Self-assembled large-area annular cavity arrays with tunable cylindrical surface plasmons for sensing

Perspective on Tailored Nanostructure-Dominated SPP Effects for SERS

Localized surface plasmon resonance (LSPR) excited by an incident light can normally produce strong surface-enhanced Raman scattering (SERS) at the nanogaps among plasmonic nano-objects (so-called hot spots), which is extensively explored. In contrast, surface plasmon polaritons (SPPs) that can be generated by an incident beam via particular structures with a conservation of wave vectors can excite SERS effects as well. SPPs actually play an indispensable role in high-performance SERS devices but receive much less attention. In this perspective, SPPs and their couplings with LSPR for SERS excitations with differing effectiveness through particular plasmonic/dielectric structures/configurations, along with relevant fabrication approaches, are profoundly reviewed and commented on from a unique perspective from in situ to ex situ excitations of SERS enabled by spatiotemporally separated multiple processes of SPPs. Quantitative design of particular configurations/architectures enabling highly efficient and effective multiple processes of SPPs is particularly emphasized as one giant leap toward ultimate full quantitative design of intrinsically high-performance SERS chips and very critical for their batch manufacturability and applications as well. The viewpoints and prospects about innovative SERS devices based on tailored structure-dominated SPPs effects and their coupling with LSPR are presented and discussed.

Plasmon Hybridizations in Compound Nanorod-Nanohole Arrays

This study shows that a hybridized plasmonic mode, represented by an additional transmission peak, in a compound structure consisting of a nanorod embedded in a nanohole can be effectively described as a quasi-dipole oscillator. When two nanorods are introduced into a nanohole, these two quasi-dipoles can couple and hybridize, giving rise to two additional transmission peaks in the enhanced optical transmission spectrum. The relative intensities of these peaks can be controlled by adjusting the incident polarization, while their separations can be tuned by modifying the length of the nanorods. The concept of quasi-dipoles in compound nanohole structures can be further extended to predict the coupling behavior of even more complex compound configurations, such as multiple nanorods within nanoholes, resulting in the generation of multiple hybridization states. Consequently, the shape and response of the transmission peaks can be precisely engineered. This strategy could be used to design nanohole-based metasurfaces for applications such as ultra-thin optical filters, waveplates, polarizers, etc.

Large-Area Ordered Palladium Nanostructures by Colloidal Lithography for Hydrogen Sensing

Reliable gas sensors are very important for hydrogen (H2) gas detection and storage. Detection methods based on palladium (Pd) metal are cost-effective and widely studied. When Pd is exposed to H2, it turns into palladium hydride with modified optical properties, which thus can be monitored for H2 sensing. Here, we fabricated large-area Pd nanostructures, including Pd nanotriangles and nanohole arrays, using colloidal lithography and systematically studied their H2-sensing performance. After hydrogen absorption, both the Pd nanoholes and nanotriangles showed clear transmittance changes in the visible-near infrared range, consistent with numerical simulation results. The influences of the structural parameters (period of the array P and diameter of the nanohole D) of the two structures are further studied, as different structural parameters can affect the hydrogen detection effect of the two structures. The nanohole arrays exhibited bigger transmittance changes than the nanotriangle arrays.

Readily tunable surface plasmon resonances in gold nanoring arrays fabricated using lateral electrodeposition

Generation and fine-tuning of surface plasmon resonances is a prerequstite for several established and emerging applications such as photovoltaics, photocatalysis, photothermal therapy, surface-enhanced spectroscopy, sensing, superlensing and lasing. We present a low-cost and scalable lateral electrodeposition method for fabrication of high aspect ratio gold nanoring arrays that exhibit multiple surface plasmon resonances in the visible to near-infrared region. Nickel disc arrays of 2 µm size were initially fabricated using maskless lithography and e-beam evaporation. Selective electrodeposition of gold on the lateral surfaces of nickel disc arrays was achieved using a 50 nm SiO₂ film as an insulating mask. Growing from miniscule 100 nm wide lateral surfaces of nickel discs, nanorings with height up to 1084 nm could be obtained with their thickness and aspect ratio governed by the duration of electrodeposition. Facile tuning of the number of plasmon resonances, their resonant wavelength and relative intensity is demonstrated with applications in plasmon mediated photocatalysis and surface-enhanced Raman scattering.

