# Comprehensive Engineering of PVA-Based Gel Probes for Scanning Gel Electrochemical Microscopy

Marouen Zammali, Mathieu Etienne, Liang Liu

► To cite this version:

Marouen Zammali, Mathieu Etienne, Liang Liu. Comprehensive Engineering of PVA-Based Gel Probes for Scanning Gel Electrochemical Microscopy. Advanced Materials Technologies, 2025, 10 (16), pp.70047. 10.1002/admt.202500147. hal-05230367

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<figure><img src="image_1.png" /></figure>
# Comprehensive engineering of PVA-based gel probes for scanning gel electrochemical microscopy

Marouen Zammali, Mathieu Etienne, Liang Liu*

Université de Lorraine, CNRS, Laboratoire de Chimie Physique et Microbiologie pour les Matériaux et l'Environnement (LCPME), F-54000 Nancy, France.

Email: liang.liu@cnrs.fr; liang.liu@univ-lorraine.fr

**Keywords:** Scanning Gel Electrochemical Microscopy (SGECM), PVA-borate, PEO, rheology, electrodeposition + pulling.

## Abstract

Scanning Gel Electrochemical Microscopy (SGECM) is a novel tool for microscale electrochemical imaging and patterning. In this study, poly(vinyl alcohol)-borate (PVA-borate) gel probes, the core component of SGECM, are developed through an integrated approach combining electrochemistry, polymer science, and fluid mechanics. Two types of gel probes are engineered with distinct fabrication strategies. Type I gel probes are formed by electrodeposition of PVA-borate on platinum micro-disk electrodes. The deposition potential controls the gel's shape, while co-deposition with poly(ethylene oxide) enhances its stiffness and toughness via the Double Network strategy. These probes enable precise topography and current mapping, with reusability ensured by re-soaking in electrolyte. Type II gel probes are fabricated on etched tungsten wires using a "deposition + pulling" technique. By exploiting the viscous-to-viscoelastic transition of the precursor solution at high pulling velocities, precise shape control is achieved despite the slow electrodeposition kinetics of PVA-borate. The optimized Type II probes achieve a spatial resolution of 2.1 µm, demonstrated through SGECM patterning of silver. This work underscores the powerful synergy between electrochemistry, polymer science, and fluid mechanics in advancing electrodeposition processes engineering,
ultimately leading to the creation of novel gel probes for spatially localized electrochemical
analysis.

**1. Introduction**

Scanning electrochemical probe techniques are powerful for measuring and applying electrochemical reactions at micro- and nanoscale. These techniques generally take two pathways: (1) localizing the electrode or (2) localizing the electrolyte. In the first pathway we find the Scanning Electrochemical Microscopy (SECM).[1] It relies on a micro- or nanoelectrode that is positioned close to the sample and laterally scans over it. The whole sample is immersed in the electrolyte during the measurement. The second pathway involving localized electrolyte was initially referred as scanning droplet cell,[2] or electrochemical pen,[3] and now is more accepted as scanning electrochemical cell microscopy (SECCM).[4-7] The concept is based on holding the electrolyte in a pulled capillary, then approaching the capillary until the electrolyte wets the sample. In this manner, the electrical circuit is bridged (with counter and/or reference electrodes in the capillary) allowing electrochemical reactions occurring locally in the wetted area. The capillary-based probe can also laterally scan with intermittent contact. Thanks to the matured protocols for pulling glass capillaries (which was widely used in electrophysiology), SECCM measurements can reach nanometer resolution with relatively easy probe fabrication. So far, both SECM and SECCM have been well developed for spatially localized electrochemistry, from mapping reactivity based on charge transfer kinetics to patterning materials by enforcing electrochemical reactions,[4,8] leading to applications in energy conversion and storage,[9,10] biology,[11] surface engineering and 3D printing.[12,13]

While been widely implemented with great success, SECCM faces an intrinsic issue of
electrolyte wetting the sample surface. On one hand, this is the key for localizing the
measurement, but on the other hand it may also affect the quantification due to the variation of
the electrolyte boundary. Several attempts have been reported to tackle this issue, including
performing measurements in oil,[14,15] and monitoring the electrolyte by IRM.[16] Besides, a
remarkable effort is to immobilize the electrolyte in the form of gel.[17] Kang et al. reported the
electrochemical pen based on 8.3 wt.% agarose for 3D printing of platinum nanostructures.[18]
The pyramid-shaped gel pen was prepared through a casting-gelation-peeling process of
viscous agarose solution. On the other hand, the gel electrolyte could also be electrodeposited
on micro-disk electrodes or etched metal wires forming gel probes. This gets rid of the capillary
and allows flexible resolution based on the elastic response of the gel probe in soft contact with
the sample. The technique is denoted as scanning gel electrochemical microscopy (SGECM).
As compared with SECCM based on liquid electrolyte, SGECM may inhibit the electrolyte
wetting as supported by *in-situ* QCM monitoring and *ex-situ* SEM observations.[19]

