Proximity Hybridization-Regulated Immunoassay for Cell Surface Protein and Protein-Overexpressing Cancer Cells via Electrochemiluminescence

A quantitative method for aquaporin-1 protein using magnetic preconcentration and probe-based immunoassay coupling to inductively coupled plasma mass spectrometry in urine analysis

BACKGROUND

Aquaporin-1 (AQP1) protein plays a crucial role in intracellular and extracellular water homeostasis and fluid transport in organs and tissues associated with diverse life activities and is extremely abundant in the kidney. Accurate detection of AQP1 in urine can be applied as screening of early-stage disease. Application of magnetic preconcentration and probe-based signal amplification strategy coupling to inductively coupled plasma mass spectrometry (ICP-MS) is a more accurate, sensitive and specific detection method for AQP1 in complex biological samples compared to conventional methods.

RESULTS

We described an element-labelling strategy based on magnetic preconcentration and probe-based immunoassay coupling to ICP-MS detection. The magnetic beads (MBs) modified with epoxy groups were capable of enriching AQP1 proteins and separating them from complex matrices. The probe constructed by conjugating anti-AQP1 antibody molecules on the surface of gold nanoparticles could specifically recognize AQP1 proteins attached on MBs and be analyzed by ICP-MS. The concentration of AQP1 protein could be precisely quantified and amplified by 14,000 times through the corresponding signal of Au atoms. This assay for AQP1 protein quantification achieved a detection limit down to 0.023 ng mL-1, a broad linear calibration curve between 0.3 ng mL-1 and 30 ng mL-1, as well as outstanding specificity.

SIGNIFICANCE

The proposed method was successfully applied to detect AQP1 protein in human urine samples, showing the potential for its applications concerning accurate AQP1 quantification. It can also screen a wide range of proteins provided the antibodies specific to these target proteins are available.

A homogeneous electrochemiluminescence immunoassay platform based on carbon quantum dots and magnetic beads enrichment for detection of thyroglobulin in serum

Considering the high probability of recurrence or metastasis after thyroidectomy, it is meaningful to develop a rapid, sensitive and specific method for monitoring thyrophyma-related biomarkers. In this study, a homogeneous electrochemiluminescence immunoassay (HO-ECLIA) coupled with magnetic beads (MBs)-based enrichment tactic was established for the determination of thyrophyma-related thyroglobulin (Tg). Importantly, owing to the abundant surface groups and good biocompatibility of carbon quantum dots (CQDs), the incorporation of CQDs onto the Tg antigen surface was achieved, resulting in the formation of Tg-encapsulated CQDs (CQDs-Tg), which served not only as an ECL probe but as a biorecognition element. Under optimal experimental conditions, the proposed platform demonstrated a wide linear range from 0.01 to 100 ng·mL-1 with a detection limit of 6.9 pg·mL-1 (S/N = 3), and performed well in real serum sample analysis against interference. Collectively, the proposed platform exhibited the rapid response, satisfactory sensitivity and specificity toward Tg in complex serum milieu, and held a considerable potential for clinical prognosis monitoring of thyrophyma.

Laser-Induced Electrochemical Biosensor Modified with Graphene-Based Ink for Label-Free Detection of Alpha-Fetoprotein and 17β-Estradiol

In this research, a novel electrochemical biosensor is proposed based on inducing graphene formation on polyimide substrate via laser engraving. Graphene polyaniline (G-PANI) conductive ink was synthesized by planetary mixing and applied to the working zone of the developed sensor to effectively enhance the electrical signals. The laser-induced graphene (LIG) sensor was used to detect alpha-fetoprotein (AFP) and 17β-Estradiol (E2) in the phosphate buffer saline (PBS) buffer and human serum. The electrochemical performance of the biosensor in determining these biomarkers was investigated by differential pulse voltammetry (DPV) and chronoamperometry (CA). In a buffer environment, alpha-fetoprotein (AFP) and 17β-Estradiol detection range were 4-400 ng/mL and 20-400 pg/mL respectively. The experimental results showed a limit of detection (LOD) of 1.15 ng/mL and 0.96 pg/mL for AFP and estrogen, respectively, with an excellent linear range (R2 = 0.98 and 0.99). In addition, the designed sensor was able to detect these two types of biomarkers in human serum successfully. The proposed sensor exhibited excellent reproducibility, repeatability, and good stability (relative standard deviation, RSD = 0.96%, 1.12%, 2.92%, respectively). The electrochemical biosensor proposed herein is easy to prepare and can be successfully used for low-cost, rapid detection of AFP and E2. This approach provides a promising platform for clinical detection and is advantageous to healthcare applications.

