Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates

In-operando control of sum-frequency generation in tip-enhanced nanocavities

Sum-frequency generation (SFG) is a second-order nonlinear process widely used for characterizing surfaces and interfaces with monolayer sensitivity. Recently, optical field enhancement in plasmonic nanocavities has enabled SFG with continuous wave (CW) lasers from nanoscale areas of molecules, promising applications like nanoscale SFG spectroscopy and coherent upconversion for mid-infrared detection at visible frequencies. Here, we demonstrate CW SFG from individual nanoparticle-on-mirror (NPoM) cavities, which are resonant at visible frequencies and filled with a monolayer of molecules, when placed beneath a metal scanning probe tip. The tip acts as an efficient broadband antenna, focusing incident CW infrared illumination onto the nanocavity. The cascaded near-field enhancement within the NPoM nanocavity yields nonlinear optical responses across a broad range of infrared frequencies, achieving SFG enhancements of up to 14 orders of magnitude. Further, nanomechanical positioning of the tip allows for in-operando control of SFG by tuning the local field enhancement rather than the illumination intensities. The versatility of tip-enhanced nanocavities allows for SFG studies of a wide range of molecular species in the few-molecule regime without the need for complex nanofabrication. Our results also promise SFG nanoimaging with tips providing strong visible and IR field enhancement at their apex, offering a robust platform for future applications in nonlinear nanooptics.

Molecular spectroscopies with semiconductor metasurfaces: towards dual optical/chemical SERS

Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful technique for the ultra-sensitive detection of molecules and has been widely applied in many fields, ranging from biomedical diagnostics and environmental monitoring to trace-level detection of chemical and biological analytes. While traditional metallic SERS substrates rely predominantly on electromagnetic field enhancement, emerging semiconductor SERS materials have attracted growing interest because they offer the additional advantage of simultaneous chemical and electromagnetic enhancements. Here, we review some of the recent advancements in the design and optimization of semiconductor SERS substrates, with a focus on their dual enhancement mechanisms. We also discuss the transition from nanoparticle-based platforms to more advanced nanoresonator-based SERS metasurfaces, highlighting their superior sensing performance.

Integrative plasmonics: optical multi-effects and acousto-electric-thermal fusion for biosensing, energy conversion, and photonic circuits

Surface plasmons, a unique optical phenomenon arising at the interface between metals and dielectrics, have garnered significant interest across fields such as biochemistry, materials science, energy, optics, and nanotechnology. Recently, plasmonics is evolving from a focus on "classical plasmonics," which emphasizes fundamental effects and applications, to "integrative plasmonics," which explores the integration of plasmonics with multidisciplinary technologies. This review explores this evolution, summarizing key developments in this technological shift and offering a timely discussion on the fusion mechanisms, strategies, and applications. First, we examine the integration mechanisms of plasmons within the realm of optics, detailing how fundamental plasmonic effects give rise to optical multi-effects, such as plasmon-phonon coupling, nonlinear optical effects, electromagnetically induced transparency, chirality, nanocavity resonance, and waveguides. Next, we highlight strategies for integrating plasmons with technologies beyond optics, analyzing the processes and benefits of combining plasmonics with acoustics, electronics, and thermonics, including comprehensive plasmonic-electric-acousto-thermal integration. We then review cutting-edge applications in biochemistry (molecular diagnostics), energy (harvesting and catalysis), and informatics (photonic integrated circuits). These applications involve surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), surface-enhanced fluorescence (SEF), chirality, nanotweezers, photoacoustic imaging, perovskite solar cells, photocatalysis, photothermal therapy, and triboelectric nanogenerators (TENGs). Finally, we conclude with a forward-looking perspective on the challenges and future of integrative plasmonics, considering advances in mechanisms (quantum effects, spintronics, and topology), materials (Dirac semimetals and hydrogels), technologies (machine learning, edge computing, in-sensor computing, and neuroengineering), and emerging applications (5G, 6G, virtual reality, and point-of-care testing).

Continuous Anisotropic Growth of Plasmonic CsxWO3-δ Nanocrystals into Rods and Platelets

Shape control during nanocrystal synthesis enables tunable physicochemical properties that emerge at the nanoscale. While extensive efforts have been devoted to controlling shapes in various systems such as plasmonic metal nanoparticles or semiconductor quantum dots, the shape control of plasmonic doped semiconductor nanocrystals remains less explored and limited. Here, we report the synthesis of CsxWO3-δ nanocrystals with exquisite shape control achieved through a continuous injection synthesis combined with precursor-mediated facet-selective growth. We demonstrate that the anisotropic growth of CsxWO3-δ nanocrystals is strongly dependent on the precursor injection rate, which we attribute to the material's intrinsic structural anisotropy and the contrasting reaction kinetics on different crystallographic facets. Furthermore, we reveal that the presence of halide ions in the reaction medium is critical for passivating and suppressing the growth of Cs-exposed basal planes. By systematically modulating the shape aspect ratio, we achieved an extended range of nanocrystal morphologies, leading to a broad tunability of LSPR spectra, spanning the entire near-infrared region and extending into the mid-infrared. Computational simulations effectively reproduce the observed shape-dependent optical properties and highlight the size-dependent damping behavior consistent with the free electron model. These findings provide a robust experimental methodology for shape control in structurally anisotropic nanocrystals and offer theoretical insights into the tunable LSPR properties of heavily doped plasmonic semiconductor systems.

Quick Freezing-Induced Au Nanoparticle Aggregates (QFIAAs) for Near-IR (NIR) Surface-Enhanced Raman Scattering (SERS) Substrates

Here, we report a simple method to prepare near-IR (NIR) surface-enhanced Raman scattering (SERS) substrates by quickly freezing a citrate-capped Au nanoparticle (AuNP) solution in liquid nitrogen, followed by thawing it at room temperature. This process aggregates AuNPs in a controlled manner by forming ice crystals with smaller grain sizes when compared to a slow freezing process. The resulting smaller AuNP aggregates remain suspended in solution long enough to conduct high-throughput chemical analysis in a microwell plate using the NIR SERS spectroscopy. We named these aggregates quick freezing-induced AuNP aggregates (QFIAAs). The aggregation state of QFIAAs in solution is stable for at least three months when stored at 4 °C. Several QFIAAs were prepared using monodisperse citrate-capped AuNPs of various sizes. QFIAAs prepared from AuNPs with an average diameter of 70 nm (70 nm QFIAAs) showed the best performance, considering both NIR SERS activity and the repeatability of the results. The NIR SERS enhancement factor of the 70 nm QFIAAs measured using 57 nM Rhodamine 6G (R6G) was 5 × 104. The R6G molecules could not displace the citrates present in the hotspots of QFIAAs, indicating that the long-term stability of QFIAAs originates from the tight interparticle binding through the citrates. The limit of detection (LOD) of R6G was 2 × 101 nM using the 70 nm QFIAAs. We anticipate that the QFIAA system can be used not only to screen reporter molecules for the NIR SERS bioimaging but also to detect analytes with background fluorescence that can be suppressed with NIR excitation wavelengths.

