Target-Induced 3D DNA Network Structure as a Novel Signal Amplifier for Ultrasensitive Electrochemiluminescence Detection of MicroRNAs

Multispatially Localized DNA Walker Coupling Covalent Organic Framework for Dual-Mode Detection of Nucleocapsid Protein Using the Walking-Recycling-Conversion Strategy

DNA walkers have emerged as a powerful tool in bioanalysis; however, many existing approaches are still restricted by low reaction kinetics and inaccurate single-mode detection. Herein, a fluorescence (FL) and electrochemical (EC) dual-mode biosensor was proposed based on a multispatially localized DNA walker (m-DNA walker) coupling covalent organic framework (COF) using the walking-recycling-conversion strategy. Specifically, the functionalized COF not only served as a three-dimensional nanocarrier but also acted as an effective quencher of the walking tracks. In the presence of the target, the activated m-DNA walker moved fast along the numerous quenching tracks, leading to the cleavage of Cy3-H1 and the recovery of the FL signal. To further improve the detection sensitivity, the Cy3-H1 fragments' recycling process was implemented with the generation of a large amount of S1 and S2, which caused the assembly of DNA-Fe3+-polydopamine network amplifiers on the electrode. The rapid electrochemical conversion was introduced to convert DNA-Fe3+-polydopamine into electroactive Prussian Blue, providing a significant EC signal output. Using nucleocapsid protein (N-protein) as the model target, the designed biosensing platform produced a FL/EC dual-mode readout with the detection limits of 65.0 fg/mL for FL mode and 2.3 fg/mL for EC mode, which could eliminate the interference from different reactive pathways and improve the detection accuracy, holding potential application in early disease diagnosis and treatment.

Label-free fluorescent biosensor based on AuNPs etching releasing signal for miRNA-155 detection

Quantitative microRNA (miRNA) detection is crucial for early breast cancer diagnosis and prognosis. However, quick and stable fluorescence sensing for miRNA identification is still challenging. This work developed a novel label-free detection method based on AuNPs etching for quantitatively detecting miRNA-155. A layer of AuNPs was grown on the surface of mesoporous silica nanoparticles (MSN) loaded with Rhodamine 6G (R6G) using seed-mediated growth, followed by probe attachment. In the presence of miRNA-155, the MSN@R6G@AuNP surface loses the protection of the attached probe, rendering AuNPs susceptible to etching by hydrochloric acid. This results in a significant fluorescent signal being released in the free space. The encapsulation with AuNPs effectively reduces signal leakage, while the rapid etching process shortens detection time. This strategy enables sensitive and fast detection with a detection range of 100 fM to 100 nM, a detection limit of 2.18 fM, and a detection time of 30 min. The recovery rate in normal human serum ranges from 99.02 % to 106.34 %. This work presents a simple biosensing strategy with significant potential for application in tumor diagnosis.

Recent Advances in Electrochemiluminescence Biosensors for MicroRNA Detection

Electrochemiluminescence (ECL) as an analytical technology with a perfect combination of electrochemistry and spectroscopy has received considerable attention in bioanalysis due to its high sensitivity and broad dynamic range. Given the selectivity of bio-recognition elements and the high sensitivity of the ECL analysis technique, ECL biosensors are powerful platforms for the sensitive detection of biomarkers, achieving the accurate prognosis and diagnosis of diseases. MicroRNAs (miRNAs) are crucial biomarkers involved in a variety of physiological and pathological processes, whose aberrant expression is often related to serious diseases, especially cancers. ECL biosensors can fulfill the highly sensitive and selective requirements for accurate miRNA detection, prompting this review. The ECL mechanisms are initially introduced and subsequently categorize the ECL biosensors for miRNA detection in terms of the quenching agents. Furthermore, the work highlights the signal amplification strategies for enhancing ECL signal to improve the sensitivity of miRNA detection and finally concludes by looking at the challenges and opportunities in ECL biosensors for miRNA detection.

Heavy Metal-Induced Assembly of DNA Network Biosensor from Double-Loop Hairpin Probes for Ultrasensitive Detection of UO22+ in Water and Soil Samples

The uranyl ion (UO22+) is the most stable form of uranium, which exhibits high toxicity and bioavailability posing a severe risk to human health. The construction of ultrasensitive, reliable, and robust sensing techniques for UO22+ detection in water and soil samples remains a challenge. Herein, a DNA network biosensor was fabricated for UO22+ detection using DNAzyme as the heavy metal recognition element and double-loop hairpin probes as DNA assembly materials. UO22+-activated specific cleavage of the DNAzyme will liberate the triggered DNA fragment, which can be utilized to launch a double-loop hairpin probe assembly among Hab, Hbc, and Hca. Through multiple cyclic cross-hybridization reactions, hexagonal DNA duplex nanostructures (n[Hab•Hbc•Hca]) were formed. This DNA network sensing system generates a high fluorescence response for UO22+ monitoring. The biosensor is ultrasensitive, with a detection limit of 2 pM. This sensing system also displays an excellent selectivity and robustness, enabling the DNA network biosensor to work even in complex water and soil samples with excellent accuracy and reliability. With the advantages of enzyme-free operation, outstanding specificity, and high sensitivity, our proposed DNA network biosensor provides a reliable, simple, and robust method for trace levels of UO22+ detection in environmental samples.

