Electrical Tuning of the SERS Enhancement by Precise Defect Density Control.

The discovery of surface-enhanced Raman spectroscopy (SERS) presents an opportunity for the development of ultrasensitive biosensors. SERS allows for fingerprint detection of biomolecules at low concentrations and can perform single-molecule detection. The most used SERS substrates are Plasmonic metal nanostructures such as Au and Ag mainly because of their superior electrical conductivity. The Plasmonic metal nanoparticles enhance the Raman signal of the target biomolecules through the electromagnetic field mechanism. Despite the successful use of metal nanostructures, alternative materials have been sought which can exhibit similar or better properties than their metal counterparts. Materials such as graphene which has excellent electrical properties and high surface area are good candidates for SERS applications. Graphene is a 2D nanomaterial consisting of sp2 hybridized carbon atoms. The use of graphene in SERS is termed graphene-enhanced Raman spectroscopy (GERS) and is reported to use a charge transfer mechanism. Both graphene and metal nanostructures can be used together as SERS substrate and the synergistic effect of their mechanism can improve the enhancement factor and the sensitivity of the biosensor, however, the use of graphene as a SERS substrate is still at an early stage. GERS can be applied in disease diagnosis, detection of food additives, and biological species biosensors. The use of GERS in biosensors such as glucose biosensors presents an opportunity for ultrasensitive biosensors which can greatly improve the lives of individuals living with diabetes. In this review insights will be given on how to successfully circumvent the challenges associated with small Raman cross-section of glucose detection for use in medical application.

Applicable surface enhanced Raman scattering (SERS) active substrates require high enhancement factor (EF), excellent spatial reproducibility, and low-cost fabrication method on a large area. Although several SERS substrates with high EF and relative standard deviation (RSD) of signal less than 5% were reported, reliable fabrication for large area SERS substrates with both high sensitivity and high reproducibility via low-cost routes remains a challenge. Here, we report a facile and cost-effective fabrication process for large-scale SERS substrate with Ag inter-nanoparticle (NP) gaps of 5 nm based on ultrathin alumina mask (UTAM) surface pattern technique. Such closely packed Ag NP arrays with high density of electromagnetic field enhancement ("hot spots") on large area exhibit high SERS activity and excellent reproducibility, simultaneously. Rhodamine 6G molecules with concentration of 1 × 10(-7) M are used to determine the SERS performance, and an EF of ∼10(9) is obtained. It should be noted that we obtain RSDs about 2% from 10 random spots on an area of 1 cm(2), which implies the highly reproducible signals. Finite-difference time-domain simulations further suggest that the enhanced electric field originates from the narrow gap, which agrees well with the experimental results. The low value of RSD and the high EF of SERS signals indicate that the as-prepared substrate may be promising for highly sensitive and uniform SERS detection.

The investigation of Au-Ag nanoalloys as SERS (Surface Enhanced Raman Scattering) substrate in enhancement of Raman Signal was interested. In this work, we reported the performance of Au-Ag nanoalloys as SERS substrate on silicon wafer in enhancement Deltamethrin Pesticides Signal. Au0Ag100, Au50Ag50 and Au100Ag0 was synthesized using photochemical reduction from salt metals using Femtosecond Laser. The colloidal Au-Ag nanoalloys was fabricated as SERS substrate on silicon wafer using drop casting method on hot plate in 100°C. All SERS substrates of Au0Ag100, Au50Ag50 and Au100Ag0 were tested the performance in enhancement Raman signal of 500 ppm deltamethrin pesticides. The Raman peak of deltamethrin have successfully detected in 563 nm, 665 nm, 806 nm, 1031 nm, 1363 nm, 1589 nm, 2859 nm and 2989 nm. The result showed that the EF (Enhancement Factor) of Au0Ag100, Au50Ag50 and Au100Ag0 were 103, 102 and 101, respectively.

Improving the SERS effect of van der Waals material by intercalation strategy

Neodymium doping MoS2 nanostructures with remarkable surface-enhanced Raman scattering activity

This study investigated the effect of molar concentration of colloidal silver nanoparticles that enhanced Raman signal on Surface-Enhanced Raman Scattering (SERS) substrate performance. Colloidal silver nanoparticles were fabricated as SERS substrate on a glass microscope using drop-casting method. Colloidal silver nanoparticles have been synthesized separately in different concentrations of 5 x 10−4 M (sample 1) and 20 x 10−4 M (sample 2) from silver nitrate (AgNO3) irradiated by femtosecond laser in 5 minutes irradiation times. SERS substrates were fabricated on glass using a hot plate in around 100°C. The performance of SERS substrates was tested for the detection of 500 ppm deltamethrin pesticides using Raman spectroscopy. The result shows that there were increasing Raman signal in sample 2. This indicated that the increased molar concentration of colloidal silver nanoparticles enhances Raman signal. The samples were also characterized using FE SEM to know the particle size of nanoparticles in SERS substrate. Sample 2 that have a higher molar concentration of silver nanoparticles showed bigger particle size than sample 1. The particle size in sample 1 and 2 were 27.46 nm and 37.27 nm, respectively.

