Concluding Remarks : Surface enhanced Raman scattering

Reassessing SERS enhancement factors: using thermodynamics to drive substrate design

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.

Self-Assembled Large-Scale Monolayer of Au Nanoparticles at the Air/Water Interface Used as a SERS Substrate

Self-assembly of metal nanoparticles has attracted considerable attention because of its unique applications in technologies such as plasmonics, surface-enhanced optics, sensors, and catalysts. However, fabrication of ordered nanoparticle structures remains a significant challenge. Thus, developing an efficient approach for the assembly of large-scale Au nanoparticles films for theoretical studies and for various applications is highly desired. In this paper, a facial approach for fabricating a monolayer film of Au nanoparticles was developed successfully. Using the surfactant polyvinylpyrrolidone (PVP), a large-scale monolayer film of well-ordered, uniform-sized Au nanoparticles was fabricated at the air/water interface. The film exhibited a two-dimensional (2D) hexagonal close-packed (HCP) structure having interparticle gaps smaller than 2 nm. These gaps generated numerous uniform "hot spots" for surface-enhanced Raman scattering (SERS) activity. The as-prepared monolayer film could be transferred to a solid substrate for use as a suitable SERS substrate with high activity, high uniformity, and high stability. The low spot-to-spot and substrate-to-substrate variations of intensity (<10%), the large surface enhancement factor (∼10(6)), and the high stability (∼45 days) make the substrate suitable for SERS measurements. Transfer of the monolayer film onto a glassy carbon electrode produced an Au electrode with clean, well-defined nanostructure suitable for electrochemical SERS measurements. The adsorption process of ionic liquids on the electrode with the monolayer film is similar to that on bulk metal electrodes. The present strategy provides an effective way for self-assembly of Au nanoparticles into well-defined nanostructures that may form optimal reproducible SERS substrates for quantitative analysis. It also provides an electrode with clean, well-defined nanostructure for electrochemical investigations.

Magnetic-Polaron-Induced Enhancement of Surface Raman Scattering

The studies of the effects of magnetic field on surface enhanced Raman scattering (SERS) have been so far limited to the case of ferromagnetic/noble-metal, core/shell nano-particles, where the influence was always found to be negative. In this work, we investigate the influence of magnetic field on a diluted magnetic semiconductor/metal SERS system. Guided by three dimensional finite-difference time-domain simulations, a high efficient SERS substrate was obtained by diluting Mn into Au-capped ZnO, which results in an increase of the dielectric constant and, therefore, an enhancement of Raman signals. More remarkably, an increase of intensities as well as a reduction of the relative standard deviation (RSD) of Raman signals have been observed as a function of the external magnetic strength. We ascribe these positive influences to magnetic-field induced nucleation of bound magnetic polarons in the Mn doped ZnO. The combination of diluted magnetic semiconductors and SERS may open a new avenue for future magneto-optical applications.

Au nanocluster arrays on self-assembled block copolymer thin films as highly active SERS substrates with excellent reproducibility

We demonstrate the fabrication of uniform Au nanocluster arrays utilizing a self-assembled polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) thin film as the template and their application as a surface-enhanced Raman scattering (SERS) substrate.

Geometrical and morphological optimizations of plasmonic nanoarrays for high-performance SERS detection

Here we present an in-depth and comprehensive study of the effect of the geometry and morphology of nanoarray (NA) substrates on their surface-enhanced Raman scattering (SERS) performance. The high-quality SERS-active NA substrates of various unit shapes and pitches are assembled through electron beam lithography and fabricated by electron beam physical vapor deposition. Good agreement is found on comparing the Raman scattering results with the integrals of the fourth power of local electric fields from the three-dimensional numerical simulations. A novel type of hybrid NA substrate composed of disordered nanoparticles and a periodic NA is fabricated and characterized. The morphology of NAs has little influence on the SERS performance of hybrid NA substrates and they perform better than both their counterparts pure NA and disordered nanoparticle substrates.

