Sensitive Surface-enhanced Raman Scattering Research Articles

Freeze Surface-Enhanced Raman Scattering Coupled with Thin-Layer Chromatography: Pesticide Detection and Quantification Case.

Thin-layer chromatography (TLC) is widely used in various branches of chemical science to separate components in complex mixtures because of its simplicity. In most cases, analyte spots are visually detected by fluorescence, and the retention factor (Rf) is determined from the distance traveled by the analyte. Further characterizations are often necessary to identify separated chemicals because molecular information other than Rf is not available. Surface-enhanced Raman scattering (SERS) has been coupled with TLC to complement molecular information. In previously reported TLC-SERS, metal nanoparticle suspension was dropped onto analyte spots to obtain SERS spectra. This approach is simple and efficient for SERS measurements on the TLC plate but has limited sensitivity for several reasons, such as the low solubility of analytes in the dropped solution, difficult control of nanoparticle aggregation, and interference from the stationary phase. We recently showed that freezing enhances SERS sensitivity by a factor of ∼103. Freezing simultaneously concentrates analytes and silver nanoparticles (AgNPs) in a freeze concentrated solution, where aggregation of AgNPs is facilitated, allowing sensitive freeze SERS (FSERS) measurements. Here, we discuss FSERS measurements on TLC plates to demonstrate the superiority of this combination, i.e. TLC-FSERS. Freezing enhances SERS sensitivity by freeze concentration and facilitated aggregation of AgNPs and, in addition, eliminates interference from the stationary phase. Under the optimized condition, TLC-FSERS enables the on-site detection of pesticides at the nM level. The use of the SERS signal from adenine added as the internal standard allows us to quantify pesticides. Applications to a commercial green tea beverage are also demonstrated.

The Origin of Mo2C Films for Surface-Enhanced Raman Scattering Analysis: Electromagnetic or Chemical Enhancement?

The relatively weak Raman enhanced factors of semiconductor-based substrate limit its further application in surface-enhanced Raman scattering (SERS). Here, a kind of two-dimensional (2D) semimetal material, molybdenum carbide (Mo2C) film, is prepared via a chemical vapor deposition (CVD) method, and the origin of SERS is investigated for the first time. The detection limits of the prepared Mo2C films for crystal violet (CV) and rhodamine 6G (R6G) molecules are low at 10-6 M and 10-8 M, respectively. Our detailed theoretical analysis, based on density functional theory and the finite element method, demonstrates that the enhancement of the 2D Mo2C film is indeed CM in nature rather than the EM effects. Besides, the basic doping strategies are proposed to further optimize the SERS sensitivity of Mo2C for Fermi level regulation. We believe this work will provide a helpful guide for developing a highly sensitive semimetal SERS substrate.

Facile Regulation of Shell Thickness of the Au@MOF Core-Shell Composites for Highly Sensitive Surface-Enhanced Raman Scattering Sensing.

Metal-organic frameworks (MOFs)-based core-shell composites have advanced the development of surface-enhanced Raman scattering (SERS) analysis, which originates from the promising structural characteristics of the outer framework material as well as the inherent plasmonic properties of the novel metal structure core (for example, nanoparticle, MNP). However, the SERS effect only exists directly in the surface of MNP or restricted around the plasmonic MNP surface. Consequently, the nanoscale control of the thickness of MOF shell in hybrid core-shell substrates is highly desirable. Despite the great effects which have been made to integrate various MOF matrices with MNP for the purpose of improving the SERS activity, the nanoscale thickness control of MOF shell remains a significant challenge. Here, we report a facile regulation method that enables the Au NP to be encapsulated by a zirconium-based MOF (BUT-17) with different thickness through the controlling of synthesis parameters. This method provides a promising strategy for optimizing the activity of core-shell SERS substrates for potential trace detection.

Detection of 3,4-Methylene Dioxy Amphetamine in Urine by Magnetically Improved Surface-Enhanced Raman Scattering Sensing Strategy.

