Plasmon-Coupled Nanostructures Comprising Finite Number of Gold Particles

We report a simple method for preparation of plasmonic nanostructures containing two, three, four, and five closely spaced 15-nm gold particles. The structures were separated from each other and purified to greater than 90 % by electrophoresis. The plasmon absorption spectra of the structures are redshifted with respect to the spectrum of gold nanoparticles not connected to each other. The magnitude of the redshift is directly proportional to the number of nanoparticles in the structure.     

Gamma–Radiation-Assisted Synthesis of Luminescent ZnO/Ag Heterostructure Core–Shell Nanocomposites

There is increasing interest in tuning the physical properties of semiconductor nanostructures using metal nanoparticles. In this work, ZnO nanosphere covered with Ag nanoparticles were synthesized using gamma–radiation-assisted method. The amount of deposited Ag nanoparticles is controlled by changing irradiation dose in the range of 30–100 kGy in order to tune the semiconductor–metal interaction. The successful deposition of Ag on the ZnO nanoparticles is examined by analyzing the morphology, microstructure, optical, and magnetic properties of ZnO/Ag nanoparticles through field emission scanning electron (FESEM), microscopy X-ray diffraction spectra, UV-visible absorption, photoluminescence measurement, and vibrating sample magnetometer. FESEM and elemental mapping results confirmed that Ag nanoparticles have been concentrated at the surface of spherical ZnO particles. Moreover, formation of pure metallic Ag nanoparticles has been confirmed by XRD analysis. UV-visible absorption spectra of obtained ZnO/Ag showed two combined peaks, a weak peak at the shoulder around 360 nm corresponds to ZnO and a sharp absorption at 420 nm refers to spherical Ag nanoparticles. Obtained results from photoluminescence revealed that the near-band-edge emission and defect-related visible emission bands of ZnO could be enhanced dramatically at the same time by deposition of Ag nanoparticles, which was ascribed to localized surface plasmon–exciton coupling and surface plasmon scattering. Controlling the semiconductor and metal coupling effect is interesting because of its application in highly efficient optoelectronic devices and biosensor.     

Synthesis,Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.     

Experimental Study on Localized Surface Plasmon Mode Hybridization in the Near and Mid Infrared

We investigate plasmon excitations within a regular grating of double-layered gold/insulator nanoparticles in the infrared and visible spectral region. Provided a flat gold film as substrate, strong coupling between the localized surface plasmon modes and their image-like excitations in the metal is observed. The interaction results in a strong red shift of the plasmon mode as well as the splitting of the modes into levels of different angular momenta, often referred to as plasmon hybridization. The diameters of the nanoparticles are designed in a way that the splitting of the resonances occurs in the spectral region between 0.1 and 1 eV, thus being accessible using an infrared microscope. Moreover, we investigated the infrared absorption signal of gratings that contain two differently sized nanoparticles. The interaction between two autonomous localized surface plasmon excitations is investigated by analyzing their crossing behavior. In contrast to the interaction between localized surface plasmons and propagating plasmon excitations which results in pronounced anticrossing, the presented structures show no interaction between two autonomous localized surface plasmons. Finally, plasmon excitations of the nanostructured surfaces in the visible spectral region are demonstrated through photographs acquired at three different illumination angles. The change in color of the gratings demonstrates the complex interaction between propagating and localized surface plasmon modes.     

Nanogold-plasmon-resonance-based glucose sensing

Noble metal nanoparticles are well known for their strong interactions with light through the resonant excitations of the collective oscillations of the conduction electrons on the particles, the so-called surface plasmon resonances. The close proximity of two nanoparticles is known to result in a red-shifted resonance wavelength peak, due to near-field coupling. We have subsequently employed this phenomenon and developed a new approach to glucose sensing, which is based on the aggregation and disassociation of 20-nm gold particles and the changes in plasmon absorption induced by the presence of glucose. High-molecular-weight dextran-coated nanoparticles are aggregated with concanavalin A (Con A), which results in a significant shift and broadening of the gold plasmon absorption. The addition of glucose competitively binds to Con A, reducing gold nanoparticle aggregation and therefore the plasmon absorption when monitored at a near-red arbitrary wavelength. We have optimized our plasmonic-type glucose nanosensors with regard to particle stability, pH effects, the dynamic range for glucose sensing, and the observation wavelength to be compatible with clinical glucose requirements and measurements. In addition, by modifying the amount of dextran or Con A used in nanoparticle fabrication, we can to some extent tune the glucose response range, which means that a single sensing platform could potentially be used to monitor microM --> mM glucose levels in many physiological fluids, such as tears, blood, and urine, where the glucose concentrations are significantly different.     

