2D superlattices via interfacial self-assembly of polymer-grafted Au nanoparticles

Nanoparticle (NP) superlattices are periodic arrays of nanoscale building blocks. Because of the collective effect between functional NPs, NP superlattices can exhibit exciting new properties that are distinct from those of individual NPs or corresponding bulk materials. In particular, two-dimensional (2D) NP superlattices have attracted increasing attention due to their emerging applications in micro/opto-electronics, catalysis, sensing, and other fields. Among various preparation methods, evaporation-induced interfacial self-assembly has become the most popular method for preparing 2D NP superlattices because it is a simple, low-cost, and scalable process that can be widely applied to various NPs. Introducing soft ligands, such as polymers, can not only provide convenience in controlling the self-assembly process and tuning superlattice structures but also improve the properties of 2D NP superlattices. This feature article focuses on the methods of evaporation-induced self-assembly of polymer-grafted Au NPs into free-standing 2D NP superlattice films at air/liquid interfaces and 2D NP superlattice coatings on substrates, followed by studies on in situ tracking of the self-assembly evolution process through small-angle X-ray scattering. Their application in nano-floating gate memory devices is also included. Finally, the challenges and perspectives of this direction are discussed.

O-phthalaldehyde assisted surface enhanced Raman spectroscopy selective determination of trace homocysteine in serum

High plasma homocysteine (Hcy) levels may indicate cardiovascular disease. However, sensitive and selective determination of Hcy remains a major challenge. Herein, we present a sensing strategy for Hcy by surface enhanced Raman scattering (SERS) method along with a specific reaction of o-phthalaldehyde (OPA) and Hcy. The obtained adduct 2-(1-carboxyl-3-thiopropyl)-1-isoindolinone (Hcy-OPA) can be directly detected by SERS using gold nanoparticles (Au NPs) as the substrate. The developed SERS method displays superior sensitivity (low detection limit of 2.50 × 10^-12 mol L^-1) with a broad linear range (5.00 × 10^-10 -5.00 × 10^-6 mol L^-1). As a proof of real application, it can be used to detect Hcy in bovine serum samples with a concentration as low as 5.00 × 10^-9 mol L^-1, which is free from the interference of the other amino acids and glutathione.

Electric, magnetic, and shear field-directed assembly of inorganic nanoparticles

Ordered assemblies of inorganic nanoparticles (NPs) have shown tremendous potential for wide applications due to their unique collective properties, which differ from those of individual NPs. Various assembly methods, such as external field-directed assembly, interfacial assembly, template assembly, biomolecular recognition-mediated assembly, confined assembly, and others, have been employed to generate ordered inorganic NP assemblies with hierarchical structures. Among them, the external field-directed assembly method is particularly fascinating, as it can remotely assemble NPs into well-ordered superstructures. Moreover, external fields (e.g., electric, magnetic, and shear fields) can introduce a local and/or global field intensity gradient, resulting in an additional force on NPs to drive their rotation and/or translation. Therefore, the external field-directed assembly of NPs becomes a robust method to fabricate well-defined functional materials with the desired optical, electronic, and magnetic properties, which have various applications in catalysis, sensing, disease diagnosis, energy conversion/storage, photonics, nano-floating-gate memory, and others. In this review, the effects of an electric field, magnetic field, and shear field on the organization of inorganic NPs are highlighted. The methods for controlling the well-ordered organization of inorganic NPs at different scales and their advantages are reviewed. Finally, future challenges and perspectives in this field are discussed.

Interfacial-Shear-Mediated Snowball Assembly of Hotspot-Rich Silver Pompon Architectures for Tailored Surface-Enhanced Raman Scattering Responses

To gain superior signal-enhanced performance, metal nanocrystals serving as building blocks can be collectively assembled into a hierarchically ordered structure for creating multiple hotspots. However, the collaborative assembly of anisotropic crystals to form a hotspot-rich structure remains a challenging task. In this study, controllable shear was introduced to a soft liquid-liquid interface to provide a unique environment for the snowball assembly of silver pompon architectures (Ag-PAs). Micrometer-scale 3D plasmonic Ag pompon architectures composed of densely packed nanoparticles (NPs) are fabricated using shear-mediating crystal growth dynamics. The crystal morphology and size are easily controlled by tuning the interfacial shear and diffusion pathways. The hotspot-rich Ag-PAs with high sensitivity (LOD = 1.1 × 10^-13 mol/L) exhibit a superior Raman enhancement performance, which is comparable to some bimetals.

