Molecular imprinted polymers on Electrospun fibers

Molecular imprinted polymers (MIP) are gaining interest due to its relative ease of construction to form binding sites towards targeted analytes. This allows it to be used in a wide range of applications where molecular detection or removal is needed such as sensors and fluid purifications. High surface area of electrospun nanofibers are ideal carrier for MIP or made of MIP. MIP are formed by binding monomers around a template molecule followed by the removal of the molecule. Electrospinning may be used in conjunction with MIP in several ways.

MIP may be formed into microspheres for encapsulation into electrospun fibers. Liu et al (2012) used precipitation polymerization to prepare molecularly (rhodamine) imprinted microspheres and these were added into polyethylene terephthalate (PET) solution for electrospinning. The resultant membrane was tested for adsorption of rhodamine (rhB) compared to a control without molecular imprint. As expected, electrospun PET membrane containing MIP for rhB performed much better than the membrane without MIP. The adsorbed rhB may be released by immersion in methanol with membrane recovery of more than 97%. Using the same concept, Tonglairoum et al (2013) prepared propranolol (PPL) imprinted microspheres (MIP) via oil/water polymerization using methyl methacrylate (MMA) monomer, PPL template, and divinylbenzene (DVB) cross-linker and added into Eudragit-RS100 solution for electrospinning. Up to 50% w/w of MIP in Eudragit-RS100 fibers may be added for optimal analyte adsorption. The resultant electrospun membrane with MIP showed higher selectivity to PPL than other β-blockers (atenolol, metoprolol, and timolol). As with most blending technique, the functional surface of the MIP may be covered by the polymer matrix thus rendering it less effective.

To ensure that the MIP is on or near the surface of the electrospun fiber, post-electrospinning treatment may be carried out to form MIP on the fiber surface. Zhai et al (2015) first prepared cellulose acetate (CA)/multi-walled carbon nanotubes (MWCNTs)/polyvinylpyrrolidone (PVP)-based nanofiber membranes by electrospinning. The nanofiber surface was functionalized by polymerizing pyrrole through electrochemical cyclic voltammetry (CV) with the polypyrrole acting as the monomer and the cross-linking agent for molecularly imprinting ascorbic acid (AA). The resultant membrane exhibited high adsorption and strong affinity toward AA.

To simplify the process of forming electrospun fibers with MIP, a base polymer, monomers and template molecules may be mixed to form the solution for electrospinning. After electrospinning to form the fibers, the template molecules are removed to leave behind MIP fibers. Chronakis et al (2006) demonstrated this concept using poly(ethylene terephthalate) (PET) as the base polymer, polyallylamine as the functional group and template molecule, 2,4-dichlorophenoxyacetic acid (2,4-D). The resultant electrospun MIP fiber following solvent extraction of the template was able to selectively rebind the target molecule. Using the same concept, Ruggieri et al (2015) was able to create an electrospun MIP polystyrene nanofibers for adsorption of six pesticides, atrazine, atrazine desisopropyl, atraton, carboxin, linuron, and chlorpyrifos. Comparing with commercial adsorbents, their tests showed that electrospun MIP nanofibers performs better at higher analyte concentrations while commercial adsorbents were more efficient at low analyte concentrations.

Molecular imprinting has also been attempted for longer chain polymers using electrospun fibers. Perez-Puyana et al (2021) demonstrated the feasibility of using poly(ε-caprolactone) (PCL) as the base polymer and natural proteins (gelatin, collagen, and elastin) as the template to create imprints on PCL. To create the imprinted PCL fiber, a solution of the protein and PCL was blended and electrospun into fibers. Solvent extraction was used to remove the protein from the blended fibers. With gelatin imprinted on PCL fibers, the resultant fibers showed good site specificity towards gelatin. The gelatin imprinted PCL fibers also showed better adsorption of collagen and elastin compared to non-imprinted PCL. Collagen-imprinted PCL fibers did not show any site specificity towards collagen, gelatin and elastin although general adsorption of these three proteins were better than non-imprinted PCL fibers. The non-specificity of collagen-imprinted PCL may be due to denaturation of collagen protein on the hydrophobic PCL fibers or the solvent. The harmful effect of solvents on collagen has been discussed in several references. Hence the collagen imprint may be structurally different from pristine collagen. Elastin-imprinted PCL fibers did not show any better protein adsorption compared to non-imprinted PCL. This has been attributed to interchain cross-linking of elastin which may have altered the protein structure and reduced protein rebinding on the fiber. For both collagen and gelatin-printed PCL fibers, the better protein adsorption may be advantages in cell culture. Storage duration of the fibrous scaffold may be improved with the absence of easily degradable proteins.