Synthesis and Optical Properties of Hybrid and Alloy Plasmonic Nanoparticles

Green Synthesis of Gold, Silver, and Iron Nanoparticles for the Degradation of Organic Pollutants in Wastewater

The green synthesis of nanoscale materials is of special interest to researchers all over the world. We describe a simple, robust, inexpensive, and environmentally friendly approach to the synthesis of gold, silver, and iron nanoparticles using a variety of biomolecules/phytochemicals as potential reducers and stabilizers. The green approach to the controlled synthesis of nanoparticles with different morphologies is based on the use of plant extracts. Green synthesized nanoparticles can be used as catalysts, photocatalysts, adsorbents, or alternative agents for the elimination of various organic dyes. The kinetic enhancement of nanoparticles for the degradation/removal of dyes could provide significant and valuable insights for the application of biochemically functionalized nanoparticles in engineering. In this review, current plant-mediated strategies for preparing nanoparticles of gold, silver, and iron are briefly described, and morphologically dependent nanoparticles for the degradation of organic pollutants in wastewater are highlighted. Overall, the approach presented in the article supports environmental protection and is a promising alternative to other synthesis techniques.

Dark-field microscopic real-time monitoring the growth of Au on Cu2O nanocubes for ultra-sensitive glucose detection

Nanoparticle plasmon scattering can provide real-time imaging information on the formation process of noble metal-based nanomaterials. Due to the synergistic effect of the interface between metal and oxide supporting pores, metal nanoparticles (NPs), especially Au NPs, generally exhibit higher catalytic activity on oxide carriers than single-component NPs. Here, we use the dark field scattering microscope to in situ monitor the growth of Au on Cu₂O surface by oxidation-reduction reactions and the nanostructures could be precisely controlled via the scattering signal. The prepared Cu₂O/Au nanocomposite has a higher electrocatalytic activity toward Glucose. When being used as a potential biosensor for nonenzyme glucose detection, excellent performance, such as high sensitivity with a detection limit of 4 μM, high selectivity and outstanding stability, was obtained. The scattering imaging strategy is a convenient and universal approach in controllable synthesis of plasmonic heterostructures, and leads to the improvement of electrocatalysts in biosensing.

Revealing the etching process of water-soluble Au25 nanoclusters at the molecular level

Etching (often considered as decomposition) is one of the key considerations in the synthesis, storage, and application of metal nanoparticles. However, the underlying chemistry of their etching process still remains elusive. Here, we use real-time electrospray ionization mass spectrometry to study the reaction dynamics and size/structure evolution of all the stable intermediates during the etching of water-soluble thiolate-protected gold nanoclusters (Au NCs), which reveal an unusual "recombination" process in the oxidative reaction environment after the initial decomposition process. Interestingly, the sizes of NC species grow larger and their ligand-to-metal ratios become higher during this recombination process, which are distinctly different from that observed in the reductive growth of Au NCs (e.g., lower ligand-to-metal ratios with increasing sizes). The etching chemistry revealed in this study provides molecular-level understandings on how metal nanoparticles transform under the oxidative reaction environment, providing efficient synthetic strategies for new NC species through the etching reactions.

Dual Active Center-Assembled Cu31S16-Co9-xNixS8 Heterodimers: Coherent Interface Engineering Induces Multihole Accumulation for Light-Enhanced Electrocatalytic Oxygen Evolution

The design of low-cost yet highly efficient electrocatalysts plays a critical role in energy storage and conversion reactions. The oxygen evolution reaction (OER) is considered a bottleneck of electrochemical water splitting for hydrogen fuel generation. It is still challenging to extract a high density of charge carriers in noble-metal-free alternative catalysts to facilitate sluggish kinetics. Herein, we report the rational design and coherent interface engineering for combining light-harvesting Cu₃₁S₁₆ with electroactive Co_9-xNi_xS₈ (x = 0-9) to form novel Cu₃₁S₁₆-Co_9-xNi_xS₈ heterodimers. By delicately controlling the kinetic growth in a seed-mediated growth method, the bifunctional centers, even with two distinct crystal phases, were integrated into a synergistic architecture, which achieved full-spectrum solar energy capture and light conversion to drive and activate the electrochemical reaction. Benefiting from the well-defined structure, high-quality interface, oriented attachment, and optimal Co/Ni bimetal ratio, Cu₃₁S₁₆-Co_7.2Ni_1.8S₈ produces a dramatically reduced overpotential (242 mV at 10 mA cm^-2) with a shift of 83 mV under visible-light excitation, achieving a 4.5-fold higher turnover frequency than that of its unirradiated Co_7.2Ni_1.8S₈ counterpart. This enhanced performance also far exceeds commercial RuO₂ (358 mV at 10 mA cm^-2) and most nonprecious-metal nanocatalysts. Further mechanistic studies reveal that coherent interface engineering leads to a strong photo/electricity coupling effect and efficient spatial charge separation, which induces sufficient hot holes that eventually accumulate at the electroactive sites to accelerate the multihole-involved OER. This work would open up new opportunities for the fabrication of non-noble metal electrocatalysts and management of charge carriers.

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.

