The Meniscus in SECCM: Formation, Stability, and Its Impact on Quantitative Electrochemistry

Pipette fabrication

Pipette fabrication is a critical step in SECCM because the geometry of the pipette directly determines the size of the meniscus droplet and the spatial resolution of the measurement. Pipettes are typically pulled from borosilicate or quartz capillaries using either a CO₂ laser puller or resistive coil puller, where fine adjustment of fabrication parameters enables accurate tuning of the pipette aperture and taper profile [9,20,32,33]. Key variables include the laser heating power, the delay time before pulling (cooling phase), and the pulling strength and velocity. Detailed descriptions and optimization guidelines for these parameters are provided by the instrument manufacturer [34]. In general, higher laser power softens a larger portion of the capillary, usually yielding finer tips with smaller radii, whereas lower laser power tends to produce pipettes with large tip diameters [3]. Because the final tip shape may deviate from nominal values due to local temperature gradients, instrumental conditions of the puller, and ambient factors such as humidity, it is essential that each laboratory establish and periodically verify its own pulling parameters to ensure reproducibility. Following fabrication, pipettes should be inspected under optical microscopy for routine screening and, when high-resolution confirmation is required, under scanning electron microscopy (SEM) [5,9,32,35,36]. For tips with sub-200 nm apertures, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) can provide complementary characterization of the tip geometry and integrity [37–41]. To accurately assess the true tip radius, imaging should be performed at an angle that reveals the circular cross section rather than the pipette’s side profile.

Operation

A typical SECCM configuration consists of a conductive or semiconductive substrate acting as the working electrode and a QRCE housed within an electrolyte-filled pipette. The QRCE, commonly an Ag/AgCl wire, serves simultaneously as the reference and counter electrode, and is fabricated by chloridizing a silver wire [42]. The electrolyte within the pipette establishes ionic continuity between the QRCE and the meniscus droplet formed at the pipette tip. Proper substrate preparation is essential for stable meniscus contact and reliable quantitative measurements. Common substrates include highly oriented pyrolytic graphite (HOPG) [43], Au [9] or Pt [44] films, carbon plates [45], and various catalyst-coated materials [46]. Depending on the application, substrates may be freshly cleaved, polished, plasma-cleaned, or used as supports for drop-cast catalysts. Maintaining consistent surface cleanliness and appropriate wettability is critical because these factors strongly influence droplet stability and current response.

A complete SECCM system integrates precise mechanical control, electrochemical instrumentation, and environmental stabilization to reliably position a pipette and maintain a stable meniscus. Three–axis position control—achieved through piezoelectric or motorized stages—allows nanometer-scale movement of the pipette in the x, y, and z directions, which is essential for accurate droplet placement, topographical compensation, and reproducible mapping. The potentiostat applies and measures electrochemical signals, while vibration isolation and controlled environmental conditions (e.g., humidity, temperature) reduce mechanical disturbances and fluctuations in meniscus geometry. Together, these components ensure that the pipette approaches the substrate smoothly and that the confined cell is formed without perturbing the droplet shape or position.

When the pipette approaches the substrate, the meniscus droplet makes contact with the surface, forming a confined electrochemical cell. In this geometry, the droplet–substrate junction defines the reaction area, enabling localized electrochemical measurements. During the approach step, the pipette is lowered gradually toward the substrate surface while the system monitors the current as feedback. Before contact, the current remains close to zero because no electrochemical pathway exists between the QRCE and the substrate. When the meniscus first touches the surface, a shar_p increase in current is observed, marking the completion of the electrical circuit and the formation of the confined cell. Following this initial spike, the current stabilizes as the meniscus reaches an equilibrium shape on the substrate. This sequence constitutes the form-cell event and indicates that the droplet has been established for measurements.

After the approach step establishes a stable meniscus–substrate junction, surface imaging is carried out by controlled movement of the pipette while continuously acquiring electrochemical and positional data. SECCM is inherently suited for high-throughput imaging because each measurement represents an independent, localized electrochemical cell. By repeating this sequence across many locations, large datasets can be rapidly accumulated under identical experimental conditions.

