SERS-Based Plasmon-Driven Reaction and Molecule Detection on a Single Ag@MoS2 Microsphere: Effect of Thickness and Crystallinity of MoS2

Recent progress in SERS monitoring of photocatalytic reactions

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique renowned for its ultra-high sensitivity. Extensive research in SERS has led to the development of a wide range of SERS substrates, including plasmonic metals, semiconductors, metal organic frameworks, and their assemblies. Some of these materials are also excellent photocatalysts, and by taking advantage of their bifunctional characteristics, the photocatalytic processes that occur on their surface can be monitored in situ via SERS. This provides us with unique opportunities to gain valuable insights into the intricate details of the photocatalytic processes that are challenging to access using other techniques. In this review, we highlight key development in in situ and/or real-time SERS-tracking of photocatalytic reactions. We begin by providing a brief account of recent developments in SERS substrates, followed by discussions on how SERS can be used to elucidate crucial aspects of photocatalytic processes, including: (1) the influence of the surrounding media on charge carrier extraction; (2) the direction of charge carrier transfer; (3) the pathway of photocatalytic activation; and (4) differentiation between the effects of photo-thermal and energetic electrons. Additionally, we discuss the benefits of tip-enhanced Raman spectroscopy (TERS) due to the ability to achieve high-spatial-resolution measurements. Finally, we address major challenges and propose potential directions for the future of SERS monitoring of photocatalytic reactions. By leveraging the capabilities of SERS, we can uncover new insights into photocatalytic processes, paving the way for advancements in sustainable energy and environmental remediation.

Annealing temperature effects on monolayer WS2-veiled Ag nanoparticle array for surface catalytic reaction

Plasmonic-WS2 hybrids have attracted widespread interest for plasmon driven catalytic reactions. In this work, a Ag nanoparticles (NPs)/WS2 hybrid was fabricated by utilizing a one-step anodized Al template-assisted vacuum thermal evaporation technique and wet transfer method. To optimize the catalytic performance, the morphological evolution and corresponding changes in the catalytic properties of the Ag NPs/WS2 hybrid at different thermal annealing temperatures were investigated. It was found that the surface-enhanced Raman scattering (SERS) and catalytic activity of the hybrid were optimized by tuning the annealing temperature, with the optimal SERS and catalytic properties observed at 290 °C. These results may open new avenues for improving the efficiency and expanding the research field of plasmon-driven reactions.

Toward low-cost and sustainable SERS substrate: novel ultrasensitive AMS5 nanoflowers

Surface-enhanced Raman scattering (SERS) is an analytical technique for the rapid detection of low-concentration analytes. However, the lack of uniform, stable, and recyclable substrate limits its wide applications. Here, Ag-doped MoS₂ (AMS_x) was prepared by the hydrothermal method. Band structures, LSV, and EIS characteristics confirmed that Ag doping can reduce the indirect band gap and increase the charge transfer between substrates and molecules. As a SERS substrate, AMS_x displays excellent reproducibility, stability, and recyclability, which is beneficial for the application of the SERS substrate. Meanwhile, AMS_x has excellent sensitivity with an enhancement factor of 4.07 × 10⁶, comparable to that of precious metals. In addition, AMS_x exhibits ultrahigh sensitivity in sensing bilirubin and Bisphenol A (BPA); the corresponding detection limit of both is 10^-9 M, also better than that of previously reported semiconductors. This work provided a novel idea to synthesize low-cost ultrasensitive SERS substrates and the strategy of improving metal-chalcogenide semiconductor sensing.

Nanomaterials meet surface-enhanced Raman scattering towards enhanced clinical diagnosis: a review

Surface-enhanced Raman scattering (SERS) is a very promising tool for the direct detection of biomarkers for the diagnosis of i.e., cancer and pathogens. Yet, current SERS strategies are hampered by non-specific interactions with co-existing substances in the biological matrices and the difficulties of obtaining molecular fingerprint information from the complex vibrational spectrum. Raman signal enhancement is necessary, along with convenient surface modification and machine-based learning to address the former issues. This review aims to describe recent advances and prospects in SERS-based approaches for cancer and pathogens diagnosis. First, direct SERS strategies for key biomarker sensing, including the use of substrates such as plasmonic, semiconductor structures, and 3D order nanostructures for signal enhancement will be discussed. Secondly, we will illustrate recent advances for indirect diagnosis using active nanomaterials, Raman reporters, and specific capture elements as SERS tags. Thirdly, critical challenges for translating the potential of the SERS sensing techniques into clinical applications via machine learning and portable instrumentation will be described. The unique nature and integrated sensing capabilities of SERS provide great promise for early cancer diagnosis or fast pathogens detection, reducing sanitary costs but most importantly allowing disease prevention and decreasing mortality rates.

