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Electron tomography has become a commonly used tool to investigate the three-dimensional (3D) structure of nanomaterials, including colloidal nanoparticle assemblies. However, electron microscopy is typically done under high-vacuum conditions, requiring sample preparation for assemblies obtained by wet colloid chemistry methods. This involves solvent evaporation and deposition on a solid support, which consistently alters the nanoparticle organization. Here, we suggest using electron tomography to study nanoparticle assemblies in their original colloidal liquid environment. To address the challenges related to electron tomography in liquid, we devise a method that combines fast data acquisition in a commercial liquid-cell with a dedicated alignment and reconstruction workflow. We present the advantages of this methodology in accurately characterizing two different systems. 3D reconstructions of assemblies comprising polystyrene-capped Au nanoparticles encapsulated in polymeric shells reveal less compact and more distorted configurations for experiments performed in a liquid medium compared to their dried counterparts. A similar expansion can be observed in quantitative analysis of the surface-to-surface distances of self-assembled Au nanorods in water rather than in a vacuum, in agreement with bulk measurements. This study, therefore, emphasizes the importance of developing high-resolution characterization tools that preserve the native environment of colloidal nanostructures. Drying force-induced deformation complicates the characterization of the 3D structure of colloidal assemblies. Here, the authors develop a liquid electron tomography method for unravelling the 3D structures of small colloidal assemblies under native conditions. [ABSTRACT FROM AUTHOR]

Handedness is an essential attribute of chiral nanocrystals, having a major influence on their properties. During chemical growth, the handedness of nanocrystals is usually tuned by selecting the corresponding enantiomer of chiral molecules involved in asymmetric growth, often known as chiral inducers. We report that, even using the same chiral inducer enantiomer, the handedness of chiral gold nanocrystals can be reversed by using Au nanorod seeds with either single crystalline or pentatwinned structure. This effect holds for chiral growth induced both by amino acids and by chiral micelles. Although it was challenging to discern the morphological handedness for L‐cystine‐directed particles, even using electron tomography, both cases showed circular dichroism bands of opposite sign, with nearly mirrored chiroptical signatures for chiral micelle‐directed growth, along with quasi‐helical wrinkles of inverted handedness. These results expand the chiral growth toolbox with an effect that might be exploited to yield a host of interesting morphologies with tunable optical properties. [ABSTRACT FROM AUTHOR]

Chiral metal nanoparticles (NPs) offer a powerful means of inducing and harnessing optical activity. However, due to the incomplete knowledge of the underlying growth mechanisms, there is still limited control over the achievable morphological detail and, consequently, over the resulting optical activity. Therefore, theoretical modeling is needed to guide experimental development toward optimizing the plasmonic chiroptical response. Toward filling this gap, herein an extensive parametric analysis is presented, via computer‐aided‐design (CAD) models and full‐wave electrodynamic simulations, which aims at systematically analyzing the influence of structural changes on the plasmonic circular dichroism (CD) spectra of rod‐shaped gold NPs comprising helical indentations on achiral nanorod cores. From this analysis, interesting patterns in the plasmon‐mediated resonant behavior are identified and cause–effect relationships are drawn that may serve as a go‐to recipe for the understanding and fabrication of these NPs and their applications, such as spectroscopic (bio)detection including CD spectral shifts and surface‐enhanced Raman optical activity. [ABSTRACT FROM AUTHOR]

An essential element in the synthesis of nanomaterials based on gold nanoparticles comprises the control over parameters such as size, shape and composition, due to their strong influence on the properties of the particles. However, it is the coating material which often plays the primary role in tuning the size, morphology, and even plasmon resonance wavelength or mode multiplicity, as well as colloidal stability and functional versatility, ultimately determining the physical, chemical, optical, electronic and catalytic properties of the nanoparticles. Therefore, it is utterly important to select the adequate wet chemistry synthetic approach with the most suitable coating material for the preparation of gold nanoparticles with the desired requirements. Within this context, this review is focused on describing various types of organic and inorganic coating materials for gold nanoparticles that may notably affect their optical properties by either directly influencing the synthesis procedure or by changing their chemical and physical properties upon post-synthetic modifications, such that they exhibit novel and useful optical properties.

