High Surface-enhanced Raman Scattering Enhancement Factor Research Articles

Amorphous Nitrogen-Doped Carbon Nanocages with Excellent SERS Sensitivity and Stability for Accurate Identification of Tumor Cells.

The surface-enhanced Raman scattering (SERS) bioprobe's strategy for identifying tumor cells always depended on the intensity difference of the Raman signal compared with that of normal cells. Hence, exploring novel SERS nanostructure with excellent spectra stability, a high enhancement factor (EF), and good biocompatibility is a primary premise for boosting SERS signal reliability and accuracy of tumor cells. Here, high SERS EF (5.52 × 106) is acquired by developing novel amorphous nitrogen-doped carbon (NDC) nanocages (NCs), whose EF value was in a leading position among carbon-based SERS substrates. In addition, a uniform SERS signal was obtained on NDC NCs due to homogeneous morphology and size. The delocalized carbon-conjugated systems of graphitic-N, pyrrole-N, and pyridine-N with lone pair electrons increase the electronic density of states and reduce the electron localization function of NDC NCs, thereby promoting the charge transfer process. The electron-donor platform of the NDC NCs facilitates the thermodynamic process of charge transfer, resulting in multimode vibrational coupling in the surface complexes, which greatly amplifies the molecular polarizability. Importantly, the good biocompatibility and signal stability endow these NDC NC SERS bioprobes unique superiority in distinguishing tumor cells, and quantitative recognition of two triple-negative breast cancer cells based on SERS detection mode has been successfully realized.

Fast tailoring of gold nanoflowers by an interface-modified reverse microdroplet strategy

Gold nanoflowers (Au NFs) have received great attention for surface-enhanced Raman scattering (SERS) applications, due to the high surface area, availability of conjugation sites on surface and the localized surface plasmon resonance. However, their further application is much hampered by the complicated and time-consuming synthesis process, which remains better responded. Herein, we propose a novel & straightforward interface-modified reverse microdroplet strategy to configure homogeneously dispersed Au NFs within two hours, where Au3+ was reduced by excess aniline as two-in-one medium of reductant and solvent. It is demonstrated that the reaction system is robustly directed by the dynamic built-in reaction interface, thanks to the capping effect of the in-situ formed polyaniline (PANI) and polyvinyl pyrrolidone (PVP). It is further identified that the final morphology of Au NFs is strongly determined by the nucleation & growth rates. Consequently, such a well-tailored microstructure endows the Au NFs with sound performance for SERS applications. When using methylene blue (MB) as a molecular probe, a high SERS enhancement factor above 105 is delivered. This work provides a feasible interface-assisted methodology to investigate multi-level structures, and sheds light toward simply advancing versatile nanomaterials for broad applications.

AuNanostar@4-MBA@Au Core-Shell Nanostructure Coupled with Exonuclease III-Assisted Cycling Amplification for Ultrasensitive SERS Detection of Ochratoxin A.

The "turn-on" mode surface-enhanced Raman scattering (SERS) aptasensor for ultrasensitive ochratoxin A (OTA) detection was developed based on the SERS "hot spots" of AuNanostar@4-MBA@Au core-shell nanostructures (AuNS@4-MBA@Au) and exonuclease III (Exo III)-assisted target cycle amplification strategy. Compared with conventional gold nanoparticles, AuNS@4-MBA@Au provides a much higher SERS enhancement factor because AuNS exhibits a larger surface roughness and the lightning rod effect, as well as an excellent electromagnetic field between the AuNS core and the Au shell, which contribute to the superstrong SERS signal. Meanwhile, Exo III-assisted target cycle amplification can be used as an effective method for the further amplified detection of OTA. Additionally, the utilization of streptavidin magnesphere paramagnetic particles offers a green, economical, and facile technology for the accumulation and separation of the signal probe AuNS@4-MBA@Au from solution. All these factors lead to a significant enhancement of detectable signals and superhigh sensitivity. As a result, the limit of detection as low as 0.25 fg mL-1 could be achieved, which was lower than that in the other reported literatures on SERS methods for OTA detection as we know. The developed SERS aptasensor also provides a promising tool for foodstuff detection.

