Strong dependence of surface plasmon resonance and surface enhanced Raman scattering on the composition of Au-Fe nanoalloys

Magnetic-Plasmonic Nanocomposites as Versatile Substrates for Surface-enhanced Raman Scattering (SERS) Spectroscopy

Surface-enhanced Raman scattering (SERS) spectroscopy, a highly sensitive technique for detecting trace-level analytes, relies on plasmonic substrates. The choice of substrate, its morphology, and the excitation wavelength are crucial in SERS applications. To address advanced SERS requirements, the design and use of efficient nanocomposite substrates have become increasingly important. Notably, magnetic-plasmonic (MP) nanocomposites, which combine magnetic and plasmonic properties within a single particle system, stand out as promising nanoarchitectures with versatile applications in nanomedicine and SERS spectroscopy. In this review, we present an overview of MP nanocomposite fabrication methods, explore surface functionalization strategies, and evaluate their use in SERS. Our focus is on how different nanocomposite designs, magnetic and plasmonic properties, and surface modifications can significantly influence their SERS-related characteristics, thereby affecting their performance in specific applications such as separation, environmental monitoring, and biological applications. Reviewing recent studies highlights the multifaceted nature of these materials, which have great potential to transform SERS applications across a range of fields, from medical diagnostics to environmental monitoring. Finally, we discuss the prospects of MP nanocomposites, anticipating favorable developments that will make substantial contributions to various scientific and technological areas.

Intrinsic Visible Plasmonic Properties of Colloidal PtIn2 Intermetallic Nanoparticles

Materials that intrinsically exhibit localized surface plasmon resonance (LSPR) in the visible region have been predominantly researched on nanoparticles (NPs) composed of coinage metals, namely Au, Ag, and Cu. Here, as a coinage metal-free intermetallic NPs, colloidal PtIn2 NPs with a C1 (CaF2 -type) crystal structure are synthesized by the liquid phase method, which evidently exhibit LSPR at wavelengths similar to face-centered cubic (fcc)-Au NPs. Computational simulations pointed out differences in the electronic structure and photo-excited electron dynamics between C1-PtIn2 and fcc-Au NPs; reduces interband transition and stronger screening with smaller number of bound d-electrons compare with fcc-Au are unique origins of the visible plasmonic nature of C1-PtIn2 NPs. These results strongly indicate that the intermetallic NPs are expected to address the development of alternative plasmonic materials by tuning their crystal structure and composition.

Accurate prediction of the optical properties of nanoalloys with both plasmonic and magnetic elements

The alloying process plays a pivotal role in the development of advanced multifunctional plasmonic materials within the realm of modern nanotechnology. However, accurate in silico predictions are only available for metal clusters of just a few nanometers, while the support of modelling is required to navigate the broad landscape of components, structures and stoichiometry of plasmonic nanoalloys regardless of their size. Here we report on the accurate calculation and conceptual understanding of the optical properties of metastable alloys of both plasmonic (Au) and magnetic (Co) elements obtained through a tailored laser synthesis procedure. The model is based on the density functional theory calculation of the dielectric function with the Hubbard-corrected local density approximation, the correction for intrinsic size effects and use of classical electrodynamics. This approach is built to manage critical aspects in modelling of real samples, as spin polarization effects due to magnetic elements, short-range order variability, and size heterogeneity. The method provides accurate results also for other magnetic-plasmonic (Au-Fe) and typical plasmonic (Au-Ag) nanoalloys, thus being available for the investigation of several other nanomaterials waiting for assessment and exploitation in fundamental sectors such as quantum optics, magneto-optics, magneto-plasmonics, metamaterials, chiral catalysis and plasmon-enhanced catalysis.

Exploring the formation of gold/silver nanoalloys with gas-phase synthesis and machine-learning assisted simulations

While nanoalloys are of paramount scientific and practical interest, the main processes leading to their formation are still poorly understood. Key structural features in the alloy systems, including the crystal phase, chemical ordering, and morphology, are challenging to control at the nanoscale, making it difficult to extend their use to industrial applications. In this contribution, we focus on the gold/silver system that has two of the most prevalent noble metals and combine experiments with simulations to uncover the formation mechanisms at the atomic level. Nanoparticles were produced using a state-of-the-art inert-gas aggregation source and analyzed using transmission electron microscopy and energy-dispersive X-ray spectroscopy. Machine-learning-assisted molecular dynamics simulations were employed to model the crystallization process from liquid droplets to nanocrystals. Our study finds a preponderance of nanoparticles with five-fold symmetric morphology, including icosahedra and decahedra which is consistent with previous results on mono-metallic nanoparticles. However, we observed that gold atoms, rather than silver atoms, segregate at the surface of the obtained nanoparticles for all the considered alloy compositions. These segregation tendencies are in contrast to previous studies and have consequences on the crystallization dynamics and the subsequent crystal ordering. We finally showed that the underpinning of this surprising segregation dynamics is due to charge transfer and electrostatic interactions rather than surface energy considerations.

