# Role of Functional Monomers upon the Properties of Bisphenol A Molecularly Imprinted Silica Films
## 3 Results and Discussion
### Optical Properties of BPA-MIP and NIP Films  
The spectroscopic ellipsometry (SE) measurements were performed to highlight the effect of imprinting on the optical properties of films, in terms of refractive index and transmittance. The refractive index of the films was measured in the 200-1000 nm wavelength domenium and the transmittance profiles were evaluated up to 900 nm spectral range, respectively, in different points of the films surface. In this respect, the Cauchy dispersion model was used for modelling the ellipsometric experimental data (Figures S5.1-S5.4, SM) which delivered reliable values for the films thickness and surface roughness.  
The thickness and surface roughness for both prepared sets of films, summarized in Table 2, revealed major differences as function of the various employed monomers. For MPTES-based films the thickness varied between 230 and 290 nm, while for the DAMO-T-based films was 10-fold higher. A similar trend was registered for the roughness, as well. The thickness and roughness discrepancy between the two types of films was attributed to the different growth mechanism of the organosilica matrix upon BPA addition and to the presence of TEOS, supporting the results attained in the morphological assessment.  
Figure 6 shows the refractive index, n, and the transmittance spectra of BPA-MIP\({}_{\text{DAMOT}}\) and BPA-MIP\({}_{\text{MPTTES}}\) films against their corresponding NIPs. Pronounced modifications of the optical features (transmittance and refractive index) for both type of films were recorded particularly in the 300-550 nm wavelength region. The variance of refractive index and transmittance presented similar profiles for both MIP films, indicating the presence of BPA in both film structures. The refractive index (Figure 6a) showed great fluctuations for both types of films. For example, the n values registered for the MIP/NIP films with DAMO-T were affected by thickness and, thus, their whole profile is shifted towards higher n values  
\begin{table}
\begin{tabular}{c c c c} \hline
**Film** & **d\({}_{\text{layer}}\) [nm]** & **Roughness [nm]** & **MSE** \\ \hline BPA-MIP\({}_{\text{DAMOT}}\) & 2897.58 & 0.62 & 0.49 \\ NIP\({}_{\text{DAMOT}}\) & 3156.57 & 0.32 & 0.52 \\ BPA-MIP\({}_{\text{MPTTES}}\) & 237.36 & 1.31 & 0.46 \\ NIP\({}_{\text{MPTTES}}\) & 290.33 & 0.97 & 1.63 \\ \hline \end{tabular}
\end{table}
Table 2: Data on films thickness (d\({}_{\text{layer}}\)), roughness and mean square error (MSE) for thickness determined by ellipsometry.  
compared to the MIP/NIP pair with MPTES. However, the same trend is maintained between the imprinted and non-imprinted films suggesting that light travels faster through the non-imprinted material (as lower n values were registered in both cases compared to the ones of MIPs).  
In accordance with the refractive index, the transmittance profiles of the BPA-MIP films registered a drop of about 25% T compared to the NIPs (Figure 6b). This sudden decrease from 90% T to around 66% T was again attributed to the presence of BPA. Yet, the BPA-MIP\({}_{\text{DAMOT}}\) film led to a more obvious decrease of transmittance as the film thickness is 10-fold higher than the one of BPA-MIP\({}_{\text{MPTES}}\).  
Both the refractive index and transmittance profiles are consistent with the findings from previous studies [22,34]. These results strengthen the belief that ellipsometric data may better underline the effect of imprinting and the presence of the template when preparing thin and transparent MIP films. Hence, it can be used as a complementary method to characterize the optical features of MIP films and decide upon the potential of such films to be used as sensitive elements for building optical sensors. In addition, the small values of the roughness for the MPTES-based films obtained in this work are considered suitable for optical sensing applications [42].