Monodisperse gold nanotriangles: size control, large-scale self-assembly, and performance in surface-enhanced Raman scattering

Open-Nanogap-Induced Strong Electromagnetic Enhancement in Au/AgAu Monolayer as a Stable and Uniform SERS Substrate for Ultrasensitive Detection

Nanogap-based plasmonic metal nanocrystals have been applied in surface-enhanced Raman scattering detection, while the closed and insufficient electromagnetic fields as well as the nonreproducible Raman signal of the substrate greatly restrict the actual application. Herein, a highly uniform Au/AgAu monolayer with abundant nanogaps and huge electromagnetic enhancement is prepared, which shows ultrasensitive and reproducible SERS detection. Au/AgAu with an inner nanogap is first prepared based on Au nanotriangles, and the nanogap is opened from the three tips via a subsequent etching process. The open-gap Au/AgAu displays much higher SERS efficiency than Au and Au/AgAu with an inner nanogap on detecting crystal violet due to the open-gap induced electromagnetic enhancement and improved molecular absorption. Furthermore, the open-gap Au/AgAu monolayer is prepared via interfacial self-assembly, which shows further improved SERS due to the dense and strong hotspots in the nanocavities induced by the electromagnetic coupling between adjacent open gaps. The monolayer possesses excellent signal stability, uniformity, and reproducibility. The analytic enhancement factor and relative standard deviation reach to 2.12 × 108 and 4.65% on detecting crystal violet, respectively. Moreover, the monolayer achieves efficient detection of thiram in apple juice, biphenyl-4-thiol, 4-mercaptobenzoic, melamine, and a mixed solution of four different molecules, showing great promise in practical detection.

High-performance, large-area flexible SERS substrates prepared by reactive ion etching for molecular detection

In the realm of molecular detection, the surface-enhanced Raman scattering (SERS) technique has garnered increasing attention due to its rapid detection, high sensitivity, and non-destructive characteristics. However, conventional rigid SERS substrates are either costly to fabricate and challenging to prepare over a large area, or they exhibit poor uniformity and repeatability, making them unsuitable for inspecting curved object surfaces. In this work, we present a flexible SERS substrate with high sensitivity as well as good uniformity and repeatability. First, the flexible polydimethylsiloxane (PDMS) substrate is manually formulated and cured. SiO2/Ag layer on the substrate can be obtained in a single process by using ion beam sputtering. Then, reactive ion etching is used to etch the upper SiO2layer of the film, which directly leads to the desired densely packed nanostructure. Finally, a layer of precious metal is deposited on the densely packed nanostructure by thermal evaporation. In our proposed system, the densely packed nanostructure obtained by etching the SiO2layer directly determines the SERS ability of the substrate. The bottom layer of silver mirror can reflect the penetrative incident light, the spacer layer of SiO2and the top layer of silver thin film can further localize the light in the system, which can realize the excellent absorption of Raman laser light, thus enhancing SERS ability. In the tests, the prepared substrates show excellent SERS performance in detecting crystalline violet with a detection limit of 10-11M. The development of this SERS substrate is anticipated to offer a highly effective and convenient method for molecular substance detection.

Evolutionary Optimized, Monocrystalline Gold Double Wire Gratings as a Novel SERS Sensing Platform

Achieving reliable and quantifiable performance in large-area surface-enhanced Raman spectroscopy (SERS) substrates poses a formidable challenge, demanding signal enhancement while ensuring response uniformity and reproducibility. Conventional SERS substrates often made of inhomogeneous materials with random resonator geometries, resulting in multiple or broadened plasmonic resonances, undesired absorptive losses, and uneven field enhancement. These limitations hamper reproducibility, making it difficult to conduct comparative studies with high sensitivity. This study introduces an innovative approach that addresses these challenges by utilizing monocrystalline gold flakes to fabricate well-defined plasmonic double-wire resonators through focused ion-beam lithography. Inspired by biological strategy, the double-wire grating substrate (DWGS) geometry is evolutionarily optimized to maximize the SERS signal by enhancing both excitation and emission processes. The use of monocrystalline material minimizes absorption losses and ensures shape fidelity during nanofabrication. DWGS demonstrates notable reproducibility (RSD = 6.6%), repeatability (RSD = 5.6%), and large-area homogeneity > 104 µm2. It provides a SERS enhancement for sub-monolayer coverage detection of 4-Aminothiophenol analyte. Furthermore, DWGS demonstrates reusability, long-term stability on the shelf, and sustained analyte signal stability over time. Validation with diverse analytes, across different states of matter, including biological macromolecules, confirms the sensitive and reproducible nature of DWGSs, thereby establishing them as a promising platform for future sensing applications.

Template-confined assembly of Ag nanocubes: An approach to fabricate SERS substrate with good performance

Surface-enhanced Raman scattering (SERS) is an important analytical technique. Its detection sensitivity and reproducibility depend on the density and distribution of SERS hotspots. Self-assembly is an efficient method to produce of SERS substrates due to its easy accessibility. However, the assembled defects can hardly be avoided on large area, which could lower the density and uniformity of the hotspots, leading to poor SERS performance. Herein, we report a method to reduce the defects by taking a patterned substrate as template to confine the assembly of Ag nanocubes. The template was prepared based on the combination of photo lithography and self-assembly. Confined by the template, the Ag nanocubes were assembled closely in each dots of the pattern. The limit of detection (LOD) is down to 3.42 × 10-17 M and the enhanced factor (EF) is up to 3.44 × 1010 on the prepared substrate for detecting rhodamine 6G (R6G). In addition, the relative standard deviation (RSD) of the different substrates is 8.75 %. The assembled Ag nanocubes exhibits high sensitivity and reproducibility as SERS substrate, which are contributed by the formation of high-density and uniform hotspots. The prepared substrate can be used for detecting trace amounts of melamine in milk with LOD of 2.06 × 10-7 M and RSD of 6.91 %, so the substrate is applicable for analyzing various analytes.

Unexpectedly high thermal stability of Au nanotriangle@mSiO2 yolk-shell nanoparticles

The shape of Au nanoparticles (NPs) plays a crucial role for applications in, amongst others, catalysis, electronic devices, biomedicine, and sensing. Typically, the deformation of the morphology of Au NPs is the most significant cause of loss of functionality. Here, we systematically investigate the thermal stability of Au nanotriangles (NTs) coated with (mesoporous) silica shells with different morphologies (core-shell (CS): Au NT@mSiO2/yolk-shell (YS): Au NT@mSiO2) and compare these to 'bare' nanoparticles (Au NTs), by a combination of in situ and/or ex situ TEM techniques and spectroscopy methods. Au NTs with a mesoporous silica (mSiO2) coating were found to show much higher thermal stability than those without a mSiO2 coating, as the mSiO2 shell restricts the (self-)diffusion of surface atoms. For the Au NT@mSiO2 CS and YS NPs, a thicker mSiO2 shell provides better protection than uncoated Au NTs. Surprisingly, the Au NT@mSiO2 YS NPs were found to be as stable as Au NT@mSiO2 CS NPs with a core-shell morphology. We hypothesize that the only explanation for this unexpected finding was the thicker and higher density SiO2 shell of YS NPs that prevents diffusion of Au surface atoms to more thermodynamically favorable positions.