Incorporation of Metal Phosphide Domains into Colloidal Hybrid Nanoparticles

Colloidal hybrid nanoparticles have generated considerable attention in the inorganic nanomaterials community. The combination of different materials within a single nanoparticle can lead to synergistic properties that can enable new properties, new applications, and the discovery of new phenomena. As such, methodologies for the synthesis of hybrid nanoparticles that integrate metal-metal, metal chalcogenide, metal oxide, and oxide-chalcogenide domains have been extensively reported in the literature. However, colloidal hybrid nanoparticles containing metal phosphide domains are rare, despite being attractive systems for their potentially unique catalytic, photocatalytic, and optoelectronic properties. In this Forum Article, we report a study of the synthesis of colloidal hybrid nanoparticles that couple the metal phosphides Ni₂P and Co_xP_y with Au, Ag, PbS, and CdS using heterogeneous seeded-growth reactions. We also investigate the transformation of Au-Ni heterodimers to Au-Ni₂P, where phosphidation of preformed metal-metal hybrid nanoparticles offers an alternative route to metal phosphide systems. We also study sequential cation-exchange reactions to target specific metal phosphide hybrids, i.e., the transformation of Ni₂P-PbS into Ni₂P-Ag₂S and then Ni₂P-CdS. Throughout all of these pathways, the accompanying discussion emphasizes the synthetic rationale, as well as the challenges in synthesis and characterization that are unique to these systems. In particular, the observation of oxide shells that surround the phosphide domains has implications for the potential photocatalytic applications of these hybrid nanoparticles.

Organic multicomponent microparticle libraries

Multimetallic nanostructures can be synthesized by integrating up to seven or eight metallic elements into a single nanoparticle, which represent a great advance in developing complex multicomponent nanoparticle libraries. Contrary, organic micro- and nanoparticles beyond three π-conjugated components have not been explored because of the diversity and structural complexity of molecular assemblies. Here, we report a library of microparticles composed of an arbitrary combination of four luminescent organic semiconductors. We demonstrate that the composition and emission color of each domain as well as its spatial distribution can be rationally modulated. Unary, binary, ternary, and quaternary microparticles are thus realized in a predictable manner based on the miscibility of the components, resulting in mixed-composition phases or alloyed or phase separated heterostructures. This work reports a simple yet available synthetic methodology for rational modulation of organic multicomponent microparticles with complex architectures, which can be used to direct the design of functional microparticles.

The synthesis of alloyed complex multicomponent micro- and nanoparticles composed of organic molecules is challenging. Here, the authors demonstrate a library of mixed composition organic microparticles in comprising up to four compounds in their structure.

Structural Control over Bimetallic Core-Shell Nanorods for Surface-Enhanced Raman Spectroscopy

Bimetallic nanorods are important colloidal nanoparticles for optical applications, sensing, and light-enhanced catalysis due to their versatile plasmonic properties. However, tuning the plasmonic resonances is challenging as it requires a simultaneous control over the particle shape, shell thickness, and morphology. Here, we show that we have full control over these parameters by performing metal overgrowth on gold nanorods within a mesoporous silica shell, resulting in Au-Ag, Au-Pd, and Au-Pt core-shell nanorods with precisely tunable plasmonic properties. The metal shell thickness was regulated via the precursor concentration and reaction time in the metal overgrowth. Control over the shell morphology was achieved via a thermal annealing, enabling a transition from rough nonepitaxial to smooth epitaxial Pd shells while retaining the anisotropic rod shape. The core-shell synthesis was successfully scaled up from micro- to milligrams, by controlling the kinetics of the metal overgrowth via the pH. By carefully tuning the structure, we optimized the plasmonic properties of the bimetallic core-shell nanorods for surface-enhanced Raman spectroscopy. The Raman signal was the most strongly enhanced by the Au core-Ag shell nanorods, which we explain using finite-difference time-domain calculations.

Selective Epitaxial Growth of Rh Nanorods on 2H/fcc Heterophase Au Nanosheets to Form 1D/2D Rh-Au Heterostructures for Highly Efficient Hydrogen Evolution

Phase engineering of nanomaterials (PEN) enables the preparation of metal nanomaterials with unconventional phases that are different from their thermodynamically stable counterparts. These unconventional-phase nanomaterials can serve as templates to construct precisely controlled metallic heterostructures for wide applications. Nevertheless, how the unconventional phase of templates affects the nucleation and growth of secondary metals still requires systematic explorations. Here, two-dimensional (2D) square-like Au nanosheets with an unconventional 2H/face-centered cubic (fcc) heterophase, composing of two pairs of opposite edges with 2H/fcc heterophase and fcc phase, respectively, and two 2H/fcc heterophase basal planes, are prepared and then used as templates to grow one-dimensional (1D) Rh nanorods. The effect of different phases in different regions of the Au templates on the overgrowth of Rh nanorods has been systematically investigated. By tuning the reaction conditions, three types of 1D/2D Rh-Au heterostructures are prepared. In the type A heterostructure, Rh nanorods only grow on the fcc defects including stacking faults and/or twin boundaries (denoted as fcc-SF/T) and 2H phases in two 2H/fcc edges of the Au nanosheet. In the type B heterostructure, Rh nanorods grow on the fcc-SF/T and 2H phases in two 2H/fcc edges and two 2H/fcc basal planes of the Au nanosheet. In the type C heterostructure, Rh nanorods grow on four edges and two basal planes of the Au nanosheet. Furthermore, the type C heterostructure shows promising performance toward the electrochemical hydrogen evolution reaction (HER) in acidic media, which is among the best reported Rh-based and other noble-metal-based HER electrocatalysts.

