Thin layer cell behavior of CNT yarn and cavity carbon nanopipette electrodes: Effect on catecholamine detection

Carbon nanotube yarn microelectrodes (CNTYMEs) are an alternative to carbon-fiber microelectrodes (CFMEs) with interesting electrochemical properties because analyte is momentarily trapped in cavities between the CNTs. Here, we compare fast-scan cyclic voltammetry (FSCV) detection of catecholamines, including dopamine, norepinephrine, and epinephrine, at CNTYMEs, CFMEs, as well as cavity carbon nanopipette electrodes (CNPEs). At CFMEs, current decreases dramatically at high FSCV repetition frequencies. At CNTYMEs, current is almost independent of FSCV repetition frequency because the analytes are trapped in the crevices between CNTs, and thus the electrode acts like a thin-layer cell. At CFMEs, small cyclization product peaks are observed due to an intramolecular cyclization reaction to form leucocatecholamine, which is electroactive, and these peaks are largest for the secondary amine epinephrine. At CNTYMEs, more of the leucocatecholamine cyclization product is detected for all catecholamines because of the enhanced trapping effects, particularly at higher repetition rates where the reaction occurs more frequently and more product is accumulated. For epinephrine, the secondary peaks have larger currents than the primary oxidation peaks at 100 Hz, and similar trends are observed with faster scan rates and 500 Hz repetition frequencies. Finally, we examined CNPEs, which also momentarily trap neurotransmitters. Similar to CNTYMEs, at CNPEs, catecholamines have robust cyclization peaks, particularly at high repetition rates. Thus, CNTYMEs and CNPEs have thin layer cell behavior that facilitates high temporal resolution measurements, but catecholamines CVs are complicated by cyclization reactions. However, those additional peaks could be useful in discriminating the analytes, particularly epinephrine and norepinephrine.