Stability of dodecyl sulphate-doped poly (pyrrole)/glucose oxidase modified electrodes exposed in human blood serum

The conductivity and stability behaviour of dodecyl sulphate doped poly (pyrrole)/glucose oxidase coated electrodes has been studied. Poly (pyrrole) charge transfer resistance (conductivity) during serum exposure was found to depend on the initial redox (oxidation) state of films. Exposure of a semi-oxidized (partially conductive) film to blood serum led to a 15% increase in film conductivity; with solutions representing different blood serum constituents, deproteinized serum, high ionic strength electrolyte and albumin, similar reduction in resistance was found. The effect of albumin was unexpected, given the lack of penetration of the 65 kDa protein into the film interior, however, attenuated total reflectance transform IR spectroscopy and spectral reflectance provided indirect evidence that even with albumin surface adsorption some local reordering of the poly(pyrrole) structure may occur with a change in chain conjugation length and hence conductivity. The corresponding fully oxidized (conductive) films became overoxidized and lost conductivity irreversibly in blood serum. This effect was also observed in films exposed to deproteinated blood serum but not high ionic strength electrolyte nor albumin solution. The overoxidation was confirmed by an increased amperometric response towards cationic dopamine and a decreased response to anionic ascorbate. This is due to the insertion of hydroxyl and/or carbonyl groups during the overoxidation process which leads to a net negative charge on the polymer film hence facilitating dopamine partitioning. Following serum exposure the amperometric glucose responses were lowered. This was attributed to the loss of glucose oxidase from the film as the polymer became overoxidized and adopted a more porous structure. A possible reason for the loss of poly (pyrrole) conductivity is the reaction of conducting bipolarons on the polymer with low molecular weight species in serum. The retention of conductivity at partially oxidized poly(pyrrole) may reside in the low level of reactive bipolaron sites on the polymer backbone. Implications on the development of poly(pyrrole) based blood glucose sensors are discussed, and means by which adverse serum interactions might be minimized are suggested.