Interaction of bioactive compounds on capillary inner surfaces bearing a dense thermoresponsive polymer brush.

To confirm the modification of a dense thermoresponsive polymer brush on a silica surface by surface-initiated (SI) atom transfer radical polymerization (ATRP), poly(N-isopropylacrylamide) (PNIPAAm) was polymerized from a free ATRP initiator and an ATRP initiator immobilized silica particle (SiP) via ATRP under various initiator concentrations. The obtained PNIPAAm exhibited a well-controlled molecular weight and modification of the concentrated PNIPAAm brush on the SiP surface was confirmed. Based on the optimal ATRP conditions, the PNIPAAm- and poly(NIPAAm-co-butyl methacrylate [BMA]) (PIB)-grafted capillary lumen were prepared by SI-ATRP and used as capillary columns for micro high-performance liquid chromatography for the modulation of biomolecular interactions. The hydrophobic interactions of steroids on the thermoresponsive polymer brush grafted capillary lumens increased with increasing polymer length above the lower critical solution temperature for each polymer. In addition, an increase in the retention factor of steroids with the PIB-grafted capillary began at lower temperatures than that observed for the PNIPAAm-grafted capillary. Thus, the separation of steroids was achieved using a stepwise temperature gradient and was enhanced by modulating the aqueous eluent flow rate during thermoresponsive capillary chromatography. These results indicated that the chain lengths of the concentrated thermoresponsive polymer brushes on the capillary lumen have a crucial influence on the interaction and separation of bioactive compounds in an aqueous mobile phase.