Optimal data quality depends on maintaining stable meniscus formation across all measurement sites. The landing interval, approach/retract speed, and feedback threshold must be carefully tuned to prevent droplet distortion during movement. Environmental factors, particularly humidity, also play a major role. At low relative humidity, solvent evaporation at the pipette tip can dramatically concentrate electrolytes, leading to salt crystallization and deformation of the meniscus. Even slight evaporation can alter droplet size, footprint shape, and contact angle, thereby reducing the reproducibility of the form-cell event. Thus, humidity control and electrolyte stability are essential for maintaining reliable SECCM imaging performance.

Steady-state current

Understanding electrochemical behavior within the confined droplet at the pipette tip requires consideration of both geometric and physicochemical factors that govern mass transport. The pipette probe provides a high mass-transfer rate because the wetted reaction zone is extremely small, functioning analogously to an ultramicroelectrode (UME) [8]. When the electrode dimensions (typically 10 nm–25 μm) approach the diffusion layer thickness, radial diffusion dominates, enabling rapid replenishment of electroactive species and producing a characteristic steady-state current [3,8,47–50].

However, the SECCM configuration differs fundamentally from classical UME systems because mass transport occurs through a conical diffusion pathway defined by the pipette lumen (Fig. 2B), rather than the full-hemispherical diffusion field typically assumed for a hemispherical UME (Fig. 2A). In SECCM, the electrolyte-filled pipette serves as a conical reservoir, and diffusion toward the meniscus–substrate junction is governed by quasi-radial transport within this confined geometry (Fig. 2B). This geometry allows the system to reach near steady-state conditions more readily when small pipettes are used [11]. Under diffusion-limited conditions, the current can be approximated as that flowing to an electrode in contact with an effectively infinite conical volume of solution [51]. In this situation, the meniscus–substrate contact region is sometimes approximated as a hemispherical UME electrode for analytical convenience. Although this approximation does not capture the full geometric complexity of the SECCM droplet, it introduces only minor deviations in mass-transport resistance compared with more realistic planar geometries, making it a reasonable and widely used simplification (Fig. 2A, B) [47].

Fig. 2.

Schematic illustrations of mass transport (A) to a hemispherical electrode and (B) through a conical pipette geometry used to approximate a SECCM tip. (C) Definition of effective electrode radii employed in analytical models. The figure was redrawn based on Ref [47]. Copyright 2023 American Chemical Society.

For a hemispherical electrode, the steady-state current is given by Eq. (1) [52]:

where R is the hemispherical radius, n is the number of electrons transferred, F is the Faraday constant, D is the diffusion coefficient, and C is the bulk concentration of the redox-active species.

In SECCM, because diffusion originates from the pipette interior, the expression must be modified to account for the conical transfer pathway. Incorporating the area ratio defined by the inner half-cone angle of the pipette (θ_p), the current becomes Eq. (2) [47]:

The pipet half cone angle (θ_p) typically ranges from 1.5° to 13°, depending strongly on pipette length and pulling conditions [11].

The effective electrode radius in SECCM can be defined in several ways (Fig. 2C), all centered at the pipette apex:

where r_p is the inner pipette radius and h is the meniscus height. Notably, only R₃ accounts for meniscus height, making it more sensitive to experimental conditions [47]. Most early SECCM studies adopted R₂ to estimate diffusion-limited current [2,8,11,51], yielding:

When θ_p 8°, a commonly used geometry, the resulting steady-state current is approximately ten times smaller than the current expected for a disk UME of comparable radius [11]:

In Ref [47], the analytical expressions for SECCM steady-state current were systematically evaluated by comparison with numerical modeling of mass transport and electron-transfer behavior. The authors examined the dependence of the analytical current on the nanopipette half angle (with r_p = 200 nm) for the three geometric definitions of the effective radius (R1–R3) and showed that R3, which incorporates the meniscus height, provides significantly better agreement with the simulated results than the commonly used R2 expression. However, although R3 provides the most rigorous description when precise modeling of the system is required, it relies on knowledge of the meniscus height, which is not always readily accessible in typical SECCM experiments. Accordingly, R3 is most rigorous when h can be independently constrained, whereas, in most routine analyses, the R2-based expression (Eq. 4) provides a practical and widely applicable estimate without requiring explicit determination of height measurements of the meniscus.