Abuse of illicit drugs has become a major issue of global concern. As a synthetic amphetamine analog, 3,4-Methylene Dioxy Amphetamine (MDA) causes serotonergic neurotoxicity, posing a serious risk to human health. In this work, a two-dimensional substrate of ITO/Au is fabricated by transferring Au nanoparticle film onto indium–tin oxide glass (ITO). By magnetic inducing assembly of Fe3O4@Au onto ITO/Au, a sandwich-based, surface-enhanced Raman scattering (SERS) detection strategy is designed. Through the use of an external magnet, the MDA is retained in the region of hot spots formed between Fe3O4@Au and ITO/Au; as a result, the SERS sensitivity for MDA is superior compared to other methods, lowering the limit of detection (LOD) to 0.0685 ng/mL and attaining a corresponding linear dynamic detection range of 5–105 ng/mL. As an actual application, this magnetically improved SERS sensing strategy is successfully applied to distinguish MDA in urine at trace level, which is beneficial to clinical and forensic monitors.

Bimetallic Gold Nanostars Having High Aspect Ratio Spikes for Sensitive Surface-Enhanced Raman Scattering Sensing.

There has been increasing interest in evolution of plasmonic nanoplatforms based on noble metal nanoparticles to achieve ultrasensitive detection of trace analyte molecules through solution-based surface-enhanced Raman spectroscopy (SERS). This work presents a surfactant-free synthesis method of bimetallic gold nanostars coated with silver (BGNS-Ag) having sharp, high aspect-ratio spikes for achieving ultrahigh detection sensitivity and high reproducibility. Specifically, the unique BGNS-Ag platform combines both the strong SERS enhancement effects of gold nanostar sharp spikes and the high scattering feature of the silver–gold bimetallic structure. To achieve SERS reproducibility, this solution-based SERS measurement requires minimal sample preparation without addition of any external reagents, which can cause irregular aggregation of nanoparticles and reduce the reproducibility of SERS measurements. Moreover, we have streamlined our SERS sensing procedure by using standard well-plates and a portable Raman device for SERS measurements, which could be utilized for rapid on-site detection. This solution-based SERS performance was studied using methylene blue (MB) as a model analyte system. The detection limit of MB was as low as 42 pM, indicating high sensitivity of detection using BGNS-Ag. To illustrate the usefulness for environmental sensing, we showed that the SERS sensor can detect a pesticide, thiram, at a concentration as low as 0.8 nM. This study demonstrated that the BGNS-Ag system could serve as an effective and versatile plasmonic-active platform for reproducible, fast, and in-field detection of small organic analytes at trace levels.

Graphene oxide/Ag nanoparticle/WS2 nanosheet heterostructures for surface-enhanced Raman spectroscopy

Combing the merits of metals and semiconductors,with superior plasmon resonance effects and high charge mobility, 3D-nanocomposite structures consisting of graphene oxide (GO), noble metal nanostructures, and two-dimensional transition metal sulfides (2D-TMDS) are an important topic in surface-enhanced Raman scattering (SERS) research. This paper presents a novel GO/Ag NPs (silver nanoparticles)/WS2 composite SERS substrate, and electric field simulation by COMSOL software. The GO/Ag/WS2 composite substrate shows very high SERS detective sensitivity and stability to probe molecules such as rhodamine 6 g (R6G), crystal violet (CV), methylene blue (MB) and deoxyribonucleic acid (DNA). The SERS sensitivity can reach 10−12 M, the relative standard deviation (RSD) is 8.24%, and the enhancement factor (EF) is approximately 6.60 × 1010 for R6G, which promoted the implementation of the SERS technique in the area of quantitative profiling and testing.

