Chemical Enhancement Mechanism Research Articles

SERS-Based Sensor Using Subnanometric Copper-Silver Mixed Clusters Ag(8-n)Cun (n = 0-8) for Pyridine: A DFT Study.

Surface-enhanced Raman spectroscopy (SERS) is a powerful Raman technique that provides high selectivity and sensitivity in analyzing the intermolecular interaction of a target compound adsorbed on the surface of a noble nanomaterial, i.e., silver, gold, or copper. Although copper presents a better SERS enhancement than gold and silver, its oxidation in the air is much easier than that of gold and silver. A mixed material between these metals may potentially improve the SERS signal enhancement in this context. In this work, we evaluated the SERS spectra of pyridine (Py) adsorbed on the copper-silver mixed clusters Ag(8-n)Cun (n = 0-8) using density functional theory (DFT) at the PBE functional. The cc-pVDZ-PP basis set was chosen for Ag and Cu, while the cc-pVDZ basis set was used for C, N, and H atoms. Geometrical and electronic structures of the mixed clusters and the Py adsorption configuration on these clusters were computed. The calculated SERS spectra then revealed the influence of the Ag/Cu mixing ratio on the SERS enhancement. As a result, the substituted copper atoms on the silver cluster turned out to be favorable adsorption sites for Py. Interestingly, when the number of Cu atoms increased from n = 0 (pure Ag8 cluster) to n = 5 (Ag3Cu5 cluster), the ring stretching peak (1590 cm-1) of Py significantly increased from 20 to 120 au and then saturated around this value despite increasing the Cu atom number to 8 (pure Cu8 cluster). This observation was extended for other ligands such as pyrazine and 3H-pyrrole. TD-DFT was then employed to clarify the chemical enhancement mechanism. The results obtained hopefully provide helpful information for the design of analytical sensors with lower costs.

Transferable G/Au Film for Constructing a Variety of SERS Substrates.

Surface-enhanced Raman scattering (SERS), as one of the most powerful analytical methods, undertakes important inspection tasks in various fields. Generally, the performance of an SERS-active substrate relies heavily on its structure, which makes it difficult to integrate multiple-functional detectability on the same substrate. To address this problem, here we designed and constructed a film of graphene/Au nanoparticles (G/Au film) through a simple method, which can be conveniently transferred to different substrates to form various composite SERS substrates subsequently. By means of the combination of the electromagnetic enhancement mechanism (EM) and the chemical enhancement mechanism (CM) of this structure, the film realized good SERS performance experimentally, with the enhancement factor (EF) approaching ca. 1.40 × 105. In addition, the G/Au film had high mechanical strength and had large specific surface area and good biocompatibility that is beneficial for Raman detection. By further transferring the film to an Ag/Si composite substrate and PDMS flexible film, it showed enhanced sensitivity and in situ detectability, respectively, indicating high compatibility and promising prospect in Raman detection.

Nanoimprinted cellulose acetate-TiO2 composite thin film

Cellulose acetate is a safe, sustainable, and cost-effective material that is capable of forming nanostructures through facial processing methods such as surface imprinting. Forming optically active structures using cellulose acetate can advance green photonic device design. In this work, we create a hybrid material consisting of nanoscale plasmon active metal–semiconductor Schottky junctions. Demonstrating that such a hybrid material possesses improved performance when applied to Raman-based sensing. Boosting surface-enhanced Raman detection sensitivity through electromagnetic and chemical enhancement mechanisms from the metal-semiconductor junction, in addition to photonic resonances created via the imprinted nanoscale metamaterial array surface features. This work expands the use of cellulose-based materials for sensing-based applications.

