Synthesis of caffeic acid molecularly imprinted polymer microspheres and high-performance liquid chromatography evaluation of their sorption properties

In the current work, a molecularly imprinted polymer (MIP) has been synthesized and used to enable the extn. of a naturally-occurring antioxidant from complex media. More specifically, we describe the first example of a caffeic acid (CA) MIP which has been synthesized in the form of well-defined polymer microspheres, and its use for the extn. of CA from fruit juice sample. The CA MIP was synthesized by pptn. polymn. using 4-vinylpyridine as functional monomer, divinylbenzene-80 as crosslinker and acetonitrile:toluene (75/25, vol./vol.) as porogen. The particle sizing and morphol. characterization of the polymers was carried out by means of SEM (narrow particle size distribution; ∼5 and 1.5 μm particle diams. for the MIP and NIP [non-imprinted polymer], resp.) and nitrogen sorption porosimetry (sp. surface areas of 340 and 350 m2 g-1, and specific pore vols. of 0.17 and 0.19 cm3 g-1 for the MIP and NIP, resp.). The polymers were evaluated further by batch rebinding expts., and from the derived isotherms their binding capacity and binding strength were detd. (no. of binding sites (N K) = 0.6 and 0.3 mmol g-1 for the MIP and NIP, resp., and apparent av. adsorption const. (K N) = 10.0 and 1.6 L mmol-1 for the MIP and NIP, resp.). To evaluate the mol. recognition character of the MIP it was packed into a stainless steel column (50 mm × 4.6 mm i.d.) and evaluated as an HPLC-stationary phase. The mobile phase compn., flow rate, and the elution profile were then optimized in order to improve the peak shape without neg. affecting the imprinting factor (IF). Very interesting, promising properties were revealed. The imprinting factor (IF) under the optimized conditions was 11.9. Finally, when the imprinted LC column was used for the selective recognition of CA over eight related compds., very good selectivity was obtained. This outcome enabled the direct extn. of CA in com. apple juice samples with recoveries in excess of 81% and, rather significantly, without any need for a clean-up step prior to the extn