Anisotropic overgrowth of metal heterostructures induced by a site-selective silica coating

Surface Modification of Gold Nanorods: Multifunctional Strategies and Application Prospects

Gold nanorods (AuNRs), as an important type of gold nanomaterials, have attracted much attention in the nano field. Compared with gold nanoparticls, AuNRs have broader application potential due to their tunable localized surface plasmon resonance properties and anisotropic shapes. Yet, conventional synthesis methods using surfactants have limited the use of AuNRs in a variety of fields such as biomedical applications, plasma-enhanced fluorescence, optics and optoelectronic devices. To solve this problem and improve the stability and biocompatibility of AuNRs, researchers in recent years have used surface modification and functionalization to modify AuNRs, among which the introduction of organic ligands to prepare organic/gold hybrid nanorods has become an effective strategy. Organic materials have better toughness and easy processing, and by introducing organic ligands into the surface of AuNRs, the molecular-level composite of organic and inorganic materials can be realized, thus obtaining hybrid nanorods with excellent properties. This paper reviews the research progress of hybrid nanocomposites, and introducing the synthesis methods of AuNRs and the development of surface ligand modification, then summarises the applications of a wide variety of ligands. Also, the advantages and disadvantages of different ligands and their roles in further self-assembly processes are discussed.

Symmetry-Breaking of Nanoparticle Surface Function Via Conformal DNA Design

Asymmetric surface functionalization of complex nanoparticles to control their directional self-assembly remains a considerable challenge. Here, we demonstrated a conformal DNA design strategy for flexible remodeling of the surface of complex nanoparticles, taking Au nanobipyramids (AuNBPs) as a model. We sheathed one or both tips of AuNBPs into conformal DNA origami with an exceptionally accurate orientation control. Such asymmetrically and symmetrically distributed surface patches possess regioselective, sequence, and site-specific DNA binding capabilities. As a result, we realized a series of prototypical multicomponent "colloidal molecules" made of AuNBPs and Au nanospheres (AuNSs) with defined directionality and number of "bonding valence" as well as 1D and 3D hierarchical assemblies, e.g., inverse core-satellites of AuNBPs and AuNSs, side-by-side and tip-to-tip linear assemblies of AuNBPs, and 3D helical superstructures of AuNBPs with tunable twists. These findings inspire new opportunities for nanoparticle surface engineering and the high-order self-assembly of nanoarchitectures with higher complexity and broadened functionalities.

Synthesis and photocatalytic property of Au-TiO2 nanocomposites with controlled morphologies in microfluidic chips

We present an efficient approach for the consecutive synthesis of Au-TiO2 nanocomposites with controlled morphologies in a microfluidic chip. The seed-mediated growth method was employed to synthesize Au nanorods as the core, and TiO2 layers of varying thicknesses were deposited on the surface or tip of the Au nanorods. Au-TiO2 nanocomposites with core-shell, dumbbell, and dandelion-like structures can be precisely synthesized in a one-step manner within the microfluidic chip by finely tuning the flow rate of NaHCO3 and the amount of hexadecyl trimethyl ammonium bromide. Furthermore, we have investigated the photocatalytic activity of the synthesized nanocomposites, and it was found that Au NR-TiO2 core-shell nanostructure with a thin TiO2 shell exhibits superior catalytic performance. This work provides an effective and controlled method for the microscale preparation and photocatalytic application of various Au-TiO2 nanocomposite structures.

Step-by-Step Nanoscale Top-Down Blocking and Etching Lead to Nanohexapods with Cartesian Geometry

In this research, we designed a stepwise synthetic method for Au@Pt hexapods where six elongated Au pods are arranged in a pairwise perpendicular fashion, sharing a common point (the central origin in a Cartesian-coordinate-like hexapod shape), featured with tip-selectively decorated Pt square nanoplates. Au@Pt hexapods were successfully synthesized by applying three distinctive chemical reactions in a stepwise manner. The Pt adatoms formed discontinuous thin nanoplates that selectively covered six concave facets of a Au truncated octahedron and served as etching masks in the succeeding etching process, which prevented underlying Au atoms from being oxidized. The subsequent isotropic etching proceeded radially, starting from the bare Au surface, carving the central nanocrystal in a concave manner. By controlling the etching conditions, Au@Pt hexapods were successfully fabricated, wherein the core Au domain is connected to six protruding arms, which hold Pt nanoplates at the ends. Due to their morphology, Au@Pt hexapods feature distinctive optical properties in the near-infrared region, as a proof of concept, allowing for surface-enhanced Raman spectroscopy (SERS)-based monitoring of in situ CO electrooxidation. We further extended our synthetic library by tailoring the size of the Pt nanoplates and neck widths of Au branches, demonstrating the validity of selective blocking and etching-based colloidal synthesis.

Recent advances in surface modified gold nanorods and their improved sensing performance

Gold nanorods (AuNRs) have received tremendous attention recently in the fields of sensing and detection applications due to their unique characteristic of surface plasmon resonance. Surface modification of the AuNRs is a necessary path to effectively utilize their properties for these applications. In this Article, we have focused both on demonstrating the recent advances in methods for surface functionalization of AuNRs as well as their use for improved sensing performance using various techniques. The main surface modification methods discussed include ligand exchange with the assistance of a thiol-group, the layer by layer assembly method, and depositing inorganic materials with the desired surface and morphology. Covered techniques that can then be applied for using these functionalized AuNRs include colourimetric sensing, refractive index sensing and surface enhance Raman scattering sensing. Finally, the outlook on the future development of surface modified AuNRs for improved sensing performance is considered.