Semiconductor Enhanced rGO/Ag0/ZnO Ternary Hybrid Revealing Charge Transfer Enhancement in SERS Detection of Raman Reporter Molecules

AbstractIn this work, we have investigated the role of the chemical enhancement mechanism in SERS detection on a ternary hybrid substrate fabricated by incorporating ZnO nanostructures into an in situ synthesized reduced graphene oxide/silver composite. The sensing performance of the ternary substrate at various ZnO concentrations is investigated using Raman reporter molecules (RRM) such as Rhodamine 6G and 4‐Mercaptobenzoic acid. The SERS signals show enhancement by adding varied amounts of ZnO, revealing a new way of SERS enhancement by incorporating semiconductor nanostructures rather than conventional hotspot engineering. To probe chemical enhancement, we kept the electromagnetic contribution constant by keeping the same concentration of Ag in all the substrates investigated in this study. The obtained enhanced SERS signals of the probe molecules reveal the contribution from the chemical enhancement mechanism with the addition of ZnO. This vital observation reveals a new gateway in the field of SERS detection for developing a semiconductor‐enhanced, economical, and sustainable SERS substrate.

Multiconfigurational Calculations and Experimental Resonant Raman/SERRS of a Donor-Acceptor Thiadiazole Dye.

The resonant Raman (RR) and resonant SERS spectra of the thiadiazole-based dye dibromobenzo[c]-1,2,5-thiadiazole (DBTD) were studied through multiconfigurational XMS-CASPT2/CASSCF and experimental methods in solution. The results indicate that the S1 excited state of DBTD is described by π → π* with internal CT from the benzene ring to the thiadizole. In resonance conditions at 364 nm, the RR spectrum shows intensifications in modes that describe extensive geometrical changes at both the benzene ring and the thiadiazole region, indicating an internal CT character to the S1. The SERS spectra observed on gold and silver nanoparticles indicate different adsorption geometries, which leads to distinct enhancement patterns on the spectra with varying excitation energy. It evidences the major contribution of the chemical enhancement mechanism on the spectra from a metal → DBTD CT state, as confirmed by the simulated spectra. This theoretical approach proved strong in the prediction of the main features of the observed experimental resonant Raman and SERS spectra indicating a potential for adequate description of the chemical mechanism of SERS.

A rapid detection method of three foodborne pathogens based on physical and chemical Raman spectroscopy enhanced

Escherichia coli, Listeria monocytogenes, and Salmonella typhi are three pathogens commonly found in food. Label-free enhanced substrates have limitations in achieving high sensitivity in three bacteria detection. To enable low-concentration detection and differentiation of foodborne pathogens, this research presents an optimized detection strategy using Au @Ag NPs as the enhancing substrate for SERS technology. The impact of the particle size of Au @Ag NPs and the pH of the borate buffer solution on enhancing the Raman signals of these bacteria was investigated through electromagnetic and chemical enhancement mechanisms. By evaluating the intensity of bacterial Raman spectra, and employing chemometric techniques, the concentration and classification of the three bacterial species were predicted and analyzed. The research findings revealed that the optimized detection method was able to detect three pathogens at the concentration lower than 3 lg CFU/mL. Logarithmic fitting of the bacteria enabled prediction correlations above 0.98 and prediction root mean square errors below 0.17. After normalizing, efficient discrimination of low-concentration bacteria was achieved using the PLS-DA, with a classification prediction correlation greater than 0.94. The fabrication process of the proposed enhancement substrate is simple, but the stability of signal detection needs further improvement in subsequent experimental testing steps.

Preparation and SERS applications of Ta2O5 composite nanostructures.

Noble metal and semiconductor composite substrates possess high sensitivity, excellent stability, good biocompatibility, and selective enhancement, making them an important research direction in the field of surface-enhanced Raman scattering (SERS). Ta2O5, as a semiconductor material with high thermal stability, corrosion resistance, outstanding optical properties, and catalytic performance, has great potential in SERS research. This study aims to design and fabricate a composite SERS substrate based on Ta2O5 nanostructures, achieving optimal detection performance by combining the urchin-like structure of Ta2O5 with silver nanoparticles (Ag NPs). The urchin-like Ta2O5 nanostructures were prepared using a hydrothermal reaction method. The bandgap was modulated through structure design and the self-doping technique, the charge transfer efficiency and surface plasmon resonance effects were improved, thereby achieving better SERS performance. The composite substrate enables highly sensitive quantitative detection. This composite SERS substrate combines the electromagnetic enhancement mechanism (EM) and chemical enhancement mechanism (CM), achieving ultra-low detection limits of 10-13 M for R6G. Within the concentration range above 10-12 M, there is a good linear relationship between concentration and peak intensity, demonstrating excellent quantitative analysis capabilities. Furthermore, this composite SERS substrate is capable of precise detection of analytes such as crystal violet (CV) and methylene blue (MB), holding broad application prospects in areas such as food safety and environmental monitoring.

