Magnetically Assembled SERS Substrates Composed of Iron-Silver Nanoparticles Obtained by Laser Ablation in Liquid

Structural evolution under physical and chemical stimuli of metastable Au-Fe nanoalloys obtained by laser ablation in liquid

Metastable alloy nanoparticles are investigated for their variety of appealing properties exploitable for photonics, magnetism, catalysis and nanobiotechnology. Notably, nanophases out of thermodynamic equilibrium feature a complex "ultrastructure" leading to a dynamic evolution of composition and atomic arrangement in response to physical-chemical stimuli. In this manuscript, metastable Au-Fe alloy nanoparticles were produced by laser ablation in liquid, an emerging versatile synthetic approach for freezing multielement nanosystems in non-equilibrium conditions. The Au-Fe nanoalloys were characterized through electron microscopy, elemental analysis, X-ray diffraction and Mössbauer spectroscopy. The dynamics of the structure of the Au-Fe system was tracked at high temperature under vacuum and atmospheric conditions, evidencing the intrinsic transformative nature of the metastable nanoalloy produced by laser ablation in liquid. This dynamic structure is relevant to possible application in several fields, from photocatalysis to nanomedicine, as demonstrated through an experiment of magnetic resonance imaging in biological fluids.

Multigram-Scale Production of Hybrid Au-Si Nanomaterial by Laser Ablation in Liquid (LAL) for Temperature-Feedback Optical Nanosensing, Light-to-Heat Conversion, and Anticounterfeit Labeling

Recent progress in hybrid optical nanomaterials composed of dissimilar constituents permitted an improvement in the performance and functionality of novel devices developed for optoelectronics, catalysis, medical diagnostics, and sensing. However, the rational combination of contrasting materials such as noble metals and semiconductors within individual hybrid nanostructures via a ready-to-use and lithography-free fabrication approach is still a challenge. Here, we report on a two-step synthesis of hybrid Au-Si microspheres generated by laser ablation of silicon in isopropanol followed by laser irradiation of the produced Si nanoparticles in the presence of HAuCl₄. Thermal reduction of [AuCl₄]^- species to a metallic gold phase, along with its subsequent mixing with silicon under laser irradiation, creates a nanostructured material with a unique composition and morphology, as revealed by electron microscopy, tomography, and elemental analysis. A combination of basic plasmonic and nanophotonic materials such as gold and silicon within a single microsphere allows for efficient light-to-heat conversion, as well as single-particle SERS sensing with temperature-feedback modality and expanded functionality. Moreover, the characteristic Raman signal and hot-electron-induced nonlinear photoluminescence coexisting within the novel Au-Si hybrids, as well as the commonly criticized randomness of the nanomaterials prepared by laser ablation in liquid, were proved to be useful for the realization of anticounterfeiting labels based on a physically unclonable function approach.

Recent Developments in Plasmonic Alloy Nanoparticles: Synthesis, Modelling, Properties and Applications

Despite the traditional plasmonic materials are counted on one hand, there are a lot of possible combinations leading to alloys with other elements of the periodic table, in particular those renowned for magnetic or catalytic properties. It is not a surprise, therefore, that nanoalloys are considered for their ability to open new perspectives in the panorama of plasmonics, representing a leading research sector nowadays. This is demonstrated by a long list of studies describing multiple applications of nanoalloys in photonics, photocatalysis, sensing and magneto-optics, where plasmons are combined with other physical and chemical phenomena. In some remarkable cases, the amplification of the conventional properties and even new effects emerged. However, this field is still in its infancy and several challenges must be overcome, starting with the synthesis (control of composition, crystalline order, size, processability, achievement of metastable phases and disordered compounds) as well as the modelling of the structure and properties (accuracy of results, reliability of structural predictions, description of disordered phases, evolution over time) of nanoalloys. To foster the research on plasmonic nanoalloys, here we provide an overview of the most recent results and developments in the field, organized according to synthetic strategies, modelling approaches, dominant properties and reported applications. Considering the several plasmonic nanoalloys under development as well as the large number of those still awaiting synthesis, modelling, properties assessment and technological exploitation, we expect a great impact on the forthcoming solutions for sustainability, ultrasensitive and accurate detection, information processing and many other fields.

