# Recent Advances in Plasmonic Sensor-Based Fiber Optic Probes for Biological Applications
## 6 FOPS Geometrical Configuration and Techniques
### Novel Techniques for Optical Fiber Sensors  
With the new developments in fiber fabrication, novel SPR sensors using optical fibers were reported in the last few years. In 2001, both magnesium fluoride and Teflon AF-1600 were successfully utilized as dielectric buffer materials for long-range surface plasmons (LRSPR) wavelength-modulated sensor design by Nenninger with a maximum sensitivity of 9200 nm/RIU [82]. In 2004, [PERSON] and [PERSON] presented an improved theoretical analysis method for the LRSPR device by taking the angular interrogation into account, in addition to the spectral interrogation that is usually considered for the conventional SPR configuration [83]. In the same year, [PERSON] et al. explained their SPR spectroscopy experiment using the characteristic matrix approach and the total transmission coefficients [84]. Later on, the SPR imaging (SPRI) technique became the most promising tool for high-throughput detection and [PERSON]'s group reported SPR imaging with a special multilayer structure in 2005 [85]. A review of thin metal strip waveguides for LRSPR and their applications was presented by [PERSON] in 2009 [86]. Further, [PERSON] et al. reported the existence of multiple phases in epithelial cell adhesion and showed that the penetration depth and propagation length of near-infrared SPR can be enhanced to more than 1 mm [87]. [PERSON] summarized new sensing materials, such as negative index materials,  
Figure 7: Schematic illustration of various geometries of SPR sensors. In I: (**a–c**) are conventional geometry modified fiber sensors [22; 56; 61]; In II: (**d–f**) are grating-assisted fiber sensors [66; 67; 68]; In III: (**g–h**) are specialty fiber sensors [70; 79].  
ZnO, and other analytical techniques combined with SPR, in his review paper [88]. In 2017, [PERSON] et al. proposed a spectro-polarimetric SPR sensor with high RI resolution and high tolerance that depends on the Ag film thickness [89]. CNT/Cu-nanoparticles with good dielectric properties were used for the design of a highly sensitive fiber SPR sensor based on the nanocomposite technique for the sensing of nitrates [90]. Recently, a review article by [PERSON] discussed different configurations for high-performance sensors via an extended surface electromagnetic wave (SEW) [91]. Moreover, cholesterol oxidase was employed as a sensing material in FOPS for measuring the concentration of cholesterol by [PERSON]'s group [92].  
To date, the conventional coating methods of SPR-based FOPS have been reported based on RF sputtering, thermal evaporation, electrolysis plating or wet-chemistry deposition, and chemical vapor deposition. There is no unanimously recognized method to list the characteristics influencing sensor performance. However, sensitivity, resolution, and detection limit are considered to be a few important parameters that characterize the performance of the sensor. Hence, the sensor performance of recent SPR-based FOPS has to be compared with standard prism-based systems, which are bulkier and costlier.