Ionic liquid-based liposome for selective SERS detection

An ionic liquid (IL)-based liposome was utilized as a substrate to construct a SERS platform. The isotropy of the IL outer surface together with its ion-exchange property led to the array-like growth of Au nanoparticles (NPs), generating hot-spots and resulting in anionic probes being present on the hot-spot regions. The simultaneous strategy of enrichment and localization endowed the platform with ability to detect trace amounts of anionic probes.

Surface-Enhanced Raman Scattering Activity of ZrO2 Nanoparticles: Effect of Tetragonal and Monoclinic Phases

The effect of the ZrO₂ crystal form on surface-enhanced Raman scattering (SERS) activity was studied. The ratio of the tetragonal (T) and monoclinic (M) phases of ZrO₂ nanoparticles (ZrO₂ NPs) was controlled by regulating the ratio of two types of additives in the hydrothermal synthesis method. The SERS intensity of 4-mercaptobenzoic acid (4–MBA) was gradually enhanced by changing the M and T phase ratio in ZrO₂ NPs. The degree of charge transfer (CT) in the enhanced 4–MBA molecule was greater than 0.5, indicating that CT was the main contributor to SERS. The intensity of SERS was strongest when the ratio of the T crystal phase in ZrO₂ was 99.7%, and the enhancement factor reached 2.21 × 10⁴. More importantly, the proposed study indicated that the T and M phases of the ZrO₂ NPs affected the SERS enhancement. This study provides a new approach for developing high-quality SERS substrates and improving the transmission efficiency of molecular sensors.

Nanostructure-Based Surface-Enhanced Raman Spectroscopy Techniques for Pesticide and Veterinary Drug Residues Screening

Pesticide and veterinary drug residues in food and environment pose a threat to human health, and a rapid, super-sensitive, accurate and cost-effective analysis technique is therefore highly required to overcome the disadvantages of conventional techniques based on mass spectrometry. Recently, the surface-enhanced Raman spectroscopy (SERS) technique emerges as a potential promising analytical tool for rapid, sensitive and selective detections of environmental pollutants, mostly owing to its possible simplified sample pretreatment, gigantic detectable signal amplification and quick target analyte identification via finger-printing SERS spectra. So theoretically the SERS detection technology has inherent advantages over other competitors especially in complex environmental matrices. The progress in nanostructure SERS substrates and portable Raman appliances will promote this novel detection technology to play an important role in future rapid on-site assay. This paper reviews the advances in nanostructure-based SERS substrates, sensors and relevant portable integrated systems for environmental analysis, highlights the potential applications in the detections of synthetic chemicals such as pesticide and veterinary drug residues, and also discusses the challenges of SERS detection technique for actual environmental monitoring in the future.

Silver nanoparticle-decorated TiO2 nanotube array for solid-phase microextraction and SERS detection of antibiotic residue in milk

The excessive use or abuse of antibiotics on dairy cows leads to residues in milk, which can represent a public health risk. However, in recent years the β-Lactamase was illegally used to degrade residual antibiotics in milk, which makes the traditional antibiotic detection methods ineffective. Therefore, there is an extremely urgent need for multi-analyte analysis techniques for the detection of antibiotic residues. Herein, we reported an ultra-fast, facile, and sensitive solid-phase microextraction (SPME)-surface enhanced Raman scattering (SERS) platform for the detection of degraded antibiotics-2-mercapto-5-methyl-1,3,4-thiadiazole (MMT). The results showed that the log-log plot of SERS intensity to MMT concentration exhibits a superior linear relationship (R² = 0.992) in the concentration range of 0.5-1000 μM, with a detection limit of 0.11 μM. The silver nanoparticle-decorated TiO₂ nanotube array was successfully used as an all-in-one SPME-SERS substrate in the extraction and identification of the antibiotic degradation products in real milk. Due to the rapid pre-treatment, good reproducibility, and self-cleaning, the proposed SPME-SERS method has a great promise to be applied as a powerful tool for on-site detection in the field of food safety.

