High Surface-enhanced Raman Scattering Sensitivity Research Articles

Nickel foam–caged Ag-Au bimetallic nanostructure as a highly rugged and durable SERS substrate

A three-dimensional nickel foam (NF) caging Ag-Au bimetallic nanostructure (Ag-Au@NF) has been demonstrated as a highly rugged and sensitive surface-enhanced Raman scattering (SERS) substrate. The main concept employed in designing this substrate was the protection of SERS-active nanostructures by holding them within a sturdy frame. For this purpose, NF was readily chosen as a strong porous frame to secure the internal nanostructures owing to its excellent structural strength and ability to easily incorporate analytes due to its porous structure. The next issue was the construction of stable and SERS-efficient nanostructures within the NF frame. To meet this need, an Ag-Au bimetallic nanostructure was chosen, combining the high SERS efficiency of Ag and the good surface stability of Au. To fabricate the Ag-Au nanostructure, first a Sn-Ag alloy was constructed on the NF frame by means of electrodeposition (producing Sn-Ag@NF) and then Sn was selectively leached from the alloy by means of free corrosion in NaOH solution (producing Ag@NF). This selective leaching was adopted to make the surface rough to enhance the SERS efficiency in the final substrate. Finally, Au was incorporated into the Ag@NF by means of a galvanic replacement reaction (producing Ag-Au@NF). The use of Ag-Au@NF enabled identification of rhodamine 6G at concentrations as low as 0.1 nM. The Ag-Au@NF substrates were rugged, showing small Raman signal drops after 100 cycles of sandpaper abrasion or 30 min of sonication. Also, the signal decrease was only 4.26% upon 3 days of exposure to ambient air without special care. Overall, the Ag-Au@NF developed herein shows practical merits for adoption in routine and field SERS analyses.

A needle-like reusable surface-enhanced Raman scattering substrate, and its application to the determination of acetamiprid by combining SERS and thin-layer chromatography

A micro surface-enhanced Raman scattering (SERS) substrate has been fabricated by electrochemical deposition of dendrite-like gold on carbon fiber needles (Au-CFNs). Scanning electron microscopy and energy dispersive spectroscopy were used to confirm the presence of the gold nanostructure on the CFNs. This substrate has a Raman scattering enhancement factor as high as 3.3 × 10^7 when using rhodamine 6G as the reporter molecule. The high SERS sensitivity is attributed to the massive hotspots on gold bulges that enhance the local surface plasmon resonance. The Au-CFN substrate was reproduced 10 times after electrochemically wiping off the analytes from the needle-like electrode. The substrate has attractive features such as convenient sampling, low sample dosage, and minimal invasion. It was applied, in combination with thin-layer chromatography, for the determination of acetamiprid on vegetables. The result was more accurate because the sample information of both the surface and the bulk can be obtained at the same time after inserting the tip of this needle substrate into the TLC plate. The limit of detection for acetamiprid is 0.05μg⋅mL-1 and the linear range is 0.1-10μg⋅mL-1. Graphical abstract A reusable micro needle-like SERS substrate was fabricated and applied for pesticide residue analysis. The SERS signal of acetamiprid can be acquired on the tip of this micro needle-like Au-CFN substrate. This substrate can be reused for 10 times.

Development of a Nanostructured Platform for Identifying HER2-Heterogeneity of Breast Cancer Cells by Surface-Enhanced Raman Scattering.

