Direct Fabrication Route to Plastic-Supported Gold Nanoparticles for Flexible NIR-SERS

Stable gold nanoparticles with surface plasmon resonance tunable from visible (Vis) to near-infrared (NIR) are deposited via a direct sputtering methodology on large area polyethylene terephthalate (PET) to be used as effective, flexible NIR surface-enhanced Raman scattering (SERS) substrates. An O₂ plasma treatment of PET is used to tailor growth dynamics, geometry, and plasmonic properties of nanoparticles. The O₂ plasma treatment of PET results also in effective improvement of nanoparticle anchoring on the plastic substrate, providing more stable, flexible SERS systems. The functionality of fabricated SERS substrates has been tested using benzylthiol, and SERS enhancement factors in the range 10⁴ have been achieved, which are comparable with those reported in literature for gold nanostructures fabricated on silicon substrate. These results attest the great potentiality of this methodology for the production of cost-effective flexible and reusable large-scale SERS substrates.     

Higher Order Tunable Fano Resonances in Multilayer Nanocones

We present a computational study of the plasmonic response of a gold–silica–gold multilayered nanostructure based on truncated nanocones. Symmetry breaking is introduced by rotating the nanostructure and by offsetting the layers. Nanocones with coaxial multilayers show dipole–dipole Fano resonances with resonance frequencies depending on the polarization of the incident light, which can be changed by rotating the nanostructure. By breaking the axial symmetry, plasmonic modes of distinct angular momenta are strongly mixed, which provide a set of unique and higher order tunable Fano resonances. The plasmonic response of the multilayered nanocones is compared to that of multishell nanostructures with the same volume and the former are discovered to render visible high-order dark modes and to provide sharp tunable Fano resonances. In particular, higher order tunable Fano resonances arising in non-coaxial multilayer nanocones can vary the plasmon lines at various spectral regions simultaneously, which makes these nanostructures greatly suitable for plasmon line shaping both in the extinction and near field spectra.     

Optimized Electroless Silver Coating for Optical and Plasmonic Applications

Electroless metal deposition is a simple and convenient technique to fabricate metallic films and to provide isotropic metal functionalization of 3D structures with complex geometries. In this work, we describe the synthesis of silver coatings by means of a modified Tollens reaction and their use as optical coating. The chemical composition of the metallization bath is here addressed to optimize the metal coating deposition. The synthesis parameters have been tailored in order to deposit very smooth films which were characterized by scanning electron microscopy, atomic force microscopy, and optical spectroscopy. 2D diffraction gratings and sinusoidal plasmonic gratings were produced with the proposed method. Optical characterization confirmed the plasmonic activities of the resultant structures, proving the efficiency of the described method for optical applications. Thermal annealing was found to improve the surface roughness of the coating and therefore the optical properties of the plasmonic gratings.     

Design and Analysis of Infrared Tunable All-Optical Filters Based on Plasmonic Hybrid Nanostructure Using Periodic Nanohole Arrays

A tunable high transmission optical bandpass filter based on a plasmonic hybrid nanostructure, composed of a periodic array of nanocircles and nanoholes combining two isolated waveguides is introduced in this paper. The presented design improves the coupling, which results in a higher transmission peak. To study the filtering operation, different topologies are investigated. The transmission properties and the resonance wavelengths are adjusted by sweeping various geometrical parameters. A multimode spectrum for each of the topologies is obtained. A tunable bandgap and bandwidth can be obtained by adjusting the refractive index of the periodic nanostructure. We have reached a maximum quality factor and a small full width at half-maximum bandwidth with the maximum transmission values greater than 80%. The advantages of the presented structures which include the benefits of both plasmonic and periodic nanostructures are tunability, high detection resolution, and integrability at the nanoscale for optical applications.

     

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.     

Scaffold-Based Multi-nanoparticle Clusters as Tunable LSPR Substrates for SERS Applications

Tunable local surface plasmon resonance (LSPR) enhancement properties of scaffold-based multi-nanoparitcle clusters were investigated using finite-difference time-domain (FDTD) method with calculated optical spectra, near-field distribution, and average enhancement of hybrid nanostructures as slab/nanoparticls, cylinder/nanoparticles, and sphere/nanoparticles. Focusing on influence factors including surface curvature, coupling effect, and decorated particle number, several models were built for further understanding on the dominate contribution in complicate multi-particle nanostructure and to explore their potential for plasmonic enhancement applications such as surface-enhanced Raman spectroscopy (SERS), solar cells material, LSPR sensor, and nanoantenna.     

