Plasmonic and Nonlinear Optical Absorption Properties of Ag:ZrO2 Nanocomposite Thin Films

Ag nanoparticles (NPs) embedded in a zirconium oxide matrix in the form of Ag:ZrO₂ nanocomposite (NC) thin films were synthesized by using the sol–gel technique followed by thermal annealing. With the varying of the concentration of Ag precursor and annealing conditions, average sizes (diameters) of Ag nanoparticles (NPs) in the nanocomposite film have been varied from 7 to 20 nm. UV–VIS absorption studies reveal the surface plasmon resonance (SPR)-induced absorption in the visible region, and the SPR peak intensity increases with the increasing of the Ag precursor as well as with the annealing duration. A red shift in SPR peak position with the increase in the Ag precursor concentration confirms the growth of Ag NPs. Surface topographies of these NC films showed that deposited films are dense, uniform, and intact during the variation in annealing conditions. The magnitude and sign of absorptive nonlinearities were measured near the SPR of the Ag NPs with an open-aperture z-scan technique using a nanosecond-pulsed laser. Saturable optical absorption in NC films was identified having saturation intensities in the order of 10¹² W/m². Such values of saturation intensities with the possibility of size-dependent tuning could enable these NC films to be used in nanophotonic applications.     

Modulation of Visible and Near-Infrared Surface Plasmon Resonance of Au Nanoparticles Based on Highly Doped Graphene

We study an active modulation of surface plasmon resonance (SPR) of Au nanoparticles based on highly doped graphene in visible and near-infrared regions. We find that compared to the traditional metal SPR, the SPR of Au nanoparticles based on graphene causes a remarkable blue shift. The field intensity in the gap is redistributed to standing wave. The field intensity of standing wave is about one order of magnitude higher than the traditional model. Moreover, the SPR of Au nanoparticles can be actively modulated by varying the graphene Fermi energy. We find the maximum modulation of field intensity of absorption spectra is more than 21.6 % at λ?=?822?nm and the amount of blue shift is 17.4 nm, which is about 2.14 % of the initial wavelength λ ₀?=?813.4?nm, with increasing monolayer graphene Fermi energy from 1.0 to 1.5 ev. We find that the SPR sensitivity to the refractive index n of the environment is about 642 nm per refractive index unit (RIU). The SPR wavelengths have a big blue shift, which is about 33 nm, with increasing number of graphene layers from 1 to 3, and some shoulders on the absorption spectra are observed in the models with multilayer graphene. Finally, we study the Au nanorod array based on monolayer graphene. We find that the blue shift caused by the graphene increases from 14 to 24 nm, with increasing gap g _y from 10 to 20 nm. Then, it decreases from 24 to 14 nm, with increasing gap g _y from 20 to 50 nm. This study provides a new way for actively modulating the optical and optoelectronic devices.     

Enhancing LSPR Sensitivity of Au Gratings through Graphene Coupling to Au Film

A particular interesting plasmonic system is that of metallic nanostructures interacting with metal films. As the localized surface plasmon resonance (LSPR) behavior of gold nanostructures (Au NPs) on the top of a gold thin film is exquisitely sensitive to the spacer distance of the film-Au NPs, we investigate in the present work the influence of a few-layered graphene spacer on the LSPR behavior of the NPs. The idea is to evidence the role of few-layered graphene as one of the thinnest possible spacer. We first show that the coupling to the Au film induces a strong lowering at around 507 nm and sharpening of the main LSPR of the Au NPs. Moreover, a blue shift in the main LSP resonance of about 13 nm is observed in the presence of a few-layered graphene spacer when compared to the case where gold nanostructures are directly linked to a gold thin film. Numerical simulations suggest that this LSP mode is dipolar and that the hot spots of the electric field are pushed to the top corners of the NPs, which makes it very sensitive to surrounding medium optical index changes and thus appealing for sensing applications. A figure of merit of such a system (gold/graphene/Au NPs) is 2.8, as compared to 2.1 for gold/Au NPs. This represents a 33 % gain in sensitivity and opens-up new sensing strategies.     