Multifunctional magnetic Fe3O4/Cu2O-Ag nanocomposites with high sensitivity for SERS detection and efficient visible light-driven photocatalytic degradation of polycyclic aromatic hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) with carcinogenic, teratogenic and mutagenic properties are persistent organic pollutants in the environment. Herein, the novel multifunctional Fe3O4/Cu2O-Ag nanocomposites (NCs) have been established for ultra-sensitive surface-enhanced Raman scattering (SERS) detection and visible light-driven photocatalytic degradation of PAHs. Fe3O4/Cu2O-Ag NCs with different amounts of Ag nanocrystals were synthesized, and the effect of Ag contents on SERS performance was studied by finite-difference time-domain (FDTD) algorithm. The synergistic interplay of electromagnetic and chemical enhancement was responsible for excellent SERS sensitivity of Fe3O4/Cu2O-Ag NCs. The limit of detection (LOD) of optimal SERS substrates (FCA-2 NCs) for Nap, BaP, Pyr and Ant was as low as 10-9, 10-9, 10-9 and 10-10 M, respectively. The SERS detection of PAHs in actual soil environment was also studied. Moreover, a simple SERS method was used to monitor the photocatalytic process of PAHs. The recovery and reuse of Fe3O4/Cu2O-Ag NCs were achieved through magnetic field, and the outstanding SERS and photocatalytic performance were still maintained even after eight cycles. This magnetic multifunctional NCs provide a unique idea for the integration of ultra-sensitive SERS detection and efficient photocatalytic degradation of PAHs, and thus will have more hopeful prospects in the field of environmental protection.

Controllable fabrication of silver-deposited polyurethane acrylate nanopillar array film as a flexible surface-enhanced Raman scattering (SERS) substrate with high sensitivity and reproducibility.

Acontrollable method for fabricating flexible surface-enhanced Raman scattering (SERS) substrates is demonstratedby depositing silver onto a flexible nanopillar array film. The flexible nanopillar array film was cost-effectively prepared by replicating an anodic aluminum oxide (AAO) template with UV-curable polyurethane acrylate (PUA) over a large area. Then, the deposition of silver was done by an Ar-assisted thermal evaporation. In the deposition process, the partial pressure of Ar was optimized because it has a significant influence on the SERS intensity through the microstructural changes of silver deposited on PUA nanopillars. In addition, the increase in the nanopillar diameter and height enhanced the SERS intensity obtained at 785-nm excitation because of the increased number of hot spots. However, the agglomeration of Ag-deposited nanopillars, which is caused by high aspect ratios, negatively affected the SERS performance in terms of intensity and standard deviation. The optimized Ag-deposited nanopillar array film with nanopillar diameters and heights of 80nm and 200nm exhibited excellent SERS sensitivity and signal reproducibility with stable mechanical flexibility. For application in food and biomedical analysis, it was used for detecting saccharin and peptide and showed a good linear relationship between the SERS intensity and concentration. These findings demonstrate the suitability of our method for the controllable fabrication and optimization of flexible SERS substrates with high sensitivity and reproducibility.

Fabrication of Au nanostar/MIL-101(Fe) architecture for surface-enhanced Raman scattering detections

In the surface-enhanced Raman scattering (SERS) measurements, it is very important to fabricate “hot spots” and concentrate the analytes into these regions. In this work, a hybrid structure of Au nanostars (NSs) with high SERS sensitivity and metal-organic framework MIL-101(Fe) with high molecular adsorption capability was constructed as high performance SERS substrate. The MIL-101(Fe) shell between AuNSs is several nanometers, promoting the construct of “hot spots” and the aggregation of target molecules. The as-prepared AuNS/MIL-101(Fe) has achieved high SERS sensitivity, long-time stability, reproducibility, and capability of universal detections. The limit of detection for rhodamine 6 G is down to 0.29 nM. Meanwhile, the SERS activity of AuNS/MIL-101(Fe) is kept for at least 30 days. Furthermore, the relative standard deviations of signals on five substrates are all less than 10 %. The as-prepared AuNS/MIL-101(Fe) also exhibit the SERS activities for various molecules including dyes, organic contaminants, pesticides, and amino acids, indicating that the AuNS/MIL-101(Fe) substrates have wide applications in chemical analysis and environmental monitoring.