Similar as other scanning probe techniques, the heart of SGECM is the probe, and more
specifically the gel material that constitutes the probe. It is essential to tune not only the shape
but also the chemical and mechanical properties of the gel, which is an engineering challenge
that involves polymer physics and chemistry as well as electrochemistry. Ideally, the gel probe
for SGECM should be engineered in the following aspects: (1) shape, (2) elasticity, (3) fatigue
resistance, (4) solvent evaporation and (5) ionic conductivity. As for the shape, the gel should
be as sharp as possible at the tip to reach high spatial resolution of the measurements. Besides,
the shape of gel must be precisely controlled as it determines the boundary condition of the
analysis. Meanwhile, a good elastic capability ensures that the probe may recover to the original
shape (free form) after touching the sample, so that the measurement can continue laterally with
intermittent contact. Moreover, a fatigue resistance property is crucial as gel probe has to
undergo pressing-stretching for each pixel. For electrochemistry, the generation of a map may
require at least thousands of pixels, which is very harsh for micrometer sized gel from the
mechanical aspect. Furthermore, as the measurement is usually carried out in air and
intrinsically takes long time due to the necessity of feedback loop for each pixel in the
intermittent contact,[20] the evaporation of solvent in the gel probe must be minimized to
negligible extent at least for several hours. On the other hand, a reasonable ionic conductivity
is highly required since it qualifies the gel as an electrolyte for electrochemical measurements.
This may also simplify the quantification by ignoring the potential drop in the gel. Among these
points, several contradictions can be foreseen. For example, the miniaturized shape is essential
for high spatial resolution, but it also favors the solvent evaporation. From the mechanical point
of view, employing gel composites may lead to good fatigue resistance, which usually relies on
relatively high solid content. However, this would negatively impact the ionic conductivity by
limiting the diffusion of the species. All these shall be considered in a balanced manner for the
process engineering towards gel probes.

In previous work of SGECM, chitosan was chosen as polymer matrix for fabricating gel
probes due to the ease of control in electrodeposition.[21] It was electrodeposited on Pt and Ag
micro-disk electrodes (denoted as Type I gel probe), and could also form sharp gel probes by
"electrodeposition + pulling" on various metals (e.g. W, Pt, Au, ..., denoted as Type II gel
probe). As a natural polymer, chitosan is highly biocompatible under the conditions for
fabricating gel probes, yet its mechanical strength is limited especially at low concentration.
Effort was also made to increase the stiffness of the gel, including co-electrodeposition with
gelatin or tetraethoxysilane (TEOS),[19] and post-treatment by glutaraldehyde.[22] However,
using both gelatin and TEOS can create challenges in controlling the process. This is because
elevated temperature is required to keep gelatin in its sol state for chitosan electrodeposition,
while the TEOS precursor undergoes dynamic hydrolysis and condensation thus imposes
narrow time window for deposition. Glutaraldehyde would induce chemical cross-linking of
chitosan and improve the network stiffness, yet it would also affect the shape (shrinking),
hydrophobicity and swelling (for soaking functional species) of the gel. Besides, the batch
difference of chitosan also required fine tuning of the deposition conditions for each series of experiments.

Chitosan has clearly shown its advantages but also limitations as gel probes. Therefore in this work we attempt to explore a new polymer network based on poly(vinyl alcohol)-borate hydrogel. PVA can reach 99% saponification degree making it very hydrophilic, so that it can form hydrogel with high water content. It is also highly stable with high concentration of supporting electrolyte thanks to the neutral nature. In the presence of boric acid, PVA can also be electrodeposited due to the local pH increase on the cathode that triggers the transformation of boric acid to borate anions that serve as cross-linker. Here, the electrodeposition process is well controlled for the elaboration of Type I gel probes. With the strategy of double-network, poly(ethylene oxide) (PEO) with high molecular weight is co-deposited alongside PVA-borate to reinforce the mechanical strength of gel probe. Moreover, “electrodeposition + pulling” is applied for preparing Type II gel probes with higher spatial resolution. The process is rationally guided by the rheology of the deposition solution, yielding fast convergence towards the optimal conditions for targeted shape of gel probes. The gel probes are tested for SGECM applications through approach-retract, imaging and patterning measurements. This work highlights the importance of rheology and polymer physics in the electrodeposition process engineering at microscale, leading to a new polymer-based gel probe for SGECM.

## 2. Results and discussions

The electrodeposition of PVA with borate at macroscale was reported by Jiang *et al.*.[23] By applying cathodic potential, H₂O is reduced to generate OH⁻ ions that would increase the pH near the cathode. The latter ionizes boric acid forming borate B(OH)₄⁻ anions, which serve as crosslinker for PVA chains triggering gelation by forming a three-dimensional soft network.
The negative charge of borate in the gel is neutralized by Li⁺ cations from the supporting electrolyte. The process is illustrated in **Figure 1**.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Electrodeposition mechanism of PVA-borate gel.</figcaption></figure>

From the mechanism above, it is clearly seen that the driving force for the electrodeposition of PVA-borate is the electrochemical generation of OH⁻. This offers an advantage of controlling the shape of deposition by tuning the pH gradient near the cathode, which is highly useful for the engineering of gel probes at microscale. The latter is achieved by considering three aspects: (1) polymer properties; (2) electrodeposition; (3) fluid mechanics. With the rational control, Type I and Type II gel probes with desired shape and mechanical strength are elaborated for SGECM measurements.