Aptamer–Au NPs conjugates-enhanced SPR sensing for the ultrasensitive sandwich immunoassay

The goal of this work is to explore the amplification effect of aptamer–gold nanoparticles (Au NPs) conjugates for ultrasensitive detection of large biomolecules by surface plasmon resonance (SPR). A novel sandwich immunoassay is designed to demonstrate the amplification effect of aptamer–Au NPs conjugates by using human immunoglobulin E (IgE) as model analyte. Human IgE, captured by immobilized goat anti-human IgE on SPR gold film, is sensitively detected by SPR spectroscopy with a lowest detection limit of 1 ng/ml after anti-human IgE aptamer–Au NPs conjugates is used as amplification reagent. Meanwhile, the non-specific adsorption of aptamer–Au NPs conjugates on goat anti-human IgE is confirmed by SPR spectroscopy and then it is minimized by treating aptamer–Au NPs conjugates with 6-mercaptohexan-1-ol (MCH). These results confirm that aptamer–Au NPs conjugates is a powerful sandwich element and an excellent amplification reagent for SPR-based sandwich immunoassay.     

Temperature Sensor Based on Hollow Fiber Filled with Graphene-Ag Composite Nanowire and Liquid

A temperature sensor based on hollow fiber (HF) filled with graphene-Ag composite nanowire and liquid is presented. The coupling properties and sensing performance are numerically analyzed by finite element method using wavelength and amplitude interrogations. Results show that the sensor exhibiting strong birefringence with x-polarized peak provides much higher sensitivities and better signal-to-noise ratio (SNR) than y-polarized, which is more suitable for temperature detection. The graphene-Ag composite nanowire can not only solve the oxidation problem but also avoid the metal coating. Moreover, it provides better performance than other similar works like Au-Ag nanowire-filled, Au nanowire-filled, and Ag nanowire-filled sensors. Contrary to the blue shift of traditional SPR temperature sensors, the resonance peak shifts to the longer wavelength in our device when temperature increases and the high sensitivity 9.44 nm/ °C is obtained. The influences of nanowire diameter, grapheme-layer thickness on the designed sensor, are also investigated. This work can provide a reference for developing a high sensitivity, real-time, remote sensing, and distributed temperature SPR sensor.     

Dopamine-Induced Growth of Au and Ag Nanoparticles on ITO Substrate and Their Application in PCPDTBT-Based Polymer Solar Cell

The direct attachment and growth of gold or silver nanoparticles (NPs) on indium tin oxide (ITO) surfaces was demonstrated using a simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method by chemical reduction of the precursor metal salts with dopamine aqueous solution. Ag NPs on ITO substrate were approximately spherical with an average particle size of about 57 nm, but had a wide particle size distribution. Compared with Ag NPs, under the same 10 SILAR cycles, Au NPs have higher density packing and smaller average particle size of about 36 nm. XRD characterization and surface chemistry analysis confirmed the formation of Ag and Au NPs on ITO substrate with small amounts of dopamine-quinone adsorbed on the surface of them. Although Au NPs showed characteristic plasmon absorption, this did not result in performance enhancement in solar cell with the structure of ITO/ZnO/PCPDTBT:[6,6]-phenyl C₇₁/MoO₃/Ag because of the energy level mismatch between ZnO and dopamine molecules adsorbed on the surface of metal NPs.     

