Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> As alternatives to conventional metals, new plasmonic materials offer many advantages in the rapidly growing fields of plasmonics and metamaterials. These advantages include low intrinsic loss, semiconductor-based design, compatibility with standard nanofabrication processes, tunability, and others. Transparent conducting oxides such as Al:ZnO, Ga:ZnO and indium-tin-oxide (ITO) enable many high-performance metamaterial devices operating in the near-IR. Transition-metal nitrides such as TiN or ZrN can be substitutes for conventional metals in the visible frequencies. In this paper we provide the details of fabrication and characterization of these new materials and discuss their suitability for a number of metamaterial and plasmonic applications. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> This paper reviews the state-of-the-art of grating fabrication in silica and polymer microstructured optical fibres. It focuses on the difficulties and challenges encountered during photo-inscription of such gratings and more specifically on the effect of the air hole lattice microstructure in the cladding of the fibre on the transverse coupling of the coherent writing light to the core region of the fibre. Experimental and computational quantities introduced thus far to assess the influence of the photonic crystal lattice on grating writing efficiency are reviewed as well, together with techniques that have been proposed to mitigate this influence. Finally, early proposals to adapt the microstructure in view of possibly enhancing multi-photon grating fabrication efficiency are discussed. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> A graphene coated microfiber Bragg grating (GMFBG) for gas sensing is reported in this Letter. Taking advantage of the surface field enhancement and gas absorption of a GMFBG, we demonstrate an ultrasensitive approach to detect the concentration of chemical gas. The obtained sensitivities are 0.2 and 0.5 ppm for NH3 and xylene gas, respectively, which are tens of times higher than that of a GMFBG without graphene for tiny gas concentration change detection. Experimental results indicate that the GMFBG-based NH3 gas sensor has fast response due to its highly compact structure. Such a miniature fiber-optic element may find applications in high sensitivity gas sensing and trace analysis. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> We demonstrate that etched fiber Bragg gratings (eFBGs) coated with single walled carbon nanotubes (SWNTs) and graphene oxide (GO) are highly sensitive and accurate biochemical sensors. Here, for detecting protein concanavalin A (Con A), mannose-functionalized poly(propyl ether imine) (PETIM) dendrimers (DMs) have been attached to the SWNTs (or GO) coated on the surface modified eFBG. The dendrimers act as multivalent ligands, having specificity to detect lectin Con A. The specificity of the sensor is shown by a much weaker response (factor of similar to 2500 for the SWNT and similar to 2000 for the GO coated eFBG) to detect non specific lectin peanut agglutinin. DM molecules functionalized GO coated eFBG sensors showed excellent specificity to Con A even in the presence of excess amount of an interfering protein bovine serum albumin. The shift in the Bragg wavelength (Delta lambda(B)) with respect to the lambda(B) values of SWNT (or GO)-DM coated eFBG for various concentrations of lectin follows Langmuir type adsorption isotherm, giving an affinity constant of similar to 4 x 10(7) M-1 for SWNTs coated eFBG and similar to 3 x 10(8) M-1 for the GO coated eFBG. (C) 2014 Elsevier B.V. All rights reserved. <s> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> We report the first excitation of surface plasmon waves at near-infrared telecommunication wavelengths using polymer optical fibers (POFs) made of poly(methyl methacrylate) (PMMA). For this, weakly tilted fiber-Bragg gratings (TFBGs) have been photo-inscribed in the core of step-index POFs and the fiber coated with a thin gold layer. Surface plasmon resonance is excited with radially polarized modes and is spectrally observed as a singular extinction of some cladding-mode resonances in the transmitted amplitude spectrum of gold-coated TFBGs. The refractometric sensitivity can reach ∼550 nm/RIU (refractive index unit) with a figure of merit of more than 2000 and intrinsic temperature self-compensation. This kind of sensor is particularly relevant to in situ operation. <s> BIB005 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> We report on the first inscription of fiber Bragg gratings (FBGs) in cyclic transparent optical polymer (CYTOP)-perfluorinated polymer optical fibers (POFs). We have used a direct write method with a femtosecond laser operating in the visible. The FBGs have a typical reflectivity of 70%, a bandwidth of 0.25 nm, a 3-mm length, and an index change of $\sim 10^{-4}$ . The FBGs operate in the $C$ -band, where CYTOP offers key advantages over polymethyl methacrylate optical fibers, displaying significantly lower optical loss in the important near-infrared (NIR) optical communications window. In addition, we note that CYTOP has a far lower affinity for water absorption and a core-mode refractive index that coincides with the aqueous index regime. These properties offer several unique opportunities for POF sensing at NIR wavelengths, such as compatibility with existing optical networks, the potential for POF sensor multiplexing and suitability for biosensing. We demonstrate compatibility with a commercial Bragg grating demodulator. <s> BIB006 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> A