Layer-by-layer self-assembled multi-walled carbon nanotubes/silica microsphere composites as stationary phase for high-performance liquid chromatography

Multi-walled carbon nanotubes (MWCNTs) were layer-by-layer assembled onto silica microspheres to form MWCNTs/SiO2 composites and were characterized by surface analysis, elemental analysis, and contact angle. The composite was used as a stationary phase in HPLC for the separation of many kinds of aromatic compounds. The separation of polycyclic aromatic hydrocarbons (PAHs) affected by the number of MWCNTs layers on SiO2 was investigated. Retention times and resolutions of PAHs were affected by the assembled layer of MWCNTs and the type of organic additive in the mobile phase. When the MWCNTs layer increased to 5 (MWCNTs/SiO2-5), PAHs can be completely separated. Aromatic compounds with different substituents can be well separated when only water was used as the eluent on the MWCNTs/SiO2-5 column. Decreasing the pH of the eluent can increase the retention factors of organic acids. Organic amines can also be separated with acetonitrile as organic additive, which were eluted in the order of the electronic cloud density of their aromatic rings. Comparing the MWCNTs/SiO2-5 column with a commercial HPLC column, it was clearly shown that the commercial column exhibited the characteristic of hydrophobicity, and the MWNCTs/SiO2-5 column exhibited the characteristic of a large pi-electron system.