Another study has demonstrated that voltammetric responses vary significantly with the extent of droplet spreading [53]. In that work, the electrode surface was partitioned into three regions according to the mass-transport pathways—the pipette, wall, and meniscus zones (Fig. 3A)—and voltammograms were evaluated as a function of droplet diameter while keeping the tip diameter constant. Differences in droplet spreading were represented using the wetting factor (WF), defined as the ratio of the droplet diameter (d_drp) to the tip diameter (d_tip). According to the simulations, only the area directly beneath the tip experienced a direct mass-transport flux. This region exhibited a sigmoidal voltammetric response that remained essentially invariant with respect to WF (Fig. 3C, D). This behavior reflects the spherical diffusion arising from the pipette orifice, which is largely insensitive to lateral droplet spreading. In contrast, the voltammetric response from the meniscus region displayed pronounced WF-dependent peak shapes (Fig. 3E). As illustrated in Fig. 3B–E, increasing droplet spreading transforms the overall voltammogram from sigmoidal to peak-shaped. This phenomenon arises because a “pancake-like” droplet geometry—typical under alkaline conditions—induces thin-layer cell behavior within the wetted region. The confined diffusion within this thin layer enhances the contribution of surface-limited mass transport, thereby altering the current–potential profile. Taken together, these observations indicate that SECCM voltammograms arise from a convolution of two diffusion regimes: spherical diffusion near the pipette tip (with a limited semi-angle) and confined diffusion of a thin-layer geometry within the spread meniscus. Consequently, explicit consideration of droplet spreading is essential for accurately interpreting SECCM electrochemical data and for distinguishing intrinsic electrochemical kinetics from mass-transport artifacts imposed by meniscus geometry.

Fig. 3.

SECCM voltammograms as a function of droplet diameter at a fixed tip diameter of 900 nm. (A) Division of the electrode surface into three regions according to the dominant mass transport flux. (B) Overall voltammogram of 1 mM Fc(MeOH)₂ with a scan rate of 1 V s^–1 in dependence of the wetting factor (WF). (C-E) The contribution of (C) pipette, (D) wall, and (E) meniscus components to the overall voltammogram. Reprinted with permission from Ref [53]. Copyright 2024 Royal Society of Chemistry.

Verification of cell formation

Accurate interpretation of SECCM measurements requires precise knowledge of the actual droplet–substrate contact area, because the true wetted area and meniscus shape beyond pipette geometry strongly influence mass transport, current magnitude, and the reproducibility of the localized electrochemical cell. Although pipette geometry provides a first approximation of the expected footprint, the true wetted area can deviate significantly due to interfacial chemistry, substrate wettability, electrolyte properties, or the dynamics of the electrochemical reaction itself [20,54–56]. Traditionally, most studies have inferred this footprint indirectly from post-experiment droplet residues or from assumptions relating tip size to contact area [2,13,23,35,57,58]. While the residual imprint is often similar to the pipette opening, this is not always reliable: depending on environmental conditions and substrate–electrolyte interactions, the droplet may spread irregularly, shrink asymmetrically, or undergo reaction-driven wetting changes that obscure the true cell dimensions during measurement [36,55,59,60]. Furthermore, once the pipette retracts, solvent residue can continue to spread before evaporation, artificially enlarging the final footprint and complicating interpretation of the actual reaction zone [17,61].