Dielectric Nanoparticles Coated upon Silver Hollow Nanosphere as an Integrated Design to Reinforce SERS Detection of Trace Ampicillin in Milk Solution

Surface-enhanced Raman scattering (SERS) technique is competent to trace detection of target species, down to the single molecule level. The detection sensitivity is presumably degraded by the presence of non-specific binding molecules that occupy a SERS-active site (or hot spot) on the substrate surface. In this study, a silver hollow nano-sphere (Ag HNS) with cavity has been particularly designed, followed by depositing dielectric nanoparticles (Di NPs) upon Ag HNS. In the integrated nanostructures, Di NPs/Ag HNS were furthermore confirmed by cutting through the cross sections using the Focused Ion Beam (FIB) technique, which provides the Scanning Electron Microscope (SEM) with Energy-dispersive Spectroscope (EDS) mode for identifying the distribution of Di NPs upon Ag HNS. The results have indicated that Di NPs/Ag HNS exhibits small diameter of cavity, and among Di NPs in this study, Al2O3 with lower dielectric constant provides a much higher SERS enhancement factor (e.g., ~6.2 × 107). In this study, to detect trace amounts (e.g., 0.01 ppm) of Ampicillin in water or milk solution, Al2O3 NPs/Ag HNS was found to be more efficient and less influenced by non-specific binding molecules in milk. A substrate with integrated plasmonic and dielectric components was designed to increase the adsorption of target species and to repulse non-specific binding molecules from SERS-active sites.

Ag/ZnO/Au 3D hybrid structured reusable SERS substrate as highly sensitive platform for DNA detection

Ultra-sensitive hybrid Silver/Zinc oxide/Gold (Ag/ZnO/Au) structure based three dimensional (3D) surface enhanced Raman scattering (SERS) substrates have been prepared by three step fabrication process using thermal evaporation, hydrothermal growth, and sputtering techniques. The size and inter-particle (IP) gap of decorated Au nanoparticles (NPs) on ZnO nanorods (NRs) in the sub-nanometer range have been achieved through varying the sputtering time of Au. The superhydrophobic nature, the formation of the Schottky barrier at ZnO/Au interface and the broad optical absorption spectrum facilitated towards higher SERS activity of Ag/ZnO/Au hybrid structures. The higher SERS activity of 3D SERS substrate as compared with two dimensional (2D) SERS substrate has been studied. The good SERS signal reproducibility of 3D hybrid structures have been explored through Raman mapping. Higher SERS enhancement factor (EF) of 1 × 1010 has been achieved with a limit of detection (LOD) up to 10−16 M and 10 ng/μL for Rhodamine-6 G (Rh6G) and lambda DNA (λ-DNA), respectively. The degradation of Rh6G and λ-DNA molecules have been studied through photocatalytic degradation process to explore the reusability of the SERS substrates up to 10 and 4 times, respectively, with maintaining good SERS signal reproducibility. This metal/semiconductor/metal hybrid structure based SERS substrate with reusable capability indicates potential application towards biosensor for the detection of biologically important molecules at very low concentration level.

3D Plasmon Coupling Assisted Sers on Nanoparticle-Nanocup Array Hybrids

Unique colorimetric optical properties of nanomaterials can effectively influence the light absorption or emission of molecules. Here, we design plasmonic substrate for surface-enhanced Raman scattering (SERS) by inducing three-dimensional (3D) hot spots on the sensing surface. The 3D hot spots are formed by the self-assembly of plasmonic nanoparticles (NPs) on a 3D plasmonic nanocup array structure. This 3D hot spot formation on the periodic nanocup arrays achieves much higher SERS enhancement factor than the 2D NP arrays, which have been conventionally sought SERS substrates. We also utilize the colorimetric properties of the nanocup arrays for an additional degree of SERS enhancement. Colorimetry, achieved by tunable plasmon resonance wavelength by controlling dielectric property on the nanocup array surface, eases the modulation of the plasmonic resonance condition without modifying the nanostructure design. By continuously monitoring the shifts of the plasmon resonance condition and its effect on the light absorption and emission of the nearby molecules, we verify that larger SERS enhancement is achieved when the plasmon resonance wavelength is matched with the Raman excitation wavelength. The ease of plasmon resonance tuning of this nanocup array-nanoparticle hybrid structure allows versatile SERS enhancement for a variety of different Raman measurement conditions.

