That's the gap Scanning Electrochemical Microscopy (SECM) fills. It's a scanning probe technique that maps local electrochemical reactivity instead of physical structure, using a tiny biased electrode instead of a mechanical tip. Researchers in battery R&D, corrosion science, and bio-electrochemistry increasingly rely on it to see what's happening chemically, not just physically, at a surface.
This article covers how SECM works, its main operating modes, where it's applied, and how it compares—and pairs—with AFM.
Key Takeaways
- Maps local electrochemical activity by measuring Faradaic current with a biased ultramicroelectrode (UME) probe
- Introduced by Allen J. Bard in 1989; now the primary scanning probe method for local electrochemistry
- Works on conducting and insulating samples: no bulk conductivity requirement
- Constant-height and constant-distance (feedback) modes trade spatial resolution against setup complexity
- Core uses span corrosion, battery SEI, fuel-cell catalysts, and bio-electrochemistry
What Is Scanning Electrochemical Microscopy (SECM)?
SECM belongs to the scanning probe microscopy (SPM) family, but it swaps the mechanical tip for a microelectrode probe. Instead of feeling surface contours, the probe senses chemistry.
The core principle: Researchers scan a biased ultramicroelectrode near a sample immersed in an electrolyte that contains a redox mediator.
As the probe approaches the surface, it detects Faradaic current from that mediator diffusing between probe and sample. Where the sample is more reactive, the current signature changes.
This makes SECM fundamentally different from topographic SPM techniques like AFM or STM:
- AFM/STM reveal physical structure: height, roughness, and mechanical or electrical properties
- SECM reveals chemical and electrochemical reactivity: where reactions happen and how fast
Allen J. Bard, along with Fu-Ren Fan, Juhyoun Kwak, and Ovadia Lev, introduced the technique in a landmark 1989 Analytical Chemistry paper. That work built on earlier microelectrode diffusion-layer studies from the 1980s.
Corrosion science, enzyme and bioactivity research, and materials characterization were among the first fields to adopt it. Those three areas remain core use cases today.
How Does SECM Work?
The measurement hinges on three things: probe potential, a redox mediator in solution, and the resulting Faradaic current.
When the biased UME probe sits far from a sample, it generates a steady diffusion-limited current from the mediator. Bring it close to a surface, and that current changes with how the surface interacts with the mediator:
- Positive feedback — over a conductive sample, the mediator gets regenerated at the surface and recycled back to the probe. Current increases.
- Negative feedback — over an insulating sample, diffusion is blocked. Current decreases.
That distance-dependent current is what creates image contrast. The catch is that topography and surface reactivity both shift the same signal, so you have to separate the two when you interpret a scan.
The Probe Itself
Because the current depends so strongly on tip–sample distance, probe geometry matters. Typical SECM probes share a few defining traits:
- Diameter: 25 µm or smaller, often down to 1 µm
- RG ratio: the ratio of insulating sheath radius to active electrode radius (RG = R/r), commonly around 10, though published work has used values as low as 5
- Material: platinum is the most common choice, prized for chemical inertness and ease of surface prep
The steady-state current at the UME follows a simple relationship:
i_ss = 4nFDCr
Where:
- i_ss = steady-state current
- n = number of electrons transferred
- F = Faraday's constant
- D = diffusion coefficient of the mediator
- C = bulk mediator concentration
- r = electrode radius
Two Basic Experiment Types
With the tip characterized, most SECM work falls into two experiment types:
- Approach curves — the probe moves straight toward the sample to set tip–sample distance and extract kinetic information before imaging
- Line/area scans — the probe rasters in x-y to build a 2D map of local reactivity
Modes of SECM Operation
Constant Height Mode
This is the simplest setup: the probe's z-position stays fixed while it scans across x and y. No feedback control needed.
The tradeoff? If the sample has any surface relief, that relief gets mixed into the electrochemical signal. Accurate topography imaging only works if the probe stays above the sample's highest point throughout the scan. That limit makes constant height a poor fit for rough or heterogeneous surfaces.
Constant Distance Mode
Here, feedback control actively adjusts probe position to maintain a fixed gap above the sample, following its contour as it scans.
Why it matters: This separates topographic effects from electrochemical activity. For rough or heterogeneous samples (corroded metal, biological tissue), that separation is often essential for a clean reactivity map rather than a conflated signal.