Combination of Ternary Electrochemiluminescence System of BNQDs/AgMOG-K2S2O8 and Electrochemiluminescence Resonance Energy Transfer Strategy for Ultrasensitive Immunoassay of Amyloid-β Protein

In this work, based on boron nitride quantum dots (BNQDs) as energy donors and MnO2@MWCNTs-COOH as energy receptors, we designed an efficient electrochemiluminescence resonance energy transfer (ECL-RET) immunosensor for the detection of amyloid-β (Aβ42) protein, a biomarker of Alzheimer's disease (AD). First, the signal amplification of a ternary ECL system composed of BNQDs (as the ECL emitter), K2S2O8 (as the coreactant), and silver metal-organic gels (AgMOG, as the coreaction accelerator) was realized, and PDDA as stabilizer was added, a strong and stable initial ECL signal was obtained. AgMOG could not only support a large amount of BNQDs and Aβ42 capture antibody (Ab1) through Ag-N bond but also exhibit excellent ECL catalytic performance and enhance the luminescent intensity of BNQDs@PDDA-K2S2O8 system. In addition, due to the broad absorption spectrum of MnO2@MWCNTs-COOH and the extensive overlap with the ECL emission spectrum of BNQDs, the quenching probe Ab2-MnO2@MWCNTs-COOH could be introduced into the ternary system through a sandwich immune response. On this basis, the signal on-off ECL immunosensor was constructed to achieve the ultrasensitive detection of Aβ42 through signal transformation. Under the optimal conditions, the prepared ECL biosensor manifested a wide linear range (10 fg/mL-100 ng/mL) with a detection limit of 2.89 fg/mL and showed excellent stability, selectivity, and repeatability, which provided an effective strategy for the ultrasensitive detection of biomarkers in clinical analysis.

Self-enhanced Electrochemiluminescence Luminophore Based on Pd Nanocluster-Anchored Metal Organic Frameworks via Ion Annihilation for Sensitive Cell Assay of Human Lung Cancer

Electrochemiluminescence (ECL) has attracted significant interest in the analysis of cancer cells, where the ruthenium(II)-based emitter demonstrates urgency and feasibility to improve the ECL efficiency. In this work, the self-enhanced ECL luminophore was prepared by covalent anchoring of Pd nanoclusters on aminated metal organic frameworks (Pd NCs@MOFs), followed by linkage with bis(2,2'-bipyridine)-5-amino-1,10-phenanthroline ruthenium(II) (RuP). The resultant luminophore showed 214-fold self-magnification in the ECL efficiency over RuP alone, combined by promoting the interfacial photoelectron transfer. The enhanced mechanism through ion annihilation was critically proved by controlled experiments and density functional theory (DFT) calculations. Based on the above, a "signal off" ECL biosensor was built by assembly of tyrosine kinase 7 (PTK-7) aptamer (Apt) on the established sensing platform for analysis of human lung cancer cells (A549). The built sensor showed a lower detection limit of 8 cells mL-1, achieving the single-cell detection. This work reported a self-enhanced strategy for synthesis of advanced ECL emitters, combined by exploring the ECL biosensing devices in the single-cell analysis of cancers.

Sensitive detection of microRNA by dynamic light scattering based on DNAzyme walker-mediated AuNPs self-assembly

As an important biomarker, microRNAs (miRNAs) play an important role in gene expression, and their detection has attracted increasing attention. In this study, a DNAzyme walker that could provide power to perform autonomous movement was designed. Based on the continuous mechanical motion characteristics of DNAzyme walker, a miRNA detection strategy for the self-assembly of AuNPs induced by the hairpin probe-guided DNAzyme walker "enzyme cleavage and walk" was established. In this strategy, DNAzyme walker continuously cleaved and walked on the hairpin probe on the surface of AuNPs to induce the continuous shedding of some segments of the hairpin probe. The remaining hairpin sequences on the surface of the AuNP pair with each other, causing the nanoparticles to self-assemble. This strategy uses the autonomous movement mechanism of DNAzyme walker to improve reaction efficiency and avoid the problem of using expensive and easily degradable proteases. Secondly, using dynamic light scattering technology as the signal output system, ultra-sensitive detection with a detection limit of 3.6 fM is achieved. In addition, this strategy has been successfully used to analyze target miRNAs in cancer cell samples.

An electrochemical biosensor based on network-like DNA nanoprobes for detection of mesenchymal circulating tumor cells

The identification and detection of mesenchymal circulating tumor cells （mCTCs） is important for early warning of tumor metastasis. The majority of conventional detection methods for CTCs rely on the recognition of epithelial biomarkers, which is technically challenging for detecting CTCs with epithelial-mesenchymal transition (EMT)-induced phenotypic alteration. In this work, we have constructed a label-free biosensor for sensitive electrochemical assay of mCTCs. In our design, the capture probe can recognize the vimentin overexpressed on the surface of mCTCs with high specificity. Meantime, the network-like DNA nanoprobes with multiple G-quadruplex/hemin complexes and multiple cholesterol molecules can be grafted to the cell surface based on the high affinity between cholesterol molecules and cell membrane. Owing to the mimic horseradish peroxidase of G-quadruplex/hemin complexes, strong electrochemical responses will be obtained for sensitive quantification of mCTCs with a detection limit of 8 cell mL-1. Moreover, the biosensor can effectively overcome the interference of vimentin negative cells or secretory vimentin, and realize the recovery tests in whole blood with high accuracy, thereby may further promoting the diagnosis and personalized treatment of cancer in clinic.

Entropy-driven DNA circuit with two-stage strand displacement for elegant and robust detection of miRNA let-7a

MicroRNAs (miRNAs) research in cancer diagnosis is expanding, on account of miRNAs were demonstrated to be key indicator of gene expression and hopeful candidates for biomarkers. In this study, a stable miRNA-let-7a fluorescent biosensor was successfully designed based on an exonuclease Ⅲ-assisted two-stage strand displacement reaction (SDR). First, an entropy-driven SDR containing a three-chain structure of the substrate is used in our designed biosensor, leading to reduce the reversibility of the target recycling process in each step. The target acts on the first stage to start the entropy-driven SDR, which generates the trigger used to stimulate the exonuclease Ⅲ-assisted SDR in the second stage. At the same time, we design a SDR one-step amplification strategy as a comparison. Expectly, this developed two-stage strand displacement system has a low detection limit of 25.0 pM as well as a broad detection range of 4 orders of magnitude, making it more sensitive than the SDR one-step sensor, whose detection limit is 0.8 nM. In addition, this sensor has high specificity across members of the miRNA family. Therefore, we can take advantage of this biosensor to promote miRNA research in cancer diagnosis sensing systems.

