Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates

Elevated gold ellipse nanoantenna dimers as sensitive and tunable surface enhanced Raman spectroscopy substrates

We demonstrate large area arrays of elevated gold ellipse dimers with precisely controlled gaps for use as sensitive and highly controllable surface enhanced Raman scattering (SERS) substrates. The enhanced Raman signal observed with SERS arises from both localized and long range plasmonic effects. By controlling the geometry of a SERS substrate, in this case the size and aspect ratio of individual ellipses, the plasmon resonance can be tuned in a broad wavelength range, providing a method for designing the response of SERS substrates at different excitation wavelengths. Plasmon effects exhibited by the elevated gold ellipse dimer substrates are also demonstrated and confirmed through finite difference time domain (FDTD) simulations. A plasmon resonance red shift with an increase of the ellipse aspect ratio is observed, allowing systematic control of the resulting SERS signal intensity. Optimized elevated ellipse dimer substrates with 10 ± 2 nm gaps exhibit uniform SERS enhancement factors on the order of 10(9) for adsorbed p-mercaptoaniline molecules.

Plasmonic trapping and tuning of a gold nanoparticle dimer

We demonstrate theoretically the trapping and manipulating of a gold nanoparticle dimer, using surface plasmon excited by a focused linearly-polarized laser beam on a silver film. We use both finite-difference time-domain force analysis and Maxwell stress tensor to show that the gold nanoparticle dimer can be trapped by a virtual probe pair. A formula is derived to represent the plasmonic field, suggesting that the gap between the two gold nanoparticles in the dimer can be controlled, for example, by tuning the excitation-laser wavelength. We further test our theory by successfully trapping nanoparticle dimers formed by nanospheres and nanorods. The controllable gap in between the nanoparticles can lead to tunable localized surface plasmon resonances, and this may find new exciting applications in plasmonic sensing or in lab-on-a-chip devices.

Anisotropic Nanoparticles and Anisotropic Surface Chemistry

Anisotropic nanoparticles are powerful building blocks for materials engineering. Unusual properties emerge with added anisotropy-often to an extraordinary degree-enabling countless new applications. For bottom-up assembly, anisotropy is crucial for programmability; isotropic particles lack directional interactions and can self-assemble only by basic packing rules. Anisotropic particles have long fascinated scientists, and their properties and assembly behavior have been the subjects of many theoretical studies over the years. However, only recently has experiment caught up with theory. We have begun to witness tremendous diversity in the synthesis of nanoparticles with controlled anisotropy. In this Perspective, we highlight the synthetic achievements that have galvanized the field, presenting a comprehensive discussion of the mechanisms and products of both seed-mediated and alternative growth methods. We also address recent breakthroughs and challenges in regiospecific functionalization, which is the next frontier in exploiting nanoparticle anisotropy.

Highly narrow nanogap-containing Au@Au core-shell SERS nanoparticles: size-dependent Raman enhancement and applications in cancer cell imaging

Cellular imaging technologies employing metallic surface-enhanced Raman scattering (SERS) tags have gained much interest toward clinical diagnostics, but they are still suffering from poor controlled distribution of hot spots and reproducibility of SERS signals. Here, we report the fabrication and characterization of high narrow nanogap-containing Au@Au core-shell SERS nanoparticles (GCNPs) for the identification and imaging of proteins overexpressed on the surface of cancer cells. First, plasmonic nanostructures are made of gold nanoparticles (∼15 nm) coated with gold shells, between which a highly narrow and uniform nanogap (∼1.1 nm) is formed owing to polyA anchored on the Au cores. The well controlled distribution of Raman reporter molecules, such as 4,4'-dipyridyl (44DP) and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), are readily encoded in the nanogap and can generate strong, reproducible SERS signals. In addition, we have investigated the size-dependent SERS activity of GCNPs and found that with the same laser wavelength, the Raman enhancement discriminated between particle sizes. The maximum Raman enhancement was achieved at a certain threshold of particle size (∼76 nm). High narrow nanogap-containing Au@Au core-shell SERS tags (GCTs) were prepared via the functionalization of hyaluronic acid (HA) on GCNPs, which recognized the CD44 receptor, a tumor-associated surface biomarker. And it was shown that GCTs have a good targeting ability to tumour cells and promising prospects for multiplex biomarker detection.