Fast assembling microarrays of superparamagnetic Fe3O4@Au nanoparticle clusters as reproducible substrates for surface-enhanced Raman scattering

It is currently a very active research area to develop new types of substrates which integrate various nanomaterials for surface-enhanced Raman scattering (SERS) techniques. Here we report a unique approach to prepare SERS substrates with reproducible performance. It features silicon mold-assisted magnetic assembling of superparamagnetic Fe3O4@Au nanoparticle clusters (NCs) into arrayed microstructures on a wafer scale. This approach enables the fabrication of both silicon-based and hydrogel-based substrates in a sequential manner. We have demonstrated that strong SERS signals can be harvested from these substrates due to an efficient coupling effect between Fe3O4@Au NCs, with enhancement factors >10(6). These substrates have been confirmed to provide reproducible SERS signals, with low variations in different locations or batches of samples. We investigate the spatial distributions of electromagnetic field enhancement around Fe3O4@Au NCs assemblies using finite-difference-time-domain (FDTD) simulations. The procedure to prepare the substrates is straightforward and fast. The silicon mold can be easily cleaned out and refilled with Fe3O4@Au NCs assisted by a magnet, therefore being re-useable for many cycles. Our approach has integrated microarray technologies and provided a platform for thousands of independently addressable SERS detection, in order to meet the requirements of a rapid, robust, and high throughput performance.

Towards improved precision in the quantification of surface-enhanced Raman scattering (SERS) enhancement factors: a renewed approach

This paper demonstrates a renewed procedure for the quantification of surface-enhanced Raman scattering (SERS) enhancement factors with improved precision. The principle of this method relies on deducting the resonance Raman scattering (RRS) contribution from surface-enhanced resonance Raman scattering (SERRS) to end up with the surface enhancement (SERS) effect alone. We employed 1,8,15,22-tetraaminophthalocyanato-cobalt(II) (4α-Co(II)TAPc), a resonance Raman- and electrochemically redox-active chromophore, as a probe molecule for RRS and SERRS experiments. The number of 4α-Co(II)TAPc molecules contributing to RRS and SERRS phenomena on plasmon inactive glassy carbon (GC) and plasmon active GC/Au surfaces, respectively, has been precisely estimated by cyclic voltammetry experiments. Furthermore, the SERS substrate enhancement factor (SSEF) quantified by our approach is compared with the traditionally employed methods. We also demonstrate that the present approach of SSEF quantification can be applied for any kind of different SERS substrates by choosing an appropriate laser line and probe molecule.

Reliable Quantitative SERS Analysis Facilitated by Core-Shell Nanoparticles with Embedded Internal Standards

Quantitative analysis is a great challenge in surface-enhanced Raman scattering (SERS). Core-molecule-shell nanoparticles with two components in the molecular layer, a framework molecule to form the shell, and a probe molecule as a Raman internal standard, were rationally designed for quantitative SERS analysis. The signal of the embedded Raman probe provides effective feedback to correct the fluctuation of samples and measuring conditions. Meanwhile, target molecules with different affinities can be adsorbed onto the shell. The quantitative analysis of target molecules over a large concentration range has been demonstrated with a linear response of the relative SERS intensity versus the surface coverage, which has not been achieved by conventional SERS methods.

Electrodeposition of nanoflake Pd structures: structure-dependent wettability and SERS activity

The characteristic properties of metal surfaces, i.e., wettability and surface-enhanced Raman scattering (SERS) activity, have been the subject of intensive research because of their useful applications. In the present work, we report a simple electrodeposition of nanoflake Pd structures onto clean Au surfaces without the use of additives. The fine structure of the nanoflake Pd surfaces was regulated by controlling the deposition charge, and the effect of the structural variations on the wettability and SERS activity was examined. The wettability of nanoflake Pd structures in terms of water contact angle was closely related to the fine structures of Pd deposits and their surface roughness. The SERS activity of the nanoflake Pd surfaces was highly dependent on the presence of sharp edge sites on the Pd structures. Well-defined nanoflake Pd structures prepared using a deposition charge of 0.04 C exhibited superhydrophobic natures and reproducible SERS activity. The effect of the metal surface structures on the wettability and the SERS activity demonstrated in this work provides insight into the fabrication of functional metal nanostructures.