Figures of merit of plasmon lattice resonance sensors: shape and material matters

The figure of merit (FOM) of plasmon lattice resonance (PLR) sensors based on the array of metal/Si/SiO₂nanoparticles has been investigated. We demonstrate the shape and material of metal nanoparticles have remarkable effects on the PLR and FOM. FOM is governed by full-widths at half maximum (FWHM) and sensitivity of the PLR. Three different types of PLR can be generated by changing Ag nanoparticles' shapes (pillars, cubes, spheres). One (named PLR₁) is mainly originated from the coupling between Mie resonance of individual Si nanopillars and diffraction waves. PLR₁of Ag/Si/SiO₂nanoparticle arrays is limited in sensing applications due to lower intensity (for Ag pillars and Ag cubes), or smaller FOM (for Ag spheres). The other two are named PLR₂. PLR₂of Ag/Si/SiO₂nanoparticle array with Ag pillars (or Ag cubes) is mainly originated from the coupling between the quadrupole resonance of individual Ag nanopillars (or Ag cubes) and diffraction waves. While PLR₂of Ag/Si/SiO₂nanoparticle array with Ag spheres is mainly originated from the coupling between dipole resonance of individual Ag nanospheres and diffraction waves. The optimal Ag nanoparticles' shape in FOM is pillar due to the smallest FWHM of PLR₂of Ag/Si/SiO₂nanoparticle array with Ag pillars. Meanwhile, a comparison of FOM between Au, Ag and Al nanopillars of fixed size is made. The optimal material of metal nanopillars to obtain a high FOM is Ag due to higher sensitivity and narrower FWHM.

Enhanced Refractive Index Sensitivity through Combining a Sol-Gel Adsorbate with a TiO2 Nanoimprinted Metasurface for Gas Sensing

We combine a gas-adsorbent microporous hybrid silica layer and a dense TiO2 Mie resonator array (metasurface), both obtained by sol-gel deposition and nanoimprint lithography, to form nanocomposite systems with high sensitivity for refractive index (RI) variations induced by gas adsorption. Using optical transduction based on direct specular reflection, we show spectral shifts of 4470 nm/RIU corresponding to 0.2 nm/ppm gas (air concentration) and reflection intensity changes of R* = 17 (R/RIU) and 0.55 × 10-3 R/ppm (air concentration). The metasurface is composed of hexagonally arranged TiO2 nanopillar arrays, whereas the surrounding sensitive material is a class II microporous hybrid silica, containing methyl and phenyl covalently bonded organic functions. This hybrid layer shows efficient adsorption capability of volatile organic molecules such as isopropanol, which is used to induce slight variations of RI around the TiO2 antennas. Specular reflectance variations at 45° incidence and refractive index measurements are performed using a spectroscopic ellipsometer. The presence of the titania metasurface enhances the signal by almost an order of magnitude with respect to the 2D counterpart (simulated as an effective medium approximation) and is attributed to the antenna effect, enhancing the interaction of the confined electromagnetic wave with the sensitive microporous medium. This sol-gel nanocomposite system presents many advantages such as high throughput and low-cost elaboration of elements and a high chemical, mechanical, and thermal resistance, ensuring high stability as a potential gas-sensitive nanocomposite layer for long periods. This work is a case study of improving the sensitivity of sol-gel gas-sensitive materials in optical transduction, which will be exploited in further works to develop artificial noses.

Plasmon lattice resonances induced by an all-dielectric periodic array of Si nanopillars on SiO2nanopillars

An all-dielectric periodic array is proposed to form plasmon lattice resonances (PLR). In the array, Si nanopillars are on top of SiO₂nanopillars, and SiO₂nanopillars are on top of quartz substrates. The simulated results show that the line-width of the PLR can be as small as 3.3 nm. This can be attributed to the coupling between the Mie resonances of Si nanopillars and the diffracted waves. While the PLR can't be formed by the periodic Si nanopillar array directly sitting on quartz substrates. The diameter and height of Si nanopillars, the period of the array and the height of SiO₂nanopillars have significant impacts on the PLR. This work extends the application of PLR.