Au-Based Thin-Film Metallic Glasses for Propagating Surface Plasmon Resonance-Based Sensor Applications

We deposited Au-Cu-Si, an Au-based thin-film metallic glass (TFMG) of ∼50 nm thickness, as the activation layer for propagating surface plasmon resonance (PSPR)-based sensors on a BK7 glass substrate to substitute the commonly used gold layer. The film composition was tuned to yield the maximum Au content (∼65 at %), while the structure remained amorphous. The results showed that the Au-based TFMG could support surface plasmon resonance and gave rise to the extinction in the angle-resolved reflection spectrum. Using deionized water and ethyl alcohol with the refractive index difference of ∼0.03 as the analytes, the angle shift given by Au-based TFMG was 4° compared to 5° given by the Au film. Hence, Au-based TFMG is feasible to be used as the activation layer in PSPR-based sensors. Compared to the Au film, Au-based TFMG has the advantages of being less expensive, lacking grain boundary scattering, better adhesion to the substrate, and higher resistance to scratch and corrosion because of its amorphous structure with excellent mechanical properties.

Assembly of long silver nanowires into highly aligned structure to achieve uniform "Hot Spots" for Surface-enhanced Raman scattering detection

Silver nanowires (AgNWs) as a promising surface-enhanced Raman spectroscopy (SERS) substrate could be used in the analytical science due to its high sensitivity. However, it is difficult for the randomly-distributed silver nanowires to offer uniform "hot spots" to achieve the SERS signal reproducibility of small molecules detection. Herein, the evaporation-induced aggregation had been used to assemble long silver nanowires into highly aligned structure to achieve uniform "hot spots" for SERS detection. The normal glass slide with well-aligned silver nanowires could act as a high sensitivity and excellent reproducibility SERS substrate to provide a versatile platform for detecting analytes. Rhodamine 6G (R6G) is used to evaluate the sensitivity and reproducibility of these AgNWs SERS substrates. Even the low concentration of the R6G was 10^-10 mol/L, the SERS features of R6G could be still observed clearly, and the uniform distribution of enhancement factor (EF) was higher than 0.8 × 10⁴ accounting for about 75 % in the observed mapping area. Moreover, the relative standard deviation (RSD) of SERS intensity at the band of 610 cm^-1 was used to estimate the signal reproducibility, and the calculated RSD value of aligned AgNWs substrate was about 3.6%, which was much higher than that of the randomly distributed AgNWs (26.8%) because of the highly aligned structure of silver nanowires with abundant and uniform inherent "hot spots". In addition, potential SERS detection of other small molecule, e.g. melamine was also demonstrated in the micromolar range.

MoS2/graphene van der Waals heterojunctions combined with two-layered Au NP for SERS and catalysis analyse

MoS₂-plasmonic hybrid platforms have attracted significant interest in surface-enhanced Raman scattering (SERS) and plasmon-driven photocatalysis. However, direct contact between the metal and MoS₂ creates strain that deteriorates the electron transport across the metal/ MoS₂ interfaces, which would affect the SERS effect and the catalytic performance. Here, the MoS₂/graphene van der Waals heterojunctions (vdWHs) were fabricated and combined with two-layered gold nanoparticles (Au NP) for SERS and plasmon-driven photocatalysis analyse. The graphene film is introduced to provide an effective buffer layer between Au NP and MoS₂, which not only eliminates the inhomogeneous contact on MoS₂ but also benefits the electron transfer. The substrate exhibits excellent SERS capability realizing ultra-sensitive detection for 4-pyridinethiol molecules. Also, the surface catalytic reaction of p-nitrothiophenol (PNTP) to p,p-dimercaptobenzene (DMAB) conversion was in situ monitored, demonstrating that the vdWHs-plasmonic hybrid could effectively accelerate reaction process. The mechanism of the SERS and catalytic behaviors are investigated via experiments combined with theoretical simulations (finite element method and quantum chemical calculations).

Enhanced solid-state plasmon catalyzed oxidation and SERS signal in the presence of transition metal cations at the surface of gold nanostructures

The effect of several metal cations (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) on the photochemical conversion of 4-aminothiophenol (4-ATP) into 4,4'-dimercaptoazobenzene (DMAB) is probed using surface enhanced Raman scattering (SERS). The coupling reaction is carried out on the surface of Au nanoparticles and Au nanorods using 532 nm and 632.8 nm laser excitations, respectively, in the absence and presence of metal cations. Here, we report that DMAB formation on the surface of Au nanostructures - when carried out in the solid state - is augmented significantly (by a factor of 1.98 to 4.07 and 3.34 to 5.74 for Au nanoparticle and Au nanorod substrates, respectively, and depending on the metal). Furthermore, the SERS signal is also markedly enhanced. Thus, the results underpin a new way of carrying out a photochemical reaction with a higher yield along with a higher SERS signal.