Synthesis and characterisation of dual plasmonic gold nanostars as high‐performance surface‐enhanced Raman spectroscopy substrate

Synthesising gold nanoprobes in the near infrared (NIR) region is of particular interest in developing nanosensors due to the minimal light attenuation from biomolecules. Here, the controlled synthesis and tunability of gold nanostars’ two distinct localised surface plasmon resonances (LSPRs) at around 700 and 1100 nm is reported. By using UV–Vis–NIR absorption measurements and finite-difference time-domain calculations, the induction of the LSPR and the multipolar nature of the resonances have been investigated experimentally and theoretically. Simulation results demonstrate that large electric fields are confined at the tips of the branches, where the LSPR can be induced specifically by controlling the polarisation of the incident electric field. The surface-enhanced Raman scattering (SERS) capability of these dual plasmonic gold nanostars (DPGNS) has also been demonstrated using a Raman reporter, diethylthiatricarbocyanine iodide and high SERS enhancement factor (EF) of 2 × 107 is obtained with 785 nm excitation. With ease of synthesis, LSPR at NIR and high SERS EF, DPGNS demonstrated the capability to be an effective SERS substrate and the potential to elicit the highest SERS EF ever reported for gold nanoparticles, with further longer wavelength excitations at and beyond 1064 nm.

Ultra-small rhenium nanoparticles immobilized on DNA scaffolds: An excellent material for surface enhanced Raman scattering and catalysis studies

Highly Sensitive and ultra-small Rhenium (Re) metal nanoparticles (NPs) were successfully stabilized in water by the staging and fencing action of the versatile biomolecule DNA that resulted in two distinct aggregated chain-like morphologies with average grain sizes of 1.1±0.1nm and 0.7±0.1nm for the very first time within a minute of reaction time. Re NPs are formed by the borohydride reduction of ammonium perrhenate (NH4ReO4) in the presence of DNA at room temperature (RT) under stirring. The morphologies were controlled by carefully monitoring the molar ratio of NH4ReO4 and DNA. The synthesized material was employed in two potential applications: as a substrate for surface enhanced Raman scattering (SERS) studies and as a catalyst for the reduction of aromatic nitro compounds. SERS study was carried out by taking methylene blue (MB) as the probe and the highest SERS enhancement factor (EF) of 2.07×107 was found for the aggregated chain-like having average grain size of 0.7±0.1nm. Catalytic reduction of 4-nitro phenol (4-NP), 2-nitro phenol (2-NP) and 4-nitroaniline (4-NA) with a rate constant value of 6×10−2min−1, 33.83×10−2min−1 and 37.4×10−2min−1 have testified the excellent catalytic performance of our Re NPs immobilized on DNA. The overall study have revealed the capability of DNA in stabilizing the highly reactive Re metal at nanoscale and made them applicable in practice. The present route can also be extended to prepare one dimensional (1-D), self-assembled NPs of other reactive metals, mixed metals or even metal oxides for specific applications in water based solutions.

Two-Step Centrifugation Method for Subpicomolar Surface-Enhanced Raman Scattering Detection.

The potentiality of surface-enhanced Raman scattering (SERS) to detect ultralow concentrations of analyte has attracted much attention in detection of trace components in both medicinal and environmental samples. However, detection at trace concentration usually requires sophisticated systems. Here, we present an ultrasensitive and facile SERS approach, a two-step centrifugation method, which achieved a detection limit of 500 fM with phenformin hydrochloride and risperidone as acidic and alkaline analyte, respectively. This method consists of two steps: (1) centrifuging colloidal silver to increase nanoparticles' concentration and to remove small-size nanoparticles, thus increasing the chance of analyte adsorption on large nanoparticles that have strong SERS activity; (2) centrifuging samples after the analytes were mixed with nanoparticles. After the first centrifugation and mixing with aqueous analyte, the colloidal silver is either flocculated (for high-concentration samples) or forms a nanoparticle-analyte complex (for low-concentration samples). Until the second centrifugation, the concentration of analyte and hot-spot formation is significantly increased, and thus a high SERS enhancement factor is obtained. In short, the two-step centrifugation method overcomes drawbacks of the traditional method, which demands not only sophisticated operation but also expensive instruments, to fully exploit the potential of SERS detection.

Synthesis of Silver Decorated Silica Nanospheres for Surface Enhanced Raman Scattering (SERS) Substrates

A sol-gel technique has been used to synthesize silver decorated silica nanospheres for surface enhanced Raman scattering (SERS) substrates. X- Ray diffraction (XRD) spectra shows peak at 2θ = 38.1, 44.6, 64.7, and 77.5°confirming the presence of Ag nanoparticles on the substrates. The Ag- decorated silica nanospheres were applied as SERS substrates using Rhodamine 6G (R6G) as probe molecule. From Raman analysis, the highest SERS enhancement factor at R6G concentration of 102was calculated around ~1010.