Separation of Nanoparticle Seed Pseudoisomers via Amplification of Their Crystallographic Differences

Seed-mediated syntheses rely on small nanoparticle (NP) precursors that act as templates for growth but are often inhomogeneous with respect to their internal twinning structures (e.g., single crystalline, multiply twinned), leading to nonuniform product morphologies. To address this, we developed a method for separating seed NPs of the same approximate size (∼ 10 nm) but with different interior twinning (i.e., NP "pseudoisomers") by exaggerating their crystallographic differences through heteroexpitaxial metal overgrowth. Specifically, single crystalline and pentatwinned Au seeds that are natively inseparable via traditional methods exhibit drastically different Ag shell morphologies that allow for their selective precipitation through colloidal depletion forces. Oxidation of the Ag shell from separated particles results in seeds that are both size uniform and crystallographically pure (>99%), allowing for the controlled synthesis of a library of Oh- and D5h-symmetric gold NPs bearing {111}, {110}, {730}, {310}, {720}, and {100} facets, several of which have no precedent in the literature. These results lay the foundation for precision nanosynthesis by establishing a new paradigm for the purification of NP precursors.

Surface-enhanced Raman scattering from Au nanorods, nanotriangles, and nanostars with tuned plasmon resonances

Electromagnetic theory predicts that the optimal value of the localized plasmon resonance (LPR) wavelength for the maximal SERS enhancement factor (EF) is half the sum of the laser and Raman wavelengths. For small Raman shifts, the theoretical EF scales as the fourth power of the local field. However, experimental data often disagree with these theoretical conclusions, leaving the question of choosing the optimal plasmon resonance for the maximal SERS signal unresolved. Here, we present experimental data for gold nanorods (AuNRs), gold nanotriangles (AuNTs), and gold nanostars (AuNSTs) simulating 1D, 2D, and 3D plasmonic nanostructures, respectively. The LPR wavelengths were tuned by chemical etching within 580-1020 nm at a constant concentration of the particles. The particles were functionalized with Cy7.5 and NBT, and the dependence of the intensity at 940 cm-1 (Cy7.5) and 1343 cm-1 (NBT) on the LPR wavelength was examined for laser wavelengths of 633 nm and 785 nm. The electromagnetic SERS EFs were calculated by averaging the product of the local field intensities at the laser and Raman wavelengths over the particle surface and their random orientations. The calculated SERS plasmonic profiles were redshifted compared to the laser wavelength. For 785 nm excitation, the calculated EFs were five to seven times higher than those for 633 nm excitation. With AuNR@Cy7.5 and AuNT@ Cy7.5, the experimental SERS was 35-fold stronger than it was with NBT-functionalized particles, but with AuNST@Cy7.5 and AuNST@NBT, the SERS responses were similar. With all nanoparticles tested, the SERS plasmonic profiles after 785 nm excitation were slightly blue-shifted, as compared with the laser wavelength, possibly owing to the inner filter effect. After 633 nm excitation, the SERS profiles were red-shifted, in agreement with EM theory. In all cases, the plasmonic EF profiles were much broadened compared to the calculated ones and did not follow the four-power law.

Confinement Effects on the Structure of Entropy-Induced Supercrystals

Depletion-induced self-assembly is routinely used to separate plasmonic nanoparticles (NPs) of different shapes, but less often for its ability to create supercrystals (SCs) in suspension. Therefore, these plasmonic assemblies have not yet reached a high level of maturity and their in-depth characterization by a combination of in situ techniques is still very much needed. In this work, gold triangles (AuNTs) and silver nanorods (AgNRs) are assembled by depletion-induced self-assembly. Small Angle X-ray Scattering (SAXS) and scanning electron microscopy (SEM) analysis shows that the AuNTs and AgNRs form 3D and 2D hexagonal lattices in bulk, respectively. The colloidal crystals are also imaged by in situ Liquid-Cell Transmission Electron Microscopy. Under confinement, the affinity of the NPs for the liquid cell windows reduces their ability to stack perpendicularly to the membrane and lead to SCs with a lower dimensionality than their bulk counterparts. Moreover, extended beam irradiation leads to disassembly of the lattices, which is well described by a model accounting for the desorption kinetics highlighting the key role of the NP-membrane interaction in the structural properties of SCs in the liquid-cell. The results shed light on the reconfigurability of NP superlattices obtained by depletion-induced self-assembly, which can rearrange under confinement.

Depletion force optimization for high-purity gold nanotriangles prepared using different growth methods

A homogeneous structural distribution in metal nanoparticle is commonly required for their application, and despite high-yield growth techniques, unavoidable structural heterogeneity remains a concern in metal nanoparticle synthesis. Gold nanotriangles (AuNTs) were synthesized using seed-mediated and seedless growth methods. Recent advancements in high-yield synthesis processes have enabled easy handling of AuNTs, which exhibit unique localized surface plasmon resonance characteristics due to their anisotropic triangular form. The flocculation and subsequent precipitation technique was used to purify AuNTs of different sizes synthesized using seed-mediated and seedless growth methods. The optimal conditions for obtaining high-purity AuNTs were explored by introducing a high concentration of cetyltrimethylammonium chloride. Additionally, the depletion force necessary for achieving high-purity AuNTs was calculated to reveal variations in the required depletion forces for AuNTs synthesized using different growth techniques. The alternations in the size distribution of AuNTs during the flocculation step were tracked using dynamic light scattering, and the surface charge of AuNTs synthesized through different growth methods was evaluated by ζ-potential. The high purity of the AuNTs produced using the seedless growth method required a larger depletion force than the seed-mediated grown AuNTs. The difference in the required depletion forces results from the difference in the electrostatic forces caused by the different growth methods.

