Piecing puzzle pieces together: Selective separation of vanillic acid with molecularly imprinted polypyrrole

Molecular imprinting of the conductive polymer polypyrrole was investigated with the objective of creating an electrically-responsive material for the selective capture and release of vanillic acid. The reversibility was driven by switching between the oxidation states of polypyrrole, which affected, for example, charge and wettability. The synthesis of imprinted (PPy-MIP) and non-imprinted (PPy) polypyrrole films was performed through galvanostatic deposition on gold surfaces in a surface plasmon resonance (SPR) system, allowing real-time evaluation of layer formation kinetics, as well as interaction with the target molecules. The difference in polymerization was limited between the PPy and PPy-MIP, with 25–30 % of adsorption sites being specific cavities created by imprinting. This increased the adsorption capacity and selectivity for vanillic acid in both the oxidized and reduced states. Simultaneously, imprinting appeared to decrease the adsorption capacity of structural analogues eugenol and gallic acid. This resulted in an increase in the calculated selectivity ratio from 0.4 to 1.1 for eugenol and from 1.1 to 1.8 for gallic acid in the reduced polypyrrole film. These insights suggest that both the interaction between the molecule and polypyrrole, as well as steric hindrance, co-determine adsorption, which can be considered a first step toward designing materials with enhanced selectivity. We also tried reversible binding of vanillic acid and eugenol, but this resulted in the build-up of target molecules over time. To bring these materials toward application, a stronger switch is required, allowing desorption and reusability of the material. • Molecular imprinting enhances the adsorption selectivity of polypyrrole in both redox states. • The characteristics of the template molecule influence its effect on electropolymerization. • Polypyrrole can interact with charged and uncharged molecules. • The oxidation state of polypyrrole influences its adsorption selectivity.