Porous Aluminium Oxide Coating for the Development of Spectroscopic Ellipsometry Based Biosensor: Evaluation of Human Serum Albumin Adsorption

An electrochemically synthesised porous anodic aluminium oxide (pAAO) layer has been analysed by means of spectroscopic ellipsometry. The determined thickness of the formed pAAO layer obtained from spectroscopic ellipsometry measurements and modelling was 322.75 ± 0.12 nm. The radius of the nanopores estimated from SEM images was 39 ± 5 nm and the distance between nanopores was 107 ± 6 nm. The investigation of human serum albumin (HSA) adsorption on the pAAO coating showed that: (i) the protein concentration inside nanopores, depending on exposure time, approximately was from 200 up to 600 times higher than that determined in buffer solution; (ii) the initial phase of the adsorption process is 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; (iii) conventional washing with PBS solution and deionised 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. Thus, due to such binding ability, HSA can be successfully used for the blocking of the remaining free surface, which is applied for the reduction in non-specific binding after the immobilisation of biorecognition molecules on the pAAO surface. It was determined that some desorption of HSA molecules from the pAAO layer occurred during the sensor’s surface washing step; however, HSA concentration inside the nanopores still remained rather high. These results recommend the continued application of pAAO in the development of biosensors.