Signal propagation in multi-layer DNAzyme cascades using structured chimeric substrates

Recent Advances in DNA-Based Nanoprobes for In vivo MiRNA Imaging

As a post transcriptional regulator of gene expression, microRNAs (miRNA) is closely related to many major human diseases, especially cancer. Therefore, its precise detection is very important for disease diagnosis and treatment. With the advancement of fluorescent dye and imaging technology, the focus has shifted from in vitro miRNA detection to in vivo miRNA imaging. This concept review summarizes signal amplification strategies including DNAzyme catalytic reaction, hybrid chain reaction (HCR), catalytic hairpin assembly (CHA) to enhance detection signal of lowly expressed miRNAs; external stimuli of ultraviolet (UV) light or near-infrared region (NIR) light, and internal stimuli such as adenosine triphosphate (ATP), glutathione (GSH), protease and cell membrane protein to prevent nonspecific activation for the avoidance of false positive signal; and the development of fluorescent probes with emission in NIR for in vivo miRNA imaging; as well as rare earth nanoparticle based the second near-infrared window (NIR-II) nanoprobes with excellent tissue penetration and depth for in vivo miRNA imaging. The concept review also indicated current challenges for in vivo miRNA imaging including the dynamic monitoring of miRNA expression change and simultaneous in vivo imaging of multiple miRNAs.

Split aptazyme-based signal amplification for AβO analysis

Aptazyme is a chimera of functional nucleic acids, which integrates recognition and amplification elements to simplify the assay process and improve sensing efficiency. However, its application may be limited by signal leakage. In this work, we innovatively integrate the AβO aptamer and an MNAzyme (multicomponent nucleic acid enzyme) for highly efficient detection of AβO. The aptamer and half of the MNAzyme are positioned at one strand, and the other half of the MNAzyme is integrated with a toehold sequence. These two sequences cannot hybridize to activate the MNAzyme until the target is added. The background signal is significantly reduced by the split format and the secondary structure of DNA probes formed in the absence of the target. The proposed aptazyme can not only achieve amplification through enzymatic catalysis but also greatly improve the efficiency of signal transduction and output. We systematically investigated the influence of different DNA probes on the detection performance, and the optimized aptazyme can detect as low as 26.5 pM targets in 1h. The stability of this method was also investigated by detection targets in real biological samples.

Programmable Molecular Signal Transmission Architecture and Reactant Regeneration Strategy Driven by EXO λ for DNA Circuits

The characteristics of DNA hybridization enable molecular computing through strand displacement reactions, facilitating the construction of complex DNA circuits, which is an important way to realize information interaction and processing at a molecular level. However, signal attenuation in the cascade and shunt process hinders the reliability of the calculation results and further expansion of the DNA circuit scale. Here, we demonstrate a novel programmable exonuclease-assisted signal transmission architecture, where DNA strand with toehold employed to inhibit the hydrolysis process of EXO λ is applied in DNA circuits. We construct a series circuit with variable resistance and a parallel circuit with constant current source, ensuring excellent orthogonal properties between input and output sequences while maintaining low leakage (<5%) during the reaction. Additionally, a simple and flexible exonuclease-driven reactant regeneration (EDRR) strategy is proposed and applied to construct parallel circuits with constant voltage sources that could amplify the output signal without extra DNA fuel strands or energy. Furthermore, we demonstrate the effectiveness of the EDRR strategy in reducing signal attenuation during cascade and shunt processes by constructing a four-node DNA circuit. These findings offer a new approach to enhance the reliability of molecular computing systems and expand the scale of DNA circuits in the future.

Modular Assembly of a Concatenated DNA Circuit for In Vivo Amplified Aptasensing

Probing endogenous molecular profiles in living entities is of fundamental significance to decipher biological functions and exploit novel theranostics. Despite programmable nucleic acid-based aptasensing systems across the breadth of molecular imaging, an aptasensing system enabling in vivo imaging with high sensitivity, accuracy, and adaptability is highly required yet is still in its infancy. Artificial catalytic DNA circuits that can modularly integrate to generate multiple outputs from a single input in an isothermal autonomous manner, have supplemented powerful toolkits for intracellular biosensing research. Herein, a multilayer nonenzymatic catalytic DNA circuits-based aptasensing system is devised for in situ imaging of a bioactive molecule in living mice by assembling branched DNA copolymers with high-molecular-weight and high-signal-gain based on avalanche-mimicking hybridization chain reactions (HCRs). The HCRs aptasensing circuit performs as a general and powerful sensing platform for precise analysis of a series of bioactive molecules due to its inherent rich recognition repertoire and hierarchical reaction accelerations. With tumor-targeting capsule encapsulation, the HCRs aptasensing circuit is specifically delivered into tumor cells and allowed the high-contrast imaging of intracellular adenosine triphosphate in living mice, highlighting its potential for visualizing these clinically important biomolecules and for studying the associated physiological processes.

