Graphitic Carbon Fiber Microelectrode pH Sensors

Graphitic carbon fiber microelectrodes (CFMEs) have been used as biosensors for the detection of biomolecules such as neurotransmitters using fast scan cyclic voltammetry (FSCV). The electrodes are relatively small (7 microns in diameter), biocompatible, have high spatiotemporal resolution, and do not generally illicit an immune response. CFMEs were characterized with SEM imaging for surface features and EDS/EDX for chemical surface functionalization. Traditionally, CFMEs have been used to detect neurotransmitters such as dopamine, serotonin, norepinephrine, and others. The shape and position of the cyclic voltammogram (CV) is a chemical fingerprint for neurotransmitter detection with the peak oxidative current being proportional to concentration. The method is ideal for measuring fast, subsecond changes of neurotransmitters such as the phasic firing of dopaminergic neurons. Recently, the method has been expanded to measure other molecules such as metals, amino acids, peptides (including neuropeptides), hormones, proteins, neuromodulators, and other biomolecules.<br/><br/>In this study, we demonstrate the use of CFMEs as both FSCV sensors and field-effect transistor (FET) transducers for dynamic pH measurements. The electrochemical oxidation and reduction of functional groups on the surface of CFMEs affect their response over a physiologically relevant pH range. When measured with FET transducers, the sensitivity of the measurements over the measured pH range was found to be (101 ± 18) mV, which exceeded the Nernst value at room temperature of 59 mV by approximately 70 %. Finally, we validated the functionality of CFMEs as pH sensors with FSCV ex vivo in rat brain coronal slices with exogenously applied solutions of varying pH indicating that potential in vivo study is feasible. The sensitive, fast, biocompatible, and selective detection of pH fluctuations provides for a multitude of potential future applications such as the optimization of biomolecule measurement and measurement of pH in specific extracellular tumor microenvironments for cancer studies in addition to many others.