Effect of pH and surface functionalities on the cyclic voltammetric responses of carbon-fiber microelectrodes

Carbon electrodes are useful for the detection of oxidizable species with c yclic voltammetry. In particular, carbon-fiber microelectrodes have been em ployed for the measurement of several neurotransmitters in brain tissue. Ho wever, during cyclic voltammetry with carbon-fiber electrodes the current v aries with changes in concentration of some inorganic cations as a result o f their interaction with surface functional groups. The electrode's respons e to the hydronium ion is a particular concern because its voltammetric res ponse occurs over a broad range of potentials that overlap those of neurotr ansmitters of interest such as dopamine. This is especially a problem in vi vo because simultaneous changes of dopamine and pH frequently occur in brai n tissue. In this work, voltammetric current changes are shown to arise fro m pH dependent shifts in the peak potentials of background voltammetric wav es that arise from species confined to the carbon-fiber electrode surface. Polishing the electrode with alumina suspended in cyclohexane in an environ ment containing lowered oxygen, a method previously demonstrated to remove oxides from the carbon surface, leads to a substantial reduction in the sen sitivity to pH changes. However, this is accompanied by a loss in signal am plitude for dopamine. The dopamine response can be restored using the catio n exchanger Nation without significantly increasing the pH response. To inv estigate which oxide functional groups play a direct role in the electrode' s current responses to changes in pH, surface-confined carbonyl and alcohol functionalities were chemically modified. In both cases, the modification did not affect the carbon-fiber electrode's responsiveness to changes in pH . Nonetheless, the polishing technique proved to be effective in reducing p H interferences in in vivo applications.