Emergence of magnetic nanoparticles in photothermal and ferroptotic therapies

With their distinctive physicochemical features, nanoparticles have gained recognition as effective multifunctional tools for biomedical applications, with designs and compositions tailored for specific uses. Notably, magnetic nanoparticles stand out as first-in-class examples of multiple modalities provided by the iron-based composition. They have long been exploited as contrast agents for magnetic resonance imaging (MRI) or as anti-cancer agents generating therapeutic hyperthermia through high-frequency magnetic field application, known as magnetic hyperthermia (MHT). This review focuses on two more recent applications in oncology using iron-based nanomaterials: photothermal therapy (PTT) and ferroptosis. In PTT, the iron oxide core responds to a near-infrared (NIR) excitation and generates heat in its surrounding area, rivaling the efficiency of plasmonic gold-standard nanoparticles. This opens up the possibility of a dual MHT + PTT approach using a single nanomaterial. Moreover, the iron composition of magnetic nanoparticles can be harnessed as a chemotherapeutic asset. Degradation in the intracellular environment triggers the release of iron ions, which can stimulate the production of reactive oxygen species (ROS) and induce cancer cell death through ferroptosis. Consequently, this review emphasizes these emerging physical and chemical approaches for anti-cancer therapy facilitated by magnetic nanoparticles, combining all-in-one functionalities.

Composite ligand shells on gold nanoprisms - an ensemble and single particle study

The attachment of thiolated molecules onto gold surfaces is one of the most extensively used and robust ligand exchange approaches to exploit the nanooptical features of nanoscale and nanostructured plasmonic materials. In this work, the impact of thiol adsorption on the optical properties of wet-chemically synthesized gold nanoprisms is studied both at the ensemble and single particle level to investigate the build-up of more complex ligand layers. Two prototypical ligands with different lengths have been investigated ((16-mercaptohexadecyl)trimethylammonium bromide - MTAB and thiolated polyethylene glycol - mPEG-SH). From ensemble experiments it is found that composite ligand layers are obtained by the sequential addition of the two thiols, and an island-like surface accumulation of the molecules can be anticipated. The single particle experiment derived chemical interface damping and resonance energy changes further support this and show additionally that when the two thiols are used simultaneously, a higher density, intermixed layer is formed. Hence, when working with more than a single type of ligand during surface modification, sequential adsorption is preferred for the combination of accessible essential surface functionalities, whereas for high overall loading the simultaneous use of the different ligand types is favourable.

Apparent symmetry rising induced by crystallization inhibition in ternary co-crystallization-driven self-assembly

The concept of apparent symmetry rising, opposite to symmetry breaking, was proposed to illustrate the unusual phenomenon that the symmetry of the apparent morphology of the multiply twinned particle is higher than that of its crystal structure. We developed a unique strategy of co-crystallization-driven self-assembly of amphiphilic block copolymers PEO-b-PS and the inorganic cluster silicotungstic acid to achieve apparent symmetry rising of nanoparticles under mild conditions. The triangular nanoplates triply twinned by orthogonal crystals (low symmetry) have an additional triple symmetry (high symmetry). The appropriate crystallization inhibition of short solvophilic segments of the block copolymers favors the oriented attachment of homogeneous domains of hybrid nanoribbons, and consequently forms kinetic-controlled triangular nanoplates with twin grain boundaries.

Morphology-Controlled Growth of Crystalline Ag-Pt-Alloyed Shells onto Au Nanotriangles and Their Plasmonic Properties

The surface plasmon resonance of noble-metal nanoparticles depends on nanoscale size, morphology, and composition, and provides great opportunities for applications in biomedicine, optoelectronics, (photo)catalysis, photovoltaics, and sensing. Here, we present the results of synthesizing ternary metallic or trimetallic nanoparticles, Au nanotriangles (Au NTs) with crystalline Ag-Pt alloyed shells, the morphology of which can be adjusted from a yolk-shell to a core-shell structure by changing the concentration of AgNO3 or the concentration of Au NT seeds, while the shell thickness can be precisely controlled by adjusting the concentration of K2PtCl4. By monitoring the growth process with UV-vis spectra and scanning transmission electron microscopy (STEM), the shells on the Au NT-Ag-Pt yolk-shell nanoparticles were found to grow via a galvanic replacement synergistic route. The plasmonic properties of the as-synthesized nanoparticles were investigated by optical absorbance measurements.

Flexible SERS sensor using AuNTs-assembled PDMS film coupled chemometric algorithms for rapid detection of chloramphenicol in food

The misuse of chloramphenicol (CAP) has led to the development of drug-resistant strains that pose significant threats to public health. Here, we propose a universal flexible surface-enhanced Raman spectroscopy (SERS) sensor utilizing gold nanotriangles (AuNTs) and polydimethylsiloxane (PDMS) film for rapid detection of CAP in food samples. Initially, AuNTs@PDMS with unique optical and plasmonic properties were used to collect spectra of CAP. Afterward, four chemometric algorithms were executed and compared. Accordingly, random frog-partial least squares (RF-PLS) exhibited optimum results with correlation coefficient of prediction (Rp = 0.9802) and the lowest root-mean-square error of prediction (RMSEP = 0.348 µg/mL). Furthermore, the sensor's efficacy to detect CAP in milk samples was confirmed, and the findings were compatible with the conventional HPLC approach (P > 0.05). Therefore, the proposed flexible SERS sensor could effectively be used to monitor milk quality and safety.

Controlling Atomic-Scale Restructuring and Cleaning of Gold Nanogap Multilayers for Surface-Enhanced Raman Scattering Sensing

We demonstrate the reliable creation of multiple layers of Au nanoparticles in random close-packed arrays with sub-nm gaps as a sensitive surface-enhanced Raman scattering substrate. Using oxygen plasma etching, all the original molecules creating the nanogaps can be removed and replaced with scaffolding ligands that deliver extremely consistent gap sizes below 1 nm. This allows precision tailoring of the chemical environment of the nanogaps which is crucial for practical Raman sensing applications. Because the resulting aggregate layers are easily accessible from opposite sides by fluids and by light, high-performance fluidic sensing cells are enabled. The ability to cyclically clean off analytes and reuse these films is shown, exemplified by sensing of toluene, volatile organic compounds, and paracetamol, among others.

Microfluidic-Generated Seeds for Gold Nanotriangle Synthesis in Three or Two Steps

Nanoparticle synthesis has drawn great attention in the last decades. The study of crystal growth mechanisms and optimization of the existing methods lead to the increasing accessibility of nanomaterials, such as gold nanotriangles which have great potential in the fields of plasmonics and catalysis. To form such structures, a careful balance of reaction parameters has to be maintained. Herein, a novel synthesis of gold nanotriangles from seeds derived with a micromixer, which provides a highly efficient mixing and simple parameter control is reported. The impact of the implemented reactor on the primary seed characteristics is investigated. The following growth steps are studied to reveal the phenomena affecting the shape yield. The use of microfluidic seeds led to the formation of well-defined triangles with a narrower size distribution compared to the entirely conventional batch synthesis. A shortened two-step procedure for the formation of triangles directly from primary seeds, granting an express but robust synthesis is further described. Moreover, the need for a thorough study of seed crystallinity depending on the synthesis conditions, which - together with additional parameter optimization - will bring a new perspective to the use of micromixers which are promising for scaling up nanomaterial production is highlighted.

