High-Sensitivity Refractive Index Sensor Based on D-Shaped Photonic Crystal Fiber with Rectangular Lattice and Nanoscale Gold Film

We propose and investigate a D-shaped photonic fiber refractive index sensor with rectangular lattice based on surface plasmon resonance. In such sensor, the nanoscale gold metal film is deposited on the flat surface where it is side polished. Numerical results show that the average sensitivity of Au-metalized surface plasmon resonance (SPR) sensor could reach as high as 8,129 nm/refractive index unit (RIU) in the dynamic index range from 1.35 to 1.41 as well as 2,000 nm/RIU from 1.33 to 1.35. Compared to conventional Au-metalized SPR sensors, the performance of our device is obviously better, and the production process is greatly simplified.     

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.     

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.     

Coupled plasmon-waveguide resonators: a new spectroscopic tool for probing proteolipid film structure and properties.

A variant of surface plasmon resonance (SPR) spectroscopy has been developed that involves a coupling of plasmon resonances in a thin metal film and waveguide modes in a dielectric overcoating. This new technique is referred to as coupled plasmon-waveguide resonance (CPWR) spectroscopy. It combines a greatly enhanced sensitivity (due to increased electromagnetic field intensities at the dielectric surface) and spectral resolution (due to decreased resonance linewidths), with the ability to directly measure anisotropies in refractive index and optical absorption coefficient in a dielectric film adsorbed onto the surface of the overcoating. Experimental data obtained with an egg phosphatidylcholine bilayer are presented to document these properties.     

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.

     

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.     

Improvement of Figure of Merit for Gold Nanobar Array Plasmonic Sensors

We report a strategy to improve two types of the figure of merit (FOM and FOM*) of the refractive index sensitivity of a gold nanobar array localized surface plasmon resonance (LSPR) biosensor by simply placing it close to a thin gold film with a dielectric spacer. The thickness of the dielectric spacer determines the plasmon coupling strength between the gold nanobars and the gold film and consequently the FOM and FOM* of the biosensor. From our calculations, when the spacer thickness is 20 nm, the FOM and FOM* reach maximal (4.68 and 310, respectively) and the sensitivity remains at a high value of 600 nm per refractive index unit. This biosensor scheme is practically realizable, and this strategy is also potentially applicable to the LSPR biosensors with other geometries.     

Ambiguous Refractive Index Sensitivity of Fano Resonance on an Array of Gold Nanoparticles

We investigate the optical response to refractive index changes of a Fano resonance occurring in a random array of gold nanoparticles supported on a glass substrate. The Fano resonance results from the interference between localized surface plasmon on a gold nanoparticle and the light reflected at the boundary of the glass substrate. We demonstrate that the sensitivity of the resonance to the refractive index of the surrounding medium is highly dependent on the excitation geometry and can assume either positive or negative values. We furthermore present a theoretical analysis explaining this behavior based on the rigorous coupled wave analysis (RCWA) as well as the island film theory.

     

Parallel-scan based microarray imager capable of simultaneous surface plasmon resonance and hyperspectral fluorescence imaging

With the development of the microarray technology, demands for array detection techniques become higher and higher. For many microarrays, several biomolecular interactions occur simultaneously and the interplay of various factors that affect these interactions remains poorly understood. Detecting such interactions with a single technique can often be a difficult and complicated process. In this work we propose a combined technique which enables simultaneous angle-interrogation surface plasmon resonance (SPR) sensing and hyperspectral fluorescence imaging. This tandem technique offers two-dimensional imaging of the whole array plane. The refractive index information obtained from SPR sensing and the physicochemical properties obtained from fluorescence imaging provide a comprehensive analysis of biological events on the array-chip. In addition, SPR and fluorescence detection techniques confirm each other in experimental results to exclude false-positive or false-negative cases. In terms of SPR sensing performance, the refractive index resolution is 3.86 × 10⁻⁶ refractive index units (RIU), and the detection limit is 10⁴ cfu/ml of Escherichia coli bacteria. The resolving power and detection sensitivity of fluorescence imaging are approximately 20 μm and 0.61 fluors/μm², respectively. Finally, two model experiments, detecting the DNA hybridization and biotin–avidin interactions respectively, demonstrate the biomedical application of this system.     