Cooperative Amplification of Au@FeCo as Mimetic Catalytic Nanozymes and Bicycled Hairpin Assembly for Ultrasensitive Electrochemical Biosensing

Exploring the cooperative amplification of peroxidase-like metal nanocomposites and cycled hairpin assembly is intriguing for sensitive bioanalysis. Herein, we report the first design of a unique electrochemical biosensor based on mimicking Au@FeCo nanozymes and bicycled hairpin assembly (BHA) for synergistic signal amplification. By loading the enzyme-like FeCo alloy in Au nanoparticles (AuNPs), the as-synthesized Au@FeCo hybrids display great improvement of electronic conductivity and active surface area and excellent mimic catalase activity to H₂O₂ decomposition into ^•OH radicals. The immobilization of Au@FeCo in an electrode sensing interface is stabilized via the resulting electrodeposition in HAuCl₄ while efficiently accelerating the electron transfer of electroactive ferrocene (Fc). Upon the immobilization of a helping hairpin (HH) via Au-S bonds, a specific DNA trigger (T*) is introduced to activate BHA operation through competitive strand displacement reactions among recognizing hairpin (RH), signaling hairpin (SH), and HH. T* and RH are rationally released to catalyze two cycles, in which the transient depletion of dsDNA intermediates rapidly drives the progressive hairpin assemblies to output more products SH·HH. Thus, the efficient amplification of Au@FeCo mimic catalase activity combined with BHA leads to a significantly increased current signal of Fc dependent on miRNA-21 analogous to T*, thereby directing the creation of a highly sensitive electrochemical biosensor having applicable potential in actual samples.

Gold Nanoparticles as a Biosensor for Cancer Biomarker Determination

Molecular biology applications based on gold nanotechnology have revolutionary impacts, especially in diagnosing and treating molecular and cellular levels. The combination of plasmonic resonance, biochemistry, and optoelectronic engineering has increased the detection of molecules and the possibility of atoms. These advantages have brought medical research to the cellular level for application potential. Many research groups are working towards this. The superior analytical properties of gold nanoparticles can not only be used as an effective drug screening instrument for gene sequencing in new drug development but also as an essential tool for detecting physiological functions, such as blood glucose, antigen-antibody analysis, etc. The review introduces the principles of biomedical sensing systems, the principles of nanomaterial analysis applied to biomedicine at home and abroad, and the chemical surface modification of various gold nanoparticles.

Highly Efficient Aggregation-Induced Electrochemiluminescence of Al(III)-Cbatpy Metal-Organic Gels Obtained by Ultrarapid Self-Assembly for a Biosensing Application

Aggregation-induced electrochemiluminescence (AIECL) has attracted extensive interest due to the significant increase in ECL response by restricting free intramolecular rotation and torsion, but traditional AIECL emitters suffer from limited ECL efficiency, high cost, and complex synthetic steps, dramatically limiting their applications. Herein, novel Al(III)-Cbatpy metal-organic gels (Al(III)-Cbatpy-MOGs) with nanofiber morphology and ultrarapid coordination of Al³⁺ and 4'-carboxylic acid-2,2':6',2″-terpyridine (Cbatpy) are developed, which demonstrates an excellent AIECL enhancement behavior far beyond that reported in ECL supramolecular gels. In view of the strong affinity of N and O atoms in Cbatpy toward Al³⁺, Al(III)-Cbatpy-MOGs with high viscosity and stability can be assembled in one step within about 15 s, easily conquering the main predicaments of current AIECL emitters: complicated synthesis steps and poor film formation. Impressively, the ECL efficiency of Al(III)-Cbatpy-MOGs with superemission is about 20 times higher than that of individual Cbatpy molecules, which is attributed to the aggregation of the organic ligand Cbatpy restricting intramolecular rotation and torsion to reduce nonradiative relaxation. Furthermore, compared with traditional metal complexes, Al(III)-Cbatpy-MOGs show the benefits of remarkable biocompatibility and low cost without the involvement of any organic solvents, noble metals, and rare metals. As proof, a "signal-off" sensing platform based on an Al(III)-Cbatpy-MOGs/S₂O₈^2- system was constructed for the sensitive detection of dopamine (DA) with a low detection limit of 0.34 nM. This strategy provides a novel method to prepare cheap metal-organic gels as a highly efficient AIECL emitter, which is promising as a luminescent molecular device and biosensor for clinical diagnostic applications.

Enzyme-free nucleic acid dual-amplification strategy combined with mimic enzyme catalytic precipitation reaction for the photoelectrochemical detection of microRNA-21

A novel photoelectrochemical (PEC) biosensor based on an enzyme-free nucleic acid dual-amplification strategy combined with a mimic enzyme to catalyze the deposition of a quencher is reported for the ultrasensitive detection of miRNA-21. A limited amount of target miRNA-21 can trigger the formation of long DNA duplexes on the electrode, owing to the synergistic effect of the enzyme-free nucleic acid dual-amplification strategy of entropy-driven strand displacement reaction (ESDR) amplification and hybridization chain reaction (HCR) amplification. The embedded manganese porphyrin (MnPP) in the long DNA duplexes acts as a horseradish peroxidase (HRP)-mimicking enzyme to catalyze the transformation of benzo-4-chlorohexadienone on the electrode surface, resulting in a significant reduction in photocurrent intensity. As a photosensitive material, BiOCl-BiOI is used as a tag to provide strong initial PEC signals. Based on the cascade integration of the enzyme-free nucleic acid dual-amplification strategy and the mimic enzyme-catalyzed precipitation reaction, the current PEC biosensor exhibits outstanding performance for miRNA-21 detection with an ultralow detection limit (33 aM) and a wide quantification range (from 100 aM to 1 nM). This work provides a new avenue toward the ultrasensitive detection of miRNAs, and is expected to be used for clinical and biochemical samples. A unique PEC biosensor with the BiOCl-BiOI composite, as the photosensitive material, has been developed for ultrasensitive miRNA-21 determination based on the combination of an enzyme-free nucleic acid dual-amplification strategy and mimic enzyme catalytic precipitation reaction.

