Silver Double Nanorings with Circular Hot Zone

Au-Ag controllable composition nanoalloying of hexagonal nanoplates: heterogeneous interfacial nanogaps enhance near-field focusing

In this study, we present a novel strategy for fabricating binary-array surface-enhanced Raman scattering (SERS) substrates composed of gold (Au) and silver (Ag) plasmonic hexagonal nanoplates (h-NPLs), functioning as a "nanoalloy" system. Using Au h-NPLs as scaffolds, we synthesized Ag h-NPLs of closely identical sizes and shapes, facilitating the construction of a mixed plasmonic system. The flat morphology of h-NPLs enables their face-to-face assembly into parallel "wire-like string" arrays, referred to as "columnar superpowders (SPs)", which expose nanogaps perpendicular to the incident light and maximize near-field focusing. We achieved anisotropic superstructures of Au-Ag core-shell h-NPLs through epitaxial Ag growth on Au surfaces, controlled by the interplay of halide ions and surface crystal energy differences. Free-standing columnar SPs were fabricated via an upside-down assembly method, forming dense face-to-face nanogaps that act as hotspots for SERS enhancement. The thickness of the Ag shell was critical in optimizing plasmonic coupling between the Au core and Ag shell, thereby enhancing near-field effects. The observed strong near-field focusing originates from synergistic intra- and inter-interface plasmonic coupling, which induces a mirror charge effect, amplifying near-field polarization and SERS sensitivity. Theoretical simulations and experimental bulk SERS analyses validated these effects, underscoring the potential of binary-component nanoalloy structures for advanced optical sensing technologies. This work highlights the critical role of heterogeneous interfacial nanogaps in enhancing both near- and far-field plasmonic effects, paving the way for next-generation SERS applications.

Nature-inspired surface modification strategies for implantable devices

Medical and implantable devices are essential instruments in contemporary healthcare, improving patient quality of life and meeting diverse clinical requirements. However, ongoing problems such as bacterial colonization, biofilm development, foreign body responses, and insufficient device-tissue adhesion hinder the long-term effectiveness and stability of these devices. Traditional methods to alleviate these issues frequently prove inadequate, necessitating the investigation of nature-inspired alternatives. Biomimetic surfaces, inspired by the chemical and physical principles found in biological systems, present potential opportunities to address these challenges. Recent breakthroughs in manufacturing techniques, including lithography, vapor deposition, self-assembly, and three-dimensional printing, now permit precise control of surface properties at the micro- and nanoscale. Biomimetic coatings can diminish inflammation, prevent bacterial adherence, and enhance stable tissue integration by replicating the antifouling, antibacterial, and adhesive properties observed in creatures such as geckos, mussels, and biological membranes. This review emphasizes the cutting-edge advancements in biomimetic surfaces for medical and implantable devices, outlining their design methodologies, functional results, and prospective clinical applications. Biomimetic coatings, by integrating biological inspiration with advanced surface engineering, have the potential to revolutionize implantable medical devices, providing safer, more lasting, and more effective interfaces for prolonged patient benefit.

Advances, Challenges, and Opportunities in Plasmonic Nanogap-Enhanced Raman Scattering with Nanoparticles

Surface-enhanced Raman scattering has been widely used for molecular/material characterization and chemical and biological sensing and imaging applications. In particular, plasmonic nanogap-enhanced Raman scattering (NERS) is based on the highly localized electric field formed within the nanogap between closely spaced metallic surfaces to more strongly amplify Raman signals than the cases with molecules on metal surfaces. Nanoparticle-based NERS offers extraordinarily strong Raman signals and a plethora of opportunities in sensing, imaging and many different types of biomedical applications. Despite its potential, several challenges still remain for NERS to be widely useful in real-world applications. This Perspective introduces various plasmonic nanogap configurations with nanoparticles, discusses key advances and critical challenges while addressing possible misunderstandings in this field, and provides future directions for NERS to generate stronger, more uniform, and stable signals over a large number of structures for practical applications.

