DNA-mediated homogeneous binding assays for nucleic acids and proteins

Specific and relative detection of urinary microRNA signatures in bladder cancer for point-of-care diagnostics

We present a dual-isothermal cascade strategy assisted by a lateral flow peptide nucleic acid biosensor for point-of-care detection of urinary microRNAs without a temperature protocol and complex instruments. The proposed assay is expected to be of great promise for bladder cancer diagnosis and point-of-care diagnostics.

Optical nano-biosensing interface via nucleic acid amplification strategy: construction and application

Modern optical detection technology plays a critical role in current clinical detection due to its high sensitivity and accuracy. However, higher requirements such as extremely high detection sensitivity have been put forward due to the clinical needs for the early finding and diagnosing of malignant tumors which are significant for tumor therapy. The technology of isothermal amplification with nucleic acids opens up avenues for meeting this requirement. Recent reports have shown that a nucleic acid amplification-assisted modern optical sensing interface has achieved satisfactory sensitivity and accuracy, high speed and specificity. Compared with isothermal amplification technology designed to work completely in a solution system, solid biosensing interfaces demonstrated better performances in stability and sensitivity due to their ease of separation from the reaction mixture and the better signal transduction on these optical nano-biosensing interfaces. Also the flexibility and designability during the construction of these nano-biosensing interfaces provided a promising research topic for the ultrasensitive detection of cancer diseases. In this review, we describe the construction of the burgeoning number of optical nano-biosensing interfaces assisted by a nucleic acid amplification strategy, and provide insightful views on: (1) approaches to the smart fabrication of an optical nano-biosensing interface, (2) biosensing mechanisms via the nucleic acid amplification method, (3) the newest strategies and future perspectives.

Spatial organization based reciprocal switching of enzyme-free nucleic acid circuits

We report a new nucleic acid sensing strategy through an intelligent design of spatial organization based reciprocal switching of catalytic hairpin assembly (CHA). The so-called SORS-CHA not only turns a well-designed CHA circuit into a relatively universal detector for any targeting sequence, but also guarantees a much enhanced signal resolution and a believability to minimize the misreading induced by unexpected signal drifts. With more trustworthy results, but a simpler sequence design, nucleic acid circuits could become competitive in real-world applications.

Allosteric kissing complex-based electrochemical biosensor for sensitive, regenerative and versatile detection of proteins

Herein, an allosteric kissing complex-based electrochemical biosensor was ingeniously proposed for the simple, sensitive, regenerative and versatile detection of proteins. Two hairpins (Hp1 and Hp2) were designed and the Hp1 was immobilized on the electrode surface, which could form a kissing complex with Hp2 through the apical loop-loop or kissing interaction of the RNA-RNA base sequences. The Hp2 possesses the appended single-stranded tails on each end, which hybridize with the recognition element-conjugated DNA strands to construct a protein responsive switch of Hp2 scaffold. After kissing complex formation between the Hp2 scaffold and the immobilized Hp1, the streptavidin-labeled alkaline phosphatase (SA-ALP) can be introduced onto the electrode surface for the generation of electrochemical signal. In the presence of target protein, its binding to the recognition elements linked onto the Hp2 scaffold endows the steric strain to open the Hp2 stem, propagated by the disruption of the kissing complex structure, resulting into a decreased electrochemical signal related with the protein quantification. Also, the Hp1 immobilized electrode can be directly regenerated after protein-induced kissing complex dissociation. The current kissing complex-based electrochemical biosensing strategy can be easily extended for the detection toward different protein targets of interest by simply changing the recognition elements conjugated onto the Hp2 scaffold. The sensitive and selective detection toward proteins could be achieved with the detection limits toward Anti-Dig antibody and thrombin of about 1ng/mL and 10pM, respectively. The developed kissing complex-based protein biosensing strategy should be a beneficial supplement in current biosensor field, providing a promising means for the applications in bioanalysis, disease diagnostics, and clinical biomedicine.

Homogeneous electrochemical aptasensor based on a dual amplification strategy for sensitive detection of profenofos residues

A homogeneous type of electrochemical aptasensor was designed based upon the principle of target-induced and tool enzyme-assisted signal amplification, which was employed for the detection of profenofos residues.

Highly sensitive detection of nucleic acids using a cascade amplification strategy based on exonuclease III-assisted target recycling and conjugated polyelectrolytes

A ratiometric and cascade amplification strategy that combines the signal amplification and effecitive FRET property of CPEs with the Exo III-assisted target recycling method has been developed for DNA detection.

A novel steric effect-regulated isothermal exponential amplification technology for the one-step homogeneous sensing of proteins

A novel approach utilizing the steric effect and isothermal exponential amplification for one-step homogeneous sensing of proteins.

Fluorescence Sensing Using DNA Aptamers in Cancer Research and Clinical Diagnostics

Among the various advantages of aptamers over antibodies, remarkable is their ability to tolerate a large number of chemical modifications within their backbone or at the termini without losing significant activity. Indeed, aptamers can be easily equipped with a wide variety of reporter groups or coupled to different carriers, nanoparticles, or other biomolecules, thus producing valuable molecular recognition tools effective for diagnostic and therapeutic purposes. This review reports an updated overview on fluorescent DNA aptamers, designed to recognize significant cancer biomarkers both in soluble or membrane-bound form. In many examples, the aptamer secondary structure switches induced by target recognition are suitably translated in a detectable fluorescent signal using either fluorescently-labelled or label-free aptamers. The fluorescence emission changes, producing an enhancement (“signal-on”) or a quenching (“signal-off”) effect, directly reflect the extent of the binding, thereby allowing for quantitative determination of the target in bioanalytical assays. Furthermore, several aptamers conjugated to fluorescent probes proved to be effective for applications in tumour diagnosis and intraoperative surgery, producing tumour-type specific, non-invasive in vivo imaging tools for cancer pre- and post-treatment assessment.

