Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Overview of Plasmonic Fiber-Grating Immunosensors <s> This article reviews the recent progress in optical biosensors that use the label-free detection protocol, in which biomolecules are unlabeled or unmodified, and are detected in their natural forms. In particular, it will focus on the optical biosensors that utilize the refractive index change as the sensing transduction signal. Various optical label-free biosensing platforms will be introduced, including, but not limited to, surface plasmon resonance, interferometers, waveguides, fiber gratings, ring resonators, and photonic crystals. Emphasis will be given to the description of optical structures and their respective sensing mechanisms. Examples of detecting various types of biomolecules will be presented. Wherever possible, the sensing performance of each optical structure will be evaluated and compared in terms of sensitivity and detection limit. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Overview of Plasmonic Fiber-Grating Immunosensors <s> We report an original design approach based on the modal dispersion curves for the development of long period gratings in transition mode near the dispersion turning point exhibiting ultrahigh refractive index sensitivity. The theoretical model predicting a giant sensitivity of 9900 nm per refractive index unit in a watery environment was experimentally validated with a result of approximately 9100 nm per refractive index unit around an ambient index of 1.3469. This result places thin film coated LPGs as an alternative to other fiber-based technologies for high-performance chemical and biological sensing applications. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Overview of Plasmonic Fiber-Grating Immunosensors <s> Abstract We report an innovative fiber optic nano-optrode based on Long Period Gratings (LPGs) working in reflection mode for the detection of human Thyroglobulin (TG), a protein marker of differentiated thyroid cancer. The reflection-type LPG (RT-LPG) biosensor, coated with a single layer of atactic polystyrene (aPS) onto which a specific, high affinity anti-Tg antibody was adsorbed, allowed the label-free detection of Tg in the needle washouts of fine-needle aspiration biopsies, at concentrations useful for pre- and post-operative assessment of the biomarker levels. Analyte recognition and capture were confirmed with a parallel on fiber ELISA-like assay using, in pilot tests, the biotinylated protein and HRP-labeled streptavidin for its detection. Dose-dependent experiments showed that the detection is linearly dependent on concentration within the range between 0 and 4 ng/mL, while antibody saturation occurs for higher protein levels. The system is characterized by a very high sensitivity and specificity allowing the ex-vivo detection of sub ng/ml concentrations of human Tg from needle washouts of fine-needle aspiration biopsies of thyroid nodule from different patients. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Overview of Plasmonic Fiber-Grating Immunosensors <s> The manufacturing and spectral features of different types of long period fiber gratings (LPFGs), ranging from phase-shifted, turn-around point, and internally tilted gratings, to pseudo-random gratings, are described and discussed in detail. LPFGs were manufactured on boron-germanium co-doped photosensitive optical fibers with the point-by-point technique using an excimer KrF laser operating at 248 nm. The developed experimental setup to manufacture high-quality LPFGs was designed to totally customize any type of gratings with the possibility of setting different parameters, such as the grating period (or pitch), the number of grating planes, the number of laser shots for each plane, etc. Some important spectral features of the LPFGs’ spectra were taken into account. This allows realizing homemade devices useful in several fiber-based applications, such as optical filtering, coupling systems, random lasers, physical and chemical sensing, and biosensing. <s> BIB004
In this section, a general survey of the recent literature in this topic is presented in Table 1 in which the main characteristics and performance indicators of experimental demonstrations are given. Hence, whenever possible, the limit of detection of the target molecules is specified. The latter strongly depends on the characteristics of the analytes, especially their nature and mass. So, making a fair comparison between the relative performances of these various immunosensing experiments is particularly tough and will not be covered by this article. Also, while non-metalized configurations have demonstrated very good performances for biosensing and immunosensing BIB001 BIB002 BIB004 BIB003 , this table focuses on grating configurations where metals are used, either in the form of sheath or nanoparticles, as specified in the dedicated table entry. 