Self-assembly of colloidal nanoparticles into 2D arrays at water-oil interfaces: rational construction of stable SERS substrates with accessible enhancing surfaces and tailored plasmonic response

Self-assembly at water-oil interfaces has been shown to be a cheap, convenient and efficient route to obtain densely packed layers of plasmonic nanoparticles which have small interparticle distances. This creates highly plasmonically active materials that can be used to give strong SERS enhancement and whose structure means that they are well suited to creating the highly stable, reproducible and uniform substrates that are needed to allow routine and accurate quantitative SERS measurements. A variety of methods have been developed to induce nanoparticle self-assembly at water-oil interfaces, fine tune the surface chemistry and adjust the position of the nanoparticles at the interface but only some of these are compatible with eventual use in SERS, where it is important that target molecules can access the active surface unimpeded. Similarly, it is useful to transform liquid plasmonic arrays into easy-to-handle free-standing solid films but these can only be used as solid SERS substrates if the process leaves the surface nanoparticles exposed. Here, we review the progress made in these research areas and discuss how these developments may lead towards achieving rational construction of tailored SERS substrates for sensitive and quantitative SERS analysis.

Fabrication of homogeneous waffle-like silver composite substrate for Raman determination of trace chloramphenicol

Waffle-like anodized aluminum oxide homogeneously immobilized with Ag nanoparticles (AAO/Ag) is rationally designed and fabricated as surface-enhanced Raman scattering (SERS) substrate. The as-prepared SERS substrate is characterized with transmission electron microscope (TEM), scanning electron microscopy (SEM), UV-Vis spectrophotometer, and Fourier transform infrared spectrometer (FT-IR). The AAO/Ag substrate shows good uniformity of the Raman signals (RSD = 7.02%) due to waffle-like AAO supporting the well-dispersed Ag nanoparticles. For real application, the AAO/Ag substrate is used for rapid determination of chloramphenicol (CAP) in honey with low detection limit (4.0 × 10^-9 mol L^-1) and good linearity from 1.0 × 10^-5 to 1.0 × 10^-8 mol L^-1 based on the SERS peak at 1348 cm^-1. The better accumulation in the short pore path of AAO improves the target molecule approaching into the vicinity of hot spots of Ag nanoparticles. The high selectivity for CAP is attributed to the strong interaction between -NO₂ group in CAP and the composite substrate. Schematic representation of the preparation of SERS substrate, AAO₁₅₀/Ag₁₀-5 composite nanoparticles, and antibiotic determination.

Synergistic plasmon resonance coupling and light capture in ordered nanoarrays as ultrasensitive and reproducible SERS substrates

An effective SERS substrate for on-field detection needs to satisfy high sensitivity to analyte and signal reproducibility even in the special case of tilting or bending of substrates. Herein, we transferred monolayer AuNPs into a nanocavity to construct a Au particle-in-hemispherical honeycomb nanoarray (PIHHN) as an ultrasensitive and spatially reproducible SERS substrate. The capacity of detection for R6G in an optimal PIHHN substrate is as low as a concentration of 10^-15 M, and the RSD of signal deviation is no more than 5.6%. FDTD simulations explain that placing AuNPs into a metallic nanocavity can capture and focus the light field to improve the interaction between the light and the substrate and provide the collective effect of multiple plasmon coupling, which can induce a stronger electromagnetic field. In addition, the system can generate more hot spots between AuNPs and between AuNPs and the metallic nanocavity. In particular, when the substrate is tilted or bent at an angle from 0° to 60°, the SERS performance remains stable due to the rotational symmetry of the PIHHN structure, which demonstrates the capability of on-field detection. Furthermore, the PIHHN substrate is employed as a highly sensitive multiplex sensor in on-field analysis for contaminated soil, achieving the detection of analytes down to 0.5 ppb.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

