In Situ SERS Monitoring of the Plasmon-Driven Catalytic Reaction by Using Single Ag@Au Nanowires as Substrates

Plasmon-driven substitution of 4-mercaptophenylboronic acid to 4-nitrothiophenol monitored by surface-enhanced Raman spectroscopy

Plasmon-driven reactions on plasmonic nanoparticles (NPs) occur under significantly different conditions from those of classical organic synthesis and provide a promising pathway for enhancing the efficiency of various chemical processes. However, these reactions can also have undesirable effects, such as 4-mercaptophenylboronic acid (MPBA) deboronation. MPBA chemisorbs well to Ag NPs through its thiol group and can subsequently bind to diols, enabling the detection of various biological structures by surface-enhanced Raman scattering (SERS), but not upon its deboronation. To avoid this reaction, we investigated the experimental conditions of MPBA deboronation on Ag NPs by SERS. Our results showed that the level of deboronation strongly depends on both the morphology of the system and the excitation laser wavelength and power. In addition, we detected not only the expected products, namely thiophenol and biphenyl-4,4-dithiol, but also 4-nitrothiophenol (NTP). The crucial reagent for NTP formation was an oxidation product of hydroxylamine hydrochloride, the reduction agent used in Ag NP synthesis. Ultimately, this reaction was replicated by adding NaNO2 to the system, and its progress was monitored as a function of the laser power, thereby identifying a new reaction of plasmon-driven -B(OH)2 substitution for -NO2.

New nanosensor fabricated on single nanopore electrode filled with prussian blue and graphene quantum dots coated by polypyrrole for hydrogen peroxide sensing

Hydrogen peroxide (H2O2) is a common oxidant that plays an important role in many biological processes and is also an important medium analysis in various fields. In this work, a new electrochemical nanosensor capable of detecting and quantifying hydrogen peroxide was introduced. This nanosensor was fabricated by electrodepositing prussian blue (PB)/graphene quantum dots (GQDs)/polypyrrole (PPy) on single nanopore electrode etched from single gold nanoelectrode. This prepapred nanosensor exhibits good electrochemical response to hydrogen peroxide with high sensitivity and stability, with a linear response in the 2.0 and 80 μM by using amperometric method and differential pulse voltammetry (DPV) method. The limit of detections are 0.33 μM (S/N = 3) for amperometric method and 0.67 μM (S/N = 3) for differential pulse voltammetry (DPV) method, respectively. This nanosensor can be used for the determination of hydrogen peroxide in human urine, and can serve as a new electrochemical platform to monitor H2O2 release from single living cells due to its small overal dimension and high sensitivity.

Core-Shell Gold Nanoparticles@Pd-Loaded Covalent Organic Framework for In Situ Surface-Enhanced Raman Spectroscopy Monitoring of Catalytic Reactions

A core-shell nanostructure of gold nanoparticles@covalent organic framework (COF) loaded with palladium nanoparticles (AuNPs@COF-PdNPs) was designed for the rapid monitoring of catalytic reactions with surface-enhanced Raman spectroscopy (SERS). The nanostructure was prepared by coating the COF layer on AuNPs and then in situ synthesizing PdNPs within the COF shell. With the respective SERS activity and catalytic performance of the AuNP core and COF-PdNPs shell, the nanostructure can be directly used in the SERS study of the catalytic reaction processes. It was shown that the confinement effect of COF resulted in the high dispersity of PdNPs and outstanding catalytic activity of AuNPs@COF-PdNPs, thus improving the reaction rate constant of the AuNPs@COF-PdNPs-catalyzed hydrogenation reduction by 10 times higher than that obtained with Au/Pd NPs. In addition, the COF layer can serve as a protective shell to make AuNPs@COF-PdNPs possess excellent reusability. Moreover, the loading of PdNPs within the COF layer was found to be in favor of avoiding intermediate products to achieve a high total conversion rate. AuNPs@COF-PdNPs also showed great catalytic activities toward the Suzuki-Miyaura coupling reaction. Taken together, the proposed core-shell nanostructure has great potential in monitoring and exploring catalytic processes and interfacial reactions.