Environmental photochemistry with thiol- and silica-modified plasmonic nanocomposites: SERS sensing of municipal solid waste and tannery waste leachate from groundwater.

Mismanagement of obsolete solid waste generates a massive deteriorating effect on the environment. There is a high level of open trash disposal contaminating its neighboring water bodies. This despoliation trash causes an endangerment to the living environment. The waste management act is to hinder harmful effects on human beings, animals, plants, and their natural environment through the principles of waste prevention, waste processing, and waste disposal. Surface-enhanced Raman scattering (SERS) enhances the hazardous chemical sensing of environmental pollutants. To vigorously focus on the leaching of a couple of landfills in groundwater and surface water, an unusual combination of SERS-based poly vinyl thiol and silica-modified silver nanocomposites (PVT/SiO2@Ag NCs) was synthesized. The optical, crystalline, and structural properties of PVT/SiO2@Ag NCs were described with UV-visible spectroscopy (UV-Vis), X-ray diffractometer (XRD), transmission electron microscope (TEM), and energy-dispersive X-ray analysis (EDX). The surface plasmon resonance (SPR) is detected at 403nm from the PVT/SiO2@Ag NPs. The average crystallite size of PVT/SiO2 @ Ag NCs is estimated using the Scherrer formula as 11nm. The calculated specific surface area (SSA), strains, and dislocation densities demonstrate the improved mechanical properties of the substrate. The well-separated spherical shape of NPs is also observed, and the composition of silica and sulfur element in addition of Ag was confirmed by EDAX. Negatively charged SiO2 were bound strongly with the SH group and Ag NPs through electrostatic interaction mechanism as S-Ag-O-Si-O-Ag-S. SERS sensitivity is demonstrated by the prepared nanoparticles using an environmentally ignored leachate of municipal solid waste (MSW) and tannery waste (TW) landfill. PVT/SiO2@Ag NCs has detected the presence of innards of MSW leachate viz., aromatic hydrocarbon, phenols, phthalates, and pesticide from the groundwater. Furthermore, the TW leachate compositions of benzenes, hydrocarbons, amines, and chromium VI were analytically identified. Also, the leaching of TW leachate was confirmed in the water samples referred. Hence, this study provides a novel SERS sensor of PVT/SiO2@Ag NCs in the tile to detect and analyze environmentally ignored organic and inorganic compounds.

Design of high SERS sensitive substrates based on branched Ti nanorods

This paper reports a rational design of branched titanium (Ti) nanorods formed by glancing angle physical vapor deposition and their applications as substrates for surface-enhanced Raman scattering (SERS). Ti nanorods with branches have larger surface areas than non-branched nanorods. However, Ti surface oxidizes easily resulting in very little SERS effect. The SERS sensitivity of the branched titanium nanorod is improved by annealing Ti nanorods in nitrogen in an effort to reduce oxidation. Additionally, the plasmonic resonance of the branched titanium nanorod is further improved by coating the top of the nanorods and branches with silver (Ag). The sensitivity of the SERS substrates is about 3700% that of as-deposited branched Ti nanorods with a native oxide layer. Our investigation provides a mechanism to fabricate sensitive SERS sensors of Ti nanorods that are known to be thermally and chemically stable and compatible with silicon-based electronics.

Development of jellyfish-like ZnO@Ag substrate for sensitive SERS detection of melamine in milk

Melamine has received enormous attention because of being illegally adulterated into milk, posing potential threats to human health. It is vital for developing sensitive analytical methods for detecting melamine in milk at the lowest possible concentration. In this context, we proposed a highly sensitive surface-enhanced Raman scattering (SERS) substrate based on hybridization of jellyfish-like zinc oxide microparticles and silver nanoparticles (JF ZnO@Ag) for detecting melamine in milk. The JF ZnO with a rough surface facilitates the deposition of Ag nanoparticles and adsorbing analytes, providing the abundant “hot spots” for the electromagnetic enhancement and the suitable charge transfer path to induce the chemical enhancement. Using the rhodamine 6G (R6G) as the Raman probe, the enhancement factor is as high as 7.58 × 106. As a real application, the substrate was successfully utilized in the detection of melamine in milk. The limit of detection (LOD) was down to 0.098 ppm, which is lower than the safety limit in the USA (2.5 ppm). The results manifest that the proposed substrate is an effective strategy to be applied as an analytical platform for the efficient detection of melamine in milk.

