Electrogenerated Chemiluminescence of Luminol Mediated by Carbonate Electrochemical Oxidation at a Boron-Doped Diamond

Comparison between electrochemiluminescence of luminol and electrocatalysis by Prussian blue for the detection of hydrogen peroxide

Electrochemiluminescence (ECL) of luminol and electrocatalysis by Prussian blue were compared for the selective detection of H2O2 at the boron-doped diamond (BDD) electrodes. The H2O2 detection was optimized by various parameters such as the applied potential at pH 7.4, which is a physiological value usually used for H2O2 detection in enzymatic reactions. At an optimum applied potential of +0.5 V, a linear increase in the ECL signals (R2 = 0.99) was achieved for H2O2 concentrations ranging between 0 to 100 μM with an estimated limit of detection (LOD) of 2.59 μM. This LOD was better than that obtained with electrocatalysis measurements using the same electrode modified with Prussian blue. Furthermore, the interference study in the presence of glucose, Fe3+, Cl-, Ca2+, CO32-, Na+, and F- ions showed a comparable selectivity of the luminol ECL and PB-BDD electrochemical current. Nevertheless, the ECL method exhibited significant advantage in the high stability of its signal response.

Electrogenerated chemiluminescence from luminol-labelled microbeads triggered by in situ generation of hydrogen peroxide

We developed a sensing strategy that mimics the bead-based electrogenerated chemiluminescence immunoassay. However, instead of the most common metal complexes, such as Ru or Ir, the luminophore is luminol. The electrogenerated chemiluminescence of luminol was promoted by in situ electrochemical generation of hydrogen peroxide at a boron-doped diamond electrode. The electrochemical production of hydrogen peroxide was achieved in a carbonate solution by an oxidation reaction, while at the same time, microbeads labelled with luminol were deposited on the electrode surface. For the first time, we proved that was possible to obtain light emission from luminol without its direct oxidation at the electrode. This new emission mechanism is obtained at higher potentials than the usual luminol electrogenerated chemiluminescence at 0.3-0.5 V, in conjunction with hydrogen peroxide production on boron-doped diamond at around 2-2.5 V (vs Ag/AgCl).

From theory to practice: understanding the challenges in the implementation of electrogenerated chemiluminescence for analytical applications

Electrogenerated chemiluminescence (ECL) stands out as a remarkable phenomenon of light emission at electrodes initiated by electrogenerated species in solution. Characterized by its exceptional sensitivity and minimal background optical signals, ECL finds applications across diverse domains, including biosensing, imaging, and various analytical applications. This review aims to serve as a comprehensive guide to the utilization of ECL in analytical applications. Beginning with a brief exposition on the theory at the basis of ECL generation, we elucidate the diverse systems employed to initiate ECL. Furthermore, we delineate the principal systems utilized for ECL generation in analytical contexts, elucidating both advantages and challenges inherent to their use. Additionally, we provide an overview of different electrode materials and novel ECL-based protocols tailored for analytical purposes, with a specific emphasis on biosensing applications.

A Carbonate-Involved Amplification Strategy for Cathodic Electrochemiluminescence of Luminol Triggered by the Catalase-like CoO Nanorods

The lumiol-O₂ electrochemiluminescence (ECL) system constantly emits bright light at positive potential. Notably, compared with the anodic ECL signal of the luminol-O₂ system, the great virtues of cathodic ECL are that it is simple and causes minor damage to biological samples. Unfortunately, little emphasis has been paid to cathodic ECL, owing to the low reaction efficacy between luminol and reactive oxygen species. The state-of-the-art work mainly focuses on improving the catalytic activity of the oxygen reduction reaction, which remains a significant challenge. In this work, a synergistic signal amplification pathway is established for luminol cathodic ECL. The synergistic effect is based on the decomposition of H₂O₂ by catalase-like (CAT-like) CoO nanorods (CoO NRs) and regeneration of H₂O₂ by a carbonate/bicarbonate buffer. Compared with Fe₂O₃ nanorod (Fe₂O₃ NR)- and NiO microsphere-modified glassy carbon electrodes (GCEs), the ECL intensity of the luminol-O₂ system is nearly 50 times stronger when the potential ranged from 0 to -0.4 V on the CoO NR-modified GCE in a carbonate buffer solution. The CAT-like CoO NRs decompose the electroreduction product H₂O₂ into OH^· and O₂^·-, which further oxidize HCO₃^- and CO₃^2- to HCO₃^· and CO₃^·-. These radicals very effectively interact with luminol to form the luminol radical. More importantly, H₂O₂ can be regenerated when HCO₃^· dimerizes to produce (CO₂)₂*, which provides a cyclic amplification of the cathodic ECL signal during the dimerization of HCO₃^·. This work inspires developing a new avenue to improve cathodic ECL and deeply understand the mechanism of a luminol cathodic ECL reaction.