A Nanostructured SERS Switch Based on Molecular Beacon-Controlled Assembly of Gold Nanoparticles

In this paper, highly purified and stable gold nanoparticle (AuNP) dimers connected at the two ends of DNA linkage were prepared by a versatile method. A nanostructured, surface-enhanced Raman scattering (SERS) switching sensor system was fabricated based on the controlled organization of gold nanoparticles (AuNPs) by a DNA nanomachine through the controlled formation/deformation of SERS “hotspots”. This strategy not only opens opportunities in the precise engineering of gap distances in gold-gold nanostructures in a highly controllable and reproducible fashion, but also provides a unique ability to research the origin of SERS and sequence-specific DNA detection.

Electrostatically tunable plasmonic devices fabricated on multi-photon polymerized three-dimensional microsprings

Electrostatically tunable plasmonic devices on three-dimensional (3D) microsprings were fabricated using multi-photon polymerization followed by metal deposition. These plasmonic devices comprised a nanostructured Au microplate and two 3D microsprings. The maximum plasmon excitation efficiency was 35%, a value achieved with incident light of wavelength 632.8 nm. The efficiency could be continuously changed from almost zero to maximum by inclining the microplates with the application of DC voltage up to 50 V. Such dynamic functionality is useful for the realization of highly integrated optoelectronic devices and tunable metamaterials.

Ordered arrays of Ag nanodendrite clusters as effective surface-enhanced Raman scattering substrates

Large-area ordered Ag nanodendrite cluster tetragonal arrays have been achieved on a Zn plate via galvanic replacement reaction.

A color-changing plasmonic actuator based on silver nanoparticle array/liquid crystalline elastomer nanocomposites

The color-changing Ag NPs/LCE actuators can be used for smart environmental responsive devices by coupling the LSPR of Ag NPs with the deformation of the LCE.

One-pot synthesis of gold nanodimers and their use as surface-enhanced Raman scattering tags

Gold dimers consisting of 40 nm-diameter nanospheres show enhancement factors as high as ∼1.8 × 10⁷at the hot-spot level.

All-dielectric metamaterials

The ideal material for nanophotonic applications will have a large refractive index at optical frequencies, respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, confine and guide light at the nanoscale, and be able to modify the phase and amplitude of incoming radiation in a fraction of a wavelength. Artificial electromagnetic media, or metamaterials, based on metallic or polar dielectric nanostructures can provide many of these properties by coupling light to free electrons (plasmons) or phonons (phonon polaritons), respectively, but at the inevitable cost of significant energy dissipation and reduced device efficiency. Recently, however, there has been a shift in the approach to nanophotonics. Low-loss electromagnetic responses covering all four quadrants of possible permittivities and permeabilities have been achieved using completely transparent and high-refractive-index dielectric building blocks. Moreover, an emerging class of all-dielectric metamaterials consisting of anisotropic crystals has been shown to support large refractive index contrast between orthogonal polarizations of light. These advances have revived the exciting prospect of integrating exotic electromagnetic effects in practical photonic devices, to achieve, for example, ultrathin and efficient optical elements, and realize the long-standing goal of subdiffraction confinement and guiding of light without metals. In this Review, we present a broad outline of the whole range of electromagnetic effects observed using all-dielectric metamaterials: high-refractive-index nanoresonators, metasurfaces, zero-index metamaterials and anisotropic metamaterials. Finally, we discuss current challenges and future goals for the field at the intersection with quantum, thermal and silicon photonics, as well as biomimetic metasurfaces.

Bridging the Nanogap with Light: Continuous Tuning of Plasmon Coupling between Gold Nanoparticles

The control of nanogaps lies at the heart of plasmonics for nanoassemblies. The plasmon coupling sensitively depends on the size and the shape of the nanogaps between nanoparticles, permitting fine-tuning of the resonance wavelength and near-field enhancement at the gap. Previously reported methods of molecular or lithographic control of the gap distance are limited to producing discrete values and encounter difficulty in achieving subnanometer gap distances. For these reasons, the study of the plasmon coupling for varying degrees of interaction remains a challenge. Here, we report that by using light, the interparticle distance for gold nanoparticle (AuNP) dimers can be continuously tuned from a few nanometers to negative values (i.e., merged particles). Accordingly, the plasmon coupling between the AuNPs transitions from the classical electromagnetic regime to the contact regime via the nonlocal and quantum regimes in the subnanometer gap region. We find that photooxidative desorption of alkanedithiol linkers induced by UV irradiation causes the two AuNPs in a dimer to approach each other and eventually merge. Light-driven control of the interparticle distance offers a novel means of exploring the fundamental nature of plasmon coupling as well as the possibility of fabricating nanoassemblies with any desired gap distance in a spatially controlled manner.