Complexing deoxyribozymes with RNA aptamers for detection of the small molecule theophylline

Biointegrative information processing systems offer a great advantage to autonomous biodevices, as their capacity for biological computation provides the ability to sense the state of more complex environments and better integrate with downstream biological regulation systems. Deoxyribozymes (DNAzymes) and aptamers are of interest to such computational biosensing systems due to the enzymatic properties of DNAzymes and the ligand-inducible conformational structures of aptamers. Herein, we describe a novel method for providing ligand-responsive allosteric control to a DNAzyme using an RNA aptamer. We designed a NOT-logic-compliant E6 DNAzyme to be complementary to an RNA aptamer targeting theophylline, such that the aptamer competitively interacted with either theophylline or the DNAzyme, and disabled the DNAzyme only when theophylline concentration was below a given threshold. Out of our seven designed "complexing aptazymes," three demonstrated effective theophylline-responsive allosteric regulation (2.84 ± 3.75%, 4.97 ± 2.92%, and 8.91 ± 4.19% activity in the absence of theophylline; 46.29 ± 3.36%, 50.70 ± 10.15%, and 61.26 ± 6.18% activity in the presence of theophylline). Moreover, the same three complexing aptazymes also demonstrated the ability to semi-quantitatively determine the concentration of theophylline present in solution, successfully discriminating between therapeutically ineffective (<20 μM), safe (20-100 μM), and toxic (>100 μM) theophylline concentrations. Our method of using an RNA aptamer for ligand-responsive allosteric control of a DNAzyme expands the way aptamers can be configured for biosensing, and suggests a pathway for embedding DNAzymes to provide enhanced information processing and control of biological systems.

Programmable allosteric DNA regulations for molecular networks and nanomachines

Structure-based molecular regulations have been widely adopted to modulate protein networks in cells and recently developed to control allosteric DNA operations in vitro. However, current examples of programmable allosteric signal transmission through integrated DNA networks are stringently constrained by specific design requirements. Developing a new, more general, and programmable scheme for establishing allosteric DNA networks remains challenging. Here, we developed a general strategy for programmable allosteric DNA regulations that can be finely tuned by varying the dimensions, positions, and number of conformational signals. By programming the allosteric signals, we realized fan-out/fan-in DNA gates and multiple-layer DNA cascading networks, as well as expanding the approach to long-range allosteric signal transmission through tunable DNA origami nanomachines ~100 nm in size. This strategy will enable programmable and complex allosteric DNA networks and nanodevices for nanoengineering, chemical, and biomedical applications displaying sense-compute-actuate molecular functionalities.

Constructing DNA logic circuits based on the toehold preemption mechanism

Strand displacement technology and ribozyme digestion technology have enriched the intelligent toolbox of molecular computing and provided more methods for the construction of DNA logic circuits. In recent years, DNA logic circuits have developed rapidly, and their scalability and accuracy in molecular computing and information processing have been fully demonstrated. However, existing DNA logic circuits still have some problems such as high complexity of DNA strands (number of DNA strands) hindering the expansion of practical computing tasks. In view of the above problems, we presented a toehold preemption mechanism and applied it to construct DNA logic circuits using E6-type DNAzymes, such as half adder circuit, half subtractor circuit, and 4-bit square root logic circuit. Different from the dual-track logic expressions, all the signals in the circuits of this study were monorail which substantially reduced the number of DNA strands in the DNA logic circuits. The presented preemption mechanism provides a way to simplify the implementation of large and complex DNA integrated circuits.

A Novel DNAzyme Signal Amplification-based Colorimetric Method for RNase H Assays

A simple visual strategy was developed for the RNase H colorimetric measurement using DNAzyme-mediated signal amplification. When RNase H was presented, the RNA strand of the duplex formed by the G-rich DNA sequence (G-Rich) and its complementary RNA sequence (cp-RNA) was digested, releasing G-Rich to form HRP-mimicking DNAzymes of the G-quadruplex/hemin complexes in the presence of hemin. These DNAzymes catalyze the oxidation reaction of the substrate of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) to produce green-color products of ABTS^•-, allowing for the detection of RNase H. A horseradish peroxidase (HRP)-mimicking DNAzyme of the G-quadruplex/hemin complex was used to mediate the signal amplification in the sensing strategy, resulting in high selectivity and sensitivity. This proposed colorimetric method shows a low detection limit of 0.04 U/mL, with a detection range of 0.1 to 3 U/mL. Moreover, this colorimetric method has been successfully used for RNase H assays in complicated biosamples, such as cell lysates. These results indicate that our colorimetric method not only detects RNase H in an ideal system, but also in real samples.

Stimuli-Responsive Autonomous-Motion Molecular Machine for Sensitive Simultaneous Fluorescence Imaging of Intracellular MicroRNAs

