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.     

Self-Referencing Plasmonic Array Sensors

A self-referencing plasmonic platform is proposed and analyzed. By introducing a thin gold layer below a periodic two-dimensional nano-grating, the structure supports multiple modes including localized surface plasmon resonance (LSPR), surface plasmon resonance (SPR), and Fabry-Perot resonances. These modes get coupled to each other creating multiple Fano resonances. A coupled mode between the LSPR and SPR responses is spatially separated from the sensor surface and is not sensitive to refractive index changes in the surrounding materials or surface attachments. This mode can be used for self-referencing the measurements. In contrast, the LSPR dominant mode shifts in wavelength when the refractive index of the surrounding medium is changed. The proposed structure is easy to fabricate using conventional lithography and electron beam deposition methods. A bulk sensitivity of 429 nm/RIU is achieved. The sensor also has the ability to detect nanometer thick surface attachments on the top of the grating.

     

Compact and low-cost biosensor based on novel approach to spectroscopy of surface plasmons

A high-performance surface plasmon resonance (SPR) sensor based on a novel approach to spectroscopy of surface plasmons is reported. This approach employs a special diffraction grating structure (referred to as surface plasmon resonance coupler and disperser, SPRCD) which simultaneously couples light into a surface plasmon and disperses the diffracted light for spectral readout of SPR signal. The developed SPRCD sensor consists of a miniature cartridge integrating the diffraction grating and microfluidics and a compact optical system which simultaneously acquires data from four independent sensing channels in the cartridge. It is demonstrated that the SPRCD sensor is able to measure bulk refractive index changes as small as 3 × 10⁻⁷ RIU (refractive index units) and to detect short oligonucleotides in concentrations down to 200 pM.     

Metallic Grating-Assisted Fiber Optic SPR Sensor with Extreme Sensitivity in IR Region

We theoretically propose a surface plasmon resonance (SPR)-based fiber optic refractive index (RI) sensor. A surface plasmon exciting metallic grating formed with the alternation of indium tin oxide (ITO) and silver (Ag) stripes is considered on the core of the fiber. A thin film of silicon is used as an overlay. Silicon film not only protects the metallic grating from oxidation but also enhances the field to improve the device sensitivity. The sensor is characterized in terms of sensitivity, detection accuracy (DA), figure of merit (FoM), and quality factor (QF). The maximum sensitivity in the RI range 1.33 to 1.38 refractive index unit (RIU) is reported to be?~25 µm/RIU in infra-red region of investigation.

     

SPR Label-Free Biosensor with Oxide-Metal-Oxide-Coated D-Typed Optical Fiber: a Theoretical Study

In this article, a surface plasmon resonance (SPR) biosensor based on D-typed optical fiber coated by Al₂O₃/Ag/Al₂O₃ film is investigated numerically. Resonance in near infrared with an optimized architecture is achieved. Refractive index sensitivity of 6558 nm/RIU (refractive index unit) and detection limit of 1.5 × 10⁻⁶ RIU, corresponding to 0.4357 nm/μM and detection limit of 23 nM in BSA (bovine serum albumin) concentration sensing, are obtained. The analysis of the performance of the sensor in gaseous sensing indicates that this proposed SPR sensor is much suitable for label-free biosensing in aqueous media.

     

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.     

A new surface plasmon resonance sensor for high-throughput screening applications

We report a new high-throughput surface plasmon resonance (SPR) sensor based on combination of SPR imaging with polarization contrast and a spatially patterned multilayer SPR structure. We demonstrate that this approach offers numerous advantageous features including high-contrast SPR images suitable for automated computer analysis, minimum crosstalk between neighboring sensing channels and inherent compensation for light level fluctuations. Applications of a laboratory prototype of the high-throughput SPR sensor with 108 sensing channels for refractometry and biosensing are described. In refractometric experiments, the noise-limited refractive index resolution of the system has been established to be 3 x 10(-6) refractive index unit (RIU). Experimental data on detection of human choriogonadotropin (hCG) suggest that in conjunction with monoclonal antibodies against hCG, the reported SPR imaging sensor is capable of detecting hCG at concentrations lower than 500 ng/ml.     

High-throughput SPR sensor for food safety

High-throughput surface plasmon resonance (SPR) biosensor for rapid and parallelized detection of nucleic acids identifying specific bacterial pathogens is reported. The biosensor consists of a high-performance SPR imaging sensor with polarization contrast and internal referencing (refractive index resolution 2 x 10(-7) RIU) and an array of DNA probes microspotted on the surface of the SPR sensor. It is demonstrated that short sequences of nucleic acids (20-23 bases) characteristic for bacterial pathogens such as Brucella abortus, Escherichia coli, and Staphylococcus aureus can be detected at 100 pM levels. Detection of specific DNA or RNA sequences can be performed in less than 15 min by the reported SPR sensor.     

Highly Sensitive Plasmonic Refractive Index Sensor Based on Dual D-Shaped Photonic Crystal Fiber with Aluminum Nitride-Silver Films

A dual-core and dual D-shaped photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor with silver and aluminum nitride (AlN) films is designed. The distribution characteristics of the electromagnetic fields of core and plasmon modes, as well as the sensing properties, are numerically studied by finite element method (FEM). The structure parameters of the designed sensor are optimized by the optical loss spectrum. The results show the resonance wavelength variation of 489 nm for the refractive index (RI) range of 1.36?~?1.42. In addition, a maximum wavelength sensitivity of 13,400 nm/RIU with the corresponding RI resolution of 7.46?×?10^?6 RIU is obtained in the RI range of 1.41?~?1.42. The proposed sensor with the merits of high sensitivity, low cost, and simple structure has a wide application in the fields of RI sensing, such as hazardous gas detection, environmental monitoring, and biochemical analysis.