Fabrication of Gold Nanoparticles/Graphene-PDDA Nanohybrids for Bio-detection by SERS Nanotechnology

In this research, graphene nanosheets were functionalized with cationic poly (diallyldimethylammonium chloride) (PDDA) and citrate-capped gold nanoparticles (AuNPs) for surface-enhanced Raman scattering (SERS) bio-detection application. AuNPs were synthesized by the traditional citrate thermal reduction method and then adsorbed onto graphene-PDDA nanohybrid sheets with electrostatic interaction. The nanohybrids were subject to characterization including X-ray diffraction (XRD), transmission electron microscopy (TEM), zeta potential, and X-ray photoelectron spectroscopy (XPS). The results showed that the diameter of AuNPs is about 15-20 nm immobilized on the graphene-PDDA sheets, and the zeta potential of various AuNPs/graphene-PDDA ratio is 7.7-38.4 mV. Furthermore, the resulting nanohybrids of AuNPs/graphene-PDDA were used for SERS detection of small molecules (adenine) and microorganisms (Staphylococcus aureus), by varying the ratios between AuNPs and graphene-PDDA. AuNPs/graphene-PDDA in the ratio of AuNPs/graphene-PDDA = 4:1 exhibited the strongest SERS signal in SERS detection of adenine and S. aureus. Thus, it is promising in the application of rapid and label-free bio-detection of bacteria or tumor cells.

Raman fingerprinting of single dielectric nanoparticles in plasmonic nanopores

Plasmonic nano-apertures are commonly used for the detection of small particles such as nanoparticles and proteins by exploiting electrical and optical techniques. Plasmonic nanopores are metallic nano-apertures sitting on a thin membrane with a tiny hole. It has been shown that plasmonic nanopores with a given geometry identify internal molecules using Surface Enhanced Raman Spectroscopy (SERS). However, label-free identification of a single dielectric nanoparticle requires a highly localized field comparable to the size of the particle. Additionally, the particle's Brownian motion can jeopardize the amount of photons collected from a single particle. Here, we demonstrate that the combination of optical trapping and SERS can be used for the detection and identification of 20 nm polystyrene nanoparticles in plasmonic nanopores. This work is anticipated to contribute to the detection of small bioparticles, optical trapping and nanotribology studies.

Z-scan studies of the nonlinear optical properties of gold nanoparticles prepared by electron beam deposition

This paper details the fabrication process for placing single-layer gold (Au) nanoparticles on a planar substrate, and investigation of the resulting optical properties that can be exploited for nonlinear optics applications. Preparation of Au nanoparticles on the substrate involved electron beam deposition and subsequent thermal dewetting. The obtained thin films of Au had a variation in thicknesses related to the controllable deposition time during the electron beam deposition process. These samples were then subjected to thermal annealing at 600°C to produce a randomly distributed layer of Au nanoparticles. Observation from field-effect scanning electron microscope (FESEM) images indicated the size of Au nanoparticles ranges from ∼13 to ∼48  nm. Details of the optical properties related to peak absorption of localized surface plasmon resonance (LSPR) of the nanoparticle were revealed by use of UV-Vis spectroscopy. The Z-scan technique was used to measure the nonlinear effects on the fabricated Au nanoparticle layers where it strongly relates LSPR and nonlinear optical properties.

Computationally Guided Assembly of Oriented Nanocubes by Modulating Grafted Polymer-Surface Interactions

The bottom-up fabrication of ordered and oriented colloidal nanoparticle assemblies is critical for engineering functional nanomaterials beyond conventional polymer-particle composites. Here, we probe the influence of polymer surface ligands on the self-orientation of shaped metal nanoparticles for the formation of nanojunctions. We examine how polymer graft-surface interactions dictate Ag nanocube orientation into either edge-edge or face-face nanojunctions. Specifically, we investigate the effect of end-functionalized polymer grafts on nanocube assembly outcomes, such as interparticle angle and interparticle distance. Our assembly results can be directly mapped onto our theoretical phase diagrams for nanocube orientation, enabling correlation of experimental variables (such as graft length and metal binding strength) with computational parameters. These results represent an important step toward unifying modeling and experimental approaches to understanding nanoparticle-polymer self-assembly.

