Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Additional Considerations <s> A novel class of fiber-optic evanescent-wave sensor was constructed on the basis of modification of the unclad portion of an optical fiber with self-assembled gold colloids. The optical properties and, hence, the attenuated total reflection spectrum of self-assembled gold colloids on the optical fiber changes with different refractive index of the environment near the colloidal gold surface. With sucrose solutions of increasing refractive index, the sensor response decreases linearly. The colloidal gold surface was also functionalized with glycine, succinic acid, or biotin to enhance the selectivity of the sensor. Results show that the sensor response decreases linearly with increasing concentration of each analyte. When the colloidal gold surface was functionalized with biotin, the detection limit of the sensor for streptavidin was 9.8 x 10(-11) M. Using this approach, we demonstrate proof-of-concept of a class of refractive index sensor that is sensitive to the refractive index of the environment near the colloidal gold surface and, hence, is suitable for label-free detection of molecular or biomolecular binding at the surface of gold colloids. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Additional Considerations <s> Gold nanorods are deposited on the core surface of an etched optical fiber, and their surface plasmon resonance is excited by the evanescent wave from the optical fiber. The excitation of the surface plasmon produces strong light scattering from individual nanorods, allowing for single-particle scattering-based imaging and spectroscopy. We systematically characterize the dependence of the scattering spectra of individual nanorods on the refractive index of the surrounding medium. As the refractive index is increased, the scattering spectra red shift steadily. The refractive index sensitivity and sensing figure of merit, which is the index sensitivity divided by the scattering peak line width, are found to reach 200 nm/RIU (RIU = refractive index unit) and 3.8, respectively. We further investigate the ensemble response of the plasmon resonance of Au nanorods to varying dielectric environments by measuring the intensity and spectrum of the light transmitted through the fiber. The ensemble index sensitivity ... <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Additional Considerations <s> We demonstrate an improvement by more than 1 order of magnitude of the figure of merit (FoM) of plasmonic nanoparticle sensors by means of the diffractive coupling of localized surface plasmon resonances. The coupling in arrays of nanoparticles leads to Fano resonances with narrow line widths known as surface lattice resonances, which are very suitable for the sensitive detection of small changes in the refractive index of the surroundings. We focus on the sensitivity to the bulk refractive index and find that the sensor FoM scales solely with the frequency difference between the surface lattice resonance and the diffracted order grazing to the surface of the array. This result, which can be extended to other systems with coupled resonances, enables the design of plasmonic sensors with a high FoM over broad spectral ranges with unprecedented accuracy. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Additional Considerations <s> Abstract Two types of localized surface plasmon resonance (LSPR)-based optical fiber biosensors using gold nanospheres (GNSs) and gold nanorods (GNRs) have been developed and their performance characteristics evaluated and cross-compared successfully in this work. Based on the results obtained from the optimization of each of these types of biosensor and reported by the authors elsewhere, GNSs with a diameter of 60 nm and GNRs with an aspect ratio of 4.1 were specifically chosen in this work for the fabrication of two representative sensor probes, with an aim to create a highly sensitive and wavelength-based LSPR sensor to overcome the limitations arising from other intensity-based sensors. In order to develop effective LSPR biosensors, both GNSs and GRNs respectively were immobilized on an unclad surface of an optical fiber, prior to the functionalization with human IgG in order to create a device for the detection of anti-human IgG, at different concentrations. The experimental results obtained from tests carried out show that the sensitivities of GNSs and GNRs-based LSPR sensors to refractive index variation are 914 and 601 nm/RIU respectively; however as biosensors they have demonstrated the same detection limit of 1.6 nM for the detection of anti-human IgG. <s> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Additional Considerations <s> This review presents an overview of “Lab on Fiber” technologies and devices with special focus on the design and development of advanced fiber optic nanoprobes for biological applications. Depending on the specific location where functional materials at micro and nanoscale are integrated, “Lab on Fiber Technology” is classified into three main paradigms: Lab on Tip (where functional materials are integrated onto the optical fiber tip), Lab around Fiber (where functional materials are integrated on the outer surface of optical fibers), and Lab in Fiber (where functional materials are integrated within the holey structure of specialty optical fibers). ::: ::: This work reviews the strategies, the main achievements and related devices developed in the “Lab on Fiber” roadmap, discussing perspectives and challenges that lie ahead, with special focus on biological sensing applications. <s> BIB005 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Additional Considerations <s> A metrological assessment of grating-based optical fiber sensors is proposed with the aim of providing an objective evaluation of the performance of this sensor category. Attention was focused on the most common parameters, used to describe the performance of both optical refractometers and biosensors, which encompassed sensitivity, with a distinction between volume or bulk sensitivity and surface sensitivity, resolution, response time, limit of detection, specificity (or selectivity), reusability (or regenerability) and some other parameters of generic interest, such as measurement uncertainty, accuracy, precision, stability, drift, repeatability and reproducibility. Clearly, the concepts discussed here can also be applied to any resonance-based sensor, thus providing the basis for an easier and direct performance comparison of a great number of sensors published in the literature up to now. In addition, common mistakes present in the literature made for the evaluation of sensor performance are highlighted, and lastly a uniform performance assessment is discussed and provided. Finally, some design strategies will be proposed to develop a grating-based optical fiber sensing scheme with improved performance. <s> BIB006 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Additional Considerations <s> An excessively tilted LPFG-assisted surface plasmon polaritons sensor (Ex-TLPFG