Specific Oxygen-Containing Functional Groups on the Carbon Surface Underlie an Enhanced Sensitivity to Dopamine at Electrochemically Pretreated Carbon Fiber Microelectrodes

The in vivo use of carbon-fiber microelectrodes for neurochemical investigation has proven to be selective and sensitive when coupled with background-subtracted fast-scan cyclic voltammetry (FSCV). Various electrochemical pretreatments have been established to enhance the sensitivity of these sensors; however, the fundamental chemical mechanisms underlying these enhancement strategies remain poorly understood. We have investigated an electrochemical pretreatment in which an extended triangular waveform from -0.5 to 1.8 V is applied to the electrode prior to the voltammetric detection of dopamine using a more standard waveform ranging from -0.4 to 1.3 V. This pretreatment enhances the electron-transfer kinetics and significantly improves sensitivity. To gain insight into the chemical mechanism, the electrodes were studied using common analytical techniques. Contact atomic force microscopy (AFM) was used to demonstrate that the surface roughness was not altered on the nanoscale by electrochemical pretreatment. Raman spectroscopy was utilized to investigate oxide functionalities on the carbon surface and confirmed that carbonyl and hydroxyl functional groups were increased by electrochemical conditioning. Spectra collected after the selective chemical modification of these groups implicate the hydroxyl functionality, rather than the carbonyl, as the major contributor to the enhanced electrochemical signal. Finally, we have demonstrated that this electrochemical pretreatment can be used to create carbon microdisc electrodes with sensitivities comparable to those associated with larger, conventionally treated cylindrical carbon fiber microelectrodes.