Multiscale Tunable Nanorings Based on Bi-Component Micellar-Configuration-Transformation Induced by Hydrophobicity

Ringy nanostructures are amazing materials, displaying unique optical, magnetic, and electronic properties highly related to their dimensions. A strategy capable of continuously tailoring the diameter of nanorings is the key to elucidating their structure-function relationship. Herein, a method of bi-component micellar-configuration-transformation induced by hydrophobicity for the synthesis of nanorings with diameters ranging from submicron (≈143 nm) to micron (≈4.8 µm) and their carbonaceous analogs is established. Remarkably, the nanorings fabricated with this liquid phase strategy achieve the record for the largest diameter span. Through varying the molecular lengths of fatty alcohols and copolymers, shortening the molecular length of fatty alcohol can swell the primary micelles, improve the exposure of hydrophobic component and boost the assembly kinetics for ultra-large nanorings is shown here. On the other hand, shortening the molecular length of the copolymer will give rise to ultra-small nanorings by reducing the size of primary micelles and shortening the assembly time. When assembling the nanorings into monolayer arrays and then depositing Au, such substrate displays enhanced surface-enhanced Raman scattering (SERS) performance. This research develops a facile method for the controllable synthesis of ringy materials with multiscale tunable diameters and may inspire more interesting applications in physics, optical, and sensors.

Crystal Engineering and Self-Assembled Nanoring Formation with Purine-CdII /HgII Supramolecular Frameworks

We report three complexes of CdII and HgII with two purine rare tautomers, N9-(pyridin-2-ylmethyl)-N6 -methoxyadenine, L1 and N7-(pyridin-2-ylmethyl)-N6 -methoxyadenine, L2, highlighting diverse crystallographic signatures exhibited by them. Influence of substituents, binding sites, steric effects and metal salts on the different modes of binding enabled an insight into metal-nucleobase interactions. L1 interacted with two and three equivalents of Cd(NO3 )2 .4H2 O and HgCl2 , respectively, while L2 interacted with two equivalents of HgCl2 , altogether leading to three different complexes (1 [C48 H48 Cd6 N34 O50 ], 2 [C12 H12 Cl4 Hg2 N6 O] and 3 [C12 H12 Cl2 HgN6 O]) possessing varied dimensionality and stabilising interactions. The photoluminescent properties of these coordination frameworks have also been probed. Notably, nanoring-like structures were obtained, as a result of self-assembly of 3 when investigated by transmission electron microscopy, additionally supported by molecular dynamics simulations.

Photothermal Nanomaterials: A Powerful Light-to-Heat Converter

All forms of energy follow the law of conservation of energy, by which they can be neither created nor destroyed. Light-to-heat conversion as a traditional yet constantly evolving means of converting light into thermal energy has been of enduring appeal to researchers and the public. With the continuous development of advanced nanotechnologies, a variety of photothermal nanomaterials have been endowed with excellent light harvesting and photothermal conversion capabilities for exploring fascinating and prospective applications. Herein we review the latest progresses on photothermal nanomaterials, with a focus on their underlying mechanisms as powerful light-to-heat converters. We present an extensive catalogue of nanostructured photothermal materials, including metallic/semiconductor structures, carbon materials, organic polymers, and two-dimensional materials. The proper material selection and rational structural design for improving the photothermal performance are then discussed. We also provide a representative overview of the latest techniques for probing photothermally generated heat at the nanoscale. We finally review the recent significant developments of photothermal applications and give a brief outlook on the current challenges and future directions of photothermal nanomaterials.

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.

