Super-resolution optical imaging of single-molecule SERS hot spots

We present the first super-resolution optical images of single-molecule surface-enhanced Raman scattering (SM-SERS) hot spots, using super-resolution imaging as a powerful new tool for understanding the interaction between single molecules and nanoparticle hot spots. Using point spread function fitting, we map the centroid position of SM-SERS with +/-10 nm resolution, revealing a spatial relationship between the SM-SERS centroid position and the highest SERS intensity. We are also able to measure the unique position of the SM-SERS centroid relative to the centroid associated with nanoparticle photoluminescence, which allows us to speculate on the presence of multiple hot spots within a single diffraction-limited spot. These measurements allow us to follow dynamic movement of the SM-SERS centroid position over time as it samples different locations in space and explores regions larger than the expected size of a SM-SERS hot spot. We have proposed that the movement of the SERS centroid is due to diffusion of a single molecule on the surface of the nanoparticle, which leads to changes in coupling between the scattering dipole and the optical near field of the nanoparticle.

Optimally designed nanolayered metal-dielectric particles as probes for massively multiplexed and ultrasensitive molecular assays

An outstanding challenge in biomedical sciences is to devise a palette of molecular probes that can enable simultaneous and quantitative imaging of tens to hundreds of species down to ultralow concentrations. Addressing this need using surface-enhanced Raman scattering-based probes is potentially possible. Here, we theorize a rational design and optimization strategy to obtain reproducible probes using nanospheres with alternating metal and reporter-filled dielectric layers. The isolation of reporter molecules from metal surfaces suppresses chemical enhancement, and consequently signal enhancements are determined by electromagnetic effects alone. This strategy synergistically couples interstitial surface plasmons and permits the use of almost any molecule as a reporter by eliminating the need for surface attachment. Genetic algorithms are employed to optimize the layer dimensions to provide controllable enhancements exceeding 11 orders of magnitude and of single molecule sensitivity for nonresonant and resonant reporters, respectively. The strategy also provides several other opportunities, including a facile route to tuning the response of these structures to be spectrally flat and localization of the enhancement within a specific volume inside or outside the probe. The spectrally uniform enhancement for multiple excitation wavelengths and for different shifts enables generalized probes, whereas enhancement tuning permits a large dynamic range by suppression of enhancements from outside the probe. Combined, these theoretical calculations open the door for a set of reproducible and robust probes with controlled sensitivity for molecular sensing over a concentration range of over 20 orders of magnitude.

Resolving single molecules in surface-enhanced Raman scattering within the inhomogeneous broadening of Raman peaks

We demonstrate both the observation of either a single or a few molecules resolved within the inhomogeneous broadening of a peak in surface-enhanced raman scattering (SERS). Our results demonstrate a fundamental aspect of spectroscopy and also a possible technique to learn more about the varying interactions that single molecules can have with a given SERS substrate. Resolving more than one molecule within the inhomogeneous broadening is only possible thanks to the combination of (i) high-resolution measurements, and (ii) low temperatures (to narrow down the intrinsic homogeneous broadening as much as possible). Besides being a textbook-like example of laser spectroscopy, this result provides yet another confirmation of single molecule sensitivity in SERS. We show specific experimental examples for these effects in single molecule SERS spectra of the molecules nile blue (NB) and rhodamine 800 (RH800). The possible physical origins of the fluctuations in terms of (i) interactions with the substrate, (ii) isotopic effects, or (iii) instrumental contributions, are explained and discussed.

Surface-enhanced Raman scattering biomedical applications of plasmonic colloidal particles

This review article presents a general view of the recent progress in the fast developing area of surface-enhanced Raman scattering spectroscopy as an analytical tool for the detection and identification of molecular species in very small concentrations, with a particular focus on potential applications in the biomedical area. We start with a brief overview of the relevant concepts related to the choice of plasmonic nanostructures for the design of suitable substrates, their implementation into more complex materials that allow generalization of the method and detection of a wide variety of (bio)molecules and the strategies that can be used for both direct and indirect sensing. In relation to indirect sensing, we devote the final section to a description of SERS-encoded particles, which have found wide application in biomedicine (among other fields), since they are expected to face challenges such as multiplexing and high-throughput screening.

Probing surface and interface structure using optics

Optical techniques for probing surface and interface structure are introduced and recent developments in the field are discussed. These techniques offer significant advantages over conventional surface probes: all pressure ranges of gas-condensed matter interfaces are accessible and liquid-liquid, liquid-solid and solid-solid interfaces can be probed, due to the large penetration depth of the optical radiation. Sensitivity and discrimination from the bulk are the two challenges facing optical techniques in probing surface and interface structure. Where instrumental improvements have resulted in enhanced sensitivity, conventional optical techniques can be used to characterize heterogeneous adsorbed layers on a substrate, often with sub-monolayer resolution. Nanoscale lateral resolution is possible using scanning near-field optics. A separate class of techniques, which includes reflection anisotropy spectroscopy, and nonlinear optical probes such as second-harmonic and sum-frequency generation, uses the difference in symmetry between the bulk and the surface or interface to suppress the bulk contribution. A perspective is presented of likely future developments in this rapidly expanding field.

