Cross-catalytic hairpin assembly-based exponential signal amplification for CRET assay with low background noise

A sensitive and versatile electrochemical sensor based on hybridization chain reaction and CRISPR/Cas12a system for antibiotic detection

A sensitive electrochemical platform was constructed with NH2-Cu-MOF as electrochemical probe to detect antibiotics using CRISPR/Cas12a system triggered by hybridization chain reaction (HCR). The sensing system consists of two HCR systems. HCR1 occurred on the electrode surface independent of the target, generating long dsDNA to connect signal probes and producing a strong electrochemical signal. HCR2 was triggered by target, and the resulting dsDNA products activated the CRISPR/Cas12a, thereby resulting in effective and rapid cleavage of the trigger of HCR1, hindering the occurrence of HCR1, and reducing the number of NH2-Cu-MOF on the electrode surface. Eventually, significant signal change depended on the target was obtained. On this basis and with the help of the programmability of DNA, kanamycin and ampicillin were sensitively detected with detection limits of 60 fM and 10 fM (S/N = 3), respectively. Furthermore, the sensing platform showed good detection performance in milk and livestock wastewater samples, demonstrating its great application prospects in the detection of antibiotics in food and environmental water samples.

Signal amplification strategy of DNA self-assembled biosensor and typical applications in pathogenic microorganism detection

Biosensors have emerged as ideal analytical devices for various bio-applications owing to their low cost, convenience, and portability, which offer great potential for improving global healthcare. DNA self-assembly techniques have been enriched with the development of innovative amplification strategies, such as dispersion-to-localization of catalytic hairpin assembly, and dumbbell hybridization chain reaction, which hold great significance for building biosensors capable of realizing sensitive, rapid and multiplexed detection of pathogenic microorganisms. Here, focusing primarily on the signal amplification strategies based on DNA self-assembly, we concisely summarized the strengths and weaknesses of diverse isothermal nucleic acid amplification techniques. Subsequently, both single-layer and cascade amplification strategies based on traditional catalytic hairpin assembly and hybridization chain reaction were critically explored. Furthermore, a comprehensive overview of the recent advances in DNA self-assembled biosensors for the detection of pathogenic microorganisms is presented to summarize methods for biorecognition and signal amplification. Finally, a brief discussion is provided about the current challenges and future directions of DNA self-assembled biosensors.

Recent advances in cascade isothermal amplification techniques for ultra-sensitive nucleic acid detection

Nucleic acid amplification techniques have always been one of the hot spots of research, especially in the outbreak of COVID-19. From the initial polymerase chain reaction (PCR) to the current popular isothermal amplification, each new amplification techniques provides new ideas and methods for nucleic acid detection. However, limited by thermostable DNA polymerase and expensive thermal cycler, PCR is difficult to achieve point of care testing (POCT). Although isothermal amplification techniques overcome the defects of temperature control, single isothermal amplification is also limited by false positives, nucleic acid sequence compatibility, and signal amplification capability to some extent. Fortunately, efforts to integrating different enzymes or amplification techniques that enable to achieve intercatalyst communication and cascaded biotransformations may overcome the corner of single isothermal amplification. In this review, we systematically summarized the design fundamentals, signal generation, evolution, and application of cascade amplification. More importantly, the challenges and trends of cascade amplification were discussed in depth.

Recent advances of catalytic hairpin assembly and its application in bioimaging and biomedicine

Catalytic hairpin assembly (CHA) appears to be a particularly appealing nucleic acid circuit because of its powerful amplification capability, simple protocols, and enzyme-free and isothermal conditions, and can combine with various signal output modes for the biosensing of various analytes. Especially in the last five years, vast CHA related studies have sprung up. With the deep exploration of the CHA mechanism, some novel and excellent CHA strategies have been proposed; meanwhile the CHA cascade strategies with various amplification techniques further improve the analysis performance. Furthermore, diverse CHA based biosensors have been tactfully engineered and extensively employed in imaging applications in living cells and in vivo ascribed to its gentle reaction, efficient amplification and universality. Hence, we present a comprehensive and systematic summary of the progress in CHA and its application in bioimaging and biomedicine to date. At first, we introduced the mechanism and diversification of CHA in detail, including the newly developed CHA and its ingenious combination with a variety of other technologies. Concurrently, we summarized the latest application progress of different CHA strategies in bioimaging and biomedicine, highlighting the merits and drawbacks of representative approaches. Finally, we put forward some views on the challenges and prospects of CHA in bioimaging and biomedicine in the future.

