Porous Au-Ag Alloy Particles Inlaid AgCl Membranes As Versatile Plasmonic Catalytic Interfaces with Simultaneous, in Situ SERS Monitoring

Solar light driven atomic and electronic transformations in a plasmonic Ni@NiO/NiCO3 photocatalyst revealed by ambient pressure X-ray photoelectron spectroscopy

This work employs ambient pressure X-ray photoelectron spectroscopy (APXPS) to delve into the atomic and electronic transformations of a core-shell Ni@NiO/NiCO3 photocatalyst - a model system for visible light active plasmonic photocatalysts used in water splitting for hydrogen production. This catalyst exhibits reversible structural and electronic changes in response to water vapor and solar simulator light. In this study, APXPS spectra were obtained under a 1 millibar water vapor pressure, employing a solar simulator with an AM 1.5 filter to measure spectral data under visible light illumination. The in situ APXPS spectra indicate that the metallic Ni core absorbs the light, exciting plasmons, and creates hot electrons that are subsequently utilized through hot electron injection in the hydrogen evolution reaction (HER) by NiCO3. Additionally, the data show that NiO undergoes reversible oxidation to NiOOH in the presence of water vapor and light. The present work also investigates the contribution of carbonate and its involvement in the photocatalytic reaction mechanism, shedding light on this seldom-explored aspect of photocatalysis. The APXPS results highlight the photochemical reduction of carbonates into -COOH, contributing to the deactivation of the photocatalyst. This work demonstrates the APXPS efficacy in examining photochemical reactions, charge transfer dynamics and intermediates in potential photocatalysts under near realistic conditions.

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.

Metal-Organic Framework (MOF) Derivatives as Promising Chemiresistive Gas Sensing Materials: A Review

The emission of harmful gases has seriously exceeded relative standards with the rapid development of modern industry, which has shown various negative impacts on human health and the natural environment. Recently, metal-organic frameworks (MOFs)-based materials have been widely used as chemiresistive gas sensing materials for the sensitive detection and monitoring of harmful gases such as NO_x, H₂S, and many volatile organic compounds (VOCs). In particular, the derivatives of MOFs, which are usually semiconducting metal oxides and oxide-carbon composites, hold great potential to prompt the surface reactions with analytes and thus output amplified resistance changing signals of the chemiresistors, due to their high specific surface areas, versatile structural tunability, diversified surface architectures, as well as their superior selectivity. In this review, we introduce the recent progress in applying sophisticated MOFs-derived materials for chemiresistive gas sensors, with specific emphasis placed on the synthesis and structural regulation of the MOF derivatives, and the promoted surface reaction mechanisms between MOF derivatives and gas analytes. Furthermore, the practical application of MOF derivatives for chemiresistive sensing of NO₂, H₂S, and typical VOCs (e.g., acetone and ethanol) has been discussed in detail.

Nanoenzyme Reactor-Based Oxidation-Induced Reaction for Quantitative SERS Analysis of Food Antiseptics

Nanoenzyme reactors based on shell-isolated colloidal plasmonic nanomaterials are well-established and widely applied in catalysis and surface-enhanced Raman scattering (SERS) sensing. In this study, a "double wing with one body" strategy was developed to establish a reduced food antiseptic sensing method using shell-isolated colloidal plasmonic nanomaterials. Gold nano particles (Au NPs) were used to synthesize the colloidal plasmonic nanomaterials, which was achieved by attaching ferrous ions (Fe²⁺), ferric ions (Fe³⁺), nitroso (NO^-) group, cyanogen (CN^-) group, and dopamine (DA) via coordinative interactions. The oxidation-induced reaction was utilized to generate •OH following the Fe²⁺-mediated Fenton reaction with the shell-isolated colloidal plasmonic nanomaterials. The •OH generated in the cascade reactor had a high oxidative capacity toward acid preservatives. Importantly, with the introduction of the signal molecule DA, the cascade reactor exhibited also induced a Raman signal change by reaction with the oxidation product (malondialdehyde) which improved the sensitivity of the analysis. In addition, the stable shell-isolated structure was effective in realizing a reproducible and quantitative SERS analysis method, which overcomes previous limitations and could extend the use of nanoenzymes to various complex sensing applications.

