<div>Glutamate is the principal excitatory neurotransmitter in the central nervous system. As one of the most abundant neurotransmitters, glutamate plays an essential role in many processes of the central nervous system and beyond. As a result, any disruption that causes an abnormal glutamate level can significantly impact the central nervous system's neurological functions. Glutamate excitotoxicity is a neuropathology that persists in many neurodegenerative disorders such as Parkinson's and Alzheimer's disease as well as in the traumatic brain and spinal cord injuries. Thus, the ability to obtain precise information about the extracellular glutamate level in the living brain and spinal cord tissue may provide new insights into the fundamental understanding of glutamate in neurological disorders and neurophysiological phenomena.</div><div><br></div><div>Conventional bioanalytical techniques that characterize glutamate levels <i>in vivo</i> have a low spatiotemporal resolution that has impeded our understanding of this dynamic event. The electrochemical sensor has emerged as a promising solution that can satisfy the requirement for highly reliable and continuous monitoring methods with an excellent spatiotemporal resolution for the characterization of extracellular glutamate concentration. In this thesis, I present various amperometric biosensors fabricated using a simple direct ink writing technique for<i> ex vivo </i>and <i>in vivo</i> glutamate monitoring.</div><div><br></div><div>The amperometric biosensor is fabricated by immobilizing glutamate oxidase on nanocomposite electrodes made of platinum nanoparticles, multiwalled carbon nanotubes, and a conductive polymer. The biosensors demonstrate good sensitivity and selectivity that can be inserted into a spinal cord and measure extracellular glutamate concentration. Additionally, another type of glutamate biosensor is fabricated from commercially available activated carbon with platinum microparticles. We utilize astrocyte cell culture to demonstrate our biosensor's ability to monitor the glutamate uptake process. We also present a direct measurement of glutamate release from optogenetic stimulation in mouse primary visual cortex brain slides. </div><div><br></div><div>Moreover, we explore a new type of material, perovskite nickelate-Nafion heterostructure, to fabricate biosensors and measure glutamate inside the mouse brain. Finally, by utilizing the nanocomposite ink and direct ink writing technique, we also fabricate the gold-ruthenium non-enzymatic glucose biosensor. We apply a modified Butler-Volmer non-linear model to evaluate the impact of geometrical and chemical design parameters of non-enzymatic biosensor performance. </div><div><br></div>