Reversible Ratiometric Electrochemiluminescence Biosensor Based on DNAzyme Regulated Resonance Energy Transfer for Myocardial miRNA Detection

Myocardial miRNAs in peripheral blood are closely related to the pathogenic process of myocardial infarction. Rapid identification and accurate quantification of myocardial miRNAs are of great significance to clinical interventions for treating cardiovascular lesions. Therefore, a ratiometric electrochemiluminescence (ECL) biosensor integrating DNAzyme with a resonance energy transfer (RET) system was designed to detect myocardial miRNA. The dual-signal system was composed of rA marked substrate strand functionalized CdTe quantum dots (QDs) as reductive-oxidative (R-O) emitters and Cy5-labeled strand-functionalized Ru(bpy)₃²⁺-filled silica nanoparticles (RuSi NPs) as oxidative-reductive (O-R) emitters. In the presence of target miRNA, DNAzyme was activated to cut substrate strands on the CdTe QDs and release triggers for opening hairpin probes. Then, the Cy5 molecule-labeled hairpin DNA on the RuSi NPs was opened to introduce Cy5 molecules and RuSi NPs into the system. The R-O ECL was quenched by ECL-RET between CdTe QDs and Cy5 molecules and the O-R ECL was introduced by the RuSi NPs. In this way, based on the simultaneous changing of the ECL signal, the dual-potential dynamic signal ratiometric ECL sensing platform was developed. By measuring the ratio of O-R ECL signal to R-O ECL signal, the concentration of miRNA-499 was accurately quantified in the range of 10 fM to 10 nM, and the detection limit was as low as 2.44 fM (S/N = 3). This DNAzyme guided dual-potential ratiometric ECL method provides a sensitive and reliable method for myocardial miRNA detection, and it has great potential in clinical diagnosis and treatment.

Two-Dimensional Quantum Dot-Based Electrochemical Biosensors

Two-dimensional quantum dots (2D-QDs) derived from two-dimensional sheets have received increasing interest owing to their unique properties, such as large specific surface areas, abundant active sites, good aqueous dispersibility, excellent electrical property, easy functionalization, and so on. A variety of 2D-QDs have been developed based on different materials including graphene, black phosphorus, nitrides, transition metal dichalcogenides, transition metal oxides, and MXenes. These 2D-QDs share some common features due to the quantum confinement effects and they also possess unique properties owing to their structural differences. In this review, we discuss the categories, properties, and synthetic routes of these 2D-QDs and emphasize their applications in electrochemical biosensors. We deeply hope that this review not only stimulates more interest in 2D-QDs, but also promotes further development and applications of 2D-QDs in various research fields.

Conductive Covalent Organic Frameworks with Conductivity- and Pre-Reduction-Enhanced Electrochemiluminescence for Ultrasensitive Biosensor Construction

Covalent organic frameworks (COFs) have attracted widespread attention in the electrochemiluminescence (ECL) field owing to their high load capacity of ECL luminophores and porous structures, but their ECL performance is still limited by the intrinsic poor conductivity (generally <10^-8 S m^-1). To address this shortcoming, we used 2,3,6,7,10,11-hexaaminotriphenylene (HATP) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) to synthesize a conductive COF (HHTP-HATP-COF, conductivity = 3.11 × 10^-4 S m^-1). Compared with HATP, HHTP, and low-conductive HHTP-DABZ-COF, HHTP-HATP-COF exhibited superior ECL performance, not only because HHTP-HATP-COF possessed massive ECL luminophores but also because its conductive porous framework accelerated charge transport in the whole framework and improved the utilization ratio of ECL luminophores. More interestingly, the ECL intensity of the HHTP-HATP-COF/S₂O₈^2- system was further improved after pre-reduction electrolysis due to the accumulation of HHTP-HATP-COF cation radicals. The experimental results showed that the ECL intensity of the HHTP-HATP-COF/S₂O₈^2- system after pre-reduction was about 1.64-, 3.96-, 6.88-, and 8.09-fold higher than those of HHTP-HATP-COF/S₂O₈^2-, HHTP-DABZ-COF/S₂O₈^2-, HHTP/S₂O₈^2-, and HATP/S₂O₈^2- systems, respectively. Considering the superior ECL property of the HHTP-HATP-COF/S₂O₈^2- system after pre-reduction, it was used as a high-efficient ECL beacon together with an aptamer/protein proximity binding-induced three-dimensional bipedal DNA walker to construct an ultrasensitive biosensor for thrombin detection, which displayed broad linearity (100 aM to 1 nM) with a detection limit of 62.1 aM. Overall, the work offered effective ways to increase ECL performance by the enhancement of conductivity and by the pre-reduction, proposing new ideas to design high-efficiency COF-based ECL materials and endowing conductive COFs with ECL biosensor application for the first time.

Programmable High-Speed and Hyper-Efficiency DNA Signal Magnifier

Herein, a programmable dual-catalyst hairpin assembly (DCHA) for realizing the synchronous recycle of two catalysts is developed, displaying high reaction rate and outstanding conversion efficiency beyond traditional nucleic acid signal amplifications (NASA). Once catalyst I interacts with the catalyst II, the DCHA can be triggered to realize the simultaneous recycle of catalysts I and II to keep the highly concentrated intermediate product duplex I-II instead of the steadily decreased one in typical NASA, which can accomplish in about only 16 min and achieves the outstanding conversion efficiency up to 4.54 × 108 , easily conquering the main predicaments of NASA: time-consuming and low-efficiency. As a proof of the concept, the proposed DCHA as a high-speed and hyper-efficiency DNA signal magnifier is successfully applied in the rapid and ultrasensitive detection of miRNA-21 in cancer cell lysates, which exploits the new generation of universal strategy for the applications in biosensing assay, clinic diagnose, and DNA nanobiotechnology.