Visible-Light Photocatalytic Application of Hierarchical Au-ZnO Flower-Rod Heterostructures via Surface Plasmon Resonance

The semiconductor metal oxide, ZnO, is limited to application in photocatalysis because of large bandgap. Metallic nanostructures are utilized to enhance visible-light absorption and improve photocatalytic activity via surface plasmon resonance (SPR). In this work, through a facile thermal decomposition method, the Au nanoparticles (NPs) were successfully loaded on oriented hierarchical ZnO flower-rod (ZFRs) heterostructures, which have a higher areal proportion of exposed active (001) crystal faces. The results indicate that ZFRs are wurtzite phase grown along the [001] direction and the Au NPs spread on the surface of ZFRs in metallic form. It reveals that there is an electronic interaction between Au NPs and ZFRs, and more oxygen molecules are adsorbed on the surface of Au-ZnO flower-rod heterostructures (AZFRs). The AZFRs show a strong absorbance in visible region due to the SPR and enhance the separation of electron–hole pair, resulting in an improvement of photocatalytic activity under visible-light irradiation. During 80 min, the degradation efficiency of rhodamine B for AZFRs-20 is about 3.7 times as the pure ZFRs. Moreover, the AZFRs exhibit predominant photoelectrochemical properties, and the photocurrent can reach 1.2?×?10^?4?A. Finally, the as-prepared photocatalytic device composed of AZFRs based on indium doped tin oxide glass is convenient to recycle without centrifugation.     

Determination of the Optical Thickness of sub 10-nm Thin Metal Films by SPR Experiments

We discuss the experimental data of surface plasmon resonance (SPR) occurring at the interface between air and single and bimetallic thin layers of Au and Ag prepared on glass substrates. The bilayer configuration allowed for the measurements of the optical constants of metallic films that are ultra thin; e.g., below 10 nm of thickness since SPR modes on such thin films in a single-layer configuration are shallow. We also discuss the effect of film thickness on SPR coupling. Thickness and refractive index of the films were determined by matching experimental SPR curves to the theoretical ones. Thickness and roughness of the films were also measured by atomic force microscopy. The results obtained by experimental measurements are in good agreement with AFM analysis.     

Fabrication of Monodispersed Silver Nanoparticles and their Collective Sharp Plasmonic Response

Monodispersed silver (Ag) nanoparticles (NPs) were obtained by applying an electric field on unipolar-charged Ag NPs fabricated using a heterogeneous condensation technique in gas media. Well defined and charged Ag NPs were separated based on their sizes and were collected on quartz substrates. Thin films consisting of monodispersed Ag NPs with size ranging from 35 to 120 nm were prepared by varying an applied electric field during the fabrication process. Scanning electron microscope results showed that the samples have uniform size distribution. Coherent oscillations of conduction band electrons in gas medium induced by electromagnetic field and coupling of all similar plasmon resonances due to uniform Ag NPs size produced unique and interesting optical properties. Narrow extinction widths (～41 to ～69 nm) were observed compared to the width of polydispersed Ag sample. The ability to prepare samples in gas media and tune the plasmon resonance by merely varying an electric field during fabrication makes the method simple, fast, and highly economical.     

In situ growth of gold nanoparticles by enzymatic glucose oxidation within alginate gel matrix

In situ growth of gold nanoparticles (Au NPs) was performed in an alginate gel matrix co‐encapsulating Au NP seeds and glucose oxidase (GOx) for the development of a glucose‐sensing platform. We observed a simultaneous growth of Au NPs in the alginate matrix through the reduction of AuCl by H₂O₂ on Au NP seeds. The detection of the glucose level was carried out successfully via the coupling of Au NP enlargement with the oxidation of glucose catalyzed by the immobilized GOx. The enlargement of Au NPs in the alginate matrix exhibited only a red‐shift in absorbance maxima, while the generation of small Au NPs in a free solution caused a blue‐shift in higher glucose concentrations. This study shows that the co‐encapsulation of metal NPs and bioreceptor in a gel matrix may provide a simple route for the fabrication of an optical biosensor. Biotechnol. Bioeng. 2010;105: 210–214. © 2009 Wiley Periodicals, Inc.     

