Increased Sensitivity and Extended Range of Distance Measurements in Spin-Labeled Membrane Proteins: Q-Band Double Electron-Electron Resonance and Nanoscale Bilayers

We report a significant methodological advance in the application of double electron-electron resonance (DEER) spectroscopy to measure long-range distances in spin-labeled membrane proteins. In the pseudo two-dimensional environment of proteoliposomes, a steep intermolecular background shapes DEER signals leading to long accumulation times, complicating data analysis and reducing the maximal measurable distances from 70 Å down to ∼40-50 Å. To eliminate these limitations, we took advantage of the homogeneity and monodispersity of a class of discoidal nanoscale phospholipid bilayers in conjunction with the micromolar DEER sensitivity at Q-band (34 GHz) microwave frequency. Spin-labeled mutants of the ABC transporter MsbA were functionally reconstituted at a ratio of one functional dimer per nanoscale apolipoprotein-bound bilayer (NABB). DEER echo intensities from NABB-reconstituted MsbA have linear baselines reflecting a three-dimensional spatial distribution. This results in an order-of-magnitude higher sensitivity at Q-band relative to proteoliposomes and restores the maximal observable distance effectively increasing experimental throughput. The advances described here set the stage for the use of DEER spectroscopy to analyze conformational dynamics of sample-limited eukaryotic membrane proteins.     

Ultra-High Sensitivity Nanosensor Based on Multiple Fano Resonance in the MIM Coupled Plasmonic Resonator

This paper proposes a compact plasmonic structure that is composed of a metal-insulator-metal (MIM) waveguide coupled with a groove and stub resonators, and then investigates it by utilizing the finite element method (FEM). Simulation results show that the interaction between the local discrete state caused by the stub resonator and the continuous spectrum caused by the groove resonator gives rise to one of the two Fano resonances, while the generation of the other resonance relies only on the groove. Meanwhile, the asymmetrical linear shape and the resonant wavelength can be easily tuned by changing the parameters of the structure. By adding stubs on the groove, we excited multiple Fano resonances. The proposed structure can serve as an excellent plasmonic sensor with a sensitivity of 2000 nm/RIU and a figure of merit of about 3.04?×?10³, which can find extensive applications for nanosensors.     

Actively Tunable Fano Resonance in H-Like Metal-Graphene Hybrid Nanostructures

Actively tunable Fano resonance has obvious advantages in applications such as chemical or biological sensors, switches, modulators, and optical filters. In this paper, we studied theoretically the actively tunable Fano resonance in H-like metal-graphene hybrid nanostructures at visible and near-infrared wavelengths. We found that the absorption spectrum of H-like metal-graphene hybrid nanostructures has two resonance peaks, and the absorption spectrum has an obvious blue shift compared with that of the H-like metal nanostructures without graphene. The optical properties of different nanostructures are explained by the electric field distribution. Then, the dependence of the Fano resonance on the nanostructure parameters, refractive index of host materials, and graphene Fermi energy is studied. The wavelength and intensity of absorption spectrum can be manipulated by adjusting the structure parameters and host materials. In addition, the wavelength and intensity of absorption spectrum can be manipulated actively by changing the Fermi energy levels of graphene. This study provides a method for designing the actively tunable Fano resonance in H-like metal-graphene hybrid nanostructures.

     

Design of Sub wavelength-Grating-Coupled Fano Resonance Sensor in Mid-infrared

Plasmonics - By introducing the sub-wavelength grating (SWG) waveguide in the long-range surface phonon resonance (LRSPhR) device, a mid-infrared Fano resonance is formed due to the coupling...     

Tunable Fano Resonance Based Mode Interference in Waveguide-Cavity-Graphene Hybrid Structure

In this paper, a graphene strip is introduced into a metal-insulator-metal (MIM)-integrated square cavity hybrid structure; the transmission spectra are theoretically investigated by the finite different time domain (FDTD) methods. An asymmetric Fano resonance dip that has high figure of merit (FOM) value appears in the transmission band. According to the multimode interference coupled mode theory (MICMT) analytical method, the Fano resonance originates from the coherent coupling between TM₁₀ cavity magnetic mode and graphene plasmonic resonance electric mode. The center wavelength, full width at half maximum (FWHM), and FOM value of the Fano resonance can be tuned dynamically by altering the Fermi level of the graphene. Through breaking the symmetry of the hybrid structure or introducing double graphene strips with different Fermi level into hybrid structure, double Fano resonance are realized. This study can provide some theoretical basis and design reference for designing ultrahigh sensitivity plasmonic sensor.

     

Transverse Magneto-Optical Kerr Effect in Strongly Coupled Plasmon Gratings

Enhancement of magneto-optical response by coupling of propagating with localized plasmons in a structure based on silver and bismuth-substituted ferrite garnet has been numerically studied. It is shown that the absolute value of the magneto-optical response in the examined structure reaches a high value of 0.04, and the structure has a reflection coefficient sufficiently high for a number of practical applications. The strong coupling between localized and propagating plasmons, which caused the significant enhancement of the magneto-optical response, was manifested in the reflection spectrum of the structure in the form of an asymmetric Fano-like resonance. The proposed structure, intended for operation in the near infrared range, is a promising one for solving various problems in magnonics and bionanophotonics.     

