<figure><img src="image_1.png" /></figure>

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# Wireless Multimodal Light-Emitting Arrays Operating on the Principles of LEDs and ECL

Miaoxia Liu+,<sup>[a]</sup> Leslie R. Arias-Aranda+<sup>[a]</sup> Haidong Li,<sup>b</sup> Laurent Bouffier,<sup>[a]</sup> Alexander Kuhn,<sup>[a]</sup> Neso Sojic,<sup>[a]</sup> and Gerardo Salinas*<sup>[a]</sup>

Electrochemistry-based light-emitting devices have gained considerable attention in different applications such as sensing and optical imaging. In particular, such systems are an interesting alternative for the development of multimodal light-emitting platforms. Herein we designed a multicolor light-emitting array, based on the electrochemical switch-on of light-emitting diodes (LEDs) with a different intrinsic threshold voltage. Thermodynamically and kinetically favored coupled redox reactions, i.e. the oxidation of Mg and the reduction of

protons on Pt, act as driving force to power the diodes. Moreover, this system enables to trigger an additional light emission based on the interfacial reductive-oxidation electrochemiluminescence (ECL) mechanism of the Ru(bpy)₃²⁺/S₂O₈²⁻ system. The synergy between these light-emission pathways offers a multimodal platform for the straightforward optical readout of physico-chemical information based on composition changes of the solution.

## Introduction

Electrochemically driven light-emitting systems have gained considerable attention as optical platforms for the study of physico-chemical information.[1-4] These devices take advantage of the current or voltage generated during an interfacial electron transfer in order to trigger light emission either by fluorescence,[5-7] electrochemiluminescence (ECL),[8-14] or by using light-emitting diodes (LEDs).[15-20] Thus, the physicochemical information is encoded in the corresponding flux of electrons across the electrochemical device and its light emission. Such systems enable interesting applications from sensing to optical mapping/imaging. Recently, the possible combination and correlation of two or more optical readouts, within a single device, has become an interesting alternative to classic imaging systems.[21,22] The design of these so-called multimodal imaging platforms is of utmost importance for the characterization of biological and chemical processes.[23-29] However, the majority of the available electrochemically driven light-emitting multimodal platforms require a direct electric connection to a power supply. Thus, even though ECL and LEDs

produce rather strong light emission, providing high spatial and temporal resolution, their use for the design of wireless electrochemically driven multimodal systems remains a challenge.

Recently, the concept of bipolar electrochemistry (BE)[30,31] was explored as an alternative to design wireless light-emitting devices.[32-34] Briefly, BE is based on the asymmetric polarization of conducting objects, or bipolar electrodes (BPE), either by applying an external electric field (no spontaneous redox processes)[35-38] or by harvesting thermodynamically favored sources of electrons (spontaneous redox processes).[39,40] These wireless multimodal light-emitting devices take advantage of two different light sources based on different physico-chemical principles, ECL and LEDs simultaneously triggered by BE. In particular, the spontaneous, or so-called endogenous, BE approach is a promising alternative for the local imaging of physico-chemical information, since no external power source is required. Such a system takes advantage of the fundamental principles of a battery in order to induce an in-built electric field across the BPE. In addition, according to the Nernst equation, it is possible to fine-tune the thermodynamic potential, hence the induction of light, by changing the nature and concentration of the electroactive species in solution.[41] An interesting alternative to evaluate such changes of potential is to take advantage of the intrinsic threshold potential, or forward voltage (V~F~) associated with a given color produced by a LED. Typically, the V~F~ value is found between 1.8 and 3.3 V, and is strongly related to the nature of the semiconductor composing the diode. For example, to power the light emission, red AlGaNP LEDs require around 1.7 V, whereas white InGaN LEDs approximately 2.8 V.[42]

In this work, we design a wireless multimodal light-emitting array by taking advantage of the intrinsic threshold voltage of three LEDs with different colors, namely yellow, green and blue. The efficient coupling of thermodynamically and kinetically favored redox reactions triggers a flux of electrons from the anodic to the cathodic pole of the diodes, switching on each

<footnote>[a] M. Liu,¹ L. R. Arias-Aranda,¹ Dr. L. Bouffier, Prof. A. Kuhn, Prof. N. Sojic, Dr. G. Salinas
Univ. Bordeaux, CNRS UMR 5255, Bordeaux INP, ISM, Site ENSMAC
33607 Pessac, France
E-mail: Gerardo.salinassanchez@enscbp.fr</footnote>