Surface mode enhanced by avoided crossing in microstructure fibers for improved SERS sensing

Photonic crystal fibers (PCFs), providing ingenious microfluidic channels and intense light guidance, have many advantages in the detection of three-dimensional surface-enhanced Raman scattering (SERS). In solid-core PCFs, the light interacts with the sample in the form of an evanescent field, which is weak and limits the SERS detection performance. In this paper, we propose a new mechanism based on avoided crossings between the fiber core mode and the surface mode, which greatly enhances the laser power ratio in the liquid channels and improves the SERS sensitivity. Numerical results show that the intensity in the liquid channels can be increased from 2.8 % to 42 %. To realize this avoided-crossing effect in PCFs experimentally, the silver mirror reaction was optimized which can decorate discrete and densely distributed silver nanoparticles on the hydroxylated surface of the microfluidic channels in less than 15 min. The resulting SERS-active suspended-core PCF has ultra-high sensitivity, with a detection limit of 10−12 M, and an enhancement factor of 1010 for crystal violet solution. It also provides good stability and reproducibility with a relative standard deviation of 4.8 %. This ultra-sensitive and highly reproducible PCF-SERS probe has great potential for application in rapid in-situ detection.

Fabrication of in situ magnetic capturing and Raman enhancing nanoplatelets for detection of bacteria and biomolecules

In situ magnetic capturing and surface-enhanced Raman scattering (SERS) detection techniques can be used for detecting microbial contamination and uremic toxins. Magnetic SERS substrates were fabricated using immobilized gold nanoparticles (AuNPs) and iron-oxide (Fe3O4) nanoparticles on exfoliated nanoscale silicate platelets (NSPs). The prepared magnetic SERS nanosheets (Fe3O4@AuNPs@NSP nanosheets) were able to not only magnetically capture bacteria and biomolecules but also effectively induce the hot-spot effect and enhance the Raman signal by the surface plasmon resonance of the AuNP arrays. The results showed that both the ratio of AuNPs to Fe3O4 nanoparticles and the order of immobilization on the NSP nanosheets are important factors for inducing magnetic capturing behavior and SERS sensitivity. A method that yielded magnetic capturing behavior was to graft AuNPs on NSP nanosheets first, followed by the immobilization of Fe3O4 nanoparticles. Fe3O4@AuNPs@NSP nanosheets prepared using 0.9 mM HAuCl4 displayed the highest magnetic capturing behavior and SERS enhancement effect, showing an optimal interparticle gap. The bacteria (Escherichia coli) were captured and separated by the magnetic SERS substrates using an applied magnetic field, and then the magnetically captured samples were monitored by Raman spectroscopy for rapid SERS detection. Furthermore, the SERS sensitivity increased by ~2 times after magnetic capturing, and the limit of detection for sensing bacteria was below 103 CFU/mL. The novel magnetic SERS substrates provide ultrasensitive, rapid, and reproducible SERS detection for magnetically capturing biomolecules (bacteria, uremic toxins, and parathyroid hormone) and can be applied in environmental, water, and biomedical detection.

DNAzyme signal amplification based on Au@Ag core-shell nanorods for highly sensitive SERS sensing miRNA-21.

