Chitosan-coated anisotropic silver nanoparticles as a SERS substrate for single-molecule detection

Surface-enhanced Raman spectroscopy (SERS) is a technique that has become widely used for identifying and providing structural information about molecular species in low concentration. There is an ongoing interest in finding optimum particle size, shape and spatial distribution for optimizing the SERS substrates and pushing the sensitivity toward the single-molecule detection limit. This work reports the design of a novel, biocompatible SERS substrate based on small clusters of anisotropic silver nanoparticles embedded in a film of chitosan biopolymer. The SERS efficiency of the biocompatible film is assessed by employing Raman imaging and spectroscopy of adenine, a significant biological molecule. By combining atomic force microscopy with SERS imaging we find that the chitosan matrix enables the formation of small clusters of silver nanoparticles, with junctions and gaps that greatly enhance the Raman intensities of the adsorbed molecules. The study demonstrates that chitosan-coated anisotropic silver nanoparticle clusters are sensitive enough to be implemented as effective plasmonic substrates for SERS detection of nonresonant analytes at the single-molecule level.     

Highly sensitive surface enhanced Raman scattering substrates based on filter paper loaded with plasmonic nanostructures

We report a novel surface enhanced Raman scattering (SERS) substrate platform based on a common filter paper adsorbed with plasmonic nanostructures that overcomes many of the challenges associated with existing SERS substrates. The paper-based design results in a substrate that combines all of the advantages of conventional rigid and planar SERS substrates in a dynamic flexible scaffolding format. In this paper, we discuss the fabrication, physical characterization, and SERS activity of our novel substrates using nonresonant analytes. The SERS substrate was found to be highly sensitive, robust, and amiable to several different environments and target analytes. It is also cost-efficient and demonstrates high sample collection efficiency and does not require complex fabrication methodologies. The paper substrate has high sensitivity (0.5 nM trans-1,2-bis(4-pyridyl)ethene (BPE)) and excellent reproducibility (~15% relative standard deviation (RSD)). The paper substrates demonstrated here establish a novel platform for integrating SERS with already existing analytical techniques such as chromatography and microfluidics, imparting chemical specificity to these techniques.     

Silver nanoparticles deposited on anodic aluminum oxide template using magnetron sputtering for surface-enhanced Raman scattering substrate

Low-cost and highly sensitive surface-enhanced Raman scattering (SERS) substrates have been fabricated by a simple anodizing process and a magnetron sputtering deposition. The substrates, which consist of silver nanoparticles embedded on anodic aluminum oxide (AAO) templates, are investigated by a scanning electron microscope and a confocal Raman spectroscopy. The SERS activities are demonstrated by Raman scattering from adsorbed solutions of methylene blue and pyridine on the SERS substrate surface. The most optimized SERS substrate contains the silver nanoparticles, with a size distribution of 10-30 nm, deposited on the AAO template. From a calculation, the SERS enhancement factor is as high as 8.5 × 10⁷, which suggests strong potentials for direct applications in the chemical detection and analyses.     

High-rate synthesis of microcrystalline silicon films using high-density SiH4/H2 microwave plasma

A high electron density (> 10¹¹ cm^− 3) and low electron temperature (1-2 eV) plasma is produced by using a microwave plasma source utilizing a spoke antenna, and is applied for the high-rate synthesis of high quality microcrystalline silicon (μc-Si) films. A very fast deposition rate of ∼ 65 Å/s is achieved at a substrate temperature of 150 °C with a high Raman crystallinity and a low defect density of (1-2) × 10¹⁶ cm^− 3. Optical emission spectroscopy measurements reveal that emission intensity of SiH and intensity ratio of H_α/SiH are good monitors for film deposition rate and film crystallinity, respectively. A high flux of film deposition precursor and atomic hydrogen under a moderate substrate temperature condition is effective for the fast deposition of highly crystallized μc-Si films without creating additional defects as well as for the improvement of film homogeneity.     

Photonic Crystal Hydrogel Enhanced Plasmonic Staining for Multiplexed Protein Analysis

Plasmonic nanoparticles are commonly used as optical transducers in sensing applications. The optical signals resulting from the interaction of analytes and plamsonic nanoparticles are influenced by surrounding physical structures where the nanoparticles are located. This paper proposes inverse opal photonic crystal hydrogel as 3D structure to improve Raman signals from plasmonic staining. By hybridization of the plasmonic nanoparticles and photonic crystal, surface‐enhanced Raman spectroscopy (SERS) analysis of multiplexed protein is realized. It benefits the Raman analysis by providing high‐density “hot spots” in 3D and extra enhancement of local electromagnetic field at the band edge of PhC with periodic refractive index distribution. The strong interaction of light and the hybrid 3D nanostructure offers new insights into plasmonic nanoparticle applications and biosensor design.     

