Proximity hybridization-regulated electrogenerated chemiluminescence bioassay of α-fetoprotein via target-induced quenching mechanism

Tetrahedral DNA-linked aptamer-antibody-based sandwich-type electrochemical sensor with Ag@Au core-shell nanoparticles as a signal amplifier for highly sensitive detection of α-fetoprotein

The conventional electrochemical detection strategy for alpha-fetoprotein (AFP) is limited by the antigen-antibody (Ag-Ab) reactions and suffers from low sensitivity and poor reproducibility due to the inconsistency of Ab-modified electrodes. Herein, we designed and explored a sandwich-type electrochemical sensor for highly sensitive detection of AFP based on aptamer (Apt)-AFP-Ab interaction mode with silver@gold (Ag@Au) core-shell nanoparticles (NPs) as a signal amplifier. AuNPs were electrodeposited onto MXene (Ti3C2TX)-modified glassy carbon electrode (GCE) to get AuNPs/MXene/GCE and further used as the signal amplification substrate. The tetrahedral DNA-linked AFP aptamers were immobilized onto AuNPs/MXene/GCE surface via Au-S bonds and used as the sensing and recognition platform for AFP capturing. Ag@AuNPs with core-shell structures were synthesized, characterized, and bound with Ab as detection elements by catalyzing H2O2 reduction. In the presence of AFP, a stable Apt-AFP-Ab sandwich structure was formed owing to the high affinities of aptamer and Ab toward the target AFP. The catalytic current produced by H2O2 reduction increased linearly with the logarithm of AFP concentration from 5 × 10-4 ng/mL to 1 × 105 ng/mL, accompanied by a low detection limit (1.6 × 10-4 ng/mL). Moreover, the novel sandwich-type electrochemical sensor shows high sensitivity, outstanding selectivity, and promising performance in the analysis of actual samples, displaying a broad application prospect in bioanalysis.

Ultrasensitive Electrochemiluminescence Biosensor to Detect Ampicillin Resistance Gene (ARGAMP) Based on a Novel Near-Infrared Ruthenium Carbene Complex/TPrA/PEI Ternary ECL System

The establishment of rapid target identification and analysis methods for antibiotic resistance genes (ARGs) is urgently needed. In this study, we unprecedently designed a target-catalyzed hairpin assembly (CHA) electrochemiluminescent (ECL) biosensor for the ultrasensitive detection of ampicillin resistance genes (ARGAMP) based on a novel, efficient near-infrared ruthenium carbene complex/TPrA/PEI ternary ECL system with low oxidation potential. The ternary NIR-ECL system illustrated in this work displayed double ECL intensity in comparison with their corresponding traditional binary ECL system. The as-prepared ECL biosensor illustrated in this work demonstrates highly selective and sensitive determination of ARGAMP from 1 fM to 1 nM and a low detection limit of 0.23 fM. Importantly, it also exhibits good accuracy and stabilities to identify ARGAMP in plasmid and bacterial genome DNA, which demonstrates its excellent reliability and great potential in detecting ARGAMP in real environmental samples.

Signal-on electrogenerated chemiluminescence detection of gonyautoxin 1/4 based on proximity ligation-induced an electrode-bound pseudoknot DNA

A "signal on" electrogenerated chemiluminescence (electrochemiluminescence, ECL) aptasensor based on proximity ligation-induced an electrode-bound pseudoknot DNA for sensitive detection of gonyautoxin 1/4 (GTX1/4) was developed on basis of the competitive type reaction mode. Aptamer was adopted as recognition element. Ru(bpy)32+ as ECL signal, was attached on the glassy carbon electrode (GCE) surface modified with nafion and gold nanoparticles (AuNPs) by electrostatic attraction to obtain the ECL platform. The pseudoknot DNA as capture probe, was immobilized onto the ECL platform via Au-S bond to obtain the ECL aptasensor. In the absence of GTX1/4, Y-shape proximate cooperative complex among aptamer, pseudoknot DNA and DNA1 was formed, drawing the ferrocene groups Fc, as ECL quencher) of both pseudoknot DNA and DNA1 near the electrode surface and resulting in low ECL signal. In the presence of GTX1/4, GTX1/4 competed with pseudoknot DNA and DNA1 for aptamer in homogeneous solution, preventing the formation of proximate cooperative complex and keeping the capture DNA in the pseudoknot conformation with Fc groups far away from the electrode surface, generating a high ECL signal. The recovery of ECL intensity increased with the GTX1/4 concentration and allowed the detection of GTX1/4 in the range of 0.01 ng/mL to 10 ng/mL with a detection of limit as low as 6.56 pg/mL. Additionally, the accuracy of this method was validated for analysis of spiked sea water samples with good recoveries, which indicates great potential in commercial application.