Versatile and robust synthesis process for the fine control of the chemical composition and core-crystallinity of spherical core-shell Au@Ag nanoparticles

Au nanoparticles (NPs) characterized by distinct surface chemistry (including dodecanethiol or oleylamine as capping agent), different sizes (∼5 and ∼10 nm) and crystallinities (polycrystalline or single crystalline), were chosen as seeds to demonstrate the versatility and robustness of our two-step core-shell Au@Ag NP synthesis process. The central component of this strategy is to solubilize the shell precursor (AgNO₃) in oleylamine and to induce the growth of the shell on selected seeds under heating. The shell thickness is thus controlled by the temperature, the annealing time, the (shell precursor)/(seed) concentration ratio, seed size and crystallinity. The shell thickness is thus shown to increase with the reactant concentration and to grow faster on polycrystalline seeds. The crystalline structure and chemical composition were characterized by HRTEM, STEM-HAADF, EELS and Raman spectroscopy. The plasmonic response of Au@Ag core-shell NPs as a function of core size and shell thickness was assessed by spectrophotometry and simulated by calculations based on the discrete dipole approximation (DDA) method. Finally, the nearly monodisperse core-shell Au@Ag NPs were shown to form micrometer-scale facetted 3D fcc-ordered superlattices (SLs) after solvent evaporation and deposition on a solid substrate. These SLs are promising candidates for applications as a tunable surface-enhanced Raman scattering platform.

Visualizing Ligand-Mediated Bimetallic Nanocrystal Formation Pathways with in Situ Liquid-Phase Transmission Electron Microscopy Synthesis

Colloidal synthesis of alloyed multimetallic nanocrystals with precise composition control remains a challenge and a critical missing link in theory-driven rational design of functional nanomaterials. Liquid-phase transmission electron microscopy (LP-TEM) enables direct visualization of nanocrystal formation mechanisms that can inform discovery of design rules for nanocrystal synthesis, but it remains unclear whether the salient flask synthesis chemistry is preserved under electron beam irradiation during LP-TEM. Here, we demonstrate controlled in situ LP-TEM synthesis of alloyed AuCu nanocrystals while maintaining the molecular structure of electron beam sensitive metal thiolate precursor complexes. Ex situ flask synthesis experiments formed alloyed nanocrystals containing on average 70 atomic% Au using heteronuclear metal thiolate complexes as a precursor, while gold-rich alloys with nearly no copper formed in their absence. Systematic dose rate-controlled in situ LP-TEM synthesis experiments established a range of electron beam synthesis conditions that formed alloyed AuCu nanocrystals that had statistically indistinguishable alloy composition, aggregation state, and particle size distribution shape compared to ex situ flask synthesis, indicating the flask synthesis chemistry was preserved under these conditions. Reaction kinetic simulations of radical-ligand reactions revealed that polymer capping ligands acted as effective hydroxyl radical scavengers during LP-TEM synthesis and prevented oxidation of metal thiolate complexes at low dose rates. Our results revealed a key role of the capping ligands aside from their well-known functions, which was to prevent copper oxidation and facilitate formation of prenucleation cluster intermediates via formation of metal thiolate complexes. This work demonstrates that complex ion precursor chemistry can be maintained during LP-TEM imaging, enabling probing nonclassical nanocrystal formation mechanisms with LP-TEM under reaction conditions representative of ex situ flask synthesis.

Symmetric and asymmetric epitaxial growth of metals (Ag, Pd, and Pt) onto Au nanotriangles: effects of reductants and plasmonic properties

The surface plasmon resonance of noble metals can be tuned by morphology and composition, offering interesting opportunities for applications in biomedicine, optoelectronics, photocatalysis, photovoltaics, and sensing. Here, we present the results of the symmetrical and asymmetrical overgrowth of metals (Ag, Pd, and Pt) onto triangular Au nanoplates using l-ascorbic acid (AA) and/or salicylic acid (SA) as reductants. By varying the reaction conditions, various types of Au nanotriangle-metal (Au NT-M) hetero-nanostructures were easily prepared. The plasmonic properties of as-synthesized nanoparticles were investigated by a combination of optical absorbance measurements and Finite-Difference Time-Domain (FDTD) simulations. We show that specific use of these reductants enables controlled growth of different metals on Au NTs, yielding different morphologies and allowing manipulation and tuning of the plasmonic properties of bimetallic Au NT-M (Ag, Pd, and Pt) structures.

Corner-, edge-, and facet-controlled growth of nanocrystals

The ability to precisely control nanocrystal (NC) shape and composition is useful in many fields, including catalysis and plasmonics. Seed-mediated strategies have proven effective for preparing a wide variety of structures, but a poor understanding of how to selectively grow corners, edges, and facets has limited the development of a general strategy to control structure evolution. Here, we report a universal synthetic strategy for directing the site-specific growth of anisotropic seeds to prepare a library of designer nanostructures. This strategy leverages nucleation energy barrier profiles and the chemical potential of the growth solution to control the site-specific growth of NCs into exotic shapes and compositions. This strategy can be used to not only control where growth occurs on anisotropic seeds but also control the exposed facets of the newly grown regions. NCs of many shapes are synthesized, including over 10 here-to-fore never reported NCs and, in principle, many others are possible.

Controlling ultrasmall gold nanoparticles with atomic precision

Gold nanoparticles are probably the nanoparticles that have been best studied for the longest time due to their stability, physicochemical properties and applications. Controlling gold nanoparticles with atomic precision is of significance for subsequent research on their structures, properties and applications, which is a dream that has been pursued for many years since ruby gold was first obtained by Faraday in 1857. Fortunately, this dream has recently been partially realized for some ultrasmall gold nanoparticles (nanoclusters). However, rationally designing and synthesizing gold nanoparticles with atomic precision are still distant goals, and this challenge might rely primarily on rich atomically precise gold nanoparticle libraries and the in-depth understanding of metal nanoparticle chemistry. Herein, we review general synthesis strategies and some facile synthesis methods, with an emphasis on the controlling parameters determined from well-documented results, which might have important implications for future nanoparticle synthesis with atomic precision and facilitate related research and applications.

