Natural Deposition Strategy for Interfacial, Self-Assembled, Large-Scale, Densely Packed, Monolayer Film with Ligand-Exchanged Gold Nanorods for In Situ Surface-Enhanced Raman Scattering Drug Detection

Emergence of Voronoi-Patterned Cellular Membranes via Confinement Transformation of Self-Assembled Metal-Organic Frameworks

The self-assembly of nanoparticles allows the fabrication of complex, nature-inspired architectures. Among these, Voronoi tessellations─intricate patterns found in many natural systems such as insect wings and plant tissues─have broad implications across materials science, biology, and geography. However, replicating these irregular yet organized features at the nanoscale through nanoparticle self-assembly remains challenging. Here, we introduce a confinement transformation method to generate two-dimensional (2D) Voronoi patterns by converting metal-organic frameworks, specifically zeolitic imidazolate framework-8 (ZIF-8), into layered hydroxides. The process begins with the self-assembly of ZIF-8 particles into densely packed monolayers at the liquid-air interface, driven by the Marangoni effect. Subsequent Ni2+-induced etching converts the floating ZIF-8 monolayer into a freestanding membrane composed of interconnected polygonal cells, closely resembling the geometric characteristics of Voronoi tessellations. We systematically investigate the parameters affecting the transformation of ZIF-8 particles, shedding light on the mechanism governing Voronoi pattern formation. Mechanical testing and simulations demonstrate that the resulting cellular membranes exhibit enhanced stress distribution and crack resistance, attributed to their Voronoi-patterned architecture. These robust, monolithic membranes composed of Ni-based hydroxides, when serving as catalyst support materials, can synergistically enhance the intrinsic activity of Pt catalysts for alkaline hydrogen evolution reaction by facilitating water dissociation. This work presents a promising approach for creating nature-inspired materials with optimal stress management, superior mechanical properties, and potential catalytic applications.

Rational side-by-side self-assembly of gold nanorods with short and medium aspect ratios via the self-evaporation method to boost their potential as a surface-enhanced Raman scattering (SERS) substrate

Surface-enhanced Raman scattering (SERS) represents a compelling detection methodology centered on the electromagnetic fields, commonly termed "hot spots", generated around noble nanoparticles. Nonetheless, the efficacy of electromagnetic field (EMF) amplification is constrained when utilizing individual nanoparticles. There has been a notable lack of experimental and theoretically simulated studies regarding the increase of the electromagnetic field when gold nanorods with different aspect ratios undergo self-assembly in either perpendicular or parallel orientations to substrates. This research presents a novel and facile methodology for fabricating SERS nanosubstrates. This method entails self-assembling gold nanorods (AuNRs) with short and medium aspect ratios (ARs) through natural evaporation. By manipulating the water-to-ethanol ratios, we ascertain the appropriate conditions for the rational alignment of the nanorods in both perpendicular and parallel orientations relative to the silicon substrate. These nanosubstrates have been experimentally evaluated for their ability to improve the Surface-Enhanced Raman Scattering (SERS) performance, presenting a novel perspective in this field. In addition, a computational analysis employing the finite-difference time-domain (FDTD) method was conducted to elucidate the electromagnetic field generated by nanoarrays when subjected to incident light of varying wavelengths, including 532 nm, 638 nm, and 785 nm. Notably, the FDTD simulation outcomes indicated that gold nanorods (AuNRs) possessing an aspect ratio of 3.0 and nanogaps of 2.0 nm exhibited exceptional electromagnetic field characteristics when aligned parallel to the substrate under 532 nm laser illumination. Conversely, when the AuNRs were oriented perpendicular to the substrates, they produced lower EMFs upon interaction with excitation laser light. These findings can potentially contribute to the advancement of SERS nanosubstrate design.

