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Molecularly imprinted polymers selective for Di(2-ethylhexyl)phthalate

Phthalate MIPs were prepared using only one kind of functional monomer (methacrylic acid - MAA), but different templates (bis(2-ethylhexyl)phthalate - DEHP, benzylbutylphthalate - BBP, and dibutylphthalate - DBP), cross-linkers (ethylene glycol dimethacrylate - EDMA and TRIM), porogens (cyclohexane - CH and acetonitrile - ACN), and polymerization methods (bulk polymerization - BP, precipitation polymerization - PP, miniemulsion polymerization - MEP).

While with BBP and DBP only a very weak imprinting effect could be observed with corresponding MIPs (Imprinting Factor ~2.5), the use of DEHP as template molecule in combination with MAA and EDMA led to a polymer, which exhibited most significant selectivity (IF 8.3).

The lacking binding of the template (DEHP) in the rather polar porogen (ACN) indicates that the interaction of DEHP with the MIP is not based on hydrogen bonding but moreover on hydrophobic interaction between template and polymer. Using HPLC and ACN-water (60:40, v/v) as mobile phase, almost baseline separation of the three different phthalate esters was obtained.

This was not possible with MIPs, which were prepared accordingly but using DBP and BBP as templates and, therefore, is a clear indication that this different binding is not caused by a non-specific interaction of these analytes to the polymer.

If the different polymerization methods are compared, significantly higher capacity factors were obtained using PP in comparison to BP. This should be based on the difference in surface area between both polymers. However, the IF were almost the same (8.3 vs. 9.8).

Using PP and MEP, nano-sized MIPs were prepared. Significantly smaller particles were obtained by MEP (~250 nm compared to ~500 nm). With both methods, size of NIPs was higher than corresponding MIPs. The effect was most obvious with MEP. Therefore, an influence of the template molecule on the size of the resulting particles seems to be clear.

Further, nano-sized MIPs prepared by both methods showed an imprinting effect on pure ACN and ACN-water (60:40, v/v). Compared to the MIPs prepared by PP, the rebinding capacity of MEP-MIPs was significantly higher in both solvents.

As revealed from BET experiments, for the latter the highest surface area was calculated (75.83 m2 g-1 compared to 32.71 m2 g-1 for the MEP-NIP), however, this difference is not as big as the observed difference in the amount of bond analyte (DEHP) (factor was about 6).

Therefore, it can be concluded that MEP-MIPs contain a considerable number of specific binding sites for DEHP. DEHP-MIPs prepared by BP were used as sorbents for MISPE and selectivity was evaluated using a mixture of 19 EDCs.

While a volume of 8 ml of ACN-water (60:40, v/v) removed ~95% of DEHP from the blank column it was almost totally retained on the MIP column. Only a loss of about 5% was observed which could be attributed to non-specific binding of the analyte. However, it could be completely eluted from the MIP with the same volume of pure ACN.

Interestingly, beside DEHP also p,p -DDT and Benzo[a]pyrene were almost completely found in the eluate fraction. From the investigated EDCs, these two analytes exhibited by far the highest capacity factors, which should be attributed to their extremely high hydrophobicity.

However, the IF were significantly lower compared to DEHP (IF 7.1 and 6.8 for p,p-DDT and BAP vs. 8.3 for DEHP). Based on the findings, a LOD can be calculated as three times the signal-to-noise ratio, i.e. 240 ng of DEHP in 1 l of water.

Reported by

Institut für Wasserchemie, TU München
Marchioninistrasse 17
81377 München
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