SERS spectral evolution of azo-reactions mediated by plasmonic Au@Ag core-shell nanorods

The mechanism and application of localized surface plasmon resonance induced photocatalytic reactions remain an issue of interest. In this work, we used Au@Ag core-shell nanorods as a platform for plasmon-driven photocatalysis, which was in situ investigated by surface-enhanced Raman scattering (SERS) spectroscopy. The para-aminothiophenol (PATP) and para-nitrothiophenol (PNTP) adsorbed on the nanorods were irradiated with different excitation wavelengths (633 nm, 785 nm) and transformed into 4,4'-dimercaptoazobenzene (DMAB) as evidenced by the emerging Raman peaks at 1142 cm^-1, 1390 cm^-1, 1440 cm^-1, and 1477 cm^-1, corresponding to hot carrier dominated oxidation of PATP and reduction of PNTP. Preliminary azo-reaction kinetics and in situ SERS measurements were conducted by comparing the relative intensity ratio of SERS peaks at 1440 cm^-1 (DMAB stretching of N[double bond, length as m-dash]N) and 1080 cm^-1 (C-S stretching of PATP and PNTP). These results indicate that the catalytic efficiency was dominated by the excitation wavelength as well as the resonance condition between the plasmon band of the nanorods and the excitation line. As a proof of concept, the Au@Ag core-shell nanorods were used to catalyze 4-nitrophenol molecules, and 4-hydroxyazobenzene molecules as the product were confirmed by in situ SERS spectra as well theoretical predictions, showing potential in plasmon driven catalysis and degradation of organic molecules.

Shape controlled synthesis of concave octahedral Au@AuAg nanoparticles to improve their surface-enhanced Raman scattering performance

In this work, a seed mediated strategy has been proposed to design and fabricate uniform octahedral shaped gold@gold-silver nanoparticles (Au@AuAg NPs) with unique concave structure and an AuAg alloy shell. The morphology and Au/Ag ratio of the Au@AuAg nanostructures can be delicately controlled by varying the concentration of reagents, namely the Au nanorod (NR) seeds, HAuCl₄ and AgNO₃ precursor. Besides, the investigation of the growth mechanism revealed that the morphology of the product also can be controlled by tuning the growth time. Furthermore, uniformly arranged assemblies of concave octahedral Au@AuAg NPs were prepared through a solvent evaporation self-assembly strategy and employed as surface-enhanced Raman scattering (SERS) substrates, effectively applied to the analysis of R6G for the examination of SERS performance. Satisfyingly, owing to the synergistic effect between the Au and Ag elements and concave structure, concave octahedral Au@AuAg NPs exhibit significantly higher SERS enhancement compared with traditional octahedral Au NPs, which have an enhancement factor of ∼1.3 × 10⁷ and a detection limit as low as 10⁻¹⁰ M. Meanwhile, the SERS substrate reveals an excellent uniformity and reproducibility of the SERS performance. This work opens a new avenue toward bimetallic NPs with concave structure, which have broad application prospects in optics, SERS detection and other fields.

In this work, a seed mediated strategy has been proposed to design and fabricate uniform octahedral shaped gold@gold-silver nanoparticles (Au@AuAg NPs) with unique concave structure and an AuAg alloy shell.