Controlled assembly of gold nanoparticles (AuNPs) into clusters is a promising avenue for the development of sensitive bioimaging and diagnostic tools based on surface‐enhanced Raman scattering (SERS). However, several challenges, such as biocompatibility or colloidal and structural stability in biological environments, remain before AuNPs can be used as a tool for in vivo bioimaging. A versatile strategy for the preparation of colloidally stable and biocompatible AuNP clusters (AuNPCs) is introduced with high SERS signals, which are used as SERS contrast bioimaging agents (SERS tags). By tuning the ligand ratio of Raman reporter molecules to stabilizing polymer on the surface of each AuNP, aggregation can be carefully controlled. The resulting AuNPCs exhibit redshifted surface plasmon resonances in the near‐infrared (NIR) region, as well as distinct electromagnetic hotspots that give rise to SERS analytical enhancement factors above 104, compared to non‐clustered spherical AuNPs. Thanks to the protective polymer shell, high levels of cellular uptake with low cytotoxicity are observed, allowing 3D SERS mapping of cells with sufficiently high spatial resolution to detect AuNPCs within intracellular organelles. [ABSTRACT FROM AUTHOR]

Thin films sustaining plasmonic circular dichroism (PCD) have acquired high scientific relevance and a great potential for applications. While most efforts in PCD thin film structures focus on lithographically fabricated static metasurfaces, the bottom‐up fabrication of active chiral plasmonic films constitutes an alternative approach. Herein, the preparation of PCD thin films by melting and freezing a mixture of liquid crystal (LC), a chiral dopant, and gold nanoparticles (Au NPs), serving as helical matrix, symmetry breaking inducer, and plasmonic component, respectively is reported. UV–vis and circular dichroism spectroscopies, as well as theoretical modeling are used to disclose the interactions among thin film components, toward maximizing the PCD dissymmetry factor (g‐factor). Variation of substrate temperature affords reversible off/on switching of the chiroptical response. The soft nature of LC matrix enables patterning of the films via a thermal nanoimprinting method, using a polydimethylsiloxane mold for transfer‐printing onto a flexible substrate, leading to stretchable PCD films. The PCD wavelengths can be readily tuned by varying the geometry of the Au NPs. This work provides an efficient technique to produce PCD thin films with active plasmonic properties and mechanical tunability. [ABSTRACT FROM AUTHOR]

Lattice plasmons, i.e., diffractively coupled localized surface plasmon resonances, occur in long‐range ordered plasmonic nanostructures such as 1D and 2D periodic lattices. Such far‐field coupled resonances can be employed for ultrasensitive surface‐enhanced Raman spectroscopy (SERS), provided they are spectrally matched to the excitation wavelength. The spectral positions of lattice plasmon modes critically depend on the lattice period and uniformity, owing to their pronounced sensitivity to structural disorder. We report the fabrication of superlattices by templated self‐assembly of gold nanoparticles on a flexible support, with tunable lattice‐plasmon resonances by means of macroscopic strain. We demonstrate that the highest SERS performance is achieved by matching the lattice plasmon mode to the excitation wavelength, by post‐assembly fine‐tuning of long‐range structural parameters. Both asymmetric and symmetric lattice deformations can be used to adapt a single lattice structure to both red‐shifted and blue‐shifted excitation lines, as exemplified by lattice expansion and contraction, respectively. This proof‐of‐principle study represents a basis for alternative designs of adaptive functional nanostructures with mechanically tunable lattice resonances using strain as a macroscopic control parameter. [ABSTRACT FROM AUTHOR]

Raman spectroscopy is becoming increasingly popular as an in vitro bioimaging technique, when coupled with plasmonic substrates such as gold nanoparticles (AuNPs). Plasmonic AuNPs not only display excellent biocompatibility but can also induce the surface‐enhanced Raman scattering (SERS) effect, which can be exploited for cell labeling, as an interesting alternative to fluorescence‐based techniques. SERS bioimaging requires the use of so‐called SERS tags or SERS‐encoded AuNPs. A remaining difficulty toward the general implementation of this method is the difficulty to correlate the SERS signal (spectral intensity) with the number of SERS tags. Therefore, a general correlation method, suitable for arbitrary AuNP morphologies and Raman‐active molecules (Raman reporters or RaRs), should largely improve the quantitative character of SERS as an imaging technique. We propose a protocol, with an associated app (SERSTEM), which enables the user to determine the average SERS intensity per nanoparticle from transmission electron microscopy (TEM) and SERS data. As a proof of concept, we demonstrated the method for Au nanostars and nanorods, carrying four different RaRs, and implemented the SERSTEM app, which is publicly available from an open‐source platform. [ABSTRACT FROM AUTHOR]

3D printing strategies have acquired great relevance toward the design of 3D scaffolds with precise macroporous structures, for supported mammalian cell growth. Despite advances in 3D model designs, there is still a shortage of detection tools to precisely monitor in situ cell behavior in 3D, thereby allowing a better understanding of the progression of diseases or to test the efficacy of drugs in a more realistic microenvironment. Even if the number of available inks has exponentially increased, they do not necessarily offer the required functionalities to be used as internal sensors. Herein the potential of surface‐enhanced Raman scattering (SERS) spectroscopy for the detection of biorelevant analytes within a plasmonic hydrogel‐based, 3D‐printed scaffold is demonstrated. Such SERS‐active scaffolds allow for the 3D detection of model molecules, such as 4‐mercaptobenzoic acid. Flexibility in the choice of plasmonic nanoparticles is demonstrated through the use of gold nanoparticles with different morphologies, gold nanorods showing the best balance between SERS enhancement and scaffold transparency. Detection of the biomarker adenosine is also demonstrated as a proof‐of‐concept toward the use of these plasmonic scaffolds for SERS sensing of cell‐secreted molecules over extended periods of time. [ABSTRACT FROM AUTHOR]

