OPTIMIZATION OF AMPEROMETRIC MICROSENSORS FOR MONITORING CHOLINE IN THE EXTRACELLULAR FLUID OF BRAIN-TISSUE

Microsensors for choline with selectivity over ascorbate have been prepared by immobilizing horseradish peroxidase and choline oxidase within a cross-linkable redox polymer deposited onto 10-mu m diameter carbon fiber microcylinder electrodes, ca. 300-400 mu m in length. To eliminate ascorbate interference, microsensors are modified by incorporating ascorbate oxidase into the sensing membrane and by applying a Nafion overlayer. When operated at 37 degrees C at an applied potential of -0.1 V vs. Ag/AgCl, the microsensors detect 10 mu M to 1 mM choline with no interference from 0-400 mu M ascorbate. The modified choline microsensors detect 0-100 mu M choline in a linear fashion (r = 0.99), again with no ascorbate interference. At 37 degrees C the microsensors detect 10 mu M Ch with a signal-to-noise ratio of 12 and a response time of ca. 15 s (10-90% of the change in signal). In order to evaluate their performance in vivo, the microsensors were implanted into the brain of a rat alongside a micropipet from which small volumes (75-250 nl) of a choline solution were injected into the tissue.