Highly sensitive, selective, and rapid detection of miRNA-21 using an RCA/G-quadruplex/QnDESA probing system

In this study we developed a very simple and rapid miRNA 21 detection system using a novel quinolinium diethylamino salicylaldehyde (QnDESA) probe for sensing the 22AG hybrid G-quadruplex with a single-step rolling circle amplification (RCA) reaction. We synthesized a circular DNA padlock template containing a sequence complementary to the 22AG hybrid G-quadruplex, used SplintR ligase to ensure perfect hybridization with miRNA 21, applied this circular DNA and phi-29 DNA polymerase for tandem amplification of the 22AG hybrid G-quadruplex sequence, and then probed the product using QnDESA. This combination of RCA-G-quadruplex and QnDESA allowed the rapid (1 h) and simple one-pot detection of miRNA 21 based on a change in fluorescence. In addition, this system displayed high sensitivity (limit of detection: 1.37 fM) and selectivity. This probing system should also be useful for identifying a diverse range of DNA- and RNA-based biomarkers.

Recent advances in catalytic hairpin assembly signal amplification-based sensing strategies for microRNA detection

Accumulative evidences have indicated that abnormal expression of microRNAs (miRNAs) is closely associated with many health disorders, making them be regarded as potentialbiomarkers for early clinical diagnosis. Therefore, it is extremely necessary to develop a highly sensitive, specific and reliable approach for miRNA analysis. Catalytic hairpin assembly (CHA) signal amplification is an enzyme-free toehold-mediated strand displacement method, exhibiting significant potential in improving the sensitivity of miRNA detection strategies. In this review, we first describe the potential of miRNAs as disease biomarkers and therapeutics, and summarize the latest advances in CHA signal amplification-based sensing strategies for miRNA monitoring. We describe the characteristics and mechanism of CHA signal amplification and classify the CHA-based miRNA sensing strategies into several categories based on the "signal conversion substance", including fluorophores, enzymes, nanomaterials, and nucleotide sequences. Sensing performance, limit of detection, merits and disadvantages of these miRNA sensing strategies are discussed. Moreover, the current challenges and prospects are also presented.

Contrary Logic Pair Library, Parity Generator/Checker and Various Concatenated Logic Circuits Engineered by a Label-Free and Immobilization-Free Electrochemiluminescence Resonance Energy Transfer System

Herein, a label-free and immobilization-free electrochemiluminescence resonance energy transfer (ECL-RET) system based on graphitic carbon nitride nanosheets (GCNNs)/Ru(phen)32+ donor/acceptor pair is developed, in which the ECL-RET is regulated by regulating the diffusivity of Ru(phen)32+ molecules toward the negatively charged GCNNs through logically programmed DNA hybridization reactions. The two optical signals of GCNNs (445 nm) and Ru(phen)32+ (593 nm) show completely opposite changes through the same one-time DNA hybridization reaction. Based on this ECL-RET system, a contrary logic pair (CLP) library, a parity generator/checker system for differentiating the erroneous bits during data transmission, the parity checker to identify the even/odd natural numbers from 0 to 9, and a series of concatenated logic circuits including a six-input logic gate capable of implementing of 64 input combinations for meeting the needs of computational complexity are developed. The ECL-RET-based molecular logic system avoids the time-consuming, costly and inefficient labeling procedures and the laborious processes of immobilization, presenting great potential for building more complicated and advanced logic gates, and providing a refreshing inspiration for the construction of combinatorial logic circuits based on ECL method.

Highly Selective and Sensitive microRNA-210 Assay Based on Dual-Signaling Electrochemical and Photocurrent-Polarity-Switching Strategies

Highly sensitive and selective microRNA (miRNA) assay is of great significance for disease diagnosis and therapy. Herein, a magnetic-assisted electrochemistry (EC)-photoelectrochemistry (PEC) dual-mode biosensing platform was developed for miRNA-210 detection based on dual-signaling EC and photocurrent-polarity-switching PEC strategies. Porous magnetic Fe₃O₄ octahedra with a large surface area were synthesized by calcining Fe-based metal-organic frameworks. Subsequently, the magnetic photoelectric materials (Fe₃O₄@CdS) were developed by the successive ionic layer adsorption and reaction method in Cd²⁺ and S^2- solutions. Then, the self-assembled DNA nanoprisms contained three thiols/hanging arms that could capture miRNA-210 efficiently and were anchored to the Fe₃O₄@CdS octahedra via the Cd-S bond. When miRNA-210 was present, the double-stranded DNA concatemers [the self-assembled duplex helixes based on a pair of methylene blue (MB)-labeled single-stranded DNAs (AP1 and AP2) through the hybridization chain reaction and then intercalated with adriamycin (Dox) into their grooves] were connected with the Fe₃O₄@CdS-DNA nanoprisms. MB and Dox not only acted as the electrochemical probes but also synergistically switched the photocurrent polarity of the Fe₃O₄@CdS octahedra. Thus, miRNA-210 was assayed sensitively and selectively via the proposed EC-PEC dual-mode biosensing platform. Additionally, the abovementioned recognition steps occurred in a homogeneous system, and the effects of the impurities and interferences on the miRNA-210 assay could be easily avoided by magnetic separation due to the good magnetic properties of Fe₃O₄ octahedra. The proposed EC-PEC dual-mode biosensing platform showed a wide range of potential applications in bioanalysis and early diagnosis of disease.

