Optimization of Surface Plasmon Resonance Biosensor with Ag/Au Multilayer Structure and Fiber-Optic Miniaturization

In this paper, we report a novel wavelength interrogation-based surface plasmon resonance (SPR) system, in which a film of three Ag layers and three Au layers are alternately deposited on a Kretschmann configuration as sensing element. This multilayer film shows higher sensitivity for refractive index (RI) measurement by comparing with single Au layer structure, which is consistent with its theoretical calculation. A sensitivity range of 2056–5893 nm/RIU can be achieved, which is comparable to RI sensitivities of other wavelength-modulated SPR sensors. Compared with Ag film, this Ag/Au multilayer arrangement offers anti-oxidant protection. This SPR biosensor based on a cost-effective Ag/Au multilayer structure is applicable to the real-time detection of specific interactions and dissociation of low protein concentrations. To extend the application of this highly-sensitive metal film device, we integrated this concept on an optical fiber. The range of RI sensitivities with Ag/Au multilayer was 1847–3309 nm/RIU. This miniaturized Ag/Au multilayer-based fiber optic sensor has a broad application in chemical and biological sensing.     

A High Performance Plasmonic Sensor Based on Metal-Insulator-Metal Waveguide Coupled with a Double-Cavity Structure

A high performance plasmonic sensor based on a metal-insulator-metal (MIM) waveguide coupled with a double-cavity structure consisting of a side-coupled rectangular cavity and a disk cavity is proposed. The transmission characteristics of the rectangular cavity and disk cavity are analyzed theoretically and the improvements of performance for the double-cavity structure compared with a single cavity are studied. The influence of structural parameters on the transmission spectra and sensing performance are investigated in detail. A sensitivity of 1136 nm/RIU with a high figure of merit of 51,275 can be achieved at the resonant wavelength of 1148.5 nm. Due to the high performance and easy fabrication, the proposed structure may be applied in integrated optical circuits and on-chip nanosensors.     

Tunable Surface Plasmon Resonance Sensor Based on Photonic Crystal Fiber Filled with Gold Nanoshells

We present a photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor, whose operating wavelength range is tunable. Gold nanoshells, consisting of silica cores coated with thin gold shells, are designed to be the functional material of the sensor because of their attractive optical properties. It is demonstrated that the resonant wavelength of the sensor can be precisely tuned in a broad range, 660 nm to 3.1 μm, across the visible and near-infrared regions of the spectrum by varying the diameter of the core and the thickness of the shell. Furthermore, the effects of structural parameters of the sensor on the sensing properties are systematically analyzed and discussed based on the numerical simulations. It is observed that a high spectral sensitivity of 4111.4 nm/RIU with the resolution of 2.45 × 10^?5 RIU can be achieved in the sensing range of 1.33–1.38. These features make the sensor of great importance for a wide range of applications, especially in biosensing.     

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.     

Highly Sensitive SPR Sensor Based on D-shaped Photonic Crystal Fiber Coated with Indium Tin Oxide at Near-Infrared Wavelength

A surface plasmon resonance (SPR) sensor based on D-shaped photonic crystal fiber (PCF) coated with indium tin oxide (ITO) film is proposed and numerically investigated. Thanks to the adjustable complex refractive index of ITO, the sensor can be operated in the near-infrared (NIR) region. The wavelength sensitivity, amplitude sensitivity, and phase sensitivity are investigated with different fiber structure parameters. Simulation results show that ～6000 nm/refractive index unit (RIU), ～148/RIU, and ～1.2?×?10⁶ deg/RIU/cm sensitivity can be achieved for wavelength interrogation, amplitude interrogation, and phase interrogation, respectively, when the environment refractive index varies between 1.30 and 1.31. It is noted that the wavelength sensitivity and phase sensitivity are more pronounced with larger refractive index. The proposed SPR sensor can be used in various applications, including medicine, environment, and large-scale targets detection.     

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.     

Search of Extremely Sensitive Near-Infrared Plasmonic Interfaces: A Theoretical Study

The sensitivity of the wavelength position of localized surface plasmon resonance (LSPR) in metal nanostructures to local changes in the refractive index has been widely used for label-free detection strategies. Tuning the optical properties of the nanostructures from the visible to the infrared region is expected to have a drastic effect on the refractive index sensitivity. Here, we theoretically investigate the optical response of a newly designed plasmonic interface to changes in the bulk refractive index by the finite difference time domain method. It consists of a structured interface, where the planar interface is superposed with dielectric pillars 30 nm in height and 125 nm in length with a separation distance of 15 nm. The pillars are covered with U-shaped gold nanostructures of 50 nm in height, 125 nm in length, and 5 nm of gold base thickness. The whole structure is finally covered with a 5-nm thick dielectric layer of n ₂?=?2.63. This plasmonic structure shows bulk refractive index sensitivities up to 1750 nm/RIU (RIU : refractive index unit) in the near infrared (λ?=?2621 nm). The enhanced sensitivity is a consequence of the extremely enhanced electrical field between the gold nanopillars of the plasmonic interface.     