Biosensor technology has great potential for the detection of cancer through tumor-associated molecular biomarkers. In this work, we describe the immobilization of the recombinant humanized anti-HER2 monoclonal antibody (trastuzumab) on a silver nanostructured plate made by pulsed laser deposition (PLD), over a thin film of Au(111). Immobilization was performed via 4-mercapto benzoic acid self-assembled monolayers (4-MBA SAMs) that were activated with coupling reagents. A combination of immunofluorescence images and z-stack analysis by confocal laser scanning microscopy (CLSM) allowed us to detect HER2 presence and distribution in the cell membranes. Four different HER2-expressing breast cancer cell lines (SKBR3 +++, MCF-7 +/−, T47D +/−, MDA-MB-231 −) were incubated during 24 h on functionalized silver nanostructured plates (FSNP) and also on Au(111) thin films. The cells were fixed by means of an ethanol dehydration train, then characterized by atomic force microscopy (AFM) and surface-enhanced Raman scattering (SERS). SERS results showed the same tendency as CLSM findings (SKBR3 > MCF-7 > T47D > MDA-MB-231), especially when the Raman peak associated with phenylalanine amino acid (1002 cm−1) was monitored. Given the high selectivity and high sensitivity of SERS with a functionalized silver nanostructured plate (FSNP), we propose this method for identifying the presence of HER2 and consequently, of breast cancer cells.

Graphene nano-mesh-Ag-ZnO hybrid paper for sensitive SERS sensing and self-cleaning of organic pollutants

Rationally designed self-assemble, functional 3D architecture of graphene nano-meshes with unique plasmonic resonance metal nanoparticles can be excellent nanohybrid Raman scattering (SERS) substrate in spotting the low levels of environmental organic pollutants. In this study, graphene nano-mesh-Ag-ZnO metal (GNMM) nanohybrid was prepared by self-assembled hydrothermal reduction of graphene oxide (GO) followed by thermal annealing of site-localized metal-salts as a highly sensitive surface-enhanced Raman scattering (SERS) substrate. The crumpled graphene nano-meshes decorated with Ag-ZnO nanoparticles created SERS hot spots (holes) that manifested high SERS sensitivity to model organic pollutants such as methyl orange (MO), rhodamine 6G (Rh-6G) and paraquat (PQ) with detection limit as low as 10−11, 10−13 and 10−14 M, respectively. A maximum enhancement factor (EF) of 1.97 × 1011 was obtained with PQ molecules on synthesized GNMM substrate. The enhancement in signal can be supported through charge transfer mechanism that induced strong enhancement of the local electric field on free available graphene nano-mesh edges abundant with hotspots. This novel nanohybrid SERS substrate realized self-cleaning by photocatalytic degradation of model organic Rh-6G molecules adsorbed on the nanohybrid paper and shows reusability during the detection process. Meanwhile, GNMM nanohybrid also possesses highly effective antimicrobial activities for Escherichia coli. The detection of organic pollutants by GNMM nanohybrid SERS substrate has several advantages such as low cost, easy and simple preparation method, sensitive detection and reusability that can be potentially applied for detection of variety of environmental pollutants and antimicrobial agent applications.

Low-cost and large-scale flexible SERS-cotton fabric as a wipe substrate for surface trace analysis

The large-scale surface enhanced Raman scattering (SERS) cotton fabrics were fabricated based on traditional woven ones using a dyeing-like method of vat dyes, where silver nanoparticles (Ag NPs) were in-situ synthesized by ‘dipping-reducing-drying’ process. By controlling the concentration of AgNO3 solution, the optimal SERS cotton fabric was obtained, which had a homogeneous close packing of Ag NPs. The SERS cotton fabric was employed to detect p-Aminothiophenol (PATP). It was found that the new fabric possessed excellent reproducibility (about 20%), long-term stability (about 57 days) and high SERS sensitivity with a detected concentration as low as 10−12M. Furthermore, owing to the excellent mechanical flexibility and good absorption ability, the SERS cotton fabric was employed to detect carbaryl on the surface of an apple by simply swabbing, which showed great potential in fast trace analysis. More importantly, this study may realize large-scale production with low cost by a traditional cotton fabric.