Novel optoelectronic metal organic framework material perylene tetracarboxylate magnesium: preparation and biosensing

The pursuit for improving photoelectrochemical (PEC) performances of organic materials remains an urgent need. Here, we have proposed an envision of the preparation the metal-organic frameworks (MOFs) with arenes to realize high photo-to-current conversion efficiency and excellent PEC performances. Magnesium 3,4,9,10-perylene tetracarboxylic acid metal-organic frameworks (Mg-PTCA MOFs) were synthesized for the first time. The uniformly distributed and regular-shaped Mg-PTCA MOFs showed a much more stable and higher photocurrent than the single PTCA and its derivatives, which confirmed our hypothesis. A regenerated-biosensor was designed for microRNA analysis based on Mg-PTCA MOFs as a novel photoelectric material, target-triggered three-dimensional DNA Scaffold (3D-Sca) as an efficient signal amplifier, and gold nanoclusters (Au NCs) as quencher. The elaborately designed biosensor achieved ultrasensitive detection for miRNA 21 with a dynamic range from 10 aM to 10 pM and a detection limit of 2.8 aM. This biosensor showed good analytical performance in the extracts of different cancer cells, indicating the possibility for early diagnosis, timely staging assessment, and accurate prognostic judgment for diseases. The recommendable performances of Mg-PTCA MOFs highlight the significance of organic MOFs in PEC sensing.

Rapid and highly sensitive hairpin structure-mediated colorimetric detection of miRNA

Herein, we report a novel hairpin structure-mediated diagnostic method for the simple and rapid colorimetric detection of miRNA through the sensing of pyrophosphate. When the hairpin structure of the template DNA (h-Probe) was hybridized with the primer, the DNA primer extension mediated by nPfu special enzyme was blocked. However, this h-Probe was extended using nPfu special enzyme, upon the structural change of the template DNA, from a hairpin to a linear structure, in the presence of the target miRNA. The miRNA-hybridized template DNA sequence was cleaved by a duplex-specific nuclease (DSN), which cleaved the DNA from the RNA-DNA hybrid, thereby allowing the target miRNA to be recycled. Primer extension using nPfu special enzyme produced pyrophosphate when nucleotide triphosphate was incorporated into the DNA; this pyrophosphate was sensed in terms of a color change, from pink to colorless, when using pp Probe, a probe developed previously by our group. This novel system for the colorimetric detection of target miRNA operated with high sensitivity (LOD = 132 aM) and selectivity, with the whole detection process requiring only 30 min. Furthermore, this system could also detect miRNA fluorimetrically with similar sensitivity (LOD = 105 aM), highlighting the dual-sensing properties of pp Probe. This unique, extremely simple, and rapid system for the detection of miRNA through a highly sensitive color change would presumably be useful in applications requiring point-of-care detection.

A DNAzyme-driven random biped DNA walking nanomachine for sensitive detection of uracil-DNA glycosylase activity

Highly specific and ultrasensitive detection of uracil-DNA glycosylase (UDG) activity is of great significance for maintaining genomic integrity and medical research of related diseases. Here, we constructed a random DNA walking nanomachine based on a DNAzyme for UDG activity detection on the AuNP (Au nanoparticle) surface. When UDG is present, the U bases in the Y structure are removed, resulting in AP sites, which will be cleaved by Endo-IV to generate a 3' concave end for Exo-III, causing the locking strand of the DNAzyme to be completely hydrolyzed by the Exo-III and release the walking strand to randomly pair with the substrate strand on the AuNP surface; then, the walking strand exerts its cleavage activity with the assistance of Mg2+ to cleave the substrate strand and keep the fluorophore 6-carboxyfluorescein (FAM) away from the surface of the AuNP, which restores the fluorescence signal of this system. In this way, sensitive detection of UDG can be realized, and the detection limit is as low as 3.69 × 10-6 U mL-1. In addition, we found that this method is highly specific to UDG and can be used to detect UDG specifically in complex samples, which has certain application prospects in biomedical research and clinical diagnosis related to UDG.

Gold Nanoparticles Photosensitization towards 3,4,9,10-Perylenetetracarboxylic Dianhydride Integrated with a Dual-Particle Three-Dimensional DNA Roller: A General "ON-OFF-ON" Photoelectric Plasmon-Enhanced Biosensor

A high initial signal for the sensitive detection of analytes is critical in photoelectrochemical (PEC) biosensing systems. As a semiconductor, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) possesses an appropriate optical band gap of 2.5 eV and inherently intense and stable PEC response. When gold nanoparticles (Au NPs) are electrodeposited on the surface of PTCDA to form a Schottky junction (Au NPs/PTCDA), a surprising and satisfactory PEC performance is unfolded before our eyes. Considering the outstanding PEC behaviors of Au NPs/PTCDA and the great quenching effect of gold nanoclusters (Au NCs), the "ON-OFF-ON" PEC sensing platform has been developed for microRNA 1246 (miRNA 1246) detection combined with the cascaded quadratic amplification strategy of the polymerization/nicking reaction and dual-particle 3D DNA roller. The higher initial PEC signals of the system can be acquired by regulating the deposition time for 35 s (-0.2 V), which is derived from the synergetic effect of localized surface plasmon resonance of Au NPs and the formation of a Schottky junction. The dual-particle 3D DNA roller has been designed to guarantee wide walking space, remarkable operation performances, and inhibition of derailment. The proposed biosensor shows a dynamic range from 10 aM to 1 pM at a low detection limit of 3.1 aM and exhibits good analytical behaviors while analyzing miRNA 1246 in healthy human serum samples. This work not only expands the application of organic photoelectric materials in bioanalysis but also provides potential possibility of detecting other biomarkers by choosing appropriate target units.