Comparison of Fabrication Methods Based on Nanoimprinting Lithography for Plasmonic Color Filter Fabrication

The angle-variable tunable optical filter was strictly fabricated by two strategies of nanoimprint-coupled metal nanopatterning with improved cost-effectiveness and accessibility. The tunable optical properties and the performances of two strategies were experimentally examined and turned out to be well matched to numerical results. Tunable properties are obtained by three factors: size of fabricated Ag nanodisks, incident illumination angle, and fabrication strategies. The resonant extinction peak shifts were identified to show a large increase along with the increase in fabricated Ag disk size and increase in the incidence angle of illumination. When comparing a fabrication strategy, it was confirmed that the sample fabricated by the strip-off method has better stability on color changes with a consistent dependency on the incident angle. The presented strategies of fabrication are technically viable for obtaining well-defined plasmonic nanostructures so that it has the feasibility to apply for fascinating optical applications including display or tunable optical filters.

     

Surface Enhanced Raman Spectroscopy Detection of Biomolecules Using EBL Fabricated Nanostructured Substrates

Fabrication and characterization of conjugate nano-biological systems interfacing metallic nanostructures on solid supports with immobilized biomolecules is reported. The entire sequence of relevant experimental steps is described, involving the fabrication of nanostructured substrates using electron beam lithography, immobilization of biomolecules on the substrates, and their characterization utilizing surface-enhanced Raman spectroscopy (SERS). Three different designs of nano-biological systems are employed, including protein A, glucose binding protein, and a dopamine binding DNA aptamer. In the latter two cases, the binding of respective ligands, D-glucose and dopamine, is also included. The three kinds of biomolecules are immobilized on nanostructured substrates by different methods, and the results of SERS imaging are reported. The capabilities of SERS to detect vibrational modes from surface-immobilized proteins, as well as to capture the protein-ligand and aptamer-ligand binding are demonstrated. The results also illustrate the influence of the surface nanostructure geometry, biomolecules immobilization strategy, Raman activity of the molecules and presence or absence of the ligand binding on the SERS spectra acquired.     

Ag Nanostructures Produced by Glancing Angle Deposition with Remarkable Refractive Index Sensitivity

Glancing angle deposition is a powerful method for direct fabrication of nanostructures on various substrates. In this research, GLAD method has been used to fabricate Ag nanostructures with columnar morphology for refractive index sensing applications. The morphology and plasmonic properties of the nanostructures are controlled by changing deposition parameters such as glancing angle, speed of azimuthal rotation of the substrate, and the height of deposited nanostructures. The results show that increasing the deposition thickness from 200 to 500 nm leads to narrowing the plasmonic peak, which mainly relates to increment of the distance between larger nanostructures. By changing the glancing angle between 86° to 80°, the narrowest plasmonic peak corresponding to the greatest sensitivity has been obtained for the film deposited at the angle of 82°. Also, increment of the rotation speed of the samples leads to narrowing of the plasmonic peaks. By measuring the refractive index sensitivity (RIS) of the nanostructures, a best sensitivity of 154 nm/RIU has been obtained. Finally, we investigated the stability of Ag nanostructures in deionized water by introducing a new stabilizing technique in which a thin Au layer is coated on the Ag nanostructures. This technique has the merits of simultaneously protecting the Ag nanostructures against oxidation and keeping their refractive index sensitivity high enough for long time usages.     

Fabrication of Plasmonic Nanoparticles on a Wave Shape PDMS Substrate

Plasmonics - In this paper, we report on detailed fabrication process and analytical studies of plasmonic nanoparticles on flexible substrates of polydimethylsiloxane (PDMS) that are fabricated...     

Intracellular Localized Surface Plasmonic Sensing for Subcellular Diagnosis

This paper proposes a method for diagnosing intracellular conditions and organelles of cells with localized surface plasmonic resonance (LSPR) by directly internalizing the gold nanoparticles (AuNPs) into the cells and measuring their plasmonic properties through hyperspectral imaging. This technique will be useful for direct diagnosis of cellular organelles, which have potential for cellular biology, proteomics, pharmaceuticals, drug discovery etc. Furthermore, localization and characterization of citrate-capped gold nanoparticles in HeLa cells were studied, by hyperspectral microscopy and other imaging techniques. Here, we present the method of internalizing the gold nanoparticles into the cells and subcellular organelles to facilitate subcellular plasmonic measurements. An advanced label-free visualization technique, namely hyperspectral microscopy providing images and spectral data simultaneously, was used to confirm the internalization of gold nanoparticles and to reveal their optical properties for possible intracellular plasmonic detection. Hyperspectral technology has proved to be effective in the analysis of the spectral profile of gold nanoparticles, internalized under different conditions. Using this relatively novel technique, it is possible to study the plasmonic properties of particles, localized in different parts of the cell. The position of the plasmon bands reflects the interactions of gold nanoparticles with different subcellular systems, including particle-nucleus interactions. Our results revealed the effect of the different intracellular interactions on the aggregation pattern of gold nanoparticles, inside the cells. This novel technique opens the door to intracellular plasmonics, an entirely new field, with important potential applications in life sciences. Similarly, the characterization of AuNP inside the cell was validated using traditional methods such as light microscopy and scanning electron microscopy. Under the conditions studied in this work, gold nanoparticles were found to be non-toxic to HeLa (cervical cancer) cells.     

