SERS detection of biomolecules using lithographed nanoparticles towards a reproducible SERS biosensor

In this paper we highlight the accurate spectral detection of bovine serum albumin and ribonuclease-A using a surface-enhanced Raman scattering (SERS) substrate based on gold nanocylinders obtained by electron-beam lithography (EBL). The nanocylinders have diameters from 100 to 180 nm with a gap of 200 nm. We demonstrate that optimizing the size and the shape of the lithographed gold nanocylinders, we can obtain SERS spectra of proteins at low concentration. This SERS study enabled us to estimate high enhancement factors (10(5) for BSA and 10(7) for RNase-A) of important bands in the protein Raman spectrum measured for 1 mM concentration. We demonstrate that, to reach the highest enhancement, it is necessary to optimize the SERS signal and that the main parameter of optimization is the LSPR position. The LSPR have to be suitably located between the laser excitation wavelength, which is 632.8 nm, and the position of the considered Raman band. Our study underlines the efficiency of gold nanocylinder arrays in the spectral detection of proteins.     

Gold nanorods with finely tunable longitudinal surface plasmon resonance as SERS substrates

Advances in nanophotonics have shown the potential of colloidal metal nanoparticles with sharp tips, such as rods, to focalize plasmonic electromagnetic fields. We report on the synthesis of Au nanorods via a seed mediated approach and the influence of silver ions on the aspect ratio of the Au nanorods. The longitudinal surface plasmon resonance (LSPR) of the Au nanorods was successfully tuned with the concentration of silver ions. The surface enhanced Raman scattering (SERS) effect of 2-aminothiophenol (2-ATP) as a probe molecule on Au nanorods was systematically studied by varying the longitudinal surface plasmon resonance of the nanorods. The highest electromagnetic enhancement was observed when the longitudinal surface plasmon resonance of the Au nanorods overlapped with the laser excitation wavelength. The variation of the SERS enhancement factor with the longitudinal surface plasmon resonance and laser excitation lines is also discussed in detail.     

Controlled assemblies of gold nanorods in PVA nanofiber matrix as flexible free-standing SERS substrates by electrospinning

Under control: Controlled assemblies of gold nanorods in a poly(vinyl alcohol) (PVA) nanofiber matrix with tunable optical properties can be achieved by using electrospinning. The resultant assemblies can be used as substrates for surface-enhanced Raman spectroscopy (SERS). This work provides a facile way to control alignment of anisotropic nanostructures in a polymer nanofiber matrix and generates new assemblies with interesting properties.     

Fabrication and characterization of nanostructures on insulator substrates by electron-beam-induced deposition

Abstract

The fabrication, characterization, and decoration with metallic nanoparticles of nanostructures such as nanowhiskers, nanodendrites, and fractal-like nanotrees on insulator substrates by electron-beam-induced deposition (EBID) are reviewed. Nanostructures with different morphologies of whiskers, dendrites, or trees are fabricated on insulator (Al₂O₃ or SiO₂) substrates by EBID in transmission electron microscopes by controlling the irradiation conditions such as the electron beam intensity. The growth of the nanostructure is related to the accumulation of charges on the surface of a substrate during electron-beam irradiation. A high concentration of the target metallic element and nanocrystal grains of the element are contained in the fabricated nanostructures. The process of growth of the nanostructures is explained qualitatively on the basis of mechanisms in which the formation of the nanostructures is considered to be related to the nanoscaled unevenness of the charge distribution on the surface of the substrate, the movement of the charges to the convex surface of the substrate, and the accumulation of charges at the tip of the grown nanostructure. Novel composite structures of Pt nanoparticle/tungsten (W) nanodendrite or Au nanoparticle/W nanodendrite are fabricated by the decoration of W nanodendrites with metallic elements. Because they have superior features, such as a large specific surface area, a freestanding structure on substrates, a typical size of several nanometers of the tip or the branch, and high purity, the nanostructures may have applications in technologies such as catalysts, sensors, and electron emitters. However, there are still some subjects that should be further studied before their application.     