**2.1. Electrodeposition for Type I gel probe: Enhancement by PEO towards mapping.**

The most important parameter for controlling electrodeposition is the potential. **Figure 2(a)** exhibits cyclic voltammogram of Pt micro-disk electrode in PVA-borate deposition solution. It is seen that the current starts to significantly increase at ca. -1.1 V (vs. Ag/AgCl QRE). This may indicate the onset of electrochemical reduction of water, which is favorable for the electrodeposition of PVA. After 2 cycles, excessive deposition of gel is observed on the micro-
disk electrode. From this point, the deposition potential is tuned in the range of -1.1 to -1.4 V (vs. Ag/AgCl), as shown in **Figure 2(b)**. Generally, the gel deposit has ellipsoid shape that fits the pH gradient on the electrode, which is similar as the electrodeposition of silane and chitosan on microdisk electrodes.[19,21,22,24] For SGECM, the shape of gel probe that corresponds to -1.15 and -1.2 V (vs. Ag/AgCl QRE) is desired, while excess deposition at the side wall of the glass shield (-1.4V (vs. Ag/AgCl QRE), and the inset of **Figure 2(a)**) shall be avoided. The extra negative potential may also induce heavy evolution of H₂, leaving bubbles in the gel that is unacceptable (Supporting information, **Figure S1**). Note that the deposition conditions may need to slightly adapt to the R_g of the micro-disk electrode.[22]

<figure><img src="image_3.png" /><figcaption>**Figure 2.** (a) Cyclic voltammogram of 25 µm diameter Pt micro-disk electrode at scan rate of 10 mV/s. Inset demonstrates the formation of PVA-Borate gel on the electrode after 2 cycles; (b) Electrodeposition of Type I gel probes at -1.1, -1.15, -1.2 and -1.4 V (vs. Ag/AgCl) for 300 s. Inset: optical microscope images of the fabricated gel probes. Deposition solution: 2 wt.% of PVA, 20 mM of BA and 1 M of LiCl in 1-1 H₂O-NMP. Scale bars: 25 µm.</figcaption></figure>

The optimal gel probe above is examined for SGECM experiments. The first step is to perform approach-retract curves. It could validate the current feedback and can also stabilize
the gel probe by removing absorbed free electrolyte in liquid form.[19] **Figure 3(a)** shows clear current spikes when the PVA-borate probe touches the Pt sample, indicating that the approaching is reliable by current feedback. During the retraction, the probe stays in contact with the sample for *ca.* 8.8 µm before being completely detached. This hysteresis originates from the adhesion between the gel probe and the sample. Moreover, the approach-retract curves almost overlap starting from the 5th measurement, indicating that the gel probe is well stabilized which is a crucial step before reliable SGECM measurements.[19] Note that in this work, the gel probes are always stabilized by multiple approach-retract cycles until getting the same current response before further measurement.

<figure><img src="image_4.png" /></figure>

**Figure 3.** (a) Approach-retract curves at a potential -0.5 V (*E*<sub>probe</sub> vs. *E*<sub>substrate</sub>). Inset: scheme of SGECM setup with current feedback; (b) Cyclic voltammogram of PVA-borate gel probe with and without ferrocenedimethanol in contact with Pt substrate at a scan rate of 5 mV/s.

Another important requirement for gel probes is the ability of incorporating functional species, especially the redox probes. Here, 1 mM ferrocenedimethanol is added in the deposition solution to co-deposit with PVA-borate. **Figure 3(b)** demonstrates the CV of these gel probes in contact with a Pt surface. As compared with the blank gel, the probe containing
ferrocenedimethanol shows clear redox peaks. This indicates that ferrocenedimethanol is successfully incorporated in the gel probe, and the PVA-borate gel can act as electrolyte with reasonable ionic conductivity. Note that the CV is performed in two-electrode system with the two electrodes positioned very close and in the confined electrolyte, thus it shall not be interpreted in the same way as that measured from classical three-electrode systems in bulk solution. Here, the reactions on the counter electrode, i.e. the gel/tip interface, shall also be considered. The main reaction would be the reduction of ferrocenium generated on the gel/sample interface, which is known as redox cycling. This is favourable for the measurement as it would increase the current like the positive feedback of SECM.[25,26] Meanwhile, other reactions, such as $O_2$ reduction, could also occur due to the ease of diffusion of $O_2$ through the micrometer-sized gel.

The PVA-borate gel probe is further tested for electrochemical imaging taking a model sample of ITO partially covered by copper tape. The sample has two zones with different topography and oxidation reactivity. As illustrated in **Figure 4(a)**, the gel probe undergoes repeated approach-hold-retract while moving the sample laterally in a gap of 50 µm between adjacent sampling points. The hold time for each sampling point is 5 s in order to record a stable current.
<figure><img src="image_5.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_8.png" /></figure>

**Figure 4.** (a) Scheme of SGECM setup with current feedback for topography and current mapping measurements of a Cu/ITO sample; (b) Viscoelastic spectrum of 1.5 wt.% of PEO dissolved in 1-1 H₂O-NMP; (c) PVA-borate-PEO gel probe before imaging experiment; (d) Approach-retract curves of gel probes performed on Pt substrate in the presence and absence of PEO entanglement network. Scale bar: 25 µm.

