Raman Enhancement Research Articles

A General Approach to Hybrid Platform of AU Nanoparticles on Monolayer Semiconductor for Ultrasensitive Raman Enhancement of 2d Materials and Molecule Detection

A General Approach to Hybrid Platform of AU Nanoparticles on Monolayer Semiconductor for Ultrasensitive Raman Enhancement of 2d Materials and Molecule Detection

A dual-responsive nanozyme sensor with ultra-high sensitivity and ultra-low cross-interference towards metabolic biomarker monitoring.

Accurate, sensitive and selective detection of metabolic biomarkers in biofluids are of vital significance for health self-monitoring and chronic disease prevention. Here, for the first time, a smart dual-responsive nanozyme sensor (DNS) was developed for simultaneous analysis of glucose and caffeine utilizing stimuli-responsive yolk-shell gold nanoparticles (GNPs)-embedded MIL-53 (Al) (GNPs@MIL-53) structures. After the introduction of glucose, GNPs@MIL-53 displays excellent glucose oxidase (GOx)-like activity to induce the conversion of glucose to gluconic acid and H2O2. H2O2 can oxidize 3,3',5,5'-tetramethylbenzidine (TMB) with the generation a bright-blue color, enabling in-field visualization and surface enhanced Raman scattering (SERS) detection of glucose. Upon the addition of caffeine, 2-aminoterephthalic acid modified MIL-53 can react with the caffeine to form intermolecular hydrogen-bonded complexes, leading to strong cyan fluorescence and significant Raman enhancements. The DNS with multi-channel signal outputs can simultaneously determine glucose and caffeine at concentrations of as low as 3 × 10-8 M and 1.2 × 10-11 M, respectively. Importantly, the DNS-based analytical system not only enables visual discrimination and accurate assay of glucose and caffeine in biofluids, but also exhibits negligible cross-interference between glucose and caffeine determination. The combined characteristics of high selectivity, enhanced accuracy and superior quantitative performance make our platform suitable for the point-of-care monitoring of chronic-disease-related metabolic biomarkers.

Synthesis and SERS activity of niobium nitride thin films via reduction nitridation of sol-gel derived films

The niobium nitride thin films as the novel substrates for surface-enhanced Raman spectroscopy (SERS) were synthesized via reduction nitridation of Nb2O5 thin films derived by sol-gel method in NH3 gas. The composition, morphology, energy level structure, optical properties and SERS performance of the films were characterized. The XRD and XPS results show that the solid solution niobium oxynitride (NbOxNy) is formed, rather than the stoichiometric Nb4N5 in the thin films. The surface plasmon resonance absorption peak occurred a red shift and became widen, and the band gap of niobium nitride thin films became narrow with the reduction nitridation temperature increasing. Meanwhile, the films surface became rough, which could provide a lot of “hot spots”. As a result, the niobium nitride films have the surface-enhanced Raman spectroscopy activity via electromagnetic field enhancement and charge transfer mechanism. The niobium nitride films obtained at 800 °C show the optimal Raman enhancement performance, for which detection limit of probe molecule R6G is 10−7 M, and the enhancement factor is 4 × 103. In addition, the niobium nitride films possess good uniformity, long-term stability, and high temperature resistance.

A multifunctional nanozyme-based enhanced system for tert-butyl hydroquinone assay by surface-enhanced Raman scattering.

An Au-based nanozyme composite (AuNPs/Cu,I) was constructed by using Cu,I-doped carbon dots (Cu,I-CDs) as the reducing agent as well as the nanozyme. Notably, AuNPs/Cu,I nanozyme not only possessed the intrinsic activity of mimicking enzymes of superoxide dismutase, peroxidase, and catalase at different conditions but was also employed as surface-enhanced Raman spectroscopy (SERS) enhancer. The combination of Cu,I-CDs and AuNPs promoted the electron transferability, leading to increased peroxidase-like activity and superoxide-like activity. Compared to the individual Cu,I-CDs and AuNPs nanozyme, the AuNPs/Cu,I composite demonstrated promising peroxidase-like activity by transferring electrons instead of generating OH. Interestingly, the multienzyme-like activity of AuNPs/Cu,I nanozyme could be finely tuned by changing the composition of Cu0/Cu+ and Au. Thetert-butyl hydroquinone (TBHQ) as the substrate could be catalyzed with AuNPs/Cu,I nanozyme to produce red substances, resulting in a significant Raman enhancement effect at the same time, showing good linear range from 0.11 to 10mg L-1. Overall, the current investigation provides a flexible and controllable way to design multifunctional nanozymes along with the Raman enhancement strategy based on the catalysis of nanozyme.

