Single Cell Electrochemiluminescence Imaging: From the Proof-of-Concept to Disposable Device-Based Analysis

Real-Time Visualization of Endogenous H2O2 Production in Mammalian Spheroids by Electrochemiluminescence

Two-dimensional cell culture may be insufficient when it comes to understanding human disease. The redox behavior of complex, three-dimensional tissue is critical to understanding disease genesis and propagation. Unfortunately, few measurement tools are available for such three-dimensional models to yield quantitative insight into how reactive oxygen species (ROS) form over time. Here, we demonstrate an imaging platform for the real-time visualization of H2O2 formation for mammalian spheroids made of noncancerous human embryonic kidney cells (HEK-293) and metastatic breast cancer cells (MCF-7 and MDA-MB-231). We take advantage of the luminol and H2O2 electrochemiluminescence reaction on a transparent tin-doped indium oxide electrode. The luminescence of this reaction as a function of [H2O2] is linear (R 2 = 0.98) with a dynamic range between 0.5 μM to 0.1 mM, and limit of detection of 2.26 ± 0.58 μM. Our method allows for the observation of ROS activity in growing spheroids days in advance of current techniques without the need to sacrifice the sample postanalysis. Finally, we use our procedure to demonstrate how key ROS pathways in cancerous spheroids can be up-regulated and downregulated through the addition of common metabolic drugs, rotenone and carbonyl cyanide-p-trifluoromethoxyphenylhydrazone. Our results suggest that the Warburg Effect can be studied for single mammalian cancerous spheroids, and the use of metabolic drugs allows one to implicate specific metabolic pathways in ROS formation. We expect this diagnostic tool to have wide applications in understanding the real-time propagation of human disease in a system more closely related to human tissue.

Pre-Label-Free Three-Dimensional Imaging of Cellular Bottom Topography with a Charge-Lock Electrochemiluminescence Microscope

Cellular bottom structure plays a vital role in cell mobility, but its narrow space makes it difficult to study. Currently, it remains challenging to panoramically map the cellular bottom topography variations due to the potential cellular damage and off-target effects of luminophores when using current ultra-high-resolution optical microscopy. Here, we present charge-lock spatiotemporal transformation electrochemiluminescence (CL-STTECL) microscopy. The ECL is "locked" by the charge exclusion between cationic luminophores and the proton film, while amine co-reactants simultaneously "unlock" this process. Consequently, the uneven distribution of luminophores at the cellular base triggers a gap distance-dependent ECL emission trajectory, allowing the identification of cell-matrix gap distance heterogeneity at an axial spatial resolution of ∼10 nm. Using CL-STTECL microscopy, we demonstrated changes in cellular bottom topography in response to various external stimulations and uncovered heterogeneity in panoramic features between normal tissue cells and metastatic cancer cells, offering a novel approach for early cancer screening.

In Situ "Confocal" Electrochemiluminescence 3D Imaging: From Cell to Tissue Section

Endowing electrochemiluminescence (ECL) imaging technique with three-dimensional (3D) resolution to investigate specimens at varying axial depths poses a challenging yet significant objective. Herein, a "confocal" 3D ECL imaging method was developed using luminol as ECL probe, in which excited luminophore was formed in the vicinity of electrode surface through homogeneous chemical reactions between oppositely diffusing ECL precursors, luminol diazaquinone intermediate (L), and hydrogen peroxide (H2O2), confining the ECL emission in a thin plane (ECL focal plane) parallel to electrode surface at their intersection. The regulating ability of electrochemical method on the reaction fluxes of L and H2O2 was validated, regulating the axial location of the ECL focal plane from 0 to 63 µm, which can even extend to 400 µm by using the stable coreactant of ClO-. Leveraging the optical sectioning capability of the ECL focal plane, the "confocal" 3D ECL imaging method was applied to bioimaging, from cells to tissue sections. It revealed cellular morphology changes during cell polarity establishment and the heterogeneous distribution of complex tubule structure in kidney tissue sections. The optical sectioning capability of "confocal" 3D ECL imaging makes it a powerful tool for studying complex biological samples.

Tomography Electrogenerated Chemiluminescence Imaging from Magnetic Microbeads

Electrogenerated chemiluminescence (ECL) bioassay has witnessed remarkable growth in biosensing and diagnostics. An understanding of the reaction mechanism within ECL is important for improving and designing sensitive ECL methods. Herein, a sidelong ECL microscopy and finite element simulation are developed for tomography imaging from single magnetic microbeads and deciphering reaction mechanism within bead-based coreactant ECL system. Ruthenium derivative labeled magnetic beads (Ru1-Mag@MB) and tri-n-propylamine are chosen as typical coreactant ECL systems. ECL emissions from Ru1-Mag@MB in different conditions are imaged via the sidelong ECL microscopy, which can allow the spatially resolved measurement of the ECL emitting layer from single microbeads in the vertical direction of the electrode surface. Surface-confined ECL emissions are mainly observed at or near the interface between the electrode surface and the Ru1-Mag@MB. The surface distribution plots of ECL intensity, and tomography ECL, across single Ru1-Mag@MB display "first increase and then decrease" phenomena. The ECL intensity and emission extension are varied with the change of the radical lifetime, the kinetics of chemical reactions, and the optical parameters of microbeads. The sidelong ECL microscopy with the tomography of the ECL emitting layer can provide a promising way for insightful mechanistic information within a bead-based ECL system.

Spatial Profiling of Multiple Enzymatic Activities at Single Tissue Sections via Fenton-Promoted Electrochemiluminescence

Profiling multiple enzymatic activities in tissue is crucial for understanding complex metabolic and signaling networks, yet remains a challenge with existing optical microscopies. Here, we developed a Fenton-promoted luminol electrochemiluminescence (ECL) imaging method to achieve the spatial mapping of multiple enzymatic activities within a single tissue section. This method quantitatively visualizes individual enzymatic activity by combining the enzymatic conversion of substrates with the chemical confinement of the locally produced hydrogen peroxide. To achieve high-resolution spatial imaging by limiting the diffusion (∼500 μm) of hydrogen peroxide, iron oxide nanoparticles were coated on the tissue surface to initiate the Fenton process, locally converting hydrogen peroxide into short-lived hydroxyl radicals with a nanometer-scale diffusion range. The Fenton-promoted ECL emission is confined at the enzymatic conversion sites, offering unprecedented spatial visualization of four tumor-associated oxidases within a single tissue section. Colocalization revealed a synergistic effect between lysyl oxidase and quiescin sulfhydryl oxidase on post-translational modifications of tumor extracellular matrix proteins, along with a previously undiscovered interaction with amiloride-sensitive amine oxidase, which could not be distinguished based on expressions or single enzymatic activity alone. This approach offers a novel activity-based protein profiling tool at the tissue level, providing new data for future enzynomic research and multimodal imaging.

