Three-dimensional plasmonic nanoclusters

Directional Self-Assembly of Programmable Atom-like Nanoparticles into Colloidal Molecules

Colloidal molecules, novel nanoparticle clusters with molecular-like structures, can enhance material performance and broaden applications in nanotechnology and materials science. However, constructing them with a precise, controllable architecture remains challenging. Inspired by the concepts of chemical reaction, we theoretically design a novel type of nanoparticles bifunctionalized by DNA strands and polymer chains and propose a stepwise strategy to hierarchically program the assembly of bifunctionalized nanoparticles into well-defined colloidal molecules by virtue of coarse-grained molecular dynamics simulations. This method leverages the synergistic effects of polymers and DNA to create programmable atom-like nanoparticles with various valence domains. By carefully designing strands, these nanoparticles are programmed to coassemble into various colloidal molecules with distinct symmetries and coordination numbers, which can be finely tuned by the molecular design of nanoparticles as well as the composition design of the coassembly system. Our strategy provides a novel protocol for the controlled coassembly of nanoparticles into customized colloidal molecules, expanding nanomaterial manufacturing techniques.

Emerging trends in SERS-based veterinary drug detection: multifunctional substrates and intelligent data approaches

Veterinary drug residues in poultry and livestock products present persistent challenges to food safety, necessitating precise and efficient detection methods. Surface-enhanced Raman scattering (SERS) has been identified as a powerful tool for veterinary drug residue analysis due to its high sensitivity and specificity. However, the development of reliable SERS substrates and the interpretation of complex spectral data remain significant obstacles. This review summarizes the development process of SERS substrates, categorizing them into metal-based, rigid, and flexible substrates, and highlighting the emerging trend of multifunctional substrates. The diverse application scenarios and detection requirements for these substrates are also discussed, with a focus on their use in veterinary drug detection. Furthermore, the integration of deep learning techniques into SERS-based detection is explored, including substrate structure design optimization, optical property prediction, spectral preprocessing, and both qualitative and quantitative spectral analyses. Finally, key limitations are briefly outlined, such as challenges in selecting reporter molecules, data imbalance, and computational demands. Future trends and directions for improving SERS-based veterinary drug detection are proposed.

Self-Assembly of Colloidal Dumbbell Isomers and Plasmonic Properties for Optical Metamaterials

In this study, we explore the self-assembly of various colloidal symmetric dumbbell (DB) isomers, including dipole Janus, cis-Janus, trans-Janus, apolar-inward and polar-inward perpendicular Janus, and alternating perpendicular Janus DBs. Using dissipative particle dynamics (DPD) simulations under conditions mimicking experimental setups, we investigate cluster formation driven by emulsion droplet evaporation. Our findings reveal a diverse set of cluster structures, which are in good agreement with experimental and simulation results reported in the literature while also predicting the formation of novel cluster configurations. These structures, characterized by well-defined and predictable patterns, are potentially applicable to creating colloidal molecules and crystals. Furthermore, we examine the dynamics of cluster formation to gain insight into the mechanisms guiding the self-assembly of these diverse colloidal DBs. The study highlights the impact of particle isomerism on the resulting assembly structures. We further select a set of typical nanoclusters obtained, including a tetrahedral cluster, which is the simplest, to study its plasmonic properties. Our findings indicate that increasing the nanoparticle (NP) radius or decreasing the gap between NPs leads to a red shift in the plasmonic resonance wavelength and enhances the resonance strength. We identify critical parameter regions where the electric-dipole and magnetic-dipole resonances can be engineered to achieve negative dielectric permittivity and magnetic permeability, which are essential for developing negative-index metamaterials.

Influence of Solvophobicity of Biphenol-Derived Small Surface Ligands on the Formation of Size-Controllable Gold Nanoparticle Vesicles

The self-assembly of gold nanoparticles (GNPs) into gold nanoparticle vesicles (GNVs) has been a topic of significant interest in recent years. However, the formation mechanism of GNVs is still not fully understood. In this article, we report that the new oligo(ethylene glycol)-terminated biphenol ligands (OBLs) show different solubility in tetrahydrofuran (THF) depending upon the number of terminal ethylene glycol units, resulting in a differential solvophobicity. The fluorine-free OBLs have the ability to self-assemble with GNPs into GNVs driven by the solvophobic feature of the ligands. The size of GNVs can be precisely controlled by tuning the interparticle attraction through changes in the unit number of terminal ethylene glycol or the water content in THF. Time-dependent studies revealed that the vesicle formation process consists of two stages: the rapid generation of vesicles, followed by their fusion to form thermodynamically stable GNVs with a saturated size. These two rapid processes are primarily influenced by the pronounced solvophobic attraction exerted by the surface ligands.

Nanogap engineering of 3D nanoraspberries into 2D plasmonic nanoclusters toward improved SERS performance

3D raspberry-like core/satellite nanostructures were prepared by controlled surface functionalization of silica spheres using crosslinked poly(4-vinylpyridine) (P4VP) chains with known binding affinity for gold nanoparticles (AuNPs). The 3D SiO2-g-P(4VP-co-DVB)/AuNP nanoraspberries can be further transformed into 2D plasmonic nanoclusters by etching the silica core with hydrofluoric acid (HF). After the transformation, the interparticle distance between the AuNPs dramatically reduced from a 10 nm scale to sub 2 nm. Owing to the strong electromagnetic field generated by the plasmonic coupling between AuNPs in very close proximity, the established P(4VP-co-DVB)/AuNP nanoclusters provided strong and undisturbed Raman signals as a SERS substrate. In addition, benefiting from the stabilizing effect of the crosslinked P(4VP-co-DVB) network, the prepared SERS substrate has the advantages of good uniformity, stability and reproducibility, as well as strong SERS enhancement, endowing it with great potential for rapid and efficient SERS detection.

