Transparent carbon nanotube network for efficient electrochemiluminescence devices

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.

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.

Recent Progress in Flexible and Wearable All Organic Photoplethysmography Sensors for SpO2 Monitoring

Flexible and wearable biosensors are the next-generation healthcare devices that can efficiently monitor human health conditions in day-to-day life. Moreover, the rapid growth and technological advancements in wearable optoelectronics have promoted the development of flexible organic photoplethysmography (PPG) biosensor systems that can be implanted directly onto the human body without any additional interface for efficient bio-signal monitoring. As an example, the pulse oximeter utilizes PPG signals to monitor the oxygen saturation (SpO2 ) in the blood volume using two distinct wavelengths with organic light emitting diode (OLED) as light source and an organic photodiode (OPD) as light sensor. Utilizing the flexible and soft properties of organic semiconductors, pulse oximeter can be both flexible and conformal when fabricated on thin polymeric substrates. It can also provide highly efficient human-machine interface systems that can allow for long-time biological integration and flawless measurement of signal data. In this work, a clear and systematic overview of the latest progress and updates in flexible and wearable all-organic pulse oximetry sensors for SpO2 monitoring, including design and geometry, processing techniques and materials, encapsulation and various factors affecting the device performance, and limitations are provided. Finally, some of the research challenges and future opportunities in the field are mentioned.

Electrochemiluminescence Distance and Reactivity of Coreactants Determine the Sensitivity of Bead-Based Immunoassays

Herein we report the study of electrochemiluminescence (ECL) generation by tris(2,2'-bipyridyl)ruthenium (Ru(bpy)₃ ²⁺ ) and five tertiary amine coreactants. The ECL distance and lifetime of coreactant radical cations were measured by ECL self-interference spectroscopy. And the reactivity of coreactants was quantitatively evaluated in terms of integrated ECL intensity. By statistical analysis of ECL images of single Ru(bpy)₃ ²⁺ -labeled microbeads, we propose that ECL distance and reactivity of coreactant codetermine the emission intensity and thus the sensitivity of immunoassay. 2,2-bis(hydroxymethyl)-2,2',2''-nitrilotriethanol (BIS-TRIS) can well balance ECL distance-reactivity trade-off and enhance the sensitivity by 236 % compared with tri-n-propylamine (TPrA) in the bead-based immunoassay of carcinoembryonic antigen. The study brings an insightful understanding of ECL generation in bead-based immunoassay and a way of maximizing the analytical sensitivity from the aspect of coreactant.

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.

Electrochemiluminescent immunoassay enhancement driven by carbon nanotubes

Electrochemiluminescence (ECL) is a leading analytical technique for clinical monitoring and early disease diagnosis. Carbon nanotubes are used as efficient nanomaterials for ECL signal enhancement providing new insights into the mechanism for the ECL generation but also affording application in bead-based immunoassay and ECL microscopy-based bioimaging.

Determination of ascorbic acid using electrochemiluminescence sensor based on nitrogen and sulfur doping graphene quantum dots with luminol as internal standard

A novel internal standard electrochemiluminescence (ECL) sensor has been designed for the detection of ascorbic acid (AA). The adopted dual-emission luminophore (NSGQDs-PEI-luminol-Pt) is composed of nitrogen and sulfur double-doped graphene quantum dots (NSGQDs, as the main luminophore), luminol (as the auxiliary luminophore and internal standard), platinum nanoparticles (Pt NPs, as the co-reaction accelerator), and polyetherimide (PEI, as the linker of NSGQDs and luminol). The results suggest obviously enhanced  ECL intensities by the Förster resonance energy transfer (FRET) between luminol (donor) and NSGQDs (acceptor). In this sensing system, the cathodic ECL intensities of NSGQDs (ECL-1, -1.8 V vs. Ag/AgCl) gradually decrease with increasing concentration of AA, while the anodic ECL intensities of luminol (ECL-2, 0.3 V vs. Ag/AgCl) almost remain essentially constant at a potential window from -2.0 to 0.4 V. The natural logarithm of the ratio between ECL-1 and ECL-2 (ln I _{(ECL-1/ECL-2)}) shows a good linear relationship with AA concentration ranging from 10 to 360 nM. The regression equation is ln I _{(ECL-1/ECL-2)} = - 0.0059 c_AA + 3.55 (R² = 0.992) with a limit of detection of 3.3 nM. Such sensor has also been applied for monitoring AA in human serum. The recovery range was 96.5-105.3% and the relative standard deviation was  1.3-3.3%.

