In situ SERS study of surface plasmon resonance enhanced photocatalytic reactions using bifunctional Au@CdS core-shell nanocomposites

Cascade Bowl Multicavity Structure for In Situ Surface-Enhanced Raman Scattering Detection of Organic Gas Molecules

With the increasing emphasis on atmospheric environmental protection, it is crucial to find an efficient, direct, and accurate method to identify pollutant species in the atmosphere. To solve this problem, we designed and prepared the cascade multicavity (CMC) structure composed with silver nanoparticles (Ag NPs) as a surface-enhanced Raman scattering (SERS) substrate with favorable light transmittance and flexibility. The multicavity structure distributed on the surface introducing the homogeneous connecting holes endows the structure to more fully utilize the incident light while slowing the gas movement rate. Theoretical and experimental results have demonstrated that the Ag NPs/cascade multicavity (Ag-CMC) SERS substrate is a highly sensitive SERS substrate that can be used for in situ detection of gases under non-perpendicularly incident laser irradiation or bending of the substrate. We believe that the SERS substrate can provide a more efficient and feasible way for in situ detection of gaseous pollutants.

Plasmon-enhanced photocatalysis using gold nanoparticles encapsulated in nanoscale molybdenum oxide shell

Using solar energy to enhance the transformation rate of organic molecules is a promising strategy to advance chemical synthesis and environmental remediation. Plasmonic nanoparticles responsive to sunlight show great promise in the catalysis of chemical reactions. In this work, we used a straightforward wet-chemistry method to synthesize plasmonic octahedral gold nanoparticles (NPs) coated with thin molybdenum oxide (MoO_3-x), Au@MoO_3-xNPs, which exhibited strong surface plasmon resonance in a broad wavelength range. The synthesized Au@MoO_3-xNPs were characterized by UV-vis, SEM, TEM, EDS, XPS, and the electrochemical technique of cyclic voltammetry (CV). The catalytic performance of Au@MoO_3-xNPs under visible light irradiation was investigated using the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction. The presence of a thin capping layer of MoO_3-xon our Au NPs contributed to the broadening of their range of absorption of visible light, resulting in a stronger intra-particle plasmonic resonance and the modulation of surface energy and electronic state. Accordingly, the kinetics of plasmon photocatalytic transformation of 4-NP to 4-AP was significantly accelerated (by a factor of 8.1) under visible light, compared to uncapped Au NPs in the dark. Our as-synthesized Au@MoO_3-xNPs is an example that the range of plasmonic wavelengths of NPs can be effectively broadened by coating them with another plasmon-active (semiconducting) material, which substantially improves their plasmonic photocatalytic performance. Meanwhile, the synthesized Au@MoO_3-xNPs can be used to accelerate the transformation of organic molecules under visible light irradiation.

Microelectrode-Based Electrochemical Impedance Determination of Brain-Derived Neurotrophic Factor in Aqueous Humor for Diagnosis of Glaucoma

The abundance of brain-derived neurotrophic factor (BDNF) in aqueous humor (AH) is an ideal biomarker for the diagnosis of glaucoma, a chronic progressive optic neuropathy and the most frequent cause of irreversible blindness. The difficulty of AH-based BDNF detection is from the small amount of extracted AH in a paracentesis (<100 μL) and the ultra-low abundance of BDNF. In this work, we systematically studied the non-specific adsorption of biofluids on the bare gold electrode by electrochemistry and Raman spectroscopy techniques, revealing the unexpected negative correlation of the extent of non-specific adsorption with the size of the electrode. Based on it, a simple microelectrode-based sensor without the introduction of the blocking layer was developed for the detection of BDNF in the AH sample. Using electrochemical impedance spectroscopy (EIS) and extracting the changes of electron-transfer resistance of the electrochemical probe [Fe(CN)6]3-/4- on the sensor surface, the BDNF was quantified. The dynamic range was from 0.5 to 50 pg·mL-1, with a detection limit of 0.3 pg·mL-1 and a sample consumption of 5 μL. The real AH sample analysis confirmed the significant decrease of BDNF abundance in the AH of glaucoma patients. Our microelectrode-based EIS sensor displayed prominent advantages on simplified preparation, sensitive response, and low sample consumption. This AH-based BDNF analysis is expected to be used for the screening and diagnosis of glaucoma, especially for the high-risk population who have ocular diseases and have to undergo surgeries.