In Situ Optical Extinction Measurement for Locally Control of Surface Plasmon Resonance During Nanosecond Laser Irradiation of Silver Ion Exchanged Silicate Glass

Online UV/visible extinction measurement have been achieved during nanosecond Nd:YAG laser irradiation at 532 nm of a silver-exchanged silicate glass after each shot. We have explained the evolution of the integrated spectral evolution with the help of a few observed spots after the laser/glass interaction and completed them by optical and surface measurements. This optical method allows to in situ follow silver ions precipitation in nanoparticles (NPs) and the consequently surface plasmon resonance evolution (SPR). In this study, we focus on the interest of this method for one silver-exchanged soda-lime glass by direct observation of the sample surface. Scanning electron microscopy measurement and optical microscopy were used to identify the various ablation mechanisms. Profilometry was used to evaluate the material distribution and the surface roughness evolution (Rms parameter). Thus, we explain the silver NPs effect on glass matrix at various micrometric scales according to the deposited fluence and silver concentration.     

Spectral Investigations on the Fluorescence Quenching of 1,4-dihydroxy-2,3-dimethylanthracene-9,10-dione by Plasmonic Silver Nanoparticles

Silver nanoparticles (Ag NPs) of different sizes have been prepared by Lee and Meisel’s method using trisodium citrate as reducing agent under ultra sonication. Optical absorption and fluorescence emission techniques were employed to investigate the interaction of 1,4-dihydroxy-2,3-dimethyl anthracene-9,10-dione (DHDMAD) with silver nanoparticles. In fluorescence spectroscopic study, we used the DHDMAD and Ag NPs as component molecules for construction of Förster Resonance Energy Transfer (FRET), whereas DHDMAD serve as donor and Ag NPs as acceptor. The surface plasmon resonance (SPR) peak of the prepared silver colloidal solution was observed from 419 nm to 437 nm. The synthesized silver nanoparticles at different heating time intervals were spherical in shape about the size of 25 nm and 55 nm. The fluorescence interaction between silver nanoparticles and DHDMAD confirms the FRET mechanism. According to Förster theory, the distance between silver nanoparticles and DHDMAD and the critical energy transfer distance were calculated and it is increase with heating time.     

Development of uniform density control with self-assembled colloidal gold nanoparticles on a modified silicon substrate

Here, we present a simple method for controlling the density of Au nanoparticles (Au NPs) on a modified silicon substrate, by destabilizing the colloidal Au NPs with 3-mercaptopropyltrimethoxylsilane (3-MPTMS) for microelectromechanical-system-based applications to reduce tribological issues. A silicon surface was pretreated with a 3-MPTMS solution, immediately after which thiolated Au NPs were added to it, resulting in their uniform deposition on the silicon substrate. Without any material property change of the colloidal Au NPs, we observed the formation of large clusters Au NPs on the modified silicon surface. Analysis by scanning electron microscopy with energy dispersive X-ray spectroscopy indicated that the addition of 3-MPTMS resulted in an alternation of the chemical characteristics of the solution. Atomic force microscopy imaging supported the notion that silicon surface modification is the most important factor on tribological properties of materials along with ligand-modified Au NPs. The density of Au NPs on a silicon surface was significantly dependent on several factors, including the concentration of colloidal Au NPs, deposition time, and concentration of 3-MPTMS solution, while temperature range which was used throughout experiment was determined to have no significant effect. A relatively high density of Au NPs forms on the silicon surface as the concentrations of Au NPs and 3-MPTMS are increased. In addition, the maximum deposition of Au NPs on silicon wafer was observed at 3 h, while the effects of temperature variation were minimal.     

The Tunable and Well-Controlled Surface Plasmon Resonances of Au Hollow Nanostructures by a Chemical Route

Au plasmonic hollow spherical nanostructures were synthesized by electrochemical reduction (GRR, the Galvanic Replacement Reaction) using Ag nanoparticles as templates. From UV-visible absorption spectroscopy, it was found that the surface plasmon resonance (SPR) of gold hollow spherical nanostructures first showed red shift and then blue shift. However, further addition of gold precursor (HAuCl₄) resulted into a red shift of SPR peak. The morphological changes from Ag nanoparticles to Au hollow nanostructures were assessed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX)analysis. The Mie Scattering theory based simulations of SPR of Au hollow nanostructures were performed which are in good agreement with the experimental observations. Based on the experimental observations and theoretical calculations, a complete growth mechanism for Au hollow nanostructures is proposed.     

