Verification of surfactant CHAPS effect using AFM for making biomemory device consisting of recombinant azurin monolayer

In this study, a protein-based biomemory device was developed using a surface modified recombinant azurin layer and its surface characteristics were analyzed by atomic force microscopy. The cysteine-modified azurin used for this purpose was a metalloprotein that had redox properties. To immobilize the metalloprotein on the Au substrates, the cysteine-modified azurin layer was self-assembled on the Au surface through a covalent bond between the thiol group on the cysteine and the Au surface. In our previous work, we showed that this protein layer was formed as cohesive clusters on Au surface through physical adsorption. To reduce the formation of these cohesion clusters, a zwitterionic surfactant, (3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate) (CHAPS) was introduced to modify the surface properties. Using this approach, we found that CHAPS significantly reduced the amount of cysteine-modified azurin aggregates that nonspecifically adsorbed to the Au substrate. Atomic force microscopy was used to analyze the modified-surface. Based on this analysis, the size of the recombinant azurin clusters when CHAPS was used were about 15–25 nm whereas aggregates of 150–200 nm were observed in the absence of CHAPS. In addition, Raman spectroscopy was performed to confirm the retention of azurin molecules self-assembled on the Au surface. Electrochemical results using cyclic voltammetry indicated that recombinant azurin was successfully immobilized onto the Au surface with CHAPS and its redox property remained intact. Chronoamperometry was then used to demonstrate the memory characteristics of this azurin-based fabricated memory device. The combined results of this study show that CHAPS can significantly reduce the size of protein aggregates that become immobilized on the surface without a loss of the electrochemical properties of the protein.