Switched recognition and release ability of temperature responsive molecularly imprinted polymers based on magnetic halloysite nanotubes

Abstract

CoFe₂O₄/halloysite nanotube magnetic composites (MHNTs) were firstly achieved via a wet impregnation technique, and then a thermal polymerization under (NH₄)₂S₂O₈ chain initiation in water was employed to obtain methacrylic acid-functionalized MHNTs (MAA-MHNTs). By decorating the MAA-MHNTs with the temperature responsive monomer N-isopropylacrylamide (NIPAM), a novel temperature responsive molecularly imprinted material based on the obtained NIPAM-MHNTs (TMMIPs) was synthesized by a surface imprinting technique. The results of characterization indicated that the TMMIPs exhibited magnetic sensitivity (M_s = 1.758 emu g⁻¹), magnetic stability (in the pH range of 2.0–8.0), thermal stability (especially below 200 °C) and contained a temperature responsive imprinted layer (the lower critical solution temperature was 33.96 °C). Then the TMMIPs were applied to switched recognition and release of 2,4,5-trichlorophenol molecules (5-TCP) by changing the medium temperature. TMMIPs showed outstanding recognition ability towards the imprinted species under high temperature conditions (such as 60 °C), due to the loss of hydration and a collapsed state of inter-poly(N-isopropylacrylamide), which resulted in the formation of a specific structure between 5-TCP and the polymer network. In contrast, at relatively low temperatures (such as 20 °C), the captured 5-TCP was released from the swelled TMMIPs, which resulted from increasing the distance between 5-TCP and the polymer network. The selective analysis demonstrated the high affinity and selectivity of TMMIPs towards 5-TCP over competitive phenolic compounds, and the specific recognition of binding sites may be based on the distinct size, structure and functional groups of the template molecules.