Electrochemical Redox Cycling with Pyrolytic Carbon Stacked-Layer Nanogap Electrodes

Redox cycling (RC) amplification has been introduced as an efficient strategy to enhance signals in electrochemical sensing at low analyte concentrations of relevant biomarkers such as dopamine. RC amplification requires closely spaced and electrically separate electrodes, preferably with nanogaps. The aim of this study was to establish a method enabling the microfabrication of carbon-based stacked-layer nanogap electrodes (SLNE) designed for RC amplification. Pyrolytic carbon was employed as the electrode material and Al₂O₃ deposited by atomic layer deposition as the insulating layer in between the two electrodes. SLNE with 89 nm nanogaps were realized without the need for high-resolution lithography methods, and access to the bottom generator electrode was enabled by dry etching of the insulating layer. Electrical separation between collector and generator electrodes was confirmed using resistance measurements, cyclic voltammetry, and electrochemical impedance spectroscopy. Different SLNE designs and redox cycling modes were investigated in terms of capacitive background current, amplification factors, and collection efficiency using the neurotransmitter dopamine as model analyte. A redox cycling mode, here termed differential chronoamperometry (DCA) combining chronoamperometry with differential cyclic voltammetry, was proposed to minimize the effect of background current drift while still operating with steady-state currents. With DCA, a limit of detection (LOD) of 21 nM, a sensitivity of 83 nA μM^-1, a linear range from 25 nM to 10 μM, and actual detection at low concentrations of 25 nM were demonstrated for dopamine.