Electrochemical
impedance spectroscopy (EIS) is a versatile tool
for electrochemistry, particularly when applied locally to reveal
the properties and dynamics of heterogeneous interfaces. A new method
to generate local electrochemical impedance spectra is outlined, by
applying a harmonic bias between a quasi-reference counter electrode
(QRCE) placed in a nanopipet tip of a scanning ion conductance microscope
(SICM) and a conductive (working electrode) substrate (two-electrode
setup). The AC frequency can be tuned so that the magnitude of the
impedance is sensitive to the tip-to-substrate distance, whereas the
phase angle is broadly defined by the local capacitive response of
the electrical double layer (EDL) of the working electrode. This development
enables the surface topography and the local capacitance to be sensed
reliably, and separately, in a single measurement. Further, self-referencing
the probe impedance near the surface to that in the bulk solution
allows the local capacitive response of the working electrode substrate
in the overall AC signal to be determined, establishing a quantitative
footing for the methodology. The spatial resolution of AC-SICM is
an order of magnitude larger than the tip size (100 nm radius), for
the studies herein, due to frequency dispersion. Comprehensive finite
element method (FEM) modeling is undertaken to optimize the experimental
conditions and minimize the experimental artifacts originating from
the frequency dispersion phenomenon, and provides an avenue to explore
the means by which the spatial resolution could be further improved.