Investigation of inhibition effect of rhodanine-N-acetic acid on mild steel corrosion in HCl solution

Corrosion inhibition effect of rhodanine-N-acetic acid (R-NA) on mild steel (MS) corrosion in 0.1 M HCl solution was investigated. For this purpose, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR) as well as hydrogen gas evolution (VH₂−t) and the change of open circuit potential as a function of immersion time (E_ocp − t) were used. The MS surfaces exposed to 0.1 M HCl solution in the absence and presence of inhibitor were examined by scanning electron microscopy (SEM). The thermodynamic parameters of adsorption were calculated and discussed. In order to gain more information about the adsorption mechanism, the EIS technique was used to evaluate the potential of zero charge (PZC) and a mechanism of adsorption process was proposed. It was found that, R-NA is a good corrosion inhibitor for the MS corrosion in 0.1 M HCl solution. The inhibition efficiency increased with increasing inhibitor concentration and reached 98% at 1.0 × 10⁻² M R-NA. The high inhibition efficiency was related to adsorption of R-NA on steel surface. Surface SEM images showed a good surface coverage of inhibitor on the metal surface.     

Plasmonic dimer antennas for surface enhanced Raman scattering

Electron beam induced deposition (EBID) has recently been developed into a method to directly write optically active three-dimensional nanostructures. For this purpose a metal-organic precursor gas (here dimethyl-gold(III)-acetylacetonate) is introduced into the vacuum chamber of a scanning electron microscope where it is cracked by the focused electron beam. Upon cracking the aforementioned precursor gas, 3D deposits are realized, consisting of gold nanocrystals embedded in a carbonaceous matrix. The carbon content in the deposits hinders direct plasmonic applications. However, it is possible to activate the deposited nanostructures for plasmonics by coating the EBID structures with a continuous silver layer of a few nanometers thickness. Within this silver layer collective motions of the free electron gas can be excited. In this way, EBID structures with their intriguing precision at the nanoscale have been arranged in arrays of free-standing dimer antenna structures with nanometer sized gaps between the antennas that face each other with an angle of 90°. These dimer antenna ensembles can constitute a reproducibly manufacturable substrate for exploiting the surface enhanced Raman effect (SERS). The achieved SERS enhancement factors are of the order of 10? for the incident laser light polarized along the dimer axes. To prove the signal enhancement in a Raman experiment we used the dye methyl violet as a robust test molecule. In future applications the thickness of such a silver layer on the dimer antennas can easily be varied for tuning the plasmonic resonances of the SERS substrate to match the resonance structure of the analytes to be detected.     

Performance and impact damage of a three dimensionally integrated microstrip feeding antenna structure

A three dimensionally integrated microstrip antenna (3DIMA) with microstrip feeding was fabricated and analyzed. In this design concept, a microstrip antenna was woven into the three-dimensional orthogonal woven composite for the load-bearing while functioning as an antenna. The simulation work was done using the antenna simulation software before fabrication. The measured voltage standing wave ratio (VSWR) of the antenna was 1.18 at the resonant frequency of 1.31 GHz. The measured radiation pattern had a maximum value at 0°, which agreed well with the simulation results. In addition, impact tests were performed with the impact energy ranging from 0 J to 15 J. The results showed that the VSWR changed little even at 15 J impact energy demonstrating good impact resistance and structural integrity of the antenna structure.     

Generation of ultralarge surface enhanced Raman spectroscopy (SERS)-active hot-spot volumes by an array of 2D nano-superlenses

Surface Enhanced Raman Spectroscopy (or SERS) has received tremendous attention in the past three decades. However, the extremely-confined probe volume (1 nm) of the plasmonic hot-spots occurring on a conventional roughened SERS-active metallic surface has limited value in macro-molecular studies. In this article, we show the plausibility of generating large SERS hot-spot volumes on an atomically-flat metal surface based upon a special 3D adiabatic plasmonic nano-focusing effect brought about by an array of nano-scale superlenses. We experimentally demonstrate the feasibility of this particular approach and report, for the first time, the acquisition of whole-protein SERS spectra of a layer of test protein, Cytochrome-c, using a custom-made Otto-Raman spectroscopy system equipped with nano-fluidics. Our study shows the potential of whole-protein SERS spectroscopy as a useful analytical tool that complements surface probe microscopies.     

