Surface-enhanced Raman Scattering Technique Research Articles

Recent Advances in Metallic Nanoparticle Assemblies for Surface-Enhanced Spectroscopy.

Robust and versatile strategies for the development of functional nanostructured materials often focus on assemblies of metallic nanoparticles. Research interest in such assemblies arises due to their potential applications in the fields of photonics and sensing. Metallic nanoparticles have received considerable recent attention due to their connection to the widely studied phenomenon of localized surface plasmon resonance. For instance, plasmonic hot spots can be observed within their assemblies. A useful form of spectroscopy is based on surface-enhanced Raman scattering (SERS). This phenomenon is a commonly used in sensing techniques, and it works using the principle that scattered inelastic light can be greatly enhanced at a surface. However, further research is required to enable improvements to the SERS techniques. For example, one question that remains open is how to design uniform, highly reproducible, and efficiently enhancing substrates of metallic nanoparticles with high structural precision. In this review, a general overview on nanoparticle functionalization and the impact on nanoparticle assembly is provided, alongside an examination of their applications in surface-enhanced Raman spectroscopy.

In situ SERS monitoring of plasmon-driven catalytic reaction on gap-controlled Ag nanoparticle arrays under 785 nm irradiation

Plasmon-enhanced photocatalysis has attracted considerable attention due to its low energy consumption and high energy throughput. Surface-enhanced Raman scattering (SERS) is a highly sensitive and label-free nondestructive tool to investigate plasmon-driven photocatalytic reactions. Herein, we present a facile method to fabricate gap-controlled Ag nanoparticle (NP) arrays with uniform and high-density distribution of hot spots, which can be employed as both efficient plasmonic photocatalysts and stable SERS platforms. The plasmon-driven catalytic reaction of 4-nitrobenzenethiol (4NBT), which transforms it into p, p′-dimercaptoazobenzene (DMAB), is detected by using an in situ SERS technique at the excited wavelength of 785 nm. According to the temperature and laser power density dependent photocatalytic reaction rates observed on the Ag NP arrays, we quantitatively determined that the reductive coupling of 4NBT is more likely to occur as the gap decreases. The finite-difference time-domain (FDTD) simulation results demonstrate that the plasmonic hot spots are significantly enhanced with a decrease in gap, which in turn reduces activation energy. The gap-controlled Ag NP arrays are efficient for both promotion and detection of plasmon-driven catalytic reactions, and may pave a pathway for implementing efficient plasmonic photocatalytic platforms.

Modified Ag nanoparticles on the regular array structure to improve the optical properties

The surface-enhanced Raman scattering (SERS) technique strongly depended on the excellent optical properties of the substrate. However, simply roughing of the metal surfaces to support SERS techniques suffer from the lack of uniform, and reproducibility to apply the SERS substrate. In this paper, we report that Ag nanoparticles are modified after sputtering Ag film on the cicada wing (CW) to preparate the SERS substrate for AgNPs/Ag-20/CW. Compared with Ag-20/CW, the AgNPs/Ag-20/CW has better polarization independence, and the RSD value is 6.1% which represents the excellent uniform of the substrate. The optical properties of SERS substrate are improved by modifying a large number of dense and uniform Ag nanoparticles on the Ag@CW.

Optically active plasmonic cellulose fibers based on Au nanorods for SERS applications

Cellulose might be a promising material for surface-enhanced Raman scattering (SERS) substrates due to its wide availability, low cost, ease of fabrication, high flexibility and low optical activity. This work shows, for the first time development of the cellulose-based substrate, that owes its SERS activity to the presence of gold nanorods in its internal structure, and not only on the surface, as it is shown elsewhere, thus ensuring superior stability of the obtained material. This flexible cellulose-based substrate exhibiting plasmonic activity, provide easy and reproducible detection of different analytes via SERS technique. The substrate was prepared by introduction of gold nanorods into the cellulose fibers matrix using an eco-friendly process based on N-Methylmorpholine-N-Oxide. Au-modified cellulose fibers were used for the detection of p-Mercaptobenzoic acid and Bovine Serum Albumin by the SERS method. The obtained results show that this substrate offers large signal enhancement of 6-orders of magnitude, and high signal reproducibility with a relative standard deviation of 8.3%. Additionally, washing tests (90 °C, 20 h) showed superior stability of the as prepared plasmonic fibers, thus proving the good reusability of the substrates and the long shelf life.