Hairpin-functionalized DNA tetrahedra for miRNA imaging in living cells via self-assembly to form dendrimers

A DNA tetrahedron-based intramolecular catalytic hairpin self-assembly platform that uses fluorescence signals to image miRNAs in live cells for accurate tumor cell identification.

A novel strategy for programmable DNA tile self-assembly with a DNAzyme-mediated DNA cross circuit

The proposed strategy promotes the controllability and modularization of trigger elements, realizes programmable molecular self-assembly, and has broad applications for the construction of DNA nanodevices.

Endogenous microRNA triggered enzyme-free DNA logic self-assembly for amplified bioimaging and enhanced gene therapy via in situ generation of siRNAs

BACKGROUND

Small interfering RNA (siRNA) has emerged as a kind of promising therapeutic agents for cancer therapy. However, the off-target effect and degradation are the main challenges for siRNAs delivery. Herein, an enzyme-free DNA amplification strategy initiated by a specific endogenous microRNA has been developed for in situ generation of siRNAs with enhanced gene therapy effect on cervical carcinoma.

METHODS

This strategy contains three DNA hairpins (H1, H2/PS and H3) which can be triggered by microRNA-21 (miR-21) for self-assembly of DNA nanowheels (DNWs). Notably, this system is consistent with the operation of a DNA logic circuitry containing cascaded "AND" gates with feedback mechanism. Accordingly, a versatile biosensing and bioimaging platform is fabricated for sensitive and specific analysis of miR-21 in HeLa cells via fluorescence resonance energy transfer (FRET). Meanwhile, since the vascular endothelial growth factor (VEGF) antisense and sense sequences are encoded in hairpin reactants, the performance of this DNA circuit leads to in situ assembly of VEGF siRNAs in DNWs, which can be specifically recognized and cleaved by Dicer for gene therapy of cervical carcinoma.

RESULTS

The proposed isothermal amplification approach exhibits high sensitivity for miR-21 with a detection limit of 0.25 pM and indicates excellent specificity to discriminate target miR-21 from the single-base mismatched sequence. Furthermore, this strategy achieves accurate and sensitive imaging analysis of the expression and distribution of miR-21 in different living cells. To note, compared to naked siRNAs alone, in situ siRNA generation shows a significantly enhanced gene silencing and anti-tumor effect due to the high reaction efficiency of DNA circuit and improved delivery stability of siRNAs.

CONCLUSIONS

The endogenous miRNA-activated DNA circuit provides an exciting opportunity to construct a general nanoplatform for precise cancer diagnosis and efficient gene therapy, which has an important significance in clinical translation.

Proximity-Induced Hybridization Chain Reaction-Based Photoacoustic Imaging System for Amplified Visualization Protein-Specific Glycosylation in Mice

Glycosylation is a key cellular mechanism that regulates several physiological and pathological functions. Therefore, identification and characterization of specific-protein glycosylation in vivo are highly desirable for studying glycosylation-related pathology and developing personalized theranostic modalities. Herein, we demonstrated a photoacoustic (PA) nanoprobe based on the proximity-induced hybridization chain reaction (HCR) for amplified visual detection of protein-specific glycosylation in vivo. Two kinds of functional DNA probes were designed. A glycan probe (DBCO-GP) was attached to glycans through metabolic oligosaccharide engineering (MOE) and protein probe (PP)-targeted proteins by aptamer recognition. Proximity-induced hybridization of the complementary domain between the two kinds of probes promoted conformational changes in the protein probes and in situ release of the HCR initiator domain. Gold nanoparticles (AuNPs) modified by complementary sequences (Au-H1 and Au-H2) self-assembled into Au aggregates via the HCR, thereby converting DNA signals to photoacoustic signals. Due to the high contrast and deep penetration of photoacoustic imaging, this strategy enabled in situ detection of Mucin 1 (MUC1)-specific glycosylation in mice with breast cancer and successfully monitored its dynamic states during tunicamycin treatment. This imaging technique provides a powerful platform for studying the effects of glycosylation on the protein structure and function, which helps to elucidate its role in disease processes.