Multi-Layered PtAu Nanoframes and Their Light-Enhanced Electrocatalytic Activity via Plasmonic Hot Spots

Here, the rational design of complex PtAu double nanoframes (DNFs) for plasmon-enhanced electrocatalytic activity toward the methanol oxidation reaction (MOR) is reported. The synthetic strategy for the DNFs consists of on-demand multiple synthetic chemical toolkits, including well-faceted Au growth, rim-on selective Pt deposition, and selective Au etching steps. DNFs are synthesized by utilizing Au truncated octahedrons (TOh) as a starting template. The outer octahedral (Oh) nanoframes (NFs) nest the inner TOh NFs, eventually forming DNFs with a tunable intra-nanogap distance. Residual Au adatoms on Pt skeletons act as light entrappers and produce plasmonic hot spots between inner and outer frames through localized surface plasmon resonance (LSPR) coupling, which promotes enhanced electrocatalytic activity for the MOR. Importantly, the correlation between the gap-induced hot carriers and electrocatalytic activity is evaluated. The highest catalytic activity is achieved when the gap is the narrowest. To further harness their light-trapping capability, hierarchically structured triple NFs (TNFs) are synthesized, wherein three NFs are entangled in a single entity with a high density of hot regions, exhibiting superior electrocatalytic activity toward the MOR with a sixfold larger current density under light irradiation compared to the dark conditions.

Multiple Stepwise Synthetic Pathways toward Complex Plasmonic 2D and 3D Nanoframes for Generation of Electromagnetic Hot Zones in a Single Entity

ConspectusRational design of nanocrystals with high controllability via wet chemistry is of critical importance in all areas of nanoscience and nanotechnology research. Specifically, morphologically complex plasmonic nanoparticles have received considerable attention because light-matter interactions are strongly associated with the size and shape of nanoparticles. Among many types of nanostructures, plasmonic nanoframes (NFs) with controllable structural intricacy could be excellent candidates as strong light-entrappers with inner voids as well as high surface area, leading to highly effective interaction with light and analytes compared to their solid counterparts. However, so far studies on single-rim-based NFs have suffered from insufficient near-field focusing capability due to their structural simplicity (e.g., a single rim or NF molded from simple platonic solids), which necessitates a conceptually new NF architecture. If one considers a stereoscopic nanostructure with dual, triple, and multiple resonant intra-nanogaps on each crystallographic facet of nanocrystals, unprecedented physicochemical properties could be expected. Realizing such complex multiple NFs with intraparticle surface plasmon coupling via localized surface plasmon resonance is very challenging due to the lack of synthetic strategic principles with systematic structural control, all of which require a deep understanding of surface chemistry. Moreover, realizing those complex architectures with high homogeneity in size and shape via a bottom-up method where diverse particle interactions are involved is more challenging. Although there have been several reports on NFs used for catalysis, techniques for production of structurally complex NFs with high uniformity and an understanding of the correlation between such complexity in a single plasmonic entity and electromagnetic near-field focusing have remained highly elusive.In this Account, we will summarize and highlight the rational synthetic pathways for the design of complex two-dimensional (2D) and three-dimensional (3D) NFs with unique inner rim structures and characterize their optical properties. This systematic strategy is based on publications from our group during the last 10 years. First, we will introduce a chemical step of shape transformation of triangular Au nanoplates to circular and hexagonal plates, which are used as sacrificial layers for the formation of NFs. Then, we will describe the methods on how to synthesize monorim-based plasmonic NFs using Pt scaffolds with different shapes and correlate with their electromagnetic near-field. Then, we will describe a multiple stepwise synthetic method for the formation of 2D complex NFs wherein different starting Au nanocrystals evolved from systematic shape transformation are used to produce circular, triangular, hexagonal, crescent, and Y-shaped inner hot zones. Then, we will discuss how one can synthesize NFs with multiple rims wherein rims with different diameters are concentrically connected, by exploiting chemical toolkits such as eccentric and concentric growth of Au, borrowing the concept of total synthesis that is frequently adopted in organic chemistry. We then introduce dual-rim-faceted NFs and frame-in-frame 3D matryoshka NF geometries via well-faceted growth of Au with high control of intra-nanogaps. Finally, and importantly, we will provide examples of more advanced hierarchical NF architectures produced by controlling geometrical shapes of nanoparticles, number of rims, and different components, leading to the expansion of the NF library.