Fabrication and characterization of nanostructures on insulator substrates by electron-beam-induced deposition

The fabrication, characterization, and decoration with metallic nanoparticles of nanostructures such as nanowhiskers, nanodendrites, and fractal-like nanotrees on insulator substrates by electron-beam-induced deposition (EBID) are reviewed. Nanostructures with different morphologies of whiskers, dendrites, or trees are fabricated on insulator (Al₂O₃ or SiO₂) substrates by EBID in transmission electron microscopes by controlling the irradiation conditions such as the electron beam intensity. The growth of the nanostructure is related to the accumulation of charges on the surface of a substrate during electron-beam irradiation. A high concentration of the target metallic element and nanocrystal grains of the element are contained in the fabricated nanostructures. The process of growth of the nanostructures is explained qualitatively on the basis of mechanisms in which the formation of the nanostructures is considered to be related to the nanoscaled unevenness of the charge distribution on the surface of the substrate, the movement of the charges to the convex surface of the substrate, and the accumulation of charges at the tip of the grown nanostructure. Novel composite structures of Pt nanoparticle/tungsten (W) nanodendrite or Au nanoparticle/W nanodendrite are fabricated by the decoration of W nanodendrites with metallic elements. Because they have superior features, such as a large specific surface area, a freestanding structure on substrates, a typical size of several nanometers of the tip or the branch, and high purity, the nanostructures may have applications in technologies such as catalysts, sensors, and electron emitters. However, there are still some subjects that should be further studied before their application.     

Monodisperse, micrometer-scale, highly crystalline, nanotextured Ag dendrites: rapid, large-scale, wet-chemical synthesis and their application as SERS substrates

In this letter, we report on our interesting finding that the direct mixing of aqueous AgNO(3) and NH(2)OH solutions at room temperature leads to rapid, high-yield production of monodisperse, micrometer-scale, highly crystalline, nanotextured Ag dendrites. The surface-enhanced Raman scattering (SERS) effect of these Ag dendrites was evaluated by using 4-aminothiophenol (p-ATP) as the Raman probe and the results demonstrate that they exhibit strong SERS effects.     

Chitosan-coated anisotropic silver nanoparticles as a SERS substrate for single-molecule detection

Surface-enhanced Raman spectroscopy (SERS) is a technique that has become widely used for identifying and providing structural information about molecular species in low concentration. There is an ongoing interest in finding optimum particle size, shape and spatial distribution for optimizing the SERS substrates and pushing the sensitivity toward the single-molecule detection limit. This work reports the design of a novel, biocompatible SERS substrate based on small clusters of anisotropic silver nanoparticles embedded in a film of chitosan biopolymer. The SERS efficiency of the biocompatible film is assessed by employing Raman imaging and spectroscopy of adenine, a significant biological molecule. By combining atomic force microscopy with SERS imaging we find that the chitosan matrix enables the formation of small clusters of silver nanoparticles, with junctions and gaps that greatly enhance the Raman intensities of the adsorbed molecules. The study demonstrates that chitosan-coated anisotropic silver nanoparticle clusters are sensitive enough to be implemented as effective plasmonic substrates for SERS detection of nonresonant analytes at the single-molecule level.     

Directed assembly of gold nanorods using aligned electrospun polymer nanofibers for highly efficient SERS substrates

Nonspherical metal nanoparticles are very attractive plasmonic nanostructures owing to the facile tunability of the plasmonic properties and the presence of sharp corners and edges, which act as electromagnetic hot spots for surface enhanced Raman scattering (SERS). However, such anisotropic nanostructures exhibit strong polarization dependence in their plasmonic properties, exhibiting significantly higher SERS intensity in certain orientations. In this paper, we demonstrate a facile strategy to achieve directed assembly of aligned gold nanorods using highly aligned electrospun nanofibers. We believe that the interstices between the nanofibers act as micro-and nanochannels, resulting in hydrodynamic drag forces on the gold nanorods, thus inducing massive alignment of the same on the nanofibers. Apart from exhibiting nearly 50 times higher SERS intensity compared to a planar SERS substrate with randomly oriented nanorods, our results highlight the importance of the orientation of anisotropic nanostructures. Finite difference time domain (FDTD) simulations employed to understand the electromagnetic field distribution around an aligned nanorod array showed excellent agreement with the experimental observations.     