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.

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.

Single-Step Fabrication of Au-Fe-BaTiO3 Nanocomposite Thin Films Embedded with Non-Equilibrium Au-Fe Alloyed Nanostructures

Nanocomposite thin film materials present great opportunities in coupling materials and functionalities in unique nanostructures including nanoparticles-in-matrix, vertically aligned nanocomposites (VANs), and nanolayers. Interestingly the nanocomposites processed through a non-equilibrium processing method, e.g., pulsed laser deposition (PLD), often possess unique metastable phases and microstructures that could not achieve using equilibrium techniques, and thus lead to novel physical properties. In this work, a unique three-phase system composed of BaTiO₃ (BTO), with two immiscible metals, Au and Fe, is demonstrated. By adjusting the deposition laser frequency from 2 Hz to 10 Hz, the phase and morphology of Au and Fe nanoparticles in BTO matrix vary from separated Au and Fe nanoparticles to well-mixed Au-Fe alloy pillars. This is attributed to the non-equilibrium process of PLD and the limited diffusion under high laser frequency (e.g., 10 Hz). The magnetic and optical properties are effectively tuned based on the morphology variation. This work demonstrates the stabilization of non-equilibrium alloy structures in the VAN form and allows for the exploration of new non-equilibrium materials systems and their properties that could not be easily achieved through traditional equilibrium methods.

Thermally Stable Magneto-Plasmonic Nanoparticles for SERS with Tunable Plasmon Resonance

Bifunctional magneto-plasmonic nanoparticles that exhibit synergistically magnetic and plasmonic properties are advanced substrates for surface-enhanced Raman spectroscopy (SERS) because of their excellent controllability and improved detection potentiality. In this study, composite magneto-plasmonic nanoparticles (Fe3O4@AgNPs) were formed by mixing colloid solutions of 50 nm-sized magnetite nanoparticles with 13 nm-sized silver nanoparticles. After drying of the layer of composite Fe3O4@AgNPs under a strong magnetic field, they outperformed the conventional silver nanoparticles during SERS measurements in terms of signal intensity, spot-to-spot, and sample-to-sample reproducibility. The SERS enhancement factor of Fe3O4@AgNP-adsorbed 4-mercaptobenzoic acid (4-MBA) was estimated to be 3.1 × 107 for a 633 nm excitation. In addition, we show that simply by changing the initial volumes of the colloid solutions, it is possible to control the average density of the silver nanoparticles, which are attached to a single magnetite nanoparticle. UV-Vis and SERS data revealed a possibility to tune the plasmonic resonance frequency of Fe3O4@AgNPs. In this research, the plasmon resonance maximum varied from 470 to 800 nm, suggesting the possibility to choose the most suitable nanoparticle composition for the particular SERS experiment design. We emphasize the increased thermal stability of composite nanoparticles under 532 and 442 nm laser light irradiation compared to that of bare Fe3O4 nanoparticles. The Fe3O4@AgNPs were further characterized by XRD, TEM, and magnetization measurements.

Solvent-particles interactions during composite particles formation by pulsed laser melting of α-Fe2O3

This work thoroughly investigates chemical solvent-particles interactions during the formation of composite particles by pulsed laser melting of α-Fe₂O₃. Two solvents, with different dielectric constants, such as ethyl acetate (ε_r = 6) and ethanol (ε_r = 24.6), were examined in terms of their effect on the morphology, size, and phase composition of iron oxide composites. We calculated the laser fluence curves using the heating-melting-evaporation approach to identify the critical particle size that undergoes the phase changes first. We assessed the temperature of the particles irradiated with 390 mJ/pulse^.cm² in both solvents, including the heat dissipation between the particles and the liquid. The phase diagram of the Fe-O-C-H system was calculated to determine the temperature-pressure relationship of the system in equilibrium. We also employed an in situ GC-MS analysis to identify the volatile products during irradiation. Based on our experimental results, we concluded that the final diameter of the composites increases from 400 to 600 nm, along with the decreasing dielectric constant of the solvent, which is related to the different polarization of the organic liquid and the degree of particle agglomeration. The reduction of hematite in ethanol proceeded much faster, ending up with Fe/FeC_x, while in ethyl acetate, it ended up with Fe₃O₄. Among all the particles, those with a diameter of 200 nm have the highest temperature and undergo the phase transition first. The temperature of a 200 nm composite particle in ethanol is slightly lower than in ethyl acetate, i.e. 1870 K as compared to 1902 K. Phase equilibrium diagrams proved the existence of Fe, FeO, and Fe₃O₄ as the preferred phases at about 1900 K. Our research provides a new insight into the process of submicron particle formation during pulsed laser irradiation and allows proposing a mechanism for the growth of particles of different size and phase composition depending on the solvent.