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. (2) Classifying four pivotal phases from the view of innovative methodologies in the fifty-year progression: initial development (mid-1970s to mid-1980s), downturn (mid-1980s to mid-1990s), nano-driven transformation (mid-1990s to mid-2010s), and recent boom (mid-2010s onwards). (3) Illuminating the entire journey and framework of SERS and its family members such as tip-enhanced Raman spectroscopy (TERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and highlighting the trajectory. (4) Emphasizing the importance of innovative methods to overcome developmental bottlenecks, thereby expanding the material, morphology, and molecule generalities to leverage SERS as a versatile technique for broad applications. (5) Extracting the invaluable spirit of groundbreaking discovery and perseverant innovations from the pioneers and trailblazers. These key inspirations include proactively embracing and leveraging emerging scientific technologies, fostering interdisciplinary cooperation to transform the impossible into reality, and persistently searching to break bottlenecks even during low-tide periods, as luck is what happens when preparation meets opportunity.

Hierarchical structures of surface-accessible plasmonic gold and silver nanoparticles for SERS detection

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive analytical technique with excellent molecular specificity. However, separate pristine nanoparticles produce relatively weak Raman signals. It is necessary to focus on increasing the "hot-spot" density generated at the nanogaps between the adjacent nanoparticles (second-generation SERS hotspot), thus significantly boosting the Raman signal by creating an electromagnetic field. This study employed a self-assembly method without using modifiers based on promoter-induced self-assembly to synthesize stable and plasmonically active surfaces from citrate-reduced Ag and Au nanoparticles. Hierarchical structures like Pickering emulsions (PEs) and stable plasmonic aggregates (SPAs) were studied, focusing on controlling their sizes using "promoters" (TBANO3). The sizes of the SPAs were also adjusted from 85.5 nm to 136 nm by regulating the ratio of the water to the oil phase. Furthermore, to understand the distribution of "hot-spots" on these Au or Ag hierarchical structures, the electric field was simulated using the finite difference time domain (FDTD) software. Third-generation hotspots were also created using hybrid structures of plasmonic nanomaterials and surfaces to significantly improve SERS detection by depositing the colloidosome structure on Cu foil (AgSPAs/Cu substrate). The SERS signal was amplified by achieving an enhancement factor of 7 × 107, compared to an enhancement factor of 2 × 106 when using the AgSPA/glass substrate. Significantly, the limits of detection (LOD) and quantification (LOQ) for the colloidosome substrate to detect crystal violet were found to be 4.51 ppb and 13.66 ppb, respectively. The reproducibility of the prepared substrates was demonstrated to be commendably high, characterized by relative standard deviations (RSDs) of 8.00% for the 1177 cm-1 peak, 7.61% for the 1588 cm-1 peak, and 9.35% for the 1619 cm-1 peak. The AgSPAs/Cu substrate's demonstrated reliability made it suitable for detecting and quantifying analytes, potentially for determining trace amounts of pesticide residues. The LOD and LOQ for thiram detection were calculated to be 0.1 ppm and 0.3 ppm, respectively. These findings highlight the effectiveness of increasing electromagnetic field density for SERS enhancement.

Mixed atomistic-implicit quantum/classical approach to molecular nanoplasmonics

A multiscale quantum mechanical (QM)/classical approach is presented that is able to model the optical properties of complex nanostructures composed of a molecular system adsorbed on metal nanoparticles. The latter is described by a combined atomistic-continuum model, where the core is described using the implicit boundary element method (BEM) and the surface retains a fully atomistic picture and is treated employing the frequency-dependent fluctuating charge and fluctuating dipole (ωFQFμ) approach. The integrated QM/ωFQFμ-BEM model is numerically compared with state-of-the-art fully atomistic approaches, and the quality of the continuum/core partition is evaluated. The method is then extended to compute surface-enhanced Raman scattering within a time-dependent density functional theory framework.

Review of molecular dynamics simulations in laser-based micro/nano-fabrication

Laser technology is integral to the advancement of micro/nano-fabrication. While laser machining offers numerous advantages over alternative micro/nano-fabrication techniques, several challenges and bottlenecks continue to impede its large-scale industrial implementation. In response to these constraints, the molecular dynamics (MD) method has emerged as a formidable tool for optimizing the process parameters of laser micro/nano-fabrication and investigating alternative laser-based micro/nano-fabrication techniques. In this review, the application of MD in laser-based micro/nano-fabrication is comprehensively examined, including numerical simulations of short-pulse, long-pulse, continued laser and hybrid laser machining. The corresponding MD simulation schemes for lasers with different pulse widths are outlined. The mechanisms of laser-material interactions across diverse processing scenarios and the complete process of laser-based micro/nano-fabrication are also elucidated. Furthermore, the prevailing challenges in this domain and potential solutions are discussed, with future research directions being charted based on current knowledge and technological advancements.

Simultaneous SERS-decoding detection of multiple pathogens in drinking water with home-made portable double-layer filtration and concentration device

The engineering of a home-made portable double-layer filtration and concentration device with the common syringe for rapid analysis of water samples is reported. The core elements of the device were two installed filtration membranes with different pore sizes for respective functions. The upper filtration membrane was used for preliminary intercepting large interfering impurities (interception membrane), while the lower filtration membrane was used for collecting multiple target pathogens (enrichment membrane) for determination. This combination can make the contaminated environmental water, exemplified by surface water, filtrated quickly through the device and just retained the target bacteria of Escherichia coli O157:H7, Staphylococcus aureus, and Listeria monocytogenes on the lower enrichment membrane. Integrating with surface-enhanced Raman spectra (SERS) platform to decode the SERS-Tags (SERS-TagCVa, SERS-TagR6G, and SERS-TagMB) already labeled on each of the enriched bacteria based the antibody-mediated immuno-recognition effect, fast separation, concentration, and detection of multiple pathogenic bacteria from the bulk of contaminated environmental water were realized. Results show that within 30 min, all target bacteria in the lake water can be simultaneously and accurately measured in the range from 101 to 106 CFU mL-1 with detection limit of 10.0 CFU mL-1 without any pre-culture procedures. This work highlights the simplicity, rapidness, cheapness, selectivity, and the robustness of the constructed method for simultaneous detecting multiple pathogens in aqueous samples. This protocol opens a new avenue for facilitating the development of versatile analytical tools for drinking water and food safety monitoring in underdeveloped or developing countries.

