Assaying with PCF-based SPR refractive index biosensors: From recent configurations to outstanding detection limits

Optical sensing has undergone tremendous advancements with the advent of photonic crystal fibers (PCFs). Similarly, optical sensing utilizing surface plasmon resonance (SPR) operation has produced remarkable outcomes and new paradigms for providing prompt and label-free platforms for ultra-sensitive and high-resolution determinations in chemical, biological, and related sensing applications. Here we review refractive index photonic crystal fiber surface plasmon resonance biosensors (RI PCF SPRBs) with a focus on low, high, and wide-ranged RI PCF SPRBs. A performance comparison based on operating refractive index range amongst various RI PCF SPRBs shows that, to date, most of the RI PCF SPRBs proposed and/or fabricated belong to the high index category. Due to their extensive operating range, wide-ranged RI PCF SPRBs can function both as low and as high RI PCF SPRBs making them more attractive for lab-on-photonic crystal fiber (LOPCF), lab-in-photonic crystal fiber (LIPCF), and allied applications, where one or more laboratory functions are scaled to a single device for in vitro or in vivo screening and analysis. It is suggested that future research should continue to improve upon sensing metrics based on innovativeness not only in core and cladding designs but also in optimization of structural parameters and implementation of new plasmonic materials such as silicene, germanene, and phosphorene whose growths have been demonstrated recently.