novel fiber optical refractive index sensor based on gold nanoshells immobilized on the surface of an etched single-mode fiber including a Bragg grating is demonstrated. The nanoparticle coating induces refractive index dependent waveguide losses, because of the variation of the evanescently guided part of the light. Hence the amplitude of the Bragg reflection is highly sensitive to refractive index changes of the surrounding medium. The nanoshell functionalized fiber optical refractive index sensor works in reflectance mode, is suitable for chemical and biochemical sensing, and shows an intensity dependency of 4400% per refractive index unit in the refractive index range between 1.333 and 1.346. Furthermore, the physical length of the sensor is smaller than 3 mm with a diameter of 6 μm, and therefore offers the possibility of a localized refractive index measurement. <s> BIB007 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> The authors demonstrated the fabrication of a fiber Bragg grating-like plasmonic nanostructure on the surface of a tapered optical fiber using gold nanorods (GNRs). A multimode optical fiber with core and cladding diameters of 105 and 125 μm, respectively, was used to make a tapered fiber using a dynamic etching process. The tip diameter was ∼100 nm. Light from a laser was coupled to the untapered end of the fiber, which produced a strong evanescent field around the tapered section of the fiber. The gradient force due to the evanescent field trapped the GNRs on the surface of the tapered fiber. The authors explored possible causes of the GNR distribution. The plasmonic structure will be a good candidate for sensing based on surface enhanced Raman scattering. <s> BIB008 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> Abstract A single mode fiber Bragg grating (FBG) is used to generate Surface Plasmon Resonance (SPR). The uniform gratings of the FBG are used to scatter light from the fiber optic core into the cladding thus enabling the interaction between the light and a thin gold film in order to generate SPR. Applying this technique, the cladding around the FBG is left intact, making this sensor very robust and easy to handle. A thin film of graphene oxide (GO) is deposited over a 45 nm gold film to enhance the sensitivity of the SPR sensor. The gold coated sensor demonstrated high sensitivity of approximately 200 nm/RIU when tested with different concentrations of ethanol in an aqueous medium. A 2.5 times improvement in sensitivity is observed with the GO enhancement compared to the gold coated sensor. <s> BIB009 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> We report a graphene-assisted all-optical control of a fiber Bragg grating (FBG), which enables in-fiber optical bistability and switching. With an optical pump, a micro-FBG wrapped by graphene evolves into chirped and phase-shifted FBGs, whose characteristic wavelengths and bandwidths could be controlled by the pump power. Optical bistability and multistability are achieved in the controlled FBG based on a shifted Bragg reflection or Fabry–Perot-type resonance, which allow the implementation of optical switching with an extinction ratio exceeding 20 dB and a response time in tens of milliseconds. <s> BIB010 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> This study reports on the development of a surface plasmon resonance (SPR) optical fiber biosensor based on tilted fiber Bragg grating technology for direct detection of small biomarkers of interest for lung cancer diagnosis. Since SPR principle relies on the refractive index modifications to sensitively detect mass changes at the gold coated surface, we have proposed here a comparative study in relation to the target size. Two cytokeratin 7 (CK7) samples with a molecular weight ranging from 78 kDa to 2.6 kDa, respectively CK7 full protein and CK7 peptide, have been used for label-free monitoring. This work has first consisted in the elaboration and the characterization of a robust and reproducible bioreceptor, based on antibody/antigen cross-linking. Immobilized antibodies were then utilized as binding agents to investigate the sensitivity of the biosensor towards the two CK7 antigens. Results have highlighted a very good sensitivity of the biosensor response for both samples diluted in phosphate buffer with a higher limit of detection for the larger CK7 full protein. The most groundbreaking nature of this study relies on the detection of small biomolecule CK7 peptides in buffer and in the presence of complex media such as serum, achieving a limit of detection of 0.4 nM. <s> BIB011 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Conclusions <s> In this work, a novel and simple optical fiber hot-wire anemometer based on single-walled carbon nanotubes (SWCNTs) coated tilted fiber Bragg grating (TFBG) is proposed and demonstrated. For the hot-wire wind speed sensor design, TFBG is an ideal in-fiber sensing structure due to its unique features. It is utilized as both light coupling and temperature sensing element without using any geometry-modified or uncommon fiber, which simplifies the sensor structure. To further enhance the thermal conversion capability, SWCNTs are coated on the surface of the TFBG instead of traditional metallic materials, which have excellent thermal characteristics. When a laser light is pumped into the sensor, the pump light propagating in the core will be easily coupled into cladding of the fiber via the TFBG and strongly absorbed by the SWCNTs thin film. This absorption acts like a hot-wire raising the local temperature of the fiber, which is accurately detected by the TFBG resonance shift. In the experiments, the sensor’s performances were investigated and controlled by adjusting the inherent angle of the TFBG, the thickness of SWCNTs film, and the input power of the pump laser. It was demonstrated that the developed anemometer exhibited significant light absorption efficiency up to 93%, and the maximum temperature of the local area on the fiber was heated up to 146.1°C under the relatively low pump power of 97.76 mW. The sensitivity of −0.3667 nm/(m/s) at wind speed of 1.0 m/s was measured with the selected 12° TFBG and 1.6 μm film. <s> BIB012
