Optical Fiber-Based Surface-Enhanced Raman Scattering Sensor Using Au Nanovoid Arrays

Plasmonic properties of gold nanovoid array substrates for fiber-based surface-enhanced Raman scattering (SERS) sensing are studied numerically and experimentally. In the nanovoid arrays, each void has openings on both sides, bottom hole facing the fiber tip for introducing incident light and collecting scattered light and the top hole exposed to the analyte solution for interrogating analyte molecules in the voids. Electromagnetic field modes are confined strongly in and around these nanovoids, acting as localized plasmon resonators. The enhanced electric field extends hundreds of nanometers into the voids, resulting in a large SERS-active zone several orders of magnitude larger than nanoparticle-based structures. The effect of structural parameters of the nanovoid arrays, including void diameter, Au film thickness, and bottom hole diameter, on electric field confinement in the voids is investigated using three-dimensional finite difference time domain simulation. Au nanovoid arrays are fabricated using a scalable, inexpensive nanosphere lithography fabrication method. The largest SERS signal is realized by tuning the localized plasmon resonance peak of Au nanovoid arrays to the laser excitation wavelength. Multiplexed detection capability with the fiber-based SERS sensor using Au nanovoid arrays is demonstrated by measuring the Raman spectrum of a mixture solution of diethylthiatricarbocyanine and crystal violet.     

Surface Plasmon-Enhanced Spontaneous Emission from InGaN/GaN Multiple Quantum Wells by Indium Nanoparticles Fabricated Using Nanosphere Lithography

Economic nanofabrication of Ag, Al, Au nanotriangle arrays and In nanoparticle arrays are demonstrated using nanosphere lithography. The sizes of the nanoparticles are precisely tuned when nanospheres with different diameters are used. Localized surface plasmon resonances (LSPR) of the nanoparticle arrays are observed to be strongly dependent on their sizes and the LSPR of In nanoparticle arrays with various sizes cover the whole visible spectrum. By placing In nanoparticle arrays near the InGaN/GaN multiple quantum wells (MQWs), enhanced spontaneous emission is observed when the LSPR of the In nanoparticles matches the emission wavelength of InGaN/GaN MQWs.     

Ultrasensitive electrochemical detection of Bacillus thuringiensis transgenic sequence based on in situ Ag nanoparticles aggregates induced by biotin-streptavidin system

A novel electrochemical biosensor was developed for detecting short DNA oligonucleotide of Bacillus thuringiensis (Bt) transgenic sequence based on Ag nanoparticle aggregates. To fabricate this DNA biosensor, the thiol-modified capture DNA (cDNA) was first anchored on gold (Au) electrode, and then the target DNA (tDNA) was hybridized with the immobilized cDNA. Subsequently, the probe DNA (pDNA) functionalized by biotinylated Ag nanoparticle was associated with the fixed tDNA, and the single Ag nanoparticle label was obtained (cited as SAg label). Finally, dissociative biotinylated Ag nanoparticle was bound to the resultant biotinylated SAg label assembled on Au electrode by virtue of bridge molecule streptavidin (SA) through biotin-SA specific interaction, which could lead to in situ aggregate of Ag nanoparticles on Au electrode and induce a novel tag including multiple Ag nanoparticles (cited as MAg tag). The novel tag exhibited excellent electroactive property in the solid-state Ag/AgCl process and was successfully applied to Bt transgenic sequence assay. A detection limit of 10 fM was achieved, which was improved by three orders of magnitude as compared to the SAg label. Furthermore, this novel DNA biosensor demonstrated a good selectivity towards tDNA.     

Plasmonic Resonance Excited Extinction Spectra of Cross-Shaped Ag Nanoparticles

Plasmonic properties of cross-shaped Ag nanoparticles are investigated theoretically using finite-difference time-domain algorithm. Electric field (E-field) distribution of a single cross-shaped Ag nanoparticle with different shape parameters and patterned nanoparticles with different periods were presented. Both red shift and blue shift of the extinction spectra were observed. The simulation results demonstrated that the strong E-field intensity is located at sharp corner of the nanoparticles. And E-field intensity of the nanoparticle array is much stronger than that of a single Ag nanoparticle. Enhancement of the large localized E-field originating from the nanoparticles was analyzed. Corresponding influence of “hot spots” effect on enhancing Raman scattering was discussed as well.     