Effective Distribution of Gold Nanorods in Ordered Thick Mesoporous Silica: A Choice of Noninvasive Theranostics

Porous silica coated gold nanorod core-shell structures demonstrate a multifunctional role in bioimaging, drug delivery, and cancer therapeutics applications. Here, we address a new approach for effective distribution of gold nanorods (GNRs) in a mesoporous silica (MS) shell, viz., one nanorod in one silica particle (GMS). We have studied that silica coating presents major advantages for the better biocompatibility and stability of GNRs. In this study, two different thicknesses of silica shell over GNRs have been discussed as per the application's need; GNRs in thin silica (11 nm) are fit for phototherapy and bioimaging, whereas thick and porous silica (51 nm) coated gold nanorods are suitable for triggered drug delivery and theranostics. However, effective distribution of GNRs in ordered architecture of thick mesoporous silica (MS, more than 50 nm thickness) with high surface area (more than 1000 m2/g) is not well understood so far. Here, we present methodical investigations for uniform and highly ordered mesoporous silica coating over GNRs with tunable thickness (6 to 51 nm). Judicious identification and optimization of different reaction parameters like concentrations of silica precursor (TEOS, 1.85-43.9 mM), template (CTAB, 0.9-5.7 mM), effect of temperature, pH (8.6-10.8), stirring speed (100-400 rpm), and, most importantly, the mode of addition of TEOS with GNRs have been discussed. Studies with thick, porous silica coated GNRs simplify the highest ever reported surface area (1100 m2/g) and cargo capacity (57%) with better product yield (g/batch). First and foremost, we report a highly scalable (more than 500 mL) and rapid direct deposition of an ordered MS shell around GNRs. These engineered core-shell nanoparticles demonstrate X-ray contrast property, synergistic photothermal-chemotherapeutics, and imaging of tumor cell (96% cell death) due to released fluorescent anticancer drug molecules and photothermal effect (52 °C) of embedded GNRs. A deeper insight into their influence on the architectural features and superior theranostics performances has been illustrated in detail. Hence, these findings indicate the potential impact of individual GMS for image guided combination therapeutics of cancer.

Gold nanohexagrams via active surface growth under sole CTAB control

The synthesis of homochiral nanostructures involves not only the chiral ligand, but also CTAB. The latter is often treated as a weak ligand unable to compete with the thiol-based chiral ligand. Here, we show that CTAB alone is able to induce Active Surface Growth on Au nanoplates, giving curved tips and steep ridges in the resulting nano-hexagrams. The growth materials (Au0) are diverted to a few active sites, whereas the rest of the Au surfaces are inhibited. Modulation of the growth rate by the ratio of ascorbic acid to Au precursor gives a continuous change of the growth modes, explaining the main trends of shape evolution and the inequivalent growth of the equivalent surfaces. With only CTAB as the ligand, the fact that ridges and spikes could be formed in defiance of facet control suggests that the role of CTAB cannot be ignored in the chiral synthesis and that the precise modulation of the Active Surface Growth could be the key to rational synthetic controls.

Gold-semiconductor nanohybrids as advanced phototherapeutics

Phototherapeutics is gaining momentum as a mainstream treatment for cancer, with gold-semiconductor nanocomposites showing promise as potent phototherapeutic agents due to their structural tunability, biocompatibility and functional diversity. Such nanohybrids possess plasmonic characteristics in the presence of gold and the catalytic nature of semiconductor units, as well as the unexpected physicochemical properties arising from the contact interface. This perspective provides an overview of the latest research on gold-semiconductor nanocomposites for photodynamic, photothermal and photocatalytic therapy. The relationship between the spatial configuration of these nanohybrids and their practical performance was explored to deliver comprehensive insights and guidance for the design and fabrication of novel composite nanoplatforms to enhance the efficiency of phototherapeutics, promoting the development of nanotechnology-based advanced biomedical applications.

Strong Ligand Control for Noble Metal Nanostructures

ConspectusNanosynthesis is the art of creating nanostructures, with on-demand synthesis as the ultimate goal. Noble metal nanoparticles have wide applications, but the available synthetic methods are still limited, often giving nanospheres and symmetrical nanocrystals. The fundamental reason is that the conventional weak ligands are too labile to influence the materials deposition, so the equivalent facets always grow equivalently. Considering that the ligands are the main synthetic handles in colloidal synthesis, our group has been exploring strong ligands for new growth modes, giving a variety of sophisticated nanostructures. The model studies often involve metal deposition on seeds functionalized with a certain strong ligand, so that the uneven distribution of the surface ligands could guide the subsequent deposition.In this Account, we focus on the design principles underlying the new growth modes, summarizing our efforts in this area along with relevant literature works. The basics of ligand control are first revisited. Then, the four major growth modes are summarized as follows: (1) The curvature effects would divert the materials deposition away from the high-curvature tips when the ligands are insufficient. With ligands fully covering the seeds, the sparser ligand packing at the tips would then promote the initial nucleation thereon. (2) The strong ligands may get trapped under the incoming metal layer, thus modulating the interfacial energy of the core-shell interface. The evidence for embedded ligands is discussed, along with examples of Janus nanostructures arising from the synthetic control, including metal-metal, metal-semiconductor, and metal-C₆₀ systems using a variety of ligands. (3) Active surface growth is an unusual mode with divergent growth rates, so that part of the emerging surface is inhibited, and the growth is focused onto a few active sites. With seeds attached to oxide substrates, the selective deposition at the metal-substrate interface produces ultrathin nanowires. The synthesis can be generally applied to grow Au, Ag, Pd, Pt, and hybrid nanowires, with straight, spiral, or helical structures, and even rapid alteration of segments via electrochemical methods. In contrast, active surface growth for colloidal nanoparticles has to be more carefully controlled. The rich growth phenomena are discussed, highlighting the role of strong ligands, the control of deposition rates, the chiral induction, and the evidence for the active sites. (4) An active site with sparse ligands could also be exploited in etching, where the freshly exposed surface would promote further etching. The result is an unusual sharpening etching mode, in contrast to the conventional rounding mode for minimized surface energy.Colloidal nanosynthesis holds great promise for scalable on-demand synthesis, providing the crucial nanomaterials for future explorations. The strong ligands have delivered powerful synthetic controls, which could be further enhanced with in-depth studies on growth mechanisms and synthetic strategies, as well as functions and properties.

Engineering the Au-Cu2 O Crystalline Interfaces for Structural and Catalytic Integration

Precise structural control has attracted tremendous interest in pursuit of the tailoring of physical properties. Here, this work shows that through strong ligand-mediated interfacial energy control, Au-Cu₂ O dumbbell structures where both the Au nanorod (AuNR) and the partially encapsulating Cu₂ O domains are highly crystalline. The synthetic advance allows physical separation of the Au and Cu₂ O domains, in addition to the use of long nanorods with tunable absorption wavelength, and the crystalline Cu₂ O domain with well-defined facets. The interplay of plasmon and Schottky effects boosts the photocatalytic performance in the model photodegradation of methyl orange, showing superior catalytic efficiency than the AuNR@Cu₂ O core-shell structures. In addition, compared to the typical core-shell structures, the AuNR-Cu₂ O dumbbells can effectively electrochemically catalyze the CO₂ to C₂₊ products (ethanol and ethylene) via a cascade reaction pathway. The excellent dual function of both photo- and electrocatalysis can be attributed to the fine physical separation of the crystalline Au and Cu₂ O domains.