Biotags Based on Surface-Enhanced Raman Can Be as Bright as Fluorescence Tags

Surface enhanced Raman spectroscopy (SERS) is a powerful analytical technique that has been proposed as a substitute for fluorescence for biological imaging and detection but is not yet commercially utilized. The reason lies primarily in the lower intensity and poor reproducibility of most metal nanoparticle-based tags as compared to their fluorescence-based counterparts. Here, using a technique that scrupulously preserves the same number of dye molecules in both the SERS and fluorescence measurements, we show that SERS-based biotags (SBTs) with highly reproducible optical properties can be nanoengineered such that their brightness is at least equal to that of fluorescence-based tags.

Tunable plasmon resonance and enhanced second harmonic generation and upconverted fluorescence of hemispheric-like silver core/shell islands

We investigate tunable plasmon resonance and enhanced second harmonic generation (SHG) and up-converted fluorescence (UCF) of the hemispheric-like silver core/shell islands. The Ag, Ag/Ag2O, and Ag/Ag2O/Ag island films are prepared by using a sputtering technique. The SHG and UCF of the Ag/Ag2O/Ag core/shell islands near the percolating regime is enhanced 2.34 and 3.94 times compared to the sum of two individual counterparts of Ag/Ag2O core/shell and Ag shell islands. The ratio of SHG intensity induced by p- and s-polarization is 0.86 for the initial Ag islands and increase to 1.61 for the Ag/Ag2O/Ag core/shell samples. The tunable intensity ratio of SHG to UCF of the Ag islands treated by thermal and laser annealing processes is also observed. The physical mechanism of the enhanced SHG and UCF in the Ag/Ag2O/Ag core/shell islands is discussed. Our observations provide a new approach to fabricate plasmon-enhanced optical nonlinear nanodevices with tunable SHG and UCF.

High-Density 2D Homo- and Hetero- Plasmonic Dimers with Universal Sub-10-nm Gaps

Fabrication of high-density plasmonic dimers on a large (wafer) scale is crucial for applications in surface-enhanced spectroscopy, bio- and molecular sensing, and optoelectronics. Here, we present an experimental approach based on nanoimprint lithography and shadow evaporation that allows for the fabrication of high-density, large-scale homo- (Au-Au and Ag-Ag) and hetero- (Au-Ag) dimer substrates with precise and consistent sub-10-nm gaps. We performed scanning electron, scanning transmission electron, and atomic force microscopy studies along with a complete electron energy-loss spectroscopy (EELS) characterization. We observed distinct plasmonic modes on these dimers, which are well interpreted by finite-difference time-domain (FDTD) and plasmon hybridization calculations.

Remarkably enhanced red-NIR broad spectral absorption via gold nanoparticles: applications for organic photosensitive diodes

For organic films, remarkably enhanced red-NIR broad spectral absorption was achieved via the incorporation of gold nanoparticles (AuNPs) using a simple and facile preparation. The relevant thermal evaporation method has produced size-controllable AuNPs in the range of 0-20 nm diameter. The potential use of localized surface plasmon resonance (LSPR) enhanced organic photosensitive diodes (OPDs) as sensitive broadband sensors was discussed in this context. Here we showed that, by combining organic heterojunctions with size-controllable plasmonic AuNPs, the efficiency of organic photodetectors could be increased by up to one order of magnitude, because of LSPR and scattering effects of the AuNPs. Fabricated OPD devices showed a large photoresponse under radiation from wavelengths between 650 and 830 nm, accompanied by a low power consumption profile. A schematic energy level model combined with theoretical simulation analysis was proposed to explain the experimental data. More importantly, to the best of our knowledge, this work demonstrated the broadest photosensitivity with high responsivity from AuNP-based photodetectors, proving the potential of AuNPs as a promising material for efficient optoelectronic devices.