Magnetic-Based Double Fano Resonances in Au-SiO2-Si Multilayer Nanoshells

Plasmonics - Compared to metallic nanostructures employed in plasmonics, dielectric materials with high refractive index can directly engineer magnetic responses in addition to the electric...     

Magneto-Electric Double Fano Resonances in Hybrid Split Ring/Disk Hetero-Cavity

In this work, we conceive and demonstrate the magneto-electric double Fano resonances of a hetero-cavity composed of Si disk and Au split ring, where Si disk can provide additional magnetic responses besides electric responses. The interference between electric and magnetic responses in proposed hetero-cavity gives rise to magneto-electric double Fano resonances with magnetic and electric near-field enhancements. Dipole radiative enhancement is used to analyze magnetic and electric responses of hetero-cavity and the spectral features of hetero-cavity can be used to quantitatively characterize by coupled oscillator model. And the spectral tunability of magneto-electric double Fano resonances is investigated, highlighting a potential for applications in low-loss sensing and nanophotonic devices.     

Coupled SPP Modes on 1D Plasmonic Gratings in Conical Mounting

Plasmonics - Plasmonic nanostructures exhibit a variety of surface plasmon polariton (SPP) modes, with different characteristic properties. While a single metal dielectric interface supports a...     

A High-Performance Refractive Index Sensor Based on Fano Resonance in Si Split-Ring Metasurface

We present a high-performance refractive index sensor based on Fano resonance with a figure of merit (FOM) about 56.5 in all-dielectric metasurface which consists of a periodically arranged silicon rings with two equal splits dividing them into pairs of arcs of different lengths. A Fano resonance with quality factor ~133 and spectral contrast ratio ~100% arises from destructive interference of two antiphase electric dipoles in the two arcs of the split-ring. We can turn on and/or off the Fano resonance with a modulation depth nearly 100% at the operating wavelength of 1067 nm by rotating the polarization of incident light. We believe that our results will open up avenues for the development of applications using Fano resonance with dynamically controllability such as biochemical sensors, optical switching, and modulator.     

Ultra-high Sensitivity Plasmonic Nanosensor Based on Multiple Fano Resonance in the MDM Side-Coupled Cavities

We propose a compact plasmonic structure comprising a metal-dielectric-metal (MDM) waveguide coupled with a side cavity and groove resonators. The proposed system is investigated by the finite element method. Simulation results show that the side-coupled cavity supports a local discrete state and the groove provides a continuous spectrum, the interaction between them, gives rise to the Fano resonance. The asymmetrical line shape and the resonant wavelength can be easily tuned by changing the geometrical parameters of the structure. Moreover, we can extend this plasmonic structure by the double side-coupled cavities to gain the multiple Fano resonances. The proposed structure can serve as an excellent plasmonic sensor with a sensitivity of ～1900 nm/RIU and a figure of merit of about ～3.8?×?10⁴, which can find wide applications for nanosensors.     

Synchronous Tuning of Twined Resonance Modes with Controllable Spectral Separation in Plasmonic Gratings

We investigated angle-resolved tuning performance of the double resonance modes of waveguide metallic grating structures, where the incident angle was changed in the plane formed by the extending direction of the grating lines and the wave vector with the grating plane tilted with respect to the vertical axis. Double resonance modes were observed due to tilting of the grating, which were tuned simultaneously to the blue with increasing the angle of incidence. The spectral separation between the resonance modes can be adjusted simply by changing the tilting angle of the grating. Such a double resonance device is important for exploring multichannel optical filters, optical switching device, or sensors.     

Plasmonic Fano Resonance in Homotactic Aluminum Nanorod Trimer: the Key Role of Coupling Gap

Recently, the Fano effect of aluminum nanostructures has attracted a lot of attentions in several detector and sensor applications, but the role of coupling gap in it remains unintuitive. In this paper, a homotactic aluminum rod trimer (HART) is designed to form the plasmonic Fano resonances and visualize the important role of coupling gap size. The plasmon hybridization model and far field images were used to qualitatively describe the formation mechanism of Fano resonance. The simulation results intuitively show that the Fano dip of HART with a smaller coupling gap size has a higher red-shift speed when increasing the refractive index of surrounding environment or the length of HART with a fixed axial ratio (L_S/L_L = 0.6). Our study provides the insights to the key role of coupling gap in the performance of Fano structures.