Influence of two-dimensional molybdenum disulfide on the surface enhanced Raman scattering effect in a silver nanocavity

Surface-enhanced Raman scattering (SERS) is commonly associated with metal nanostructures. However, recently large Raman enhancement has also been reported for two-dimensional (2D) semiconductors. In the present work, we exam systematically the influence of thin 2D molybdenum disulfide (MoS2) films on the SERS properties of a typical silver (Ag) nanocavity formed by a single Ag nanoparticle (NP) and a flat Ag surface. The dependence of the localized electric field, Raman enhancement and spatial resolution on the NP-MoS2 film distance, the thickness of MoS2 film, as well as the NP radius were quantitatively calculated and analyzed. The results show that the presence of MoS2 film could efficiently enhance the plasmonic coupling between the NP and the substrate, leading to more significant SERS enhancement. Thinner MoS2 film and shorter NP-film distance will further improve the localized plasmonic enhancement. The calculated Raman enhancement factor reaches 9.1 ×1010 with monolayer MoS2 film under the 660-nm excitation. We also show that the spatial resolution for Raman spectroscopy could be significantly improved by reducing the NP radius, and the maximum resolutions is 4.5 nm at the radius size of 20 nm. Our results offer a theoretical guidance for better understanding the SERS properties of the novel 2D material and its further applications.

Excited state charge transfer promoted Raman enhancement of copper phthalocyanine by twisted bilayer graphenes

Few atom thick, twisted bilayer graphene (tBLG) possesses a rotation angle (θ) dependent van Hove singularity (vHs). Fine-tuning vHs serves a potential method to enhance charge transfer (CT) in surface enhanced Raman spectroscopy. This study shows that tBLG having a specific θ promotes as high as a 1.7 times enhancement of the Raman signals of copper phthalocyanine (CuPc) as compared to that caused by single layer graphene (SLG). The results of a combination of reflection imaging spectroscopy and widefield Raman provide spatial and spectral information about both tBLG with θ ranging from 10.9 to 13.7° and the corresponding vHs. Comparison of Raman spectra of CuPc in presence and absence of tBLG demonstrates that a significant enhancement of certain CuPc vibrational modes occurs when the underlying tBLG possesses a θ = 12.2°, showing as high as 6.8 and 1.7 times enhancements of certain vibrational mode as compared to those of CuPc on bare and SLG substrates, respectively. Theoretical calculations indicate that a match between the energies of vHs of tBLG with those of frontier orbitals of CuPc facilitates CT from the distant SLG to CuPc.

Effects of chloride ions on flower-like Ag-coated ZnO nanorod arrays and the Raman enhancement

Chloride solutions are used to modify flower-like Ag-coated ZnO nanorod (Ag-ZnO-NR) arrays, leading to the formation of Ag-nanosphere decorated ZnO-NR arrays, which have a morphology with each sphere-like Ag nanoparticle independently lying at the top of ZnO-NR and thus can be used as a new kind of substrates for the study of surface-enhanced Raman spectroscopy (SERS). The chloride modification is dominated by Ag abnormal migration on ZnO NRs in the environment of chloride solution. The concentration of chloride ions for effective modification is higher than ∼1 × 10−4 M and the final morphology is nearly steady as the soaking time is longer than 10 min. On the modified SERS substrates, the thermalization of probing molecules during Raman measurement is substantially depressed. Using the SERS samples prepared by loading the probing molecules in the presence of chloride ions, the Raman signals are obviously enhanced with an extremely high signal-to-noise ratio. The Raman enhancement is evidenced to be due to the enhanced adsorption of probing molecules on Ag surface in the presence of chloride ions.