Detection of cholesterol by digitonin conjugated gold nanoparticles

Functionalized colloidal gold is widely used for qualitative and quantitative detection of specific analytes. We report here a novel modification of gold nanoparticles by digitonin, a glycoside used for precipitating membrane cholesterol. The specific molecular recognition of cholesterol by digitonin gold nanoparticles (DGNP), make it an attractive alternative to the existing enzymatic methods for cholesterol sensing. To enable cholesterol binding, we modified mercapto modified GNPs with digitonin, by a simple esterification reaction. The blue shift in the plasmon absorption spectra of DGNP with different cholesterol concentrations accompanied by a decrease in the absorbance is the principle applied here for the estimation. The observed size reduction followed by cholesterol binding is reasoned due to the enhanced hydophobicity of the surface which in turn expels the water layers associated with the particles prior to cholesterol binding. The method exhibited linearity between concentration of cholesterol and the corresponding absorbance of the plasmon peak, in the range of 160-600 ng/mL with a detection limit of 100±9 ng/mL. Other steroids did not show any binding affinity towards DGNP. The method depicted here has potential for development as an enzyme free sensor for cholesterol although many factors need to be addressed to transform it for assaying samples like blood.     

Absorption Enhancements in Plasmonic Solar Cells Coated with Metallic Nanoparticles

We find that three mechanisms lead to the absorption enhancements of light in a thin-film amorphous silicon solar cell coated with a periodic array of silver nanoparticles on the rear surface according to our simulation. They are localized surface plasmon modes of the silver nanoparticles, Fabry–Pérot resonant cavity modes and waveguide effects. Each enhancing mechanism can yield a maximum absorption enhancement of over two times at the corresponding resonant wavelengths when the nanoparticles cover 20 % of the solar cell surface, and an average absorption enhancement of up to 57 % can be achieved in the AM 1.5 G solar spectrum. The absorption enhancements can also be tuned in spectrum to optimize the total absorption in a plasmonic solar cell.     

Plasmonic Spectral Detection of Carcinoembryonic Antigen by Preventing the Direct Binding of Rhodamine 6G with Au Nanoparticles

Carcinoembryonic antigen (CEA) was used as a separator to prevent the Rhodamine 6G (R6G)-induced aggregation of colloidal gold nanoparticles. The destroyed aggregation has been monitored by measuring the absorption and resonance light scattering peaks corresponding to the longitudinal surface plasmon resonance (SPR) of the chain-like aggregated gold nanoparticles (AuNPs). It was found that the pre-adding of CEA with different concentrations to the gold colloids before mixing them with R6G could lead to the longitudinal SPR peak decrease and blue shift. By analysing the intensity changing and wavelength shifting of the absorption spectra, CEA could be detected in a linear range from 0.2 to 4 ng/mL, and the limit of detection reaches to 0.1 ng/mL. The sensitivity of the CEA concentration dependent shifting and quenching of the plasmonic absorption and scattering corresponding to the AuNPs aggregation presents a well potential application of biologic spectral sensing.     

Femtomolar DNA detection by parallel colorimetric darkfield microscopy of functionalized gold nanoparticles

We introduce a sensing platform for specific detection of DNA based on the formation of gold nanoparticles dimers on a surface. The specific coupling of a second gold nanoparticle to a surface bound nanoparticle by DNA hybridization results in a red shift of the nanoparticle plasmon peak. This shift can be detected as a color change in the darkfield image of the gold nanoparticles. Parallel detection of hundreds of gold nanoparticles with a calibrated true color camera enabled us to detect specific binding of target DNA. This enables a limit of detection below 1.0×10(-14) M without the need for a spectrometer or a scanning stage.     