Bimetallic alloy Ag@Au nanorings with hollow dual-rims focus near-field on circular intra-nanogaps

Here, we report a highly sensitive and reliable surface enhanced Raman scattering (SERS)-based immunoassay using bimetallic alloy Ag@Au hollow dual-rim nanorings (DRNs) where two hollow nanorings with different diameters are concentrically overlapped and connected by thin metal ligaments, forming circular hot-zones in the intra-nanogaps between the inner and outer rims. Pt DRNs were first prepared, and then Ag was deposited on the surface of the Pt skeleton, followed by Au coating, resulting in alloy Ag@Au hollow DRNs. The chemical stability of Au and the high optical properties of Ag are incorporated into a single entity, Ag@Au hollow DRNs, enabling strong single-particle SERS activity and biocompatibility through surface modification with thiol-containing functionalities. When Ag@Au hollow DRNs were utilized as nanoprobes for detecting human chorionic gonadotropin (HCG) hormone through a SERS-based immunoassay, a very low limit of detection of 10 pM with high reliability was achieved, strongly indicating their advantage as ultrasensitive SERS nanoprobes.

Chiral plasmonic Au-Ag core shell nanobipyramid for SERS enantiomeric discrimination of biologically relevant small molecules

The identification of enantiomers is of great importance in chiral separations and medicinal chemistry. While Surface-enhanced Raman spectroscopy (SERS) is a technique that provides vibrational fingerprints of analytes. The enantiomers identification relies on the SERS difference between left and right-handed circularly polarized light or additional selectors for indirect distinction. In this work, Au-Ag core shell nanobipyramid (L/D-Au@Ag BPs) were synthesized guiding by chiral encoder of L/D-cysteine. L/D-Au@Ag BPs produced plasmon-induced circular dichroism signals in the plasmon resonance absorption band, which can be tuned by modulation the amount of cysteine. Moreover, the chiral anisotropy factor of L/D-Au@Ag BPs at 532 nm can reach 5.11 × 10^-3. Due to the selective resonance coupling between L/D-Au@Ag BPs and different enantiomers, L/D-Au@Ag BPs were further used as SERS substrates for efficient discrimination of biologically relevant small molecules. Chiral Au@Ag BPs display the potential for chiral drug identification.

Asymmetric seed passivation for regioselective overgrowth and formation of plasmonic nanobowls

Plasmonic nanoparticles (NPs) have garnered excitement over the past several decades stemming from their unique optoelectronic properties, leading to their use in various sensing applications and theranostics. Symmetry dictates the properties of many nanomaterials, and nanostructures with low, but still defined symmetries, often display markedly different properties compared to their higher symmetry counterparts. While numerous methods are available to manipulate symmetry, surface protecting groups such as polymers are finding use due to their ability to achieve regioselective modification of NP seeds, which can be removed after overgrowth as shown here. Specifically, poly(styrene-b-polyacrylic acid) (PSPAA) is used to asymmetrically passivate cubic Au seeds through competition with hexadecyltrimethylammonium bromide (CTAB) ligands. The asymmetric passivation via collapsed PSPAA causes only select vertices and faces of the Au cubes to be available for deposition of new material (i.e., Au, Au-Ag alloy, and Au-Pd alloy) during seeded overgrowth. At low metal precursor concentrations, deposition follows observations from unpassivated seeds but with new material growing from only the exposed seed portions. At high metal precursor concentrations, nanobowl-like structures form from interaction between the depositing phase and the passivating PSPAA. Through experiment and simulation, the optoelectronic properties of these nanobowls were probed, finding that the interiors and exteriors of the nanobowls can be functionalized selectively as revealed by surface enhanced Raman spectroscopy (SERS).