Because of these limitations, real-time visualization of the meniscus during operation has emerged as a critical tool for validating cell formation. Recent advances integrate SECCM probes with inverted optical microscopy platforms by employing semi-transparent conductive substrates—such as ITO-coated or thin Au-coated glass—that permit simultaneous electrochemical probing and optical imaging (Fig. 4) [20,62–65]. By adjusting substrate reflectivity and illumination geometry, this configuration enables the use of interference reflection microscopy (IRM), an optical method highly sensitive to variations in refractive index and changes in separation distance between interfaces. The operando SECCM–IRM approach allows tracking of cell formation throughout the approach, landing, measurement, and retraction steps with high temporal resolution. As shown in Fig. 5A, the pixel intensity at the center of the landing site evolves dynamically as the droplet first contacts and then stabilizes on the substrate, reflecting both optical path length changes and variations in meniscus curvature. IRM produces contrast through interference between light reflected from the substrate and light reflected from the droplet, causing the wetted region under the meniscus to appear distinctly darker than the surrounding surface (Fig. 5B). Because the characteristic dimensions of the meniscus are comparable to the illumination wavelength, IRM captures a mixed interferometric response that is strongly dependent on droplet thickness and lateral extent. Line-profile analysis of IRM images (Fig. 5C) enables quantitative extraction of the full width at half–maximum (FWHM), which corresponds to the diameter of the wetted area for each landing event (Fig. 5D). This real-time quantification reveals subtle fluctuations in the meniscus—including spreading, contraction, or asymmetric deformation—that cannot be captured through ex-situ inspection.

Fig. 4.

Schematic of the double-channel SECCM pipette tip and meniscus in contact with the ITO substrate with IRM. Reprinted with permission from Ref [20]. Copyright 2025 American Chemical Society.

Fig. 5.

Quantitative optical analysis of meniscus behavior during approach and retraction. (A) Normalized pixel intensity at the center of the landing spot (orange points, left vertical axis) shown along with the simulated relative variation of reflected light (green trace, right vertical axis – considering the reflectivity of a planar interface and the PSF of the microscope). (B) A typical IRM movie frame during meniscus contact with the surface, showing the position of the line profile (orange dotted line) used for analysis. (C) The intensity profile (orange points) and FWHM measure obtained from the Gaussian fit (black line). (D) FWHM from the fitted intensity line profiles of the IRM images taken around the period that the pipette was held in meniscus contact with the surface. Reprinted with permission from Ref [20]. Copyright 2025 American Chemical Society.

Overall, operando SECCM–IRM provides a robust and sensitive means to verify proper cell formation, directly correlate electrochemical signals with the true contact geometry, and detect deviations from ideal wetting behavior. These capabilities significantly enhance the reliability of SECCM measurements, particularly when investigating materials or electrolytes where droplet stability is challenged. Because the IRM module can be readily integrated into existing SECCM setups, it has already been applied across a diverse range of systems, including the hydrogen evolution reaction (HER), ferrocenedimethanol (FcDM) oxidation, silver nanoparticle electrodeposition, lithium nucleation, and calcium carbonate precipitation [20,63,65–67]. This coupling of optical and electrochemical information establishes a powerful methodology for understanding localized interfacial processes.

In addition to optical approaches such as SECCM–IRM, electrochemical strategies have also been developed to estimate the actual droplet size and verify cell formation in situ. Among these, investigation of the retraction curve has gained particular attention because it enables simultaneous electrochemical measurement and quantitative estimation of the wetted area without requiring additional microscopy [17]. To obtain a retraction curve, the pipette was first approached toward the substrate while a potential more negative than the open-circuit potential (OCP) was applied. Once the meniscus made contact, the OCP within the wetted area was measured and subsequently applied as the working potential. The pipette was then withdrawn while continuously recording the current as a function of the z-position. As the droplet at the tip fully detached from the surface, the current eventually reached zero (Fig. 6). The distance–current profile provided a quantitative signature of the confined droplet geometry, and the wetted diameter inferred from the retraction curve has been shown to agree well with ex situ measurements of SECCM footprints. However, the wetted area estimated from retraction curves may not correspond exactly to the electrochemically active area during the scan. The meniscus formed during pipette landing is associated with the advancing contact angle, whereas the wetted area inferred during retraction reflects the receding contact angle; the resulting contact-angle hysteresis can be amplified on rough or chemically heterogeneous surfaces. Accordingly, retraction-curve–based diameters should be interpreted as operational estimates of the wetted area under the specific withdrawal conditions, rather than a direct measure of the active area at landing. Nonetheless, because the method measures the meniscus shape while the droplet is still in contact with the surface, it can track how the droplet wets the surface over time and help determine when droplet spreading reduces measurement reproducibility.