Proximity hybridization induced molecular machine for signal-on electrochemical detection of α-synuclein oligomers

α-synuclein oligomer is a marker of Parkinson's disease. The traditional enzyme-linked immunosorbent assay for α-synuclein oligomer detection is not conducive to large-scale application due to its time-consuming, high cost and poor stability. Recently, DNA-based biosensors have been increasingly used in the detection of disease markers due to their high sensitivity, simplicity and low cost. In this study, based on the DNAzyme-driven DNA bipedal walking method, we developed a signal-on electrochemical sensor for the detection of α-syn oligomers. Bipedal DNA walkers have a larger walking area and faster walking kinetics, providing higher amplification efficiency compared to conventional DNA walkers. The DNA walker is driven via an Mg2+-dependent DNAzyme, and the binding-induced DNA walker will continuously clamp the MB, resulting in the proliferation of Fc confined near the GE surface. The linear range and limit of detection were 1 fg/mL to 10 pg/mL and 0.57 fg/mL, respectively. The proposed signal-on electrochemical sensing strategy is more selective. It will play a significant role in the sensitive and precise electrochemical analysis of other proteins.

Antibody-Protein-Aptamer Electrochemical Biosensor based on Highly Efficient Proximity-Induced DNA Hybridization on Tetrahedral DNA Nanostructure for Sensitive Detection of Insulin-like Growth Factor-1

Herein, an antibody-protein-aptamer electrochemical biosensor was designed by highly efficient proximity-induced DNA hybridization on a tetrahedral DNA nanostructure (TDN) for ultrasensitive detection of human insulin-like growth factor-1 (IGF-1). Impressively, the IGF-1 antibody immobilized on the top vertex of the TDN could effectively capture the target protein with less steric effect, and the ferrocene-labeled signal probe (SP) bound on the bottom vertex of the TDN was close to the electrode surface for generating a strong initial signal. In the presence of target protein IGF-1 and an aptamer strand, an antibody-protein-aptamer sandwich could be formed on the top vertex of TDN, which would trigger proximity-induced DNA hybridization to release the SP on the bottom vertex of TDN; therefore, the signal response would decrease dramatically, enhancing the sensitivity of the biosensor. As a result, the linear range of the proposed biosensor for target IGF-1 was 1 fM to 1 nM with the limit of detection down to 0.47 fM, which was much lower than that of the traditional TDN designs on electrochemical biosensors. Surprisingly, the use of this approach offered an innovative approach for the sensitive detection of biomarkers and illness diagnosis.

Rationally Designed DNA-Based Scaffolds and Switching Probes for Protein Sensing

The detection of a protein analyte and use of this type of information for disease diagnosis and physiological monitoring requires methods with high sensitivity and specificity that have to be also easy to use, rapid and, ideally, single step. In the last 10 years, a number of DNA-based sensing methods and sensors have been developed in order to achieve quantitative readout of protein biomarkers. Inspired by the speed, specificity, and versatility of naturally occurring chemosensors based on structure-switching biomolecules, significant efforts have been done to reproduce these mechanisms into the fabrication of artificial biosensors for protein detection. As an alternative, in scaffold DNA biosensors, different recognition elements (e.g., peptides, proteins, small molecules, and antibodies) can be conjugated to the DNA scaffold with high accuracy and precision in order to specifically interact with the target protein with high affinity and specificity. They have several advantages and potential, especially because the transduction signal can be drastically enhanced. Our aim here is to provide an overview of the best examples of structure switching-based and scaffold DNA sensors, as well as to introduce the reader to the rational design of innovative sensing mechanisms and strategies based on programmable functional DNA systems for protein detection.

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.