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.     

Comparison of Gold and Silver/Gold Bimetallic Surface for Highly Sensitive Near-infrared SPR Sensor at 1550 nm

A large majority of surface plasmon resonance (SPR) sensors reported in the literature are designed to operate in the visible electromagnetic spectrum. However, the near-infrared, particularly at the telecommunications wavelength of 1550 nm, is also especially attractive for SPR sensing applications. In fact, SPR sensors operating in this region benefit from narrower resonance and deeper field penetration. In this paper, we report a theoretical and experimental study of an SPR sensor operating at a fixed wavelength of 1550 nm. The influence of the choice of metals and the interrogation methods on the sensitivity of the resulting SPR sensor is investigated. Two types of sensor chips (simple gold (Au) and bimetallic silver/Au structure) and three interrogation methods (monitoring of the position of the reflectivity minimum, the position of the centroid, and the intensity evolution of the reflectivity) are examined. We show that a refractive index resolution of 2.7?×?10^?6 refractive index unit can be easily obtained, and with further optimization of the measurement system, the ultimate limit of detection is expected to be even lowered. Therefore, the approach discussed here already shows a promising potential for highly sensitive SPR sensors.     

Metallic Nanowire Array–Polymer Hybrid Film for Surface Plasmon Resonance Sensitivity Enhancement and Spectral Range Enlargement

In this paper, the coupling interaction is investigated between a metallic nanowire array and a metal film under the Kretschmann condition. The plasmonic multilayer is composed of a metallic nanowire array embedded in a polymer layer positioned above a metal film, exploiting the classical surface plasmon resonance (SPR) configuration. We analyze the influence of various structural parameters of the metallic nanowire array on the SPR spectrum of thin metal film. The results show that the coupling interactions of nanowires with the metal film can greatly affect SPR resonance wavelength and increase SPR sensitivity. The coupling strength of metallic nanowire array and metal film also impacts resonance wavelength, which can be used to adjust SPR range but have little effect on its sensitivity. The results are confirmed using a dipole coupling resonance model of metallic nanowire. We demonstrated that this nanostructured hybrid structure can be used for high sensitivity SPR monitoring in a large spectral range, which is important for advanced SPR measurement including fiber-optic SPR sensing technology.     

Improving the Sensitivity of Fiber Surface Plasmon Resonance Sensor by Filling Liquid in a Hollow Core Photonic Crystal Fiber

Inspired by the classic theory, we suggest that the performance of a D-shaped fiber optical surface plasmon resonance (SPR) sensor can be improved by manipulating the fiber core mode. To demonstrate this, we propose a novel fiber SPR sensor based on a hollow core photonic crystal fiber with liquid mixture filled in the core. The fiber sensor design involves a side-polished fiber with gold film deposited on the polished plane and liquid filling. Numerical simulation results suggest that by tuning the refractive index of the liquid mixture, the predicted sensitivity will be over 6,430 nm/refractive index unit for an aqueous environment, which is competitive for fiber chemical sensing. This optimization method may lead to an ultrahigh sensitivityfiber optical biosensor.     

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.     

Real-time protein biosensor arrays based on surface plasmon resonance differential phase imaging

This paper reports the application of differential phase surface plasmon resonance (SPR) imaging in two-dimensional (2D) protein biosensor arrays. Our phase imaging approach offers a distinct advantage over the conventional angular SPR technique in terms of utilization efficiency of optical sensor elements in the imaging device. In the angular approach, each biosensor site in the biosensor array requires a linear array of optical detector elements to locate the SPR angular dip. The maximum biosensor density that a two-dimensional imaging device can offer is a one-dimensional SPR biosensor array. On the other hand, the phase-sensitive SPR approach captures data in the time domain instead of the spatial domain. It is possible that each pixel in the captured interferogram represents one sensor site, thus offering high-density two-dimensional biosensor arrays. In addition, our differential phase approach improves detection resolution through removing common-mode disturbances. Experimental results demonstrate a system resolution of 8.8 x 10(-7)RIU (refractive index unit). Real-time monitoring of bovine serum albumin (BSA)/anti-BSA binding interactions at various concentration levels was achieved using a biosensor array. The detection limit was 0.77 microg/ml. The reported two-dimensional SPR biosensor array offers a real-time and non-labeling detection tool for high-throughput protein array analysis. It may find promising applications in protein therapeutics, drug screening and clinical diagnostics.     