Single Molecule Surface Enhanced Raman Scattering in a Single Gold Nanoparticle-Driven Thermoplasmonic Tweezer.

Surface enhanced Raman scattering (SERS) is optically sensitive and chemically specific to detect single-molecule spectroscopic signatures. Facilitating this capability in optically trapped nanoparticles at low laser power remains a significant challenge. In this letter, we show single molecule SERS signatures in reversible assemblies of trapped plasmonic nanoparticles using a single laser excitation (633 nm). Importantly, this trap is facilitated by the thermoplasmonic field of a single gold nanoparticle dropcasted on a glass surface. We employ the bianalyte SERS technique to ascertain the single molecule statistical signatures and identify the critical parameters of the thermoplasmonic tweezer that provide this sensitivity. Furthermore, we show the utility of this low power (≈ 0.1 mW/μm2) tweezer platform to trap a single gold nanoparticle and transport assembly of nanoparticles. Given that our configuration is based on a dropcasted gold nanoparticle, we envisage its utility to create reconfigurable plasmonic metafluids in physiological and catalytic environments and to be potentially adapted as an in vivo plasmonic tweezer.

Label-free Au NRs-based SERS coupled with chemometrics for rapid quantitative detection of thiabendazole residues in citrus

Citrus is a highly consumed fruit worldwide. However, the excessive use of thiabendazole (TBZ) pesticides during citrus cultivation poses a health risk to people. Hence, a rapid and quantitative method has been established for TBZ determination in citrus by coupling gold nanorods (Au NRs) based surface enhanced Raman scattering (SERS) coupling chemometrics. The results show that support vector machine (SVM) can distinguish TBZ residues of different orders of magnitude with 99.1667% accuracy and that genetic algorithm-partial least squares (GA-PLS) had the best performance in the quantitative prediction of TBZ residues (Rp2 = 0.9737, RMSEP = 0.1179 and RPD = 5.85) in citrus. The limit of detection (LOD) was 0.33 μg mL−1. Furthermore, the proposed method was validated by a standard HPLC method using t-test with no significant difference. Therefore, the proposed Au NRs-based SERS technique can be used for the rapid quantitative analysis of TBZ residues in citrus.

Metal-organic framework engineered corn-like SERS active Ag@Carbon with controllable spacing distance for tracking trace amount of organic compounds

Probing water-soluble organic compounds via Surface-enhanced Raman scattering (SERS) technique could be helpful to prevent harmful impacts of polluted water. A key limitation of restraining SERS technique in probing these pollutants is the difficulty to control the spacing distance of plasmonic nanoparticles within 10 nm so that SERS effect can be efficiently induced. Herein, a strategy of mass-producing Ag-based SERS active material with tunable spacing distance is reported. In brevity, metal-organic framework (MOF) engineered corn-like Ag@Carbon is synthesized by simply thermal treating Ag-MOF. The thermal treatment in-situ turns Ag+ into Ag nanoparticles (NPs), resulting in Ag NPs well-dispersed on the surface of the carbonized MOF and forming ordered SERS hotspots. Due to the spatial distance of Ag+ directly depends on the molecular diameter of MOF organic ligands, spacing distance of Ag NP is fixed at around 7 nm. Theoretical analysis and experimental study confirm that the uniformly distributed Ag NPs lead to desirable SERS activity. Further study evidences the presented corn-like Ag@Carbon could be a good candidate for tacking organic compounds with satisfactory sensitivity, specificity and low detection limit (10−8 M). Conclusively, these impressive results indicate a bright future of adopting the proposed strategy to design future SERS active materials.