Mixed valence Ce-doped TiO2 with multiple energy levels and efficient charge transfer for boosted SERS performance

Considering the variable valence characteristics of rare earth elements, they can be in a variety of valence forms coexistence. Doping of rare earth element with different valence states may produce different energy levels to tune the semiconductor energy band structure. We utilize rare earth element Ce doping TiO₂ for the development of high-performance semiconductor surface-enhanced Raman scattering (SERS) substrates based on an energy-level tuning strategy. Ce doping not only forms multiple energy levels including Ce³⁺ and Ce⁴⁺ metal doping energy levels in the bandgap of TiO₂, but also enriches the surface state level of TiO₂ itself, which together promote the separation of photogenerated carriers and improve charge transfer efficiency between substrates and absorbed molecules. This endows TiO₂ semiconductor substrate with a higher SERS enhancement factor, which can reach 2.2 × 10⁶. The detectable concentration of methylene blue can be as low as 10^-10 mol/L. Moreover, the semiconductor substrate exhibits excellent uniformity and stability. This study not only provides a new strategy to develop excellent semiconductor SERS substrate with multiple energy levels, but also lays the foundation for promising practical application of semiconductor substrate.

Enhanced photocatalytic activity of nano-silica/copper plasmon by aminofunctional silane for dye pollutant degradation

The synthesis of silica gel nanostructures and loading it with copper specie via a hydrothermal process were performed. The sample is treated with an amino-functional reagent 3-aminopropyl triethoxysilane (APTES). The products were characterized by X-ray diffraction (XRD), FT-IR, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), TGA/DSC measurements, and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of the nanostructures were studied for degradation of methylene blue dye (as a classic dye contaminant) in aqueous solution utilizing visible light source. The results displayed that the sample treated with APTES is much more effective in photocatalytic degradation of methylene blue. This modified catalyst could eliminate methylene blue dye (50 mL, 18 µg mL^-1) within 60 min under visible light. The degradation efficiency was increased by shortening the degradation time to 30 min in the alkaline medium. The pseudo-first-order model well describes the kinetics of the reaction.

High-Density-Nanotips-Composed 3D Hierarchical Au/CuS Hybrids for Sensitive, Signal-Reproducible, and Substrate-Recyclable SERS Detection

Surface-enhanced Raman scattering (SERS) provides an unprecedented opportunity for fingerprinting identification and trace-level detection in chemistry, biomedicine, materials, and so on. Although great efforts have been devoted to fabricating sensitive plasmonic nanomaterials, it is still challenging to batch-produce a SERS substrate with high sensitivity, good reproducibility, and perfect recyclability. Here, we describe a facile fabrication of three-dimensional (3D) hierarchical Au/CuS nanocomposites, in which high-density Au nanotips enable highly SERS-active sensing, and the well-defined microflower (MF) geometry produces perfect signal reproducibility (RSD < 5%) for large laser spot excitations (>50 μm2), which is particularly suitable for practical on-site detection with a handheld Raman spectrometer. In addition, a self-cleaning ability of this Au/CuS Schottky junction photocatalyst under sunlight irradiation allows complete removal of the adsorbed analytes, realizing perfect regeneration of the SERS substrates over many cycles. The mass-production, ultra-sensitive, high-reproducibility, and fast-recyclability features of hierarchical Au/CuS MFs greatly facilitate cost-effective and field SERS detection of trace analytes in practice.

