Molecular aptamer beacon tuned DNA strand displacement to transform small molecules into DNA logic outputs

Harnessing DNA computing and nanopore decoding for practical applications: from informatics to microRNA-targeting diagnostics

DNA computing represents a subfield of molecular computing with the potential to become a significant area of next-generation computation due to the high programmability inherent in the sequence-dependent molecular behaviour of DNA. Recent studies in DNA computing have extended from mathematical informatics to biomedical applications, with a particular focus on diagnostics that exploit the biocompatibility of DNA molecules. The output of DNA computing devices is encoded in nucleic acid molecules, which must then be decoded into human-recognizable signals for practical applications. Nanopore technology, which utilizes an electrical and label-free decoding approach, provides a unique platform to bridge DNA and electronic computing for practical use. In this tutorial review, we summarise the fundamental knowledge, technologies, and methodologies of DNA computing (logic gates, circuits, neural networks, and non-DNA input circuity). We then focus on nanopore-based decoding, and highlight recent advances in medical diagnostics targeting microRNAs as biomarkers. Finally, we conclude with the potential and challenges for the practical implementation of these techniques. We hope that this tutorial will provide a comprehensive insight and enable the general reader to grasp the fundamental principles and diverse applications of DNA computing and nanopore decoding, and will inspire a wide range of scientists to explore and push the boundaries of these technologies.

Integrating Ligands into Nucleic Acid Systems

Signal transduction from non-nucleic acid ligands (small molecules and proteins) to structural changes of nucleic acids plays a crucial role in both biomedical analysis and cellular regulations. However, how to bridge between these two types of molecules without compromising the expandable complexity and programmability of the nucleic acid nanomachines is a critical challenge. Compared with the previously most widely applied transduction strategies, we review the latest advances of a kinetically controlled approach for ligand-oligonucleotide transduction in this Concept article. This new design works through an intrinsic conformational alteration of the nucleic acid aptamer upon the ligand binding as a governing factor for nucleic acid strand displacement reactions. The functionalities and applications of this transduction system as a ligand converter on biosensing and DNA computation are described and discussed. Furthermore, we propose some potential scenarios for utilization of this ligand transduction design to regulate gene expression through synthetic RNA switches in the cellular contexts. Finally, future perspectives regarding this ligand-oligonucleotide transduction platform are also discussed.

Analysis of Nanopore Data: Classification Strategies for an Unbiased Curation of Single-Molecule Events from DNA Nanostructures

Nanopores are versatile single-molecule sensors that are being used to sense increasingly complex mixtures of structured molecules with applications in molecular data storage and disease biomarker detection. However, increased molecular complexity presents additional challenges to the analysis of nanopore data, including more translocation events being rejected for not matching an expected signal structure and a greater risk of selection bias entering this event curation process. To highlight these challenges, here, we present the analysis of a model molecular system consisting of a nanostructured DNA molecule attached to a linear DNA carrier. We make use of recent advances in the event segmentation capabilities of Nanolyzer, a graphical analysis tool provided for nanopore event fitting, and describe approaches to the event substructure analysis. In the process, we identify and discuss important sources of selection bias that emerge in the analysis of this molecular system and consider the complicating effects of molecular conformation and variable experimental conditions (e.g., pore diameter). We then present additional refinements to existing analysis techniques, allowing for improved separation of multiplexed samples, fewer translocation events rejected as false negatives, and a wider range of experimental conditions for which accurate molecular information can be extracted. Increasing the coverage of analyzed events within nanopore data is not only important for characterizing complex molecular samples with high fidelity but is also becoming essential to the generation of accurate, unbiased training data as machine-learning approaches to data analysis and event identification continue to increase in prevalence.

Molecular Computation for Molecular Classification

DNA as an informational polymer has, for the past 30 years, progressively become an essential molecule to rationally build chemical reaction networks endowed with powerful signal-processing capabilities. Whether influenced by the silicon world or inspired by natural computation, molecular programming has gained attention for diagnosis applications. Of particular interest for this review, molecular classifiers have shown promising results for disease pattern recognition and sample classification. Because both input integration and computation are performed in a single tube, at the molecular level, this low-cost approach may come as a complementary tool to molecular profiling strategies, where all biomarkers are quantified independently using high-tech instrumentation. After introducing the elementary components of molecular classifiers, some of their experimental implementations are discussed either using digital Boolean logic or analog neural network architectures.