Photocarrier Recombination Dynamics in Highly Scattering Cu2O Nanocatalyst Clusters

Inversion analysis of transient absorption data to capture the photoexcited charge carrier population rate dynamics is a powerful technique for extracting realistic lifetimes and identifying recombination pathways. However, for highly scattering samples such as Cu2O nanoparticles (NPs) with associated dielectric Mie scattering, the scattering leads to an inaccurate measure of the excited photocarrier. This work studies methods to correct for the scattering to generalize the use of inversion analysis and provide secondary information about the nature of the scattering NPs. Scattering profiles of semitransparent disks containing Cu2O NPs with different shapes and sizes are measured to demonstrate that the inclusion of scattering in analysis reduces the photoexcited carrier density by 1 order of magnitude. It is found that the photocarrier density response is affected by shape rather than size. A Fourier transform of the scattering profiles produces a distribution of length scales within the sample characteristic of the mean separation of scatterers. This analysis reveals that NPs are forming clusters. Links are made between the scattering and carrier dynamics.

Advances in fundamentals and application of plasmon-assisted CO2 photoreduction

Artificial photosynthesis of hydrocarbons from carbon dioxide (CO2) has the potential to provide renewable fuels at the scale needed to meet global decarbonization targets. However, CO2 is a notoriously inert molecule and converting it to energy dense hydrocarbons is a complex, multistep process, which can proceed through several intermediates. Recently, the ability of plasmonic nanoparticles to steer the reaction down specific pathways and enhance both reaction rate and selectivity has garnered significant attention due to its potential for sustainable energy production and environmental mitigation. The plasmonic excitation of strong and confined optical near-fields, energetic hot carriers and localized heating can be harnessed to control or enhance chemical reaction pathways. However, despite many seminal contributions, the anticipated transformative impact of plasmonics in selective CO2 photocatalysis has yet to materialize in practical applications. This is due to the lack of a complete theoretical framework on the plasmonic action mechanisms, as well as the challenge of finding efficient materials with high scalability potential. In this review, we aim to provide a comprehensive and critical discussion on recent advancements in plasmon-enhanced CO2 photoreduction, highlighting emerging trends and challenges in this field. We delve into the fundamental principles of plasmonics, discussing the seminal works that led to ongoing debates on the reaction mechanism, and we introduce the most recent ab initio advances, which could help disentangle these effects. We then synthesize experimental advances and in situ measurements on plasmon CO2 photoreduction before concluding with our perspective and outlook on the field of plasmon-enhanced photocatalysis.

Fabrication of silver nanostructure array patterns (SNAPs) on silicon wafer for highly sensitive and reliable SERS substrates

Metal nanostructure arrays with large amounts of nano-gaps are important for surface enhanced Raman scattering applications, though the fabrications of such nanostructures are difficult due to the complex and multiple synthetic steps. In this research, we report silver nanostructure array patterns (SNAPs) on silicon wafer, which is fabricated with semiconductor manufacturing technology, Cu2O electrochemistry deposition, and Ag In-situ oxidation-reduction growth. Benefiting from the dense and uniform distribution of Ag nanowires, the fabricated SNAPs demonstrate a very strong and uniform surface-enhanced Raman scattering (SERS) effect. The efficiency of SNAPs was investigated by using rhodamine 6G (R6G) dye as an analyte molecule. The results show that the minimum detectable concentration of R6G can reach as low as 10-11 M, and the Raman signals in the random region show good signal homogeneity with a low relative standard deviation (RSD) of 4.77 %. These results indicate that the SNAPs perform a great sensitivity and uniformity as a SERS substrate. Furthermore, we used the SNAPs substrate to detect antibiotic sulfadiazine. The main peaks in sulfadiazine Raman and vibration modes assignments were obtained and the quantitative analysis model was established by principal component analysis (PCA). The detection and application results of sulfadiazine indicate that the SNAPs substrate can be applied for trace detection of antibiotics. In addition, we have cited the application of the SNAPs substrate in anti-counterfeiting labels. These practical applications demonstrate that the fabricated SNAPs can potentially provide a way to develop low-cost SERS platforms for environmental detections, biomedicine analysis, and commodities anti-counterfeiting.

Chemical and Physical Properties of Photonic Noble-Metal Nanomaterials

Colloidal noble metal nanoparticles (NPs) are composed of metal cores and organic or inorganic ligand shells. These NPs support size- and shape-dependent plasmonic resonances. They can be assembled from dispersions into artificial metamolecules which have collective plasmonic resonances originating from coupled bright and dark optical electric and magnetic modes that form depending on the size and shape of the constituent NPs and their number, arrangement, and interparticle distance. NPs can also be assembled into extended 2D and 3D metamaterials that are glassy thin films or ordered thin films or crystals, also known as superlattices and supercrystals. The metamaterials have tunable optical properties that depend on the size, shape, and composition of the NPs, and on the number of NP layers and their interparticle distance. Interestingly, strong light-matter interactions in superlattices form plasmon polaritons. Tunable interparticle distances allow designer materials with dielectric functions tailorable from that characteristic of an insulator to that of a metal, and serve as strong optical absorbers or scatterers, respectively. In combination with lithography techniques, these extended assemblies can be patterned to create subwavelength NP superstructures and form large-area 2D and 3D metamaterials that manipulate the amplitude, phase, and polarization of transmitted or reflected light.