Effects of Au6 and Au20 Adsorption Sites of Cyromazine-Au Complexes by Raman Spectroscopy and Density Functional Theory.

Cyromazine, when used as an insect growth regulator and low-toxicity insecticide, may degrade into melamine and pose a potential threat to the environment and soil health, which has thus attracted extensive research on eliminating such a harmful effect. In this paper, density functional theory (DFT)/LC-BLYP/6-311G(d,p) is used to optimize the geometric structure and analyze the vibration of cyromazine. The DFT/LC-BLYP/def2-SVP is used for the cyromazine-Au complex optimization and vibration analysis. The molecular electrostatic potential (MEP), frontier molecular orbitals (FMOs), vibration frequency, electrophilicity-based charge transfer (ECT) descriptor, binding energy (BE), polarizability, normal Raman spectroscopy (NRS), and surface-enhanced Raman spectroscopy (SERS) of cyromazine adsorbing on Au6 and Au20 are calculated. The study of the chemical enhancement mechanism of SERS of cyromazine at different adsorption sites of Au6 or Au20 confirms the existence of a charge transfer between cyclopromazine and Au6 and Au20, which can adsorb and form stable cyromazine-Au complexes. The results show that N2, H13, and N4 are the adsorption sites of Au6 and Au20. The Raman spectra of the cyromazine-Au complex can be selectively enhanced with a factor up to 9.07. Compared with those of cyromazine-Au6, the Raman spectra of cyromazine-Au20 are enhanced more significantly.

Near-Neighbor Electron Orbital Coupling Effect of Single-Atomic-Layer Au Cluster Intercalated Bilayer 2H-TaS2 for Surface Enhanced Raman Scattering Sensing.

It is difficult to perfectly analyze the enhancement mechanism of two-dimensional (2D) materials and their combination with precious metals as surface enhanced Raman scattering (SERS) substrates using chemical enhancement mechanisms. Here, we propose a new mentality based on the coupling effect of neighboring electron orbitals to elucidate the electromagnetic field enhancement mechanism of single-atom-layer Au clusters embedded in double-layer 2H-TaS2 for SRES sensing. The insertion of Au atoms into the 2H-TaS2 interlayer was verified by XRD, AFM, and HRTEM, and a SERS signal enhancement of 2 orders of magnitude was obtained compared to the pure 2H-TaS2. XPS and micro-UV/vis-NIR spectra indicate that the outer electrons of neighboring Au and 2H-TaS2 overlap and migrate from Au to 2H-TaS2. First-principles calculations suggest strong electronic coupling between Au and 2H-TaS2. This study offers insights into SERS enhancement in nonprecious metal compounds and guides the development of new SERS substrates.

Green Synthesis for Carbon Quantum Dots via Opuntia ficus-indica and Agave maximiliana: Surface-Enhanced Raman Scattering Sensing Applications.