Metal-Organic-Framework-Derived Ball-Flower-like Porous Co3O4/Fe2O3 Heterostructure with Enhanced Visible-Light-Driven Photocatalytic Activity

A porous ball-flower-like Co3O4/Fe2O3 heterostructural photocatalyst was synthesized via a facile metal-organic-framework-templated method, and showed an excellent degradation performance in the model molecule rhodamine B under visible light irradiation. This enhanced photocatalytic activity can be attributed to abundant photo-generated holes and hydroxyl radicals, and the combined effects involving a porous structure, strong visible-light absorption, and improved interfacial charge separation. It is notable that the ecotoxicity of the treated reaction solution was also evaluated, confirming that an as-synthesized Co3O4/Fe2O3 catalyst could afford the sunlight-driven long-term recyclable degradation of dye-contaminated wastewater into non-toxic and colorless wastewater.

Facile fabrication of PS/Cu2S/Ag sandwich structure as SERS substrate for ultra-sensitive detection

In this work, a serials of PS(polystyrene)/Cu₂S/Ag sandwich substrates were successfully constructed using the magnetic sputtering method by adjusting the Ag sputtering time (0 min, 2 min, 4 min, 6 min, 8 min and 10 min) and used as the surface-enhanced Raman scattering (SERS) substrates. When the Ag sputtering time was 6 min, the strongest SERS signal was observed. The optimized SERS substrate has strong SERS activity on 4-mercaptobenzoic acid (4-MBA), the minimum detection limit was 10^-13 M and the enhancement factor was as high as 4.7 × 10⁷. In addition, the SERS signals were highly reproducible with small standard deviation. The SERS enhancement mechanism of the PS/Cu₂S/Ag system was attributed to the synergistic effect of the chemical mechanism and the electromagnetic enhancement mechanism. This strategy has find a new way for manufacturing SERS activity sensor with high sensitivity and reproducibility.

Silver@mesoporous Anatase TiO2 Core-Shell Nanoparticles and Their Application in Photocatalysis and SERS Sensing

Nanostructured noble metal-semiconductor materials have been attracting increasing attention because of their broad application in the field of environmental remediation, sensing and photocatalysis. In this study, a facile approach for fabricating silver@mesoporousanataseTiO2 (Ag@mTiO2) core-shell nanoparticles employing sol-gel and hydrothermal reaction is demonstrated. The Ag@mTiO2nanoparticles display excellent surface-enhanced Raman scattering (SERS) sensitivity and they can detect the methylene blue (MB) molecules with the concentration of as low as 10−8 M. They also exhibit outstanding photocatalytic activity compared with mTiO2, due to the efficient separation and recombination restrain of electron–hole pairs under ultraviolet light. The Ag@mTiO2nanoparticles also present good stability and they can achieve recyclable photocatalytic degradation experiments for five times without loss of activity. Subsequently, the nanoparticles with dual functions were successfully used to in situ monitor the photodegradation process of MB aqueous solution. These results, demonstrating the multifunctional Ag@mTiO2 nanoparticles, hold promising applications for simultaneous SERS analysis and the removal of dye pollutants in environmental field.

One-dimensional necklace-like assemblies of inorganic nanoparticles: Recent advances in design, preparation and applications

One-dimensional (1D) necklace-like assembly of inorganic nanoparticles exhibits unique collective properties, which are critical to open up new and remarkable opportunities in the field of nanotechnology. This review focuses on the recent advances in the production of these types of assemblies employing two strategies: colloidal synthesis and self-assembly procedures. After a brief description of the forces guiding nanoparticles towards the assembly, the main features of both strategies are discussed. Examples of approaches, typically involved in colloidal synthesis, are highlighted. The peculiar properties of 1D nanostructures are strictly associated with the nanoparticle arrangement in the form of highly ordered assemblies, which are attained during the synthesis both in the solution and using a template, as well as under the action of an external force. The various 1D necklace-like structures, created through nanoparticle self-assembly, demonstrate aligned, oriented nanoparticle organization. Diverse nature, size and shape of preformed particles as building blocks, along with utilizing different linkers, templates or external field lead to fabrication of 1D chain nanostructures with properties responsible for their wide applications. The unique structure-property relationship, both in colloidal synthesis, and self-assembly, offers broad spectrum of 1D necklace-like nanostructure implementations, illustrated by their use in photonics, electronics, electrocatalysis, magnetics.