Synthesis of Silver and Gold Nanoparticles Using Cashew Nut Shell Liquid and Its Antibacterial Activity Against Fish Pathogens

This study reveals a green process for the production of multi-morphological silver (Ag NPs) and gold (Au NPs) nanoparticles, synthesized using an agro-industrial residue cashew nut shell liquid. Aqueous solutions of Ag⁺ ions for silver and chloroaurate ions for gold were treated with cashew nut shell extract for the formation of Ag and Au NPs. The nano metallic dispersions were characterized by measuring the surface plasmon absorbance at 440 and 546 nm for Ag and Au NPs. Transmission electron microscopy showed the formation of nanoparticles in the range of 5–20 nm for silver and gold with assorted morphologies such as round, triangular, spherical and irregular. Scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction analyses of the freeze-dried powder confirmed the formation of metallic Ag and Au NPs in crystalline form. Further analysis by Fourier transform infrared spectroscopy provided evidence for the presence of various biomolecules, which might be responsible for the reduction of silver and gold ions. The obtained Ag and Au NPs had significant antibacterial activity, minimum inhibitory concentration and minimum bactericidal concentration on bacteria associated with fish diseases.     

Spectral Tuning of Plasmon Resonances of Bimetallic Noble Metal Alloy Nanoparticles Through Compositional Changes

The optical absorption properties of the bimetallic noble metal alloy (viz. Au-Ag, Au-Cu, and Ag-Cu) nanoparticles (NPs) of radii of 10 nm and 20 nm embedded in silica glass have been studied theoretically using a simple model based on the effective medium theory. Our study reveals that the spectra of the above bimetallic alloy NPs exhibit single but composition-sensitive surface plasmon resonance (SPR) peak which indicates the successful formation of alloys. The position of the SPR peak that appeared corresponding to alloy NPs is different from that of the component metals. The study further reveals that the Ag-Au and Au-Cu alloy systems are completely miscible over the entire concentration range but Ag-Cu is miscible up to a certain extent, although, their SPR peak shows a linear shift with molar concentration. It has been further observed that the phase of the Ag-Au alloy system changes with concentration of Au during the alloy formation but no such change is seen in the other two systems. Thus, our study shows that the Ag-Cu system which otherwise does not form alloy in bulk may form alloy in nanoscale with limited miscibility. A shift of the SPR peak positions from ~ 405 to ~ 535 nm for Au-Ag, from ~ 535 to ~ 590 nm for Au-Cu, and from ~ 405 to ~ 436 nm for Ag-Cu NP systems has been observed for different composition of constituent monometals. The compositional changes lead to a spectral tuning of the SPR of the system under studies.

     

Enhanced surface plasmon resonance with the modified catalytic growth of Au nanoparticles

The catalytic growth of Au nanoparticles (AuNPs) has been employed in several analytical methods for improving the detection sensitivity, or integrated with the enzyme reactions for the quantitative detection of the respective substrates. However, the catalytic growth of Au nanoparticles do not work in some situations, such as surface plasmon resonance (SPR), electrochemistry, where metal matrices were used, because metal matrices used in these techniques, e.g. Au, are susceptible to metal deposition, which increased the background seriously. In this work, a SiO(2) layer was vapor-deposited on the gold film. The inhibition of metal deposition by this SiO(2) layer was investigated by SPR sensor. The results showed that the SiO(2) layer could avoid the deposition of metal on Au film. With the low background achieved by SiO(2)-coated Au films, sensitive detection of DNA hybridization using the catalytic growth of Au nanoparticles enhanced SPR was demonstrated. The work described here maybe helpful for the development of sensitive bioanalytical methods.     