Colloidal anisotropic gold nanocrystals play a central role in the field of plasmonics owing to their tunable optical activity across a wide spectral range. However, achieving sufficient optical quality for practical implementation requires advanced synthetic protocols yielding gold nanocrystals with the desired morphology and plasmonic properties. This Minireview focuses on a fundamental step during the growth of anisotropic nanocrystals, namely symmetry breaking. In connection with thermodynamic and kinetic control of nanocrystal growth, we discuss the complex interplay between the role of seed morphology and that of surfactants, shape‐directing additives and reducing agents. We revisit some iconic syntheses of anisotropic gold nanoparticles, including nanorods, nanotriangles, and nanobipyramids. Finally, we analyze the use of co‐surfactants as an emerging strategy to disconnect the symmetry breaking event from the anisotropic growth process, overcoming current limitations in the synthesis of anisotropic gold nanocrystals. [ABSTRACT FROM AUTHOR]

Abstract: The presence of iodide ions during the growth of gold nanorods strongly affects the shape of the final products, which is proposed to be due to selective iodide adsorption on certain crystallographic facets. Therefore, a detailed structural and morphological characterization of the starting rods is crucial toward understanding this effect. Electron tomography is used to determine the crystallographic indices of the lateral facets of gold nanorods, as well as those present at the tips. Based on this information, density functional theory calculations are used to determine the surface and interface energies of the observed facets and provide insight into the relationship between the amount of iodide ions in the growth solution and the final morphology of anisotropic gold nanoparticles. [ABSTRACT FROM AUTHOR]

Abstract: Polymers and nanoparticles can be combined into different materials with applications in various fields like catalysis, biotechnology, or drug delivery, to cite just a few. Colloidal composites may vary significantly, ranging from a single nanoparticle stabilized by a polymer shell through a polymeric carrier decorated with hundreds of particles. We review here composite colloids comprising gold nanoparticles, with an emphasis in systems with potential application in surface enhanced Raman scattering (SERS). The focus is on selected strategies for synthesis and functionalization, such as: encapsulation of gold nanoparticles by amphiphilic polymers, polymeric matrices as nanoparticle carriers and smart polymer based composites. We stress the benefits derived from the combination of polymers and metal particles toward SERS, such as chemical and colloidal stabilization in complex environments, and collective optical effects through hot spot generation for optimized SERS enhancement or improved imaging tags. [ABSTRACT FROM AUTHOR]

Abstract: A limiting factor of solvent‐induced nanoparticle self‐assembly is the need for constant sample dilution in assembly/disassembly cycles. Changes in the nanoparticle concentration alter the kinetics of the subsequent assembly process, limiting optical signal recovery. Herein, we show that upon confining hydrophobic nanoparticles in permeable silica nanocapsules, the number of nanoparticles participating in cyclic aggregation remains constant despite bulk changes in solution, leading to highly reproducible plasmon band shifts at different solvent compositions. [ABSTRACT FROM AUTHOR]

Oxidative etching was used to produce gold seeds of different sizes and crystal habits. Following detailed characterization, the seeds were grown under different conditions. Our results bring new insights toward understanding the effect of size and crystallinity on the growth of anisotropic particles, whilst identifying guidelines for the optimisation of new synthetic protocols of predesigned seeds. [ABSTRACT FROM AUTHOR]

The incorporation of gold nanoparticles in biodegradable polymeric nanostructures with controlled shape and size is of interest toward different applications in nanomedicine. Properties of the polymer such as drug loading and antibody functionalization can be combined with the plasmonic properties of gold nanoparticles, to yield advanced hybrid materials. This study presents a new way to synthesize multicompartmental microgels, fibers, or cylinders, with embedded anisotropic gold nanoparticles. Gold nanoparticles dispersed in an organic solvent can be embedded within the poly(lactic- co-glycolic acid) (PLGA) matrix of polymeric microstructures, when prepared via electrohydrodynamic co-jetting. Prior functionalization of the plasmonic nanoparticles with Raman active molecules allows for imaging of the nanocomposites by surface-enhanced Raman scattering (SERS) microscopy, thereby revealing nanoparticle distribution and photostability. These exceptionally stable hybrid materials, when used in combination with 3D SERS microscopy, offer new opportunities for bioimaging, in particular when long-term monitoring is required. [ABSTRACT FROM AUTHOR]

