Ionic liquid-functionalized graphene quantum dot-bonded silica as multi-mode HPLC stationary phase with enhanced selectivity for acid compounds

Graphene quantum dots (GQDs), which can form hydrophobic, π–π stacking, hydrogen bond and hydrophilic interactions with analytes, are a type of multifunctional separation materials. GQD-based stationary phases have great prospects in HPLC. In this study, 1-aminoethyl-3-methylimidazolium bromide ionic liquids (ILs) were chosen as functional molecules to improve the chromatographic performance of GQD-bonded silica stationary phase. This new phase could be applied in multiple separation modes including normal phase, reverse phase, ionic exchange and hydrophilic chromatography modes. Anilines, phenols and polycyclic aromatic hydrocarbons were efficiently separated on the IL/GQD/SiO2 column within 10 minutes in the normal phase and reverse phase modes. Five inorganic anions were baseline separated in the ionic exchange mode. Hydrophilic compounds including alkaloids, glycosides, amino acids and aromatic acids achieved satisfactory resolutions on this stationary phase in the hydrophilic chromatography mode. The separation performance of the IL/GQD/SiO2 column was compared to those of GQD/SiO2 column and IL/SiO2 column; it was found that the IL/GQD/SiO2 column showed better separation performance than the other two columns, which indicated ILs and GQDs had certain synergistic and complementary effects. Introduction of ILs enhanced the separation performance of the GQD-based phase, especially for acid compounds. By investigating the effects of buffer pH, buffer concentration and water content in the mobile phase, it was found that the electrostatic adsorption retention mechanism played a major role in the hydrophilic chromatography mode. This stationary phase, which allows the selection of appropriate separation modes on the basis of analytes, possesses practical prospects after further improvement.