Current strategies of plasmonic nanoparticles assisted surface-enhanced Raman scattering toward biosensor studies

With the progressive nanofabrication technology, plasmonic nanoparticles (PNPs) have been increasingly deployed in the field of biosensing. PNPs have favorable biocompatibility, conductivity, and tunable optical properties. In addition, the localized surface plasmon resonance (LSPR) of PNPs plays a vital role in surface-enhanced Raman scattering (SERS). PNPs-based SERS biosensing enables wide-ranging applications for sensitive detection and high spatial and temporal resolution imaging. Numerous reviews of PNPs in the field of SERS biosensing highlight the fabrication or applications in one or more fields. However, the specific strategies for the SERS biosensor construction had not been summarized systematically. Thus, this work offers a comprehensive overview of SERS enhancement strategies based on PNPs, with a focus on SERS label-free detection along with label detection sensing construction, as well as its challenges and future trends.

Stimuli-responsive SERS biosensor for ultrasensitive tetracycline sensing using EDTA-driven PEI@CaCO3 microcapsule and CS@FeMMs

In this work, a stimuli-responsive SERS biosensor was fabricated for tetracycline (TTC) by "signal-on" strategy using (EDTA)-driven polyethyleneimine grafted calcium carbonate (PEI@CaCO₃) microcapsule and chitosan-Fe magnetic microbeads (CS@FeMMs). Initially, aptamer conjugated magnetic-bead CS@FeMMs@Apt with superparamagnetism and excellent biocompatibility was employed as capture probe, which facilitated the rapid and easy magnetic separation. Subsequently, the PEI cross-linked layer and aptamer network layer were constructed onto the outer layer of CaCO₃@4-ATP microcapsule to form sensing probes (PEI@CaCO₃@4-ATP@Apt) via the layer-by-layer assembly method. In the presence of TTC, a sandwich SERS-assay was exploited by aptamer recognition induced target-bridged strategy. When the solution of EDTA was added, the core layer of CaCO₃ would be dissolved quickly, destroying the microcapsule to release 4-ATP. The released 4-ATP could be quantitatively monitored by dripping the supernatant onto the AuNTs@PDMS SERS platform, resulting in a strong Raman "signal-on". Under the optimal conditions, a good linear relationship was established with a correlation coefficient (R²) of 0.9938 and a LOD of 0.03 ng/mL. Additionally, the application capacity of the biosensor to detect TTC was also affirmed in food matrixes, and the results were consistent with the standard ELISA method (P > 0.05). Hence, this SERS biosensor affords extensive application prospects for TTC detection with multiple merits such as high sensitivity, environment friendliness, and high stability.

Programming Nucleation and Growth in Colloidal Crystals Using DNA

Colloidal crystal engineering with DNA has advanced beyond controlling the lattice symmetry and parameters of ordered crystals to now tuning crystal habit and size. However, the predominately used slow-cooling procedure that enables faceted crystal habits also limits control over crystal size and uniformity because nucleation and growth cannot be separated. Here, we explore how DNA sequence design can be used to deliberately separate nucleation and growth in a given crystallization process. Specifically, two batches of complementary particles are created with one batch exhibiting perfectly complementary base pairs while the other has a strategically introduced mismatch. This design enables the weaker binding "growth" particles to participate in heterogeneous growth on the nucleates formed from the stronger binding "seed" particles, effectively eliminating secondary nucleation pathways. By eliminating secondary nucleation events, this approach improves crystal uniformity, as measured by polydispersity (from PDI = 0.201 to 0.091). By using this approach with two different particle cores (gold and silver), we show how core-shell colloidal crystals can be synthesized in a one-pot fashion. This work shows how tuning DNA interaction strength can profoundly impact crystal size, uniformity, and structure, parameters central to using such materials as device components.

Site-Selective Chiral Growth of Anisotropic Au Triangular Nanoplates for Tuning the Optical Chirality

Site-selective chiral growth of anisotropic nanoparticles is of great importance to realize the plasmonic nanostructures with delicate geometry and desired optical chirality; however, it remains largely unexplored. This work demonstrates a controlled site-selective chiral growth system based on the seed-mediated growth of anisotropic Au triangular nanoplates. The site-selective chiral growth involves two distinct underlying pathways, faceted growth and island growth, which are interswitchable upon maneuvering the interplay of chiral molecules, surfactants, and reducing agents. The pathway switch governs the geometric and chirality evolution of Au triangular nanoplates, giving rise to tailorable circular dichroism spectra. The ability to tune the optical chirality in a controlled manner by manipulating the site-selective chiral growth pathway opens up a promising strategy for exploiting chiral metamaterials with increasing architectural complexity in chiroptical applications.

ZnO nanorods decorated with Ag nanoflowers as a recyclable SERS substrate for rapid detection of pesticide residue in multiple-scenes

Pesticide residues threaten the ecological environment and human health. Therefore, developing high performance SERS substrate to achieve highly sensitive detection of pesticide residues is meaningful. In this study, based on the strategy of combining "hot spots" engineering and material hybridization, we construct a novel hybrid SERS substrate by depositing Ag nanoflowers (NFs) on ZnO nanorods (NRs). Benefiting from the synergistic effect of electromagnetic enhancement and charge transfer effect, the Ag NFs@ZnO NRs substrate exhibits a low detection limit (10^-13 M) for crystal violet molecules. This SERS substrate has good uniformity with a relative standard deviation of 7.463 %. Besides, owning to the photocatalytic property of ZnO NRs, the hybrid substrate can degrade probe molecules after SERS detection and realize recyclability. As a demonstration, we employed our SERS substrate for the trace detection of pesticide residues on apple surface and in river water. This study provides a new idea for improving the SERS performance of hybrid substrates.