The preparation of silver nanoparticle decorated silica nanowires on fused quartz as reusable versatile nanostructured surface-enhanced Raman scattering substrates

We introduce a platform, comprised of silver nanoparticle decorated silica nanowires (SiONWs) dispersed on fused quartz substrates, for high sensitivity surface-enhanced Raman scattering (SERS) measurements using both frontal (through the analytes) and back-face (through the transparent substrate) excitation. Quasi-quantitative SERS performances on the specialized substrate, vis-à-vis a silver deposited bare fused quartz plate, showed: (i) the suitability of the Ag modified SiONW substrate for frontal as well as back-face excitation; (ii) a wider detection range with high sensitivity to Rhodamine 6G; and (iii) good underwater metal-oxide adhesion of the specialized substrates. Capable of surviving ultrasonic cleaning, the substrate introduced is one of the few reusable low-cost Ag-based nanostructured SERS substrates, requiring only a simple silver reload process (the silver mirror reaction).

Free-standing carbon nanotube films as optical accumulators for multiplex SERRS attomolar detection

A novel hybrid material comprising silver aggregates supported on the porous structure of a free-standing carbon nanotube film was devised and fabricated. This material readily allows filtration of large volumes of fluids, while retaining the active analytes on silver aggregates so that their characteristic surface-enhanced resonance Raman scattering signals could be registered. The direct identification of multiple analytes at the attomolar regime was readily achieved through their single-molecule spectra.

Measuring the SERS Enhancement Factors of Dimers with Different Structures Constructed from Silver Nanocubes

We describe a systematic investigation on the SERS enhancement factors of individual dimers (EF(dimer)) constructed from two Ag nanocubes that display a face-to-face, edge-to-face, or edge-to-edge structure. The highest field-enhancements were obtained for the dimers displaying a face-to-face and edge-to-face configuration. In these two systems, EF(dimer) was insensitive the dimer geometry and corresponded to 2.0×10(7) and 1.5×10(7), respectively. However, EF(dimer) was decreased to 5.6×10(6) for the edge-to-edge structure. These variations in the detected field-enhancements could be explained based on the relative orientation of the nanocubes and the number of probe molecules enclosed in the hot-spot region for each dimer configuration.

Combinatorial synthesis of a triphenylmethine library and their application in the development of surface enhanced Raman scattering (SERS) probes

The first synthesis of a triphenylmethine (TM) library of compounds and screening of their Surface Enhanced Raman Scattering (SERS) capability was carried out to identify novel Raman reporters with high sensitivity. We identified three novel SERS reporters (B2, B7, and C7) with higher signal intensity than that of commonly used crystal violet (CV). These reporters may find potential applications in developing sensitive SERS based biosensors.

Epitaxially driven formation of intricate supported gold nanostructures on a lattice-matched oxide substrate

A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of lattice-matched MgAl(2)O(4) substrates. The nanostructures were fabricated through a synthesis route where a thin gold film dewets, liquefies, and then slowly self-assembles. The supported nanostructures are intricately shaped, crystalline, and epitaxially aligned. Simulations based on a continuum elastic theory indicate that the self-assembly is driven by strained epitaxy and minimization of the surface free energy.

Measuring the surface-enhanced Raman scattering enhancement factors of hot spots formed between an individual Ag nanowire and a single Ag nanocube

This paper describes a systematic study of the surface-enhanced Raman scattering (SERS) activity of hot spots formed between a Ag nanowire and a Ag nanocube with sharp corners. We investigated two distinct dimer structures: (i) a nanocube having one side face nearly touching the side face of a nanowire, and (ii) a nanocube having one edge nearly touching the side face of a nanowire. The field enhancements for the dimers displayed a strong dependence on laser polarization, and the strongest SERS intensities were observed for polarization along the hot-spot axis. Moreover, the detected SERS intensities were dependent on the hot-spot structure, i.e., the relative orientation of the Ag nanocube with respect to the nanowire's side face. When the dimer had a face-to-face configuration, the enhancement factor EF(dimer) was 1.4 x 10(7). This corresponds to 22-fold and 24-fold increases compared to those for individual Ag nanowires and nanocubes, respectively. Conversely, when the dimer had an edge-to-face configuration, EF(dimer) was 4.3 x 10(6). These results demonstrated that the number of probe molecules adsorbed at the hot spot played an important role in determining the detected SERS intensities. EF(dimer) was maximized when the dimer configuration allowed for a larger number of probe molecules to be trapped within the hot-spot region.