Laser-Ablative Synthesis of Ultrapure Magneto-Plasmonic Core-Satellite Nanocomposites for Biomedical Applications

The combination of magnetic and plasmonic properties at the nanoscale promises the development of novel synergetic image-guided therapy strategies for the treatment of cancer and other diseases, but the fabrication of non-contaminated magneto-plasmonic nanocomposites suitable for biological applications is difficult within traditional chemical methods. Here, we describe a methodology based on laser ablation from Fe target in the presence of preliminarily ablated water-dispersed Au nanoparticles (NPs) to synthesize ultrapure bare (ligand-free) core-satellite nanostructures, consisting of large (several tens of nm) Fe-based core decorated by small (mean size 7.5 nm) Au NPs. The presence of the Fe-based core conditions a relatively strong magnetic response of the nanostructures (magnetization of >12.6 emu/g), while the Au NPs-based satellite shell provides a broad extinction peak centered at 550 nm with a long tale in the near-infrared to overlap with the region of relative tissue transparency (650-950 nm). We also discuss possible mechanisms responsible for the formation of the magnetic-plasmonic nanocomposites. We finally demonstrate a protocol to enhance colloidal stability of the core-satellites in biological environment by their coating with different polymers. Exempt of toxic impurities and combining strong magnetic and plasmonic responses, the formed core-satellite nanocomposites can be used in biomedical applications, including photo- and magneto-induced therapies, magnetic resonance imaging or photoacoustic imaging.

Gold-iron oxide nanohybrids: insights into colloidal stability and surface-enhanced Raman detection

Nanoparticles are acquiring an ever increasing role in analytical technologies for enhanced applications such as signalling of hazardous dyes. One challenge for the synthesis of hybrid nanomaterials is to control their shape, size and properties. The colloidal and interfacial properties of initial nanoparticles are decisive for the formation, growth and characteristics of nanohybrids. Our objective is to combine the advantages of iron oxide nanoparticles for magnetic separation with nanoscale gold for a surface enhanced Raman scattering (SERS) effect which could be used e.g. for improved detection of dye molecules. We synthesized iron oxide nanoparticles (∼10 nm) with a high saturation magnetization of around 80 Am2 kg-1 and coupled nanoscale gold to these particles. The focus was set in testing multiple approaches to combine these two materials with the goal of understanding and discussing the effect of the colloidal stability of iron oxide nanoparticles on the properties of the hybrid material. Stability is a seldom addressed issue; however, it plays a critical role for guaranteeing a homogeneous distribution of the gold on the iron oxide surface. We characterized the produced materials with UV/Vis spectroscopy, dynamic light scattering, and transmission electron microscopy, and their capability to enhance Raman signals is investigated. The seed-mediated growth method of oleate and PEG-stabilized magnetic particles yielded the best enhancement of Raman scattering for identification of the dye Rhodamin 6G. This approach can be used to couple gold nanoparticles to other surfaces and microfluidic devices. The presented method might pave the way to further applications in diagnostics or also in environmental approaches and beyond.

Room temperature synthesized solid solution AuFe nanoparticles and their transformation into Au/Fe Janus nanocrystals

Solid solution AuFe nanoparticles were synthesized for the first time under ambient conditions by an adapted method previously established for the Fe₃O₄-Au core-shell morphology. These AuFe particles preserved the fcc structure of Au incorporated with paramagnetic Fe atoms. The metastable AuFe can be segregated by transformation into Janus Au/Fe particles with bcc Fe and fcc Au upon annealing. The ferromagnetic Fe was epitaxially grown on low index fcc Au planes. This preparation route delivers new perspective materials for magnetoplasmonics and biomedical applications and suggests the reconsideration of existing protocols for magnetite-gold core-shell synthesis.

