Plasmonic Metasurfaces Based on Nanopin-Cavity Resonator for Quantitative Colorimetric Ricin Sensing

Surface Functionalization and Texturing of Optical Metasurfaces for Sensing Applications

Optical metasurfaces are planar metamaterials that can mediate highly precise light-matter interactions. Because of their unique optical properties, both plasmonic and dielectric metasurfaces have found common use in sensing applications, enabling label-free, nondestructive, and miniaturized sensors with ultralow limits of detection. However, because bare metasurfaces inherently lack target specificity, their applications have driven the development of surface modification techniques that provide selectivity. Both chemical functionalization and physical texturing methodologies can modify and enhance metasurface properties by selectively capturing analytes at the surface and altering the transduction of light-matter interactions into optical signals. This review summarizes recent advances in material-specific surface functionalization and texturing as applied to representative optical metasurfaces. We also present an overview of the underlying chemistry driving functionalization and texturing processes, including detailed directions for their broad implementation. Overall, this review provides a concise and centralized guide for the modification of metasurfaces with a focus toward sensing applications.

Flexible, robust, and high-performance gas sensors based on lignocellulosic nanofibrils

Gas detection in flexible electronics demands novel materials with superior sensing performance that have high mechanically strength, are flexible, low-cost, and sustainable. We explore a composite sensing nanopaper based on lignocellulosic cellulose nanofibrils (LCNF) as a renewable and mechanically strong substrate that enables the fabrication of flexible, and highly sensitive gas sensors. In the system the hydrophobic lignin covalently bonds to cellulose in the nanofibrils, increasing the nanopaper water-resistance and limiting sensing materials response to humidity. The sensor is composed of polyaniline (PANI) grown on flexible LCNF and reduced graphene oxide (rGO) nanosheets. The proposed structure, at 10 wt% rGO, demonstrated a 10-fold improvement in sensitivity to volatile amines (i.e. ammonia detection down to 1 ppm) while maintaining an acceptable selectivity. Furthermore, we demonstrated the application of the sensing nanopaper in a microwave sensor that paves the path toward flexible, wireless, and high-performance sensing devices.

Self-driving dynamic plasmonic colors based on needle steering for simultaneous control of transition direction and time on metallic nanogroove metasurfaces

Dynamically tunable plasmonic colors hold great promise for a wide range of applications including color displays, colorimetric sensing, and information encryption. However, dynamic control speed of plasmonic colors is still slow to date. Herein, we propose to use a needle to direct the flow of water and gas pressure to drive water, realizing a simultaneous direction-controllable and fast plasmonic color transition. The highly reflected background light of the metallic nanogroove metasurface is suppressed to generate high-purity plasmonic colors through the cross-polarized input and output configuration. When the environment is changed from air to water, a giant color change from cyan to red (a wavelength shift of 156 nm) is experimentally observed. More importantly, by utilizing a needle to steer the flow of water, direction-controllable and fast plasmonic color transition is achieved by controlling gas pressure to drive water. Compared with current state-of-the-art plasmonic color scanning technology, the color transition time via water driven by gas pressure decreases by three orders of magnitude for the same scanning length. The multi-degrees of freedom dynamic structural colors could have potential applications in dynamic displays, anti-counterfeiting, and information security.

A Simple, Fast and Portable Method for Electrochemical Detection of Adenine Released by Ricin Enzymatic Activity

International authorities classify ricin toxin present in castor seed as a potential agent for use in bioterrorism. Therefore, the detection, identification, and characterization of ricin in various sample matrices are considered necessary actions for risk assessment during a suspected exposure. This study reports a portable electrochemical assay for detecting active ricin based on the adenine electro-oxidation released from herring sperm DNA substrate by its catalytic action. Also, kinetic parameters were calculated, and the values were K_m of 3.14 µM and K_cat 2107 min⁻¹. A linear response was found in optimized experimental conditions for ricin concentrations ranging from 8 to 120 ng/mL, and with a detection limit of 5.14 ng/mL. This proposed detection strategy emphasizes the possibility of field detection of active ricin in food matrices and can be applied to other endonucleolytic activities.

