Surface Enhanced Raman Spectroscopy Enhancement Factor Research Articles

Two-dimensional silver nanonetwork on Ag4Ti5O12 film as highly efficient SERS substrate

Surface Enhanced Raman Spectroscopy (SERS) is a powerful technique to study and detect single molecules adsorbed onto a surface. In this work, molar concentrations were varied over three consecutive orders of magnitude (10-3 M, 10-6 M, 10-9 M) of Methylene Blue (MB) to study SERS on different thicknesses of silver (6, 8, 10, 12 nm) coated on Ag4Ti5O12 deposited onto glass substrates as the base material. The Ag4Ti5O12 thin films were prepared by a unique ion-exchange method where Li4Ti5O12 thin films were initially deposited by the sol–gel method followed by the exchange of Li+ ion with Ag+ ion. The varied thicknesses of thermally deposited Ag-thin films were grown with unaltered Ag+ ions nucleation sites atop the Ag4Ti5O12 layer to create the two-dimensional (2D) nanonetwork. The structure and morphology of the as-prepared thin-film samples were characterized using X-ray diffraction, Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM) techniques. The SERS enhancement factor (EF), calculated using Raman spectroscopy, showed the systematic changes in the Raman signal enhancement as a function of Ag-thickness. The 10 nm thick as-deposited Ag sample showed the highest signal enhancement in Raman spectra than the other Ag thickness films. The successive signal enhancement was obtained due to forming the critical nuclei size of Ag on top of Ag4Ti5O12, which was further bolstered with film surface roughness and surface energy measurements. The as-prepared Ag-10 nm sample showed the lowest film roughness of ∼2.07 nm, in tune with the maximum enhancement factor (EF). We believe this work could open wide opportunities for developing efficient SERS substrates with sensing capabilities for ultra-low analyte concentration coupled with low cost and high enhancement factors.

Oblique-Incidence-Excited Localized Hot Spots in Plasmonic Particle-on-Film Nanocavities

Regarding plasmonic nanocavities formed by particle-on-film, the nanogap hot spots play an essential role in wide applications including plasmon-enhanced spectroscopy, ultrasensitive sensing, photovoltaics, and photocatalysis. However, hot spots at the nanogap are symmetrically distributed on both sides of the junction under normal incidence. Herein, under the condition of oblique incidence, properties of plasmonic hot spots in the Au@SiO2 particle-on-film have been investigated theoretically. We find that when the plane wave is incident obliquely, the hot spots in the gap are not only symmetrically distributed on both sides of the contact point but also circularly distributed around the junction at specific excitation wavelengths. With the adjustment of the incidence angle, the maximum surface-enhanced Raman spectroscopy enhancement factor from this circularly distributed hot spot reaches 11.3 orders in magnitude. Moreover, the resonant mode at 785 nm has been readily matched by optimizing the size of the Au core, and the optimal thickness of the silica shell with 1 nm for achieving the maximum electromagnetic (EM) field enhancement was further demonstrated. These findings offer a new strategy for manipulating locations of localized hot spots with strong near-field enhancement in plasmonic nanocavities.

Robust, reproducible, recyclable SERS substrates: monolayers of gold nanostars grafted on glass and coated with a thin silica layer

We prepared and characterized recyclable surface enhanced Raman spectroscopy (SERS) active glass chips. Gold nanostars were grafted on properly functionalized glasses by means of electrostatic interactions and then they were coated with a silica layer of controllable thickness in the nanometer range. The SERS activity of the obtained substrates were tested in terms of reproducibility and homogeneity intra-samples and inter-samples from different batches using the Raman reporter as the model compound rhodamine 6G. The uncoated substrates were used as reference to evaluate the effect of silica spacers on SERS enhancement factors (EFs). The chemical route to obtain silica-coated SERS chips is described in detail, and the morphology and the optical response of substrates have been characterized. We demonstrate that SERS substrates coated with 1 nm silica conserve a good EF, and that the coating confers to the SERS platform an extreme robustness leading to reusability of the substrates.

Three-dimensional (3D) plasmonic hot spots for label-free sensing and effective photothermal killing of multiple drug resistant superbugs.