Bilayer composites consisting of gold nanorods and titanium dioxide as highly sensitive and self-cleaning SERS substrates

The authors describe a novel recyclable substrate for highly sensitive surface-enhanced Raman scattering (SERS) studies. It consists of gold nanorods (AuNRs) self-assembled on a TiO2 film and combines the high SERS enhancement of assembled AuNRs with the self-cleaning property of the TiO2 film. It overcomes limitations of conventional SERS substrates such as limited sensitivity, high costs, and poor recyclability. The packing density of the AuNRs on the TiO2 film can be well adjusted, and therefore the SERS enhancement factor of the substrates can be tuned. For the optimized substrate, the SERS enhancement factor is as high as 5.4 × 1010 (Raman laser at 782 nm with 10% of 1.15 mW). This indicates extremely high sensitivity and is promising in terms of single molecule detection. Raman mapping experiments showed the SERS signal to be homogeneously distributed over the whole surface, with a maximal deviation of 3.5%. The SERS activity of this substrate can be restored by degradation of the probe molecules through TiO2 photocatalysis under UV irradiation. In our perception, these findings open a new venue for developing ultra-sensitive and recyclable SERS substrates.

Ordered array of Ag semishells on different diameter monolayer polystyrene colloidal crystals: An ultrasensitive and reproducible SERS substrate.

Ag semishells (AgSS) ordered arrays for surface-enhanced Raman scattering (SERS) spectroscopy have been prepared by depositing Ag film onto polystyrene colloidal particle (PSCP) monolayer templates array. The diversified activity for SERS activity with the ordered AgSS arrays mainly depends on the PSCP diameter and Ag film thickness. The high SERS sensitivity and reproducibility are proved by the detection of rhodamine 6G (R6G) and 4-aminothiophenol (4-ATP) molecules. The prominent enhancements of SERS are mainly from the “V”-shaped or “U”-shaped nanogaps on AgSS, which are experimentally and theoretically investigated. The higher SERS activity, stability and reproducibility make the ordered AgSS a promising choice for practical SERS low concentration detection applications.

Facile Shape-Controlled Fabrication of Copper Nanostructures on Borophosphate Glasses: Synthesis, Characterization, and Their Highly Sensitive Surface-Enhanced Raman Scattering (SERS) Properties

We demonstrate air-stable copper-doped nanostructured borophosphate samples, which were prepared by a facile, low cost, and green synthesis method. The thermal annealing, in a reducing hydrogen atmosphere, enables the formation of metallic copper nanostructures, which was confirmed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and optical absorption. The optical spectra show a main intense surface plasmon resonance (SPR) band centered at 579 nm. The shapes of the nanostructures, morphology, and thickness of the copper nanostructures coating are chosen to be suitable for SERS applications. These samples exhibited very high SERS enhancement factors (EF), depending on thermal annealing time, with excellent reproducibility. The estimated EFs have been found in the range between 107 and 108.

Porous Silicon Covered with Silver Nanoparticles as Surface-Enhanced Raman Scattering (SERS) Substrate for Ultra-Low Concentration Detection.

Microporous and macro-mesoporous silicon templates for surface-enhanced Raman scattering (SERS) substrates were produced by anodization of low doped p-type silicon wafers. By immersion plating in AgNO3, the templates were covered with silver metallic film consisting of different silver nanostructures. Scanning electron microscopy (SEM) micrographs of these SERS substrates showed diverse morphology with significant difference in an average size and size distribution of silver nanoparticles. Ultraviolet-visible-near-infrared (UV-Vis-NIR) reflection spectroscopy showed plasmonic absorption at 398 and 469 nm, which is in accordance with the SEM findings. The activity of the SERS substrates was tested using rhodamine 6G (R6G) dye molecules and 514.5 nm laser excitation. Contrary to the microporous silicon template, the SERS substrate prepared from macro-mesoporous silicon template showed significantly broader size distribution of irregular silver nanoparticles as well as localized surface plasmon resonance closer to excitation laser wavelength. Such silver morphology has high SERS sensitivity that enables ultralow concentration detection of R6G dye molecules up to 10(-15) M. To our knowledge, this is the lowest concentration detected of R6G dye molecules on porous silicon-based SERS substrates, which might even indicate possible single molecule detection.