DNAzymes with enzymatic activity identified from random DNA pools by in vitro selection have recently attracted considerable attention. In this work, a DNAzyme-based autonomous-motion (AM) molecular machine is demonstrated for sensitive simultaneous imaging of different intracellular microRNAs (miRNAs). The AM molecular machine consists of two basic elements, one of which is a target-analogue-embedded double-stem hairpin substrate (TDHS) and the other is a locking-strand-silenced DNAzyme (LSDz). LSDz can be activated by target miRNA and catalytically cleave TDHS, generating Clv-TDHS and releasing free target analogue capable of triggering the next round of cleavage reaction. As such, the molecular machine can exert sustainable autonomous operation, producing an enhanced signal. Because the active target analogue comes from the machine itself and offers cyclical stimulation in a feedback manner, this target-induced autonomous cleavage circuit is termed a self-feedback circuit (SFC). The SFC-based molecular machine can be used to quantify miRNA-21 down to 10 pM without interference from nontarget miRNAs, indicating a substantial improvement in assay performance compared with its counterpart system without an SFC effect. Moreover, due to the enzyme-free process, the AM molecular machine is suitable for miRNA imaging in living cells, and the quantitative results are consistent with the gold standard PCR assay. More interestingly, the AM molecular machine can be used for the simultaneous fluorescence imaging of several intracellular miRNAs, enabling the accurate discrimination of cancerous cells (e.g., HeLa and MCF-7) from healthy cells. The SFC-based autonomous-motion machine is expected to be a promising tool for the research of molecular biology and early diagnosis of human diseases.

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.

RNA-Cleaving DNA Thresholder Controlled by Concentrations of miRNA Cancer Marker

Oligonucleotide gene therapy (OGT) agents suppress specific mRNAs in cells and thus reduce the expression of targeted genes. The ability to unambiguously distinguish cancer from healthy cells can solve the low selectivity problem of OGT agents. Cancer RNA markers are expressed in both healthy and cancer cells with a higher expression level in cancer cells. We have designed a DNA-based construct, named DNA thresholder (DTh) that cleaves targeted RNA only at high concentrations of cancer marker RNA and demonstrates low cleavage activity at low marker concentrations. The RNA-cleaving activity can be adjusted within one order of magnitude of the cancer marker RNA concentration by simply redesigning DTh. Importantly, DTh recognizes cancer marker RNA, while cleaving targeted RNA; this offers a possibility to suppress vital genes exclusively in cancer cells, thus triggering their death. DTh is a prototype of computation-inspired molecular device for controlling gene expression and cancer treatment.

Scalable Logic Circuits with Multiple Outputs and an Automatic Reset Function Based on DNAzyme-Mediated Branch Migration

A scalable logic platform made up of multilayer DNA circuits was constructed using Pb²⁺, Cu²⁺, and Zn²⁺ as the three inputs and three different fluorescent signals as the outputs. DNAzyme-guided cyclic cleavage reactions and DNA toehold-mediated strand branch migration were utilized to organize and connect nucleic acid probes for building the high-level logic architecture. The sequence communications between each circuit enable the logic network to work as a keypad lock, which is an information protection model at the molecular level. The multi-output mode was used to monitor the gradual unlocking process of the security system, from which one can determine which password is correct or not immediately. The autocatalytic cleavage of DNAzyme makes the biocomputing circuit feasible to realize the reset function automatically without external stimuli. Importantly, the logic platform is robust and can work effectively even in complex environmental samples.

Domain-Specific Programming Languages for Computational Nucleic Acid Systems

The construction of models of system behavior is of great importance throughout science and engineering. In bioengineering and bionanotechnology, these often take the form of dynamic models that specify the evolution of different species over time. To ensure that scientific observations and conclusions are consistent and that systems can be reliably engineered on the basis of model predictions, it is important that models of biomolecular systems can be constructed in a reliable, principled, and efficient manner. This review focuses on efforts to address this need by using domain-specific programming languages as the basis for custom design tools for researchers working on computational nucleic acid devices, where a domain-specific language is simply a programming language tailored to a particular application domain. The underlying thesis of our review is that there is a continuum of practical implementation strategies for computational nucleic acid systems, which can all benefit from appropriate domain-specific languages and software design tools. We emphasize the need for specialized yet flexible tools that can be realized using domain-specific languages that compile to more general-purpose representations.

A Thioxanthone Sensitizer with a Chiral Phosphoric Acid Binding Site: Properties and Applications in Visible Light-Mediated Cycloadditions

A chiral phosphoric acid with a 2,2'-binaphthol core was prepared that displays two thioxanthone moieties at the 3,3'-position as light-harvesting antennas. Despite its relatively low triplet energy, the phosphoric acid was found to be an efficient catalyst for the enantioselective intermolecular [2+2] photocycloaddition of β-carboxyl-substituted cyclic enones (e.r. up to 93:7). Binding of the carboxylic acid to the sensitizer is suggested by NMR studies and by DFT calculations to occur by means of two hydrogen bonds. The binding event not only enables an enantioface differentiation but also modulates the triplet energy of the substrates.

A Smart, Autocatalytic, DNAzyme Biocircuit for in Vivo, Amplified, MicroRNA Imaging

DNAzymes have been recognized as promising transducing agents for visualizing endogenous biomarkers, but their inefficient intracellular delivery and limited amplification capacity (including insufficient cofactor supply) preclude their extensive biological application. Herein, an autocatalytic DNAzyme (ACD) biocircuit is constructed for amplified microRNA imaging in vivo based on a hybridization chain reaction (HCR) and DNAzyme biocatalysis, sustained by a honeycomb MnO₂ nanosponge (hMNS). The hMNS not only delivers DNA probes, but also supplies Mn²⁺ as a DNAzyme cofactor and magnetic resonance imaging (MRI) agent. Through the subsequent cross-activation of HCR and DNAzyme amplicons, the ACD amplifies the limited signal resulting from miRNA recognition. The hMNS/ACD system was used to image microRNA in vivo, thus demonstrating its great promise in cancer diagnosis.

