A Highly Sensitive Dual-Core Photonic Crystal Fiber Based on a Surface Plasmon Resonance Biosensor with Silver-Graphene Layer

A highly sensitive dual-core photonic crystal fiber based on a surface plasmon resonance (PCF-SPR) biosensor with a silver-graphene layer is described. The silver layer with a graphene coating not only prevents oxidation of the silver layer but also can improve the silver sensing performance due to the large surface-to-volume ratio of graphene. The dual-core PCF-SPR biosensor is numerically analyzed by the finite-element method (FEM). An average spectral sensitivity of 4350 nm/refractive index unit (RIU) in the sensing range between 1.39 and 1.42 and maximum spectral sensitivity of 10,000 nm/RIU in the sensing range between 1.43 and 1.46 are obtained, corresponding to a high resolution of 1 × 10⁻⁶ RIU as a biosensor. Our analysis shows that the optical spectra of the PCF-SPR biosensor can be optimized by varying the structural parameters of the structure, suggesting promising applications in biological and biochemical detection.

     

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.

     

Coupled waveguide-surface plasmon resonance biosensor with subwavelength grating

This study develops a coupled waveguide-surface plasmon resonance (CWSPR) biosensor with a subwavelength grating structure for the real-time analysis of biomolecular interactions. In the proposed optical metrology system, normally incident white light is coupled into the waveguide layer through the subwavelength grating structure thereby enhancing the wave vector which excites the surface plasmons on the metal sensing surface. The proposed CWSPR biosensor not only retains the same sensing sensitivity as that of a conventional surface plasmon resonance device, but also yields a sharper dip in the reflectivity spectrum and therefore provides an improved measurement precision. Moreover, the metrology setup overcomes the limitations of the conventional Kretschmann attenuated total reflection approach and is less sensitive to slight variations in the angle of the incident light. The experimental results confirm that the current CWSPR biosensor provides a straightforward yet powerful technique for real-time biomolecular interaction analysis.     

Resonance of Different Waveguide Structures with Various Vertical Indirect Coupled Cavities

In the paper, resonances of different waveguide structures with various vertical indirect coupled cavities were investigated by FDTD (finite difference-time domain). In the silicon cavity, Fano resonance could be observed at about 1430 nm. The coupling distance for the gold cavity/air cavity had less effect on the transmittance of the main waveguide but had a great influence on the transmission for water cavity in the visible region, which showed that water cavity could adjust resonance of waveguide structures. In addition, with the increment of refractive index n, the resonance peak at about 850 nm moved to the long wavelength (redshift). Dispersion rate about 2 × 10^–3/nm indicated that the transparent dielectric selectively absorbed the surface plasmon polariton wave and the sensitivity of the waveguide structure designed in this paper has high stability for the refractive index of the main waveguide cavity. Obvious Fano resonance could be observed with the increase of refractive index for silicon cavity. Among the four dielectrics, silicon and water are suitable for studying Fano resonance and filter dielectrics.

     

Surface plasmon resonance based biosensor technique: A review

Optical Surface plasmon resonance (SPR) biosensors represent the most advanced and developed optical label‐free biosensor technology. Optical SPR biosensors are a powerful detection and analysis tool that has vast applications in environmental protection, biotechnology, medical diagnostics, drug screening, food safety and security. This article reviews the recent development of SPR biosensor techniques, including bulk SPR and localized SPR (LSPR) biosensors, for detecting interactions between an analyte of interest in solution and a biomolecular recognition. The concepts of bulk and localized SPs and the working principles of both sensing techniques are introduced. Major sensing advances on biorecognition elements, measurement formats, and sensing platforms are presented. Finally, the discussions on both biosensor techniques as well as comparison of both SPR sensing techniques are made. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Portable Capillary Sensor Integrated with Plasmonic Platform for Monitoring Water Pollutants

In this work, a label-free and inexpensive method for the monitoring of water pollutants is demonstrated. We introduce a localized surface plasmon resonance (LSPR) based plasmonic capillary optical biosensor to detect microalgae cells. Here, the plasmonic capillary biosensor was prepared by decorating the inner walls of a glass capillary with gold nanoparticles that were employed for investigations. Since the gold nanoparticle has the potential to sense pollutants in water rapidly with high sensitivity and they are expected to perform a significant role in environmental monitoring. Our proposed plasmonic capillary sensor has a detection limit of 25 algal cells (Chlorella sp. CB4). Furthermore, the plasmonic capillary sensing platform significantly simplifies sensor fabrication and reduces the cost of the device. We believe that the presented plasmonic sensor could stand as a potential candidate for developing a cost-effective, label-free, and rapid sensing platform to detect microalgae pollutants present in the water at very low concentrations.