Kinetically Stable Nonequilibrium Gold-Cobalt Alloy Nanoparticles with Magnetic and Plasmonic Properties Obtained by Laser Ablation in Liquid

Nonequilibrium nanoalloys are metastable solids obtained at the nanoscale under nonequilibrium conditions that allow the study of kinetically frozen atoms and the discovery of new physical and chemical properties. However, the stabilization of metastable phases in the nanometric size regime is challenging and the synthetic route should be easy and sustainable, for the nonequilibrium nanoalloys to be practically available. Here we report on the one-step laser ablation synthesis in solution (LASiS) of nonequilibrium Au-Co alloy nanoparticles (NPs) and their characterization on ensembles and at the single nanoparticle level. The NPs are obtained as a polycrystalline solid solution stable in air and water, although surface cobalt atoms undergo oxidation to Co(II). Since gold is a renowned plasmonic material and metallic cobalt is ferromagnetic at room temperature, these properties are both found in the NPs. Besides, surface conjugation with thiolated molecules is possible and it was exploited to obtain colloidally stable solutions in water. Taking advantage of these features, an array of magnetic-plasmonic dots was obtained and used for surface-enhanced Raman scattering experiments. Overall, this study confirms that LASiS is an effective method for the formation of kinetically stable nonequilibrium nanoalloys and shows that Au-Co alloy NPs are appealing magnetically responsive plasmonic building blocks for several nanotechnological applications.

Laser-Assisted Synthesis of Non-Equilibrium Nanoalloys

Vincenzo Amendola is Professor of Physical Chemistry at Padova University, where he established and directs the Laser-Assisted Synthesis and Plasmonics (LASP) lab. He obtained a PhD in Materials Science and Engineering in 2008 and the Italian qualification as Full Professor in 2017, after research experience at Massachusetts Institute of Technology and Cambridge University. He is part of the Program Committee of the ANGEL conference series and he is a current member of the ChemPhysChem Editorial Advisory Board.

Electronic Structure-Dependent Surface Plasmon Resonance in Single Au-Fe Nanoalloys

The relationship between composition and plasmonic properties in noble metal nanoalloys is still largely unexplored. Yet, nanoalloys of noble metals, such as gold, with transition elements, such as iron, have unique properties and a number of potential applications, ranging from nanomedicine to magneto-plasmonics and plasmon-enhanced catalysis. Here, we investigate the localized surface plasmon resonance at the level of the single Au-Fe nanoparticle by applying a strategy that combines experimental measurements using near field electron energy loss spectroscopy with theoretical studies via a full wave numerical analysis and density functional theory calculations of electronic structure. We show that, as the iron fraction increases, the plasmon resonance is blue-shifted and significantly damped, as a consequence of the changes in the electronic band structure of the alloy. This allows the identification of three relevant phenomena to be considered in the design and realization of any plasmonic nanoalloy, specifically: the appearance of new states around the Fermi level; the change in the free electron density of the metal; and the blue shift of interband transitions. Overall, this study provides new opportunities for the control of the optical response in Au-Fe and other plasmonic nanoalloys, which are useful for the realization of magneto-plasmonic devices for molecular sensing, thermo-plasmonics, bioimaging, photocatalysis, and the amplification of spectroscopic signals by local field enhancement.

Magnetic tuning of SERS hot spots in polymer-coated magnetic-plasmonic iron-silver nanoparticles

Plasmonic 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.

Correlation of surface-enhanced Raman scattering (SERS) with the surface density of gold nanoparticles: evaluation of the critical number of SERS tags for a detectable signal

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/µm² 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/µm² when the acquisition time is extended to 10 s, but must be increased to 3 tags/µm² 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.

SERS based detection of multiple analytes from dye/explosive mixtures using picosecond laser fabricated gold nanoparticles and nanostructures

Surface enhanced Raman spectroscopy (SERS) is a cutting edge analytical tool for trace analyte detection due to its highly sensitive, non-destructive and fingerprinting capability. Herein, we report the detection of multiple analytes from various mixtures using gold nanoparticles (NPs) and nanostructures (NSs) as SERS platforms. NPs and NSs were achieved through the simple approach of laser ablation in liquids (LAL) and their morphological studies were conducted with a UV-Visible absorption spectrometer, a high resolution transmission electron microscope (HRTEM) and a field emission scanning electron microscope (FESEM). The fabricated NPs/NSs allowed the sensitive and selective detection of different mixed compounds containing (i) rhodamine 6G (Rh6G) and methylene blue (MB), (ii) crystal violet (CV) and malachite green (MG), (iii) picric acid (explosive) and MB (dye), (iv) picric acid and 3-nitro-1,2,4- triazol-5-one (explosive, NTO) and (v) picric acid and 2,4-dinitrotoluene (explosive, DNT) using a portable Raman spectrometer. Thus, the obtained results demonstrate the capability of fabricated SERS substrates in identifying explosives and dyes from various mixtures. This could pave a new way for simultaneous detection of multiple analytes in real field applications.

