Plasmonic Annular Nanotrenches with 1 nm Nanogaps for Detection of SARS-CoV-2 Using SERS-Based Immunoassay.

This study represents the synthesis of a novel class of nanoparticles denoted as annular Au nanotrenches (AANTs). AANTs are engineered to possess embedded, narrow circular nanogaps with dimensions of approximately 1 nm, facilitating near-field focusing for detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) via a surface-enhanced Raman scattering (SERS)-based immunoassay. Notably, AANTs exhibited an exceedingly low limit of detection (LOD) of 1 fg/mL for SARS-CoV-2 spike glycoproteins, surpassing the commercially available enzyme-linked immunosorbent assay (ELISA) by 6 orders of magnitude (1 ng/mL from ELISA). To assess the real-world applicability, a study was conducted on 50 clinical samples using an SERS-based immunoassay with AANTs. The results revealed a sensitivity of 96% and a selectivity of 100%, demonstrating the significantly enhanced sensing capabilities of the proposed approach in comparison to ELISA and commercial lateral flow assay kits.

Ready-to-Use Free-Standing Super-Powder Made with Complex Nanoparticles for SERS.

This study presents a straightforward and efficient synthetic approach for producing high-yield, ready-to-use, free-standing super-powder. The synthesis protocol demonstrates versatility, enabling the creation of assemblies from various nanoparticle morphologies and compositions without the need for specific substrates. Au nanorings are employed as building blocks for fabricating the super-powder, which can be used in surface-enhanced Raman spectroscopy (SERS). The distinctive aspect ratio of the ring nanoframes allows the formation of densely packed columnar assemblies on the substrate, aligning the exposed gaps perpendicular to the laser beam. This arrangement significantly enhances the charge separation among nanorings, leading to a highly focused near-field that is applicable to SERS analysis. The SERS detection feasibility of this powder in both pre- and post-contamination conditions is demonstrated. Using a wide range of building blocks, encompassing various shapes (for instance, rods, hexagons, cubes, cuboctahedrons, elongated dodecahedrons, triangular rings, double-rings, elongated dodecahedra frames, cuboctahedra frames, and double-walled frames), the generalizability of the process for synthesizing super-powders with diverse morphologies is substantiated.

Combinatorial Effect of Tricomponent Dual-Rim Nanoring Building Blocks: Label-Free SERS Detection of Biomolecules.

Detecting weakly adsorbing molecules via label-free surface-enhanced Raman scattering (SERS) has presented a significant challenge. To address this issue, we propose a novel approach for creating tricomponent SERS substrates using dual-rim nanorings (DRNs) made of Au, Ag, and CuO, each possessing distinct functionalities. Our method involves depositing different metals on Pt nanoring skeletons to obtain each nanoring with varying surface compositions while maintaining a similar size and shape. Next, the mixture of these nanorings is transferred into a monolayer assembly with homogeneous intermixing on a solid substrate. The surface of the CuO DRNs has dangling bonds (Cu2+) that facilitate the strong adsorption of carboxylates through the formation of chelating bonds, while the combination of Au and Ag DRNs significantly enhances the SERS signal intensity through a strong coupling effect. Notably, the tricomponent assemblies enable the successful SERS-based analysis of biomolecules such as amino acids, proteins, nucleobases, and nucleotides.

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.

Octahedron in a Cubic Nanoframe: Strong Near-Field Focusing and Surface-Enhanced Raman Scattering.

Here, we describe the synthesis of a plasmonic particle-in-a-frame architecture in which a solid Au octahedron is enclosed by a Au cubic nanoframe. The octahedra are positioned inside and surrounded by outer Au cubic nanoframes, creating intra-nanogaps within a single entity. Six sharp vertexes in the Au octahedra point toward the open (100) facets of the cubic nanoframes. This allows not only efficient interactions with the surroundings but also tip-enhanced electromagnetic near-field focusing at the sharp tips of the octahedra, combined with intraparticle coupling. The solid core-frame shell structure enhances near-field focusing, giving rise to a heightened concentration of "hot spots". This effect enables highly sensitive detection of 2-naphthalenethiol and thiram, indicating these substrates for use in surface-enhanced Raman spectroscopy-related applications.

Three-Dimensional Au Octahedral Nanoheptamers: Single-Particle and Bulk Near-Field Focusing for Surface-Enhanced Raman Scattering.