Controlled synthesis of a PS/Au/ZIF-8 hybrid structure as a SERS substrate for ultrasensitive detection

A PS/Au/ZIF-8 hybrid was successfully prepared and used as a SERS active substrate. Meanwhile, the hybrid SERS substrate exhibited good SERS reproducibility, and we successfully obtained an ultrahigh enhancement factor of 1.67 × 10⁶.

In situ monitoring of catalytic reaction on single nanoporous gold nanowire with tuneable SERS and catalytic activity

Single nanoporous gold nanowire was introduced as a tunable one-dimensional nano-sensor platform with both SERS and catalytic activity, and it precisely fit the requirement of materials for in situ SERS monitoring of plasmon-assisted catalytic reaction. The nanoporous gold nanowires exhibited much more "hot spots" on their surface and much better SPR effect than the smooth nanowires. We demonstrated that these nanowires could be used as a SERS substrate assuring the sensitivity and reproducibility of Raman signals. Besides, they could be applied as a kind of heterogeneous catalyst for in situ SERS monitoring of the plasmon-assisted catalytic reaction-reduction of p-nitrothiophenol (p-NTP) to p,p-dimercaptoazobenzene (DMAB) at their surface. The SERS and catalytic activity of the nanowires could be respectively optimized by adjusting their dealloying time, similar to the procedure of catalyst screening.

Wrapping AgCl Nanostructures with Trimetallic Nanomeshes for Plasmon-Enhanced Catalysis and in Situ SERS Monitoring of Chemical Reactions

Selective chemical control of multiple reactions is incredibly important for the fabrication of sophisticated nanostructures for functional applications. A representative example is the synthesis of plasmonic nanomaterial-silver chloride (AgCl) conjugates, where metal ions should be selectively reduced into metallic nanostructures for plasmon-enhanced catalytic activity, while the reducible AgCl nanomaterials remain intact despite the presence of a chemical reductant. In addition to the selectively controlled reduction, the plasmonic nanostructures should be appropriately designed for the high stability and photoefficiency of catalysts. In this study, we demonstrate how AgCl nanocubes and nanospheres could be comprehensively wrapped by plasmonic three-dimensional nanomesh structures consisting of gold, silver, and palladium by the selective reduction of their ionic precursors while the AgCl nanostructures remain intact. Complete trimetallic wrapping provided the absorption of visible light, while the porosity of the nanomesh structures exposed the photocatalytic AgCl surface to catalyze desired reactions. Platinum in place of palladium was examined to demonstrate the versatility of the wrapping scheme, resulting in an extraordinary catalytic activity. Importantly, the detailed chemical mechanism behind the trimetallic wrapping of the AgCl nanostructures was systematically investigated to understand the roles of each reaction component in controlling the chemical selectivity. The synthesized AgCl-trimetal nanoconjugates excellently exhibit both metal-based and plasmon-enhanced catalytic properties for the removal of environmentally harmful Cr⁶⁺. Moreover, their applications as surface-enhanced Raman-scattering (SERS) probes for the in situ monitoring of catalytic reduction in real-time and as single-nanoparticle SERS probes for molecular detection are thoroughly demonstrated.

A porous Au-Ag hybrid nanoparticle array with broadband absorption and high-density hotspots for stable SERS analysis

Constructing high-density hotspots is of crucial importance in surface enhanced Raman scattering (SERS). In this paper, we present a large-area and broadband porous Au-Ag hybrid nanoparticle array which was fabricated by an ultra-thin alumina mask (UTAM) technique incorporated with annealing and galvanic replacement techniques. Experimental results and numerical simulations demonstrated that the porous Au-Ag hybrid nanoparticle array possessed enormous hotspots for high sensitivity, uniformity, and stability in SERS analysis. A large Raman enhancement factor of 2.2 × 107 was achieved with a relative standard deviation (RSD) of 7.7%, leading to excellent reliability for Raman detection. Furthermore, this novel substrate exhibited a long shelf time in an ambient environment and promising practical applications in many SERS-based quantitative analytical and biomedical sensing techniques.