Highly Sensitive Side-Polished Birefringent PCF-Based SPR Sensor in near IR

We propose a highly sensitive side-polished birefringent photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR). The polished surface of the proposed structure is coated with indium tin oxide (ITO) to excite plasmon and the analytes can be placed on the flat surface easily instead of filling the voids. The birefringent nature of the structure helps in coupling more fields to the ITO-dielectric interface. With the optimum thickness of 110 nm of ITO, the structure shows a maximum wavelength sensitivity of 17000 nm/RIU with a resolution of 5.8?×?10^?6 RIU. Further this also showed an amplitude sensitivity of 74 RIU^?1 along with a resolution of 1.35?×?10^?5 RIU. Moreover, the effect of bending on this low loss structure is also analyzed.     

Utilizing the Metallic Nano-Rods in Hexagonal Configuration to Enhance Sensitivity of the Plasmonic Racetrack Resonator in Sensing Application

A high sensitive plasmonic refractive index sensor based on metal-insulator-metal (MIM) waveguides with embedding metallic nano-rods in racetrack resonator has been proposed. The refractive index changes of the dielectric material inside the resonator together with temperature changes can be acquired from the detection of the resonance wavelength, based on their linear relationship. With optimum design and considering a tradeoff among detected power, structure size, and sensitivity, the finite difference time domain simulations show that the refractive index and temperature sensitivity values can be obtained as high as 2610 nm per refractive index unit (RIU) and 1.03 nm/°C, respectively. In addition, resonance wavelengths of resonator are obtained experimentally by using the resonant conditions. The effects of nano-rods radius and refractive index of racetrack resonator are studied on the sensing spectra, as well. The proposed structure with such high sensitivity will be useful in optical communications that can provide a new possibility for designing compact and high-performance plasmonic devices.     

A Novel Diamond Ring Fiber-Based Surface Plasmon Resonance Sensor

We propose a highly sensitive novel diamond ring fiber (DRF)-based surface plasmon resonance (SPR) sensor for refractive index sensing. Chemically active plasmonic material (gold) layer is coated inside the large cavity of DRF, and the analyte is infiltrated directly through the fiber instead of selective infiltration. The light guiding properties and sensing performances are numerically investigated using the finite element method (FEM). The proposed sensor shows a maximum wavelength and amplitude interrogation sensitivity of 6000 nm/RIU and 508 RIU^?1, respectively, over the refractive index range of 1.33–1.39. Additionally, it also shows a sensor resolution of 1.67 × 10^?5 and 1.97 × 10^?5 RIU by following the wavelength and amplitude interrogation methods, respectively. The proposed diamond ring fiber has been fabricated following the standard stack-and-draw method to show the feasibility of the proposed sensor. Due to fabrication feasibility and promising results, the proposed DRF SPR sensor can be an effective tool in biochemical and biological analyte detection.     

Temperature Sensor Based on Hollow Fiber Filled with Graphene-Ag Composite Nanowire and Liquid

A temperature sensor based on hollow fiber (HF) filled with graphene-Ag composite nanowire and liquid is presented. The coupling properties and sensing performance are numerically analyzed by finite element method using wavelength and amplitude interrogations. Results show that the sensor exhibiting strong birefringence with x-polarized peak provides much higher sensitivities and better signal-to-noise ratio (SNR) than y-polarized, which is more suitable for temperature detection. The graphene-Ag composite nanowire can not only solve the oxidation problem but also avoid the metal coating. Moreover, it provides better performance than other similar works like Au-Ag nanowire-filled, Au nanowire-filled, and Ag nanowire-filled sensors. Contrary to the blue shift of traditional SPR temperature sensors, the resonance peak shifts to the longer wavelength in our device when temperature increases and the high sensitivity 9.44 nm/ °C is obtained. The influences of nanowire diameter, grapheme-layer thickness on the designed sensor, are also investigated. This work can provide a reference for developing a high sensitivity, real-time, remote sensing, and distributed temperature SPR sensor.     