Hairpin DNA-Mediated isothermal amplification (HDMIA) techniques for nucleic acid testing

Nucleic acid detection is of great importance in a variety of areas, from life science and clinical diagnosis to environmental monitoring and food safety. Unfortunately, nucleic acid targets are always found in trace amounts and their response signals are difficult to be detected. Amplification mechanisms are then practically needed to either duplicate nucleic acid targets or enhance the detection signals. Polymerase chain reaction (PCR) is one of the most popular and powerful techniques for nucleic acid analysis. But the requirement of costly devices for precise thermo-cycling procedures in PCR has severely hampered the wide applications of PCR. Fortunately, isothermal molecular reactions have emerged as promising alternatives. The past decade has witnessed significant progress in the research of isothermal molecular reactions utilizing hairpin DNA probes (HDPs). Based on the nucleic acid strand interaction mechanisms, the hairpin DNA-mediated isothermal amplification (HDMIA) techniques can be mainly divided into three categories: strand assembly reactions, strand decomposition reactions, and strand creation reactions. In this review, we introduce the basics of HDMIA methods, including the sensing principles, the basic and advanced designs, and their wide applications, especially those benefiting from the utilization of G-quadruplexes and nanomaterials during the past decade. We also discuss the current challenges encountered, highlight the potential solutions, and point out the possible future directions in this prosperous research area.

Ultrasensitive electrochemical detection of miRNA coupling tetrahedral DNA modified gold nanoparticles tags and catalyzed hairpin assembly

MicroRNAs (miRNAs) play key regulatory roles in a number of biological processes, which act as critical biomarkers for clinical diagnosis. There are urgent needs to develop advanced tools for accurate and convenient analysis of miRNA in biological circumstances. In this study, an ultrasensitive electrochemical biosensor for miRNA assay is fabricated. Tetrahedral DNA modified gold nanoparticles tags are applied with optimized orientation, which are able to recruit a large number of electrochemical species for remarkable signal responses. Benefiting from the excellent amplification efficiency of the association of strand displacement amplification and catalyzed hairpin assembly, the established method shows ultrahigh sensitivity with the limit of detection as low as 10 aM. A wide linear range from 10^-17 to 10^-7 M is achieved. In addition, this method is capable to specifically discriminate interfering miRNAs with slightly different sequences. The successful assessment of miRNA levels in human serum samples also demonstrates good practical utility. Therefore, the proposed method has great potential to the applications of miRNA expression profiling and biological studies.

Boolean logic gate based on DNA strand displacement for biosensing: current and emerging strategies

DNA computers are considered one of the most prominent next-generation molecular computers that perform Boolean logic using DNA elements. DNA-based Boolean logic gates, especially DNA strand displacement-based logic gates (SDLGs), have shown tremendous potential in biosensing since they can perform the logic analysis of multi-targets simultaneously. Moreover, SDLG biosensors generate a unique output in the form of YES/NO, which is contrary to the quantitative measurement used in common biosensors. In this review, the recent achievements of SDLG biosensing strategies are summarized. Initially, the development and mechanisms of Boolean logic gates, strand-displacement reaction, and SDLGs are introduced. Afterwards, the diversified input and output of SDLG biosensors are elaborated. Then, the state-of-the-art SDLG biosensors are reviewed in the classification of different signal-amplification methods, such as rolling circle amplification, catalytic hairpin assembly, strand-displacement amplification, DNA molecular machines, and DNAzymes. Most importantly, limitations and future trends are discussed. The technology reviewed here is a promising tool for multi-input analysis and lays a foundation for intelligent diagnostics.

A review of aptamer-based SERS biosensors: Design strategies and applications

Surface-enhanced Raman spectroscopy, due to its high sensitivity, unique vibrational fingerprint identification of molecules and easy operation, has been extensively applied in different fields. Aptamers, being the unique single stranded DNA/RNA sequences that can specifically recognize and seize the target analytes, combined with Surface-enhanced Raman spectroscopy (SERS), can offer potent multiplex detection capacity with high specificity and sensitivity. In this review, we summarize and classify the general working strategies of different types of aptamer-based SERS biosensors with diversified protocols which either take aptamer conformational change as intrinsic reporter, or make use of various extrinsic Raman reporters in different sensor designs via on/off approach, sandwich-type and magnetic nanoparticles (NPs)-assisted approach, and catalytic reaction assisted approach with amplification of alternative Raman signals. The advantages, applications and perspectives of these aptamer-based SERS biosensors are also discussed.