Dynamically Switchable Polarization-Independent Triple-Band Perfect Metamaterial Absorber Using a Phase-Change Material in the Mid-Infrared (MIR) Region

A tunable metamaterial absorber (MMA) by reversible phase transitions in a mid-infrared regime is theoretically investigated. The absorber is composed of a molybdenum (Mo)-germanium-antimony-tellurium (Ge₂Sb₂Te₅, GST)-Mo nanodisk structure superimposed on the GST-Al₂O₃ (aluminum oxide)-Mo film. Studies have shown that the combination of the inlaid metal-medium dielectric waveguide mode and the resonant cavity mode and the excitation of the propagating surface plasmon mode are the main reasons for the formation of the triple-band high absorption. Additionally, through the reversible phase change, the transition from high absorption to high reflection in the mid-infrared region is realized. The symmetry of the absorber eliminates the polarization dependence, and the near unity absorption efficiency can be maintained by incidence angles up to 60°. The presented method will enhance the functionality of the absorber and has the potential for the applications that require active control over light absorption.

Coupled plasmonic systems: controlling the plasmon dynamics and spectral modulations for molecular detection

This review describes recent studies on coupled plasmonic systems for controlling plasmon dynamics and molecular detection using spectral modulations. The plasmon dephasing time can be controlled by weak and strong coupling regimes between the plasmonic nanostructures or localized surface plasmon resonances (LSPRs) and the other optical modes such as microcavities. The modal coupling induces near-field enhancement by extending the plasmon dephasing time to increase the near-field enhancement at certain wavelengths resulting in the enhancement of molecular detection. On the other hand, the interaction between LSPR and molecular excited or vibrational states also modulates the resonance spectrum, which can also be used for detecting a small number of molecules with a subtle change in the spectrum. The spectral modulation is induced by weak and strong couplings between LSPRs and the electronic or vibrational states of molecules, and this method is sensitive enough to measure a single molecule.

Optical confinement in the nanocoax: coupling to the fundamental TEM-like mode

The nanoscale coaxial cable (nanocoax) has demonstrated optical confinement in the visible and the near infrared. We report on a novel nanofabrication process which yields optically addressable, sub-µm diameter, and high aspect ratio metal-insulator-metal nanocoaxes made by atomic layer deposition of Pt and Al₂O₃. We observe sub-diffraction-limited optical transmission via the fundamental, TEM-like mode by excitation with a radially polarized optical vortex beam. Our experimental results are based on interrogation with a polarimetric imager. Finite element method numerical simulations support these results, and their uniaxial symmetry was exploited to model taper geometries with both an electrically large volume, (15λ)³, and a nanoscopic exit aperture, (λ/200)².

Synergistic plasmon resonance coupling and light capture in ordered nanoarrays as ultrasensitive and reproducible SERS substrates

An effective SERS substrate for on-field detection needs to satisfy high sensitivity to analyte and signal reproducibility even in the special case of tilting or bending of substrates. Herein, we transferred monolayer AuNPs into a nanocavity to construct a Au particle-in-hemispherical honeycomb nanoarray (PIHHN) as an ultrasensitive and spatially reproducible SERS substrate. The capacity of detection for R6G in an optimal PIHHN substrate is as low as a concentration of 10^-15 M, and the RSD of signal deviation is no more than 5.6%. FDTD simulations explain that placing AuNPs into a metallic nanocavity can capture and focus the light field to improve the interaction between the light and the substrate and provide the collective effect of multiple plasmon coupling, which can induce a stronger electromagnetic field. In addition, the system can generate more hot spots between AuNPs and between AuNPs and the metallic nanocavity. In particular, when the substrate is tilted or bent at an angle from 0° to 60°, the SERS performance remains stable due to the rotational symmetry of the PIHHN structure, which demonstrates the capability of on-field detection. Furthermore, the PIHHN substrate is employed as a highly sensitive multiplex sensor in on-field analysis for contaminated soil, achieving the detection of analytes down to 0.5 ppb.