SERRS for single-molecule detection of dye-labeled phospholipids in Langmuir-Blodgett monolayers

The coupling of molecular excitations to localized surface plasmon resonances (LSPR) in silver or gold nanostructures is at the center of single-molecule detection (SMD) using surface-enhanced Raman scattering (SERS). The effect is attributed to the enhanced scattering power caused by coupling with the surface plasmons of the metal. The most efficient coupling is attained when the excitation is in resonance with the molecule and the nanostructure, the case of surface-enhanced resonance Raman scattering (SERRS). This incredible effect has the potential to be a powerful optical tool when used in conjunction with vibrationally differentiable chromophores. Here we present a unique study where the targeted system is a phospholipid that is tagged with a xanthene dye (the SERRS probe), a chromophore that dominates the Raman signal when the laser is in resonance with its absorption. The labeled phospholipid was incorporated into a single fatty acid Langmuir monolayer at varying concentrations and transferred onto a silver nanoparticle film to form Langmuir-Blodgett (LB) monolayers. Because the xanthene dye is tagged to a much larger molecule, the chances of dye aggregation (formation of dimers or higher aggregates) is negligible. Single-molecule detection of the dye tag (SERRS probe monomer) is readily achieved and demonstrated through the use of doped LB monolayers, Raman microscopy, spectral mapping, and efficient coupling of the laser line into the dye absorption band and plasmon resonances.

Revealing the spatial distribution of the site enhancement for the surface enhanced Raman scattering on the regular nanoparticle arrays

The spatial distribution of the site enhancement for the surface-enhanced Raman scattering (SERS) on the regular nanoparticle arrays has been investigated by the confocal Raman microscopy. It was found that the spatial distribution of the Raman signals on the well-ordered nanoparticle arrays was very inhomogeneous and concentrated on the defects of the nanoparticle arrays. The SERS signals were also observed to depend on the thickness of silver film and the defect density. It has been demonstrated that the number of SERS active sites can be increased ten folds by trimming the size of nanoparticles using oxygen plasma.

Characteristics of surface-enhanced Raman scattering and surface-enhanced fluorescence using a single and a double layer gold nanostructure

This paper reports the characteristics of surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) using a unique SERS-active substrate comprised of a single layer and a double layer of two-dimensional (2D) gold nanostructure. Colloidal gold nanoparticles were immobilized on a glass substrate and a multi-purpose experimental setup was adopted to obtain surface plasmon resonance (SPR), SERS and SEF on a single platform. Inhomogeneous intensity distribution was observed in correlated images of SPR and SERS. Several laser lines were used as excitation sources for further SERS measurements. Higher SERS intensities were observed with longer wavelength excitations at the same spatial position. Fluorescence measurements were carried out using 514 nm line and SEF images were obtained using the same sample. Fluorescence emissions were found to be enhanced in the presence of 2D gold nanostructure. A series of SERS spectra were recorded by conducting ensemble SERS measurements at a periodic interval of 2 microm, crossing bare substrates, the single layer and the double layer of gold nanostructure. The double layer provides higher enhancement in SERS than that of the single layer. Polarization-selective SERS measurements obtained at the single layer and double layer showed a clear difference in their dispersions. SERS intensities of the analytes adsorbed at the single layer were fitted well with cos(4)theta dependence; however, for the double layer, the relationship was quite uncertain.

Silicon nanowires coated with silver nanostructures as ultrasensitive interfaces for surface-enhanced Raman spectroscopy

Silver nanoparticles (Ag NPs) were chemically deposited on silicon nanowires (SiNWs), prepared using the vapor-liquid-solid (VLS) growth mechanism, using an in situ electroless metal deposition technique. The resulting SiNWs/Ag NPs composite interfaces showed large Raman scattering enhancement for rhodamine 6G (R6G) with a detection limit of 10(-14) M and an enhancement factor of 2.3 x 10(8). This large enhancement factor was attributed to the presence of "hot" spots on the SiNWs/Ag NPs substrate.

Surface-enhanced Raman spectroscopy of dyes: from single molecules to the artists' canvas

This perspective presents recent surface-enhanced Raman spectroscopy (SERS) studies of dyes, with applications to the fields of single-molecule spectroscopy and art conservation. First we describe the development and outlook of single-molecule SERS (SMSERS). Rather than providing an exhaustive review of the literature, SMSERS experiments that we consider essential for its future development are emphasized. Shifting from single-molecule to ensemble-averaged experiments, we describe recent efforts toward SERS analysis of colorants in precious artworks. Our intention is to illustrate through these examples that the forward development of SERS is dependent upon both fundamental (e.g., SMSERS) and applied (e.g., on-the-specimen SERS of historical art objects) investigations and that the future of SERS is very bright indeed.