Calix[4]arene-Functionalised Silver Nanoparticles as Hosts for Pyridinium-Loaded Gold Nanoparticles as Guests

A series of lipophilic gold nanoparticles (AuNPs) circa 5 nm in diameter and having a mixed organic layer consisting of 1-dodecanethiol and 1-(11-mercaptoundecyl) pyridinium bromide was synthesised by reacting tetraoctylammonium bromide stabilised AuNPs in toluene with different mixtures of the two thiolate ligands. A bidentate ω-alkylthiolate calix[4]arene derivative was instead used as a functional protecting layer on AgNPs of approximately 3 nm. The functionalised nanoparticles were characterised by transmission electron microscopy (TEM), and by UV/Vis and X-ray photoelectron spectroscopy (XPS). Recognition of the pyridinium moieties loaded on the AuNPs by the calix[4]arene units immobilised on the AgNPs was demonstrated in solution of weakly polar solvents by UV/Vis titrations and DLS measurements. The extent of Au-AgNPs aggregation, shown through the low-energy shift of their surface plasmon bands (SPB), was strongly dependent on the loading of the pyridinium moieties present in the organic layer of the AuNPs. Extensive aggregation between dodecanethiol-capped AuNPs and the Ag calix[4]arene-functionalised NPs was also promoted by the action of a simple N-octyl pyridinium difunctional supramolecular linker. This linker can interdigitate through its long fatty tail in the organic layer of the dodecanethiol-capped AuNPs, and simultaneously interact through its pyridinium moiety with the calix[4]arene units at the surface of the modified AgNPs.

Thickness of a metallic film, in addition to its roughness, plays a significant role in SERS activity

In this paper we evaluate the effect of roughness and thickness of silver film substrates, fabricated on glass and polydimethylsiloxane (PDMS) templates, on surface-enhanced Raman Spectroscopy (SERS) activity. While the silver substrates obtained on glass templates exhibit nm-scale roughness, the silver substrates on PDMS templates show larger roughness, on the order of 10 s of nm. These roughness values do not change significantly with the thickness of the silver film. The SERS intensities of 4-aminothiophenol (ATP) deposited on these substrates strongly depend on both roughness and thickness, with more significant contribution from the roughness on thinner films. FEM simulations of the electric field intensities on surfaces of different thicknesses for rough and flat surfaces suggest higher localized plamons on thinner, rough surfaces. This study indicates that, besides roughness, the thickness of the metallic layer plays a significant role in the SERS activity.

Nanoparticles as Nonfluorescent Analogues of Fluorophores for Optical Nanoscopy

Optical microscopy modalities that achieve spatial resolution beyond the resolution limit have opened up new opportunities in the biomedical sciences to reveal the structure and kinetics of biological processes on the nanoscale. These methods are, however, mostly restricted to fluorescence as contrast mechanism, which limits the ultimate spatial resolution and observation time that can be achieved by photobleaching of the fluorescent probes. Here, we demonstrate that Raman scattering provides a valuable contrast mechanism for optical nanoscopy in the form of super-resolution structured illumination microscopy. We find that nanotags, i.e., gold and silver nanoparticles that are capable of surface-enhanced Raman scattering (SERS), can be imaged with a spatial resolution beyond the diffraction limit in four dimensions alongside and with similar excitation power as fluorescent probes. The highly polarized nature of super-resolution structured illumination microscopy renders these nanotags elliptical in the reconstructed super-resolved images, which enables us to determine their orientation within the sample. The robustness of nanotags against photobleaching allows us to image these particles for unlimited periods of time. We demonstrate this by imaging isolated nanotags in a dense layer of fluorophores, as well as on the surface of and after internalization by osteosarcoma cells, always in the presence of fluorescent probes. Our results show that SERS nanotags have the potential to become highly multiplexed and chemically sensitive optical probes for optical nanoscopy that can replace fluorophores in applications where fluorescence photobleaching is prohibitive for following the evolution of biological processes for extended times.

Multifunctional self-assembled composite colloids and their application to SERS detection

We present a simple method for the co-encapsulation of gold nanostars and iron-oxide nanoparticles into hybrid colloidal composites that are highly responsive to both light and external magnetic fields. Self-assembly was driven by hydrophobic interactions between polystyrene capped gold nanostars and iron oxide nanocrystals stabilized with oleic acid, upon addition of water. A block copolymer was then used to encapsulate the resulting spherical colloidal particle clusters, which thereby became hydrophilic. Electron microscopy analysis unequivocally shows that each composite particle comprises a single Au nanostar surrounded by a few hundreds of iron oxide nanocrystals. We demonstrate that this hybrid colloidal system can be used as an efficient substrate for surface enhanced Raman scattering, using common dyes as model molecular probes. The co-encapsulation of iron oxide nanoparticles renders the system magnetically responsive, so that application of an external magnetic field leads to particle accumulation and limits of detection are in the nM range.