Stimuli-Responsive DNA Nanomachines for Intracellular Targeted Electrochemiluminescence Imaging in Single Cells

Electrochemiluminescence (ECL) microscopy has emerged as a powerful technique for single-cell imaging owing to its unparalleled background-free imaging advantages. However, controlled intracellular ECL imaging remains challenging. Here, we developed a stimuli-responsive self-assembled DNA nanomachine that enables the ECL imaging of intracellular target biomolecules in single cells. The ECL nanoprobe consists of an ECL nanoemitter constructed from Ru(bpy)3 2+-doped metal-organic framework as the nanoreactor core, with a DNA polymer hydrogel (DNAgel) shell acting as the stimuli-gated layer. The outer functionalized DNAgel of the ECL nanoprobe was specifically designed to block ECL generation and to dissociate by ATP molecules, thereby enabling selective recovery of ECL emission capability. Such an engineered stimuli-responsive nanomachine successfully achieved the targeted ECL imaging of intracellular ATP distribution with spatial resolution. In addition, ECL imaging of various intracellular biomolecules should be generalizable by simply changing the switching DNA sequence of the probe. Our research provided a reliable strategy for ECL microscopy within cells, which will broaden the application of ECL in single-cell and single-molecule profiling.

Proline-Selective Electrochemiluminescence Detecting a Single Amino Acid Variation Between A1 and A2 β-Casein Containing Milks

The proline amino acid and prolyl residues of peptides/proteins confer unique biological and biochemical properties that motivates the development of proline-selective analysis. The study focuses on one specific class of problem, the detection of single amino acid variants involving proline, and reports a Pro-selective electrochemiluminescence (ECL) method. To develop this method, the A1-/A2- variants of milk's β-casein protein are investigated because it is a well-established example and abundant samples are readily available. Specifically, β-casein has 209 amino acids with 34 (or 35) proline residues: the A1-variant has a Pro-to-His substitution at position 67 (relative to the A2 variant). The study shows that proline's strong luminescence allows the generic discrimination of: Pro from other amino acids; an A2-oligopeptide from an A1-oligopeptide; the A2-β-casein variant from the A1-variant; and commercially-available A2 milks from A1-containing regular milks. The evidence indicates that luminescence depends on proline content and accessibility, as well as signal quenching. Compared to conventional immunoassays, the ECL method is simple, rapid, and inexpensive. Further, the ECL-method is Pro-selective (vs molecularly-selective like typical immunoassays) which should make it broadly useful for studying the role of proline in biology and especially useful for tracking the digestion of proline-rich proteins in the diet.

Frontiers in fluorescence imaging: tools for the in situ sensing of disease biomarkers

Fluorescence imaging has been recognized as a powerful tool for the real-time detection and specific imaging of biomarkers within living systems, which is crucial for early diagnosis and treatment evaluation of major diseases. Over the years, significant advancements in this field have been achieved, particularly with the development of novel fluorescent probes and advanced imaging technologies such as NIR-II imaging, super-resolution imaging, and 3D imaging. These technologies have enabled deeper tissue penetration, higher image contrast, and more accurate detection of disease-related biomarkers. Despite these advancements, challenges such as improving probe specificity, enhancing imaging depth and resolution, and optimizing signal-to-noise ratios still remain. The emergence of artificial intelligence (AI) has injected new vitality into the designs and performances of fluorescent probes, offering new tools for more precise disease diagnosis. This review will not only discuss chemical modifications of classic fluorophores and in situ visualization of various biomarkers including metal ions, reactive species, and enzymes, but also share some breakthroughs in AI-driven fluorescence imaging, aiming to provide a comprehensive understanding of these advancements. Future prospects of fluorescence imaging for biomarkers including the potential impact of AI in this rapidly evolving field are also highlighted.

Electrochemiluminescence Detecting and Imaging of Yeast Metabolism Indicated by Endogenetic Co-reactant

Glycolysis, a pivotal step in yeast metabolism, plays an indispensable role as a carbohydrate utilization process crucial for cellular survival. Developing advanced technologies to elucidate this fundamental physiological process holds significant scientific implications. Electrochemiluminescence (ECL) imaging exhibits the advantage of negligible background interference and facilitates straightforward visualization, thereby conferring significant value in biomolecular observation. In this study, we present an ECL imaging method for investigating yeast metabolism by utilizing the endogenetic NADH as an efficient coreactant for ECL generation. The yeast glycolysis process drives the conversion of NAD+ to NADH, resulting in enhanced ECL response as well as the increased brightness of ECL images that can be used for quantification of yeast activity. There was a linear correlation between the reciprocal of both the gray value of ECL image and yeast concentration within the range of 6.25 × 106 - 6.25 × 108 CFU/mL. Due to the highly efficient coreactant behavior of NADH, our method demonstrated excellent selectivity with minimal interference. Furthermore, we employed this approach to investigate some toxic inhibitors on yeast metabolism, yielding reliable results. This ECL imaging method not only avoids the use of additional coreactants but also provides a sensitive and intuitive approach for monitoring yeast metabolism, demonstrating great potential in revealing various complex biological processes.

Regulating the work function and surface hydrophobicity of an indium tin oxide electrode for enhanced electrochemiluminescence analysis

The electrochemical properties of the indium tin oxide (ITO) electrode were improved significantly by surface modification with ethephon and an ultrathin polydimethylsiloxane (PDMS) layer to regulate the work function and surface hydrophobicity of ITO. Based on this strategy, the electrochemiluminescence (ECL) intensity of tris(2,2'-bipyridyl)ruthenium (Ru(bpy)32+) and tri-n-propylamine (TPrA) in solution and on a microbead surface can be enhanced by 110 and 2 times, respectively. When using the modified electrode to detect nicotinamide adenine dinucleotide (NADH), the linear range (5-1000 μM) was increased dramatically in comparison with a bare ITO electrode, with a limit of detection of 1.65 μM. The modified electrode with improved electrochemical properties holds great potential for applications in ECL bioassays and imaging analysis.

Recent Advances in Electrochemiluminescence Visual Biosensing and Bioimaging

Electrochemiluminescence (ECL) is one of the most powerful techniques that meet the needs of analysis and detection in a variety of scenarios, because of its highly analytical sensitivity and excellent spatiotemporal controllability. ECL combined with microscopy (ECLM) offers a promising approach for quantifying and mapping a wide range of analytes. To date, ECLM has been widely used to image biological entities and processes, such as cells, subcellular structures, proteins and membrane transport properties. In this review, we first introduced the mechanisms of several classic ECL systems, then highlighted the progress of visual biosensing and bioimaging by ECLM in the last decade. Finally, the characteristics of ECLM were summarized, as well as some of the current challenges. The future research interests and potential directions for the application of ECLM were also outlooked.

Electrochemiluminescent imaging of a NADH-based enzymatic reaction confined within giant liposomes

Herein, transient releases either from NADH-loaded liposomes or enzymatic reactions confined in giant liposomes were imaged by electrochemiluminescence (ECL). NADH was first encapsulated with the [Ru(bpy)3]2+ luminophore inside giant liposomes (around 100 µm in diameter) made of DOPC/DOPG phospholipids (i.e., 1,2-dioleolyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycerol-3-phospho-(1'-rac-glycerol) sodium salt) on their inner- and outer-leaflet, respectively. Then, membrane permeabilization triggered upon contact between the liposome and a polarized ITO electrode surface and ECL was locally generated. Combination of amperometry, photoluminescence, and ECL provided a comprehensive monitoring of a single liposome opening and content release. In a second part, the work is focused on the ECL characterization of NADH produced by glucose dehydrogenase (GDH)-catalyzed oxidation of glucose in the confined environment delimited by the liposome membrane. This was achieved by encapsulating both the ECL and catalytic reagents (i.e., the GDH, glucose, NAD+, and [Ru(bpy)3]2+) in the liposome. In accordance with the results obtained, NADH can be used as a biologically compatible ECL co-reactant to image membrane permeabilization events of giant liposomes. Under these conditions, the ECL signal duration was rather long (around 10 s). Since many enzymatic reactions involve the NADH/NAD+ redox couple, this work opens up interesting prospects for the characterization of enzymatic reactions taking place notably in artificial cells and in confined environments.

