# Surface Plasmon Resonance-Based Fiber Optic Sensors Utilizing Molecular Imprinting
## 4 Localized Surface Plasmon Resonance  
The light getting trapped within the metallic nanoparticles having a size smaller than the wavelength of light causes the collective oscillations of conduction electrons, resulting in localized surface plasmons. The displacement of electron clouds with respect to the nuclei occur due to this oscillation which in turn causes attractive and repulsive forces. This allows the transfer of energy to result in strong absorption, scattering and hence enhancement in the EM field. The field due to the resonance inside the nanoparticle creates a dipolar field outside the nanoparticle as shown in Figure 3[59; 60]. The frequency at which resonance occurs is called the resonance frequency. In the case of nanostructures, it is the absorbance that matters. Using the wavelength interrogation method, the change in refractive index can be measured with evaluation of the changing absorbance wavelength [61].  
Figure 2: The general optic fiber-based configuration for excitation of SPR.  
For the implementation of LSPR-based fiber optic sensor, over the unclad bare core of the sensing region, metallic nanoparticles are immobilized. The most used nanoparticles for sensing applications are those of silver and gold because of their sensitivity and biocompatibility. There are several key factors in the nanoparticles/nanostructures which affect the sensor performance. Some of these are the shape and size of the nanostructures, the alignment and separation between nanoparticles and thickness of the nanostructure layer. The shape of the nanoparticle/nanostructure is an important parameter which decides the aspect ratio. Aspect ratio, which is the surface to volume ratio of a nanostructure helps in tuning the sensor's performance and application. The higher the aspect ratio, the more the absorption is. The sensitivity can be improved by increasing the number of nanoparticles and their size, while limiting the distance between them [62]. The thickness of the nanostructure film also plays a significant role in sensor performance as it decides the interaction of the evanescent field with the sensing medium [63]. The field created by a nanoparticle at its space is affected by the surrounding nanoparticles and it will be different for each nanoparticle. Further, in the case of a single metal nanoparticle, there will be a fast decay in the field with distance from it [64] because the metal nanoparticle acts as a dipole, and for a dipole the electric field decay is inversely proportional to the square of the distance from the dipole [64]. Considering a nanoparticle, the ability of the particle to absorb light which transfers electrons from the ground to an excited state is called absorption cross section while the efficiency of a particle to scatter the photons approaching it is called scattering cross section. The combination of these is termed as extinction cross section which adds both the absorption and scattering efficiency of the particle. The extinction cross section of nanoparticles has a maximum value of \(\pi\)R\({}^{2}\). Nanostructures such as nanowires, nanorods, nanospheres which will be having different aspect ratios can be used to exploit the LSPR phenomenon and can be fabricated accordingly for different analytes to give high performance sensors. LSPR-based sensors have been reported to have applications in the field of energy, biomedicine, environment, food safety, water control, pharmaceuticals and agriculture [64]. A combination of SPR and LSPR in designing fiber probes for sensing applications has also been reported which greatly enhances the field to improve the sensitivity and limit of detection of the sensor [65; 66].