Multifunctional SERS chip mediated by black phosphorus@gold-silver nanocomposites inserted in bilayer membrane for in-situ detection and degradation of hazardous materials

Self-cleaning surface-enhanced Raman scattering (SERS) substrates dependent on versatile two-dimensional semiconductors offer an efficient channel for the sensitive monitoring and timely degradation of hazardous molecules. Herein, a kind of sophisticated SERS-active nanocomposites was developed by incorporating Au-Ag nanoparticles onto black phosphorus (BP) nanosheets via photo-induced self-reduction. Combining the substantial electromagnetic "hot spots" triggered by bimetallic plasma coupling effect and the efficient charge transfer from BP to probe molecules, the proposed nanocomposites featured attractive SERS enhancement, facilitating a limit of detection down to 4.5 × 10^-10 M. Attributed to the remarkable restriction of electron-hole recombination stemming from "Schottky contact", the photocatalytic activity of BP was prominently boosted, demonstrating a complete degradation time as short as 65 min. Furthermore, the disgusting instability of BP was considerably hindered by inserting the nanocomposites into various bilayer matrices with diverse hardness and viscosity inspired by cling film principle. Moreover, a significantly elevated collection rate high to 93.1% for in-situ detection was also achieved by the as-manufactured flexible SERS chips based on tape. This study illustrates a clear perspective for the development of versatile BP-based SERS chips which might facilitate sensitive analysis and treatment of perilous contaminants in complicated real-life scenarios.

In Situ Raman Probing of Hot-Electron Transfer at Gold-Graphene Interfaces with Atomic Layer Accuracy

Plasmonic metals under photoexcitation can generate energetic hot electrons to directly induce chemical reactions. However, the capability and fundamental insights of the transportation of these hot electrons at plasmonic metal-2D material interfaces remain unclear. Herein, hot-electron transfer at Au-graphene interfaces has been in situ studied using surface-enhanced Raman spectroscopy (SERS) with atomic layer accuracy. Combining in situ SERS studies with density functional theory calculations, it is proved that hot electrons can be injected from plasmonic Au nanoparticles to graphene and directly penetrate graphene to trigger photocatalytic reactions. With increasing graphene layers, the transportation of hot electrons decays rapidly and would be completely blocked after five layers of graphene. Moreover, the transfer of hot electrons can be modulated by applying an external electric field, and the hot-electron transfer efficiency under electrochemical conditions is improved by over three times in the presence of a monolayer of graphene.

Strong Visible Light Absorption and Abundant Hotspots in Au-Decorated WO3 Nanobricks for Efficient SERS and Photocatalysis

Metal/semiconductor hybrids show potential application in fields of surface-enhanced Raman spectroscopy (SERS) and photocatalysis due to their excellent light absorption, electric field, and charge-transfer properties. Herein, a WO3-Au metal/semiconductor hybrid, which was a WO3 nanobrick decorated with Au nanoparticles, was prepared via a facile hydrothermal method. The WO3-Au hybrids show excellent visible light absorption, strong plasmon coupling, high-performance SERS, and good photocatalytic activity. In particular, on sensing rhodamine B (RhB) under 532 nm excitation, bare WO3 nanobricks have a Raman enhancement factor of 2.0 × 106 and a limit of detection of 10-8 M due to the charger-transfer property and abundant oxygen vacancies. WO3-Au metal/semiconductor hybrids display a largely improved Raman enhancement factor compared to pure Au and WO3 components owing to the synergistic effect of electromagnetic enhancement and charge transfer. The Raman enhancement factor and limit of detection are further improved, reaching 5.3 × 108 and 10-12 M, respectively, on increasing the content of Au to 2.1 wt %, owing to the strong plasmon coupling between the Au nanoparticles. Additionally, the WO3-Au hybrids also exhibit excellent photocatalytic activity toward degradation of RhB under visible light irradiation. WO3-Au (2.1 wt %) possesses the fastest photocatalytic rate, which is 6.1 and 2.0 times that of pure WO3 nanobricks and commercial P25, respectively. The enhanced photocatalytic activity is attributed to the strong plasmon coupling and the efficient charge transfer between Au and WO3 nanobricks. The as-prepared materials show great potential in detecting and degrading pollutants in environmental treatment.