In this study, we present an alternative method for synthesizing carbon quantum dots (CQDs) using a green synthesis approach via extracts from Agave maximiliana and Opuntia ficus-indica(Ofi). The extracts from both plants were used as the carbon source for the CQDs. The synthesis method employs mesoporous zeolite 4A as a refractory for the thermal treatment of the samples. Transmission electron microscopy analysis established that the size of the CQDs shows a narrow distribution centered around 2 nm with a maximum size of less than 3 nm for both cases. The CQDs exhibit absorption bands associated with π-π* transitions located around 220 nm. In both cases, photoluminescence (PL) phenomenon was detected by irradiating the samples with a UV wavelength and detecting emissions close to the blue wavelength. Additionally, both kinds of CQDs were tested as surface-enhanced Raman scattering (SERS) substrates against methylene blue (MB), indicating an enhancement associated with ring deformation and stretching modes of the v(C-C) and v(C-N) bonds located around 1400 and 1620 cm-1, respectively. Complementarily, in the framework of density functional theory, H2nC2(2m+1) structures (with n = 3-5 and m = 1-3) were used as a theoretical representation of CQDs in interaction with the MB molecule. It is used for developing the analysis of charge transfer effects between both systems and for specifying elements that generate the SERS effect associated with the chemical enhancement mechanism.

Highly sensitive detection of cationic pollutants on molybdenum carbide (MXene)/Fe2O3/Ag as a SERS substrate

Recent studies reported that the two-dimensional transition-metal carbides (MXenes) present promising results for surface-enhanced Raman spectroscopy (SERS) application, permitting sensitive detection of analyte molecules thanks to rich surface chemistries, surface plasmon resonances, and electrical features. However, Mxene materials, especially Mo-based ones, still suffer from low concentrated sensing activity and lack specific sensing suited to surface chemistries. To overcome those issues, for the first time, we propose a simple and scalable approach to fabricate Mo2CTx/Fe2O3/Ag (MFA) hybrid nanostructure via the hydrothermal treatment of MXene and Fe2O3, and subsequent in-situ decorated of Ag NPs via seed-mediated growth processes. As-prepared MFA presents a 1 × 10−9 M detection limit with a 1.46 × 106 enhancement factor for cationic MB molecules. Here, in addition to the contribution of the electromagnetic enhancement mechanism due to the presence of Ag, the chemical enhancement mechanism also comes into play as a result of the specific binding of the cationic molecule, MB, to the negatively charged MXene surface. This result is also observed for harmful cationic molecules found in wastewater such as CTAC and arsenic. MFA exhibits a low detection limit for CTAB, where a very low concentration of 0.075 M was achieved probably owing to its cationic character. Last but not least, the MFA hybrid nanostructure shows promising results for arsenic detection in tap water with 10 μg/L. Overall, an as-prepared MFA is a promising option for creating a SERS-active surface that can be customized for particular uses, due to its adjustable surface chemistries and elevated charge carrier densities, while also being low-cost and simple to manufacture.

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.

Ultra-small gold particles via Citrus × sinensis: Theoretical and experimental SERS study

In this study, we have addressed the green synthesis of sub-nanostructured gold species using Citrus × sinensis. Absorption bands in the ultraviolet region (338 nm) were observed. Analysis by atomic resolution microscopy revealed the identification of sub-nanostructured Au species with particle sizes of about 0.9 nm. With favorable results, these systems were tested as substrates for surface-enhanced Raman spectroscopy (SERS) using pyridine. The chemical enhancement mechanism was studied as the source of the SERS effect. Complementarily, the interaction between gold clusters (Au2n, with n = 1–5) and pyridine was modeled using Density Functional at the Becke-3-parameter-Lee-Yang-Parr level of approximation in combination with the LANL2DZ basis set. A correlation between the adsorption energy and the SERS enhancement factor was analyzed as a function of the Au-N interaction distance.

Interpreting chemical enhancements of surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) provides orders of magnitude of enhancements to weak Raman scattering. The improved sensitivity and chemical information conveyed in the spectral signatures make SERS a valuable analysis technique. Most of SERS enhancements come from the electromagnetic enhancement mechanism, and changes in spectral signatures are usually attributed to the chemical enhancement mechanism. As the electromagnetic mechanism has been well studied, we will give an overview of models related to the chemical mechanism, which explain the Raman response in terms of electronic transitions or induced electron densities. In the first class of models based on electronic transitions, chemical enhancements are attributed to changes in transitions of the molecule and new charge transfer transitions. The second class of models relate chemical enhancements to charge flows near the molecule–metal interface by partitioning the induced electron density of the SERS system in real space. Selected examples will be given to illustrate the two classes of models, and connections between the models are demonstrated for prototypical SERS systems.