Halogen Bonding-Driven Reversible Self-Assembly of Plasmonic Colloidal Molecules

Colloidal molecules (CMs) assembled from plasmonic nanoparticles are an emerging class of building blocks for creating plasmonic materials and devices, but precise yet reversible assembly of plasmonic CMs remains a challenge. This communication describes the reversible self-assembly of binary plasmonic nanoparticles capped with complementary copolymer ligands into different CMs via halogen bonding interactions at high yield. The coordination number of the CMs is governed by the number ratio of complementary halogen donor and acceptor groups on the interacting nanoparticles. The reversibility of the halogen bonds allows for controlling the repeated formation and disassociation of the plasmonic CMs and hence their optical properties. Furthermore, the CMs can be designed to further self-assemble into complex structures in selective solvents. The precisely engineered reversible nanostructures may find applications in sensing, catalysis, and smart optoelectronic devices.

Formation of kinetically trapped small clusters of PEGylated gold nanoparticles revealed by the combination of small-angle X-ray scattering and visible light spectroscopy

Gold nanoparticles coated with polyethylene glycol (PEG) are able to form clusters due to the collapse of the surface-grafted polymer chains when the temperature and ion concentration of the aqueous medium are increased. The chain collapse reduces the steric repulsion, leading to particle aggregation. In this work, we combine small angle X-ray scattering (SAXS) and visible light spectroscopy to elucidate the structure of the developing clusters. The structure derived from the SAXS measurements reveals a decrease in interparticle distance and drastic narrowing of its distribution in the cluster, indicating restricted particle mobility and displacement within the cluster. Surprisingly, instead of forming a large crystalline phase, the evolving clusters are composed of about a dozen particles. The experimental optical extinction spectra measured during cluster formation can be very well reproduced by optical simulations based on the SAXS-derived structural data.

Dimensional Design for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that enables specific identification of target analytes with sensitivity down to the single-molecule level by harnessing metal nanoparticles and nanostructures. Excitation of localized surface plasmon resonance of a nanostructured surface and the associated huge local electric field enhancement lie at the heart of SERS, and things will become better if strong chemical enhancement is also available simultaneously. Thus, the precise control of surface characteristics of enhancing substrates plays a key role in broadening the scope of SERS for scientific purposes and developing SERS into a routine analytical tool. In this review, the development of SERS substrates is outlined with some milestones in the nearly half-century history of SERS. In particular, these substrates are classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional substrates according to their geometric dimension. We show that, in each category of SERS substrates, design upon the geometric and composite configuration can be made to achieve an optimized enhancement factor for the Raman signal. We also show that the temporal dimension can be incorporated into SERS by applying femtosecond pulse laser technology, so that the SERS technique can be used not only to identify the chemical structure of molecules but also to uncover the ultrafast dynamics of molecular structural changes. By adopting SERS substrates with the power of four-dimensional spatiotemporal control and design, the ultimate goal of probing the single-molecule chemical structural changes in the femtosecond time scale, watching the chemical reactions in four dimensions, and visualizing the elementary reaction steps in chemistry might be realized in the near future.

Nanoparticle cluster formation mechanisms elucidated via Markov state modeling: Attraction range effects, aggregation pathways, and counterintuitive transition rates

Nanoparticle clusters are promising candidates for developing functional materials. However, it is still a challenging task to fabricate them in a predictable and controllable way, which requires investigation of the possible mechanisms underlying cluster formation at the nanoscale. By constructing Markov state models (MSMs) at the microstate level, we find that for highly dispersed particles to form a highly aggregated cluster, there are multiple coexisting pathways, which correspond to direct aggregation, or pathways that need to pass through partially aggregated, intermediate states. Varying the range of attraction between nanoparticles is found to significantly affect pathways. As the attraction range becomes narrower, compared to direct aggregation, some pathways that need to pass through partially aggregated intermediate states become more competitive. In addition, from MSMs constructed at the macrostate level, the aggregation rate is found to be counterintuitively lower with a lower free-energy barrier, which is also discussed.

Bottom-up synthesis of meta-atoms as building blocks in self-assembled metamaterials : Recent advances and perspectives

Meta-atoms interact with light in interesting ways and offer a large range of exciting properties. They exhibit optical properties inaccessible by natural atoms but their fabrication is notoriously difficult because of the precision required. In this perspective, we present the current research landscape in making meta-atoms, with a focus on the most promising self-assembly approaches and main challenges to overcome, for the development of materials with novel properties at optical frequencies.

Confinement Assembly in Polymeric Micelles Enables Nanoparticle Superstructures with Tunable Molecular-Like Geometries

The self-assembly of a small number of nanoparticles into superstructures that mimic the geometry of molecules provides an unprecedented route for creating materials with precisely defined structures and potentially programmable functionalities. Such nanoparticle clusters (NPCs), also known as colloidal molecules, have a wide range of applications due to the decisive ensemble effect. Here, a universal and straightforward strategy is developed to construct NPCs with tunable molecular-like geometries by confining the self-assembly of hydrophobic nanoparticles within micelles formed by amphiphilic copolymers. It is found that confinement assembly of both spherical and anisotropic nanoparticles can lead to NPCs, the molecular-like conformation of which is widely tunable by adjusting the ratio between copolymers and nanoparticles. Mechanistic studies reveal the formation of large-vesicle intermediates along the path toward forming NPCs. This work establishes a facile and general strategy of assembling finite nanoparticles with precisely tunable geometries without introducing any directional interactions, which can accelerate the exploration of clustered superstructures toward broad applications.