Fig. 6.

Schematic illustrating the steps of the SECCM experiment: (1) approach, (2) landing and OCP measurement, (3) retraction curve, and (4) complete detachment followed by droplet spreading. Graph showing the current measured as a function of micropipet displacement. Reprinted with permission from Ref [17]. Copyright 2025 American Chemical Society.

When this method is implemented in hopping–mode SECCM, appropriate selection of landing separation distance and retraction rate becomes essential for reproducible cell formation (Fig. 7). If the lateral distance between landing sites is too small relative to the pipette diameter, adjacent droplets may overlap, as reflected in enlarged and inconsistent contact areas extracted from the retraction curves (Fig. 7A, B). This effect is particularly pronounced on reactive substrates where local corrosion or surface modification alters the wettability of previously interrogated regions. In contrast, when an adequately spaced landing grid is used, each meniscus forms independently and produces stable, uniform OCP readings, confirming consistent cell formation across the array (top, Fig. 7C).

Fig. 7.

Effects of landing separation distance and retraction rate on droplet footprints and extracted electrochemical metrics. (A–C) Three scan matrices acquired with landing separation distances of 8 μm (bottom), 16 μm (middle), and 24 μm (top): (A) SEM image of the scan area (inset: magnified region), (B) droplet contact diameters inferred from the retraction curves, and (C) open-circuit potential (OCP) measured at each landing site, with the displayed value representing the average of the last 5% of the data. (D) SEM image showing the effect of retraction rate on the size of the resulting droplet footprints for each mapped matrix. Reprinted with permission from Ref [17]. Copyright 2025 American Chemical Society.

The retraction rate also strongly influences meniscus behavior (Fig. 7D). Rapid retraction induces elongational deformation of the droplet due to high upward acceleration, generating a pulling force in the solution inside the pipette. This process can artificially enlarge the contact diameter and lead to significant spread of the wetted region, ultimately causing footprint overlap. Conversely, excessively slow retraction minimizes distortional forces but greatly increases experimental duration. Therefore, optimizing the retraction speed yields consistent and minimally expanded droplet footprints, improving the accuracy and reproducibility of extracted geometric parameters.

Overall, the retraction-curve method provides a robust electrochemical means of tracking meniscus geometry during SECCM measurements, complementing optical techniques and enabling rigorous control of droplet-scale wetting behavior. When combined with optimized landing spacing and retraction rates, this approach significantly enhances the reliability of cell formation across large SECCM maps.

Electrolyte solutions

One of the most frequently encountered challenges in SECCM is maintaining a stable and reproducible meniscus at the substrate interface. Solvent evaporation can change the meniscus geometry as well as the local solution concentration, which in turn affects mass transport and current responses, complicating quantitative analysis. In addition, evaporation-driven concentration shifts can perturb the QRCE potential, introducing apparent potential drift. This sensitivity becomes particularly evident when the volatility of the electrolyte solvent is high, because rapid evaporation at the pipette tip can lead to concentration changes, salt precipitation, or even crystallization, which disrupts the continuity of the meniscus.