Semiconductor composite materials have attracted interest from surface-enhanced Raman scattering (SERS) substrate research. Here, we investigate an organic-inorganic semiconductor heterojunction P3HT@Ag2NCN composite film as a recyclable SERS substrate for molecule detection application. Our study shows that the SERS substrate of the composite P3HT@Ag2NCN composite film has high sensitivity, excellent signal reproducibility, and is reusable. Significant π-stacking of the probe molecules with the thiophene π-cores molecules from P3HT plays an important role in the large SERS enhancement by the charge transfer mechanism. Due to physical interaction between P3HT and Ag2NCN, the organic-inorganic semiconductor heterojunction structure further improves charge transfer efficiency and the SERS property. Our results show that the enhancement factor (EF) of P3HT@Ag2NCN composite films (EF = 6147 ± 300) for the probe molecule methylene blue is more than 7 times that of P3HT substrate (EF = 848 ± 85) and is about 75 times that of Ag2NCN nanorods (EF = 82 ± 8). In addition, the SERS substrates of the P3HT@Ag2NCN composite film also display excellent reusability and signal reproducibility (RSD < 4.8%). Our study opens up a new opportunity for designing an ideal SERS substrate with high sensitivity, selectivity, long-term stability, low cost, and reusability.

Surface-enhanced Raman scattering (SERS) is a powerful technique for trace chemical analysis and single molecule detection in the application of biochemical monitoring and food safety due to its ability to enhance the Raman scattering of molecules near the metallic surface or nanostructures. Here, we present a comprehensive study of the SERS enhancement by the periodically nanostructured surface, where the thin film of silver is deposited onto the surface, except the sidewall of posts, of 1-D lamellar gratings with varying pitch to forming metal-dielectric composite nanostructures. By enhancing the localized and surface-propagating mode in the vicinity of the concaves, the SERS signal can be improved by amplifying the intensity of electric field and increasing the optical path length of the incident light. Experimental investigations show that the enhancement factor can be manipulated by varying the polarization of incident light and the pitch size of gratings. To demonstrate the SERS effects of the proposed structures, thin layers of benzoic acid, which is commonly used as a food preservative, are deposited on the SERS substrates by spin-coating a solution of benzoic acid and dried at room temperature. A Confocal Raman microscope with a 532 nm laser source is used to illuminate light and measure the Raman spectrum of benzoic acid. We demonstrate the Raman signal of benzoic acid can be enhanced on the order of 10 2 on the SERS substrates.

This work investigates the effect of gold nanoparticle (AuNP) addition to paper substrate and examines the ability of these composite materials to amplify the surface enhanced Raman scattering (SERS) signal of a dye adsorbed. Paper has a three-dimensional (3D), porous, and heterogeneous morphology. The manner in which paper adsorbs the nanoparticles is crucial to its SERS properties, particularly with regards to aggregation. In this work, we sought to maintain the same degree of aggregation, while changing the concentration of nanoparticles deposited on paper. We achieved this by dipping paper into AuNP solutions of different, known concentration and found that the initial packing density of AuNPs in solutions was retained on paper with the same degree of aggregation. The surface coverage of AuNPs on paper was found to scale linearly to their concentration profile in solutions. The SERS performances of the AuNP-treated papers were evaluated with 4-aminothiophenol (4-ATP) as the Raman molecule, and their SERS intensities increased linearly with the AuNPs' concentration. Compared to AuNP-treated silicon, the Raman enhancement factor (EF) from paper was relatively higher due to a more uniform and greater degree of adsorption of AuNPs. The effect of the spatial distribution of AuNPs in their substrates on SERS activity was also investigated. In this experiment, the number of AuNPs was kept constant (a 1 μL droplet of AuNPs was deposited on all substrates), and the distribution profile of AuNPs was controlled by the nature of the substrate: paper, silicon, and hydrophobized paper. The AuNP droplet on paper showed the most reproducible and sensitive SERS signal. This highlighted the role of the z-distribution (through film) of AuNPs within the bulk of the paper, producing a 3D multilayer structure to allow inter- and intralayer plasmon coupling, and hence amplifying the SERS signal. The SERS performance of nanoparticle-functionalized paper can thus be optimized by controlling the 3D distribution of the metallic nanoparticles, and such control is critical if these systems are to be implemented as a low-cost and highly sensitive bioassay platform.

Generally the fabrication, assembly and evaluation of plasmonic nanostructures for surface enhanced Raman scattering (SERS) substrates has focused on static rigid substrates such as glass and silicon. However, these static substrates severely limit the application of plasmonic nanostructures as (i) they provide no means to alter the state of assembly of the nanostructures once they are formed or anchored on the surface i.e., not reconfigurable; and (ii) preclude applications which demand non-planar, flexible or conformal surfaces. The above considerations has led to the development of a novel class of SERS substrates based on flexible substrates such paper, polymer membranes and electrospun fibers. These flexible SERS media based on unconventional substrates such as paper offer distinct advantages compared to the conventional SERS substrates in that (i) flexible nature of the substrate enables conformal contact with the surfaces under investigation leading to efficient sample collection; (ii) porous nature of the SERS substrate (interstices between the fibers) provides efficient access to the analytes; (iii) high surface area of the 3D paper substrate results in large dynamic range of the chemical sensors; (iv) intricate network of fibers decorated with metal nanoparticles can provide potentially high density of electromagnetic hotspots; (v) intense light scattering caused by the fibrous structure of the substrate (e.g., paper) enables efficient light-metal interaction; and (vi) facile fabrication leads to efficient, robust, reliable, reusable and cost-effective SERS substrates. In this presentation, we will focus on the Army need for a more flexible (substrate surface and application) SERS substrate for universal sensing. This presentation will leverage from material presented at a flexible SERS (May 2014) workshop hosted by Dr. Srikanth Singamaneni at Washington University.