Unfortunately, the first attempts of using PVA-borate probes above are not successful. The gel is completely dissociated from the probe only after two lines of scan, which corresponds to *ca.* 80 approach-retract cycles. As a consequence, the contact between the probe and the sample may not be sensed by current feedback, leading to over-approaching that damages the probe (Supporting information, **Figure S2a**). Besides, residue of gel is observed on the copper tape after the measurements (Supporting information, **Figure S2b**). This suggests that the PVA-borate gel is mechanically not strong enough for electrochemical imaging by SGECM, considering that the latter usually requires thousands of touching-stretching (approach-retract)
cycles. It is likely due to the relatively weak physical network of PVA and low cross-linking
density. Thus, for SGECM imaging, it is highly desired to enhance the mechanical strength of
the PVA-borate gel while keeping the ionic conductivity, which is an engineering challenge.

Considering that the electrodeposited PVA-borate has a soft and ductile nature due to the
concerns of minimizing the cross-linking density for maximizing the ionic conductivity, a
relatively denser network is required for reinforcing PVA. This is referred as double network
strategy (DN), which was first proposed by J. P. Gong and her coworkers in 2003.[27] The
concept is to introduce two interpenetrated networks where one is dense and the other is
loose.[28,29] Here, PEO is chosen as the dense network. **Figure 4(b)** shows the frequency sweep
of the viscoelastic moduli of 1.5 wt.% of PEO in 1-1 H₂O-NMP. At low frequencies (< 0.05
rad/s), the solution shows a viscous flow behavior by G'' > G', while at high frequencies it
shows elastic behavior (G'' < G'). Note that the crossover frequency 0.05 rad/s is rather low,
which means that the elastic behavior is dominant almost through the whole frequency range.
This suggests that 1.5 wt.% of PEO contains high density of entanglements and topological
crosslinks, making it a good candidate for improving the mechanical strength of PVA through
DN. **Figure 4(c)** shows that the gel probe can still be electrodeposited from PVA-borate
solution with 1.5% wt of PEO, despite the visibly high viscosity of the solution. The one-pot
electrodeposition of PVA-borate-PEO gel double network ensures the interpenetration between
both networks. Note that for the sake of not sacrificing ionic conductivity, we did not try higher
concentration of PEO.

**Figure 4(d)** compares the approach-retract behavior of PVA-borate gel probes with and without PEO, **measured on Pt surface**. The deposition is made on the same Pt micro-disk electrode to avoid the effect of $R_g$, and the deposition conditions are tuned so that the gel has similar geometry (Supporting information, **Figure S3**). It is seen from the curves that the current at contact position $z = 0$ is much higher without PEO. The physical meaning of this current is
not fully clear, but it might indicate that the addition of PEO reduces the conductivity of the gel. Moreover, the hysteresis which originates from the adhesion between the gel probe and the sample is significantly different. The distance between touching and detaching, as quantified from the curves, is affected by the geometry, the adhesion (with the sample), and the stiffness of the gel probe. Considering that the geometry of the two probes is similar and the adhesion is unlikely to greatly differ, the results may suggest an increase in the stiffness of gel probe by incorporating PEO, which can be well explained by PEO as a first sacrificial network in the double network with PVA-borate. Therefore, the PVA-borate-PEO gel probes are used for SGECM imaging.

The PVA-borate-PEO gel probes are approached to the surfaces of ITO and Cu, and CV is carried out at the local contact (Figure 5(a)). In the measured range between -0.5 and 0.5 V *E*<sub>probe</sub> vs. *E*<sub>substrate</sub>), ITO appears to be inert with very low current, while Cu shows a clear oxidation peak at ~ 0.1 V (*E*<sub>probe</sub> vs. *E*<sub>substrate</sub>) during the forward scan (-0.5 → 0.5 V (*E*<sub>probe</sub> vs. *E*<sub>substrate</sub>)). The latter may correspond to the oxidation of metallic Cu to Cu<sup>2+</sup>. During the reverse scan (0.5 → -0.5 V (*E*<sub>probe</sub> vs. *E*<sub>substrate</sub>)), the small reduction peak at ~ -0.1 to 0 V (*E*<sub>probe</sub> vs. *E*<sub>substrate</sub>) might be related to the reduction of Cu<sup>2+</sup> to Cu. Note that the quantitative interpretation of the CV shall consider also the reactions at the counter electrode (gel/tip interface), including the effect of redox cycling and possibly also other reactions such as O<sub>2</sub> reduction. This is beyond the focus of this work, and here we mainly pay attention to the differentiation of Cu and ITO.

Based on this, imaging experiment is carried out by applying 0.15 V (*E*<sub>probe</sub> vs. *E*<sub>substrate</sub>) using an optimized and stabilized gel probe. Figure 5(b) shows the topography map of the sample based on the recording of touching positions. A step of ca. 70-80 µm is measured between Cu and ITO, which agrees well with the profilometer measurements in Supporting Information (Figure S4). Meanwhile, the current response is found to be considerably higher for Cu than for ITO as shown in Figure 5(c). This phenomenon is consistent with CV. Since both ITO and
Cu are conductive, the difference is mainly originating from the electrochemical reactivity rather than the electrical conductivity. More importantly, we would draw attention that the array scan of 40 × 40 is successfully completed, which is not possible for PVA-borate gel probes without PEO. We recall that the gel probe undergoes approach-hold-retract cycle for each sampling point, which means that the probe has experienced 1600 cycles of stretching test for generating an image in **Figure 5**. This is very harsh test for gel especially at microscale. The results suggest that PEO entanglement network has effectively improved the mechanical properties of the PVA-borate gel probe and thus its lifetime and resilience towards approach-retract processes. The PVA-borate-PEO gel probe can also re-soak ferrocenedimethanol after mapping **Figure 5**, as supported by CV (Supporting information, **Figure S5**) and optical images (Supporting information, **Figure S6**). The soaked probe can be used to perform another imaging experiment at the same conditions (Supporting information, **Figure S7**).

