Capillary electrophoretic separation of biologically active amines and acids using nanoparticle-coated capillaries

This manuscript describes dynamic coating of capillaries with poly(L-lysine) (PLL) and silica nanoparticles (SiO₂ NPs) and use of the as-prepared capillaries for the separation of biogenic amines and acids by CE in conjunction with LIF detection. The directions of EOF are controlled by varying the outmost layer of the capillaries with PLL and SiO₂ NPs, respectively. Over the pH range 3.0-5.0, the (PLL-SiO₂NP)_n-PLL capillaries have an EOF toward the anodic end and are more suitable for the separation of acids with respect to speed, while the (PLL-SiO₂NP)_n capillaries have an EOF toward the cathodic end and are more suitable for the separation of biogenic amines regarding speed and sensitivity. The separations of standard solutions containing five amines and two acids by CE with LIF detection using (PLL-SiO₂NP)₂-PLL and (PLL-SiO₂NP)₃ capillaries were accomplished within 10 and 7 min, providing plate numbers of 3.8 and 5.0 × 10⁴ plates/m for 5-hydroxytryptamine (5-HT), respectively. The LODs for 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) are 32 and 2 nM and 0.2 and 1.5 nM when using the (PLL-SiO₂NP)₂-PLL and (PLL-SiO₂NP)₃ capillaries, respectively. Identification and quantification of 5-HIAA, homovanillic acid, and DL-vanillomandelic acid in urine samples from a male before and after drinking green tea were tested to validate practicality of the present approach. The results show that the (PLL-SiO₂NP)₂-PLL capillary provides greater resolving power, while the (PLL-SiO₂NP)₃ capillary provides better sensitivity, higher efficiency, and longer durability for the separation of the amines and acids.