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# ECL-Based Readout of Electric Field-Induced Fine-Tuning of Redox Activity

Changlin Zhou, Leslie R Arias-aranda, Gerardo Salinas, Laurent Bouffier,

Zonghua Wang, Alexander Kuhn, Neso Sojic

► To cite this version:

Changlin Zhou, Leslie R Arias-aranda, Gerardo Salinas, Laurent Bouffier, Zonghua Wang, et al., ECL-Based Readout of Electric Field-Induced Fine-Tuning of Redox Activity. ChemElectroChem, 2025, 12 (11), pp.e202500012. 10.1002/celc.202500012. hal-05379214

HAL Id: hal-05379214

https://hal.science/hal-05379214v1

Submitted on 24 Nov 2025

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# ECL-Based Readout of Electric Field-Induced Fine-Tuning of Redox Activity

Changlin Zhou,[a] Leslie R. Arias-Aranda,[a] Gerardo Salinas,[a] Laurent Bouffier,[a] Zonghua Wang,[b] Alexander Kuhn,*[a] and Neso Sojic*[a]

Magnesium, a highly reactive alkaline earth metal, acts spontaneously as a strong reducing agent. In this work, the redox reactivity of Mg is fine-tuned selectively in a wireless manner by means of bipolar electrochemistry (BE). This is demonstrated by mapping the spatial electroactivity of Mg via the electrochemiluminescence (ECL) emission of the $Ru(bpy)_3^{2+}/S_2O_8^{2-}$ system. Asymmetric ECL patterns are obtained by controlling the applied external electric field. The ECL intensity is increased

by up to approximately 80% at the cathodic extremity whereas a 50% decrease is observed at the anodic side of a bipolar electrode. Additionally, the influence of the spatial distribution of the electric field on the reactivity of Mg and its concomitant ECL emission is evaluated, producing complex luminescent patterns. These findings, illustrate the wireless modulation of redox reactivity by BE.

## Introduction

The ordinary meaning of chemical reactivity is related to the driving force with which a species reacts. It is well-established that this is intimately related to the thermodynamic and kinetic properties of the involved reactions. For example, alkaline metals such as sodium and lithium, exhibit high intrinsic chemical reactivity under environmental conditions due to their high thermodynamic standard redox potentials ($E°_{M⁺/M} > -2.5$ V vs NHE). Among highly reactive metals, magnesium (Mg), is a well-known strong reducing agent ($E°_{Mg²⁺/Mg} = -2.34$ V vs NHE), which retains a reasonable chemical stability under ambient conditions. This feature allows the use of Mg as an efficient reagent and power source in organic synthesis[1-3] and for dynamic[4-7] and light-emitting systems.[8,9] However, due to its high reactivity, the entire surface of a piece of Mg that is immersed inside an electrolyte acts exclusively as a reducing agent with homogeneous strengths, limiting its use for alternative redox processes. Hence, the possible fine-tuning of the reactivity of this metal remains challenging.

In this context, bipolar electrochemistry (BE) is an interesting wireless method for the possible induction of asymmetric reactivity on conductive substrates.[10-17] This remote approach

is based on the induction of a polarization potential difference (ΔV) across a conducting object, i.e. bipolar electrode (BPE), due to the presence of an external electric field (ε), triggering non-spontaneous redox reactions (exogenous, ΔrG > 0) or by coupling thermodynamically spontaneous redox reactions (endogenous, ΔrG < 0). However, it is important to point out that in BE, the oxidation and reduction of a given set of electroactive species occurs simultaneously at the opposite extremities of the BPE when ΔV overcomes the associated thermodynamic threshold potential (ΔVmin). The induction of asymmetric reactivity facilitates the monitoring of redox reactions on a single object and has been exploited in different applications such as electroanalysis,[18-21] organic synthesis[22-25] and surface modification.[26-29] For example, redox intercalation gradients[30,31] or doping gradients[32-35] across metallic substrates and conducting polymers were generated, respectively, by controlling the potential distribution across the BPE established by the applied electric field. More recently, localized resistance patterns were generated in polypyrrole films by fine-tuning the spatial arrangement of the BPE.[36]