Charge-Transfer Induced by the Oxygen Vacancy Defects in the Ag/MoO3 Composite System

In this paper, an Ag/MoO3 composite system was cosputtered by Ar plasma bombardment on a polystyrene (PS) colloidal microsphere array. The MoO3 formed by this method contained abundant oxygen vacancy defects, which provided a channel for charge transfer in the system and compensated for the wide band gap of MoO3. Various characterization methods strongly demonstrated the existence of oxygen vacancy defects and detected the properties of oxygen vacancies. 4-Aminothiophenol (p-aminothiophenol, PATP) was used as a candidate surface-enhanced Raman scattering (SERS) probe molecule to evaluate the contribution of the oxygen vacancy defects in the Ag/MoO3 composite system. Interestingly, oxygen vacancy defects are a kind of charge channel, and their powerful effect is fully reflected in their SERS spectra. Increasing the number of charge channels and increasing the utilization rate of the channels caused the frequency of SERS characteristic peaks to shift. This interesting phenomenon opens up a new horizon for the study of SERS in oxygen-containing semiconductors and provides a powerful reference for the study of PATP.

Au/CdS Core-Shell Sensitized Actinomorphic Flower-Like ZnO Nanorods for Enhanced Photocatalytic Water Splitting Performance

Herein, a novel actinomorphic flower-like ZnO/Au/CdS nanorods ternary composite photocatalyst is prepared to extend the light-responsive range, reduce the photogenerated charge carriers recombination, and ultimately improve the water splitting performance. Flower-like ZnO nanorods are synthesized by a chemical bath method and the CdS nanoparticles are sensitized by successive ionic layer adsorption and reaction method. Then the Au nanoparticles as co-catalysts are introduced by the photodeposition method to modify the interface of ZnO/CdS for reducing the photogenerated electron recombination rate and further improving the performance of water splitting. Detailed characterizations and measurements are employed to analyse the crystallinity, morphology, composition, and optical properties of the flower-like ZnO/Au/CdS nanorods samples. As a result, the flower-like ZnO/Au/CdS nanorod samples present significantly enhanced water splitting performance with a high gas evolution rate of 502.2 μmol/g/h, which is about 22.5 and 1.5 times higher than that of the pure ZnO sample and ZnO/CdS sample. The results demonstrate that the flower-like ZnO/Au/CdS nanorods ternary composite materials have great application potential in photocatalytic water splitting for the hydrogen evolution field.

Measuring nanoparticle-induced resonance energy transfer effect by electrogenerated chemiluminescent reactions

Electrogenerated chemiluminescence (ECL) efficiencies, redox potentials, photoluminescent (PL) (quenching and coupling) effects, and AFM images for the [Ru(bpy)₃]²⁺/Au@tiopronin system were determined in aqueous solutions of the gold nanoparticles (NPs) at pH 7.0. The most remarkable finding was that ECL measurements can display the nanoparticle-induced resonance energy transfer (NP-RET) effect. Its effectiveness was quantified through a coefficient, K_(NP-RET)ECL, which measures how much an ECL reaction has been enhanced. Moreover, the NP-RET effect was also checked using PL measurements, in such a way that a coefficient, K_(NP-RET)PL, was determined; both constants, K_(NP-RET)ECL and K_(NP-RET)PL being in close agreement. It is important to highlight the fact that the NP-RET effect is only displayed in diluted solutions in which there is no NPs self-aggregation. The existence of the NPs self-aggregation behavior is revealed through AFM measurements.

Electrogenerated chemiluminescence efficiencies, redox potentials, photoluminescent (quenching and coupling) effects, and AFM images for the [Ru(bpy)₃]²⁺/Au@tiopronin system were determined in aqueous solutions of the gold nanoparticles at pH 7.0.

Collective excitation of plasmon-coupled Au-nanochain boosts photocatalytic hydrogen evolution of semiconductor

Localized surface plasmon resonance (LSPR) offers a valuable opportunity to improve the efficiency of photocatalysts. However, plasmonic enhancement of photoconversion is still limited, as most of metal-semiconductor building blocks depend on LSPR contribution of isolated metal nanoparticles. In this contribution, the concept of collective excitation of embedded metal nanoparticles is demonstrated as an effective strategy to enhance the utilization of plasmonic energy. The contribution of Au-nanochain to the enhancement of photoconversion is 3.5 times increase in comparison with that of conventional isolated Au nanoparticles. Experimental characterization and theoretical simulation show that strongly coupled plasmonic nanostructure of Au-nanochain give rise to highly intensive electromagnetic field. The enhanced strength of electromagnetic field essentially boosts the formation rate of electron-hole pair in semiconductor, and ultimately improves photocatalytic hydrogen evolution activity of semiconductor photocatalysts. The concept of embedded coupled-metal nanostructure represents a promising strategy for the rational design of high-performance photocatalysts.