DNA-based customized functional modules for signal transformation

Information on the temporal and spatial scale of cellular molecules in biological systems is crucial for estimating life processes and may be conducive to an improved understanding of disease progression. This intracellular and extracellular information is often difficult to obtain at the same time due to the limitations of accessibility and sensing throughput. DNA is an excellent material for in vivo and in vitro applications, and can be used to build functional modules that can transform bio-information (input) into ATCG sequence information (output). Due to their small volume and highly amenable programming, DNA-based functional modules provide an opportunity to monitor a range of information, from transient molecular events to dynamic biological processes. Over the past two decades, with the advent of customized strategies, a series of functional modules based on DNA networks have been designed to gather different information about molecules, including the identity, concentration, order, duration, location, and potential interactions; the action of these modules are based on the principle of kinetics or thermodynamics. This paper summarizes the available DNA-based functional modules that can be used for biomolecular signal sensing and transformation, reviews the available designs and applications of these modules, and assesses current challenges and prospects.

Wavelength-Dependent, Orthogonal Photoregulation of DNA Liberation for Logic Operations

In this study, we synthesized two phosphoramidites based on 2,7-bis-{4-nitro-8-[3-(2-propyl)-styryl]}-9,9-bis-[1-(3,6-dioxaheptyl)]-fluorene (BNSF) and 4,4'-bis-{8-[4-nitro-3-(2-propyl)-styryl]}-3,3'-di-methoxybiphenyl (BNSMB) structures as visible light-cleavable linkers for oligonucleotide conjugation. In addition to the commercial ultraviolet (UV) photocleavable (PC) linker, the BNSMB linker was further applied as a building component to construct photoregulated DNA devices as duplex structures, which are functionalized with fluorophores and quenchers. Selective cleavage of PC and BNSMB is achieved in response to ultraviolet (UV) and visible light irradiations as two inputs, respectively. This leads to controllable dissociation of pieces of DNA fragments, which is followed by changes of fluorescence emission as signal outputs of the system. By tuning the number and position of the photocleavable molecules, fluorophores, and quenchers, various DNA devices were developed, which mimic the functions of Boolean logic gates and achieve logic operations in AND, OR, NOR, and NAND gates in response to two different wavelengths of light inputs. By sequence design, the photolysis products can be precisely programmed in DNA devices and triggered to release in a selective and/or sequential manner. Thus, this photoregulated DNA device shows potential as a wavelength-dependent drug delivery system for selective control over the release of multiple individual therapeutic oligonucleotide-based drugs. We believe that our work not only enriches the library of photocleavable phosphoramidites available for bioconjugation but also paves the way for developing spatiotemporal-controlled, orthogonal-regulated DNA-based logic devices for a range of applications in materials science, polymers, chemistry, and biology.

Catalyst-Based Biomolecular Logic Gates

Regulatory processes in biology can be re-conceptualized in terms of logic gates, analogous to those in computer science. Frequently, biological systems need to respond to multiple, sometimes conflicting, inputs to provide the correct output. The language of logic gates can then be used to model complex signal transduction and metabolic processes. Advances in synthetic biology in turn can be used to construct new logic gates, which find a variety of biotechnology applications including in the production of high value chemicals, biosensing, and drug delivery. In this review, we focus on advances in the construction of logic gates that take advantage of biological catalysts, including both protein-based and nucleic acid-based enzymes. These catalyst-based biomolecular logic gates can read a variety of molecular inputs and provide chemical, optical, and electrical outputs, allowing them to interface with other types of biomolecular logic gates or even extend to inorganic systems. Continued advances in molecular modeling and engineering will facilitate the construction of new logic gates, further expanding the utility of biomolecular computing.