Bio-Coreactant-Enhanced Electrochemiluminescence Microscopy of Intracellular Structure and Transport

A bio-coreactant-enhanced electrochemiluminescence (ECL) microscopy realizes the ECL imaging of intracellular structure and dynamic transport. This microscopy uses Ru(bpy)₃ ²⁺ as the electrochemical molecular antenna connecting extracellular and intracellular environments, and uses intracellular biomolecules as the coreactants of ECL reactions via a "catalytic route". Accordingly, intracellular structures are identified without using multiple labels, and autophagy involving DNA oxidative damage is detected using nuclear ECL signals. A time-resolved image sequence discloses the universal edge effect of cellular electroporation due to the influence of the geometric properties of cell membranes on the induced transmembrane voltage. The dynamic transport of Ru(bpy)₃ ³⁺ in the different cellular compartments unveils the heterogeneous intracellular diffusivity correlating with the actin cytoskeleton. In addition to single-cell studies, the bio-coreactant-enhanced ECL microscopy is used to image a slice of a mouse liver and a colony of Shewanella oneidensis MR-1.

Spatially resolved electrochemiluminescence through a chemical lens

Electrochemiluminescence (ECL) microscopy is an emerging technique with a wide range of imaging applications and unique properties in terms of high spatial resolution, surface confinement and favourable signal-to-noise ratio. Despite its successful analytical applications, tuning the depth of field (i.e., thickness of the ECL-emitting layer) is a crucial issue. Indeed, the control of the thickness of this ECL region, which can be considered as an "evanescent" reaction layer, limits the development of cell microscopy as well as bioassays. Here we report an original strategy based on chemical lens effects to tune the ECL-emitting layer in the model [Ru(bpy)3]2+/tri-n-propylamine (TPrA) system. It consists of microbeads decorated with [Ru(bpy)3]2+ labels, classically used in bioassays, and TPrA as the sacrificial coreactant. In particular we exploit the buffer capacity of the solution to modify the rate of the reactions involved in the ECL generation. For the first time, a precise control of the ECL light distribution is demonstrated by mapping the luminescence reactivity at the level of single micrometric bead. The resulting ECL image is the luminescent signature of the concentration profiles of diffusing TPrA radicals, which define the ECL layer. Therefore, our findings provide insights into the ECL mechanism and open new avenues for ECL microscopy and bioassays. Indeed, the reported approach based on a chemical lens controls the spatial extension of the "evanescent" ECL-emitting layer and is conceptually similar to evanescent wave microscopy. Thus, it should allow the exploration and imaging of different heights in substrates or in cells.

Electrochemiluminescence Mechanisms Investigated with Smartphone-Based Sensor Data Modeling, Parameter Estimation and Sensitivity Analysis

The present study introduces a unified framework combining a mechanistic model with a genetic algorithm (GA) for the parameter estimation of electrochemiluminescence (ECL) kinetics of the Ru(bpy)32+/TPrA system occurring in a smartphone-based sensor. The framework allows a straightforward solution for simultaneous estimation of multiple parameters which can be, otherwise, time-consuming and lead to non-convergence. Model parameters are estimated by achieving a high correlation between the model prediction and the measured ECL intensity from the ECL sensor. The developed model is used to perform a sensitivity analysis (SA), which provides quantitative effects of the model parameters on the concentrations of chemical species involved in the system. The results demonstrate that the GA-based parameter estimation and the SA approaches are effective in analyzing the kinetics of the ECL mechanism. Therefore, these approaches can be incorporated as analysis tools in the ECL kinetics study with practical application in the calibration of mechanistic models for any required sensing condition.

Reactivity mapping of luminescence in space: Insights into heterogeneous electrochemiluminescence bioassays

Electrochemiluminescence (ECL) is a powerful (bio)analytical method based on an optical readout. It is successfully applied in the heterogeneous format for immunoassays and imaging using the model and most widely used ECL system, which consists of the immobilized [Ru(bpy)₃]²⁺ label with tripropylamine (TPA) as a coreactant. However, a major drawback is the significant decrease of the ECL intensity over time. Herein, to decipher the process responsible for this progressive loss of ECL signal, we investigated its electrochemical and photophysical properties by mapping the luminescence reactivity at the level of single micrometric beads. Polystyrene beads were functionalized by the [Ru(bpy)₃]²⁺ dye via a sandwich immunoassay or a peptide bond. ECL emission was generated in presence of the very efficient TPA coreactant. Imaging both photoluminescence and ECL reactivities of different regions (located near or far from the electrode surface) of a [Ru(bpy)₃]²⁺-decorated bead allows us to demonstrate the remarkable photophysical stability of the ECL label, even in presence of the very reactive electrogenerated TPA radicals. We show that the ECL vanishing correlates directly with the lower TPA oxidation current. Finally, we propose a simple electrochemical treatment, which allows to regenerate the electrode surface and thus to recover several times the strong initial ECL signal. The reactivity imaging approach provides insights into the ECL mechanism and the main factors governing the stability of the emission, which should find promising ECL applications in bioassays and microscopy.

