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Keysight Technologies
Introduction to SECM and
Combined AFM-SECM




                                                          Application Note




                     Introduction
                     Scanning electrochemical microscopy (SECM) is a powerful
                     scanning probe technique, which is suitable for investigating
                     surface reactivity, and processes at the solid/liquid as well as
                     liquid/liquid interface. Redox reactions and their kinetics involving
                     active species are of fundamental importance in emerging research
                     and application areas ranging from the analysis of biochemical
                     signaling processes e.g., at live cells and tissues to relevant
                     questions in material sciences including e.g., fuel cell technology,
                     catalysis, sensing, and environmental chemistry.

                     PD Dr. Christine Kranz, Institute of Analytical and
                     Bioanalytical Chemistry, University of Ulm
                     Dr. Shijie Wu, Keysight Technologies, Inc.
While electrochemical scanning tunneling microscopy (EC-STM) and
electrochemical atomic force microscopy (EC-AFM) are predominantly based
on imaging structural/topographical and electronic changes at a biased
macroscopic sample surfaces, SECM advantageously combines fundamental
microelectrochemical information via the entire palette of electroanalytical
techniques with imaging modalities. The scanning electrochemical microscope
was introduced by Bard and coworkers with the fundamental principle
entailing scanning a biased ultra-microelectrode (UME) as an imaging probe
across the sample surface, while recording Faradaic (redox) currents at the
UME, and optionally, also at the sample.1 In contrast to EC-AFM and EC-
STM, SECM is not limited to conductive or biased samples. In addition, due
to thoroughly developed theoretical descriptions and models SECM readily
allows the quantification and prediction of experimental data. Since the
introduction of SECM, a multitude of imaging modalities have been developed
and experimentally demonstrated based on a wide variety of electrochemical
analysis techniques including DC voltammetry2 , redox competition mode3, and
alternating current (AC)-SECM imaging4.

The so-called feedback mode is the
most commonly applied imaging mode
in SECM5. Here, the probe and the
sample are immersed in a solution
containing a redox active species
(e.g., R providing a reversible redox
behavior governed by diffusion). If
an appropriate potential is applied at
the probe, R is oxidized to O, thereby
resulting in a steady state Faradaic
current, which is proportional to
the radius, r of the UME and the
concentration, c of the redox species
following I = 4nFDcr (n = number
of transferred electrons, F = Faraday
constant, D diffusion constant of the
redox active species).

If now the biased probe (i.e., the UME)
is moved toward a sample surface
(Fig. 1) at a distance of several         Figure 1. Feedback mode. Left: negative feedback effect due to hindered diffusion of the redox active species
electrode radii, the faradaic current     towards the UME. Right: positive feedback effect due to the regeneration of the redox mediator at the sample
measured at the probe is increasingly     surface.
affected by the sample surface
properties. An insulating sample
surface or surface feature leads to a
diffusion limitation of electroactive
species (here, R) towards the
electrode, and hence, results in a
reduced faradaic current vs. the
steady state current (Fig. 1 left).
A conductive sample surface or
surface feature results in a locally
increased concentration of the redox
species, as the oxidized species can
be regenerated at the sample surface
(here by reduction), which leads to an
instantaneously increased faradaic
current (Fig. 1 right) in comparison to   Figure 2. Generation collection mode. Left: substrate generation/tip collection mode. Right: tip generation/
                                          substrate collection mode .
the steady state current.
                                                                    2
Figure 3. Combined AFM-SECM measurements based on AFM tip-integrated electrodes. Bottom: Simultaneously recorded images showing the topography (left) of the
Agilent logo deposited from platinum/carbon composite by an ion beam-induced deposition (SEM image, middle) and the electrochemical image recorded in feedback
mode SECM (right).

The generation/collection mode (Fig 2)6 is based on detecting electro-active
species at the UME, which are generated at the substrate surface (substrate
generation/tip collection mode SG/TC, Fig. 2 left). This mode has been used for
imaging transport phenomena through membranes, investigation of corrosion
processes and imaging of biological samples such as immobilized enzymes.
Tip generation/substrate collection mode (TG/SC mode, Fig. 2 right) is based
on the generation of an electroactive species at the UME, which is detected at
macroscopic electrode surface. This mode is mainly applied for time to flight
experiments in order to study fast homogeneous reactions.

The following schemes present the principle of feedback mode (Fig. 1) and
generator collector mode (Fig. 2):

Applications of SECM
SECM has evolved from an expert tool into a versatile scanning probe technique,
which is reflected in a steadily increasing number of applications. As SECM is
extremely versatile for investigating dynamic surface processes and is not limited
to certain sample types or sizes, SECM has been applied to:

Investigation of homogeneous and heterogeneous electron transfer reactions
  Imaging of biologically active processes
  

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