Signal-off photoelectrochemical aptasensor for kanamycin: Strand displacement reaction combines p-n competition

A"signal-off" photoelectrochemical aptasensor based on p-n type semiconductor competitive quenching effect and strand displacement reaction was constructed for the determination of kanamycin. Au NPs@MgIn₂S₄-graphene composite was used as n-type photoactive semiconductor material. In the presence of the kanamycin, strand displacement reaction was triggered and the p-type CuInS₂ quantum dots labeled aptamer was introduced on the Au NPs@MgIn₂S₄-graphene surface. The CuInS₂ quantum dots can competitive consume the electron donors (AA) and light energy of the PEC system, thus quenched the anodic photocurrent of Au NPs@MgIn₂S₄-graphene. The photocurrent decreased with the increase of kanamycin concentration. The linear range of kanamycin was 1.0 pM-10 μM, and the detection limit was 1.7 pM. In addition, the method can be used for the determination of kanamycin in milk and honey.

Dynamically elongated associative toehold for tuning DNA circuit kinetics and thermodynamics

Associative toehold is a powerful concept enabling efficient combinatorial computation in DNA circuit. A longer association length boosts circuit kinetics and equilibrium signal but results in higher leak rate. We reconcile this trade-off by using a hairpin lock design to dynamically elongate the effective associative toehold length in response to the input target. Design guidelines were established to achieve robust elongation without incurring additional leakages. Three hairpin initiators with different combinations of elongated associative toehold (4 → 6 nt, 5 → 8 nt and 6 → 9 nt) were shortlisted from the design framework for further discussion. The circuit performance improved in terms of reaction kinetics, equilibrium signal generated and limit of detection. Overall, the elongated associative toehold served as a built-in function to stabilize and favour the forward, desired reaction when triggered.

Cofactor-assisted three-way DNA junction-driven strand displacement

Toehold-mediated strand displacement is widely used to construct and operate DNA nanodevices. Cooperative regulation of strand displacement with diverse factors is pivotal in the design and construction of functional and dynamic devices. Herein, a cofactor-assisted three-way DNA junction-driven strand displacement strategy was reported, which could tune the reaction kinetics by the collaboration of DNA and other types of stimulus. This strategy is responsive to various inputs by incorporation of the specific sequence into the three-way junction structure. Specifically, the cooperation of multiple factors changes the conformation of the specific domain and promotes the reaction. To demonstrate the strategy, adenosine triphosphate (ATP), HG²⁺, and pH were used as cofactors to modulate the displacement reaction. The electrophoresis and fluorescence experiments showed that the cooperative regulation of the strand displacement reaction could be achieved by diverse factors using this strategy. The proposed strategy provides design flexibility for dynamic DNA devices and may have potential in biosensing and biocomputing.

Cooperative regulation of strand displacement with diverse factors was achieved by a cofactor-assisted three-way DNA junction-driven strategy. Using this strategy nanodevices reacted to various inputs by incorporating a specific sequence into the three-way junction structure.

DNAr: An R Package to Simulate and Analyze CRN and DSD Networks

Chemical reaction networks (CRNs) have been proposed as an abstraction for molecular computing. DNA strand displacement (DSD) reactions are good candidates to realize this endeavor, since DNA strands can be wired to implement the desired dynamic behavior in a test tube. Specialists use simulators to help them design such chemical systems before experimental implementation. In this sense, we present the DNAr package, an alternative open-source tool, developed in R language, for users from multidisciplinary areas. The current version of our tool offers functions to simulate CRNs, convert a formal CRN into a DSD network, interpret results, export to Visual DSD, and create libraries. Here, we use the consensus CRN to show DNAr features and a neural network model to demonstrate scalability, simulating more than 600 chemical reactions in a few minutes.

Logic Gates Based on DNA Aptamers

DNA bio-computing is an emerging trend in modern science that is based on interactions among biomolecules. Special types of DNAs are aptamers that are capable of selectively forming complexes with target compounds. This review is devoted to a discussion of logic gates based on aptamers for the purposes of medicine and analytical chemistry. The review considers different approaches to the creation of logic gates and identifies the general algorithms of their creation, as well as describes the methods of obtaining an output signal which can be divided into optical and electrochemical. Aptameric logic gates based on DNA origami and DNA nanorobots are also shown. The information presented in this article can be useful when creating new logic gates using existing aptamers and aptamers that will be selected in the future.