Novel and Sensitive Core-Shell Nanoparticles Based on Surface Plasmon Resonance

In this paper, a rough silver core-shell nanoparticle with strong electric field enhancement in the vicinity of a bumpy structure on the silver core-shell surface is reported. A dipolar plasmonic mode of the silver nanoshell is investigated by using the quasi-static approach and plasmon hybridization theory, which analytical results identify the electric field enhancement spectra in which the enhancement is optimized. As the silver shell thickness is small, the hot spots play an important role in the plasmonic field enhancement. In addition, the deposition of a rough silver shell can generate a stronger near-field enhancement near the silver surface which is more desirable than that of a smooth silver shell for sensitive detection based on SPR and surface enhanced Raman scattering (SERS). The plasmonic field enhancement of a bumpy silver core-shell nanoparticle permits the detection and characterization of bovine serum albumin (BSA) protein molecule and hemoglobin solution with a high sensitivity.     

Remote-Excitation Time-Dependent Surface Catalysis Reaction Using Plasmonic Waveguide on Sites of Single-Crystalline Crossed Nanowires

The remote-excitation polarization-dependent surface enhanced Raman scattering (SERS) induced by plasmonic waveguide is used to investigate the surface catalysis reaction of 4-nitrobenzenethiol converting to p,p′-dimercaptoazobenzene. The propagating surface plasmon polaritons along single-crystalline nanowires can be coupled by the crossed nanowire as nanoantenna for generating massive electromagnetic field enhancement in the nanogap. The remote-excitation SERS spectra in the nanogap reveal the occurrence of a surface catalysis reaction. The time-dependent remote-excitation SERS spectra further confirmed such surface catalysis reaction. This novel sensitive technology could lead to miniaturized photonics and realize high-resolution microscopy/spectroscopy used in the field of remote catalysis reaction.     

Direct Electrodeposition of Hollowed Ag Nanostructures on ITO Glass for Reproducible SERS Application

Hollowed Ag nanostructures are, for the first time, electrodeposited on ITO glass without use of surfactant. The hollowed Ag nanostructure was investigated via a collaboration of scanning electron microscopy (SEM), XRD, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), XRD, and UV-vis. Results exhibited that the formation of the hollowed Ag nanostructure can be interpreted as the synergy effect of twin defect and low nucleation driving force. Surface-enhanced Raman scattering (SERS) spectra of rhodamine 6G and adenine molecules adsorbed on the surface of these Ag nanostructures were recorded. The smallest RSD of 1651 cm^?1 Raman bands of rhodamine 6G was 14.7 %, indicating that the hollowed Ag nanostructures can be utilized for reproducible SERS application. Through comparison, it was found the good crystallinity was beneficial for SERS.     

Plasmonic Spectral Engineering via Interferometric Illumination of Colloid Sphere Monolayers

A novel method is presented for complex structure fabrication, which is capable of breaking the hexagonal symmetry of the conventional colloid sphere lithography via the interferometric illumination of colloid sphere monolayers (IICSM). It is demonstrated that the perfect lateral synchronization of a linear intensity modulation originating from two-beam interference with respect to a hexagonal colloid sphere monolayer makes it possible to tune four complex structure parameters independently. Based on comparative study of hexagonal and rectangular hole doublet-arrays, which can be generated by linearly polarized light via homogeneous illumination and via IICSM, it is shown that the novel IICSM method enables plasmonic spectral engineering with higher degrees of freedom. The unique spectral properties of the complex patterns attainable via IICSM are more precisely tunable by properly selected azimuthal orientation during illumination and by the surrounding medium. It is shown that coupling phenomena between propagating and localized plasmonic modes on appropriately designed complex structures result in unique charge and near-field distribution accompanied by narrow Fano lines. Optimal configurations of complex plasmonic structures consisting of a rectangular array of hole doublets with different geometrical size parameters are presented, which ensure enhanced sensitivity in bio-detection.     

