In-electrode vs. on-electrode: ultrasensitive Faraday cage-type electrochemiluminescence immunoassay

Emerging trends and recent advances in MXene/MXene-based nanocomposites toward electrochemiluminescence sensing and biosensing

Electrochemiluminescence (ECL) sensing systems have surged in popularity in recent years, making significant strides in sensing and biosensing applications. The realization of high-throughput ECL sensors hinges on the implementation of novel signal amplification strategies, propelling the field toward a new era of ultrasensitive analysis. A key strategy for developing advanced ECL sensors and biosensors involves utilizing novel structures with remarkable properties. The past few years have witnessed the emergence of MXenes as a captivating class of 2D materials, with their unique properties leading to exploitation in diverse applications. This review provides a comprehensive summary of the latest advancements in MXene-modified materials specifically engineered for ECL sensing and biosensing applications. We thoroughly analyze the structure, surface functionalization, and intrinsic properties of MXenes that render them exceptionally suitable candidates for the development of highly sensitive ECL sensors and biosensors. Furthermore, this study explores the broad spectrum of applications of MXenes in ECL sensing, detailing their multifaceted roles in enhancing the performance and sensitivity of ECL (bio)sensors. By providing a comprehensive overview, this review is expected to promote progress in related areas.

Faraday cage-type self-powered immunosensor based on hybrid enzymatic biofuel cell

Self-powered immunosensors (SPIs) based on enzymatic biofuel cell (EBFC) have low sensitivity and poor stability due to the high impedance of the immune sandwich and the vulnerability of enzymes to environmental factors. Here, we applied the Faraday cage-type sensing mode on a hybrid biofuel cell (HBFC)-based SPI for the first time, which exhibited high sensitivity and stability. Cytokeratin 19 fragment (CYFRA 21-1) was used as a model analyte. Au nanoparticle-reduced graphene oxide (Au-rGO) composite was used as the supporting matrix for immunoprobe immobilized with detection antibody and glucose dehydrogenase (GDH), also the builder for Faraday cage structure on the bioanode in the presence of antigen. After the combination of immunoprobe, antigen, and the antibody on the bioanode, the Faraday cage was constructed in case the AuNP-rGO was applied as a conductive cage for electron transfer from GDH to the bioanode without passing through the poorly conductive protein. With the assistance of the Faraday cage structure, the impedance of the bioanode decreased significantly from 4000 to 300 Ω, representing a decline of over 90%. The sensitivity of the SPI, defined as the changes of open circuit voltage (OCV) per unit concentration of the CYFRA 21-1, was 68 mV [log (ng mL-1)]-1. In addition, Fe-N-C was used as an inorganic cathode material to replace enzyme for oxygen reduction reaction (ORR), which endowed the sensor with 4-week long-term stability. This work demonstrates a novel sensing platform with high sensitivity and stability, bringing the concept of hybrid biofuel cell-based self-powered sensor.

Faraday cage-type ECL biosensor for the detection of circulating tumor cell MCF-7

Herein, a Faraday cage-type electrochemiluminescence biosensor was designed for the detection of human breast cancer cell MCF-7. Two kinds of nanomaterials, Fe₃O₄-APTs and GO@PTCA-APTs, were synthesized as capture unit and signal unit, respectively. In presence of the target MCF-7, the Faraday cage-type electrochemiluminescence biosensor was constructed by forming a complex "capture unit-MCF-7-signal unit". In this case, lots of electrochemiluminescence signal probes were assembled and could participate in the electrode reaction, achieving a significant increase in sensitivity. In addition, the double aptamer recognition strategy was adopted to improve the capture, enrichment efficiency and detection reliability. Under optimal experimental conditions, the limit of detection was 3 cells/mL. And, the sensor could afford the detection of actual human blood samples, which is the first report on the detection of intact circulating tumor cells by the Faraday cage-type electrochemiluminescence biosensor.

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.

Accurate Capture and Identification of Exosomes: Nanoarchitecture of the MXene Heterostructure/Engineered Lipid Layer

Recently, exosome detection has become an important breakthrough in clinical diagnosis. However, the effective capture and accurate identification of cancer exosomes in a complex biomatrix are still a tough task. Especially, the large size and non-conductivity of exosomes are not conducive to highly sensitive electrochemical or electrochemiluminescence (ECL) detection. Therefore, we have developed a Ti₃C₂T_x-Bi₂S_3-x heterostructure/engineered lipid layer-based nanoarchitecture to overcome the limitations. The engineered lipid layer not only specifically captured and efficiently fused CD63 positive exosomes but also showed excellent antifouling property in the biological matrix. Moreover, the MUC1 aptamer-modified Ti₃C₂T_x-Bi₂S_3-x heterostructure further identified and covered the gastric cancer exosomes that have been trapped in the engineered lipid layer. In the self-luminous Faraday cage-type sensing system, the Ti₃C₂T_x-Bi₂S_3-x heterostructure with sulfur vacancies extended the outer Helmholtz plane and amplified the ECL signal. Therefore, this sensor can be used to detect tumor exosomes in ascites of cancer patients without additional purification. It provides a new pathway to detect exosomes and other large-sized vesicles with high sensitivity.