In this work, electrochemiluminescence (ECL)[37] was used to probe and image the local reactivity of a Mg foil that was polarized in a wireless manner by BE. ECL is a chemically-induced light emission based on the electrochemical generation of the excited state of a luminophore, which, upon relaxation, emits light.[38-40] In contrast to photoluminescence, ECL does not require any light excitation, providing multiple advantages, such as a fast response, a high sensitivity and also a superior signal/background ratio.[41-46] Such features enable the use of ECL in a broad range of applications, from optical imaging[47-49] to sensing[50-52] and bioanalysis.[53-55] As a strong reductant, Mg spontaneously triggers the reductive-oxidation mechanism of the $Ru(bpy)_3^{2+}/S_2O_8^{2-}$ ECL system in acid media, generating the corresponding red light.[56-60] In addition, such endogenous ECL emission has recently been used for the design of sophisticated multimodal light-emitting devices.[61,62]

<footnote>[a] C. Zhou, L. R. Arias-Aranda, Dr. G. Salinas, Dr. L. Bouffier, Prof. Dr. A. Kuhn, Prof. Dr. N. Sojic
Univ. Bordeaux, CNRS UMR 5255, Bordeaux INP, ENSMAC, 33607 Pessac, France
E-mail: kuhn@enscbp.fr
sojic@enscbp.fr</footnote>

<footnote>[b] Prof. Dr. Z. Wang
College of Chemistry and Chemical Engineering, Qingdao University,
Qingdao 266071, China</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/celc.202500012</footnote>

<footnote>© 2025 The Authors. ChemElectroChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Illustration of the operating principle of the wireless induction of asymmetric electrochemiluminescence (ECL) taking place at the surface of a Mg foil acting as bipolar electrode (BPE). The part on the right shows the endogenous process (i.e. without applying an electric field). The anodically and cathodically polarized extremities are displayed with the associated redox reactions and the corresponding electron flux. The feeder electrodes, used to apply the electric field (ε), are represented by the black cylinders.</figcaption></figure>

Herein, we take advantage of the versatility of BE to asymmetrically modify the redox reactivity of Mg. This is monitored by the spontaneous ECL-like reductive-oxidation mechanism of the $Ru(bpy)_3^{2+}/S_2O_8^{2-}$ system in acid media. Thus, when a high enough ε is applied, the symmetry of the ECL emission generated on the surface of Mg changes, leading to a site-selective enhancement or inhibition of the light emission at the cathodic and anodic extremities of the BPE, respectively (i.e. exogenous contribution, Scheme 1). Furthermore, by fine-tuning the spatial distribution of the applied electric field across the BPE, it is possible to generate different and complex ECL emission patterns by locally shifting the reactivity of Mg.

## Results and Discussion

To test the possible induction of asymmetric reactivity of Mg by BE, we monitored the intensity of ECL emission. We selected the $Ru(bpy)_3^{2+}/S_2O_8^{2-}$ system that follows the reductive-oxidation mechanism in acid media. The ECL intensity was imaged as a function of ε imposed by the feeder electrodes. For this, a 1 cm² Mg foil, acting as BPE, was placed at the center of a classic BE cell containing a 1 mM $Ru(bpy)_3Cl_2$, 20 mM $K_2S_2O_8$, 20 mM $H_2SO_4$ aqueous solution (pH=1.8). As stated above, under these conditions and in the absence of an external electric field, Mg acts as a strong reducing agent, triggering the spontaneous reduction of both $Ru(bpy)_3^{2+}$ and $S_2O_8^{2-}$ (endogenous process, ΔrG < 0). The reduced species produced by these coupled reactions generate the excited state of the lumophore $Ru(bpy)_3^{2+*}$, which emits a photon upon its radiative relaxation (Equations (1)–(5)). In addition to these reactions, spontaneous proton reduction occurs, leading to the continuous formation of $H_2$ bubbles on the surface of the BPE.