A Metallic Niobium Nitride with Open Nanocavities for Surface-Enhanced Raman Spectroscopy

The construction of open hot-spot structures that facilitate the entry of analytes is crucial for surface-enhanced Raman spectroscopy. Here, metallic niobium nitride (NbN) three-dimensional (3D) hierarchical networks with open nanocavity structure are first found to exhibit a strong visible-light localized surface plasmon resonance (LSPR) effect and extraordinary surface-enhanced Raman scattering (SERS) performance. The unique nanocavity structure allows easy entry of molecules, promoting the utilization of electromagnetic hot spots. The NbN substrate has a lowest detection limit of 1.0 × 10^-12 M and a Raman enhancement factor (EF) of 1.4 × 10⁸ for contaminants. Furthermore, the NbN hierarchical networks possess outstanding environmental durability, high signal reproducibility, and detection universality. The remarkable SERS sensitivity of the NbN substrate can be attributed to the joint effect of LSPR and interfacial charge transport (CT).

Three-dimensional nanoframes with dual rims as nanoprobes for biosensing

Three-dimensional (3D) nanoframe structures are very appealing because their inner voids and ridges interact efficiently with light and analytes, allowing for effective optical-based sensing. However, the realization of complex nanoframe architecture with high yield is challenging because the systematic design of such a complicated nanostructure lacks an appropriate synthesis protocol. Here, we show the synthesis method for complex 3D nanoframes wherein two-dimensional (2D) dual-rim nanostructures are engraved on each facet of octahedral nanoframes. The synthetic scheme proceeds through multiple executable on-demand steps. With Au octahedral nanoparticles as a sacrificial template, sequential processes of edge-selective Pt deposition and inner Au etching lead to Pt octahedral mono-rim nanoframes. Then, adlayers of Au are grown on Pt skeletons via the Frank-van der Merwe mode, forming sharp and well-developed edges. Next, Pt selective deposition on both the inner and outer boundaries leads to tunable geometric patterning on Au. Finally, after the selective etching of Au, Pt octahedral dual-rim nanoframes with highly homogeneous size and shape are achieved. In order to endow plasmonic features, Au is coated around Pt frames while retaining their geometric shape. The resultant plasmonic dual-rim engraved nanoframes possess strong light entrapping capability verified by single-particle surface-enhanced Raman scattering (SERS) and show the potential of nanoprobes for biosensing through SERS-based immunoassay.

Most SERS-active nanostructures suffer from low robustness against misalignment to field polarization. Here, the authors demonstrate three-dimensional nanoframes of octahedral geometry, with two rims engraved on each facet, as polarization-independent SERS nanoprobes.

Nesting of multiple polyhedral plasmonic nanoframes into a single entity

The development of plasmonic nanostructures with intricate nanoframe morphologies has attracted considerable interest for improving catalytic and optical properties. However, arranging multiple nanoframes in one nanostructure especially, in a solution phase remains a great challenge. Herein, we show complex nanoparticles by embedding various shapes of three-dimensional polyhedral nanoframes within a single entity through rationally designed synthetic pathways. This synthetic strategy is based on the selective deposition of platinum atoms on high surface energy facets and subsequent growth into solid platonic nanoparticles, followed by the etching of inner Au domains, leaving complex nanoframes. Our synthetic routes are rationally designed and executable on-demand with a high structural controllability. Diverse Au solid nanostructures (octahedra, truncated octahedra, cuboctahedra, and cubes) evolved into complex multi-layered nanoframes with different numbers/shapes/sizes of internal nanoframes. After coating the surface of the nanoframes with plasmonically active metal (like Ag), the materials exhibited highly enhanced electromagnetic near-field focusing embedded within the internal complicated rim architecture.

The spatial configuration of nanostructure building blocks determines the physical and optical properties of their superstructures. Here, the authors report on complex nanoparticles in which different geometric forms of nanoframes are nested into a single entity by multistep chemical reactions.