Herein, we present a synthetic approach to fabricate Au nanoheptamers composed of six individual Au nanospheres interconnected through thin metal bridges arranged in an octahedral configuration. The resulting structures envelop central Au nanospheres, producing Au nanosphere heptamers with an open architectural arrangement. Importantly, the initial Pt coating of the Au nanospheres is a crucial step for protecting the inner Au nanospheres during multiple reactions. As-synthesized Au nanoheptamers exhibit multiple hot spots formed by nanogaps between nanospheres, resulting in strong electromagnetic near-fields. Additionally, we conducted surface-enhanced Raman-scattering-based detection of a chemical warfare agent simulant in the gas phase and achieved a limit of detection of 100 ppb, which is 3 orders lower than that achieved using Au nanospheres and Au nanohexamers. This pseudocore-shell nanostructure represents a significant advancement in the realm of complex nanoparticle synthesis, moving the field one step closer to sophisticated nanoparticle engineering.

Plasmonic Dodecahedral-Walled Elongated Nanoframes for Surface-Enhanced Raman Spectroscopy.

Here, elongated pseudohollow nanoframes composed of four rectangular plates enclosing the sides and two open-frame ends with four ridges pointing at the tips for near-field focusing are reported. The side facets act as light-collecting domains and transfer the collected light to the sharp tips for near-field focusing. The nanoframes are hollow inside, allowing the gaseous analyte to penetrate through the entire architecture and enabling efficient detection of gaseous analytes when combined with Raman spectroscopy. The resulting nanostructures are named Au dodecahedral-walled nanoframes. Synthesis of the nanoframes involves shape transformation of Au nanorods with round tips to produce Au-elongated dodecahedra, followed by facet-selective Pt growth, etching of the inner Au, and regrowth steps. The close-packed assembly of Au dodecahedral-walled nanoframes exhibits an attomolar limit of detection toward benzenethiol. This significant enhancement in SERS is attributed to the presence of a flat solid terrace for a large surface area, sharp edges and vertices for strong electromagnetic near-field collection, and open frames for effective analyte transport and capture. Moreover, nanoframes are applied to detect chemical warfare agents, specifically mustard gas simulants, and 20 times higher sensitivity is achieved compared to their solid counterparts.

Spike Growth on Patterned Gold Nanoparticle Scaffolds.

This work reports a scaffold-templated, bottom-up synthesis of 3D anisotropic nanofeatures on periodic arrays of gold nanoparticles (AuNPs). Our method relies on substrate-bound AuNPs as large seeds with hemispherical shapes and smooth surfaces after the thermal annealing of as-fabricated particles. Spiky features were grown by immersing the patterned AuNPs into a growth solution consisting of a gold salt and Good's buffer; the number and length of spikes could be tuned by changing the solution pH and buffer concentration. Intermediate structures that informed the growth mechanism were characterized as a function of time by correlating the optical properties and spike features. Large-area (cm2) spiky AuNP arrays exhibited surface-enhanced Raman spectroscopy enhancement that was associated with increased numbers of high-aspect-ratio spikes formed on the AuNP seeds.

Au Octahedral Nanosponges: 3D Plasmonic Nanolenses for Near-Field Focusing.

Here, we report the synthesis of three-dimensional plasmonic nanolenses for strong near-field focusing. The nanolens exhibits a distinctive structural arrangement composed of nanoporous sponge-like networks within their interior. We denote these novel nanoparticles as "Au octahedral nanosponges" (Au Oh NSs). Employing a carefully planned multistep synthetic approach with Au octahedra serving as sacrificial templates, we successfully synthesized Au Oh NSs in solution. The porous domains resembling sponges contributed to enhanced scattering and absorption of incident light within metal ligaments. This optical energy was subsequently transferred to the nanospheres at the vertex, where near-field focusing was maximized. We named this observed enhancement a "lightning-sphere effect". Using single particle-by-particle surface-enhanced Raman scattering (SERS), we optimized the morphological dimensions of the spheres and porous domains to achieve the most effective near-field focusing. In the context of bulk SERS measurements targeting weakly adsorbing analytes (2-chloroethyl phenyl sulfide) in the gas phase, we achieved a low detection limit of 10 ppb. For nonadsorbing species (dimethyl methyl phosphonate), utilization of hybrid SERS substrates consisting of Au Oh NSs and metal-organic frameworks as gas-adsorbing intermediate layers was highly effective for successful SERS detection.

Plasmonic Double-Walled Nanoframes with Face-to-Face Nanogaps for Strong SERS Activity.

A synthesis method for plasmonic double-walled nanoframes was developed, where single-walled truncated octahedral nanoframes with (111) open facets and (100) solid flat planes are nested in a core-shell manner. By applying multiple chemical toolkits to Au cuboctahedrons as a starting template, Au double-walled nanoframes with controllable face-to-face nanogaps were successfully synthesized in high homogeneity in size and shape. Importantly, when the gap distance between inner and outer flat walled frames became closer, augmentation of electromagnetic near-field focusing was achieved, leading to generation of hot-zones, which was verified by surface-enhanced Raman spectroscopy. The unique optical property of Au double-walled nanoframes with high structural intricacy was carefully investigated and the SERS substrates comprising Au double-walled nanoframes with the narrowest nanogaps exhibited much improved near-field enhancement toward strongly and/or weakly adsorbing analytes, allowing for gas phase detection in chemical warfare agents, which is a huge challenge in early warning systems.