$$2Ru(bpy)_3^{2+} + Mg \rightarrow 2Ru(bpy)_3^{+} + Mg^{2+} \quad (1)$$

$$2S_2O_8^{2-} + Mg \rightarrow 2SO_4^{2-} + 2SO_4^{2-} + Mg^{2+} \quad (2)$$

$$Ru(bpy)_3^{+} + S_2O_8^{2-} \rightarrow SO_4^{2-} + SO_4^{2-} + Ru(bpy)_3^{2+} \quad (3)$$

$$Ru(bpy)_3^{+} + SO_4^{2-} \rightarrow Ru(bpy)_3^{2+*} + SO_4^{2-} \quad (4)$$

$$Ru(bpy)_3^{2+*} \rightarrow Ru(bpy)_3^{2+} + h\nu \ (620 \ nm) \quad (5)$$

In the absence of the external electric field, the characteristic ECL emission takes place homogeneously over the entire surface of the Mg foil (Figure 1a and Figure S1). However, when an external ε is applied, an asymmetric polarization of the BPE occurs, triggering a flux of electrons from the positive extremity (δ⁺) to the negative (δ⁻) pole (Scheme 1). A gradual increase of the ECL intensity at the cathodic extremity with a concomitant decrease on the anodic side of the Mg foil, was observed as a function of the applied electric field (Figures 1a and Figure S1). To provide a more quantitative analysis, the ECL profile across

<figure><img src="image_6.png" /><figcaption>**Figure 1.** (a) ECL images of the emission generated at the surface of a Mg foil-BPE in an aqueous 1 mM $Ru(bpy)_3Cl_2$, 20 mM $K_2S_2O_8$, 20 mM $H_2SO_4$ solution at different applied ε values (indicated in the figure), with a representation of the anodically (δ⁺) and cathodically (δ⁻) polarized extremities. (b) ECL profiles as a function of the applied ε (indicated in the figure). (c) ECL spectra collected in three different regions across the Mg foil-BPE; the cathodically (yellow line) and anodically polarized (brown line) extremities and the center of the foil (red line), at a constant electric field (1.31 V cm⁻¹).</figcaption></figure>
<figure><img src="image_7.png" /></figure>

the Mg plate-BPE was evaluated for all the ε values tested. This was performed by analyzing a defined region of interest parallel to the orientation of the electric field (Figure S2). From these plots, it is possible to map the polarization gradient profile across the BPE *via* changes in the ECL intensity. In the absence of external ε, a rather symmetric ECL emission profile across the BPE was obtained (Figure 1b, black curve). However, as ε increases, the symmetry of the profile breaks, producing a concomitant increase and decrease of the ECL emission at the cathodic and anodic extremities, respectively (Figure 1b, red and blue curves). For example, at ε = 1.31 V cm⁻¹, the ECL intensity increases by up to 100% on the δ⁻ side, whereas it decreases by 45% on the δ⁺ side. ECL imaging shows that the spontaneous reactivity of the Mg is tuned locally by the external electric field. Such a non-spontaneous behavior can be explained by the fundamentals of exogenous BE. The asymmetric polarization of the Mg foil induces an increase of the cathodic polarization on the δ⁻ side of the BPE, enhancing the kinetics of the ECL reactions, which translates into an increase of the local ECL intensity. Concomitantly, the δ⁺ side is more anodically polarized. In other words, this extremity is depleted of available electrons to trigger the reductive-oxidation ECL mechanism, thus, the oxidation of Mg is favored, leading to a decrease of the light intensity (Scheme 1). Furthermore, the plot of the ECL intensity at three different positions along the BPE as a function of the electric field reveals an approximately 80% increase of the ECL emission at the cathodic side, whereas a loss of 50% occurs at the anodic extremity (Figure S3). One can notice a loss of around 15% of the initial endogenous ECL intensity after this set of experiments (Figure S4). This can be associated with the formation of a thin MgSO₄ passivation layer along the BPE,[8] since sulfate and magnesium ions are the sub-products of the redox reactions involved in the wireless ECL process (Equations (2)–(4)). It is important to highlight that, at rather high electric field values, the formation of H₂ and O₂ at the cathode and anode of the BPE, respectively, may block the active sites of the Mg surface, limiting other redox processes.