Highly sensitive SERS assay of DENV gene via a cascade signal amplification strategy of localized catalytic hairpin assembly and hybridization chain reaction

Pathogenic viruses with worldwide distribution, high incidence and great harm are significantly and increasingly threatening human health. However, there is still lack of sufficient, highly sensitive and specific detection methods for on-time and early diagnosis of virus infection. In this work, taking dengue virus (DENV) as an example, a highly sensitive SERS assay of DENV gene was proposed via a cascade signal amplification strategy of localized catalytic hairpin assembly (LCHA) and hybridization chain reaction (HCR). The SERS assay was performed by two steps, i.e., the operation of cascade signal amplification strategy and the following SERS measurements by transferring the products on SERS-active AgNRs arrays. The sensitivity of the cascade signal amplification strategy is significantly amplified, which is 4.5 times that of individual CHA, and the signal-to-noise ratio is also improved to 5.4 relative to 1.8 of the CHA. The SERS sensing possesses a linear calibration curve from 1 fM to 10 nM with the limit of detection low to 0.49 fM, and has good specificity, uniformity and recovery, which indicates that the highly sensitive SERS assay provides an attractive tool for reliable, early diagnosis of DENV gene and is worth to be popularized in a wide detection of other viruses.

Expanding the analytical applications of nucleic acid hybridization using junction probes

Nucleic acid hybridization is crucial in target recognition with respect to in vitro and in vivo nucleic acid biosensing. Conventional linear probes and molecular beacons encounter challenges in multiplexing and specific recognition of intractable nucleic acids. Advances in nucleic acid nanotechnologies have resulted in a set of novel structural probes: junction probes (JPs), which make full use of the advantages of specificity, stability, programmability and predictability of Watson-Crick base pairing. In recent years, junction probes have been regularly implemented in constructing systems related to biosensing, synthetic biology and gene regulation. Herein, we summarize the latest advances in JP designs as potential nucleic acid biosensing systems and their expansive applications, and provide some general guidelines for developing JP based sensing strategies for implementation of such systems.

Target-Catalyzed Assembly of Pyrene-Labeled Hairpins for Exponentially Amplified Biosensing

Rapid and sensitive detection of nucleic acids is vital for disease diagnosis. This work designed an enzyme-free isothermal strategy for rapid exponential signal amplification through target-triggered catalytic hairpin assembly (CHA) to induce the spatially sensitive fluorescent signal of the pyrene excimer. Functionally, this system consisted of three pyrene labelled hairpins (H1, H2, and H3) and one catalyst DNA C. In the presence of C, the CHA was activated to generate intermediate I, which contained a single-stranded region identical to the C sequence for initiating the second cycle of CHA to obtain 2I and thus achieved the exponential formation of I along with the switching of pyrene excimer. The fluorescent signal of the pyrene excimer could be further enhanced via the inclusion of γ-cyclodextrin and showed a linear increase upon increasing logarithm of C concentration. Through the introduction of a helping hairpin H4-containing C sequence and a region specific to the target, this strategy could be extended to realize the quick and sensitive detection of different analytes. Using dengue virus RNA as an analyte model, the proposed fluorescent method showed a linear range from 0.1 to 50 nM with a limit of detection of 0.048 nM at 3σ and good selectivity. The excellent performance and convenient operation demonstrated its promising application in clinical disease diagnosis.

Dual catalytic DNA circuit-induced gold nanoparticle aggregation: An enzyme-free and colorimetric strategy for amplified detection of nucleic acids

An enzyme-free dual catalytic DNA circuit for amplified detection of nucleic acids has been developed. The system functions based on a cyclic self-assembly of two auxiliary hairpins (H1 and H2) and three biotinylated hairpin oligonucleotides (H3, H4 and H5), in the format of two molecular circuits. In the upstream circuit, a target initiator (I) besides H1 and H2 hairpins constructs H1-H2 duplexes that trigger the operation of a subsequent circuit. In the downstream circuit, the H1-H2 duplex initiates cascaded self-assembly reactions, produces triplex H3-H4-H5, as sensing system, and releases the H1-H2 duplex as the catalyst for the self-assembly of additional hairpins. The H3-H4-H5 triplex acts as the scaffolds for assembling and orienting the streptavidin-functionalized gold nanoparticles (SA-AuNPs) into a lattice-like arrangement that generates a DNA-SA-AuNP cross-linked network, resulting in a dramatic pale red-to-blue color change. By ingeniously engaging two catalytic circuits with feedback amplification capabilities, the system can detect the target nucleic acid with an LOD value of 5 femtomolar and unambiguously discriminate spurious targets (i.e. targets containing substitution, insertion, and deletion nucleotides) without instrumentation. Simple and convenient operation of the assay makes the DNA circuit appropriate for point-of-care monitoring in resource-constrained settings.