Quick mode contrast:
- Constant height — fixed z; simple setup; topography bleeds into the faradaic signal
- Constant distance — active gap control; cleaner reactivity maps on uneven samples
- Best default for rough samples — constant distance when surface relief would otherwise dominate the current
AC-SECM and Other Advanced Modes
Beyond the two primary scan geometries, alternating current SECM (AC-SECM) applies a sinusoidal bias instead of a constant DC potential. That enables impedance-based measurements without a redox mediator.
A 2007 study in Electrochemistry Communications used constant-distance AC-SECM to visualize corrosion pits, resolving nanoscale features with shear-force distance control. Other specialized variants (for example hopping-mode or combined SECM-AFM setups) are chosen when a standard DC feedback experiment cannot separate topography, kinetics, or local impedance cleanly.
Applications of Scanning Electrochemical Microscopy
SECM's reach spans several research domains, each reading a different electrochemical signal.
Corrosion research. SECM maps localized activity at defects and grain boundaries where standard imaging sees only uniform metal. A 2025 study in Corrosion Communications used SECM alongside EBSD on pure zinc and found reactivity differed by crystal orientation. Current was highest at ⟨20⟩ grains and lowest at ⟨0001⟩ grains, with the deepest corrosion at grain boundaries.
Battery research. SECM tracks Solid Electrolyte Interphase (SEI) formation on graphite anodes as batteries cycle. A 2014 Angewandte Chemie study followed SEI evolution in real time, revealing volume changes, gas evolution, and areal decomposition of the insulating SEI layer during delithiation.
Fuel cell and electrocatalyst screening. Researchers use SECM to screen combinatorial libraries of Pt-Ru thin films (up to 256 compositions at once) for hydrogen oxidation activity, compressing catalyst discovery into far fewer experiments.
Biological applications. Because SECM is non-contact and low-damage, it's well-suited to living systems:
- Neurotransmitter secretion mapped at single PC12 cells
- Localized nitric oxide release from endothelial cells
- Secreted enzymes outside a cell detected alongside fluorescent protein inside it
Emerging materials. SECM probes lithium-ion intercalation behavior in few-layer graphene, revealing that ion channel behavior changes measurably as layer count increases from one to six layers.
SECM vs. AFM: Complementary Techniques for Nanoscale Research
AFM and SECM measure different properties of the same surface, so labs often use them together.
| Aspect | AFM | SECM |
|---|---|---|
| Probe type | Mechanical tip | Biased ultramicroelectrode |
| Primary output | Topography, mechanical/electrical properties | Faradaic current, chemical reactivity |
| Best for | High-resolution structural imaging | Localized electrochemical activity |
| Sample requirement | Works on virtually any surface | No conductivity requirement |
Because the outputs differ, many labs run both. A 2000 study in Analytical Chemistry showed simultaneous AFM-SECM imaging on track-etched polycarbonate membranes, matching pore topography (1.5 ± 0.3 µm) with electrochemical response in a single pass—linking structure from AFM to reactivity from SECM.
For labs already running modular AFM/STM platforms, that correlation does not require a new instrument stack. Molecular Imaging's PicoPlus 5500 series supports in-situ electrochemical AFM and STM work, with liquid-cell designs compatible with acidic, basic, and organic-solvent environments.
Those configurations give corrosion, battery, and bio-imaging groups a practical path to add electrochemical mapping onto existing structural imaging workflows without rebuilding the platform from scratch.
Frequently Asked Questions
What is the basic principle of SECM?
SECM uses a biased microelectrode probe to measure Faradaic current from a redox mediator diffusing between the probe and sample. Changes in that current reveal local electrochemical activity across the surface.
Who invented SECM and when?
Allen J. Bard introduced SECM in 1989, along with co-authors Fu-Ren Fan, Juhyoun Kwak, and Ovadia Lev, building on earlier microelectrode diffusion studies from the 1980s.
What is the difference between SECM and AFM?
AFM measures physical topography and mechanical properties using a mechanical tip. SECM measures chemical and electrochemical activity using a biased electrode probe instead.
What types of samples can be studied with SECM?
SECM works on fully conducting, fully insulating, or mixed samples—including biological materials and 2D materials like graphene. There is no sample-conductivity requirement.
What is the RG ratio in an SECM probe?
RG is the ratio of the insulating sheath radius to the active electrode radius (RG = R/r). It's commonly around 10, and it directly affects image contrast and resolution.
What industries commonly use SECM?
Battery R&D, corrosion science, fuel-cell development, and biomedical research all use SECM for localized electrochemical insight at the microscale.
