Hollow Porous Gold Nanoshells with Controlled Nanojunctions for Highly Tunable Plasmon Resonances and Intense Field Enhancements for Surface-Enhanced Raman Scattering

Enhanced Molecular Interaction of 3D Plasmonic Nanoporous Gold Alloys by Electronic Modulation for Sensitive Molecular Detection

Three-dimensional gold and its alloyed nanoporous structures possess high surface areas and strong local electric fields, rendering them ideal substrates for plasmonic molecular detection. Despite enhancing plasmonic properties and altering molecular interactions, the effect of alloy composition on molecular detection capability has not yet been explored. Here, we report molecular interactions between nanoporous gold alloys and charged molecules by controlling the alloy composition. We demonstrate enhanced adsorption of negatively charged molecules onto the alloy surface due to positively charged gold atoms and a shifted d-band center through charge transfer between gold and other metals. Despite similar EM field intensities, nanoporous gold with silver (Au/Ag) achieves SERS enhancement factors (EF) up to 6 orders of magnitude higher than those of other alloys for negatively charged molecules. Finally, nanoporous Au/Ag detects amyloid-beta at concentrations as low as approximately 1 fM, with SERS EF up to 10 orders of magnitude higher than that of a monolayer of Au nanoparticles.

Reusable Wrinkled Nanoporous Silver Film Fabricated by Plasma Treatment for Surface-Enhanced Raman Scattering Applications

A nanoporous silver film (npAgF), a promising structure for surface-enhanced Raman spectroscopy (SERS), can be fabricated by using successive O2 and Ar plasma treatments on a planar silver film. The common dealloying method for producing an npAgF involves annealing at high temperatures to produce an alloy film, as well as harsh etching using corrosive chemicals. By contrast, the plasma-based method can be applied directly to various functional substrates to produce more sophisticated npAgF structures. Herein, we report a facile fabrication method for a wrinkled npAgF (w-npAgF) for SERS applications using a thermally contractible polystyrene substrate. The w-npAgF had 3D wrinkles of the nanoporous structure and showed approximately 8 times higher SERS enhancement than did the flat npAgF. Moreover, the w-npAgF could be reused for multiple SERS measurements of different molecules by mild O2 and Ar plasma treatments after each use, in which the O2 plasma effectively removed the adsorbed organic molecules and the Ar plasma reduced silver oxide to pristine silver for subsequent SERS measurements. The wrinkled nanoporous structure was maintained after multiple mild plasma treatments for reuse. The simplicity of plasma-based fabrication and high sensitivity of w-npAgFs are promising features for the green production of low-cost and reusable 3D SERS substrates.

Seed-assembly-mediated fabrication and application of highly branched gold nanoshells having hollow and porous morphologies

Highly branched gold nanoshells (BAuNSs) having hollow and porous morphologies have been fabricated via a seed-assembly-mediated strategy. Gold seed assemblies can be prepared by removal of SiO₂nanotemplates with help of polyvinylpyrrolidone (PVP) molecules, which weakly link gold nanoparticles together even after SiO₂etching. L-3,4-dihydroxy phenylalanine (L-DOPA) and AgNO₃are employed as shape-directing agents to induce the anisotropic growth of gold. BAuNSs exhibit 7.4 and 4.4 times stronger activities than SiO₂@Au nanoparticles in catalysis and surface-enhanced Raman scattering (SERS) applications, respectively, due to their large surface areas and numerous hot spots. It is necessary to find the optimal amount of gold deposition in fabrication to effectively utilize the hollow and porous morpologies of BAuNSs for catalysis and SERS applications. Overgrown nanobranches can fill the nanopores and nanogaps of BAuNSs, resulting in decrease of activities in applications. Overall, the seed-assembly-mediated fabrciation can be employed to produce plasmonic nanostructures having unique morphologies and high application activities.

Playing with sizes and shapes of colloidal particles via dry etching methods

Monolayers of self-assembled quasi-spherical colloidal particles are essential building blocks in the field of materials science and engineering. More typically, they are used as a template for the fabrication of nanostructures if they serve, for instance, as a mask for deposition of new material on the surface on which particles are assembled or for etching of the material underneath; in this case, they are removed afterwards. This is what occurs in colloidal or nanosphere lithography. In some other cases, they are not used as a sacrificial material but they are incorporated in the final structure because they are inherently interesting for their properties. Independently of their specific use and application, different strategies have been devised in order to modify size and shape of colloidal particles, so as to enrich the variety of attainable patterns and to tailor the properties of the final structures and materials. In this review, we will focus on one of the most widespread methods to shape spherical colloidal particles, i.e. dry etching techniques. We will follow the development of such approaches until recent days, so as to trace an extensive panorama of the diverse parameters that can be harnessed to achieve specific morphological changes and highlight the characteristic features of the variants of this method. We will finally discuss how particles modified via dry etching can be used for patterning or can be resuspended in solvents for very diverse applications.