Nanozyme-Catalyzed Metasurface Plasmon Sensor-Based Portable Ultrasensitive Optical Quantification Platform for Cancer Biomarker Screening

Developing plasmonic biosensors that are low-cost, portable, and relatively simple to operate remains challenging. Herein, a novel metasurface plasmon-etch immunosensor is described, namely a nanozyme-linked immunosorbent surface plasmon resonance biosensor, for the ultrasensitive and specific detection of cancer biomarkers. Gold-silver composite nano cup array metasurface plasmon resonance chip and artificial nanozyme-labeled antibody are used in two-way sandwich analyte detection. Changes in the biosensor's absorption spectrum are measured before and after chip surface etching, which can be applied to immunoassays without requiring separation or amplification. The device achieved a limit of alpha-fetoprotein (AFP) detection < 21.74 fM, three orders of magnitude lower than that of commercial enzyme-linked immunosorbent assay kits. Additionally, carcinoembryonic antigen (CEA) and carbohydrate antigen 125 (CA125) are used for quantitative detection to verify the universality of the platform. More importantly, the accuracy of the platform is verified using 60 clinical samples; compared with the hospital results, the three biomarkers achieve high sensitivity (CEA: 95.7%; CA125: 90.9%; AFP: 86.7%) and specificity (CEA: 97.3%; CA125: 93.9%; AFP: 97.8%). Due to its rapidity, ease of use, and high throughput, the platform has the potential for high-throughput rapid detection to facilitate cancer screening or early diagnostic testing in biosensing.

Aptamer/proximity hybridization-based label-free and highly sensitive colorimetric detection of methotrexate via polymerization/nicking recycling amplifications

Methotrexate (MTX) is one of the commonly used therapeutic drugs for treating various tumors and autoimmune diseases. However, high dose usage of MTX may cause severe side effects and the monitoring of MTX is therefore critical. By coupling a new MTX aptamer-based proximity hybridization with polymerization/nicking reaction (PNR) recycling amplifications, we develop here a sensitive and label-free colorimetric approach for MTX detection in diluted human serums. The MTX molecules can bind and switch the conformation of aptamers in the DNA duplex probes to initiate subsequent proximity hybridization-induced PNR recycling processes for the yield of a great deal of G-quadruplexes with the assistance of two single-stranded assistant DNA sequences. Hemin subsequently combines with these G-quadruplexes to produce lots of G-quadruplex/hemin horseradish peroxidase (HRP) mimicking DNAzymes, which then catalyze intensified color transition of the substrate solution to exhibit highly magnified UV-Vis absorption for label-free and ultrasensitive detection of MTX at concentration as low as 5.66 nM in the range of 10 nM to 1 μM. High selectivity of the developed method also enables it to monitor low levels of MTX in diluted serum samples, which offers such a method enormous potentials for convenient and highly sensitive detection of other small molecule drugs for various clinical applications.

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.

Electrochemiluminescence nanoemitters for immunoassay of protein biomarkers

The family of electrochemiluminescent luminophores has witnessed quick development since the electrochemiluminescence (ECL) phenomenon of silicon nanoparticles was first reported in 2002. Moreover, these developed ECL nanoemitters have extensively been applied in sensitive detection of protein biomarker by combining with immunological recognition. This review firstly summarized the origin and development of various ECL nanoemitters including inorganic and organic nanomaterials, with an emphasis on metal-organic frameworks (MOFs)-based ECL nanoemitters. Several effective strategies to amplify the ECL response of nanoemitters and improve the sensitivity of immunosensing were discussed. The application of ECL nanoemitters in immunoassay of protein biomarkers for diagnosis of cancers and other diseases, especially lung cancer and heart diseases, was comprehensively presented. The recent development of ECL imaging with the nanoemitters as ECL tags for detection of multiplex protein biomarkers on single cell membrane also attracted attention. Finally, the future opportunities and challenges in the ECL biosensing field were highlighted.

Dual-mode electrochemiluminescence and electrochemical sensor for alpha-fetoprotein detection in human serum based on vertically ordered mesoporous silica films

In this study, we demonstrated the highly sensitive detection of alpha-fetoprotein (AFP) by electrochemiluminescence (ECL) and electrochemistry (EC) based on the gated transport of the bifunctional probe (tris(1,10-phenanthroline) ruthenium (II) chloride, Ru (phen)₃Cl₂) into the nanochannels of vertically ordered mesoporous silica films (VMSFs). Due to the negatively charged surface and ultrasmall pore size, VMSF displays a signal amplification effect on Ru (phen)₃Cl₂ and is suitable for the construction of sensors with excellent sensitivity. With the linkage of (3-glycidyloxypropyl) trimethoxysilane, the anti-AFP antibody could covalently bind to the external surface of VMSF, generating a highly specific recognized sensing interface toward AFP. When AFP is presented, the formed immunocomplex hinders the diffusion of Ru (phen)₃Cl₂ to the underlying electrode surface, resulting in a decreased ECL or EC response. The dual-mode detection of AFP is achieved with a relatively low limit of detection (0.56 fg/ml for ECL and 4.5 pg/ml for EC) and a wide linear range (10 fg/ml∼1 μg/ml for ECL and 10 pg/ml∼1 μg/ml for EC). Moreover, owing to the inherent anti-fouling property of VMSF, satisfactory results in the analysis of human serum were obtained, showing the great potential of the designed strategy in clinical diagnosis.

