Novel Separation Media with Metal Oxide Nanostructures for Capillary Electrochromatography

Zinc oxide nanowires (ZnO nanowire, ZnO NWs) are nanostructures that have drawn attention as separation media for efficient biomolecules because of high biological compatibility and low cost. Development of the capillary column (ZnO column) using a ZnO NW to an inner wall has been reported, although there are only a few studies about molecular recognition of a ZnO NW regardless of numerous studies reporting ZnO NWs. In our previous studies, we conducted fundamental research to elucidate molecular recognition of ZnO NW and develop a novel liquid phase separation field. Consequently, we achieved baseline separation of mixed adenosine phosphate analytes using a phosphate buffer in the mobile phase. In this study, to improve the low resistance of ZnO NW toward a solvent, we covered a surface of ZnO NW with titanium oxide (TiO₂) thin layers using atomic layer deposition. As a result, the column (TiO₂ NW column) showed high affinity toward acidic compounds like the ZnO column, strongly interacting with especially phosphate groups. Resistance of ZnO NW to a weak acidic buffer solution was then dramatically improved. This is because multipoint electrostatic interaction between the phosphate groups and the NW surface occurred. Next, we conducted capillary electrochromatography to examine the possibility for application of separation analysis. The elution order of the phosphorylated compound was successfully controlled by the migration solution containing aqueous acetonitrile with weak acids.