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-μm diameter carbon fiber microcylinder electrodes, ca. 300–400 μm 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 ° C at an applied potential of −0.1 V vs. Ag/AgCl, the microsensors detect 10 μm to 1 mM choline with no interference from 0–400 μM ascorbate. The modified choline microsensors detect 0–100 μM choline in a linear fashion (r = 0.99), again with no ascorbate interference. At 37 ° C the microsensors detect 10 μM 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.