Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Introduction <s> Mode coupling at bends in optical fibers supporting one or only a few guided modes is analyzed by considering the local normal modes for the corresponding straight waveguide. Matrix elements giving the strength of coupling between guided modes at a corner bend, and for coupling between guided modes and radiation modes, are calculated as a function of guiding strength for this "geometrical" effect. The correction to these matrix elements due to the longitudinal strain in a bent fiber is also determined. The increase in propagation constant for the fundamental mode of a fiber wrapped in a coil of constant radius is calculated from information on the coupling strengths and mode propagation constants. The phase shift and attenuation of the fundamental mode caused by a spatially periodic microbending of the fiber axis are also considered. Finally, potential applications of these effects in fiber-optic devices such as mode converters, phase shifters, switches, and sensors are discussed. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Introduction <s> Abstract A fiber-optic chemical sensor is presented which utilizes surface plasmon resonance excitation. The sensing element of the fiber has been fabricated by removing a section of the fiber cladding and symmetrically depositing a thin layer of highly reflecting metal onto the fiber core. A white-light source is used to introduce a range of wavelengths into the fiber optic. Changes in the sensed parameters (e.g., bulk refractive index, film thinkness and film refractive index) are determined by measuring the transmitted spectral-intensity distribution. Experimental results of the sensitivity and the dynamic range in the measurement of the refractive indices of aqueous solutions are in agreement with the theoretical model of the sensor. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Introduction <s> The optical probe of a scanning near-field optical microscope is shown to act as a point source of surface plasmon (SP) polaritons on gold and silver films. Plasmon excitation manifests itself by emission of light in the direction of the SP resonance angle, originating from an area with the shape of a dipole radiation pattern whose extension is given by the SP decay length. Interaction with selected, individual surface inhomogeneities gives rise to characteristic modifications of the emitted radiation, which provide detailed information about SP scattering, reflection, and interference phenomena. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Introduction <s> A fibre-optic evanescent field absorption sensor based on a U-shaped sensing probe is described. The influences of fibre core diameter, bending radius of the probe and the refractive index of the fluid on the sensitivity of the sensor are evaluated experimentally. The results are compared with the theoretical results obtained using geometrical optics and based on two-dimensional treatment. A good qualitative agreement is found between them. <s> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Introduction <s> Abstract Since the first application of the surface plasmon resonance (SPR) phenomenon for sensing almost two decades ago, this method has made great strides both in terms of instrumentation development and applications. SPR sensor technology has been commercialized and SPR biosensors have become a central tool for characterizing and quantifying biomolecular interactions. This paper attempts to review the major developments in SPR technology. Main application areas are outlined and examples of applications of SPR sensor technology are presented. Future prospects of SPR sensor technology are discussed. <s> BIB005 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Introduction <s> Surface plasmons are waves that propagate along the surface of a conductor. By altering the structure of a metal's surface, the properties of surface plasmons—in particular their interaction with light—can be tailored, which offers the potential for developing new types of photonic device. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics. <s> BIB006 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Introduction <s> Since the introduction of optical fiber technology in the field of sensor based on the technique of surface plasmon resonance (SPR), fiber-optic SPR sensors have witnessed a lot of advancements. This paper reports on the past, present, and future scope of fiber-optic SPR sensors in the field of sensing of different chemical, physical, and biochemical parameters. A detailed mechanism of the SPR technique for sensing purposes has been discussed. Different new techniques and models in this area that have been introduced are discussed in quite a detail. We have tried to put the different advancements in the order of their chronological evolution. The content of the review article may be of great importance for