Insights into the mechanism of coreactant electrochemiluminescence facilitating enhanced bioanalytical performance

Electrochemiluminescence (ECL) is a powerful transduction technique with a leading role in the biosensing field due to its high sensitivity and low background signal. Although the intrinsic analytical strength of ECL depends critically on the overall efficiency of the mechanisms of its generation, studies aimed at enhancing the ECL signal have mostly focused on the investigation of materials, either luminophores or coreactants, while fundamental mechanistic studies are relatively scarce. Here, we discover an unexpected but highly efficient mechanistic path for ECL generation close to the electrode surface (signal enhancement, 128%) using an innovative combination of ECL imaging techniques and electrochemical mapping of radical generation. Our findings, which are also supported by quantum chemical calculations and spin trapping methods, led to the identification of a family of alternative branched amine coreactants, which raises the analytical strength of ECL well beyond that of present state-of-the-art immunoassays, thus creating potential ECL applications in ultrasensitive bioanalysis.

Oxygen-Resistant Electrochemiluminescence System with Polyhedral Oligomeric Silsesquioxane

We report the oxygen-resistant electrochemiluminescence (ECL) system from the polyhedral oligomeric silsesquioxane (POSS)-modified tris(2,2'-bipyridyl)ruthenium(II) complex (Ru-POSS). In electrochemical measurements, including cyclic voltammetry (CV), it is shown that electric current and ECL intensity increase in the mixture system containing Ru-POSS and tripropylamine (TPrA) on the indium tin oxide (ITO) working electrode. The lower onset potential (E_onset) in CV is observed with Ru-POSS compared to tris(2,2'-bipyridyl)ruthenium(II) complex (Ru(bpy)₃²⁺). From the series of mechanistic studies, it was shown that adsorption of Ru-POSS onto the ITO electrode enhances TPrA oxidation and subsequently the efficiency of ECL with lower voltage. Moreover, oxygen quenching of ECL was suppressed, and it is proposed that the enhancement to the production of the TPrA radical could contribute to improving oxygen resistance. Finally, the ECL-based detection for water pollutant is demonstrated without the degassing treatment. The commodity system with Ru(bpy)₃²⁺ is not applicable in the absence of degassing with the sample solutions due to critical signal suppression, meanwhile the present system based on Ru-POSS was feasible for estimating the amount of the target even under aerobic conditions by fitting the ECL intensity to the standard curve. One of critical disadvantages of ECL can be solved by the hybrid formation with POSS.

Electrochemiluminescence as emerging microscopy techniques

The use of electrochemiluminescence (ECL), i.e., chemiluminescence triggered by electrochemical stimulus, as emitting light source for microscopy is an emerging approach with different applications ranging from the visualization of nanomaterials to cell mapping. In this trend article, we give an overview of the state of the art in this new field with the purpose to illustrate all the possible applications so far explored as well as describing the mechanism underlying this transduction technique. The results discussed here would highlight the great potential of the combination between ECL and microscopy and how this marriage can turn into an innovative approach with specific application in analytical sciences. Graphical abstract.

Surface-Confined Electrochemiluminescence Microscopy of Cell Membranes

Herein is reported a surface-confined microscopy based on electrochemiluminescence (ECL) that allows to image the plasma membrane of single cells at the interface with an electrode. By analyzing photoluminescence (PL), ECL and AFM images of mammalian CHO cells, we demonstrate that, in contrast to the wide-field fluorescence, ECL emission is confined to the immediate vicinity of the electrode surface and only the basal membrane of the cell becomes luminescent. The resulting ECL microscopy reveals details that are not resolved by classic fluorescence microscopy, without any light irradiation and specific setup. The thickness of the ECL-emitting regions is ∼500 nm due to the unique ECL mechanism that involves short-lifetime electrogenerated radicals. In addition, the reported ECL microscopy is a dynamic technique that reflects the transport properties through the cell membranes and not only the specific labeling of the membranes. Finally, disposable transparent carbon nanotube (CNT)-based electrodes inkjet-printed on classic microscope glass coverslips were used to image cells in both reflection and transmission configurations. Therefore, our approach opens new avenues for ECL as a surface-confined microscopy to develop single cell assays and to image the dynamics of biological entities in cells or in membranes.