Since all the redox reactions take place spontaneously due to the high intrinsic reactivity of Mg, in a first-order approximation, the enhancement/weakening of the ECL light intensity is exclusively associated with the local polarization and the corresponding flux of electrons from the anode to the cathode through the BPE, triggered by the external electric field. To corroborate this behavior, the ECL spectra across the BPE were recorded at a constant applied electric field (1.31 V cm⁻¹). For this, three different regions across the Mg foil-BPE were monitored through an optical fiber connected to the spectrometer. The fiber was positioned in front of the cathodic or anodic extremities as well as at the center of the Mg-foil. In the absence of ε, the ECL spectrum at each position along the BPE exhibits the characteristic emission peak centered at 620 nm, associated with photons in the red visible region, with a rather similar intensity (Figure S5). On the contrary, when applying the external electric field, a 2-fold increase and a 4-fold decrease of the ECL emission peak at the cathode and anode of the BPE, respectively, were obtained (Figure 1c). These results confirm that the same excited state of the luminophore Ru(bpy)₃²⁺* is generated independently of the longitudinal position on the magnesium foil.

After this first set of experiments, we investigated the influence of the spatial arrangement of the electric field on the reactivity of Mg and its concomitant ECL emission. For that, the Mg foil-BPE was placed between two sets of feeder electrodes (Figure 2a), positioned in such a configuration that the applied electric fields are orthogonal to each other (ε₁⊥ε₂). In a first order of approximation, such a double electric field arrangement induces a global electric field (ε<sub>global</sub>) whose direction follows the diagonal of the BPE (Scheme S1). Hence, in the presence of the Ru(bpy)₃²⁺/S₂O₈²⁻ system, the electrochemical reactions will be confined to the corresponding corners of the Mg foil (Figure 2a). As observed previously, in the absence of ε, the ECL emission is homogenous along the surface of the BPE (Figures 2b and Figure S6). When the applied ε increases, an arrowhead-shaped ECL emission is generated at the cathodic extremity of the BPE (Figure 2b). For this configuration the ECL

<figure><img src="image_8.png" /><figcaption>**Figure 2.** (a) Illustration of the operating principle of the asymmetric ECL emission induced on the surface of a Mg foil-BPE in a double electric field configuration. The anodically (δ⁺) and cathodically (δ⁻) polarized extremities are displayed with the associated redox reactions and the corresponding electron flux. (b) Optical images of the ECL light generated at the surface of a Mg foil-BPE in an aqueous 1 mM Ru(bpy)₃Cl₂, 20 mM K₂S₂O₈, 20 mM H₂SO₄ solution at different applied ε values (indicated in the figure), with a representation of the anodically (δ⁺) and cathodically (δ⁻) polarized extremities. (c) ECL profiles as a function of the applied ε (indicated in the figure).</figcaption></figure>
<figure><img src="image_9.png" /></figure>

profile was evaluated by defining a region of interest parallel to the global electric field, which is directed from the cathodic corner to the anodic tip of the BPE (Figure S7).