Exploration of cucurbituril-mediated SERS plasmonic nanoarrays with sub-nanometer gaps

The uneven distribution of hotspots and the challenges associated with precise analyte localization within these hotspots present significant hurdles in the field of surface-enhanced Raman scattering (SERS). Here, at the water-oil interface, gold nanoparticles (AuNPs) interconnected by cucurbiturils[8] (CB[8]) with sub-nanometer gaps (AuNPs:CB[8]) were organized into plasmonic arrays. This arrangement was engineered to generate highly efficient hotspots. The CB[8] molecules, serving a dual role, not only facilitated the assembly of AuNPs with sub-nanometer (~ 1 nm) gaps to create intense plasmonic hotspots but also acted as molecular traps, enabling the precise localization of molecules within these hotspots. By comparing the enhancement effect of probe molecule on Au nanofilm, AuNPs:CB[8] colloids, and AuNPs:CB[8] nanofilm, it was found that the SERS intensity of the E1 characteristic peak in AuNPs:CB[8] nanofilm is five times higher than that on Au nanofilm, and more than 104 times higher than that of AuNPs:CB[8] colloids. The gaps are also accessible to different electronegativite molecules, such as estrone, p-aminoazobenzene, or methylene blue, which are captured at the plasmonic hotspots by the interaction of CB[8]. The method was employed for the practical detection of artificial antioxidant butylated hydroxyanisole (BHA), which has a weak Raman scattering cross-section, by coupling it with a reaction to enhance its SERS effect. The detection limit of BHA in soybean oil sample is 5.89 × 10-8 mol/L, with the recovery range 85.1-115%. In conclusion, this hot-spot design and molecular capture approach will offer a highly effective method for detecting weak Raman scattering cross-section molecules and holds great promise for practical applications in the future.

Frequency dependence of nanorod self-alignment using microfluidic methods

Dielectrophoresis is a potential candidate for aligning nanorods on electrodes, in which the interplay between electric fields and microfluidics is critically associated with its yield. Despite much of previous work on dielectrophoresis, the impact of frequency modulation on dielectrophoresis-driven nanorod self-assembly is insufficiently understood. In this work, we systematically explore the frequency dependence of the self-alignment of silicon nanorod using a microfluidic channel. We vary the frequency from 1kHz to 1000 kHz and analyze the resulting alignments in conjunction with numerical analysis. Our experiment reveals an optimal alignment yield at approximately 100 kHz, followed by a decrease in alignment efficiency. The nanorod self-alignments are influenced by multiple consequences, including the trapping effect, induced electrical double layer, electrohydrodynamic flow, and particle detachment. This study provides insights into the impact of frequency modulation of electric fields on the alignment of silicon nanorods using dielectrophoresis, broadening its use in various future nanotechnology applications.

Self-assembled core-shell nanoparticles with embedded internal standards for SERS quantitative detection and identification of nicotine released from snus products

Surface enhanced Raman spectroscopy (SERS) is a unique analytical technique with excellent performance in terms of sensitivity, non-destructive detection and resolution. However, due to the randomness and poor repeatability of hot spot distribution, SERS quantitative analysis is still challenging. Meanwhile, snus is a type of tobacco product that can release nicotine and other components in the mouth without burning, and the rapid detection technique based on SERS can reliably evaluate the amount of nicotine released from snus, which is of great significance for understanding its characteristics and regulating its components. Herein, the strategy was proposed to solve the feasibility of SERS quantitative detection based on self-assembled core-shell nanoparticles with embedded internal standards (EIS) due to EIS signal can effectively correct SERS signal fluctuations caused by different aggregation states and measurement conditions, thus allowing reliable quantitative SERS analysis of targets with different surface affinity. By means of process control, after the Au nanoparticles (Au NPs) were modified with 4-Mercaptobenzonitrile (4-MBN) as internal standard molecules, Ag shell with a certain thickness was grown on the surface of the AuNP@4-MBN, and then the Au@4-MBN@Ag NPs were used to regulate and control the assembly of liquid-liquid interface. The high-density nano-arrays assembled at the liquid-liquid interface ensure high reproducibility as SERS substrates, and which could be used for SERS detection of nicotine released from snus products. In addition, time-mapping research shows that this method can also be used to dynamically monitor the release of nicotine. Moreover, such destruction-free evaluation of the release of nicotine from snus products opens up new perspectives for further research about the impact of nicotinoids-related health programs.