the research community who are to take the field of fiber-optic SPR sensors as its research endeavors. <s> BIB007 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Introduction <s> Surface plasmon resonance technique in collaboration with optical fiber technology has brought tremendous advancements in sensing of various physical, chemical, and biochemical parameters. In this review article, we present the principle of SPR technique for sensing and various designs of the fiber optic SPR probe reported for the enhancement of the sensitivity of the sensor. In addition, we present few examples of the surface plasmon resonance- (SPR-) based fiber optic sensors. The present review may provide researchers valuable information regarding fiber optic SPR sensors and encourage them to take this area for further research and development. <s> BIB008 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Introduction <s> In this paper we present a fiber optic surface plasmon resonance (SPR) sensor as a reusable, cost-effective and label free biosensor for measuring DNA hybridization and DNA-protein interactions. This is the first paper that combines the concept of a fiber-based SPR system with DNA aptamer bioreceptors. The fibers were sputtered with a 50nm gold layer which was then covered with a protein repulsive self-assembled monolayer of mixed polyethylene glycol (PEG). Streptavidin was attached to the PEG's carboxyl groups to serve as a versatile binding element for biotinylated ssDNA. The ssDNA coated SPR fibers were first evaluated as a nucleic acid biosensor through a DNA-DNA hybridization assay for a random 37-mer ssDNA. This single stranded DNA showed a 15 nucleotides overlap with the receptor ssDNA on the SPR fiber. A linear calibration curve was observed in 0.5-5 microM range. A negative control test did not reveal any significant non-specific binding, and the biosensor was easily regenerated. In a second assay the fiber optic SPR biosensor was functionalized with ssDNA aptamers against human immunoglobulin E. Limits of detection (2nM) and quantification (6nM) in the low nanomolar range were observed. The presented biosensor was not only useful for DNA and protein quantification purposes, but also to reveal the binding kinetics occurring at the sensor surface. The dissociation constant between aptamer and hIgE was equal to 30.9+/-2.9nM. The observed kinetics fully comply with most data from the literature and were also confirmed by own control measurements. <s> BIB009
Biosensors bring a solution to the demand for direct, accurate and in situ monitoring in numerous fields such as genomics, proteomics, medical diagnosis, environmental monitoring, food analysis and security. Label-free optical biosensors enable real-time and direct observation of molecular interactions without using labels, since they sense binding-induced refractive index changes. To this aim, they usually combine a biological receptor compound and a physical or physicochemical transducer. Optical methods of transduction are usually minimally invasive, safe and provide multi-dimensional detection based on wavelength, intensity, phase or polarization metrology. They rely on well-established technologies (light sources, detectors, etc.) available from both telecommunication and micro-nano technologies industries, at optical frequencies in the visible and near-infrared regions. These ranges also coincide with a wide range of physical properties of bio-related materials, which is therefore perfectly suited for effective interrogation. Within the available biosensor configurations (based on absorbance, reflectance, fluorescence, refractive index changes, among others), plasmonic devices combining dielectric and metal interfaces are particularly attractive. The strong sensitivity of the plasmon (oscillation of electrons at the metal-dielectric interface) propagation constant to the permittivity of bioreceptors grafted on the metal surface enables the detection and quantification of biochemical changes resulting from molecules binding. Practically, biochemical reactions are therefore measured by monitoring the effective refractive index shift of the so-called surface plasmon resonance (SPR). In a standard bulk approach, this is achieved with the Kretschmann prism that couples light beams from a glass medium to a thin metal layer above the critical angle so that light is totally reflected, as sketched in Figure 1 . In this condition, an evanescent wave extends in both the metal layer and the surrounding dielectric medium, with a penetration depth not exceeding a fraction of the light wavelength (λ)  . Under phase-matching conditions BIB003 BIB005 , part of the light couples to the plasmon, which decreases the reflection at a given angle (θ 0 in Figure 1 , where θ c denotes the critical angle of incidence). This device is usually interrogated either by varying the wavelength and keeping the incidence angle constant or by using