Reproducible and Bendable SERS Substrates with Tailored Wettability Using Block Copolymers and Anodic Aluminum Oxide Templates

Surface properties are essential for substrates exhibiting high sensitivity in surface-enhanced Raman scattering (SERS) applications. In this work, novel SERS hybrid substrates using polystyrene-block-poly(methyl methacrylate) and anodic aluminum oxide templates is presented. The hybrid substrates not only possess hierarchical porous nanostructures but also exhibit superhydrophilic surface properties with the water contact angle ≈0°. Such surfaces play an important role in providing uniform enhanced intensities over large areas (relative standard deviation ≈10%); moreover, these substrates are found to be highly sensitive (limit of detection ≈10^-12 m for rhodamine 6G (R6G)). The results show that the hybrid SERS substrates can achieve the simultaneous detection of multicomponent mixtures of different target molecules, such as R6G, crystal violet, and methylene blue. Furthermore, the bending experiments show that about 70% of the SERS intensities are maintained after bending from ≈30° to 150°.

Tape-Imprinted Hierarchical Lotus Seedpod-Like Arrays for Extraordinary Surface-Enhanced Raman Spectroscopy

A novel two-side-activatable high-performance surface-enhanced Raman spectroscopy (SERS) substrate is developed based on the tape-imprinting method. It features 3D full-space-distributed hot spots originating from the hierarchical lotus seedpod-like silver arrays, which offer ultrasensitive, uniform, reproducible, and reliable quantitative measurements with an inherent internal standard. This excellent SERS substrate also holds great promise in practical in situ molecule detection on curved surfaces, such as pesticides on fruit, which is not yet possible with the traditional rigid or flexible material-based SERS counterparts.

Ultralarge Area Sub-10 nm Plasmonic Nanogap Array by Block Copolymer Self-Assembly for Reliable High-Sensitivity SERS

Effective surface enhancement of Raman scattering (SERS) requires strong near-field enhancement as well as effective light collection of plasmonic structures. To this end, plasmonic nanoparticle (NP) arrays with narrow gaps or sharp tips have been suggested as desirable structures. We present a highly dense and uniform Au nanoscale gap array enabled by the customized design of NP shape and arrangement employing block copolymer self-assembly. Block copolymer self-assembly in thin films offers uniform hexagonally packed nanopost template arrays over the entire surface of a 2 in. wafer. Conventional evaporative metal deposition over the nanotemplate surface allows precise geometric control and positional arrangement of metal NPs, constituting tunable, strong plasmonic near-field enhancement particularly at the "hot spots" near interparticular nanoscale gaps. Underlying field distribution has been investigated by a finite-difference time-domain simulation. In the detection of thiophenol, our Au nanogap array shows a remarkable enhancement of Raman intensity greater than ∼10⁴, a standard deviation as small as 12.3% compared to that of the planar Au thin film. In addition, adenine biomolecules can be detected with a detection limit as low as 100 nM. Our approach proposes highly sensitive and reliable SERS on the basis of a scalable, low-cost bottom-up strategy.

Paper-Based Surface-Enhanced Raman Spectroscopy for Diagnosing Prenatal Diseases in Women

We report the development of a surface-enhanced Raman spectroscopy sensor chip by decorating gold nanoparticles (AuNPs) on ZnO nanorod (ZnO NR) arrays vertically grown on cellulose paper (C). We show that these chips can enhance the Raman signal by 1.25 × 10⁷ with an excellent reproducibility of <6%. We show that we can measure trace amounts of human amniotic fluids of patients with subclinical intra-amniotic infection (IAI) and preterm delivery (PTD) using the chip in combination with a multivariate statistics-derived machine-learning-trained bioclassification method. We can detect the presence of prenatal diseases and identify the types of diseases from amniotic fluids with >92% clinical sensitivity and specificity. Our technology has the potential to be used for the early detection of prenatal diseases and can be adapted for point-of-care applications.