Site-Selective Chiral Growth of Anisotropic Au Triangular Nanoplates for Tuning the Optical Chirality

Site-selective chiral growth of anisotropic nanoparticles is of great importance to realize the plasmonic nanostructures with delicate geometry and desired optical chirality; however, it remains largely unexplored. This work demonstrates a controlled site-selective chiral growth system based on the seed-mediated growth of anisotropic Au triangular nanoplates. The site-selective chiral growth involves two distinct underlying pathways, faceted growth and island growth, which are interswitchable upon maneuvering the interplay of chiral molecules, surfactants, and reducing agents. The pathway switch governs the geometric and chirality evolution of Au triangular nanoplates, giving rise to tailorable circular dichroism spectra. The ability to tune the optical chirality in a controlled manner by manipulating the site-selective chiral growth pathway opens up a promising strategy for exploiting chiral metamaterials with increasing architectural complexity in chiroptical applications.

Plasmonic Dual-Gap Nanodumbbells for Label-Free On-Particle Raman DNA Assays

Metal nanostructures with a tunable plasmonic gap are useful for photonics, surface-enhanced spectroscopy, biosensing, and bioimaging applications. The use of these structures as chemical and biological sensing/imaging probes typically requires an ultra-precise synthesis of the targeted nanostructure in a high yield, with Raman dye-labeling and complex assay components and procedures. Here, a plasmonic nanostructure with tunable dual nanogaps, Au dual-gap nanodumbbells (AuDGNs), is designed and synthesized via the anisotropic adsorption of polyethyleneimine on Au nanorods to facilitate tip-selective Au growths on nanorod tips for forming mushroom-shaped dumbbell-head structures at both tips and results in dual gaps (intra-head and inter-head gaps) within a single particle. AuDGNs are synthesized in a high yield (>90%) while controlling the inter-head gap size, and the average surface-enhanced Raman scattering (SERS) enhancement factor (EF) value is 7.5 × 10⁸ with a very narrow EF distribution from 1.5 × 10⁸ to 1.5 × 10⁹ for >90% of analyzed particles. Importantly, AuDGNs enable label-free on-particle SERS detection assays through the diffusion of target molecules into the intraparticle gap for different DNA sequences with varying ATGC combinations in a highly specific and sensitive manner without a need for Raman dyes.

An ultrasensitive FRET-based fluorescent low molecular weight heparin nanoprobe for quantifying heparanase activity

Heparanase (HPA) is a multifaceted endo-β-glucuronidase, and its dysregulation facilitates cancer metastasis. Developing techniques for fast and sensitively monitoring HPA enzymatic activity is crucial for searching for molecular therapies targeting HPA. Herein, we developed a novel fluorescence resonance energy transfer (FRET)-based nanoprobe AuNCs-LMWH-AuNRs, with AuNCs@GSH-cys and AuNRs/end-NH₂/side-SiO₂ attached to the non-reducing terminus and reducing terminus of low molecular weight heparin (LMWH), respectively. AuNCs@GSH-cys exhibited an absolute quantum yield of 1.1%. The absorption spectra of AuNRs/end-NH₂/side-SiO₂ (825 nm for maximum longitudinal absorption) and the emission spectra of AuNCs@GSH-cys (824 nm for maximum emission) were precisely overlapping, further enhancing the efficiency of FRET. In the presence of HPA, the LMWH nanoprobe exhibited an ultrasensitive response with excitation/emission wavelength (lambda (ex) = 560 nm, lambda (em) = 824 nm). The probe presented a wide linear dynamic detection range (LDR) of 0.125 ng/μL - 0.01 μg/μL in vitro with a limit of detection (LODs) of 82.15 pM (0.43 pg/μL). The excellent selectivity and good fluorescence turn-on efficiency of the probe made it possible for one-step detection of cellular heparanase activity. High throughput screening of HPA inhibitors also can be accomplished using the highly efficient LMWH nanoprobe.

Examining Silver Deposition Pathways onto Gold Nanorods with Liquid-Phase Transmission Electron Microscopy

Liquid-phase transmission electron microscopy (LP-TEM) enables one to directly visualize the formation of plasmonic nanoparticles and their postsynthetic modification, but the relative contributions of plasmonic hot electrons and radiolysis to metal precursor reduction remain unclear. Here we show silver deposition onto plasmonic gold nanorods (AuNRs) during LP-TEM is dominated by water radiolysis-induced chemical reduction. Silver was observed with LP-TEM to form bipyramidal shells at higher surfactant coverage and tip-preferential lobes at lower surfactant coverage. Ex situ silver photodeposition formed nanometer-thick shells on AuNRs with preferential deposition in inter-rod gaps, while chemical reduction deposited silver at AuNR tips at low surfactant coverage and formed pyramidal shells at higher surfactant coverage, consistent with LP-TEM. Silver deposition locations during LP-TEM were inconsistent with simulated near-field enhancement and hot electron generation hot spots. Collectively, the results indicate chemical reduction dominated during LP-TEM, indicating observation of plasmonic hot electron-induced photoreduction will necessitate suppression of radiolysis.

