Coupled Fluorometer-Potentiostat System and Metal-Free Monochromatic Luminophores for High-Resolution Wavelength-Resolved Electrochemiluminescent Multiplex Bioassay

Phase-Resolved Electrochemiluminescence with a Single Luminophore

Multiphase chemical systems are greatly different than bulk solutions, as they provide a unique environment for reactions to proceed and have unique physicochemical properties. Thus, new tools need to be developed to gain a more detailed understanding of these systems. Here, we use electrogenerated chemiluminescence (ECL) to elucidate phase boundaries precisely and comprehensively between aqueous droplets and an organic continuous phase owing to ECL's unprecedented spatial resolution (a few micrometers) confined at the electrode surface. Phase-resolved mapping was accomplished by selecting a luminophore that is soluble in both phases while selecting two coreactants that are exclusively soluble in one phase or the other. This type of system allows us to map the complex liquid|electrode and the liquid|liquid interfaces in a multiphase system. We show that electrical connectivity is not conserved throughout solvent inclusions, which result from neighboring droplet coalescence, indicating an unexpected initial lack of electronic communication. These results have great importance to energy storage and conversion devices and wearable/implantable sensors, which are dominated by complex, multiphase environments.

Paper-based multiplex biosensors for inexpensive healthcare diagnostics: a comprehensive review

Multiplex detection is a smart and an emerging approach in point-of-care testing as it reduces analysis time and testing cost by detecting multiple analytes or biomarkers simultaneously which are crucial for disease detection at an early stage. Application of inexpensive substrate such as paper has immense potential and matter of research interest in the area of point of care testing for multiplexed analysis as it possesses several unique advantages. This study presents the use of paper, strategies adopted to refine the design created on paper and lateral flow strips to enhance the signal, increase the sensitivity and specificity of multiplexed biosensors. An overview of different multiplexed detection studies performed using biological samples has also been reviewed along with the challenges and advantages offered by multiplexed analysis.

Dual-target nucleic acid sequences responsive electrochemiluminescence biosensor using single type carbon dots as probe for SARS-CoV-2 detection based on series catalytic hairpin assembly amplification

Severe acute respiratory syndrome coronavirus (SARS-CoV-2) is rampant all over the world, and rapid and effective virus detection is the best auxiliary to curb the spread of the epidemic. A diagnosis can only be made if two or more different nucleic acid sequences are confirmed at the same time, and in most of traditional detection technologies, these target sequences have been detected separately. In this work, an electrochemiluminescent (ECL) biosensor employing a single ECL probe as signal output and responding to dual-target simultaneously is proposed for the first time. Taking the two sequences located in ORF 1ab region and N region of SARS-CoV-2 gene sequence as the model target and nitrogen doped carbon quantum dots (CDs) as ECL beacon, supplemented with catalytic hairpin assembly (CHA) reaction for signal amplification, the presented strategy has been successfully applied to the rapid detection of SARS-CoV-2. The developed SARS-CoV-2 biosensor based on the series CHA systems can realize the quantitative determination of SARS-CoV-2 in the range of 50 fM to 200 pM within 40 min. Moreover, the clinical validity of this method has been verified by the high consistency between the detection results of using this method and those using RT-qPCR for seven clinical pharyngeal swab samples.

Gold nanoparticle-based signal amplified electrochemiluminescence for biosensing applications

Since the content levels of biomarkers at the early stage of many diseases are generally lower than the detection threshold concentration, achieving ultrasensitive and accurate detection of these biomarkers is still one of the major goals in bio-analysis. To achieve ultrasensitive and reliable bioassay, it requires developing highly sensitive biosensors. Among all kinds of biosensors, electrogenerated chemiluminescence (ECL) based biosensors have attracted enormous attention due to their excellent properties. In order to improve the performance of ECL biosensors, gold nanoparticles (Au NPs) have been widely utilized as signal amplification tags. The introduction of Au NPs could dramatically enhance the performance of the constructed ECL biosensors via diverse ways such as electrode modification material, efficient energy acceptor in ECL resonant energy transfer (ECL-RET), reaction catalyst, surface plasmon resonance (SPR) enhancer, and as nanocarrier. Herein, we summarize recent developments and progress of ECL biosensors based on Au NPs signal amplification strategies. We will cover ECL applications of Au NPs as a signal amplification tag in the detection of proteins, metal ions, nucleic acids, small molecules, living cells, exosomes, and cell imaging. Finally, brief summary and future outlooks of this field will be presented.