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
**Figure 5.** (a) CV comparison of PVA-borate-PEO gel probe on Cu and ITO at a scan rate of 10 mV/s; (b) SGECM imaging of topography of Cu/ITO sample; (c) SGECM current map of Cu/ITO sample at 0.15 V (*E*<sub>probe</sub> vs. *E*<sub>substrate</sub>).

From the results above, it is seen that PEO can toughen PVA-borate gel and greatly improve
its resilience to stretching, thanks to the double network. Here, the PVA-borate is a loose
network formed by **covalent ester bonds**,[30–32] while the PEO formed by topological cross-
linking acts as the dense sacrificial network. During stretching, the PEO network absorbs and
dissipates energy around the cracks by disentanglement process (Supporting information,
**Figure S8**).[33] This prevents the stress accumulation and the crack propagation, and allows the
PVA-borate network to survive the stretching and maintain the whole gel structure intact.[34–41]
Moreover, the PVA-borate-PEO gel also opens the door for the preparation of Type II gel
probes by “electrodeposition + pulling”.

**2.2. Rheology guided “electrodeposition + pulling” for Type II gel probes**

Besides Type I gel probe prepared by electrodeposition on micro-disk electrodes, the gel
probe can also be prepared on etched metal wires by “electrodeposition + pulling”, denoted as
Type II. It is achieved by approaching a sharp metal wire to a viscous deposition solution, then
holding at a fixed potential to obtain “seed” electrodeposition, and finally pulling the wire out
of the solution while keeping the potential applied. During the pulling, a meniscus is formed
and simultaneously turns to gel due to the electrodeposition. In our previous work, it is shown
that Type II gel probe may greatly and reliably improve the spatial resolution of SGECM to *ca.*
<5 µm.

The beauty of the “electrodeposition + pulling” process is on the synergy of
electrochemical processes with fluid mechanics. However, it also brings high complexity to the
process control, although empirical study has shown that the shape of the gel can be rationally
tuned by deposition and pulling conditions.[42] As discussed in,[42] the key of the control of
"electrodeposition + pulling" is matching the kinetics of electrodeposition and the meniscus
formation. While the former has been well studied,[42] here we attempt to push further the
process engineering by evaluating the lifetime of meniscus from rheological properties of the
deposition solution.

**Figure 6a** shows the rheological frequency sweeps of PVA-borate electrodeposition solutions containing different concentrations of PEO. Generally, the evolution of the viscoelastic moduli against the angular frequency ω displays a typical behavior of an entangled polymer solution for all the three systems. At relatively low PEO concentration of 0.6 wt.%, the viscous modulus G'' is higher than the elastic modulus G' within the whole tested frequency range. Moreover, the viscoelastic spectrum follows a Maxwellian behavior where the terminal flow regime is characterized by G' ~ ω² and G'' ~ ω. These results suggest that at 0.6 wt.% of PEO, the amount of polymer chains in solution is not high enough to form any sort of transient entangled network by topological crosslinks and the system is showing a purely flow behavior within the investigated frequency range. As the concentration of PEO increases to 1 wt.%, the behavior is completely different where an elastic signature emerges in the viscoelastic spectrum. A crossover frequency of ca. 0.2 rad/s is observed, above which the elastic behavior is dominant by G' > G''. The crossover frequency is an indication of reptation behavior of the PEO chains in solution. It shifts towards lower values (around 0.05 rad/s) for the solution with 1.5 wt.% PEO. Meanwhile, the gap between G' and G'' is also more significant at high frequencies. These suggest a more dominant elastic behavior for PVA-borate solution with 1.5 wt.% PEO.

In order to quantitatively analyse the viscoelasticity of PVA-borate-PEO solutions,
several rheological parameters have been calculated: the reptation time $\tau_r$, the zero-shear
viscosity η and the equilibrium elastic modulus $G_0$ (Figure 6b). $τ_r$ equals to $1/ω_r$ where $ω_r$ is the crossover frequency between G' and G''. It clearly increases from 0.07 to 20 s by increasing PEO concentration from 0.6 to 1.5 wt.%. The zero-shear viscosity η is calculated by extrapolating the complex viscosity at zero shear values. It drastically increases from 0.85 Pa·s at 0.6 wt.% of PEO to 339 Pa·s at 1.5 wt.% of PEO. Moreover, the equilibrium elastic modulus $G_0$ (which corresponds to the values of G' where the loss factor $tan δ = G''/G'$ is minimum) is found to increase from 2.7 Pa at 0.6 wt.% to 53.8 Pa at 1.5 wt.% of PEO. The results indicate that the concentration of PEO has clear impact on the rheology of the deposition solution. The high degree of overlapping of PEO chains increases the reptation time $τ_r$, the friction (zero-shear viscosity η), and the density of active topological crosslinks (equilibrium elastic modulus $G_0$). The trend is consistent with flip test of the solutions, where the deposition solution containing 1.5 wt.% of PEO shows an elastic behavior which is different from immediate flow of the solutions with lower concentration of PEO (Supporting Information, Figure S9).