Anti-Fouling Magnetic Beads Combined with Signal Amplification Strategies for Ultra-Sensitive and Selective Electrochemiluminescence Detection of MicroRNAs in Complex Biological Media

Herein, an electrochemiluminescence (ECL) microRNA biosensor based on anti-fouling magnetic beads (MBs) and two signal amplification strategies was developed. The newly designed anti-fouling dendritic peptide was wrapped on the surfaces of MBs to make them resistant to nonspecific adsorption of biomolecules in complex biological samples so as to realize accurate and selective target recognition. One of the amplification strategies was achieved through nucleic acid cycle amplification based on the DNAzyme on the surfaces of MBs. Then, the output DNA generated by the nucleic acid cycle amplification program stimulated the hybrid chain reaction (HCR) process on the modified electrode surface to generate the other amplification of the ECL response. Titanium dioxide nanoneedles (TiO₂ NNs), as a co-reaction accelerator of the Ru(bpy)₂(cpaphen)²⁺ and tripropylamine (TPrA) system, were wrapped with the electrodeposited polyaniline (PANI) on the electrode surface to enhance the ECL intensity of Ru(bpy)₂(cpaphen)²⁺. The conducting polymer PANI can not only immobilize the TiO₂ NNs but also improve the conductivity of the modified electrodes. The biosensor exhibited ultra-high sensitivity and excellent selectivity toward the detection of miRNA 21, with a detection limit of 0.13 fM. More importantly, with the anti-fouling MBs as a unique separation tool, this ECL biosensor was capable of assaying targets in complex biological media such as serum and cell lysate.

Target-swiped DNA lock for electrochemical sensing of miRNAs based on DNAzyme-assisted primer-generation amplification

As an extremely important post-transcriptional regulator, miRNAs are involved in a variety of crucial biological processes, and the abnormal expressions of miRNAs are closely related to a variety of diseases. In this work, for the first time, we designed a nucleic acid lock nanostructure for specific detection of miRNA-21, which changes the self-structure to "active conformation" by binding the target, in order to generate triggers to initiate the subsequent reaction. Emphatically, this flexible nucleic acid lock is capable of self-cleaving without the assistance of external component, overcoming the disadvantages of the complex design and requiring protease assistance in traditional nanostructure. Moreover, the combination of DNAzyme and RCA technology not only greatly improves the efficiency of signal amplification but also enables primer generation to simultaneous cascade RCA amplification. Additionally, the electrochemical detection technology based on silver nanoclusters overcomes the shortcomings of traditional detection methods such as low sensitivity and complex operation. The detection limit achieved was 9.3 aM with a wide dynamic response ranging from 10 aM to 100 pM (at the DPV peak of - 0.5 V), which is comparable to most of the reported studies. Therefore, our work provided an ultra-sensitive way for the detection of miRNAs using nanostructures and revealed an effective means for disease theranostics and cancer diagnosis. In this work, for the first time, we designed a nucleic acid lock nanostructure based on its self-structural transformation for the specific detection of miRNA. And the combination of DNAzyme and cascade RCA reaction greatly improved the signal amplification efficiency.

Exploring the Trans-Cleavage Activity of CRISPR/Cas12a on Gold Nanoparticles for Stable and Sensitive Biosensing

Taking advantage of the excellent trans-cleavage activity, CRISPR-based diagnostics (CRISPR-Dx) has shown great promise in molecular diagnostics. However, the single-stranded DNA reporter of the current CRISPR-Dx suffers from poor stability and limited sensitivity, which make their application in complex biological environments difficult. Herein, we, for the first time, explore the trans-cleavage activity of CRISPR/Cas12a toward the substrate on gold nanoparticles and apply the new phenomenon to develop a spherical nucleic acid (SNA) reporter for stable and sensitive CRISPR-Dx biosensing. By anchoring the DNA substrate on gold nanoparticles, we discovered different trans-cleavage activities of different types of the Cas12a system (e.g., LbCas12a and AsCas12a) on a nanoparticle surface. The further study suggests that the trans-cleavage activity of LbCas12a on the nanoparticle surface is highly dependent on the density and length of DNA strands. Based on these interesting discoveries, we furthermore develop SNA reporter-based fluorescent CRISPR-Dx for stable and sensitive biosensing application. Compared to traditional ssDNA reporters, the SNA reporter exhibits improved stability, which enables the stable application in a complex serum environment. In addition, the SNA reporter system with tunable density exhibits high sensitivity with a detection limit of 10 fM, which is about 2 orders of magnitude lower than that of the ssDNA reporter system. Finally, the practical application of SNA reporter-based CRISPR-Dx in clinical serum was successfully achieved. These results indicate their significant potential in future research on biology science and medical diagnoses.

Electrochemiluminescence immunosensor based on the quenching effect of CuO@GO on m-CNNS for cTnI detection

A sandwich-type electrochemiluminescence (ECL) immunosensor based on the resonance energy transfer (RET) was proposed for ultrasensitive detection of cardiac troponin I (cTnI). The RET behavior could be generated between graphite carbon nitride nanosheets (m-CNNS) as donor and copper oxide@graphene oxide (CuO@GO) as acceptor, achieving the quenching effect of CuO@GO on m-CNNS for cTnI detection. The m-CNNS synthesized by mechanical grinding of the graphite carbon nitride (CN) not only has better dispersion and higher specific surface area, but also has high luminous efficiency and stable chemical properties. Therefore, m-CNNS was used as the matrix material and luminophore. As the acceptor, CuO@GO prepared by in-situ chemical synthesis of CuO NPs onto GO sheets also has a high specific surface area, which could be used as a label of secondary antibody (Ab₂). Under optimal conditions, cTnI could be determined within the linear range of 0.1 pg mL^-1 to 100 ng mL^-1 and had a low detection limit (0.028 pg mL^-1, S/N = 3). Meanwhile, the prepared ECL immunosensor possessed great stability, specificity and reproducibility, providing a new method for detecting cTnI and other biomarkers.