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.

Photothermal-Induced Electrochemical Interfacial Region Regulation Enables Signal Amplification for Dual-Mode Detection of Ovarian Cancer Biomarkers

Detection sensitivity of an electrochemical immunosensor mainly depends on the accessible distance toward the sensing interface; regulating the electrochemical interfacial region thereon is an effective strategy for signal amplification. Herein, a photothermal-regulated sensing interface was designed based on a near-infrared (NIR)-responsive hydrogel probe for ultrasensitive detection of human epididymis protein 4 (HE4). Silver nanoparticle-deposited graphene oxide nanosheet (AgNPs@GO) hybrids as electrochemical signal tags and a photothermal transducer, which were encapsulated in the poly(N-isopropylacrylamide) (pNIPAM) hydrogel, were used to develop the NIR-responsive GO@AgNPs-pNIPAM hydrogel probe. Under NIR light irradiation, the excellent photothermal effect of AgNPs@GO hybrids not only rapidly converted NIR light into heat for temperature readout but also triggered the shrinkage behavior of the hydrogel for electrochemical signal amplification. Compared with the conventional sandwich immunoassay, the shrinkage behavior of the hydrogel signal probe endowed itself with interface regulation capability to improve the electrochemical reaction efficiency; on the basis of ensuring the extended outer Helmholtz plane (OHP) region, the proposed photothermal-induced interface regulation also shortened the OHP, leading to higher sensitivity. Moreover, the obtained dual-mode signals provided satisfactory accuracy for the detection of tumor markers. Therefore, this detection scheme provided an opportunity for the broad applications of the photothermal effect in clinical diagnosis.

Electrochemiluminescent immunoassay for the determination of CA15-3 and CA72-4 using graphene oxide nanocomposite modified with CdSe quantum dots and Ru(bpy)3 complex

A novel immunoassay is introduced based on co-reactant enhancing strategy for the electrochemiluminescent (ECL) determination of CA15-3 and CA72-4 tumor markers in real samples. For the preparation of the signaling probe, CA15-3 and CA72-4 antibodies first were labeled using Ru(bpy)₃²⁺-N-hydroxysuccinimide ester (Ru(bpy)₃²⁺-NHS) and conjugated with L-cysteine capped cadmium selenide (CdSe) quantum dots. Finally, it was cross-linked with chitosan-grafted graphene oxide (GO@CS) nanocomposite. The capture probe was constructed by deposition of multi-walled carbon nanotubes (MWCNT) at the surface of dual-working gold screen-printed electrodes (MWCNT-dwSPE) and covalent attachment of capture CA15-3 and CA72-4 antibodies to MWCNT-dwSPE. ECL signals were recorded by applying cyclic potential ranging from 0.3 to 1.1 V (vs. pseudo-reference Ag/AgCl) at the scan rate of 100 mV.s^-1. This immunoassay was used for determination of CA15-3 and CA72-4 in real samples the detection limits of 9.2 μU.ml^-1 and 89 μU.ml^-1 within linear ranges of 10 μU.ml^-1-500 U.ml^-1 and 100 μU.ml^-1-150 U.ml^-1, respectively. This immunoassay also showed acceptable accuracy with recoveries in the range 96.5-108 % and high reproducibility with RSD of 3.1 and 4.9.

Rational Fabrication of a Smart Electrochemiluminescent Sensor: Synergistic Effect of a Self-Luminous Faraday Cage and Biomimetic Magnetic Vesicles

A smart electrochemiluminescent (ECL) sensor has been designed in this work. The sensing system consisted of Ag NPs-Ti₃AlC₂ nanosheets (Ag-TACS) as the self-luminous Faraday cage and biomimetic magnetic vesicles as the functional substrate. By engineering the structure and properties of Ti₃AlC₂ nanosheets to induce the Faraday-cage effect, the outer Helmholtz plane (OHP) was extended to contribute to ECL enhancement. Compared with the Faraday cage that further incorporated luminous materials, the self-luminous Faraday cage in the "direct label" model kept all the luminous materials on the OHP. Meanwhile, biomimetic magneticvesicles with highly efficient fluidity were used to improve the sensing efficiency and obtain a perfect Faraday-cage structure to enhance the ECL signals. The highest ECL enhancement (ca. 25 times) has been achieved by the synergistic effect of the Faraday cage and biomimetic magnetic vesicles. This sensing system was used to detect the wild-type K-ras gene in the colorectal tumor tissue. It provides not only an important guide for the novel ECL sensing concept but also a smart modulation system of the electromagnetic field.