Lensless On-Chip Chemiluminescence Imaging for High-Throughput Single-Cell Heterogeneity Analysis

High-throughput single-cell heterogeneity imaging and analysis is essential for understanding complex biological systems and for advancing personalized precision disease diagnosis and treatment. Here, we present a miniaturized lensless chemiluminescence chip for high-throughput single-cell functional imaging with subcellular resolution. With the sensitive chemiluminescence sensing and wide field of view of contact lensless imaging, we demonstrated the chemiluminescent imaging of over 1000 single cells, and their membrane glycoprotein and the high-throughput single-cell heterogeneity of membrane protein imaging were examined for precision analysis. Furthermore, the functional adhesion and heterogeneity of single live cells were imaged and explored. This miniaturized lensless on-chip CL-CMOS imaging platform enables high-throughput single-cell imaging and analysis with high sensitivity and subcellular resolution, providing new techniques for the cellular study of biological heterogeneity and has potential application in precision disease diagnosis and treatment at the point-of-care settings.

Wireless single-electrode electrochemiluminescence device based on wireless reverse charging or on-the-go USB transmission for multiplex analysis

A wireless electrochemiluminescence (ECL) device employing the wireless reverse charging function or on-the-go (OTG) USB transmission function of smartphones is designed. It was coupled with a multi-channel single-electrode electrochemical system based on the resistance-induced potential difference for multiple ECL analyses using a smartphone as the detector.

Recent Advances in Luminophores for Enhanced Electrochemiluminescence Analysis

Electrochemiluminescence (ECL) detection is widely applied in many fields, including chemical measurement, biological analysis, and clinic tests, due to its high sensitivity. Currently, the fast development of many new electrochemical luminophores is continuously improving the ECL-based detection ability. Besides the enhancement of luminescence emission for a high detection sensitivity, minimizing the effect of co-reactants on ECL detection and achieving multiple analysis in one sample are also the main directions in this field. This review focuses on a summary of recently prepared new luminophores to achieve the three aims mentioned above. Especially, the review is composed by three parts, focusing on the luminophores or materials with high ECL efficiency, self-enhancing properties, and multi-color ECL luminophores. The fabrication of biosensors using these molecules is also reviewed to exhibit the advances in biological applications.

Automated ECL Aptasensing Platform from an Intrarticular Radical Annihilation Route for Distinguishing Glioma Stages

Nowadays, continuous efforts have been devoted to designing stable and high-efficiency electrochemiluminescence (ECL) emitters as alternatives for tris(2,2'-bipyridine)-ruthenium(II) (Ru(bpy)32+) in medical research. Herein, a novel ECL emitter was obtained by coordinating crystalline covalent triazinyl frameworks (cCTFs) with Ru2+ (termed Ru-cCTFs), which exhibited strong ECL emission by the ligand to metal charge transfer (LMCT) route. After its integration with 4-mercaptopyridine (SH-Py), the resultant SH-Py-Ru-cCTFs achieved 2.3-fold enhancement in the ECL efficiency by employing Ru(bpy)32+ as a standard, which involved a dynamic "intrarticular radical annihilation" ECL pathway. On such foundation, an automated ECL (A-ECL) aptasensor was constructed with an "on-off-on" model and magnetic separation upon linkage of the SH-Py-Ru-cCTFs with streptavidin (SA) magnetic beads (MBs). This automatic assay of miRNA-182 showed a wider linear range from 1.0 to 100.0 fM with a correlation coefficient (R2) of 0.994, a lower limit of detection (LOD) down to 0.28 fM, and faster operation within 41 min. Impressively, this bioassay facilely distinguished the stages of glioma disease from clinical blood samples with high accuracy. Hence, this research sheds light on how to develop advanced ECL luminophores and an automatic method, showing substantial insights into pathogenesis research of gliomas.

Electroprecipitating the Sulfate Radical Anion Amplifies Electrochemiluminescence in Space and Time

We have discovered a strategy to synthesize reactive radical salts, effectively bottling up radicals in space and time for future use. We apply this new principle to electrochemiluminescence (ECL) through the simultaneous electro-reduction of peroxydisulfate, S2O82-, and tris(bipyridine)ruthenium(II), [Ru(bpy)3]2+ in a water/acetonitrile mixture. The electrode generates a concentration profile exceeding the solubility of the cation and anion pair, promoting precipitation. After the application of a potential, leads are disconnected, and the crystals electrolessly chemiluminesce during dissolution and can be transported to other solutions for later chemiluminescence uses. Our method extends ECL hundreds of micrometers from the electrode surface and increases the ECL lifetime by orders of magnitude. Control experiments, including electron spin resonance, validate the crystallization of SO4•-, allowing detailed mechanistic insight. We demonstrate platform generalizability by precipitating a radical salt made of calcium and SO4•-, and we demonstrate the salt's ability to drive chemiluminescence. Our results emphasize the elegant chemical tenet that extremely reactive radicals can be bottled up as solids to be used as future reagents if precipitation occurs more quickly than the radical lifetime.

Facilitating intrinsic delayed fluorescence of conjugated emitters by inter-chromophore interaction

Delayed fluorescence (DF) is a unique emitting phenomenon of great interest for important applications in organic optoelectronics. In general, DF requires well-separated frontier orbitals, inherently corresponding to charge transfer (CT)-type emitters. However, facilitating intrinsic DF for local excited (LE)-type conjugated emitters remains very challenging. Aiming to overcome this obstacle, we demonstrate a new molecular design strategy with a DF-inactive B,N-multiple resonance (MR) emitter as a model system. Without the necessity of doping with heavy atoms, we synthesized a co-facial dimer in which an excimer-like state (Sexc) was expected to facilitate efficient reverse intersystem crossing (RISC, T1 → Sexc) and intrinsic DF. Benefiting from greatly enhanced SOC and reduced ΔE ST, the proof-of-concept emitter Np-2CzB exhibited k RISC up to 6.5 × 105 s-1 and intrinsic DF with >35% contribution (Φ DF/Φ F) in dilute solution. Further investigation indicated that Sexc state formation relies on an optimized co-facial distance (d = ∼4.7 Å), strong inter-chromophore interaction (J coul > 450 cm-1) and a rigid structure (Γ S1→S0 < 350 cm-1). Although our strategy was demonstrated with a B,N-MR emitter, it can be applicable to many LE-type conjugated emitters without intrinsic DF. By triggering potential DF emission, many classic emitters might play a more important role in optoelectronics.

