Surface plasmon refractive index biosensors: A review of optical fiber, multilayer 2D material and gratings, and MIM configurations

•Comprehensive compilation of surface plasmon (SP) refractive index (RI) biosensors developed since 2015 are presented.•PCF, MCF, uncladded, hollow core, multimode, and other optical fiber-based SP RI biosensors are outlined.•Prism-integrated 2D material, and grating-based SP RI multilayer biosensors are summarized.•Progress on small-footprint, metal-insulator-metal SP RI biosensors are also discussed.•In chronological tables, in-depth information on the design and performance parameters of SP RI biosensors is provided. Surface Plasmon (SP) based biosensors have attracted the scientific community owing to their wide range of applications and remarkable performance in the past few years. Exploiting the principle of SP propagation at metal-dielectric and the subsequent surface plasmon resonance (SPR) has established these sensors as a promising candidates in biosensing applications with sensitivities reaching as high as 67000 nm/refractive index unit (nm/RIU). In this work, we have extensively discussed and summarized SP refractive index (RI) based biosensors from the literature and their performance since 2015. Moreover, we have categorized the biosensors into three classes based on SP excitation and its deployment in sensing RI of biomaterials or analytes. Firstly, fiber based, which includes photonic crystal fiber (PCF) and other optical fibers, where extensive work has been accomplished. Following are the multilayer structure based SP RI biosensors engaging either 2D materials or gratings and have reported sensitivities of 243 deg/RIU and 2300 nm/RIU, respectively. Lastly, metal–insulator-metal (MIM) biosensors exhibit small footprint features and the possibility of photonic integration, exhibiting sensitivity as high as 7564 nm/RIU. In general, the state-of-the-art topologies in these categories, employed materials, design parameters, and their performances metrics in terms of sensitivity, the figure of merit (FOM), detection range (DR), detection limit (DL), etc., are incorporated and may serve as an extensive assemblage of SP RI biosensors for scientific and industry committees. Index Terms—Surface Plasmon, Refractive index sensors, Biosensors, Metal-insulator–metal, Photonic crystal fiber, Multicore fiber, 2D materials, Gratings.