Trends in sensor development toward next-generation point-of-care testing for mercury

Mercury is one of the most common heavy metals and a major environmental pollutant that affects ecosystems. Since mercury and its compounds are toxic to humans, even at low concentrations, it is very important to monitor mercury contamination in water and foods. Although conventional mercury detection methods, including inductively coupled plasma mass spectrometry, atomic absorption spectroscopy, and gas chromatography-mass spectrometry, exhibit excellent sensitivity and accuracy, they require operation by an expert in a sophisticated and fully controlled laboratory environment. To overcome these limitations and realize point-of-care testing, many novel methods for direct sample analysis in the field have recently been developed by improving the speed and simplicity of detection. Commonly, these unconventional sensors rely on colorimetric, fluorescence, or electrochemical mechanisms to transduce signals from mercury. In the case of colorimetric and fluorescent sensors, benchtop methods have gradually evolved through technology convergence to give standalone platforms, such as paper-based assays and lab-on-a-chip systems, and portable measurement devices, such as smartphones. Electrochemical sensors that use screen-printed electrodes with carbon or metal nanomaterials or hybrid materials to improve sensitivity and stability also provide promising detection platforms. This review summarizes the current state of sensor platforms for the on-field detection of mercury with a focus on key features and recent developments. Furthermore, trends for next-generation mercury sensors are suggested based on a paradigm shift to the active integration of cutting-edge technologies, such as drones, systems based on artificial intelligence, machine learning, and three-dimensional printing, and high-quality smartphones.

A Comparison of Commercially Available Screen-Printed Electrodes for Electrogenerated Chemiluminescence Applications

We examined a series of commercially available screen-printed electrodes (SPEs) for their suitability for electrochemical and electrogenerated chemiluminescence (ECL) detection systems. Using cyclic voltammetry with both a homogeneous solution-based and a heterogeneous bead-based ECL assay format, the most intense ECL signals were observed from unmodified carbon-based SPEs. Three commercially available varieties were tested, with Zensor outperforming DropSens and Kanichi in terms of sensitivity. The incorporation of nanomaterials in the electrode did not significantly enhance the ECL intensity under the conditions used in this evaluation (such as gold nanoparticles 19%, carbon nanotubes 45%, carbon nanofibers 21%, graphene 48%, and ordered mesoporous carbon 21% compared to the ECL intensity of unmodified Zensor carbon electrode). Platinum and gold SPEs exhibited poor relative ECL intensities (16% and 10%) when compared to carbonaceous materials, due to their high rates of surface oxide formation and inefficient oxidation of tri-n-propylamine (TPrA). However, the ECL signal at platinum electrodes can be increased ∼3-fold with the addition of a surfactant, which enhanced TPrA oxidation due to increasing the hydrophobicity of the electrode surface. Our results also demonstrate that each SPE should only be used once, as we observed a significant change in ECL intensity over repeated CV scans and SPEs cannot be mechanically polished to refresh the electrode surface.

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.

Faraday-Cage-Type Electrochemiluminescence Immunoassay: A Rise of Advanced Biosensing Strategy

Electrochemiluminescence immunoassays are usually carried out through "on-electrode" strategy, i.e., sandwich-type immunoassay format, the sensitivity of which is restricted by two key bottlenecks: (1) the number of signal labels is limited and (2) only a part of signal labels could participate in the electrode reaction. In this Perspective, we discuss the development of an "in-electrode" Faraday-cage-type concept-based immunocomplex immobilization strategy. The biggest difference from the traditional sandwich-type one is that the designed "in-electrode" Faraday-cage-type immunoassay uses a conductive two-dimensional (2-D) nanomaterial simultaneously coated with signal labels and a recognition component as the detection unit, which could directly overlap on the electrode surface. In such a case, electrons could flow freely from the electrode to the detection unit, the outer Helmholtz plane (OHP) of the electrode is extended, and thousands of signal labels coated on the 2-D nanomaterial are all electrochemically "effective." Thus, then, the above-mentioned bottlenecks obstructing the improvement of the sensitivity in sandwich-type immunoassay are eliminated, and as a result a much higher sensitivity of the Faraday-cage-type immunoassay can be obtained. And, the applications of the proposed versatile "in-electrode" Faraday-cage-type immunoassay have been explored in the detection of target polypeptide, protein, pathogen, and microRNA, with the detection sensitivity improved tens to hundreds of times. Finally, the outlook and challenges in the field are summarized. The rise of Faraday-cage-type electrochemiluminescence immunoassay (FCT-ECLIA)-based biosensing strategies opens new horizons for a wide range of early clinical identification and diagnostic applications.