Highly Sensitive Electro‐Plasmonic Switches Based on Fivefold Stellate Polyhedral Gold Nanoparticles

Electron–photon coupling in metal nanostructures has raised a new trend for active plasmonic switch devices in both fundamental understanding and technological applications. However, low sensitivity switches with an on/off ratio less than 5 have restricted applications. In this work, an electrically modulated plasmonic switch based on a surface‐enhanced Raman spectroscopy (SERS) system with a single fivefold stellate polyhedral gold nanoparticle (FSPAuNP) is reported. The reversible switch of the SERS signal shows high sensitivity with an on/off ratio larger than 30. Such a high on/off ratio arises primarily from the plasmonic resonance shift of the FSPAuNP with the incident laser due to the altered free electron density on the nanoparticle under an applied electrochemical potential. This highly sensitive electro‐plasmonic switch may enable further development of plasmonic devices.     

Optical and magneto-optical properties of erbium doped InGaN and GaN epilayers

Using combined excitation-emission spectroscopy we have studied the erbium incorporation into GaN and InGaN for in situ doped MOCVD-grown layers and compared them to samples grown by MBE. A smaller up-conversion efficiency for the main site is observed compared to minority sites in the same sample as well as versus all sites from MBE grown samples. Furthermore, we show that the 1.54 μm emission efficiency is reduced with increasing In-content both under excitation above the bandgap in the UV as well as under resonant excitation at around 980 nm. This indicates that non-radiative decay channels for the Er ion are the largest contributing factor for this behavior. Finally, the Zeeman splitting of the excitation and emission lines of Er:GaN under application of magnetic fields up to 6.6 T with B||c-axis is shown.     

Robust SERS substrates with massive nanogaps derived from silver nanocubes self-assembled on massed silver mirror via 1,2-ethanedithiol monolayer as linkage and ultra-thin spacer

Robust SERS substrates with hotspots on a large scale from massive nanogaps can be fabricated by assembling Ag nanocubes on the massed Ag mirror via 1,2-ethanedithiol monolayer as linkage and ultra-thin spacer. X-ray spectroscopy was used to confirm the existence of nanogaps. The plasmonic interaction between Ag nanocubes and the massed Ag mirror was corroborated by UV–Vis spectra. The strength and frequency change of plasmon resonance indicates the electrical field coupling between Ag nanocubes and the massed Ag surface. The 1,2-ethanedithiol spacer prevents the plasmon quench from complete contact between Ag nanocubes and the massed Ag surface, being corroborated by both simulations and SERS results. The SERS results prove the supreme performance of the robust substrate by detecting 10⁻⁹ M rhodamine 6G solution with high sensitivity (analytical enhancement factor 2.8 × 10⁸), high reliability (6.6% standard deviation from 20-sites measurements), and high precision (calibration line with 99.9% correlation coefficient).     

Application to cleaning of waste plastic surfaces using atmospheric non-thermal plasma jets

The removal of paint on the surface of waste plastics is difficult by the conventional process; in this research, a new cleaning mechanism using atmospheric plasmas was examined through optical emission spectroscopy, electron spectroscopy for chemical analysis, and scanning electron microscopy. Results indicate that an increase of pulse frequency enables for a short processing time for the removal of the paint film, signifying that the production of radicals in plasma, especially oxygen radicals, can be controlled by pulse frequency.Plasma jets were generated under the experimental conditions of an input power of 250 W to 400 W, a pulse frequency of 2 kHz to 12 kHz, and a plasma gas flow rate of 30 L/min. Examination of the intensity ratio of the reactive species, as measured by emission spectroscopy, showed that the O/N value increased with an increase in pulse frequency. Results of analysis with electron spectroscopy for chemical analysis show that nitrogen atoms and molybdenum in only the paint film decreased through plasma processing.     

Synthesis of high glass transition temperature fluorescent polyarylene ether nitrile copolymers

A novel series of fluorescent copolymers were synthesized by the nucleophilic aromatic substitution polymerization of 2, 6-dichlorobenzonitrile with different ratios of phenolphthalein to phenolphthalin. The derived copolymers showed good solubility in common organic solvents at room temperature and good film-forming properties. These copolymers were characterized by Fourier transform infrared, proton nuclear magnetic resonance, ultraviolet-visible absorption, fluorescence spectroscopy, differential scanning calorimetry and thermogravimetric analysis. They had high glass transition temperatures ranging from 237 °C to 260 °C and good thermal stability with the 5% weight loss temperatures in the range of 373-455 °C. Furthermore, the copolymers exhibited characteristic bimodal ultraviolet-visible absorption and unimodal fluorescence emission in both solution and film states.     

Synthesis and properties of solution-processible oligomers for the use of organic light-emitting diodes (OLEDs)

Two oligomers with X-shaped repeating units bearing anthracene and fluorene units were synthesized in a facile procedure, and exhibited high photoluminescence efficiencies, thermal stabilities and good solubility. Their ease of processing enabled spin coating with an electron-transporting bitriazine layer to afford organic light-emitting diodes which displayed a light blue emission with the maximum luminance of 3650 cd/m² and the current efficiency of 0.69 cd/A at an operation voltage of 10 V.     