Colorimetric-SERS dual-mode sensing of Pb(II) ions in traditional Chinese medicine samples based on carbon dots-capped gold nanoparticles as nanozyme

Peroxidase (POD)-mimicking nanozymes have got great progress in the sensing field, but most nanozyme assaying systems are built with a single-signal output mode, which is vulnerable to the effect of different factors. Thus, establishment of a dual-signal output mode is necessary for acquiring dependable and durable performance. This work described an Fe doped noradrenaline-based carbon dots and Prussian blue (Fe,NA-CDs/PB) nanocomposite as a POD-like nanozyme and modified gold nanoparticles (AuNPs) for the colorimetric and surface-enhanced Raman scattering (SERS) dual-mode sensor of Pb(II) in traditional Chinese medicine samples. With 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and 3,3',5,5'-tetramethylbenzidine (TMB) as the substrates, it was found that the addition of Pb(II) inhibited the POD-like activity of Fe,NA-CDs/PB and AuNPs, so it was used for colorimetric and SERS dual-mode assays. The POD-like activity was shown to be a "ping-pong" catalytic mechanism, whereas the addition of Pb(II) produced noncompetitive inhibition with modulatory effects on Fe,NA-CDs/PB. The linear response range for colorimetric and SERS sensor detection of Pb(II) was 0.01-1.00 mg/L with the detection limit of 5 μg/L and 8 μg/L, respectively. This dual-mode detection system shows excellent selectivity. More importantly, the Pb(II) in traditional Chinese medicine samples have successfully assayed with good recovery from 90.4 to 108.9 %.

Plasmon-enhanced second harmonic generation of metal nanostructures

As the most common nonlinear optical process, second harmonic generation (SHG) has important application value in the field of nanophotonics. With the rapid development of metal nanomaterial processing and chemical preparation technology, various structures based on metal nanoparticles have been used to achieve the enhancement and modulation of SHG. In the field of nonlinear optics, plasmonic metal nanostructures have become potential candidates for nonlinear optoelectronic devices because of their highly adjustable physical characteristics. In this article, first, the basic optical principles of SHG and the source of surface symmetry breaking in metal nanoparticles are briefly introduced. Next, the related reports on SHG in metal nanostructures are reviewed from three aspects: the enhancement of SHG efficiency by double resonance structures, the SHG effect based on magnetic resonance and the harmonic energy transfer. Then, the applications of SHG in the sensing, imaging and in situ monitoring of metal nanostructures are summarized. Future opportunities for SHG in composite systems composed of metal nanostructures and two-dimensional materials are also proposed.

Multicomponent chiral plasmonic hybrid nanomaterials: recent advances in synthesis and applications

Chiral hybrid nanomaterials with multiple components provide a highly promising approach for the integration of desired chirality with other functionalities into one single nanoscale entity. However, precise control over multicomponent chiral plasmonic hybrid nanomaterials to enable their application in diverse and complex scenarios remains a significant challenge. In this review, our focus lies on the recent advances in the preparation and application of multicomponent chiral plasmonic hybrid nanomaterials, with an emphasis on synthetic strategies and emerging applications. We first systematically elucidate preparation methods for multicomponent chiral plasmonic hybrid nanomaterials encompassing the following approaches: physical deposition approach, galvanic replacement reaction, chiral molecule-mediated, chiral heterostructure, circularly polarized light-mediated, magnetically induced, and chiral assembly. Furthermore, we highlight emerging applications of multicomponent chiral plasmonic hybrid nanomaterials in chirality sensing, enantioselective catalysis, and biomedicine. Finally, we provide an outlook on the challenges and opportunities in the field of multicomponent chiral plasmonic hybrid nanomaterials. In-depth investigations of these multicomponent chiral hybrid nanomaterials will pave the way for the rational design of chiral hybrid nanostructures with desirable functionalities for emerging technological applications.

Combinatorial Approach to Find Nanoparticle Assemblies with Maximum Surface-Enhanced Raman Scattering

Plasmonic nanoparticles exhibit unique properties that distinguish them from other nanomaterials, including vibrant visible colors, the generation of local electric fields, the production of hot charge carriers, and localized heat emission. These properties are particularly enhanced in the narrow nanogaps formed between nanostructures. Therefore, creating nanogaps in a controlled fashion is the key to achieving a fundamental understanding of plasmonic phenomena originating from the nanogaps and developing advanced nanomaterials with enhanced performance for diverse applications. One of the most effective approaches to creating nanogaps is to assemble individual nanoparticles into a clustered structure. In this study, we present a fast, facile, and highly efficient method for preparing core@satellite (CS) nanoassembly structures using gold nanoparticles of various shapes and sizes, including nanospheres, nanocubes (AuNCs), nanorods, and nanotriangular prisms. The sequential assembly of these building blocks on glass substrates allows us to obtain CS nanostructures with a 100% yield within 4 h. Using 9 different building blocks, we successfully produce 16 distinct CS nanoassemblies and systematically investigate the combinations to search for the highest Raman enhancement. We find that the surface-enhanced Raman scattering (SERS) intensity of AuNC@AuNC CS nanoassemblies is 2 orders of magnitude larger than that of other CS nanoassemblies. Theoretical analyses reveal that the intensity and distribution of the electric field induced in the nanogaps by plasmon excitation, as well as the number of molecules in the interfacial region, collectively contribute to the unprecedentedly large SERS enhancement observed for AuNC@AuNC. This study not only presents a novel assembly method that can be extended to produce many other nanoassemblies but also identifies a highly promising SERS material for sensing and diagnostics through a systematic search process.

