## 5 Experimental Results
### SPR-TPOF Sensors without Buffer Layer and with Different Taper Ratios  
In Figure 6, three pictures of different configurations of SPR-TPOF sensors are presented. After having obtained different TPOFs in terms of taper ratio using the developed system described in Section 2 to make the SPR sensors, we used the same procedure already described in Section 3 (configurations without the buffer layer) to obtain these three SPR-TPOF sensors by fixing different TPOFs in resin blocks, polishing them, and sputtering the same gold film on their D-shaped area (1 cm long). The images in Figure 6 were acquired by Dino-Lite digital microscope (AnMo Electronics Corporation, Taiwan). Configurations \"A\", \"B\", and \"C\" present taper ratios of about 1.3, 1.4, and 1.5, respectively. Figure 5: SPR spectra, normalized to reference spectrum, of (**a**) SPR-TPOF configuration with buffer layer and (**b**) without buffer layer. (**c**) Resonance wavelength variations (\(\Delta\lambda\)) with respect to the water (1.332) vs. the refractive index for the configurations with and without buffer layer, along with the linear fitting to the data and with the error bars. Linear fitting equation for the configuration with buffer layer: \(\Delta\lambda=1801.8\) n \(-2414.5\) (R\({}^{2}\) = 0.95). Linear fitting equation for the configuration without buffer layer: \(\Delta\lambda=1774.2\) n \(-2336.3\) (R\({}^{2}\) = 0.97). The performances of this kind of sensor are usually determined by parameters like sensitivity and signal-to-noise ratio (SNR) [17], so different water-glycerin mixtures with refractive indices ranging from 1.332 to 1.381 were used to test the produced SPR-TPOF sensor configurations. Figure 6: SPR-TPOF sensor images acquired from a digital microscope. (**a**) Configuration “A”, taper ratio 1.3; (**b**) configuration “B”, taper ratio 1.4; (**c**) configuration “C”, taper ratio 1.5. Figure 7 shows the SPR spectra, normalized to the reference spectrum, for (1) configuration \"A\", (2) configuration \"B\", and (3) configuration \"C\" when several water-glycerin mixtures are used in contact with the gold film. Figure 7: SPR spectra, normalized to reference spectrum, of (**a**) configuration “A”, (**b**) configuration “B”, and (**c**) configuration “C”. (**d**) Plasmon resonance wavelength variation (\(\Delta\lambda\)) with respect to water (1.332) as a function of the refractive index, along with quadratic fitting to the data, for each configuration. In these SPR-TPOF sensor configurations, when the refractive index increases, the resonance wavelength increases as well, and the shape of the SPR curve changes too. However, the shape and the resonance wavelength change in a different way when the taper ratio changes, as shown in Figure 7. In particular, when the taper ratio increases as the refractive index increases, both the resonance variation and the full width at half maximum of the SPR curve increase as well. The variation in resonance wavelength with respect to the water (1.332) as a function of the refractive index, along with the quadratic fitting to the experimental data, is reported in Figure 7d for all three configurations. In Figure 7d, each experimental value is the average of five subsequent measurements, and the respective standard deviations (error bars) are shown too. In Figure 7d, instead of the linear fitting, the quadratic fitting of data has been shown in order to improve the accuracy. In particular, for the three tested configurations, the fitting curve and the parameters are reported in Equation (2) and in Table 1, respectively. \[\Delta\lambda=B2\ n^{2}+B1\ n+intercept \tag{2}\]  
So, from Equation (1) and the fitting Equation (2), also reported in Figure 7d, it is possible to calculate the sensitivity for each configuration, as reported in Figure 8. Configuration \"C\", which corresponds to the higher taper ratio (about 1.5), presents the higher sensitivity. This result was expected because as the taper ratio increases, the sensitivity increases as well, such as reported in the theoretical approach [22].