Electrochemiluminescence in Microfluidic Channels: Influence of Mass Transport on the Tris(2,2'-bipyridyl)ruthenium(II)/Tripropylamine System at Semitransparent Electrodes

Mass transport in laminar flow can improve the electrochemiluminescence (ECL) performance at the microchannel electrodes, depending on the device geometry and operating regimes. The known Ru(bpy)32+/tripropylamine (TPA) system was selected and studied in continuous microfluidics on semitransparent platinum electrodes. With this electrode material, ECL is mainly generated via a catalytic pathway. This mechanism was characterized under flow conditions by monitoring the ECL emission using linear sweep voltammetry and chronoamperometry. In parallel, ECL imaging of the electrode surface was conducted in order to characterize the ECL profiles above the electrode in the flow direction. Numerical simulations were carried out and then compared to experimental data to both confirm the ECL mechanism and assess the main kinetic parameters. A good agreement was obtained, demonstrating the influence of the operating regimes of the microchannel electrodes on the ECL performances. In the thin-layer regime, due to TPA depletion, ECL is located at the upstream edge of the electrode, while it is homogeneously distributed over the electrode surface in convective regimes. These characteristics will necessarily have to be taken into account in the design of dedicated ECL analytical microfluidic devices operating under continuous flow.

Enhanced Electrochemiluminescence at the Gas/Liquid Interface of Bubbles Propelled into Solution

Electrochemiluminescence (ECL) is typically confined to a micrometric region from the electrode surface. This study demonstrates that ECL emission can extend up to several millimeters away from the electrode employing electrogenerated chlorine bubbles. The mechanism behind this bubble-enhanced ECL was investigated using an Au microelectrode in chloride-containing and chloride-free electrolyte solutions. We discovered that ECL emission at the gas/solution interface is driven by two parallel effects. First, the bubble corona effect facilitates the generation of hydroxyl radicals capable of oxidizing luminol while the bubble is attached to the surface. Second, hypochlorite generated from chlorine sustains luminol emission for over 200 s and extends the emission range up to 5 mm into the solution, following bubble detachment. The new approach can increase the emission intensity of luminol-based assays 5-fold compared to the conventional method. This is demonstrated through a glucose bioassay, using a midrange mobile phone camera for detection. These findings significantly expand the potential applications of ECL by extending its effective range in time and space.

Electrochemiluminescence Microscopy

Electrochemiluminescence (ECL) is rapidly evolving from an analytical method into an optical microscopy. The orthogonality of the electrochemical trigger and the optical readout distinguishes it from classic microscopy and electrochemical techniques, owing to its near-zero background, remarkable sensitivity, and absence of photobleaching and phototoxicity. In this minireview, we summarize the recent advances in ECL imaging technology, emphasizing original configurations which enable the imaging of biological entities and the improvement of the analytical properties by increasing the complexity and multiplexing of bioassays. Additionally, mapping the (electro)chemical reactivity in space provides valuable information on nanomaterials and facilitates deciphering ECL mechanisms for improving their performances in diagnostics and (electro)catalysis. Finally, we highlight the recent achievements in imaging at the ultimate limits of single molecules, single photons or single chemical reactions, and the current challenges to translate the ECL imaging advances to other fields such as material science, catalysis and biology.

Thickness-Resolved Electrochemiluminescence Microscopy of Extracellular Matrix at Tumor Tissues for Rapid Cancer Diagnosis

The traditional recognition of extracellular matrix (ECM) at tissue sections relies on the time-consuming immunofluorescence that could not meet the demand of rapid diagnosis. Herein, we introduce a thickness-resolved electrochemiluminescence (ECL) microscopy to image thin-layer ECM at tissue sections for fast histopathological analysis. The unique surface-confined ECL mechanism enables to unveil the diversity and complexity of multiple tissue structures with varying thicknesses. Notably, the short lifetimes and the limited diffusion of electrogenerated coreactant radicals combined with their chemical reactivity result in a 2-fold increase in ECL intensity on ECM structures compared to the remaining tissue, enabling ECM visualization without specific labeling. The further quantitation of the ECM localization within tissue sections furnishes crucial insights into tumor progression and, more importantly, differentiates carcinoma and paracancerous tissues from patients in less than 30 min. Moreover, the reported electrochemistry-based microscopy is a dynamic approach allowing to investigate the transport, tortuosity, and trafficking properties through the tissues. This thickness-resolved recognition strategy not only opens new avenues for imaging complex samples but also holds promise for expediting tissue pathologic diagnosis, offering a more automated protocol with enhanced quantitative data compared to current intraoperative pathology methods.

An Encyclopedic Compendium on Chemosensing Supramolecular Metal-Organic Gels

Chemosensing, an interdisciplinary scientific domain, plays a pivotal role ranging from environmental monitoring to healthcare diagnostics and (inter)national security. Metal-organic gels (MOGs) are recognized for their stability, selectivity, and responsiveness, making them valuable for chemosensing applications. Researchers have explored the development of MOGs based on different metal ions and ligands, allowing for tailored properties and sensitivities, and have even demonstrated their applications as portable sensors such as paper-based test strips for practical use. Herein, several studies related to MOGs development and their applications in the chemosensing field via UV-visible or luminance along with electrochemical approach are presented. These papers explored MOGs as versatile materials with their use in sensing bio or environmental analytes. This review provides a foundational understanding of key concepts, methodologies, and recent advancements in this field, fostering the scientific community.

Shadow electrochemiluminescence imaging of giant liposomes opening at polarized electrodes

In this work, the release of giant liposome (∼100 μm in diameter) content was imaged by shadow electrochemiluminescence (ECL) microscopy. Giant unilamellar liposomes were pre-loaded with a sucrose solution and allowed to sediment at an ITO electrode surface immersed in a solution containing a luminophore ([Ru(bpy)3]2+) and a sacrificial co-reactant (tri-n-propylamine). Upon polarization, the electrode exhibited illumination over its entire surface thanks to the oxidation of ECL reagents. However, as soon as liposomes reached the electrode surface, dark spots appeared and then spread over time on the surface. This observation reflected a blockage of the electrode surface at the contact point between the liposome and the electrode surface, followed by the dilution of ECL reagents after the rupture of the liposome membrane and release of its internal ECL-inactive solution. Interestingly, ECL reappeared in areas where it initially faded, indicating back-diffusion of ECL reagents towards the previously diluted area and thus confirming liposome permeabilization. The whole process was analyzed qualitatively and quantitatively within the defined region of interest. Two mass transport regimes were identified: a gravity-driven spreading process when the liposome releases its content leading to ECL vanishing and a diffusive regime when ECL recovers. The reported shadow ECL microscopy should find promising applications for the imaging of transient events such as molecular species released by artificial or biological vesicles.

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.

