Electrochemical sensor for folic acid based on a hyperbranched molecularly imprinted polymer-immobilized sol–gel-modified pencil graphite electrode

Contrary to the ‘substructure imprinting approach’ of larger molecule like folic acid, which often leads molecular recognition for both folic acid and structural analogues containing pteridine and glutamic acid substructures, a molecularly imprinted polymer capable of binding specifically folic acid has been prepared by stoichiometric imprinting process (template/monomer molar ratio, 1:3) creating multiple binding sites within the cross-linked hyperbranched polymer. Dendrimer-like chains were obtained by an ‘initiator-fragment incorporation radical polymerization’ technique involving a new trifunctional monomer, 2,4,6-trisacrylamido-1,3,5-triazine. An electrochemical sensor was developed for the selective and quantitative recognition of folic acid, using a preanodized sol–gel coated pencil graphite (grade 2B) electrode with imprinted polymer immobilized to its exterior surface. During preconcentration step at +0.8 V (with respect to Ag/AgCl), the analyte recapture at pH 2.5 in aqueous environment simultaneously involved mixed hydrophobically driven hydrogen bondings and ionic interactions with pteridine substructure and purely hydrogen bonding interactions with glutamic acid residue of folic acid. The encapsulated analyte was instantly oxidized and then cathodically stripped off responding differential pulse cathodic stripping voltammetric signal. The folic acid was selectively detected with a limit of detection of 0.002 μg mL−1(3σ, RSD ≤3.0%), without any cross-reactivities and real matrix complications.