# Photonic Crystal-Based Sensors for Detecting Alcohol Concentration
## 3 Results and Discussion
### Ethanol Concentration Examined by PS Opal and TiO\({}_{2}\) Inverse Opal Structures  
The photonic band gap of inverse opals depends on the refractive index contrast between the crystal and the pore fluid. A change of refractive index of the pore fluid causes variations in the photonic band gap which can be observed as change in the wavelength of reflected light of the crystal (color change). Based on this principle, we used for identification of ethanol concentration [28]. The reflectance peak shift (\(\Delta\lambda\) = 50 nm) of the PS opal sensor response with different ethanol concentrations in Figure 7a is unremarkable. On the other hand, the red shift for the 600 \({}^{\circ}\)C sintered TiO\({}_{2}\) sensor is 170 nm, as shown in Figure 7c, which is higher than that for the sensor sintered at 500 \({}^{\circ}\)C (\(\Delta\lambda\) = 100 nm), as shown in Figure 7b. That is, the 600 \({}^{\circ}\)C sintered sensor could detect the ethanol concentration better than the sensor sintered at 500 \({}^{\circ}\)C. Therefore, the TiO\({}_{2}\) inverse opal structure is more suitable for quantitatively detecting the ethanol concentration than PS opal structure. It is evident that the 600 \({}^{\circ}\)C sintered TiO\({}_{2}\) film is the best candidate for an ethanol concentration sensor. Moreover, because the TiO\({}_{2}\) inverse opal structure in our work has a reflectance peak shift of \(\Delta\lambda\) = 170 nm, it should more easily recognize the ethanol concentration previously developed sensors (\(\Delta\lambda\) = 15 nm in [3]). The main peaks reflected from the PS opal and TiO\({}_{2}\) inverse opal sensors with different ethanol concentrations are listed in Table 1. In addition, the weaker reflectance might be resulted from the random scattering of the imperfect films after TiO\({}_{2}\) sols sintering. Figure 6: The XRD patterns of the titania inverse opal sintered at (**a**) 500 \({}^{\circ}\)C and (**b**) 600 \({}^{\circ}\)C.  
determined the peak maximum from the spectra in Figure 7 at the measuring range (450-750 nm) of the apparatus. In our measurement, a piece of a microscope glass substrate was used as the background reference. A nonlinear relationship between the ethanol concentrations and the corresponding reflectance peak positions was fitted by a second degree polynomial function also shown in Figure 8. The reflectance peak shift with the ethanol concentration from the PS opal is smallest, as shown in Figure 8a. This result revealed that the resolving power greatly declined for ethanol concentration detecting. Therefore, the more difference of the refractive indices between the inverse opal sensor and ethanol concentration, the higher reflectance peak shift can be obtained, as shown in Figure 8b,c. The red shift for the 600 \({}^{\circ}\)C sintered TiO\({}_{2}\) sensor is greater than that for the sensor sintered at 500 \({}^{\circ}\)C, because the former with a higher refractive index. Furthermore, the curve (c) in Figure 8 shown that the 600 \({}^{\circ}\)C sintered TiO\({}_{2}\) inverse opal sensor retains a strong but not a perfect linear relationship between the red shift and the ethanol concentration in the region between 10% and 80% (\(v/v_{0}\)). The red shifts of the peak position were observed with the increasing the ethanol concentration because the average of refractive index (\(n_{\text{a}}\)) of inverse opal are increasing and cause a red shift reflectance peak position. The trend curve in Figure 8c is consistent with the refractive index of ethanol-water mixture [29]. At higher ethanol concentration (>80%), the red shift reaches a plateau, which defines the detection limit of the TiO\({}_{2}\) inverse opal film sensor. These results are also consistent with the previously reported results using a surface plasmon resonance (SPR) sensor [30]. In this study, we found that 600 \({}^{\circ}\)C sintered TiO\({}_{2}\) sensor not only retains a good fit but also possesses high precision.