Electrochemiluminescence-Based Single-Particle Tracking of the Biomolecules Moving along Intercellular Membrane Nanotubes between Live Cells

Electrochemiluminescence (ECL) imaging, a rapidly evolving technology, has attracted significant attention in the field of cellular imaging. However, its primary limitation lies in its inability to analyze the motion behaviors of individual particles in live cellular environments. In this study, we leveraged the exceptional ECL properties of quantum dots (QDs) and the excellent electrochemical properties of carbon dots (CDs) to develop a high-brightness ECL nanoprobe (CDs-QDs) for real-time ECL imaging between living cells. This nanoprobe has excellent signal-to-noise ratio imaging capabilities for the single-particle tracking (SPT) of biomolecules. Our finding elucidated the enhanced ECL mechanism of CDs-QDs in the presence of reactive oxygen species through photoluminescence, electrochemistry, and ECL techniques. We further tracked the movement of single particles on membrane nanotubes between live cells and confirmed that the ECL-based SPT technique using CD-QD nanoparticles is an effective approach for monitoring the transport behaviors of biomolecules on membrane nanotubes between live cells. This opens a promising avenue for the advancement of ECL-based single-particle detection and the dynamic quantitative imaging of biomolecules.

Recent Advances in Electrochemiluminescence Biosensors for MicroRNA Detection

Electrochemiluminescence (ECL) as an analytical technology with a perfect combination of electrochemistry and spectroscopy has received considerable attention in bioanalysis due to its high sensitivity and broad dynamic range. Given the selectivity of bio-recognition elements and the high sensitivity of the ECL analysis technique, ECL biosensors are powerful platforms for the sensitive detection of biomarkers, achieving the accurate prognosis and diagnosis of diseases. MicroRNAs (miRNAs) are crucial biomarkers involved in a variety of physiological and pathological processes, whose aberrant expression is often related to serious diseases, especially cancers. ECL biosensors can fulfill the highly sensitive and selective requirements for accurate miRNA detection, prompting this review. The ECL mechanisms are initially introduced and subsequently categorize the ECL biosensors for miRNA detection in terms of the quenching agents. Furthermore, the work highlights the signal amplification strategies for enhancing ECL signal to improve the sensitivity of miRNA detection and finally concludes by looking at the challenges and opportunities in ECL biosensors for miRNA detection.

Bimodal Oxidation Electrochemiluminescence Mechanism of Coreactant-Embedded Covalent Organic Frameworks via Postsynthetic Modification

Electrochemiluminescence (ECL) efficiency is determined by charge transfer between coreactants and emitters in coreactant systems, which are usually limited by their slow intermolecular charge transfer. In this study, a covalent organic framework (COF) with aldehyde residue was synthesized, and then coreactants were covalently integrated into the skeleton through the postsynthetic modification strategy, resulting in a crystalline coreactant-embedded COF nanoemitter (C-COF). Compared to the pristine COF with an equivalent external coreactant, C-COF exhibited an extraordinary 1008-fold enhancement of ECL intensity due to the rapid intrareticular charge transfer. Significantly, with the pH increase, C-COF shows protonation-induced ECL enhancement for the first ECL peaked at +1.1 V and an opposite trend for the second ECL at +1.4 V, which were attributed to the antedating oxidation of coreactant in framework and COF self-oxidation, respectively. The resulting bimodal oxidation ECL mechanism was rationalized by spectral characterization and density functional theory calculations. The postsynthetic coreactant-embedded nanoemitters present innovative and universal avenues for advancing ECL systems.

Controllable and Low-Loss Electrochemiluminescence Waveguide Supported by a Micropipette Electrode

One-dimensional molecular crystal waveguide (MCW) can transmit self-generated electrochemiluminescence (ECL), but heavy optical loss occurs because of the small difference in the refractive index between the crystal and its surroundings. Herein, we report a micropipette electrode-supported MCW (MPE/MCW) for precisely controlling the far-field transmission of ECL in air with a low optical loss. ECL is generated from one terminal of the MCW positioned inside the MPE, which is transmitted along the MCW to the other terminal in air. In comparison with conventional waveguides on solid substrates or in solutions, the MPE/MCW is propitious to the total internal reflection of light at the MCW/air interface, thus confining the ECL efficiently in MCW and improving the waveguide performance with an extremely low-loss coefficient of 4.49 × 10-3 dB μm-1. Moreover, by regulation of the gas atmosphere, active and passive waveguides can be resolved simultaneously inside MPE and in air. This MPE/MCW offers a unique advantage of spatially controlling and separating ECL signal readout from its generation, thus holding great promise in biosensing without or with less electrical/chemical disturbance.

Infrared photoinduced electrochemiluminescence microscopy of single cells

Electrochemiluminescence (ECL) is evolving rapidly from a purely analytical technique into a powerful microscopy. Herein, we report the imaging of single cells by photoinduced ECL (PECL; λem = 620 nm) stimulated by an incident near-infrared light (λexc = 1050 nm). The cells were grown on a metal-insulator-semiconductor (MIS) n-Si/SiOx/Ir photoanode that exhibited stable and bright PECL emission. The large anti-Stokes shift allowed for the recording of well-resolved images of cells with high sensitivity. PECL microscopy is demonstrated at a remarkably low onset potential of 0.8 V; this contrasts with classic ECL, which is blind at this potential. Two imaging modes are reported: (i) photoinduced positive ECL (PECL+), showing the cell membranes labeled with the [Ru(bpy)3]2+ complex; and (ii) photoinduced shadow label-free ECL (PECL-) of cell morphology, with the luminophore in the solution. Finally, by adding a new dimension with the near-infrared light stimulus, PECL microscopy should find promising applications to image and study single photoactive nanoparticles and biological entities.

Electrochemiluminescence Analysis of Multiple Glycans on Single Living Cell with a Closed Bipolar Electrode Array Chip

The profiling of multiple glycans on a single cell is important for elucidating glycosylation mechanisms and accurately identifying disease states. Herein, we developed a closed bipolar electrode (BPE) array chip for live single-cell trapping and in situ galactose and sialic acid detection with the electrochemiluminescence (ECL) method. Methylene blue-DNA (MB-DNA) as well as biotin-DNA (Bio-DNA) codecorated AuNPs were prepared as nanoprobes, which were selectively labeled on the cell surface through chemoselective labeling techniques. The individual cell was captured and labeled in the microtrap of the cathodic chamber, under an appropriate potential, MB molecules on the cellular membrane underwent oxidation, triggering the reduction of [Ru(bpy)3]2+/TPA and consequently generating ECL signals in the anodic chamber. The abundance of MB groups on the single cell enabled selective monitoring of both sialic acid and galactosyl groups with high sensitivity using ECL. The sialic acid and galactosyl content per HepG2 cell were detected to be 0.66 and 0.82 fmol, respectively. Through comprehensive evaluation of these two types of glycans on a single cell, tumor cells, and normal cells could be effectively discriminated and the accuracy of single-cell heterogeneous analysis was improved. Additionally, dynamic monitoring of variations in galactosyl groups on the surface of the single cell was also achieved. This work introduced a straightforward and convenient approach for heterogeneity analysis among single cells.