A novel composite material based on metal nanoparticles (NP) and mesoporous thin films (MPTF), more specifically bilayers of MPTF with a submonolayer of NP deposited between them is introduced. By controlling the deposition conditions, it is possible to build a variety of multilayers including MPTF with different compositions (TiO2, SiO2) and pore sizes (templated with cetyltrimethylammonium bromide, F127, Brij 58) and NP of different characteristics and sizes (Au spheres, Au decorated SiO2). By means of 2D small angle X-ray scattering, transmission electron microscopy, UV-vis spectroscopy, and ellipsometric porosimetry it is demonstrated that NP and MPTF retain their structure and properties within the new material. Importantly, the NP remain accessible for further reactions, a feature that is established by Au overgrowth. Interestingly, the extent of Au growth depends on the mesoporous size and the position of the MPTF with respect to the NP. This new architecture allows the NP to be in direct contact with two different chemical environments and, as a consequence, opens up the possibility to control identity and size of the molecules that can reach them. Thus, the obtained materials may be applied in the development of highly specific sensors and catalysts, in which spatial position plays a fundamental role. [ABSTRACT FROM AUTHOR]

The cell possesses the remarkable ability to perceive and process chemical and physical stimuli, which in turn modulate cellular behavior. One of the great wonders about nanotechnology is that it enables the fabrication of tools on the same length scale as biomolecules, thereby providing us with a unique handle for characterizing and controlling basic cellular processes. Owing to their tunable size and shape dependent physical properties, biocompatibility and facile surface modification, gold nanoparticles become potentially powerful tools to probe fundamental aspects of cell biology. Consequently, innovative approaches based on gold nanoparticles are under development toward the manipulation of cell function and improvement of techniques currently used in biomedicine and biotechnology. In this review we provide an overview of recent applications based on gold nanoparticles and nanostructured materials for the modulation of cellular activity and behavior, mediated by their interactions with cell surface receptors. [ABSTRACT FROM AUTHOR]

The combination of cell microenvironment control and real-time monitoring of cell signaling events can provide key biological information. Through precise multipatterning of gold nanoparticles (GNPs) around cells, sensing and actuating elements can be introduced in the cells' microenviroment, providing a powerful substrate for cell studies. In this work, a combination of techniques are implemented to engineer complex substrates for cell studies. Alternating GNPs and bioactive areas are created with micrometer separation by means of a combination of vacumm soft-lithography of GNPs and protein microcontract printing. Instead of conventional microfluidics that need syringe pumps to flow liquid in the microchannels, degas driven flow is used to fill dead-end channels with GNP solutions, rendering the fabrication process straightforward and accessible. This new combined technique is called Printing and Vacuum lithography (PnV lithography). By using different GNPs with various organic coating ligands, different macroscale patterns are obtained, such as wires, supercrystals, and uniformly spread nanoparticle layers that can find different applications depending on the need of the user. The application of the system is tested to pattern a range of mammalian cell lines and obtain readouts on cell viability, cell morphology, and the presence of cell adhesive proteins. [ABSTRACT FROM AUTHOR]

Understanding protein amyloidogenesis is an important topic in protein science, fueled by the role of amyloid aggregates, especially oligomers, in the etiology of a number of devastating human degenerative diseases. However, the mechanisms that determine the formation of amyloid oligomers remain elusive due to the high complexity of the amyloidogenesis process. For instance, gold nanoparticles promote or inhibit amyloid fibrillation. We have functionalized gold nanorods with a metal-chelating group to selectively immobilize soluble RepA-WH1, a model synthetic bacterial prionoid, using a hexa-histidine tag (H6). H6-RepA-WH1 undergoes stable amyloid oligomerization in the presence of catalytic concentrations of anisotropic nanoparticles. Then, in a physically separated event, such oligomers promote the growth of amyloid fibers of untagged RepA-WH1. SERS spectral changes of H6-RepA-WH1 on spherical citrate-AuNP substrates provide evidence for structural modifications in the protein, which are compatible with a gradual increase in β-sheet structure, as expected in amyloid oligomerization. [ABSTRACT FROM AUTHOR]

Reliable quantification of 3D results obtained by X-ray energy-dispersive spectroscopy (XEDS) tomography is currently hampered by the presence of shadowing effects and poor spatial resolution. Here, a method is presented which overcomes these problems by synergistically combining quantified XEDS data and high angle annular dark field-scanning transmission electron microscopy tomography. As a proof of principle, the approach is applied to characterize a complex Au/Ag nanorattle obtained through a galvanic replacement reaction. However, the technique that is proposed here is widely applicable to a broad range of nanostructures. [ABSTRACT FROM AUTHOR]