Electromagnetic field enhancement and spectrum shaping through plasmonically integrated optical vortices

We introduce a new design approach for surface-enhanced Raman spectroscopy (SERS) substrates that is based on molding the optical powerflow through a sequence of coupled nanoscale optical vortices "pinned" to rationally designed plasmonic nanostructures, referred to as Vortex Nanogear Transmissions (VNTs). We fabricated VNTs composed of Au nanodiscs by electron beam lithography on quartz substrates and characterized their near- and far-field responses through combination of computational electromagnetism, and elastic and inelastic scattering spectroscopy. Pronounced dips in the far-field scattering spectra of VNTs provide experimental evidence for an efficient light trapping and circulation within the nanostructures. Furthermore, we demonstrate that VNT integration into periodic arrays of Au nanoparticles facilitates the generation of high E-field enhancements in the VNTs at multiple defined wavelengths. We show that spectrum shaping in nested VNT structures is achieved through an electromagnetic feed-mechanism driven by the coherent multiple scattering in the plasmonic arrays and that this process can be rationally controlled by tuning the array period. The ability to generate high E-field enhancements at predefined locations and frequencies makes nested VNTs interesting substrates for challenging SERS applications.     

Focusing Plasmons in Nanoslits for Surface‐Enhanced Raman Scattering

The focusing of plasmons to obtain a strong and localized electromagnetic‐field enhancement for surface‐enhanced Raman scattering (SERS) is increasing the interest in using plasmonic devices as molecular sensors. In this Full Paper, we report the successful fabrication and demonstration of a solid‐state plasmonic nanoslit–cavity device equipped with nanoantennas on a freestanding thin silicon membrane as a substrate for SERS. Numerical calculations predict a strong and spatially localized enhancement of the optical field in the nanoslit (6 nm in width) upon irradiation. The predicted enhancement factor of SERS was 5.3 × 10⁵, localized in an area of just 6 × 1.5 nm². Raman spectroscopy and imaging confirm an enhancement factor of ≈10⁶ for SERS from molecules chemisorbed at the nanoslit, and demonstrate the electromagnetic‐field‐enhancing function of the plasmonic nanoantennas. The freestanding membrane is open on both sides of the nanoslit, offering the potential for through‐slit molecular translocation studies, and opening bright new perspectives for SERS applications in real‐time (bio)chemical analysis.     

Broad-band tunable visible emission of sol-gel derived SiBOC ceramic thin films

Strong broad band tunable visible emission of SiBOC ceramic films is reported and the results are compared with one of boron free SiOC ceramic films. The insertion of boron into the SiOC network is verified by Fourier-Transform Infrared Spectroscopy. Optical properties are studied by photoluminescence and ultraviolet-visible spectroscopy measurements. Boron addition causes a decrease in the emission intensity attributed to defect states and shifts the emission to the visible range at lower temperatures (800-900 °C) leading to a very broad tunable emission with high external quantum efficiency.     

Synthesis of carbon nanotubes at low temperature by filament assisted atmospheric CVD and their field emission characteristics

Multiwalled carbon nanotubes (MWNTs) were synthesized using a hot filament assisted chemical vapor deposition (CVD) at the atmospheric pressure at a substrate temperature of 550 °C. The size of nanotubes was controlled by changing the size of catalyst particles. The structure and composition of these nanotubes were investigated using scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The electron field emission current of MWNTs was also measured. It was found that the nanotubes with smaller the diameter had higher the emission current levels though synthesis conditions except catalyst particles were the same. These as-grown MWNTs had emission current densities of 6.5 mA/cm² and 2.5 mA/cm² at 1 V/μm for 5-8 nm and 20 nm size carbon nanotube samples, respectively. The results indicated that the MWNTs synthesized had low emission threshold voltages and high emission current levels that are favorable properties for field emission-based display device applications.     

Synthesis of large-area single-walled carbon nanotube films on glass substrate and their field electron emission properties

Large-area and homogeneous single-walled carbon nanotube (SWCNT) films have been deposited via arc discharge directly on glass substrate coated with a layer of indium tin oxide film. The characterization, by means of electron microscopy and Raman spectroscopy, shows that the as-grown films are uniformly woven and consist of SWCNT with diameters ranging from 0.82 to 1.15 nm. As a cathode material, the field emission test indicates the films have low turn-on field of ∼1.2 V/μm at 10 μA/cm² emission current, and high emission intensity causing luminance of about 7000 cd/cm² with fine uniformity. The best performing sample exhibits a constant degradation of less than 3% per hour at an emission current of around 1 mA. Measuring with the high voltage (2000 V) on the films for 2.0 h increased the field enhancement factor from 4500 to 5400 at the high field region. The results are of significance to the development of field emission display using nanoemitters.