Fluorophore-assisted green fabrication of flexible and cost-effective Ag nanoparticles decorated PVA nanofibers for SERS based trace detection

Over the past 40 years fundamental and application research into Surface-Enhanced Raman Scattering (SERS) has been explored by academia, industry, and government laboratories. To date however, SERS has achieved little commercial success as an analytical technique. Researchers are tackling a variety of paths to help break through the commercial barrier by addressing the reproducibility in both the SERS substrates and SERS signals as well as continuing to explore the underlying mechanisms. To this end, investigators use a variety of methodologies, typically studying strongly binding analytes such as aromatic thiols and azarenes, and report SERS enhancement factor calculations. However a drawback of the traditional SERS enhancement factor calculation is that it does not yield enough information to understand substrate reproducibility, application potential with another analyte, or the driving factors behind the molecule-metal interaction. Our work at the US Army Edgewood Chemical Biological Center has focused on these questions and we have shown that thermodynamic principles play a key role in the SERS response and are an essential factor in future designs of substrates and applications. This work will discuss the advantages and disadvantages of various experimental techniques used to report SERS enhancement with planar SERS substrates and present our alternative SERS enhancement value. We will report on three types of analysis scenarios that all yield different information concerning the effectiveness of the SERS substrate, practical application of the substrate, and finally the thermodynamic properties of the substrate. We believe that through this work a greater understanding for designing substrates will be achieved, one that is based on both thermodynamic and plasmonic properties as opposed to just plasmonic properties. This new understanding and potential change in substrate design will enable more applications for SERS based methodologies including targeting molecules that are traditionally not easily detected with SERS due to the perceived weak molecule-metal interaction of substrates.

Flexible, graphene protected Ag nanoparticles–polyimide tape for use as a transparent Surface-Enhanced Raman Scattering (SERS) substrate and its application in pesticide detection

Graphene-based noble metal nanocomposites have attracted tremendous research interest in the fields of surface-enhanced Raman scattering (SERS) recently. However, efficient utilization as SERS substrates has been impeded by the difficulty of tuning SERS enhancement effects induced from chemical and plasmonic enhancement by different preparation methods of graphene. Herein, three kinds of graphene-based Au hybrid, CVD-G/Au, GO/Au and rGO/Au, which were synthesized by physical sputtering and chemical in-situ crystallization growth methods, were fabricated to evaluate as SERS substrates. Simple methods were developed to enhance the Raman signals effectively by tuning both plasmonic and chemical enhancement, respectively. Besides, the prepared rGO/Au nanocomposites were used as SERS substrates to monitor the process of plasmon-driven surface-catalyzed reaction from 4-nitrobenzenethiol (4-NBT) to p,p'-dimercaptoazobenzene (DMAB). According to systematic comparisons during power- and time-dependent SERS experiments, rGO/Au was demonstrated to be with lower power threshold and higher catalytic efficiency than Au nanoparticles (NPs) toward the reaction, which provide clues to understand the interaction between metal and graphene as well as further study of plasmon-driven chemical reactions for further.

A two-dimensional flexible surface-enhanced Raman scattering (SERS) filter substrate provides an alternative strategy for the highly sensitive portable detection of various toxic molecules and biomaterials. Herein, we developed a solid-liquid interfacial reduction reaction to post-engineer a solid Au nanostructure surface on filter paper to improve the SERS effect. Among four reductants (ascorbic acid, l-dopamine, hydroquinone (HQ), and formaldehyde), HQ possessed a larger oxidation overpotential and facilitated homogeneous growth, forming small Au branch-structure nanoparticles from HAuCl4 solution. Due to the surface effect by exposing abundant -OH groups and intrinsic aromatic rings from TNA/HQ on nano-gold, the SERS effect on positively charged analytes near the plasmonic Au surface was enhanced, while forming a protective layer against severe water interruption. The resulting SERS substrate with branched nano-gold provided several SERS-enhanced sites, increased the enhancement by more than 6 times compared to original SERS sensing, and displayed a 1.4-7.4 × 105 analytical enhancement factor, which leads to a limit of detection down to several ppb. Less than 6% of deviation in the SERS intensity at different sensing sites was observed. We successfully improved the primary SERS substrate using a high overpotential reductant. Owing to its soft and flexible properties, the paper-based SERS substrate can be used conveniently in different sizes, pasting on curved materials, detecting additives in fish, and preventing the coffee-ring effect, showing high practicality and potential commercial value in the future.