Gold nanoisland films as reproducible SERS substrates for highly sensitive detection of fungicides

A wet-chemical approach is used to fabricate centimeter-scale gold nanoisland films (NIFs) with tunable morphology of islands and with strong electromagnetic coupling between them. The approach consists in a uniform seeding of small gold nanoparticles on a glass or silicon substrate, followed by controllable growth of the seeds into small nanoislands. A special technique for TEM sampling was developed to follow the gradual formation of larger-sized isolated nanoparticles, nanoislands of sintered overgrown seeds, and a complete gold layer with nanoscale cracks. The electromagnetic field distribution inside the fabricated NIFs was calculated by FDTD simulations applied to actual TEM images of the fabricated samples rather than to artificial models commonly used. SERS measurements with 1,4-aminothiophenol (ATP) molecules demonstrated the analytical enhancement factor about of 10(7) and the fundamental enhancement factor about of 10(8) for optimized substrates. These values were at least 1 order of magnitude higher than that for self-assembled arrays of gold nanostars and silver nanocubes. SERS spectra of independent samples demonstrated good sample-to-sample reproducibility in terms of the relative standard deviation (RSD) of the main peaks less than 20%. Additionally, Raman maps with 1 μm increment in X-Y directions of NIFs (800 spectral spots) demonstrated good point-to-point repeatability in the intensity of the main Raman vibration modes (RSD varied from 5% to 15% for 50 randomly selected points). A real-life application of the fabricated SERS substrates is exemplified by the detection of the thiram fungicide in apple peels within the 5-250 ppb linear detection range. Specifically, the NIF-based SERS technology detected thiram on apple peel down to level of 5 ng/cm(2).

Reagent- and separation-free measurements of urine creatinine concentration using stamping surface enhanced Raman scattering (S-SERS)

We report a novel reagent- and separation-free method for urine creatinine concentration measurement using stamping surface enhanced Raman scattering (S-SERS) technique with nanoporous gold disk (NPGD) plasmonic substrates, a label-free, multiplexed molecular sensing and imaging technique recently developed by us. The performance of this new technology is evaluated by the detection and quantification of creatinine spiked in three different liquids: creatinine in water, mixture of creatinine and urea in water, and creatinine in artificial urine within physiologically relevant concentration ranges. Moreover, the potential application of our method is demonstrated by creatinine concentration measurements in urine samples collected from a mouse model of nephritis. The limit of detection of creatinine was 13.2 nM (0.15 µg/dl) and 0.68 mg/dl in water and urine, respectively. Our method would provide an alternative tool for rapid, cost-effective, and reliable urine analysis for non-invasive diagnosis and monitoring of renal function.

Physical deposition improved SERS stability of morphology controlled periodic micro/nanostructured arrays based on colloidal templates

An effective and inexpensive method is developed to fabricate periodic arrays by sacrificial colloidal monolayer template route by chemical deposition and further physical deposition. By a colloidal template induced precursor solution dipping strategy, different periodic arrays of semi-hollow sphere array, inverse opal with monolayer pore arrays and hole arrays are obtained under different conditions. After magnetron sputtering deposition, their morphologies are changed to novel micro/nanostructured arrays of honeycomb-shaped arrays, hollow cavity arrays, and regular network arrays due to multiple direction deposition of sputtering deposition and shadow effect. After coating a gold thin layer, these periodic micro/nanostructured arrays are used as SERS active substrates and demonstrate a very stable SERS performance compared with periodic arrays achieved by direct colloidal template-induced chemical deposition. Additionally, a honeycomb-shaped array displays better SERS enhancement than that of a hollow cavity array or a regular network array. After optimization of honeycomb-shaped arrays with different periodicities, an array with periodicity of 350 nm demonstrates much stronger SERS enhancement and possesses a low detection limit of 10(-11) M R6G. Such stable SERS performance is useful for practical application in portable Raman detecting devices to detect organic molecules.

Bottom-up growth of Ag/a-Si@Ag arrays on silicon as a surface-enhanced Raman scattering substrate with high sensitivity and large-area uniformity

Bottom-up growth of Ag/a-Si@Ag arrays on Si, which worked as a highly sensitive SERS substrate.

Highly sensitive SERS sensor for mercury ions based on the catalytic reaction of mercury ion decorated Ag nanoparticles

Schematic diagram of the SERS sensing mechanism for mercury ions by a catalytic oxidation reaction.

A Au nanoparticle-incorporated sponge as a versatile transmission surface-enhanced Raman scattering substrate

We report a sponge-based transmission surface-enhanced Raman scattering (TSERS) substrate that combines the bulk sampling capabilities of a transmission measurement to improve the quantitative representation of sample concentration with several sponge properties useful for analysis such as fast sample uptake, easy sample enrichment, and a stable polymeric structure.