Construction of a Dual-Mode Immune Platform Based on the Photothermal Effect of AgCo@NC NPs for the Detection of α-Fetoprotein

Compared with the accuracy of a single signal and the limitation of environmental applicability, the application value of dual-mode detection is gradually increasing. To this end, based on the photothermal effect of Ag/Co embedded N-rich mesoporous carbon nanomaterials (AgCo@NC NPs), we designed a dual-mode signal response system for the detection of α-fetoprotein (AFP). First, AgCo@NC NPs act as a photothermal immunoprobe that converts light energy into heat driven by a near-infrared (NIR) laser and obtains temperature changes corresponding to the analyte concentration on a hand-held thermal imager. In addition, this temperature recognition system can significantly improve the efficiency of Fenton-like reactions. AgCo@NC NPs act as peroxidase mimics to initiate the generation of poly N-isopropylacrylamide (PNIPAM, resistance enhancer) by cascade catalysis and the degradation of methylene blue (MB), thus enabling electrochemical testing. The dual-mode assay ranges from 0.01 to 100 and 0.001-10 ng/mL, with lower limits of detection (LOD) of 3.2 and 0.089 pg/mL, respectively, and combines visualization, portability, and high efficiency, opening new avenues for future clinical diagnostics and inhibitor studies.

Electrochemiluminescence bioassay with anti-fouling ability for determination of matrix metalloproteinase 9 secreted from living cells under external stimulation

An electrochemiluminescence (ECL) bioassay with high sensitivity and anti-fouling ability was developed for determination of matrix metalloproteinase 9 (MMP-9) secreted from living cells under external stimulation. A peptide with sequence of CLGRMGLPGK and a new cyclometalated iridium(III) complex bearing carboxyl group, (pq)2Ir(dcbpy) (pq = 2-phenylquinoline, dcbpy = 2,2'-bipyridyl-4,4'-dicarboxyli acid, abbreviated as Ir) were employed as molecular recognition substrate and ECL emitter, respectively. The peptide was labelled with the Ir to form Ir-peptide as ECL probe. Ir-peptide was self-assembled onto Nafion and gold nanoparticles (AuNPs) modified glassy carbon electrode (AuNPs/Nafion/GCE) and then both of 6-mercapto-1-hexanol (MCH) and zwitterionic peptide as blocking reagents were co-assembled on Ir-peptide/AuNPs/Nafion/GCE to form an anti-fouling ECL peptide-based biosensor. MMP-9 can be quantified in the range 1.0-50 ng·mL-1 with a detection limit of 0.50 ng·mL-1 based on the decreased ECL intensity. Relative standard derivation was 2.3% for six fabricated anti-fouling ECL peptide-based biosensors after reaction with 50 ng·mL-1 MMP-9. The anti-fouling ECL peptide-based biosensor can be used to monitor MMP-9 secreted from living cells under external stimulation. 96.0%-108.0% of recoveries were obtained in 60-diluted cell culture media. This study demonstrates that the ECL biosensor by the combination of iridium(III) complex-based sensitive ECL method and the anti-fouling interface provides a promising way for the determination of MMP-9 in biological sample, which is viable in clinical diagnosis and point-of-care test of protease.

Unique Electron-Transfer-Mediated Electrochemiluminescence of AuPt Bimetallic Nanoclusters and the Application in Cancer Immunoassay

Noble Metal nanoclusters (NCs) are promising electrochemiluminescence (ECL) emitters due to their amazing optical properties and excellent biocompatibility. They have been widely used in the detection of ions, pollutant molecules, biomolecules, etc. Herein, we found that glutathione-capped AuPt bimetallic NCs (GSH-AuPt NCs) emitted strong anodic ECL signals with triethylamine as co-reactants which had no fluorescence (FL) response. Due to the synergistic effect of bimetallic structures, the ECL signals of AuPt NCs were 6.8 and 94 times higher than those of monometallic Au and Pt NCs, respectively. The electric and optical properties of GSH-AuPt NCs differed from those of Au and Pt NCs completely. An electron-transfer mediated ECL mechanism was proposed. The excited electrons may be neutralized by Pt(II) in GSH-Pt and GSH-AuPt NCs, resulting in the vanished FL. Furthermore, abundant TEA radicals formed on the anode contributed electrons to the highest unoccupied molecular orbital of GSH-Au_2.5Pt NCs and Pt(II), booming intense ECL signals. Because of the ligand effect and ensemble effect, bimetallic AuPt NCs exhibited much stronger ECL than GSH-Au NCs. A sandwich-type immunoassay for alpha fetoprotein (AFP) cancer biomarkers was fabricated with GSH-AuPt NCs as signal tags, which displayed a wide linear range from 0.01 to 1000 ng·mL^-1 and a limit of detection (LOD) down to 1.0 pg·mL^-1 at 3S/N. Compared to previous ECL AFP immunoassays, this method not only had a wider linear range but also a lower LOD. The recoveries of AFP in human serum were around 108%, providing a wonderful strategy for fast, sensitive, and accurate cancer diagnosis.