Repeated Reuse of Deoxyribozyme-Based Logic Gates

Deoxyribozymes (DNAzymes) have demonstrated a significant capacity for biocomputing and hold promise for information processing within advanced biological devices if several key capabilities are developed. One required capability is reuse-having DNAzyme logic gates be cyclically, and controllably, activated and deactivated. We designed an oligonucleotide-based system for DNAzyme reuse that could (1) remove previously bound inputs by addition of complementary oligonucleotides via toe-hold mediated binding and (2) diminish output signal through the addition of quencher-labeled oligonucleotides complementary to the fluorophore-bound substrate. Our system demonstrated, for the first time, the ability for DNAzymes to have their activity toggled, with activity returning to 90-125% of original activity. This toggling could be performed multiple times with control being exerted over when the toggling occurs, with three clear cycles observed before the variability in activity became too great. Our data also demonstrated that fluorescent output of the DNAzyme activity could be actively removed and regenerated. This reuse system can increase the efficiency of DNAzyme-based logic circuits by reducing the number of redundant oligonucleotides and is critical for future development of reusable biodevices controlled by logical operations.

Target-induced structure switching of aptamers facilitates strand displacement for DNAzyme recycling amplification detection of thrombin in human serum

To monitor the thrombin concentration under the condition of abnormal blood coagulation is of clinical significance for the diagnosis of various diseases. Here, on the basis of the aptamer structure switching induced by the target molecules and the signal amplification strategy via recycling of metal-ion dependent DNAzymes, we have established a sensitive and simple fluorescent aptasensor for detecting thrombin in human serum. The thrombin target specifically binds to the aptamer sequence and causes a corresponding conformational structure switching, which leads to the formation of a toehold sequence to facilitate the strand migration displacement reaction for the generation of functional metal-ion dependent DNAzymes. These DNAzymes further cleave the fluorescently quenched hairpin substrates cyclically to yield substantially amplified fluorescence recovery for sensitively detecting thrombin in the dynamic range from 0.01 nM to 50 nM. Such an aptasensor shows a detection limit of 6.9 pM and can achieve the monitoring of thrombin in diluted human serum with high selectivity, offering a universal sensing strategy for the construction of various sensitive and simple aptasensors to detect different biomarker molecules.

Entropy-driven DNA logic circuits regulated by DNAzyme

The catalytic DNA circuits play a critical role in engineered biological systems and molecular information processing. Actually, some of the natural or synthetic DNA circuits were triggered by covalent modifications, where conformational changes were induced to facilitate complex DNA engineering functions and signal transmissions. However, most of the reported artificial catalytic DNA circuits were regulated by the toehold-mediated reaction. Therefore, it is significant to propose a strategy to regulate the catalytic DNA circuit not only by the toehold-mediated mechanism, but also by involving the conformational changes induced by the covalent modification. In this study, we developed the catalytic DNA logic circuits regulated by DNAzyme. Here, a regulation strategy based on the covalent modification was proposed to control the DNA circuit, combing two reaction mechanisms: DNAzyme digestion and entropy-driven strand displacement. The DNAzyme and DNA catalyst can participate into the reactions alternatively, thus realizing the cascading catalytic circuits. Using the DNAzyme regulation, a series of logic gates (YES, OR and AND) were constructed. In addition, a two-layer cascading circuit and a feedback self-catalysis circuit were also established. The proposed DNAzyme-regulated strategy shows great potentials as a reliable and feasible method for constructing more complex catalytic DNA circuits.

A Universal Paper-Based Electrochemical Sensor for Zero-Background Assay of Diverse Biomarkers

This paper describes a universal paper-based electrochemical sensing platform that uses a paper modified with signal molecule-labeled DNA and a screen-printed electrode along with target recognition solutions to achieve the detection of multiple types of biomarkers. These assays rely on the target-induced synthesis of Mg²⁺-dependent DNAzyme for catalyzing the cleavage of substrate DNA from paper, which have been demonstrated by using microRNA recognition probe for miR-21, a phosphorylated hairpin probe for alkaline phosphatase, and a DNA aptamer for carcinoembryonic antigen assays, respectively. Taking advantages of the high specific target-triggered polymerization/nicking and DNAzyme-catalyzed signal amplification, the present assays enable highly sensitive and selective detection of these targets with zero-background. These assays can also be applied to detect target in spiked serum samples, demonstrating the potential for point-of-care detection of clinical samples.

Microcompartments for Protection and Isolation of Nanoscale DNA Computing Elements

Physical isolation of molecular computing elements holds the potential for increasing system complexity by enabling the reuse of standardized components and by protecting the components from environmental degradation. However, once elements have been compartmentalized, methods for communicating into these compartments are needed. We report the compartmentalization of steroid-responsive DNA aptamers within giant unilamellar vesicles (GUVs) that are permeable to steroid inputs. Monodisperse GUVs are loaded with aptamers using a microfluidic platform. We demonstrate the target-specific activation of individual aptamers within the GUVs and then load two noninterfering aptamers into the same GUV and demonstrate specific responses to all possible combinations of the two input steroids. Crucially, GUVs prevent the degradation of DNA components by nucleases, providing a potential mechanism for deploying nucleic acid components in vivo. Importantly, our compartments also prevent nonspecific cross-talk between complementary strands, thereby providing a method for parallel execution of cross-reacting molecular logic components. Thus, we provide a mechanism for spatially organizing molecular computing elements, which will increase system modularity by allowing standardized components to be reused.