Solid state electrochemiluminescence from homogeneous and patterned monolayers of bifunctional spirobifluorene

Electrochemiluminescence (ECL) from self-assembled monolayers of a spirobifluorene dye covalently linked to a transparent ITO surface is reported.

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

We report here the development of coreactant-based electrogenerated chemiluminescence (ECL) as a surface-confined microscopy to image single cells and their membrane proteins. Labeling the entire cell membrane allows one to demonstrate that, by contrast with fluorescence, ECL emission is only detected from fluorophores located in the immediate vicinity of the electrode surface (i.e., 1-2 μm). Then, to present the potential diagnostic applications of our approach, we selected carbon nanotubes (CNT)-based inkjet-printed disposable electrodes for the direct ECL imaging of a labeled plasma receptor overexpressed on tumor cells. The ECL fluorophore was linked to an antibody and enabled to localize the ECL generation on the cancer cell membrane in close proximity to the electrode surface. Such a result is intrinsically associated with the unique ECL mechanism and is rationalized by considering the limited lifetimes of the electrogenerated coreactant radicals. The electrochemical stimulus used for luminescence generation does not suffer from background signals, such as the typical autofluorescence of biological samples. The presented surface-confined ECL microscopy should find promising applications in ultrasensitive single cell imaging assays.

Highly sensitive electrochemiluminescence detection of a prostate cancer biomarker

Prostate-specific membrane antigen (PSMA), a glycoprotein expressed in the prostatic epithelium endowed with enzymatic activity, is a very promising diagnostic marker for the early detection of prostate cancer. In this study, we report a novel electrochemiluminescence ELISA-like immunosensor based on carbon nanotubes and a highly specific sandwich immunoassay for the PSMA detection. To fabricate the device, an optically transparent electrode was modified with doubly functionalized multi-walled carbon nanotubes carrying amine groups and a monoclonal anti-PSMA antibody. Subsequently, to complete the sandwich immunosensing device, a second specific monoclonal anti-PSMA antibody was labelled with a electrochemiluminescent probe. Under optimized experimental conditions, the proposed sensing device exhibits a performance exceeding that of the state of-the-art in terms of the limit of detection (LOD) and limit of quantification (LOQ) as good as 0.88 ng mL^-1 and 2.60 ng mL^-1, respectively, in real complex samples such as cell lysates. In addition, the unique role of carbon nanotubes is also discussed by comparison with an analogue sensor assembled without the nanocarbon-based material.

Electrochemiluminescence of Supramolecular Nanorods and Their Application in the "On-Off-On" Detection of Copper Ions

In this work, an "on-off-on" switch system has been successfully applied through the construction of an electrochemiluminscent biosensor for copper ion (Cu(2+) ) detection based on a new electrochemiluminescence (ECL) emitter of supramolecular nanorods, which was achieved through supramolecular interactions between 3,4,9,10-perylenetetracarboxylic acid (PTCA) and aniline. The initial "signal-on" state with strong and stable ECL emission was obtained by use of the supramolecular nanorods with a new signal amplification strategy involving a co-reaction accelerator. In addition, ECL quencher probes (Fc-NH2 /Cu-Sub/nano-Au) were fabricated by immobilizing aminoferrocene (Fc-NH2 ) on Cu-substrate strand modified Au nanoparticles. The quencher probes were hybridized with the immobilized Cu-enzyme strand to form Cu(2+) -specific DNAzyme. Similarly, the "signal-off" state was obtained by the high quenching effect of Fc-NH2 on the ECL of the excited-state PTCA ((1) PTCA*). As expected, the second "switch-on" state could achieved by incubating with the target Cu(2+) , owing to the Cu(2+) -specific DNAzyme, which was irreversibly cleaved, resulting in the release of the quencher probes from the sensor interface. Herein, on the basis of the ECL intensity changes (ΔIECL ) before and after incubating with the target Cu(2+) , the prepared Cu(2+) -specific DNAzyme-based biosensor was used for the determination of Cu(2+) concentrations with high sensitivity, excellent selectivity, and good regeneration.