Design and Simulation of a Novel Tunable Terahertz Biosensor Based on Metamaterials for Simultaneous Monitoring of Blood and Urine Components

In this essay, a tunable metamaterial-based biosensor is proposed for simultaneous monitoring of blood components including cells, plasma, water, thrombus, and urine components as well as glucose, albumin, and urea. The proposed biosensor is based on optical sensors, and it provides real-time, label-free, and direct detection, small size, and cost-effectiveness that can be an alternative tool to other conventional methods. The influence of operating frequency, sample thickness, temperature, and radiation angle on the performance of the sensor is investigated by the finite element method (FEM). Numerical results show that the maximum sensitivity and figure of merit (FoM) in the high frequency are 500 (nm/RIU) and 2000, and for low frequency are 136 (µm/RIU) and 155, respectively. The footprint of the structure is 0.34 µm2, which is remarkably smaller than the other reported biosensing structures. The proposed biosensor has the potential to provide high sensitivity, high FoM, and a wide operating range for biomedical applications.

     

Design Analysis of Refractive Index Sensor with High Quality Factor Using Au-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Grating on Aluminum

We propose a surface plasmon resonance (SPR)-based refractive index sensor using gold-alumina grating over aluminum film for biosensing. Conventional SPR sensor based on gold grating exhibits broader SPR dips whereas that based on aluminum grating exhibits narrow reflection dip. A narrow reflection dip is desirable as it provides good resolution and improves the accuracy of measurement. Aluminum is less stable and generally is not preferred for an SPR-based sensor. It is more prone to being oxidized, which reduces the sensitivity and increases the width of the reflection dip of the sensor. While gold cannot provide narrow SPR reflection dips, but is used as an SPR active metal due to its more chemical stability. In order to improve the accuracy of gold grating-based sensor while taking care of oxidation problem of aluminum, in this paper, we propose a gold grating over aluminum film for SPR-based sensor and show that this configuration improves the sensitivity and the detection accuracy of the conventional sensor. Moreover, the oxidation problem is reduced to some extent as a part of aluminum is covered with gold. In order to completely avoid the oxidation of aluminum, we further propose to cover the exposed part of the aluminum with alumina and show that this configuration further improves the accuracy by reducing the width of the SPR reflection dip without affecting the sensitivity significantly. Numerical simulations show that sensitivity of proposed sensor is 270.33°/RIU with quality factor of more than 267.65 RIU^?1.     

Highly Sensitive Surface Plasmon Resonance Based D-Shaped Photonic Crystal Fiber Refractive Index Sensor

In this article, a D-shaped photonic crystal fiber based surface plasmon resonance sensor is proposed for refractive index sensing. Surface plasmon resonance effect between surface plasmon polariton modes and fiber core modes of the designed D-shaped photonic crystal fiber is used to measure the refractive index of the analyte. By using finite element method, the sensing properties of the proposed sensor are investigated, and a very high average sensitivity of 7700 nm/RIU with the resolution of 1.30 × 10^?5 RIU is obtained for the analyte of different refractive indices varies from 1.43 to 1.46. In the proposed sensor, the analyte and coating of gold are placed on the plane surface of the photonic crystal fiber, hence there is no necessity of the filling of voids, thus it is gentle to apply and easy to use.     

Coupled-Resonator-Induced Fano Resonances for Plasmonic Sensing with Ultra-High Figure of Merits

Fano resonances are numerically predicted in an ultracompact plasmonic structure, comprising a metal-isolator-metal (MIM) waveguide side-coupled with two identical stub resonators. This phenomenon can be well explained by the analytic model and the relative phase analysis based on the scattering matrix theory. In sensing applications, the sensitivity of the proposed structure is about 1.1?×?10³ nm/RIU and its figure of merit is as high as 2?×?10⁵ at λ?=?980 nm, which is due to the sharp asymmetric Fano line-shape with an ultra-low transmittance at this wavelength. This plasmonic structure with such high figure of merits and footprints of only about 0.2 μm² may find important applications in the on-chip nano-sensors.     