Rapid testing for coronavirus disease 2019 (COVID-19)

Rapid testing, generally refers to the paper-based diagnostic platform known as "lateral flow assay" (LFA), has emerged as a critical asset to the containment of coronavirus disease 2019 (COVID-19) around the world. LFA technology stands out amongst peer platforms due to its cost-effective design, user-friendly interface, and low sample-to-readout times. This article aims to introduce its design, use, and practicality for the purpose of diagnosing SARS-CoV-2 infection. A connection is made from the normal COVID-19 immune response to the design and efficacy of rapid testing. Interference in test results is a challenge shared by most diagnostic platforms and can be rooted in various underlying issues. The current knowledge and situation about interference in rapid COVID-19 tests due to variant strains as well as vaccination are discussed. The cost and societal impact are reviewed as they play important roles in determining how to properly implement public testing practices. Perspectives on improving the performance, especially detection sensitivity, of LFA for COVID-19 are provided.

The impact of surface chemistry on the interfacial evaporation-driven self-assembly of thermoplasmonic gold nanoparticles

This paper reports an interfacial evaporation-driven approach for self-assembly of a gold nanoparticle (AuNP) film at the interface of liquid/air. We have designed colloidal plasmonic AuNPs capped with different types and surface coverage densities of ligands (i.e. purified and unpurified oleylamine-capped or thiol-protected AuNPs) and studied the impact of surface chemistry on the self-assembly of AuNPs using the optically excited plasmonic heating effect. By employing the extended DerjaguinLandau-Verwey-Overbeek model, the calculated lowest potential energies of the assembled AuNPs capped with purified oleylamine or alkyl thiols are between -1 k_BT and -2 k_BT, which is close to the room temperature thermal energy and represents a meta-stable assembly, indicating the reversible self-assembly of the AuNP film observed from the experiment. Furthermore, we observed the superheating phenomenon in well-dispersed nanoparticle solution while normal boiling occurred in the solutions with AuNP assemblies. The SERS activity of the as-prepared AuNP film has also been studied using rhodamine 6G as a molecular probe. This work not only provides a new aspect of the boiling phenomena of optically heated colloidal plasmonic nanoparticle solutions, but also provides inspiration for a new approach in designing surface ligands on the nanoparticles to realize reversible self-assembly via interfacial evaporation.

Spiky yolk-shell AuAg bimetallic nanorods with uniform interior gap for the SERS detection of thiram residues in fruit juice

By using gold nanorods with silver coating as the sacrificial templates, we prepared spiky yolk-shell AuAg bimetallic nanorods with uniform interior gap via galvanic replacement reaction. The length and number of Au tips of the spiky yolk-shell AuAg nanorods can be tuned simultaneously by altering HAuCl₄ volume. The influence of HAuCl₄ volume and the sliver layer thickness on the SERS activity of spiky yolk-shell AuAg nanorods are studied. When the sliver layer is thin, the interior gap has not been shielded completely and the outer shell has obvious tips, thus the surface-enhanced Raman scattering (SERS) activity has the strongest enhancement with an enhancement factor (EF) of 4.9 × 10⁵. The spiky yolk-shell AuAg nanorods with the strongest SERS activity are used as SERS substrates to detect thiram. The results demonstrate that the SERS intensity increases linearly with the logarithmic concentration of thiram in the range of 10^-3 M to 10^-7 M. The detection limit is as low as 97 nM, which is lower than the maximum pesticide residue limit (29 µM) in fruits stipulated by the US Environmental Protection Agency (EPA). Therefore, the spiky yolk-shell AuAg bimetallic nanorods have important practical application value in pesticide detection.

Biosensing Using SERS Active Gold Nanostructures

Surface-enhanced Raman spectroscopy (SERS) has become a powerful tool for biosensing applications owing to its fingerprint recognition, high sensitivity, multiplex detection, and biocompatibility. This review provides an overview of the most significant aspects of SERS for biomedical and biosensing applications. We first introduced the mechanisms at the basis of the SERS amplifications: electromagnetic and chemical enhancement. We then illustrated several types of substrates and fabrication methods, with a focus on gold-based nanostructures. We further analyzed the relevant factors for the characterization of the SERS sensor performances, including sensitivity, reproducibility, stability, sensor configuration (direct or indirect), and nanotoxicity. Finally, a representative selection of applications in the biomedical field is provided.