K-Doped Graphitic Carbon Nitride with Obvious Less Electrode Passivation for Highly Stable Electrochemiluminescence and Its Sensitive Sensing Analysis of MicroRNA

In this study, upon potassium (K) element doping, the electrochemiluminescence (ECL) excitation potential of graphitic carbon nitride (g-C₃N₄) obviously shifted from -1.57 to -0.74 V. Compared with other reported methods, this work was the first one that could reduce the ECL excitation potential of g-C₃N₄ to below the critical value of -0.9 V. It could more effectively overcome electrode passivation and significantly improve the ECL intensity and stability. Meanwhile, the lower excitation potential could significantly reduce other side reactions caused by high voltage, and the introduction of the K element could obviously increase the water solubility to shorten the preparation time. The apparent decrease of the excitation potential was due to the doping of the K element, which could reduce the band gap, increase the in-plane spacing, and expand π-conjugated systems. Furthermore, using K-doped g-C₃N₄ with highly stable electrochemiluminescence at lower potential as an emitter, a biosensor for microRNA-141 (miRNA-141) sensitive detection was constructed with the assistance of an innovative nicking enzyme-assisted strand displacement amplification (N-SDA). Compared to the traditional SDA, a nicking enzyme was introduced to obviously improve the utilization rate of the fuel chain and increase the number of cycles, finally resulting in higher signal amplification efficiency. Therefore, the constructed biosensor showed excellent performance in the ultrasensitive detection of miRNA-141 with the limit of detection (LOD) being 44.8 aM. This work gave a more effective means to obviously improve the ECL property of g-C₃N₄ caused by electrode passivation and provided a more efficient and convenient detection method for biochemical analysis.

Photoluminescence and Energy Transfer in Double- and Triple-Lanthanide-Doped YVO4 Nanoparticles

Optical materials doped with several lanthanides are unique in their properties and are widely used in various fields of science and technology. The study of these systems provides solutions for noncontact thermometry, bioimaging, sensing technology, and others. In this paper, we report on the demonstration of YVO₄ nanoparticles doped with one, two, and three different rare earth ions (Tm³⁺, Er³⁺, and Nd³⁺). We discuss the morphology, structural properties, and luminescence behavior of particles. Luminescence decay kinetics reveal the energy transfer efficiency (up to 78%) for different ions under the selective excitation of individual ions. Thus, we found that the energy transition from Tm³⁺ is more favorable than from Er³⁺ while we did not observe any significant energy rearrangement in the samples under the excitation of Nd³⁺. The observed strong variation of REI lifetimes makes the suggested nanoparticles promising for luminescent labeling, anticounterfeiting, development of data storage systems, etc.

Lighting Up Electrochemiluminescence-Inactive Dyes via Grafting Enabled by Intramolecular Resonance Energy Transfer

Due to near-zero optical background and photobleaching, electrochemiluminescence (ECL), an optical phenomenon excited by electrochemical reactions, has drawn extensive attention, especially for ultrasensitive bioassays. Developing diverse ECL emitters is crucial to unlocking their multiformity and performances but remains a formidable challenge due to the rigorous requirements for ECL. Herein, we report a general strategy to light up ECL-inactive dyes in an aqueous solution via grafting, a well-developed concept for plant propagation since 500 BCE. As a proof of concept, a series of luminol donor-dye acceptor-based ECL emitters were grafted with near-unity resonance energy transfer (RET) efficiency and coarse/fine-tunable emission wavelengths. Rather than the sophisticated design of new skeleton-based molecules to meet all of the prerequisites for ECL in a constrained manner, each unit in the proposed ECL ensemble performed its functions maximally. As a result, beyond traditional two-dimensional (2D) ones, a three-dimensional (3D) coordinate biosensing system, simultaneously showing a calibration curve and selectivity, was established using the new ECL emitter. This lighting up strategy would generally address the scarcity of ECL emitters and enable unprecedented functions.