Review of Some Interesting Surface Plasmon Resonance-enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems

Noble metal, especially gold (Au) and silver (Ag) nanoparticles exhibit unique and tunable optical properties on account of their surface plasmon resonance (SPR). In this review, we discuss the SPR-enhanced optical properties of noble metal nanoparticles, with an emphasis on the recent advances in the utility of these plasmonic properties in molecular-specific imaging and sensing, photo-diagnostics, and selective photothermal therapy. The strongly enhanced SPR scattering from Au nanoparticles makes them useful as bright optical tags for molecular-specific biological imaging and detection using simple dark-field optical microscopy. On the other hand, the SPR absorption of the nanoparticles has allowed their use in the selective laser photothermal therapy of cancer. We also discuss the sensitivity of the nanoparticle SPR frequency to the local medium dielectric constant, which has been successfully exploited for the optical sensing of chemical and biological analytes. Plasmon coupling between metal nanoparticle pairs is also discussed, which forms the basis for nanoparticle assembly-based biodiagnostics and the plasmon ruler for dynamic measurement of nanoscale distances in biological systems.     

Simultaneous Photoluminescence and SERS Observation of Nanodiamond at Laser Deposition on Noble Metals

The metal-modified luminescence and surface-enhanced Raman scattering (SERS) occurring near nanostructured surfaces of noble metals recently have been observed for different kinds of nanocrystals associated with the metal nanostructures. In the present work, the photoluminescence and Raman scattering of diamond nanocrystals of sizes 100 and 300 nm patterned on Ag and Au thin nanostructured films via laser accelerated deposition using a femtosecond laser are discussed. The laser accelerated deposition forms ordered periodical nanodiamond–metal nanostructures and allows adjusting the interaction between nanodiamond and metal by varying the laser acceleration parameters as well as by using different metals (Ag and Au), and varying the structure of the metal film. Correspondingly, the spectroscopic properties of the system determined by interaction between nanoparticles and metal are tuned. The enhancement of nanodiamond photoluminescence together with SERS of graphite fraction and disordered carbon of nanodiamonds are observed for nanodiamond–Ag structures at 488- and 532-nm excitations, while for the nanodiamond–Au structure some characteristic SERS effects are observed at 785-nm excitation. The mechanisms of enhancement are discussed considering the nanodiamond–metal interaction and laser acceleration effect on nanodiamond.     

Investigating the Effect of Ag and Au Nanostructures with Spherical and Rod Shapes on the Emission Wavelength of OLED

Noble metals, especially Ag and Au nanostructures, have unique and adjustable optical attributes in terms of surface plasmon resonance. In this research, the effect of Ag and Au nanoparticles with spherical and rod shapes on the light extraction efficiency and the FWHM of OLED structures was investigated using the finite difference time domain (FDTD) method. The simulation results displayed that by changing the shape and size of Ag and Au nanostructures, the emission wavelength can be adjusted, and the FWHM can be reduced. The presence of Ag and Au nanoparticles in the OLEDs showed a blue and red shift of the emission wavelength, respectively. Also, the Ag and Au nanorods caused a significant reduction in the FWHM and a shift to the longer wavelengths in the structures. The structures containing Ag nanorods showed the narrowest FWHM and longer emission wavelength than the other structures.

     