Plasmonic Heating-Promoted Photothermal Synthesis of α-Cyanoacrylonitriles Over Au/h-BN Catalysts

Plasmonic nanoparticle-involved materials play an essential role in the field of photothermal conversion. Herein, we report the application of photothermal heterogeneous catalysts consisting of gold nanoparticles decorated on defect-rich h-BN sheets (Au/h-BN) for the photocatalytic synthesis of α-cyanoacrylonitriles under mild conditions. It has been demonstrated the–NH₂ groups present in the defect-rich h-BN act as the catalytically active sites, while plasmonic heating from the gold nanoparticles can drive the reaction by providing local heat. Au/h-BN catalyst can work for a broad substrate scope in the synthesis of α-cyanoacrylonitriles, and a plausible –NH₂ group-involved reaction mechanism has been proposed. This work may open up new avenues in photothermal catalysis by combining plasmonic materials and catalytic sites in one system.

Recent Advances in Plasmonic Nanostructures for Enhanced Photocatalysis and Electrocatalysis

Plasmonic nanomaterials coupled with catalytically active surfaces can provide unique opportunities for various catalysis applications, where surface plasmons produced upon proper light excitation can be adopted to drive and/or facilitate various chemical reactions. A brief introduction to the localized surface plasmon resonance and recent design and fabrication of highly efficient plasmonic nanostructures, including plasmonic metal nanostructures and metal/semiconductor heterostructures is given. Taking advantage of these plasmonic nanostructures, the following highlights summarize recent advances in plasmon-driven photochemical reactions (coupling reactions, O₂ dissociation and oxidation reactions, H₂ dissociation and hydrogenation reactions, N₂ fixation and NH₃ decomposition, and CO₂ reduction) and plasmon-enhanced electrocatalytic reactions (hydrogen evolution reaction, oxygen reduction reaction, oxygen evolution reaction, alcohol oxidation reaction, and CO₂ reduction). Theoretical and experimental approaches for understanding the underlying mechanism of surface plasmon are discussed. A proper discussion and perspective of the remaining challenges and future opportunities for plasmonic nanomaterials and plasmon-related chemistry in the field of energy conversion and storage is given in conclusion.

Recent Advances in Plasmon-Promoted Organic Transformations Using Silver-Based Catalysts

Plasmonics has emerged as a promising methodology to promote chemical reactions and has become a field of intense research effort. Ag nanoparticles (NPs) as plasmonic catalysts have been extensively studied because of their remarkable optical properties. This review analyzes the emergence and development of localized surface plasmon resonance (LSPR) in organic chemistry, mainly focusing on the discovery of novel reactions with new mechanisms on Ag NPs. Initially, the basics of LSPR and LSPR-promoted photocatalytic mechanisms are illustrated. Then, the recent advances in plasmonic nanosilver-mediated photocatalysis in organic transformations are highlighted with an emphasis on the related reaction mechanisms. Finally, a proper perspective on the remaining challenges and future directions in the field of LSPR-promoted organic transformations is proposed.

Nanoscale optical imaging in chemistry

Single-molecule-level measurements are bringing about a revolution in our understanding of chemical and biochemical processes. Conventional measurements are performed on large ensembles of molecules. Such ensemble-averaged measurements mask molecular-level dynamics and static and dynamic fluctuations in reactivity, which are vital to a holistic understanding of chemical reactions. Watching reactions on the single-molecule level provides access to this otherwise hidden information. Sub-diffraction-limited spatial resolution fluorescence imaging methods, which have been successful in the field of biophysics, have been applied to study chemical processes on single-nanoparticle and single-molecule levels, bringing us new mechanistic insights into physiochemical processes. However, the scope of chemical processes that can be studied using fluorescence imaging is considerably limited; the chemical reaction has to be designed such that it involves fluorophores or fluorogenic probes. In this article, we review optical imaging modalities alternative to fluorescence imaging, which expand greatly the range of chemical processes that can be probed with nanoscale or even single-molecule resolution. First, we show that the luminosity, wavelength, and intermittency of solid-state photoluminescence (PL) can be used to probe chemical transformations on the single-nanoparticle-level. Next, we highlight case studies where localized surface plasmon resonance (LSPR) scattering is used for tracking solid-state, interfacial, and near-field-driven chemical reactions occurring in individual nanoscale locations. Third, we explore the utility of surface- and tip-enhanced Raman scattering to monitor individual bond-dissociation and bond-formation events occurring locally in chemical reactions on surfaces. Each example has yielded some new understanding about molecular mechanisms or location-to-location heterogeneity in chemical activity. The review finishes with new and complementary tools that are expected to further enhance the scope of knowledge attainable through nanometer-scale resolution chemical imaging.