Interfacial electron transfer promotes photo-catalytic reduction of 4-nitrophenol by Au/Ag2O nanoparticles confined in dendritic mesoporous silica nanospheres

Manipulating the electron transfer in composite metals or/and metal oxides on the nanoscale is crucial for the development of improved heterogeneous catalysts.

Preparation of carbon-based AuAg alloy nanoparticles by using the heterometallic [Au4Ag4] cluster for efficient oxidative coupling of anilines

We herein report the preparation of unique heteroatom-doped and carbon-based AuAg alloy nanoparticles (NPs) via the pyrolysis of a structurally defined octanuclear heterometallic Au(i)-Ag(i) cluster [Au4Ag4(Dppy)4(Tab)4(MeCN)4](PF6)8 (2, Dppy = diphenylphosphine-2-pyridine and Tab = 4-(trimethylammonio)benzenethiolate). This cluster-precursor approach exerts a fine control over the spatial arrangement, size and uniformity of the AuAg alloy NPs as well as the doped heteroatoms (P, N, F and S). The optimized material prepared at 450 °C efficiently catalyzes the oxidative coupling of anilines to yield azobenzenes under mild conditions.

Reversible light-dependent molecular switches on Ag/AgCl nanostructures

Nanostructured Ag/AgCl substrates were used to generate reversible and highly efficient light-dependent chemical switches based on adsorbed 4,4'-dimercaptoazobenzene (DMAB). DMAB was formed in situ via laser-induced dimerization either from 4-nitrothiophenol (4-NTP) or 4-aminothiophenol (4-ATP). The subsequent reaction pathways of DMAB, however, were quite different as monitored by surface enhanced Raman spectroscopy. In the 4-NTP/DMAB system, AgCl catalyses the reversal of the dimerization. Conversely, irradiation of adsorbed 4-ATP first generated cis-DMAB attached to the surface via two Ag-S bonds, followed by AgCl-catalysed cleavage of one Ag-S bond and cis → trans photoisomerisation of DMAB. In the dark, the trans-isomer thermally reverts to cis-DMAB. The here presented light-dark chemical switches, which work without changing other parameters (e.g., pH, anaerobic vs. aerobic), are based on the (photo)catalytic properties of the Ag/AgCl substrate and do not function on pure metal surfaces.

Laser-induced photochemical synthesis of branched Ag@Au bimetallic nanodendrites as a prominent substrate for surface-enhanced Raman scattering spectroscopy

The project of wielding laser light as a versatile tool for sculpting branched Ag@Au bimetallic nanocrystals with mean size of ~50 nm has been developed in this work. The moderate overgrowth of Ag species with negligible damage effect on the branched Ag@Au nanostructures was achieved by laser-induced photo-oxidation. The final Ag@Au nanodendrites exhibit superior surface enhanced Raman scattering (SERS) activities with an enhancement factor up to ~10¹¹ and a detection limit as low as ~10^-14 M. The pronounced feature should be attributed to the noticeable small-sized branches (<10 nm) and unique pronounced inter-metallic synergies. Our results have a promising potential for developing SERS-based ultrasensitive probes in biomedical application.

Chemical Sensing on a Single SERS Particle

We report a new chemical sensing platform on a single surface-enhanced Raman scattering (SERS) particle. A cabbage-like Au microparticle (CLMP) with high SERS enhancement was applied as an ultrasensitive SERS substrate. A new Raman reporter bis[4,4'-[dithiodiphenyl azo-phenol] (DTDPAP) was synthesized to display multiple fingerprints and high reactivity toward sodium dithionite. The reaction of DTDPAP with sodium dithionite was in situ monitored by SERS on a single CLMP. The DTDPAP fingerprint change is dependent on the sodium dithionite concentration, providing a simple and sensitive method for sodium dithionite profiling.