Highly Sensitive and Contactless Ammonia Detection Based on Nanocomposites of Phosphate-Functionalized Reduced Graphene Oxide/Polyaniline Immobilized on Microstrip Resonators

Ammonia is a key compound in a variety of industrial sectors, including automotive, chemical, and food. Its hazardous effects on the environment and human health require the implementation of proper safety guidelines and monitoring techniques. An attractive approach is to add sensing functionality to low-cost wireless communication devices to allow for the monitoring/mapping of the chemical environment across a large area. This study outlines a highly sensitive contactless ammonia gas sensor with the potential for continuous and wireless mapping of ammonia emissions by integrating an antenna on the device. The devices were fabricated by casting a novel advanced sensing nanocomposite, polyaniline (PANI), and phosphate-functionalized reduced graphene oxide (P-rGO) on split-ring resonators (SRRs). P-rGO incorporation in PANI produced a positive-sensing synergistic effect to multiply the sensing response severalfold to ammonia and dimethylamine gases. Furthermore, we identified that the modification of the semiconductive behavior of the nanosheets, achieved via phosphate functionalization, is the key factor to the positive-sensing synergy observed in the nanocomposites because of the formation of localized heterojunctions. The prepared SRRs exhibited remarkably a low detection limit, ∼1 ppm, to ammonia gas, as well as good stability and selectivity, which paves the path for a novel generation of wireless, chipless, potentially fully printable, and passive sensor platforms.

Three-dimensional cavity-coupled metamaterials for plasmonic color and real-time colorimetric biosensors

Plasmonic structure color has significant potential for visual biochemical sensing by simple instrumentation or even naked eye detection. Herein, we present a visual and real-time sensing strategy for refraction index sensing and detection of the biotin-avidin system based on three-dimensional cavity-coupled metamaterials. These metamaterials composed of a top array of gold disks, aluminium pillars and a bottom reflection film of aluminium have structures similar to the metal-insulator-metal structure. The insulating layer comprises air-gap cavities that are easily filled with gaseous or liquid dielectrics. Therefore, analytes can permeate into the nano-scale cavities and produce strong light-matter interactions. The sensor shows that any tiny change in the refraction index will induce a significant color variation and the sensitivity reaches 683.5 nm per refraction index unit with a figure of merit of 3.5. The color of the metamaterials changes from rose-red to violet and then loden after a monomolecular layer of thiolated biotin and streptavidin bind to the surface of the nanostructure successively. This sensing strategy offers new opportunities for the convenient detection of proteins, nucleic acids, and lipids.

Designable Spectrometer-Free Index Sensing Using Plasmonic Doppler Gratings

Typical nanoparticle-based plasmonic index sensors detect the spectral shift of localized surface plasmon resonance (LSPR) upon the change of the environmental index. Therefore, they require broadband illumination and spectrometers. The sensitivity and flexibility of nanoparticle-based index sensors are usually limited because LSPR peaks are usually broad and the spectral position cannot be freely designed. Here, we present a fully designable index sensing platform using plasmonic Doppler gratings (PDGs), which provide broadband and azimuthal angle dependent grating periodicity. Different from LSPR sensors, PDG index sensors are based on the momentum matching between photons and surface plasmons via the lattice momentum of the grating. Therefore, the index change is translated into the variation of the in-plane azimuthal angle for photon-to-plasmon coupling, which manifests as directly observable dark bands in the reflection image. The PDG can be freely designed to optimally match the range of index variation for specific applications. In this work, we demonstrate PDG index sensors for large (n = 1.00-1.52) and small index variations (n = 1.3330-1.3650). The tiny and nonlinear index change of the water-ethanol mixture has been clearly observed and accurately quantified. Since the PDG is a dispersive device, it enables on-site and single-color index sensing without a spectrometer and provides a promising spectroscopic platform for on-chip analytical applications.