Janus gold nanoparticles are of high interest because they allow directed self-assembly and display plasmonic properties. We succeeded in coating gold nanoparticles with two different polymers that form a Janus shell. The spontaneous segregation of two immiscible polymers at the surface of the nanoparticles was verified by NOESY NMR and most importantly by electron microscopy analysis in two and three dimensions. The Janus structure is additionally shown to affect the aggregation behavior of the nanoparticles. [ABSTRACT FROM AUTHOR]

Methods for efficient detachment of cells avoiding damage are required in tissue engineering and regenerative medicine. We introduce a bottom-up approach to build plasmonic substrates using micellar block copolymer nanolithography to generate a 2D array of Au seeds, followed by chemical growth leading to anisotropic nanoparticles. The resulting plasmonic substrates show a broad plasmon band covering a wide part of the visible and near-infrared (NIR) spectral ranges. Both human and murine cells were successfully grown on the substrates. A simple functionalization step of the plasmonic substrates with the cyclic arginylglycylaspartic acid (c-RGD) peptide allowed us to tune the morphology of integrin-rich human umbilical vein endothelial cells (HUVEC). Subsequent irradiation with a NIR laser led to highly efficient detachment of the cells with cell viability confirmed using the MTT assay. We thus propose the use of such plasmonic substrates for cell growth and controlled detachment using remote near-IR irradiation, as a general method for cell culture in biomedical applications. [ABSTRACT FROM AUTHOR]

Simple wet chemistry has been applied to control the vertical growth of gold nanowires on a glass substrate. As a consequence, the longitudinal localized surface plasmon band position can be tuned from 656 to 1477 nm in a few minutes by simply controlling the growth rate and time. This allowed us to select the optimum conditions for maximum electromagnetic enhancement and performance in surface enhanced Raman scattering (SERS) detection. SERS measurements confirmed the uniform and reproducible distribution of the nanowires on the substrate, with the subsequent high reproducibility of hot spot formation. Detection of malachite green in water and of 1-naphthalenethiol from the gas phase are demonstrated as proof-of-concept applications of these three-dimensional SERS substrates. [ABSTRACT FROM AUTHOR]

We present a simple procedure for the synthesis of quasi-spherical Au nanoparticles in a wide size range mediated by macrocyclic host molecules, ammonium pillar[5]arene (AP[5]A). The strategy is based on a seeded growth process in which the water-soluble pillar[5]arene undergoes complexation of the Au salt through the ammonium groups, thereby avoiding Au nucleation, while acting as a stabilizer. The presence of the pillar[5]arene onto the Au nanoparticle particle surface is demonstrated by surface-enhanced Raman scattering (SERS) spectroscopy, and the most probable conformation of the molecule when adsorbed on the Au nanoparticles surface is suggested on the basis of theoretical calculations. In addition, we analyze the host-guest interactions of the AP[5]A with 2-naphthoic acid (2NA) by using 1H NMR spectroscopy and the results are compared with theoretical calculations. Finally, the promising synergetic effects of combining supramolecular chemistry and metal nanoparticles are demonstrated through SERS detection in water of 2NA and a polycyclic aromatic hydrocarbon, pyrene (PYR). [ABSTRACT FROM AUTHOR]

A new type of gold nanoparticle—called “spiky nanodumbbells”—is introduced. These particles combine the anisotropy of nanorods with sharp nanoscale features of nanostars, which are important for SERS applications. Both the morphology and the optical response of the particles are characterized in detail, and the experimental results are compared with FDTD simulations, showing good agreement. [ABSTRACT FROM AUTHOR]

Photothermal anticancer therapy based on plasmonic nanoparticles is proposed to enhance treatment efficacy while mitigating unintended side effects. However, most studies blindly rely on the accumulation of nanoparticles at the tumor site, which may result in inefficient treatment. In this study, the aim is to evaluate relevant parameters to improve plasmonic photothermal therapy. Gold nanorods (AuNRs) with an optimized aspect ratio and either amino or carboxylic acid surface functionalization are selected as photothermal agents. AuNR biocompatibility and photothermal activity in 2D and 3D human MDA‐MB‐231 triple‐negative breast cancer cell models, evaluating localized hyperthermal cell death upon irradiation with resonant near‐infrared (NIR) light, are analyzed first. To ensure reliable tracking of biodistribution in vivo, AuNRs are labeled with the positron emitter copper‐64 (64Cu), and their distribution in a murine MDA‐MB‐231 tumor model is studied via positron emission tomography (PET) imaging. PET images reveal enhanced tumor accumulation of carboxylic acid‐functionalized AuNRs compared to amino‐functionalized AuNRs post‐intravenous administration. Relatively low NIR laser power densities (0.5 W cm−2) are used for controlled heating – keeping local temperature below 50 °C – upon irradiation of intravenously and intratumorally administered AuNRs. As a result, tumor growth is significantly decelerated, even 9 days after application of photothermal therapy. [ABSTRACT FROM AUTHOR]