Plasmonic effect offers a valuable opportunity to improve the efficiency of semiconductor, photocatalysts. Here, the authors show that the collective excitation of plasmonic metal, nanoparticles is more favorable for enhancing the utilization of plasmonic energy by, semiconductors.

SERS monitoring of the Fenton degradation reaction based on microfluidic droplets and alginate microparticles

Surface-enhanced Raman scattering (SERS) spectroscopy as a powerful tool has been used to explore different catalysis degradation reactions, whereas some drawbacks caused by ferric ions still exist in the current SERS monitoring of the Fenton reaction process. In this work, microfluidic droplet- and alginate microparticle-based methods were, respectively, applied to realize SERS monitoring of the Fenton degradation process in a relatively stable environment, which benefited from reduction of the loss of ferrous ions and the aggregation of the SERS substrate. As expected, the spectroscopic evidence at the molecular level directly revealed the degradation mechanism of rhodamine dyes, showing that the chemical bonds between xanthene and carboxybenzene broke continuously during the reaction. Afterward, the degradation mechanism determined by SERS was verified via mass spectrometry detection, which confirmed the validity of the SERS-based method. More broadly, the microfluidic droplet- and microparticle-based methods are potentially applicable for SERS monitoring of more Fenton degradation reactions.

Ultrathin and Isotropic Metal Sulfide Wrapping on Plasmonic Metal Nanoparticles for Surface Enhanced Ram Scattering-Based Detection of Trace Heavy-Metal Ions

A facile and general strategy is presented for homogenous and ultrathin metal sulfide wrapping on plasmonic metal (PM) nanoparticles (NPs) based on a thiourea-induced isotropic shell growth. This strategy is typically implemented just via adding the thiourea into pre-formed PM colloidal solutions containing target metal ions. The validity of this strategy is demonstrated by taking the wrapped NPs with Au core and CuS shell or Au@CuS NPs as an example. They are successfully fabricated via adding the thiourea and Cu²⁺ solutions into pre-formed Au NP colloidal solution. The CuS shell layer is highly homogenous (<10% in relative standard deviation of shell thickness), regardless of the NPs' shape or curvature. The shell thickness can be controlled from tens down to 0.5 nm just by the addition of different amounts of shell precursors. The formation of the shell layer on the Au NPs can be attributed to the alternative deposition of Cu²⁺ and S^2- ions on the thiourea-modified surface of Au NPs in the solution, which induces the isotropic shell growth. Further, this strategy is of good universality. Many other sulfide-wrapped PM NPs, such as Ag@CuS, Au@PtS₂, Au@HgS, Ag@Ag₂S NPs, and Ag@CuS nanorods, have been successfully obtained with homogeneous and ultrathin shells. Importantly, such ultrathin sulfide-wrapped PM NPs can be used for surface enhanced Raman scattering (SERS)-based detection of trace heavy-metal ions with strong anti-interference via the ion exchange process between the metal sulfide shell and heavy-metal ions. This study provides a simple and controllable route for wrapping the homogenous and ultrathin sulfide layers on the PM NPs, and such wrapped NPs have good practical applications in the SERS-based detection of trace heavy-metal ions.

A composite prepared from gold nanoparticles and a metal organic framework (type MOF-74) for determination of 4-nitrothiophenol by surface-enhanced Raman spectroscopy