Optical Absorption in Overcoats of Nanoparticle Arrays on a Metallic Substrate

Surface plasma oscillations in metallic particles as well as in thin metallic films have been studied extensively in the past decades. New features regarding surface plasma excitations are, however, constantly discovered, leading, for example, to surface-enhanced Raman scattering studies and enhanced optical transmission though metal films with nanohole arrays. In the present work, the role of a metallic substrate is examined in two cases, one involving an overcoat of dielectric nanoparticles and the other an overcoat of metallic nanoparticles. Theoretical results are obtained by modeling the nanoparticles as forming a two-dimensional, hexagonal lattice of spheres. The scattered electromagnetic field is then calculated using a variant of the Green function method. Comparison with experimental results is made for nanoparticles of tungsten oxide and tin oxide deposited on either gold or silver substrates, giving qualitative agreement on the extra absorption observed when the dielectric nanoparticles are added to the metallic surfaces. Such absorption would be attributed to the mirror image effects between the particles and the substrate. On the other hand, calculations of the optical properties of silver or gold nanoparticle arrays on a gold or a silver substrate demonstrate very interesting features in the spectral region from 400 to 1,000 nm. Interactions between the nanoparticle arrays surface plasmons and their images in the metallic substrate would be responsible for the red shift observed in the absorption resonance. Moreover, effects of particle size and ambient index of refraction are studied, showing a great potential for applications in biosensing with structures consisting of metallic nanoparticle arrays on metallic substrates.     

Surface-enhanced resonance Raman spectroscopy of porphyrin and metalloporphyrin species in systems with Ag nanoparticles and their assemblies

The advantages of systems with Ag nanoparticles and their assemblies for surface-enhanced resonance Raman scattering (SERRS) spectral investigation, detection and determination of porphyrin species are demonstrated. SERRS spectral detection limits of the testing porphyrin species (including porphyrin aggregates) in these systems are shown to be, on average, 10(2)-10(3) lower than detection limits by resonance Raman scattering (RRS). Systems with Ag nanoparticles modified by anionic organosulfur spacers enable us to obtain SERRS spectra of unperturbed cationic porphyrin species. In the case of thiopheneacetate-modified Ag particles prepared by laser ablation, no negative effect of the spacer on the spectral detection limit of the porphyrin was observed. Systems with isolated Ag nanoparticles allow for obtaining SERRS spectra of porphyrin species upon excitation into the Soret electronic absorption band which leads to at least a 10-fold decrease in the detection limit.     

Scattering Efficiency and LSPR Tunability of Bimetallic Ag,Au, and Cu Nanoparticles

Scattering efficiencies of Ag–Cu, Ag–Au, and Au–Cu alloy nanoparticles are studied based on Mie theory for their possible applications in solar cells. The effect of size (radius), surrounding medium, and alloy composition on the scattering efficiency at the localized surface plasmon resonance (LSPR) wavelengths has been reported. In the alloy nanoparticles of Ag_1?x Cu _x , Au_1?x Cu _x and Ag_1?x Au _x ; the scattering efficiency gets red-shifted with increase in x. Moreover, the scattering efficiency enhancement can be tuned and controlled with both the alloy composition and the surrounding medium refractive index. A linear relationship which is in good agreement to the experimental observations between the scattering efficiency and metal composition in the alloys are found. The effect of nanoparticle size and LSPR wavelength (scattering peak position) on the full width half maxima and scattering efficiency has also been studied. Comparison of Au–Ag, Au–Cu, and Ag–Cu alloy nanoparticles with 50-nm radii shows the optical response of Ag–Cu alloy nanoparticle with wide bandwidth in the visible region of the electromagnetic spectrum making them suitable for plasmonic solar cells. Further, the comparison of Ag–Cu alloy and core@shell nanoparticles of similar size and surrounding medium shows that Cu@Ag nanoparticle exhibits high scattering efficiency with nearly the same bandwidth.     

Biotechnological synthesis of Pd/Ag and Pd/Au nanoparticles for enhanced Suzuki–Miyaura cross-coupling activity

Bimetallic nanoparticle catalysts have attracted considerable attention due to their unique chemical and physical properties. The ability of metal-reducing bacteria to produce highly catalytically active monometallic nanoparticles is well known; however, the properties and catalytic activity of bimetallic nanoparticles synthesized with these organisms is not well understood. Here, we report the one-pot biosynthesis of Pd/Ag (bio-Pd/Ag) and Pd/Au (bio-Pd/Au) nanoparticles using the metal-reducing bacterium, Shewanella oneidensis, under mild conditions. Energy dispersive X-ray analyses performed using scanning transmission electron microscopy (STEM) revealed the presence of both metals (Pd/Ag or Pd/Au) in the biosynthesized nanoparticles. X-ray absorption near-edge spectroscopy (XANES) suggested a significant contribution from Pd(0) and Pd(II) in both bio-Pd/Ag and bio-Pd/Au, with Ag and Au existing predominately as their metallic forms. Extended X-ray absorption fine-structure spectroscopy (EXAFS) supported the presence of multiple Pd species in bio-Pd/Ag and bio-Pd/Au, as inferred from Pd–Pd, Pd–O and Pd–S shells. Both bio-Pd/Ag and bio-Pd/Au demonstrated greatly enhanced catalytic activity towards Suzuki–Miyaura cross-coupling compared to a monometallic Pd catalyst, with bio-Pd/Ag significantly outperforming the others. The catalysts were very versatile, tolerating a wide range of substituents. This work demonstrates a green synthesis method for novel bimetallic nanoparticles that display significantly enhanced catalytic activity compared to their monometallic counterparts.     