Surface-enhanced Raman and optical scattering in coupled plasmonic nanoclusters

Optical excitation of small Au nanoparticle (NP) clusters of appropriate wavelength is known to generate intense electromagnetic fields localized uniquely at NP junction sites within the nanoclusters. These intense and localized field hot-sites can induce intense surface-enhanced Raman scattering (SERS) of molecules residing at the junction hot-sites. In this paper, we present a series of electromagnetic simulations, experimental SERS and extinction data obtained from small self-assembled Au NP clusters coated to saturation with a Raman reporter molecule. Our experimental data show that the SERS enhancement factor remains relatively constant despite the heterogeneity of the nanocluster and this is supported by the simulation results. Furthermore, our simulation results show significant variations in the localized electric field intensities of the junction hot-sites in different nanocluster geometries. This explains the observation that increasing the number of hot-sites does not necessarily result in a higher SERS enhancement factor.

SILVER NANOPARTICLES FILMS DEPOSITED ON AAO TEMPLATES BY THERMAL EVAPORATION FOR SURFACE-ENHANCED RAMAN SCATTERING OF R6G

Silver nanoparticles (NPs) films with different nominal thicknesses were deposited on the anodic aluminum oxide (AAO) templates with pore diameter about 200 nm by thermal evaporation. These NPs films were used as the surface enhanced Raman scattering (SERS) active substrates. The microstructure and surface morphology of the films were studied by X-ray diffraction and scanning electron microscopy, respectively. The SERS activity of the films was investigated by Raman scattering of adsorbed rhodamine 6G (R6G) at different concentrations. The results revealed that the average diameter of Ag NPs in different samples is 58 nm, 75 nm, 93 nm and 108 nm, respectively. Ag NPs film on AAO template is very suitable as a SERS active substrate and can detect R6G molecules with a concentration of 10-8 M. The intensity of the SERS spectra is related to both the Ag NPs size and interparticle distance. The sample with an average particle diameter of 93 nm has the highest SERS enhancement factor (1.3 × 106).

Silver Nanoparticle Arrays on a DVD-Derived Template: An easy&cheap SERS Substrate

Commercially available digital versatile discs (DVDs) contain a silver-coated spiral distribution of rectangular-shaped grooves (AgDVD): for the first time, they have been used to produce surface-enhanced Raman scattering (SERS) substrates by electrochemical deposition of silver nanoparticles (AgNPs@AgDVD). The overall procedure only requires cheap and widely available materials and can be easily accomplished. Scanning electron microscopy images of AgNPs@AgDVD revealed that small AgNPs (average diameter about 15 nm) are present within the valleys of AgDVD, whereas over the ridges, the AgNPs are bigger, more densely packed and with a dendrite-like morphology somewhere. The SERS properties of these substrates have been studied in terms of the enhancement factor (EF), point-to-point reproducibility and sample-to-sample repeatability. It turned out that high SERS EF and good reproducibility requirements are both fulfilled. As for repeatability, remarkably better results than typical literature values have been achieved. Such an easy&cheap preparation along with efficient SERS properties make DVD-derived SERS substrates very good candidates for the development of convenient and disposable sensing platforms.

Templated fabrication of metal half-shells for surface-enhanced Raman scattering

Here we report a scalable colloidal templating approach for producing metal half-shells as efficient surface-enhanced Raman scattering (SERS) substrates. Nonclose-packed monolayer colloidal crystals created by a spin-coating technology are used as structural template to fabricate both water-dispersed half-shells and disordered arrays of half-shells with preferential upright orientation. The sharp edges of the half-shells and the small gaps between adjacent shells can significantly enhance the local electromagnetic field, resulting in high SERS enhancement factor (up to 10(10)) which is nearly 2 orders of magnitude higher than those of periodic substrates produced by other colloidal templating approaches. We have also demonstrated that the thickness of the half-shells determines the surface plasmon resonance and the resulting SERS enhancement. Counterintuitively, the disordered arrays of oriented half-shells show reproducible enhancement with a standard deviation of less than 20%. This new bottom-up approach enables the large-scale production of SERS substrates that are at least 2 orders of magnitude larger in area than those made by other colloidal lithography technologies. The resulting substrates with high and reproducible SERS enhancement are promising for ultrasensitive chemical and biological sensing.