The synthesis strategy, methods and parameters for atomically precise gold nanoclusters were reviewed, and future outlook was also proposed.

IRMPD spectroscopy and quantum chemistry calculations on mono- and bi-metallic complexes of acetylacetonate ligands with aluminum, iron, and ruthenium ions

Metal-ligand cluster ions are structurally characterized by means of gas-phase infrared multiple photon dissociation spectroscopy. The mass-selected complexes consist of one or two metal cations M³⁺ (M = Al, Fe, or Ru) and two to five anionic bidentate acetylacetonate ligands. Experimental IR spectra are compared with different density functional theory calculations, namely, PBE/TZVP, B3LYP/6-31G^*, and M06/6-31+G^**. Frequency analysis was also performed at different levels, namely, scaled static harmonic and unscaled static anharmonic, or with ab initio molecular dynamics simulations at the PBE/TZVP level. All methods lead to simulated spectra that fit rather well with experimental data, and the spectral red shifts of several main bands, in the 1200 cm^-1-1800 cm^-1 range, are sensitive to the strength of the metal-ligand interaction and to the spin state of the ion. Due to the rigidity of those complexes, first principles molecular dynamics calculations provide spectra similar to that produced by static calculations that are already able to catch the main spectral signatures using harmonic calculations at the B3LYP/6-31G^* level.

Resonant plasmon enhancement of light emission from CdSe/CdS nanoplatelets on Au nanodisk arrays

Semiconducting nanoplatelets (NPLs) have attracted great attention due to the superior photophysical properties compared to their quantum dot analogs. Understanding and tuning the optical and electronic properties of NPLs in a plasmonic environment is a new paradigm in the field of optoelectronics. Here, we report on the resonant plasmon enhancement of light emission including Raman scattering and photoluminescence from colloidal CdSe/CdS nanoplatelets deposited on arrays of Au nanodisks fabricated by electron beam lithography. The localized surface plasmon resonance (LSPR) of the Au nanodisk arrays can be tuned by varying the diameter of the disks. In the case of surface-enhanced Raman scattering (SERS), the Raman intensity profile follows a symmetric Gaussian shape matching the LSPR of the Au nanodisk arrays. The surface-enhanced photoluminescence (SEPL) profile of NPLs, however, follows an asymmetric Gaussian distribution highlighting a compromise between the excitation and emission enhancement mechanisms originating from energy transfer and Purcell effects. The SERS and SEPL enhancement factors depend on the nanodisk size and reach maximal values at 75 and 7, respectively, for the sizes, for which the LSPR energy of Au nanodisks coincides with interband transition energies in the semiconductor platelets. Finally, to explain the origin of the resonant enhancement behavior of SERS and SEPL, we apply a numerical simulation to calculate plasmon energies in Au nanodisk arrays and emission spectra from NPLs in such a plasmonic environment.

DNA-Based Plasmonic Heterogeneous Nanostructures: Building, Optical Responses, and Bioapplications

The integration of multiple functional nanoparticles into a specific architecture allows the precise manipulation of light for coherent electron oscillations. Plasmonic metals-based heterogeneous nanostructures are fabricated by using DNA as templates. This comprehensive review provides an overview of the controllable synthesis and self-assembly of heterogeneous nanostructures, and analyzes the effects of structural parameters on the regulation of optical responses. The potential applications and challenges of heterogeneous nanostructures in the fields of biosensors and bioanalysis, in vivo monitoring, and phototheranostics are discussed.

Composition-Dependent Antimicrobial Ability of Full-Spectrum AuxAg25-x Alloy Nanoclusters

Alloying is an efficient chemistry to diversify the properties of metal nanoparticles; however, the atomic-level understandings of the composition-dependent physicochemical properties and their related biological performance are presently lacking. Here, we developed a full spectrum of alloy metal nanoclusters (NCs), Au_xAg_25-x(MHA)₁₈ (MHA = 6-mercaptohexanoic acid) with x = 0-25, and investigated their composition-dependent antimicrobial performance. Interestingly, we observed a U-shape antimicrobial behavior of Au_xAg_25-x(MHA)₁₈ NCs, where the alloy NCs showed decreased antimicrobial ability instead of the common trend of increasing. Detailed atomic-level characterizations of the AuAg NCs suggest that the decreased performance of alloy NCs is due to their enhanced stability after alloying, which can deactivate their capability in generating reactive oxygen species (ROS) that can kill the bacteria. More interestingly, the transition point of the antimicrobial performance was only obtained with our full-spectrum Au_xAg_25-x(MHA)₁₈ NCs, which indicates the importance of exploring the composition-dependent properties and application performance in a full-spectrum composition range. A library of full-spectrum alloy NCs also provides a good platform to investigate other composition-dependent physicochemical and biological properties of metal NCs.