Generalization of Self-Assembly Toward Differently Shaped Colloidal Nanoparticles for Plasmonic Superlattices

Periodic superlattices of noble metal nanoparticles  have demonstrated superior plasmonic properties compared to randomly distributed plasmonic arrangements due to near-field coupling and constructive far-field interference. Here, a chemically driven, templated self-assembly process of colloidal gold nanoparticles is investigated and optimized, and the technology is extended toward a generalized assembly process for variously shaped particles, such as spheres, rods, and triangles. The process yields periodic superlattices of homogenous  nanoparticle clusters on a centimeter scale. Electromagnetically simulated absorption spectra and corresponding experimental extinction measurements demonstrate excellent agreement in the far-field for all particle types and different lattice periods. The electromagnetic simulations reveal the specific nano-cluster near-field behavior, predicting the experimental findings provided by surface-enhanced Raman scattering measurements. It turns out that periodic arrays of spherical nanoparticles produce higher surface-enhanced Raman scattering enhancement factors than particles with less symmetry as a result of very well-defined strong hotspots.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Biomimetic mineralization based on self-assembling peptides

Biomimetic science has attracted great interest in the fields of chemistry, biology, materials science, and energy. Biomimetic mineralization is the process of synthesizing inorganic minerals under the control of organic molecules or biomolecules under mild conditions. Peptides are the motifs that constitute proteins, and can self-assemble into various hierarchical structures and show a high affinity for inorganic substances. Therefore, peptides can be used as building blocks for the synthesis of functional biomimetic materials. With the participation of peptides, the morphology, size, and composition of mineralized materials can be controlled precisely. Peptides not only provide well-defined templates for the nucleation and growth of inorganic nanomaterials but also have the potential to confer inorganic nanomaterials with high catalytic efficiency, selectivity, and biotherapeutic functions. In this review, we systematically summarize research progress in the formation mechanism, nanostructural manipulation, and applications of peptide-templated mineralized materials. These can further inspire researchers to design structurally complex and functionalized biomimetic materials with great promising applications.

Gold Triangular Nanoprisms: Anisotropic Plasmonic Materials with Unique Structures and Properties

As a novel type of anisotropic inorganic nanomaterials, gold triangular nanoprisms (AuTNPs) have been widely studied for their well-defined structures and excellent plasmonic properties. This review starts with synthetic methodology, combing through the early thermal solution method to the mature seed-mediated method and seedless method. The possible mechanisms proposed by predecessors and the problems needed to be solved are also arranged. Along with this, the important morphological evolution process of AuTNPs during synthesis and post-synthesis stages are revealed, which is of great significance for further understanding the structure of AuTNPs and developing new synthesis strategies. Finally, the applications of AuTNPs, especially associated with plasmonic properties, are listed and summarized where surface-enhanced Raman scattering (SERS), catalysis, phototherapy and biosensor are included, so that researchers can quickly comprehend the current situation, and provide a basis for further development and exploration of AuTNPs.

Light-Induced Clusterization of Gold Nanoparticles: A New Photo-Triggered Antibacterial against E. coli Proliferation

Metallic nanoparticles show plasmon resonance phenomena when irradiated with electromagnetic radiation of a suitable wavelength, whose value depends on their composition, size, and shape. The damping of the surface electron oscillation causes a release of heat, which causes a large increase in local temperature. Furthermore, this increase is enhanced when nanoparticle aggregation phenomena occur. Local temperature increase is extensively exploited in photothermal therapy, where light is used to induce cellular damage. To activate the plasmon in the visible range, we synthesized 50 nm diameter spherical gold nanoparticles (AuNP) coated with polyethylene glycol and administered them to an E. coli culture. The experiments were carried out, at different gold nanoparticle concentrations, in the dark and under irradiation. In both cases, the nanoparticles penetrated the bacterial wall, but a different toxic effect was observed; while in the dark we observed an inhibition of bacterial growth of 46%, at the same concentration, under irradiation, we observed a bactericidal effect (99% growth inhibition). Photothermal measurements and SEM observations allowed us to conclude that the extraordinary effect is due to the formation, at low concentrations, of a light-induced cluster of gold nanoparticles, which does not form in the absence of bacteria, leading us to the conclusion that the bacterium wall catalyzes the formation of these clusters which are ultimately responsible for the significant increase in the measured temperature and cause of the bactericidal effect. This photothermal effect is achieved by low-power irradiation and only in the presence of the pathogen: in its absence, the lack of gold nanoparticles clustering does not lead to any phototoxic effect. Therefore, it may represent a proof of concept of an innovative nanoscale pathogen responsive system against bacterial infections.

Seed-Mediated Synthesis of Thin Gold Nanoplates with Tunable Edge Lengths and Optical Properties

Thin Au nanoplates show intriguing localized surface plasmon resonance (LSPR) properties with potential applications in various fields. The conventional synthesis of Au nanoplates usually involves the formation of spherical nanoparticles or produces nanoplates with large thicknesses. Herein, we demonstrate a synthesis of uniform thin Au nanoplates by using Au-Ag alloy nanoframes obtained by the galvanic replacement of Ag nanoplates with HAuCl₄ as the seeds and a sulfite (SO₃^2-) as a ligand. The SO₃^2- ligand not only complexes with the Au salt for the controlled reduction kinetics but also strongly adsorbs on Au {111} facets for effectively constraining the crystal growth on both basal sides of the Au nanoplates for controlled shape and reduced thicknesses. This seed-mediated synthesis affords Au nanoplates with a thickness of only 7.5 nm, although the thickness increases with the edge length. The edge length can be customizable in a range of 48-167 nm, leading to tunable LSPR bands in the range of 600-1000 nm. These thin Au nanoplates are applicable not only to surface-enhanced Raman spectroscopy with enhanced sensitivity and reliability but also to a broader range of LSPR-based applications.

Tunable reactivity of silver nanoclusters: a facile route to synthesize a range of bimetallic nanostructures

Quantized energy levels and unique optoelectronic properties of atomically precise noble metal nanoclusters (NCs) have made them important in materials science, catalysis, sensors, and biomedicine. Recent studies on the profound chemical interactions of such NCs within themselves and with ultrasmall plasmonic nanoparticles (NPs) indicate that depending on the size, shape, and composition of the second reactant, NCs can either take part in colloidal assembly without any chemical modifications or lead to products with atoms exchanged. Anisotropic NPs are a unique class of plasmonic nanomaterials as their sharp edges and protrusions show higher chemical reactivity compared to flat surfaces, often leading to site-specific growth of foreign metals and metal oxide shells. Here, using chemical interactions between gold nanotriangles (AuNTs) and Ag NCs of different compositions, we show for the first time that metal atom etching, alloying/atom exchange, and colloidal assembly can all happen at a particular length scale. Specifically, Ag₂₅(DMBT)₁₈ NCs (denoted as 1), upon reacting with AuNTs of ∼57 nm edge length, etch gold atoms from their sharp tips and edges. Simultaneously, the two nanosystems exchange metal atoms, resulting in Ag-doped AuNTs and Au_xAg_24-x(DMBT)₁₈ (x = 1, 2). However, another Ag NC with the same metallic core, but a different ligand shell, namely, Ag₂₅H₂₂(DPPE)₈ (denoted as 2), creates dendritic shells made of Ag, surrounding these AuNTs under the same reaction conditions. Furthermore, we show that in the case of a more reactive thiol-protected Ag NC, namely, Ag₄₄(pMBA)₃₀ (denoted as 3), gold etching is faster from the edges and tips, which drastically alters the identities of both the reactants. Interestingly, when the AuNTs are protected by pMBA, 3 systematically assembles on AuNTs through H-bonding, resulting in an AuNT core-Ag NC shell nanocomposite. Thus, while shedding light on various factors affecting the reactivity of Ag NCs towards AuNTs, the present study proposes a single strategy to obtain a number of bimetallic nanosystems of targeted morphology and functionality.