Au Nanoflowers for Catalyzing and In Situ Surface-Enhanced Raman Spectroscopy Monitoring of the Dimerization of p-Aminothiophenol

In this work, we demonstrated a facile approach for fabrication of Au nanoflowers (Au NFs) using an amino-containing organosilane, 3-aminopropyltriethoxysilane (APTES), as a shape-directing agent. In this approach, the morphology of the Au particles evolved from sphere-like to flower-like with increasing the concentration of APTES, accompanied by a red shift in the localized surface plasmon resonance peak from 520 to 685 nm. It was identified that the addition of APTES is profitable to direct the preferential growth of the (111) plane of face-centered cubic gold and promote the formation of anisotropic Au NFs. The as-prepared Au NFs, with APTES on their surface, presented effective catalytic and surface-enhanced Raman scattering (SERS) performances, as evidenced by their applications in catalyzing the dimerization of p-aminothiophenol and monitoring the reaction process via in situ SERS analysis.

Porous Inorganic Materials for Bioanalysis and Diagnostic Applications

Porous inorganic materials play an important role in adsorbing targeted analytes and supporting efficient reactions in analytical science. The detection performance relies on the structural properties of porous materials, considering the tunable pore size, shape, connectivity, etc. Herein, we first clarify the enhancement mechanisms of porous materials for bioanalysis, concerning the detection sensitivity and selectivity. The diagnostic applications of porous material-assisted platforms by coupling with various analytical techniques, including electrochemical sensing, optical spectrometry, and mass spectrometry, etc., are then reviewed. We foresee that advanced porous materials will bring far-reaching implications in bioanalysis toward real-case applications, especially as diagnostic assays in clinical settings.

Coupling enhancement mechanisms, materials, and strategies for surface-enhanced Raman scattering devices

Surface-enhanced Raman scattering (SERS) has become one of the most sensitive analytical techniques for identifying the chemical components, molecular structures, molecular conformations, and the interactions between molecules. However, great challenges still need to be addressed until it can be widely accepted by the absolute quantification of analytes. Recently, many efforts have been devoted to addressing these issues via various electromagnetic (EM), chemical (CM), and EM-CM hybrid coupling enhancement strategies. In comparison with uncoupled SERS devices, they offer key advantages in terms of sensitivity, reproducibility, uniformity, stability, controllability and reliability. This review provides an in-depth analysis of coupled SERS devices, including coupling enhancement mechanisms, materials and approaches. Finally, we also discuss the remaining bottlenecks and possible strategies for the development of coupling-enhanced SERS devices in the future.

Recent Advances in Hollow Gold Nanostructures for Biomedical Applications

The localized surface plasmon resonance of metallic nanoparticles has attracted much attention owing to its unique characteristics, including the enhancement of signals in sensors and photothermal effects. In particular, hollow gold nanostructures are highly promising for practical applications, with significant advantages being found in their material properties and structures: 1) the interaction between the outer surface plasmon mode and inner cavity mode leads to a greater resonance, allowing it to absorb near-infrared light, which can readily penetrate tissue; 2) it has anti-corrosiveness and good biocompatibility, which makes it suitable for biomedical applications; 3) it shows a reduced net density and large surface area, allowing the possibility of nanocarriers for drug delivery. In this review, we present information on the classification, characteristics, and synthetic methods of hollow gold nanostructures; discuss the recent advances in hollow gold nanostructures in biomedical applications, including biosensing, bioimaging, photothermal therapy, and drug delivery; and report on the existing challenges and prospects for hollow gold nanostructures.

Au nanolenses for near-field focusing

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

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

Synthesis of porous Au–Ag alloy nanorods with tunable plasmonic properties and intrinsic hotspots for surface-enhanced Raman scattering

The tunability of longitudinal plasmonic bands of P-AuAgNRs is realized to cover a wide range of wavelengths. P-AuAgNRs exhibit numerous internal hotspots which favor highly sensitive surface-enhanced Raman scattering detection.