<footnote>[b] Dr. H. Li
College of Chemistry and Chemical Engineering. Yangzhou University
225002 Yangzhou, China</footnote>

<footnote>[+] Miaoxia Liu and Leslie R. Arias-Aranda contributed equally</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cphc.202400133</footnote>

<footnote>© 2024 The Authors. ChemPhysChem 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>
LED. Furthermore, an additional light source based on an ECL mechanism,[43,44] taking place at the anode of the hybrid BPE, was integrated. Such a multicolor light-emitting array was designed by coupling the spontaneous oxidation of Mg with the kinetically favored reduction of protons on Pt and the interfacial reductive-oxidative mechanism of the Ru(bpy)₃²⁺/S₂O₈²⁻ system (Scheme 1).[45,46] The synergy between these ingredients results in a multimodal platform for the straightforward optical readout of chemical information based on compositional changes of the solution.

## Results and Discussion

As stated above the wireless multimodal light-emitting array takes advantage of a thermodynamically favorable source of electrons, in this case the spontaneous oxidation of Mg. However, in order to switch-on each LED, it is necessary to overcome its corresponding threshold potential difference. In principle, this can be achieved by coupling the kinetically favored reduction of protons on Pt with the spontaneous oxidation of Mg (eqn. 1).

$$2H^+ + Mg \rightarrow Mg^{2+} + H_2 \quad (1)$$

Thus, the hybrid array was designed by connecting a Mg and Pt wire to the anodic and cathodic pole of each miniaturized LED. In theory, these reactions take place spontaneously in acidic media with a redox potential difference of 2.34 V. Once again, in a first order of approximation, this spontaneous system can be considered as a short-circuited battery, explaining the polarity of the electrodes and the direction of the current indicated in Scheme 1. As it can be seen from the electric characterization of each diode (Figure S1) this value is high enough to overcome the threshold potential required to switch on the yellow and green diodes ($V_F \approx 1.7$ V and $\approx 2.1$ V, respectively), but is rather low in comparison with the measured $V_F$ value to power the emission of the blue LED ($\approx 2.4$ V). Since, according to the Nernst equation, it is possible to fine-tune the potential difference of the global reaction by changing the concentration of protons in solution, at first, the light emission of each BPE within the array was tested as a

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Scheme presenting the operating principle of the hybrid wireless light-emitting array powered by endogenous bipolar electrochemistry (BE). The associated redox and electrochemiluminescence (ECL) reactions involving the Ru(bpy)₃²⁺/S₂O₈²⁻ system, the electron flux and the color of the resulting light emission are shown.</figcaption></figure>

function of the H₂SO₄ concentration. As it can be seen, only the yellow LED emits light in the absence of acid, whereas, the green and blue diodes remain switched-off (Figure S2). However, the green diode switches on with the first addition of acid (0.5 mM). A gradual increase of the light intensity of both, yellow and green LEDs, as a function of the acid concentration was observed, reaching a plateau at concentrations above 4 mM (Figure 1a).

Interestingly along the full range of acid concentrations, the blue diode remains in its off-state (Figure S2 and Figure 1a). Thus, under these conditions, the induced potential difference is high enough to only trigger light emissions from the yellow and green diodes. To corroborate this, we measured the potential difference associated with the spontaneous redox reactions, by connecting a voltammeter between the Pt and Mg

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

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Integrals of the yellow, green and blue LED light intensity as a function of H₂SO₄ concentration in the absence (a) and presence (b) of 10 mM S₂O₈²⁻. (c) Optical pictures of the LED light generated by the multimodal array in 10 mM H₂SO₄ in the absence (left) and presence (right) of 10 mM S₂O₈²⁻. Scale bar 0.5 cm.</figcaption></figure>
wires. In the absence of acid, a potential difference of 1.79 V is induced by the redox reactions, which is high enough to power exclusively the yellow LED (Figure S3, black dots, yellow dotted line). As expected, the potential difference increases as a function of the acid concentration, following a similar trend than the produced light intensity, reaching a maximum value of 2.1 V in 4 mM H₂SO₄ (Figure S3, black dots). Hence, a high enough electromotive force is achieved in this range of acid concentration, powering in an endogenous manner both yellow and green emissions.

After this set of experiments, we evaluated the possible use of an alternative oxidant agent in order to increase the potential difference between the anode and the cathode of the LEDs, consequently powering the blue emission. For example, the two-electron reduction of S₂O₈²⁻ to SO₄²⁻ is characterized by a thermodynamic potential of 1.99 V vs NHE.[47] In theory, by coupling this reduction with the oxidation of Mg, a redox potential difference of 4.33 V can be induced (eqn. 2).