     

DNA Detection Based on Localized Surface Plasmon Resonance Spectroscopy of Ag@Au Biocomposite Nanoparticles

Noble metal nanoparticles (NPs) have attracted much attention due to their unique physical and chemical properties such as tunable surface plasmonics, high-efficiency electrochemical sensing, and enhanced fluorescence. We produced two biosensor chips consisting of Ag@Au bimetallic nanoparticles (BNPs) on a carbon thin film by simple RF-sputtering and RF-plasma-enhanced chemical vapor co-deposition. We deposited Au NPs with average size of 4 nm (Au₁ NPs) or 11 nm (Au₂ NPs) on a sensor chip consisting of Ag NPs with mean size of 15 nm, and we investigated the effect of shell size (Au NPs) on the chemical activities of the resulting Ag@Au₁ BNPs and Ag@Au₂ BNPs. We estimated the average size and morphology of Ag@Au BNPs by scanning electron microscopy (SEM) and atomic force microscopy (AFM) images. X-ray diffraction (XRD) patterns revealed that Ag NPs and Au NPs had face-centered cubic (FCC) structure. We studied aging of the biosensor chips consisting of Ag@Au BNPs by localized surface plasmon resonance (LSPR) spectroscopy for up to 3 months. UV–visible aging of the prepared samples indicated that Ag@Au₁ BNPs, which corresponded to Ag NPs covered with smaller Au NPs, were more chemically active than Ag@Au₂ BNPs. Furthermore, we evaluated changes in the LSPR absorption peaks of Ag@Au₁ BNPs and bare Ag NPs in the presence of a DNA primer decamer at fM concentrations, to find that Ag@Au₁ BNPs were more sensitive biosensor chips within a short response time as compared to bare Ag NPs.

     

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.     

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.     

A label-free immunosensor for determination of salbutamol based on localized surface plasmon resonance biosensing

We developed a localized surface plasmon resonance (LSPR)-based label-free optical biosensor for detection of salbutamol (Sal). Hollow gold nanoparticles (HGNs) which deposited on transparent indium tin oxide (ITO) film coated glass was used to sensing platform. Antibody against Sal was immobilized on HGN surface to recognize the target Sal molecules. Thus, the change of LSPR peak was proportional to the concentration of Sal in the solution. The experimental results demonstrated that the LSPR immunosensor possessed a good sensitivity and a high selectivity for Sal. The detection range for Sal was from 0.05 to 0.8 μg/mL with a correlation coefficient of 0.996. The biosensor was applied for the detection for Sal in spiked animal feed and pork liver samples, and the recoveries were in the range of 97–105 %. Therefore, it is expected that this approach may offer a new method in designing label-free LSPR immunosensor for detection of small molecules.     

Remote Excitation Surface Plasmon and Consequent Enhancement of Surface-Enhanced Raman Scattering Using Evanescent Wave Propagating in Quasi-One-Dimensional MoO3 Ribbon Dielectric Waveguide

We report experimentally the remote excitation surface plasmon and consequent enhancement of surface-enhanced Raman scattering (SERS) using evanescent wave propagating in quasi-one-dimensional (Q1D) MoO₃ ribbon dielectric waveguide. The propagating dielectric waveguide along Q1D MoO₃ ribbon is realized experimentally, when the 632.8 nm laser radiates on the one side edge of Q1D MoO₃ ribbon. The remote excitation SERS spectra-enhanced by chemical and electromagnetic field mechanisms are measured, respectively, where silver (Ag) nanoparticles are excited by electromagnetic field after propagating 7.3 μm in the ribbon. The chemical mechanism for the remote excitation SERS is contributed from the charge transfer between the analyte molecule and MoO₃ ribbon. The electromagnetic field mechanism for the remote excitation SERS arises from the energy conversion from the propagating dielectric waveguide to the surface plasmon of Ag nanoparticles on the Q1D MoO₃ ribbon. It is important to reveal the mechanism of energy conversion from the propagating dielectric waveguide to the surface plasmon for potential applications in micro- and nanoscale devices.     