Homogeneous large-scale crystalline nanoparticle-covered substrate with high SERS performance

This article details the surface-enhanced Raman scattering (SERS) performance of plasmonic substrates fabricated by a physical metal evaporation technique that uses no precursor or intermediate coating. We outline a cost-effective nanofabrication protocol that uses common laboratory equipment to produce homogeneously covered crystalline nanoparticle substrates. Our fabrication yields a homogeneous SERS response over the whole surface. The platform is tested with methylene blue diluted at various concentrations to estimate the sensitivity, homogeneity, and reproducibility of the process. The capacity of the substrates is also confirmed with spectroscopic investigations of human microsomal cytochrome b5.

Coupling Mediated Coherent Control of Localized Surface Plasmon Polaritons

We investigate the phase-dependent excitation of localized surface plasmon polaritons in coupled nanorods by using nonlinear spectroscopy. Our design of a coupled three-nanorod structure allows independent excitation with cross-polarized light. Here, we show that the excitation of a particular plasmon mode can be coherently controlled by changing the relative phase of two orthogonally polarized light fields. Furthermore, we observe a phase relation for the excitation that is dominantly caused by damping effects.

High Aspect-Ratio Iridium-Coated Nanopillars for Highly Reproducible Surface-Enhanced Raman Scattering (SERS)

A variety of different gold and silver nanostructures have been proposed over the years as high sensitivity surface-enhanced Raman scattering (SERS) sensors. However, efficient use of SERS has been hindered by the difficulty of realizing SERS substrates that provide reproducible SERS response over the whole active area. Here, we show that atomic layer deposition (ALD) grown iridium can be used to produce highly reliable SERS substrates. The substrates are based on a periodic array of high aspect-ratio iridium coated nanopillars that feature efficient and symmetrically distributed hot spots within the interpillar gaps (gap width<10 nm). We show that the enhancement with the iridium based nanostructures is of significant magnitude and it equals the enhancement of silver based reference substrates. Most notably, we demonstrate that the ordered and well-defined plasmonic nanopillars offer a measurement-to-measurement variability of 5%, which paves the way for truly quantitative SERS measurements.

The Coupling between Gold or Silver Nanocubes in Their Homo-Dimers: A New Coupling Mechanism at Short Separation Distances

Using the DDA method, we investigated the near-field coupling between two excited Au or Ag 42 nm nanocubes in a face-to-face dimer configuration at small separation distances where the exponential coupling behavior distinctly changes. This could be due to the failure of the dipole approximation at short distances or a change in the electromagnetic field distribution between the adjacent monomers. A detailed calculation of the plasmonic field distribution strongly suggests that the latter mechanism is responsible for the failure of the expected exponential coupling behavior at small separation distances. The results suggest that the observed optical properties of the pair of Au or Ag nanocubes separated by distances larger than 6 nm, result from the electromagnetic coupling between the oscillating dipoles at the corners of the adjacent facets of the nanocubes. At separations smaller than 6 nm, the distribution of the plasmonic dipoles along both the facets and the corners of the adjacent monomers control the plasmonic spectra and the distance dependent optical properties of the dimer.

Tunable Light-Matter Interaction and the Role of Hyperbolicity in Graphene-hBN System

Hexagonal boron nitride (hBN) is a natural hyperbolic material, which can also accommodate highly dispersive surface phonon-polariton modes. In this paper, we examine theoretically the mid-infrared optical properties of graphene-hBN heterostructures derived from their coupled plasmon-phonon modes. We find that the graphene plasmon couples differently with the phonons of the two Reststrahlen bands, owing to their different hyperbolicity. This also leads to distinctively different interaction between an external quantum emitter and the plasmon-phonon modes in the two bands, leading to substantial modification of its spectrum. The coupling to graphene plasmons allows for additional gate tunability in the Purcell factor and narrow dips in its emission spectra.