Pulsed Laser in Liquids Made Nanomaterials for Catalysis

Catalysis is essential to modern life and has a huge economic impact. The development of new catalysts critically depends on synthetic methods that enable the preparation of tailored nanomaterials. Pulsed laser in liquids synthesis can produce uniform, multicomponent, nonequilibrium nanomaterials with independently and precisely controlled properties, such as size, composition, morphology, defect density, and atomistic structure within the nanoparticle and at its surface. We cover the fundamentals, unique advantages, challenges, and experimental solutions of this powerful technique and review the state-of-the-art of laser-made electrocatalysts for water oxidation, oxygen reduction, hydrogen evolution, nitrogen reduction, carbon dioxide reduction, and organic oxidations, followed by laser-made nanomaterials for light-driven catalytic processes and heterogeneous catalysis of thermochemical processes. We also highlight laser-synthesized nanomaterials for which proposed catalytic applications exist. This review provides a practical guide to how the catalysis community can capitalize on pulsed laser in liquids synthesis to advance catalyst development, by leveraging the synergies of two fields of intensive research.

Deposition pressure-induced microstructure control and plasmonic property tuning in hybrid ZnO-Ag x Au1-x thin films

Self-assembled oxide-metallic alloy nanopillars as hybrid plasmonic metamaterials (e.g., ZnO-Ag x Au1-x ) in a thin film form have been grown using a pulsed laser deposition method. The hybrid films were demonstrated to be highly tunable via systematic tuning of the oxygen background pressure during deposition. The pressure effects on morphology and optical properties have been investigated and found to be critical to the overall properties of the hybrid films. Specifically, low background pressure results in the vertically aligned nanocomposite (VAN) form while the high-pressure results in more lateral growth of the nanoalloys. Strong surface plasmon resonance was observed in the UV-vis region and a hyperbolic dielectric function was achieved due to the anisotropic morphology. The oxide-nanoalloy hybrid material grown in this work presents a highly effective approach for tuning the binary nanoalloy morphology and properties through systematic parametric changes, important for their potential applications in integrated photonics and plasmonics such as sensors, energy harvesting devices, and beyond.

First-Principles Insights into Plasmon-Induced Catalysis

The size- and shape-controlled enhanced optical response of metal nanoparticles (NPs) is referred to as a localized surface plasmon resonance (LSPR). LSPRs result in amplified surface and interparticle electric fields, which then enhance light absorption of the molecules or other materials coupled to the metallic NPs and/or generate hot carriers within the NPs themselves. When mediated by metallic NPs, photocatalysis can take advantage of this unique optical phenomenon. This review highlights the contributions of quantum mechanical modeling in understanding and guiding current attempts to incorporate plasmonic excitations to improve the kinetics of heterogeneously catalyzed reactions. A range of first-principles quantum mechanics techniques has offered insights, from ground-state density functional theory (DFT) to excited-state theories such as multireference correlated wavefunction methods. Here we discuss the advantages and limitations of these methods in the context of accurately capturing plasmonic effects, with accompanying examples.

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.

4D Multimodal Nanomedicines Made of Nonequilibrium Au-Fe Alloy Nanoparticles

Several examples of nanosized therapeutic and imaging agents have been proposed to date, yet for most of them there is a low chance of clinical translation due to long-term in vivo retention and toxicity risks. The realization of nanoagents that can be removed from the body after use remains thus a great challenge. Here, we demonstrate that nonequilibrium gold-iron alloys behave as shape-morphing nanocrystals with the properties of self-degradable multifunctional nanomedicines. DFT calculations combined with mixing enthalpy-weighted alloying simulations predict that Au-Fe solid solutions can exhibit self-degradation in an aqueous environment if the Fe content exceeds a threshold that depends upon element topology in the nanocrystals. Exploiting a laser-assisted synthesis route, we experimentally confirm that nonequilibrium Au-Fe nanoalloys have a 4D behavior, that is, the ability to change shape, size, and structure over time, becoming ultrasmall Au-rich nanocrystals. In vivo tests show the potential of these transformable Au-Fe nanoalloys as efficient multimodal contrast agents for magnetic resonance imaging and computed X-ray absorption tomography and further demonstrate their self-degradation over time, with a significant reduction of long-term accumulation in the body, when compared to benchmark gold or iron oxide contrast agents. Hence, Au-Fe alloy nanoparticles exhibiting 4D behavior can respond to the need for safe and degradable inorganic multifunctional nanomedicines required in clinical translation.