High performance SERS active substrates fabricated by directly growing graphene on Ag nanoparticles

An efficient surface enhanced Raman scattering (SERS) substrate of graphene-isolated Ag nanoparticle (G/AgNP) has been developed by using excimer laser to ablate the ordered pyrolytic graphite in high vacuum onto Ag nanoparticles.

Electrodeposition of Pt nanostructures with reproducible SERS activity and superhydrophobicity

A simple one-step electrodeposition of nanostructured Pt structures exhibiting both reproducible SERS activity and superhydrophobicity was demonstrated.

Surface-enhanced Raman scattering on a hierarchical structural Ag nano-crown array in different detection ways

A hierarchical structural Ag nano-crown array SERS substrate that is more efficiently excited from the back side.

Smart liquid SERS substrates based on Fe3O4/Au nanoparticles with reversibly tunable enhancement factor for practical quantitative detection

There is a strong correlation between the surface enhanced Raman scattering (SERS) enhancement factor (EF), the excitation wavelength, and the feature properties (composition, size, geometry, and analytes). The prediction of the EF of specific substrates, crucial to the quantitative SERS detection, is however still very difficult. The present work presents smart liquid SERS substrates consisting of suspensions of Fe₃O₄/Au nanoparticles, which provide high spot-to-spot uniformity, reproducibility and good reversibility. The EF of these substrates can be reversibly tuned by applying an external magnetic field. The EF magnetic tuning is within 2 orders of magnitude per substrate in the range of 10⁴–10⁷. The ability to reversibly adjust the SERS EF enables to reduce EF variations caused by external effects such as substrate-to-substrate differences and long-term-storage degradation. This improves the quantitative detection of analytes and might be a significant step forward in employing SERS for practical applications.

Large-scale uniform Au nanodisk arrays fabricated via x-ray interference lithography for reproducible and sensitive SERS substrate

Large-scale Au nanodisk arrays on Si substrate are successfully fabricated via x-ray interference lithography and followed by electron-beam vapor deposition. The Au nanodisk arrays exhibit a significant, uniform, and reproducible surface enhancement on Raman scattering signal, which enables the detection of R6G as low as 10(-8) M with an enhancement factor of 10(6). Importantly, the Au nanodisk arrays SERS-active substrates with uniformly high sensitivity also have high reproducibility and stability. The diameters of the nanodisks and the inter-disk distance can be simply optimized to obtain high enhancement in Raman signal by varying exposure time and development time in XIL process. The electric fields of the Au nanodisks with various diameters and inter-disk distance simulated by the finite difference time domain (FDTD) techniques further confirm that the Raman signal enhancement of Au nanodisks is determined by the diameters of nanodisks and the inter-disk distance of nanodisks. The Au/Ag double-layer bimetal nanodisk arrays are also fabricated which show a significant increase in the Raman signal enhancement than that of the Au nanodisk arrays. XIL nanofabrication appears to be a feasible approach to prepare uniform and reproducible SERS-active substrates with high sensitivity for practical SERS applications.

Metal double layers with sub-10 nm channels

Double-layer plasmonic nanostructures are fabricated by depositing metal at normal incidence onto various resist masks, forming an antenna layer on top of the resist post and a hole layer on the substrate. Antenna plasmon resonances are found to couple to the hole layer, inducing image charges which enhance the near-field for small layer spacings. For continued evaporation above the resist height, a sub-10 nm gap channel develops due to a self-aligned process and a minimal undercut of the resist sidewall. For such double layers with nanogap channels, the average surface-enhanced Raman scattering intensity is improved by a factor in excess of 60 in comparison to a single-layer antenna with the same dimensions. The proposed design principle is compatible with low-cost fabrication, straightforward to implement, and applicable over large areas. Moreover, it can be applied for any particular antenna shape to improve the signals in surface-enhanced spectroscopy applications.