Detection of recombinant growth hormone by evanescent cascaded waveguide coupler on silica‐on‐silicon

An evanescent wave based biosensor is developed on the silica‐on‐silicon (SOS) with a cascaded waveguide coupler for the detection of recombinant growth hormone. So far, U ‐bends and tapered waveguides are demonstrated for increasing the penetration depth and enhancing sensitivity of the evanescent wave sensor. In this work, a monolithically integrated sensor platform containing a cascaded waveguide coupler with optical power splitters and combiners designed with S ‐bends and tapper waveguides is demonstrated for an enhanced detection of recombinant growth hormone. In the cascaded waveguide coupler, a large surface area to bind the antibody with increased penetration depth of evanescent wave to excite the tagged‐rbST is obtained by splitting the waveguide into multiple paths using Y splitters designed with s ‐bends and subsequently combining them back to a single waveguide through tapered waveguide and combiners. Hence a highly sensitive fluoroimmunoassay sensor is realized. Using the 2D FDTD (Finite‐difference time‐domain method) simulation of waveguide with a point source in Rsoft FullWAVE, the fluorescence coupling efficiency of straight and bend section of waveguide is analyzed. The sensor is demonstrated for the detection of fluorescently‐tagged recombinant growth hormone with the detection limit as low as 25 ng/ml. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Integrated-optic biosensor by electro-optically modulated surface plasmon resonance

We present a new integrated-optic surface plasmon resonance (SPR) biosensor based on electro-optical modulation. The SPR characteristics for the analyte concentration detection can be electro-optically modulated by applying the voltage on the electrodes of the biosensor fabricated on lithium niobate, which is an excellent electro-optic material. Two measurement methods, electro-optically modulated SPR spectral measurement and electro-optically modulated SPR intensity measurement, are demonstrated and their measurands are the SPR wavelength and the output optical intensity, respectively. Human serum albumin is coated on the gold film surface of the proposed biosensor to detect the concentration of beta-blocker, which is a remedy for heart disease. As the applied voltage increases such that the effective index of guided mode rises, the SPR wavelength shifts toward the long wavelength side and the output optical intensity at the wavelength of 632.8 nm diminishes. The linear regression slope of the relation between the measurand and the applied voltage is dependent on the analyte concentration and can be used to determine the concentration variation. Experimental results measured by the electro-optically modulated SPR methods are compared with those measured by the conventional spectral and intensity methods, and the effects of waveguide width on the biosensor performance are discussed.     

Nanoscale porous silicon waveguide for label-free DNA sensing

Porous silicon (PSi) is an excellent material for biosensing due to its large surface area and its capability for molecular size selectivity. In this work, we report the experimental demonstration of a label-free nanoscale PSi resonant waveguide biosensor. The PSi waveguide consists of pores with an average diameter of 20nm. DNA is attached inside the pores using standard amino-silane and glutaraldehyde chemistry. Molecular binding in the PSi is detected optically based on a shift of the waveguide resonance angle. The magnitude of the resonance shift is directly related to the quantity of biomolecules attached to the pore walls. The PSi waveguide sensor can selectively discriminate between complementary and non-complementary DNA. The advantages of the PSi waveguide biosensor include strong field confinement and a sharp resonance feature, which allow for high sensitivity measurements with a low detection limit. Simulations indicate that the sensor has a detection limit of 50nM DNA concentration or equivalently, 5pg/mm2.     

Resonant TE Transmission Through a Continuous Metal Film: Perspectives for Low-Loss Plasmonic Elements

Whereas resonant transverse magnetic transmission across an undulated continuous metal film is achieved with the mediation of plasmon modes excited by the undulation, it is shown here that transverse electric (TE) resonant transmission through a continuous metal film can also be achieved with the mediation of the second-order TE₁ mode of a dielectric slab waveguide having the metal film sandwiched at its middle. The demonstration is made by using the materials currently used in the domain of optical security and counterfeit deterrence: ZnS is shown to possibly be a lossless interface/adhesion layer between a polymer and a noble metal for plasmonic resonant elements.     