Once again when ε = 0 V cm⁻¹, the ECL profile exhibits a homogeneous light intensity along the diagonal of the BPE. However, as the applied ε increases, a gradual localized enhancement or inhibition of the ECL emission at the cathodic and anodic corners of the BPE is observed (Figure 2c). With such a double electric field configuration, a 30% increase of the ECL emission at the cathodic side is produced, whereas a 70% decay of the light intensity at the anodic extremity is obtained (red line). However, in this configuration, only 50% of the initial endogenous ECL intensity was recovered (grey line by comparison to black line) (Figure S8). Once again, this loss can be attributed to the possible formation of a MgSO₄ layer, which does inhibit the endogenous ECL process. It is important to highlight that a similar arrowhead-shaped ECL profile can be produced by simply orientating the Mg-BPE with the corners facing the feeder electrodes (Figure S9a). However, for the same external electric field value (1.05 V cm⁻¹), the intensity of the ECL emission benefits from the produced ε<sub>global</sub> which, under this condition, is found to be around 1.48 V cm⁻¹ (Figure S9b and S9c). This simple experimental set-up allows to break the symmetry of the endogenous ECL emission but, in theory, it is possible to focus and/or confine the ECL emission in multiple regions along the Mg-foil BPE.

In this context, the double electric field system was used,
and two setups were created in which feeder electrodes with
opposite polarities were alternately arranged around the Mg-
BPE foil: one with the electrodes positioned on the sides and
the other at the corners (Figure 3a). The difference between
these configurations directly impacts the magnitude of the
applied ε required to induce asymmetric reactivity on the Mg
foil. Therefore, to polarize the Mg-foil, a significantly higher
electric field is now required. In theory, in such a configuration,
the maximum polarization regions are strictly located in front of
each feeder electrode; with each of the δ⁻ and δ⁺ poles
exhibiting an opposite polarization compared to the feeder
ones (Figure 3a). This is an interesting feature, since with this
spatial arrangement, it is possible to localize the ECL emission
simultaneously at two regions of the BPE. Furthermore, by fine-
tuning the spatial arrangement of the BPE, it should be possible
to restrict the ECL emission either at the edges or the corners of
the Mg-foil (Figure 3a). For that, we first evaluated the influence
of the electric field on the reactivity of Mg and its concomitant
ECL emission by positioning the feeder electrodes facing the
edges of the BPE (Figure 3a left). In the absence of electric field,
a homogeneous ECL emission was obtained (Figure 3b, left
image). As ε increases, two well-defined ECL patterns are
formed at the edges facing the feeder anodes (Figure 3b, center
image and Figure S10a). In addition, a gradual decrease in light
emission at the center of the Mg-foil BPE and at the edges
facing the feeder cathodes was observed. This is corroborated
by the ECL profile in the absence and presence of an external
electric field. With this configuration a 70% increase and a 45%
decrease of the ECL emission at the edges and center of the
BPE, respectively, were obtained. Finally, the localized gener-

<figure><img src="image_10.png" /><figcaption>**Figure 3.** (a) Schematic illustration of the double electric field set-ups with a representation of the spatial distribution of the feeder electrodes facing the edges or corners of the Mg foil-BPE. (b) Optical pictures of the ECL light generated at the surface of a Mg foil-BPE in an aqueous 1 mM Ru(bpy)₃Cl₂, 20 mM K₂S₂O₈, 20 mM H₂SO₄ solution in the absence (left image) and presence of a constant applied ε (3.85 Vcm⁻¹), produced by two sets of feeder electrodes facing the edges (center image) or corners (right image) of the Mg foil-BPE, with a representation of the anodically (δ⁺) and cathodically (δ⁻) polarized extremities. (c) ECL profiles obtained in the absence (black line) and presence of a constant applied ε (3.85 Vcm⁻¹) (red line), produced by two sets of feeder electrodes facing the edges (top) or corners (bottom) of the Mg foil-BPE.</figcaption></figure>

ation of ECL patterns was demonstrated, simply by changing
the orientation of the BPE with respect to the feeder electrodes
by a rotation of 45° (Figure 3a, right). Once again when the ε =
0 V cm⁻¹, the ECL emission remains constant across the entire
surface of the BPE (Figure S10b). As expected, when ε increases,
two arrowhead-shaped ECL patterns are formed at the corners
facing the feeder anodes (Figure 3b right image and Fig-
ure S10b), with a gradual decrease of the light emission at the
center of the Mg-foil BPE and at the corners facing the feeder
<figure><img src="image_11.png" /></figure>

