Abstract
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.
Highlights
Electrochemical impedance spectroscopy (EIS) has become an important tool in a wide range of applications, including labelfree detection for biosensors and drug screening,[1] measurement of charge-transfer kinetics and the corrosion of metals,[2,3] assessing passive layers with thicknesses down to the nanometer level,[4] analysis of the state of charge in batteries,[5] and determining the electronic structure of semiconductors in photovoltaics.[6]
We show that this simplified two-electrode setup is attractive for generating local EIS: the magnitude of impedance can be used for precise probe positioning, while phase angle values can be utilized to quantify the properties of the electrical double layer (EDL)
The technique is most powerfully implemented in a self-referencing mode, where the impedance is measured with the tip in the bulk and near the surface at each pixel in an image, allowing the local capacitive response of the substrate to be isolated at a single frequency, establishing a quantitative footing for the technique
Summary
Electrochemical impedance spectroscopy (EIS) has become an important tool in a wide range of applications, including labelfree detection for biosensors and drug screening,[1] measurement of charge-transfer kinetics and the corrosion of metals,[2,3] assessing passive layers with thicknesses down to the nanometer level,[4] analysis of the state of charge in batteries,[5] and determining the electronic structure of semiconductors in photovoltaics.[6]. We use a two-electrode configuration, wherein the QRCE placed in the tip is biased with respect to a grounded conductive (electrode) substrate, so as to channel the AC bias through the electrical double layer (EDL) (Figure 1a) We show that this simplified two-electrode setup is attractive for generating local EIS: the magnitude of impedance can be used for precise probe positioning (i.e., topographical sensing), while phase angle values can be utilized to quantify the properties of the EDL. Underpinned by complementary numerical analysis using finite element method (FEM) modeling, this work highlights the future prospects of this new mode of SICM for the functional characterization of electrochemical interfaces
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