Large and Ultrafast Optical Response of a One-Dimensional Plasmonic–Photonic Cavity

The resonant coupling of a localized surface plasmon mode and a cavity mode in a photonic crystal has been recently shown to strongly tailor the stationary optical response of gold nanoparticles. Here, we demonstrate that this can be further exploited for controlling light on an ultrashort time scale. The stationary and ultrafast optical responses of such a plasmonic–photonic cavity are investigated numerically. We show that the transient photo-induced change of the optical transmittance of a bare nanocomposite thin film can be amplified up to 60 times once resonantly coupled to the cavity mode in the hybrid device, despite the degradation of this mode due to absorption losses. In addition, different all-optical, ultrafast, efficient, and reversible photonic functions (increase or decrease of the signal intensity, transient spectral shift of the cavity mode) can be achieved depending on the spectral position of the transmitted mode tuned by varying the angle of incidence. The transient modification of the signal intensity is predicted to reach about 300 % after a subpicosecond rise time when the defect mode matches the plasmon resonance.     

Tailored Aggregate-Free Au Nanoparticle Decorations with Sharp Plasmonic Peaks on a U-Type Optical Fiber Sensor by Nanosecond Laser Irradiation

A novel experimental methodology is presented for fabricating U-shaped optical fiber probes decorated with aggregate-free Au nanoparticles exhibiting sharp localized surface plasmon resonance (LSPR) spectra. The U-type tip is coated with gold nanoparticles (AuNPs) using a simple and time-efficient dip-coating procedure, without initially taking any care to prevent the formation of nanoparticle aggregates in the coated area. In a second step, the coating was irradiated with a few tens of laser pulses of 5-ns duration at 532 nm with intensities in the range of 2–14 MW/cm², leading to the formation of aggregate-free LSPR optical fiber probes. The process was monitored and controlled in real time through the changes induced into the fiber’s extinction spectra by the laser irradiation, and the coated fibers were characterized by electron microscopy. The proposed methodology resulted into the fabrication of U-type optical fiber probes coated with AuNPs exhibiting a sharp plasmon peak, which is a perquisite for their application as sensing devices.     

Multiple Surface Plasmon Resonances in Compound Structure with Metallic Nanoparticle and Nanohole Arrays

The optical properties of a compound structure with metallic nanoparticle and nanohole arrays are numerically investigated by the means of finite-difference time domain method. We report on the observation of multi-valleys in the reflection spectra due to the excitation of surface plasmon (SP) resonant modes of the compound structure. Simulation results show that multiple SP resonances consist of surface plasmon polaritons on the gold film, localized surface plasmons on the nanoparticles, and coupling mode between them. These findings are important for applications utilizing multiple surface plasmon resonances.     

Fluorescent (Au@SiO2)SiC Nanohybrids: Influence of Gold Nanoparticle Diameter and SiC Nanoparticle Surface Density

Gold@silica core–shell nanoparticles were prepared with various gold core diameters (ranging from 20 to 150 nm) and silica thicknesses (ranging from 10 to 30 nm). When the gold diameter is increased, the size dispersion became larger, leading to a broader plasmon band. Then, silicon carbide (SiC) nanoparticles were covalently immobilized onto silica to obtain hybrid (Au@SiO₂) SiC nanoparticles. The absorption properties of these hybrid nanoparticles showed that an excess of SiC nanoparticles in the dispersion can be identified by a strong absorption in the UV region. Compared to SiC reference samples, a blue shift of the fluorescence emission, from 582 to 523 nm, was observed, which was previously attributed to the strong surface modification of SiC when immobilized onto silica. Finally, the influence of several elaboration parameters (gold diameter, silica thickness, SiC concentration) on fluorescence enhancement was investigated. It showed that the highest enhancements were obtained with 10 nm silica thickness, low concentration of SiC nanoparticles, and surprisingly, with a 20-nm gold core diameter. This last result could be attributed to the broad plasmon band of big gold colloids. In this case, SiC emission strongly overlapped gold absorption, leading to possible quenching of SiC fluorescence by energy transfer.     

Preparation and Optical Characterization of Core–Shell Bimetal Nanoparticles

Chemical approaches allow for the synthesis of highly defined metal heteronanostructures, such as core–shell nanospheres. Because the material in the metal nanoparticles determines the plasmon resonance-induced absorption band, control of particle composition results in control of the position of the absorption band. Metal deposition on gold or silver nanoparticles yielded core–shell particles with modified optical properties. UV–vis spectroscopy on solution-grown, as well as surface-grown, particles was conducted and provided ensemble measurements in solution. Increasing the layers of a second metal leads to a shift in the absorption band. A shell diameter comparable to the original particle diameter leads to a predominant influence by the shell material. Extent of shell growth could be controlled by reaction time or the concentration of metal salt or reducing agent. Besides optical characterization, the utilization of atomic force microscopy, scanning electron microscopy, and transmission electron microscopy yielded important information about the ultrastructure of nanoparticle complexes. Surface-grown core–shell particles were superior in terms of achievable shell thickness, because of difficulties encountered with solution-grown particles due to salt-induced aggregation.     