This review of recent developments in fiber grating-based SPR immunosensors confirms the increasing level of maturity obtained in the field, essentially resulting from the complementary efforts made by photonics and biochemistry groups all around the world. This development comes along with the use of more sophisticated experimental protocols and more realistic error analyses. We can now assert that grating-based SPR sensor platforms have passed the proof-of-principle level, as they have been tested in complex media that replicate the final application environment. Grating-based SPR sensors can operate easily in the mid-infrared wavelength range, via the resonant coupling between the cladding modes and the surface plasmon wave. Hence, they are compatible with the use of cost-effective and high-resolution FBG interrogators that can be easily computer-driven to process the spectral information in real time. Most grating architectures also provide a response that is inherently immune to ambient temperature fluctuations, which is highly desirable for practical implementation. Of course, in addition to work on relevant clinical applications, some new developments also offer exciting possibilities for future research, notably in the areas of microstructured fibers BIB002 and plastic optical fibers-that are inherently much more biocompatible than their silica counterparts- BIB005 BIB006 or even the use of natural fibers  . Another important path of exploration is the realization of nanostructures in metals BIB008 or the inclusion of carbon nanotubes, graphene and other novel plasmonic materials, such as oxides and nitrides, in sensor fabrication BIB001 BIB012 BIB003 . Recent work indicates that combining graphene with noble metal particles and layers promises a wealth of new physics and sensing modes, in addition to intrinsic tunable and adjustable plasmonic properties BIB004 BIB009 [140] BIB010 . It is hoped that this review will help in fostering further research in the field of fiber-grating SPR biosensors. The study conducted in BIB011 BIB007 has demonstrated that these immunosensors reach a limit of detection of 1 pM in phosphate buffered saline (PBS) media supplemented with 10 % of serum. Then, the detection of cytokeratins trapped in the crosslinked polymer network of a porous gel matrix was reported, despite the non-liquid nature of the hosting medium. These results have allowed the sensors to be inserted in fresh tissues obtained from a biopsy. Such measurements have been conducted at the hospital and have demonstrated a positive biosensor response in tumoral tissues. These first results constitute an important milestone towards the possible demonstration of in vivo diagnosis using plasmonic fiber-grating biosensors. To this end, numerous developments and validation tests remain to be done, both in terms of extensive testing of the devices in different kinds of tumors and to fully demonstrate the biocompatibility and compatibility with standard practices in the sterilization of the probes. This review of recent developments in fiber grating-based SPR immunosensors confirms the increasing level of maturity obtained in the field, essentially resulting from the complementary efforts made by photonics and biochemistry groups all around the world. This development comes along with the use of more sophisticated experimental protocols and more realistic error analyses. We can now assert that grating-based SPR sensor platforms have passed the proof-of-principle level, as they have been tested in complex media that replicate the final application environment. Grating-based SPR sensors can operate easily in the mid-infrared wavelength range, via the resonant coupling between the cladding modes and the surface plasmon wave. Hence, they are compatible with the use of cost-effective and high-resolution FBG interrogators that can be easily computer-driven to process the spectral information in real time. Most grating architectures also provide a response that is inherently immune to ambient temperature fluctuations, which is highly desirable for practical implementation. Of course, in addition to work on relevant clinical applications, some new developments also offer exciting possibilities for future research, notably in the areas of microstructured fibers BIB002 and plastic optical fibers-that are inherently much more biocompatible than their silica counterparts- BIB005 BIB006 or even the use of natural fibers  . Another important path of exploration is the realization of nanostructures in metals BIB008 or the inclusion of carbon nanotubes, graphene and other novel plasmonic materials, such as oxides and nitrides, in sensor fabrication BIB001 BIB012 BIB003 . Recent work indicates that combining graphene with noble metal particles and layers promises a wealth of new physics and sensing modes, in addition to intrinsic tunable and adjustable plasmonic properties BIB004 BIB009 [140] BIB010 . It is hoped that this review will help in fostering further research in the field of fiber-grating SPR biosensors.