Recognition of Helicobacter pylori by protein-targeting aptamers

BACKGROUND

Helicobacter pylori (H pylori) is a disease-causing pathogen capable of surviving under acidic conditions of the human stomach. Almost half of the world's population is infected with H pylori, with gastric cancer being the most unsatisfactory prognosis. Although H pylori has been discovered 30 years ago, the effective treatment and elimination of H pylori continue to be problematic.

MATERIALS AND METHODS

In our study, we screened nucleic acid aptamers using H pylori surface recombinant antigens as targets. Trypsin was used for separating aptamers that were bound to proteins. Following nine rounds of screening, we performed sequence similarity analyses to assess whether the aptamers can recognize the target protein. Two sequences with desirable recognition ability were selected for affinity detection. Aptamer Hp4 with the strongest binding ability to the H pylori surface recombinant antigen was chosen. After optimization of the binding conditions, we conducted specificity tests for Hp4 using Escherichia coli, Staphylococcus aureus, Vibrioanguillarum, and H pylori.

RESULTS

The data indicated that the aptamer Hp4 had an equilibrium dissociation constant (K_d ) of 26.48 ± 5.72 nmol/L to the target protein. This aptamer was capable of exclusively detecting H pylori cells, without displaying any specificity for other bacteria.

CONCLUSIONS

We obtained a high-affinity aptamer for H pylori, which is expected to serve as a new molecular probe for detection of H pylori.

Duplexed aptamers: history, design, theory, and application to biosensing

Nucleic acid aptamers are single stranded DNA or RNA sequences that specifically bind a cognate ligand. In addition to their widespread use as stand-alone affinity binding reagents in analytical chemistry, aptamers have been engineered into a variety of ligand-specific biosensors, termed aptasensors. One of the most common aptasensor formats is the duplexed aptamer (DA). As defined herein, DAs are aptasensors containing two nucleic acid elements coupled via Watson-Crick base pairing: (i) an aptamer sequence, which serves as a ligand-specific receptor, and (ii) an aptamer-complementary element (ACE), such as a short DNA oligonucleotide, which is designed to hybridize to the aptamer. The ACE competes with ligand binding, such that DAs generate a signal upon ligand-dependent ACE-aptamer dehybridization. DAs possess intrinsic advantages over other aptasensor designs. For example, DA biosensing designs generalize across DNA and RNA aptamers, DAs are compatible with many readout methods, and DAs are inherently tunable on the basis of nucleic acid hybridization. However, despite their utility and popularity, DAs have not been well defined in the literature, leading to confusion over the differences between DAs and other aptasensor formats. In this review, we introduce a framework for DAs based on ACEs, and use this framework to distinguish DAs from other aptasensor formats and to categorize cis- and trans-DA designs. We then explore the ligand binding dynamics and chemical properties that underpin DA systems, which fall under conformational selection and induced fit models, and which mirror classical SN1 and SN2 models of nucleophilic substitution reactions. We further review a variety of in vitro and in vivo applications of DAs in the chemical and biological sciences, including riboswitches and riboregulators. Finally, we present future directions of DAs as ligand-responsive nucleic acids. Owing to their tractability, versatility and ease of engineering, DA biosensors bear a great potential for the development of new applications and technologies in fields ranging from analytical chemistry and mechanistic modeling to medicine and synthetic biology.

Solution of Equations Based on Analog DNA Strand Displacement Circuits

Deoxyribonucleic acid (DNA) strand displacement can be used to build complex functional circuits due to its highly modular and programmable properties. While DNA strand displacement is most often used to solve logic problems, it can also be used to compute the roots of equations. In this paper, we present the design of novel architectures for catalysis, degradation, and annihilation in ideal formal reaction modules, and we translate these reaction modules to DNA networks. These ideal formal or DNA reaction modules are suitable for building analog circuits for solving tasks. The computing analog DNA circuits are assessed by solving a linear equation, a one-variable quadratic equation, and a set of two simultaneous linear equations. The results were evaluated by simulation.