Monitoring of Anthracene Using Nanoscale Au-Cu Bimetallic Alloy Nanoparticles Synthesized with Various Compositions

Bimetallic alloy Au-Cu nanoparticles (Au-Cu alloy NPs) were synthesized using a chemical reduction method for sensing applications. Electronic absorption spectroscopy (UV-visible spectroscopy), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used for the confirmation and morphological studies of the synthesized nanoparticles. The composition of Au-Cu alloy NPs was studied by energy-dispersive spectroscopy (EDS). The high crystallinity of Au-Cu alloy NPs was demonstrated by XRD analysis. Both XRD and SEM analyses revealed that the nanoparticles' size ranges from 15 to 25 nm. Pyrrole was polymerized into polypyrrole (PPy) over a neat and clean glassy carbon electrode (GCE) by potentiodynamic polymerization. The sensitivity of GCE was improved by modifying it into a composite electrode. The composite electrode was developed by coating GCE with an overoxidized PPy polymer followed by Au-Cu alloy NPs. The ratio of Au and Cu was carefully controlled. The composite electrode (PPyox/Au-Cu/GCE) successfully detected an environmental toxin anthracene with a detection limit of 0.15 μM, as evidenced by cyclic voltammetry (CV), square-wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS).

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.

Heterophase fcc-2H-fcc gold nanorods

The crystal phase-based heterostructures of noble metal nanomaterials are of great research interest for various applications, such as plasmonics and catalysis. However, the synthesis of unusual crystal phases of noble metals still remains a great challenge, making the construction of heterophase noble metal nanostructures difficult. Here, we report a one-pot wet-chemical synthesis of well-defined heterophase fcc-2H-fcc gold nanorods (fcc: face-centred cubic; 2H: hexagonal close-packed with stacking sequence of "AB") at mild conditions. Single particle-level experiments and theoretical investigations reveal that the heterophase gold nanorods demonstrate a distinct optical property compared to that of the conventional fcc gold nanorods. Moreover, the heterophase gold nanorods possess superior electrocatalytic activity for the carbon dioxide reduction reaction over their fcc counterparts under ambient conditions. First-principles calculations suggest that the boosted catalytic performance stems from the energetically favourable adsorption of reaction intermediates, endowed by the unique heterophase characteristic of gold nanorods.

Nanofabrication within unimolecular nanoreactors

Nanoparticles (NPs) have been a research focus over the last three decades owing to their unique properties and extensive applications. It is crucial to precisely control the features of NPs including topology, architecture, composition, size, surface and assembly because these features will affect their properties and then applications. Ingenious nanofabrication strategies have been developed to precisely control these features of NPs, especially for templated nanofabrication within predesigned nanoreactors. Compared with conventional nanoreactors (hard templates and supramolecular nanoreactors), unimolecular nanoreactors exhibit (1) covalently stable nanostructures uninfluenced by environmental variations, (2) extensively regulated features of the structure including topology, composition, size, surface and valence due to the rapid development of polymer chemistry, and (3) effective encapsulation of abundant guests with or without strong interaction to achieve the function of loading, delivery and conversion of guests. Thus, unimolecular nanoreactors have shown fascinating prospects as templates for nanofabrication. Various NPs with expected topologies (sphere, rod, tube, branch, and ring), architectures (compact, hollow, core-shell, and necklace-like), compositions (metal, metal oxide, semiconductor, doping, alloy, silica, and composite), sizes (generally 1-100 nm), surface properties (hydrophilic, hydrophobic, reactivity, valence and responsivity) and assemblies (oligomer, chain, and aggregate) can be fabricated easily within reasonably designed unimolecular nanoreactors in a programmable way. In this review, we provide a brief introduction of the properties and types of unimolecular nanoreactors, a condensed summary of representative methodologies of nanofabrication within various unimolecular nanoreactors and a predicted outlook of the potential further developments of this charming nanofabrication approach.

PbS Quantum Dots Decorating TiO2 Nanocrystals: Synthesis, Topology, and Optical Properties of the Colloidal Hybrid Architecture

Fabrication of heterostructures by merging two or more materials in a single object. The domains at the nanoscale represent a viable strategy to purposely address materials’ properties for applications in several fields such as catalysis, biomedicine, and energy conversion. In this case, solution-phase seeded growth and the hot-injection method are ingeniously combined to fabricate TiO₂/PbS heterostructures. The interest in such hybrid nanostructures arises from their absorption properties that make them advantageous candidates as solar cell materials for more efficient solar light harvesting and improved light conversion. Due to the strong lattice mismatch between TiO₂ and PbS, the yield of the hybrid structure and the control over its properties are challenging. In this study, a systematic investigation of the heterostructure synthesis as a function of the experimental conditions (such as seeds’ surface chemistry, reaction temperature, and precursor concentration), its topology, structural properties, and optical properties are carried out. The morphological and chemical characterizations confirm the formation of small dots of PbS by decorating the oleylamine surface capped TiO₂ nanocrystals under temperature control. Remarkably, structural characterization points out that the formation of heterostructures is accompanied by modification of the crystallinity of the TiO₂ domain, which is mainly ascribed to lattice distortion. This result is also confirmed by photoluminescence spectroscopy, which shows intense emission in the visible range. This originated from self-trapped excitons, defects, and trap emissive states.

Quantum Leap from Gold and Silver to Aluminum Nanoplasmonics for Enhanced Biomedical Applications

Nanotechnology has been used in many biosensing and medical applications, in the form of noble metal (gold and silver) nanoparticles and nanostructured substrates. However, the translational clinical and industrial applications still need improvements of the efficiency, selectivity, cost, toxicity, reproducibility, and morphological control at the nanoscale level. In this review, we highlight the recent progress that has been made in the replacement of expensive gold and silver metals with the less expensive aluminum. In addition to low cost, other advantages of the aluminum plasmonic nanostructures include a broad spectral range from deep UV to near IR, providing additional signal enhancement and treatment mechanisms. New synergistic treatments of bacterial infections, cancer, and coronaviruses are envisioned. Coupling with gain media and quantum optical effects improve the performance of the aluminum nanostructures beyond gold and silver.

