# Recent Advances in Plasmonic Sensor-Based Fiber Optic Probes for Biological Applications
## 9 Conclusions and Future Directions  
In this paper, we have reviewed the theoretical model and experimental techniques that culminated in the successful realization of FOPS for biological applications. In this review, we have explained the SPR technique in different media, namely, prism-based SPR FOPS, grating-based SPR FOPS, and optical fiber-based SPR FOPS. Currently, the novel SPR-based FOPS geometries and approaches are being explored by several groups to model sensors for achieving high sensitivity and high resolution with better performance. It is noted that the optical fiber-based SPR FOPS are enormously attractive for the possibility of sensor miniaturization. Nowadays, the unique properties of the metal nanoparticles are used to explain the phenomenon of LSPR in FOPS. Several functionalization strategies for metal-based LSPR have been established. In this line, engineering the optical properties and thin-film thickness are carried for such lamellar SPR-based FOPS for point-of-care biosensing applications.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Type of Metal Coating** & **Advantage** & **Disadvantage** & **FOPS Geometric Structure** \\ \hline Selective metal coating [76] & Analyte is filled in the chosen air hole cell & Metal coating is challenging on the circular air holes & \\ External metal coating [50] & Analyte is filled in the external surface of the fiber & Different air hole sizes are required. & \\ Internal nanowire filling [133] & Analyte is filled in the nanowire & Selective filling of air hole in nano size is challenging & \\ Side-polished [125] & A flow of analyte is allowed at the outer surface of the fiber & A precise etching and polishing exertion are needed & \\ \hline \end{tabular}
\end{table}
Table 2: Various metal-coating techniques in the different geometry of FPOS (Fiber Optic Plasmonic Sensor).  
Recent trends concerning several geometries of the FOPS have been discussed. We have found that the applications of FOPS in medical diagnosis, such as characteristics of particular biomolecular species, have been analyzed on buffered samples instead of clinical samples. With the SPR-based FOPS, the penetration depth is limited. Identifying a specific molecule among groups of molecules with less interaction between the unwanted molecules remains difficult. Besides high sensitivity and better performance, compactness is also a challenging task for future research. For the early detection of cancer and cardiovascular problems such as tissue perfusion, blood pressure, heart rate, etc., novel biosensors can be developed by exploiting the optical properties of recently proposed materials such as TMDC, hBN, black phosphorus, etc. Furthermore, new fiber designs may also be introduced for achieving low-cost biosensors. FOPS shall also be considered for understanding the role of several characterizations of optical fibers during the sensing process. Finally, we believe that this review will help encourage further research in the field of SPR-based FOPS.  
Conceptualization, All authors; Methodology, All Authors; Software, M.S.A.G. and S.C.; Validation, K.S., P.R.B., K.N. and Q.L.; Formal Analysis, All Authors; Investigation, M.S.A.G. and S.C.; Resources, All Authors; Data Curation, M.S.A.G. and S.C.; Writing--Original Draft Preparation, M.S.A.G. and S.C.; Writing--Review and Editing, All Authors; Visualization, K.S., P.R.B., K.N. and Q.L.; Supervision, K.S., P.R.B., K.N. and Q.L.; Project Administration, K.N. and Q.L. Funding Acquisition, K.N. and Q.L.  
This research was funded by National Natural Science Foundation of China (NSFC), grant number [61675008].  
KN wishes to thank The Royal Society Kan Tong Po International Fellowship 2018 for the travel fund to visit Hong Kong Polytechnic University and Shenzhen Science and Technology Innovation Commission (Project GJHZ20180411185015272).  
The authors declare no conflict of interest.