     

Conformational Cycle of the ABC Transporter MsbA in Liposomes: Detailed Analysis Using Double Electron-Electron Resonance Spectroscopy

Driven by the energy of ATP binding and hydrolysis, ATP-binding cassette transporters alternate between inward- and outward-facing conformations, allowing vectorial movement of substrates. Conflicting models have been proposed to describe the conformational motion underlying this switch in access of the transport pathway. One model, based on three crystal structures of the lipid flippase MsbA, envisions a large-amplitude motion that disengages the nucleotide-binding domains and repacks the transmembrane helices. To test this model and place the crystal structures in a mechanistic context, we use spin labeling and double electron-electron resonance spectroscopy to define the nature and amplitude of MsbA conformational change during ATP hydrolysis cycle. For this purpose, spin labels were introduced at sites selected to provide a distinctive pattern of distance changes unique to the crystallographic transformation. Distance changes in liposomes, induced by the transition from nucleotide-free MsbA to the highest energy intermediate, fit a simple pattern whereby residues on the cytoplasmic side undergo 20-30 Å closing motion while a 7- to 10-Å opening motion is observed on the extracellular side. The transmembrane helices undergo relative movement to create the outward opening consistent with that implied by the crystal structures. Double electron-electron resonance distance distributions reveal asymmetric backbone flexibility on the two sides of the transporter that correlates with asymmetric opening of the substrate-binding chamber. Together with extensive accessibility analysis, our results suggest that these structures capture features of the motion that couples ATP energy expenditure to work, providing a framework for the mechanism of substrate transport.     

Characterization of Metabolites in IntactStreptomyces citricolorCulture Supernatants Using High-Resolution Nuclear Magnetic Resonance and Directly Coupled High-Pressure Liquid Chromatography–Nuclear Magnetic Resonance Spectroscopy

A novel NMR spectroscopic approach to the direct biochemical characterization of bacterial culture broths is presented. A variety of one- and two-dimensional 1H NMR spectroscopic methods were used to characterize low-molecular-weight organic components of broth supernatants from cultures of Streptomyces citricolor. By applying 1H NMR spectroscopy to analyze whole, untreated culture supernatants, it was possible to identify and monitor simultaneously a range of media substrates and excreted metabolites. Identified metabolites include 2-phenylethylamine, trehalose, succinate, acetate, uridine, and aristeromycin, a secondary metabolite with antibiotic properties. Directly coupled HPLC-NMR spectroscopy was also applied to the analysis of broth supernatants for the first time, to aid spectral assignments, especially where signals were extensively overlapped in the 1H NMR spectra of the whole broth mixtures. Two-dimensional NMR methods such as 1H-1H correlation spectroscopy, 1H-13C heteronuclear single quantum correlation, and 1H-13C heteronuclear multiple bond correlation aided the structure elucidation and peak assignments of individual components in the mixtures by providing information on 1H-1H coupling networks and 13C chemical shifts. This work shows that high-resolution NMR spectroscopic methods provide a rapid and efficient means of investigating microbial metabolism directly without invasive or destructive sample pretreatment.     

Surface Phonon Resonance Assisted Refractive Index Sensing in Mid-Infrared Wavelength Range

We propose a highly sensitive refractive index sensor based on the surface phonon resonance (SPhR) in the mid-IR spectral range. Surface phonon polaritons (SPhPs) are formed on polar dielectrics such as SiC in mid-IR wavelength range and can be excited with the help of a metallic grating at specific wavelength termed as resonance wavelength. The resonance wavelength of SPhP is significantly affected by the refractive index of the analyte medium placed over the grating. This forms the basis of a refractive index sensor. We have numerically evaluated the performance of such an SPhP-based refractive index sensor by using rigorous coupled wave analysis (RCWA) in terms of sensitivity, detection accuracy, and quality factor. The quality factor and detection accuracy of the sensor formed on SiC substrate are found to be 225.1 RIU^–1 (inverse of refractive index unit) and 6.75, respectively. We have also extended the study for other polar dielectric substrates cBN and GaN and observed considerable enhancement in the performance of the sensor for GaN. The values of quality factor and detection accuracy could be increased to 361.2 RIU^–1 and 10.84, respectively, by using GaN substrate. The proposed sensor finds applications in refractive index sensing of liquids and biomolecules having refractive index in the range 1.33–1.36.

     

Effect of Light Incident Angle on Fano Resonance Loss Mediation in Silver Nanoparticles Integrated Thin Silicon Wafers

In this work, we have studied the role of light incidence angle in the suppression of Fano resonance loss for mediating broadband light incoupling. At light normal incidence angle, the transmission loss of silver nanoparticles (Ag NPs) integrated 100 μm silicon (Si) wafer is reduced to ~?4 from ~?11%, and total reflectance is reduced to 32.7 from 38.7%. The reduction in reflectance is observed only in surface plasmon off-resonance region of the NPs, but the reflectance is enhanced slightly in surface plasmon resonance (SPR) region. With the change in light incident angles, we have observed a reduction in reflectance from NPs integrated silicon wafer at SPR region. At 40° light incident angle, the reflectance is reduced to 21 from 38.7% due to minimization of the Fano resonance in SPR region of the NPs. The Fano resonance loss reduction is explained based on modification in NPs’ dipole and quadrupole modes hybridization at the silicon interface with different light incident angles instead of normal incidence. Experimental observations are validated by simulating Ag NPs’ near-fields and angular distribution of far-fields at the silicon interface, scattering efficiency spectra at different light incident angles by finite difference time domain calculations.