Tunable reactivity of silver nanoclusters: a facile route to synthesize a range of bimetallic nanostructures

Quantized energy levels and unique optoelectronic properties of atomically precise noble metal nanoclusters (NCs) have made them important in materials science, catalysis, sensors, and biomedicine. Recent studies on the profound chemical interactions of such NCs within themselves and with ultrasmall plasmonic nanoparticles (NPs) indicate that depending on the size, shape, and composition of the second reactant, NCs can either take part in colloidal assembly without any chemical modifications or lead to products with atoms exchanged. Anisotropic NPs are a unique class of plasmonic nanomaterials as their sharp edges and protrusions show higher chemical reactivity compared to flat surfaces, often leading to site-specific growth of foreign metals and metal oxide shells. Here, using chemical interactions between gold nanotriangles (AuNTs) and Ag NCs of different compositions, we show for the first time that metal atom etching, alloying/atom exchange, and colloidal assembly can all happen at a particular length scale. Specifically, Ag₂₅(DMBT)₁₈ NCs (denoted as 1), upon reacting with AuNTs of ∼57 nm edge length, etch gold atoms from their sharp tips and edges. Simultaneously, the two nanosystems exchange metal atoms, resulting in Ag-doped AuNTs and Au_xAg_24-x(DMBT)₁₈ (x = 1, 2). However, another Ag NC with the same metallic core, but a different ligand shell, namely, Ag₂₅H₂₂(DPPE)₈ (denoted as 2), creates dendritic shells made of Ag, surrounding these AuNTs under the same reaction conditions. Furthermore, we show that in the case of a more reactive thiol-protected Ag NC, namely, Ag₄₄(pMBA)₃₀ (denoted as 3), gold etching is faster from the edges and tips, which drastically alters the identities of both the reactants. Interestingly, when the AuNTs are protected by pMBA, 3 systematically assembles on AuNTs through H-bonding, resulting in an AuNT core-Ag NC shell nanocomposite. Thus, while shedding light on various factors affecting the reactivity of Ag NCs towards AuNTs, the present study proposes a single strategy to obtain a number of bimetallic nanosystems of targeted morphology and functionality.

Ultrabroad spectral response and excellent SERS performance of PbS-assisted Au/PbS/Au nanostars

Noble metal nanomaterials have many excellent optical properties due to localized surface plasmon resonance induced by external electric and magnetic fields. The plasmon-enhanced optical properties of nanomaterials can be controlled by changing their shapes or compositions. Here, we use a gentle approach to synthesize Au/PbS/Au nanostars with multiple tips and explore the surface-enhanced Raman scattering (SERS) activity, the second harmonic generation (SHG), and photocatalytic performance. The Au/PbS/Au nanostars have ultrabroad spectral responses and significantly enhanced local electric fields near the sharp tips. The size and tip length of the Au/PbS/Au nanostars can be adjusted by changing the amount of HAuCl₄. The Au/PbS/Au nanostars exhibit largely enhanced SERS activity and photocatalytic degradation efficiency compared with the Au bipyramids and the Au BPs@PbS nanocrystals. In addition, the SHG of Au/PbS/Au nanostars is also significantly enhanced due to asymmetry and local field enhancement. This research shows potential in many applications ranging from photophysics to photochemistry.

Tipping Gold Nanobipyramids with Titania for the Use of Plasmonic Hotspots to Drive Amine Coupling

Designing plasmonic photocatalysts with spatially controlled catalytic sites is an effective strategy to boost the sunlight-driven chemical transformation efficiency through plasmonic enhancement. Herein, we describe a facile method for the synthesis of TiO₂-tipped Au nanobipyramids (NBPs) to give (Au NBP)/t-TiO₂ nanodumbbells. The surfactant cetyltrimethylammonium bromide concentration is the key factor in the construction of this type of unique nanostructure. The photocatalytic aerobic oxidative coupling of amines using the plasmonic photocatalysts with the dumbbell-like and core@shell structures indicates that the TiO₂-tipped ends for the photo-reduction and the exposed adjacent Au surface for the photo-oxidation on (Au NBP)/t-TiO₂ can significantly improve the photocatalytic activity. The underlying mechanism of the photocatalytic oxidative coupling of benzylamine over (Au NBP)/t-TiO₂ has been thoroughly investigated. Both experimental and simulation results for (Au NBP)/t-TiO₂ and (Au nanorod)/t-TiO₂ confirm the important effect of the plasmonic hotspots on the enhancement of the photocatalytic activity.

Symmetry-breaking synthesis of Janus Au/CeO2 nanostructures for visible-light nitrogen photofixation

Precise manipulation of the reactive site spatial distribution in plasmonic metal/semiconductor photocatalysts is crucial to their photocatalytic performance, but the construction of Janus nanostructures through symmetry-breaking synthesis remains a significant challenge. Here we demonstrate a synthetic strategy for the selective growth of a CeO2 semi-shell on Au nanospheres (NSs) to fabricate Janus Au NS/CeO2 nanostructures with the assistance of a SiO2 hard template and autoredox reaction between Ag+ ions and a ceria precursor. The obtained Janus nanostructures possess a spatially separated architecture and exhibit excellent photocatalytic performance toward N2 photofixation under visible-light illumination. In this scenario, N2 molecules are reduced by hot electrons on the CeO2 semi-shell, while hole scavengers are consumed by hot holes on the exposed Au NS surface, greatly promoting the charge carrier separation. Moreover, the exposed Au NS surface in the Janus structures offers an additional opportunity for the fabrication of ternary Janus noble metal/Au NS/CeO2 nanostructures. This work highlights the genuine superiority of the spatially separated nanoarchitectures in the photocatalytic reaction, offering instructive guidance for the design and construction of novel plasmonic photocatalysts.

Site-selective modification of metallic nanoparticles

Surface patterning of inorganic nanoparticles through site-selective functionalization with mixed-ligand shells or additional inorganic material is an intriguing approach to developing tailored nanomaterials with potentially novel and/or multifunctional properties. The unique physicochemical properties of such nanoparticles are likely to impact their behavior and functionality in biological environments, catalytic systems, and electronics applications, making it vital to understand how we can achieve and characterize such regioselective surface functionalization. This Feature Article will review methods by which chemists have selectively modified the surface of colloidal nanoparticles to obtain both two-sided Janus particles and nanoparticles with patchy or stripey mixed-ligand shells, as well as to achieve directed growth of mesoporous oxide materials and metals onto existing nanoparticle templates in a spatially and compositionally controlled manner. The advantages and drawbacks of various techniques used to characterize the regiospecificity of anisotropic surface coatings are discussed, as well as areas for improvement, and future directions for this field.