Combined Surface-Enhanced Raman and Infrared Absorption Spectroscopies for Streamlined Chemical Detection of Polycyclic Aromatic Hydrocarbon-Derived Compounds

Polycyclic aromatic hydrocarbons (PAHs) constitute a class of universally prevalent carcinogenic environmental contaminants. It is increasingly recognized, however, that PAHs derivatized with oxygen, sulfur, or nitrogen functional groups are frequently more dangerous than their unfunctionalized counterparts. This much larger family of chemicals─polycyclic aromatic compounds─PACs─is far less well characterized than PAHs. Using surface-enhanced Raman and IR Absorption spectroscopies (SERS + SEIRA) combined on a single substrate, along with density functional theoretical (DFT) calculations, we show that direct chemical detection and identification of PACs at sub-parts-per-billion concentration can be achieved. Focusing our studies on 9,10-anthraquinone, 5,12-tetracenequinone, 9-nitroanthracene, and 1-nitropyrene as model PAC contaminants, detection is made possible by incorporating a hydroxy-functionalized self-assembled monolayer that facilitates hydrogen bonding between analytes and the SERS + SEIRA substrate. 5,12-Tetracenequinone was detected at 0.3 ppb, and the limit of detection was determined to be 0.1 ppb using SEIRA alone. This approach is straightforwardly extendable to other families of analytes and will ultimately facilitate fieldable chemical detection of these dangerous yet largely overlooked environmental contaminants.

Light-induced synthesis of silver nanoprisms as a surface-enhanced Raman scattering substrate for N-acetyl procainamide drug quantification

Triangle-shaped silver nanoprisms (AgNPMs) were prepared by a photo-induced method through a seed-mediated growth process and were successfully employed as an ultra-sensitive surface-enhanced Raman scattering (SERS) substrate for the detection of the chemotherapeutic N-acetyl procainamide (NAPA) compound. The transformation of the morphology of the nanoprisms substrate could be noted with a remarkable change in color, possessing an average size of 95 nm. The shape-modified AgNPMs exhibited interesting optical characteristics owing to the truncated dual edges, which led to a pronounced longitudinal localized surface plasmonic resonance (LLSPR) behavior. The nanoprisms-based SERS substrate demonstrated an outstanding sensitivity for NAPA in aqueous solutions with the lowest ever reported detection limit of 0.5 × 10-13 M corresponding to excellent recovery and stability. A steady linear response with a broad dynamic range (10-4-10-12 M) and an R2 of 0.945 was also achieved. The results proved that the NPMs demonstrated excellent efficiency, reproducibility (97%), and stability (30 days) with a superior Raman signal enhancement reaching an ultralow detection limit of 0.5 × 10-13 M compared to the nanosphere particles which could show an LOD of 0.5 × 10-9 M.

Tuning the Plexcitonic Optical Chirality Using Discrete Structurally Chiral Plasmonic Nanoparticles

Constructing chiral plexcitonic systems with tunable plasmon-exciton coupling may advance the scientific exploitation of strong light-matter interactions. Because of their intriguing chiroptical properties, chiral plasmonic materials have shown promising applications in photonics, sensing, and biomedicine. However, the strong coupling of chiral plasmonic nanoparticles with excitons remains largely unexplored. Here we demonstrate the construction of a chiral plasmon-exciton system using chiral AuAg nanorods and J aggregates for tuning the plexcitonic optical chirality. Circular dichroism spectroscopy was employed to characterize chiral plasmon-exciton coupling, in which Rabi splitting and anticrossing behaviors were observed, whereas the extinction spectra exhibited less prominent phenomena. By controlling the number of molecular excitons and the energy detuning between plasmons and excitons, we have been able to fine-tune the plexcitonic optical chirality. The ability to fine-tune the plexcitonic optical chirality opens up unique opportunities for exploring chiral light-matter interactions and boosting the development of emerging chiroptical devices.

Thermal dynamics of gold nanoshell dimers under femtosecond laser pulse irradiation: A numerical approach

We present a numerical investigation of the photothermal response of gold nanoshell (AuNS) dimers when subjected to femtosecond laser pulse irradiation. The time-varying temperature fields for core-shell AuNS dimers are quantified by implementing finite element modeling, integrating the electromagnetic and thermal dual-physics simulations. Given the ultrafast nature of laser pulses, we employ a two-temperature model to accurately portray the energy transfer from excited electrons to the lattice system, a process typically completed post pulse-termination. The temporal analysis of the temperature in the AuNS and the surrounding medium, together with the spatial temperature distribution under different separation distances, elucidates the processes that drive the AuNS dimers' transient temperature distribution and heat dissipation. We report on the critical effects of geometrical parameters on the photothermal response, demonstrating that thinner shells maximize the total deposited energy per unit volume, resulting in increased temperature fields, while decreasing separation distances result in excessive field amplification due to plasmonic modes' production. Our robust numerical approach, enabling simulations with tunable material properties and configurations, may help design nanomaterials with desired features for photothermal cancer treatment and imaging.

The Effect of Nanoparticle Composition on the Surface-Enhanced Raman Scattering Performance of Plasmonic DNA Origami Nanoantennas

A versatile generation of plasmonic nanoparticle dimers for surface-enhanced Raman scattering (SERS) is presented by combining a DNA origami nanofork and spherical and nonspherical Au or Ag nanoparticles. Combining different nanoparticle species with a DNA origami nanofork to form DNA origami nanoantennas (DONAs), the plasmonic nanoparticle dimers can be optimized for a specific excitation wavelength in SERS. The preparation of such nanoparticle dimers is robust enough to enable the characterization of SERS intensities and SERS enhancement factors of dye-modified DONAs on a single dimer level by measuring in total several thousands of dimers from five different dimer designs, each functionalized with three different Raman reporter molecules and measured at four different excitation wavelengths. Based on these data, SERS enhancement factor (EF) distributions have been determined for each dimer design and excitation wavelengths. The structures and measurement conditions with the highest EFs are suitable for single-molecule SERS (SM-SERS), which is realized by placing single dye molecules into hot spots. We demonstrate that the probability of placing single molecules in a strongly enhancing hot spot for SM-SERS can be increased by using anisotropic nanoparticles with several sharp edges, such as nanoflowers. Combining a Ag nanoparticle with a Au particle in one dimer structure allows for a broadband excitation covering almost the whole visible range. The most versatile plasmonic dimer structure for SERS combines a spherical Ag nanoparticle with a Au nanoflower. Employing the discontinuous Galerkin time domain method, we numerically investigate the bare, symmetric dimers with respect to spectral and near-field properties, showing that, indeed, the nanoflowers induce multiple hot spots located at the edges which surpass the intensity of the spherical dimers, indicating the possibility for SM-SERS. The presented DONA structures and SERS data provide a robust basis for applying such designs as versatile SERS tags and as substrates for SM-SERS measurements.