cathodes. The ECL profile in the absence and presence of an external electric field corroborates the increase of ECL intensity at the vertices (Figure 3c bottom). However, with this configuration only a 40% increase of the ECL emission at the vertices was obtained. This can be associated with the relatively small area of the BPE electrode being affected by the applied electric field, which limits the efficient polarization and the triggered ECL reactions. Nonetheless, this set of results illustrates the possibility to locally fine-tune the reactivity of Mg at multiple regions of the BPE in a wireless manner.

## Conclusions

In summary, the wireless nature of BE was exploited to locally tune the reactivity of Mg. This was monitored by the formation of ECL patterns produced by the classic $Ru(bpy)_3^{2+}/S_2O_8^{2-}$ system. A direct correlation of the enhancement or decrease of ECL intensity across the Mg-foil BPE with the applied $\epsilon$ was observed. For a given applied external electric field, a strong modulation of the ECL amplitude was observed with an increase of up to 80% at the cathodic extremities and a decrease of 50% at the anodic sides of the BPEs, respectively. The formation of more complex ECL emission patterns evidences the impact of the spatial distribution of the electric field around the BPE. These results illustrate the localized wireless tuning of reactivity of Mg, which not only highlights the versatility of the double electric field configuration, but also provides new opportunities for future research on the behavior of reactive metals and their applications in advanced electrochemical systems.

## Experimental Section

Potassium peroxodisulfate ($K_2S_2O_8$, Fluka, > 98%), sulfuric acid and Tris(2,2'-bipyridyl) dichlororuthenium (II) hexahydrate ($Ru(bpy)_3Cl_2 \cdot 6H_2O$, Aldrich, 97%), sulfuric acid ($H_2SO_4$, J. T. Baker, 95%-97%) and Mg foil (0.125 mm thick, Alfa Aesar, 99.9%) were used as received. All solutions were prepared with deionized water (MilliQ Direct-Q, resistivity of 18.2 MΩ.cm at 25 °C). The bipolar electrodes were prepared by properly cutting the Mg foil to the desired dimension ($A = 1 cm^2$) and immobilizing it at the bottom of the bipolar cell. All the bipolar electrochemical measurements were carried out in 10 mL of an aqueous 1 mM $Ru(bpy)_3Cl_2$, 20 mM $K_2S_2O_8$, 20 mM $H_2SO_4$ solution. A power supply (Rigol DG1032Z) coupled to a high voltage amplifier (Pendulum instruments A800DI) was used to apply different voltages, in a range between 0 V to 5 V, between two graphite feeder electrodes positioned at a distance of 3.8 cm. The applied electric field was calculated by the ratio between each applied voltage and the distance between the feeder electrodes.[63,64] Spectroelectrochemical measurements were performed with a spectrophotometer SpectraPro 2300i (Princeton instruments) coupled with the power supply. Light emission was monitored by using a CCD camera (CANON EOS 70D, Objective Canon Macro Lens 100 mm 1:2.8) and the images were processed with Image J software by using a multicolor Look-Up-Table ranging from white (0) to red (256). From the optical images, the light emission profile was extracted by using a constant region of interest.

## Acknowledgements

C. Z. acknowledges the China Scholarship Council for his PhD scholarship. L. R. A.-A. acknowledges the University of Bordeaux for her PhD scholarship. The authors thank the Agence National de la Recherche (2PhotonECL, ANR-23-CE06-0024-02) for the financial support.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Keywords: Bipolar electrochemistry
Electrochemiluminescence · Redox reactivity imaging ·
Corrosion mapping

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Manuscript received: January 16, 2025

Revised manuscript received: February 11, 2025

Version of record online: February 21, 2025