Designed and Evolved Nucleic Acid Nanotechnology: Contrast and Complementarity

In this review, we explore progress on DNA aptamers (evolved DNA), DNA circuits (designed DNA), and the newest projects that integrate both. Designed DNA nanotechnology includes static nanostructures, dynamic nanodevices, and reaction networks (sometimes called DNA circuits). DNA circuits are dynamic DNA reactions that perform computations and sequence-specific amplification. Directed evolution can be used to produce DNA that can recognize specific targets. Aptamers are evolved nucleic acids; they are produced artificially with an in vitro selection process. DNA aptamers are molecular recognition elements made of single-stranded DNA (ssDNA) with the potential to interact with proteins, small molecules, viruses, and even cells. Designed molecular structures can incorporate aptamers for applications with immediate practical impact.

Enzyme-free ultrasensitive fluorescence detection of epithelial cell adhesion molecules based on a toehold-aided DNA recycling amplification strategy

Epithelial cell adhesion molecules (EpCAMs) play a significant role in tumorigenesis and tumor development. EpCAMs are considered to be tumor signaling molecules for cancer diagnosis, prognosis and therapy. Herein, an enzyme-free and highly sensitive fluorescent biosensor, with a combined aptamer-based EpCAM recognition and toehold-aided DNA recycling amplification strategy, was developed for sensitive and specific fluorescence detection of EpCAMs. Due to highly specific binding between EpCAMs and corresponding aptamers, strand a, which is released from the complex of aptamer/strand a in the presence of EpCAMs which is bound to the corresponding aptamer, triggered the toehold-mediated strand displacement process. An amplified fluorescent signal was achieved by recycling strand a for ultrasensitive EpCAM detection with a detection limit as low as 0.1 ng mL-1, which was comparable or superior to that of reported immunoassays and biosensor strategies. In addition, high selectivity towards EpCAMs was exhibited when other proteins were selected as control proteins. Finally, this strategy was successfully used for the ultrasensitive fluorescence detection of EpCAMs in human serum samples with satisfactory results. Importantly, the present strategy may be also expanded for the detection of other targets using the corresponding aptamers.

A universal split spinach aptamer (USSA) for nucleic acid analysis and DNA computation

We demonstrate how a single universal spinach aptamer (USSA) probe can be used to detect multiple (potentially any) nucleic acid sequences. USSA can be used for cost-efficient and highly selective analysis of even folded DNA and RNA analytes, as well as for the readout of outputs of DNA logic circuits.

Identifying Structure in Short DNA Scaffolds Using Solid-State Nanopores

The identification of molecular tags along nucleic acid sequences has many potential applications in bionanotechnology, disease biomarker detection, and DNA sequencing. An attractive approach to this end is the use of solid-state nanopores, which can electrically detect molecular substructure and can be integrated into portable lab-on-a-chip sensors. We present here a DNA origami-based approach of molecular assembly in which solid-state nanopores are capable of differentiating 165 bp scaffolds containing zero, one, and two dsDNA protrusions. This highly scalable technique requires minimal sample preparation and is customizable for a wide range of targets and applications. As a proof-of-concept, an aptamer-based DNA displacement reaction is performed in which a dsDNA protrusion is formed along a 255 bp scaffold in the presence of ATP. While ATP is too small to be directly sensed using conventional nanopore methods, our approach allows us to detect ATP by identifying molecular substructure along the DNA scaffold.

Facile conversion of ATP-binding RNA aptamer to quencher-free molecular aptamer beacon

We have developed RNA-based quencher-free molecular aptamer beacons (RNA-based QF-MABs) for the detection of ATP, taking advantage of the conformational changes associated with ATP binding to the ATP-binding RNA aptamer. The RNA aptamer, with its well-defined structure, was readily converted to the fluorescence sensors by incorporating a fluorophore into the loop region of the hairpin structure. These RNA-based QF-MABs exhibited fluorescence signals in the presence of ATP relative to their low background signals in the absence of ATP. The fluorescence emission intensity increased upon formation of a RNA-based QF-MAB·ATP complex.

Experimental Realization of a High-Quality Biochemical XOR Gate

We report an experimental realization of a biochemical XOR gate function that avoids many of the pitfalls of earlier realizations based on biocatalytic cascades. Inputs-represented by pairs of chemicals-cross-react to largely cancel out when both are nearly equal. The cross-reaction can be designed to also optimize gate functioning for noise handling. When not equal, the residual inputs are further processed to result in the output of the XOR type, by biocatalytic steps that allow for further gate-function optimization. The quality of the realized XOR gate is theoretically analyzed.