Washing-free electrogenerated chemiluminescence magnetic microbiosensors based on target assistant proximity hybridization for multiple protein biomarkers

This work reports washing-free electrogenerated chemiluminescence (ECL) magnetic microbiosensors based on target assistant proximity hybridization (TAPH) for multiple protein biomarkers for the first time. As a principle-of-proof, alpha-fetoprotein (AFP) was chosen as a model analyte, and biotin-DNA1 bound streptavidin-coated magnetic microbeads (MMB@SA⋅biotin-DNA1) were designed as the universal capture MMB, while the corresponding two antibodies tagged with DNA2 or DNA3 were utilized as hybrid recognition probes, and ruthenium complex-tagged DNA4-10A was designed as a universal ECL signal probe. When the capture MMB was added into the mixture solution (containing the analyte, hybrid recognition probes, signal probe and tri-n-propylamine), biocomplexes were formed on the MMB. After the resulting MMB was efficiently brought to the surface of a magnetic glassy carbon electrode (MGCE), ECL measurement was performed without a washing step, resulting in an increase in the ECL intensity. A model for ECL measuring the second-order rate constants of hybridization reactions on MMB was derived. It was found that the rate constants for hybridization reactions on MMB in rotating mode are 1.6-fold higher than those in shaking mode, and a suitable DNA length of the signal probe can improve the signal-to-noise ratio. The washing-free ECL method was developed for the determination of AFP with a much lower detection limit (LOD) of 0.04 ng mL^-1. The developed flexible strategy has been extended to determine D-dimer with an LOD of 0.1 ng mL^-1 and myoglobinglobin with an LOD of 1.1 ng mL^-1. This work demonstrated that the proposed strategy of ECL TAPH on MMB at MGCE is a washing-free and flexible promising strategy, and can be extended to qualify other multiple protein biomarkers in real clinical assays.

Recent advances in electrochemical proximity ligation assay

Proximity ligation assay (PLA) is a vigorously developed homogeneous immunoassay assisted by DNA combining dual recognition of target protein by pairs of proximity probes, in which the detection of protein is tactfully converted to the detection of DNA. The booming developments in PLA have enabled a variety of ultrasensitive assays for the detection of protein and this concept of PLA is also extended to the detection of nucleic acids and some small molecule. The association between PLA and electrochemical method, defined as electrochemical proximity ligation assay (ECPLA), has gained much interests in disease diagnosis, food safety and environmental assays with the advantages, such as broad range of targets, simplicity, low cost and rapid response. In this review, we took a different perspective to present the history of PLA, the classical ECPLA biosensing methodology as well as the developments of ECPLA based on several key parameters, such as sensitivity, selectivity, reusability and generalization. In addition, the developments of PLA with electrochemiluminescence as readout are also presented. Finally, perspective and some unresolved challenges in ECPLA that can potentially be addressed have also been discussed.

A solid-state electrochemiluminescence aptasensor for β-lactoglobulin using Ru-AuNP/GNP/Naf nanocomposite-modified printed sensor

An electrochemiluminescence (ECL) aptasensor for the detection of the milk protein allergen β-lactoglobulin (β-LG) using nanocomposite as luminophore was fabricated. The Ru-AuNPs/GNP/Naf complex was formed by combining the Rubpy₃²⁺-AuNPs complex (Ru-AuNPs), prepared by modifying the negatively charged surface of gold nanoparticles (AuNPs) with positively charged Rubpy₃²⁺ through electrostatic interactions and the graphene nanoplatelets-Nafion (GNP/Naf) at a ratio of 2:1. The nanocomposite was coated on the surface of the screen-printed electrode (SPCE) through the film-forming properties of Nafion. A layer of chitosan (CS) was coated onto this modified electrode, and later amine-terminated β-LG aptamers were covalently attached to the CS/Ru-AuNP/GNP/Naf via glutaraldehyde (GLUT) cross-linking. When β-LG was incubated with the aptasensor, a subsequent decrease in ECL intensity was recorded. Under the optimal conditions, the ECL intensity of the aptasensor changed linearly with the logarithmic concentration of β-LG, in the range 0.1 to 1000 pg/ml, and the detection limit was 0.02 pg/mL (3σ/m). The constructed aptasensor displayed simple and fast determination of β-LG with excellent reproducibility, stability, and high specificity. Additionally, the proposed ECL aptasensor displayed high recoveries (92.5-112%) and low coefficients of variation (1.6-7.8%), when β-LG fortified samples were analyzed. Integrating Ru-AuNPs/GNP/Naf nanocomposite in the ECL aptasensor paves the way towards a cost-effective and sensitive detection of the milk allergen β-LG.