<figure><img src="image_12.png" /></figure>
**Figure 6.** (a) Rheological frequency sweeps of PEO solution at 0.6 wt.%, 1 wt.% and 1.5 wt.%; (b) Evolution of characteristic rheological parameters against PEO concentration: elastic modulus $G_0$, zero-shear viscosity $\eta$ and reptation time $\tau_r$.

From the cross-over frequency, or the reptation time, one may estimate the condition of
formation and the lifetime of the meniscus during the pulling step, since the latter can be
regarded as a shear stimuli. Specifically, the pulling velocity is **analogous** to the shear frequency
(angular velocity) in the rheology measurement, considering the isotropic nature of the
deposition solution with regard to the direction of the applied deformation. That being said,
from the solution with 1.5 wt.% PEO, one may pull the etched metal wire at high velocity to
trigger the elastic response of the solution yielding a viscoelastic meniscus, thanks to the coil-
stretch conformational transition of PEO chains and the entanglement effect.[43-45] This gives
time for electrodeposition which jellifies the meniscus forming sharp Type II gel probe.
Moreover, the rheology tests also confirm the shear thinning behavior of the deposition solution
(Supporting information, **Figure S10**). That being said, when pulling at high velocity, one may
expect a very thin meniscus that would be ideal for obtaining sharp gel probes. Thus, for the
“electrodeposition + pulling” process engineering, the rheological information of the deposition
solution may guide the choice of pulling velocity, which is a critical parameter for shaping the
gel.

We recall that the “electrodeposition + pulling” process consists of three steps: (1) Approaching the probe with $E_a$ that provides a current feedback for contact with the deposition solution but does not induce electrodeposition; (2) Holding the probe at a desired immersion length ($l = 30$ µm for all the experiments of this work) at $E_h$ for $t_h$, for the sake of depositing a “seed” on the etched metal wire. (3) Withdrawing the electrode out of the solution at a set velocity $v_w$. The key of the process, especially for the third step, is the matching of kinetics for
electrodeposition and fluid mechanics. Thus, rational choice of the deposition and pulling conditions is based on these two aspects.

The most straightforward way to test the kinetics of electrodeposition is by CV of the electrode in the deposition solution. The results indicate the potential range (-1.25 to -1.55 V) where one can expect electrochemically induced gelation without heavy $H_2$ evolution. This is further evaluated by examining the deposition when holding the etched W wire for long time. Deposition can be observed mainly at the side of the etched W wire by holding at -1.4 V for 30 s due to the radial diffusion of $OH^-$ (Supporting Information, **Figure S11**). The potential is more negative and the time is longer as compared with depositing chitosan of similar size, indicating that the electrodeposition kinetics is more sluggish for PVA-borate-PEO. According to literature, this could be attributed to the necessity of forming didiol bridges between PVA chains, which ideally requires reacting with all the four B-OH groups of $B(OH)_4^-$ as illustrated in **Figure 1**.[30,46–48] In this sense, following the previous work, the pulling velocity should be even lower than that for chitosan, which is unfavorable for shape control as the lateral deposition cannot be avoided. We attempted to use 0.5 µm/s pulling velocity with various deposition potential and PEO concentrations but did not get satisfactory deposition (blunt without shape control, Supporting Information, **Figure S12**). This forces us to look for new strategies to facilitate the deposition.

As discussed in previous text, the driving force for the electrodeposition of PVA-borate-PEO is the electrochemically generated $OH^-$ ions. In this sense, in order to promote the electrodeposition, one may either increase the rate of production or reduce the diffusion of $OH^-$ . The former may reach a bottleneck due to the evolution of $H_2$, so it is rational to consider the latter. Here, we take advantage of the shear thinning effect that is observed from rheology measurements. The concept is that by pulling the etched wire at high velocity out of the solution, a linear meniscus is rapidly formed that confines the diffusion of $OH^-$ in one dimension. **Figure**
**7a** shows that during the pulling step, the meniscus takes long distance (*ca.* 2000 µm) before being fully detached from the solution. This gives sufficient time for the gelation of PVA in 1D, which is key to the success of deposition (**Figures 7b and 7c**). **Figure 7d** shows the gel deposition at pulling velocity from 20 to 40 µm/s. It is seen that the shape of gel transforms from thick, short and blunt to thin, long and sharp as the pulling velocity increases up to 35 µm/s. This can be explained by the necking effect during pulling, similar as pulling glass capillaries. At 40 µm/s, the deposition is still very thin, yet is shorter. This could be associated to the elastic strain recovery of the highly stretched PEO chains due to the fast pulling velocity.[45] It should be noted that all the deposited gels are clearly much shorter than the detach distance. This suggests that only upper part of the meniscus turns to gel due to the local pH increase, and the bottom part returns to the solution after detaching. Therefore, one may not use the detach distance, or the detach time multiplied by the pulling velocity, to estimate the length of deposition. This is very different from our previous work, where chitosan was deposited with a different mechanism of fluid mechanics.[42] With the full understanding of rheology, the electrodeposition + pulling process is highly reproducible for preparing Type II gel probes based on PVA-borate-PEO (Supporting Information, **Figure S13**).
<figure><img src="image_13.png" /><figcaption>**Figure 7.** (a) Current response during the fabrication of Type II gel probe at $v_w$ = 35 µm/s; (b) Scheme of the creation of 1D long and thin meniscus/filament by pulling at high velocities, the symbols "+" and "-" denote $Li^+$ and $Cl^-$ in the solution, respectively; (c). Real picture demonstrating the formation of 1D long and thin meniscus/filament by pulling at $v_w$ = 35 µm/s; (d) Effect of pulling velocity $v_w$ on the deposition of PVA-borate-PEO type II gel probe. Scale bars: 25 µm.</figcaption></figure>