Wearable SERS Sensor Based on Omnidirectional Plasmonic Nanovoids Array with Ultra-High Sensitivity and Stability

Surface-enhanced Raman spectroscopy (SERS) is a promising technology for wearable sensors due to its fingerprint spectrum and high detection sensitivity. However, since SERS-activity is sensitive to both the distribution of "hotspots" and excitation angle, it is profoundly challenging to develop a wearable SERS sensor with high stability under various deformations during movements. Herein, inspired by omnidirectional light-harvesting of the compound eye of Xenos Peckii, a wearable SERS sensor is developed using omnidirectional plasmonic nanovoids array (OPNA), which is prepared by assembling a monolayer of metal nanoparticles into the artificial plasmonic compound-eye (APC). Specifically, APC is an interconnected frame containing omnidirectional "pockets" and acts as an "armour", not only rendering a broadband and omnidirectional enhancement of "hotspots" in the delicate nanoparticles array, but also maintaining an integrity of the "hotspots" against external mechanical deformations. Furthermore, an asymmetry super-hydrophilic pattern is fabricated on the surface of OPNA, endowing the hydrophobic OPNA with the ability to spontaneously extract and concentrate the analytes from sweat. Such an armored SERS sensor can enable the wearable and in situ analysis with high sensitivity and stability, exhibiting great potential in point-of-care analysis.

Cu2-x Sx Capped AuCu Nanostars for Efficient Plasmonic Photothermal Tumor Treatment in the Second Near-Infrared Window

Plasmonic nanohybrids are promising photo energy conversion materials in photoelectronics and biomedicine, due to their unique surface plasmon resonance (SPR). Au and Cu_2-x S_x nanostructures with strong SPR in the near-infrared (NIR) spectral region are classic plasmonic systems used to convert NIR photons into heat for photothermal therapy (PTT). The rational design of the Au/Cu_2-x S_x nanohybrids is expected to induce better photothermal conversion; however, the construction of such hybrids via wet-chemistry methods with a well-controlled interfacial structure is still challenging. Here, the synthesis of an AuCu Star/Cu_2-x S_x nanohybrid is reported, where the Cu_2-x S_x components are selectively grown on the AuCu nanostar tips to form "caps". The spatial formation of the Cu_2-x S_x caps on star tips is mainly governed by surfactant concentration, tip curvature, and experimental manipulation. The nanohybrids show low cytotoxicity and superior photothermal conversion efficiency, enabling robust PTT to kill cancer cells in the second NIR window. Numerical simulation reveals that the coupling of Cu_2-x S_x on nanostar tips generates strong interfacial electric field, improving photothermal conversion. Moreover, the spatial separation structure favors the continuous flow of hot charge carriers to produce active radicals, further promoting the tumor treatment effect.

Regulating and Directionally Controlling Electron Emission from Gold Nanorods with Silica Coatings

Dielectric coatings offer a versatile means of manipulating hot carrier emission from nanoplasmonic systems for emerging nanocatalysis and photocathode applications, with uniform coatings acting as regulators and nonuniform coatings providing directional photocurrent control. However, the mechanisms for electron emission through dense and mesoporous silica (SiO₂) coatings require further examination. Here, we present a systematic investigation of photoemission from single gold nanorods as a function of dense versus mesoporous silica coating thicknesses. Studies with dense coatings on gold nanostructures clarify the short (∼1 nm) attenuation length responsible for severely reduced transmission through the silica conduction band. By contrast, mesoporous silica is much more transmissive, and a simple geometric model quantitatively recapitulates the electron escape probability through nanoscopic porous channels. Finally, photoelectron velocity map imaging (VMI) studies of nanorods with coating defects verify that photoemission occurs preferentially through the thinner regions, illustrating new opportunities for designing photocurrent distributions on the nanoscale.

Recent Progress on Asymmetric Carbon- and Silica-Based Nanomaterials: From Synthetic Strategies to Their Applications

HIGHLIGHTS

The synthetic strategies and fundamental mechanisms of various asymmetric carbon- and silica-based nanomaterials were systematically summarized. The advantages of asymmetric structure on their related applications were clarified by some representative applications of asymmetric carbon- and silica-based nanomaterials. The future development prospects and challenges of asymmetric carbon- and silica-based nanomaterials were proposed.

ABSTRACT

Carbon- and silica-based nanomaterials possess a set of merits including large surface area, good structural stability, diversified morphology, adjustable structure, and biocompatibility. These outstanding features make them widely applied in different fields. However, limited by the surface free energy effect, the current studies mainly focus on the symmetric structures, such as nanospheres, nanoflowers, nanowires, nanosheets, and core–shell structured composites. By comparison, the asymmetric structure with ingenious adjustability not only exhibits a larger effective surface area accompanied with more active sites, but also enables each component to work independently or corporately to harness their own merits, thus showing the unusual performances in some specific applications. The current review mainly focuses on the recent progress of design principles and synthesis methods of asymmetric carbon- and silica-based nanomaterials, and their applications in energy storage, catalysis, and biomedicine. Particularly, we provide some deep insights into their unique advantages in related fields from the perspective of materials’ structure–performance relationship. Furthermore, the challenges and development prospects on the synthesis and applications of asymmetric carbon- and silica-based nanomaterials are also presented and highlighted. [Image: see text]

Bimetallic Janus Nanocrystals: Syntheses and Applications

Bimetallic Janus nanocrystals have received considerable interest in recent years owing to their unique properties and niche applications. The side-by-side distribution of two distinct metals provides a flexible platform for tailoring the optical and catalytic properties of nanocrystals. First, a brief introduction to the structural features of bimetallic Janus nanocrystals, followed by an extensive discussion of the synthetic approaches, is given. The strategies and experimental controls for achieving the Janus structure, as well as the mechanistic understandings, are specifically discussed. Then, a number of intriguing properties and applications enabled by the Janus nanocrystals are highlighted. Finally, this article is concluded with future directions and outlooks with respect to both syntheses and applications of this new class of functional nanomaterials.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Site-Selective Deposition of Metal-Organic Frameworks on Gold Nanobipyramids for Surface-Enhanced Raman Scattering

Site-selective deposition of metal-organic frameworks (MOFs) on metal nanocrystals has remained challenging because of the difficult control of the nucleation and growth of MOFs. Herein we report on a facile wet-chemistry approach for the selective deposition of zeolitic imidazolate framework-8 (ZIF-8) on anisotropic Au nanobipyramids (NBPs) and nanorods. ZIF-8 is selectively deposited at the ends and waist and around the entire surface of the elongated Au nanocrystals. The NBP-based nanostructures with end-deposited ZIF-8 exhibit the best surface-enhanced Raman scattering (SERS) performance, implying that molecules can be concentrated by ZIF-8 at the hot spots. In addition, the SERS signal exhibits good selectivity for small molecules because of the molecular sieving effect of ZIF-8. This study opens up a promising route for constructing plasmonic nanostructures with site selectively deposited ZIF-8, which hold enormous potential for molecular sensing, optical switching, and plasmonic catalysis.