Allosteric DNAzyme-based DNA logic circuit: operations and dynamic analysis

Recently, due to the dual roles of DNA and enzyme, DNAzyme has been widely used in the field of DNA circuit, which has a wide range of applications in bio-engineered system, information processing and biocomputing. In fact, the activity of DNAzymes was regulated by subunits assembly, pH control and metal ions triggers. However, those regulations required to change the sequences of whole DNAzyme, as separating parts and inserting extra DNA sequence. Inspired by the allosteric regulation of proteins in nature, a new allosteric strategy is proposed to regulate the activity of DNAzyme without DNA sequences changes. In this strategy, DNA strand displacement was used to regulate the DNAzyme structure, through which the activity of DNAzyme was well controlled. The strategy was applied to E6-type DNAzymes, and the operations of DNA logic circuit (YES, OR, AND, cascading and feedback) were established and simulated with the dynamic analyses. The allosteric regulation has potential to construct more complicated molecular systems, which can be applied to bio-sensing and detection.

Dual-Analyte Chronopotentiometric Aptasensing Platform Based on a G-Quadruplex/Hemin DNAzyme and Logic Gate Operations

Conventional potentiometric ion sensors that rely on a specific ion carrier in a polymeric membrane can hardly achieve multianalyte detection. Inspired by the remarkable ability of built-in logic gate sensors for multianalyte detection, herein we report a potentiometric aptasensing platform based on a G-quadruplex/hemin DNAzyme and logic gate operations for determination of two analytes using a single membrane electrode. A bifunctional probe with two aptamer units and a signal reporter oligonucleotide with a DNAzyme sequence are assembled on the magnetic beads to form a DNA hybrid structure. The "OR" and "INHIBIT" logic functions can be performed by using the two aptamers and their targets as inputs, and using the chronopotentiometric response based on the G-quadruplex/hemin DNAzyme-H₂O₂-mediated oxidation of 3,3',5,5'-tetramethylbenzidine as output. Kanamycin and oxytetracycline, as commonly used antibiotics, have been employed as the models and successfully measured.

Addressable activated cascade DNA sequential logic circuit model for processing identical input molecules

A 3-bit register sequential logic circuit, constructed based on a state and activation mechanism, has a sequential storage function.

Detecting Biothreat Agents: From Current Diagnostics to Developing Sensor Technologies

Although a fundamental understanding of the pathogenicity of most biothreat agents has been elucidated and available treatments have increased substantially over the past decades, they still represent a significant public health threat in this age of (bio)terrorism, indiscriminate warfare, pollution, climate change, unchecked population growth, and globalization. The key step to almost all prevention, protection, prophylaxis, post-exposure treatment, and mitigation of any bioagent is early detection. Here, we review available methods for detecting bioagents including pathogenic bacteria and viruses along with their toxins. An introduction placing this subject in the historical context of previous naturally occurring outbreaks and efforts to weaponize selected agents is first provided along with definitions and relevant considerations. An overview of the detection technologies that find use in this endeavor along with how they provide data or transduce signal within a sensing configuration follows. Current "gold" standards for biothreat detection/diagnostics along with a listing of relevant FDA approved in vitro diagnostic devices is then discussed to provide an overview of the current state of the art. Given the 2014 outbreak of Ebola virus in Western Africa and the recent 2016 spread of Zika virus in the Americas, discussion of what constitutes a public health emergency and how new in vitro diagnostic devices are authorized for emergency use in the U.S. are also included. The majority of the Review is then subdivided around the sensing of bacterial, viral, and toxin biothreats with each including an overview of the major agents in that class, a detailed cross-section of different sensing methods in development based on assay format or analytical technique, and some discussion of related microfluidic lab-on-a-chip/point-of-care devices. Finally, an outlook is given on how this field will develop from the perspective of the biosensing technology itself and the new emerging threats they may face.

Multiple advanced logic gates made of DNA-Ag nanocluster and the application for intelligent detection of pathogenic bacterial genes

The integration of multiple DNA logic gates on a universal platform to implement advance logic functions is a critical challenge for DNA computing. Herein, a straightforward and powerful strategy in which a guanine-rich DNA sequence lighting up a silver nanocluster and fluorophore was developed to construct a library of logic gates on a simple DNA-templated silver nanoclusters (DNA-AgNCs) platform. This library included basic logic gates, YES, AND, OR, INHIBIT, and XOR, which were further integrated into complex logic circuits to implement diverse advanced arithmetic/non-arithmetic functions including half-adder, half-subtractor, multiplexer, and demultiplexer. Under UV irradiation, all the logic functions could be instantly visualized, confirming an excellent repeatability. The logic operations were entirely based on DNA hybridization in an enzyme-free and label-free condition, avoiding waste accumulation and reducing cost consumption. Interestingly, a DNA-AgNCs-based multiplexer was, for the first time, used as an intelligent biosensor to identify pathogenic genes, E. coli and S. aureus genes, with a high sensitivity. The investigation provides a prototype for the wireless integration of multiple devices on even the simplest single-strand DNA platform to perform diverse complex functions in a straightforward and cost-effective way.

