Surface-enhanced Raman spectroscopy: a half-century historical perspective.

Surface-enhanced Raman spectroscopy (SERS) has evolved significantly over fifty years into a powerful analytical technique. This review aims to achieve five main goals. (1) Providing a comprehensive history of SERS's discovery, its experimental and theoretical foundations, its connections to advances in nanoscience and plasmonics, and highlighting collective contributions of key pioneers.

Integrating, Validating, and Expanding Information Space in Single-Molecule Surface-Enhanced Raman Spectroscopy for Biomolecules.

Single-molecule surface-enhanced Raman spectroscopy (SM-SERS) is an ultrahigh-resolution spectroscopic method for directly obtaining the complex vibrational mode information on individual molecules. SM-SERS offers a wide range of submolecular information on the hidden heterogeneity in its functional groups and varying structures, dynamics of conformational changes, binding and reaction kinetics, and interactions with the neighboring molecule and environment. Despite the richness in information on individual molecules and potential of SM-SERS in various detection targets, including large and complex biomolecules, several issues and practical considerations remain to be addressed, such as the requirement of long integration time, challenges in forming reliable and controllable interfaces between nanostructures and biomolecules, difficulty in determining hotspot size and shape, and most importantly, insufficient signal reproducibility and stability.

Open Cross-gap Gold Nanocubes with Strong, Large-Area, Symmetric Electromagnetic Field Enhancement for On-Particle Molecular-Fingerprint Raman Bioassays.

Plasmonic nanoparticles with an externally open nanogap can localize the electromagnetic (EM) field inside the gap and directly detect the target via the open nanogap with surface-enhanced Raman scattering (SERS). It would be beneficial to design and synthesize the open gap nanoprobes in a high yield for obtaining uniform and quantitative signals from randomly oriented nanoparticles and utilizing these particles for direct SERS analysis. Here, we report a facile strategy to synthesize open cross-gap (X-gap) nanocubes (OXNCs) with size- and EM field-tunable gaps in a high yield.

Selective Flocculation and HO-Free Oxidative Etching-Based Synthesis of Highly Monodisperse Ag Nanospheres for Uniform Quantum Dot Photoluminescence-Enhancing Plasmonic Cavity Applications.

Ag nanoparticles have garnered significant attention for their excellent plasmonic properties and potential use as plasmonic cavities, primarily because of their intrinsically low ohmic losses and optical properties in the visible range. These are particularly crucial in systems involving quantum dots that absorb light at low wavelengths, where the need for a high threshold energy of interband transitions necessitates the incorporation of Ag nanostructures. However, the synthesis of Ag nanoparticles still encounters challenges in achieving structural uniformity and monodispersity, along with chemical stability, consequentially inducing inconsistent and poorly reliable optical responses.

Cascade Catalytic Nanoparticles Selectively Alkalize Cancerous Lysosomes to Suppress Cancer Progression and Metastasis.

Lysosomes are critical in modulating the progression and metastasis for various cancers. There is currently an unmet need for lysosomal alkalizers that can selectively and safely alter the pH and inhibit the function of cancer lysosomes. Here an effective, selective, and safe lysosomal alkalizer is reported that can inhibit autophagy and suppress tumors in mice.

A Chemist's View on Electronic and Steric Effects of Surface Ligands on Plasmonic Metal Nanostructures.

ConspectusPlasmonic metal nanostructures have been extensively developed over the past few decades because of their ability to confine light within the surfaces and manipulate strong light-matter interactions. The light energy stored by plasmonic nanomaterials in the form of surface plasmons can be utilized to initiate chemical reactions, so-called plasmon-induced catalysis, which stresses the importance of understanding the surface chemistry of the plasmonic materials. Nevertheless, only physical interpretation of plasmonic behaviors has been a dominant theme, largely excluding chemical intuitions that facilitate understanding of plasmonic systems from molecular perspectives.

DNA origami-designed 3D phononic crystals.

Moulding the flow of phononic waves in three-dimensional (3D) space plays a critical role in controlling the sound and thermal properties of matter. To this end, 3D phononic crystals (PnCs) have been considered the gold standard because their complete phononic bandgap (PnBG) enables omnidirectional inhibition of phononic wave propagation. Nevertheless, achieving a complete PnBG in the high-frequency regime is still challenging, as attaining the correspondingly demanded mesoscale 3D crystals consisting of continuous frame networks with conventional fabrications is difficult.

Nanotechnologies for the Diagnosis and Treatment of SARS-CoV-2 and Its Variants.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for the global coronavirus disease 2019 (COVID-19) pandemic, has caused well over 750 million infections and 6.8 million deaths. Rapid diagnosis and isolation of infected patients are the primary aims of the concerned authorities to minimize the casualties.

Plasmonic Cross-Linking Colorimetric PCR for Simple and Sensitive Nucleic Acid Detection.

Simple, low-cost, and accurate nucleic acid assay platforms hold great promise for point-of-care (POC) pathogen detection, disease surveillance, and control. Plasmonic photothermal polymerase chain reaction (PPT-PCR) is a powerful and efficient nucleic acid amplification technique, but it lacks a simple and convenient analysis method for POC applications. Herein, we propose a novel plasmonic cross-linking colorimetric PCR (PPT-ccPCR) assay by integrating plasmonic magnetic nanoparticle (PMN)-based PPT-PCR with gold nanoparticle (AuNP)-based cross-linking colorimetry.