Silver Nanoparticle-Decorated Silica Nanospheres and Arrays as Potential Substrates for Surface-Enhanced Raman Scattering.

Poly(vinylpyrrolidone) (PVP) was used as both a modifier and reductant to in situ deposit silver nanoparticles (denoted Ag NPs) on the surface of silica nanospheres (nanosilica or nano-SiO2), affording Ag-decorated nanosilica (denoted SiO2@Ag). The as-obtained SiO2@Ag composite can form silver nanoparticle-decorated silica nanosphere arrays (denoted SiO2@Ag arrays) via evaporation-induced self-assembly. The as-prepared SiO2@Ag composite and SiO2@Ag array were used as the SERS substrates to measure the Raman signals of the dilute solutions of rhodamine 6G (denoted R6G), an organic dye that is a potential pollutant to the environment. The findings indicate that the as-prepared SiO2@Ag composite and SiO2@Ag array as potential SERS substrates simultaneously exhibit a high degree of metal coverage and small size of Ag NPs as well as good stability and abundant “hot spots”, which contributes to their desired Raman enhancement capacities. For the detection of trace R6G, they provide a limit of detection of as low as 10–9–10–11 M as well as good reproducibility, showing promising potential for monitoring chemical and biological molecules.

Multimodal Gold Nanostars as SERS Tags for Optically-Driven Doxorubicin Release Study in Cancer Cells.

Surface Enhanced Raman Scattering (SERS) active gold nanostars represent an opportunity in the field of bioimaging and drug delivery. The combination of gold surface chemical versatility with the possibility to tune the optical properties changing the nanoparticles shape constitutes a multimodal approach for the investigation of the behavior of these carriers inside living cells. In this work, SERS active star-shaped nanoparticles were functionalized with doxorubicin molecules and covered with immuno-mimetic thiolated polyethylene glycol (PEG). Doxorubicin-conjugate gold nanoparticles show an intense Raman enhancement, a good stability in physiological conditions, and a low cytotoxicity. The strong adsorption of the anticancer drug doxorubicin in close contact with the gold nanostars surface enables their use as SERS tag imaging probes in vivo. Upon laser irradiation of the nanoparticles, a strong SERS signal is generated by the doxorubicin molecules close to the nanostars surface, enabling the localization of the nanoparticles inside the cells. After long time irradiation, the SERS signal drops, indicating the thermally driven delivery of the drug inside the cell. Therefore, the combination of SERS and laser scanning confocal microscopy is a powerful technique for the real-time analysis of drug release in living cells.

Silver Flowerlike Structures for Surface-Enhanced Raman Spectroscopy.

Micro- and nanoflowers are a class of materials composed of particles with high surface-to-volume ratio. They have been extensively studied in the last decade due to simple preparation protocols and promising applications in biosensing, as drug delivery agents, for water purification, and so on. Flowerlike objects, due to their highly irregular surface, may act also as plasmonic materials, providing resonant coupling between optical waves and surface plasmon excitations. This fact allows us to infer the possibility to use micro- and nanoflowers as effective surface-enhanced Raman scattering (SERS) substrate materials. Here, we report on the design and Raman enhancement properties of silver flowerlike structures, deposited on aluminum surface. A simple and cost-effective fabrication method is described, which leads to SERS substrates of high developed surface area. The morphology of the silver flowers on a nanoscale is characterized by self-organized quasiperiodic stacks of nanosheets, which act as plasmonic cavity resonators. The substrates were tested against rhodamine-6G (R6G) water solutions of concentration varying between 10−3 M and 10−7 M. Optimal SERS enhancement factors of up to 105 were established at R6G concentrations in the 10−6–10−7 M range.