Restriction Enzymes as a Target for DNA-Based Sensing and Structural Rearrangement

DNA nanostructures have been shown viable for the creation of complex logic-enabled sensing motifs. To date, most of these types of devices have been limited to the interaction with strictly DNA-type inputs. Restriction endonuclease represents a class of enzyme with endogenous specificity to DNA, and we hypothesize that these can be integrated with a DNA structure for use as inputs to trigger structural transformation and structural rearrangement. In this work, we reconfigured a three-arm DNA switch, which utilizes a cyclic Förster resonance energy transfer interaction between three dyes to produce complex output for the detection of three separate input regions to respond to restriction endonucleases, and investigated the efficacy of the enzyme targets. We demonstrate the ability to use three enzymes in one switch with no nonspecific interaction between cleavage sites. Further, we show that the enzymatic digestion can be harnessed to expose an active toehold into the DNA structure, allowing for single-pot addition of a small oligo in solution.

Enzyme-free amplified detection of circulating microRNA by making use of DNA circuits, a DNAzyme, and a catalytic hairpin assembly

A homogeneous and enzyme-free fluorometric assay is described for the determination of microRNA-182. It is based on the use of DNA circuits and DNAzyme. The DNA circuits warrant strong signal amplification by virtue of catalytic hairpin assembly, a system that consists of two hairpin substrates. A part of the DNAzyme sequence is programmed to sequester into one of the two hairpin substrates. In the presence of target microRNA-182, the two hairpin substrates undergo catalytic assembling. This results in the formation of a DNA duplex and the release of the DNAzyme from the hairpin structure. Upon cyclic amplification, one target catalyzes the formation of Mg (II)-dependent DNAzymes. These bind to, and hydrolyze, the fluorescently labeled substrates for signal amplification and transduction. Based on nucleic acid programmability, this engineered assay has a limit of detection as low as 6.8 f. and a dynamic range that covers the 10 f. to10 nM microRNA-182 concentration range. Detection can be performed within 60 min. The assay is simple, rapid, homogenous, cost-effective, and enzyme-free. These features make the method an attractive tool in routine microRNA diagnosis and, conceivably, in point of care uses. Graphical abstract Schematic of a homogeneous and enzyme-free fluorometric assay for the determination of microRNA-182. It is based on the use of DNA circuits and DNAzymes. The DNA circuits warrant strong signal amplification by virtue of catalytic hairpin assembly that uses two hairpin substrates. The method represents an attractive tool for routine microRNA diagnosis and, conceivably, point of care uses.

A Microsphere-Supported Lipid Bilayer Platform for DNA Reactions on a Fluid Surface

We report a versatile microsphere-supported lipid bilayer system that can serve as a general-purpose platform for implementing DNA nanotechnologies on a fluid surface. To demonstrate our platform, we implemented both toehold-mediated strand displacement (TMSD) and DNAzyme reactions, which are typically performed in solution and which are the cornerstone of DNA-based molecular logic and dynamic DNA nanotechnology, on the surface. We functionalized microspheres bearing supported lipid bilayers (μSLBs) with membrane-bound nucleic acid components. Using functionalized μSLBs, we developed TMSD and DNAzyme reactions by optimizing reaction conditions to reduce nonspecific interactions between DNA and phospholipids and to enhance bilayer stability. Additionally, the physical and optical properties of the bilayer were tuned via lipid composition and addition of fluorescently tagged lipids to create stable and multiplexable μSLBs that are easily read out by flow cytometry. Multiplexed TMSD reactions on μSLBs enabled the successful operation of a Dengue serotyping assay that correctly identified all 16 patterns of target sequences to demonstrate detection of DNA strands derived from the sequences of all four Dengue serotypes. The limit of detection for this assay was 3 nM. Furthermore, we demonstrated DNAzyme reactions on a fluid lipid surface, which benefit from free diffusion on the surface. This work provides the basis for expansion of both TMSD and DNAzyme based molecular reactions on supported lipid bilayers for use in molecular logic and DNA nanotechnology. As our system is multiplexable and results in fluid surfaces, it may be of use in compartmentalization and improved kinetics of molecular logic reactions and as a useful building block in a variety of DNA nanotechnology systems.

Instrument-free visual detection of tetracycline on an autocatalytic DNA machine using a caged G-quadruplex as the signal reporter

An instrument-free visual biosensor for the amplified detection of tetracycline has been successfully constructed using an autocatalytic DNA machine as the signal amplifier and a caged G-quadruplex as the signal reporter. The assay is ultrasensitive, enabling the visual detection of trace levels of tetracycline as low as 1 pM without instrumentation.

Self-Assembling Molecular Logic Gates Based on DNA Crossover Tiles

DNA-based computational hardware has attracted ever-growing attention due to its potential to be useful in the analysis of complex mixtures of biological markers. Here we report the design of self-assembling logic gates that recognize DNA inputs and assemble into crossover tiles when the output signal is high; the crossover structures disassemble to form separate DNA stands when the output is low. The output signal can be conveniently detected by fluorescence using a molecular beacon probe as a reporter. AND, NOT, and OR logic gates were designed. We demonstrate that the gates can connect to each other to produce other logic functions.