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.

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.

Plasmonic-Magnetic Active Nanorheology for Intracellular Viscosity.

We demonstrate active plasmonic systems where plasmonic signals are repeatedly modulated by changing the orientation of nanoprobes under an external magnetic field, which is a prerequisite for in situ active nanorheology in intracellular viscosity measurements. Au/Ni/Au nanorods act as "nanotransmitters", which transmit the mechanical motion of nanorods to an electromagnetic radiation signal as a periodic sine function. This fluctuating optical response is transduced to frequency peaks via Fourier transform surface plasmon resonance (FTSPR). As a driving frequency of the external magnetic field applied to the Au/Ni/Au nanorods increases and reaches above a critical threshold, there is a transition from the synchronous motion of nanorods to asynchronous responses, leading to the disappearance of the FTSPR peak, which allows us to measure the local viscosity of the complex fluids. Using this ensemble-based method with plasmonic functional nanomaterials, we measure the intracellular viscosity of cancer cells and normal cells in a reliable and reproducible manner.

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.

Plasmonic Cyclic Au Nanosphere Hexamers.

Rational design of plasmonic colloidal assemblies via bottom-up synthesis is challenging but would show unprecedented optical properties that strongly relate to the assembly's shape and spatial arrangement. Herein, the synthesis of plasmonic cyclic Au nanosphere hexamers (PCHs) is reported, wherein six Au nanospheres (Au NSs) are connected via thin metal ligaments. By tuning Au reduction, six dangling Au NSs are interconnected with a core hexagon nanoplate (NPL). Then, Pt atoms are selectively deposited on the edges of the spheres. After etching of the core, necklace-like nanostructures of Pt framework are obtained. Deposition of Au is followed, leading to PCHs in high yield (≈90%). Notably, PCHs exhibit the combinatorial plasmonic characteristics of individual Au NSs and the in-plane coupling of the six linked Au NSs. They yield highly uniform, reproducible, and polarization-independent single-particle surface-enhanced Raman scattering signals, which are attributed to the 2-dimensional isotropic alignment of the Au NSs. Those are applied to a SERS-based immunoassay as quantitative and qualitative single particle SERS nanoprobes. This assay shows a low limit-of-detection, down to 100 pm, which is orders of magnitude lower than those based on Au NSs, and one order of magnitude lower than an assay using analogous particles of smooth Au nanorings.

Synthesis of Pt Double-Walled Nanoframes with Well-Defined and Controllable Facets.

In this paper, we demonstrate the synthesis of morphologically complex nanoframes wherein a mixture of frames and thin solid planes, which we refer to as walled-nanoframes, are present in a single particle. By applying multiple chemical steps including shape evolution of Au nanocrystals and controlling chemical potential of solution for selective deposition, we successfully designed a variety of Pt nanoframes including Pt cuboctahedral nanoframes and Pt single-walled nanoframes. The rationale for on-demand chemical steps with well-faceted Au overgrowth allowed for the synthesis of double-walled nanoframes where two Pt single-walled nanoframes are concentrically overlapped in a single entity with a clearly discernible gap between the two nanoframes. Given the coexistence of an open structure of nanoframe and thin plates within one entity, the double-walled nanoframes showed a dramatic increase in catalytic activity toward the methanol oxidation reaction, acting as high-surface area, carbon-free, and volume-compact nanocatalysts.

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.

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.

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.

All-Hot-Spot Bulk Surface-Enhanced Raman Scattering (SERS) Substrates: Attomolar Detection of Adsorbates with Designer Plasmonic Nanoparticles.

Here we report a synthetic pathway toward Au truncated octahedral dual-rim nanoframes wherein two functional facets are formed including (1) eight hot nanogaps formed by hexagonal nanoframes embracing core circular nanorings for near-field focusing and (2) six flat squares that facilitate the formation of well-ordered arrays of nanoframes through self-assembly. The existence of intra-nanogaps in a single entity enables strong electromagnetic near-field focusing, allowing single-particle surface-enhanced Raman spectroscopy. Then, we built "all-hot-spot bulk SERS substrates" with those entities, wherein the presence of truncated terraces with high homogeneity in size and shape facilitate spontaneous self-assembly into a highly ordered and uniform superlattice, exhibiting a limit of detection of attomolar concentrations toward 2-naphthalenethiol, which is 6 orders lower than that of monorim counterparts. The observed low limit of detection originates from the combined synergic effect from both inter- and intraparticle coupling in a superlattice, which we dubbed "all-hot-spot bulk SERS substrates".