Improving ascorbic acid rejection using beta-cyclodextrin as modifier of permeselective films on amperometric biosensor design

Introduction of permselective films in biosensor designs is a common and reliable way to avoid interference during amperometric analysis. Monomers as 2-methoxy phenols and phenylendiamine isomers can electro-polymerize on Pt electrodes [1,2]. Different conditions applied during film formation influence permeabilities toward analytes and interferents [3]. Selective interference rejection can be enhanced by introducing non-electro active molecules during electro-deposition. Globular or fibrous proteins and oligo-saccharides can influence the ortho-phenylendiamine polymerization when added to monomer solution. As a consequence of modifier addition permselective properties of the film are improved [4]. Among modifiers, ?-cyclodextrin (?CD) is a cyclic oligosaccharide composed of seven glucose units; the spacial disposition of glucosidic residues confers to ?CD a toroidal shape with an hydrophobic cavity and an hydrophilic outer. Ascorbic acid (AA) and some 2-methoxy phenols can be hosted into ?CD cavity [5,6]. AA is an electro-active species and a common interferent in amperometric detection. A modifier like ?CD activates supramolecular recognition and makes AA rejection more effective improving biosensor performances. In this work different amounts of ?CD or ?CD-eugenol inclusion complex are used as modifiers in the electro-polymerization of two natural phenols: eugenol and magnolol. The improvement in permselectivity due to modifier addition on poly-eugenol and poly-magnolol films was evaluated by scanning electron microscopy and in vitro characterization. Performances where compared to poly-orthophenylendiamine (PPD) a permselective film very effective in AA-rejection. During magnolol polymerization the presence of ?CD negatively affected the formation of a performing film resulting in a worsening in permselectivity compared to poly-magnolol alone. Conversely, the polymerization of eugenol with ?CD-based modifiers enhanced the AA interference rejection, leading to better polymer performances respect to the poly-eugenol without modifier. The more effective AA rejection due to introduction of ?CD can be due to supramolecular recognition between modifier-monomer as well as modifier-interferent. The inclusion of eugenol in ?CD can affect the resulting polymeric features by two distinct behaviours that evidence a "directing" and "trapping" role of ?CD, respectively. ?CD can release eugenol directly to the electrode in a precise spatial orientation. Moreover ?CD embedded into the polymer matrix can trap a small amount of AA molecules that can self-reject other AA by same charge repulsion. This kind of ?CD-mediated interactions seems to be influenced by the availability of ?CD cavity. Different concentration of ?CD and ?CD-Eugenol inclusion complex were evaluated. A poly-eugenol film co-polymerized with 2 mM ?CD-eugenol inclusion complex showed a permselectivity equal to PPD film and a lower permeability to AA, likely to be related with a self-rejecting mechanism. Based on these results, the poly-eugenol film polymerized with 2 mM ?CD-Eugenol inclusion complex was successfully integrated in a glutamate biosensor design with satisfactory permselective and enzymatic parameters. Microsensors and biosensor studies clearly show how the introduction of ?CD or ?CD-eugenol inclusion complex as modifier can offer a high performing and sustainable way to enhance biosensors performances.