Controllable preparation of Ti3C2Tx/Ag composite as SERS substrate for ultrasensitive detection of 4-nitrobenzenethiol

Currently, metal carbonitride (MXene) has been identified as a hot research topic in the research area of surface-enhanced Raman scattering (SERS). In this study, Ti3C2Tx/Ag composite was fabricated as SERS substrate with different Ag contents. The fabricated Ti3C2Tx/Ag composites show good SERS behavior by detecting 4-Nitrobenzenethiol (4-NBT) probe molecules. Through calculation, the SERS enhancement factor (EF) of the Ti3C2Tx/Ag substrate was as high as 4.15 × 106. It is worth noting that the detection limit of 4-NBT probe molecules can be achieved ultralow concentration of 10-11 M. In this system, electromagnetic enhancement mechanism and chemical enhancement mechanism have synergistic effects on SERS phenomenon. Meanwhile, the Ti3C2Tx/Ag composite substrate exhibited good SERS reproducibility. In addition, the SERS detection signal hardly changed after 6 months of natural standing, and the substrate showed good stability. This work suggests that the Ti3C2Tx/Ag substrate could be used as a sensitivity SERS sensor for practical application, and could be applied in the field of environmental monitoring.

Differences between surfactant-free Au@Ag and CTAB-stabilized Au@Ag star-like nanoparticles in the preparation of nanoarrays to improve their surface-enhanced Raman scattering (SERS) performance

In this study, we assessed the controlled synthesis and efficacy of surface-enhanced Raman scattering (SERS) on two distinct types of star-like Au@Ag core-shell nanoarrays. These nanoarrays were designed based on gold nanostars (AuNSs), which were synthesized with and without CTAB surfactant (AuNSs-CTAB and AuNSs-FS, respectively). The AuNS-FS nanoparticles were synthesized via a novel modification process, which helped overcome the previous limitations in the free-surfactant preparation of AuNSs by significantly increasing the number of branches, increasing the sharpness of the branches and minimizing the adsorption of the surfactant on the surface of AuNSs. Furthermore, the differences in the size and morphology of these AuNSs in the created nanoarrays were studied. To create the nanoarrays, a three-step method was employed, which involved the controlled synthesis of gold nanostars, covering them with a silver layer (AuNSs-FS@Ag and AuNSs-CTAB@Ag), and finally self-assembling the AuNS@Ag core-shelled nanoparticles via the liquid/liquid self-assembly method. AuNSs-FS@Ag showed higher ability in forming self-assembled nanoarrays than the nanoparticles prepared using CTAB, which can be attributed to the decrease in the repulsion between the nanoparticles at the interface. The nano-substrates developed with AuNSs-FS@Ag possessed numerous "hot spots" on their surface, resulting in a highly effective SERS performance. AuNSs-FS featured a significantly higher number of sharp branches than AuNSs-CTAB, making it the better choice for creating nanoarrays. It is worth mentioning that AuNSs-CTAB did not exhibit the same benefits as AuNSs-FS. The morphology of AuNSs with numerous branches was formed by controlling the seed boiling temperature and adding a specific amount of silver ions. To compare the SERS activity between the as-prepared nano-substrates, i.e., AuNS-CTAB@Ag and AuNS-FS@Ag self-assembled nanoarrays, low concentrations of crystal violet aqueous solution were characterized. The results showed that the developed AuNSs-FS@Ag could detect CV at trace concentrations ranging from 1.0 ng mL-1 to 10 ng mL-1 with a limit of detection (LOD) of 0.45 ng mL-1 and limit of quantification (LOQ) of 1.38 ng mL-1. The nano-substrates remained stable for 42 days with a decrease in the intensity of the characteristic Raman peaks of CV by less than 7.0% after storage. Furthermore, the spiking method could detect trace amounts of CV in natural water from the Dong Nai River with concentrations as low as 1 to 100 ng mL-1, with an LOD of 6.07 ng mL-1 and LOQ of 18.4 ng mL-1. This method also displayed good reproducibility with an RSD value of 5.71%. To better understand the impact of CTAB stabilization of the Au@Ag star-like nanoparticles on their surface-enhanced Raman scattering (SERS) performance, we conducted density functional theory (DFT) calculations. Our research showed that the preparation of AuNSs-FS@Ag via self-assembly is an efficient, simple, and fast process, which can be easily performed in any laboratory. Furthermore, the research and development results presented herein on nanoarrays have potential application in analyzing and determining trace amounts of organic compounds in textile dyeing wastewater.