Nanofibrillar cellulose/Au@Ag nanoparticle nanocomposite as a SERS substrate for detection of paraquat and thiram in lettuce

A nanocomposite based on nanofibrillar cellulose (NFC) coated with gold-silver (core-shell) nanoparticles (Au@Ag NPs) was developed as a novel surface-enhanced Raman spectroscopy (SERS) substrate. SERS performance of NFC/Au@Ag NP nanocomposite was tested by 4-mercaptobenzoic acid. The cellulose nanofibril network was a suitable platform that allowed Au@Ag NPs to be evenly distributed and stabilized over the substrate, providing more SERS hotspots for the measurement. Two pesticides, thiram and paraquat, were successfully detected either individually or as a mixture in lettuce by SERS coupled with the nanocomposite. Strong Raman scattering signals for both thiram and paraquat were obtained within a Raman shift range of 400-2000 cm-1 and a Raman intensity ~ 8 times higher than those acquired by NFC/Au NP nanocomposite. Characteristic peaks were clearly observable in all SERS spectra even at a low concentration of 10 μg/L of pesticides. Limit of detection values of 71 and 46 μg/L were obtained for thiram and paraquat, respectively. Satisfactory SERS performance, reproducibility, and sensitivity of NFC/Au@Ag NP nanocomposite validate its applicability for real-world analysis to monitor pesticides and other contaminants in complex food matrices within a short acquisition time. Graphical abstract.

Photopolarimetrical properties of coronavirus model particles: Spike proteins number influence

Coronavirus virions have spherical shape surrounded by spike proteins. The coronavirus spike proteins are very effective molecular mechanisms, which provide the coronavirus entrance to the host cell. The number of these spikes is different; it dramatically depends on external conditions and determines the degree of danger of the virus. A larger number of spike proteins makes the virus infectivity stronger. This paper describes a mathematical model of the shape of coronavirus virions. Based on this model, the characteristics of light scattered by the coronavirus virions were calculated. It was found two main features of coronavirus model particles in the spectral region near 200 nm: a minimum of intensity and a sharp leap of the linear polarization degree. The effect of the spike protein number on the intensity and polarization properties of the scattered light was studied. It was determined that when the number of spike proteins decreases, both the intensity minimum and the position of the linear polarization leap shift to shorter wavelengths. This allows us to better evaluate the shape of the coronavirus virion, and, therefore, the infectious danger of the virus. It was shown that the shorter the wavelength of scattered light, the more reliably one can distinguish viruses from non-viruses. The developed model and the light scattering simulations based on it can be applied not only to coronaviruses, but also to other objects of a similar structure, for example, pollen.

A Noble-Transition Alloy Excels at Hot-Carrier Generation in the Near Infrared

Above-equilibrium "hot"-carrier generation in metals is a promising route to convert photons into electrical charge for efficient near-infrared optoelectronics. However, metals that offer both hot-carrier generation in the near-infrared and sufficient carrier lifetimes remain elusive. Alloys can offer emergent properties and new design strategies compared to pure metals. Here, it is shown that a noble-transition alloy, Au_x Pd_{1- x} , outperforms its constituent metals concerning generation and lifetime of hot carriers when excited in the near-infrared. At optical fiber wavelengths (e.g., 1550 nm), Au₅₀ Pd₅₀ provides a 20-fold increase in the number of ≈0.8 eV hot holes, compared to Au, and a threefold increase in the carrier lifetime, compared to Pd. The discovery that noble-transition alloys can excel at hot-carrier generation reveals a new material platform for near-infrared optoelectronic devices.

Manipulating Bimetallic Nanostructures With Tunable Localized Surface Plasmon Resonance and Their Applications for Sensing

Metal nanocrystals with well-controlled shape and unique localized surface plasmon resonance (LSPR) properties have attracted tremendous attention in both fundamental studies and applications. Compared with monometallic counterparts, bimetallic nanocrystals endow scientists with more opportunities to precisely tailor their LSPR and thus achieve excellent performances for various purposes. The aim of this mini review is to present the recent process in manipulating bimetallic nanostructures with tunable LSPR and their applications for sensing. We first highlight several significant strategies in controlling the elemental ratio and spatial arrangement of bimetallic nanocrystals, followed by discussing on the relationship between their composition/morphology and LSPR properties. We then focus on the plasmonic sensors based on the LSPR peak shift, which can be well-controlled by seed-mediated growth and selective etching. This review provides insights of understanding the “rules” involving in the formation of bimetallic nanocrystals with different structures and desired LSPR properties, and also forecasts the development directions of plasmonic sensors in the future.

The Quest for Zero Loss: Unconventional Materials for Plasmonics

There has been an ongoing quest to optimize the materials used to build plasmonic devices: first the elements were investigated, then alloys and intermetallic compounds, later semiconductors were considered, and, most recently, there has been interest in using more exotic materials such as topological insulators and conducting oxides. The quality of the plasmon resonances in these materials is closely correlated with their structure and properties. In general gold and silver are the most commonly specified materials for these applications but they do have weaknesses. Here, it is shown how, in specific circumstances, the selection of certain other materials might be more useful. Candidate alternatives include Ti_x N, VO₂ , Al, Cu, Al-doped ZnO, and Cu-Al alloys. The relative merits of these choices and the many pitfalls and subtle problems that arise are discussed, and a frank perspective on the field is provided.