Electrodeposition of triangular Pd rod nanostructures and their electrocatalytic and SERS activities

We report a simple one-step electrodeposition of triangular Pd rod nanostructures on clean Au substrates without additives. Scanning electron microscopy, transmission electron microscopy, and electrochemical techniques were utilized to characterize the structural features of the triangular Pd rod nanostructures. The regulation of the electrodeposition rate by optimizing the electrolyte concentration and applied potential was critical for the anisotropic growth of Pd in the vertical direction. The triangular Pd rod structures exhibited electrocatalytic activities for oxygen reduction and methanol oxidation reactions. These surfaces could be effectively utilized as reproducible surface-enhanced Raman scattering (SERS) active substrates to produce stable SERS signals under electrochemical systems. A simple preparation of well-defined triangular Pd rod structures would allow new opportunities in various areas utilizing Pd-based nanostructured surfaces.

The effect of dielectric constants on noble metal/semiconductor SERS enhancement: FDTD simulation and experiment validation of Ag/Ge and Ag/Si substrates

The finite-difference time-domain (FDTD) method was employed to simulate the electric field distribution for noble metal (Au or Ag)/semiconductor (Ge or Si) substrates. The simulation showed that noble metal/Ge had stronger SERS enhancement than noble metal/Si, which was mainly attributed to the different dielectric constants of semiconductors. In order to verify the simulation, Ag nanoparticles with the diameter of ca. 40 nm were grown on Ge or Si wafer (Ag/Ge or Ag/Si) and employed as surface-enhanced Raman scattering substrates to detect analytes in solution. The experiment demonstrated that both the two substrates exhibited excellent performance in the low concentration detection of Rhodamine 6G. Besides, the enhancement factor (1.3 × 10⁹) and relative standard deviation values (less than 11%) of Ag/Ge substrate were both better than those of Ag/Si (2.9 × 10⁷ and less than 15%, respectively), which was consistent with the FDTD simulation. Moreover, Ag nanoparticles were grown in-situ on Ge substrate, which kept the nanoparticles from aggregation in the detection. To data, Ag/Ge substrates showed the best performance for their sensitivity and uniformity among the noble metal/semiconductor ones.

Surface-enhanced Raman scattering (SERS) chips made from metal nanoparticle-doped polymer fibers

We employed an electrospinning method to prepare metal nanoparticle (NP) doped polymer nanofiber mats, which can be easily cut to size and fixed on slides or in microfluidic channels for surface-enhanced Raman scattering (SERS) measurements.

A highly sensitive microfluidics system for multiplexed surface-enhanced Raman scattering (SERS) detection based on Ag nanodot arrays

We present a microfluidics system with Ag nanodot arrays as the enhancement substrate for multiplexed SERS detection of low-concentration mixtures of thiram and adenine.

Single-molecule surface-enhanced Raman spectroscopy with nanowatt excitation

Bi-analyte experiments demonstrate that single-molecule detection via SERS can be achieved at ultra-low excitation powers.

Preparation of gold nanoparticles-agarose gel composite and its application in SERS detection

Agarose gel/gold nanoparticles hybrid was prepared by adding gold nanoparticles to preformed agarose gel. Nanocomposite structures and properties were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and UV-Vis-NIR absorption spectroscopy. Based on the swelling-contraction characteristics of agarose gel and the adjustable localized surface plasmon resonance (LSPR) of the gold nanoparticles, the nanocomposites were used as surface enhanced Raman scattering (SERS) substrate to detect the Raman signal molecules (NBA, MBA, 1NAT). Results revealed that the porous structure of the agarose gel provided a good carrier for the enrichment of the gold nanoparticles. The gold nanoparticles dynamic hot-spot effect arising from the agarose gel contraction loss of water in the air greatly enhanced the Raman signal. Furthermore, the gel could be cleaned with washing solution and recycling could be achieved for Raman detection.

Green synthesis and electrophoretic deposition of Ag nanoparticles on SiO₂/Si(100)

Plasmonic substrates were prepared by electrophoretic deposition of Ag nanoparticles on SiO2/Si(100). The Ag nanoparticles were obtained using [Ag(NH3)2](+) as the Ag precursor and d-glucose as the reducing agent. Under optimized conditions, this simple and green synthesis method furnished a suspension of Ag nanoparticles with a narrow dimensional dispersion (centered around 27 nm) and a negative z-potential, suitable for electrophoretic deposition. Samples were chemically, optically and morphologically characterized by photoemission and UV-vis spectroscopy and electron microscopy, and tested as substrates for surface enhanced Raman spectroscopy. Despite being a very simple procedure, good enhancement factors were measured thanks to the formation of hot spots, formed by sandwiching the analyte (benzenethiol) between sequentially deposited Ag nanoparticles.