Surface enhanced Raman spectroscopic studies on the adsorption behaviour of nitric oxide on a Ru covered Au nanoparticle film

Nitric oxide (NO) is very interesting because of its effects on air pollution and especially biological systems. The adsorption behavior of NO molecules has fundamental importance with great technical challenges due to complex processes and species identification. Herein, the NO adsorption behavior on a Ru surface has been investigated using well-designed surface enhanced Raman spectroscopy (SERS) substrates. A Au nanoparticle monolayer film on ITO was employed as the electrode and Ru layers were electrochemically deposited. The internal SERS effect from the Au nanoparticles with high sensitivity and the metallic surfaces of Ru with practical application were integrated into a composite Au/Ru substrate. The molecular adsorption and dissociation of NO were observed simultaneously by SERS. A competitive relationship between adsorption and dissociation was observed at higher NO pressure, and the 3-fold and 2-fold bridge and top adsorption configurations appeared on the surface and were associated with different ν_NO vibrational frequencies. The results indicated that 3-fold bridge sites are preferred for dissociation over other structures. The dissociation of NO produced adsorbed atomic nitrogen and oxygen species to form Ru–N and Ru–O bonds, respectively. The dissociation process, especially for linear NO, was site dependent and blocked at higher pressure or coverage. Due to the change in adsorption energy and coverage, a conversion of the adsorption configuration from bridge to top was observed in the initial stage of NO adsorption, and this was followed by a mixture of bridge and top configurations of NO and dissociated species. A two-step dissociation mechanism and the steps of NO adsorption were proposed. The present study suggested that the SERS technique with appropriate attractive metal overlayers provided a significant and possibly even a valuable approach to explore adsorption behavior and kinetics at gas–solid interfaces.

A SERS borrowing strategy with well-designed substrates has been developed to monitor the adsorption and dissociation of NO at Au/Ru surfaces.

A multiple signal amplification sandwich-type SERS biosensor for femtomolar detection of miRNA

MicroRNAs are widely used as tumor markers for cancer diagnosis and prognosis. Herein, a multiple signal amplification sandwich-type SERS biosensor for femtomolar detection of miRNA is reported. The signal unit consisted of giant Au vesicles, DNA sequences and deposited silver nanoparticles. The giant Au vesicles provided large-volume hot spots because of sharp tips and abundant hotspot gaps, thus enhancing the electromagnetic intensity for the SERS performance. Further silver stain would easily lead to second-stage amplification of Raman signal. In addition, more SERS signal molecules R6G adsorbed on the signal unit with the aid of HCR and the controlled nanogaps between adjacent AgNPs, brought about the third-stage amplification. The capture unit, prepared by immobilizing the capture probe (CP) on the Fe₃O₄@AuNPs, could easily capture target miRNA and greatly simplify the separation step to improve reproducibility. The higher concentration of target miRNA definitely formed more sandwich-type structures with combination of capture unit and signal unit, resulting in multiple amplification of SERS signals. The proposed multiple signal amplification sandwich-type SERS biosensor could detect miRNA-141 at the femtomolar level with a low detection limit of 0.03 fM. Meanwhile, it exhibited high selectivity and accuracy, even for practical analysis in human serum. Therefore, the designed multiple signal amplification sandwich-type SERS biosensor would be a very promising alternative tool for the detection of miRNA and analogs in the field of biomedical diagnosis.