Through-Space Electrochemiluminescence Reveals Bubble Forces at Remote Phase Boundaries

Several groups have reported on the curious chemistry and reaction acceleration in confined volumes. These complex multiphase systems most closely resemble natural processes, and new measurement tools are necessary to probe chemistry in such environments. Generally, electrochemiluminescence (ECL) reports on processes immediately near (within a few micrometers) the electrode surface. Here, we introduce through-space ECL, reporting on dynamics of processes far away (100s of μm) from the electrode surface. We achieved this by collecting reflected ECL light. During the heterogeneous oxidation of C2O42- in an aqueous phase adjacent to a 1,2-dichlorethane droplet, CO2 accumulates in the 1,2-dichloroethane droplet. Upon buildup, we demonstrate that a CO2 bubble forms in the nonaqueous phase and is surprisingly trapped at the water|1,2-dichloroethane interface and continues to grow. The co-oxidation of tris(bipyridine)ruthenium(II) in the aqueous phase lights up the electrode surface and reflects off the edges of the bubble, revealing the bubble growth over time even when the bubble is fractions of a millimeter from the surface. We extend our results to quantifying bubble forces at the water-oil interface at remote distances from the electrode surface.

Dual-Signal Imaging Mode Based on Fluorescence and Electrochemiluminescence for Ultrasensitive Visualization of SARS-CoV-2 Spike Protein

Accurate and reliable detection of SARS-CoV-2 is critical for the effective prevention and rapid containment of COVID-19. Current approaches suffer from complex procedures or a single signal readout, resulting in an increased risk of false negatives and low sensitivity. Here, we developed a fluorescence (FL) and electrochemiluminescence (ECL) dual-mode imaging platform based on a self-powered DNAzyme walker to achieve accurate surveillance of SARS-CoV-2 spike protein at the single-molecule level. The specific activation of the DNAzyme walker by the target protein provides the power for the system's continuous running, enabling the simultaneous recording of the reduction in fluorescence spots and the appearance of ECL spots generated by the Ru-doped metal-organic framework (MOF) emitter. Therefore, the constructed imaging platform can achieve dual-mode detection of spike protein via reverse dual-signal feedback, which could effectively eliminate false-positive or false-negative signals and improve the detection accuracy and sensitivity with a low detection limit. In particular, the dual-mode accuracy of spike protein diagnosis in samples has been significantly improved compared to single-signal output means. In addition, this dual-mode imaging platform may become a prospective diagnostic device for other infectious viruses.

Optics Determines the Electrochemiluminescence Signal of Bead-Based Immunoassays

Electrochemiluminescence (ECL) is an optical readout technique that is successfully applied for the detection of biomarkers in body fluids using microbead-based immunoassays. This technology is of utmost importance for in vitro diagnostics and thus a very active research area but is mainly focused on the quest for new dyes and coreactants, whereas the investigation of the ECL optics is extremely scarce. Herein, we report the 3D imaging of the ECL signals recorded at single microbeads decorated with the ECL labels in the sandwich immunoassay format. We show that the optical effects due to the light propagation through the bead determine mainly the spatial distribution of the recorded ECL signals. Indeed, the optical simulations based on the discrete dipole approximation compute rigorously the electromagnetic scattering of the ECL emission by the microbead and allow for reconstructing the spatial map of ECL emission. Thus, it provides a global description of the ECL chemical reactivity and the associated optics. The outcomes of this 3D imaging approach complemented by the optical modeling provide insight into the ECL optics and the unique ECL chemical mechanism operating on bead-based immunoassays. Therefore, it opens new directions for mechanistic investigations, ultrasensitive ECL bioassays, and imaging.

Application and outlook of electrochemical technology in single-cell analysis

Cellular heterogeneity, especially in some important diseased cells like tumor cells, acts as an invisible driver for disease development like cancer progression in the tumor ecosystem, contributing to differences in the macroscopic and microscopic detection of disease lesions like tumors. Traditional analysis techniques choose group information masked by the mean as the analysis sample, making it difficult to achieve precise diagnosis and target treatment, on which could be shed light via the single-cell level determination/bioanalysis. Hence, in this article we have reviewed the special characteristic differences among various kinds of typical single-cell bioanalysis strategies and electrochemical techniques, and then focused on the recent advance and special bio-applications of electrochemiluminescence and micro-nano electrochemical sensing mediated in single-cell bioimaging & bioanalysis. Especially, we have summarized the relevant research exploration of the possibility to establish the in-situ single-cell electrochemical methods to detect cell heterogeneity through determination of specific biomolecules and bioimaging of some important biological processes. Eventually, this review has explored some important advances of electrochemical single-cell detection techniques for the real-time cellular bioimaging and diagnostics of some disease lesions like tumors. It raises the possibility to provide the specific in-situ platform to exploit the versatile, sensitive, and high-resolution electrochemical single-cell analysis for the promising biomedical applications like rapid tracing of some disease lesions or in vivo bioimaging for precise cancer theranostics.

T Cell Antigen Recognition and Discrimination by Electrochemiluminescence Imaging

Adoptive T lymphocyte (T cell) transfer and tumour-specific peptide vaccines are innovative cancer therapies. An accurate assessment of the specific reactivity of T cell receptors (TCRs) to tumour antigens is required because of the high heterogeneity of tumour cells and the immunosuppressive tumour microenvironment. In this study, we report a label-free electrochemiluminescence (ECL) imaging approach for recognising and discriminating between TCRs and tumour-specific antigens by imaging the immune synapses of T cells. Various T cell stimuli, including agonistic antibodies, auxiliary molecules, and tumour-specific antigens, were modified on the electrode's surface to allow for their interaction with T cells bearing different TCRs. The formation of immune synapses activated by specific stimuli produced a negative (shadow) ECL image, from which T cell antigen recognition and discrimination were evaluated by analysing the spreading area and the recognition intensity of T cells. This approach provides an easy way to assess TCR-antigen specificity and screen both of them for immunotherapies.

Electrochemiluminescence sensing of HeLa cells labeled with biotinylated ruthenium complex using bipolar electrode based on microwell modified optical fiber

Biotinylated ruthenium complexes exhibit improved photoluminescent (PL) properties when they bind with streptavidin, making them useful labels or probes in bio-related analysis. However, their ECL properties are still unknown to date. Herein, we reported the use of [Ru(bpy)2(biot-bpy)]2+ complexes as a new ECL luminophore, which was functionalized with biotin moiety and exhibited higher ECL efficiency after binding to streptavidin. Moreover, [Ru(bpy)2(biot-bpy)]2+ complexes could be attached to HeLa cells through the biotin-streptavidin binding. A microwell bipolar electrode (MBE) prepared at one end of an optical fiber bundle was applied to produce ECL of the labeled HeLa cells, which was remotely detected at the other end. The [Ru(bpy)2(biot-bpy)]2+-streptavidin binding effect together with the high surface/volume ratio of MBE promoted the ECL generation on HeLa cells, which was applied to sensitively detect HeLa cells with a linear range from 1.56 × 102 to 6.74 × 106 cells/mL and a detection limit of 83 cells/mL. Moreover, ECL images were successfully acquired to resolve the emission on each HeLa cell. Such cytosensor based on [Ru(bpy)2(biot-bpy)]2+ and MBE may extend the applications of ECL for cell detections.