Tailoring plasmonic nanostructures for optimal SERS sensing of small molecules and large microorganisms

Local electric fields can be tuned dramatically by varying the diameter of quasi-3D gold plasmonic nanostructure arrays, as indicated by 3D finite-difference time-domain calculations. Utilizing quasi-3D arrays that exhibit a maximum electric field intensity (i.e., a "hot" spot) either at the bottom (gold nanodisks) or on the top (gold film patterned with nanoholes), the optimal surface-enhanced Raman scattering (SERS) sensitivity for the detection of small molecules or large microorganisms can be achieved. The precisely fabricated and optimized SERS-active quasi-3D nanostructure arrays make it possible to quantitatively and reproducibly detect chemical and biological species using SERS, leading to a new sensing platform with molecular specificity based on SERS for many important applications.     

Synthesis of SERS active Ag2S nanocrystals using oleylamine as solvent,reducing agent and stabilizer

Semiconductor Ag₂S nanocrystals have been prepared via a facile solution growth method, in which AgNO₃ and sulfur (S) powder are used as precursors, oleylamine is used and can function as both reducing agent and stabilizer during the synthetic process. The experimental results demonstrate that the as-synthesized Ag₂S nanocrystals, which have uniform size are monoclinic Ag₂S. Furthermore, surface-enhanced Raman scattering (SERS) spectrum of rhodamine 6G can be obtained on the Ag₂S nanocrystals modified substrate, indicating novel optical response of Ag₂S nanocrystals. The SERS effect of Ag₂S nanocrystals should attract much interest in fundamental physics as well as device application points of view.     

Polyacrylic acid sodium salt film entrapped Ag-nanocubes as molecule traps for SERS detection

Surface-enhanced Raman spectroscopy （SERS） is a fast analytical technique for trace chemicals; however, it requires the active SERS-substrates to adsorb analytes, thus limiting target species to those with the desired affinity for substrates. Here we present networked polyacrylic acid sodium salt （PAAS） film entrapped Ag-nanocubes （denoted as Ag-nanocubes@PAAS） as an effective SERS-substrate for analytes with and without high affinity. Once the analyte aqueous solution is cast on the dry Ag-nanocubes@PAAS substrate, the bibulous PAAS becomes swollen forcing the Ag-nanocubes loose, while the analytes diffuse in the interstices among the Ag-nanocubes. When dried, the PAAS shrinks and pulls the Ag-nanocubes back to their previous aggregated state, while the PAAS network ＂detains＂ the analytes in the small gaps between the Ag-nanocubes for SERS detection. The strategy has been proven effective for not only single- analytes but also multi-analytes without strong affinity for Ag, showing its potential in SERS-based simultaneous multi-analyte detection of both adsorbable and non-adsorbable pollutants in the environment.     

Synthesis of porous ZnO nanostructures using bamboo fibers as templates

In this study, we fabricated ZnO nanostructures using bamboo fibers as templates. The starting material used was zinc acetate, and the nanostructures were synthesized by soaking and calcining the bamboo fibers. The fabricated nanostructures were characterized using X-ray powder diffraction (XRD) analysis, scanning electron microscopy (SEM), and ultraviolet-visible spectrophotometry. The results showed that the size of the ZnO nanoparticles was approximately 20–100 nm. When the ZnO nanoparticles were used as the catalyst in the photodegradation of methyl orange, the dye degraded by 95.98 % in 80 min. The response and recovery times of a gas sensor based on the ZnO nanoparticles were 25 and 24 s, respectively, during the detection of C₂H₅OH in a concentration of 10 ppm at 270 °C.     

Rapid and sensitive detection of respiratory virus molecular signatures using a silver nanorod array SERS substrate

A spectroscopic assay based on surface enhanced Raman scattering (SERS) using silver nanorod array substrates has been developed that allows for rapid detection of trace levels of viruses with a high degree of sensitivity and specificity. This novel SERS assay can detect spectral differences between viruses, viral strains, and viruses with gene deletions in biological media. The method provides rapid diagnostics for detection and characterization of viruses generating reproducible spectra without viral manipulation.     

