Investigating protein adsorption and desorption from nanoporous materials is an important task due to the wide application of nanostructured materials in protein separation, purification, and biosensor development. Nanoporous material ability to adsorb large molecules within the pores, comes from the pore size and is significantly dependent on the hydrophobic or hydrophilic properties of the material. Biosensors based on nanomaterial coatings attract significant attention, especially those that are highly sensitive to a specific protein or molecule [1]. Nanoporous metal oxide coatings can be used in the development of such biosensors due to the physical and chemical properties that they possess.
Porous anodic aluminium oxide (pAAO) coating, which is formed by electrochemical anodization of aluminium in an acidic electrolyte, is one of the typical self-organised fine structures with nanohole arrays. The pore diameter can be changed by varying the concentration and composition of the acidic electrolyte as well as the voltage of anodization [2]. pAAO exhibits such properties as biocompatibility, highly ordered self-assembled honeycomb pore structure, high mechanical and thermal stability, and chemical resistance. Various biosensing platforms based on porous aluminium oxide have been applied in optical and electrochemical biosensors [3].
Spectroscopic ellipsometry (SE) can be successfully used for optical biosensing because it requires no labelling and the light used for the measurement does not affect or destroy the samples. Furthermore, this method makes it possible to perform simultaneous measurement of both s and p polarisation, thus obtaining a better agreement between the experiment and model data [4].
In this work, the investigation of human serum albumin (HSA) adsorption on the pAAO coating was analysed using SE method. The thickness of the formed pAAO layer determined from SE results was 322.75 ± 0.12 nm. The estimated radius of the nanopores was 39 ± 5 nm and the distance between nanopores was 107 ± 6 nm. The obtained results showed that the protein concentration inside the nanochannels was several hundred times higher than in the buffer solution; the initial phase of the adsorption process was slow (3.23 mg·cm-3·min-1 ) in comparison with the protein desorption rate (21.2 mg·cm-3·min-1 ) by means of pAAO layer washing; conventional washing with PBS solution and deionized water does not completely remove HSA molecules from pAAO pores and, therefore, the HSA concentration inside nanopores after 16 h of washing still remains almost 100 times higher than that present in PBS solution.