Gold-Nanoparticle-Functionalized Cobalt-Nickel Phosphate 3D Nanoice Creams to Fabricate Stable and Sensitive Biosensors for the Cytochrome c Assay

Designing a stable and sensitive luminol-based electrochemiluminescence (ECL) analytical platform in the neutral condition has attracted a lot of attention. Here, gold-nanoparticle-functionalized cobalt-nickel phosphate three-dimensional nanoice creams (Au@Co₃Ni₃(PO₄)₄ NICs) are successfully prepared via electrostatic interaction. Generally, cobalt-nickel phosphate nanoice creams (Co₃Ni₃(PO₄)₄ NICs) are synthesized via a mild hydrothermal method and functionalized via polyethylenimine (PEI). Then, Au NPs are adsorbed on the surface of Co₃Ni₃(PO₄)₄ NICs via Au-N weak interaction to fabricate Au@Co₃Ni₃(PO₄)₄ NICs. Owing to the important roles of Au@Co₃Ni₃(PO₄)₄ in exhibiting excellent electrocatalytic activity, as well as preventing the deposition of negatively charged oxidation product induced electrode passivation, luminol in the nanohybrids (LH-Au@Co₃Ni₃(PO₄)₄) gives strong and stable ECL intensity in the neutral conditions. Moreover, the ECL emission of luminol is obviously quenched based on the resonance energy transfer (RET) between luminol as donor and cytochrome c (Cyt c) as acceptor. Hence, a sensitive ECL biosensor is successfully fabricated for the quantitative determination of Cyt c in cell lysates and exhibits wide linear ranges of 1.0 × 10^-4-0.5 × 10^-5 and 0.5 × 10^-5-1.0 × 10^-8 M as well as a low detection limit of 2.48 nM. This novel sensing strategy will broaden the application of transition metal (Co, Ni) phosphates in bioassays.

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.

An intelligent nanodevice based on the synergistic effect of telomerase-triggered photodynamic therapy and gene-silencing for precise cancer cell therapy

The development of intelligent and precise cancer therapy systems that enable accurate diagnosis and specific elimination of cancer cells while protecting normal cells to improve the safety and effectiveness of the treatment is still a challenge. Herein, we report a novel activatable nanodevice for precise cancer therapy. The nanodevice is constructed by adsorbing a DNA duplex probe onto MnO2 nanosheets. After cellular uptake, the DNA duplex probe undergoes telomerase-triggered conformation switching, resulting in a Ce6 "turn-on" signal for the identification of cancer cells. Furthermore, Deoxyribozyme (DNAzyme) is activated to catalyse the cleavage of survivin mRNA, actualizing a precise synergistic therapy in cancer cells involving photodynamic therapy and gene-silencing. The MnO2 nanosheets provide Mn2+ for the DNAzyme and relieve hypoxia to improve the efficiency of the photodynamic therapy. Live cell studies reveal that this nanodevice can diagnose cancer cells and specifically eliminate them without harming normal cells, so making the treatment safer and more effective. The developed DNA-MnO2 nanodevice provides a valuable and general platform for precise cancer therapy.

3D DNA nanonet structure coupled with target-catalyzed hairpin assembly for dual-signal synergistically amplified electrochemical sensing of circulating microRNA

DNA nanomaterials are reliable and powerful tools in the development of a variety of biosensors owing to their notable self-assembly ability and precise recognition capability. Here, we propose a DNA nanomaterial-based system for the dual-amplified electrochemical sensing of circulating microRNAs by a coupled cascade of catalyzed hairpin assembly (CHA) and three-dimensional (3D) DNA nanonet structure. In the target-assisted CHA process, the stable hairpin structures H1 and H2 act as probes for the recognition and recycling of circulating microRNAs, leading to the formation of abundant H1-H2 duplexes with tails. Subsequently, a 3D DNA nanonet structure was introduced, which was assembled using three DNA strands constructed X-DNA monomers as the building blocks, and hybridized to the tails of H1-H2 duplexes. The successful integration of target-assisted CHA and 3D DNA nanonet structure induced the second signal amplification. The designed biosensor performed under optimized experimental conditions, and exposed admirable analytical performance for the detection of circulating miR-21, with a wide linear range from 10 fM to 1 nM, high sensitivity of limit of detection (LOD) of 3.6083 fM, good specificity in the face of single nucleotides and other microRNAs, satisfactory stability and reproducibility for practical analysis. Furthermore, the clinical applicability for circulating miR-21 detection was verified in human serum samples without additional treatment. We hope that this elaborated biosensor will provide new opportunities for bioassays based on DNA nanomaterials.

An AIEgen-based 2D ultrathin metal-organic layer as an electrochemiluminescence platform for ultrasensitive biosensing of carcinoembryonic antigen

In this work, a novel two-dimensional (2D) ultrathin metal-organic layer (MOL) based on the aggregation-induced emission (AIE) ligand H4ETTC (H4ETTC = 4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-4-carboxylic acid))) was developed and used to construct a novel electrochemiluminescence (ECL) aptasensor for ultrasensitive detection of carcinoembryonic antigen (CEA). The newly synthesized AIE luminogen (AIEgen)-based MOL (Hf-ETTC-MOL) yielded a higher ECL intensity and efficiency than did H4ETTC monomers, H4ETTC aggregates and 3D bulk Hf-ETTC-MOF. This improvement occurred not only because the ETTC ligands were coordinatively immobilized in a rigid MOL matrix, which restricted the intramolecular free rotation and vibration of these ligands and then reduced the non-radiative transition, but also because the porous ultrathin 2D MOL greatly shortened the transport distances of ions, electrons, coreactant (triethylamine, TEA) and coreactant intermediates (TEA˙ and TEA˙+), which made more ETTC luminophores able to be excited and yielded a high ECL efficiency. On the basis of using the Hf-ETTC-MOL as a novel ECL emitter and rolling circle amplification (RCA) as a signal amplification strategy, the constructed ECL aptasensor exhibited a linear range from 1 fg mL-1 to 1 ng mL-1 with a detection limit of 0.63 fg mL-1. This work has opened up new prospects for developing novel ECL materials and is expected to lead to increased interest in using AIEgen-based MOLs for ECL sensing.