Electrochemically Controlled Atom Transfer Radical Polymerization for Electrochemical Aptasensing of Tumor Biomarkers

Tumor biomarkers are of great value in the liquid biopsy of malignant tumors. In this work, a simple and cost-friendly electrochemical aptasensor was presented for the highly sensitive and selective detection of glycoprotein tumor biomarkers. The DNA aptamer-modified electrode was used as the sensing interface to specifically capture the target glycoprotein tumor biomarkers, to which the alkyl halide initiators for atom transfer radical polymerization (ATRP) were then attached via the esterification crosslinking between the boronic acid group and the cis-dihydroxyl sites of the conjugated oligosaccharide chains on glycoprotein tumor biomarkers followed by the growth of long-chain polymers through electrochemically controlled ATRP (eATRP) to efficiently recruit the ferrocene detection tags. As there are tens to hundreds of cis-dihydroxyl sites on a glycoprotein tumor biomarker for attaching ATRP initiators while each long-chain polymer can recruit hundreds to thousands of ferrocene detection tags, a significantly high current signal can be generated even in the presence of ultralow-abundance targets. Hence, the eATRP-based electrochemical aptasensor is capable of sensitively and selectively detecting glycoprotein tumor biomarkers. Using alpha-fetoprotein as the model target, the limit of detection was demonstrated to be 0.32 pg/mL. Moreover, the aptasensor has been successfully applied to detect glycoprotein tumor biomarkers in human serum samples. In view of its high sensitivity and selectivity, simple operation, and cost-friendliness, the eATRP-based electrochemical aptasensor shows great promise in the glycoprotein-based liquid biopsy of malignant tumors, even at the early stage of development.

Hemin-graphene oxide-gold nanoflower-assisted enhanced electrochemiluminescence immunosensor for determination of prostate-specific antigen

An ultrasensitive luminol electrochemiluminescence (ECL) immunosensor was constructed for the detection of prostate specific antigen (PSA) using glucose oxidase-decorated hemin-graphene oxide-gold nanoflowers ternary nanocomposites as probes. Graphene oxide was first modified with hemin and then with gold nanoflowers through an in situ growth method, which has significantly boosted the catalytic activity of this graphene oxide-based peroxidase mimetics. The biocatalytical activity of this ECL immunosensor was thoroughly investigated to achieve selective recognition of the analyte molecules (PSA) by specific binding between antigens and antibodies. The limit of detection was calculated to be 0.32 pg mL^-1 with a signal-to-noise ratio of 3. A broad linear range from 7.5 × 10^-4 to 2.5 ng mL^-1 was obtained. Such step-by-step assembled biosensor showed controlled nanostructure and exhibited promising application in analysis of human serum samples with a recovery range of 90.6-111.8% and a RSD range of 3.9-5.5%.

Gold Nanowires Array-Based Closed Bipolar Nanoelectrode System for Electrochemiluminescence Detection of α-Fetoprotein on Cell Surface

Inspired by the promising applications of a closed bipolar electrodes (c-BPEs) system in electrochemiluminescence (ECL) detection of cell adhesion and disease-related biomarkers, here, a gold nanowires array-based c-BPEs system was constructed for cell surface protein detection. Regular and uniform gold nanowires array were prepared by intermittent potentiostatic deposition. Then, two poly(dimethylsiloxane) (PDMS) chips with a hole diameter of 2 mm as a reservoir were placed at both sides of Au nanowires array to construct c-BPEs system. Thionine-functionalized silicon dioxide nanoparticles conjugated to antibody (Ab2-Th@SiO₂) were used as the electrochemical probe, while [Ru(bpy)₃]²⁺-wrapped SiO₂ nanoparticles (Ru(II)@SiO₂) were employed as the ECL signal readout. Taking α-fetoprotein (AFP) as model, the gold nanowires array-based c-BPEs system allowed sensitive detection of AFP at a linear range from 0.002 to 50.0 ng/mL and at least 6 living cells ascribing to the synergetic amplification effect at both sensing and reporting chambers. Besides, the amount of AFP expressed by HepG2 cells was calculated to be 6.71 pg/cell. The presented strategy with high sensitivity provided a promising and universal platform for the detection of other cancer cells and disease-related biomarkers (such as proteins, glycan, miRNA).