A dual signal amplification strategy for the highly sensitive fluorescence detection of nucleic acids

A dual signal amplification strategy comprising target-triggered recycling and DSN-mediated amplifications was designed and proposed for a highly sensitive fluorescence assay of nucleic acids.

Applications of Catalytic Hairpin Assembly Reaction in Biosensing

Nucleic acids are considered as perfect programmable materials for cascade signal amplification and not merely as genetic information carriers. Among them, catalytic hairpin assembly (CHA), an enzyme-free, high-efficiency, and isothermal amplification method, is a typical example. A typical CHA reaction is initiated by single-stranded analytes, and substrate hairpins are successively opened, resulting in thermodynamically stable duplexes. CHA circuits, which were first proposed in 2008, present dozens of systems today. Through in-depth research on mechanisms, the CHA circuits have been continuously enriched with diverse reaction systems and improved analytical performance. After a short time, the CHA reaction can realize exponential amplification under isothermal conditions. Under certain conditions, the CHA reaction can even achieve 600 000-fold signal amplification. Owing to its promising versatility, CHA is able to be applied for analysis of various markers in vitro and in living cells. Also, CHA is integrated with nanomaterials and other molecular biotechnologies to produce diverse readouts. Herein, the varied CHA mechanisms, hairpin designs, and reaction conditions are introduced in detail. Additionally, biosensors based on CHA are presented. Finally, challenges and the outlook of CHA development are considered.

Chemiluminescence and Bioluminescence Imaging for Biosensing and Therapy: In Vitro and In Vivo Perspectives

Chemiluminescence (CL) and bioluminescence (BL) imaging technologies, which require no external light source so as to avoid the photobleaching, background interference and autoluminescence, have become powerful tools in biochemical analysis and biomedical science with the development of advanced imaging equipment. CL imaging technology has been widely applied to high-throughput detection of a variety of analytes because of its high sensitivity, high efficiency and high signal-to-noise ratio (SNR). Using luciferase and fluorescent proteins as reporters, various BL imaging systems have been developed innovatively for real-time monitoring of diverse molecules in vivo based on the reaction between luciferin and the substrate. Meanwhile, the kinetics of protein interactions even in deep tissues has been studied by BL imaging. In this review, we summarize in vitro and in vivo applications of CL and BL imaging for biosensing and therapy. We first focus on in vitro CL imaging from the view of improving the sensitivity. Then, in vivo CL applications in cells and tissues based on different CL systems are demonstrated. Subsequently, the recent in vitro and in vivo applications of BL imaging are summarized. Finally, we provide the insight into the development trends and future perspectives of CL and BL imaging technologies.

Silver nanoparticle@DNA tetrahedron-based colorimetric detection of HIV-related DNA with cascade strand displacement amplification

DNA tetrahedron-modified silver nanoparticles (AgNPs) were achieved via amino-silver chemistry for the first time and were applied as a colorimetric biosensor for detecting HIV-related DNA. Target DNA initiated strand displacement polymerization and nicking endonuclease-aided cycles were involved to link DNA tetrahedron-modified AgNPs, reporting colorimetric responses. This developed method showed excellent specificity and sensitivity. A wide linear range from 1 to 15 000 nM was achieved with a limit of detection of 0.84 nM. Moreover, it was successfully applied to determine DNA in blood serum samples.