Graphene oxide-encoded Ag nanoshells with single-particle detection sensitivity towards cancer cell imaging based on SERRS

Developing ultrasensitive Raman nanoprobes is one of the emerging interests in the field of biosensing and bioimaging. Herein, we constructed a new type of surface-enhanced resonance Raman scattering nanoprobe composed of an Ag nanoshell as a surface-enhanced Raman scattering-active nanostructure, which was encapsulated with 4,7,10-trioxa-1,13-tridecanediamine-functionalized graphene oxide as an ultrasensitive Raman reporter exhibiting strong resonance Raman scattering including distinct D and G modes. The designed nanoprobe was able to produce much more intense and simpler Raman signals even at a single particle level than the Ag nanoshell bearing a well-known Raman reporter, which is beneficial for the sensitive detection of a target in a complex biological system. Finally, this ultrasensitive nanoprobe successfully demonstrated its potential for bioimaging of cancer cells using Raman spectroscopy.

Quantum effects in the optical response of extended plasmonic gaps: validation of the quantum corrected model in core-shell nanomatryushkas

Electron tunneling through narrow gaps between metal nanoparticles can strongly affect the plasmonic response of the hybrid nanostructure. Although quantum mechanical in nature, this effect can be properly taken into account within a classical framework of Maxwell equations using the so-called Quantum Corrected Model (QCM). We extend previous studies on spherical cluster and cylindrical nanowire dimers where the tunneling current occurs in the extremely localized gap regions, and perform quantum mechanical time dependent density functional theory (TDDFT) calculations of the plasmonic response of cylindrical core-shell nanoparticles (nanomatryushkas). In this axially symmetric situation, the tunneling region extends over the entire gap between the metal core and the metallic shell. For core-shell separations below 0.5 nm, the standard classical calculations fail to describe the plasmonic response of the cylindrical nanomatryushka, while the QCM can reproduce the quantum results. Using the QCM we also retrieve the quantum results for the absorption cross section of the spherical nanomatryushka calculated by V. Kulkarni et al. [Nano Lett. 13, 5873 (2013)]. The comparison between the model and the full quantum calculations establishes the applicability of the QCM for a wider range of geometries that hold tunneling gaps.

Enhancement and extinction effects in surface-enhanced stimulated Raman spectroscopy

We address the optical physics of surface-enhanced stimulated Raman spectroscopy (SESRS) from the microscopic to macroscopic scales to provide experimental design criteria in colloidal-suspension SESRS. The nanoparticles that provide local field enhancement also extinguish the Raman signal. We compute the total Raman signal detected from a suspension of Raman-active molecules and nanoparticles due to the cumulative effects of enhancement and extinction and find optimum operating parameters for pump frequency and nanoparticle concentration.

Bi-SERS sensing and enhancement by Au-Ag bimetallic non-alloyed nanoparticles on amorphous and crystalline silicon substrate

We have demonstrated Au-Ag bimetallic non-alloy nanoparticles (BNNPs) on thin a-Si film and c-Si substrate for high SERS enhancement, low cost, high sensitivity and reproducible SERS substrate with bi-SERS sensing properties where two different SERS peak for Au NPs and Ag NPs are observed on single SERS substrate. The isolated Au-Ag bimetallic NPs, with uniform size and spacing distribution, are suitable for uniform high density hotspot SERS enhancement. The SERS enhancement factor of Au-Ag BNNPs is 2.9 times higher compared to Ag NPs on similar substrates due to the increase of the localized surface plasmon resonance effect. However there is a decrement of SERS peak intensity at specific wavenumbers when the surrounding refractive index increases due to out-phase hybridization of Au NPs. The distinct changes of the two different SERS peaks on single Au-Ag BNNPs SERS substrate due to Au and Ag NPs independently show possible application for bi-molecular sensing.

Synthesis and characterization of PVK/AgNPs nanocomposites prepared by laser ablation

Nanocomposites of Poly (n-vinylcarbazole) PVK/Ag nanoparticles were prepared by laser ablation of a silver plate in aqueous solution of chlorobenzene. The influences of laser parameters such as; time of irradiation, source power and wavelength (photon energy) on structural, morphological and optical properties have been investigated using X-ray diffraction (XRD), Transmission electron microscopy (TEM), Ultraviolet-visible (UV-Vis) and Photoluminescence (PL). A correlation between the investigated properties has been discussed. XRD, TEM and PL indicated that the complexation between AgNPs and PVK in the composite system is possible. Only the reflection peak at 2θ=38° of AgNPs appeared in the composite nanoparticles while the other reflection peaks were destroyed. The nanoparticles shape and size distribution were evaluated from TEM images. TEM analysis revealed a lower average particle size at long laser irradiation time 40min and short laser wavelength 532nm together with high laser power 570mW. From UV-Visible spectra the values of absorption coefficient, absorption edge and energy tail were calculated. The reduction of band tail value with increasing the laser ablation parameters confirms the decrease of the disorder in such composite system. The PL and UV-Vis. spectra confirm that nanocomposite samples showed quantum confinement effect.