Composition and structure of magnetic high-temperature-phase, stable Fe-Au core-shell nanoparticles with zero-valent bcc Fe core

Advanced quantitative TEM/EDXS methods were used to characterize different ultrastructures of magnetic Fe-Au core-shell nanoparticles formed by laser ablation in liquids. The findings demonstrate the presence of Au-rich alloy shells with varying composition in all structures and elemental bcc Fe cores. The identified structures are metastable phases interpreted by analogy to the bulk phase diagram. Based on this, we propose a formation mechanism of these complex ultrastructures. To show the magnetic response of these magnetic core nanoparticles protected by a noble metal shell, we demonstrate the formation of nanostrands in the presence of an external magnetic field. We find that it is possible to control the lengths of these strands by the iron content within the alloy nanoparticles.

Preparation of multi-functional magnetic-plasmonic nanocomposite for adsorption and detection of thiram using SERS

Magnetic materials have been widely used for constructing substrate in surface enhanced Raman scattering (SERS) sensing due to the magnetic responsibility. Here, we reported a facile and effective approach to construct multi-functional SERS substrate based on assembling Ag nanoparticles (NPs) on porous Fe microspheres. The porous Fe microspheres were prepared through hydrogen reduction of Fe₂O₃ NPs with porous structure, in which the size and morphology of Fe could be well controlled. The surface of Fe was grafted with amino group, and then decorated with Ag NPs. The surface area and pore size of Fe microsphere were characterized by nitrogen adsorption and desorption. The Fe@Ag nanocomposite illustrated a good SERS activity. Furthermore, this substrate could be used for pesticide monitoring by portable Raman spectrometer. Especially, the porous Fe microsphere could adsorb analyte from target sample and the Fe@Ag could be concentrated by magnetic force to amplify the SERS signal for thiram detection.

Picosecond laser-induced surface structures on alloys in liquids and their influence on nanoparticle productivity during laser ablation

The productivity of nanoparticles formed by laser ablation of gold-silver and iron-gold alloy as well as copper and iron-nickel alloy targets in water is correlated with the formation of laser-induced surface structures. At a laser fluence optimized for maximum nanoparticle productivity, it is found that a binary alloy with an equimolar ratio forms laser-induced periodic surface structures (LIPSS) after ablation, if one of the constituent metals also form LIPSS. The ablation rate of nanoparticles linearly depends on the laser fluence if LIPSS is not formed, while a logarithmic trend and a decrease in productivity is evident when LIPSS is formed. To cancel LIPSS formation and recover from this decrease, a change to circularly polarized light is performed and an increase in nanoparticle productivity of more than 30% is observed.

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.

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.

A Review on Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Carbon Nanotubes Modified With Au for Electrochemical Detection of Prostate Specific Antigen: Effect of Au Nanoparticle Size Distribution

Different functionalized Multi-Wall Carbon Nanotube and gold nanoparticles (AuNPs) were synthesized as biosensor electrodes. These materials have been applied to the detection of the Prostate Specific Antigen (PSA). The synthesis of AuNPs was carried out using polyvinylpyrrolidone (PVP) as protecting agent. The PVP/Au molar ratio (0.5 and 50) controls the nanoparticle size distribution, obtaining a wide and narrow distribution with an average diameter of 9.5 and 6.6 nm, respectively. Nanoparticle size distribution shows an important effect in the electrochemical performance of the biosensor, increasing the electrochemical active surface area (EASA) and promoting the electron-transfer from the redox probe (Ferrocene/Ferrocenium) to the electrode. Furthermore, a narrow and small nanoparticle size distribution enhances the amount of antibodies immobilized on the transducer material and the performance during the detection of the PSA. Significant results were obtained for the quantification of PSA, with a limit of detection of 1 ng·ml⁻¹ and sensitivities of 0.085 and 0.056 μA·mL·ng⁻¹ for the two transducer materials in only 5 min of detection.

Microfluidic Schottky-junction photovoltaics with superior efficiency stimulated by plasmonic nanoparticles and streaming potential