Core-shell nanoparticles (NPs) consisting of a gold core and a metal-organic framework shell (type MOF-74) were synthesized via one-pot synthesis. The NPs exhibit highly sensitive and stable SERS activity for the detection of 4-nitrothiophenol, with a specific band at 1337 cm^-1. The method has a linear response in 0.10-10 μmol·L^-1 analyte concentration range and a lower detection limit of 69 nmol·L^-1. The potential application of this novel SERS substrate was evaluated by two model reactions involving 4-nitrothiophenol. The first involves in-situ SERS monitoring of the surface plasmon-induced nitration of aromatic rings without adding conventional acid catalyst. The second involves the photocatalytic reduction of 4-nitrothiophenol to 4-thioaminophenol in the presence of Au/MOF-74 under 785-nm laser irradiation. The plasmon-assisted dimerization of 4-nitrothiophenol to form 4,4'-dimercaptoazobenzene can also be monitored simultaneously. Graphical abstract Schematic presentation of a nanoparticle SERS substrate consisting of gold core and MOF-74 shell, which was applied to detection of 4-nitrothiophenol. The Au/MOF-74 was successfully used for in-situ monitoring of two model reactions involving 4-nitrothiophenol by SERS.

An in situ SERS study of plasmonic nanochemistry based on bifunctional "hedgehog-like" arrays

An in situ SERS (surface-enhanced Raman scattering) study of plasmonic nanochemistry is realized on hierarchical Ag nanocone arrays ("hedgehog-like" arrays, denoted as HLAs) without any conventional catalyst. Ag nanocones are designed on 3D polystyrene (PS) microsphere arrays to provide a high density of hot spots within the laser-illumination area. Both experiments and numerical simulations demonstrate that the remarkable SERS and plasmonic catalytic performance of HLAs arise from the improved utilization rate of irradiation light in the third dimension and the tip enhancement effect of the nanocone arrays. On further combining their inherent SERS and catalytic properties, the in situ SERS study of plasmon-induced photocatalytic degradation reactions is realized. In this paper, not only the decomposition of methylene blue (MB) molecules is observed, but also the detailed molecular mechanisms of the reactions are revealed. Based on the bifunctional properties of the membrane-material interface, the HLAs are believed to be promising candidates in SERS and in situ SERS studies.

Stepwise Synthesis of Au@CdS-CdS Nanoflowers and Their Enhanced Photocatalytic Properties

Fabrication of hybrid nanostructures with complex morphologies and high photocatalytic activity is a difficult challenge because these particles require extremely high preparation skills and are not always practical. Here, hierarchical flower-like Au@CdS-CdS nanoparticles (Au@CdS-CdS nanoflowers) have been synthesized using a stepwise method. The Au@CdS-CdS nanoflowers are consisted of Au core, CdS shell, and CdS nanorods. The UV-Vis absorption range of the Au@CdS-CdS nanoflowers reaches up to 850 nm which covers the whole visible range (400-760 nm). Photoinduced charge transfer property of Au@CdS-CdS nanoflowers was demonstrated using photoluminescence (PL) spectroscopy. Compared to CdS counterparts and Au@CdS counterparts, Au@CdS-CdS nanoflowers demonstrated the highest photocatalytic degradation rate under irradiation of λ = 400-780 nm and λ = 600-780 nm, respectively. Based on structure and morphology analyses, we have proposed a possible formation mechanism of the hybrid nanostructure which can be used to guide the design of other metal-semiconductor nanostructures with complex morphologies.

In Situ Analysis of Surface Catalytic Reactions Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Electrochemistry and heterogeneous catalysis continue to attract enormous interest. In situ surface analysis is a dynamic research field capable of elucidating the catalytic mechanisms of reaction processes. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is a nondestructive technique that has been cumulatively used to probe and analyze catalytic-reaction processes, providing important spectral evidence about reaction intermediates produced on catalyst surfaces. In this perspective, we review recent electrochemical- and heterogeneous-catalysis studies using SHINERS, highlight its advantages, summarize the flaws and prospects for improving the SHINERS technique, and give insight into its future research directions.

A green and general strategy for the synthesis of hollow Ag/CdS nanocomposites for superior SERS performance

In this study, we developed a convenient, environmentally friendly approach for the fabrication of hollow Ag/CdS composites, which presented superior SERS performance.

Metal-free SERS substrate based on rGO–TiO2–Fe3O4 nanohybrid: contribution from interfacial charge transfer and magnetic controllability

We proposed a new ternary nanohybrid rGO–TiO₂–Fe₃O₄ as a magnetically controllable, ultra-sensitive SERS substrate with ultra-high SERS activity and applicability.