Forced Convection during Feedback Approach Curve Measurements in Scanning Electrochemical Microscopy: Maximal Displacement Velocity with a Microdisk

Abstract

In scanning electrochemical microscopy (SECM), an approach curve performed in feedback mode involves the downward displacement of a microelectrode toward a substrate while applying a bias to detect dissolved electroactive species at a diffusion-limited rate. The resulting measured current is said to be at steady state. In order to reduce the required measurement time, the approach velocity can be increased. In this paper, we investigate experimentally and theoretically the combination of diffusion and convection processes related to a moving microdisk electrode during feedback approaches. Transient modeling and numerical simulations with moving boundaries are performed, and the results are compared to the experimental approach curves obtained in aqueous solution. The geometry and misalignment of the microelectrode influence the experimental approach curves recorded at high approach velocities. The effects are discussed through the decomposition of the current into transient diffusional, radial convectional, and axial convectional contributions. Finally a ready-to-use expression is provided to rapidly evaluate the maximal approach velocity for steady state measurements as a function of the microelectrode geometry and the physical properties of the media. This expression holds for the more restrictive case of negative feedback as well as other modes, such as SECM approach curves performed at substrates displaying first order kinetics.

High-Speed Scanning Electrochemical Microscopy Method for Substrate Kinetic Determination: Method and Theory

“…16,17 The determination of substrate kinetics using SECM approach curves is very possible 14,18 , however, in addition to the previously mentioned disadvantages of physical contact between electrode and cell, this method holds the risk of damaging or deforming a soft cell sample. Lateral SECM imaging is commonly used for live cell analysis.…”

“…Furthermore it is even possible to extract a substrate's kinetic rate by numerical modeling. SECM instruments, scanning at high velocities are not standardized and the overall quantitative analysis of samples at high scan rates is difficult as shown in literature 14 . Our group demonstrated previously that a tip angle misalignment can hardly be avoided and has also an impact on the measured current.…”

Nanoscale Intelligent Imaging Based on Real-Time Analysis of Approach Curve by Scanning Electrochemical Microscopy

“…Importantly, this high velocity is still slow enough to measure the current response of SECM nanotips under diffusion-limited steady-state conditions, which were also assumed for SECM theory (e.g., eq 2). These conditions are satisfied at up to the maximum velocity of tip approach to an insulating substrate, v max , given by 37 vmax=DaRG115+22RG1.9 where an error of 2% is anticipated. Eq 6 with D = 1 × 10 −5 cm 2 /s gives a high v max value of ~35 μm/s for Pt tips with a = 0.25 μm and RG = 1.5 as used in this study.…”

Local probe investigation of electrocatalytic activity

“…35 The d can be established by obtaining approach curves, 36 which can then be compared with theory. 37,38 The microelectrodes commonly used in SECM display different phenomena from macroelectrodes typically used by the RDE method. While in at macroelectrodes the diffusion of species occurs in a perpendicular manner, in microelectrodes hemispherical diffusion around the electrode occurs, promoting higher mass transfer and limiting current densities 40 owing to the geometry (which can be diverse 41 ) and the small size of the UME.…”