A droplet energy harvester (DEH) composed of aqueous salt solution could generate electrical energy from light when placed on a metal-semiconductor Schottky-junction emulating the principles of electrochemical photovoltaics (ECPV). The maximum potential difference generated was ∼95 mV under sun, which was enhanced by ∼1.5 times after the addition of gold nanoparticles (AuNPs) in the droplet because of the generation of additional charge carriers from the localized surface plasmon resonance (LSPR). Focusing the solar illumination through a bi-convex lens on five such droplets increased the voltage to ∼320 mV with a power density of ∼0.25 mW cm^-2. When the DEH was converted to a microfluidic energy harvester (MEH) by flowing the AuNP laden salt solution through a microchannel integrated with an array of Schottky-junction electrodes, at an optimal flow rate, another two-fold increase in the power density was observed. In the MEH, because the ECPV aided by the LSPR converted the solar energy into electrical energy, the streaming potential (SP) generated across the electrodes because of the fluid flow converted the mechanical energy into electrical energy. Increase in the number of electrode pairs improved the voltage generation, which suggested that the MEH had potential for microscale-very-large-scale-integration (μ-VLSI). The combined effects of ECPV, LSPR, and SP in the MEH could show an efficiency ∼2.5%, which was one of the highest ones reported, for Schottky-junction energy harvesters. This study shows some simple and efficient pathways to harvest high-density electrical power using microchannels and droplets from the naturally abundant solar or hydroelectric (hydel) energy resources.

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.

Determining the Composite Structure of Au-Fe-Based Submicrometre Spherical Particles Fabricated by Pulsed-Laser Melting in Liquid

Submicrometre spherical particles made of Au and Fe can be fabricated by pulsed-laser melting in liquid (PLML) using a mixture of Au and iron oxide nanoparticles as the raw particles dispersed in ethanol, although the detailed formation mechanism has not yet been clarified. Using a 355 nm pulsed laser to avoid extreme temperature difference between two different raw particles during laser irradiation and an Fe₂O₃ raw nanoparticle colloidal solution as an iron source to promote the aggregation of Au and Fe₂O₃ nanoparticles, we performed intensive characterization of the products and clarified the formation mechanism of Au-Fe composite submicrometre spherical particles. Because of the above two measures (Fe₂O₃ raw nanoparticle and 355 nm pulsed laser), the products-whether the particles are phase-separated or homogeneous alloys-basically follow the phase diagram. In Fe-rich range, the phase-separated Au-core/Fe-shell particles were formed, because quenching induces an earlier solidification of the Fe-rich component as a result of cooling from the surrounding ethanol. If the particle size is small, the quenching rate becomes very rapid and particles were less phase-separated. For high Au contents exceeding 70% in weight, crystalline Au-rich alloys were formed without phase separation. Thus, this aggregation control is required to selectively form homogeneous or phase-separated larger submicrometre-sized particles by PLML.

Two-dimensional amorphous NiO as a plasmonic photocatalyst for solar H2 evolution

Amorphous materials are usually evaluated as photocatalytically inactive due to the amorphous nature-induced self-trapping of tail states, in spite of their achievements in electrochemistry. NiO crystals fail to act as an individual reactor for photocatalytic H₂ evolution because of the intrinsic hole doping, regardless of their impressive cocatalytic ability for proton/electron transfer. Here we demonstrate that two-dimensional amorphous NiO nanostructure can act as an efficient and robust photocatalyst for solar H₂ evolution without any cocatalysts. Further, the antenna effect of surface plasmon resonance can be introduced to construct an incorporate antenna-reactor structure by increasing the electron doping. The solar H₂ evolution rate is improved by a factor of 19.4 through the surface plasmon resonance-mediated charge releasing. These findings thus open a door to applications of two-dimensional amorphous NiO as an advanced photocatalyst.

While photocatalysis offers a means to store solar energy as chemical fuels, photocatalysts typically require crystalline structures and expensive noble-metal cocatalysts. Here, authors prepare 2D amorphous nano-nickel oxide capable of plasmonic, photodriven H₂ evolution without cocatalysts.

Au–Fe nanoparticles visited by MD simulation: structural and thermodynamic properties affected by chemical composition

In this work, Fe–Au nanoalloys and Fe@Au core–shell nanoclusters are investigated via classical molecular dynamics simulation to determine the effect of their composition on their thermodynamic stability and melting mechanism.

Prediction of a low-temperature N2 dissociation catalyst exploiting near-IR-to-visible light nanoplasmonics

Despite more than a century of advances in catalyst and production plant design, the Haber-Bosch process for industrial ammonia (NH₃) synthesis still requires energy-intensive high temperatures and pressures. We propose taking advantage of sunlight conversion into surface plasmon resonances in Au nanoparticles to enhance the rate of the N₂ dissociation reaction, which is the bottleneck in NH₃ production. We predict that this can be achieved through Mo doping of the Au surface based on embedded multireference correlated wave function calculations. The Au component serves as a light-harvesting antenna funneling energy onto the Mo active site, whereby excited-state channels (requiring 1.4 to 1.45 eV, near-infrared-to-visible plasmon resonances) may be accessed. This effectively lowers the energy barriers to 0.44 to 0.77 eV/N₂ (43 to 74 kJ/mol N₂) from 3.5 eV/N₂ (335 kJ/mol N₂) in the ground state. The overall process requires three successive surface excitation events, which could be facilitated by amplified resonance energy transfer due to plasmon local field enhancement.