Exploiting Plasmonic Hot Spots in Au-Based Nanostructures for Sensing and Photocatalysis

ConspectusLocalized surface plasmon resonance is a unique property appearing in certain metal nanostructures, which can generate hot carriers (electrons and holes) and bring about an intense electromagnetic field localized near the surface of nanostructures. Specific locations, such as the rough surfaces and gaps in nanostructures, where a strong electromagnetic field is formed are referred to as hot spots. Hot-spot-containing plasmonic nanostructures have shown great promise in molecular sensing and plasmon-induced catalytic applications by exploiting the unique optical properties of hot spots. In this Account, we will review our recent developments in the synthesis of Au nanostructures consisting of multiple hot spots and Au-based heteronanostructures combining secondary active metals or semiconductors with Au nanostructures as promising plasmonic platforms for hot-spot-induced sensing and photocatalysis. We first provide a brief introduction to Au nanocrystals and Au nanoparticle assemblies with multiple hot spots. High-index-faceted hexoctahedral Au nanocrystals having multiple high-curvature vertices and edges are beneficial for the generation of an intense and reproducible electromagnetic field, which can enhance the performance of surface-enhanced Raman scattering-based molecular sensing. In addition, the engineering of interparticle gaps in Au nanoparticle assemblies to have a controlled size and a certain number of gaps can lead to the enhancement of plasmonic properties due to the significant amplification of the electromagnetic field at interparticle gaps. We then discuss hot-spot-containing Au-based heteronanostructures prepared by growing secondary components on the aforementioned Au nanostructures. With a combination of merit from strong plasmon energy formed by hot spots and catalytically active secondary materials, Au-based heteronanostructures have emerged as an attractive and versatile catalyst platform for various photocatalytic reactions. Through the control of key factors governing the photocatalysis of Au-based heteronanostructures, such as the coupling manner, shell thickness of secondary materials, and intimacy of contact, the plasmon energy formation of heteronanostructures and its transfer to catalytically active materials can be optimized, leading to the promotion of photocatalysis, such as photocatalytic hydrogen evolution. The rational design of Au nanostructures and Au-based heteronanostructures with multiple hot spots not only could realize enhanced sensing and photocatalysis but also could enable the understanding of the geometry-performance relationship. It is envisioned that the developed strategies can offer new opportunities for the design of various high-efficiency catalytic platforms.

Controlling the Shrinkage of 3D Hot Spot Droplets as a Microreactor for Quantitative SERS Detection of Anticancer Drugs in Serum Using a Handheld Raman Spectrometer

Quantitative measurement is one of the ultimate targets for surface-enhanced Raman spectroscopy (SERS), but it suffers from difficulties in controlling the uniformity of hot spots and placing the target molecules in the hot spot space. Here, a convenient approach of three-phase equilibrium controlling the shrinkage of three-dimensional (3D) hot spot droplets has been demonstrated for the quantitative detection of the anticancer drug 5-fluorouracil (5-FU) in serum using a handheld Raman spectrometer. Droplet shrinkage, triggered by the shaking of aqueous nanoparticle (NP) colloids with immiscible oil chloroform (CHCl₃) after the addition of negative ions and acetone, not only brings the nanoparticles in close proximity but can also act as a microreactor to enhance the spatial enrichment capability of the analyte in plasmonic sites and thereby realize simultaneously controlling 3D hot spots and placing target molecules in hot spots. Moreover, the shrinking process of Ag colloid droplets has been investigated using a high-speed camera, an in situ transmission electron microscope (in situ TEM), and a dark-field microscope (DFM), demonstrating the high stability and uniformity of nanoparticles in droplets. The shrunk Ag NP droplets exhibit excellent SERS sensitivity and reproducibility for the quantitative analysis of 5-FU over a large range of 50-1000 ppb. Hence, it is promising for quantitative analysis of complex systems and long-term monitoring of bioreactions.

Collective Plasmon Coupling in Gold Nanoparticle Clusters for Highly Efficient Photothermal Therapy

Plasmonic nanomaterials with strong absorption at near-infrared frequencies are promising photothermal therapy agents (PTAs). The pursuit of high photothermal conversion efficiency has been the central focus of this research field. Here, we report the development of plasmonic nanoparticle clusters (PNCs) as highly efficient PTAs and provide a semiquantitative approach for calculating their resonant frequency and absorption efficiency by combining the effective medium approximation (EMA) theory and full-wave electrodynamic simulations. Guided by the theoretical prediction, we further develop a universal strategy of space-confined seeded growth to prepare various PNCs. Under optimized growth conditions, we achieve a record photothermal conversion efficiency of up to ∼84% for gold-based PNCs, which is attributed to the collective plasmon-coupling-induced near-unity absorption efficiency. We further demonstrate the extraordinary photothermal therapy performance of the optimized PNCs in in vivo application. Our work demonstrates the high feasibility and efficacy of PNCs as nanoscale PTAs.

Plasmonic Coupling Architectures for Enhanced Photocatalysis

Plasmonic photocatalysis is a promising approach for solar energy transformation. Comparing with isolated metal nanoparticles, the plasmonic coupling architectures can provide further strengthened local electromagnetic field and boosted light-harvesting capability through optimal control over the composition, spacing, and orientation of individual nanocomponents. As such, when integrated with semiconductor photocatalysts, the coupled metal nanostructures can dramatically promote exciton generation and separation through plasmonic-coupling-driven charge/energy transfer toward superior photocatalytic efficiencies. Herein, the principles of the plasmonic coupling effect are presented and recent progress on the construction of plasmonic coupling architectures and their integration with semiconductors for enhanced photocatalytic reactions is summarized. In addition, the remaining challenges as to the rational design and utilization of plasmon coupling structures are elaborated, and some prospects to inspire new opportunities on the future development of plasmonic coupling structures for efficient and sustainable light-driven reactions are raised.

Controlled Assembly of Plasmonic Nanoparticles: From Static to Dynamic Nanostructures

The spatial arrangement of plasmonic nanoparticles can dramatically affect their interaction with electromagnetic waves, which offers an effective approach to systematically control their optical properties and manifest new phenomena. To this end, significant efforts were made to develop methodologies by which the assembly structure of metal nanoparticles can be controlled with high precision. Herein, recent advances in bottom-up chemical strategies toward the well-controlled assembly of plasmonic nanoparticles, including multicomponent and multifunctional systems are reviewed. Further, it is discussed how the progress in this area has paved the way toward the construction of smart dynamic nanostructures capable of on-demand, reversible structural changes that alter their properties in a predictable and reproducible manner. Finally, this review provides insight into the challenges, future directions, and perspectives in the field of controlled plasmonic assemblies.