Surface Engineering and Controlled Ripening for Seed-Mediated Growth of Au Islands on Au Nanocrystals

Engineering the nucleation and growth of plasmonic metals (Ag and Au) on their pre-existing seeds is expected to produce nanostructures with unconventional morphologies and plasmonic properties that may find unique applications in sensing, catalysis, and broadband energy harvesting. Typical seed-mediated growth processes take advantage of the perfect lattice match between the deposited metal and seeds to induce conformal coating, leading to either simple size increases (e.g., Au on Au) or the formation of core-shell structures (e.g., Ag on Au) with limited morphology change. In this work, we show that the introduction of a thin layer of metal with considerable lattice mismatch can effectively induce the nucleation of well-defined Au islands on Au nanocrystal seeds. By controlling the interfacial energy between the seed and the deposited material, the oxidative ripening, and the surface diffusion of metal precursors, we can regulate the number of islands on the seeds and produce complex Au nanostructures with morphologies tunable from core-satellites to tetramers, trimers, and dimers.

DNA-encoded bimetallic Au-Pt dumbbell nanozyme for high-performance detection and eradication of Escherichia coli O157:H7

Infectious Escherichia coli O157:H7 threatens the health of millions people each year. Thus, it is important to establish a simple and sensitive method for bacterial detection and eradication. Herein, a DNA-programming strategy is explored to synthesize anisotropic dumbbell-like Au-Pt nanoparticles with excellent catalytic and anti-bacterial activities, which were applied in the simultaneous detection and eradication of pathogenic bacteria. The DNA sequence-dependent growth of bimetallic nanoparticles is firstly studied and polyT20 has the tendency to form dumbbell-like Au-Pt bimetallic structures based on gold nanorods seeds. PolyA20 and polyC20 can also form similar structures but only at much lower DNA concentrations, which can be explained by their much higher affinity to the metal surfaces than T20. The as-prepared nanoparticles exhibit high nanozyme catalytic activity resulting from the synergistic effect of Au and Pt. Under light irradiation, the Au-Pt nanoparticles show high photothermal conversion efficiency and enhanced catalytic activity, which can be applied for the eradication and detection of E. coli O157:H7 with a robust efficacy (95%) in 5 min and provides excellent linear detection (10-10⁷ CFU/mL), with a detection limit of 2 CFU/mL. This study offered new insights into DNA-directed synthesis of nanomaterials with excellent biosensing and antibiotic applications.

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

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

Switching to the brighter lane: pathways to boost the absorption of lanthanide-doped nanoparticles

Lanthanide-doped nanoparticles (LNPs) are speedily colonizing several research fields, such as biological (multimodal) imaging, photodynamic therapy, volumetric encoding displays, and photovoltaics. Yet, the electronic transitions of lanthanide ions obey the Laporte rule, which dramatically hampers their light absorption capabilities. As a result, the brightness of these species is severely restricted. This intrinsic poor absorption capability is the fundamental obstacle for untapping the full potential of LNPs in several of the aforementioned fields. Among others, three of the most promising physicochemical approaches that have arisen during last two decades to face the challenges of increasing LNP absorption are plasmonic enhancement, organic-dye sensitization, and coupling with semiconductors. The fundamental basis, remarkable highlights, and comparative achievements of each of these pathways for absorption enhancement are critically discussed in this minireview, which also includes a detailed discussion of the exciting perspectives ahead.

Competitive Lateral Flow Immunoassay Relying on Au-SiO2 Janus Nanoparticles with an Asymmetric Structure and Function for Furazolidone Residue Monitoring

Gold nanoparticles (AuNPs) are the most commonly used signal materials in lateral flow immunoassay (LFIA). However, the assay sensitivity of traditional AuNP-based LFIA is usually limited by the incomplete competition between free target analytes and immobilized antigens for the binding of AuNP-labeled antibodies. To unfreeze this limitation, here, asymmetric Au-SiO₂ Janus NPs (about 66 nm) were designed and synthesized. Au-SiO₂ Janus NPs can assemble into snowman-like anisotropic structures and combine two different physicochemical properties at their opposite sides, where the AuNP side mainly possesses the antibody conjugating and signal providing functions and the SiO₂ side primarily offers the stable function. In virtue of the unique asymmetric nanostructure, only the AuNP side can interact with target analytes by specific antigen-antibody interactions, which could significantly improve the efficiency of competition. Selecting furazolidone as a model analyte, the immunoassay biosensor showed a limit of detection as low as 0.08 ng/mL, 10-fold decreased than that of the AuNPs-LFIA. Moreover, the Au-SiO₂ Janus NP lateral flow immunoassay was well applied in chicken, pork, honey, and beef food samples with visual detection limits of 0.8 ng/g, 0.16 ng/g, 0.4 ng/mL, and 0.16 ng/g, respectively. The Au-SiO₂ Janus NPs possess the advantages of both materials, which will broaden their applications as a potential alternative in the rapid and sensitive detection of antibiotic residues.

Silica Nanoparticles

Silica consists of one silicon atom and two oxygen atoms (SiO₂) and is commonly used in various aspects of daily life. For example, it has been used as glass, insulator, and so on. Nowadays, silica is used as core reagents for fabricating and encapsulating nanoparticles (NPs). In this chapter, the usage of silica in nanotechnology is described. Synthesis and surface modification of silica nanoparticles (SiNPs), including via the Stöber method, reverse microemulsion method, and modified sol-gel method, are illustrated. Then, various NPs with silica encapsulation are explained. At last, the biological applications of those mentioned NPs are described.