Thermal Stability of Hollow Porous Gold Nanoparticles: A Molecular Dynamics Study

Hollow nanoparticle structures play a major role in nanotechnology and nanoscience since their surface to volume ratio is significantly larger than that of filled ones. While porous hollow nanoparticles offer a significant improvement of the available surface area, there is a lack of theoretical understanding, and scarce experimental information, on how the porosity controls or dominates the stability. Here we use classical molecular dynamics simulations to shed light on the particular characteristics and properties of gold porous hollow nanoparticles and how they differ from the nonporous ones. Adopting gold as a prototype, we show how, as the temperature increases, the porosity introduces surface stress and minor transitions that lead to various scenarios, from partial shrinkage for small filling factors to abrupt compression and the loss of spherical shape for large filling. Our work provides new insights into the stability limits of porous hollow nanoparticles, with important implications for the design and practical use of these enhanced geometries.

High-purity foam-like micron-sized gold cage material with tunable plasmon properties

Herein, by growing mono dispersed gold nanoparticles (MNPAu) on the surface of polystyrene (PS)/nanogold (Au) core–shell composites (PS@Au), we successfully synthesized a micron-sized gold cage (2.6–10.7 μm), referred to as PS@Au@MNPAu for the first time. The new micron-gold cage materials exhibit broadband absorption range from near-ultraviolet to near-infrared, which is unlike the conventional nanogold core–shell structure. The uniform growth of MNPAu on the surface forms a new photonic crystal spectrum. The strong coupling of the spectra causes anomalous absorption in the ultraviolet-near infrared band (400–900 nm). Furthermore, by removing the PS core, a nanogold cavity structure referred to as Au@MNPAu was prepared. This structure demonstrated a high purity (> 97 wt%), low density (9–223 mg/cm³), and high specific surface area (854 m²/g). As the purification process progressed, the MNPAu coupling on the surface of the micro-gold cage strengthened, resulting in the formation of peaks around 370 nm, plasma resonant peaks around 495 nm, and structural bands of photonic crystal peaks around 850 nm. The micron-sized gold cage provides hybridized and tunable plasmonic systems. The theoretical simulations indicate that this plasmon anomalous absorption phenomena can be understood as the novel form of the topological structural transitions near the percolation threshold, which is consistent experimental measurements.

Nanocatalosomes as Plasmonic Bilayer Shells with Interlayer Catalytic Nanospaces for Solar-Light-Induced Reactions

Interest and challenges remain in designing and synthesizing catalysts with nature-like complexity at few-nm scale to harness unprecedented functionalities by using sustainable solar light. We introduce "nanocatalosomes"-a bio-inspired bilayer-vesicular design of nanoreactor with metallic bilayer shell-in-shell structure, having numerous controllable confined cavities within few-nm interlayer space, customizable with different noble metals. The intershell-confined plasmonically coupled hot-nanospaces within the few-nm cavities play a pivotal role in harnessing catalytic effects for various organic transformations, as demonstrated by "acceptorless dehydrogenation", "Suzuki-Miyaura cross-coupling" and "alkynyl annulation" affording clean conversions and turnover frequencies (TOFs) at least one order of magnitude higher than state-of-the-art Au-nanorod-based plasmonic catalysts. This work paves the way towards next-generation nanoreactors for chemical transformations with solar energy.

Shape Deformation in Ion Beam Irradiated Colloidal Monolayers: An AFM Investigation

Self-assembly of colloidal monolayers represents a prominent approach to the fabrication of nanostructures. The modification of the shape of colloidal particles is essential in order to enrich the variety of attainable patterns which would be limited by the typical assembly of spherical particles in a hexagonal arrangement. Polymer particles are particularly promising in this sense. In this article, we investigate the deformation of closely-packed polystyrene particles under MeV oxygen ion irradiation at normal incidence using atomic force microscopy (AFM). By developing a procedure based on the fitting of particle topography with quadrics, we reveal a scenario of deformation more complex than the one observed in previous studies for silica particles, where several phenomena, including ion hammering, sputtering, chemical modifications, can intervene in determining the final shape due to the specific irradiation conditions. In particular, deformation into an ellipsoidal shape is accompanied by shrinkage and polymer redistribution with the presence of necks between particles for increasing ion fluence. In addition to casting light on particle irradiation in a regime not yet explored, we present an effective method for the characterization of the colloidal particle morphology which can be applied to describe and understand particle deformation in other regimes of irradiation or with different techniques.

Biosynthetically grown dendritic silver nanostructures for visible Surface Enhanced Resonance Raman Spectroscopy (v-SERRS)

A simple approach to achieve high SERS enhancement for bio-analyte detection at visible wavelength through a resonance Raman (RR) effect has been proposed in this study.