Vesicle Formation by the Self-Assembly of Gold Nanoparticles Covered with Fluorinated Oligo(ethylene glycol)-Terminated Ligands and Its Stability in Aqueous Solution

Water-stable gold nanoparticle vesicles (GNVs) with hollow interiors have attracted attention due to their great potential for biological applications; however, their preparation through the self-assembly approaches has been restricted due to the limited understanding of their critical mechanistic issues. In this paper, we demonstrate that a fluorinated tetra (ethylene glycol) (FTEG)-terminated tetra (ethylene glycol) (EG4), namely, FTEG-EG4, ligand can self-assemble with gold nanoparticles (5 and 10 nm) into GNVs with a hollow structure in THF due to the solvophobic feature of the ligand. Time-dependent studies showed that the GNVs with a closely packed surface derived from the incomplete and irregular GNVs, but not through the fusion of the GNV precursors. After dialysis in water, the assemblies retained vesicular structures in water, even though GNVs aggregated together, which was initiated by the hydrophobic interactions between the FTEG heads of the surface ligands on GNVs. This study provides a new insight into the design of novel small surface ligands to produce water-stable GNVs for biological applications.

Designing New Metal Chalcogenide Nanoclusters through Atom-by-Atom Substitution

Atom-by-atom substitution is a promising strategy for designing new cluster-based materials, which has been used to generate new gold- and silver-containing clusters. Here, the first study focused on atom-by-atom substitution of Fe and Ni to the core of a well-defined cobalt sulfide superatom [Co₆ S₈ L₆ ]⁺ ligated with triethylphosphine (L = PEt₃ ) to produce [Co₅ MS₈ L₆ ]⁺ (M = Fe, Ni) is reported. Electrospray ionization mass spectrometry confirms the substitution of 1-6 Fe atoms with the single Fe-substituted cluster being the dominant species. The Fe-substituted clusters oxidize in solution to generate dicationic species. In contrast, only a single Ni-substituted cluster is observed, which remains stable as a singly charged species. Collision-induced dissociation experiments indicate the reduced stability of the [Co₅ FeS₈ L₆ ]⁺ toward ligand loss in comparison with the unsubstituted and Ni-substituted counterparts. Density functional theory calculations provide insights into the effect of metal atom substitution on the stability and electronic structures of the clusters. The results indicate that Fe and Ni have a different impact on the electronic structure, optical, and magnetic properties, as well as ligand-core interaction of [Co₆ S₈ L₆ ]. This study extends the atom-by-atom substitution strategy to the metal chalcogenide superatoms providing a direct path toward designing novel atomically precise core-tailored superatoms.

Polyhedral plasmonic nanoclusters through multi-step colloidal chemistry

We describe a new approach to making plasmonic metamolecules with well-controlled resonances at optical wavelengths. Metamolecules are highly symmetric, subwavelength-scale clusters of metal and dielectric. They are of interest for metafluids, isotropic optical materials with applications in imaging and optical communications. For such applications, the morphology must be precisely controlled: the optical response is sensitive to nanometer-scale variations in the thickness of metal coatings and the distances between metal surfaces. To achieve this precision, we use a multi-step colloidal synthesis approach. Starting from highly monodisperse silica seeds, we grow octahedral clusters of polystyrene spheres using seeded-growth emulsion polymerization. We then overgrow the silica and remove the polystyrene to create a dimpled template. Finally, we attach six silica satellites to the template and coat them with gold. Using single-cluster spectroscopy, we show that the plasmonic resonances are reproducible from cluster to cluster. By comparing the spectra to theory, we show that the multi-step synthesis approach can control the distances between metallic surfaces to nanometer-scale precision. More broadly, our approach shows how metamolecules can be produced in bulk by combining different, high-yield colloidal synthesis steps, analogous to how small molecules are produced by multi-step chemical reactions.

The moveable "hot spots" effect in an Au nanoparticles-Au plate coupled system

Surface-enhanced Raman spectroscopy (SERS) is mainly contributed by "hot spots". Due to the huge electromagnetic enhancement, "hot spots" have wide applications in surface analysis and surface catalysis. The in-depth research on the "hot spots" effect is conducive to understanding SERS enhancement mechanisms and designing substrates with high enhancement. At present, the investigation on the "hot spots" effect is mainly based on theoretical simulation and simple experimental models. However, little attention has been paid to the SERS substrates with practical applications. The main reason is that it is difficult to construct a suitable coupled model with great uniformity and sensitivity, which led to the lack of comparability of SERS intensities from different spots or substrates. In this work, Au nanoparticle mono-/bi-layer films coupled with Au single-crystal plate systems were constructed to investigate the distribution and transformation of "hot spots" dependent on the excitation wavelength by a single or dual probe-modified strategy, in which one or two types of molecules with distinct characteristic peaks were modified in different enhanced gaps. The results demonstrated that the wavelength that drove the transformation of the coupling mode from the "particle-particle" mode to the "particle-surface" mode was around 638 nm in the Au nanoparticle monolayer film (Au MLF) covered Au plate system. As the second naked Au MLF was transferred onto the first Au MLF, "hot spots" were transferred to the "particle-particle" gap between the upper and lower Au MLFs with a 638 nm laser as the excitation line. This work offers a novel avenue to investigate the "hot spots" effect in the complex multidimensional nanostructures, which is beneficial for the development of theoretical research and practical applications of SERS.