Enhanced SPR Sensitivity with Nano-Micro-Ribbon Grating—an Exhaustive Simulation Mapping

In this study, we theoretically investigate the sensing potential of 2D nano- and micro-ribbon grating structuration on the surface of Kretschmann-based surface plasmon resonance (SPR) biosensors when they are employed for detection of biomolecular binding events. Numerical simulations were carried out by employing a model based on the hybridization of two classical methods, the Fourier modal method and the finite element method. Our calculations confirm the importance of light manipulation by means of structuration of the plasmonic thin film surfaces on the nano- and micro-scales. Not only does it highlight the geometric parameters that allow the sensitivity enhancement compared with the response of the conventional SPR biosensor based on a flat surface but also describes the transition from the regime where the propagating surface plasmon mode dominates to the regime where the localized surface plasmon mode dominates. An exhaustive mapping of the biosensing potential of the 2D nano- and micro-structured biosensors surface is presented, varying the structural parameters related to the ribbon grating dimensions, i.e., the widths and thicknesses. The nano- and micro-structuration also leads to the creation of regions on biosensor chips that are characterized by strongly enhanced electromagnetic (EM) fields. New opportunities for further improving the sensitivity are offered if localization of biomolecules can be carried out in these regions of high EM fields. The continuum of nano- and micro-ribbon structured biosensors described in this study should prove a valuable tool for developing sensitive and reliable 2D-structured plasmonic biosensors.     

A "do-it-yourself" array biosensor

We have developed an array biosensor for the simultaneous detection of multiple targets in multiple samples within 15-30 min. The biosensor is based on a planar waveguide, a modified microscope slide, with a pattern of small (mm2) sensing regions. The waveguide is illuminated by launching the emission of a 635 nm diode laser into the proximal end of the slide via a line generator. The evanescent field excites fluorophores bound in the sensing region and the emitted fluorescence is measured using a Peltier-cooled CCD camera. Assays can be performed on the waveguide in multichannel flow chambers and then interrogated using the detection system described here. This biosensor can detect many different targets, including proteins, toxins, cells, virus, and explosives with detection limits rivaling those of the ELISA detection system.     

Construction of d-amino acid biosensor based on d-amino acid oxidase immobilized onto poly (indole-5-carboxylic acid)/zinc sulfide nanoparticles hybrid film

d-Amino acid oxidase (DAAO) purified from goat kidney was immobilized covalently via N-ethyl-N-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxy succinimide (NHS) chemistry onto poly indole 5-carboxylic acid (Pin5-COOH)/zinc sulfide nanoparticles (ZnSNPs) hybrid film electrodeposited on surface of an Au electrode. A highly sensitive d-amino acid biosensor was constructed using this enzyme electrode as working electrode, Ag/AgCl as reference electrode, and Pt wire as auxiliary electrode connected through potentiostat. The biosensor showed optimum response within 3 s at pH 7.5 and 35 °C, when polarized at 0.15 V vs. Ag/AgCl. There was a linear relationship between biosensor response (mA) and d-alanine concentration in the range 0.001–2.0 mM. The sensitivity of the biosensor was 58.85 μA cm^?2 mM^?1 with a detection limit of 0.001 mM (S/N = 3). The enzyme electrode was used 120 times over a period of 2 months when stored at 4 °C. The biosensor has an advantage over earlier enzyme sensors that it has no leakage of enzyme during reuse and is unaffected by the external environment due to the protective layer of poly indole-5-carboxylic acid film. The biosensor was evaluated and employed for measurement of d-amino acid level in fruits and vegetables.     

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.

     

Hybrid Dielectric-Loaded Plasmonic Waveguide-Based Power Splitter and Ring Resonator: Compact Size and High Optical Performance for Nanophotonic Circuits

The key challenge of the plasmonic waveguide is to achieve simultaneously both the long propagation length and high confinement. The hybrid dielectric-loaded plasmonic waveguide consists of a SiO₂ stripe sandwiched between a Si-nanowire and a silver film and thus promises as a best candidate to overcome this challenge. We propose to exploit this unique property of this structure to design different high-efficient silicon-based plasmonic components including waveguide, power splitter, and wavelength-selective ring resonator. As a result, the proposed power splitter with a waveguide cross section (λ ²/60) and a strong mode confinement area (~λ ²/240) features a low power transmission loss (<0.4 dB) at the optimal arm length of 4 μm with respect to different separation distances of output arms. Moreover, we also demonstrate that a plasmonic ring resonator with a compact ring radius of 2 μm may achieve high optical performance such as high-extinction ratio of 30 dB, large free spectral range of 67 nm, and small bandwidth of 0.6 nm. These superior performances make them promising building blocks for integrated nanophotonic circuits.