Gold nanoparticle superlattice monolayer with tunable interparticle gap for surface-enhanced Raman spectroscopy

Taking into account the near-field coupling interaction, non-close-packed two-dimensional nanoparticle (NP) assemblies with centimeter-scale and tunable interparticle gap (d) have attracted considerable attention due to their remarkable physicochemical properties, which show a wide range of potential applications, e.g., surface-enhanced Raman spectroscopy (SERS) active substrates. In the present work, we demonstrate that an Au NP superlattice monolayer (SM) with tunable d, playing a critical role in governing SERS activity, can be created via a rapid liquid-liquid interfacial assembly strategy. We show that the enhancement factor (EF) of SERS has an approximate 1/d^2.4 dependence on the gap of Au NP SM assemblies. This work provides a platform for the rational design of plasmon-enhanced spectroscopy active substrates for theoretical studies and for various applications, including SERS-active substrates, photoelectronic devices, biosensors and others.

Magnetic tuning of SERS hot spots in polymer-coated magnetic-plasmonic iron-silver nanoparticles

Plasmonic nanostructures are intensively studied for their ability to create electromagnetic hot spots, where a great variety of optical and spectroscopic processes can be amplified. Understanding how to control the formation of hot spots in a dynamic and reversible way is crucial to further expand the panorama of plasmon enhanced phenomena. In this work, we investigate the ability to modulate the hot spots in magnetic-plasmonic iron-doped silver nanoparticles dispersed in aqueous solution, by applying an external magnetic field. Evidence of magnetic field induction of hot spots was achieved by measuring the amplification of surface enhanced Raman scattering (SERS) from analytes dispersed in the solution containing Ag-Fe NPs. A polymeric shell was introduced around Ag-Fe NPs to confer colloidal stability, and it was found that the length and density of the polymer chains have a significant influence on SERS performance, and therefore on the formation of electromagnetic hot spots, under the action of the external magnetic field. These findings are expected to provide an important contribution to understanding the growing field of tuneable electromagnetic enhancement by external stimuli, such as magnetic fields applied to magnetic-plasmonic nanoparticles.

A Simple SERS-Based Trace Sensing Platform Enabled by AuNPs-Analyte/AuNPs Double-Decker Structure on Wax-Coated Hydrophobic Surface

In this work, a simple and versatile SERS sensing platform enabled by AuNPs-analyte/AuNPs double-decker structure on wax-coated hydrophobic surface was developed using a portable Raman spectrometer. Wax-coated silicon wafer served as a hydrophobic surface to induce both aggregation and concentration of aqueous phase AuNPs mixed with analyte of interest. After drying, another layer of AuNPs was drop-cast onto the layer of AuNPs-analyte on the substrate to form double-decker structure, thus introducing more “hot spots” to further enhance the Raman signal. To validate the sensing platform, methyl parathion (pesticide), and melamine (a nitrogen-enrich compound illegally added to food products to increase their apparent protein content) were employed as two model compounds for trace sensing demonstration. The as-fabricated sensor showed high reproducibility and sensitivity toward both methyl parathion and melamine detection with the limit of detection at the nanomolar and sub-nanomolar concentration level, respectively. In addition, remarkable recoveries for methyl parathion spiked into lake water samples were obtained, while reasonably good recoveries for melamine spiked into milk samples were achieved. These results demonstrate that the as-developed SERS sensing platform holds great promise in detecting trace amount of hazardous chemicals for food safety and environment protection.

Carrier Density-Dependent Localized Surface Plasmon Resonance and Charge Transfer Observed by Controllable Semiconductor Content

We discuss how the controllable carrier influences the localized surface plasmon resonance (LSPR) and charge transfer (CT) in the same system based on ultraviolet-visible and surface-enhanced Raman scattering (SERS) measurements. The LSPR can be easily tuned from 580 to 743 nm by changing the sputtering power of Cu₂S in the Ag and Cu₂S composite substrate. During this process, surprisingly, we find that the LSPR is proportional to the sputtering power of Cu₂S. This observation indicates that LSPR can be accurately adjusted by changing the content of the semiconductor, or even the carrier density. Moreover, we characterize the carrier density through the detection of the Hall effect to analyze the Raman shift caused by CT and obtain the relationships between them. These fundamental discussions provide a guideline for tunable LSPR and the investigation of CT.