A novel environmentally sustainable synthesis of Au–Ag@AgCl nanocomposites and their application as an efficient and recyclable catalyst for quinoline synthesis

An eco-friendly synthesis of Au–Ag@AgCl NCs was described using the tuber extract of Nephrolepis cordifolia. The synthetic utility of the NCs was demonstrated by the synthesis of pharmaceutically important quinoline derivatives.

An Au/AgBr–Ag heterostructure plasmonic photocatalyst with enhanced catalytic activity under visible light

An efficient bimetallic–semiconductor (Au/AgBr–Ag) plasmonic heterostructure with plasmon enhanced absorption and plasmonic sensitization for visible light induced photocatalysis.

A flexible conductive film prepared by the oriented stacking of Ag and Au/Ag alloy nanoplates and its chemically roughened surface for explosive SERS detection and cell adhesion

A Au–Ag alloy with oriented stacking has applications in SERS detection and cell adhesion.

Ultrafast self-assembly of silver nanostructures on carbon-coated copper grids for surface-enhanced Raman scattering detection of trace melamine

Structurally well-defined assemblies of silver nanoparticles, including the dendritic nano-flowers (NFs), planar nano-spheres (NSs) and nano-dendrites (NDs) were obtained by a surfactant-free and ultrafast (≈15min) self-assembly process on as-purchased carbon-coated copper TEM grids. The silver nano-assemblies, especially the NFs modified TEM grids, when serving as surface-enhanced Raman spectroscopy (SERS) substrates for detecting melamine molecules, demonstrated a long-lived limit of detection (LOD) of as low as 10^-11M, suggesting the potential of these silver-assemblies modified carbon-coated copper grids as novel potable and cost-effective SERS substrates for trace detection toward various food contaminants like melamine.

Holey Au-Ag alloy nanoplates with built-in hotspots for surface-enhanced Raman scattering

Plasmonic noble metal nanocrystals with interior nanogaps have attracted great attention in surface-enhanced Raman scattering (SERS) applications due to the presence of built-in hotspots. Herein, we report a synthesis route to holey Au-Ag alloy nanoplates by controlled galvanic replacement with Ag nanoplates as the sacrificial template, a sulfite-coordinated Au(i) salt as the Au source, and polyvinylpyrrolidone (PVP) as the capping agent. PVP helps regulate the anisotropic growth of nanopores on the Ag nanoplates to afford a highly holey nanostructure, and the monovalent Au(i) salt plays a critical role in stabilizing these holey nanoplates by rapidly enriching Au in the alloy nanostructures. Numerical simulations and experimental results suggest that these holey Au-Ag alloy nanoplates possess enormous internal hotspots for high sensitivity in the SERS analysis, and high stability for excellent reliability of the analysis under many harsh conditions. We believe that this strategy is potentially applicable to the synthesis of many other types of plasmonic nanostructures with inherent nanogaps for many sensing and imaging applications.

Porous Au-Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis

Colloidal plasmonic metal nanoparticles have enabled surface-enhanced Raman scattering (SERS) for a variety of analytical applications. While great efforts have been made to create hotspots for amplifying Raman signals, it remains a great challenge to ensure their high density and accessibility for improved sensitivity of the analysis. Here we report a dealloying process for the fabrication of porous Au-Ag alloy nanoparticles containing abundant inherent hotspots, which were encased in ultrathin hollow silica shells so that the need of conventional organic capping ligands for stabilization is eliminated, producing colloidal plasmonic nanoparticles with clean surface and thus high accessibility of the hotspots. As a result, these novel nanostructures show excellent SERS activity with an enhancement factor of ∼1.3 × 10(7) on a single particle basis (off-resonant condition), promising high applicability in many SERS-based analytical and biomedical applications.