# Recent Advances in Plasmonic Sensor-Based Fiber Optic Probes for Biological Applications
## 1 Introduction  
In recent years, the surface plasmon resonance (SPR)-based optical sensing of several quantities such as chemicals [1; 2], temperature [3], pressure [4], force [5], environmental monitoring [6], optofluidic detection [7], food safety [8], and biological species [9; 10], as illustrated in Figure 1[1; 12; 13; 14], has been proven to be advantageous. The optical fiber deployed in such sensors has replaced the traditional substrates for SPR sensors such as prisms, electrodes, etc. [15]. Today, SPR-based optical sensors are employed for different applications in life sciences, electrochemistry, environmental safety, and biomedical diagnostics, as illustrated in Figure 1[16; 17]. SPR is a promising and powerful optical detection technique for studying label-free biomolecular interactions in real time within a variety of diverse biomedical applications due to the label-free sensing, fast response, and high sensitivity [18; 19; 20]. Several SPR-based optical fiber sensing configurations are widely proposed, and fiber optic sensing configuration allows for simplified designs [21; 22]. Recently, researchers have found that these SPR-based FOPSs are applied in remote sensing over the prism-based SPR sensing devices [1]. The most beneficial aspect of FOPSs is their ability to analyze minute quantities of biological samples [23]. Thus, SPR-based FOPSs have received a lot of attention in the recent past [24].  
We start this review with a brief discussion of some SPR-based FOPSs that find wide application because of their simple structure, compactness, high resolution, and sensitivity. The geometrical configurations of different FOPSs with single mode and multimode fibers for sensing applications are discussed [25, 26]. As the propagation modes in multi-mode fibers are nearly random, the mode distribution at different incident angles is unstable [27]. As only part of the light beams undergoes resonance, it directly influences the sensing performance, reliability, and repeatability. Hence, for multimode optical fiber plasmonic sensors, the signal-to-noise ratio limits the measurable range of the sensitivity and resolution. Although there is some related work already reported for constant-angle SPR sensor design [28, 29, 30], a number of unexpected incident angles were still involved in their designs. In addition, multimode optical fibers are sensitive to mechanical disturbance and hence the quality of the output signal is lowered as well [31]. In contrast, a single-mode fiber exhibits a narrow resonance dip in the transmission spectrum, which results in an improved detection limit. Thus, to enhance the resolution, the single-mode optical fiber plasmonic sensor has been established [32]. Generally, the geometric structure of the FOPSs consists of two media, namely, dielectric and metal. The sensing mechanism between the dielectric silica cladding with various metal-coated surfaces is attributed to the surface plasmon resonance.  
Nowadays, in order to enhance the sensitivity and resolution of SPR-based FOPS, a thin layer of graphene and molybdenum disulfide is added to the metal layer [33, 34, 35]. Furthermore, other combinations, namely, molybdenum disulfide, molybdenum diselenide, tungsten disulphide, and tungsten diselenide layers (transition metal dichalcogenides), have also been developed [21, 36]. In this review, we briefly discuss the different geometrical configurations of the SPR-based FOPS with various combinations of transition metal dichalcogenides for biomedical applications.  
Figure 1: Biological applications of fiber optic plasmonic sensor [11, 12, 13, 14, 16, 17].