Continuous variables logic via coupled automata using a DNAzyme cascade with feedback

The concentration of molecules can be changed by chemical reactions and thereby offer a continuous readout. Yet computer architecture is cast in textbooks in terms of binary valued, Boolean variables. To enable reactive chemical systems to compute we show how, using the Cox interpretation of probability theory, one can transcribe the equations of chemical kinetics as a sequence of coupled logic gates operating on continuous variables. It is discussed how the distinct chemical identity of a molecule allows us to create a common language for chemical kinetics and Boolean logic. Specifically, the logic AND operation is shown to be equivalent to a bimolecular process. The logic XOR operation represents chemical processes that take place concurrently. The values of the rate constants enter the logic scheme as inputs. By designing a reaction scheme with a feedback we endow the logic gates with a built in memory because their output then depends on the input and also on the present state of the system. Technically such a logic machine is an automaton. We report an experimental realization of three such coupled automata using a DNAzyme multilayer signaling cascade. A simple model verifies analytically that our experimental scheme provides an integrator generating a power series that is third order in time. The model identifies two parameters that govern the kinetics and shows how the initial concentrations of the substrates are the coefficients in the power series.

DNA Computing Systems Activated by Electrochemically-triggered DNA Release from a Polymer-brush-modified Electrode Array

An array of four independently wired indium tin oxide (ITO) electrodes was used for electrochemically stimulated DNA release and activation of DNA-based Identity, AND and XOR logic gates. Single-stranded DNA molecules were loaded on the mixed poly(N,N-di-methylaminoethyl methacrylate) (PDMAEMA)/poly-(methacrylic acid) (PMAA) brush covalently attached to the ITO electrodes. The DNA deposition was performed at pH 5.0 when the polymer brush is positively charged due to protonation of tertiary amino groups in PDMAE-MA, thus resulting in electrostatic attraction of the negatively charged DNA. By applying electrolysis at -1.0 V(vs. Ag/AgCl reference) electrochemical oxygen reduction resulted in the consumption of hydrogen ions and local pH increase near the electrode surface. The process resulted in recharging the polymer brush to the negative state due to dissociation of carboxylic groups of PMAA, thus repulsing the negatively charged DNA and releasing it from the electrode surface. The DNA release was performed in various combinations from different electrodes in the array assembly. The released DNA operated as input signals for activation of the Boolean logic gates. The developed system represents a step forward in DNA computing, combining for the first time DNA chemical processes with electronic input signals.

Materials learning from life: concepts for active, adaptive and autonomous molecular systems

A broad overview of functional aspects in biological and synthetic out-of-equilibrium systems.

Supervised Learning in Adaptive DNA Strand Displacement Networks

The development of engineered biochemical circuits that exhibit adaptive behavior is a key goal of synthetic biology and molecular computing. Such circuits could be used for long-term monitoring and control of biochemical systems, for instance, to prevent disease or to enable the development of artificial life. In this article, we present a framework for developing adaptive molecular circuits using buffered DNA strand displacement networks, which extend existing DNA strand displacement circuit architectures to enable straightforward storage and modification of behavioral parameters. As a proof of concept, we use this framework to design and simulate a DNA circuit for supervised learning of a class of linear functions by stochastic gradient descent. This work highlights the potential of buffered DNA strand displacement as a powerful circuit architecture for implementing adaptive molecular systems.

DNAzyme-Based Logic Gate-Mediated DNA Self-Assembly

Controlling DNA self-assembly processes using rationally designed logic gates is a major goal of DNA-based nanotechnology and programming. Such controls could facilitate the hierarchical engineering of complex nanopatterns responding to various molecular triggers or inputs. Here, we demonstrate the use of a series of DNAzyme-based logic gates to control DNA tile self-assembly onto a prescribed DNA origami frame. Logic systems such as "YES," "OR," "AND," and "logic switch" are implemented based on DNAzyme-mediated tile recognition with the DNA origami frame. DNAzyme is designed to play two roles: (1) as an intermediate messenger to motivate downstream reactions and (2) as a final trigger to report fluorescent signals, enabling information relay between the DNA origami-framed tile assembly and fluorescent signaling. The results of this study demonstrate the plausibility of DNAzyme-mediated hierarchical self-assembly and provide new tools for generating dynamic and responsive self-assembly systems.

Catalytic nucleic acids (DNAzymes) as functional units for logic gates and computing circuits: from basic principles to practical applications

This feature article addresses the implementation of catalytic nucleic acids as functional units for the construction of logic gates and computing circuits, and discusses the future applications of these systems. The assembly of computational modules composed of DNAzymes has led to the operation of a universal set of logic gates, to field programmable logic gates and computing circuits, to the development of multiplexers/demultiplexers, and to full-adder systems. Also, DNAzyme cascades operating as logic gates and computing circuits were demonstrated. DNAzyme logic systems find important practical applications. These include the use of DNAzyme-based systems for sensing and multiplexed analyses, for the development of controlled release and drug delivery systems, for regulating intracellular biosynthetic pathways, and for the programmed synthesis and operation of cascades.