Biomimetic Molecular Signaling using DNA Walkers on Microparticles

We report the release of catalytic DNA walkers from hydrogel microparticles and the detection of those walkers by substrate-coated microparticles. This might be considered a synthetic biology analog of molecular signal release and reception. One type of particles was coated with components of a DNA one-step strand displacement (OSD) reaction to release the walker. A second type of particle was coated with substrate (or "track") for the molecular walker. We distinguish these particle types using fluorescence barcoding: we synthesized and distinguished multiple particle types with multicolor fluorescence microscopy and automated image analysis software. This represents a step toward amplified, multiplex, and microscopically localized detection based on DNA nanotechnology.

Surface Modification of Gold Nanoparticles with Small Molecules for Biochemical Analysis

As one of the major tools for and by chemical science, biochemical analysis is becoming increasingly important in fields like clinical diagnosis, food safety, environmental monitoring, and the development of chemistry and biochemistry. The advancement of nanotechnology boosts the development of analytical chemistry, particularly the nanoparticle (NP)-based approaches for biochemical assays. Functional NPs can greatly improve the performance of biochemical analysis because they can accelerate signal transduction, enhance the signal intensity, and enable convenient signal readout due to their unique physical and chemical properties. Surface chemistry is a widely used tool to functionalize NPs, and the strategy for surface modification is of great significance to the application of NP-mediated biochemical assays. Surface chemistry not only affects the quality of NPs (stability, monodispersity, and biocompatibility) but also provides functional groups (-COO^-, -NH₃⁺, -CHO, and so on) or charges that can be exploited for bioconjugation or ligand exchange. Surface chemistry also dictates the sensitivity and specificity of the NP-mediated biochemical assays, since it is vital to the orientation, accessibility, and bioactivity of the functionalized ligands on the NPs. In this Account, we will focus on surface chemistry for functionalization of gold nanoparticles (AuNPs) with small organic molecules for biochemical analysis. Compared to other NPs, AuNPs have many merits including controllable synthesis, easy surface modification and high molar absorption coefficient, making them ideal probes for biochemical assays. Small-molecule functionalized AuNPs are widely employed to develop sensors for biochemical analysis, considering that small molecules, such as amino acids and sulfhydryl compounds, are more easily and controllably bioconjugated to the surface of AuNPs than biomacromolecules due to their less complex structure and steric hindrance. The orientation and accessibility of small molecules on AuNPs in most cases can be precisely controlled without compromising their bioactivity as well, thus ensuring the performance, such as the specificity and sensitivity, of AuNP-based biochemical assays. This Account reviews recent progress in the surface chemistry of functionalized AuNPs for biochemical assays. The surface chemistries mainly include click chemistry, ligand exchange reaction, and coordination-based recognition. These surface-modified AuNPs allow for assaying a range of important biochemical markers including metal ions, small biomolecules, enzymes, and antigens and antibodies. Applications of these systems range from environmental monitoring to medical diagnostics. This Account highlights the advantages and limitations (sensitivity, detection efficiency, and stability) that AuNP-mediated assays still have compared with conventional analytical methods. This Account also discusses the future research directions of surface-modified AuNP-mediated biochemical analysis. The main aim of this Account is to summarize the current surface modification strategies for AuNPs and further demonstrate how to make use of surface modification strategies to effectively improve the performance of AuNP-mediated analytical methods for a wide variety of applications relating to biochemical analysis.

Stimuli-Responsive Self-Assembled DNA Nanomaterials for Biomedical Applications

Stimuli-responsive DNA-based materials represent a major class of remarkable functional nanomaterials for nano-biotechnological applications. In this review, recent progress in the development of stimuli-responsive systems based on self-assembled DNA nanostructures is introduced and classified. Representative examples are presented in terms of their design, working principles and mechanisms to trigger the response of the stimuli-responsive DNA system upon expose to a large variety of stimuli including pH, metal ions, oligonucleotides, small molecules, enzymes, heat, and light. Substantial in vitro studies have clearly revealed the advantages of the use of stimuli-responsive DNA nanomaterials in different biomedical applications, particularly for biosensing, drug delivery, therapy and diagnostic purposes in addition to bio-computing. Some of the challenges faced and suggestions for further development are also highlighted.