Multiple Surface Plasmon Resonances in a Metal–Insulator–Metal Structure Assembled Via Silver–Copper Nanoparticle Arrays

The Ag–Cu nanoparticle arrays, prepared using the electrochemical deposition method, were assembled into the metal–insulator–metal (MIM) structure with polyvinyl alcohol acting as insulating layer, the transmission spectrum of the MIM structure was observed to support the multiple surface plasmon resonances in the wavelength range 1,000 to 2,600 nm. The multiple peaks were formed due to the superposition and coupling of the surface plasmon resonance of nanoparticles with various sizes in the metal layers. The newly found MIM structure in which multiple resonances exist has a potential application in multiband-pass filters and optical magnetic metamaterials at the resonance wavelength.     

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.     

Photoreduction of Silver Salts Using Au Nanoparticles to Form a Core-Shell-Type Nanostructure: Insight into the Reaction Mechanism

In this paper, we highlight the formation of Ag/Au core-shell nanoparticles at room temperature by using a low-power laser. We have investigated the plasmon-induced reduction of Ag⁺ ions on bare Au nanoparticles synthesized by laser ablation technique, and citrate-capped Au nanoparticles synthesized by chemical method. It is demonstrated that citrate plays an important role for the reduction of silver ions. The citrate gets oxidized by the ‘hot’ holes produced due to the surface plasmon resonance (SPR) of the Au nanoparticles which then reduces the Ag⁺ ions to Ag. The importance of excitation laser wavelength is also demonstrated to facilitate the reduction process.

     

Refractive Index Sensitivity Analysis of Ag, Au, and Cu Nanoparticles

The localized surface plasmon resonance (LSPR) spectrum of noble metal nanoparticles is studied by quasi-static approximation. Taking the sensitivity of LSPR shape to the size and shape of nanoparticle along with surrounding refractive index, parameters like refractive index sensitivity and sensing figure of merit have been determined. In the present analysis from the sensing relevant parameters, it is concluded that Ag represents a better sensing behavior than Au and Cu over the entire visible to infrared regime of EM spectrum.     

Photocatalytic Activity of Ag/TiO2 Nanotube Arrays Enhanced by Surface Plasmon Resonance and Application in Hydrogen Evolution by Water Splitting

TiO₂ nanotube arrays (TiO₂ NTs) were fabricated by anodic oxidation and then Ag nanoparticles (Ag NPs) were assembled in TiO₂ NTs (Ag/TiO₂ NTs) by microwave-assisted chemical reduction. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence spectrum (PL), UV–vis absorption spectrum (UV–vis), and Raman spectrum, respectively. The results showed that Ag NPs were well dispersed on the surface of TiO₂ NTs with metallic state. The surface plasmon resonance (SPR) effect of Ag NPs could extend the visible light response and enhance the absorption capacity of TiO₂. Furthermore, Ag NPs could also restrain the recombination of photo-generated electron–hole pairs of TiO₂ NTs efficiently. The methylene blue photodegradation experiment proved that the SPR phenomenon had an effect on photoreaction enhancement. The results of photocatalytic water splitting indicated that Ag/TiO₂ NTs samples had better photocatalytic performance than pure TiO₂ NTs. The corresponding hydrogen evolution rate of Ag/TiO₂ NTs prepared with 0.002 M AgNO₃ solution was 3.3 times as that of pure TiO₂ NTs in the test condition. Additionally, the mechanism of catalyst activity enhanced by SPR effect was proposed.     