Using surface enhanced Raman scattering to analyze the interactions of protein receptors with bacterial quorum sensing modulators.

Many members of the LuxR family of quorum sensing (QS) transcriptional activators, including LasR of Pseudomonas aeruginosa, are believed to require appropriate acyl-homoserine lactone (acyl-HSL) ligands to fold into an active conformation. The failure to purify ligand-free LuxR homologues in nonaggregated form at the high concentrations required for their structural characterization has limited the understanding of the mechanisms by which QS receptors are activated. Surface-enhanced Raman scattering (SERS) is a vibrational spectroscopy technique that can be applied to study proteins at extremely low concentrations in their active state. The high sensitivity of SERS has allowed us to detect molecular interactions between the ligand-binding domain of LasR (LasRLBD) as a soluble apoprotein and modulators of P. aeruginosa QS. We found that QS activators and inhibitors produce differential SERS fingerprints in LasRLBD, and in combination with molecular docking analysis provide insight into the relevant interaction mechanism. This study reveals signal-specific structural changes in LasR upon ligand binding, thereby confirming the applicability of SERS to analyze ligand-induced conformational changes in proteins.

High-Sensitivity Surface-Enhanced Raman Scattering (SERS) Substrate Based on a Gold Colloid Solution with a pH Change for Detection of Trace-Level Polycyclic Aromatic Hydrocarbons in Aqueous Solution.

In this study, a gold colloid solution whose parameters were optimized, and without any surfactants, was developed as a surface-enhanced Raman scattering (SERS) substrate for the detection of trace-level polycyclic aromatic hydrocarbons (PAHs). A gold colloid solution with 57 nm gold particles and pH 13 was prepared to be the SERS substrate. It had impressive enhancement that was two orders of magnitude higher than that of a gold colloid solution with 57 nm gold particles and without pH change (pH 6). Even with a compact field-based Raman spectrometer, naphthalene, phenanthrene, anthracene, fluoranthene, and pyrene were detected, with limits of detection at 6.8 nM, 3.4 nM, 1.8 nM, 0.68 nM (680 pM), and 0.44 nM (440 pM), respectively. The significant enhancement was ascribed to an electromagnetic mechanism and a charge-transfer mechanism. Quantitative analyses for these five PAHs in water were also performed. The SERS intensities of PAHs were found to have good linear dependence relations with the concentrations in low concentration. This high-sensitivity, easily prepared substrate offers a promising technology for the quantitative detection of trace-level PAHs.

SERS and FDTD simulation of gold nanoparticles grafted on germanium wafer via galvanic displacement

Uniform and dense Au nanoparticles grown on Ge (Au/Ge) were fabricated by a facile galvanic displacement method and employed as surface-enhanced Raman scattering (SERS) substrates. The substrates exhibited excellent reproducibility in the detection of rhodamine 6G aqueous solution with a relative standard deviation of <20%. The substrate showed a high Raman enhancement factor of 3.44 × 106. This superior SERS sensitivity was numerical confirmed by the three-dimensional finite-difference time-domain method, which demonstrated a stronger electric field intensity (|E/E0|2) distribution around the Au nanoparticles grown on Ge. This facile and low-cost prepared Au/Ge substrate with high SERS sensitivity and reproducibility might have potential applications in monitoring in situ reaction in aqueous solution. Copyright © 2014 John Wiley & Sons, Ltd.