Repressor logic modules assembled by rolling circle amplification platform to construct a set of logic gates

Small molecule metabolites and their allosterically regulated repressors play an important role in many gene expression and metabolic disorder processes. These natural sensors, though valuable as good logic switches, have rarely been employed without transcription machinery in cells. Here, two pairs of repressors, which function in opposite ways, were cloned, purified and used to control DNA replication in rolling circle amplification (RCA) in vitro. By using metabolites and repressors as inputs, RCA signals as outputs, four basic logic modules were constructed successfully. To achieve various logic computations based on these basic modules, we designed series and parallel strategies of circular templates, which can further assemble these repressor modules in an RCA platform to realize twelve two-input Boolean logic gates and a three-input logic gate. The RCA-output and RCA-assembled platform was proved to be easy and flexible for complex logic processes and might have application potential in molecular computing and synthetic biology.

Integration of DNA and graphene oxide for the construction of various advanced logic circuits

Multiple advanced logic circuits including the full-adder, full-subtract and majority logic gate have been successfully realized on a DNA/GO platform for the first time. All the logic gates were implemented in an enzyme-free condition. The investigation provides a wider field of vision towards prototypical DNA-based algebra logical operations and promotes the development of advanced logic circuits.

Effective construction of a AuNPs–DNA system for the implementation of various advanced logic gates

Four advanced logic gates were successfully realized under enzyme-free conditions by integration of DNA and AuNPs.

Cleavage-based hybridization chain reaction for electrochemical detection of thrombin

In the present work, we constructed a new label-free "inter-sandwich" electrochemical aptasensor for thrombin (TB) detection by employing a cleavage-based hybridization chain reaction (HCR). The designed single-stranded DNA (defined as binding DNA), which contained the thrombin aptamer binding sequence, a DNAzyme cleavage site and a signal reporter sequence, was first immobilized on the electrode. In the absence of a target TB, the designed DNAzymes could combine with the thrombin aptamer binding sequence via complementary base pairing, and then Cu(2+) could cleave the binding DNA. In the presence of a target TB, TB could combine with the thrombin aptamer binding sequence to predominantly form an aptamer-protein complex, which blocked the DNAzyme cleavage site and prevented the binding DNA from being cleaved by Cu(2+)-dependent DNAzyme. As a result, the signal reporter sequence could leave the electrode surface to trigger HCR with the help of two auxiliary DNA single-strands, A1 and A2. Then, the electron mediator hexaammineruthenium (III) chloride ([Ru(NH3)6](3+)) was embedded into the double-stranded DNA (dsDNA) to produce a strong electrochemical signal for the quantitative measurement of TB. For further amplification of the electrochemical signal, graphene reduced by dopamine (PDA-rGO) was introduced as a platform in this work. With this strategy, the aptasensor displayed a wide linearity in the range of 0.0001 nM to 50 nM with a low detection limit of 0.05 pM. Moreover, the resulting aptasensor exhibited good specificity and acceptable reproducibility and stability. Because of these factors, the fabrication protocol proposed in this work may be extended to clinical application.

How to split a G-quadruplex for DNA detection: new insight into the formation of DNA split G-quadruplex

Here, we get a new insight into the formation of a split G-quadruplex from the viewpoints of the split mode and guanine base number. An unusual result is that the split mode 4 : 8 performed best in six split modes, including the frequently used mode 1 : 3 and 2 : 2 in the split G-quadruplex enhanced fluorescence assay. Circular dichroism spectra verified the conclusion. The application of the split G-quadruplex based assay in DNA detection was performed on the point mutations of the JAK2 V617F and HBB genes. A multi-target analysis method based on a pool of G-segments split from T30695 (GGGTGGGTGGGTGGGT) by the magic "law of 4 : 8" was established.