Proximity binding induced nucleic acid cascade amplification strategy for ultrasensitive homogeneous detection of PSA

In this work, based on the powerful cycle amplification cascades of proximity hybridization-induced hybridization chain reaction and catalyzed hairpin assembly, we engineered a nonenzymatic and ultrasensitive method which combined the Mg²⁺-DNAzyme recycling signal amplification for the analysis of the human prostate specific antigen. Herein, we adopted PSA-conjugates as triggers in the self-assembly process of two hairpin DNAs (H1, H2) into the products of the CHA which could activate the HCR to induce repeated hybridization. And both ends of each adjacent sequence of the HCR products could form a unit of Mg²⁺-DNAzyme which in presence of cofactor Mg²⁺ could recognize and cyclically cleave the hairpin probes in the solution and thus generate observably enhanced fluorescent signal. Benefit from the nucleic acid circuit amplification strategy, PSA of concentration low to 0.73 pg mL^-1 was detected in this system. This homogeneous sensing method in solution avoid the use of the sophisticated equipment and complex operation, as well as addition of artificial enzyme, thus greatly reducing the constraints and complexity of experimental conditions. Moreover, considering most protein biomarkers in serum don't have their corresponding aptamers, this sensing method provide a general sensing approach for homogeneous sensitive detection of these important protein biomarkers which transfer rough antigen-antibody interactivity to smart signal amplification sensing strategies, thus exhibiting a remarkable prospect in clinical application.

Ultrasensitive electrochemical detection of hepatitis C virus core antigen using terminal deoxynucleotidyl transferase amplification coupled with DNA nanowires

Early diagnosis of hepatitis C virus (HCV) infection is essential to prevent disease from spreading and progression. Herein, a novel electrochemical biosensor was developed for ultrasensitive detection of HCV core antigen (HCVcAg) based on terminal deoxynucleotidyl transferase (TdT) amplification and DNA nanowires (DNW). After sandwich-type antibody-antigen recognition, the antibody-conjugated DNA was pulled to the electrode surface and further extended into a long DNA sequence by robust TdT reaction. Then, large numbers of methylene blue-loaded DNW (MB@DNW) as signal labels are linked to the extended DNA sequence. This results in an amplified electrochemical signal for HCVcAg determination, typically measured at around -0.25 V (Ag/AgCl). Under the optimum conditions, the proposed biosensor achieved a wide linear range for HCVcAg from 0.1 to 312.5 pg/mL with a low limit of detection of 32 fg/mL. The good practicality of the biosensor was demonstrated by recovery experiment (recoveries from 98 to 104% with RSD of 2.5-4.4%) and comparison with enzyme-linked immunosorbent assay (ELISA). Given the highlighted performance, the biosensor is expected to act as a reliable sensing tool for HCVcAg determination in clinics. Schematic representation of the ultrasensitive electrochemical biosensor based on terminal deoxynucleotidyl transferase (TdT) amplification linked with methylene blue-loaded DNA nanowires (MB@DNW), which can be applied to the determination of hepatitis C virus core antigen (HCVcAg) in clinical samples. dTTPs, 2'-deoxythymidine 5'-triphosphate.

Recent advances of functional nucleic acids-based electrochemiluminescent sensing

Electroluminescence (ECL) has been used in extensive applications ranging from bioanalysis to clinical diagnosis owing to its simple device requirement, low background, high sensitivity, and wide dynamic range. Nucleic acid is a significant theme in ECL bioanalysis. The inherent versatile selective molecular recognition of nucleic acids and their programmable self-assembly make it desirable for the robust construction of nanostructures. Benefiting from their unique structures and physiochemical properties, ECL biosensing based on nucleic acids has experienced rapid growth. This review focuses on recent applications of nucleic acids in ECL sensing systems, particularly concerning the employment of nucleic acids as molecular recognition elements, signal amplification units, and sensing interface schemes. In the end, an outlook of nucleic acid-based ECL biosensing will be provided for future developments and directions. We envision that nucleic acids, which act as an essential component for both bioanalysis and clinical diagnosis, will provide a new thinking model and driving force for developing next-generation sensing systems.