monochromatic light and modifying the angle. In both cases, the polarization state of the light is set parallel to the incidence plane so that the plasmon wave is orthogonally polarized with respect to the interface. Biochemical reactions happening at the surrounding metal interface slightly modify the effective refractive index of the plasmon wave, which is measured through an SPR shift (corresponding to an angle change from θ 0 to θ 1 , as depicted in Figure 1 ). Such sensors present a surrounding refractive index (SRI) sensitivity of the order of 10 −6 -10 −7 RIU (refractive index unit)  BIB003 BIB005 BIB006 . Sensors 2017, 17, 2732 2 of 20 effective refractive index shift of the so-called surface plasmon resonance (SPR). In a standard bulk approach, this is achieved with the Kretschmann prism that couples light beams from a glass medium to a thin metal layer above the critical angle so that light is totally reflected, as sketched in Figure 1 . In this condition, an evanescent wave extends in both the metal layer and the surrounding dielectric medium, with a penetration depth not exceeding a fraction of the light wavelength (λ)  . Under phase-matching conditions BIB003 BIB005 , part of the light couples to the plasmon, which decreases the reflection at a given angle (θ0 in Figure 1 , where θc denotes the critical angle of incidence). This device is usually interrogated either by varying the wavelength and keeping the incidence angle constant or by using monochromatic light and modifying the angle. In both cases, the polarization state of the light is set parallel to the incidence plane so that the plasmon wave is orthogonally polarized with respect to the interface. Biochemical reactions happening at the surrounding metal interface slightly modify the effective refractive index of the plasmon wave, which is measured through an SPR shift (corresponding to an angle change from θ0 to θ1, as depicted in Figure 1 ). Such sensors present a surrounding refractive index (SRI) sensitivity of the order of 10 −6 -10 −7 RIU (refractive index unit)  BIB003 BIB005 BIB006 . While the Kretschmann prism configuration remains the most used today in commercial systems, optical fiber-based sensors bring numerous assets and are, therefore, the subject of intense research and development efforts. With their compactness and ease of connection, they provide remote operation in microliter volumes (or even below) of analytes and appear perfectly suited for in situ, or even possibly in vivo, diagnosis. In diagnosis, they can also assay different parameters simultaneously, either in line (sensing regions multiplexed along a single optical fiber) or in parallel, through the use of different fibers connected to a single read-out device. To excite SPR on the side of an optical fiber, a physical access to the core-guided light is locally required so that it can be brought into contact with the surrounding medium. A straightforward configuration for doing this is fiber bending BIB001 BIB004 . Other solutions expose the core to the surrounding medium through a polishing or etching of the cladding, totally or in part BIB007 BIB008 BIB009   . The first reported configuration is based on this mechanism and dates back to 1993 BIB002 . More advanced configurations make use of fiber gratings permanently photo-inscribed in the fiber core that enable light coupling to the surrounding medium. Metal-coated fiber gratings have been attracting ever-growing interest over the years, as they bring specific practical benefits such as:  tuning of the SPR mode-excitation wavelength via selective cladding mode coupling; (2) subsequent compatibility with telecommunication-grade optical fibers and equipment; (3) temperature self-compensation; (4) fiber integrity conservation; and (5) compatibility with mass production, among others. Hence, numerous developments are nowadays obtained with this technology since it enables SPR excitation in the telecommunication wavelength window of around 1550 nm. This article will describe their operating principle and review the main achievements obtained for sensing selective proteins and cells with such devices. It will focus on recent progress in this field, which slowly but surely paves the way towards the use of these devices for minimally-invasive diagnosis. While the Kretschmann prism configuration remains the most used today in commercial systems, optical fiber-based sensors bring numerous assets and are, therefore, the subject of intense research and development efforts. With their compactness and ease of connection, they provide remote operation in microliter volumes (or even below) of analytes and appear perfectly suited for in situ, or even possibly in vivo, diagnosis. In diagnosis, they can also assay different parameters simultaneously, either in line (sensing regions multiplexed along a single optical fiber) or in parallel, through the use of different fibers connected to a single read-out device.