Boosting ultralong chemiluminescence for the self-powered time-resolved immunosensor

The construction of an immunosensor based on ultralong chemiluminescence is challenged due to the shortage of highly efficient initiator for long and stable catalysis. Herein, the heterogeneous Au/Pt@CuO/Cu₂O catalyst was used to investigate the structure-activity relationship, while Au/Pt significantly promotes the activity of CuO/Cu₂O to catalyze H₂O₂ and thus produces ·OH and O₂^•- radicals in highly alkaline solutions, resulting in the strong and long chemiluminescence in the reaction with luminol (10 mL, more than 4 min with 1 μg catalyst). By using the Au/Pt@CuO/Cu₂O as the label in the immunoassay, the strong and long chemiluminescence could initiate the photocurrent of the photoelectrochemical (PEC) substrate, and the luminescence time could influence the photocurrent extinction time, thus a self-powered time-resolved PEC immunosensor was developed to detect furosemide, showing a linear relationship between the extinction time and the logarithm of concentrations from 10^-3 to 1 μg/L. This work not only experimentally verifies that the Pt-O-Cu bond in heterogeneous catalysts breaks the pH limitation of the Fenton reaction, but also realizes the chemiluminescence for self-powered time-resolved immunosensor, thereby expanding the portable applicability of chemiluminescence in food safety inspection, health monitoring, and biomedical detection without external light source.

Electrochemiluminescence in paired signal electrode (ECLipse) enables modular and scalable biosensing

Electrochemiluminescence (ECL) has an inherently low background and enables precise chemical reactions through electrical control. Here, we report an advanced ECL system, termed ECLipse (ECL in paired signal electrode). We physically separated ECL generation from target detection: These two processes were carried out in isolated chambers and coupled through an electrode. The strategy allowed us to minimize cross-chemical reactions, design electrodes for high ECL signals, and integrate multiple sensors in a chip. As a proof of concept, we implemented an eight-plex ECLipse and applied it to detect host factors in human plasma. ECLipse achieved higher signal-to-noise ratio than conventional ECL assays and was >7000-fold more sensitive than enzyme-linked immunosorbent assay. In a pilot clinical study, we could detect septic conditions by measuring host factors [i.e., interleukin-3 (IL-3), IL-6, and procalcitonin (PCT)]. ECLipse assay further revealed distinct IL-3 and IL-6 patterns in patients with severe acute respiratory syndrome coronavirus 2 infection.

Deciphering electrochemiluminescence generation from luminol and hydrogen peroxide by imaging light emitting layer

Electrochemiluminescence (ECL) of luminol is a luminescence process that proceeds in the presence of reactive oxygen species (e.g. hydrogen peroxide (H2O2)) at a suitable electrode potential, the reaction mechanism of which is complicated and remains ambiguous. In this work, we report a visualization approach for measuring the thickness of the ECL layer (TEL) of the luminol/H2O2 system to decipher the reaction process by combined use of the microtube electrode, ECL microscopy, and finite element simulations. With the increase of solution pH, the ECL image captured with the microtube electrode tends to vary from spot to ring, corresponding to the decrease of TEL from >9.1 μm to ca. 4.3 μm. We propose that different intermediates are involved in the course of ECL reaction. At a low pH (e.g. pH < 9), a relatively large TEL is most likely determined by the diffusion of oxidized and deprotonated luminol intermediate that is neutral and has a long lifetime. While at a high pH (e.g. pH in the range of 10 to 12), the ECL reaction is controlled by short-lived radical intermediates of both luminol and superoxide anion. The proposed mechanism is proved theoretically by finite element simulations and experimentally by the apparent effect of concentration ratio of luminol/H2O2.