Shell thickness-controlled synthesis of Au@Ag core–shell nanorods structure for contaminants sensing by SERS

The accessibility of contaminants detection methods is urgently required for environmental and food safety control. In this report, we developed the Au@Ag core–shell nanorod structures for contaminants sensing by surface-enhanced Raman spectroscopy (SERS). The silver shell thickness and the corresponding plasmon wavelength of Au@Ag core–shell nanorods were tuned by changing the coating time and the silver precursor amount. Moreover, these structures exhibit ultra-sensitive detection ability for Nile blue A dye and Fenobucarb pesticide sensing by SERS. Interestingly, the highest Raman enhancement factor is obtained for the Au@Ag core–shell sample with a minimal silver shell thickness leaded by the optimal enhancement of the electromagnetic field of bimetallic structures. Hence, our report demonstrates that the combination of unique features of two plasmonic metals into core–shell structures promises potential applicability in SERS-based analysis.

Hierarchical Magnetic Films for High-Performance Plasmonic Sensors

Hierarchically structured films comprise a growing section of the field of surface-enhanced Raman spectroscopy (SERS). Here, we report a novel, powerfully enhancing hierarchical plasmonic substrate featuring patterned multilayers of magnetic iron oxide nanospheres using an external magnetic field to create sets of radial ridges. This new substrate allows for effective analyte adsorption and significant Raman signal enhancement, thanks to the contribution of both the magnetic and plasmonic components to the electromagnetic hotspots. We demonstrate significant and reliable Raman enhancement for polycyclic aromatic hydrocarbons (PAHs), dilute but persistent environmental pollutants, in a complex and real-world matrix of produced water (PW). The substrate activity for PAHs is validated by gas chromatography-mass spectrometry analysis. An impressive signal-to-noise ratio (SNR) of several dB enables detection of the analyte below 1 ppm. This multilayer magnetic film sensor substrate shows remarkable stability and robustness suitable for real-world applications while boasting simple methods and strong potential to scale up fabrication.

Integrated Molecular Optomechanics with Hybrid Dielectric-Metallic Resonators.

Molecular optomechanics describes surface-enhanced Raman scattering using the formalism of cavity optomechanics as a parametric coupling of the molecule’s vibrational modes to the plasmonic resonance. Most of the predicted applications require intense electric field hotspots but spectrally narrow resonances, out of reach of standard plasmonic resonances. The Fano lineshapes resulting from the hybridization of dielectric–plasmonic resonators with a broad-band plasmon and narrow-band cavity mode allow reaching strong Raman enhancement with high-Q resonances, paving the way for sideband resolved molecular optomechanics. We extend the molecular optomechanics formalism to describe hybrid dielectric–plasmonic resonators with multiple optical resonances and with both free-space and waveguide addressing. We demonstrate how the Raman enhancement depends on the complex response functions of the hybrid system, and we retrieve the expression of Raman enhancement as a product of pump enhancement and the local density of states. The model allows prediction of the Raman emission ratio into different output ports and enables demonstrating a fully integrated high-Q Raman resonator exploiting multiple cavity modes coupled to the same waveguide.

Large-area nanoengineering of graphene corrugations for visible-frequency graphene plasmons.

Quantum confinement of the charge carriers of graphene is an effective way to engineer its properties. This is commonly realized through physical edges that are associated with the deterioration of mobility and strong suppression of plasmon resonances. Here, we demonstrate a simple, large-area, edge-free nanostructuring technique, based on amplifying random nanoscale structural corrugations to a level where they efficiently confine charge carriers, without inducing significant inter-valley scattering. This soft confinement allows the low-loss lateral ultra-confinement of graphene plasmons, scaling up their resonance frequency from the native terahertz to the commercially relevant visible range. Visible graphene plasmons localized into nanocorrugations mediate much stronger light-matter interactions (Raman enhancement) than previously achieved with graphene, enabling the detection of specific molecules from femtomolar solutions or ambient air. Moreover, nanocorrugated graphene sheets also support propagating visible plasmon modes, as revealed by scanning near-field optical microscopy observation of their interference patterns.