SERS substrates formed by gold nanorods deposited on colloidal silica films

We describe a new approach to the fabrication of surface-enhanced Raman scattering (SERS) substrates using gold nanorod (GNR) nanopowders to prepare concentrated GNR sols, followed by their deposition on an opal-like photonic crystal (OPC) film formed on a silicon wafer. For comparative experiments, we also prepared GNR assemblies on plain silicon wafers. GNR-OPC substrates combine the increased specific surface, owing to the multilayer silicon nanosphere structure, and various spatial GNR configurations, including those with possible plasmonic hot spots. We demonstrate here the existence of the optimal OPC thickness and GNR deposition density for the maximal SERS effect. All other things being equal, the analytical integral SERS enhancement of the GNR-OPC substrates is higher than that of the thick, randomly oriented GNR assemblies on plain silicon wafers. Several ways to further optimize the strategy suggested are discussed.

Graphene: a platform for surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) imparts Raman spectroscopy with the capability of detecting analytes at the single-molecule level, but the costs are also manifold, such as a loss of signal reproducibility. Despite remarkable steps having been taken, presently SERS still seems too young to shoulder analytical missions in various practical situations. By the virtue of its unique molecular structure and physical/chemical properties, the rise of graphene opens up a unique platform for SERS studies. In this review, the multi-role of graphene played in SERS is overviewed, including as a Raman probe, as a substrate, as an additive, and as a building block for a flat surface for SERS. Apart from versatile improvements of SERS performance towards applications, graphene-involved SERS studies are also expected to shed light on the fundamental mechanism of the SERS effect.

Surface-enhanced Raman scattering (SERS) evaluation protocol for nanometallic surfaces

We present the results of a three-year collaboration between the U.S. Army Edgewood Chemical Biological Center and the U.S. Army Research Laboratory-Aldelphi Laboratory Center on the evaluation of selected nanometallic surfaces developed for the Defense Advanced Research Projects Agency Surface-Enhanced Raman Scattering (SERS) Science and Technology Fundamentals program. The primary role of the two Army labs was to develop the analytical and spectroscopic figures of merit to unambiguously compare the sensitivity and reproducibility of various SERS substrates submitted by the program participants. We present the design and implementation of an evaluation protocol for SERS active surfaces enabling an enhancement value calculation from which different substrates can be directly compared. This method was established to: (1) collect physical and spectral characterization data from the small number of substrates (performer supplied) typically encountered, and (2) account for the complex fabrication technique and varying nature of the substrate platforms encountered within this program.

Surface-enhanced Raman spectroscopy studies of organophosphorous model molecules and pesticides

This work reports the analytical application of surface-enhanced Raman spectroscopy (SERS) in the trace analysis of organophosphorous pesticides (trichlorfon and glyphosate) and model organophosphorous compounds (dimethyl methylphosphonate and o-ethyl methylphosphonothioate) bearing different functional groups. SERS measurements were carried out using Ag nanocubes with an edge square dimension of ca. 100 nm as substrates. Density functional theory (DFT) with the B3LYP functional was used for the optimization of ground state geometries and simulation of Raman spectra of the organophosphorous compounds and their silver complexes. Adsorption geometries and marker bands were identified for each of the investigated compound. Results indicate the usefulness of SERS methodology for the sensitive analyses of organophosphorous compounds through the use of vibrational spectroscopy.

Plasmonic enhancement of the two photon absorption cross section of an organic chromophore using polyelectrolyte-coated gold nanorods

The effect of plasmonic enhancement on the two-photon absorption cross section of organic chromophores attached to polyelectrolyte-coated gold nanorods was investigated. The magnitudes of such enhancements were confirmed using single and two photon excitations of the chromophore molecules bound to polyelectrolyte-coated gold nanorods. By synthesizing two-, four-, six-, and eight-polyelectrolyte layer coated nanorods of a particular aspect ratio, the distance dependence of the evanescent electromagnetic field on molecular two-photon absorption was observed. Enhancements of 40-fold were observed for the chromophores nearest to the surface.