Ionic liquids as porogens for molecularly imprinted polymers: propranolol, a model study

Abstract

The selectivity and rebinding capacity of molecularly imprinted polymers selective for propranolol (1) using the room temperature ionic liquids [BMIM][BF₄], [BMIM][PF₆], [HMIM][PF₆] and [OMIM][PF₆] and CHCl₃ were examined. The observed IF (imprinting factor) values for MIP_BF4, MIP_PF6 and MIP_CHCl3 were 1.0, 1.98 and 4.64, respectively. The longer chain HMIM and OMIM systems returned lower IF values of 1.1 and 2.3, respectively. MIP_PF6 also showed a ∼25% binding capacity reduction vs. MIP_CHCl3 (5 μmol g⁻¹vs. 7 μmol g⁻¹ respectively). MIP_CHCl3 and MIP_PF6 differed in terms of BET surface area (306 m² g⁻¹vs. 185 m² g⁻¹), pore size (1.10 and 2.19 nm vs. 0.97 and 7.06 nm), the relative number of pores (Type A: 10.4 vs. 7.5%; Type B: 8.5 vs. 3.0%), and surface zeta potential (−37.9 mV vs. −20.3 mV). The MIP specificity for 1 was examined by selective rebinding studies with caffeine (2) and ephedrine (3). MIP_PF6 rebound higher quantities of 2 than MIP_CHCl3, but this was largely due to non-specific binding. Both MIP_CHCl3 and MIP_PF6 showed a higher affinity for 3 than for 2. Reduction in the Room Temperature Ionic Liquid (RTIL) porogen volume had little impact on the polymer morphology, but did result in a modest decrease in IF from 2.6 to 2.3 and in the binding capacity (30% to 19%). MIP_CHCl3 retained the highest template specificity on rebinding from CHCl₃ (IF = 4.6) dropping to IF = 0.6 in MeOH/[BMIM][PF₆]. The MIP_CHCl3 binding capacity remained constant using CHCl₃, CH₂Cl₂ and MeOH (46–52%), dropped to 6% on addition of [BMIM][PF₆] and increased to 83% in H₂O (but at the expense of specificity with IF_H2O = 1.4). MIP_PF6 rebinding from MeOH saw an increase in specific rebinding to IF = 4.9 and also an increase in binding capacity to 48% when rebinding 1 from MeOH and to 42% and 45% with H₂O and CH₂Cl₂, respectively, although in the latter case the increased capacity was at the cost of specificity with IF_CH2Cl2 = 1.2. Overall the MIP_PF6 capacity and specificity were enhanced on addition of MeOH.