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

Plasmonic Nanogap-Enhanced Tunable Three-Dimensional Nanoframes in Application to Clinical Diagnosis of Alzheimer's Disease

Advancements in nanotechnology led to significant improvements in synthesizing plasmon-enhanced nanoarchitectures for biosensor applications, and high-yield productivity at low cost is vital to step further into medical commerce. Metal nanoframes via wet chemistry are gaining attention for their homogeneous structure and outstanding catalytic and optical properties. However, nanoframe morphology should be considered delicately when brought to biosensors to utilize its superior characteristics thoroughly, and the need to prove its clinical applicability still remains. Herein, we controlled the frameworks of double-walled nanoframes (DWFs) precisely via wet chemistry to construct a homogeneous plasmon-enhanced nanotransducer for localized surface plasmon resonance biosensors. By tuning the physical properties considering the finite-difference time-domain simulation results, biomolecular interactions were feasible in the electromagnetic field-enhanced nanospace. As a result, DWF10 exhibited a 10-fold lower detection limit of 2.21 fM compared to DWF14 for tau detection. Further application into blood-based clinical and Alzheimer's disease (AD) diagnostics, notable improvement in classifying mild cognitive impairment patients against healthy controls and AD patients, was demonstrated along with impressive AUC values. Thus, in response to diverse detection methods, optimizing nanoframe dimensions such as nanogap and frame thickness to maximize sensor performance is critical to realize future POCT diagnosis.

Precise Construction of Chiral Plasmonic Nanoparticles for Enantioselective Discrimination

Chiral plasmonic nanostructures exhibit potential in the advanced manufacturing industry, due to their fascinating characteristics. However, the limitation of existing fabrication methods as difficulty to precisely manipulate chiral nanostructures at the nanoscale restricts their application and optimization of performance. In this work, we report a simple and robust route for the precise construction of chiral Au nanoparticles (NPs), employing star-like block copolymers with well-defined structures as chiral templates. The globular unimolecular micelles as nanoreactors enabled control over the size, shape, and chirality of in situ grown nanocrystals. Utilizing the chiral anisotropy property of surface-enhanced Raman scattering (SERS), the enantioselective discrimination on various substrates was accomplished with an enhancement factor over 9.3 × 106. NPs with a smaller size exhibited strengthened Raman enhancement and chiral recognition. Furthermore, these chiral unimolecular-micelle-based templates with high efficiency and strong controllability could pave the way for tailor-made chiral nanomaterials.

Intra-nanoparticle plasmonic nanogap based spatial-confinement SERS analysis of polypeptides

Sensing and characterizing water-soluble polypeptides are essential in various biological applications. However, detecting polypeptides using Surface-Enhanced Raman Scattering (SERS) remains a challenge due to the dominance of aromatic amino acid residues and backbones in the signal, which hinders the detection of non-aromatic amino acid residues. Herein, intra-nanoparticle plasmonic nanogap were designed by etching the Ag shell in Au@AgNPs (i.e., obtaining AuAg cores) with chlorauric acid under mild conditions, at the same time forming the outermost Au shell and the void between the AuAg cores and the Au shell (AuAg@void@Au). By varying the Ag to added chloroauric acid molar ratios, we pioneered a simple, controllable, and general synthetic strategy to form interlayer-free nanoparticles with tunable Au shell thickness, achieving precise regulation of electric field enhancement within the intra-nanogap. As validation, two polypeptide molecules, bacitracin and insulin B, were successfully synchronously encapsulated and spatial-confined in the intra-nanogap for sensing. Compared with concentrated 50 nm AuNPs and Au@AgNPs as SERS substrates, our simultaneous detection method improved the sensitivity of the assay while benefiting to obtain more comprehensive characteristic peaks of polypeptides. The synthetic strategy of confining analytes while fabricating plasmonic nanostructures enables the diffusion of target molecules into the nanogap in a highly specific and sensitive manner, providing the majority of the functionality required to achieve peptide detection or sequencing without the hassle of labeling.

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.

A review of recent advances in the use of complex metal nanostructures for biomedical applications from diagnosis to treatment

Complex metal nanostructures represent an exceptional category of materials characterized by distinct morphologies and physicochemical properties. Nanostructures with shape anisotropies, such as nanorods, nanostars, nanocages, and nanoprisms, are particularly appealing due to their tunable surface plasmon resonances, controllable surface chemistries, and effective targeting capabilities. These complex nanostructures can absorb light in the near-infrared, enabling noteworthy applications in nanomedicine, molecular imaging, and biology. The engineering of targeting abilities through surface modifications involving ligands, antibodies, peptides, and other agents potentiates their effects. Recent years have witnessed the development of innovative structures with diverse compositions, expanding their applications in biomedicine. These applications encompass targeted imaging, surface-enhanced Raman spectroscopy, near-infrared II imaging, catalytic therapy, photothermal therapy, and cancer treatment. This review seeks to provide the nanomedicine community with a thorough and informative overview of the evolving landscape of complex metal nanoparticle research, with a specific emphasis on their roles in imaging, cancer therapy, infectious diseases, and biofilm treatment. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Diagnostic Tools > Diagnostic Nanodevices.

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.

Plasmonic Multi-Layered Built-in Hotspots Nanogaps for Effectively Activating Analytes

Multi-layered plasmonic nanostructures are able to highly promote the near-field confinement and effectively activate analytes, which are of predominate significance but are extremely challenging. Herein, the semi-open Au core@carved AuAg multi-shell superstructure nanoparticles (multi-Au@Ag-Au NPs, multi = mono, bi, tri, tetra, and penta) are reported with a high designability on electromagnetic field and capability of effectively capturing analytes. By controlling synthetic parameters such as the number of galvanic exchange and Ag growth, multi-Au@Ag-Au NPs are successfully obtained, with tunable layer numbers and asymmetric nanoholes. Due to collective plasmon oscillations of multi-layered built-in nanogaps, the electromagnetic field strength of a single penta-Au@Ag-Au entity reach 48841. More importantly, the penta-Au@Ag-Au NPs show a remarkable light-harvesting capability, which is adaptive to different Raman lasers, supporting high-diversity detection. Additionally, the structural specificity allows analytes to be sufficiently captured into interior hotspots, and further achieve highly sensitive detection with limit of detection down to 3.22 × 10-12  M. This study not only provides an effective pathway for integrating abundant hotspots and activating target molecules in single plasmonic superstructure, but stimulates advancements in SERS substrates for various 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.