###### Abstract  
The influence of geometrical parameters (pore radii and porosity) on ion transport through two almost ideal nanoporous alumina membranes (NPAMs) coated with a thin TiO\({}_{2}\) layer by the atomic layer deposition technique (Sf-NPAM/TiO\({}_{2}\) and Ox-NPAM/TiO\({}_{2}\) samples) was analyzed by membrane potential and electrochemical impedance spectroscopy measurements. The results showed the significant effect of pore radii (10 nm for Sf-NPAM/TiO\({}_{2}\) and 13 nm for Ox-NPAM/TiO\({}_{2}\)) when compared with porosity (9% and 6%, respectively). Both electrochemical techniques were also used for estimation of protein (bovine serum albumin or BSA) static fouling, and the results seem to indicate deposition of a BSA layer on the Sf-NPAM/TiO\({}_{2}\) fouled membrane surface but pore-wall deposition in the case of the fouled Ox-NPAM/TiO\({}_{2}\) sample. Moreover, a typical and simple optical technique such as light transmission/reflection (wavelength ranging between 0 and 2000 nm) was also used for membrane analysis, showing only slight transmittance differences in the visible region when both clean membranes were compared. However, a rather significant transmittance reduction (\(\sim\)18%) was observed for the fouled Sf-NPAM/TiO\({}_{2}\) sample compared to the fouled Ox-NPAM/TiO\({}_{2}\) sample, and was associated with BSA deposition on the membrane surface, thus supporting the electrochemical analysis results. application. Consequently, possible easy membrane surface modification able to confer the membrane optimal geometrical and material characteristics for a specific application is of great interest. Different techniques such as dip coating, grafting, chemical vapor deposition, plasma polymerization, or atomic layer deposition have already been proposed for NPAM surface functionalization/modification [18; 19; 20]. In particular, the atomic layer deposition (ALD) technique is a suitable manner of modifying both membrane pore size and surface physicochemical characteristics (charge, hydrophilicity, or biocompatibility) by covering external and internal (pore-wall) surfaces of NPAMs with a homogeneous layer of a selected material (nitrides, sulfides, or metal oxides such as SiO\({}_{2}\), ZnO, TiO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), or even Al\({}_{2}\)O\({}_{3}\), when only geometrical parameters have to be modified), providing to these nanoporous alumina-based membranes (NPA-bMs) desired geometrical and material properties [20; 21]. In fact, the ALD technique is especially appropriate for modification of membranes with high aspect ratios (that is, pore length/pore diameter > 2000), and it also allows surface modification with two or more layers from different materials [22]. As indicated above, a significant problem associated with the use of planar NPA-bMs in the separation/transport of solutions containing proteins or macromolecules is membrane fouling, because it can significantly reduce the effectiveness of the process, necessitating the development of efficient cleaning procedures [9]. Streaming potential measurements have traditionally been performed for determination and analysis of membrane fouling, although the pressure difference needed for such measurements could not be appropriate for fragile samples as is the case with the NPA-bMs indicated above. Other kinds of electrochemical experiments such as membrane potential (\(\Delta\Phi_{\mathrm{mbr}}\)) or electrochemical impedance spectroscopy (EIS) have also been used for membrane fouling estimation [23; 24; 25; 26; 27], but different and specific optical techniques (ellipsometry spectroscopy or optical coherence tomography) have also been proposed [28; 29]. On the other hand, it was already reported that modification with TiO\({}_{2}\) nanoparticles of different types of membranes for diverse applications (membrane bioreactors, oily emulsion filtration, etc.) seems to reduce the fouling effect independently of membrane material and TiO\({}_{2}\) particle immobilization procedure (entrapped, deposited, or deep coating) [30; 31; 32].