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

However, in a 10 mM S₂O₈²⁻ aqueous solution, a potential difference between the Pt and Mg wires of only 2.55 V was measured. Furthermore, by the continuous addition of H₂SO₄, a maximum potential difference of 2.75 V was generated (Figure S3, grey dots). As it is well-known, the reduction of S₂O₈²⁻ involves at first the formation of the radical anion species (3) with a redox potential around 0.59 V vs NHE.

$$S_2O_8^{2-} + e^{-} \rightarrow SO_4^{2-} + SO_4^{2-} \quad (3)$$

The coupling of this reduction at the surface of Pt with the oxidation of Mg leads to a redox potential difference of 2.93 V, which is in close agreement with the experimental potential difference measurements. In theory, such a voltage is high enough to power the light emission of the three diodes present in the BPE array. In order to corroborate this, the light emission of the hybrid array was tested as a function of H₂SO₄ concentration in the presence of a 10 mM S₂O₈²⁻ aqueous solution. As expected, all the diodes of the array switch on in the presence of S₂O₈²⁻, even in the absence of acid (Figure S4). Once again, an increase of the light intensity for all the LEDs, as a function of the acid concentration was observed (Figure 1b). Remarkably, the presence of S₂O₈²⁻ leads to a considerable increase of the light intensity for all the diodes (Figure 1b and c). This was corroborated by measuring the current passing through the BPE in the presence and absence of S₂O₈²⁻, as a function of the acid concentration. As it can be seen, the S₂O₈²⁻ produces a 2-fold increase of current in the range of H₂SO₄ concentration, which is reflected by the enhancement of light intensity (Figure S5). Therefore, the thermodynamically and kinetically favored reactions induce a spontaneous and fast consumption of electrons at the cathode, which causes an increase of current passing through the LED and a concomitant enhancement of light intensity.

Finally, in order to further expand the concept of the endogenous multimodal light-emitting array, an additional light

source was included in the system. In this context, as stated above we take advantage of the reductive-oxidative ECL-like mechanism, based on the reduction of Ru(bpy)₃²⁺ and S₂O₈²⁻ at the surface of Mg, to produce the excited state of Ru(bpy)₃²⁺* and its concomitant light emission (eqns. 4–7). Such ECL emission takes place even on an isolated Mg wire, as evidenced in Figure S6. However, in this approach, the S₂O₈²⁻ reagent plays a dual role by increasing the LED emission and as a sacrificial coreactant in the ECL process:

$$2Ru(bpy)_{3}^{2+} + Mg \rightarrow 2Ru(bpy)_{3}^{+} + Mg^{2+} \quad (4)$$

$$2S_2O_8^{2-} + Mg \rightarrow 2SO_4^{2-} + 2SO_4^{2-} + Mg^{2+} \quad (5)$$

$$Ru(bpy)_{3}^{+} + S_{2}O_{8}^{2-} \rightarrow SO_{4}^{2-} + SO_{4}^{2-} + Ru(bpy)_{3}^{2+} \quad (6)$$

$$Ru(bpy)_{3}^{+} + SO_{4}^{2-} \rightarrow Ru(bpy)_{3}^{2+*} + SO_{4}^{2-} \quad (7)$$

For this, the hybrid array was placed in an aqueous solution containing 1 mM Ru(bpy)₃²⁺ and 10 mM S₂O₈²⁻. Once again, under these conditions the three diodes within the array switch on, whereas no ECL emission is perceived (Figure 2a). This can be attributed to the formation of Mg(OH)₂ at the surface of the anode, limiting the Mg available to trigger the reductive-oxidative ECL mechanism. However, once enough H₂SO₄ is present in the solution (above 1 mM), the ECL reaction takes place at the Mg surface of each BPE, generating photons in the red (620 nm) spectral region (Figure 2a). It is noteworthy that, in this endogenous configuration, the entire Mg material produces the characteristic red ECL emission and not just the extremity of the bipolar electrode as in a classic exogenous configuration. Plots of the ECL intensity as a function of the acid concentration reveal a relative linear correlation for all the hybrid BPEs in a range between 1 mM to 8 mM (Figures 2b, c and d). In contrast, the light intensity associated with the diodes increases with the first addition of H₂SO₄, reaching a plateau, once again at concentrations above 4 mM (Figures 2b–2d). Such an increase of the ECL and LED light intensity is associated with the constant removal of the passivation layer of the Mg anode, allowing the access of the solution to the strong reductant. This improves the oxidation of Mg, causing a considerable increase of the current passing from the anode to the cathode of each hybrid BPE. At high concentrations of H₂SO₄ (> 6–8 mM), the integrated light intensity of the ECL is comparable or higher than those from the LEDs. Therefore, this hybrid BPE array allows generating spontaneously four light emissions based on different operating principles (i.e. ECL and LEDs) within a single device, using light sources that can easily be triggered by physico-chemical changes in solution.