In a systematic study exploring SECCM operation in aprotic media, Bentley et al. evaluated how different solvent properties affect droplet stability and measurement reproducibility [15]. Their results show that when using a high-boiling point, low-volatility solvent such as propylene carbonate (PC, b.p. 242°C), the ferrocene/ferrocenium (Fc/Fc⁺) redox response becomes highly consistent across repeated landings, yielding clean sigmoidal voltammograms with well-defined half-wave potentials and steady-state limiting currents (Fig. 8A). In contrast, dimethyl carbonate (DMC), with its low boiling point (91°C) and high volatility, leads to substantial evaporation at the pipette tip, which manifests as increased current variability and irregular, merged footprints (Fig. 8B). To mitigate these evaporation-induced artifacts while still employing a low-boiling point solvent, two strategies were demonstrated: (i) introducing an ionic liquid (IL) as a supporting electrolyte, which suppresses crystallization and enhances droplet robustness in a manner comparable to using PC alone (Fig. 8C); and (ii) mixing DMC with a high-boiling point solvent, such as ethylene carbonate (EC, b.p. 248°C), which markedly improves meniscus persistence and enables stable, reproducible cell formation (Fig. 8D). Using ferrocene as a model redox probe, the study confirms that optimizing solvent composition—either by selecting intrinsically less volatile solvents or by modifying low-boiling point systems with additives—is essential for achieving consistent droplet formation and reliable SECCM measurements.

Fig. 8.

LSVs obtained from the oxidation of 5 mM Fc at a nanocrystalline Au electrode in (A) PC (0.1 M [NBu₄][PF₆]), (B) DMC (0.1 M [NBu₄][PF₆]), (C) DMC (1 M [C₂mim][NTf₂]) and (D) 1:1 (w/w) EC:DMC (1 M Li[ClO₄]). Shown on each plot is the average i−E curve (black trace) ± one standard devia on (red dashed traces), obtained from 121 independent measurements. Inset in (B) and (C) are optical microscopy images taken of the SECCM scan areas, after scanning (scale bar indicates 10 μm). Reprinted with permission from Ref [15]. Copyright 2020 American Chemical Society.

In contrast to nonaqueous systems, where volatility and salt crystallization frequently disrupt stable droplet formation, aqueous electrolytes generally provide more reliable conditions for SECCM measurements [41,68]. However, strongly alkaline solutions often show reduced meniscus stability, extensive droplet spreading, and lower measurement reproducibility compared with acidic or neutral environments. (Fig. 9) These instabilities originate from the intrinsically low surface tension and high wettability of alkaline solutions on metal surfaces such as Pt. Under such conditions, adsorption of OH^– and water renders the surface more hydrophilic, decreasing the contact angle and promoting lateral spreading of the meniscus far beyond the pipette diameter. In some cases, the resulting wetted area can reach up to seven times the capillary size, and this “pancake-like” droplet geometry leads to pronounced deviations in the voltammetric response, including peak-shaped rather than sigmoidal currents (analogous to the behavior shown in Fig. 3).

Fig. 9.

Representative images of 10 μL droplets of (A–D) KOH solutions and (E–G) HClO₄ solutions on a platinum surface. (H) The contact angle of KOH and HClO₄ solutions as a function of the electrolyte concentration. Reprinted with permission from Ref [53]. Copyright 2024 Royal Society of Chemistry.

To address these challenges, several studies have explored the use of electrolyte additives to tune solution properties and stabilize the SECCM meniscus [16,53]. Oliveira et al. demonstrated that incorporating polyvinylpyrrolidone (PVP), a nonionic polymer, into 1 M KOH effectively increases viscosity and reduces surface tension, thereby suppressing uncontrolled spreading at the droplet–substrate interface [53]. Introducing increasing concentrations of PVP (0.02%, 0.06%, 0.1% w/v) into 1 M KOH progressively raises the contact angle on Pt and reduces the droplet spreading upon landing. Quantitative analysis reveals that the wetting factor (WF)—defined as the ratio of droplet diameter to pipette diameter—decreases from ~2.5 to ~1.2 as PVP concentration increases, demonstrating more confined and reproducible meniscus formation. The reduced standard deviation in wetted–area measurements further confirms improved control and consistency during SECCM scans.