A system for the synthesis of nanoparticles by laser ablation in liquid that is remotely controlled with PC or smartphone

Nanoparticles find applications in multiple technological and scientific fields, and laser ablation in liquid (LAL) emerged as a versatile method for providing colloidal solutions of nanomaterials with various composition, by a low cost, simple, self-standing, and "green" procedure. However, the use of high energy and high power laser beams is harmful, especially when coupled with flammable or toxic liquids, and in situ operation is required for starting, monitoring the LAL synthesis, and stopping it at the desired point. Here we describe the hardware and software design and the test results of a system for the production of nanoparticles by laser ablation synthesis in liquid solution (LASiS), which is remotely controllable with a personal computer or a smartphone. In this system, laser energy and solution flux are selectable, and the synthesis status can be monitored and managed at any time off site. Only commercially available components and software are employed, making the whole apparatus easily reproducible in any LAL laboratory. The system has proven its reliability in various conditions, including intercontinental remote control experiments. Overall, this apparatus represents a step forward to improve the safety and to more efficiently exploit the time of people working with LASiS, thus contributing to the increasing demand for off-site real time monitoring of experimental equipment in many scientific and industrial laboratories, due to safety and efficiency requirements.

Explosives sensing using Ag–Cu alloy nanoparticles synthesized by femtosecond laser ablation and irradiation

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation. Initially, pure Ag and Cu NPs were produced individually using the laser ablation in liquid technique (with ∼50 femtosecond pulses at 800 nm) which was followed by laser irradiation of the mixed Ag and Cu NPs in equal volume. These Ag, Cu, and Ag–Cu NPs were characterised by UV-visible absorption, HRTEM and XRD techniques. The alloy formation was confirmed by the presence of a single surface plasmon resonance peak in absorption spectra and elemental mapping using FESEM techniques. Furthermore, the results from surface enhanced Raman scattering (SERS) studies performed for the methylene blue (MB) molecule suggested that Ag–Cu alloy NPs demonstrate a higher enhancement factor (EF) compared to pure Ag/Cu NPs. Additionally, SERS studies of Ag–Cu alloy NPs were implemented for the detection of explosive molecules such as picric acid (PA – 5 μM), ammonium nitrate (AN – 5 μM) and the dye molecule methylene blue (MB – 5 nM). These alloy NPs exhibited superiority in the detection of various analyte molecules with good reproducibility and high sensitivity with EFs in the range of 10⁴ to 10⁷.

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation.

Galvanic Displacement Synthesis of Monodisperse Janus- and Satellite-Like Plasmonic-Magnetic Ag-Fe@Fe3O4 Heterostructures with Reduced Cytotoxicity

The unique physicochemical properties of silver nanoparticles offer a large potential for biomedical application, however, the serious biotoxicity restricts their usage. Herein, nanogalvanic couple Ag-Fe@Fe3O4 heterostructures (AFHs) are designed to prevent Ag+ release from the cathodic Ag by sacrificial anodic Fe, which can reduce the cytotoxicity of Ag. AFHs are synthesized with modified galvanic displacement strategy in nonaqueous solution. To eliminate the restriction of lattice mismatch between Fe and Ag, amorphous Fe@Fe3O4 nanoparticles (NPs) are selected as seeds, meanwhile, reductive Fe can reduce Ag precursor directly even at as low as 20 °C without additional reductant. The thickness of the Fe3O4 shell can influence the amorphous properties of AFHs, and a series of Janus- and satellite-like AFHs are synthesized. A "cut-off thickness" effect is proposed based on the abnormal phenomenon that with the increase of reaction temperature, the diameter of Ag in AFHs decreases. Because of the interphase interaction and the coupling effect of Ag and Fe@Fe3O4, the AFHs exhibit unique optical and magnetic properties. This strategy for synthesis of monodisperse heterostructures can be extended for other metals, such as Au and Cu.

Laser Synthesis and Processing of Colloids: Fundamentals and Applications

Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPC-synthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials, thereby enabling the fabrication of bioconjugates and heterogeneous catalysts. Accurate size control of LSPC-synthesized materials ranging from quantum dots to submicrometer spheres and recent upscaling advancement toward the multiple-gram scale are helpful for extending the applicability of LSPC-synthesized nanomaterials to various fields. By discussing key reports on both the fundamentals and the applications related to laser ablation, fragmentation, and melting in liquids, this Article presents a timely and critical review of this emerging topic.