Facile fabrication of distance-tunable Au-nanorod chips for single-nanoparticle plasmonic biosensors

In this work, we present a simple and effective method to fabricate distance-controllable, Au nanorod (AuNR) chips thorough electrostatic assembly. Cetyltrimethylammonium bromide (CTAB)-capped AuNRs were immobilized on a hydroxyl-functionalized glass substrate by immersion of the glass into AuNR-suspension. The electrostatic surfacial assembly of AuNRs offers significant advantages over conventional thiol-induced chemistry, i.e., direct control of self-assembly of AuNRs, easy fabrication in ambient environment and most importantly, broad range of tunable inter-particle distance, ranging from 0.25 to 10 μm. The mechanism of time-dependant deposition process of AuNRs was described via competitive bindings of AuNRs and free CTAB molecules in AuNR-suspension. In addition, the electrostatically anchored AuNRs on a glass substrate provide sufficient stability under harsh experimental conditions with flow of basic/acidic solutions and organic solvents with different polarity. The feasibility of the AuNR-chips fabricated by the proposed method for single-nanoparticle plasmonic biosensors was demonstrated by the plasmonic measurement of aptamer-thrombin binding event. The corresponding limit of detection of thrombin molecule was found to be ～278 pM based on the signal to noise ratio of 4.     

The Effect of Negative Curvature on the Plasmonic Coupling of Concentric Core–Shell Metallic Nanostructures

Negative curvature-dependent localized surface plasmon resonance (LSPR) properties of concentric core–shell metallic nanostructure have been studied using quasistatic approach and plasmon hybridization theory. Whether in single-layered gold nanoshell or double gold nanoshells, the oscillating surface charges always concentrate close to the poles of the metal surface with negative curvature, which results in the anisotropic local electric field distribution and affects both the inter-surface plasmonic coupling and inter-shell plasmonic coupling. Therefore, the change of the radius of the gold surface with negative curvature could modulate the plasmon hybridization and lead to the LSPR shifting. The physical mechanism of the negative curvature-dependent LSPR presents a potential for design and fabrication of nanoscale optical device based on core–shell type metallic nanostructures.     

Direct Growth of Optical Antennas Using E-Beam-Induced Gold Deposition

Electron beam induced deposition (EBID) is used to grow on a transparent substrate plasmonic antennas formed by gold nanorods. We first discuss the influence of the growth parameters on the geometrical homogeneity of the structures. The optical response of optimized rods with different aspect ratios are measured using scattering spectroscopy. The optical data show antenna resonances in good agreement with 3D numerical simulations for pure gold antennas, validating EBID as a novel relevant technique for the fabrication of plasmonic nanostructures.     

Nano-patterned SERS substrate: application for protein analysis vs. temperature

We have illustrated the fabrication of nano-structures as a surface enhanced Raman scattering (SERS) substrate using electro-plating and electron-beam lithography techniques to obtain an array of gold nanograin-aggregate structures of diameter ranging between 80 and 100 nm with interstitial gap of 10-30 nm. The nanostructure based SERS substrate permits us to have better control and reproducibility on generation of plasmon polaritons. The calculation shows the possible detection of myoglobin concentration down to attomole. This SERS substrate is used to investigate the structural changes of different proteins; lysozyme, ribonuclease-B, bovin serum albumin and myoglobin in the temperature range between -65 and 90 degrees C. The in-depth analysis even for small conformational changes is performed using 2D Raman correlation analysis and difference Raman analysis in order to gain straightforward understanding of proteins undergoing thermodynamical perturbation.     

Rapid and Sensitive Detection of Rotavirus Molecular Signatures Using Surface Enhanced Raman Spectroscopy

Human enteric virus infections range from gastroenteritis to life threatening diseases such as myocarditis and aseptic meningitis. Rotavirus is one of the most common enteric agents and mortality associated with infection can be very significant in developing countries. Most enteric viruses produce diseases that are not distinct from other pathogens, and current diagnostics is limited in breadth and sensitivity required to advance virus detection schemes for disease intervention strategies. A spectroscopic assay based on surface enhanced Raman scattering (SERS) has been developed for rapid and sensitive detection of rotavirus. The SERS method relies on the fabrication of silver nanorod array substrates that are extremely SERS-active allowing for direct structural characterization of viruses. SERS spectra for eight rotavirus strains were analyzed to qualitatively identify rotaviruses and to classify each according to G and P genotype and strain with >96% accuracy, and a quantitative model based on partial least squares regression analysis was evaluated. This novel SERS-based virus detection method shows that SERS can be used to identify spectral fingerprints of human rotaviruses, and suggests that this detection method can be used for pathogen detection central to human health care.