The volatile release evaluation of nicotine from snus products under different storage conditions based on surface-enhanced Raman spectroscopy technology

Surface enhanced Raman spectroscopy (SERS) is a highly sensitive analytical detection technique that provides unique chemical and structural information on target molecules. Snus is a type of tobacco product that can release nicotine and other components under certain humidity and temperature without burning, and the evaluation of its nicotine release under different storage conditions is very important for understanding its characteristics, regulating its components, and setting reasonable storage conditions. Herein, by means of an artificial climate box and suction extraction device, the volatile release evaluations of nicotine from snus products under different storage conditions were performed based on Fe3O4 microparticles coated with Au nanorods and Au nanoparticles (Fe3O4@AuNRsNPs) as SERS substrates combined with a capillary. The Fe3O4@AuNRsNPs assemblies can be fixed in the inner wall of the capillary with the aid of an external magnetic field, which improved the maneuverability of the SERS substrates. By comparing the intensities of the spectral peaks of the symmetrical breathing of the pyridine moiety of nicotine molecules with increasing temperature and humidity, which could significantly accelerate the volatile release of a small amount of nicotine, the nicotine release under different conditions could be evaluated. Based on this strategy, it was possible to obtain the storage or placement conditions of the product. The results of this study provide a reference to clarify the volatile release of nicotine under various storage conditions, which is helpful for better regulation of the levels of nicotine in snus. Moreover, such destruction-free evaluation of the volatile release of nicotine from snus products under different storage conditions opens up new perspectives for further research about the impact of nicotinoids on smokers' health and cessation programs.

SERS-active plasmonic metal NP-CsPbX3 films for multiple veterinary drug residues detection

Sensitive and multiple veterinary drug residues detection is of important for food safety. Herein, uniform plasmonic Au nanobipyramids@Ag nanorods (Au NBP@Ag NR)-CsPbX₃ films with strong surface-enhanced Raman scattering (SERS) activity were constructed. The effects of different CsPbX₃ (X = Cl, Br, I, or mixed halogens) quantum dots (QDs) on the SERS performances of plasmonic metal NP films were investigated. CsPbI₃ QDs with large dielectric constant could be served as the dielectric media to retard the attenuation of electromagnetic evanescent wave, inducing strong electromagnetic strength for SERS enhancement. Plasmon-induced metal-to-perovskite interfacial charge transfer transition also contributed to SERS enhancement. SERS-active plasmonic Au NBP@Ag NR-CsPbI₃ films had excellent sensitivity and high reproducibility for quantitative, accurate and multiple detection of chloramphenicol, diazepam and malachite green in food matrix. This work deepened the understanding of the SERS enhancement mechanisms of plasmonic metal NP-perovskite hybrid heterostructures, showing potential prospects in food safety monitoring.

A Review on Self-Assembly of Colloidal Nanoparticles into Clusters, Patterns, and Films: Emerging Synthesis Techniques and Applications

The colloidal synthesis of functional nanoparticles has gained tremendous scientific attention in the last decades. In parallel to these advancements, another rapidly growing area is the self-assembly or self-organization of these colloidal nanoparticles. First, the organization of nanoparticles into ordered structures is important for obtaining functional interfaces that extend or even amplify the intrinsic properties of the constituting nanoparticles at a larger scale. The synthesis of large-scale interfaces using complex or intricately designed nanostructures as building blocks, requires highly controllable self-assembly techniques down to the nanoscale. In certain cases, for example, when dealing with plasmonic nanoparticles, the assembly of the nanoparticles further enhances their properties by coupling phenomena. In other cases, the process of self-assembly itself is useful in the final application such as in sensing and drug delivery, amongst others. In view of the growing importance of this field, this review provides a comprehensive overview of the recent developments in the field of nanoparticle self-assembly and their applications. For clarity, the self-assembled nanostructures are classified into two broad categories: finite clusters/patterns, and infinite films. Different state-of-the-art techniques to obtain these nanostructures are discussed in detail, before discussing the applications where the self-assembly significantly enhances the performance of the process.