The mechanism of metal exchange in non-metallic nanoclusters

We substituted gold atoms in fcc structured Au28 and Au36 nanoclusters with a Ag(i)SR complex and obtained Ag x Au28-x and Ag x Au36-x nanoclusters, respectively. The positive electrostatic potential (ESP) and dual descriptor (Δf) values were calculated for the metal cores of both nanoclusters, which indicated that the metal exchange is an electrophilic reaction.

Broadband chiral hybrid plasmon modes on nanofingernail substrates

There is significant interest in the utility of asymmetric nanoaperture arrays as substrates for the surface-enhanced detection, fluorescence, and imaging of individual molecules. This work introduces obliquely-cut, out-of-plane, coaxial layered structures on an aperture edge. We refer to these structures as nanofingernails, which emphasizes their curved, oblique, and out-of-plane features. Broadband coupling into chiral hybrid plasmon modes and helicity-dependent near-field scattering without circular dichroism are demonstrated. The unusually-broadband, multipolar modes of nanofingernail micropore structures exhibit phase retardation effects that may be useful for achieving spatial overlap at different frequencies. The nanofingernail geometry shows new potential for simultaneous polarization-enhanced hyperspectral imaging on apertured, plasmonic surfaces.

Plasmonic-Active Nanostructured Thin Films

Plasmonic-active nanomaterials are of high interest to scientists because of their expanding applications in the field for medicine and energy. Chemical and biological sensors based on plasmonic nanomaterials are well-established and commercially available, but the role of plasmonic nanomaterials on photothermal therapeutics, solar cells, super-resolution imaging, organic synthesis, etc. is still emerging. The effectiveness of the plasmonic materials on these technologies depends on their stability and sensitivity. Preparing plasmonics-active nanostructured thin films (PANTFs) on a solid substrate improves their physical stability. More importantly, the surface plasmons of thin film and that of nanostructures can couple in PANTFs enhancing the sensitivity. A PANTF can be used as a transducer for any of the three plasmonic-based sensing techniques, namely, the propagating surface plasmon, localized surface plasmon resonance, and surface-enhanced Raman spectroscopy-based sensing techniques. Additionally, continuous nanostructured metal films have an advantage for implementing electrical controls such as simultaneous sensing using both plasmonic and electrochemical techniques. Although research and development on PANTFs have been rapidly advancing, very few reviews on synthetic methods have been published. In this review, we provide some fundamental and practical aspects of plasmonics along with the recent advances in PANTFs synthesis, focusing on the advantages and shortcomings of the fabrication techniques. We also provide an overview of different types of PANTFs and their sensitivity for biosensing.

Photothermal structural modification of porous gold nanoshells via pulsed-laser irradiation: effects of laser wavelengths and surface conditions

We demonstrate the detailed effects of laser wavelengths and nanoparticle surface conditions, as well as laser fluences, in the structural modification of porous gold nanoshells induced by picosecond pulse irradiation.

Recoverable and recyclable nickel–cobalt magnetic alloy nanoparticle catalyzed reversible deactivation radical polymerization of methyl methacrylate at 25 °C

Recyclable Ni–Co alloy catalyzed synthesis of well-defined poly(methyl methacrylate) (PMMA, up to 129 500 g mol⁻¹) with narrow-dispersity (Đ = 1.30) via a reversible deactivation radical polymerization technique is reported.

Core-Shell Gold/Silver Nanoparticles for Localized Surface Plasmon Resonance-Based Naked-Eye Toxin Biosensing

The localized surface plasmon resonance (LSPR) phenomenon provides a versatile property for biodetection. Herein, this unique feature was employed to build a homogeneous optical biosensor to detect staphylococcal enterotoxin A (SEA) in solution down to very low levels by naked-eye readout. If the initial position of the LSPR band is located in the cyan region, even a small red shift (∼2-3 nm) induced by a refractive index change close to the surface of nanoparticles (NPs) could make the light absorption transit from cyan to green and become visually detectable via a concomitant change in the complementary colors. In this work, we aimed at synthesizing two types of NPs based on compositionally complex core-shell NPs-Ag shells on AuNPs (Au@AgNPs) and Ag inside gold nanoshells (Ag@AuNPs). By controlling the thickness of the shells and their surface chemistry with anti-SEA antibody (Ab), the LSPR band was tuned to near 495 and 520 nm for Ag@AuNPs and Au@AgNPs, respectively. The two particle systems were subsequently applied to spectroscopically and visually detect anti-SEA Ab-SEA interactions. Upon the addition of SEA, large red shifts of the LSPR band were observed spectroscopically and the limits of detection (LODs) were estimated to be 0.2 and 0.4 nM for Au@AgNPs and Ag@AuNPs, respectively. Although the two sets of NPs gave almost identical LODs, the Ag@AuNPs whose initial position of the LSPR band was tuned in the cyan to green region (∼500 nm) displayed a substantially more distinct color change from orange to red, as revealed by the naked eye. We foresee significant potential to this strategy in medical diagnostics and environmental monitoring, especially when basic laboratory infrastructure is sparse or nonexistent.

Single-Particle Tracking with Scattering-Based Optical Microscopy

Different from traditional ensemble measurement methods, single-particle tracking (SPT) is a powerful approach to study the distribution of dynamic processes in a complex environment, providing crucial information from individual objects. This Feature summarizes the optical microscopic techniques and data analysis methods for scattering-based SPT. Some essential SPT-based applications within the cell are also delineated.