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

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

Solvent Quality-Mediated Regioselective Modification of Gold Nanorods with Thiol-Terminated Polymers

Modification of nanorods (NRs) with functional polymer ligands is of great significance to enhance their surface chemistry and prompt their applications in many fields (e.g., photothermal therapy, bioimaging, and catalysis). However, the regioselective modification of AuNRs still remains a great challenge. Herein, we introduce a facile yet versatile strategy to achieve the regioselective modification of AuNRs through a solvent quality-mediated strategy. By employing a poor solvent of the original ligand cetyltrimethylammonium bromide (CTAB) as the medium in the modification, polymer ligands would selectively graft onto the two ends of AuNRs, while polymer ligands would graft onto the entire surface when employing a good solvent. This strategy demonstrates good reproducibility and is applicable to both hydrophilic and hydrophobic polymer ligand modifications. Moreover, by combing our strategy with the preoccupation route, the two ends and sidewall of AuNRs modified by two different polymers form an "ABA"-type building block, which can further self-assemble into well-ordered superstructures. Our finding provides a new opportunity for multifunctionalization of NRs.

Recent Advances in the Design and Photocatalytic Enhanced Performance of Gold Plasmonic Nanostructures Decorated with Non-Titania Based Semiconductor Hetero-Nanoarchitectures

Plasmonic photocatalysts combining metallic nanoparticles and semiconductors have been aimed as versatile alternatives to drive light-assisted catalytic chemical reactions beyond the ultraviolet (UV) regions, and overcome one of the major drawbacks of the most exploited photocatalysts (TiO2 or ZnO). The strong size and morphology dependence of metallic nanostructures to tune their visible to near-infrared (vis-NIR) light harvesting capabilities has been combined with the design of a wide variety of architectures for the semiconductor supports to promote the selective activity of specific crystallographic facets. The search for efficient heterojunctions has been subjected to numerous studies, especially those involving gold nanostructures and titania semiconductors. In the present review, we paid special attention to the most recent advances in the design of gold-semiconductor hetero-nanostructures including emerging metal oxides such as cerium oxide or copper oxide (CeO2 or Cu2O) or metal chalcogenides such as copper sulfide or cadmium sulfides (CuS or CdS). These alternative hybrid materials were thoroughly built in past years to target research fields of strong impact, such as solar energy conversion, water splitting, environmental chemistry, or nanomedicine. Herein, we evaluate the influence of tuning the morphologies of the plasmonic gold nanostructures or the semiconductor interacting structures, and how these variations in geometry, either individual or combined, have a significant influence on the final photocatalytic performance.

Facile Surface Modification of Mesoporous Au Nanoparticles for Highly Sensitive SERS Detection

The stability, dispersity, and surface chemical properties of colloidal nanoparticles are crucial for the reliable and desired chemical sensing in various applications. Here, we report an effective strategy to engineer the surface properties of mesoporous Au nanoparticles (meso-Au NPs) via PVP ligand modification, template removal, and surface purification. Monodispersed 3D meso-Au NPs with well-defined sizes and shapes were obtained using a general soft-enveloping strategy. During surface modification, the addition of PVP ligands and the concentration of HF solutions play key roles in the stability, shape, and size distributions of ordered Au networks. In order to obtain an improved sensing performance, the morphologies of meso-Au NPs were optimized with smaller mesopore size, and NaBH₄ solution was used to efficiently remove the adsorbed PVP ligands. Due to the characteristics of high-density porosities and large surface area, the purified meso-Au NPs could be a kind of promising plasmonic-enhanced nanomaterial and provide abundant "hot spots." Combined with the enrichment effect using a slippery liquid-infused porous surface, the lowest detection limits of crystal violet molecule could be down to 0.1 pM, demonstrating an excellent SERS sensitivity. Moreover, a realistic illegal drug containing aspirin could be sensitively detected with a limit of 2.8 × 10^-6 M, showing great potential for practical molecular sensing and applications.

Interfacing DNA with Gold Nanoparticles for Heavy Metal Detection

The contamination of heavy metals (e.g., Hg, Pb, Cd and As) poses great risks to the environment and human health. Rapid and simple detection of heavy metals of considerable toxicity in low concentration levels is an important task in biological and environmental analysis. Among the many convenient detection methods for heavy metals, DNA-inspired gold nanoparticles (DNA-AuNPs) have become a well-established approach, in which assembly/disassembly of AuNPs is used for colorimetric signaling of the recognition event between DNA and target heavy metals at the AuNP interface. This review focuses on the recent efforts of employing DNA to manipulate the interfacial properties of AuNPs, as well as the major advances in the colorimetric detection of heavy metals. Beginning with the introduction of the fundamental aspects of DNA and AuNPs, three main strategies of constructing DNA-AuNPs with DNA binding-responsive interface are discussed, namely, crosslinking, electrostatic interaction and base pair stacking. Then, recent achievements in colorimetric biosensing of heavy metals based on manipulation of the interface of DNA-AuNPs are surveyed and compared. Finally, perspectives on challenges and opportunities for future research in this field are provided.

Rationally designed dual-plasmonic gold nanorod@cuprous selenide hybrid heterostructures by regioselective overgrowth for in vivo photothermal tumor ablation in the second near-infrared biowindow

NIR-II plasmonic materials offer multiple functionalities for in vivo biomedical applications, such as photothermal tumor ablation, surface-enhanced Raman scattering biosensing, photoacoustic imaging, and drug carriers. However, integration of noble metals and plasmonic semiconductors is greatly challenging because of the large lattice-mismatch. This study reports the regioselective overgrowth of Cu_2-xSe on gold nanorods (GNRs) for preparation of dual-plasmonic GNR@Cu_2-xSe hybrid heterostructures with tunable NIR-II plasmon resonance absorption for in vivo photothermal tumor ablation.

Methods: The regioselective deposition of amorphous Se and its subsequent conversion into Cu_2-xSe on the GNRs are performed by altering capping agents to produce the GNR@Cu_2-xSe heterostructures of various morphologies. Their photothermal performances for NIR-II photothermal tumor ablation are evaluated both in vitro and in vivo.

Results: We find that the lateral one- and two-side deposition, conformal core-shell coating and island growth of Cu_2-xSe on the GNRs can be achieved using different capping agents. The Cu_2-xSe domain size in these hybrids can be effectively adjusted by the SeO₂ concentration, thereby tuning the NIR-II plasmon bands. A photothermal conversion efficiency up to 58-85% and superior photostability of these dual-plasmonic hybrids can be achieved under the NIR-II laser. Results also show that the photothermal conversion efficiency is dependent on the proportion of optical absorption converted into heat; however, the temperature rise is tightly related to the concentration of their constituents. The excellent NIR-II photothermal effect is further verified in the following in vitro and in vivo experiments.