Silver nanoparticle enhanced Raman scattering-based lateral flow immunoassays for ultra-sensitive detection of the heavy metal chromium

We report a simple and ultra-sensitive surface enhanced Raman scattering (SERS) strip sensor based on silver nanoparticles (AgNPs) and lateral flow immunoassays (LFIAs). LFIAs are inexpensive, simple, portable and robust, thus making them commonplace in medicine, agriculture and food safety. However, their applications are limited due to the low signal intensity of the color-formation reaction based on the label accumulation. SERS is a powerful molecular spectroscopy technique for ultra-detection, which is based on the enhancement of the inelastic scattering from molecules located near nanostructured metallic surfaces when the molecules are illuminated and the surface plasmons are excited. Because of the rapidity and robustness of LFIAs and the high sensitivity of SERS, we introduce SERS into LFIAs (SERS-LFIA). Our SERS-LFIA demonstrates fast, excellent performance and is suitable for the semiquantitative examination of ultratrace analytes (Cr3+), with the limit of the detection (LOD) as low as 10−5 ng mL−1, which is 105-fold more highly sensitive than those previously used to detect Cr3+ within 15 min.

Ag-nanoparticle-decorated Ge nanocap arrays protruding from porous anodic aluminum oxide as sensitive and reproducible surface-enhanced Raman scattering substrates.

We report on the fabrication of Ag nanoparticle (Ag NP) decorated germanium (Ge) nanocap (Ag-NPs@Ge-nanocap) arrays protruding from highly ordered porous anodic aluminum oxide (AAO) template as highly sensitive and uniform surface-enhanced Raman scattering (SERS) substrates. The hybrid SERS substrates are fabricated via a combinatorial process of AAO template-assisted growth of Ge nanotubes with each tube having a hemispherical nanocap on the AAO pore bottom, wet chemical etching of the remaining aluminum and the AAO barrier layer to expose the Ge nanocaps, and sputtering Ag NPs on the Ge nanocap arrays. Because sufficient SERS "hot spots" are created from the electromagnetic coupling among the Ag NPs on the Ge nanocap and the highly ordered Ge nanocap arrays also have semiconducting chemical supporting enhancement, the hybrid SERS substrates have high SERS sensitivity and good signal reproducibility. Using the hybrid SERS substrates, Rhodamine 6G with a concentration down to 10(-11) M is identified, and one congener of highly toxic polychlorinated biphenyls with a concentration as low as 10(-6) M is also recognized, showing great potential for SERS-based rapid detection of organic pollutants in the environment.

Preparation of Silver Nanoparticles/Graphene Composite Films by Dip-Coating Method and their Surface-Enhanced Raman Scattering Property

Recently, noble metal nanoparticles (Au or Ag NPs) decorated graphene (GE) sheets are used as good surface-enhanced Raman scattering (SERS) substrates for detection of molecules. Ultrathin GE film covered by Ag NPs has been designed for the SERS substrate in this paper. The films were prepared firstly by dip-coating method with graphene oxide (GO) and AgNO3 or Ag NPs colloid as raw materials and secondly reduced by thermal annealing at 600 °C in H2/Ar atmosphere. The structure and SERS activity of the obtained films have been investigated in detail. Results indicated that Ag NPs can be decorated on graphene sheets through the electrostatic adsorption between the GO sheets and positively charged Ag+ or Ag NPs. Compared with the film obtained with AgNO3, Ag NPs/GE film prepared with Ag NPs colloid showed the higher SERS sensitivity with rhodamine 6G (R6G) as the probing molecules, the enhanced factor for 1×10-5 M of R6G adsorbed on the surface of Ag NPs/GE substrate is about 9.1×107.

Multiple detection of proteins by SERS-based immunoassay with core shell magnetic gold nanoparticles

A highly selective and sensitive surface-enhanced Raman scattering (SERS)-based immunoassay for the multiple detection of proteins has been developed. The proposed core shell magnetic gold (Au) nanoparticles allow for successful protein separation and high SERS enhancement for protein detection. To selectively detect a specific protein in a mixed protein solution, we employed the sandwich type SERS immunoassay with core shell magnetic Au nanoparticles utilizing specific antigen–antibody interactions. Based on this proposed SERS immunoassay, we can successfully detect proteins in very low concentrations (∼800ag/mL of mouse IgG and ∼5fg/mL of human IgG) with high reproducibility. Magnetically assisted protein separation and detection by this proposed SERS immunoassay would provide great potential for effective and sensitive multiple protein detection. This technique allows for the straightforward SERS-based bioassays for quantitative protein detections.

