Surface Molecularly Imprinted Cellulose‐Synthetic Hybrid Particles Prepared via ATRP for Enrichment of Flavonoids in Olive Leaf

Abstract

Surface molecularly imprinted cellulose‐synthetic hybrid particles are prepared via atom transfer radical polymerization (ATRP). The two‐step process involves the immobilization of α‐bromoisobutyryl bromide in the pristine microcrystalline cellulose, to generate ATRP macroinitiator particles, and then the creation of a crosslinked‐imprinted shell in the particles surface considering ATRP of 4‐vinylpyridine (4VP) and ethylene glycoldimethacrylate (EGDMA) with quercetin as imprinting template. Among the polymerization recipes tested, a system with ethanol as solvent preserves a final size of the hybrid particles suitable for application as adsorbent, while also incorporating the 4VP/EGDMA co‐monomers. Testing of imprinted/non‐imprinted particles for sorption/desorption of standard phenolic compounds shows the modification of the surface of the pristine cellulose and also the achievement of molecular imprinting (imprinting factor ≈2.6). Particles are used for the enrichment of flavonoids in olive leaf extracts and the special features of the developed molecularly imprinted adsorbents are again highlighted with this complex mixture of phenolic compounds. It is shown that production of fractions rich in luteolin‐7‐O‐glucoside, apigenin‐7‐O‐glucoside, or quercetin, among other flavonoids is possible (estimated enrichment factors up to 4). These results point up for the usefulness of natural‐synthetic materials with processes to get high‐added value compounds in the framework of circular bio‐economy.

Competitive Adsorption of Phenolic Acids, Secoiridoids, and Flavonoids in Quercetin Molecularly Imprinted Polymers and Application for Fractionation of Olive Leaf Extracts

“…Estimated productivity in terms of the mass of phenolic compounds processed per mass of adsorbent indicates a global threshold of 27 mg/g and a value close to 1.5 mg/g if luteolin-7-O-glucoside is taken as reference. These values are much higher than those observed with hybrid cellulose-synthetic MIPs recently addressed to target the same extract (e.g., up to 0.2 mg/g) 12 but at the expenses of using a totally synthetic particles.…”

“…A Parr 5100 pressurized glass reactor with a 1 L maximum capacity was used to perform the inverse-suspension polymerization synthesis of the quercetin-MIP particles considered here. This procedure is an extension of our previous works with bulk or precipitation polymerization MIPs preparation. − Our goal was scaling up MIP production to the gram-scale in order to work with preparative columns and also control the particle size to prevent potential back-pressure issues when running continuous processes (see sections below). Two separated preparation steps were performed before the reactor charging: (a) In the reaction phase (dispersed phase), quercetin (2.72 g) and 4VP (9.64 mL) were dissolved in ACN/DMF 85/15 (140 mL) and left in an ultrasound bath for 15 min to promote the intermolecular interactions between the quercetin and the 4VP monomer.…”

“…However, an enlarged view of the HPLC-DAD analysis allows us to discern plenty other different minority compounds such as oleuroside, luteolin-4-O-glucoside, diosmetin-4-O-glucoside, apigenin-7-Oglucoside, quercetin, and luteolin, among many others. 12 Figures 7 and 8 illustrate the approach here considered for the fractionation of the OPA NATAC 20% extract through a sorption−desorption process with the developed MIP particles packed in a preparative column (25 g of adsorbent). In Figure 7 is presented the UV−vis monitoring of the loading process working with the extract in EtOH/water at 5 mg/mL and at T = 25 °C.…”

“…Due to the potential molecular recognition features of molecularly imprinted polymers (MIPs), their application in a large range of domains has markedly grown in the last decades. Biotechnology, biomedicine, environmental protection, catalysis, and general separation processes are broad examples of areas where MIPs are playing an innovative role. − Furthermore, MIPs are also being considered for the valorization of plant extracts and agricultural residues through the isolation of high-added-value compounds (see refs − and references therein), including also the specific case of olive leaf and olive oil production residues. − …”

“…Herein is reported the development of tailored polymer networks joining functionalization with 4-vinylpyridine (4VP) and molecular imprinting with quercetin as a template. 4VP was considered for functional monomer due to its special binding features that grounds the specific use for the development of advanced polymers in a wide range of applications (wastewater/industrial effluents treatment, enzyme/protein adsorption, electronics, biomedicine, or catalysis − ), including also adsorbents for plant extracts in view of the strong binding with phenolic compounds. − , The rationale for the selection of quercetin is based on its reference structure for many flavonoid molecules and because it is widely available (present in many plants) and is a less expensive molecule, therefore working as a kind of surrogate for flavonoids. Note that, in spite of a possible imperfect selectivity of MIPs and a likely high retention also observed with the correspondent NIPs (non-imprinted polymers) or functional commercial adsorbents, a positive impact of molecular imprinting was demonstrated, not only on specific binding sites formation but also on the improvement of the morphology and textural properties of the adsorbents. − …”