The PVA-borate-PEO based Type II gel probes are further tested for SGECM. They are approached to be in contact with Ag substrate, where Ag is locally oxidized to AgCl in the presence of $Cl^-$ in the gel (Figure 8a). This is an efficient way of evaluating the spatial resolution of the gel probes.[49] Figure 8b shows an example of pattern with 12 spots made at 0.8 V ($E_{sample}$ vs $E_{gel}$). The spots are round shape with diameter of ca. 5.8 µm. Furthermore, we challenged higher spatial resolution by using sharp gel probes, as shown in Figure 8c. The size
of the spot is ca. 2.1 µm, but the shape is less controlled and less reproducible. It should also
be mentioned that a much higher potential of 1.3 V (E$_{sample}$ vs E$_{gel}$) is essential for the oxidation
of Ag. This suggests lower conductivity of the gel probe, which is even more dominant at high
spatial resolution due to less contact area and may be attributed to the high concentration of
PEO. Thus, it is a trade-off between the mechanical properties and the ionic conductivity, which
is a classical question in optimizing gel electrolytes.

<figure><img src="image_14.png" /><figcaption>**Figure 8.** (a) Scheme of local chlorination and patterning of Ag surface using PVA-borate gel probes; (b) Micrograph of an array of AgCl spots using medium resolution (5.8 µm) gel probe; (c) Micrograph of an array of AgCl spot using high resolution (2.1 µm) gel probe.</figcaption></figure>
### 3. Conclusions

In summary, PVA-borate based gel probes are successfully elaborated for SGECM. The Type I gel probes are fabricated by electrodeposition on Pt micro-disk electrodes, and the Type II gel probes are prepared by “electrodeposition + pulling” on etched W wires. The engineering of both processes relies on the understanding of the chemical composition and the rheological properties of the polymers. For Type I gel probes, the shape control is achieved by tuning the deposition potential. By co-deposition with PEO, the PVA-borate gel enhances the stiffness and toughness thanks to the double network strategy, making it possible to perform SGECM imaging measurement of 1600 pixels. The measurement can be reproduced by re-soaking the PVA-borate-PEO gel probe in ferrocenedimethanol solution. Moreover, the rheology of PVA-borate-PEO deposition solution shows viscous to viscoelastic transition at high frequencies, which is a typical behavior of polymer entanglement solutions with high molecular weight. Taking advantage of this effect, Type II gel probes are prepared by “electrodeposition + pulling” at high pulling velocity, despite that the electrodeposition kinetics of PVA-borate-PEO is sluggish. The key is to trigger the formation of long living meniscus by combing the coil-stretch conformational transition and the entanglement effect. The shape of Type II gel probes is mainly controlled by the pulling velocity, and the optimal probes offer spatial resolution up to 2.1 µm in SGECM patterning measurements of local oxidation of Ag to AgCl. The beauty of the work is on the powerful combination of electrochemistry with polymer physics and fluid mechanics for rational electrodeposition process engineering, leading to new gel probes for spatially localized electrochemical measurements.

### 4. Experimental section
*Materials*: Poly(vinyl alcohol) (PVA) having a molecular weight *M*<sub>w</sub> = 146.000 – 186.000 g/mol and more than 99% degree of hydrolysis, poly(ethylene oxide) with *M*<sub>w</sub> = 8.000.000 g/mol (PEO) as well as 1,1'-ferrocenedimethanol (Fc(CH<sub>2</sub>OH)<sub>2</sub>) were purchased from Sigma Aldrich. Lithium chloride salt as well as N-Methyl-2-pyrrolidone (NMP) were purchased from Acros Organics. Boric acid (BA) was from Eurodex. All chemicals were used as received unless otherwise mentioned.

The preparation of PVA stock solution (concentration 8 wt.%) in 1-1 NMP-water mixture was performed carefully to ensure the reproducibility of the processes. Firstly, NMP and water were mixed and cooled down in an ice bath. Once the mixture's temperature reaches 1 °C, PVA powder was slowly added under vigorous stirring. This ensured homogeneous dispersion of PVA powder in the solvent mixture and avoided flocculation of PVA. Once the required amount of PVA was added, the mixture was heated to 90 °C to dissolve PVA while maintaining vigorous stirring. The heating was stopped as soon as a homogeneous solution was obtained. Subsequently, the solution was cooled down to room temperature under slow stirring for 2 hours prior to its utilization.