Rapid identification of the resistance of urinary tract pathogenic bacteria using deep learning-based spectroscopic analysis.

The resistance of urinary tract pathogenic bacteria to various antibiotics is increasing, which requires the rapid detection of infectious pathogens for accurate and timely antibiotic treatment. Here, we propose a rapid diagnosis strategy for the antibiotic resistance of bacteria in urinary tract infections (UTIs) based on surface-enhanced Raman scattering (SERS) using a positively charged gold nanoparticle planar solid SERS substrate. Then, an intelligent identification model for SERS spectra based on the deep learning technique is constructed to realize the rapid, ultrasensitive, and non-labeled detection of pathogenic bacteria. A total of 54,000 SERS spectra were collected from 18 isolates belonging to 6 species of common UTI bacteria in this work to realize identification of bacterial species, antibiotic sensitivity, and multidrug resistance (MDR) via convolutional neural networks (CNN). This method significantly simplify the Raman data processing processes without background removing and smoothing, however, achieving 96% above classification accuracy, which was significantly greater than the 85% accuracy of the traditional multivariate statistical analysis algorithm principal component analysis combined with the K-nearest neighbor (PCA-KNN). This work clearly elucidated the potential of combining SERS and deep learning technique to realize culture-free identification of pathogenic bacteria and their associated antibiotic sensitivity.

Synergistic function of Au NPs/GeO2 nanozymes with enhanced peroxidase-like activity and SERS effect to detect choline iodide

A novel Au NPs/GeO2 nanozymes are developed as Surface-Enhanced Raman Scattering (SERS) substrates with the promising prospect for detection ChI. Herein, it is discovered that both Au NPs and GeO2 nanozymes have peroxidase-like activity, catalyzing colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to produce blue TMBox. Interestingly, compared with single Au NPs or GeO2 nanozymes, the Au NPs/GeO2 nanozymes show stronger peroxidase-like activity, and significantly ameliorated SERS signal of TMBox. The mentioned two enhancements are ascribed to a positive synergistic function of Au NPs/GeO2 nanozymes. Surprisingly, choline iodide (ChI) can inhibit the positive synergy in Au NPs/GeO2 nanozymes, and slow down the reaction of TMB-H2O2-Au NPs/GeO2 system. On this foundation, a new Au NPs/GeO2 SERS technique with high sensitivity, label-free detection method of choline iodide (ChI) is established, suggesting that Au NPs/GeO2 nanozymes have the potential application of water environment.

Synthesis and characterization of reporter molecules embedded core-shell nanoparticles as SERS nanotags

Surface-enhanced Raman scattering (SERS) spectroscopy is presented as a sensitive and specific molecular tool for clinical diagnosis and prognosis monitoring of various diseases including cancer. In order for clinical application of SERS technique, an ideal method of bulk synthesis of SERS nanoparticles is necessary to obtain sensitive, stable and highly reproducible Raman signals. In this contribution, we determined the ideal conditions for bulk synthesis of Raman reporter (Ra) molecules embedded silver-gold core-shell nanoparticles (Au@Ra@ AgNPs) using hydroquinone as reducing agent of silver nitrate. By using UV-Vis spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM), we found that a 2:1 ratio of silver nitrate to hydroquinone is ideal for a uniform silver coating with a strong and stable Raman signal. Through stability testing of the optimized Au@Ra@AgNPs over a two-week period, these SERS nanotags were found to be stable with minimal signal change occurred. The stability of antibody linked SERS nanotags is also crucial for cancer and disease diagnosis, thus, we further conjugated the as-prepared SERS nanotags with anti-EpCAM antibody, in which the stability of bioconjugated SERS nanotags was tested over eight days. Both UV-Vis and SERS spectroscopy showed stable absorption and Raman signals on the anti-EpCAM conjugated SERS nanotags, indicating the great potential of the synthesized SERS nanotags for future applications which require large, reproducible and uniform quantities in order for cancer biomarker diagnosis and monitoring.