A general scattering proximity immunoassay with the formation of dimer of gold nanoparticle

In this work, we structured a colorimetric ultrasensitive detection of carcinoembryonic antigen (CEA) based on a proximity hybridization-induced gold nanoparticles (Au NPs) dimers structure. Under the dark-field microscope, this method takes advantage of the distinctive and strong distance-relative localized surface plasmon resonance (LSPR) of Au NPs and their oriented assembly. DNA served as a medium showing wonderful flexibility to label antibody and Au NPs, and tune interparticle spacing as well. Two capture probes were formed by the integration of DNA labeled antibody (DNA1-Ab1 or DNA2-Ab2) and asymmetrically assembled DNA (DNA 3 or DNA 4)- Au NPs via partly hybridization between DNA sequences. In the presence of antigen, the reaction between target protein and capture probes could trigger the generation of immunocomplex which led to the proximity hybridization of the DNA1 and DNA2, and then change the distance of interparticle to form Au NP dimers and thus showed a different color under dark-field microscope. A limit of detection of 14.25 pg/mL was obtained for the detection of CEA, which indicated a promising sensing method in clinical diagnosis of protein biomarkers.

Proximity hybridization-triggered DNA assembly for label-free surface-enhanced Raman spectroscopic bioanalysis

We have developed a versatile label-free surface-enhanced Raman spectroscopic platform for detecting various biotargets via proximity hybridization-triggered DNA assembly based on the 736 cm^-1 Raman peak of adenine breathing mode. We initially immobilized the first probe to AuNPs and modified the second with poly adenine. Presence of target DNA or protein molecules assembled a sandwich complex that brought the poly adenine close to the AuNPs surface, generating Raman signals, that were proportional to target molecule concentration. These approach exhibits high sensitivity, with a detection limit of 5.4 pM, 47 fM, and 0.51 pg/mL for target DNA, thrombin and CEA, respectively. Owing to a one step proximity dependent complex formation, this technique is simple and can be completed within 40 min, making it a promising candidate for point-of-care testing applications.

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.

Switching the Photoluminescence and Electrochemiluminescence of Liposoluble Porphyrin in Aqueous Phase by Molecular Regulation

By a facile peripheral decoration of 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (ATPP) with inherent aggregation-induced emission (AIE) active tetraphenylethene (TPE), a versatile AIEgenic porphyrin derivative (ATPP-TPE) was obtained, which greatly abolishes the detrimental π-π stacking and thus surmounts the notorious aggregation-caused quenching (ACQ) effect of ATPP in aqueous phase. The photoluminescence of ATPP-TPE is 4.5-fold stronger than ATPP at aggregation state. Moreover, an unequivocal aggregation induced electrochemiluminescence (AIECL) of ATPP-TPE was found to be seriously dependent on its aggregation property in aqueous solution with efficiency of 34 %, which is 6 times higher than pure ATPP. The versatility of this molecular structure modulation strategy along with the ACQ-to-AIE transformation in this work provides direction to guide for applying liposoluble porphyrins in aqueous phase by designs of synthetic porphyrin AIEgens.

Electrochemical Nanoaptasensor for Continuous Monitoring of ATP Fluctuation at Subcellular Level

Monitoring the fluctuation of adenosine 5'-triphosphate (ATP) at the subcellular level is important for the study of cell energy metabolism. Herein, we fabricated an electrochemical nanoaptasensor for continuously monitoring ATP fluctuation at the subcellular level. A gold nanoelectrode with a diameter of 120 nm was fabricated, and ferrocene (Fc)-labeled anti-ATP aptamer was self-assembled onto the nanoelectrode surface to form a nanoaptasensor. In the presence of ATP, the ferrocene-labeled anti-ATP aptamer bound with two ATP units to form an ATP-aptamer conjugation, resulting in the close proximity of Fc to the nanoelectrode surface and then an increase of oxidation current of Fc. ATP can be detected with a detection limit of 26 μM within 2 min. Cell viability assays indicated that the nanoaptasensor was biocompatible with negligible biological effects. By taking advantage of the good biocompatibility of the nanoaptasensor, ATP fluctuation at the subcellular level was monitored under glucose starvation and Ca²⁺ induction. This work demonstrates that the nanoaptasensor is a useful tool for investigating ATP-relevant biological processes via the electrochemical method.

