Quantitative Surface-enhanced Raman Scattering Analysis Research Articles

Surface-Enhanced Raman Scattering Quantitative Analysis of Ethanol Drop-Coating Silver Nanocubes on Gold Film.

Surface-enhanced Raman scattering (SERS) has been a sensitive tool for the accurate detection and analysis of a wide range of molecules. In the present study, a simple and repeatable approach is developed for the fabrication of a silver nanocubes/polyelectrolyte/gold film (Ag nanocubes/PE/Au film) sandwich structure as SERS substrate. An ethanol-water mixture, instead of pure water, is used as solvent to reduce the coffee ring effect by the drop coating deposition method, such that Ag nanocubes are distributed evenly on the gold film surface with polyelectrolyte as the middle layer. In 15 repeated measurements of a 10-7 M rhodamine 6G solution, the intensity of the Raman peak at 609 cm-1 exhibits a relative standard deviation less than 20%. Significantly, the sandwich substrate exhibits excellent point-to-point repeatability and sample to sample reproducibility, and may be used for real-life quantitative analysis, as demonstrated by the rapid diagnosis of dual analytes of rhodamine 6G and crystal violet.

Dual-mode fluorescence–SERS sensor for sensitive and selective detection of uranyl ions based on satellite Fe3O4-Au@CdTe nanostructure

Increasingly serious uranium pollution necessitates a new method for rapid qualitative and quantitative uranium detection. Some methods based on fluorescence spectroscopy and surface-enhanced Raman scattering (SERS) have been reported to detect uranyl ions (UO22+), the main species of uranium. However, various factors affect the fluorescence selectivity and SERS quantitative analysis. Herein we report on a composite material for a dual-mode fluorescence–SERS detection protocol for simultaneous quantitative and qualitative analysis of UO22+, based on a layer-by-layer assembly strategy, combining the advantages of the two complementary techniques. The composite material consists of a Fe3O4@SiO2 core coated by gold nanoparticles (FA) via electrostatic adsorption. The CdTe quantum dots (CdTe QDs) outside the FA are linked with amide bonds to form a satellite Fe3O4-Au@CdTe nanostructure (FA-CdTe). The fluorescence signal of FA-CdTe was selectively quenched by UO22+, with a detection limit as low as 1.2 nM; the material was also used as a SERS substrate to detect the Raman mode of uranyl at 828 cm−1. Furthermore, this method has been applied for UO22+ detection in seawater, river water, and tap water, demonstrating the broad application prospects and providing novel development concepts.

Ultra-sensitive, reusable, and superhydrophobic Ag/ZnO/Ag 3D hybrid surface enhanced Raman scattering substrate for hemoglobin detection

The small Raman scattering cross section of hemoglobin (Hb) molecules limits its application using a Raman spectroscopy based optical biosensor. Label-free surface enhanced Raman scattering (SERS) detection and degradation of Hb have been achieved using 3D reusable superhydrophobic SERS substrates based on a Ag/ZnO/Ag hybrid structure. The fabrication process follows the decoration of thermally evaporated non-spherical like Ag nanoparticles on hydrothermally grown ZnO nanorods on a catalytic ultra-thin Ag film. From SERS point of view, these 3D SERS substrates exhibit four important characteristics such as a higher surface to volume ratio, surface plasmon resonance in the broad wavelength region of the visible spectrum, a strong electric field at the Ag–ZnO interface due to the formation of a Schottky barrier, and the superhydrophobic surface. The SERS substrates not only performed an outstanding Raman enhancement effect due to the above factors but also displayed multiple recyclabilities owing to their excellent self-cleaning property via a UV light assisted photocatalytic degradation process. The quantitative SERS analysis has been performed by a linear regression method and resulted in 10−13.42M and 10−7.24M limit of detection for Rhodamine 6G (Rh6G) and Hb molecules, respectively, with an enhancement factor of 6 × 1011. The effect of the 3D hybrid structure toward higher SERS activity has been compared with that of 2D SERS substrates, and the SERS mapping of Rh6G molecules proves good homogeneity of the 3D SERS substrates. These ultra-sensitive 3D SERS substrates with reusable capability open the possibility of their use toward biosensors for the early detection of diseases.

Binary Thiol-Capped Gold Nanoparticle Monolayer Films for Quantitative Surface-Enhanced Raman Scattering Analysis.

Surface-enhanced Raman scattering (SERS) can provide fingerprint information of analyte molecules with unparalleled sensitivity. However, quantitative analysis using SERS has remained one of the major challenges owing to the difficulty of obtaining reproducible SERS substrates with high-density hotspots. Here, we report the rational design and fabrication of a binary thiol-capped gold nanoparticle (AuNP) monolayer film (MLF) as a substrate for highly sensitive and quantitative SERS analysis. The two thiol ligands chemically bonded to the AuNPs play different roles: dodecanethiol with a long alkyl chain controls the interparticle gaps and electromagnetic coupling among AuNPs and 4-mercaptopyridine works as a Raman internal standard (IS). The binary thiol-capped AuNPs can self-assemble into an ordered MLF with high-density hotspots anduniformly distributed IS. The as-prepared MLF has been demonstrated as a reliable SERS substrate for quantitative detection of fungicide malachite green in aqueous solution, with a high enhancement factor (up to 3.3 × 107) and a low detection limit (100 pM). Moreover, the MLF SERS substrate is flexible and transparent, which has enabled in situ detection of trace fungicide residues in a shrimp tissue.