Electrogenerated chemiluminescence of luminol at a boron-doped diamond electrode for the detection of hypochlorite

Electrogenerated chemiluminescence (ECL) of luminol at a boron-doped diamond electrode has been used for hypochlorite determination. The presence of H₂O₂ induces the generation of the ECL signals of luminol. In contrast, the presence of hypochlorite oxidizes luminol directly to decrease the ECL signals of luminol. Accordingly, a decrease of the ECL signals of luminol in the presence of H₂O₂ was used as the signal response for hypochlorite detection. A linear decrease of ECL signals with the NaClO concentration in the range from 0 to 20 μM was observed with a sensitivity of 18.56 a.u. μM^-1 cm^-2. An estimated detection limit of 0.88 μM was achieved, which is around one order lower than the detection limit obtained using the normal electrochemical method with the same electrode. The system also provides a good selectivity towards Cu²⁺ and Na⁺. A reproducibility of 3.40%RSD was noted for 15 repetitive measurements. The analytical performance was found to be favourable in comparison to those of other typical electrochemical and electrochemiluminescence methods, indicating that it is applicable for real sample detection.

CO2/carbonate-mediated electrochemical water oxidation to hydrogen peroxide

Electrochemical water oxidation reaction (WOR) to hydrogen peroxide (H₂O₂) via a 2e⁻ pathway provides a sustainable H₂O₂ synthetic route, but is challenged by the traditional 4e⁻ counterpart of oxygen evolution. Here we report a CO₂/carbonate mediation approach to steering the WOR pathway from 4e⁻ to 2e⁻. Using fluorine-doped tin oxide electrode in carbonate solutions, we achieved high H₂O₂ selectivity of up to 87%, and delivered unprecedented H₂O₂ partial currents of up to 1.3 A cm⁻², which represents orders of magnitude improvement compared to literature. Molecular dynamics simulations, coupled with electron paramagnetic resonance and isotope labeling experiments, suggested that carbonate mediates the WOR pathway to H₂O₂ through the formation of carbonate radical and percarbonate intermediates. The high selectivity, industrial-relevant activity, and good durability open up practical opportunities for delocalized H₂O₂ production.

Electrochemical H₂O oxidation to H₂O₂ is challenged by the competitive O₂ evolution reaction. Here, the authors report a CO₂/carbonate mediation approach to steering the H₂O oxidation pathway from O₂ evolution to H₂O₂ generation.

Boron-Doped Diamond Electrode Outperforms the State-of-the-Art Electrochemiluminescence from Microbeads Immunoassay

Electrochemiluminescence (ECL) is a powerful transduction technique where light emission from a molecular species is triggered by an electrochemical reaction. Application to biosensors has led to a wide range of electroanalytical methods with particular impact on clinical analysis for diagnostic and therapeutic monitoring. Therefore, the quest for increasing the sensitivity while maintaining reproducible and easy procedures has brought investigations and innovations in (i) electrode materials, (ii) luminophores, and (iii) reagents. Particularly, the ECL signal is strongly affected by the electrode material and its surface modification during the ECL experiments. Here, we exploit boron-doped diamond (BDD) as an electrode material in microbead-based ECL immunoassay to be compared with the approach used in commercial instrumentation. We conducted a careful characterization of ECL signals from a tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)₃²⁺)/tri-n-propylamine (TPrA) system, both homogeneous (i.e., free diffusing Ru(bpy)₃²⁺) and heterogeneous (i.e., Ru(bpy)₃²⁺ bound on microbeads). We investigated the methods to promote TPrA oxidation, which led to the enhancement of ECL intensity, and the results revealed that the BDD surface properties greatly affect the ECL emission, so it does the addition of neutral, cationic, or anionic surfactants. Our results from homogeneous and heterogeneous microbead-based ECL show opposite outcomes, which have practical consequences in ECL optimization. In conclusion, by using Ru(bpy)₃²⁺-labeled immunoglobulins bound on microbeads, the ECL resulted in an increase of 70% and a double signal-to-noise ratio compared to platinum electrodes, which are actually used in commercial instrumentation for clinical analysis. This research infers that microbead-based ECL immunoassays with a higher sensitivity can be realized by BDD.

Insights into Electrochemiluminescence Dynamics by Synchronizing Real-Time Electrical, Luminescent, and Mass Spectrometric Measurements

Electrochemiluminescence (ECL) comprises a sophisticated cascade of reactions. Despite advances in mechanistic studies by electrochemistry and spectroscopy, a lack of access to dynamic molecular information renders many plausible ECL pathways unclear or unproven. Here we describe the construction of a real-time ECL mass spectrometry (MS) platform (RT-Triplex) for synchronization of dynamic electrical, luminescent, and mass spectrometric outputs during ECL events. This platform allows immediate and continuous sampling of newly born species at the Pt wire electrode of a capillary electrochemical (EC) microreactor into MS, enabling characterization of short-lived intermediates and the multi-step EC processes. Two ECL pathways of luminol are validated by observing the key intermediates α-hydroxy hydroperoxide and diazaquinone and unraveling their correlation with applied voltage and ECL emission. Moreover, a “catalytic ECL route” of boron dipyrromethene (BODIPY) involving homogeneous oxidation of tri-n-propylamine with the BODIPY radical cation is proposed and verified.