Nanoconfined Cathodic Electrochemiluminescence for Self-Sensitized Bioimaging of Membrane Protein

Self-enhanced electrochemiluminescence (ECL) can be achieved via the confinement of coreactants and ECL emitters in a single nanostructure. This strategy has been used for the design of anodic ECL systems with amine compounds as coreactants. In this work, a novel confinement system was proposed by codoping positively charged ECL emitter tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)32+) and negatively charged coreactant peroxydisulfate (S2O82-) in silica nanoparticles. The codoping process could be performed by introducing S2O82- in cationic poly(diallyldimethylammonium chloride) (PDDA) to form PDDA@S2O82- and then encapsulating it and Ru(bpy)32+ in the Triton X-100 vesicle followed by the hydrolysis of tetraethyl ortosilicate, surface modification, and demulsification. The obtained RuSSNs exhibited good homogeneity, excellent monodispersity, acceptable biocompatibility, and 2.9-fold stronger ECL emission than Ru(bpy)32+-doped silica nanoparticles at an equal amount of nanoparticles in the presence of 0.1 M K2S2O8. Thus, an in situ self-sensitized cathodic ECL imaging method was designed for the monitoring of glycoprotein on living cell membranes. This work provides a new way for the modification, enhancement, and application of nano-ECL emitters in biological analysis.

Dynamic Mapping of Electrochemiluminescence Reactivity in Space: Application to Bead-Based Assays

As an electrochemical technique offering an optical readout, electrochemiluminescence (ECL) evolved recently into a powerful microscopy technique with the visualization of a wide range of microscopic entities. However, the dynamic imaging of transient ECL events did not receive intensive attention due to the limited number of electrogenerated photons. Here, the reaction kinetics of the model ECL bioassay system was revealed by dynamic imaging of single [Ru(bpy)3]2+-functionalized beads in the presence of the efficient tripropylamine coreactant. The time profile behavior of ECL emission, the variations of the ECL layer thickness, and the position of maximum ECL intensity over time were investigated, which were not achieved by static imaging in previous studies. Moreover, the dynamics of the ECL emission were confronted with the simulation. The reported dynamic ECL imaging allows the investigation of the ECL kinetics and mechanisms operating in bioassays and cell microscopy.

Imaging and Simulation of Ruthenium Derivative Coating Microbeads at the Opaque Electrode with Electrogenerated Chemiluminescence

Electrogenerated chemiluminescence (ECL) imaging is gaining increasing attention in various fields because of its high sensitivity, low background, and good temporal and spatial resolution. However, ECL imaging of microsized objects at the opaque electrode via top-view configuration is challenged with the reactants' diffusion and light propagation. Here, we imaged and numerically simulated ruthenium derivative coating polystyrene microbeads (Ru1-PS@MB) at the glassy carbon electrode (GCE) via top-view configuration by ECL imaging. The ruthenium derivative (bis(2,2'-bipyridine)-4'-methyl-4-carboxybipyridine-ruthenium N-succinimidyl ester-bis (hexafluorophosphate), Ru1), a typical ECL reagent, was covalently linked onto the surface of aminated PS@MBs via the amide reaction. "Strong emission in edge and weak emission in center" phenomena for fluorescence (FL) and ECL emissions were obtained from Ru1-PS@MB on GCE. Z-Stack imaging of the microsized Ru1-PS@MB luminescence was performed on GCE in the presence of tri-n-propylamine (TPA). It is found that the clear luminescence range of Ru1-PS@MB perpendicular to the electrode surface in ECL image is slightly smaller than that in the FL image. The bigger was the diameter of the microbeads (from 5 to 18 μm), the larger was the ECL luminescence range of Ru1-PS@MB perpendicular to the electrode surface (from 5 to 7 μm). Our findings, which are also supported by numerical simulation, provide insights into the ECL imaging of microsized objects at the electrode surface, which will raise promising ECL applications in bioassays and cell imaging at the microscale level.

Reversibly Tuning Electrochemiluminescence with Stimulated Emission Route for Single-Cell Imaging

Electrochemiluminescence (ECL) has established itself as an excellent transduction technique in biosensing and light-emitting device, while conventional ECL mechanism depending on spontaneous emission of luminophores lacks reversibility and tunable emission characters, limiting the universality of ECL technique in the fields of fundamental research and clinical applications. Here, we report the first observation of stimulated emission route in ECL and thus establish a reversible tuning ECL microscopy for single-cell imaging. This microscopy uses a focused red-shifted beam to transfer spontaneous ECL into stimulated ECL, which enables selective and reversible tuning of ECL emission from homogeneous solution, single particles, and single cells. After excluding other possible competitive routes, the stimulated ECL emission route is confirmed by a dual-objective system in which the suppressed spontaneous ECL is accompanied by the enhanced stimulated ECL. By incorporating a commercial donut-shaped beam, the sharpness of single-cell matrix adhesion is improved 2 to 3 times compared with the counterpart in confocal ECL mode. The successful establishment of this stimulated emission ECL will greatly advance the development of light-emitting device and super-resolution ECL microscopy.

Tris(2,2'-bipyridyl)ruthenium (II) complex as a universal reagent for the fabrication of heterogeneous electrochemiluminescence platforms and its recent analytical applications

In recent years, electrochemiluminescence (ECL) has received enormous attention and has emerged as one of the most successful tools in the field of analytical science. Compared with homogeneous ECL, the heterogeneous (or solid-state) ECL has enhanced the rate of the electron transfer kinetics and offers rapid response time, which is highly beneficial in point-of-care and clinical applications. In ECL, the luminophore is the key element, which dictates the overall performance of the ECL-based sensors in various analytical applications. Tris(2,2'-bipyridyl)ruthenium (II) complex, Ru(bpy)32+, is a coordination compound, which is the gold-standard luminophore in ECL. It has played a key role in translating ECL from a "laboratory curiosity" to a commercial analytical instrument for diagnosis. The aim of the present review is to provide the principles of ECL and classical reaction mechanisms-particularly involving the heterogeneous Ru(bpy)32+/co-reactant ECL systems, as well as the fabrication methods and its importance over solution-phase Ru(bpy)32+ ECL. Then, we discussed the emerging technology in solid-state Ru(bpy)32+ ECL-sensing platforms and their recent potential analytical applications such as in immunoassay sensors, DNA sensors, aptasensors, bio-imaging, latent fingerprint detection, point-of-care testing, and detection of non-biomolecules. Finally, we also briefly cover the recent advances in solid-state Ru(bpy)32+ ECL coupled with the hyphenated techniques.

Enhanced electrochemiluminescence imaging of single cell membrane proteins based on Co3O4 nanozyme catalysis

The development of enhanced strategies with excellent biocompatibility is critical for electrochemiluminescence (ECL) imaging of single cells. Here, we report an ECL imaging technique for a single cell membrane protein based on a Co3O4 nanozyme catalytic enhancement strategy. Due to the remarkable catalytic performance of Co3O4 nanozymes, H2O2 can be efficiently decomposed into reactive oxygen radicals, and the reaction with L012 was enhanced, resulting in stronger ECL emission. The anti-carcinoembryonic antigen (CEA) was coupled with nanozyme particles to construct a probe that specifically recognized the overexpressed CEA on the MCF-7 cell membrane. According to the locally enhanced visualized luminescence, the rapid ECL imaging of a single cell membrane protein was eventually realized. Accordingly, Co3O4 nanozymes with highly efficient activity will provide new insights into ECL imaging analysis of more biological small molecules and proteins.