Synthesis of Spiked Plasmonic Nanorods with an Interior Nanogap for Quantitative Surface-Enhanced Raman Scattering Analysis.

Realizing quantitative surface-enhanced Raman scattering (SERS) analysis is extremely helpful and challenging. Here, we utilize a facile method to synthesize spiked plasmonic nanorods with an interior gap. The Raman signal from the molecules embedded in the gap can be dramatically enhanced, leading to strong, stable, and reproducible SERS signals that can be used as an internal reference for quantitative SERS analysis. We demonstrate that the rough exterior surface has a good performance in enhancing the Raman signal of polycyclic aromatic hydrocarbon molecules adsorbed on the surface. The result shows that this method is applicable for a large range of analyte concentrations and there is an excellent linear relationship between the SERS intensity ratio and the analyte concentration (0.5–100 μM).

Giant Vesicles with Anchored Tiny Gold Nanowires: Fabrication and Surface-Enhanced Raman Scattering.

Sensitivity and reproducibility are two major concerns to improve the performance and extend the range of practical applications of surface-enhanced Raman scattering (SERS). A theoretical report reveals that hot spots formed by gold nanoparticles with a tip-to-tip configuration would generate the maximum electric field enhancement because of the lightning rod effect. In our present study, we constructed a giant vesicle consisting of anchored tiny gold nanowires to provide a high density of sharp tip-to-tip nanogaps for SERS application. The tiny gold nanowires were directly grown and anchored onto the surfaces of polystyrene (PS) microspheres by a seed-mediated method. Then, the removal of PS microspheres by tetrahydrofuran led to the formation of the giant gold vesicles with hierarchical cage structures, providing the sharp tips and high density of hot spots for improving SERS performance. Compared with the nonwire structure (island and inhibited nanoparticle), giant gold vesicles with tiny wires showed a higher SERS enhancement factor (9.90 × 107) and quantitative SERS analysis in the range of 10-4 to 10-7 M. In addition, the large-scale giant gold vesicle array on the silica substrate resulted in a high reproducibility of SERS signals with the variation of intensities less than 7.6%.

Tunable and Linker Free Nanogaps in Core-Shell Plasmonic Nanorods for Selective and Quantitative Detection of Circulating Tumor Cells by SERS.

Controlling the size, number, and shape of nanogaps in plasmonic nanostructures is of significant importance for the development of novel quantum plasmonic devices and quantitative sensing techniques such as surface-enhanced Raman scattering (SERS). Here, we introduce a new synthetic method based on coordination interactions and galvanic replacement to prepare core-shell plasmonic nanorods with tunable enclosed nanogaps. Decorating Au nanorods with Raman reporters that strongly coordinate Ag+ ions (e.g., 4-mercaptopyridine) afforded uniform nucleation sites to form a sacrificial Ag shell. Galvanic replacement of the Ag shell by HAuCl4 resulted in Au-AgAu core-shell structure with a uniform intra-nanoparticle gap. The size (length and width) and morphology of the core-shell plasmonic nanorods as well as the nanogap size depend on the concentration of the coordination complexes formed between Ag+ ions and 4-mercaptopyridine. Moreover, encapsulating Raman reporters within the nanogaps afforded an internal standard for sensitive and quantitative SERS analysis. To test the applicability, core-shell plasmonic nanorods were functionalized with aptamers specific to circulating tumor cells such as MCF-7 (Michigan Cancer Foundation-7, breast cancer cell line). This system could selectively detect as low as 20 MCF-7 cells in a blood mimicking fluid employing SERS. The linking DNA duplex on core-shell plasmonic nanorods can also intercalate hydrophobic drug molecules such as Doxorubicin, thereby increasing the versatility of this sensing platform to include drug delivery. Our synthetic method offers the possibility of developing multifunctional SERS-active materials with a wide range of applications including biosensing, imaging, and therapy.

Hydrophobic ligand-mediated hierarchical Cu nanoparticles on reduced graphene oxides for SERS platform

Hierarchical Cu nanoparticles/rGO (CuNPs/rGO) composites were synthesized through the kinetic reaction mediated by hydrophobic methyl trioctyl ammonium chloride (A336) and 2-hydroxy-5-nonyl-acetophenone oxime (LIX84I). The CuNPs/rGO composites were characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD), and used as substrates for surface-enhanced Raman scattering (SERS) measurements. The hierarchical CuNPs/rGO composites formed with A336 were assemblies of Cu nanosubunits with a crystal size of 5–10 nm and some observable pinholes, whereas compact Cu nanospheres and nanorods on rGO sheets were formed with LIX84I. Therefore, a hydrophobic ligand-mediated kinetic mechanism was proposed. A synergetic effect on SERS performance was observed for CuNPs/rGO composites and the ratio between CuNPs and rGO was optimized. Moreover, the SERS activity of CuNPs/rGO-A336 was much stronger than CuNPs/rGO-LIX84I owing to the hierarchical structure of CuNPs/rGO-A336. For the CuNPs/rGO-A336 substrate, the detection limit reached 10−8 mol L−1 for R6G molecules, and quantitative SERS analysis was achieved. The CuNPs/rGO-A336 substrate showed good reproducibility and a high enhancement factor of 2.75 × 106, which is superior to other Cu-based substrates in the literature.