Gold nanoparticles enlighten the future of cancer theranostics

Development of multifunctional nanomaterials, one of the most interesting and advanced research areas in the field of nanotechnology, is anticipated to revolutionize cancer diagnosis and treatment. Gold nanoparticles (AuNPs) are now being widely utilized in bio-imaging and phototherapy due to their tunable and highly sensitive optical and electronic properties (the surface plasmon resonance). As a new concept, termed "theranostics," multifunctional AuNPs may contain diagnostic and therapeutic functions that can be integrated into one system, thereby simultaneously facilitating diagnosis and therapy and monitoring therapeutic responses. In this review, the important properties of AuNPs relevant to diagnostic and phototherapeutic applications such as structure, shape, optics, and surface chemistry are described. Barriers for translational development of theranostic AuNPs and recent advances in the application of AuNPs for cancer diagnosis, photothermal, and photodynamic therapy are discussed.

Highly Sensitive Magnetic-SERS Dual-Function Silica Nanoprobes for Effective On-Site Organic Chemical Detection

We report magnetic silver nanoshells (M-AgNSs) that have both magnetic and SERS properties for SERS-based detection. The M-AgNSs are composed of hundreds of Fe₃O₄ nanoparticles for rapid accumulation and bumpy silver shell for sensitive SERS detection by near-infrared laser excitation. The intensity of the SERS signal from the M-AgNSs was strong enough to provide single particle-level detection. We obtained much stronger SERS signal intensity from the aggregated M-AgNSs than from the non-aggregated AgNSs. 4-Fluorothiophenol was detected at concentrations as low as 1 nM, which corresponds to 0.16 ppb. The limit of detection for tetramethylthiuram disulfide was 10 μM, which corresponds to 3 ppm. The M-AgNSs can be used to detect trace amounts of organic molecules using a portable Raman system.

Surface plasmon resonance in gold nanoparticles: a review

In the last two decades, plasmon resonance in gold nanoparticles (Au NPs) has been the subject of intense research efforts. Plasmon physics is intriguing and its precise modelling proved to be challenging. In fact, plasmons are highly responsive to a multitude of factors, either intrinsic to the Au NPs or from the environment, and recently the need emerged for the correction of standard electromagnetic approaches with quantum effects. Applications related to plasmon absorption and scattering in Au NPs are impressively numerous, ranging from sensing to photothermal effects to cell imaging. Also, plasmon-enhanced phenomena are highly interesting for multiple purposes, including, for instance, Raman spectroscopy of nearby analytes, catalysis, or sunlight energy conversion. In addition, plasmon excitation is involved in a series of advanced physical processes such as non-linear optics, optical trapping, magneto-plasmonics, and optical activity. Here, we provide the general overview of the field and the background for appropriate modelling of the physical phenomena. Then, we report on the current state of the art and most recent applications of plasmon resonance in Au NPs.

AuFePt Ternary Homogeneous Alloy Nanoparticles with Magnetic and Plasmonic Properties

Combining Au and Fe into a single nanoparticle is an attractive way to engineer a system possessing both plasmonic and magnetic properties simultaneously. However, the formation of the AuFe alloy is challenging because of the wide miscibility gap for these elements. In this study, we synthesized AuFePt ternary alloy nanoparticles as an alternative to AuFe alloy nanoparticles, where Pt is used as a mediator that facilitates alloying between Au and Fe in order to form ternary alloy nanoparticles. The relationship among composition, structure, and function is investigated and it was found that at an optimized composition (Au₅₂Fe₃₀Pt₁₈), ternary alloy NPs exhibit both magnetic and plasmonic properties simultaneously. The plasmonic properties are investigated in detail using a theoretical Mie model, and we found that it is governed by the dielectric constant of the resulting materials.

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.

Impurity effects on solid-solid transitions in atomic clusters

We use the harmonic superposition approach to examine how a single atom substitution affects low-temperature anomalies in the vibrational heat capacity (C_V) of model nanoclusters. Each anomaly is linked to competing solidlike "phases", where crossover of the corresponding free energies defines a solid-solid transition temperature (T_s). For selected Lennard-Jones clusters we show that T_s and the corresponding C_V peak can be tuned over a wide range by varying the relative atomic size and binding strength of the impurity, but excessive atom-size mismatch can destroy a transition and may produce another. In some tunable cases we find up to two additional C_V peaks emerging below T_s, signalling one- or two-step delocalisation of the impurity within the ground-state geometry. Results for Ni₇₄X and Au₅₄X clusters (X = Au, Ag, Al, Cu, Ni, Pd, Pt, Pb), modelled by the many-body Gupta potential, further corroborate the possibility of tuning, engineering, and suppressing finite-system analogues of a solid-solid transition in nanoalloys.