Heterogeneous Component Au (Outer)-Pt (Middle)-Au (Inner) Nanorings: Synthesis and Vibrational Characterization on Middle Pt Nanorings with Surface-Enhanced Raman Scattering

We report a synthetic approach for heterometallic (Au-Pt-Au) nanorings with intertwined triple rings (NITs), wherein three differently sized metal circular nanorings concentrically overlap in a single entity. The synthetic method allows one to control the component of core nanorings (Au or Pt) with a tunable gap distance. The narrow circular nanogaps between inner and outer Au rings strongly enhance the electromagnetic near-field via intraparticle coupling of localized surface plasmon resonance, which realizes surface-enhanced Raman scattering (SERS) at the single-particle level. Importantly, when the component of the middle ring is Pt, in situ SERS measurement for electrochemical reactions on Pt domains could be monitored with electrochemical potential variations due to the near-field focusing that is assisted by plasmonically active inner and outer Au nanorings, which is not feasible with pure Pt nanoparticle systems. The resulting NIT systems are robust and may benefit the synthesis of complicated nanostructures, giving myriad applications.

Aluminum sheet induced flower-like carbon nitride anchored with silver nanowires for highly efficient SERS detection of trace malachite green

The sensitive detection of malachite green (MG) in aquaculture wastewater is necessary for its residual poses a great threat to the living systems. Herein, the flower-like C₃N₄ (f-C₃N₄) nanostructure induced by Al sheet in the hydrothermal process is constructed. Subsequently, Ag nanowires (AgNWs) supported on Al/f-C₃N₄ and the strong interaction between AgNWs and Al/f-C₃N₄ are confirmed by XPS, Raman spectroscopy, UV-vis diffuse reflectance and fluorescence spectroscopy. Importantly, the portable Al/f-C₃N₄/AgNWs substrate shows the outstanding SERS response for MG, which is attributed to enhanced electromagnetic effect of AgNWs on large amount of corrugated and creviced regions in the flower-like Al/f-C₃N₄ and the charge transfer among the components. Also, the prepared Al/f-C₃N₄ nanostructure provides large specific surface area and abundant "N" active sites for AgNWs, and the high enrichment ability of Al/f-C₃N₄ towards MG molecules by the strong π-π stacking interaction. The detection limit of Al/f-C₃N₄/AgNWs for MG is as low as 8.38 × 10^-12 mol L^-1. The substrate can be reproduced and reused for at least 7 cycles, and the activity can still be kept after laid up for 49 days. Importantly, it unfolds a good sensitivity and selectivity for MG in actual water sample. Results indicate that the Al/f-C₃N₄/AgNWs substrate has a promising potential in practical application for trace detection of MG.

Clusters-based silver nanorings: An active substrate for surface-enhanced Raman scattering