SERS Study of Tetrodotoxin (TTX) by Using Silver Nanoparticle Arrays

The optical properties of tetrodotoxin (TTX), a scarce toxin with anesthetic properties, were studied using nanoparticle arrays-assisted surface-enhanced Raman scattering (SERS). The nanoparticles arrays were fabricated using nanosphere lithography and a metallic lift-off process to control the particle size, shape, and spacing in the arrays. Using density functional methods, the Raman spectrum of TTX was also calculated with Gaussian03 software. The main peaks of the spectrum are originated from the vibration of the NH₂ molecule group. In the SERS experiment, we were able to measure the Raman spectrum with a TTX concentration as less as 0.9 ng/mL. This sensitivity is comparable to that from high performance liquid chromatography.     

Blue-Shifted SPR of Au Nanoparticles with Ordering of Carbon by Dense Ionization and Thermal Treatment

Thin films of carbon-containing Au nanoparticles (NPs), prepared by the co-sputtering using a neutral Ar atom beam, were irradiated by 120 MeV Ag ions and also annealed, separately, at increasing temperatures in inert atmosphere. The surface plasmon resonance (SPR) band of the nanocomposite film was observed to be blue shifted (～50 nm) in both cases, with increasing fluence and temperature. The structural changes of Au NPs embedded in amorphous carbon matrix were investigated using X-ray diffraction and transmission electron microscopy. A growth of Au NPs was observed with increasing fluence and also with increasing temperature. A percolation of Au NPs was observed at 500 °C. A growth of Au NPs with ion irradiation is explained in the framework of a thermal spike model. Raman spectroscopy revealed the ordering of a-C thin films with increasing fluence and temperature, which is ascribed to a change of refractive index and the blue shift of the SPR band.     

Growth and fragmentation of silver nanoparticles in their synthesis with a fs laser and CW light by photo-sensitization with benzophenone.

The photo-sensitization synthetic technique of making silver nanoparticles using benzophenone is studied using both a laser and a mercury lamp as light sources. The power and irradiation time dependence of the synthesized nanoparticle absorption spectra and their size distribution [as determined by transmission electron microscopy (TEM)] are studied in each method and compared. In the laser synthesis, as either the laser power or the irradiation time increases, the intensity of the surface plasmon resonance absorption at 400 nm is found to increase linearly first, followed by a reduction of the red edge of the plasmon resonance absorption band. The TEM results showed that in the laser synthesis low powers and short irradiation times produce nanoparticles around 20 nm in diameter. Increasing the power or irradiation time produces a second population of nanoparticles with average size of 5 nm in diameter. These small particles are believed to be formed from the surface ablation of the large particles. The surface plasmon absorption band is found to be narrower when the nanoparticles are produced with laser irradiation. Throughout the exposure time with the CW lamp, the plasmon resonance absorption band of the particles formed first grows in intensity, then blue shifts and narrows, and finally red shifts while decreasing in intensity. The TEM results for lamp samples showed particle formation and growth, followed by small nanoparticle formation. The above results are discussed in terms of a mechanism in which, the excited benzophenone forms the ketal radical, which reduces Ag+ in solution and on the Ag nanoparticle surface. As the time of irradiation or the light energy increases the benzophenone is consumed, which is found to be the limiting reagent. This stops the formation of the normal large nanoparticles while their photo-ablation continues to make the small particles.     

Polymerase chain reaction of nanoparticle-bound primers

Using one or two primers respectively bound to the surface of Au nanoparticles (AuNPs) or magnetic nanoparticles (MNPs), polymerase chain reaction (PCR) based on nanoparticles was systemically studied, agarose gel electrophoresis and atomic force microscopy (AFM) were respectively used to detect and observe the PCR product. The results obtained indicated that with either one or two primers respectively bound to the nanoparticle surface, PCR can proceed successfully under optimized condition and is subject to certain rules, consequently a symmetric PCR technique and an asymmetric PCR technique based on nanoparticles have been developed. A kind of nanostructured aggregates can be constructed by a symmetric PCR using two nanoparticle-bound primers.