SERS detection of dopamine using metal-chelated Ag nanoshell

As the concentrations of different neurotransmitters can indicate the presence of certain disorders affecting brain functions, quantitative analyses of neurotransmitters have attracted increasing attention in various fields. Surface-enhanced Raman scattering (SERS) spectroscopy is an outstanding spectroscopic analytical tool that enables detection at the single molecule level with high specificity. As local field enhancement of surface plasmon is effective within nanometers, active interaction between SERS-active noble metals (gold and silver) and analyte molecules enhances the molecular detection capacity of SERS. However, neurotransmitters and noble metal nanoparticles are often not affinitive, because neurotransmitters generally have a hydroxyl group rather than a thiol group. As a result, the interaction between the two typically remains inactive, which makes detection more difficult. To overcome this limitation, in the present work we utilized metal-chelation to attract dopamine, a neurotransmitter molecule, close to the surface of silver nanoparticles. AgNS was capped with poly(vinyl alcohol) (PVA) and sequentially integrated with copper ion to bind dopamine in the form of chelate bonding between dopamine and copper. The PVA linked AgNS and metal ions through a coordinate bond between hydroxyl groups and metal ions. This metal-chelation-functionalized nanoprobe allowed us to stably detect dopamine in aqueous solution at a concentration of less than 10-6 M. Therefore, this method provides a convenient and easy-to-prepare option for the effective detection of dopamine, thus meaning it has the potential to be applied to other neurotransmitters.

Silver nanosheets self-assembled on polystyrene microspheres to form "hot spots" with different nanogap distances for high sensitive SERS detection

Herein, we reported a facile method to control the nanogap distance of silver (Ag) nanosheets to obtain high sensitive plasmonic Surface-enhanced Raman scattering (SERS) substrates. The sulfonated polystyrene (SPS) microspheres with different diameters were first fabricated using micro-emulsion synthesis, and then the SPS microspheres were coated with Ag nanosheets through chemical synthesis with citric acid/ascorbic acid to form Ag nanosheets@SPS (Ag@SPS) substrates with different nanogap distances. The results showed that the nanogap distance of Ag nanosheets self-assembled on SPS microspheres reduced from 80-150 nm to 28-68 nm when the diameter of SPS microspheres increased from 0.9 to 3.5 μm, and the enhancement factor (EF) increased from 105 to 107, the limit of detection of rhodamine 6G (R6G) for the Ag@SPS microspheres reduced from 10-10 to 10-13 mol/L. It confirmed that the Ag nanosheets coated on the surface of SPS microspheres could achieve ultra trace detection of analyte. Furthermore, the low concentration detection limit for melamine with the Ag@SPS microspheres substrate was about 10-8 mg/L, which is lower than the standard legislated by the European Union and the Food & Drug Administration. In addition, the SERS spectrum of 3-mercaptopropionic acid (3-MPA) could be also detected when its concentration was 10-8 mol/L. The prepared substrate offered a promising opportunity for SERS practical applications.

Three-Dimensional MXene-AgNP Hollow Spheres for In Situ Surface-Enhanced Raman Scattering Detection of Catalysis Reactions

MXenes are attractive candidates in the fields of surface-enhanced Raman scattering (SERS) and catalysis. However, most of the current studies on MXenes are based on blocks and nanosheets, limiting their SERS and catalytic properties. Herein, we have prepared 3D MXene hollow spheres wrapped with silver nanoparticles (Ti3C2-AgNP HSs) using a sacrificial template method, which exhibits excellent sensitivity with a low detection limit due to good light-trapping capability of the hollow sphere and strong localized surface plasmon resonance (LSPR) effect of AgNPs. Furthermore, it shows outstanding photocatalytic performance and realizes in situ SERS monitoring of the 4-nitrobenzenethiol (4-NTP) to 4-aminothiophenol (4-ATP) catalysis reaction. The finite-difference time-domain (FDTD) simulations confirm that 3D Ti3C2-AgNP hollow structures have stronger hot spots than 3D solid structures and higher SERS sensitivity for molecule detection. Therefore, it promises to be an excellent bifunctional material for highly sensitive SERS detection and the in situ monitoring of catalytic reactions.

Combining surface-accessible Ag and Au colloidal nanomaterials with SERS for in situ analysis of molecule-metal interactions in complex solution environments

The interactions between molecules and noble metal nanosurfaces play a central role in many areas of nanotechnology. The surface chemistry of noble metal surfaces under ideal, clean conditions has been extensively studied; however, clean conditions are seldom met in real-world applications. We developed a sensitive and robust characterization technique for probing the surface chemistry of nanomaterials in the complex environments that are directly relevant to their applications. Surface-enhanced Raman spectroscopy (SERS) can be used to probe the interaction of plasmonic nanoparticles with light to enhance the Raman signals of molecules near the surface of nanoparticles. Here, we explain how to couple SERS with surface-accessible plasmonic-enhancing substrates, which are capped with weakly adsorbing capping ligands such as citrate and chloride ions, to allow molecule-metal interactions to be probed in situ and in real time, thus providing information on the surface orientation and the formation and breaking of chemical bonds. The procedure covers the synthesis and characterization of surface-accessible colloids, the preliminary SERS screening with agglomerated colloids, the synthesis and characterization of interfacial nanoparticle assemblies, termed metal liquid-like films, and the in situ biphasic SERS analysis with metal liquid-like films. The applications of the approach are illustrated using two examples: the probing of π-metal interactions and that of target/ligand-particle interactions on hollow bimetallic nanostars. This protocol, from the initial synthesis of the surface-accessible plasmonic nanoparticles to the final in situ biphasic SERS analysis, requires ~14 h and is ideally suited to users with basic knowledge in performing Raman spectroscopy and wet synthesis of metal nanoparticles.