Solvent-surface interactions control the phase structure in laser-generated iron-gold core-shell nanoparticles

This work highlights a strategy for the one-step synthesis of FeAu nanoparticles by the pulsed laser ablation of alloy targets in the presence of different solvents. This method allows particle generation without the use of additional chemicals; hence, solvent-metal interactions could be studied without cross effects from organic surface ligands. A detailed analysis of generated particles via transmission electron microscopy in combination with EDX elemental mapping could conclusively verify that the nature of the used solvent governs the internal phase structure of the formed nanoparticles. In the presence of acetone or methyl methacrylate, a gold shell covering a non-oxidized iron core was formed, whereas in aqueous media, an Au core with an Fe₃O₄ shell was generated. This core-shell morphology was the predominant species found in >90% of the examined nanoparticles. These findings indicate that fundamental chemical interactions between the nanoparticle surface and the solvent significantly contribute to phase segregation and elemental distribution in FeAu nanoparticles. A consecutive analysis of resulting Fe@Au core-shell nanoparticles revealed outstanding oxidation resistance and fair magnetic and optical properties. In particular, the combination of these features with high stability magnetism and plasmonics may create new opportunities for this hybrid material in imaging applications.

Thermodynamic Constraints in Using AuM (M = Fe, Co, Ni, and Mo) Alloys as N₂ Dissociation Catalysts: Functionalizing a Plasmon-Active Metal

The Haber-Bosch process for NH3 synthesis is arguably one of the greatest inventions of the 20th century, with a massive footprint in agriculture and, historically, warfare. Current catalysts for this reaction use Fe for N2 activation, conducted at high temperatures and pressures to improve conversion rate and efficiency. A recent finding shows that plasmonic metal nanoparticles can either generate highly reactive electrons and holes or induce resonant surface excitations through plasmonic decay, which catalyze dissociation and redox reactions under mild conditions. It is therefore appealing to consider AuM (M = Fe, Co, Ni, and Mo) alloys to combine the strongly plasmonic nature of Au and the catalytic nature of M metals toward N2 dissociation, which together might facilitate ammonia production. To this end, through density functional theory, we (i) explore the feasibility of forming these surface alloys, (ii) find a pathway that may stabilize/deactivate surface M substituents during fabrication, and (iii) define a complementary route to reactivate them under operational conditions. Finally, we evaluate their reactivity toward N2, as well as their ability to support a pathway for N2 dissociation with a low thermodynamic barrier. We find that AuFe possesses similar appealing qualities, including relative stability with respect to phase separation, reversibility of Fe oxidation and reduction, and reactivity toward N2. While AuMo achieves the best affinity toward N2, its strong propensity toward oxidation could greatly limit its use.

Plasmonic nanostructures for surface enhanced spectroscopic methods

The development within the last five years in the field of surface enhanced spectroscopy methods was comprehensively reviewed.

Laser generated gold nanocorals with broadband plasmon absorption for photothermal applications

Gold nanoparticles with efficient plasmon absorption in the visible and near infrared (NIR) regions, biocompatibility and easy surface functionalization are of interest for photothermal applications. Herein we describe the synthesis and photothermal properties of gold "nanocorals" (AuNC) obtained by laser irradiation of Au nanospheres (AuNS) dispersed in liquid solution. AuNC are formed in two stages: by photofragmentation of AuNS, followed by spontaneous unidirectional assembly of gold nanocrystals. The whole procedure is performed without chemicals or templating compounds, hence the AuNC can be coated with thiolated molecules in one step. We show that AuNC coated with thiolated polymers are easily dispersed in an aqueous environment or in organic solvents and can be included in polymeric matrixes to yield a plasmonic nanocomposite. AuNC dispersions exhibit flat broadband plasmon absorption ranging from the visible to the NIR and unitary light-to-heat conversion. Besides, in vitro biocompatibility experiments assessed the absence of cytotoxic effects even at a dose as high as 100 μg mL(-1). These safe-by-designed AuNC are promising for use in various applications such as photothermal cancer therapy, light-triggered drug release, antimicrobial substrates, optical tomography, obscurant materials and optical coatings.