A real-time electrochemiluminescence mass spectrometry platform (RT-Triplex) was developed for revealing ECL mechanisms by synchronization of dynamic electrical, luminescent, and mass spectrometric signals at the electrode–electrolyte interface.

Natural catalyst for Luminol chemiluminesence - Application to validate peroxide levels in commercial hair dyes

Herein, we report hydrothermally treated green leaves (Moringa Oleifera) extract exploited as an efficient and highly-sensitive catalyst to catalyse the Chemiluminescence (CL) reaction of luminol. In the absence of enhancer, this green and hydrothermally treated catalyst (GHT) was found to significantly enhance the CL intensity about 3.5 fold as compared to the traditionally used K₃ Fe(CN)₆ catalyst. The structure and surface morphology of the catalyst was elucidated by XPS, SEM, XRD and Raman spectroscopy. The synergistic effect of the catalyst in the CL reaction was systematically investigated in the presence of hydrogen peroxide using UV Visible and chemiluminesence spectroscopy. Studies show that the sensitivity of the catalyst could be amplified by adjusting several parameters such as pH of the medium, concentrations of the base and luminol. The sensitivity of the novel-type catalyst was examined through the validation of hydrogen peroxide levels in the commercial hair dye samples. Remarkably, the catalyst displayed ultra-sensitivity to hydrogen peroxide as the limit of detection (LOD) of H₂ O₂ using this catalyst is determined to be 0.02 μM under optimized conditions. In general, the proposed inexpensive, eco-friendly, and non-toxic catalyst could enable the determination of the hydrogen peroxide for diverse analytical applications.

Enhanced Cathodic Electrochemiluminescence of Luminol on Iron Electrodes

Electrochemiluminescence (ECL) behavior of luminol derivative was investigated in reduction on different electrode materials. We found that luminol and its widely used L-012 derivative, emitting at physiological pH values, exhibit strong cathodic ECL emission on iron and stainless steel electrodes with hydrogen peroxide, whereas no ECL signal was observed with other classic electrode materials (Au, Pt, and C). On a Ni electrode, a low cathodic ECL signal was observed. This points out to the essential role of iron-containing materials to enhance the cathodic ECL emission. Under the reported conditions, the cathodic ECL signal of L-012 is comparable to the classically used anodic ECL emission. Thus, dual bright ECL emissions with L-012 were obtained simultaneously in oxidation and in reduction on iron materials as imaged in a wireless bipolar electrochemistry configuration. Such an ECL system generating light emission concomitantly in oxidation and in reduction is extremely rare and it opens appealing (bio)analytical and imaging applications, in biosensing, remote detection, bipolar ECL analysis, and ECL-based cell microscopy.

Mapping the Distribution of Potential Gradient in Bipolar Electrochemical Systems through Luminol Electrochemiluminescence Imaging

Bipolar electrochemistry has been regarded as a powerful and sustainable electrochemical process for the synthesis of novel functional materials. The appealing features of this electrochemical technology, such as the wireless nature of the bipolar electrode (BPE) and the possibility to drive simultaneously electrochemical reactions on multiple BPEs placed in the same electrochemical cell, together with the possibility to change the shape and positioning of the driving electrodes, give significant freedom to design reaction systems. Nevertheless, the cell geometry dramatically affects the distribution and intensity of the potential gradient generated on the BPE surface and its monitoring is hampered due to the wireless nature of the BPE. In the present study, we propose the use of electrochemiluminescence (ECL) as an electrochemical imaging technique to map the distribution of potential gradient in bipolar electrochemical cells with different geometries. The proposed approach exploits the strong ECL emission of luminol/hydrogen peroxide (H₂O₂) system generated at the anodic pole of the BPE, when the total applied voltage (E_tot) is strong enough to trigger the electrochemical reaction. Since luminol ECL emission is rather intense and relatively stable, the evolution of the potential distribution as a function of E_tot can be monitored using a digital camera, allowing the elucidation of the potential distribution profile in every bipolar configuration. The suggested approach represents a valuable and reliable method to map the potential gradient in bipolar electrochemical systems and can be readily employed in every type of bipolar configuration.