Investigations on the Thermal Stability of Fullerene-Based (Ag-C<Subscript>70</Subscript>) Nanocomposite Thin Films

Fullerene-based bi-functional nanocomposite thin film (Ag nanoparticles embedded in fullerene C₇₀ matrix) is synthesized by thermal co-deposition method. Thermal stability of Ag-C₇₀ nanocomposite is investigated by annealing the nanocomposite thin film at different temperatures from 80 to 350 °C for 30 min. Optical and structural properties of nanocomposite thin film with respect to high temperature are studied by UV-visible spectroscopy and x-ray diffraction, respectively. Transmission electron microscopy is performed to observe the temperature-dependent size evolution of Ag nanoparticles in fullerene C₇₀ matrix. A large growth of Ag nanoparticles is observed with temperature especially above 200 °C due to enhanced diffusion of Ag in fullerene C₇₀ at higher temperature and Ostwald ripening. The properties of metal-fullerene nanocomposite is not significantly affected up to a temperature of 150 °C. With a further increase in temperature, a major blue shift of ~?33 nm in SPR wavelength is seen at a temperature of 300 °C due to the thermal induced structural transformation of fullerene C₇₀ matrix into amorphous carbon. A very large-sized Ag nanoparticle with a wide size distribution varying from 27.8 ± 0.6 to 330.0 ± 4.5 nm is seen at 350 °C and due to which, a red shift of ~?16 nm is obtained at this temperature. This study throws light on the thermal stability of the devices based on metal-fullerene bi-functional nanocomposite.     

Effect of Light Incident Angle on Fano Resonance Loss Mediation in Silver Nanoparticles Integrated Thin Silicon Wafers

In this work, we have studied the role of light incidence angle in the suppression of Fano resonance loss for mediating broadband light incoupling. At light normal incidence angle, the transmission loss of silver nanoparticles (Ag NPs) integrated 100 μm silicon (Si) wafer is reduced to ~?4 from ~?11%, and total reflectance is reduced to 32.7 from 38.7%. The reduction in reflectance is observed only in surface plasmon off-resonance region of the NPs, but the reflectance is enhanced slightly in surface plasmon resonance (SPR) region. With the change in light incident angles, we have observed a reduction in reflectance from NPs integrated silicon wafer at SPR region. At 40° light incident angle, the reflectance is reduced to 21 from 38.7% due to minimization of the Fano resonance in SPR region of the NPs. The Fano resonance loss reduction is explained based on modification in NPs’ dipole and quadrupole modes hybridization at the silicon interface with different light incident angles instead of normal incidence. Experimental observations are validated by simulating Ag NPs’ near-fields and angular distribution of far-fields at the silicon interface, scattering efficiency spectra at different light incident angles by finite difference time domain calculations.     

Optimization of Surface Plasmon Resonance Biosensor with Ag/Au Multilayer Structure and Fiber-Optic Miniaturization

In this paper, we report a novel wavelength interrogation-based surface plasmon resonance (SPR) system, in which a film of three Ag layers and three Au layers are alternately deposited on a Kretschmann configuration as sensing element. This multilayer film shows higher sensitivity for refractive index (RI) measurement by comparing with single Au layer structure, which is consistent with its theoretical calculation. A sensitivity range of 2056–5893 nm/RIU can be achieved, which is comparable to RI sensitivities of other wavelength-modulated SPR sensors. Compared with Ag film, this Ag/Au multilayer arrangement offers anti-oxidant protection. This SPR biosensor based on a cost-effective Ag/Au multilayer structure is applicable to the real-time detection of specific interactions and dissociation of low protein concentrations. To extend the application of this highly-sensitive metal film device, we integrated this concept on an optical fiber. The range of RI sensitivities with Ag/Au multilayer was 1847–3309 nm/RIU. This miniaturized Ag/Au multilayer-based fiber optic sensor has a broad application in chemical and biological sensing.