Triphenylphosphonium conjugated gold nanotriangles impact Pi3K/AKT pathway in breast cancer cells: a photodynamic therapy approach

Although gold nanoparticles based photodynamic therapy (PDT) were reported to improve efficacy and specificity, the impact of surface charge in targeting cancer is still a challenge. Herein, we report gold nanotriangles (AuNTs) tuned with anionic and cationic surface charge conjugating triphenylphosphonium (TPP) targeting breast cancer cells with 5-aminoleuvinic acid (5-ALA) based PDT, in vitro. Optimized surface charge of AuNTs with and without TPP kill breast cancer cells. By combining, 5-ALA and PDT, the surface charge augmented AuNTs deliver improved cellular toxicity as revealed by MTT, fluorescent probes and flow cytometry. Further, the 5-ALA and PDT treatment in the presence of AuNTs impairs cell survival Pi3K/AKT signaling pathway causing mitochondrial dependent apoptosis. The cumulative findings demonstrate that, cationic AuNTs with TPP excel selective targeting of breast cancer cells in the presence of 5-ALA and PDT.

Plasmon Coupling and Efficient Charge Transfer in Rough-Surfaced Au Nanotriangles/MXene Hybrids as an Ultrasensitive Surface-Enhanced Raman Scattering Platform

The rational design of Raman substrate materials with prominent electromagnetic enhancement and charge transfer is quite important for surface-enhanced Raman scattering (SERS). Herein, an efficient SERS substrate based on two-dimensional ultrathin Ti₃C₂T _x MXene and rough-surfaced Au nanotriangles (NTs) was successfully prepared for efficient detection of organic molecules due to the synthetic effect of an optimized electromagnetic field and charge transfer. Uniform Au NTs with tunable surface roughness were controllably prepared by selectively depositing of Au on the smooth Au NTs. Due to the large surface area, tunable plasmon resonance, and abundant hotspots on the planar surface, the modified Au NTs showed much better SERS performance than initial Au NTs. By combination of the rough-surfaced Au NTs with MXene, the Ti₃C₂T _x /Au NT hybrids exhibited much better SERS performance than initial Au NTs and Au NTs with a rough surface. The detection limit is down to 10^-12 M, and the analytical enhancement factors reach 3.6 × 10⁹ (at 1174 cm^-1) on detecting crystal violet excited at 785 nm. This is because the strong plasmon coupling between the in-plane resonance of Au NTs and transversal plasmon resonance of Ti₃C₂T _x MXene around 785 nm can generate an intense interfacial electromagnetic field for amplifying SERS signals. Additionally, the efficient charge transfer between Au NTs, MXene, and molecules also plays an important role in enhancing the SERS performance. This work presents a new insight to develop high-performance SERS substrates based on plasmon.

Green Synthesis of Near-Infrared Plasmonic Gold Nanostructures by Pomegranate Extract and Their Supramolecular Assembling with Chemo- and Photo-Therapeutics

Au nanostructures exhibiting a localized surface plasmon resonance in the near-infrared spectral window are obtained in a single, green step at room temperature by pomegranate extract in the presence of a highly biocompatible β-cyclodextrin branched polymer, without the need of preformed seeds, external reducing and sacrificial agents, and conventional surfactants. The polymeric component makes the Au nanostructures dispersible in water, stable for weeks and permits their supramolecular assembling with the chemotherapeutic sorafenib and a nitric oxide (NO) photodonor (NOPD), chosen as representative for chemo- and photo-therapeutics. Irradiation of the plasmonic Au nanostructures in the therapeutic window with 808 nm laser light results in a good photothermal response, which (i) is not affected by the presence of either the chemo- or the phototherapeutic guests and (ii) does not lead to their photoinduced decomposition. Besides, irradiation of the hybrid Au nanoassembly with the highly biocompatible green light results in the NO release from the NOPD with efficiency similar to that observed for the free guest. Preliminary biological experiments against Hep-G2 hepatocarcinoma cell lines are also reported.

Microfluidic SERS devices: brightening the future of bioanalysis

A new avenue has opened up for applications of surface-enhanced Raman spectroscopy (SERS) in the biomedical field, mainly due to the striking advantages offered by SERS tags. SERS tags provide indirect identification of analytes with rich and highly specific spectral fingerprint information, high sensitivity, and outstanding multiplexing potential, making them very useful in in vitro and in vivo assays. The recent and innovative advances in nanomaterial science, novel Raman reporters, and emerging bioconjugation protocols have helped develop ultra-bright SERS tags as powerful tools for multiplex SERS-based detection and diagnosis applications. Nevertheless, to translate SERS platforms to real-world problems, some challenges, especially for clinical applications, must be addressed. This review presents the current understanding of the factors influencing the quality of SERS tags and the strategies commonly employed to improve not only spectral quality but the specificity and reproducibility of the interaction of the analyte with the target ligand. It further explores some of the most common approaches which have emerged for coupling SERS with microfluidic technologies, for biomedical applications. The importance of understanding microfluidic production and characterisation to yield excellent device quality while ensuring high throughput production are emphasised and explored, after which, the challenges and approaches developed to fulfil the potential that SERS-based microfluidics have to offer are described.