Design of a biochemical circuit motif for learning linear functions

Learning and adaptive behaviour are fundamental biological processes. A key goal in the field of bioengineering is to develop biochemical circuit architectures with the ability to adapt to dynamic chemical environments. Here, we present a novel design for a biomolecular circuit capable of supervised learning of linear functions, using a model based on chemical reactions catalysed by DNAzymes. To achieve this, we propose a novel mechanism of maintaining and modifying internal state in biochemical systems, thereby advancing the state of the art in biomolecular circuit architecture. We use simulations to demonstrate that the circuit is capable of learning behaviour and assess its asymptotic learning performance, scalability and robustness to noise. Such circuits show great potential for building autonomous in vivo nanomedical devices. While such a biochemical system can tell us a great deal about the fundamentals of learning in living systems and may have broad applications in biomedicine (e.g. autonomous and adaptive drugs), it also offers some intriguing challenges and surprising behaviours from a machine learning perspective.

Bridging the Two Worlds: A Universal Interface between Enzymatic and DNA Computing Systems

Molecular computing based on enzymes or nucleic acids has attracted a great deal of attention due to the perspectives of controlling living systems in the way we control electronic computers. Enzyme-based computational systems can respond to a great variety of small molecule inputs. They have the advantage of signal amplification and highly specific recognition. DNA computing systems are most often controlled by oligonucleotide inputs/outputs and are capable of sophisticated computing as well as controlling gene expressions. Here, we developed an interface that enables communication of otherwise incompatible nucleic-acid and enzyme-computational systems. The enzymatic system processes small molecules as inputs and produces NADH as an output. The NADH output triggers electrochemical release of an oligonucleotide, which is accepted by a DNA computational system as an input. This interface is universal because the enzymatic and DNA computing systems are independent of each other in composition and complexity.

A unified sensor architecture for isothermal detection of double-stranded DNA, oligonucleotides, and small molecules

Pathogen detection is an important problem in many areas of medicine and agriculture, which can involve genomic or transcriptomic signatures or small-molecule metabolites. We report a unified, DNA-based sensor architecture capable of isothermal detection of double-stranded DNA targets, single-stranded oligonucleotides, and small molecules. Each sensor contains independent target detection and reporter modules, enabling rapid design. We detected gene variants on plasmids by using a straightforward isothermal denaturation protocol. The sensors were highly specific, even with a randomized DNA background. We achieved a limit of detection of ∼15 pM for single-stranded targets and ∼5 nM for targets on denatured plasmids. By incorporating a blocked aptamer sequence, we also detected small molecules using the same sensor architecture. This work provides a starting point for multiplexed detection of multi-strain pathogens, and disease states caused by genetic variants (e.g., sickle cell anemia).

Divide and control: split design of multi-input DNA logic gates

Logic gates made of DNA have received significant attention as biocompatible building blocks for molecular circuits. The majority of DNA logic gates, however, are controlled by the minimum number of inputs: one, two or three. Here we report a strategy to design a multi-input logic gate by splitting a DNA construct.

Ternary DNA computing using 3 × 3 multiplication matrices

Ternary computing, beyond Boolean logic, is anticipated to enhance computational complexity. DNA-based ternary computing is demonstrated by the assembly of a 3 × 3 multiplication table, and the parallel operation of three 3 × 3 multiplication matrices is highlighted.

Non-Boolean computations implementing operations on multi-valued variables beyond base 2 allow enhanced computational complexity. We introduce DNA as a functional material for ternary computing, and in particular demonstrate the use of three-valued oligonucleotide inputs to construct a 3 × 3 multiplication table. The system consists of two three-valued inputs of –1; 0; +1 and a fluorophore/quencher functional hairpin acting as computational and reporter module. The interaction of the computational hairpin module with the different values of the inputs yields a 3 × 3 multiplication matrix consisting of nine nanostructures that are read out by three distinct fluorescence intensities. By combining three different hairpin computational modules, each modified with a different fluorophore/quencher pair, and using different sets of inputs, the parallel operation of three multiplication tables is demonstrated.

Biophysically inspired rational design of structured chimeric substrates for DNAzyme cascade engineering

The development of large-scale molecular computational networks is a promising approach to implementing logical decision making at the nanoscale, analogous to cellular signaling and regulatory cascades. DNA strands with catalytic activity (DNAzymes) are one means of systematically constructing molecular computation networks with inherent signal amplification. Linking multiple DNAzymes into a computational circuit requires the design of substrate molecules that allow a signal to be passed from one DNAzyme to another through programmed biochemical interactions. In this paper, we chronicle an iterative design process guided by biophysical and kinetic constraints on the desired reaction pathways and use the resulting substrate design to implement heterogeneous DNAzyme signaling cascades. A key aspect of our design process is the use of secondary structure in the substrate molecule to sequester a downstream effector sequence prior to cleavage by an upstream DNAzyme. Our goal was to develop a concrete substrate molecule design to achieve efficient signal propagation with maximal activation and minimal leakage. We have previously employed the resulting design to develop high-performance DNAzyme-based signaling systems with applications in pathogen detection and autonomous theranostics.