Investigation of factors that impact the label-free surface-enhanced Raman scattering (SERS) for the detection and discrimination of Salmonella Enteritidis

A simple and rapid label-free surface-enhanced Raman scattering (SERS) method involving silver nanoparticles, was developed to detect and discriminate Salmonella Enteritidis (S. Enteritidis) not only at a specie (Escherichia coli O157: H7; E. coli O157: H7), or a serovar level (Salmonella Gaminara; S. Gaminara) but also between the viable and non-viable state of bacteria. Parameters that influenced the bacterial spectra were optimized, such as particle size of silver nanosphere and type of silver nanosphere's coating agent. The signal intensity of the spectra of bacteria was dependent on the particle size of the silver nanospheres. Whereas the patterns of different bacterial spectra varied based upon the silver nanosphere's coating agent. In this study, citrate-silver nanosphere (C-AgNPs) successfully discriminated the spectral data of S. Enteritidis from those of E. coli O157: H7, S. Gaminara, and non-viable S. Enteritidis when analyzed using principal component analysis (PCA) and linear discriminant analysis (LDA) of full-spectrum contribution. The method allowed the detection of S. Enteritidis to the concentration of 4 log CFU/mL. These results indicated the efficacy of the developed label-free SERS technique for the rapid detection and discrimination of S. Enteritidis from other closely related bacteria.

Clinical computed tomography and surface-enhanced Raman scattering characterisation of ancient pigments

<p class="Abstract">A systematic and complete chemical and physical characterisation of painted pigments on wood samples was carried out using multislice X-ray computed tomography (MSCT) and surface-enhanced Raman scattering (SERS) techniques. Inorganic and organic pigments present on the wooden tablets were differentiated by MSCT determinations of Hounsfield units, a semi-quantitative method for measuring X-ray attenuation that, in turn, offers an indirect estimation of a material’s density. However, the MSCT technique is not as reliable as traditional spectroscopic techniques for recognising and classifying organic pigments. Nevertheless, a strength of the MSCT approach was its ability to simultaneously provide a volumetric view of the wood and segment the layers of the specimen using suitable reconstruction methods such as is generally done for biomedical applications. Furthermore, the SERS technique made it possible to identify the type of material present in the pigments (for both inorganic or organic materials) with a high spatial resolution, even pigments in mixtures or those applied directly on the investigated wooden support. The combined MSCT and SERS data obtained through this systematic investigation constitutes the basis for the assembly of larger reference databases that will ultimately support the development of long-term conservation protocols.</p>

A ultrasensitive SERS-active tags for GSH-triggered released based on surface-enhanced Raman scattering

Ischemic stroke is the main type of stroke, causing a large number of patients dead and disabled every year. Tribuloside (TBS) is a kind of natural saponin, which can improve cerebral blood flow and protect brain tissue. In the present study, we have successfully developed gold nanoparticle-modified TBS, which increase the aqueous solubility of TBS and named the SERS tags. Based on the label surface enhanced Raman scattering (SERS) technique, it enables the release of TBS from the nanoparticles’ surface induced by GSH can be monitored and realize real-time detection. SERS technique provided a nanomolar range release of TBS from the gold nanoparticle's surfaces after the injection of external GSH. In vivo SERS spectra of TBS was obtained from the brain of living mice after GSH treatment. The results showed that the nano-tribuloside could effectively reduce apoptosis and brain tissue damage in the neuronal cell ischemia-hypoxia injury model and the mouse focal cerebral infarction model induced by photochemical method. This study provides a new insight for ischemia treat, and provides a theoretical and experimental basis for clinical practice.

