Temperature and Time-Controlled Etching of Carbon Fiber Nanotip Electrodes: Creating Nanocavity- and Nanodot-Modified Surfaces for Enhanced Neurotransmitter Detection

Carbon fiber has become the standard electrode material to study neuronal communication. For techniques such as single-cell amperometry at synapses and intracellular recordings of neurotransmitters in secretory vesicles, nanometric electrodes are required. The smaller electrode dimension also offers the added benefit of low background noise during amperometric recordings. To fabricate nanotip electrodes with suitable size and geometry, the carbon fiber is generally etched using flame etching or electrochemical etching. However, current methods suffer from low reproducibility and poor success rates. In this work, we present a novel, simple-to-use method for fabricating highly sensitive pointy electrodes with tip diameters of ~150 nm. This method uses the controlled heat from a time- and temperature-regulated box-shaped metal heating filament of a micropipette puller. Scanning electron microscopy showed that electrodes produced by this approach exhibit a unique rough surface densely covered with carbon nanocavities and nanodots. Further, characterization using energy dispersive spectroscopy and Raman microscopy together with square wave and cyclic voltammetry revealed an increased abundance of graphitic edge plane defects at the electrode surface, resulting in a remarkable increase in sensitivity and electron transfer kinetics for the detection of redox-analytes and multiple catecholamine neurotransmitters while reducing surface fouling. Limiting the nanoelectrode tip length to ∼10 µm and inserting it into the cytoplasm of SH-SY5Y cells resulted in excellent low-noise amperometric recordings of intracellular norepinephrine content stored in individual secretory vesicles, which serves as a great proof of concept for the use of these nanostructured electrodes also for studies on neurotransmission at single cells.