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.

Off-Resonance Gold Nanobone Films at Liquid Interface for SERS Applications

Extensive effort and research are currently channeled towards the implementation of SERS (Surface Enhanced Raman Spectroscopy) as a standard analytical tool as it has undisputedly demonstrated a great potential for trace detection of various analytes. Novel and improved substrates are continuously reported in this regard. It is generally believed that plasmonic nanostructures with plasmon resonances close to the excitation wavelength (on-resonance) generate stronger SERS enhancements, but this finding is still under debate. In the current paper, we compared off-resonance gold nanobones (GNBs) with on-resonance GNBs and gold nanorods (GNRs) in both colloidal dispersion and as close-packed films self-assembled at liquid-liquid interface. Rhodamine 6G (R6G) was used as a Raman reporter in order to evaluate SERS performances. A 17-, 18-, and 55-fold increase in the Raman signal was observed for nanostructures (off-resonance GNBs, on-resonance GNBs, and on-resonance GNRs, respectively) assembled at liquid-liquid interface compared to the same nanostructures in colloidal dispersion. SERS performances of off-resonance GNBs were superior to on-resonance nanostructures in both cases. Furthermore, when off-resonance GNBs were assembled at the liquid interface, a relative standard deviation of 4.56% of the recorded signal intensity and a limit of detection (LOD) of 5 × 10^-9 M could be obtained for R6G, rendering this substrate suitable for analytical applications.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Microfluidic Transport of Hybrid Optoplasmonic Particles for Repeatable SERS Detection

For its ultrahigh sensitivity, the microfluidic system combined with surface-enhanced Raman spectroscopy (SERS) becomes one of the most interesting topics in integrated online monitoring related fields. In previous reports, the commonest surface plasmon-enhanced substrates in microfluidics consist of immobilized metal nanostructures on the channel surface to overcome the disturbance of Brownian motion. In this work, a hybrid optoplasmonic microfluidic conveyer is developed, in which the movable, highly ordered optoplasmonic particles are delivered to the detection spot for SERS detection. Here, the optoplasmonic particle is the SiO₂ microsphere with in situ photochemical reduced Ag nanoparticles on the surface. Because of the converged light at the SiO₂ microsphere surface, the SERS spectra collected at this optoplasmonic particle in the channel exhibit excellent performance, which is confirmed by the simulated electric field distribution. In addition, the experimental data also demonstrate that the quantitative analysis is achieved at 1 nM in this optoplasmonic microfluidic conveyer. Furthermore, the used optoplasmonic particle can be ejected from the microfluidic channel by modulating the velocity of injected fluid such that the new optoplasmonic particle will be delivered to the detection spot for repeatable SERS detection in the same channel. The dynamic process of optoplasmonic particle transport is investigated in this microconveyer, and the built theoretical model to predict the particle release is highly identical with the experimental data. These data point out that our hybrid optoplasmonic microfluidic conveyer has repeatable enhanced substrates with the high SERS sensitivity to overcome the cross-contamination of different target molecules in repeatable detection.

Rapid SERS Quantification of Trace Fentanyl Laced in Recreational Drugs with a Portable Raman Module

Rapid identification and quantification of opioid drugs are of significant importance and an urgent need in drug regulation and control, considering the serious social and economic impact of the opioid epidemic in the United States. Unfortunately, techniques for accurate detection of these opioids, particularly for fentanyl, an extremely potent synthetic drug of abuse and a main perpetrator in the opioid crisis, are often not readily accessible. Therefore, a fast, highly sensitive, and preferably quantitative technique, with excellent portability, is highly desirable. Such a technique can potentially offer timely and crucial information for drug control officials, as well as health professionals, about drug distribution and overdose prevention. We therefore propose a portable surface-enhanced Raman scattering (SERS) approach by pairing an easy to perform yet reliable SERS protocol with a compact Raman module suitable for rapid, on-site identification and quantification of trace fentanyl. Fentanyl spiked in urine control was successfully detected at concentrations as low as 5 ng/mL. Portable SERS also enabled detection of trace fentanyl laced in recreational drugs at mass concentrations as low as 0.05% (5 ng in 10 μg total) and 0.1% (10 ng in 10 μg total) in heroin and tetrahydrocannabinol (THC), respectively. Drug interaction with the nanoparticle surface was simulated through molecular dynamics to investigate the molecular adsorption mechanism and account for SERS signal differences observed for opioid drugs. Furthermore, resolution of fentanyl in binary and ternary opioid mixtures was readily achieved with multivariate data analysis. In sum, we developed a rapid, highly sensitive, and reliably quantitative method for trace fentanyl analysis by synergizing a streamlined SERS procedure and a portable Raman module at low cost.