Improved Absorbance and Near-Infrared Dispersion of AuGe Nanoparticles over Au Nanoparticles Prepared with Similar Thermal Annealing Environment

The difficulty in patterning the structures at sub-wavelength range leads to employ the bottom-up approach to form nanostructures of metals as well as dielectric components that disperse them in host media. The optical properties of nanoparticles are studied with UV-Vis 750 (lambda) NIR spectroscopy and fit with empirical relations. The refractive index is about the volume fraction of particles. The AuGe nanoparticles demonstrate improved absorbance, lower refractive index, and higher extinction than Au nanoparticles formed with similar thermal process. Surface plasmon resonance (SPR) phenomena are highly sensitive to the bonding between atoms, atomic structure, and the electronic configuration in atoms of the given material. If one takes into account the structure of materials, then the literature on eutectic alloys predicts that alloying gold with germanium (AuGe) with varying compositions will also change the x-ray diffraction peak positions of gold itself. The peak shift can be interpreted as the change in grain size or shift in grain boundaries implying a corresponding change in material’s atomic arrangement within lattice structure. As a result, there will be a change in the charge distribution of free electron cloud in original gold ultimately affecting a change in the plasmon resonance frequency and thereby modulating the various optical phenomena such as absorbance, reflectance, and refractive index. This alloying also brings a change in the dielectric constant of the material such that the plasmonic behavior may shift among different regions (UV, visible, NIR, MWIR, and LWIR). Metal semiconductor eutectic alloy which is widely popular as a soldering material would have scope in futuristic photonic applications due to its tuneable optical properties. In this work, we study the effects of Au and AuGe nanoparticle deposition on GaAs films grown by molecular beam epitaxy (MBE). Au and AuGe thin films (12-nm thick) were annealed in the temperature ranges of 400–800 and 300–700 °C, respectively, to form Au and AuGe nanoparticles. The formation of these nanoparticles was confirmed by scanning electron microscopy (SEM) measurements. Optical absorption spectroscopy measurements showed plasmon resonance peaks at around 670 and 535 nm for the AuGe-deposited 300 °C-annealed sample and Au-deposited 600 °C-annealed sample on sapphire, respectively, thereby confirming the plasmonic effect. Correlation of Raman spectroscopy measurement results with X-ray diffraction measurement results reveal that the transverse optical mode intensity and full width at half maximum of the GaAs (400) peak increased with an increase in annealing temperature, indicating degradation of the crystalline properties of GaAs film at higher annealing temperatures. The highest increments of the photoluminescence (PL) intensities in comparison to that of the bare GaAs film were observed to be 37 and 77% for the Au-deposited 600 °C-annealed and AuGe-deposited 300 °C-annealed samples, respectively. These enhancements of PL spectra are an indication of the significant scattering of photons by Au and AuGe nanoparticles, and they are attributed mainly to the contribution of the local surface plasmon resonance of these nanoparticles. A comparative analysis of PL enhancements revealed that AuGe nanoparticles induced a greater enhancement than Au nanoparticles. The calculated activation energies of the Au-deposited 600 °C-annealed sample, AuGe-deposited 300 °C-annealed sample, and bare GaAs sample were around 18, 24, and 33 meV, respectively. We found one-order increment in peak responsivity of AuGe plasmonic-based trilayer InAs quantum dot detector in comparison to as-grown detector at 80 K. Therefore, this study is expected to be very useful in the realization of high-performance plasmonic-based optoelectronic and sensing devices.     