Quantifying Efficiency of Remote Excitation for Surface-Enhanced Raman Spectroscopy in Molecular Junctions

Surface-enhanced Raman spectroscopy (SERS) is enabled by local surface plasmon resonances (LSPRs) in metallic nanogaps. When SERS is excited by direct illumination of the nanogap, the background heating of the lattice and electrons can prevent further manipulation of the molecules. To overcome this issue, we report SERS in electromigrated gold molecular junctions excited remotely: surface plasmon polaritons (SPPs) are excited at nearby gratings, propagate to the junction, and couple to the local nanogap plasmon modes. Like direct excitation, remote excitation of the nanogap can generate both SERS emission and an open-circuit photovoltage (OCPV). We compare the SERS intensity and the OCPV in both direct and remote illumination configurations. SERS spectra obtained by remote excitation are much more stable than those obtained through direct excitation when the photon count rates are comparable. By statistical analysis of 33 devices, the coupling efficiency of remote excitation is calculated to be around 10%, consistent with the simulated energy flow.

Geometric Control and Optical Properties of Intrinsically Chiral Plasmonic Nanomaterials

Intrinsically chiral plasmonic nanomaterials exhibit intriguing geometry-dependent chiroptical properties, which is due to the combination of plasmonic features with geometric chirality. Thus, chiral plasmonic nanomaterials have become promising candidates for applications in biosensing, asymmetric catalysis, biomedicine, photonics, etc. Recent advances in geometric control and optical tuning of intrinsically chiral plasmonic nanomaterials have further opened up a unique opportunity for their widespread applications in many emerging technological areas. Here, the recent developments in the geometric control of chiral plasmonic nanomaterials are reviewed with special attention given to the quantitative understanding of the chiroptical structure-property relationship. Several important optical spectroscopic tools for characterizing the optical chirality of plasmonic nanomaterials at both ensemble and single-particle levels are also discussed. Three emerging applications of chiral plasmonic nanomaterials, including enantioselective sensing, enantioselective catalysis, and biomedicine, are further highlighted. It is envisioned that these advanced studies in chiral plasmonic nanomaterials will pave the way toward the rational design of chiral nanomaterials with desired optical properties for diverse emerging technological applications.

Ultrasensitive iodide detection in biofluids based on hot electron-induced reduction of p-Nitrothiophenol on Au@Ag core-shell nanoparticles

Surveillance of iodine intake is important because either inadequate or excessive amount of iodine may lead to thyroid malfunctions. Herein, we report a method for fast iodide quantification based on a plasmonic hot electron-driven chemical reaction, which occurs on Au@Ag core-shell nanoparticles (NPs) coated with p-nitrothiophenol (PNTP) molecules. Upon resonant light illumination, hot electron-hole pairs are generated in the NPs. The hot holes capture iodide ions (I^-) and form AgI which decomposes under light; while the hot electrons are shifted to the electron orbital (LUMO) of PNTP and trigger its reduction to p-aminothiophenol (PATP). By measuring characteristic surface-enhanced Raman spectroscopic (SERS) peaks of PNTP and PATP, the concentration of I^- in water can be quantitatively determined, with a linear response in the 0.5-20 μM range and a detection limit of 0.30 μM. The Au@Ag nanosensor was then applied for I^- detection in various biofluids including urine, serum and saliva, exhibiting superior detection sensitivity and high selectivity. This sensing assay requires a small sample volume of ∼10 μL and completes the entire detection process in ∼2 min, and therefore holds significant potential for application in point-of-care settings.

3D plasmonic coral nanoarchitecture paper for label-free human urine sensing and deep learning-assisted cancer screening

Practical human biofluid sensing requires a sensor device to differentiate patients from the normal group with high sensitivity and specificity. Label-free molecular identification from human biofluids allows direct classification of abnormal samples, providing insights for disease diagnosis and finding of new biomarkers. Here, we introduce a label-free surface-enhanced Raman scattering sensor based on a three-dimensional plasmonic coral nanoarchitecture (3D-PCN), which has strong electromagnetic field enhancement through multiple hot spots. The 3D-PCN was synthesized on a paper substrate via direct one-step gold reduction, forming a coral-like nanoarchitecture with high absorption property for biofluids. This was fabricated as a urine test strip and then integrated with a handheld Raman system to develop an on-site urine diagnostic platform. The developed platform successfully classified the human prostate and pancreatic cancer urines in a label-free method supported by two types of deep learning networks, with high clinical sensitivity and specificity. Our technology has the potential to be utilized not only for urinary cancer diagnosis but also for various human biofluid sensing systems as a future point-of-care testing platform.

Non-amplification on-spot identifying the sex of dioecious kiwi plants by a portable Raman device

The kiwi plant is dioecious, and its sex is generally identified from flower morphology at blossoming, which takes several years. It is quite necessary but challenging to on-spot identify the plant sex in juvenile stage. Here the target DNA was obtained by screening the Friendly boy (FrBy) gene which is sex-related for different kiwi plant species. Its complementary sequence was divided into two parts as primer DNA and further attached to different gold nanoparticles (GNPs). The connection between target DNA and primer DNA will promote the formation of plasmonic dimers. Dark field microscopy (DFM) can distinguish particles in different aggregation states. Various conditions were optimized based on the standard of increasing the proportion of dimers while reducing that of large aggregates. Furthermore, two Raman reporters (RR) are separately labeled on the nanoprobes, and the plasmonic dimers lead to a tremendous Raman enhancement of two reporters located at the dimer nanogap. Double-blind tests proved the feasibility of this method on the actual samples of kiwi plant leaves. Our SERS method is sensitive, specific, and reliable for rapid sex identification analysis at the kiwi seeding stage, with great promise for decision-making in field management.

Multi-Effect Enhanced Raman Scattering Based on Au/ZnO Nanorods Structures

Surface-enhanced Raman scattering (SERS) was considered a potential spectroscopic technique for applications of molecular detection and has drawn great research interest during the past decade. So far, fabrications of cost-effective SERS substrates with high sensitivity and stability and the corresponding enhanced mechanisms are always among the list of research topics, although great progress has been made. In this work, Au particles were decorated on Si, ZnO film and ZnO nanorod arrays simultaneously by an economical method of ion sputtering, generating three kinds of SERS substrates for R6G detection. The morphology difference of Au particles on different samples and the consequent influence on Raman scattering were studied. The experiment results exhibited that substrates with Au particles decorated on ZnO nanorods had the highest Raman enhancement factor. Furthermore, multi-effect enhanced mechanisms summarized as localized surface plasmon resonance (LSPR) filed coupling, electron transferring induced by LSPR of Au particles and whispering gallery mode (WGM) effect of the ZnO cavity were presented. This work provides a convenient and efficient method of fabricating SERS substrates and indicates that such proper metal/semiconductor composite structures are promising candidates for SERS applications.