Optical properties of the crescent-shaped nanohole antenna

We present the first optical study of large-area random arrays of crescent-shaped nanoholes. The crescent-shaped nanohole antennae, fabricated using wafer-scale nanosphere lithography, provide a complement to crescent-shaped nanostructures, called nanocrescents, which have been established as powerful plasmonic biosensors. With both systematic experimental and computational analysis, we characterize the optical properties of crescent-shaped nanohole antennae and demonstrate tunability of their optical response by varying all key geometric parameters. Crescent-shaped nanoholes have reproducible sub-10-nm tips and are sharper than corresponding nanocrescents, resulting in higher local field enhancement, which is predicted to be |E|/|E(0)| = 1500. In addition, the crescent-shaped nanohole hole-based geometry offers increased integratability and the potential to nanoconfine analyte in "hot-spot" regions, increasing biomolecular sensitivity and allowing localized nanoscale optical control of biological functions.

Creating high density nanoantenna arrays via plasmon enhanced particle-cavity (PEP-C) architectures

We propose a new solution for high hot-spot density creation by coupling a particle and a cavity in a structure dubbed a plasmonic enhanced particle-cavity (PEP-C) antenna. In comparison to analogous particle-based dimer antenna structures, the PEP-C allows both a higher maximum field and an order-of-magnitude higher hot-spot density. In addition, the hot-spots of the PEP-C antenna can be precisely controlled, resulting in increased reliability. We elucidate the photonic characteristics of the PEP-C antenna and show tuning and optimization through choice of geometric parameters. These properties make the PEP-C antenna an excellent candidate for plasmonic-based biomolecular sensors.

Subdiffraction limited, remote excitation of surface enhanced Raman scattering

We demonstrate that focused laser excitation at the end of silver nanowires of 50-150 nm diameter excites SERS hot-spots at points of nanoparticle adsorption many micrometers along the wire due to the plasmon waveguide effect. The total SERS intensity detected at the hot-spots following wire-end excitation correlates with the known wavelength, polarization, and distance dependences of surface plasmon polariton (SPP) propagation in nanowires. The SERS spectra obtained at the hot-spots following wire-end excitation show very little background compared to when excitation occurs directly at the hot-spot, suggesting that a much smaller SERS excitation volume is achieved by remote, waveguide excitation. The ability to transfer SERS excitation over several micrometers, through a structure with a subdiffraction limit diameter, is discussed with respect to potential high-resolution SERS imaging applications.

Synthesis of size-controlled faceted pentagonal silver nanorods with tunable plasmonic properties and self-assembly of these nanorods

Monodisperse size-controlled faceted pentagonal silver nanorods were synthesized by thermal regrowth of decahedral silver nanoparticle (AgNPs) in aqueous solution at 95 degrees C, using citrate as a reducing agent. The width of the silver nanorods was determined by the size of the starting decahedral particle, while the length was varied from 50 nm to 2 mum by the amount of new silver added to the growth solution. Controlled regrowth allowed us to produce monodisperse AgNPs with a shape of elongated pentagonal dipyramid (regular Johnson solid, J(16)). Faceted pentagonal particles exhibited remarkable optical properties with sharp plasmon resonances precisely tunable across visible and NIR. Due to the narrow size distribution, faceted pentagonal silver nanorods readily self-assembled into the 3-D arrays similar to smectic mesophases. Hexagonal arrangement in the array completely overrode five-fold symmetry of the nanorods. Overall, our findings highlight the importance of pentagonal symmetry in metal nanoparticles and offer a facile method of the preparation of monodisperse AgNPs with controlled dimensions and plasmonic properties that are promising for optical applications and functional self-assembly.

Isolating and probing the hot spot formed between two silver nanocubes

Out of the frying pan: Hot spots can greatly increase the sensitivity of surface-enhanced Raman scattering, but they remain poorly understood. A new strategy based on plasma etching (see picture) can be used to isolate and exclusively probe the SERS-active molecules adsorbed in the hot-spot region between two silver nanocubes.

Improving surface-enhanced Raman scattering effect using gold-coated hierarchical polystyrene bead substrates modified with postgrowth microwave treatment

We report a novel postgrowth microwave heating implementation by selectively modifying hierarchical polystyrene (PS) bead substrates coated with gold (Au) films to effectively improve the surface-enhanced Raman scattering (SERS) effect on the analytes. The SERS signal of probe molecule rhodamine 6G (Rh 6G) on the microwave-treated Au-PS substrates can be improved by 10-fold, while the detection limit of Rh 6G in concentration can be enhanced by two orders of magnitude compared to the as-growth substrates. The high-quality SERS spectrum of saliva can also be acquired using the modified substrates, demonstrating the potential for the realization of the high-performance SERS substrates for biomedical applications.