Dual-channel detection of metallothioneins and mercury based on a mercury-mediated aptamer beacon using thymidine-mercury-thymidine complex as a quencher

A novel dual-channel strategy for the detection of metallothioneins (MTs) and Hg(2+) has been developed based on a mercury-mediated aptamer beacon (MAB) using thymidine-mercury-thymidine complex as a quencher for the first time. In the presence of Hg(2+), the T-rich oligonucleotide with a 6-carboxyfluorescein (TRO-FAM) can form an aptamer beacon via the formation of T-Hg(2+)-T base pairs, which results in a fluorescence quenching of the sensing system owing to the fluorescence resonance energy transfer (FRET) from the fluorophore of FAM to the terminated T-Hg(2+)-T base pair. The addition of MTs into this solution leads to the disruption of the T-Hg(2+)-T complex, resulting in an increase of the fluorescent signal of the system. In the optimizing condition, ΔF was directly proportional to the concentrations ranging from 5.63 nM to 0.275 μM for MTs, and 14.2 nM to 0.30 μM for Hg(2+) with the detection limits of 1.69 nM and 4.28 nM, respectively. The proposed dual-channel method avoids the label steps of a quencher in common molecular beacon strategies, without tedious procedure or the requirement of sophisticated equipment, and is rapid, inexpensive and sensitive.

A universal molecular translator for non-nucleic acid targets that enables dynamic DNA assemblies and logic operations

A universal molecular translator based on the target-triggered DNA strand displacement was developed, which was able to convert various kinds of non-nucleic acid targets into a unique output DNA. This translation strategy was successfully applied in directing dynamic DNA assemblies and in realizing three-input logic gate operations.

A smart tailor-made G-clip reporter for sensitive detection of G-triplet-containing sequences

Taking advantage of the intrinsic characteristics of G-triplet-containing sequences, a pioneering tailor-made clip-like reporter containing three-fourths of a G-quadruplex is established. The reporter can clip the G triplet in the target sequence through a recognition process to form a complete G-quadruplex structure.

Construction of a fuzzy and Boolean logic gates based on DNA

Logic gates are devices that can perform logical operations by transforming a set of inputs into a predictable single detectable output. The hybridization properties, structure, and function of nucleic acids can be used to make DNA-based logic gates. These devices are important modules in molecular computing and biosensing. The ideal logic gate system should provide a wide selection of logical operations, and be integrable in multiple copies into more complex structures. Here we show the successful construction of a small DNA-based logic gate complex that produces fluorescent outputs corresponding to the operation of the six Boolean logic gates AND, NAND, OR, NOR, XOR, and XNOR. The logic gate complex is shown to work also when implemented in a three-dimensional DNA origami box structure, where it controlled the position of the lid in a closed or open position. Implementation of multiple microRNA sensitive DNA locks on one DNA origami box structure enabled fuzzy logical operation that allows biosensing of complex molecular signals. Integrating logic gates with DNA origami systems opens a vast avenue to applications in the fields of nanomedicine for diagnostics and therapeutics.

Versatile G-quadruplex-mediated strategies in label-free biosensors and logic systems

This review addresses how G-quadruplex (G4)-mediated biosensors convert the events of target recognition into a measurable physical signal. The application of label-free G4-strategies in the construction of logic systems is also discussed.

Toehold strand displacement-driven assembly of G-quadruplex DNA for enzyme-free and non-label sensitive fluorescent detection of thrombin

Based on a new signal amplification strategy by the toehold strand displacement-driven cyclic assembly of G-quadruplex DNA, the development of an enzyme-free and non-label aptamer sensing approach for sensitive fluorescent detection of thrombin is described. The target thrombin associates with the corresponding aptamer of the partial dsDNA probes and liberates single stranded initiation sequences, which trigger the toehold strand displacement assembly of two G-quadruplex containing hairpin DNAs. This toehold strand displacement reaction leads to the cyclic reuse of the initiation sequences and the production of DNA assemblies with numerous G-quadruplex structures. The fluorescent dye, N-Methyl mesoporphyrin IX, binds to these G-quadruplex structures and generates significantly amplified fluorescent signals to achieve highly sensitive detection of thrombin down to 5 pM. Besides, this method shows high selectivity towards the target thrombin against other control proteins. The developed thrombin sensing method herein avoids the modification of the probes and the involvement of any enzyme or nanomaterial labels for signal amplification. With the successful demonstration for thrombin detection, our approach can be easily adopted to monitor other target molecules in a simple, low-cost, sensitive and selective way by choosing appropriate aptamer/ligand pairs.