Photonic Fractal Metamaterials: A Metal-Semiconductor Platform with Enhanced Volatile-Compound Sensing Performance

Advance of photonics media is restrained by the lack of structuring techniques for the 3D fabrication of active materials with long-range periodicity. A methodology is reported for the engineering of tunable resonant photonic media with thickness exceeding the plasmonic near-field enhancement region by more than two orders of magnitude. The media architecture consists of a stochastically ordered distribution of plasmonic nanocrystals in a fractal scaffold of high-index semiconductors. This plasmonic-semiconductor fractal media supports the propagation of surface plasmons with drastically enhanced intensity over multiple length scales, overcoming the 2D limitations of established metasurface technologies. The fractal media are used for the fabrication of plasmonic optical gas sensors, achieving a limit of detection of 0.01 vol% at room temperature and sensitivity up to 1.9 nm vol%^-1 , demonstrating almost a fivefold increase with respect to an optimized planar geometry. Beneficially to their implementation, the self-assembly mechanism of this fractal architecture allows fabrication of micrometer-thick media over surfaces of several square centimeters in a few seconds. The designable optical features and intrinsic scalability of these photonic fractal metamaterials provide ample opportunities for applications, bridging across transformation optics, sensing, and light harvesting.

Hydrochromic Smart Windows to Remove Harmful Substances by Mimicking Medieval European Stained Glasses

Medieval European stained glass windows are known to display various splendid colors and remove harmful airborne substances. At present, the functions of medieval stained glass windows are imperative, from the environment, health, and energy perspectives, to develop multi-functional windows that report/control environmental conditions and remove harmful substances by utilizing visible-near-infrared light sources. Here, we suggest a strategy to mimic medieval European stained glasses for devising plasmonic-based multi-functional smart stained glass windows. The stained glass windows are prepared from the deposition of gold nanoparticles on a glass that is preliminarily coated with a responsive colloidal nanosheet. The temperature responsiveness of the nanosheet enables the effective control the gold nanoparticle density of the stained glasses. Therefore, the windows can display blue, violet, and cranberry colors. The colors become iridescent by introducing a photonic crystal monolayer. The stained glass windows are hydrochromic: they switch the colors (blue ↔ cranberry) and modulate light transmittance depending on humidity conditions. Moreover, they can remove formaldehyde under the illumination of a low-power indoor light. These functions provide a new platform for the futuristic smart windows that clean indoor air for the human health and save energy.

Enhanced thermal effect of plasmonic nanostructures confined in discoidal porous silicon particles

The design of plasmonic nanostructures could have many exciting applications since it enhances or provides valuable control over efficient energy conversion. A three-dimensional (3D) space is a realistic hotspot matrix harvesting a wide conversion that has been shown in zero-dimensional nanoparticles, one-dimensional linear structures, or two-dimensional films. A novel 3D plasmonic nanostructure platform consisting of plasmonic metal nanoparticles in discoidal porous silicon particles is used in this study. Plasmonic gold nanoparticles are anchored inside the discoidal porous silicon (DPS) particles by in situ reduction synthesis. The novel plasmonic nanostructures can tailor the plasmon band and overcome the instability of photothermal materials. The “trapping well” for the anchored nanoparticles in 3D space can result in a huge change of plasmonic band of metal nanoparticles to the near-IR region in a novel 3D geometry. A multifunctional scaffold, Au–DPS particle, composed of doxorubicin conjugated to poly-(l-glutamic acid) (pDOX), was developed for combinatorial chemo-photothermal cancer therapy. The therapeutic efficacy was examined in treatment of the A549 cell line under near-IR laser irradiation. The highly efficient photothermal conversion can also be demonstrated in the laser desorption/ionization time-of-flight mass spectrometry detection analysis. The limit of detection was obviously improved in the detection of angiotensin II, P14R, and ACTH fragments 18-39 peptides. Overall, we envision that Au–DPS particles may be used in ultrasensitive theranostics in the future.

A 3D plasmonic nanostructure with a tunable plasmon resonance band to the near IR region enabled ultrasensitive theranostics for enhanced thermal effect.

Polymer-guided assembly of inorganic nanoparticles

The self-assembly of inorganic nanoparticles is of great importance in realizing their enormous potentials for broad applications due to the advanced collective properties of nanoparticle ensembles. Various molecular ligands (e.g., small molecules, DNAs, proteins, and polymers) have been used to assist the organization of inorganic nanoparticles into functional structures at different hierarchical levels. Among others, polymers are particularly attractive for use in nanoparticle assembly, because of the complex architectures and rich functionalities of assembled structures enabled by polymers. Polymer-guided assembly of nanoparticles has emerged as a powerful route to fabricate functional materials with desired mechanical, optical, electronic or magnetic properties for a broad range of applications such as sensing, nanomedicine, catalysis, energy storage/conversion, data storage, electronics and photonics. In this review article, we summarize recent advances in the polymer-guided self-assembly of inorganic nanoparticles in both bulk thin films and solution, with an emphasis on the role of polymers in the assembly process and functions of resulting nanostructures. Precise control over the location/arrangement, interparticle interaction, and packing of inorganic nanoparticles at various scales are highlighted.

One-pot synthesis of 3D Au nanoparticle clusters with tunable size and their application

In general, the preparation of Au nanoparticle clusters (NPCs) is more challenging than that of nanoparticles. The traditional multi-step method for preparing Au NPCs is time consuming and highly sensitive to the reaction conditions. Here, we report a simple and feasible method for the rapid preparation of Au NPCs (∼30 min), in which Au (III) is reduced to Au (0) by trisodium citrate, and assembled into NPCs in the presence of a trace amount of cysteine. The surface plasmon resonance peak of the Au NPCs is tunable and ranged from visible to near-IR regions by varying the content of cysteine added. The growth process of Au NPCs was monitored by dynamic light scattering, UV-vis absorption spectroscopy and transmission electron microscopy. Their elemental composition, chemical state and molecular structure of the sample surface were measured by x-ray photoelectron spectroscopy. The proposed synthesis mechanism has guiding significance for the preparation of other NPCs. Au NPCs used as surface-enhanced Raman spectroscopy substrate has a good enhancement effect because of its unique morphology.