assisted SPP sensor) with ultrahigh sensitivity is proposed and numerically investigated using the finite-element-method-based full-vector complex coupled mode theory. We show that the SPP mode is transited (or excited) gradually from both the p-polarized TM $_{0,j}$ and EH $_{v,j}$ ( $v\geq 1$ ) modes, and hence, the proposed SPP sensor can be tuned to achieve strong resonance of either the degenerate TM $_{0,j}$ and EH $_{2,j}$ modes or EH $_{1,j}$ mode to optimize the sensitivity for analyte refractive index sensing. The results confirm that a transition point corresponding to the phase matching curve of the SPP mode is obtained, which can be used to predict the optimized grating period. By this approach, ultrasensitive SPP refractometric sensor can be obtained and the sensitivity can be further improved through a simple method: reducing the fiber cladding combined with an optimized grating period. We demonstrate that a giant sensitivity as high as $10100\,\text{nm}/\text{RIU}$ is achieved for the degenerate TM $_{0,32}$ and EH $_{2,32}$ modes (or $7400\,\text{nm/RIU}$ for the EH $_{1,32}$ mode). These appealing characteristics make the proposed Ex-TLPFG-assisted SPP sensor ideal for biochemical analyte sensing applications. <s> BIB007
SPR optical fiber sensors can be derived from the aforementioned grating structures surrounded by a thin metal film (most often gold or silver). Sheaths of thickness ranging between 30 nm and 70 nm or nanoparticle arrays are generally used. It is known that continuous metal films yield SPR generation, spectrally manifested by the coupling between a given cladding mode resonance and the SPR mode. Nanoparticles usually excite localized SPR (LSPR), corresponding to a broader attenuation of numerous cladding modes BIB001  BIB002 . In surface refractometry, they can offer a comparable sensitivity to that of SPR-based sensors. The properties of the metal nanoparticles are highly dependent on their constitutive material, also in shape and size. Hence, controlling these three parameters allows to optimize the performances for detecting a particular target BIB004 . As depicted in Figure 7 , SPR generation is achieved when the electric field of the light modes is polarized mostly radially at the surrounding medium interface. The orthogonal polarization state is not able to excite the SPR, as the electric field of the light modes is polarized mostly azimuthally (i.e. tangentially to the metal) at the surrounding medium interface, and thus cannot couple energy to the surface plasmon waves. The SP phenomenon drastically enhances the sensitivity to surrounding refractive index changes at the outer surface of the optical fiber. Depending on the configuration, impressive refractometric sensitivities in the range [10 2 -10 4 nm/RIU] have been reported  BIB006 . Very recently, an SRI sensitivity even exceeding 10,000 nm/RIU has been theoretically reported for plasmon-assisted excessively tilted gratings BIB007 . It is worth noting that when comparing the sensor performances between different configurations, especially for immunosensing where different parameters (surface covering, antibody/antigen affinity, etc.) come into play, it is not sufficient to compare only sensitivities (i.e., wavelength shifts), without considering the wavelength-measurement accuracy. In practice, it is more convenient to refer to the figure of merit (FOM) of the device, which corresponds to the ratio between the sensitivity and the linewidth of the resonance. A narrow resonance can be measured with a high resolution so that its exact location can be computed, which is not true for a broad one BIB003 . As a result, in terms of experimentally demonstrated FOM, because gratings usually feature  SPR optical fiber sensors can be derived from the aforementioned grating structures surrounded by a thin metal film (most often gold or silver). Sheaths of thickness ranging between 30 nm and 70 nm or nanoparticle arrays are generally used. It is known that continuous metal films yield SPR generation, spectrally manifested by the coupling between a given cladding mode resonance and the SPR mode. Nanoparticles usually excite localized SPR (LSPR), corresponding to a broader attenuation of numerous cladding modes BIB001  BIB002 . In surface refractometry, they can offer a comparable sensitivity to that of SPR-based sensors. The properties of the metal nanoparticles are highly dependent on their constitutive material, also in shape and size. Hence, controlling these three parameters allows to optimize the performances for detecting a particular target BIB004 . As depicted in Figure 7 , SPR generation is achieved when the electric field of the light modes is polarized mostly radially at the surrounding medium interface. The orthogonal polarization state is not able to excite the SPR, as the electric field of the light modes is polarized mostly azimuthally (i.e. tangentially to the metal) at the surrounding medium interface, and thus cannot couple energy to the surface plasmon waves. The SP phenomenon drastically enhances the sensitivity to surrounding refractive index changes at the outer surface of the optical fiber. Depending on the configuration, impressive refractometric sensitivities in the range [10 2 -10 4 nm/RIU] have been reported  BIB006 . Very recently, an SRI sensitivity even exceeding 10,000 nm/RIU has been theoretically reported for plasmon-assisted excessively tilted gratings BIB007 . It is worth noting that when comparing the sensor performances between different configurations, especially for immunosensing where different parameters (surface covering, antibody/antigen affinity, etc.) come into play, it is not sufficient to compare only sensitivities (i.e., wavelength shifts), without considering the wavelength-measurement accuracy. In practice, it is more convenient to refer to the figure of merit (FOM) of the device, which corresponds to the ratio between the sensitivity and the linewidth of the resonance. A narrow resonance can be measured with a high resolution so that its exact Sensors 2017, 17, 2732 7 of 20 location can be computed, which is not true for a broad one BIB003 . As a result, in terms of experimentally demonstrated FOM, because gratings usually feature narrow cladding mode resonances (except for LPFGs), they outperform all other optical fiber configurations  . However, the goal of this paper is not to compare the relative performances of the grating architectures compared to other optical fiber configurations. Readers interested in additional considerations about sensing configuration and performance are encouraged to consult these complementary review papers BIB006 BIB005 . narrow cladding mode resonances (except for LPFGs), they outperform all other optical fiber configurations  . However, the goal of this paper is not to compare the relative performances of the grating architectures compared to other optical fiber configurations. Readers interested in additional considerations about sensing configuration and performance are encouraged to consult these complementary review papers BIB006 BIB005 . 