In contrast, more recent study has shown that although PVP was previously reported to enhance droplet stability, it can in fact interfere with the intrinsic electrochemical properties of polycrystalline Pt in 0.1 M KOH [16]. This raises concerns about its suitability in systems where accurate investigation of electrocatalytic behavior is essential. To address the broader challenge of droplet instability in alkaline SECCM, Janjani et al. carried out a systematic comparison of eight candidate additives, including glycerol, carboxymethyl cellulose (CMC), guar gum (GG), polyvinylpyrrolidone (PVP), propylene carbonate (PC), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄), polyethylene glycol (PEG), and polyethylene glycol dimethyl ether (PEGDME). Their evaluation was based on three key criteria: (i) electrochemical and chemical inertness, (ii) absence of specific adsorption or surface interactions, and (iii) the ability to physically stabilize the droplet at low additive concentrations. The performance of eight additives was compared directly through side-by-side SECCM measurements on polycrystalline Pt and Au, and glassy carbon electrodes in 0.1 M KOH (Fig. 10). As a result, PEG (0.2–1 wt%) emerged as the most promising additive for alkaline SECCM. Owing to its weak interactions with metal surfaces, PEG effectively regulated wetting and stabilizes the meniscus without significantly altering the intrinsic electrochemical response of the underlying electrode. Incorporating PEG into 0.1 M KOH enabled stable meniscus formation, minimized uncontrolled spreading, and yielded highly reproducible, spatially resolved electrochemical maps of OER and HER activity under conditions relevant to alkaline water splitting.

Fig. 10.

Macroscopic CVs obtained from (A) poly-Pt, (B) poly-Au and (C) GC macrodisk electrodes in a 0.1 M KOH solution in the absence (blue traces) and presence (black traces) of electrolyte additives. The tested additives (from bottom to top): 5 wt% PEG, 0.5 wt% CMC, 5 wt% PEGDME, 0.5 wt% GG, 5 wt% PVP, 5 wt% BMIMBF₄, 5 wt% PC, and 5 wt% glycerol. Reprinted with permission from Ref [16]. Copyright 2025 American Chemical Society.

Another work has demonstrated that modifying the solution viscosity can substantially enhance the cell robustness and imaging performance of SECCM [5]. In this context, the addition of PEG 200 to the electrolyte has been shown to provide several practical benefits directly associated with droplet stability and scanning reliability. Finite element simulations revealed that higher–viscosity PEG–containing solutions undergo markedly reduced deformation of the meniscus during pipette motion. This decrease in droplet shape fluctuation expands the effective feedback interval, lowering the probability of unintended contact between the pipette tip and the substrate. As a result, SECCM can be operated at faster scan speeds without sacrificing control over probe–surface interactions—an important advantage for experiments requiring high temporal resolution. Experimental validation on Au substrates further confirms these advantages. Statistical analysis shows a clear increase in the scanning success rate when PEG 200 is incorporated, along with a measurable reduction in the mean squared error (MSE) of reconstructed images. Collectively, these improvements lead to enhanced SECCM imaging quality and operational reliability.

Surface chemistry

Wettability plays a central role in determining how a SECCM droplet spreads, maintains contact, and evolves during measurement. Because the droplet–substrate interaction is governed by the surface free energy of the underlying material, variations in surface composition can lead to pronounced differences in wettability. Recent measurements on adjacent Al alloy and Cu regions provide a clear demonstration of how strongly the substrate governs local wetting behavior [56]. Static water contact angles (WCAs) were measured on polished Al alloy and Cu surfaces using identical preparation conditions to eliminate artefacts from roughness or oxide-growth time. The Al alloy displayed an average WCA of 76°, indicative of moderate hydrophilicity, whereas Cu exhibited a substantially higher WCA of 93°, corresponding to more hydrophobic behavior. This contrast reflects intrinsic differences in surface energy: a larger WCA signifies lower surface energy and weaker interaction with the aqueous phase. The hydrophobicity of Cu is commonly attributed to the formation of a low-energy oxide film and/or adsorption of organic contaminants, both of which reduce its effective surface energy [69–71].