Here, we developed a surface-enhanced Raman scattering (SERS) sensor based on functionalized Au@Ag core-shell nanorods (Au@Ag NRs) and cascade DNAzyme amplifier (CSA) for sensitive and accurate determination of microRNA-21 (miRNA-21). The as-prepared SERS nanoprobes were composed of a thiol-modification hairpin probe (HP2)-functionalized Au@Ag NRs and hairpin DNAzyme (HP1-Dz). Compared with original gold nanorods, the silver shell caused an enhancement of plasmonic properties, resulting in a significant enhancement of Raman signals. In the presence of target miRNAs, the hairpin construction of HP1-Dz changed due to DNA/RNA hybridization; subsequently, the DNAzyme-catalyzed cleaving process changed, and the Raman signals of the SERS nanoprobes gradually "turned off" with time elapse because of the dissociation of the Raman reporter from the surface of Au@Ag NRs. Hence, based on this principle, the proposed SERS sensor exhibited good linearity in the range 0.5 fM to 10nM for miRNA-21 detection with a detection limit (LOD) of 0.5 fM. The proposed SERS platform has potential application in quantitative and precise detection of miRNA-21 in human serum.

Construction of black phosphorus nanosheets and Ag nanoparticles co-sensitized TiO2 nanorod arrays as high-performance SERS substrate and photocatalyst

In this study, high-performance surface-enhanced Raman scattering (SERS) substrates and photocatalysts based on TiO2 nanorod (TNR) arrays co-sensitized with black phosphorus (BP) nanosheets and Ag nanoparticles were successfully synthesized. The detection limit of the organic pollutant rhodamine 6G (R6G) on this novel SERS substrate was as low as 1.0 × 10−14 M. The calculated enhancement factor (EF) was as high as 3.81 × 105. These results prove the excellent SERS sensitivity of the BP/Ag/TNR substrate. Furthermore, a good uniformity of the signals was observed during the detection process. The relative standard deviation of the intensity of the Raman fingerprint peak of R6G at 1650 cm−1 was only 8.41%. Moreover, the BP/Ag/TNR substrate exhibited self-cleaning abilities through the photocatalytic degradation of the adsorbed R6G molecules. The BP/Ag/TNR substrate exhibited a maximum rate of 93.5% for the photocatalytic degradation of R6G. The corresponding photodegradation rate constant was 34.2 × 10−3 min−1, which was approximately 3.98 and 2.01 times higher than those of the TNR and Ag/TNR arrays. The improved SERS performance and excellent photocatalytic activity of the BP/Ag/TNR substrate was due to the formation of high-density hot spots, expansion of the optical capture abilities, and enhancement of the charge transfer properties.

Surface-enhanced Raman scattering from an electromagnetic induced transparency substrate for the determination of hepatocellular carcinoma.

Surface-enhanced Raman scattering (SERS) is a powerful analytical method that is especially suitable for the detection of protein molecules. Detection sensitivity of SERS is directly related to the enhancement factor of the substrate, which is dependent on the strength of a local surface electric field generated by surface plasmonic resonance from substrate. In this study, an electromagnetic induced transparency like (EIT-like) metamaterial was used as the SERS substrate. The corresponding plasmonic resonance structure not only produces stronger optical near field but also reduces the spectral line broadening due to radiation damping. This is very beneficial for SERS process, which is strongly dependent on electric field intensity, to improve the sensitivity of SERS detection. Compared with the single resonance mode substrate, the enhancement factor for SERS with the double-mode substrate was increased by an order of magnitude. The obtained EIT-like substrate was used as a SERS-active substrate to detect Lens culinaris agglutinin (LCA)-reactive fraction of AFP (AFP-L3), a hepatocellular carcinoma (HCC)-specific maker. Experimental results are in good agreement with the clinical diagnosis, which demonstrates the potential application of metamaterials in the SERS-based diagnosis and biosensing.

Quantitative Detection of Mastitis Factor IL-6 in Dairy Cow Using the SERS Improved Immunofiltration Assay.