High-Throughput Fabrication of Ultradense Annular Nanogap Arrays for Plasmon-Enhanced Spectroscopy

The confinement of light into nanometer-sized metallic nanogaps can lead to an extremely high field enhancement, resulting in dramatically enhanced absorption, emission, and surface-enhanced Raman scattering (SERS) of molecules embedded in nanogaps. However, low-cost, high-throughput, and reliable fabrication of ultra-high-dense nanogap arrays with precise control of the gap size still remains a challenge. Here, by combining colloidal lithography and atomic layer deposition technique, a reproducible method for fabricating ultra-high-dense arrays of hexagonal close-packed annular nanogaps over large areas is demonstrated. The annular nanogap arrays with a minimum diameter smaller than 100 nm and sub-1 nm gap width have been produced, showing excellent SERS performance with a typical enhancement factor up to 3.1 × 10⁶ and a detection limit of 10^-11 M. Moreover, it can also work as a high-quality field enhancement substrate for studying two-dimensional materials, such as MoSe₂. Our method provides an attractive approach to produce controllable nanogaps for enhanced light-matter interaction at the nanoscale.

SERS polarization-dependent effects for an ordered 3D plasmonic tilted silver nanorod array

Hexagonal close-packed tilted Ag nanorod arrays that exhibit excellent uniformity and reproducibility were prepared. The tilt angle was easily controlled by regulating the sputtering angle, accompanied by a reduction and constancy in the gap size of adjacent nanorods, which is 30° and 90° relative to the sputtering direction. The surface enhanced Raman spectroscopy (SERS) technique was used to characterize the interaction of tilted Ag nanorod arrays with polarized laser excitation. Interestingly, the SERS polarization-dependence increased with increasing tilt angle of the Ag nanorods. To elucidate the essential factors responsible for this SERS result, three-dimensional (3D) electromagnetic enhancement distribution for the proposed system was numerically simulated based on p- and s-polarization excitation. Most importantly, the fundamental reasons for the polarization dependence of SERS were obtained by a quantitative 3D numerical simulation of hotspot distribution for adjacent nanorods.

Detailed correlations between SERS enhancement and plasmon resonances in subwavelength closely spaced Au nanorod arrays

Depending on the experimental conditions and plasmonic systems, the correlations between near-field surface enhanced Raman scattering (SERS) behaviors and far-field localized surface plasmon resonance (LSPR) responses have sometimes been accepted directly or argued or explored. In this work, we focus on the attractive subwavelength closely spaced metallic nanorod arrays and investigate in detail the complex relationship between their SERS behaviors and plasmon resonances. This is achieved utilizing a combination of array fabrication, conventional LSPR spectra, SERS measurements, electron microscopy and numerical modeling. Three key factors that may impact the correlations have been comprehensively analyzed: the intrinsic near-field to far-field red-shift is found to be rather small in the lattice; the surface roughness has actually little impact on the spectral alignment of the near- and far-field responses; the continuous dependence of individual SERS peak heights on the Stokes Raman shift has been visualized and further clarified. By 3D finite element method (FEM) plasmon mapping, the physical origin of the collective resonances in the lattice is verified directly to be the Fabry-Perot-like cavity mode. The strong near-field enhancement results from the coupling of surface plasmon polaritons (SPPs) propagating at the two sidewalls of neighbouring nanorods forming the resonant cavity. The physical principles demonstrated here benefit significantly the optimization of nano-optic devices based on closely spaced metallic nanorod arrays, as well as the fundamental understanding of the near- and far-field relationship.