On the other hand, contact angles were also measured at the Al/Cu interface by positioning the droplet such that it straddled both materials. Interestingly, the apparent WCA shifted toward intermediate values: ~86° on the Al side (higher than pure Al) and ~70° on the Cu side (lower than pure Cu). This redistribution reflects the complex wetting dynamics in heterogeneous environments, where the droplet experiences competing surface energies. Although an averaged WCA for the interface cannot be meaningfully reported due to limited spatial sampling and asymmetric geometry, the observed behavior underscores how mixed-surface systems can significantly distort droplet shape and footprint.

Overall, these findings demonstrate that substrate-dependent wettability can substantially alter SECCM droplet spreading, thereby modifying the electrochemically active area and introducing position-dependent variability in measured currents. For corrosion studies and electrocatalytic mapping—particularly on heterogeneous or multiphase substrates—accurate interpretation of SECCM data requires careful consideration of how local surface energy landscapes reshape droplet geometry during measurement.

Oil layer

Uncontrolled expansion of the contact area alters the effective probe size and introduces uncertainties in footprint definition, complicating the interpretation of localized catalytic activity. To address these limitations, oil-immersed SECCM configurations have been developed as an effective strategy for stabilizing the meniscus against evaporation and spreading, ensuring the precise contact area. The general SECCM and oil-immersed SECCM configurations are schematically illustrated in Fig. 11A and D. In oil-immersed SECCM, an inert organic liquid layer surrounds the aqueous electrolyte, suppressing solvent evaporation and limiting uncontrolled lateral spreading of the droplet.

Fig. 11.

Schematic illustration of the SECCM configuration showing (A) pipette contact landing on the stainless–steel surface in air and (D) droplet contact landing in oil. Optical micrographs of stainless steel surface after SECCM measurement performed in (B) air and (E) oil, scanned over an area of 1700 × 560 µm² (25 × 9, 209 landings), showing distinct landing droplet imprints. Measurements were acquired using a 10 µm microcapillary tip filled with an Ag/AgCl electrode in a 0.020 M NaCl solution at E_appr = –1.0 V. (C, F) Enlarged views of the regions highlighted by red boxes in (B) and (E), respectively. Reprinted with permission from Ref [73]. Copyright 2025 American Chemical Society.

A representative application of this strategy is found in studies of corrosion processes on polycrystalline zinc, where an n-dodecane oil layer was employed as a chemically inert barrier to suppress meniscus spreading during oxygen reduction reaction [61]. By stabilizing the droplet footprint, this approach enabled high-resolution mapping of anodic and cathodic processes across individual grains with different crystallographic orientations. The resulting electrochemical data showed a strong correlation with density functional theory (DFT)-calculated Zn-H binding and Zn dissolution energies, confirming that grain-specific reactivity dictates the localized corrosion kinetics of reactive metals. Building on the benefit of interfacial stabilization, an oil–immersed scanning micropipette contact method (using a pipette with a 1.65 μm tip radius) was also developed to prevent the evaporation of saline electrolytes, ensuring high droplet stability for prolonged mapping of aluminum alloy surfaces regardless of ambient humidity [72]. This technique successfully captured the localized electrochemical activities of the Al alloy, revealing that Fe–rich constituent phases act as cathodic sites that drive accelerated galvanic corrosion kinetics.

Despite these advantages, oil–immersed SECCM introduces several practical considerations. For example, post-mortem surface analysis can be complicated by residual oil contamination, affecting the reliability of ex-situ techniques such as SEM or footprint analysis [36]. In addition, ex-situ footprint analysis can be influenced by drying and surface alterations resulting from subsequent handling, complicating direct correlation between electrochemical data and residual droplet marks [17]. Furthermore, oil immersion can modify wetting behavior on certain substrates, such as stainless-steel alloys [73]. In this system, the presence of oil layer promoted the electrolyte spreading, resulting in increased cathodic currents proportional to the enlarged active area (Fig. 11). Overall, despite substrate-specific complexities and post-analysis considerations, the introduction of an oil layer remains an alternative strategy for enhancing meniscus stability and measurement reliability in SECCM studies.