Electrochemiluminescence Imaging of Cellular Contact Guidance on Microfabricated Substrates

Cells tend to align and move by following anisotropic topographical cues, namely the phenomenon known as contact guidance-an essential step in cell alignment, adhesion, and migration. The effect of topographical cues on individual cells has been investigated extensively, but that on cell aggregates still remains to be fully understood. Considering the high surface sensitivity of electrochemiluminescence (ECL) microscopy, it was used in this work to explore the impact of surface topography on cell behaviors. First, we studied the variations of cell-matrix adhesions of cells cultured on different topographical features. Both fibroblast-like and epithelial cells plated on microgrooved electrodes exhibited obvious contact guidance behavior. Then, the effect of surface topography on cellular collective migration was investigated. Topographic cues would be a barrier for cell migration if the orientation of microgrooves was perpendicular to the direction of migration; otherwise, it would be a helper. Finally, it was found that relaxation of cytoskeleton contractility or reduction in adhesion density could weaken the directed migration of leading cells, because the alteration of migration directionality was retarded. In contrast, such interactions were lost on the contact guidance response of follower cells, as they still aligned by following the topographic cues.

Materials for Electrochemiluminescence: TADF, Hydrogen-Bonding, and Aggregation- and Crystallization-Induced Emission Luminophores

Electrochemiluminescence (ECL) is a rapidly growing discipline with many analytical applications from immunoassays to single-molecule detection. At the forefront of ECL research is materials chemistry, which looks at engineering new materials and compounds exhibiting enhanced ECL efficiencies compared to conventional fluorescent materials. In this review, we summarize recent molecular design strategies that lead to high efficiency ECL. In particular, we feature recent advances in the use of thermally activated delayed fluorescence (TADF) emitters to produce enhanced electrochemiluminescence. We also document how hydrogen bonding, aggregation, and crystallization can each be recruited in the design of materials showing enhanced electrochemiluminescence.

Site-selective heat boosting electrochemiluminescence for single cell imaging

In operando visualization of local electrochemical reactions provides mechanical insights into the dynamic transport of interfacial charge and reactant/product. Electrochemiluminescence is a crossover technique that quantitatively determines Faraday current and mass transport in a straightforward manner. However, the sensitivity is hindered by the low collision efficiency of radicals and side reactions at high voltage. Here, we report a site-selective heat boosting electrochemiluminescence microscopy. By generating a micron-scale heat point in situ at the electrode-solution interface, we achieved an enhancement of luminescence intensity up to 63 times, along with an advance of 0.2 V in applied voltage. Experimental results and finite element simulation demonstrate that the fundamental reasons are accelerated reaction rate and thermal convection via a photothermal effect. The concentrated electrochemiluminescence not only boosts the contrast of single cells by 20.54 times but also enables the site-selective cell-by-cell analysis of the heterogeneous membrane protein abundance. This electrochemical visualization method has great potential in the highly sensitive and selective analysis of local electron transfer events.

All-Optical Electrochemiluminescence

Electrochemiluminescence (ECL) is widely employed for medical diagnosis and imaging. Despite its remarkable analytical performances, the technique remains intrinsically limited by the essential need for an external power supply and electrical wires for electrode connections. Here, we report an electrically autonomous solution leading to a paradigm change by designing a fully integrated all-optical wireless monolithic photoelectrochemical device based on a nanostructured Si photovoltaic junction modified with catalytic coatings. Under illumination with light ranging from visible to near-infrared, photogenerated holes induce the oxidation of the ECL reagents and thus the emission of visible ECL photons. The blue ECL emission is easily viewed with naked eyes and recorded with a smartphone. A new light emission scheme is thus introduced where the ECL emission energy (2.82 eV) is higher than the excitation energy (1.18 eV) via an intermediate electrochemical process. In addition, the mapping of the photoelectrochemical activity by optical microscopy reveals the minority carrier interfacial transfer mechanism at the nanoscale. This breakthrough provides an all-optical strategy for generalizing ECL without the need for electrochemical setups, electrodes, wiring constraints, and specific electrochemical knowledge. This simplest ECL configuration reported so far opens new opportunities to develop imaging and wireless bioanalytical systems such as portable point-of-care sensing devices.

Gold Microbeads Enabled Proximity Electrochemiluminescence for Highly Sensitive and Size-Encoded Multiplex Immunoassays

Developing highly sensitive multiplex immunoassays is urgently needed to guide medical research and improve clinical diagnosis. Here, we report the proximity electrochemiluminescence (ECL) generation enabled by gold microbeads (GMBs) for improving the detection sensitivity and multiplexing capacity of ECL immunoassays (ECLIAs). As demonstrated by microscopy and finite element simulation, GMBs can function as spherical ultramicroelectrodes for triggering ECL reactions in solutions. Employing GMBs as solid carriers in the bead-based ECLIA, the electrochemical oxidation of a coreactant can occur at both the GMB surface and the substrate electrode, allowing the coreactant radicals to diffuse only a short distance of ∼100 nm to react with ECL luminophores that are labeled on the GMB surface. The ECL generation via this proximity low oxidation potential (LOP) route results in a 21.7-fold increase in the turnover frequency of ECL generation compared with the non-conductive microbeads that rely exclusively on the conventional LOP route. Moreover, the proximity ECL generation is not restricted by the diffusion distance of short-lived coreactant radicals, which enables the simultaneous determination of multiple acute myocardial infarction biomarkers using size-encoded GMB-based multiplex ECLIAs. This work brings new insight into the understanding of ECL mechanisms and may advance the practical use of multiplex ECLIAs.

Self-Enhanced Electrochemiluminescence Imaging System Based on the Accelerated Generation of ROS under Ultrasound

Electrochemiluminescence (ECL) imaging, as an optical technology, has been developed at full tilt in the field of life science and nanomaterials. However, the relatively low ECL intensity or the high co-reactant concentration needed in the electrochemical reaction blocks its practical application. Here, we developed an ECL imaging system based on the rGO-TiO_2-x composite material, where the co-reactant, reactive oxygen species (ROS), is generated in situ under the synergetic effect of of ultrasound (US) and electric irradiation. The rGO-TiO_2-x composites facilitate the separation of electron (e^-) and hole (h⁺) pairs and inhibit recombination triggered by external US irradiation due to the high electroconductivity of rGO and oxygen-deficient structures of TiO₂, thus significantly boosting ROS generation. Furthermore, the increased defects on rGO accelerate the electron transfer rate, improving the electrocatalytic performance of the composite and forming more ROS. This high ultrasonic-electric synergistic efficacy is demonstrated through the enhancement of photon emission. Compared with the luminescence intensity triggered by US irradiation and electric field, an enhancement of ∼20-fold and 10-fold of the US combined with electric field-triggered emission is observed from this composite. Under the optimized conditions, using dopamine (DA) as a model target, the sensitivity of the US combined ECL strategy for detection of DA is two orders of magnitude higher than that of the ECL method. The successful detection of DA at low concentrations makes us believe that this strategy provides the possibility of applying ECL imaging for cellular single-molecule analysis and cancer therapy.