Understanding the role of neurochemicals in human brain function is challenging. Researchers have showed the importance of extracellular neurotransmitters (NTs) such as dopamine, glutamate, GABA, adenosine and serotonin in Epilepsy, Schizophrenia and Parkinson’s disease. Among these NTs, dopamine (DA) has significant impact on motor and cognitive functions. New age miniaturized prosthetic devices coupled with modern sensing and stimulation electrodes provide a unique opportunity to apply fast scan cyclic voltammetry (FSCV) for chronic DA monitoring with millisecond resolution in vivo. In addition to traditional carbon electrode materials such as glassy carbon and pyrolytic graphite, emerging carbon nanomaterials such as nanotubes, nanofibers and nanodiamonds have spurred immense interest due to their high signal sensitivity and adequate resistance to surface fouling in implant environments. Among carbon electrode materials, boron-doped diamond (BDD) has unique properties such as high thermal conductivity, chemical inertness, biocompatibility, high mechanical stability, and high corrosion and fouling resistance, which makes it an excellent electrode material. In this work, boron-doped ultrananocrystalline diamond (BDUNCD) with 3-5 nm diamond crystallites with wide electrochemical potential window, flexible surface chemistry, low capacitive background current and ultra-smooth conformal surface with minimal non-specific adsorption to biomolecules is used for chronic DA sensing in vitro. The current gold standard electrode for DA sensing is carbon fiber microelectrode (CFM) with ~5 – 10 μm diameters. But CFMs are susceptible to surface fouling and are limited mostly to acute studies (<4h). In this talk, we discuss the dimensional stability of BDUNCD microelectrodes during DA monitoring for ~8 million electrochemical cycles in PBS buffer solution. We developed a new protocol to study BDUNCD electrode fouling and conductive and nonconductive regions by depositing silver nanoparticles and mapping their growth and distribution. Custom-made microfluidics is used to precisely control the microenvironment around the BDUCND microelectrode array. The fouling rates in 100 µM DA solution is systematically studied at different electrochemical potential windows (-0.6 V to 1.2 V) at 60 Hz for up to 7 h. During high frequency FSCV cycling with high current density, severe oxidative fouling and passivation is observed. This results in ~56% fouling at ~1.4 million cycles, i.e. an overall monitoring time of 7 h. Finally, the effect of in situ electrochemical cleaning will be discussed.