Triangular gold nanoplates/two-dimensional nano mica platelets with a 3D lightning-rod effect as flexible nanohybrid substrates for SERS bacterial detection

Triangular gold nanoplates (TAuNPs) were prepared by a one-step rapid growth method and then reduced and stabilized on two-dimensional nano mica nanoplatelets (NMPs). We also prepared TAuNP/NMP nanohybrids with a three-dimensional lightning-rod effect by oxidative etching. The surface of the delaminated NMPs (only 1 nm thick) is highly charged and can provide a large specific surface area; thus, it can be used as a substrate for the stable growth of gold nanoplates. In addition, by controlling relevant synthesis parameters, the edge length of the TAuNPs can be easily adjusted in the range of 30-90 nm. During reduction of the TAuNPs, the cationic surfactant cetyltrimethylammonium chloride was added as a protective agent to surround the TAuNPs; consequently, the surface was positively charged, which facilitates adsorption for detecting molecules with negative charges. When nanohybrids were used in surface-enhanced Raman spectroscopy (SERS) to detect adenine molecules, the limit of detection concentration was 10^-9 M. The Raman enhancement factor was 5.7 × 10⁷, and the relative standard deviation (RSD) was 9.8%. Finally, this method was applied to the biological detection of Staphylococcus aureus, and the surface charge and hydrophilic properties of the material significantly improved the SERS signal of S. aureus. The limit of detection concentration was 10² CFU mL^-1, and the RSD was 11.2%. The TAuNP/NMP nanohybrids can provide very rapid and sensitive SERS detection of biomolecules.

Enhancement of Magnetic Surface-Enhanced Raman Scattering Detection by Tailoring Fe3O4@Au Nanorod Shell Thickness and Its Application in the On-site Detection of Antibiotics in Water

Surface-enhanced Raman scattering (SERS) has become a promising method for the detection of contaminants or biomolecules in aqueous media. The low interference of water, the unique spectral fingerprint, and the development of portable and handheld equipment for in situ measurements underpin its predominance among other spectroscopic techniques. Among the SERS nanoparticle substrates, those composed of plasmonic and magnetic components are prominent examples of versatility and efficiency. These substrates harness the ability to capture the target analyte, concentrate it, and generate unique hotspots for superior enhancement. Here, we have evaluated the use of gold-coated magnetite nanorods as a novel multifunctional magnetic-plasmonic SERS substrate. The nanostructures were synthesized starting from core-satellite structures. A series of variants with different degrees of Au coatings were then prepared by seed-mediated growth of gold, from core-satellite structures to core-shell with partial and complete shells. All of them were tested, using a portable Raman instrument, with the model molecule 4-mercaptobenzoic acid in colloidal suspension and after magnetic separation. Experimental results were compared with the boundary element method to establish the mechanism of Raman enhancement. The results show a quick magnetic separation of the nanoparticles and excellent Raman enhancement for all the nanoparticles both in dispersion and magnetically concentrated with limits of detection up to the nM range (∼50 nM) and a quantitative calibration curve. The nanostructures were then tested for the sensing of the antibiotic ciprofloxacin, highly relevant in preventing antibiotic contaminants in water reservoirs and drug monitoring, showing that ciprofloxacin can be detected using a portable Raman instrument at a concentration as low as 100 nM in a few minutes, which makes it highly relevant in practical point-of-care devices and in situ use.

A Review on Gold Nanotriangles: Synthesis, Self-Assembly and Their Applications

Gold nanoparticles (AuNPs) with interesting optical properties have attracted much attention in recent years. The synthesis and plasmonic properties of AuNPs with a controllable size and shape have been extensively investigated. Among these AuNPs, gold nanotriangles (AuNTs) exhibited unique optical and plasmonic properties due to their special triangular anisotropy. Indeed, AuNTs showed promising applications in optoelectronics, optical sensing, imaging and other fields. However, only few reviews about these applications have been reported. Herein, we comprehensively reviewed the synthesis and self-assembly of AuNTs and their applications in recent years. The preparation protocols of AuNTs are mainly categorized into chemical synthesis, biosynthesis and physical-stimulus-induced synthesis. The comparison between the advantages and disadvantages of various synthetic strategies are discussed. Furthermore, the specific surface modification of AuNTs and their self-assembly into different dimensional nano- or microstructures by various interparticle interactions are introduced. Based on the unique physical properties of AuNTs and their assemblies, the applications towards chemical biology and sensing were developed. Finally, the future development of AuNTs is prospected.

Turbulent route to two-dimensional soft crystals

We investigate the effect of a two-dimensional, incompressible, turbulent flow on soft granular particles and show the emergence of a crystalline phase due to the interplay of Stokesian drag and short-range interparticle interactions. We quantify this phase through the bond order parameter and local density fluctuations and find a sharp transition between the crystalline and noncrystalline phases as a function of the Stokes number. Furthermore, the nature of preferential concentration, characterized by the correlation dimension, is significantly different from that of particle-laden flows in the absence of repulsive potentials.

Large-Area Periodic Arrays of Atomically Flat Single-Crystal Gold Nanotriangles Formed Directly on Substrate Surfaces

The advancement of nanoenabled wafer-based devices requires the establishment of core competencies related to the deterministic positioning of nanometric building blocks over large areas. Within this realm, plasmonic single-crystal gold nanotriangles represent one of the most attractive nanoscale components but where the formation of addressable arrays at scale has heretofore proven impracticable. Herein, a benchtop process is presented for the formation of large-area periodic arrays of gold nanotriangles. The devised growth pathway sees the formation of an array of defect-laden seeds using lithographic and vapor-phase assembly processes followed by their placement in a growth solution promoting planar growth and threefold symmetric side-faceting. The nanotriangles formed in this high-yield synthesis distinguish themselves in that they are epitaxially aligned with the underlying substrate, grown to thicknesses that are not readily obtainable in colloidal syntheses, and present atomically flat pristine surfaces exhibiting gold atoms with a close-packed structure. As such, they express crisp and unambiguous plasmonic modes and form photoactive surfaces with highly tunable and readily modeled plasmon resonances. The devised methods, hence, advance the integration of single-crystal gold nanotriangles into device platforms and provide an overall fabrication strategy that is adaptable to other nanomaterials.

Chemically Bonded Plasmonic Triazole-Functionalized Au/Zeolitic Imidazole Framework (ZIF-67) for Enhanced CO2 Photoreduction

The design of functionalized metallic nanoparticles is considered an emerging technique to ensure the interaction between metal and semiconductor material. In the literature, this interface interaction is mainly governed by electrostatic or van der Waals forces, limiting the injection of electrons under light irradiation. To enhance the transfer of electrons between two compounds, close contact or chemical bonding at the interface is required. Herein, a new approach was reported for the synthesis of chemically bonded plasmonic Au NPs/ZIF-67 nanocomposites. The structure of ZIF-67 was grown on the surface of functionalized plasmonic Au NPs using 1H-1,2,4-triazole-3-thiol as the capping agent, which acted as both stabilizer of Au nanoparticles and a molecular linker for ZIF-67 formation. As a result, the synthesized material exhibited outstanding photocatalytic CO₂ reduction with a methanol production rate of 2.70 mmol h^-1  g^-1 _cat under sunlight irradiation. This work emphasizes that the diligent use of capping agents, with suitable functional groups, could facilitate the formation of intimate heterostructure for enhanced photocatalytic CO₂ reduction.

