The incorporation of a proper sensing material towards the construction of high selectivity optical sensing devices is vital. Polysaccharides, such as chitosan and carrageenan, are among the bio-based sensing materials that are extensively employed due to their remarkable physicochemical attributes. This paper highlights the critical aspects of the design of suitable polysaccharides for the recognition of specific analytes through physical and chemical modifications of polysaccharide structure. Such modifications lead to the enhancement of physicochemical properties of polysaccharides and optical sensor performance. Chitosan and carrageenan are two materials that possess excellent features which are capable of sensing target analytes via various interactions. The interaction between polysaccharides and analytes is dependent on the availability of functional groups in their structure. The integration of polysaccharides with various optical sensing techniques further improves optical sensor performance. The application of polysaccharides as sensing materials in various optical sensing techniques is also highlighted, particularly for metal ion sensing.

Hyperbolic metamaterials (HMMs) have attracted increasing attentions because of their unique dispersion properties and the flexibility to control the dispersion by changing the components and fractions of the composed materials. In this work, for the first time, we demonstrate a plasmonic sensor based on a side-polished few-mode-fiber coated with a layered of HMM, which is composed of alternating layers of Ag and TiO₂. To optimize the sensor performance, the effects of the metal filling fraction (ρ) and the number of bilayers (N_bi) on the HMM dispersion are thoroughly engineered with the effective medium theory and the finite element method. It is found that the HMM with ρ=0.7 and N_bi = 3 can provide the average sensitivity of 5114.3 nm/RIU (RIU: refractive index unit), and the highest sensitivity 9000 nm/RIU in the surrounding refractive index (SRI) ranging from 1.33 to 1.40 RIU. The corresponding figure of merit (FOM) reaches a maximum of 230.8 RIU^-1 which is much higher than that of the conventional silver film based SPR sensor. The influence of ρ and N_bi on the sensitivity are well explained from the aspects of the electrical field distribution and the dispersion relationship. This work opens a gate to significantly improve fiber plasmonic sensors performance by engineering the HMM dispersion, which is expected to meet the emergent demand in the biological, medical and clinical applications.

Polyurethane-graphene (PU-graphene) nanocomposite was utilized as the sensing material for a fiber Bragg grating (FBG) temperature sensor. The nanocomposite was characterized using a Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) to study the morphology and physical properties of the materials for FBG temperature sensing application. The physical, chemical, and conductivity of PU-graphene improve after graphene was introduced in pristine PU. The FTIR shows that the strong intermolecular interaction between -O-C = O (ester) and hydrogen in graphene in the PU-graphene was indicated by the shift to lower wavenumber of ether (C-O-C) peak at -1220 cm ^-1 to -1218 cm ^-1 . TGA shows the thermal stability of PU increases to 217 °C due to the strong intermolecular interaction with the presence of graphene flakes. EIS shows a good electrical conductivity of 1.39 × 10 ^-9 Scm ^-1 in the PU-graphene due to the electron transfer provided by the graphene. The SEM shows a rough and uneven texture on the surface of FBG coated by PU-graphene nanocomposite which shows that the graphene flakes are completely coated by polyurethane polymer. The PU-graphene was then dip-coated on the optical fiberbased Bragg grating, and the sensor performance for a temperature sensor was evaluated, where a good linearity with the sensitivity of 6 pm/°C was achieved.

Herein we propose a novel strategy to enhance surface plasmon resonance (SPR) by introducing a photonic cavity into a total-internal-reflection architecture. The photonic cavity, which is comprised of a highly reflective photonic crystal (PC), defect layers, and a gold (Au) film, enables Fabry–Perot (FP) resonances in the defect layers and therefore narrows the SPR resonance width in the metallic surface as well as increases the electric field intensity and penetration depth in the evanescent region. The fabricated sensor exhibits a 5.7-fold increase in the figure of merit and a higher linear coefficient as compared with the conventional Au-SPR sensor. The demonstrated PC/FP cavity/metal structure presents a new design philosophy for SPR performance enhancement.