Specific Adsorption of Glycine onto Atomically Flat Al₂O₃(0001) Surfaces in Aqueous Solution

Abstract

To understand the adsorption of functional proteins onto metal oxides in living bodies at the atomic/molecular scale, we examined the adsorption behaviors of amino acids, the fundamental elements of proteins, onto atomically flat surfaces of metal oxides not in ultra-high vacuum but in aqueous solutions using atomic force microscopy (AFM) as a preliminary step in the study of the adhesion processes of living cells onto various biomaterials. Two types of atomically flat Al₂O₃(0001) surface were observed in aqueous solutions of glycine and L-alanine using the AFM in contact mode. The first type (single-step surface) consisted of atomically flat terraces separated periodically by almost parallel steps of unit cell height. By contrast, the step height in the second type (multi-step surface) was about 5–10 unit cells owing to step bunching. In glycine solution, specific adsorption onto the multi-step surface was observed in the terrace positions apart from step edges, whereas no adsorbates were found in the AFM images of the single-step surface. Conversely, uniform adsorbates were found on both single- and multi-step surfaces in L-alanine solution. The multi-step surface is expected to be used as a tool for controlling glycine adsorption owing to the specificity of adsorption.