Recent advances in surface-enhanced Raman scattering technique for pollutant detection in Chinese medicinal material

Chinese medicinal material is the foundation of traditional Chinese medicine(TCM) industry. Its quality is not only closely related to the health of residents but also the key to the development of the TCM industry. Pesticide residues, heavy metals and mycotoxins are the major pollutants of Chinese medicinal materials. In recent years, quite a number of rapid detection methods for pollutants have been constructed. Among them, surface-enhanced Raman scattering(SERS), which has been widely used in food chemistry, environmental analysis, and other fields because of its speediness and non-destructiveness, shows its great potential in the pollutant detection in Chinese medicinal material. This paper firstly reviews the application of SERS for the detection of common pollutants in Chinese medicinal material. We then discussed the characteristics and advantages of SERS technique for pesticide detection, including the principle, SERS substrate design, specific recognition, etc. Finally, simultaneous detection of multiple pesticide residues in Chinese medicinal material was explored.

Biomarkers Determination Based on Surface-Enhanced Raman Scattering

An overview of noteworthy new methods of biomarker determination based on surface-enhanced Raman scattering (SERS) is presented. Biomarkers can be used to identify the occurrence and development of diseases, which furthers the understanding of biological processes in the body. Accurate detection of a disease-specific biomarker is helpful for the identification, early diagnosis and prevention of a disease and for monitoring during treatment. The search for and discovery of valuable biomarkers have become important research hotspots. Different diseases have different biomarkers, some of which are involved in metabolic processes. Therefore, the fingerprint characteristics and band intensities in SERS spectra have been used to identify metabolites and analyze markers. As a promising technique, SERS has been widely used for the quantitative and qualitative determination of different types of biomarkers for different diseases. SERS techniques provide new technologies for the diagnosis of disease-related markers and determining the basis for clinical treatment. Herein, several SERS-based methods with excellent sensitivity and selectivity for the determination of biomarkers for tumors, viruses, Alzheimer’s disease, cardiac muscle tissue injury, and cell activity are highlighted.

Flexible Surface-Enhanced Raman Scattering Chip: A Universal Platform for Real-Time Interfacial Molecular Analysis with Femtomolar Sensitivity.

We propose and demonstrate a flexible surface-enhanced Raman scattering (SERS) chip as a versatile platform for femtomolar detection and real-time interfacial molecule analysis. The flexible SERS chip is composed of a flexible and transparent membrane and embedded plasmonic dimers with ultrahigh particle density and ultrasmall dimer gap. The chip enables rapid identification for residuals on solid substrates with irregular surfaces or dissolved analytes in aqueous solution. The sensitivity for liquid-state measurement is down to 0.06 molecule per dimers for 10-14 mol·L-1 Rhodamine 6G molecule without molecule enrichment. Strong signal fluctuation and blinking are observed at this concentration, indicating that the detection limit is close to the single-molecule level. Meanwhile, the homogeneous liquid environment facilities accurate SERS quantification of analytes with a wide dynamic range. The synergy of flexibility and liquid-state measurement opens up avenues for the real-time study of chemical reactions. The reduction from p-nitrothiophenol (PNTP) to p-aminothiophenol (PATP) in the absence of the chemical reducing agents is observed at liquid interfaces by in situ SERS measurements, and the plasmon-induced hot electron is demonstrated to drive the catalytic reaction. We believe this robust and feasible approach is promising in extending the SERS technique as a general method for identifying interfacial molecular traces, tracking the evolution of heterogeneous reactions, elucidating the reaction mechanisms, and evaluating the environmental effects such as pH value and salty ions in SERS.

Immunoassay of Tumor Markers Based on Graphene Surface-Enhanced Raman Spectroscopy.

With a unique chemical enhancement capability, graphene oxide is exceptionally suitable to serve as an alternative surface-enhanced Raman scattering (SERS) substrate, which is usually defined as GERS. However, such a GERS matrix with both promising uniformity and molecule enrichment ability has not been applied in the quantitative detection of tumor markers. Herein, an ultrasensitive and specific immunoassay of carcinoembryonic antigens mediated by the GERS matrix was demonstrated. With the assistance of Au NRs as immunoprobes, a reliable limit of detection as low as 3.01 pg/mL was finally achieved because of the collective effect of chemical and electromagnetic enhancements. Meanwhile, a calibration curve with a high R2 value of 0.996 was obtained in the range from 1 μg/mL to 10 pg/mL. These results exhibit that the GERS matrix has a great promise for participating in immunoassay, which may pave a potential avenue for the real utilization of the SERS technique in clinical biomedicine.