Integrating potential-resolved electrochemiluminescence with molecularly imprinting immunoassay for simultaneous detection of dual acute myocardial infarction markers

A novel potential-resolved molecularly imprinted electrochemical luminescence (ECL) immunosensor has been developed for the first time for the dual sensitive detection of markers of acute myocardial infarction (AMI): cardiac troponin I (cTnI) and myoglobin (Mb). In this work, cost-effective and robust molecularly imprinted polymer (MIP) as biomimetic antibody was used to construct the immunosensors through electropolymerization and elution to form polydopamine (PDA)-MIP modified electrode. In the presence of AMI biomarkers, two ECL probes including Ru(bpy)₃²⁺@ MOCs and MoS₂ QDs functionalized by cTnI antibody and Mb aptamer could be specifically captured respectively. And two potential distinct ECL signals will be generated in one potential scan. The intensity of ECL reflects the concentrations of cTnI and Mb. The two ECL probes were characterized with field emission scanning electron microscopy, X-ray diffraction, FT-IR spectrum and UV-Vis diffuse reflectance spectroscopy. The prepared sensor exhibited a wide linear range (0.05-10⁴ ng/mL) and a low detection limit (0.0184 ng/mL for cTnI and 0.0492 ng/mL for Mb). Additionally, the MIP-ECL sensor displayed excellent anti-interference, sensitivity and stability to detect cTnI and Mb. Therefore, it will be conducive to accelerate more precise and credible early diagnosis for AMI.

A label-free multiplex electrochemical biosensor for the detection of three breast cancer biomarker proteins employing dye/metal ion-loaded and antibody-conjugated polyethyleneimine-gold nanoparticles

A new electrochemical immunosensor is developed for the label-free simultaneous detection of mucin1 (MUC1), cancer antigen 15-3 (CA15-3), and human epidermal growth factor receptor 2 (HER2) early breast cancer biomarkers. The biosensor is simply designed using the deposition of three different systems of redox species-antibody-conjugated polyethylenimine coated-gold nanoparticles (PEI-AuNPs), for the first time. The screen-printed carbon electrode (SPCE) comprising a three-working electrode array is modified with the conjugated PEI-AuNPs. Multiplex sensing is performed by utilizing the distinguishable electrochemical responses of the redox-active species; anthraquinone-2-carboxylic acid (AQ), thionine chloride (TH), and AgNO3 (Ag+) on the PEI-AuNPs conjugates for the detection of MUC1, CA15-3, and HER2, respectively. The single-run determination of the biomarkers by the proposed immunosensor is carried out by measuring the decrease (%) in the oxidation peak currents due to the formation of three kinds of antibody-antigen complexes. The decreased currents are logarithmically proportional to the antigen concentrations in the ranges of 0.10-100 U mL-1 CA15-3 and 0.10-100 ng mL-1 MUC1 and HER2 with detection limits of 0.21 U mL-1, 0.53 ng mL-1 and 0.50 ng mL-1, respectively, which are significantly lower than the clinically relevant cut-off levels. The sensor reveals high selectivity and satisfactory reproducibility. After storing for two weeks, the sensor retains the responses with ca. 90% of the initial currents. The immunosensor is successfully applied to detect three tumor markers in human serum and can provide a new technological platform for the development of low-cost, highly stable, sensitive, selective, and point-of-care (POC) diagnosis.

A Multiplexed Self-Powered Dual-Photoelectrode Biosensor for Detecting Dual Analytes Based on an Electron-Transfer-Regulated Conversion Strategy

Due to the inherent mechanism limitation of photocatalytic fuel cell based self-powered biosensors (PFC-SPBs), it was difficult to distinguish the power density of various photoactive materials or recognition events in one detection process, which made it lack multitarget quantitative capacity. In order to solve this problem, we proposed an electron-transfer-regulated conversion strategy for the construction of multiplexed PFC-SPBs. Herein, the n-type CdS/Fe₂O₃ nanorod array (NR) heterojunction and p-type CuBr semiconductor were used as photoactive materials to prepare the photoanode and photocathode. Based on the appropriate Fermi level differentiation between these two photoelectrodes, a self-powered sensing platform driven by visible light without external energy supply was achieved. In this design, two kinds of common and easily coexisting mycotoxins OTA and AFB1 acted as model analytes. The coupling of "signal off" and "signal on" was realized by controlling the electronic transmission on the interface between the photoanode and photocathode, so as to achieve the simultaneous detection of two mycotoxins. This work established a proof-of-concept for the integration of a dual-photoelectrode with dual-assay that could provide the innovative inspiration for the formation of a general multiplexed self-powered sensing platform.