Plasmon-driven photocatalytic reaction based on gold microsphere array

Plasma-driven photocatalytic reactions have great research value in the fields of energy utilization, environmental pollution treatment and micro-nano information encryption. In most cases, the substrates used to study photocatalytic reactions are dispersed and disordered, which leads to poor signal reproducibility and makes it difficult to realize applications in the field of quantitative analysis. In this paper, two different sizes of polystyrene (PS) microspheres were used as templates to prepare gold microsphere arrays (Au MA) with homogeneous particle size and regular arrangement. The p-Aminothiophenol (PATP) was selected as the probe molecule to systematically investigate the photocatalytic reaction on Au MA, and the dependence of the photocatalytic reaction on the particle size of the spheres was discussed. It was found that the smaller size of Au MA has higher catalytic activity. In addition, using conventional gold films as a comparison, no significant photocatalytic reaction was found under the same experimental conditions. The reason is the existence of strong surface plasma "hot spots" in the interstices of the particles on the surface of the Au MA, which promotes the reaction. The above experimental results are of theoretical and practical significance for the in-depth study of the photocatalytic effect of micro-nano array catalytic substrates.

Fingerprinting the Hidden Facets of Plasmonic Nanocavities

The optical properties of nanogap plasmonic cavities formed by a NanoParticle-on-Mirror (NPoM, or patch antenna) are determined here, across a wide range of geometric parameters including the nanoparticle diameter, gap refractive index, gap thickness, facet size and shape. Full understanding of the confined optical modes allows these nanocavities to be utilized in a wide range of experiments across many fields. We show that the gap thickness t and refractive index n are spectroscopically indistinguishable, accounted for by a single gap parameter G = n/t 0.47. Simple tuning of mode resonant frequencies and strength is found for each quasi-normal mode, revealing a spectroscopic "fingerprint" for each facet shape, on both truncated spherical and rhombicuboctahedral nanoparticles. This is applied to determine the most likely nanoscale morphology of facets hidden below each NPoM in experiment, as well as to optimize the constructs for different applications. Simple scaling relations are demonstrated, and an online tool for general use is provided.

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.

Coupling of plasmonic nanoparticles on a semiconductor substrate via a modified discrete dipole approximation method

Understanding the plasmonic coupling between a set of metallic nanoparticles (NPs) in a 2D array, and how a substrate affects such coupling, is fundamental for the development of optimized optoelectronic structures. Here, a simple semi-analytical procedure based on discrete dipole approximation (DDA) is reported to simulate the far-field and near-field properties of arrays of NPs, considering the coupling between particles, and the effect of the presence of a semiconductor substrate based on the image dipole approach. The method is validated for Ag NP dimers and single Ag NPs on a gallium nitride (GaN) substrate, a semiconductor widely used in optical devices, by comparison with the results obtained by the finite element method (FEM), indicating a good agreement in the weak coupling regime. Next, the method is applied to square and random arrays of Ag NPs on a GaN substrate. The increase in the surface density of NPs on a GaN substrate mainly results in a redshift of the dipolar resonance frequency and an increase in the near-field enhancement. This model, based on a single dipole approach, grants very low computational times, representing an advantage to predict the optical properties of large NP arrays on a semiconductor substrate for different applications.

Editorial for the Special Issue Applications of Nanomaterials in Plasmonic Sensors

Further progress in the modern sensor industry is associated with the widespread application of new solutions and principles from the field of nanooptics and nanophotonics [...].

Electrochemical Synthesis of Plasmonic Nanostructures

Thanks to their tunable and strong interaction with light, plasmonic nanostructures have been investigated for a wide range of applications. In most cases, controlling the electric field enhancement at the metal surface is crucial. This can be achieved by controlling the metal nanostructure size, shape, and location in three dimensions, which is synthetically challenging. Electrochemical methods can provide a reliable, simple, and cost-effective approach to nanostructure metals with a high degree of geometrical freedom. Herein, we review the use of electrochemistry to synthesize metal nanostructures in the context of plasmonics. Both template-free and templated electrochemical syntheses are presented, along with their strengths and limitations. While template-free techniques can be used for the mass production of low-cost but efficient plasmonic substrates, templated approaches offer an unprecedented synthetic control. Thus, a special emphasis is given to templated electrochemical lithographies, which can be used to synthesize complex metal architectures with defined dimensions and compositions in one, two and three dimensions. These techniques provide a spatial resolution down to the sub-10 nanometer range and are particularly successful at synthesizing well-defined metal nanoscale gaps that provide very large electric field enhancements, which are relevant for both fundamental and applied research in plasmonics.

Switching plasmonic nanogaps between classical and quantum regimes with supramolecular interactions

In the realm of extreme nanophotonics, nanogap plasmons support reliable field enhancements up to 1000, which provide unique opportunities to access a single molecule for strong coupling and a single atom for quantum catalysis. The quantum plasmonics are intriguing but difficult to modulate largely because of the lack of proper spacers that can reversibly actuate the sub-1-nm gaps. Here, we demonstrate that supramolecular systems made of oligoamide sequences can reversibly switch the gap plasmons of Au nanoparticles on mirror between classical and quantum tunneling regimes via supramolecular interactions. The results reveal detailed plasmon shift near the quantum tunneling limit, which fits well with both classical- and quantum-corrected models. In the quantum tunneling regime, we demonstrate that plasmonic hot electron tunneling can further blue shift the quantum plasmons because of the increased conductance in the nanogaps, making it a promising prototype of optical tunable quantum plasmonic devices.

Highly ordered arrays of hat-shaped hierarchical nanostructures with different curvatures for sensitive SERS and plasmon-driven catalysis

Regulation of hot spots exhibits excellent potential in many applications including nanolasers, energy harvesting, sensing, and subwavelength imaging. Here, hat-shaped hierarchical nanostructures with different space curvatures have been proposed to enhance hot spots for facilitating surface-enhanced Raman scattering (SERS) and plasmon-driven catalysis applications. These novel nanostructures comprise two layers of metal nanoparticles separated by hat-shaped MoS2 films. The fabrication of this hybrid structure is based on the thermal annealing and thermal evaporation of self-assembled polystyrene spheres, which are convenient to control the metal particle size and the curvature of hat-shaped nanostructures. Based on the narrow gaps produced by the MoS2 films and the curvature of space, the constructed platform exhibits superior SERS capability and achieves ultrasensitive detection for toxic molecules. Furthermore, the surface catalytic conversion of p-nitrothiophenol (PNTP) to p, p'-dimercaptobenzene (DMAB) was in situ monitored by the SERS substrate. The mechanism governing this regulation of hot spots is also investigated via theoretical simulations.