Nanocracking and metallization doubly defined large-scale 3D plasmonic sub-10 nm-gap arrays as extremely sensitive SERS substrates

Considering the technological difficulties in the existing approaches to form nanoscale gaps, a convenient method to fabricate three-dimensional (3D) sub-10 nm Ag/SiN_x gap arrays has been demonstrated in this study, controlled by a combination of stress-induced nanocracking of a SiN_x nanobridge and Ag nanofilm deposition. This scalable 3D plasmonic nanogap is specially suspended above a substrate, having a tunable nanogap width and large height-to-width ratio to form a nanocavity underneath. As a surface-enhanced Raman scattering (SERS) substrate, the 3D Ag/SiN_x nanogap shows a large Raman enhancement factor of ∼10⁸ and extremely high sensitivity for the detection of Rhodamine 6G (R6G) molecules, even down to 10^-16 M, indicating an extraordinary capability for single-molecule detection. Further, we verified that the Fabry-Perot resonance occurred in the deep SiN_x nanocavity under the Ag nanogap and contributed prominently to a tremendous enhancement of the local field in the Ag-nanogap zone and hence ultrasensitive SERS detection. This method circumvents the technological limitations to fabricate a sub-10 nm metal nanogap with unique features for wide applications in important scientific and technological areas.

Fabrication of highly ordered 2D metallic arrays with disc-in-hole binary nanostructures via a newly developed nanosphere lithography

2D metallic arrays with binary nanostructures derived from a nanosphere lithography (NSL) method have been rarely reported. Here, we demonstrate a novel NSL strategy to fabricate highly ordered 2D gold arrays with disc-in-hole binary (DIHB) nanostructures in large scale by employing a sacrificing layer combined with a three-step lift-off process. The structural parameters of the resultant DIHB arrays, such as periodicity, hole diameter, disc diameter and thicknesses can be facilely controlled by tuning the nanospheres size, etching condition, deposition angle and duration, respectively. Due to the intimate interactions between two subcomponents, the DIHB arrays exhibit both an extraordinary high surface-enhanced Raman scattering enhancement factor up to 5 × 10⁸ and a low sheet resistance down to 1.7 Ω/sq. Moreover, the DIHB array can also be used as a metal catalyzed chemical etching catalytic pattern to create vertically-aligned Si nano-tube arrays for anti-reflectance application. This strategy provides a universal route for synthesizing other diverse binary nanostructures with controlled morphology, and thus expands the applications of the NSL to prepare ordered nanostructures with multi-function.

Fabrication of plasmonic cavity arrays for SERS analysis

The plasmonic cavity arrays are ideal substrates for surface enhanced Raman scattering analysis because they can provide hot spots with large volume for analyte molecules. The large area increases the probability to make more analyte molecules on hot spots and leads to a high reproducibility. Therefore, to develop a simple method for creating cavity arrays is important. Herein, we demonstrate how to fabricate a V and W shape cavity arrays by a simple method based on self-assembly. Briefly, the V and W shape cavity arrays are respectively fabricated by taking KOH etching on a nanohole and a nanoring array patterned silicon (Si) slides. The nanohole array is generated by taking a reactive ion etching on a Si slide assembled with monolayer of polystyrene (PS) spheres. The nanoring array is generated by taking a reactive ion etching on a Si slide covered with a monolayer of octadecyltrichlorosilane before self-assembling PS spheres. Both plasmonic V and W cavity arrays can provide large hot area, which increases the probability for analyte molecules to deposit on the hot spots. Taking 4-Mercaptopyridine as analyte probe, the enhancement factor can reach 2.99 × 10⁵ and 9.97 × 10⁵ for plasmonic V cavity and W cavity array, respectively. The relative standard deviations of the plasmonic V and W cavity arrays are 6.5% and 10.2% respectively according to the spectra collected on 20 random spots.

Integration of tunable two-dimensional nanostructures on a chip by an improved nanosphere lithography method

By uniform infiltration of a different material into monolayered polystyrene colloidal crystals and by flexibly combining the two materials as etching masks, we demonstrate an improved nanosphere lithography method that possesses the ability to produce a diverse range of tunable nano-patterns in a small area with good reproducibility. The factors that affect the infiltration height and uniformity are characterized and discussed. Annular gap arrays, close-packed ring arrays, and bowl arrays are demonstrated by this method. The geometry size of these nano-patterns can be tuned over the range 10 nm to ∼500 nm with steps of ∼5 nm during the fabrication progress. Formation mechanisms of the close-packed ring arrays are experimentally investigated. Because all the fabrication processes involved in this method are adaptable to sophisticated integrated circuit fabrication techniques, most of the nano-patterns produced by this method could be integrated on thin films, which is desirable for optics integration and array sensing.