The concept of protecting groups, widely used in organic chemistry, has been applied for the synthesis of Au-silica Janus stars, in which gold branches protrude from one half of Au-silica Janus spheres. This configuration opens up new possibilities to apply the plasmonic properties of gold nanostars, as well as a variety of chemical functionalizations on the silica component. [ABSTRACT FROM AUTHOR]

In this viewpoint we discuss the early contribution from Brust et al., which is not only the most cited paper ever published in Chem. Commun., but also a landmark in gold nanoparticle research and in nanotechnology. We provide here an overview of its major contributions and how the field has evolved since its publication in 1994. [ABSTRACT FROM AUTHOR]

The implementation of plasmonic nanoparticles in vivo remains hindered by important limitations such as biocompatibility, solubility in biological fluids, and physiological stability. A general and versatile protocol is presented, based on seeded emulsion polymerization, for the controlled encapsulation of gold and silver nanoparticles. This procedure enables the encapsulation of single nanoparticles as well as nanoparticle clusters inside a protecting polymer shell. Specifically, the efficient coating of nanoparticles of both metals is demonstrated, with final dimensions ranging between 50 and 200 nm, i.e., sizes of interest for bio‐applications. Such hybrid nanocomposites display extraordinary stability in high ionic strength and oxidizing environments, along with high cellular uptake, and low cytotoxicity. Overall, the prepared nanostructures are promising candidates for plasmonic applications under biologically relevant conditions. Seeded emulsion polymerization is applied to the encapsulation of gold and silver nanoparticles, either as single nanoparticles or clustered, inside a protecting polymer shell. The hybrid nanocomposites are stable in high ionic strength and oxidizing environments, while displaying high cellular uptake and low cytotoxicity. These nanostructures are promising candidates for plasmonic applications under biologically relevant conditions. [ABSTRACT FROM AUTHOR]

Learning from classics: Crystal growth is a complex process, and there are multiple paths for going from dissolved ions to solid crystals. Highlighted herein is the application of traditional chemistry concepts to new ways for increasing the complexity of nanocrystals while maintaining a high degree of symmetry. [ABSTRACT FROM AUTHOR]

No Abstract [ABSTRACT FROM AUTHOR]

Plasmonic resonances of novel gold spiky nanodumbbells can be probed by both optical and electron microscopies, allowing for a complete characterization of their optical activity. The image illustrates an exemplary plasmon map of such a nanodumbbell, obtained from the integrated EELS signal, illustrating the longitudinal nature of the induced plasmonic mode. [ABSTRACT FROM AUTHOR]

The vast majority of the outstanding applications of metal nanoparticles (NPs) developed during the last two decades have arisen from their unique optical properties. Within this context, rational synthesis and assembly of gold NPs have been the main research focus, aiming at the design of nanoplasmonic devices with tailored optical functionalities. The progress made in this field is thus to be ascribed to the understanding of the origin of the interaction between light and such gold nanostructures, the dynamics of which have been thoroughly investigated with significant contributions from short and ultrashort pulse laser technologies We focus this Account on the potential of pulse lasers to provide new fundamental insights into the electron dynamics involved in the interaction of light with the free conduction electrons of Au NPs, that is, localized surface plasmon resonances (LSPRs). The excitation of LSPRs with a femtosecond pulse laser is followed by thermalization of the Au NP electrons and the subsequent relaxation of the nanocrystal lattice and the surrounding environment, which generally results in surface melting. By contrast, nanosecond irradiation usually induces AuNP fragmentation and uncontrolled melting due to overlapping excitation and relaxation phenomena. These concepts have been exploited toward the preparation of highly monodisperse gold nanospheres via pulse laser irradiation of polyhedral nanocrystal colloids, or in the fabrication of nanostructures with "written-in" optical properties. The applicability of pulsed coherent light has been extended toward the direct synthesis and manipulation of Au NPs. Through ablation of a gold target in a liquid with pulse lasers, spherical Au NPs can be synthesized with no need of stabilizing ligands, which is a great advantage in terms of reducing toxicity, rendering these NPs particularly suitable for medical applications. In addition, femtosecond laser irradiation has been proven a unique tool for the controlled welding of plasmonic gold nanostructures by electromagnetic field enhancement at the hot spots of assembled Au NPs. The combination of such nanostructures with pulse lasers promises significant chemical and biochemical advances, including the structural determination of organic reaction intermediates, the investigation of phase transitions in inorganic nanomaterials at mild reaction conditions, or the efficient photothermal destruction of cancer cells avoiding damage of surrounding tissue. [ABSTRACT FROM AUTHOR]