Revealing Adsorption Mechanism of p-Mercaptobenzoic Acid with TiO2 Surfaces Using Electric Field-Enhanced Semiconductor SERS

p-Mercaptobenzoic acid (4-MBA) is a typical molecular probe for a surface-enhanced Raman scattering (SERS) study of the enhancement performance of semiconductor nanoparticles. Understanding the molecular adsorption mechanism of 4-MBA on a semiconductor surface is crucial to reveal the enhancement mechanism of semiconductor SERS. Herein, two types of submicrometer-sized TiO2 particles with amorphous (denoted as a-TiO2) and anatase structures (denoted as c-TiO2) were fabricated, and their potential as SERS-active substrates with high electric-field enhancement was explored based on the near-field scattering theory and finite-element method simulation. The electric field-enhanced semiconductor SERS provide a better vision for us to study the adsorption modes of molecules on the TiO2 surface. On this basis, adsorption behaviors of 4-MBA on a-TiO2 and c-TiO2 particles were systematically studied by the semiconductor SERS and density functional theory. The results demonstrated that the adsorption mechanism of 4-MBA with TiO2 surfaces is highly dependent on the exposure of acid sites of TiO2 surfaces. 4-MBA adsorbs preferentially on Brønsted acid sites of a-TiO2 through a carboxyl group, in contrast on Lewis acid sites of c-TiO2 through a sulfhydryl group. Furthermore, 4-MBA molecules may form multilayer adsorption on TiO2 surfaces through the hydrogen bond and/or π–π stacking interaction. Research results not only provide a new insight to re-evaluate the chemical enhancement mechanism for TiO2–4-MBA systems but also provide a theoretical guidance for the modification of TiO2 surface with organic molecules containing carboxyl and sulfhydryl groups.

Surface structure effect of F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite heteromaterials on surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) has been widely used in food and drug detection, biological and medical sensing. In recent years, the study of non-metallic SERS substrates has gradually become a hot field of SERS. Here, we investigate the modulation effect on SERS activities of 2,3,5,6-tetrafluoro-7,7,8,8-tetrachyanoquindimethylene (F<sub>4</sub>TCNQ) grown on molybdenum disulfide (MoS<sub>2</sub>) films. The different nanostructures of F<sub>4</sub>TCNQ can have an effect on the bound capability of charges transferred from the surface of MoS<sub>2</sub>, which changes the electron density distribution on the surface of the F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite material. Therefore, the interface exhibits different charge localizations in the F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite. The charge transfer efficiency between the substrate and the adsorbed probe molecules leads the substrate to show a different SERS sensitivity. The enhancement factor of 4-mercaptobenzoic acid (4-MBA) molecules on the most optimized 7-min F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite substrate can reach <inline-formula><tex-math id="M2">\begin{document}$ 6.9\times {10}^{4} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221958_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221958_M2.png"/></alternatives></inline-formula>, and the detection limit concentration is as low as 10<sup>–6</sup> mol/L. The result of research on F<sub>4</sub>TCNQ/MoS<sub>2</sub> nanocomposite provides an effective optimization scheme of energy level regulation for SERS based on the chemical enhancement mechanism, and opens up a new way to further exploit its functional applications.

Efficient SERS Response of Porous-ZnO-Covered Gold Nanoarray Chips to Trace Benzene-Volatile Organic Compounds.