Vibrational Fingerprint Analysis of an Azo-based Resonance Raman Scattering Probe for Imaging Proton Distribution in Cellular Lysosomes.

Due to the fundamental mechanism of vibrational state transitions for chemical bonds, the spectra of Raman scattering are narrow-banded and photostable signals capable of probing specific reactions. In the case of protonation/deprotonation reactions, certain chemical bonds are broken and new bonds are formed. Based on the changes of the vibrational modes for the corresponding bonds, fingerprint analysis of multiple Raman bands may allow for the in situ visualization of proton distribution in live cells. However, Raman scattering faces the well-known challenge of low sensitivity. To perform the vibrational fingerprint analysis of Raman scattering by overcoming this challenge, we developed an azo-based resonance Raman pH probe. It was an azobenzene-featured small molecule responsive to protons with the inherent Raman signal ∼104-fold more intense than that of the conventional alkyne-type Raman reporter 5-ethynyl-2'-deoxyuridine. Through the substitution of the electron-donating and -withdrawing entities to the azobenzene group, the effect of resonance Raman scattering and fluorescence quenching was obtained. This effect resulted in a significant Raman enhancement factor of ∼103 compared to the counterpart molecules without the molecular design. Based on the enhanced Raman sensitivity of the azo-based resonance Raman pH probe, the identification of vibrational fingerprint changes at the azo group was achieved during the protonation/deprotonation reactions, and the vibrational fingerprint analysis resolved a pH difference of less than 0.2 unit. The method enabled sensitive hyperspectral cell imaging that clearly visualized the change of proton distribution in autophagic cells.

Porous Au-Ag Nanoparticles from Galvanic Replacement Applied as Single-Particle SERS Probe for Quantitative Monitoring.

Plasmonic nanostructures have raised the interest of biomedical applications of surface-enhanced Raman scattering (SERS). To improve the enhancement and produce sensitive SERS probes, porous Au-Ag alloy nanoparticles (NPs) are synthesized by dealloying Au-Ag alloy NP-precursors with Au or Ag core in aqueous colloidal environment through galvanic replacement reaction. The novel designed core-shell Au-Ag alloy NP-precursors facilitate controllable synthesis of porous nanostructure, and dealloying degree during the reaction has significant effect on structural and spectral properties of dealloyed porous NPs. Narrow-dispersed dealloyed NPs are obtained using NPs of Au/Ag ratio from 10/90 to 40/60 with Au and Ag core to produce solid core@porous shell and porous nanoshells, having rough surface, hollowness, and porosity around 30-60%. The clean nanostructure from colloidal synthesis exhibits a redshifted plasmon peak up to near-infrared region, and the large accessible surface induces highly localized surface plasmon resonance and generates robust SERS activity. Thus, the porous NPs produce intensely enhanced Raman signal up to 68-fold higher than 100nm AuNP enhancement at single-particle level, and the estimated Raman enhancement around 7800, showing the potential for highly sensitive SERS probes. The single-particle SERS probes are effectively demonstrated in quantitative monitoring of anticancer drug Doxorubicin release.

Structural engineering of transition-metal nitrides for surface-enhanced Raman scattering chips

Noble-metal-free surface-enhanced Raman scattering (SERS) substrates have attracted great attention for their abundant sources, good signal uniformity, superior biocompatibility, and high chemical stability. However, the lack of controllable synthesis and fabrication of noble-metal-free substrates with high SERS activity impedes their practical applications. Herein,we propose a general strategy to fabricate a series of planar transition-metal nitride (TMN) SERS chips via an ambient temperature sputtering deposition route.These planar TMN (tungsten nitride, tantalum nitride, and molybdenum nitride) chips show remarkable Raman enhancement factors (EFs) with ~105 owing to efficient photoinduced charge transfer process between TMN chips and probe molecules. Further, structural engineering of these TMN chips is used to improve their SERS activity. Benefiting from the synergistic effect of charge transfer process and electric field enhancement by constructing nanocavity structure, the Raman EF of WN nanocavity chips could be greatly improved to 1.29 * 107, which is an order of magnitude higher than that of planar chips. Moreover, we also design the WN/monolayer MoS2 heterostructure chips. With the increase of surface electron density on the upper WN and more exciton resonance transitions in the heterostructure, a 1.94 * 107 level EF and a 5 * 10-10 m level detention limit could be achieved. Our results provide important guidance for the structural design of ultrasensitive noble-metal-free SERS chips.