Plasma Fabrication and SERS Functionality of Gold Crowned Silicon Submicrometer Pillars

Sequential plasma processes combined with specific lithographic methods allow for the fabrication of advanced material structures. In the present work, we used self-assembled colloidal monolayers as lithographic structures for the conformation of ordered Si submicrometer pillars by reactive ion etching. We explored different discharge conditions to optimize the Si pillar geometry. Selected structures were further decorated with gold by conventional sputtering, prior to colloidal monolayer lift-off. The resulting structures consist of a gold crown, that is, a cylindrical coating on the edge of the Si pillar and a cavity on top. We analysed the Au structures in terms of electronic properties by using X-ray absorption spectroscopy (XAS) prior to and after post-processing with thermal annealing at 300 °C and/or interaction with a gold etchant solution (KI). The angular dependent analysis of the plasmonic properties was studied with Fourier transformed UV-vis measurements. Certain conditions were selected to perform a surface enhanced Raman spectroscopy (SERS) evaluation of these platforms with two model dyes, prior to confirming the potential interest for a well-resolved analysis of filtered blood plasma.

Solvent-controlled plasmon-assisted surface catalysis reaction of 4-aminothiophenol dimerizing to p,p'-dimercaptoazobenzene on Ag nanoparticles

A large number of literatures have investigated the selective photocatalytic reaction of 4-aminothiophenol (PATP) to p,p'-dimercaptoazobenzene (DMAB). Most of them mainly study the contribution of substrate, excitation wavelength, exposure time, pH and added cations to plasmon-assisted surface catalytic reactions. However, we mainly study focuses on the effects of solvents on the dimerization of PATP to DMAB under the action of Ag nanoparticles (NPs). In experiments, a variety of diols was selected as solvents for the probe molecule PATP, and power-dependent SERS spectra were obtained at an excitation wavelength of 532 nm. From the laser-dependent SERS spectrum, we found that the characteristic peak enhancement effect of the product DMAB in different solvents is significantly different. That is, different solvents could regulate the rate at which DMAB is produced from PATP. Based on the experimental results, we further explored how different diol solvents regulate the response of PATP to DMAB. Our conclusion is that the solvent in the system can quickly capture the hot electrons generated by the decay of the plasmon, so that the remaining holes can oxidize PATP to form DMAB. The ability to trap hot electrons is different due to the difference in the position of the functional groups in the solvent, so that the photocatalytic reaction rate of the hole-oxidized PATP is different. The ability to capture electrons varies depending on the position of the functional groups in the solvent, so the oxidation rate of the photocatalytic reaction is also different. This work not only deepens our understanding of the mechanism of hole-driven surface catalysis oxidation reaction, but also provides a convenient method for regulating the rate of catalytic oxidation.

Tuning the SERS activity and plasmon-driven reduction of p-nitrothiophenol on a Ag@MoS2 film

The combination of plasmonic metal nanostructures with semiconductors has been widely applied in plasmon-driven photocatalysis. Here, a uniform Ag@MoS2 hybrid film is fabricated by depositing MoS2 onto a thin Ag film via the pulsed laser deposition (PLD) technique. The thickness and crystallinity of MoS2 can be adjusted by controlling the PLD deposition time and temperature, respectively. With the assistance of surface-enhanced Raman scattering (SERS) analysis, the Raman enhancement uniformity of the substrate and plasmon-driven reaction of p-nitrothiophenol (PNTP) dimerizing into p,p'-dimercaptobenzene (DMAB) are carefully studied on Ag@MoS2 film substrates with different MoS2 crystallinities. The Raman enhancement decreases with increased MoS2 thickness, due to the weakened electromagnetic field enhancement as suggested by finite-difference time-domain (FDTD) simulations. The increased crystallinity of MoS2 can efficiently accelerate the hot electron transfer process, resulting in the enhancement of SERS activity and the improved efficiency of the plasmon-driven reaction. This study may pave the way for the design of other uniform metal-semiconductor hybrids for use as SERS substrates and photocatalysts.