Interleukin-6 (IL-6) is generally used as a biomarker for the evaluation of inflammatory infection in humans and animals. However, there is no approach for the on-site and rapid detection of IL-6 for the monitoring of mastitis in dairy farm scenarios. A rapid and highly sensitive surface enhanced Raman scattering (SERS) immunofiltration assay (IFA) for IL-6 detection was developed in the present study. In this assay, a high sensitivity gold core silver shell SERS nanotag with Raman molecule 4-mercaptobenzoic acid (4-MBA) embedded into the gap was fabricated for labelling. Through the immuno-specific combination of the antigen and antibody, antibody conjugated SERS nanotags were captured on the test zone, which facilitated the SERS measurement. The quantitation of IL-6 was performed by the readout Raman signal in the test region. The results showed that the detection limit (LOD) of IL-6 in milk was 0.35 pg mL−1, which was far below the threshold value of 254.32 pg mL−1. The recovery of the spiking experiment was 87.0–102.7%, with coefficients of variation below 9.0% demonstrating high assay accuracy and precision. We believe the immunosensor developed in the current study could be a promising tool for the rapid assessment of mastitis by detecting milk IL-6 in dairy cows. Moreover, this versatile immunosensor could also be applied for the detection of a wide range of analytes in dairy cow healthy monitoring.

Investigation of a Highly Sensitive Surface-Enhanced Raman Scattering Substrate Formed by a Three-Dimensional/Two-Dimensional Graphene/Germanium Heterostructure.

Three-dimensional graphene (3D-graphene) is used in surface-enhanced Raman spectroscopy (SERS) because of its plasmonic nanoresonator structure and good ability to interact with light. However, a thin (3-5 nm) layer of amorphous carbon (AC) inevitably appears as a template layer between the 3D-graphene and object substrate when the 3D-graphene layer is synthesized, weakening the enhancement factor. Herein, two-dimensional graphene (2D-graphene) is employed as a template layer to directly synthesize 3D-graphene on a germanium (Ge) substrate via plasma-assisted chemical vapor deposition, bypassing the formation of an AC layer. The interaction and photoinduced charge transfer ability of the 3D-graphene/Ge heterojunction with incident light are improved. Moreover, the high density of electronic states close to the Fermi level of the heterojunction induces the adsorbed probe molecules to efficiently couple to the 3D-graphene-based SERS substrate. Our experimental results imply that the lowest concentrations of rhodamine 6G and rhodamine B that can be detected on the 3D/2D-graphene/Ge SERS substrate correspond to 10-10 M; for methylene blue, it is 10-8 M. The detection limits of the 3D/2D-graphene/Ge SERS substrate with respect to 3-hydroxytyramine hydrochloride and melamine (in milk) are both less than 1 ppm. This work may provide a viable and convenient alternative method for preparing 3D-graphene SERS substrates. It also constitutes a new approach to developing SERS substrates with remarkable performance levels.

Plasmonic Superstructure Arrays Fabricated by Laser Near-Field Reduction for Wide-Range SERS Analysis of Fluorescent Materials.

Surface-enhanced Raman scattering (SERS) enables trace-detection for biosensing and environmental monitoring. Optimized enhancement of SERS can be achieved when the energy of the localized surface plasmon resonance (LSPR) is close to the energy of the Raman excitation wavelength. The LSPR can be tuned using a plasmonic superstructure array with controlled periods. In this paper, we develop a new technique based on laser near-field reduction to fabricate a superstructure array, which provides distinct features in the formation of periodic structures with hollow nanoclusters and flexible control of the LSPR in fewer steps than current techniques. Fabrication involves irradiation of a continuous wave laser or femtosecond laser onto a monolayer of self-assembled silica microspheres to grow silver nanoparticles along the silica microsphere surfaces by laser near-field reduction. The LSPR of superstructure array can be flexibly tuned to match the Raman excitation wavelengths from the visible to the infrared regions using different diameters of silica microspheres. The unique nanostructure formed can contribute to an increase in the sensitivity of SERS sensing. The fabricated superstructure array thus offers superior characteristics for the quantitative analysis of fluorescent perfluorooctanoic acid with a wide detection range from 11 ppb to 400 ppm.