Detection of trace amounts of insoluble pharmaceuticals in water by extraction and SERS measurements in a microfluidic flow regime

Detection of trace amounts of poorly water-soluble pharmaceuticals or related (bio)solutions represents a key challenge in environment protection and clinical diagnostics. However, this task is complicated by low concentrations of pharmaceuticals, complex sample matrices, and sophisticated sample preparative routes. In this work, we present an alternative approach on the basis of an on-line flow extraction procedure and SERS measurements performed in a microfluidic regime. The advantages of our approach were demonstrated using ibuprofen (Ibu), which is considered as a common pharmaceutical contaminant in wastewater and should be monitored in various bioliquids. The extraction of Ibu from water to the dichloromethane phase was performed with an optimized microfluidic mixer architecture. As SERS tags, lipophilic functionalized gold multibranched nanoparticles (AuMs) were added to the organic phase. After microfluidic extraction, Ibu was captured by the functionalized AuM surface and recognized by on-line SERS measurements with up to 10-8 M detection limit. The main advantages of the proposed approach can be regarded as its simplicity, lack of need for preliminary sample preparation, high reliability, the absence of sample pretreatment, and low detection limits.

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.

Wavelength-Tunable Optical Fiber Localized Surface Plasmon Resonance Biosensor via a Diblock Copolymer-Templated Nanorod Monolayer

Well-dispersed and dense layers of gold nanorods (AuNRs) on optical fibers are shown to regulate the longitudinal peak wavelength and enhance the sensing performances of localized surface plasmon resonance (LSPR) biosensors. A simple self-assembly method relying on a brush-like monolayer of poly(styrene)-b-poly(acrylic acid) (PS-b-PAA) diblock copolymer was used to immobilize AuNRs with various aspect ratios from 2.33 to 4.60 on optical fibers. Both the experimental and simulation results illustrated that the particle aspect ratio, deposition time (related to the coverage of AuNRs), and interparticle gap significantly affected the optical properties of the fiber-based LSPR biosensors. The highest refractive index (RI) sensitivity of the sensor was 753 nm/RIU, while the limit of detection for human IgG was as low as 0.8 nM. Compared with standard nanoparticle deposition methods of polyelectrolytes or alkoxysilanes, the RI sensitivity of the PS-b-PAA dip-coating method was approximately 3-fold better, a consequence of the higher particle coverage and fewer AuNR aggregates. The presented AuNR-based LSPR sensors could regulate the detection range by tuning the aspect ratios of AuNRs. Applicability is demonstrated via quantitative analysis of antigen-antibody interactions, DNA sensing, and surface-enhanced Raman scattering.

Facile Surface Modification of Mesoporous Au Nanoparticles for Highly Sensitive SERS Detection

The stability, dispersity, and surface chemical properties of colloidal nanoparticles are crucial for the reliable and desired chemical sensing in various applications. Here, we report an effective strategy to engineer the surface properties of mesoporous Au nanoparticles (meso-Au NPs) via PVP ligand modification, template removal, and surface purification. Monodispersed 3D meso-Au NPs with well-defined sizes and shapes were obtained using a general soft-enveloping strategy. During surface modification, the addition of PVP ligands and the concentration of HF solutions play key roles in the stability, shape, and size distributions of ordered Au networks. In order to obtain an improved sensing performance, the morphologies of meso-Au NPs were optimized with smaller mesopore size, and NaBH₄ solution was used to efficiently remove the adsorbed PVP ligands. Due to the characteristics of high-density porosities and large surface area, the purified meso-Au NPs could be a kind of promising plasmonic-enhanced nanomaterial and provide abundant "hot spots." Combined with the enrichment effect using a slippery liquid-infused porous surface, the lowest detection limits of crystal violet molecule could be down to 0.1 pM, demonstrating an excellent SERS sensitivity. Moreover, a realistic illegal drug containing aspirin could be sensitively detected with a limit of 2.8 × 10^-6 M, showing great potential for practical molecular sensing and applications.

