## 3 Results and Discussion
### Optical Characterization  
Optical techniques (light transmission and spectroscopic ellipsometry), which are of great interest due to their non-destructive character, have already demonstrated their suitability for characterization of nanoporous alumina membranes with different pore sizes and porosity (or even asymmetric structure) or human serum albumin adsorption on alumina supports [35; 58]. In this context, depending on geometrical parameters (pore-size/porosity) or surface material, differences in light transmission through nanoporous alumina-based membranes are expected. In fact, differences in transmission spectra through membranes obtained by coating Sf-NPAM or Ox-NPAM samples with a layer of different metal oxides have already been analyzed [20; 22]. Figure 9a shows a comparison of transmission spectra for two samples obtained by coating the Sf-NPAM support with a layer of Fe\({}_{2}\)O\({}_{3}\) or Al\({}_{2}\)O\({}_{3}\) (Sf-NPAM/Fe\({}_{2}\)O\({}_{3}\) or Sf-NPAM/Al\({}_{2}\)O\({}_{3}\) samples, with similar pore-size, porosity, and thickness values than Sf-NPAM/TiO\({}_{2}\)), and clear differences in the visible region can be observed due to the different surface material (although both membranes show high and similar transparency in the near infrared region); moreover, transmission for another membrane, obtained by coating the Ox-NPAM support with a SiO\({}_{2}\) layer (Ox-NPAM/SiO\({}_{2}\) sample), of different pore-size, porosity, and surface material than the Sf-NPAM/Fe\({}_{2}\)O\({}_{3}\) and Sf-NPAM/Al\({}_{2}\)O\({}_{3}\) samples, was also measured, and transmission differences in both visible and near infrared regions were obtained. These previous results seem to support the possibility of using light transmission spectra for changes associated with membrane fouling for this kind of sample. Figure 8: Comparison of Nyquist plot for: (\(\lozenge\)) Ox-NPAM/TiO\({}_{2}\) membrane and (\(\blacklozenge\)) Ox-NPAM/TiO\({}_{2}\)(f) membrane (C = 0.002 M NaCl). Then, optical characterization of both clean membranes was firstly considered and Figure 9b shows a comparison of the transmittance spectra, where the high transparency of both membranes (>70% in the visible region) and around 88% for near infrared region (NIR) can be seen. The similarity in light transmission exhibited by both membranes is significant, taking into account that they have different geometrical parameters. This may be due to a kind of pore-size/porosity compensation, as the sample with lower pore radii presents higher porosity and the free volume of both membranes is rather similar (Ox-NPAM/TiO\({}_{2}\)/Sf-NPAM/TiO\({}_{2}\) free volume ratio = 1.13), as indirectly established by the similarity of hydraulic permeability values already indicated. On the other hand, transmittance analysis also provides an indirect confirmation of the adequate TiO\({}_{2}\)-coverage of the nanoporous alumina support by the estimation of membrane band gap values, which are between 310 nm and 323 nm and agree with reported values for TiO\({}_{2}\)[59], differing significantly from the band gap for Al\({}_{2}\)O\({}_{3}\) materials (around 200 nm [60], and logically from Fe\({}_{2}\)O\({}_{3}\) as can be seen in Figure 9a); this point agrees with the almost total TiO\({}_{2}\)-coverage of the alumina support determined by the XPS technique [36] already mentioned (Table S1 in Supplementary Information). The effect of membrane BSA fouling on light transmission spectra can be observed in Figure 10. In the case of the Ox-NPAM/TiO\({}_{2}\)(f) membrane (Figure 10a), a certain wavelength-dependent reduction in the visible region was determined (9.0% at 600 nm), but almost no change is observed in the near infrared region. This small effect of BSA fouling on light transmission might support protein deposition on pore walls as the main fouling mechanism determined from electrochemical measurements, due to the reduced free volume of studied membranes and taking into account the high transparency exhibited by their solid structure.