We describe an approach to regulate the cellular uptake of small gold nanoparticles using supramolecular chemistry. The strategy relies on the functionalization of AuNPs with negatively charged pyranines, which largely hamper their penetration in cells. Cellular uptake can be activated in situ through the addition of cationic covalent cages that specifically recognize the fluorescent pyranine dyes and counterbalance the negative charges. The high selectivity and reversibility of the host-guest recognition activates cellular uptake, even in proteinrich biological media, as well as its regulation by rational addition of either cage or pyranine. [ABSTRACT FROM AUTHOR]

Fano resonances are central features in the responses of many systems including atoms, molecules, and nanomaterials. They arise as a consequence of interferences between two channels, most frequently associated with two system modes. In plasmonic materials, Fano interferences between optical modes have been shown, experimentally and theoretically, to induce narrow features in their scattering spectra. By investigating individual silver-gold heterodimers, we first experimentally demonstrate that Fano interference is also a key effect in the optical absorption of plasmonic nano-objects, in agreement with theoretical predictions. Conversely to previously investigated systems, the two interacting modes at the origin of absorptive Fano effect are mostly localized on either one or the other dimer component. Experimental results were obtained by selectively monitoring the optical absorption of one dimer component using a two-color nonlinear time-resolved technique. This also opens the way to full optical far-field noncontact investigations of charge or energy exchanges between nano-objects with a spatial resolution much smaller than the optical wavelength. [ABSTRACT FROM AUTHOR]

Labeled nanoparticles have attracted much interest toward applications in bioimaging and diagnostics. In particular, surface enhanced Raman scattering (SERS) nanotags have been demonstrated to be excellent candidates for multiplexed imaging and biological detection. We propose an alternative, effective method to easily prepare gold nanostars exhibiting plasmon bands in the near-infrared range, encoded with Raman reporter molecules, concomitantly acting as capping agents which are then protected with an amphiphilic polymer. The resulting nanotags are noncytotoxic and display long-term stability against aggregation and reporter leakage, while showing reproducible SERS signals suitable for multiplexing. These tags were used to distinguish five different types of breast cancer cells by imaging of a quintuple cell coculture. Time-lapse SERS imaging of the coculture was additionally performed, demonstrating the applicability of these nanotags for cell tracing over time scales above 24 h. [ABSTRACT FROM AUTHOR]

Clay minerals are abundant natural materials arising in the presence of water and are composed of small particles of different sizes and shapes. The interlamellar space between layered silicate clays can also be used to host a variety of different organic and inorganic guest molecules or particles. Recent studies of clay-metal hybrids formed by impregnation of nanoparticles into the interlayer spaces of the clays have not demonstrated the ability for templated growth following the shape of the particles. Following this line of interest, a method for the synthesis of gold nanoparticles on the synthetic layered silicate clay laponite was developed. This approach can be used to make metal-clay nanoparticles with a variety of morphologies while retaining the molecular adsorption properties of the clay. The surface enhanced Raman scattering enhancement of these particles was also found to be greater than that obtained from other metal nanoparticles of a similar morphology, likely due to increased dye adsorption by the presence of the clay. The hybrid particles presented herein will contribute to further study of plasmonic sensing, catalysis, dye aggregation, and novel composite materials. [ABSTRACT FROM AUTHOR]

Directed assembly of gold nanorods through the use of dithiolated molecular linkers is one of the most efficient methodologies for the morphologically controlled tip-to-tip assembly of this type of anisotropic nanocrystals. However, in a direct analogy to molecular polymerization synthesis, this process is characterized by difficulties in chain-growth control over nanoparticle oligomers. In particular, it is nearly impossible to favor the formation of one type of oligomer, making the methodology hard to use for actual applications in nanoplasmonics. We propose here a light-controlled synthetic procedure that allows obtaining selected plasmonic oligomers in high yield and with reaction times in the scale of minutes by irradiation with low fluence near-infrared (NIR) femtosecond laser pulses. Selective inhibition of the formation of gold nanorod n-mers (trimers) with a longitudinal localized surface plasmon in resonance with a 800 nm Ti:sapphire laser, allowed efficient trapping of the (n - 1)-mers (dimers) by hot spot mediated photothermal decomposition of the interparticle molecular linkers. Laser irradiation at higher energies produced near-field enhancement at the interparticle gaps, which is large enough to melt gold nanorod tips, offering a new pathway toward tip-to-tip welding of gold nanorod oligomers with a plasmonic response at the NIR. Thorough optical and electron microscopy characterization indicates that plasmonic oligomers can be selectively trapped and welded, which has been analyzed in terms of a model that predicts with reasonable accuracy the relative concentrations of the main plasmonic species. [ABSTRACT FROM AUTHOR]