Two-dimensional self-assembled Au-Ag core-shell nanorods nanoarray for sensitive detection of thiram in apple using surface-enhanced Raman spectroscopy

The development of substrate with high sensitivity and good reproducibility for surface-enhanced Raman scattering (SERS) detection of contaminants in foods has attracted more and more attention. Herein, a stable two-dimensional (2D) Au-Ag core-shell nanorods (Au@Ag NRs) nanoarray substrate with high-performance SERS activity was developed based on interface self-assembly strategy and successfully applied to the detection of thiram in apple sample. A broad linearity range of 0.01-10 mg/L and a low limit of detection of 0.018 mg/L were achieved for thiram solution. The substrate was stable and exhibited satisfactory sensitivity after preserving at ambient temperature for 4 weeks. Furthermore, this method presented the comparable result to that acquired from high-performance liquid chromatography (HPLC) with satisfactory recoveries of 93-116%. The study indicated that the prepared Au@Ag NRs nanoarray substrate was promising for SERS detection of contaminants such as pesticides in foods.

Marangoni Effect-Driven Transfer and Compression at Three-Phase Interfaces for Highly Reproducible Nanoparticle Monolayers

Interfacial self-assembly is a powerful technology for preparing large scale nanoparticle monolayers, but fabrication of highly repeatable large scale nanoparticle monolayers remains a challenge. Here we develop an oil/water/oil (O/W/O) three-phase system based on the Marangoni effect to fabricate highly reproducible nanoparticle monolayers. Nanoparticles could be easily transferred and compressed from the lower O/W interface to the upper O/W interface due to the interfacial tension gradient. The O/W/O system can be constructed using different kinds of organic solvents. Through this approach, good uniformity and reproducibility of the nanoparticle monolayers could be guaranteed even using a wide range of nanoparticle concentrations. Furthermore, this strategy is generally applicable to various nanoparticles with different sizes, shapes, components, and surface ligands, which offers a facile and general approach to functional nanodevices.

A review on recent advances in the applications of surface-enhanced Raman scattering in analytical chemistry

This review is focused on recent developments of surface-enhanced Raman scattering (SERS) applications in Analytical Chemistry. The work covers advances in the fabrication methods of SERS substrates, including nanoparticles immobilization techniques and advanced nanopatterning with metallic features. Recent insights in quantitative and sampling methods for SERS implementation and the development of new SERS-based approaches for both qualitative and quantitative analysis are discussed. The advent of methods for pre-concentration and new approaches for single-molecule SERS quantification, such as the digital SERS procedure, has provided additional improvements in the analytical figures-of-merit for analysis and assays based on SERS. The use of metal nanostructures as SERS detection elements integrated in devices, such as microfluidic systems and optical fibers, provided new tools for SERS applications that expand beyond the laboratory environment, bringing new opportunities for real-time field tests and process monitoring based on SERS. Finally, selected examples of SERS applications in analytical and bioanalytical chemistry are discussed. The breadth of this work reflects the vast diversity of subjects and approaches that are inherent to the SERS field. The state of the field indicates the potential for a variety of new SERS-based methods and technologies that can be routinely applied in analytical laboratories.

Polymer induced one-step interfacial self-assembly method for the fabrication of flexible, robust and free-standing SERS substrates for rapid on-site detection of pesticide residues

We have demonstrated a one-step approach for the fabrication of flexible, robust, reproducible and free-standing SERS substrates (AuNPs/polyvinyl chloride (PVC) film) through a polymer induced interfacial self-assembly method. In this method, the polymer (PVC) plays dual roles, that is, inducing the interfacial self-assembly of nanoparticles and fixing the assembled nanostructure in the PVC matrix. As the assembled nanoparticles are orderly half-embedded in the PVC film, the AuNPs/PVC film exhibits outstanding reproducibility and stability. In addition, the film could be easily regenerated by rinsing with NaBH4 solution. As a proof of concept, the film was directly wrapped on an apple surface for in situ detection of pesticide residues, and a detection limit of 10 ng cm-2 thiram was achieved. Furthermore, rapid on-site and in situ detection of multi-pesticide residues has been proved to be feasible with the aid of a portable Raman spectrometer. Due to its simple preparation, good reliability, outstanding stability and reusability, the AuNPs/PVC film has great potential in practical applications.