High-sensitivity and versatile plasmonic biosensor based on grain boundaries in polycrystalline 1L WS2 films

Structural defects play an important role in exploitation of two-dimensional layered materials (2DLMs) for advanced biosensors with the increasingly high sensitivity and low detection limit. Grain boundaries (GBs), as an important type of structural defect in polycrystalline 2DLM films, potentially provide sufficient active defect sites for the immobilization of bioreceptor units via chemical functionalization. In this work, we report the selective functionalization of high-density GBs with complementary DNA receptors, via gold nanoparticle (AuNP) linkers, in wafer-scale polycrystalline monolayer (1L) W(Mo)S₂ films as versatile plasmonic biosensing platforms. The large surface area and GB-rich nature of the polycrystalline 1L WS₂ film enabled the immobilization of bioreceptors in high surface density with spatial uniformity, while the AuNPs perform not only as bioreceptor linkers, but also promote detection sensitivity through surface plasmon resonance enhancement effect. Therefore, the presented biosensor demonstrated highly sensitive and selective sub-femto-molar detection of representative RNA sequences from the novel coronavirus (RdRp, ORF1ad and E). This work demonstrates the immense potential of AuNP-decorated GB-rich 2DLMs in the design of ultra-sensitive biosensing platforms for the detection of biological targets beyond RNA, bringing new opportunities for novel healthcare technologies.

Analytical Calculations of Scattering Amplitude of Surface Plasmon Polaritons Excited by a Spherical Nanoantenna

Since surface plasmon polaritons (SPPs) are surface waves, they cannot be excited by an incident plane wave, because free-space photons do not possess a sufficient in-plane momentum. Phase matching between the incident light and SPP can be achieved using a high-refractive-index prism, grating, or nanoantennas. In this work, we found an expression for the amplitude of SPP excited by an arbitrary 3D current distribution placed near a metal interface. The developed method is based on the well-known technique used in waveguide theory that enables finding the amplitudes of waveguide modes excited by the external currents. It reduces the SPP excitation problem to the summation of the set of emitters. As a particular example, we considered a spherical dipole nanoantenna on a metal substrate illuminated by a normally incident plane wave. The analytical calculations were in good agreement with the full-wave numerical simulations.

A novel sensing platform for the determination of alkaline phosphatase based on SERS-fluorescent dual-mode signals

Alkaline phosphatase (ALP), as an important biomarker, is closely associated with various diseases. Multi-mode sensing platforms can combine the advantages of different technologies and solve their inherent or practical limitations. Herein, we developed a sensing platform for the determination of alkaline phosphatase (ALP) in human serum based on SERS-fluorescent dual-mode assay. Based on the fact that ALP can trigger the in-situ reaction between o-phenylenediamine (OPD) and ascorbic acid (AA), we connected gold nanoparticles (AuNPs) to 3,4-diaminobenzene-thiol (OPD(SH)) through an Au-S covalent bond to synthesize a nanoprobe (OPD(S)-AuNPs). The nanoprobe provides a unique interactive ammonium group for the diol group of AA, which was then used to generate an N-heterocyclic compound that can exhibit good SERS and fluorescence signals without adding SERS reporter and fluorophores or quantum dots (QDs). When being excited at different wavelengths as 360 nm and 785 nm, the fluorescence and SERS signals can be separately generated, which can avoid the disturbance from each other. The response of the fluorescence system was linear from 1.0 to 20 mU mL^-1 (R² = 0.994) with a detection limit of 0.3 mU mL^-1, while that of the SERS system was linear from 0.5 to 10 mU mL^-1 (R² = 0.998) with a detection limit of 0.2 mU mL^-1. The sensing platform developed was further employed in ALP inhibitor evaluation.

Heterodimers of metal nanoparticles: synthesis, properties, and biological applications

Heterodimers of metal nanoparticles consist of two metals, come in many sizes and adopt various shapes. They offer unique properties due to the presence of two metals and have the extraordinary flexibility needed to serve as a multipurpose platform for diverse applications in areas including photonics, sensing, and catalysis. Heterodimer nanoparticles contain different metals that contribute to extraordinary surface plasmon resonance (SPR), surface-enhanced Raman scattering (SERS), and catalytic properties. These properties make them versatile molecules that can be used in intracellular imaging, as antibacterial agents, as photocatalytic and biological macromolecules and for the detection of chemical substances. Moreover, heterodimer nanoparticles are composed of the two metals within larger molecules that provide more choices for modification and application. In this review, we briefly summarize the lesser-known aspects of heterodimers, including some of their properties, and present concrete examples of recent progress in synthesis and applications. This review provides a perspective on achievements and suggests a framework for future research with a focus on the synthesis and application of heterodimers. We also explore the possible applications of heterodimer nanoparticles based on their unique properties.

SERS-based test strips: Principles, designs and applications

Test strips represent a class of point-of-care testing (POCT) tools for analysis of a variety of biomarkers towards diagnostics. Conventional test strips offer benefits of simple operation, visualization, and short detection time, along with the drawbacks of relatively low sensitivity and unavailability of quantitative analysis. Recently, the combination of surface-enhanced Raman scattering (SERS) and test strips have evolved to provide a powerful platform capable of ultrasensitive and multiplex detection of extensive analytes of interest. In this review, we focus on the working principles, design strategies and POCT applications of SERS-based test strips. Initially, both lateral and vertical flow test strips are briefly introduced, followed by presentation of various strategies for reforming SERS-based test strips with better detection performance. Applications of SERS-based test strips in diagnosis of disease biomarkers, nucleic acids and toxins are reviewed, with an emphasis on SERS tag design, sensitivity and analytical applicability. Finally, conclusions are made and perspectives on futuristic research directions are given.