Self-Assembly of Ultrathin Nanocrystals to Multidimensional Superstructures

The self-assembly of ultrathin nanocrystals (UTNCs) into well-organized multidimensional superstructures is one of the key topics in material chemistry and physics. Highly ordered nanocrystal assemblies also known as superstructures or synthetic structures have remained a focus for researchers over the past few years due to synergy in their properties as compared to their components. Here, we aim to present the recent progress being made in this field with highlights of our research group endeavors in the engineering of self-assembled complex multidimensional superstructures of various inorganic materials, including polyoxometalates. The driving forces for the assembly process and its kinetics along with the potential applications associated with these unique ordered and spatially complex superstructures are also discussed.

Magnetic Plasmon Networks Programmed by Molecular Self-Assembly

Nanoscale manipulation of magnetic fields has been a long-term pursuit in plasmonics and metamaterials, as it can enable a range of appealing optical properties, such as high-sensitivity circular dichroism, directional scattering, and low-refractive-index materials. Inspired by the natural magnetism of aromatic molecules, the cyclic ring cluster of plasmonic nanoparticles (NPs) has been suggested as a promising architecture with induced unnatural magnetism, especially at visible frequencies. However, it remains challenging to assemble plasmonic NPs into complex networks exhibiting strong visible magnetism. Here, a DNA-origami-based strategy is introduced to realize molecular self-assembly of NPs forming complex magnetic architectures, exhibiting emergent properties including anti-ferromagnetism, purely magnetic-based Fano resonances, and magnetic surface plasmon polaritons. The basic building block, a gold NP (AuNP) ring consisting of six AuNP seeds, is arranged on a DNA origami frame with nanometer precision. The subsequent hierarchical assembly of the AuNP rings leads to the formation of higher-order networks of clusters and polymeric chains. Strong emergent plasmonic properties are induced by in situ growth of silver upon the AuNP seeds. This work may facilitate the development of a tunable and scalable DNA-based strategy for the assembly of optical magnetic circuitry, as well as plasmonic metamaterials with high fidelity.

Time-Resolved Analysis of the Structural Dynamics of Assembling Gold Nanoparticles

The hydrophobic collapse is a structural transition of grafted polymer chains in a poor solvent. Although such a transition seems an intrinsic event during clustering of polymer-stabilized nanoparticles in the liquid phase, it has not been resolved in real time. In this work, we implemented a microfluidic 3D-flow-focusing mixing reactor equipped with real-time analytics, small-angle X-ray scattering (SAXS), and UV-vis-NIR spectroscopy to study the early stage of cluster formation for polystyrene-stabilized gold nanoparticles. The polymer shell dynamics obtained by in situ SAXS analysis and numerical simulation of the solvent composition allowed us to map the interaction energy between the particles at early state of solvent mixing, 30 ms behind the crossing point. We found that the rate of hydrophobic collapse depends on water concentration, ranging between 100 and 500 nm/s. Importantly, we confirmed that the polymer shell collapses prior to the commencement of clustering.

Self-assembled microrings of Au nanoparticle and Au nanorod clusters formed at the equators of Janus particles

A method for fabricating polymer Janus particles with microring structures at their equators has been developed. This method allows gold nanoparticles and nanorods to be aligned and densely packed along the microrings.

A method for fabricating polymer Janus particles with metal nanoparticle microring structures at their equators has been developed.

Chelation-assisted assembly of multidentate colloidal nanoparticles into metal-organic nanoparticles

We propose a chelation-assisted assembly of multidentate CNs into metal-organic nanoparticles (MONs). Multidentate CNs functionalized with coordination sites participate equally as organic linkers in MON construction, which is driven by chelation between metal ions and coordination sites. MONs assembled from Au nanoparticles display particle number- and size-dependent optical properties. In addition, the resulting CN-assembled MONs give evidence that assembly was dictated by the multidentate surface ligand rather than the size, shape or material of CNs. With this chelation-assisted strategy, it is possible to control the number of assembled CNs and build the connections between them.

Non-equilibrium anisotropic colloidal single crystal growth with DNA

Anisotropic colloidal crystals are materials with novel optical and electronic properties. However, experimental observations of colloidal single crystals have been limited to relatively isotropic habits. Here, we show DNA-mediated crystallization of two types of nanoparticles with different hydrodynamic radii that form highly anisotropic, hexagonal prism microcrystals with AB₂ crystallographic symmetry. The DNA directs the nanoparticles to assemble into a non-equilibrium crystal shape that is enclosed by the highest surface energy facets (AB₂(10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline 1$$\end{document}1¯0) and AB₂(0001)). Simulations and theoretical arguments show that this observation is a consequence of large energy barriers between different terminations of the AB₂(10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline 1$$\end{document}1¯0) facet, which results in a significant deceleration of the (10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline 1$$\end{document}1¯0) facet growth rate. In addition to reporting a hexagonal colloidal crystal habit, this work introduces a potentially general plane multiplicity mechanism for growing non-equilibrium crystal shapes, an advance that will be useful for designing colloidal crystal habits with important applications in both optics and photocatalysis.

Colloidal crystal engineering with DNA can be used to synthesize highly anisotropic hexagonal prismatic microcrystals. This manuscript introduces a plane multiplicity mechanism that can be used to deliberately design non-equilibrium Wulff shapes, a capability important in many areas, including optics and photocatalysis.