*The preparation of deposition solutions and their rheological characterizations:* The deposition solutions were prepared from the same above-mentioned stock solution of PVA. They consist of 2 wt.% of PVA (*M*<sub>w</sub> = 146.000 – 186.000 g/mol), 0-1.5 wt.% of PEO (*M*<sub>w</sub> = 8.000.000 g/mol), 1 M of LiCl and 20 mM of BA in the solvent of 1-1 H<sub>2</sub>O-NMP. The solutions without PEO were mixed for 6 h, and the solutions with PEO were mixed for 24 h under slow stirring before use. The rational reason for using NMP as co-solvent was to inhibit the solvent evaporation of the gel probe, since NMP has a boiling point of 202 °C. For the same purpose, very high concentration of 1 M LiCl was selected as supporting electrolyte for its hygroscopic nature besides the high ionic conductivity. The deposition solutions were stable for at least 3 months from their preparation, with neither precipitation nor phase separation observed.
The rheological experiments have been performed on AR2000 stress-controlled rheometer.
A cone-plate geometry has been used to perform the measurements. The diameter of the cone
was 60 mm, the angle was 2° and the truncation gap was fixed at 70 µm. Before performing the
frequency sweep experiments, amplitude sweep was carried out in order to identify the linear
viscoelastic regime (LVER) of the analysed deposition solution. For the amplitude sweep
experiments, the frequency was fixed at 1 Hz. Meanwhile, the frequency sweep experiments
were performed by applying a stress of 0.1 Pa. All rheological experiments were done at 25 °C.

*Elaboration of gel probes:* Platinum micro-disk electrodes of 25 µm diameter and Rg ~ 2 (the ratio between the diameter of insulating shield and the electrode) were used as substrate for electrodeposition of PVA-borate for the elaboration of Type I gel probe. The electrodes were prepared as described in our previous work.[19,21,22] Electrodeposition of PVA-borate was carried out in a three-electrode system with Pt microelectrode as working electrode (WE), an Ag/AgCl wire as quasi-reference electrode (QRE), and Pt wire of 1 mm diameter as counter electrode (CE), in the deposition solutions as described in section 2.2. Cathodic potentials between -1.1 and -1.4 V (vs. Ag/AgCl QRE) were applied to the working electrode for 300 s. The deposition process is illustrated in **Figure 9**. The optimization of the deposition solutions in the absence of PEO is detailed in supporting information Table S1.
<figure><img src="image_15.png" /><figcaption>**Figure 9.** Schematic description of the gel probe preparation process. Scale bar: 25 µm.</figcaption></figure>

Type II gel probes with improved spatial resolution were fabricated by “electrodeposition + pulling” on etched W wires.[42] The experiment was performed using the same homemade setup as described in,[42] in the deposition solutions described in section 2.2. The key fabrication parameters were controlled and optimized through the automated program (E_a: approach potential, l: immersion length, E_h: holding potential, t_h: holding time, E_w: withdrawal potential, v_w: withdrawal velocity). The current was recorded in real time versus vertical positioning z and time t in all experiments.

**SGECM experiments:** SGECM measurements were performed on a homemade setup.[21]

The gel probe was initially mounted in air, then was approached to be in contact with the sample at speed 1 µm/s. By applying -0.5 V (Eprobe vs. Esample), the contact was sensed by current feedback, i.e. when the current reached ±0.15 nA. Electrochemical measurements such as chronoamperometry and cyclic voltammetry were then carried out when holding the probe at the contact position. Finally, the gel probe was retracted from the sample at 0.1 µm/s to the
initial position with the current monitored. For mapping experiment, the approach-hold-retract
cycle was carried out for each sampling point, with the probe holding at **0.15 V** (*E*<sub>probe</sub> vs.
*E*<sub>sample</sub>) for 5 s for recording the stable current. The distance between adjacent sampling points
was 50 µm. Note that in this work, the contact position is defined as *z* = 0, and negative *z* means
that the probe is away from the sample. This notation is consistent with our previous works.[49]

The SGECM measurements were also performed for evaluating the spatial resolution of
Type II gel probes by patterning. The gel probes were approached to be in contact with Ag by
applying 0.8 V (E<sub>sample</sub> vs. E<sub>probe</sub>). Once the gel probe touched the Ag surface indicated by the
current spike, the position of the probe was maintained and an anodic potential bias was applied
on Ag vs. probe for 10 s, allowing Ag to oxidize to AgCl with Cl<sup>-</sup> from the gel. After that, the
gel probe was retracted to detach from the Ag surface and moved laterally to the next point.
This process was repeated to create a periodical pattern on Ag substrate. The patterned AgCl
spots were analyzed under optical microscope to estimate the lateral resolution of Type II gel
probes. Note that the gel probes were stabilized by several approach-retract cycles until having
reproducible approach-retract curves before any imaging or patterning experiment of SGECM.
This practice was proven efficient for getting reliable results.[19]

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from the author.

**Acknowledgements**

The platform of spectroscopy and microscopy of interfaces (SMI) of LCPME is acknowledged.
We thank Dr. Nicolas Louvet from Métro’NRJ – LEMTA (Université de Lorraine-CNRS –
https://lemta.univ-lorraine.fr/) for the assistance provided for the rheology experiments. Dr
Mariela Alicia Brites Helú is acknowledged for the training provided of the Electrodeposition
+ pulling technique.
**Conflict of interest**

The authors declare no conflict of interest.

**Funding**

This work is funded by the National Research Foundation of Singapore under project no. NRF2020-NRF-ANR102MEACT (ANR-20-CE09-0028).

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