Plasmonic core-shell nanoparticles for SERS detection of the pesticide thiram: size- and shape-dependent Raman enhancement

We systematically investigated the size- and shape-dependent SERS activities of plasmonic core-shell nanoparticles towards detection of the pesticide thiram. Monodisperse Au@Ag nanocubes (NCs) and Au@Ag nanocuboids (NBs) were synthesized and their Ag shell thickness was precisely adjusted from ∼1 nm to ∼16 nm. All these nanoparticles were used as SERS substrates for thiram detection, and the Raman intensities with three different lasers (514 nm, 633 nm and 782 nm) were recorded and compared. Our results clearly show that: (1) the excitation wavelength discriminated particle shapes regardless of particle sizes, and the maximized Raman enhancement was observed when the excitation wavelength approaches the SERS peak (provided there is significant local electric field confinement on the plasmonic nanostructures at that wavelength); (2) at the optimized laser wavelength, the maximum Raman enhancement was achieved at a certain threshold of particle size (or silver coating thickness). By exciting particles at their optimized sizes with the corresponding optimized laser wavelengths, we achieved a detection limit of roughly around 100 pM and 80 pM for NCs and NBs, respectively.

Regioselective plasmonic coupling in metamolecular analogs of benzene derivatives

In analogy with benzene-derived molecular structures, we construct plasmonic metamolecules by attaching Au nanospheres to specific sites on a hexagonal Au nanoplate. We employ a ligand exchange strategy that allows regioselective control of nanosphere attachment and study resulting structures using correlated electron microscopy/optical spectroscopy at the single-metamolecule level. We find that plasmonic coupling within the resulting assembly is strongly dependent on the structure of the metamolecule, in particular the site of attachment of the nanosphere(s). We also uncover a synergy in the polarizing effect of multiple nanospheres attached to the nanoplate. Regioselective control of plasmonic properties demonstrated here enables the design of novel structure-dependent electromagnetic modes and applications in three-dimensional spatial nanosensors.

Far- and near-field properties of gold nanoshells studied by photoacoustic and surface-enhanced Raman spectroscopies

Gold nanoshells, with a silica core and different core and shell dimensions, have been extensively investigated. Optical far-field properties, namely extinction and absorption, have been separately determined by means of spectrophotometry and photoacoustic spectroscopy, respectively, in the 440-900 nm range. The enhancement factor for surface-enhanced Raman scattering, which is related to near-field effects, has been measured from 568 to 920 nm. The absorption contribution to extinction decreases as the overall diameter increases. Moreover, absorption and scattering display different spectral distributions, the latter being red shifted. The Surface Enhanced Raman Scattering enhancement profile, measured using thiobenzoic acid as a Raman probe, is further shifted to the red. The latter result suggests that the enhancement is dominated by the presence of hot spots, which are possibly related to the surface roughness of gold nanoshell particles.

Equilibrium and dynamical properties of polymer chains in random medium filled with randomly distributed nano-sized fillers

The effect of randomly distributed nano-sized fillers on the equilibrium and dynamical properties of linear polymers is studied by using off-lattice Monte Carlo simulation.

Control of localized surface plasmon resonance energy in monolayer structures of gold and silver nanoparticles

The LSPR energy was successfully controlled in the wide range of 2.0–3.0 eV using Au and Ag nanoparticles.

Adjustable plasmonic optical properties of hollow gold nanospheres monolayers and LSPR-dependent surface-enhanced Raman scattering of hollow gold nanosphere/graphene oxide hybrids

Colloidal hollow gold nanospheres with adjustable localized surface plasmon resonance (LSPR) properties were synthesized and self-assembled into HGNs monolayers for investigation of LSPR-dependent surface enhanced Raman scattering (SERS) behavior.

Enhanced multi-phonon Raman scattering and nonlinear optical power limiting in ZnO:Au nanostructures

Enhanced Raman scattering and nonlinear optical power limiting in ZnO:Au nanostructures is demonstrated.