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 Performance-Enhanced Bimetallic Chip for the Detection of Cadmium Ions with Surface Plasmon Resonance

We demonstrate that a designed bimetallic chip is capable of improving the performance of a surface plasmon resonance (SPR) sensor based on angular interrogation. Through a numerical simulation and a refractometry experiment, we prove that this bimetallic chip can effectively reduce the noise level by about a factor of 2 compared to the traditional SPR sensors that only use a single gold film. The bimetallic chip presents a lower refractive index resolution of 5.3 × 10⁻⁷ refractive index units. In addition, the enhancement of the electric field intensity at the surface of the configuration by a factor of 2 makes it possible to have a high sensitivity in a larger region, which promotes the biosensing applications of the chip. Through a simple and novel method for the detection of cadmium ions (Cd²⁺) based on the bimetallic configuration, a detection level for Cd²⁺ (0.01 μM or 1.12 ppb) can be realized, which compares favorably with similar studies.

     

Analysis of protein interactions on protein arrays by a wavelength interrogation-based surface plasmon resonance biosensor

We have investigated whether surface plasmon resonance (SPR) sensors based on the wavelength interrogation are able to analyze protein interactions on protein arrays. The spectral SPR sensor was self-constructed and its detection limit, expressed as the minimal refractive index variation, was calculated to be 6.6x10(-5) with the signal fluctuation of 1.0x10(-5). The protein array surface was modified by a mixed thiol monolayer to immobilize proteins. Protein arrays were analyzed by the line-scanning mode of the SPR sensor, which scanned every 100 microm along the central line of array spots and the scanned results were presented by color spectra from blue to red. Glutathione S-transferase (GST)-rac1 caused a concentration-dependent increase of SPR wavelength shift on protein arrays. The surface structure of the protein arrays was analyzed by atomic force microscopy. Specific interactions of antigens with antibodies were analyzed on the protein arrays by using three antibodies and eight proteins. These results suggest that the wavelength interrogation-based SPR sensor can be used as the biosensor for the high-throughput analysis of protein interactions on protein arrays.     

Determination of the Optical Thickness of sub 10-nm Thin Metal Films by SPR Experiments

We discuss the experimental data of surface plasmon resonance (SPR) occurring at the interface between air and single and bimetallic thin layers of Au and Ag prepared on glass substrates. The bilayer configuration allowed for the measurements of the optical constants of metallic films that are ultra thin; e.g., below 10 nm of thickness since SPR modes on such thin films in a single-layer configuration are shallow. We also discuss the effect of film thickness on SPR coupling. Thickness and refractive index of the films were determined by matching experimental SPR curves to the theoretical ones. Thickness and roughness of the films were also measured by atomic force microscopy. The results obtained by experimental measurements are in good agreement with AFM analysis.     

Long Range Surface Plasmons for Observation of Biomolecular Binding Events at Metallic Surfaces

A long range surface plasmon (LRSP) is an electromagnetic wave propagating along a thin metal film with an order of magnitude lower damping than conventional surface plasmon (SP) waves. Thus, the excitation of LRSP is associated with a narrower resonance and it provides larger enhancement of intensity of the electromagnetic field. In surface plasmon resonance (SPR) biosensors, these features allow a more precise observation of the binding of biomolecules in the proximity to the metal surface by using the (label-free) measurement of refractive index (RI) variations and by SP-enhanced fluorescence spectroscopy. In this contribution, we investigate LRSPs excited on a layer structure consisting of a fluoropolymer buffer layer, a thin gold film, and an aqueous sample. By implementing such structure in an SPR sensor, we achieved a 2.4- and 4.4-fold improvement of the resolution in the label-free and fluorescence-based detection, respectively, of the binding of biomolecules in the close proximity to the surface. Moreover, we demonstrate that the sensor resolution can be improved by a factor of 14 and 12 for the label-free and fluorescence-based detection, respectively, if the biomolecular binding events occur within the whole evanescent field of LRSP.