Principles and Applications of Nucleic Acid Strand Displacement Reactions

Dynamic DNA nanotechnology, a subfield of DNA nanotechnology, is concerned with the study and application of nucleic acid strand-displacement reactions. Strand-displacement reactions generally proceed by three-way or four-way branch migration and initially were investigated for their relevance to genetic recombination. Through the use of toeholds, which are single-stranded segments of DNA to which an invader strand can bind to initiate branch migration, the rate with which strand displacement reactions proceed can be varied by more than 6 orders of magnitude. In addition, the use of toeholds enables the construction of enzyme-free DNA reaction networks exhibiting complex dynamical behavior. A demonstration of this was provided in the year 2000, in which strand displacement reactions were employed to drive a DNA-based nanomachine (Yurke, B.; et al. Nature 2000, 406, 605-608). Since then, toehold-mediated strand displacement reactions have been used with ever increasing sophistication and the field of dynamic DNA nanotechnology has grown exponentially. Besides molecular machines, the field has produced enzyme-free catalytic systems, all DNA chemical oscillators and the most complex molecular computers yet devised. Enzyme-free catalytic systems can function as chemical amplifiers and as such have received considerable attention for sensing and detection applications in chemistry and medical diagnostics. Strand-displacement reactions have been combined with other enzymatically driven processes and have also been employed within living cells (Groves, B.; et al. Nat. Nanotechnol. 2015, 11, 287-294). Strand-displacement principles have also been applied in synthetic biology to enable artificial gene regulation and computation in bacteria. Given the enormous progress of dynamic DNA nanotechnology over the past years, the field now seems poised for practical application.

Double amplified colorimetric detection of DNA using gold nanoparticles, enzymes and a catalytic hairpin assembly

The authors describe an isothermal and ultrasensitive colorimetric DNA assay that consists of two amplification stages using enzymes and a catalytic hairpin assembly (CHA). The first step consists in the selective amplification of DNA using Klenow fragment and nicking enzyme. The second step consists in the amplification of the optical signal by using a catalytic hairpin assembly. After two amplification steps, the DNA reaction induces the aggregation of the red gold nanoparticles to give a blue color shift. The degree of aggregation can be quantified by measurement of the ratio of the UV-vis absorbances of the solutions at 620 and 524 nm which are the wavelengths of the aggregated gold nanoparticles and bare gold nanoparticles. The detection limit is as low as 3.1 fM. Due to the use of a specific enzyme, only the desired DNAs will be detected. The method can be applied to the determination of DNA of various lengths. Despite the presence of large amounts of wildtype DNA, it can readily detect a target DNA. Conceivably, the technique has a large potential because of its high sensitivity and selectivity. Graphical abstract Schematic presentation of DNA detection using gold nanoparticles (AuNP), enzymes and catalytic hairpin assembly (CHA). Effective DNA detection is achieved through the aggregation of AuNPs which is caused by DNA amplification using enzymes and signal amplification using CHA.

Polydopamine-assisted versatile modification of a nucleic acid probe for intracellular microRNA imaging and enhanced photothermal therapy

MicroRNAs play an important role in various biological processes, and their aberrant expression is closely associated with various human diseases, especially cancer. Real-time monitoring of microRNAs in living cells may help us to understand their role in cellular processes, which can further provide a basis for diagnosis and treatment. In this study, polydopamine was used to assist the versatile modification of a nucleic acid probe for intracellular microRNA imaging and enhanced photothermal therapy. Polydopamine can be covalently linked with a thiol-terminated nucleic acid probe through the Michael addition reaction under slightly alkaline conditions. This modification is mild and can be performed directly in an aqueous solution, which can better resist hydrolysis than the traditional modification processes, resulting in a nanoprobe with better stability and higher loading of nucleic acids. This prepared nanoprobe can easily enter cells without transfection agents and then realize the imaging of intracellular miRNA through fluorescence restoration. Moreover, the coating of PDA can enhance the photothermal conversion efficiency of the nanoprobe, making it suitable for photothermal therapy of cancer. It is expected that the PDA-based versatile modification can help to construct a promising platform for tumor imaging and treatment.

Polydopamine can assist the versatile modification of a nucleic acid probe for intracellular miRNA responsed fluorescence imaging and enhanced photothermal therapy.

Sensitive detection of intracellular microRNA based on a flowerlike vector with catalytic hairpin assembly

A flowerlike nanovector with horn-shaped tips is developed for in situ detection of intracellular microRNA with multiple signal outputs.

Exonuclease-assisted target recycling amplification for label-free chemiluminescence assay and molecular logic operations

A versatile exonuclease-assisted target recycling amplification strategy is demonstrated to achieve label-free chemiluminescence detection of DNA and construction of a series of two-input molecular logic gates.