Characterization of permselective coatings electrosynthesized on Pt–Ir from the three phenylenediamine isomers for biosensor applications

The permeability of polymers, electrosynthesized at neutral pH on Pt–Ir cylinders from each of the three isomers of phenylenediamine (oPD, mPD and pPD), to H2O2 (signal transduction molecule in many oxidase-based biosensors) and ascorbic acid (AA, archetypal interference species in biological applications of biosensors) was measured, and used to determine the permselectivity of the three polymers. PmPD was the coating with the greatest permselectivity for H2O2 over AA, for low concentrations of AA. For AA levels greater than 200 μM, however, poly-ortho-phenylenediamine (PoPD) was superior. Furthermore, stability studies indicated that the permselectivity of PmPD degraded rapidly, even after 1 day, supporting the choice of PoPD as the permselective membrane for biosensor implantation where AA levels are high, such as in brain monitoring. A variety of techniques were used to gain further insight into the PPD layers, specifically electrochemical quartz crystal microbalance, mass spectrometry and scanning electron microscopy. Together these studies indicate that PmPD forms the thickest layer (∼15 nm) and is the least soluble of the polymers, that the PoPD layer is ∼4 nm thick and may consist mostly of tetramers, while PpPD is the thinnest (∼3 nm) and appears to consist of trimers.