Surface and interface control is fundamentally important for crystal growth engineering, catalysis, surface-enhanced spectroscopies, and self-assembly, among other processes and applications. Understanding the role of ligands in regulating surface properties of plasmonic metal nanocrystals during growth has received considerable attention. However, the underlying mechanisms and the diverse functionalities of ligands are yet to be fully addressed. In this contribution, we report a systematic study of ligand-mediated interface control in seeded growth of gold nanocrystals, leading to diverse and exotic nanostructures with an improved surface enhanced Raman scattering (SERS) activity. Three dimensional transmission electron microscopy revealed an intriguing gold shell growth process mediated by the bifunctional ligand 1,4-benzenedithiol (BDT), which leads to a unique crystal growth mechanism as compared to other ligands, and subsequently to the concept of interfacial energy control mechanism. Volmer-Weber growth mode was proposed to be responsible for BDT-mediated seeded growth, favoring the strongest interfacial energy and generating an asymmetric island growth pathway with internal crevices/gaps. This additionally favors incorporation of BDT at the plasmonic nanogaps, thereby generating strong SERS activity with a maximum efficiency for a core-semishell configuration obtained along seeded growth. Numerical modeling was used to explain this observation. Interestingly, the same strategy can be used to engineer the structural diversity of this system, by using gold nanoparticle seeds with various sizes and shapes, and varying the [Au3+]/[Au0] ratio. This rendered a series of diverse and exotic plasmonic nanohybrids such as semishell-coated gold nanorods, with embedded Raman-active tags and Janus surface with distinct surface functionalities. These would greatly enrich the plasmonic nanostructure toolbox for various studies and applications such as anisotropic nanocrystal engineering, SERS, and high-resolution Raman bioimaging or nanoantenna devices. [ABSTRACT FROM AUTHOR]

The library of plasmonic nanosystems keeps expanding with novel structures with the potential to provide new solutions to old problems in science and technology. We report the synthesis of a novel plasmonic system based on the growth of gold nanowires radially branching from the surface of silica particles. The nanowires length could be controlled by tuning the molar ratio between metal salt and surface-grafted seeds. Electron microscopy characterization revealed that the obtained one-dimensional nanoparticles are polycrystalline but uniformly distributed on the spherical template. The length of the nanowires in turn determines the optical response of the metallodielectric particles, so that longer wires display red-shifted longitudinal plasmon bands. Accurate theoretical modeling of these complex objects revealed that the densely organized nanowires display intrinsically coupled plasmon modes that can be selectively decoupled upon detachment of the nanowires from the surface of the colloidal silica template. [ABSTRACT FROM AUTHOR]

Poly(ethylene glycol) (PEG) has become the gold standard for stabilization of plasmonic nanoparticles (NPs) in biofluids, because it prevents aggregation while minimizing unspecific interactions with proteins. Application of Au NPs in biological environments requires the use of ligands that can target selected receptors, even in the presence of protein-rich media. We demonstrate here the stabilizing effect of low-molecular-weight glycans on both spherical and rod-like plasmonic NPs under physiological conditions, as bench-marked against the well-established PEG ligands. Glycan-coated NPs are resistant to adsorption of proteins from serum-containing media and avoid phagocytosis by macrophage-like cells, but retain selectivity toward carbohydrate-binding proteins in protein-rich biological media. These results open the way toward the design of efficient therapeutic/diagnostic glycan-decorated plasmonic nanotools for specific biological applications. [ABSTRACT FROM AUTHOR]

Plasmonmodes of the exact same individual gold nanoprisms are investigatedthrough combined nanometer-resolved electron energy-loss spectroscopy(EELS) and cathodoluminescence (CL) measurements. We show that CLonly probes the radiative modes, in contrast to EELS, which additionallyreveals dark modes. The combination of both techniques on the sameparticles thus provides complementary information and also demonstratesthat although the radiative modes give rise to very similar spatialdistributions when probed by EELS or CL, their resonant energies appearto be different. We trace this phenomenon back to plasmon dissipation,which affects in different ways the plasmon signatures probed by thesetechniques. Our experiments are in agreement with electromagneticnumerical simulations and can be further interpreted within the frameworkof a quasistatic analytical model. We therefore demonstrate that CLand EELS are closely related to optical scattering and extinction,respectively, with the addition of nanometer spatial resolution. [ABSTRACT FROM AUTHOR]

Quantum dot based thermometry, incombination with double beamconfocal microscopy, was used to investigate the absorption/heatingefficiency of gold nanoparticles with different morphologies (nanorods,nanocages, nanoshells, and nanostars), all of them with an intenselocalized surface plasmon resonance within the first biological window,ataround 808 nm. The heating efficiency was found to be strongly dependenton the geometry of the nanostructure, with the largest values foundfor gold nanorods and long-edge gold nanostars, both of them withheating efficiencies close to 100%. Gold nanorods and nanocages werefound to have the largest absorption cross section per unit mass amongall the studied geometries, emerging as optimum photothermal agentswith minimum metal loading for biosystems. [ABSTRACT FROM AUTHOR]