Fast and sensitive detection of gaseous volatile organic compounds (VOCs), based on surface-enhanced Raman spectroscopy (SERS), is still a challenge due to their weak interaction with plasmonic metals and overly small Raman scattering cross sections. Herein, we propose a simple strategy to achieve the SERS-based highly efficient detection of trace benzene-VOCs (B-VOCs) based on a composite chip. The composite chip is designed and fabricated via covering the porous zinc oxide on gold nanoarrays by a one-step solution growth method. Such composite chip shows highly selective capture of gaseous B-VOCs (benzene, toluene, nitrobenzene, xylene, and chlorobenzene, etc.), which leads to the rapid and sensitive SERS responses to them. Typically, this chip can response to gaseous toluene within 30 s, and the lowest detectable concentration is below 10 ppb. Further experiments have revealed that there exists an optimal thickness of the ZnO covering layer for the highly efficient SERS response to the B-VOCs, which is about 150 nm. Also, such a composite chip is recoverable in SERS response and hence reusable. The highly efficient SERS response of the composite chip to the B-VOCs is attributed to the porous structure-enhanced molecular adsorption and the electromagnetic-chemical dual-enhancement mechanism. This work not only presents a practical SERS chip for the efficient detection of the typical B-VOCs but also provides a deep understand the interaction between the B-VOCs and the ZnO as well as the chemical enhancement mechanism.

A Comprehensive Commentary of “Smart Design of High-Performance Surface Enhanced Raman Scattering Substrates”

Recently, our group published a review about surface enhanced Raman scattering (SERS) in SmartMat, which summarized the recent developments of noble metal and semiconductor SERS substrates, and then proposed the key factors for substrate design and future directions. The fundamental theories for SERS technique, i.e., the electromagnetic enhancement mechanism and chemical enhancement mechanism, and the design strategies for noble metal and semiconductor substrates are introduced detailed

Spin-Coated Ag NPs SERS Substrate: Role of Electromagnetic and Chemical Enhancement in Trace Detection of Methylene Blue and Congo Red

This work demonstrates a simple low-cost and reproducible spin-coating method followed by thermal reduction for the uniform growth of silver nanoparticles (Ag NPs) as surface-enhanced Raman scattering (SERS) substrate. In this work, cationic dye MB and anionic dye CR are selected as two different kinds of SERS probe molecules to investigate the differences between electromagnetic field enhancement (EME) and chemical enhancement (CE) mechanisms. The results indicate that SERS spectra of MB molecule on Ag NPs SERS substrate exhibit the shift in the Raman peak positions and high sensitivity (limit of detection, LOD ~ 10−10 M) with large GSERS (~ 109) value, which can be attributed to the contribution from both EME and CE mechanisms. It is found that Cl− halide ions present in the MB molecule are forming surface complex (Ag NPs@Cl−) on the surface of Ag NPs SERS substrate which is confirmed by the XPS investigation, which might be responsible for the CE enhancement in MB molecule. Whereas Congo red being anionic dye showed no shift in Raman peak positions and less sensitivity (LOD ~ 10−8) with GSERS (~ 107) value on Ag NPs SERS substrate, which can be attributed to EME mechanism only. The obtained results suggest that Cl− halide ions present in the MB molecule play an important role by giving large SERS enhancement for MB molecule on Ag NPs SERS substrate. It is expected that this work will provide insight into role of EME and CE mechanisms for cationic dye MB and anionic dye CR.

Atomically Thin TaSe2 Film as a High‐Performance Substrate for Surface‐Enhanced Raman Scattering

An atomically thin TaSe2 sample, approximately containing two to three layers of TaSe2 nanosheets with a diameter of 2.5cm is prepared here for the first time and applied on the detection of various Raman-active molecules. It achieves a limit of detection of 10-10 m for rhodamine 6G molecules. The excellent surface-enhanced Raman scattering (SERS) performance and underlying mechanism of TaSe2 are revealed using spectrum analysis and density functional theory. The large adsorption energy and the abundance of filled electrons close to the Fermi level are found to play important roles in the chemical enhancement mechanism. Moreover, the TaSe2 film enables highly sensitive detection of bilirubin in serum and urine samples, highlighting the potential of using 2D SERS substrates for applications in clinical diagnosis, for example, in the diagnosis of jaundice caused by excess bilirubin in newborn children.