Drug resistant superbug infection is one of the foremost threats to human health. Plasmonic nanoparticles can be used for ultrasensitive bio-imaging and photothermal killing by amplification of electromagnetic fields at nanoscale "hot spots". One of the main challenges to plasmonic imaging and photothermal killing is design of a plasmonic substrate with a large number of "hot spots". Driven by this need, this article reports design of a three-dimensional (3D) plasmonic "hot spot"-based substrate using gold nanoparticle attached hybrid graphene oxide (GO), free from the traditional 2D limitations. Experimental results show that the 3D substrate has capability for highly sensitive label-free sensing and generates high photothermal heat. Reported data using p-aminothiophenol conjugated 3D substrate show that the surface enhanced Raman spectroscopy (SERS) enhancement factor for the 3D "hot spot"-based substrate is more than two orders of magnitude greater than that for the two-dimensional (2D) substrate and five orders of magnitude greater than that for the zero-dimensional (0D) p-aminothiophenol conjugated gold nanoparticle. 3D-Finite-Difference Time-Domain (3D-FDTD) simulation calculations indicate that the SERS enhancement factor can be greater than 104 because of the bent assembly structure in the 3D substrate. Results demonstrate that the 3D-substrate-based SERS can be used for fingerprint identification of several multi-drug resistant superbugs with detection limits of 5 colony forming units per mL. Experimental data show that 785 nm near infrared (NIR) light generates around two times more photothermal heat for the 3D substrate with respect to the 2D substrate, and allows rapid and effective killing of 100% of the multi-drug resistant superbugs within 5 minutes.

Bio-Conjugated Gold Nanoparticle Based SERS Probe for Ultrasensitive Identification of Mosquito-Borne Viruses Using Raman Fingerprinting.

Dengue virus (DENV) and West Nile virus (WNV) are two well-documented mosquito-borne flaviviruses that cause significant health problems worldwide. Driven by this need, we have developed a bio-conjugated gold nanoparticle (AuNP)-based surface enhanced Raman spectroscopy (SERS) probe for the detection of both DENV and WNV. Reported data demonstrate anti-flavivirus 4G2 antibody conjugated gold nanoparticle (GNP) SERS probe can be used as a Raman fingerprint for the ultrasensitive detection of DENV and WNV selectively. Experimental data show that due to the plasmon coupling in nano-assembly, antibody conjugated GNP- based SERS is able to detect as low as 10 plaque-forming units (PFU)/ml of DENV-2 and WNV, which is comparable with the sensitivity of quantitative PCR-based assays. Selectivity of our probe was demonstrated using another mosquito-borne chikungunya virus (CHIKV) as a negative control. Experimental data demonstrate a huge enhancement of SERS intensity is mainly due to the strong electric field enhancement, which has been confirmed by the finite-difference time-domain (FDTD) simulation. Reported FDTD simulation data indicate the SERS enhancement factor can be more than 104 times, due to the assembled structure. Reported results suggest that bio-conjugated AuNP-4G2 based SERS probes have great potential to be used to screen viral particles in clinical and research-based laboratories.

Aluminum and Indium Plasmonic Nanoantennas in the Ultraviolet

We use classical electrodynamics calculations to investigate the versatility and capability of aluminum and indium dimers with small gaps as ultraviolet plasmonic nanoantennas, focusing on the particle size and wavelength range that gives optimum near-field enhancement. We find that Al and In are highly capable plasmonic materials in the ultraviolet, even with the incorporation of Al2O3 shells on the Al spheres; however, Al is strongly influenced by quadrupole modes while In is not. Al is the optimal material in the deep-UV, while In is ideal in the near-UV and near-visible spectral regions. Unlike Au and Ag, Al and In are most effective with the lowest refractive index background media possible, with vacuum being ideal. Ag outperforms both Al and In red of ∼320 nm, but optimal surface-enhanced Raman spectroscopy enhancement factors are still substantial for Al and In, with peak |E|4 values (for dimers in vacuum with a 1 nm gap) determined to be: Al, 2.0 × 109 (at 204 nm); In, 1.2 × 109 (at 359 nm); Al/Al...