Multiplex bioassaying of cancer proteins and biomacromolecules: Nanotechnological, structural and technical perspectives

Since the specific proteins (carbohydrate antigens, ligands and interleukins) get raised up in body tissue or fluids in cancer cases, early detection of them will provide an effective treatment and survival rate. Sensitive and accurate determination of multiple cancer proteins can be engaged in chorus by simultaneous/multiplex detection in the biomedical fields. Bioassaying technology is one of the non-invasive, high-sensitive, and economical methods. Currently, extensive application of nanomaterial (biocompatible polymers, metallic and metal oxide) in bioassays resulted in ultra-high sensitive and selective diagnosis. This review article focuses on types of multiplex bioassays for delicate and specific determination of cancer proteins for diagnostic aims. It also covers two modes of multiplex bioassays as multi labeled bioassays and spatially-separated test zones (multi-electrode mode). In this review, the nanotechnological, structural, and technical perspectives in the multiplex analysis of cancer proteins were discussed. Finally, the use of different types of nanomaterials, polysaccharides, biopolymers and their advantages in signal amplification are discussed.

Recent developments in electrochemiluminescence nanosensors for cancer diagnosis applications

In recent years, electrochemiluminescence (ECL) nanosensing systems have undergone rapid development and made significant progress in ultrasensitive analysis and cell imaging. Because of the unique advantages of high selectivity, ultra-sensitivity, and good reproducibility, ECL nanosensors can open new paths for cancer diagnosis. With the development of ECL nanosensors, high-throughput analysis, visual detection and spatially resolved ECL imaging of single cells are being realized. The innovations of ECL nanosensors consist of electrochemical excitation, coreactant catalysis, light radiation and luminescence signal amplification, which involve several fields such as nanotechnology, catalysis, optics, and electrochemistry. The developments of ECL instruments also relate to imaging technology. Herein, we review the construction modes, sensing strategies and cancer diagnosis applications of ECL nanosenors. Firstly, the nano-components of the ECL sensing system are discussed. The construction and signal amplification methods of the nanosensing system are emphasized. Secondly, the high-efficiency cancer identification strategies are presented, including protein tumor marker detection, nucleic acid assay, cancer cell identification and exosome detection. The recent advances in representative examples of ECL nanosenors in cancer diagnosis are highlighted, including high-throughput ECL analysis, in situ assay, visual ECL detection, single-cell imaging diagnosis, and so on. Finally, the challenges are featured based on the recent development of the ECL nanosensing system in the clinical diagnosis. The ECL nanosensors provide effective and reliable analytical methods and open new paths for cancer diagnosis. It is noteworthy that the prospects of the ECL nanosensing system in clinical diagnosis are instructive to the developments of other nanosensor research.

Perylene Diimide and Luminol as Potential-Resolved Electrochemiluminescence Nanoprobes for Dual Targets Immunoassay at Low Potential

In the field of clinical diagnosis, it is important to construct a potential-resolved multiplex electrochemiluminescence (ECL) biosensor for decreasing the false-positive rate and improving the diagnostic accuracy. However, the shortage of low-potential cathodic luminophores between -1 and 0 V (vs Ag/AgCl) severely limited the development of the biosensor. Herein, we synthesized a novel luminophore N,N-bis-(3-dimethyl aminopropyl)-3,4,9,10-perylene tetracarboxylic acid diimide (PDI), which gave dual emissions at -0.25/-0.26 V with K₂S₂O₈ as a co-reactant in aqueous solution. The ECL was assigned to excited J-type PDI dimers. Then, PDI and luminol were used as luminophores to respectively combine with graphite oxide and gold nanoparticles and form potential-resolved ECL nanoprobes. Also, this potential-resolved ECL nanoprobes were respectively functionalized by secondary antibodies (Ab₂) to construct a low-potential sandwiched ECL immunosensor for tumor markers carcinoembryonic antigen (CEA) and α-fetoprotein (AFP) simultaneous determination during linear scanning potential range from -0.6 to 0.6 V. The prepared multiplex immunosensor exhibited sensitive ECL response for CEA at -0.6 V due to PDI and that for AFP at 0.6 V due to luminol, and both linear semilogarithmical ranges were from 0.1 pg to 1 ng mL^-1. In addition, PDI with dual ECL peaks showed enticing prospect of built-in self-calibration for a precise quantitative and bioimaging analysis.

