# Surface Plasmon Resonance-Based Fiber Optic Sensors Utilizing Molecular Imprinting
## 3 Fiber Optic Surface Plasmon Resonance Sensors
### Performance Parameters  
A general fiber optic SPR sensor configuration is shown in Figure 2 with a coating of metal over the unclad bare core of the optical fiber surrounded by the dielectric sensing medium. The performance of a sensor probe is evaluated in terms of various parameters such as operating range, sensitivity, detection accuracy, selectivity, figure of merit (FOM), regeneration, stability, limit of detection (LOD) and limit of quantification (LOQ). The operating range of a sensor is an important parameter that decides the application and sustainability of sensor. The structure of the probe also needs to be stable in harsh environments as well as in varying pH and temperature for a particular application. The analysis result of the sensor needs to be stable with time, and should have the ability to recover; otherwise it will be a onetime usable probe. In the case of SPR-based fiber optic sensors, sensitivity (S) is defined as the change in resonance wavelength per unit change in the refractive index of the sensing media. In other words:  
\[S=\frac{\delta\lambda_{res}}{\delta n_{d}} \tag{3}\]  
where \(\lambda_{res}\) is the resonance wavelength at which the evanescent wave's propagation constant equalizes that of the surface plasmon wave's and \(n_{d}\) is the refractive index of the dielectric sensing medium. The higher the value of S, the better the sensor performance is. Sensitivity can also be calculated by taking the slope of the calibration curve showing the variation of resonance wavelength with analyte concentration or refractive index. The accuracy with which the resonance wavelength can be determined is called detection accuracy. The detection accuracy will be better if the width of the SPR curve is narrower. Hence it is inversely related to the full width at half maximum of the SPR curve. Figure of merit, another important parameter, depends on both full width at half maximum and sensor's sensitivity [45]. FOM should be maximum for the best performance of a sensor which is usually taken as the ratio of sensitivity to full width at half maximum. The selectivity of the sensor shows how precisely it can detect a particular analyte in the presence of interfering mixtures. Hence in the case of a SPR-based fiber optic sensor with a spectral interrogation method the total shift in resonance wavelength is observed in the presence of interfering mixtures which shows how effectively the sensor can detect the required analyte. The selectivity parameter is important in the case of biological samples and clinical applications. Selectivity is a specific nature of FO-SPR-MIP sensors when compared to all the other existing method-based sensors which will be discussed in the section on molecular imprinting. Limit of detection (LOD) shows the lowest concentration that can be detected by the sensor depending on its spectral resolution. The factor, spectral resolution (\(\Delta\lambda\)) of the spectrometer, plays an important role since it shows the minimal shift in the resonance wavelength that can be determined and hence decides the lowest concentration detection. Apart from spectral resolution, LOD depends on the sensitivity of the sensor at the near zero concentration of the analyte. It is calculated as the ratio of the spectral resolution of spectrometer to the sensitivity evaluated for the sensor near minimum/zero concentration (\(S_{c_{\min}}\)) of the analyte. Mathematically, it can be written as:  
\[LOD=\frac{\Delta\lambda}{S_{c_{\min}}} \tag{4}\]  
Thus, the LOD of the sensor depends on the spectrometer used for the calibration. The possibility of noise in the system and the limitations regarding spectral resolution affect the accuracy of the measurement values [46]. The LOD is an ideal quantity which does not take into account the noise/fluctuation in the measurement.