surface plasmon resonance and accounting for the presence of nonspherical AuNP in solution by the Gans model for spheroids. It has been successfully applied to free and functionalized gold nanoparticles in various solvents with diameters in the 4−25 nm range. Despite the differences among samples, we found an accuracy of about 6% on the nanoparticles average size with respect to sizes measured by transmission electron microscopy (TEM). Moreover, the fitting model provides other information not available from TEM like the concentration of AuNP in the sample and the fraction of nonspherical nanoparticles, which is particularly useful for measuring aggregation processes. The fitting procedure and models are thoroughly discussed in the text, and the fitting programs are freely accessible on the web.
Although oxide nanoparticles are ubiquitous in science and technology, a multitude of compositions, phases, structures, and doping levels exist, each one requiring a variety of conditions for their ...synthesis and modification. Besides, experimental procedures are frequently dominated by high temperatures or pressures and by chemical contaminants or waste. In recent years, laser synthesis of colloids emerged as a versatile approach to access a library of clean oxide nanoparticles relying on only four main strategies running at room temperature and ambient pressure: laser ablation in liquid, laser fragmentation in liquid, laser melting in liquid and laser defect‐engineering in liquid. Here, established laser‐based methodologies are reviewed through the presentation of a panorama of oxide nanoparticles which include pure oxidic phases, as well as unconventional structures like defective or doped oxides, non‐equilibrium compounds, metal‐oxide core–shells and other anisotropic morphologies. So far, these materials showed several useful properties that are discussed with special emphasis on catalytic, biomedical and optical application. Yet, given the endless number of mixed compounds accessible by the laser‐assisted methodologies, there is still a lot of room to expand the library of nano‐crystals and to refine the control over products as well as to improve the understanding of the whole process of nanoparticle formation. To that end, this review aims to identify the perspectives and unique opportunities of laser‐based synthesis and processing of colloids for future studies of oxide nanomaterial‐oriented sciences.
Laser‐assisted synthesis of colloids emerged as a versatile approach to access a library of oxide nanoparticles and related nanostructures at room temperature and ambient pressure. This review provides a critical presentation of laser‐based methodologies, of the various oxide nanoparticles achieved so far, and of their applications in catalysis, optics, biotechnology and further emerging fields.
The use of plasmonic nanotags based on the surface-enhanced Raman scattering (SERS) effect is highly promising for several applications in analytical chemistry, biotechnological assays and ...nanomedicine. To this end, a crucial parameter is the minimum number of SERS tags that allows for the collection of intense Raman signals under real operating conditions. Here, SERS Au nanotags (AuNTs) based on clustered gold nanoparticles are deposited on a substrate and analyzed in the same region using Raman spectroscopy and transmission electron microscopy. In this way, the Raman spectra and the surface density of the SERS tags are correlated directly, showing that 1 tag/µm
is enough to generate an intense signal above the noise level at 633 nm with an excitation power of only 0.65 mW and an acquisition time of just 1 s with a 50× objective. The AuNT density can be even lower than 1 tag/µm
when the acquisition time is extended to 10 s, but must be increased to 3 tags/µm
when a 20× objective is employed under the same excitation conditions. In addition, in order to observe a linear response, it was found that 10 SERS AuNTs inside the probed area are required. These findings indicate that a better signal-to-noise ratio requires high-magnification optics, while linearity versus tag number can be improved by using low-magnification optics or a high tag density. In general the suitability of plasmonic SERS labels for ultrasensitive analytical and biomedical applications is evident.
Iron-based nanoparticles can have useful magnetic and catalytic properties. We investigated the synthesis of iron-based nanostructures by laser ablation of bulk iron with 1064 nm nanosecond pulses in ...the following organic solvents: tetrahydrofuran, acetonitrile, dimethylformamide, dimethylsulfoxide, toluene, and ethanol. Structural analysis carried out by transmission electron microscopy and X-ray diffraction revealed that the solvent has a dramatic influence on both the composition and the nanostructure of nanoparticles. Various magnetic nanoparticles like iron carbide (Fe3C), magnetic iron oxide (magnetite/maghemite), metal iron (α-Fe), and iron@iron oxide are obtained by varying the solvent and keeping unchanged all the other experimental conditions. These results are the consequences of the different reactivity of solvent molecules exposed to the plasma plume generated during the ablation process.
The surface plasmon resonance (SPR) of silver nanoparticles (AgNPs) was studied with the discrete dipole approximation considering different shapes, sizes, dielectric environments, and supraparticles ...assemblies. In particular, we focused our simulations on AgNPs with sizes below 10 nm, where the correction of silver dielectric constant for intrinsic size effects is necessary. We found that AgNPs shape and assembly can induce distinctive features in the extinction spectra and that SPR is more intense when AgNPs have discoid or flat shapes and are embedded in a dielectric shell with high refractive index. However, the SPR loses much of its distinctive features when size effects and stabilizing molecules induce significant broadening of the extinction bands that is often observed in the case of thiolated AgNPs smaller than about 5 nm. These results are useful indications for in situ characterization and monitoring of AgNPs synthesis and for the engineering of AgNPs with new plasmonic properties.
Due to the surface‐enhanced Raman scattering (SERS) effect, SERS labels based on noble‐metal nanoparticles loaded with Raman‐active molecules are good candidates for ultrasensitive multiplexed assays ...and in vitro/in vivo imaging. However, understanding how to maximize the brightness of such labels is of paramount importance for their widespread application. The effective differential Raman scattering cross‐section (dσR/dΩ) of SERS labels made of pegylated gold nanoparticles loaded with various Raman active molecules (Raman reporters) is studied. It is found that proper choice of the Raman reporter and of nanoparticle size can enhance the dσR/dΩ by several orders of magnitude. The experimental results are understood by considering the molecular cross‐section for resonant Raman scattering and the local electromagnetic enhancement factor (GSERS) in the nearby of gold nanoparticles. These results are useful to guide the design of SERS labels with improved performances and to provide a reference for the comparison of the absolute value of the dσR/dΩ of SERS labels based on metal nanoparticles.
The brightness of surface enhanced Raman scattering (SERS) labels can be increased by several orders of magnitude by selecting the proper Raman active molecule and by controlling the size and the aggregation of the noble metal nanoparticles.
Gold nanoparticles (AuNPs) assisted laser desorption ionization mass spectrometry (LDI-MS) emerged as an effective technique for the detection of analytes with high sensitivity. The surface chemistry ...and the size of AuNPs are the crucial parameters for lowering the detection limits and increasing the selectivity of LDI-MS. Here we show that chemical-free size selected AuNPs, obtained by laser ablation synthesis in solution (LASiS), have very low background in the low mass region (<500 Da), contrary to citrate stabilized AuNPs (citrate-AuNPs) and dihydroxyacetophenone (DHAP). This allowed better performances for the picomole detection of low mass analytes like arginine, fructose, atrazine, anthracene and paclitaxel. The results suggest that chemical-free LASiS-AuNPs can be an excellent matrix for nanoparticle-assisted LDI-MS.
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