Electrochemiluminescence biosensor for thrombin detection based on metal organic framework with electrochemiluminescence indicator embedded in the framework

Ru(dcbpy)₃²⁺-polyethyleneimine-L-lysine (Ru-PEI-L-lys) had been immobilized on metal organic frameworks (ZIF-8) to form an electrochemiluminescent(ECL) indicator (Ru-PEI-L-lys-ZIF-8). In this ECL indicator, PEI-L-lys is used as a co-reactant. Platinum nanoparticles (PtNPs) has been mixed with Ru-PEI-L-lys-ZIF-8 to form a thin film to increase the electron transfer rate and enhanced the ECL response of the system. The prepared material had been characterized carefully and been combined with high selectivity of aptamer to develop a ECL biosensor for thrombin detection. RecJ_f exonuclease (an ssDNA specific exonuclease) assistant target recycling amplification has been adopted to enhance the sensitivity of the system. The ECL response of the system has a linear relationship with logarithm of thrombin concentration in the range of 1 fM to 10 pM with a detection limit of 0.02 aM. This work not only provides a new strategy for the design and synthesis of high performance and stable ECL indicator, but also opens up a new approach for the development of highly sensitive ECL sensors for biological analysis.

Dual-readout proximity hybridization-regulated and photothermally amplified protein analysis based on MXene nanosheets

Herein, an ingenious dual-readout sensing platform based on a proximity hybridization-regulated strategy is proposed for protein detection. For the first time, Ti₃C₂ MXene@thionine composites (MXene@Thi) with an excellent photothermal effect not only acted as an amplifier to enhance the electrochemical signal, but were also used as a converter to achieve the temperature readout.

Design and application of proximity hybridization-based multiple stimuli-responsive immunosensing platform for ovarian cancer biomarker detection

The development of convenient and sensitive multi-readout immunoassay is crucial but highly challenged for meeting the demand of exactness and diversity in early clinical diagnosis. Herein, a split-type multiple stimuli-responsive biosensor was outlined combined with the outstanding superiority of luminol probe-based electrochemiluminescence (ECL) strategy, mimicking enzyme-mediated colorimetric system and portable photothermal effect-induced temperature sensing. Especially, versatile MoS₂ nanosheets (MoS₂ NSs) with distinguished property not only acted as dual-promoter to improve the cathodic ECL of luminol because of its good electrocatalytic activity for dissolved O₂ and favorable photothermal effect for elevating electrode temperature, but also used as nanozyme to regulate subsequent split-type visual colorimetric sensing due to its peroxidase-like activity for the generation of oxidized 2,2'-azinobis(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) in ABTS-H₂O₂ colorimetric system. More importantly, the green oxidized ABTS (ABTS^•+) also exhibited strong near-infrared (NIR) laser-triggered photothermal performance, which can be innovatively employed as sensitive photothermal agent for converting biological signals into temperature under the irradiation of NIR laser, accomplishing more simpler temperature quantitative detection by a portable thermometer. Furthermore, on account of the affinity discrepancy of MoS₂ NSs to single-stranded and double-stranded nucleic acids, a label-free proximity hybridization-based multifunctional assay platform was proposed for target detection with human epididymis-specific protein 4 (HE4) as model protein, demonstrating good analytical performances. Significantly, this innovative work not only enriches the foundational study of multi-model biosensing based on the unitary material but also provides an unambiguous guideline for exploring more accurate and simpler point-of-care diagnosis.

Proximity-enabled bidirectional enzymatic repairing amplification for ultrasensitive fluorescence sensing of adenosine triphosphate

A novel fluorescence sensing strategy for ultrasensitive and highly specific detection of adenosine triphosphate (ATP) has been developed by the combination of the proximity ligation assay with bidirectional enzymatic repairing amplification (BERA). The strategy relies on proximity binding-triggered the release of palindromic tail that initiates bidirectional cyclic enzymatic repairing amplification reaction with the aid of polymerase and two DNA repairing enzymes, uracil-DNA glycosylase (UDG) and endonuclease IV (Endo IV). A fluorescence-quenched hairpin probe with a palindromic tail at the 3' end is skillfully designed that functions as not only the recognition element, primer, and polymerization template for BERA but also the indicator for fluorescence signal output. On the basis of the amplification strategy, this biosensor displays excellent sensitivity and selectivity for ATP detection with an outstanding detection limit of 0.81 pM. Through simultaneously enhancing the target response signal value and reducing nonspecific background, this work deducted the background effect, and showed high sensitivity and reproducibility. Moreover, our biosensor also shows promising potential in real sample analysis. Therefore, the proximity-enabled BERA strategy indeed creates a simple and valuable fluorescence sensing platform for ATP identification and related disease diagnosis and biomedical research.