Plasmonic nanostructures, particularly irregular surfaces of ring-like silver (Ag) nanostructures are promising candidates in surface-enhanced Raman scattering (SERS) spectroscopy. In this work, clusters-based Ag nanorings have been fabricated and characterized as SERS-active substrates. The rim of the as-fabricated Ag nanorings was found neither discontinuous nor linear aggregation of nanoparticles. High-resolution field emission scanning electron microscopy (FESEM) revealed that the individual constituent clusters were different from each other, particularly in terms of size and shape in addition to the cases how such clusters were emerged as the edge of the nanoring. Considering the dimensions of the clusters and the arrangement of such clusters as nanorings, it was speculated that the local electromagnetic (EM) near-field distributions would excel and thus enhanced SERS signals would be achieved. Indeed, the inherent features of the nanorings facilitated to achieve SERS enhancement factors as high as 2.1 × 10⁴. SERS-activity of as-fabricated Ag nanorings was confirmed using Rhodamine 6G (R6G) as Raman-active dyes and the enhancement was compared to those obtained from R6G adsorbed on Ag-ZnO/Glass and ZnO/Glass. To the best of our knowledge, this is the first attempt to explore the impact of localized EM near-field within the segments of nanorings through SERS spectroscopy. A model was designed resembling the nanorings under this investigation to simulate EM near-field distributions by finite difference time domain (FDTD) analysis. The dimensions of the model geometry were chosen according to the observations achieved by FESEM. To simplify the simulations, nanoobjects were considered spherical and organized in a periodic fashion, although the constituent clusters of Ag nanorings were found irregular in shape and arrangement. Since EM near-field distribution highly depends on interparticle gaps, three scenarios were implemented, such as, small gap in between two adjacent nanoobjects and adjacent nanoobjects in touch and overlapped. Each configuration was simulated and EM near-field distribution was extracted for s-, p- and 45⁰ of incident polarizations followed by a plausible correlation to SERS enhancements. Such correlated investigations as well as clusters-based Ag nanorings not only inspire the ones to look for cost-effective SERS-active substrate, but also understand the underlying EM mechanism in SERS enhancements.

Synthesis, Assembly, Optical Properties, and Sensing Applications of Plasmonic Gap Nanostructures

Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals. However, the reproducible and controlled preparation of highly uniform plasmonic nanogaps and the prediction, understanding, and control of their optical properties, especially for nanogaps in the nanometer or sub-nanometer range, remain challenging. This is because subtle changes in the nanogap significantly affect the plasmonic response and are of paramount importance to the desired optical performance and further applications. Here, recent advances in the synthesis, assembly, and fabrication strategies, prediction and control of optical properties, and sensing applications of PGNs are discussed, and perspectives toward addressing these challenging issues and the future research directions are presented.

Creation and Reconstruction of Thermochromic Au Nanorods with Surface Concavity

Conventional colloidal syntheses typically produce nanostructures with positive curvatures due to thermodynamic preference. Here, we demonstrate the creation of surface concavity in Au nanorods through seed-mediated growth in confined spaces and report their thermochromic responses to temperature changes. The unique surface concavity is created by templating against Fe₃O₄ nanorods, producing a new concavity-sensitive plasmonic band. Due to the high surface energy, the metastable nanorods can be reconstructed at a moderate temperature, enabling convenient and precise tuning of their plasmonic properties by aging in different solvents. Such structural reconstruction of concave Au nanorods enables the fabrication of thermochromic plasmonic films that can display images with vivid color changes or exhibit encrypted, invisible information upon aging. This templating strategy is universal in creating concave nanostructures, which may open the door to designing new nanostructures with promising applications in sensing, anticounterfeiting, information encryption, and displays.

Au Nanorings with Intertwined Triple Rings

We designed complex Au nanorings with intertwined triple rings (ANITs) in a single entity to amplify the efficacy of near-field focusing. Such a complex and unprecedented morphology at the nanoscale was realized through on-demand multistepwise reactions. Triangular nanoprisms were first sculpted into circular nanorings, followed by a series of chemical etching and deposition reactions eventually leading to ANITs wherein thin metal bridges hold the structure together without any linker molecules. In the multistepwise reaction, the well-faceted growth pattern of Au, which induces the growth of two distinctive flat facets in a lateral direction, is important to evolve the morphology from single to multiple nanorings. Although our synthesis proceeds through multiple steps in one batch without purification steps, it shows a remarkably high yield (>∼90%) at the final stage. The obtained high degree of homogeneity (in both shape and size) of the resulting ANITs allowed us to systematically investigate the corresponding localized surface plasmon resonance (LSPR) coupling with varying nanoring arrangements and observe their single-particle surface enhanced Raman scattering (SERS). Surprisingly, individual ANITs exhibited an enormously large enhancement factor (∼10⁹), which confirms their superior near-field focusing relative to other reported nanoparticles.