Linear clusters of gold nanoparticles in quasinematic layers of DNA liquid-crystalline dispersion particles

The effects of small size (～2 nm) gold nanoparticles on the properties of particles of cholesteric liquid-crystalline dispersions formed by double-stranded DNA molecules were analyzed. It has been shown that gold nanoparticles induce two different processes. First, they facilitate reorganization of the spatial cholesteric structure of dispersion particles to nematic one. This process is accompanied by the fast decrease in the amplitude of abnormal band in the CD spectrum. Second, they can form ensembles consisting of gold nanoparticles. This process is accompanied by the development and displacement of surface plasmon resonance band in the visible region of the absorption spectrum. The appearance of this band is analyzed by considering two different models of the formation of ensembles consisting of gold nanoparticles. By small-angle X-ray scattering we performed structural analysis of phases formed by DNA cholesteric liquid-crystalline dispersion particles treated with gold nanoparticles. As a result of this study it was possible to prove the formation of linear clusters of gold nanoparticles in the “free space” between the adjacent DNA molecules fixed in the quasinematic layers of liquid-crystalline particles. It has been hypothesized that the formation of linear clusters of gold nanoparticles is most likely related to DNA molecules, ordered in the spatial structure of quasinematic layers, and the toxicity of these nanoparticles in biological systems hypothesized.     

Multifactor-Controlled Non-Monotonic Plasmon Shift of Ordered Gold Nanodisk Arrays: Shape-Dependent Interparticle Coupling

Surface plasmon resonance (SPR) absorption spectra of gold nanodisks hexagonally arranged in planar arrays have been studied by using coupled dipole method and quasi-static approximation. The calculation results reveal that the increasing aspect ratio (AR) of gold disks in the close-packed nanoarray leads to SPR blue shift firstly and then red shift. The critical AR corresponding to the maximum blue shift can be controlled by tuning the interparticle distance and particle size. The physical mechanism of this non-monotonic SPR shift is investigated based on the competition between the influences from shape factor and arranging structure of the array. Although increasing the semi-minor axis of gold disk reduces the AR and leads to a blue shift of SPR, this increasing semi-minor axis also reduces the average gap between two neighboring disks and enhances their coupling. Furthermore, the coulombic attraction between two neighboring disks introduces an additional plasmon damping and results in a red shift of SPR. This competition between AR and interparticle coupling improves the tuning ability of SPR in anisotropic metallic nanoparticle arrays and presents a potential for design and fabrication of optical biochip based on SPR.     

Grating-based surface plasmon resonance detection of core-shell nanoparticle mediated DNA hybridization

In this report, we have investigated enhanced surface plasmon resonance (SPR) detection of DNA hybridization using gold core - silica shell nanoparticles in localized plasmonic fields. The plasmonic fields were localized by periodic linear gratings. Experimental results measured for hybridization of 24-mer single-stranded DNA oligomers suggest that core-shell nanoparticles (CSNPs) on gratings of 400 nm period provide enhanced optical signatures by 36 times over conventional thin film-based SPR detection. CSNP-mediated DNA hybridization produced 3 times larger angular shift compared to gold nanoparticles of the same core size. We have also analyzed the effect of structural variation. The enhancement using CSNPs was associated with increased surface area and index contrast that is combined by improved plasmon coupling with localized fields on gratings. The combined approach for conjugated measurement of a biomolecular interaction on grating structures is expected to lower the limit of detection to the order of a few tens of fg/mm(2).     

Peptide-conjugated gold nanorods for nuclear targeting

Resonant electron oscillations on the surface of noble metal nanoparticles (Au, Ag, Cu) create the surface plasmon resonance (SPR) that greatly enhances the absorption and Rayleigh (Mie) scattering of light by these particles. By adjusting the size and shape of the particles from spheres to rods, the SPR absorption and scattering can be tuned from the visible to the near-infrared region (NIR) where biologic tissues are relatively transparent. Further, gold nanorods greatly enhance surface Raman scattering of adsorbed molecules. These unique properties make gold nanorods especially attractive as optical sensors for biological and medical applications. In the present work, gold nanorods are covalently conjugated with a nuclear localization signal peptide through a thioalkyl-triazole linker and incubated with an immortalized benign epithelial cell line and an oral cancer cell line. Dark field light SPR scattering images demonstrate that nanorods are located in both the cytoplasm and nucleus of both cell lines. Single cell micro-Raman spectra reveal enhanced Raman bands of the peptide as well as molecules in the cytoplasm and the nucleus. Further, the Raman spectra reveal a difference between benign and cancer cell lines. This work represents an important step toward both imaging and Raman-based intracellular biosensing with covalently linked ligand-nanorod probes.