Nanostructured surfaces and assemblies as SERS media

Metallic nanostructures attract much interest as an efficient media for surface-enhanced Raman scattering (SERS). Significant progress has been made on the synthesis of metal nanoparticles with various shapes, composition, and controlled plasmonic properties, all critical for an efficient SERS response. For practical applications, efficient strategies of assembling metal nanoparticles into organized nanostructures are paramount for the fabrication of reproducible, stable, and highly active SERS substrates. Recent progress in the synthesis of novel plasmonic nanoparticles, fabrication of highly ordered one-, two-, and three-dimensional SERS substrates, and some applications of corresponding SERS effects are discussed.     

Silver nanoclusters films for single molecule detection using Surface Enhanced Raman Scattering (SERS)

A highly efficient Surface Enhanced Raman Scattering (SERS) substrate was prepared using a nanocluster deposition system that enabled detection of Crystal Violet molecules down to a single molecule level. The large SERS signal enhancement can be attributed to the presence of nano gaps on the surface of the nanoclusters which create abundant hot spots for electric field enhancement. Observed variations in the Raman peaks at very low molar concentrations in the range 4 × 10⁻¹⁴–3.2 × 10⁻¹⁸ M suggest that the spectra are due to a single molecule. Possible mechanisms for ultrahigh SERS sensitivity of the substrates are discussed. These substrates take the detection limit of CV down by two orders of magnitude as compared to those reported in literature.     

MOS-junction-based nanostructures by thermal oxidation of silicon wires for hydrogen detection

Heavily p-doped monocrystalline silicon wires have been fabricated by employing isotropic Si wet etch and thermal oxidation to achieve a nanometric cross section-a gate-oxide growth and a final palladium evaporation made up the MOS junction able to detect hydrogen concentration in air. Several types of wire dimensions have been designed and fabricated: length ranges from 5 to 70 /spl mu/m; the smallest widths obtained are around 250-300 nm, while the biggest are up to 7 /spl mu/m. Preliminary experimental results show a high signal/noise ratio sensor response to 100 ppm concentration of H/sub 2/ at room temperature, 1-atm air.     

Hydrothermal synthesis and characterization of TiO2 nanostructures using LiOH as a solvent

In the present study, we performed hydrothermal method as a simple and efficient route for the synthesis of rutile TiO₂ nanostructures in various concentrations of lithium hydroxide solutions. TiO₂ nanopowders with average sizes of 15 and 23 nm were prepared using 4 M and 7 M LiOH solutions. X-ray diffraction analysis (XRD), transmission electron microscope (FEG-STEM), scanning electron microscopy (SEM), and Brunauer–Emmet–Teller (BET) analyses were used in order to characterize the obtained products and comparison of the morphology of the powders obtained in different concentrations of LiOH solvent. It was shown that alkali solution concentration has affected the crystallinity, agglomeration ratio, particle size and specific surface area of the obtained rutile phases.     

Catalytic growth of nanostructures from carbonaceous substrates: Properties and model notions

The results of experimental investigations of carbon nanostructures grown direct from carbonaceous substrates are presented for the first time. A physical model explaining the shape of as-grown nanostructures is developed. The field emission characteristics of electron emitters based on the proposed structures have been studied.     

SERS biodetection using gold-silica nanoshells and nitrocellulose membranes

We have developed a rapid, reproducible, easy to execute, surface enhanced Raman scattering (SERS) method for detection of low volumes and total amounts of biological antigens using an analyte capture system derived from methods commonly used in Western blotting. Our method is a "half-sandwich" assay with an antigen detection scheme that employs a nitrocellulose (NC) membrane with 200 nm pore size to capture subnanograms of analyte and concentrate them in a small area from applied volumes as low as one microliter. The SERS probes used for detection consist of gold-silica nanoshells modified with a two-component mixed monolayer system. One component consists of a poly(ethylene glycol) (PEG)-modified Raman-active chromophore bound to the gold surface which allows for SERS detection and imparts particle stability. The second component uses (ortho-pyridyl) disulfide-PEG-succinimidyl ester to couple the recognition antibody to the particle surface. By controlling the reaction time and concentration of thiols, a mixed monolayer is prepared on the nanoshell surface with the ability to recognize low concentrations of analyte and generate reproducible SERS signals. Using this strategy, we have achieved SERS signals that are proportional to antigen present on the membrane allowing detection of total antigen amounts as low as 1.25 ng for some cases. The performance of this new SERS bioassay has been tested with a variety of potential antigens, demonstrating the potential for multiplexed detection of analytes.