Hybrid Nanoplasmonic Porous Biomaterial Scaffold for Liquid Biopsy Diagnostics Using Extracellular Vesicles.

For more effective early-stage cancer diagnostics, there is a need to develop sensitive and specific, non- or minimally invasive, and cost-effective methods for identifying circulating nanoscale extracellular vesicles (EVs). Here, we report the utilization of a simple plasmonic scaffold composed of a microscale biosilicate substrate embedded with silver nanoparticles for surface-enhanced Raman scattering (SERS) analysis of ovarian and endometrial cancer EVs. These substrates are rapidly and inexpensively produced without any complex equipment or lithography. We extensively characterize the substrates with electron microscopy and outline a reproducible methodology for their use in analyzing EVs from in vitro and in vivo biofluids. We report effective chemical treatments for (i) decoration of metal surfaces with cysteamine to nonspecifically pull down EVs to SERS hotspots and (ii) enzymatic cleavage of extraluminal moieties at the surface of EVs that prevent localization of complementary chemical features (lipids/proteins) to the vicinity of the metal-enhanced fields. We observe a major loss of sensitivity for ovarian and endometrial cancer following enzymatic cleavage of EVs’ extraluminal domain, suggesting its critical significance for diagnostic platforms. We demonstrate that the SERS technique represents an ideal tool to assess and measure the high heterogeneity of EVs isolated from clinical samples in an inexpensive, rapid, and label-free assay.

Rapid Detection of Cypermethrin by Using Surface-Enhanced Raman Scattering Technique

Cypermethrin is a toxic pesticide in the pyrethroid group. A Surface Enhanced Raman Scattering (SERS) based sensor has been developed to achieve simple pesticide sensing. In this work, rapid detection of cypermethrin by using the handheld Raman spectroscopy coupled with SERS substrate was demonstrated. SERS-active silver nanorods substrate was used to enhance Raman signals of test samples. The effect of exposure time and drop volume of sample was studied for cypermethrin measurement. The results found that the silver nanorods substrate can be used to measure cypermethrin in the range of 10-6 to 10-3 M with a handheld Raman spectrometer. Furthermore, the Raman signal of cypermethrin was confirmed by measuring solid cypermethrin with the standard Raman spectrometer. SERS substrate was competent to detect cypermethrin with a limit of detection (LOD) of 10-6 M.

Gold-Stabilized Gold–Silver Alloy Nanostructures as High-Performance SERS Substrate

Surface-enhanced Raman scattering (SERS) is a molecule-specific ultra-sensitive spectroscopic technique. With the development of portable Raman equipment, it has become possible to apply the SERS technique for rapid on-site detection. Therefore, the search for materials with excellent surface plasmon resonance (SPR) activity and high stability has become an important research focus in recent years. In this study, we successfully prepared plasmonic nanostructures with a large number of SERS “hotspots” using alloy nanoparticles (NPs) with tunable gold–silver mole ratios. These structures were composed of numerous narrow gapped gold–silver alloy NPs with diameters ranging from 10 to 100 nm. SERS property evaluation showed that the prepared alloy SERS substrates had good uniformity, and the performance of the substrates rapidly improved as the Ag content increased. Gold-stabilized Au–Ag alloy SERS substrates combine the high performance of the Ag SERS substrate and the excellent stability of Au. In addition, the wavelength of the SPR can be tuned by adjusting the mole ratio of Au and Ag to satisfy different excitation wavelengths. Thus, the Au–Ag alloy SERS substrates we prepared show good comprehensive performance. The lowest concentration of rhodamine 6G that can be homogeneously detected by the prepared Au1Ag8 alloy substrate is 5 × 10−12 M, and the enhancement factor is 5.1 × 108. Finite-difference time-domain (FDTD) theoretical calculations further confirmed our experimental results and theoretically demonstrated the unique properties of the Au–Ag alloy SERS substrates.