Fundamental Insights into Nanoconfined Devices by Using Electrochemical Impedance Spectroscopy.

Despite electrochemical impedance spectroscopy (EIS) being a powerful tool to inspect phenomena that occur at different time scales, its application to nanofluidic devices remains underexplored. In this study, we investigate the electrochemical behavior of glass nanopipettes internally coated with a carbon layer (CNPs, 60 nm radius) using EIS and electrochemical capacitance spectroscopy (ECS) to decouple ion transport from redox reactions. Through systematic measurements under varied conditions, we demonstrate herein that EIS and ECS can facilitate the estimation of key parameters, such as ion and electron transfer resistances, double-layer capacitance, and redox capacitance, by employing equivalent electrical circuits. Complementary methods, i.e., the distribution of relaxation times and differential capacitance, provide consistent values of resistances and capacitances compared to equivalent circuit analysis (<5% of difference between both methods) without predefined circuit assumptions, offering further insight into time constants and CNP resistance. Moreover, we underline the effectiveness of capacitance-based analysis in detecting redox species within the CNP domain at concentrations <10 μM, suggesting significant potential for sensing applications. Also, EIS effectively monitors surface modifications in the presence of nonredox-active compounds, as demonstrated using bovine serum albumin. Thus, protein adsorption led to a slight increase of 16 mV in peak separation in voltammetry; whereas EIS displayed a significant increase in both electron transfer resistance (from 0.88 to 18.4 MΩ) and ion resistance (from 8.55 to 10.6 MΩ). Overall, our findings underscore the potential of EIS in nanoscale electrochemistry, providing a valuable platform for fundamental studies and sensor development in nanoconfined domains.