Synthesis of sea urchin-shaped Au nanocrystals by double-strand diblock oligonucleotides for surface-enhanced Raman scattering and catalytic application

It is of great significance to construct specially designed gold nanocrystals (AuNCs) with precisely controllable size and morphology to achieve an excellent physicochemical performance. In this work, sea urchin-shaped AuNCs with tunable plasmonic property were successfully synthesized by the hybridized double-strand poly adenine (dsPolyA) DNA-directed self-assembly technique. Hybridized dsPolyA as the directing template had suitable rigidity and upright conformation, which benefited the controllable formation of these anisotropic multi-branched AuNCs with the assistance of surfactant. The effects of essential conditions influencing the synthesis and precise morphology control were investigated in detail. COMSOL simulation was used to evaluate their electromagnetic field distribution according to their morphologies, and the result suggested that sea urchin-shaped AuNCs had abundant 'hot spots' for surface-enhanced Raman scattering (SERS) detection due to their regular nanoprotuberance structure. Finally, sea urchin-shaped AuNCs with excellent SERS and catalytic performance were applied for the quantitative analysis of food colorant and catalytic degradation of potential pollutants. The SERS enhancement factor of sea urchin-shaped AuNCs was up to 5.27 × 10⁶, and the catalytic degradation rate for 4-NP by these AuNCs was up to -0.13min^-1.

Creating an Aligned Interface between Nanoparticles and MOFs by Concurrent Replacement of Capping Agents

Applying metal-organic frameworks (MOFs) on the surface of other materials to form multifunctional materials has recently attracted great attention; however, directing the MOF overgrowth is challenging due to the orders of magnitude differences in structural dimensions. In this work, we developed a universal strategy to mediate MOF growth on the surface of metal nanoparticles (NPs), by taking advantage of the dynamic nature of weakly adsorbed capping agents. During this colloidal process, the capping agents gradually dissociate from the metal surface, replaced in situ by the MOF. The MOF grows to generate a well-defined NP-MOF interface without a trapped capping agent, resulting in a uniform core-shell structure of one NP encapsulated in one single-crystalline MOF nanocrystal with specific facet alignment. The concept was demonstrated by coating ZIF-8 and UiO-66-type MOFs on shaped metal NPs capped by cetyltrimethylammonium surfactants, and the formation of the well-defined NP-MOF interface was monitored by spectroscopies. The defined interface outperforms ill-defined ones generated via conventional methods, displaying a high selectivity to unsaturated alcohols for the hydrogenation of an α,β-unsaturated aldehyde. This strategy opens a new route to create aligned interfaces between materials with vastly different structural dimensions.

Recent advances in sensitive surface-enhanced Raman scattering-based lateral flow assay platforms for point-of-care diagnostics of infectious diseases

This review reports the recent advances in surface-enhanced Raman scattering (SERS)-based lateral flow assay (LFA) platforms for the diagnosis of infectious diseases. As observed through the recent infection outbreaks of COVID-19 worldwide, a timely diagnosis of the disease is critical for preventing the spread of a disease and to ensure epidemic preparedness. In this regard, an innovative point-of-care diagnostic method is essential. Recently, SERS-based assay platforms have received increasing attention in medical communities owing to their high sensitivity and multiplex detection capability. In contrast, LFAs provide a user-friendly and easily accessible sensing platform. Thus, the combination of LFAs with a SERS detection system provides a new diagnostic modality for accurate and rapid diagnoses of infectious diseases. In this context, we briefly discuss the recent application of LFA platforms for the POC diagnosis of SARS-CoV-2. Thereafter, we focus on the recent advances in SERS-based LFA platforms for the early diagnosis of infectious diseases and their applicability for the rapid diagnosis of SARS-CoV-2. Finally, the key issues that need to be addressed to accelerate the clinical translation of SERS-based LFA platforms from the research laboratory to the bedside are discussed.

Large-Area Nanogap-Controlled 3D Nanoarchitectures Fabricated via Layer-by-Layer Nanoimprint

The fabrication of large-area and flexible nanostructures currently presents various challenges related to the special requirements for 3D multilayer nanostructures, ultrasmall nanogaps, and size-controlled nanomeshes. To overcome these rigorous challenges, a simple method for fabricating wafer-scale, ultrasmall nanogaps on a flexible substrate using a temperature above the glass transition temperature (Tg) of the substrate and by layer-by-layer nanoimprinting is proposed here. The size of the nanogaps can be easily controlled by adjusting the pressure, heating time, and heating temperature. In addition, 3D multilayer nanostructures and nanocomposites with 2, 3, 5, 7, and 20 layers were fabricated using this method. The fabricated nanogaps with sizes ranging from approximately 1 to 40 nm were observed via high-resolution transmission electron microscopy (HRTEM). The multilayered nanostructures were evaluated using focused ion beam (FIB) technology. Compared with conventional methods, our method could not only easily control the size of the nanogaps on the flexible large-area substrate but could also achieve fast, simple, and cost-effective fabrication of 3D multilayer nanostructures and nanocomposites without any post-treatment. Moreover, a transparent electrode and nanoheater were fabricated and evaluated. Finally, surface-enhanced Raman scattering substrates with different nanogaps were evaluated using rhodamine 6G. In conclusion, it is believed that the proposed method can solve the problems related to the high requirements of nanofabrication and can be applied in the detection of small molecules and for manufacturing flexible electronics and soft actuators.

Nanoengineering with RAFT polymers: from nanocomposite design to applications

Reversible addition–fragmentation chain-transfer (RAFT) polymerization is a powerful tool for the precise formation of macromolecular building blocks that can be used for the construction of well-defined nanocomposites.

Flexible Surface-Enhanced Raman Scattering Chip: A Universal Platform for Real-Time Interfacial Molecular Analysis with Femtomolar Sensitivity

We propose and demonstrate a flexible surface-enhanced Raman scattering (SERS) chip as a versatile platform for femtomolar detection and real-time interfacial molecule analysis. The flexible SERS chip is composed of a flexible and transparent membrane and embedded plasmonic dimers with ultrahigh particle density and ultrasmall dimer gap. The chip enables rapid identification for residuals on solid substrates with irregular surfaces or dissolved analytes in aqueous solution. The sensitivity for liquid-state measurement is down to 0.06 molecule per dimers for 10^-14 mol·L^-1 Rhodamine 6G molecule without molecule enrichment. Strong signal fluctuation and blinking are observed at this concentration, indicating that the detection limit is close to the single-molecule level. Meanwhile, the homogeneous liquid environment facilities accurate SERS quantification of analytes with a wide dynamic range. The synergy of flexibility and liquid-state measurement opens up avenues for the real-time study of chemical reactions. The reduction from p-nitrothiophenol (PNTP) to p-aminothiophenol (PATP) in the absence of the chemical reducing agents is observed at liquid interfaces by in situ SERS measurements, and the plasmon-induced hot electron is demonstrated to drive the catalytic reaction. We believe this robust and feasible approach is promising in extending the SERS technique as a general method for identifying interfacial molecular traces, tracking the evolution of heterogeneous reactions, elucidating the reaction mechanisms, and evaluating the environmental effects such as pH value and salty ions in SERS.