Quantifying Cell Confluency by Plasmonic Nanodot Arrays to Achieve Cultivating Consistency

The determination of cell confluency and subculture timing for cell culture consistency is crucial in the field of cell-based research, but there is no universal standard concerning optimal confluence. In this study, gold nanodot arrays on glass substrates were used as culture substrates, and their spectral shifts of localized surface plasmon resonance (LSPR) were employed to monitor cell growth and quantify cell confluency. Experiments including cell counting, metabolic activity, focal adhesion, and cell cycle were also performed to confirm the cell growth monitoring accuracy of the LSPR signals. The LSPR signal exhibited the same trends like the increase of cell numbers and cell metabolic activity and reached the maximum as the cell growth achieved confluency, suggesting its great capability as an effective indicator to predict suitable subculture timing. The proposed sensing approach is a noninterventional, nondestructive, real-time, and useful tool to help biologists quantify the optimal subculture timing, achieve cell culture consistency, and obtain reproducible experimental results efficiently.

A sensitive and low background fluorescent sensing strategy based on g-C3N4-MnO2 sandwich nanocomposite and liposome amplification for ricin detection

Ricin is an extremely potent ribosome-inactivating protein and serves as a likely food biocontaminant or biological weapon. Thus, simple, sensitive and accurate analytical assays capable of detecting ricin are urgently needed to be established. Herein, we present a novel method for ricin B-chain (RTB) detection by using two materials: (a) a highly efficient hybrid probe that was formed by linking a glucose oxidase (GOD)-encapsulated liposome (GOD-L) to magnetic beads (MBs) through hybridization between an aptamer and a blocker and (b) a new low-background g-C3N4-MnO2 sandwich nanocomposite that exhibits fluorescence resonance energy transfer (FRET) between the g-C3N4 nanosheet and MnO2. In the presence of RTB, the strong binding between RTB and the aptamer can release the blocker-linked liposome from the surface of the MBs. After magnetic separation, the decomposed liposome can release GOD to catalyze the oxidation of glucose, generating a certain amount of H2O2. Then, H2O2 can reduce MnO2 of the g-C3N4-MnO2 nanocomposite to Mn2+, which leads to the elimination of FRET. Thus, the fluorescence of the g-C3N4 nanosheet will be turned on. Because of the excellent signal amplification ability of liposome and the characteristic highly sensitive response of the g-C3N4-MnO2 nanocomposite toward H2O2, RTB could be detected sensitively based on the significantly enhanced fluorescent intensity. The linear range of detection was from 0.25 μg mL-1 to 50 μg mL-1 and the limit of detection (LOD) was 190 ng mL-1. Moreover, the proposed assay was successfully applied in the detection of the entire ricin toxin content in a castor seed.

Molecularly endowed hydrogel with an in silico-assisted screened peptide for highly sensitive small molecule harvesting

Schematic representation of in silico-assisted screening of an AFM1 binding peptide and the working principle of toxin harvesting by molecularly endowed hydrogel.

Three-dimensional cavity nanoantennas with resonant-enhanced surface plasmons as dynamic color-tuning reflectors

As plasmonic antennas for surface-plasmon-assisted control of optical fields at specific frequencies, metallic nanostructures have recently emerged as crucial optical components for fascinating plasmonic color engineering. Particularly, plasmonic resonant nanocavities can concentrate lightwave energy to strongly enhance light-matter interactions, making them ideal candidates as optical elements for fine-tuning color displays. Inspired by the color mixing effect found on butterfly wings, a new type of plasmonic, multiresonant, narrow-band (the minimum is about 45 nm), high-reflectance (the maximum is about 95%), and dynamic color-tuning reflector is developed. This is achieved from periodic patterns of plasmonic resonant nanocavities in free-standing capped-pillar nanostructure arrays. Such cavity-coupling structures exhibit multiple narrow-band selective and continuously tunable reflections via plasmon standing-wave resonances. Consequently, they can produce a variety of dark-field vibrant reflective colors with good quality, strong color signal and fine tonal variation at the optical diffraction limit. This proposed multicolor scheme provides an elegant strategy for realizing personalized and customized applications in ultracompact photonic data storage and steganography, colorimetric sensing, 3D holograms and other plasmon-assisted photonic devices.