Bioactivity-Protected Electrochemiluminescence Biosensor Using Gold Nanoclusters as the Low-Potential Luminophor and Cu2S Snowflake as Co-reaction Accelerator for Procalcitonin Analysis

The expansion of electrochemiluminescence (ECL) technology to immunoassay at the core of care emphasizes all immune molecules will not be inactivated in the analysis process. That poses a major challenge to ECL-based biosensors due to the deoxynucleotide sequences of an antigen or antibody could be oxidized through a route of excessive cyclic potential. Herein, an ultrasensitive ECL biosensor was developed based on a novel bioactivity-protected sensing strategy utilizing Au nanoclusters (Au NCs) as low-potential luminophor for detection of procalcitonin (PCT). Bovine serum albumin (BSA)-templated Au NCs exhibited a low-potential anodic ECL signal in triethylamine (TEA) solution at 0.87 V, where it is suitable for the survival of immune molecules. Taking advantage of good conductivity and high surface area, a Cu₂S snowflake not only functions as a satisfying substrate for connecting immune molecules but also acts as co-reaction accelerator to produce more cationic radicals TEA^•+, which could improve the ECL intensity needed to meet the requirements of trace analysis. Otherwise, HWRGWVC (HC-7) heptapeptide as specific antibody immobilizer for site-oriented fixation was introduced to further maintain the bioactivity of an antibody. In view of the preceding discussion, the obtained biosensor exhibited ultrahigh immune recognition to targets so that the detection limit was as low as an unprecedented value of 2.36 fg/mL, which will be of great significance to the application and development of a biosensor in the future.

Lanthanide terbium complex: synthesis, electrochemiluminescence (ECL) performance, and sensing application

In this study, a new lanthanide terbium complex, Tb(pzda)3(NO3)3·nH2O, was synthesized by a hydrothermal method and characterized by Fourier transform infrared spectroscopy (FT-IR) and energy-dispersive X-ray spectroscopy (EDS). It was found that the as-synthesized Tb-complex exhibited good electrochemiluminescence (ECL) behavior in the presence of triethanolamine (TEOA) in a HAc-NaAc buffer solution on a glassy carbon electrode. The possible reaction mechanism has been discussed based on the fluorescence spectra and ECL spectra. For sensing applications, it was found that protocatechuic acid (PCA) had an obvious quenching effect on the ECL signal of the Tb-complex, and this resulted in a decreased ECL signal associated with the concentration of PCA. Therefore, a highly sensitive method for the detection of PCA was established with a linear range of 1.283 × 10-10 M to 3.845 × 10-4 M and a detection limit of 0.085 nM at an S/N ratio of 3. This novel ECL assay strategy with an outstanding ECL efficiency offers great potential for pharmaceutical analyses.

A novel electrochemiluminescence resonance energy transfer system for simultaneous determination of two acute myocardial infarction markers using versatile gold nanorods as energy acceptors

A spatial- and potential-resolved platform coupled with the ECL-RET strategy was developed for simultaneous dual-target detection.

Spectrum-Resolved Triplex-Color Electrochemiluminescence Multiplexing Immunoassay with Highly-Passivated Nanocrystals as Tags

Nanocrystals (NCs) were extensively employed in optical-multiplexing with their size-dependent and narrow waveband photoluminescence (PL). Herein, to achieve NCs-based novel optical-multiplexing strategy with improved sensitivity, a spectrum-resolved triplex-color electrochemiluminescence (ECL) multiplexing immunoassay (MIA) was proposed for the first time by extensively exploring the color-different monochromatic ECL from dual-stabilizers-capped CdTe and CdSe NCs and the inherent lower background of ECL than PL. As a proof of concept, carcinoembryonic antigen (CEA), prostate specific antigen (PSA), and alpha fetoprotein (AFP) were adopted as model analytes and formed three antigen-NCs pairs at the glassy carbon electrode surface via a one-pot immune-reaction with CdSe(550), CdTe(650), and CdTe(776) NCs as tags, respectively. The spectral ECL responses of three antigen-NCs pairs were simultaneously measured via one-pot ECL assay, and the maximum ECL intensity of each tag was directly utilized to determine corresponding antigen without any assistance from either optical filters or signal deconvolution. This ECL-MIA displayed negligible cross-reactivity, high color-selectivity, and excellent sensitivity with limits of detection down to 1, 10, and 0.01 pg/mL for CEA, PSA, and AFP, respectively, which might provide a promising alternative to NCs PL multiplexing.