Ultra-High-Sensitive Sensor Based on Surface Plasmon Resonance Structure Having Si and Graphene Layers for the Detection of Chikungunya Virus

Chikungunya virus has been discovered in about 60 countries of the world. It leads to joint pain, joint swelling, headache, muscle pain, and fatigue of the human body. In this work, a surface plasmon resonance (SPR)based sensor is developed to detect chikungunya virus through normal and infected platelets and plasma blood cells. The proposed SPR-based sensor uses silicon and graphene layers coated over the base of a glass prism sputtered with a silver layer. The graphene layer has the advantage of enhancing the biomolecules adsorption on the metal layer. The silicon layer between silver and graphene enhances the sensor performance. The number of graphene layers along with the thicknesses of silicon and silver layers is optimized to get the highest sensitivity of the detector. To investigate the effect of the light source wavelength, simulations are performed for four different wavelengths. The highest sensitivities exhibited by the SPR-based sensor are 393 and 160 deg/RIU for the platelets and plasma cells, respectively.

     

One Dimensional Plasmonic Grating: High Sensitive Biosensor

We report a simple 1D grating device fabrication on ～50 nm gold (Au) film deposited on glass, which is employed as a high performance refractive index (RI) sensor by exploiting the surface plasmon polaritons (SPP) excited by the grating device along the Au/analyte interface. A finite element analysis (FEA) method is employed to maximize the sensitivity of the sensor for a fixed period and thickness of a gold film and its close correspondence with experiment has given the insight for high sensitivity and enhanced transmission. Significantly, in the context of economic design and performance, it is shown that an optimally designed and fabricated 1D grating can be as sensitive as 524 nm/RIU (linearity RI?=?1.33303 to 1.47399), which is remarkably higher than existing reports operating in a similar wavelength region.     

Plasmonic Biosensor Based on Triangular Au/Ag and Au/Ag/Au Core/Shell Nanoprisms onto Indium Tin Oxide Glass

Gold–silver core–shell triangular nanoprisms (Au/AgTNPs) were grown onto transparent indium tin oxide (ITO) thin film-coated glass substrate through a seed-mediated growth method without using peculiar binder molecules. The resulting Au/AgTNPs were characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, UV–vis spectroscopy, and cyclic voltammograms. The peak of dipolar plasmonic resonance was located at near infrared region of ～700 nm, which showed the refractive index (RI) sensitivity of 248 nm/RIU. Moreover, thin gold shells were electrodeposited onto the surface of Au/AgTNPs in order to stabilize nanoparticles. Compared with the Au/AgTNPs, this peak of localized surface plasmon resonance (LSPR) was a little red-shift and decreased slightly in intensity. The refractive index sensitivity was estimated to be 287 nm/RIU, which showed high sensitivity as a LSPR sensing platform. Those triangular nanoprisms deposited on the ITO substrate could be further functionalized to fabricate LSPR biosensors. Results of this research show a possibility of improving LSPR sensor by using core–shell nanostructures.     

Design of a High-Resolution Metal–Insulator–Metal Plasmonic Refractive Index Sensor Based on a Ring-Shaped Si Resonator

In this paper, a high-resolution refractive index sensor is proposed based on a novel metal–insulator–metal plasmonic topology. The structure is based on a Si nano-ring located inside a circular cavity. It acts as an optical notch filter with a quality factor equal to 269. The proposed filter topology is numerically simulated using the finite difference time domain method. It is shown that the proposed filter can also act as a refractive index sensor with a sensitivity of 636 nm/RIU and a fairly high figure of merit (FoM) equal to 211.3 RIU⁻¹. It is shown that the sensor can easily detect a refractive index change of ± 0.001 for dielectrics whose refractive index is between 1 and 1.2. For the refractive index range of 1.33 to 1.52, the maximum FoM of the sensor is 191 RIU⁻¹. The simplicity of the design and its high resolution are the two main features of the proposed sensor which make it a good candidate for biomedical applications.

     

Integrated Terahertz Surface Plasmon Resonance on Polyvinylidene Fluoride Layer for the Profiling of Fluid Reflectance Spectra

We design terahertz (THz) surface-plasmon-resonance (SPR) sensors using a ferroelectric polyvinylidene fluoride (PVDF) thin layer for biological sensing. The reflectivity properties based on SPR are described using transfer matrix method (TMM) and numerically simulated using finite-difference time domain (FDTD) method. The sensing characteristics of the structure are systematically analyzed through the examination of the reflectivity spectrum. The results reveal that the pronounced SPR resonance peak has quasi-linear relationship with the refractive index variation of the material under investigation. Through analyzing and optimizing the structural parameters of the THz SPR sensor, we achieved the theoretical value of the refractive index detection sensitivity as high as 0.393 THz/unit change of refractive index (RIU) for a 20-μm-thick liquid sample with a 10-μm PVDF layer. This work shows great promise toward realizing a THz SPR sensor with high sensitivity for identifying the signatures of biological fluid sample.