On-chip 3D SERS materials produced by self-assemble of copper microparticle and galvanic replacement reaction

Surface-enhanced Raman scattering (SERS) has powerful capabilities in the field of environmental analysis and biological diagnostics because of its instinctive properties of high sensitivity and label-free detection. However, the fabrication of SERS substrate requires complicated processes and expensive equipment. This paper proposes a simple method approach to synthesize a 3D SERS substrate via electroless galvanic replacement reaction inside a microfluidic channel. Copper microparticles are assembled in a microfluidic channel, and silver nitrate solution is pumped into the channel to in situ produce the silver nanoparticles (Ag NPs) on the surface of copper microparticles. Because the copper particles occupy the channel by stack, the 3D Cu@AgNP SERS substrate can be obtained. The probing molecule (methylene blue) was utilized to investigate with the limit of detection (1×10^-7  M). The biological molecules (urea and melamine) have been used to demonstrate its benefits in medical applications, and cancer cell detection has been implemented to demonstrate its benefits in cell biology. In addition, the device can filter and wash cells, forming a simple and fast filter. Our work on this simple fabrication method of active SERS substrate has great value for medical and biological applications.

Nanoscale Vertical Arrays of Gold Nanorods by Self-Assembly: Physical Mechanism and Application

The unique photonic effect of self-assembled metal nanoparticles is widely used in many applications. In this article, we prepared self-assembled gold nanorod (GNR) vertical arrays substrate by an evaporation method and found that the morphology of the substrate can be effectively regulated by changing the immersion time in the target molecules solution to obtain different Raman enhancement effects. We separately calculated the local electromagnetic field of the GNR vertical arrays and disorder substrate by the finite element method (FEM), which was consistent with the experimental results. Based on optimal soaking time, the sensitivity, reproducibility, and stability of substrates were separately studied. The experimental results show that the GNR vertical arrays can detect Rhodamine 6G (Rh6G) at concentrations as low as 10-11 M and exhibit good reproducibility and stability due to local electromagnetic (EM) field enhancement caused by the coupling of adjacent nanorods. Thus, our work can demonstrate that the substrate has excellent surface-enhanced Raman scattering (SERS) activity and the obtained GNR vertical arrays have great potential for biosensor and biodetection.

Highly sensitive detection of an antidiabetic drug as illegal additives in health products using solvent microextraction combined with surface-enhanced Raman spectroscopy

The combined SME-SERS approach realized the effective separation and sensitive detection of illegal drug additives spiked in different healthy products.

In Situ Synthesis of Ag@Cu₂O-rGO Architecture for Strong Light-Matter Interactions

Emerging opportunities based on two-dimensional (2D) layered structures can utilize a variety of complex geometric architectures. Herein, we report the synthesis and properties of a 2D+0D unique ternary platform-core-shell nanostructure, termed Ag@Cu₂O-rGO, where the reduced graphene oxide (rGO) 2D acting as a platform is uniformly decorated by Ag@Cu₂O core-shell nanoparticles. Cu₂O nanoparticles occupy the defect positions on the surface of the rGO platform and restore the conjugation of the rGO structure, which contributes to the significant decrease of the I_D/I_G intensity ratio. The rGO platform can not only bridge the isolated nanoparticles together but also can quickly transfer the free electrons arising from the Ag core to the Cu₂O shell to improve the utilization efficiency of photogenerated electrons, as is verified by high efficient photocatalytic activity of Methyl Orange (MO). The multi-interface coupling of the Ag@Cu₂O-rGO platform-core-shell nanostructure leads to the decrease of the bandgap with an increase of the Cu₂O shell thickness, which broadens the absorption range of the visible light spectrum.