Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Interrogation of Plasmonic Fiber-Grating (Bio)chemical Sensors <s> The high-order cladding modes of conventional single mode fiber come in semi-degenerate pairs corresponding to mostly radially or mostly azimuthally polarized light. Using tilted fiber Bragg gratings to excite these mode families separately, we show how plasmonic coupling to a thin gold coating on the surface of the fiber modifies the effective indices of the modes differently according to polarization and to mode order. In particular, we show the existence of a single “apolarized” grating resonance, with equal effective index for all input polarization states. This special resonance provides direct evidence of the excitation of a surface plasmon on the metal surface but also an absolute wavelength reference that allows for the precise localization of the most sensitive resonances in refractometric and biochemical sensing applications. Two plasmon interrogation methods are proposed, based on wavelength and amplitude measurements. Finally, we use a biotin-streptavidin biomolecular recognition experiment to demonstrate that differential spectral transmission measurements of a fine comb of cladding mode resonances in the vicinity of the apolarized resonance provide the most accurate method to extract information from plasmon-assisted Tilted fiber Bragg gratings, down to pM concentrations and at least 10−5 refractive index changes. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Interrogation of Plasmonic Fiber-Grating (Bio)chemical Sensors <s> We present a novel optical fiber surface plasmon resonance (SPR) sensor scheme using reflected guided cladding modes captured by a double-clad fiber coupler and excited in a gold-coated fiber with a tilted Bragg grating. This new interrogation approach, based on the reflection spectrum, provides an improvement in the operating range of the device over previous techniques. The device allows detection of SPR in the reflected guided cladding modes and also in the transmitted spectrum, allowing comparison with standard techniques. The sensor has a large operating range from 1.335 to 1.432 RIU, and a sensitivity of 510.5 nm/RIU. The device shows strong dependence on the polarization state of the guided core mode which can be used to turn the SPR on or off. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Interrogation of Plasmonic Fiber-Grating (Bio)chemical Sensors <s> Surface Plasmon Resonance (SPR) fiber sensor research has grown since the first demonstration over 20 year ago into a rich and diverse field with a wide range of optical fiber architectures, plasmonic coatings, and excitation and interrogation methods. Yet, the large diversity of SPR fiber sensor designs has made it difficult to understand the advantages of each approach. Here, we review SPR fiber sensor architectures, covering the latest developments from optical fiber geometries to plasmonic coatings. By developing a systematic approach to fiber-based SPR designs, we identify and discuss future research opportunities based on a performance comparison of the different approaches for sensing applications. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Interrogation of Plasmonic Fiber-Grating (Bio)chemical Sensors <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> BIB004
In the case of plasmonic optical fiber grating-based configurations, the refractive index variations are reflected on the optical spectrum in the form of both a wavelength shift and an optical power change BIB003 BIB001 BIB002 , as depicted in Figure 10 for a gold-coated TFBG. Bioreceptors are functionalized on top of the metallic coating BIB004 and the spectral modifications result from the molecular interactions happening on this outer film. Several ways of interrogating these sensors co-exist and the application of one or the other mostly depends on the requirements of the specific application. Spectral and intensity interrogation are the two main techniques widely reported, and an advantage of utmost importance with regard to other technologies is their implementation with commercial optical-fiber equipment or conventional FBG interrogators. Other techniques exist as well and have proved to exhibit even better resolution, but their applications are limited to laboratory environments due to their complexity.  In the case of plasmonic optical fiber grating-based configurations, the refractive index variations are reflected on the optical spectrum in the form of both a wavelength shift and an optical power change BIB003 BIB001 BIB002 , as depicted in Figure 10 for a gold-coated TFBG. Bioreceptors are functionalized on top of the metallic coating BIB004 and the spectral modifications result from the molecular interactions happening on this outer film. Several ways of interrogating these sensors co-exist and the application of one or the other mostly depends on the requirements of the specific application. Spectral and intensity interrogation are the two main techniques widely reported, and an advantage of utmost importance with regard to other technologies is their implementation with commercial optical-fiber equipment or conventional FBG interrogators. Other techniques exist as well and have proved to exhibit even better resolution, but their applications are limited to laboratory environments due to their complexity. 