Liquid core capillary-based Raman probe for surface-enhanced Raman scattering detection

High-sensitivity surface-enhanced Raman scattering (SERS) signal detection was achieved using a novel optical probe based on a processed capillary tube. The probe was produced by immobilizing silver nanoparticles (SNPs) in alkanethiol self-assembled monolayers on a capillary tube wall surface through a chemical coating technique. The developed probe could detect Rhodamine 6G with good sensitivity (10−9) due to enhanced interaction volume, high excitation and collection efficiency, and additional enhancement of hot nanojunctions. The proposed Raman probe provides a simple, cheap, fast, compact, and high-sensitivity SERS signal detection method for many potential applications, including chemical, biological, medical, and environmental fields.

Fabrication of free-standing and flexible silver nanoparticle-graphene films and their surface-enhanced Raman scattering activity

A free-standing and flexible silver nanoparticle-graphene (Ag NP-GE) film with high surface-enhanced Raman scattering (SERS) sensitivity has been synthesized. Firstly, the highly stable and dispersive Ag NP-GE nanocomposite was fabricated by an in situ reduction method using graphene oxide sheets as substrates and ethylene glycol as a reduction agent in a solvothermal environment. Secondly, the Ag NP-GE films can be fabricated through vacuum filtration of the Ag NP-GE suspension. The highly SERS-active Ag nanoparticles and Ag nanoparticle aggregates make a significant contribution to the high sensitivity of SERS to rhodamine 6G molecules with an enhancement factor of 5.6 × 107.

Self-assembly of noble metallic spherical aggregates from monodisperse nanoparticles: their synthesis and pronounced SERS and catalytic properties

Here, an oil-in-water emulsion method was used to assemble monodisperse noble metallic nanoparticles (NPs) in the oil phase into aggregates in the aqueous phase. The original size of the NPs is an essential factor in determining the final morphology: beyond the critical size, spherical aggregates with the original NPs can be formed; otherwise, complex aggregates with coarsing NPs come into being. The aggregation level of NPs and consequent properties of the as-constructed spherical assemblies can be optimized through tuning the evaporation rate of the organic solvent. The fabricated assemblies can serve as highly sensitive surface-enhanced Raman scattering (SERS) platforms for the detection of chemical or biological molecules, showing significantly high SERS activity toward rhodamine 6G (R6G) dye and 4-aminothiophenol (4-ATP) molecules. In addition, we further demonstrate the use of these highly sensitive SERS-active substrates to identify melamine at 1 × 10−7 M to insure food safety and biosecurity. Furthermore, the catalytic reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of excess NaBH4 was also investigated, in which the as-prepared silver assemblies exhibited excellent catalytic activities due to their special 3D architecture and large number of active catalytic sites.

Gold nanoparticle thin films fabricated by electrophoretic deposition method for highly sensitive SERS application

We report an electrophoretic deposition method for the fabrication of gold nanoparticle (GNP) thin films as sensitive surface-enhanced Raman scattering (SERS) substrates. In this method, GNP sol, synthesized by a seed-mediated growth approach, and indium tin oxide (ITO) glass substrates were utilized as an electrophoretic solution and electrodes, respectively. From the scanning electron microscopy analysis, we found that the density of GNPs deposited on ITO glass substrates increases with prolonged electrophoresis time. The films possess high mechanical adhesion strength and exhibit strong localized surface plasmon resonance (LSPR) effect by showing high SERS sensitivity to detect 1 × 10−7 M rhodamine 6 G in methanol solution. Finally, the relationship between Raman signal amplification capability and GNP deposition density has been further investigated. The results of our experiment indicate that the high-density GNP film shows relatively higher signal amplification capability due to the strong LSPR effect in narrow gap regions between the neighboring particles on the film.