Facile Preparation of WO3-x Dots with Remarkably Low Toxicity and Uncompromised Activity as Co-reactants for Clinical Diagnosis by Electrochemiluminescence

The exceptional nature of WO_3-x dots has inspired widespread interest, but it is still a significant challenge to synthesize high-quality WO_3-x dots without using unstable reactants, expensive equipment, and complex synthetic processes. Herein, the synthesis of ligand-free WO_3-x dots is reported that are highly dispersible and rich in oxygen vacancies by a simple but straightforward exfoliation of bulk WS₂ and a mild follow-up chemical conversion. Surprisingly, the WO_3-x dots emerged as co-reactants for the electrochemiluminescence (ECL) of Ru(bpy)₃ ²⁺ with a comparable ECL efficiency to the well-known Ru(bpy)₃ ²⁺ /tripropylamine (TPrA) system. Moreover, compared to TPrA, whose toxicity remains a critical issue of concern, the WO_3-x dots were ca. 300-fold less toxic. The potency of WO_3-x dots was further explored in the detection of circulating tumor cells (CTCs) with the most competitive limit of detection so far.

Fluorescent detection of Cu (II) ions based on DNAzymatic cascaded cyclic amplification

A fluorescent biosensor based on the cascaded cyclic amplification-lighted copper nanoparticles has been developed, optimized, and validated. In the double-modular cascaded cyclic amplification, a DNAzymatic cyclic amplification unit transforms metal ion signal to specific DNA sequences, and a linear/exponential integrated amplification unit converts as-prepared DNA codes to identical thymine (T)-rich DNA templates. T-rich scaffolds can induce the generation of red fluorescent copper nanoparticles, with fluorescence emission at 625 nm upon the excitation at 340 nm, as signal vehicles for quantitative detection of metal ions. Copper ions, selected as the model target, could be detected in a wide linear range from 10 to 10⁴ nM depending on the increased fluorescent intensity, and the detection limit is 5.6 ± 0.52 nM (n = 3) within 40 min, which is 4 orders of magnitude lower than the limits set in drinking water. In the detection of Cu²⁺ in real tap and lake water, the results between inductively coupled plasma mass spectrometry (ICP-MS) and our proposed biosensor were consistent, illustrating the practicability of the fabricated method. In summary, the established fluorescent biosensor compensates the deficiency of immunoassays failing to analyze metal ions, broadens ranges of biomarkers responding to cleaved DNAzymes, provides an open platform sensing different metal ions, and meets the increasing need for the ultrasensitive detection in the field of food safety, environmental monitoring, and medical diagnosis.

Electrochemiluminescence Imaging for the Morphological and Quantitative Analysis of Living Cells under External Stimulation

In this work, a simple electrochemiluminescence (ECL) imaging method based on the cell shield of the ECL emission was developed for the morphological and quantitative analysis of living cells under external stimulation. ECL images of MCF-7 cells cultured on or captured at the glassy carbon electrode (GCE) surface in a solution of tris(2,2'-bipyridyl)ruthenium(II)-tri-n-propylamine were recorded. Important morphological characteristics of living cells, including cell shape, cell area, average cell boundary, and junction distance between two adjacent cells, were directly obtained using the developed negative ECL imaging method. The ECL images revealed gradual morphological changes in cells on the GCE surface. During the course of H₂O₂ stimulation of cells on the GCE surface, cells shrunk, rounded up, disengaged from surrounding cells, and finally detached from the electrode surface. During the course of electrical stimulation (0.8 V), the cells on the GCE surface exhibited aggregation as demonstrated by increases in the average cell boundary and decreases in the junction distance between two adjacent cells. Additionally, a quantitative method for the sensitive determination of MCF-7 cells with a limit of detection of 29 cells/mL was developed using the negative ECL imaging strategy. This work demonstrates that the proposed negative ECL imaging strategy is a promising approach to assess important morphological characteristics of living cells during the course of external stimulation and to obtain quantitative information on cell concentrations in solution.

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.

Electrogenerated Chemiluminescence Biosensing

This Feature simply introduces the history and mechanism of classical electrogenerated chemiluminescence (ECL) systems for the detection of biomolecules, highlights new advances and emerging fields of the ECL biosensing with recent illustrative examples, and presents the challenges and perspectives of ECL biosensing.

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.