The performance of surface enhanced Raman scattering and spatial resolution with triangular plate dimer from ultra-ultraviolet to near-infrared range

The theoretical research on surface enhanced Raman spectroscopy (SERS) of triangular plate dimer (TPD) is of great significance for the design of experimental substrates. In this paper, the SERS properties of the TPD with Au, Ag, Al and Cu have been theoretical investigated in the ultra-ultraviolet, visible and near-infrared region. The influence of the TPD configuration, including the tip radian, the dimer distance and the aspect ratio on the electric field, Raman enhancement and spatial resolution are studied by the finite element method. The results show that there are dipole resonance band and quadruple dipole resonance band in the surface plasmon resonance band of TPD. The tip radian and dimer distance play the dominant role in the electric field enhancement, and the aspect ratio can be mainly used to tune the peak position of the electric field. The smaller tip radian and dimer distance will produce a stronger localized electric field and a small red shift of the peak position. Adjusting the aspect ratio can tune the position of electric field peak from ultraviolet (UV) to near-infrared without changing the peak value of the electric field significantly, especially for Al TPD. The maximum Raman enhancement factor of Au, Ag and Cu all reach 11 orders of magnitude, and 9 orders of magnitude for Al. The spatial resolution changes linearly with the gap distance, and the maximum spatial distributions of Au, Ag, Al and Cu achieve 0.65 nm, 0.67 nm, 0.69 nm and 0.70 nm with the dimer distance of 1 nm. Our results not only provide a better theoretical guidance for the optimization of TPD substrates in the SERS experiment, but also extend its application scope from ultra-UV to near-infrared range.

Shape-diversified silver nanostructures on Al foil fabricated in micellar template for high performance surface enhanced Raman scattering applications

Highly effective surface-enhanced Raman substrates with shape-diversified silver nanostructures on Al foil (AlF) were fabricated by galvanic replacement reaction in micellar template which is easy to control, simple and cost-effective. Due to the localized surface plasmon resonance (LSPR) of silver nanostructures, the substrates were used as surface enhanced Raman substrates. The shape-diversified silver nanostructures on AlF exhibited stronger surface enhanced Raman scattering (SERS) effects compared with flowerlike Ag nanostructures on ITO glass substrate fabricated by electrodeposition in micellar template. The maximum Raman enhancement factor of 109 fold was obtained, which is 236 times larger than the flowerlike silver nanostructures on ITO glass substrate. The scanning electron microscope (SEM) images show that the size and shape of the nanostructures and surface morphology make a valuable contribution to SERS. Raman enhancement experiments of three Raman probes were studied, which shows that different vibrational modes of Raman probe affect SERS effect. The fabricated shape-diversified silver nanostructures on AlF fabricated by galvanic replacement reaction in micellar template are cheap, controllable and simple. It could be potentially useful in SERS field such as imaging applications and highly sensitive molecule detecting.

Metal-free SERS: Where we are now, where we are heading

In the recent years, the emergence of metal-free surface-enhanced Raman scattering (SERS) substrates has provided promising alternatives for reliable, high-sensitivity Raman spectroscopy due to their potential to realize highly tuneable, biocompatible, and reproducible Raman enhancement. In this perspective article, we discuss recent developments and anticipate new opportunities for metal-free SERS. Specifically, we review the theory of metal-free SERS, including electromagnetic enhancement and chemical enhancement, and introduce promising substrates developed recently, including MXenes, transition metal dichalcogenides, and carbon-based substrates. Moreover, we clarify challenges, provide potential solutions, and discuss unexploited applications. This perspective provides not only a broad and detailed view of metal-free SERS at the current stage, but also a roadmap for its future advancement.