Conclusions: This study achieves one-side or two-side deposition, conformal core-shell coating, and island deposition of Cu_2-xSe on GNRs for GNR@Cu_2-xSe heterostructures with NIR-II plasmonic absorption, and further demonstrates their excellent NIR-II photothermal tumor ablation in vivo. This study provides a promising strategy for the rational design of NIR-II dual-plasmonic heterostructures and highlights their therapeutic in vivo potential.

Plasmonic Metallic Heteromeric Nanostructures

Binary, ternary, and other high-order plasmonic heteromers possess remarkable physical and chemical properties, enabling them to be used in numerous applications. The seed-mediated approach is one of the most promising and versatile routes to produce plasmonic heteromers. Selective growth of one or multiple domains on desired sites of noble metal, semiconductor, or magnetic seeds would form desired heteromeric nanostructures with multiple functionalities and synergistic effects. In this work, the challenges for the synthetic approaches are discussed with respect to tuning the thermodynamics, as well as the kinetic properties (e.g., pH, temperature, injection rate, among others). Then, plasmonic heteromers with their structure advantages displaying unique activities compared to other hybrid nanostructures (e.g., core-shell, alloy) are highlighted. Some of the main most recent applications of plasmonic heteromers are also presented. Finally, perspectives for further exploitation of plasmonic heteromers are demonstrated. The goal of this work is to provide the current know-how on the synthesis routes of plasmonic heteromers in a summarized manner, so as to achieve a better understanding of the resulting properties and to gain an improved control of their performances and extend their breadth of applications.

Gold nanonails for surface-enhanced infrared absorption

Surface-enhanced infrared absorption (SEIRA) can dramatically enhance the vibrational signals of analyte molecules owing to the interaction between plasmons and molecular vibrations. It has huge potential for applications in various detection and diagnostic fields. High-aspect-ratio rod-like metal nanostructures have been the most widely studied nanomaterials for SEIRA. However, nearly all of the rod-like nanostructures reported previously are fabricated using physical methods. They suffer from damping and low areal number densities. In this work, high-aspect-ratio Au nanorods are synthesized, and Au nanonails are prepared through Au overgrowth on the as-prepared Au nanorods. The aspect ratios of the Au nanorods and nanonails can be varied in the range of ∼10 to ∼60, and their longitudinal dipolar plasmon resonance wavelengths can be correspondingly tailored from ∼1.6 to ∼8.3 μm. The Au nanonails exhibit superior SEIRA performance with 4-aminothiophenol used as the probe molecules. They are further used to detect the common biomolecule l-cysteine. Numerical simulations are further performed to understand the experimental results. They match well with the experimental observations, revealing the mechanism of the SEIRA enhancement. Our study demonstrates that colloidal high-aspect-ratio Au nanonails and nanorods can function as SEIRA nanoantennas for highly sensitive molecular detection in various situations.

Gold nanorod/molybdenum sulfide core-shell nanostructures synthesized by a photo-induced reduction process

Coupling of plasmonic nanostructures and semiconductors gives promising hybrid nanostructures that can be used in different applications such as photosensing and energy conversion. In this report, we describe an approach for fabricating a new hybrid material by coupling a gold nanorod (Au NR) core and amorphous molybdenum sulfide (MoS_x) shell. The Au NR/MoS_x core-shell structure is achieved by exploiting the hot electrons generated in the plasmonic excitation of Au NRs to drive the reduction of [MoS₄]^2-, which is pre-adsorbed on the Au NR surface, producing a thin MoS_x layer. This approach allows us to control the thickness of the MoS_x coating layer on the Au NR surface. The resultant Au NR/MoS_x hybrid is characterized by absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy elemental mapping, x-ray diffraction and x-ray photoelectron spectroscopy.

Surface Ligands as Permeation Barrier in the Growth and Assembly of Anisotropic Semiconductor Nanocrystals

Because of the large surface-to-volume ratio of colloidal nanocrystals (NCs), surfactant molecules grafted at the NC surface play an important role in NC growth, interparticle interaction, processing, and application. For this reason, much progress has been made in understanding the surface chemistry of NCs along with the organic ligand shell, particularly in terms of grafted polar groups. However, most explanations of aliphatic counterparts are based on spherical NCs that usually have a dilute ligand layer. In anisotropic NCs such as nanorods and nanoplatelets, the linearly extended dimension results in a high-density aliphatic layer on the NC surface. Unlike spherical NCs, the compact organic shell could serve as a permeation membrane, effectively impeding a penetration of foreign molecules toward the NC surface. In this Perspective, we highlight the effects of ligand configuration on the properties of anisotropic NCs by exploring morphologies, assembled superstructures, and surface reaction of anisotropic NCs.

Bioinspired One-Step Synthesis of Pomegranate-like Silica@Gold Nanoparticles with Surface-Enhanced Raman Scattering Activity

Gold-silica (Au-SiO₂) nanohybrids are of great technological importance, and it is crucial to develop facile synthetic protocols to prepare Au-SiO₂ nanohybrids with novel structures. Here we report the bioinspired synthesis of pomegranate-like SiO₂@Au nanoparticles (P-SiO₂@Au NPs) via one-step aqueous synthesis from chloroauric acid and tetraethyl orthosilicate mediated by a basic amino acid, arginine. Effects of chloroauric acid, tetraethyl orthosilicate, and arginine on the morphology and optical property of the products are investigated in detail. The P-SiO₂@Au NPs achieve tunable plasmon resonance depending on the amount of chloroauric acid, which affects the size and shape of the P-SiO₂@Au NPs. Finite-difference time-domain simulations are performed, revealing that the plasmon peak red-shifts with increasing particle size. Arginine serves as the reducing and capping agents for Au as well as the catalyst for SiO₂ formation and also promotes the combination of Au and SiO₂. Formation process of the P-SiO₂@Au NPs is clarified through time-course analysis. The P-SiO₂@Au NPs show good sensitivity for both colloidal and paper-based surface-enhanced Raman scattering measurements. They achieve enhancement factors of 4.3 × 10⁷-8.5 × 10⁷ and a mass detection limit of ca. 1 ng using thiophenol as the model analyte.