Controlled assembly of AIEgens based on a super-quadruplex scaffold for detection of plasma membrane proteins

Quantification of plasma membrane proteins (PMPs) is crucial for understanding the fundamentals of cellular signaling systems and their related diseases. In this work, a super-quadruplex scaffold was designed to regulate assembly of oligonucleotide-grafted AIEgens for detection of PMPs. The nonfluorescence oligonucleotide-grafted AIEgen (Oligo-AIEgen) was firstly synthesized by attaching the AIEgen to 3'-terminus of the oligonucleotide through click chemistry. Meanwhile, the tetramolecular hairpin-conjugated super-quadruplex (THP-G4) as cleavage element and signal enhancement scaffold composited of three elements: a substrate sequence of DNAzyme in the loop region, partial hybridization region in the stem, and six guanine nucleotides to form G-quadruplex. Once the DNAzyme was anchored on the specific PMPs through aptamer-protein recognition, the substrate sequence on the loop of THP-G4 was cleaved by DNAzyme with the aid of cofactor Mn^II, resulting in the conformation switch of THP-G4 to the activated G-quadruplex scaffold. The latter could assemble Oligo-AIEgens to generate aggregation-induced emission (AIE) enhancement, resulting in a simple and sensitive strategy for detection of membrane proteins. Moreover, the DNAzyme continuously cut the next THP-G4 to achieve recycling amplification. Under the optimized conditions, this AIE-based strategy exhibited good linear relationship with the logarithm of MUC1 concentration from 0.01 to 10 μg mL^-1 with the limit of detection down to 4.3 ng mL^-1. The G4-assembled AIEgens provides a universal platform for detecting various biomolecules and a proof-of concept for AIE biosensing.

Electrochemiluminescence-Microscopy for microRNA Imaging in Single Cancer Cell Combined with Chemotherapy-Photothermal Therapy

In this work, a new technology using ECL as a microscopy to parallel image miRNA-21 in single cancer cell was built. Phorbol-12-myristate-13-acetate (PMA) loaded gold nanocages (Au NCs) was closed with DNA gate which could be recognized and opened by miRNA-21 in HeLa cell. PMA was then released and further induced HeLa cells to produce reactive oxygen species (ROS; including O^2-•, •OH and H₂O₂ etc.). With H₂O₂ as coreactant and luminol as ECL active material, ECL imaging of intracellular miRNA-21 in single HeLa cell was obtained by EMCCD. Moreover, ROS therapy and photothermal therapy (PPT) of Au NCs@PMA probe were also motivated by in situ miRNA-21 marker instead of the external source. The combined therapy leads to dramatically enhanced ability for cancer cell killing. Au NCs@PMA probe alone could not only achieve a high sensitivity and high resolution ECL-microscopy for imaging of intracellular miRNA-21 for the first time, but also realize the integrated diagnosis like ROS induced tumor damage and photothermal-induced intelligent therapy. This multifunctional platform is believed to be capable of playing an important role in future oncotherapy by the synergistic effects between chemotherapy and photothermal therapy.

Signal-Off Electrogenerated Chemiluminescence Biosensing Platform Based on the Quenching Effect between Ferrocene and Ru(bpy)32+-Functionalized Metal-Organic Frameworks for the Detection of Methylated RNA

N⁶-methyladenine (m⁶A), one of the most common chemical modifications of eukaryotic RNA, participates in many important biological processes. An effective strategy for the quantitative determination of m⁶A is of great significance. Herein, we used methylated microRNA-21 (miRNA21) as the model target to propose a simple and sensitive electrogenerated chemiluminescence (ECL) biosensing platform to detect a specific m⁶A RNA sequence. This strategy is based on the fact that the anti-m⁶A-antibody can specifically recognize and bind to the m⁶A site in the RNA sequence, resulting in a quenching effect between Ru(bpy)₃²⁺-functionalized metal-organic frameworks and ferrocene. Luminescent metal-organic frameworks (Ru@MOFs) not only act as ECL indicators but also serve as nanoreactors for the relative ECL reactions owing to their porous or multichannel structure, which overcomes the fact that Ru(bpy)₃²⁺ is easily released when used for aqueous-phase detection, thus enhancing the ECL efficiency. Moreover, the ECL method has fewer modification steps and uses only one antibody to recognize the target RNA sequence, which simplifies the operation process and reduces the detection time, presenting a wide linear range (0.001-10 nM) for m⁶A RNA determination with a low detection limit (0.0003 nM). Additionally, this developed strategy was validated for m⁶A RNA detection in human serum. Thus, the ECL biosensing method provides a new method for m⁶A RNA determination that is simple, highly specific, and sensitive.