     

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 Polarization-independent SPR Sensor Based on Photonic Crystal Fiber for Low RI Detection

A surface plasmon resonance (SPR) sensor based on a photonic crystal fiber (PCF) is proposed for low refractive index (RI) detection. The core of PCF is formed by two-layer air walls and either layer is composed of six identical sector rings with negative curvature. Plasmonic material gold (Au) is coated on the external cladding surface. Finite element method (FEM) is applied to investigate the performance of the SPR sensor. Results show that the sensor is independent of polarization due to the coincident coupling properties of the two polarized modes. Additionally, in low RI ranging from 1.20 to 1.33, the sensor keeps a high spectral sensitivity with an average value of 7738 nm/RIU. When RI varies from 1.32 to 1.33, the resolution reaches to its maximum of 8.3 × 10⁻⁶. The proposed sensor shows much significance in low RI detection, which is promising in real-time measurement for medical, water pollution, and humidity.

     

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.     

Bragg-Scattered Surface Plasmon Microscopy: Theoretical Study

We present a new approach to surface plasmon microscopy with high refractive index sensitivity and spatial resolution that is not limited by the propagation length of surface plasmons. It is based on a nanostructured metallic sensor surface supporting Bragg-scattered surface plasmons. We show that these non-propagating surface plasmon modes are excellently suited for spatially resolved observations of refractive index variations on the sensor surface owing to their highly confined field profile perpendicular to as well as parallel to the metal interface. The presented theoretical study reveals that this approach enables reaching similar refractive index sensitivity as regular surface plasmon resonance (SPR) microscopy and offers the advantage of improved spatial resolution when observing dielectric features with lateral size <10???m for the wavelength around 800?nm and gold as the SPR-active metal. This paper demonstrates the potential of Bragg-scattered surface plasmon microscopy for high-throughput SPR biosensing with high-density microarrays.     

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.     

Doped Single-Wall Carbon Nanotubes in Propagating Surface Plasmon Resonance-Based Fiber Optic Refractive Index Sensing

We propose a surface plasmon resonance (SPR)-based fiber coupled refractive index sensing probe utilizing single-wall carbon nanotubes (SWCNTs) as the upper most layer. The sensor is designed by considering indium tin oxide (ITO) film on the bare core of a multi-moded step-index fiber, followed by the deposition of silicon, and then by that of the highly doped bundled SWCNTs layers. The film thicknesses of different constituent layers are optimized with respect to the sensitivity and the detection accuracy of the sensor. The theoretical analysis results in high sensitivity of 9.78 μm per refractive index unit (μm/RIU) for the optimized probe in the infra-red (IR) region of the electromagnetic (EM) spectrum.     

Analysis of a Surface Plasmon Resonance Probe Based on Photonic Crystal Fibers for Low Refractive Index Detection

A photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) probe with gold nanowires as the plasmonic material is proposed in this work. The coupling characteristics and sensing properties of the probe are numerically investigated by the finite element method. The probe is designed to detect low refractive indices between 1.27 and 1.36. The maximum spectral sensitivity and amplitude sensitivity are 6 × 10³ nm/RIU and 600 RIU^?1, respectively, corresponding to a resolution of 2.8 × 10^?5 RIU for the overall refractive index range. Our analysis shows that the PCF-SPR probe can be used for lower refractive index detection.     

Platinum Layers Sandwiched Between Black Phosphorous and Graphene for Enhanced SPR Sensor Performance

Highly sensitive surface plasmon resonance (SPR) sensor consisting of Ag-Pt bimetallic films sandwiched with 2D materials black phosphorus (BP) and graphene over Pt layer in Kretschmann configuration is analyzed theoretically using the transfer matrix method. Numerical results show that upon suitable optimization of thickness of Ag-Pt layers and the number of layers of BP and graphene, sensitivity as high as 412°/RIU (degree/refractive index unit) can be achieved for p-polarized light of wavelength 633 nm. This performance can be tuned and controlled by changing the number of layers of BP and graphene. Furthermore, the addition of graphene and heterostructures of black phosphorus not only improved the sensitivity of the sensor but also kept the FWHM of the resonance curve much smaller than the conventional sensor utilizing Au as plasmonic metal and hence improved the resolution to a significant extent. We expect that this new proposed design will be useful for medical diagnosis, biomolecular detection, and chemical examination.

     

Proposition and Numerical Analysis of a Plasmonic Sensing Structure of Metallo-Dielectric Grating and Silver Nano-slabs in a Metal-Insulator-Metal Configuration

Plasmonics - We propose and numerically investigate a near-infrared surface plasmon resonance-based refractive index sensor having in unison an extremely high sensitivity (1719 nm/RIU) and...     

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.     

Ultra-narrow-band Infrared Absorbers Based on Surface Plasmon Resonance

Due to the advantage of improving the sensing performance, narrow-band metamaterial perfect absorbers (MPAs) have attracted much attention in the sensor field. Here, we propose an ultra-narrow-band infrared absorber (UNBIRA) based on localized surface plasmon resonance. The peak absorption of the UNBIRA exceeds 99% with the full width at half maximum (FWHM) of 1.94 nm and 6.32 nm for transverse electric (TE) wave and transverse magnetic (TM) wave in 1.5–1.8 μm. The corresponding Q-factors for TE wave and TM wave are 817 and 266, respectively. When used as an infrared refractive index sensor, the sensitivity of UNBIRA is as high as 1632.5 nm/RIU for TE wave and 1647.5 nm/RIU for TM wave. Accordingly, the figure of merits (FOMs) of 816.2/RIU for TE wave and 260.7/RIU for TM wave are achieved. This UNBIRA possesses a simple geometry structure and an excellent sensing performance, implying a great potential for application of ultra-narrow infrared sensing or detecting.