Electrochemical Droplet Sculpturing of Short Carbon Fiber Nanotip Electrodes for Neurotransmitter Detection

Carbon fiber nanotip electrodes (CFNEs) are crucial for electrochemical recordings of neurotransmission release in confined spaces, such as synapses and intracellular measurements. However, fabricating CFNEs with small surface area to minimize noise remains challenging due to inconsistent tip size control, low reproducibility, and low fabrication success rate. Here, we present a reliable, user-friendly method with high reproducibility and success rate for precise CFNE fabrication using microscopy-guided electrochemical etching of cylindrical carbon fiber microelectrodes in a potassium hydroxide droplet. The electrode positioning at the droplet's liquid-air interface determines the etched region, while manually applied time- and amplitude-controlled voltage pulses regulate material removal. Hence, real-time adjustments to electrode positioning and incremental voltage pulses enable precise sculpturing, akin to woodcarving with a knife. Using this method, we demonstrate successful fabrication of short (10 μm) CFNEs with tip diameters of 100 nm, with excellent electrochemical properties and sculptured into cone- and needle-shaped electrodes. Employing these CFNEs for low-noise amperometric dopamine (DA) detection from individual 200 nm DA-loaded liposomes, combined with in silico simulations, revealed that electrode shape influences detection efficiency based on vesicle size. These findings highlight the critical role of electrode geometry in vesicle-based electroanalysis.