Abstract
The use of polymerizable hosts in anion imprinting has led to powerful receptors with high oxyanion affinity and specificity in both aqueous and non-aqueous environments. As demonstrated in previous reports, a carefully tuned combination of orthogonally interacting binding groups, for example, positively charged and neutral hydrogen bonding monomers, allows receptors to be constructed for use in either organic or aqueous environments, in spite of the polymer being prepared in non-competitive solvent systems. We here report on a detailed experimental design of phenylphosphonic and benzoic acid-imprinted polymer libraries prepared using either urea- or thiourea-based host monomers in the presence or absence of cationic comonomers for charge-assisted anion recognition. A comparison of hydrophobic and hydrophilic crosslinking monomers allowed optimum conditions to be identified for oxyanion binding in non-aqueous, fully aqueous, or high-salt media. This showed that recognition improved with the water content for thiourea-based molecularly imprinted polymers (MIPs) based on hydrophobic EGDMA with an opposite behavior shown by the polymers prepared using the more hydrophilic crosslinker PETA. While the affinity of thiourea-based MIPs increased with the water content, the opposite was observed for the oxourea counterparts. Binding to the latter could however be enhanced by raising the pH or by the introduction of cationic amine- or Na+-complexing crown ether-based comonomers. Use of high-salt media as expected suppressed the amine-based charge assistance, whereas it enhanced the effect of the crown ether function. Use of the optimized receptors for removing the ubiquitous pesticide glyphosate from urine finally demonstrated their practical utility.
Highlights
Host guest chemistry has focused on small-molecule binders comprising macrocyclic, cleft- or cagelike receptors featuring convergent binding groups complementary in size, shape, and electronic configuration to the incoming guest.[3−6] Contrasting with these precisely defined receptors are molecularly imprinted polymers (MIPs) relying on the self-assembly principle.[7−12] Functional monomers are allowed to interact with a template followed by polymerization in the presence of a crosslinking monomer
Based on a detailed experimental design of phenylphosphonic and benzoic acid-imprinted polymer libraries using urea- or thiourea-based host monomers in the presence or absence of cationic comonomers, we demonstrated here powerful receptors capable of oxyanion recognition in non-aqueous, fully aqueous, or high-salt media
While the affinity of thioureabased MIPs increased with the water content, the opposite was observed for the oxourea counterparts
Summary
Commonly associated with the precision exerted by biomacromolecule receptors when binding a ligand, can be challenged by the action of artificial receptors designed bottom-up by synthetic organic chemistry.[1,2] Traditionally, host guest chemistry has focused on small-molecule binders comprising macrocyclic, cleft- or cagelike receptors featuring convergent binding groups complementary in size, shape, and electronic configuration to the incoming guest.[3−6] Contrasting with these precisely defined receptors are molecularly imprinted polymers (MIPs) relying on the self-assembly principle.[7−12] Functional monomers are allowed to interact with a template followed by polymerization in the presence of a crosslinking monomer. Methanol (MeOH) of HPLC receptors could be engineered to simultaneously recognize the oxyanion and its counterion.[26] This concept was used to prepare phosphate receptors compatible with high-salt media Inspired by these interesting results, we have probed here in more depth the parameters controlling receptor affinity and selectivity, notably the acidity and solubility of the urea grade and MeCN of HPLC grade were purchased from Acros (Geel, Belgium). To an ice-cooled solution of 2-aminoethyl methacrylate hydrochloride (0.663 g, 4 mmol) and triethylamine (0.558 mL, 4 mmol) in dry CH2Cl2 (30 mL), 3,5-bis (trifluoromethyl) phenyl isocyanate (0.692 mL, 4 mmol) was added slowly over 15 min under nitrogen followed by stirring of the reaction mixture at room temperature for 12 h. Solutions (1 mL) of PPA, PSA, or glyphosate (1 mM) were allowed to percolate through the columns whereafter the free anion concentrations in the receiving solution were measured using HPLC. Calibration standards were made up using the same solvent as used in the binding experiments
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