Web-above-a-Ring (WAR) and Web-above-a-Lens (WAL): Nanostructures for Highly Engineered Plasmonic-Field Tuning and SERS Enhancement

Synthetic strategies of web-above-a-ring (WAR) and web-above-a-lens (WAL) nanostructures are reported. The WAR has a controllable gap between the nanoring core and a nanoweb with nanopores for the effective confinement of electromagnetic field in the nanogap and subsequent surface-enhanced Raman scattering (SERS) of Raman dyes inside the gap with high signal reproducibility, which are attributed to the generation of circular 3D hot zones along the rim of Pt@Au nanorings with wrapping nanoweb architecture. More specifically, Pt@Au nanorings are adopted as a plasmonic core for structural rigidity and built porous nanowebs above them through a controlled combination of galvanic exchange and the Kirkendall effect. Both nanoweb and nanolens structures are also formed on Pt@Au nanoring, which is WAL. structure. Remarkably, plasmonic hot zone, nanopores, and hot lens are formed inside a single WAL nanostructure, and these structural components are orchestrated to generate stronger SERS signals.

Flexible nanocellulose-based SERS substrates for fast analysis of hazardous materials by spiral scanning

Surface-enhanced Raman scattering (SERS) has proven to be a valuable tool for assessing harmful chemicals in various substances, including water, soil, and foods. However, a fast measurement system is required for multiplexed detection to extend the range of its applications. The rotating scanning stage of the SERS substrate is considered to be a promising approach to achieving a fast measurement system. This paper reports a facile measurement system by using a flexible nanocellulose-based SERS substrate and a spiral scanning system, which rotates the cylinder sample holder and moves the stage. A flexible nanocellulose-based SERS substrate deposited with Au nanoparticles is suitable for the spiral scanning system, which requires SERS substrates to be highly flexible and durable. The well-known toxic fungicide, thiram, was tested by this system. The results revealed that the nanocellulose-based SERS substrate is well-fitted with a spiral scanning system and that the signal data from a large area substrate can be obtained within 30 s. It is noteworthy that the error of spiral scanning measurements is smaller than that of multi-spot sampling. This work provides a powerful tool for Raman spectroscopic analysis, which requires quantitative and fast testing. Furthermore, various flexible SERS substrates can be utilized in this system for toxic materials detection.

Au nanolenses for near-field focusing

We report a novel strategy for the synthesis of Pt@Au nanorings possessing near-field focusing capabilities at the center through which single-particle surface enhanced Raman scattering could be readily observed. We utilized Pt@Au nanorings as a light-absorber; the absorbed light could be focused at the center with the aid of a Au nanoporous structure. We synthesized the Au nanolens structure through a Galvanic exchange process between Au ions and Ag block at the inner domain of the Pt@Au nanoring. For this step, Ag was selectively pre-deposited at the inner domain of the Pt@Au nanorings through electrochemical potential-tuned growth control and different surface energies with regard to the inner and outer boundaries of the nanoring. Then, the central nanoporous architecture was fabricated through the Galvanic exchange of sacrificial Ag with Au ions leading to the resulting Au nanoring with a Au nanoporous structure at the center. We monitored the shape-transformation by observing their corresponding localized surface plasmon resonance (LSPR) profiles. By varying the rim thickness of the starting Pt@Au nanorings, the inner diameter of the nanolens was accordingly tuned to maximize near-field focusing, which enabled us to obtain the reproducible and light-polarization independent measurements of single-particle SERS. Through theoretical simulation, the near-field electromagnetic field focusing capability was visualized and confirmed through single-particle SERS measurement showing an enhancement factor of 1.9 × 10⁸ to 1.0 × 10⁹.

We synthesized a Au nanolens with electromagnetic near-field focusing capability by integrating a Au nanoporous structure at the center of the Pt@Au nanoring via synthetic steps of eccentric growth of Ag and nanoscale Galvanic exchange reaction.