Water-compatible surface molecularly imprinted silica nanoparticles as pseudostationary phase in electrokinetic chromatography for the enantioseparation of tryptophan

A novel approach based on surface molecularly imprinted technique is demonstrated for the synthesis of surface molecularly imprinted silica nanoparticles (MI-SiNPs) in aqueous media, using l-tryptophan (l-Trp) as template molecule. The MI-SiNPs were fully characterized by transmission electron microscopy (TEM), thermo gravimetric analysis (TGA) and atomic force microscopy (AFM), and the results showed MI-SiNPs thus prepared were uniform in particle size (∼87 nm) with a distinct core–shell structure with shell thickness of ∼7 nm. The terminal carboxyl groups endowed the MI-SiNPs with good water-compatibility, and so good suspension stability for MI-SiNPs in water-rich buffer solutions can be easily achieved without the addition of any other reagents. When MI-SiNPs were used as pseudostationary phases (PSPs) in electrokinetic chromatography (EKC) for the enantioseparation of Trp, a resolution of 2.73 can be achieved in less than 10 min with symmetric peaks. These excellent separation features are mainly attributed to the fast mass transfer and good accessibility of the interaction sites locating at the surface of the MI-SiNPs. The effects of some important separation factors, e.g., pH and concentration of buffer solution, content of MI-SiNPs added, content of organic modifier in buffer solution, on the enantioseparation of Trp were studied, and an optimum separation condition of 30% (v/v) acetonitrile in 20 mmol L−1 phosphate buffer (pH 7.0), with 0.50 mg mL−1 MI-SiNPs added was ultimately selected.