Size-Defined Cracked Vesicle Formation via Self-Assembly of Gold Nanoparticles Covered with Carboxylic Acid-Terminated Surface Ligands

The self-assembly of gold nanoparticles (GNPs) into a defined structure, particularly hollow capsule structures, provides great potential for applications in materials science and medicine. However, the complexity of the parameters for the preparation of those structures through self-assembly has limited access to critical mechanistic questions. With this in mind, we have studied GNP vesicle (GNV) formation through self-assembly by the surface modification of GNPs with low-molecular-weight ligands. Here, we successfully prepared GNVs composed of GNPs with a diameter of 30 nm by surface modification with carboxylic acid-terminated fluorinated oligo(ethylene glycol) ligands (CFLs). As the carboxylic acid has two states (protonated and deprotonated), the balance of the attraction and repulsion between GNPs covered with CFLs is tunable. Sodium carboxylate-terminated fluorinated oligo(ethylene glycol) ligands (SCFLs) provided smaller GNVs than did CFLs at 0.8 × 10¹¹ NPs/mL. Time-course study revealed that CFL-covered GNPs quickly form small aggregates and gradually grow to larger GNVs (ca. 200 nm), but no gradual growth was observed for SCFL-covered GNPs. This result indicated that the electrostatic repulsion inhibits fusion of the small GNVs. The size of the GNVs formed with the aid of CFLs was independent of the initial GNP concentration, but the extinction spectra were concentration-dependent. Electron microscopy imaging and simulations supported the defect formation in the assemblies. These results provided new insights into the vesicle formation mechanism.

Polydopamine@Gold Nanowaxberry Enabling Improved SERS Sensing of Pesticides, Pollutants, and Explosives in Complex Samples

Surface-enhanced Raman scattering (SERS) is a promising analysis technique for detecting various analytes in complex samples due to its unique vibrational fingerprints and high signal enhancement. However, impurity interference and substrate unreliability are direct suppression factors for practical application. Herein, we synthesize polydopamine@gold (PDA@Au) nanowaxberry, where Au nanoparticles are deposited on the surface of PDA sphere with high density and uniformity. Seed-mediated synthesis is used for fabrication of nanowaxberry. Au seeds are deposited on the surface of PDA sphere, then I ion coordinating ligand is employed to form stable AuI₄^- complex with AuCl₄^-, which decreases reduction potential of AuCl₄^- and avails formation of shell structure. Such nanowaxberry has high density of voids and gaps in three-dimensional space, which could absorb analytes and benefit practical SERS detection. Using malachite green as a model analyte, nanowaxberry realizes highly sensitive detection with low limit of detection (1 pM) and good reproducibility (relative standard deviation of about 10%). Meanwhile, the nanowaxberry is employed for practical detection of thiram, benzidine, and 2,4-dinitrotoluene in the environmental water, juice, apple peel, and soil. The high performance makes nanowaxberry to be potentially used for pesticides detection, pollutants monitoring, and forbidden explosives sensing in complex samples.

Controllable self-assembled plasmonic vesicle-based three-dimensional SERS platform for picomolar detection of hydrophobic contaminants

Hydrophobic contaminants in food and the environment seriously threaten human health. The ultrasensitive detection of these pollutants can minimize their damage. However, current ultrasensitive sensing strategies are limited to solid substrate-based surface-enhanced Raman spectroscopy (SERS) detection. Herein, we report a controllable and reproducible solution-based SERS detection platform for the direct and ultrasensitive detection of hydrophobic contaminants by using self-assembled three-dimensional plasmonic vesicles. To this end, amphiphilic gold nanoparticles (AuNPs) tethered with linear block copolymer (BCP) of polystyrene-b-poly (ethylene oxide) (PS-b-PEO) were designed, which display dual functions for improving detection sensitivity, including serving as building blocks for the construction of plasmonic vesicles to yield large numbers of hot-spots for SERS enhancement, and providing hydrophobic PS layers to enrich and concentrate target hydrophobic molecules for direct SERS detection with hydrophobic interaction. By modulating the AuNP size and the length of BCP chains, the ultrahigh detection sensitivity, down to the picomolar level, was obtained via using 80 nm AuNPs tethered with BCP of PEO45-b-PS900-SH. In addition, the proposed method exhibits excellent reproducibility, universality, practicability, as well as multiplexing detection capacity in actual contaminant-spiked soil samples. Briefly, the designed self-assembled plasmonic vesicle-based SERS platform provides an ideal generic methodology for the ultrasensitive detection of hydrophobic contaminants that can greatly accelerate on-site testing in food and environmental monitoring.

Density gradient ultracentrifugation for colloidal nanostructures separation and investigation

In this article, we review the advancement in nanoseparation and concomitant purification of nanoparticles (NPs) by using density gradient ultracentrifugation technique (DGUC) and demonstrated by taking several typical examples. Study emphasizes the conceptual advances in classification, mechanism of DGUC and synthesis-structure-property relationships of NPs to provide the significant clue for the further synthesis optimization. Separation, concentration, and purification of NPs by DGUC can be achieved at the same time by introducing the water/oil interfaces into the separation chamber. We can develop an efficient method "lab in a tube" by introducing a reaction zone or an assembly zone in the gradient to find the surface reaction and assembly mechanism of NPs since the reaction time can be precisely controlled and the chemical environment change can be extremely fast. Finally, to achieve the best separation parameters for the colloidal systems, we gave the mathematical descriptions and computational optimized models as a new direction for making practicable and predictable DGUC separation method. Thus, it can be helpful for an efficient separation as well as for the synthesis optimization, assembly and surface reactions as a potential cornerstone for the future development in the nanotechnology and this review can be served as a plethora of advanced notes on the DGUC separation method.