## Conclusions

In conclusion, we designed a wireless multimodal light-emitting array producing light emission at four different wavelengths, controlled exclusively by physico-chemical changes of the solution. According to the fundamental principles of a battery,
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 2.** (a) Optical pictures of the ECL and LED light generated by the multimodal array at different H₂SO₄ concentrations (indicated in the figure), in a 1 mM Ru(bpy)₃²⁺, 10 mM S₂O₈²⁻ solution. Integrals of the ECL and the b) yellow, (c) green and (d) blue LED light intensity as a function of the H₂SO₄ concentration, obtained in a 1 mM Ru(bpy)₃²⁺, 10 mM S₂O₈²⁻ solution. Scale bar 0.5 cm.</figcaption></figure>

the coupled redox reactions induce an in-built electric field across the BPEs, powering the array. The spontaneous oxidation of Mg at the anodic pole of the LED coupled with the kinetically favored reduction of protons on Pt, acts as a power source to switch on diodes with relatively low threshold potential, i.e. yellow and green LEDs. In addition, the use of strong oxidants such as S₂O₈²⁻ allows powering the light emission of diodes with higher forward voltages (i.e. blue LED). Furthermore, under these conditions, and in the presence of a luminophore, a red ECL emission is induced at the surface of the anode of each BPE, via a reductive-oxidative mechanism. The chemical tuning of the potential difference between the Pt and Mg poles, allows to control both ECL and LED emission intensities. In fact, a direct correlation between the ECL/LED light intensity as a function of the H₂SO₄ concentration was observed. The reported multimodal array provides an easy and straightforward optical acquisition of physico-chemical information with the main advantage that no specific and sometimes expensive equipment, such as a potentiostat, or additional electric connections are necessary. This particular feature is of considerable interest, for example, for the continuous monitoring of strong oxidizing agents in solution, such as permanganate or dichromate ions.

## Experimental Section

Tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate (Ru(bpy)₃Cl₂, Aldrich, 97%), sulfuric acid (H₂SO₄, J. T. Baker, 95%-97%), potassium peroxodisulfate (K₂S₂O₈, Fluka, > 98%), agar silver paste (Agar scientific Ltd.), yellow, green and blue light-emitting diodes (1608 SMD diode, WAYJUN technology, 1.70 mm×0.8 mm), Mg wire

(ø = 0.5 mm) and Pt wire (ø = 0.25 mm, 99.9%, Alfa Aesar) were used as received. All solutions were prepared with deionized water (MilliQ Direct-Q, resistivity of 18.2 MΩ.cm at 25 °C). All the bipolar electrodes were assembled as follows: at first the Mg and Pt wires (l ≈ 1 cm) and the LEDs were immobilized at the bottom of a Petri dish. Afterwards, the electric connection was established at the anode and cathode of each LED by using a small volume of agar silver paint, followed by an insulating layer to avoid short-circuiting. The total length of all the bipolar electrodes is around 2.1 cm. 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. Potential difference and current measurements were carried out by using a Keithley electrometer/ High resistance meter connected to a personal computer.

## Acknowledgements

The authors thank the International Research Network ELEC-TROSENS funded by the CNRS. M. L. acknowledges the China Scholarship Council (CSC) for her PhD funding. L. R. A.-A. acknowledges the University of Bordeaux for her PhD scholar-ship. The work has also been supported by the European Research Council under the European Union's Horizon 2020 research and innovation program (grant agreement no 741251, ERC Advanced grant ELECTRA).

## Conflict of Interests

The authors declare no conflict of interest.
<figure><img src="image_9.png" /></figure>

Data Availability Statement

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

Keywords: Light-emitting diodes · electrochemiluminescence · bipolar electrochemistry · optical readout · multimodal device

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Manuscript received: February 6, 2024

Revised manuscript received: April 15, 2024

Accepted manuscript online: April 16, 2024

Version of record online: May 22, 2024