New type high-index dielectric nanosensors based on the scattering intensity shift

Sensing is regarded as one of the most important applications of noble metal-based nanoplasmonics. However, all previous designs have been based on the wavelength-shift of the localized surface plasmon resonance, in which the sensitivity is intrinsically limited by the low quality factors induced by metal losses, and meanwhile the large ohmic loss, high cost and inevitable toxicity and biofouling for detection in vivo greatly hinder their further applications in biosensors. Beyond noble metals, high-refractive index dielectric materials (HRDMs) like silicon with low-loss and strong magnetic response have drawn more attention. Here, for the first time, we proposed a HRDM nanosphere as a new nanosensor for biomolecule detection, and experimentally demonstrated a HRDM sensor working on the intensity-shift but not wavelength-shift of the scattering. The sensing mechanism based on the synergistic effect of the broadening electric mode shift of HRDMs and the Kerker's scattering intensity-shift is beneficial to achieve higher sensitivity. We validated the efficacy of our sensor to detect refractive index changes and trace amounts of streptavidin molecules, and the sensitivity can reach 27 times as high as the highest sensitivity reported to date for nanoplasmonic structures. These findings showed that monitoring the change of the scattering intensity of HRDM nanostructures is superior to monitoring the wavelength-shift of nanoplasmonic structures, as is widely used in nanoplasmonic sensors, for biosensing, meaning HRDM nanosensors could be an important tool in biomolecule detection.

Tunable Lattice Coupling of Multipole Plasmon Modes and Near-Field Enhancement in Closely Spaced Gold Nanorod Arrays

Considering the nanogap and lattice effects, there is an attractive structure in plasmonics: closely spaced metallic nanoarrays. In this work, we demonstrate experimentally and theoretically the lattice coupling of multipole plasmon modes for closely spaced gold nanorod arrays, offering a new insight into the higher order cavity modes coupled with each other in the lattice. The resonances can be greatly tuned by changes in inter-rod gaps and nanorod heights while the influence of the nanorod diameter is relatively insignificant. Experimentally, pronounced suppressions of the reflectance are observed. Meanwhile, the near-field enhancement can be further enhanced, as demonstrated through surface enhanced Raman scattering (SERS). We then confirm the correlation between the near-field and far-field plasmonic responses, which is significantly important for maximizing the near-field enhancement at a specific excitation wavelength. This lattice coupling of multipole plasmon modes is of broad interest not only for SERS but also for other plasmonic applications, such as subwavelength imaging or metamaterials.

Wireless Hydrogen Smart Sensor Based on Pt/Graphene-Immobilized Radio-Frequency Identification Tag

Hydrogen, a clean-burning fuel, is of key importance to various industrial applications, including fuel cells and the aerospace and automotive industries. However, hydrogen gas is odorless, colorless, and highly flammable; thus, appropriate safety protocol implementation and monitoring are essential. Highly sensitive hydrogen-gas leak detection and surveillance systems are needed; additionally, the ability to monitor large areas (e.g., cities) via wireless networks is becoming increasingly important. In this report, we introduce a radio frequency identification (RFID)-based wireless smart-sensor system, composed of a Pt-decorated reduced graphene oxide (Pt_rGO)-immobilized RFID sensor tag and an RFID-reader antenna-connected network analyzer to detect hydrogen gas. The Pt_rGOs, produced using a simple chemical reduction process, were immobilized on an antenna pattern in the sensor tag through spin coating. The resulting Pt_rGO-based RFID sensor tag exhibited a high sensitivity to hydrogen gas at unprecedentedly low concentrations (1 ppm), with wireless communication between the sensor tag and RFID-reader antenna. The wireless sensor tag demonstrated flexibility and a long lifetime due to the strong immobilization of Pt_rGOs on the substrate and battery-independent operation during hydrogen sensing, respectively.