Highly Sensitive Surface Enhanced Raman Spectroscopy from Ag Nanoparticles Decorated Graphene Sheet

Surface enhanced Raman spectroscopy (SERS) is a powerful analytical technique and has been most intensively studied. In this work, electroless deposition is proposed for Ag nanoparticles (NPs) decorated on chemical vapor deposition (CVD) growth graphene sheets (GS) to create hybrid SERS substrate. From three aspects of size distribution, morphology, and coverage, Ag NPs controllable decoration on GS and SERS enhancement factors of the hybrid SERS substrate is investigated. 200–300 times enhanced SERS intensities are detected from the Ag NPs on GS hybrid as compared to pure GS. Controllable decoration is crucial for improving SERS enhancement factorsβEF, becauseβEFfrom quasi cubic Ag NPs on GS is 6.53 times stronger than that from spheric one; 1.6 timesβEFis detected while the Ag NPs size distribution is reduced to half, and when the coverage is doubled,βEFis nearly doubled. This controllable Ag NPs/GS hybrid is capable of serving as a high performance SERS substrate for efficient chemical and biological sensing applications.

Wide-Band Spectral Control of Au Nanoparticle Plasmon Resonances on a Thermally and Chemically Robust Sensing Platform

Gold nanoparticles on Al2O3-coated gold films are presented as a chemically and thermally robust platform for molecular sensing. Single particle spectroscopy as a function of Al2O3 coating thickness shows reproducible gold nanoparticle scattering spectra in the range from 690 to 610 nm as the Al2O3 thickness increases from 0 to 3.4 nm. Numerical simulation of these structures indicates that surface-enhanced Raman spectroscopy enhancement factors in excess of 106 can be achieved. The stability of the Al2O3-coated structures under high-power laser irradiation was tested, revealing stable scattering spectra upon irradiation with 100 W/mm2 at the particle resonance wavelength. The presented structure solves challenges with thermal stability, wavelength tuning range, and Raman background signal associated with previously attempted approaches.

Chemical Enhancement of the Surface Enhanced Raman Scattering Signals of Anilines via Their <i>Ortho</i>-Substituents

While Raman spectroscopy is a useful method for analyzing many organic compounds, it is limited by relatively low sensitivity. Therefore, Surface Enhanced Raman Spectroscopy (SERS) based on the adsorption of organic analytes onto gold or silver nanostructures has been used to enhance the signal of chemicals presented at very low concentrations. Although the plasmonic effect of SERS has been shown to play a large role in signal enhancement, the significance of the chemical effect due to the analyte chemisorption on the gold or silver surface is less well understood. In this study, the role of aniline substituents is examined by probing the SERS intensities of various anilines in silver and gold colloids using a Raman spectrometer with an excitation wavelength of 785 nm. The SERS enhancement factors and detection limits for aniline and its mono- and di-substituted ortho derivatives are determined and compared. Both the steric requirements of chemisorption and the inductive effects of electron-withdrawal due to the substituents affect the signal intensities of various vibrational modes of the amino group and the aromatic ring. The degree of enhancement is also related to the methods for preparing the silver and gold colloids, which are characterized by probing the nanoparticle morphology and its degree of aggregation using transmission electron microscopy (TEM).

In situ formation of curcumin stabilized shape-selective Ag nanostructures in aqueous solution and their pronounced SERS activity

The in situ formation of shape-selective silver nanoparticles (Ag NPs) in an aqueous solution has been demonstrated within one hour. Ag NPs were synthesized by the reduction of Ag(I) ions in the presence of a new reducing and stabilizing reagent, curcumin, under continuous stirring and heating at 80 °C. Different sizes and shapes of Ag NPs, such as small and large spheres, nanowires and anisotropic nanoflakes, are formed just by tuning the molar ratio of curcumin to Ag(I) ions and controlling the other reaction parameters. The mechanism of formation, the role of curcumin and other reaction parameters for the formation of shape-selective Ag NPs have been examined in detail. The different shaped Ag NPs have been successfully employed for the application in surface enhanced Raman spectroscopy (SERS) studies using methylene blue (MB) as a Raman probe. The shape effect of the NPs has been examined in detail and the corresponding SERS enhancement factor (EF) values have been calculated. The observed EF values are in the range ∼106 to 107. It was found that the anisotropic nanoflake structure has the highest EF compared to other shapes. The synthesis process is straightforward, simple, reproducible, cost-effective and robust. The synthesized Ag NPs are stable for more than two months while kept in a sealed bottle in the refrigerator. In the future, the present process might be helpful for the synthesis of other types of nanomaterials in a small time scale with higher yields. Moreover, the high EF value of the materials should be useful for trace detection of environmentally important biological species, DNA and RNA sequences or even in medical therapeutics.