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

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

Self-Assembly of Silver Nanowire Films for Surface-Enhanced Raman Scattering Applications

The development of SERS detection technology is challenged by the difficulty in obtaining SERS active substrates that are easily prepared, highly sensitive, and reliable. Many high-quality hotspot structures exist in aligned Ag nanowires (NWs) arrays. This study used a simple self-assembly method with a liquid surface to prepare a highly aligned AgNW array film to form a sensitive and reliable SERS substrate. To estimate the signal reproducibility of the AgNW substrate, the RSD of SERS intensity of 1.0 × 10^-10 M Rhodamine 6G (R6G) in an aqueous solution at 1364 cm^-1 was calculated to be as low as 4.7%. The detection ability of the AgNW substrate was close to the single molecule level, and even the R6G signal of 1.0 × 10^-16 M R6G could be detected with a resonance enhancement factor (EF) as high as 6.12 × 10¹¹ under 532 nm laser excitation. The EF without the resonance effect was 2.35 × 10⁶ using 633 nm laser excitation. FDTD simulations have confirmed that the uniform distribution of hot spots inside the aligned AgNW substrate amplifies the SERS signal.

Peroxidase-like Nanozymes for Point-of-Care SERS Sensing and Wound Healing

The emergence of nanozymes provides a potential method for combating multidrug-resistant bacteria resulted from the abuse of antibiotics. However, in nanozyme-catalyzed systems, few studies have addressed the actual hydrogen peroxide (H₂O₂) level involved in sterilization. Herein, we designed a high-efficiency peroxidase-mimicking nanozyme with surface-enhanced Raman scattering (SERS) property by assembling gold nanoparticles on single-layer Cu²⁺-C₃N₄ (AuNP-Cu²⁺-C₃N₄). The nanozyme effectively converts the low-active Raman reporter 3,3',5,5'-tetramethylbenzidine (TMB) into its oxidized form with H₂O₂, resulting in SERS signal changes, thereby achieving highly sensitive quantification of H₂O₂ with limit of detection as low as 0.60 μM. More importantly, the nanozyme can specifically catalyze H₂O₂ into antibacterial hydroxyl radicals. In vitro and in vivo evaluations demonstrate the remarkable antibacterial efficacy of the nanozyme/H₂O₂ combination against Staphylococcus aureus (up to 99.9%), which could promote wound healing in mice and allow point-of-care monitoring the amount of H₂O₂ participated in effective sterilization. This study not only displays great potential in combining multiple functionalities of nanomaterials for versatile bioassays but also provides a promising approach to design nanozymes for biomedical and catalytic applications.

An adhesive SERS substrate based on a stretched silver nanowire-tape for the in situ multicomponent analysis of pesticide residues

Two essential factors in powerful surface-enhanced Raman spectroscopy analysis of trace pesticide residues are viz., high sensitivity and efficient sampling. Herein, owing to elastic properties, a stretched Ag nanowire (Ag NW)-tape under the strain of 15% formed a wrinkled structure with periodic microridges and microgrooves, where abundant nanogaps were generated by the aggregated Ag NWs. Compared with the unstretched Ag NW-tape substrate, an appreciable signal enhancement of the modified 4-mercaptobenzoic acid (4-MBA) molecules with a ratio of 2.6 was discerned from the sophisticated SERS substrate due to the electromagnetic enhancement induced by the relatively high density of "hot spots" around the Ag NW aggregates. The as-fabricated Ag NW-tape substrate performed admirably in detecting 4-MBA and demonstrated an enhancement factor of 1.16 × 10⁶. Moreover, for the in situ detection of tetramethylthiuram disulfide, thiabendazole, and their mixture, the relatively high recovery rates of over 88% were favorably realized by the Ag NW-tape substrate with superior sensitivity, distinct flexibility, and adhesiveness. This fascinating SERS substrate, dependent on the flexible and adhesive Ag NW-tape, is promising for application in SERS analysis of trace residues on various practical surfaces.