Ultrasensitive mushroom-like electrochemical immunosensor for probing the activity of histone acetyltransferase

A novel mushroom-like electrochemical immunoassay for the ultrasensitive detection of histone acetyltransferase activity (HAT p300) has been established on account of the new composite graphene oxide (GO) nanolayer. The immunoassay involves immobilization of substrate peptide onto Au electrode, acetylation in lysine of substrate peptide, and the interaction between acetyl group of lysine and acetyl-antibody (Ab_Ac) of the GO nanolayer. The GO nanolayer comprises large amounts of methylene blue molecules (MB), giving rise to large signal amplification. Only in the presence of HAT p300, an obvious electrochemical signal appears and the peak linear current is proportion to the HAT p300 concentrations ranging from 0.01 to 150 nM with a detection limit of 0.0036 nM. The great enhancement on sensitivity of the proposed mushroom-like immunosensor derives from both the constructed Faraday cage and the extended outer Helmholtz plane (OHP). Further, the immunosensor with excellent sensitivity and selectivity can be applied for the HAT p300 activity detection in Hela cell lysates, serum and urine, hinting an improved and splendid analytical performance. Briefly, this stable, simple and ultrasensitive electrochemical immunoassay has considerable promise for further applications in the HATs-interrelated epigenetic studies and drug development.

Potential-resolved Faraday cage-type electrochemiluminescence biosensor for simultaneous determination of miRNAs using functionalized g-C3N4 and metal organic framework nanosheets

Here, a novel Faraday cage-type electrochemiluminescence (ECL) biosensor was presented for simultaneous determination of miRNA-141 and miRNA-21 based on the potential-resolved strategy. In this work, capture units were prepared by immobilizing hairpin DNA1 (HP1) and hairpin DNA2 (HP2) on Fe₃O₄ @Au nanocomposites, while g-C₃N₄ @AuNPs nanocomposites labelled by signal DNA1 (sDNA1) and ruthenium-based metal organic framework (Ru-MOF) nanosheets labelled by signal DNA2 (sDNA2) were used as signal units. In this proposed biosensor, signal units g-C₃N₄ @AuNPs-sDNA1 and Ru-MOF-sDNA2 could exhibit two strong and stable ECL emissions at - 1.4 V and + 1.5 V respectively, which could be used as effective potential-resolved signal tags. Moreover, taking advantage of the proposed Faraday cage-type model, all electrochemiluminophores in the signal units could take part in electrode reactions, the signal units became part of the electrode surface and extended the outer Helmholtz plane (OHP) of the proposed electrode, and then the detection sensitivity was improved greatly. Accordingly, dual targets miRNA-141 and miRNA-21 could be detected within the linear range of 1 fM to 10 pM, with the detection limit of 0.3 fM. Meanwhile, the proposed miRNA assay exhibited high selectivity and sensitivity, even for practical analysis in human serum. So, this potential-resolved ECL biosensor is proved to be a feasible tool for dual targets detection of miRNAs in clinical diagnosis.

Faraday cage-type electrochemiluminescence immunosensor for ultrasensitive detection of Vibrio vulnificus based on multi-functionalized graphene oxide

A novel Faraday cage-type electrochemiluminescence (ECL) immunosensor devoted to the detection of Vibrio vulnificus (VV) was fabricated. The sensing strategy was presented by a unique Faraday cage-type immunocomplex based on immunomagnetic beads (IMBs) and multi-functionalized graphene oxide (GO) labeled with (2,2'-bipyridine)(5-aminophenanthroline)ruthenium (Ru-NH2). The multi-functionalized GO could sit on the electrode surface directly due to the large surface area, abundant functional groups, and good electronic transport property. It ensures that more Ru-NH2 is entirely caged and become "effective," thus improving sensitivity significantly, which resembles extending the outer Helmholtz plane (OHP) of the electrode. Under optimal conditions, the developed immunosensor achieves a limit of detection as low as 1 CFU/mL. Additionally, the proposed immunosensor with high sensitivity and selectivity can be used for the detection of real samples. The novel Faraday cage-type method has shown potential application for the diagnosis of VV and opens up a new avenue in ECL immunoassay. Graphical abstract Faraday cage-type immunoassay mode for ultrasensitive detection by extending OHP.