Multifunctional plasmonic-magnetic nanoparticles for bioimaging and hyperthermia

Multicompartment nanoparticles have raised great interest for different biomedical applications, thanks to the combined properties of different materials within a single entity. These hybrid systems have opened new avenues toward diagnosis and combination therapies, thus becoming preferred theranostic agents. When hybrid nanoparticles comprise magnetic and plasmonic components, both magnetic and optical properties can be achieved, which are potentially useful for multimodal bioimaging, hyperthermal therapies and magnetically driven selective delivery. Nanostructures comprising iron oxide and gold are usually selected for biomedical applications, as they display size-dependent properties, biocompatibility, and unique physical and chemical characteristics that can be tuned through highly precise synthetic protocols. We provide herein an overview of the most recent synthetic protocols to prepare magnetic-plasmonic nanostructures made of iron oxide and gold, to then highlight the progress made on multifunctional magnetic-plasmonic bioimaging and heating-based therapies. We discuss the advantages and limitations of the various systems in these directions.

Construction of Spatially Separated Gold Nanocrystal/Cuprous Oxide Architecture for Plasmon-Driven CO2 Reduction

Plasmonic hot electrons have shown great potential in photocatalysis, but little is known about the hot hole-driven chemical reactions due to the lack of desired plasmonic metal/p-type semiconductor architectures. Herein, we describe a general and robust strategy for the site-selective growth of a p-type semiconductor, Cu₂O on Au nanocrystals (NCs), to produce diverse spatially separated Au/Cu₂O heterostructures. The preferential growth of Cu₂O on the tips/ends/edges of Au NCs is directed by the sparse coverage of the surfactant molecules at the high-curvature sites of Au NCs. The obtained dumbbell-shaped nanostructures serve as the ideal platforms for probing the hot-hole-mediated CO₂ reduction reaction. Benefiting from the hot-hole injection, a new reaction pathway is unlocked, and the C₂ product activity and selectivity are significantly improved. This study demonstrates the genuine superiority of the dumbbell-shaped nanostructures in photocatalysis, offering a new unique avenue to explore the underlying mechanism of hot-hole-mediated chemical reactions.

Expanding the Multiplexing Capabilities of Raman Imaging to Reveal Highly Specific Molecular Expression and Enable Spatial Profiling

Profiling the heterogeneous landscape of cell types and biomolecules is rapidly being adopted to address current imperative research questions. Precision medicine seeks advancements in molecular spatial profiling techniques with highly multiplexed imaging capabilities and subcellular resolution, which remains an extremely complex task. Surface-enhanced Raman spectroscopy (SERS) imaging offers promise through the utilization of nanoparticle-based contrast agents that exhibit narrow spectral features and molecular specificity. The current renaissance of gold nanoparticle technology makes Raman scattering intensities competitive with traditional fluorescence methods while offering the added benefit of unsurpassed multiplexing capabilities. Here, we present an expanded library of individually distinct SERS nanoparticles to arm researchers and clinicians. Our nanoparticles consist of a ∼60 nm gold core, a Raman reporter molecule, and a final inert silica coating. Using density functional theory, we have selected Raman reporters that meet the key criterion of high spectral uniqueness to facilitate unmixing of up to 26 components in a single imaging pixel in vitro and in vivo. We also demonstrated the utility of our SERS nanoparticles for targeting cultured cells and profiling cancerous human tissue sections for highly multiplexed optical imaging. This study showcases the far-reaching capabilities of SERS-based Raman imaging in molecular profiling to improve personalized medicine and overcome the major challenges of functional and structural diversity in proteomic imaging.

New Trends in Nanoarchitectured SERS Substrates: Nanospaces, 2D Materials, and Organic Heterostructures

This article reviews recent fabrication methods for surface-enhanced Raman spectroscopy (SERS) substrates with a focus on advanced nanoarchitecture based on noble metals with special nanospaces (round tips, gaps, and porous spaces), nanolayered 2D materials, including hybridization with metallic nanostructures (NSs), and the contemporary repertoire of nanoarchitecturing with organic molecules. The use of SERS for multidisciplinary applications has been extensively investigated because the considerably enhanced signal intensity enables the detection of a very small number of molecules with molecular fingerprints. Nanoarchitecture strategies for the design of new NSs play a vital role in developing SERS substrates. In this review, recent achievements with respect to the special morphology of metallic NSs are discussed, and future directions are outlined for the development of available NSs with reproducible preparation and well-controlled nanoarchitecture. Nanolayered 2D materials are proposed for SERS applications as an alternative to the noble metals. The modern solutions to existing limitations for their applications are described together with the state-of-the-art in bio/environmental SERS sensing using 2D materials-based composites. To complement the existing toolbox of plasmonic inorganic NSs, hybridization with organic molecules is proposed to improve the stability of NSs and selectivity of SERS sensing by hybridizing with small or large organic molecules.

Shape- and Size-Dependent Refractive Index Sensing and SERS Performance of Gold Nanoplates

Plasmonic sensors are promising for ultrasensitive chemical and biological analysis. Gold nanoplates (Au NPLs) show unique geometrical structures with high ratios of surface to bulk atoms, which display fascinating plasmonic properties but require optimization. This study presented a systematic investigation of the influence of different parameters (shape, aspect ratio, and resonance mode) on localized surface plasmon resonance properties, refractive index (RI, n) sensitivities, and surface-enhanced Raman scattering (SERS) enhancement ability of different types of Au NPLs through finite-difference time-domain (FDTD) simulations. As a proof of concept, triangular, circular, and hexagonal Au NPLs with varying aspect ratios were fabricated via a three-step seed-mediated growth method by the experiment. Both FDTD-simulated and measured experimental results confirm that the RI sensitivities increase with the aspect ratio. Furthermore, choosing a lower order resonance mode of Au NPLs benefits higher RI sensitivities. The SERS enhancement abilities of Au NPLs also predicted to be highly dependent on the shape and aspect ratio. The triangular Au NPLs showed the highest SERS enhancement ability, while it drastically decreased for circular Au NPLs after the rounding process. The SERS enhancement ability gradually became more intense as the hexagonal Au NPLs overgrown on circular Au NPLs with increasing volumes of HAuCl₄ solution. The results are expected to help develop effective biosensors.