Magneto-controlled flow-injection device for electrochemical immunoassay of alpha-fetoprotein on magnetic beads using redox-active ferrocene derivative polymer nanospheres

A new electrochemical immunosensing protocol by coupling with a magneto-controlled flow-through microfluidic device was developed for the sensitive detection of alpha-fetoprotein (AFP) on magnetic beads (MB) using ferrocene derivative polymer nanospheres (FDNP) as the electroactive mediators.

An ultrasensitive electrochemical immunoassay based on a proximity hybridization-triggered three-layer cascade signal amplification strategy

An ultrasensitive electrochemical immunoassay based on a proximity hybridization-triggered three-layer cascade signal amplification strategy.

Voltammetric immunoassay of human IgG based on the release of cadmium(II) from CdS nanocrystals deposited on mesoporous silica nanospheres

The authors describe a nanocomposite that was obtained by in-situ deposition of CdS nanocrystals on mesoporous silica nanospheres (MSNs), and its use in an electrochemical immunoassay of human immunoglobulin G (HIgG). The MCN/CdS nanocomposite was covalently modified with the antibodies against HIgG and then employed in a voltammetric immunoassay at antibody-functionalized magnetic beads. Through sandwich immunoreaction, the MCN/CdS nanoprobes are quantitatively captured onto the magnetic beads where numerous Cd(II) ions are released in an acidic solution. The Cd(II) can be detected by anodic stripping voltammetry at a typical working potential of -0.78 V (vs. Ag/AgCl). In combination with the high loading of CdS on MSNs, the use of the stripping voltammetric analysis renders the method high sensitivity. A wide linear range varying from 0.01 to 100 ng mL^-1 is obtained for HIgG detection with a lower detection limit at 2.9 pg mL^-1. In addition, the preparation of the nanoprobe is inexpensive. The magnetic bead-based assay does not require complex manipulations. Therefore, this method is deemed to possess a wide scope in that it may be applied to other immunoassays. Graphical abstract Graphical Abstract contains poor quality and small text inside the artwork. Please do not re-use the file that we have rejected or attempt to increase its resolution and re-save. It is originally poor, therefore, increasing the resolution will not solve the quality problem. We suggest that you provide us the original format. We prefer replacement figures containing vector/editable objects rather than embedded images. Preferred file formats are eps, ai, tiff and pdf.A TIFF file at 900 dpi resolution of the Graphical Abstract has been attached via this online system. Schematic presentation of the preparation of the mesoporous silica nanosphere (MSN)/CdS nanocomposite for the electrochemical immunoassay of human IgG at magnetic beads. The high decoration of CdS on MSN and the stripping voltammetric analysis of Cd(II) ions render the method high sensitivity.

Highly Sensitive and Quality Self-Testable Electrochemiluminescence Assay of DNA Methyltransferase Activity Using Multifunctional Sandwich-Assembled Carbon Nitride Nanosheets

DNA methylation catalyzed by methylase plays a key role in many biological activities. However, developing a highly sensitive, simple, and reliable way for evaluation of DNA methyltransferase (MTase) activity is still a challenge. Here, we report a sandwich-assembled electrochemiluminescence (ECL) biosensor using multifunctional carbon nitride nanosheets (CNNS) to evaluate the Dam MTase activity. The CNNS could not only be used as an excellent substrate to conjugate a large amount of hairpin probe DNA to improve the sensitivity but also be utilized as an internal reliability checker and an analyte reporter in the bottom and top layers of the biosensor, respectively. Such a unique sandwich configuration of CNNS well coupled the advantages of ECL luminophor that were generally assembled in the bottom or top layer in a conventional manner. As a result, the biosensor exhibited an ultralow detection limit down to 0.043 U/mL and a linear range between 0.05 and 80 U/mL, superior to the MTase activity assay in most previous reports. We highlighted the great potential of emerging CNNS luminophor in developing highly sensitive and smart quality self-testable ECL sensing systems using a sandwiched configuration for early disease diagnosis, treatment, and management.