Plasmonic quenching and enhancement: metal-quantum dot nanohybrids for fluorescence biosensing.

Plasmonic metal nanoparticles and semiconductor quantum dots (QDs) are two of the most widely applied nanomaterials for optical biosensing and bioimaging. While their combination for fluorescence quenching nanosurface energy transfer (NSET) or Förster resonance energy transfer (FRET) offers powerful ways of tuning and amplifying optical signals and is relatively common, metal-QD nanohybrids for plasmon-enhanced fluorescence (PEF) have been much less prevalent. A major reason is the competition between fluorescence quenching and enhancement, which poses important challenges for optimizing distances, orientations, and spectral overlap toward maximum PEF.

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.

Toward Quantitative Surface-Enhanced Raman Scattering with Plasmonic Nanoparticles: Multiscale View on Heterogeneities in Particle Morphology, Surface Modification, Interface, and Analytical Protocols.

Surface-enhanced Raman scattering (SERS) provides significantly enhanced Raman scattering signals from molecules adsorbed on plasmonic nanostructures, as well as the molecules' vibrational fingerprints. Plasmonic nanoparticle systems are particularly powerful for SERS substrates as they provide a wide range of structural features and plasmonic couplings to boost the enhancement, often up to >10-10. Nevertheless, nanoparticle-based SERS is not widely utilized as a means for reliable quantitative measurement of molecules largely due to limited controllability, uniformity, and scalability of plasmonic nanoparticles, poor molecular modification chemistry, and a lack of widely used analytical protocols for SERS.

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).

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 intraparticle coupling of localized surface plasmon resonance, which realizes surface-enhanced Raman scattering (SERS) at the single-particle level.

Ring-in-a-Triangle Nanoframes: Integrating with Intra- and Interhotspots for Highly Amplified Near-Field Focusing.

The development of a stepwise synthetic strategy for Au ring-in-a-triangle nanoframes with a high degree of structural solidity is essential to the advancement of highly amplified near-field focusing. This strategy leads to the formation of an inscribed nanoring in a triangular metal frame with stability to withstand elevated temperatures and an oxidizing environment, which is critical for successful single-particle surface-enhanced Raman scattering (SERS). The existence of inscribed nanorings plays an important role in enhancing the so-called "lightning rod effect," whereby the electromagnetic near-field enhancement occurs on the highly curved curvature of a metallic interface.

Multiplex SNP Genotyping Using SWITCH: Sequence-Specific Nanoparticle with Interpretative Toehold-Mediated Sequence Decoding in Hydrogel.

Single nucleotide polymorphisms (SNPs) that can alter phenotypes of individuals play a pivotal role in disease development and, more importantly, responses to therapy. However, SNP genotyping has been challenging due to the similarity of SNP alleles and their low concentration in biological samples. Sequence-specific nanoparticle with interpretative toehold-mediated sequence decoding in hydrogel (SWITCH) for multiplex SNP genotyping is presented.

Polysorbate- and DNA-Mediated Synthesis and Strong, Stable, and Tunable Near-Infrared Photoluminescence of Plasmonic Long-Body Nanosnowmen.

Direct photoluminescence (PL) from metal nanoparticles (NPs) without chemical dyes is promising for sensing and imaging applications since this offers a highly tunable platform for controlling and enhancing the signals in various conditions and does not suffer from photobleaching or photoblinking. It is, however, difficult to synthesize metal NPs with a high quantum yield (QY), particularly in the near-infrared (NIR) region where deep penetration and reduced light scattering are advantageous for bioimaging. Herein, we designed and synthesized Au-Ag long-body nanosnowman structures (LNSs), facilitated by polysorbate 20 (Tween 20).

Synthesis of morphology controlled PtAu@Ag nanorings through concentric and eccentric growth pathways.

We report the synthetic methodology for silver nanorings with controlled nanoscale morphology. The morphology of Ag nanorings was kinetically controlled by electrochemical potential tuning of Ag deposition using halide counter-ions, which resulted in concentric PtAu@Ag nanorings (, Ag homogeneously wrapped around the Pt nanorings) and eccentric PtAu@Ag nanorings (, Ag selectively deposited at the inner boundary of the Pt nanorings). The resulting high quality of each Ag nanoring allowed us to systematically investigate their corresponding localized surface plasmon resonance (LSPR) profiles as a function of their geometrical parameters.

Nontrivial, Unconventional Electrochromic Behaviors of Plasmonic Nanocubes.

Plasmonic electrochromism, a change in the localized surface plasmon resonance (LSPR) with an applied electric potential, has been attracting increasing attention for the development of spectroscopic tools or optoelectronic systems. There is a consensus on the mechanism of plasmonic electrochromism based on the classical capacitor and the Drude model. However, the electrochromic behaviors of metallic nanoparticles in narrow optical windows have been demonstrated only with small monotonic LSPR shifts, which limits the use of the electrochromism.

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.

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.