Ultrasmall Designed Plasmon Resonators by Fused Colloidal Nanopatterning

This work presents a procedure for large-area patterning of designed plasmon resonators that are much smaller than possible with conventional lithography techniques. Fused Colloidal Nanopatterning combines directed self-assembly and controlled fusing of spherical colloidal nanoparticles. The two-step approach first patterns a surface covered with hydrogen silsesquioxane, an electron beam resist, forming traps into which the colloidal gold nanoparticles self-assemble. Second, the patterned nanoparticles are controllably fused to form plasmon resonators of any 2D designed shape. The heights and widths of the plasmon resonators are determined by the diameter of the nanoparticle building blocks, which can be well below 10 nm. By performing the fusing step with UV ozone and heat exposure, we demonstrate that the process is easily scalable to cover large areas on silicon wafers with designed gold nanostructures. The procedure neither requires adhesion layers nor a lift-off process, making it ideally suited for plasmonics, in comparison with regular electron beam lithography. We use monochromated electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy and boundary element method simulations to demonstrate that the designed plasmon resonators are directly tunable via the pattern design. We foresee future expansion of this approach for applications such as plasmon-enhanced photocatalysis and for large-scale patterning where chemical, optical, or confinement properties require sub-10 nm metal lines.

Multicomponent Plasmonic Nanoparticles: From Heterostructured Nanoparticles to Colloidal Composite Nanostructures

Plasmonic nanostructures possessing unique and versatile optoelectronic properties have been vastly investigated over the past decade. However, the full potential of plasmonic nanostructure has not yet been fully exploited, particularly with single-component homogeneous structures with monotonic properties, and the addition of new components for making multicomponent nanoparticles may lead to new-yet-unexpected or improved properties. Here we define the term "multi-component nanoparticles" as hybrid structures composed of two or more condensed nanoscale domains with distinctive material compositions, shapes, or sizes. We reviewed and discussed the designing principles and synthetic strategies to efficiently combine multiple components to form hybrid nanoparticles with a new or improved plasmonic functionality. In particular, it has been quite challenging to precisely synthesize widely diverse multicomponent plasmonic structures, limiting realization of the full potential of plasmonic heterostructures. To address this challenge, several synthetic approaches have been reported to form a variety of different multicomponent plasmonic nanoparticles, mainly based on heterogeneous nucleation, atomic replacements, adsorption on supports, and biomolecule-mediated assemblies. In addition, the unique and synergistic features of multicomponent plasmonic nanoparticles, such as combination of pristine material properties, finely tuned plasmon resonance and coupling, enhanced light-matter interactions, geometry-induced polarization, and plasmon-induced energy and charge transfer across the heterointerface, were reported. In this review, we comprehensively summarize the latest advances on state-of-art synthetic strategies, unique properties, and promising applications of multicomponent plasmonic nanoparticles. These plasmonic nanoparticles including heterostructured nanoparticles and composite nanostructures are prepared by direct synthesis and physical force- or biomolecule-mediated assembly, which hold tremendous potential for plasmon-mediated energy transfer, magnetic plasmonics, metamolecules, and nanobiotechnology.

Metal Nano Networks by Potential-Controlled In Situ Assembling of Gold/Silver Nanoparticles

Non-spherical Au/Ag nanoparticles can be generated by chemical reduction of silver ions in the presence of preformed gold nanoparticles. The process of particle formation can be controlled by concentrations of ligands and reducing agent. The formation of ellipsoidal, nanorod- and peanut-shaped nanoparticles as well as of more complex fractal nanoassemblies can be explained by changes in particle surface state, electrochemical potential formation and particle-internal self-polarization effects. It is possible to create highly fractal nanoassemblies with sizes between the mid-nanometer and the lower micrometer range. The assemblies are marked by high optical absorption and complex nano-networks of very high surface-to-volume ratios and a granular base structure.

High-resolution manipulation of gold nanorods with an atomic force microscope

The controlled manipulation and precise positioning of nanoparticles on surfaces is a critical requisite for studying interparticle interactions in various research fields including spintronics, plasmonics, and nanomagnetism. We present here a method where an atomic force microscope operating in vacuum is used to accurately rotate and displace CTAB-coated gold nanorods on silica surfaces. The method relies on operating an AFM in a bimodal way which includes both dynamic and contact modes. Moreover, the phase of the oscillating probe is used to monitor the nanoparticle trajectory, which amplitude variations are employed to evaluate the energy dissipation during manipulation. The nanoscale displacement modes involve nanorod in-plane rotation and sliding, but no rolling events. The transitions between these displacement modes depend on the angle between the scan axis direction and the nanorod long axis. The findings reveal the importance of mean tip-substrate distance and of oscillation amplitude of the tip. The role of substrate surface and of CTAB molecular bi-layer at nanorod surface is also discussed.

Valence Engineering via Dual-Cation and Boron Doping in Pyrite Selenide for Highly Efficient Oxygen Evolution

Valence engineering has been proved an effective approach to modify the electronic property of a catalyst and boost its oxygen evolution reaction (OER) activity, while the limited number of elements restricts the structural diversity and the active sites. Also, the catalyst performance and stability are greatly limited by cationic dissolution, ripening, or crystal migration in a catalytic system. Here we employed a widely used technique to fabricate heteroepitaxial pyrite selenide through dual-cation substitution and a boron dopant to achieve better activity and stability. The overpotential of Ni-pyrite selenide catalyst is decreased from 543 mV to 279.8 mV at 10 mA cm^-2 with a Tafel slope from 161 to 59.5 mV dec^-1. Our theoretical calculations suggest both cation and boron doping can effectively optimize adsorption energy of OER intermediates, promote the charge transfer among the heteroatoms, and improve their OER property. This work underscores the importance of modulating surface electronic structure with the use of multiple elements and provides a general guidance on the minimization of activity loss with valence engineering.