Interface Design of 3D Flower-like Ag@ZnSe Composites: SERS and Photocatalytic Performance

In this work, we cosputtered Ag and ZnSe on a polystyrene template to form a three-dimensional (3D) Ag@ZnSe (x) structure. The 3D surface morphology and material composition that provided abundant "hot spots" were controlled by adjusting the sputtering power of the ZnSe, which was confirmed by finite-difference time-domain (FDTD) simulation. The introduction of ZnSe into the noble metal Ag also introduced a charge-transfer (CT) effect into the system, and the CT path was proven with the two-dimensional correlation spectroscopy (2D-COS)-surface-enhanced Raman scattering (SERS) technique. In addition, the substrate exhibited excellent catalytic activity due to the CT effect. The catalyzed degradation of malachite green (MG) was due to the CT effect in the system, and the catalytic process was successfully monitored by in situ SERS. Most importantly, the catalytic degradation by Ag@ZnSe (x) with different parameters was proportional to the degree of CT (ρ_CT). The SERS and catalytic mechanisms were analyzed in depth with the 2D-COS-SERS technique, which was also useful in verifying the CT process. The catalytic sites for MG were successfully monitored with the 2D-COS-SERS technique. This study provides a reference for studies of the synergistic effects of the electromagnetic mechanism and CT, as well as a new perspective on photocatalysis with dye molecules and monitoring of the catalytic processes.

Single-Molecule Investigation of the Protein-Aptamer Interactions and Sensing Application Inside the Single Glass Nanopore

Solid-state nanopores offer a nanoconfined space for a single-molecule sensing strategy. Evaluating the behavior of proteins and protein-related interactions at the single-molecule level is becoming more and more important for a better understanding of biological processes and diseases. In this work, the aptamer-functionalized nanopore was prepared as the sensing platform for kinetic analysis of the carcinoembryonic antigen (CEA) with its aptamers, which is an important cancer biomarker. CEA molecules were captured by the aptamers immobilized on the inner surface of the nanopore, and there was a complicated interaction between the CEA molecules and the aptamer, which is the process of association and dissociation. This could be used to measure the dynamics of aptamer-protein interactions without labeling. The kinetic analysis could be evaluated at the single-molecule level to interpret the dissociation constants of the binding and dissociation processes. Results showed that the translocation of CEA molecules in a functionalized nanopore had a deep blockades degree and long duration compared with nanopore modified with bare gold, which could be used for CEA sensing. This protein and protein-related interaction we designed provides new insights for evaluating the binding affinity, which will be beneficial for protein sensing and immunoassays.

Hierarchically Assembled Plasmonic Metal-Dielectric-Metal Hybrid Nano-Architectures for High-Sensitivity SERS Detection

In this work, we designed and prepared a hierarchically assembled 3D plasmonic metal-dielectric-metal (PMDM) hybrid nano-architecture for high-performance surface-enhanced Raman scattering (SERS) sensing. The fabrication of the PMDM hybrid nanostructure was achieved by the thermal evaporation of Au film followed by thermal dewetting and the atomic layer deposition (ALD) of the Al₂O₃ dielectric layer, which is crucial for creating numerous nanogaps between the core Au and the out-layered Au nanoparticles (NPs). The PMDM hybrid nanostructures exhibited strong SERS signals originating from highly enhanced electromagnetic (EM) hot spots at the 3 nm Al₂O₃ layer serving as the nanogap spacer, as confirmed by the finite-difference time-domain (FDTD) simulation. The PMDM SERS substrate achieved an outstanding SERS performance, including a high sensitivity (enhancement factor, EF of 1.3 × 10⁸ and low detection limit 10^-11 M) and excellent reproducibility (relative standard deviation (RSD) < 7.5%) for rhodamine 6G (R6G). This study opens a promising route for constructing multilayered plasmonic structures with abundant EM hotspots for the highly sensitive, rapid, and reproducible detection of biomolecules.

Au-on-Ag nanostructure forin-situSERS monitoring of catalytic reactions

Dual-functionality Au-on-Ag nanostructures (AOA) were fabricated on a silicon substrate by first immobilizing citrate-reduced Ag nanoparticles (Ag NPs, ∼43 nm in diameter), followed by depositing ∼7 nm Au nanofilms (Au NFs) via thermal evaporation. Au NFs were introduced for their catalytic activity in concave-convex nano-configuration. Ag NPs underneath were used for their significant enhancement factor (EF) in surface-enhanced Raman scattering (SERS)-based measurements of analytes of interest. Rhodamine 6G (R6G) was utilized as the Raman-probe to evaluate the SERS sensitivity of AOA. The SERS EF of AOA is ∼37 times than that of Au NPs. Using reduction of 4-nitrothiophenol (4-NTP) by sodium borohydride (NaBH₄) as a model reaction, we demonstrated the robust catalytic activity of AOA as well as its capacity to continuously monitor via SERS the disappearance of reactant 4-NTP, emergence and disappearance of intermediate 4,4'-DMAB, and the appearance of product 4-ATP throughout the reduction process in real-time andin situ.

Construction of Ag nanowire@Au nanoparticle nano nests with densely stacked small gaps for actively trapping molecules to realize diversity SERS detection

The SERS detection method based nano nest model actively traps molecules by the capillary action to detect different kinds of molecules, such as pesticide residues, bioenergy substances, antibiotic residues and allergic reactions.