Ultrasensitive colorimetric detection of NF-κB protein at picomolar levels using target-induced passivation of nanoparticles

We developed a highly sensitive and selective sensor based on the nanoprobe conjugates of catalytic nanoparticles and double-stranded DNA (dsDNA) for the colorimetric detection of NF-κB protein. The sensing mechanism takes advantage of the catalytic activity of nanoparticle surfaces and the specific binding of NF-κB to a dsDNA sequence. In the presence of NF-κB, the highly selective interactions between dsDNA and NF-κB lead to the passivation of the catalytic nanoparticle surfaces, impeding the sodium borohydride-mediated reduction rate of 4-nitrophenol. The correlation between the NF-κB concentration and the visualized reduction rate of 4-nitrophenol from yellow to colorless clearly demonstrates the highly quantitative nature of the sensor. Importantly, this sensor can conclusively detect concentrations as low as 6.39 pM of NF-κB, which to best of our knowledge is the lowest limit of detection for a colorimetric NF-κB detection system. The excellent sensitivity of this sensor relies on the high binding constant of NF-κB to dsDNA and the catalytic activity of nanoparticle surfaces for the signal amplification. This sensor allows visual detection without the need for any spectrometric instrumentation. We also determined the various parameters such as the pH, temperature, incubation time, and salt concentration for optimal NF-κB-dsDNA interactions. Finally, we demonstrated the performance of the sensor with simulated sample analysis. Graphical abstract A highly sensitive and selective colorimetric detection of protein NF-κB using the nanoprobeconjugates of catalytic gold nanoparticles and double-stranded DNA (dsDNA) has been developed.

Y-Shaped DNA Duplex Structure-Triggered Gold Nanoparticle Dimers for Ultrasensitive Colorimetric Detection of Nucleic Acid with the Dark-Field Microscope

Herein, we present a novel gold nanoparticle (AuNP) enumeration-based colorimetric aptamer biosensor for ultrasensitive detection of nucleic acid. This AuNP enumeration-based colorimetric method takes advantages of the distinctive and strong localized surface plasmon resonance light scattering with the dark-field microscope. In our model system, first, cost-effective DNA1 instead of expensive 2-thioethyl ether acetic acid was capped on the surface of AuNPs to form a dense DNA1 layer. Then, two DNA strands (DNA2 and DNA3) in two different solutions were separately asymmetrically functionalized on the AuNPs capped dense DNA1 layer. The subsequent binding of the target DNA could trigger the formation of perfect complementary DNA with a Y shape and adjust the distance between nanoparticles to form AuNP dimers, accompanied by a color change from green to yellow as observed, and thereby modulated the performance of the sensor, which resulted in the ultrahigh sensitivity. With this design, a 43 aM limit of detection was obtained, which exhibited an increase of at least 5-9 orders of magnitude in sensitivity over other colorimetric sensors fabricated using conventional strategies.

Construction of an autonomously concatenated hybridization chain reaction for signal amplification and intracellular imaging

The concatenated hybridization chain reaction (C-HCR) was constructed as a versatile and robust tool for signal amplification and intracellular imaging, which was attributed to the synergistic amplification effect between HCR-1 and HCR-2.

Biomolecular self-assembly has spurred substantial research efforts for the development of low-cost point-of-care diagnostics. Herein, we introduce an isothermal enzyme-free concatenated hybridization chain reaction (C-HCR), in which the output of the upstream hybridization chain reaction (HCR-1) layer acts as an intermediate input to activate the downstream hybridization chain reaction (HCR-2) layer. The initiator motivates HCR-1 through the autonomous cross-opening of two functional DNA hairpins, yielding polymeric dsDNA nanowires composed of numerous tandem triggers T as output of the primary sensing event. The reconstituted amplicon T then initiates HCR-2 and transduces the analyte recognition into an amplified readout, originating from the synergistic effect between HCR-1 and HCR-2 layers. Native gel electrophoresis, atom force microscopy (AFM) and fluorescence spectra revealed that C-HCR mediated the formation of frond-like branched dsDNA nanowires and the generation of an amplified FRET signal. As a versatile and robust amplification strategy, the unpreceded C-HCR can discriminate DNA analyte from its mutants with high accuracy and specificity. By incorporating an auxiliary sensing module, the integrated C-HCR amplifier was further adapted for highly sensitive and selective detection of microRNA (miRNA), as a result of the hierarchical and sequential hybridization chain reactions, in human serum and even living cells through an easy-to-integrate “plug-and-play” procedure. In addition, the C-HCR amplifier was successfully implemented for intracellular miRNA imaging by acquiring an accurate and precise signal localization inside living cells, which was especially suitable for the ex situ and in situ amplified detection of trace amounts of analyte. The C-HCR amplification provides a comprehensive and smart toolbox for highly sensitive detection of various biomarkers and thus should hold great promise in clinical diagnosis and assessment. The infinite layer of multilayered C-HCR is anticipated to further strengthen the amplification capacity and reliability (anti-invasion performance) of intracellular imaging approach, which is of great significance for its bioanalytical applications.

New insights into a classic aptamer: binding sites, cooperativity and more sensitive adenosine detection

The DNA aptamer for adenosine (also for AMP and ATP) is a highly conserved sequence that has recurred in a few selections. It it a widely used model aptamer for biosensor development, and its nuclear magnetic resonance structure shows that each aptamer binds two AMP molecules. In this work, each binding site was individually removed by rational sequence design, while the remaining site still retained a similar binding affinity and specificity as confirmed by isothermal titration calorimetry. The thermodynamic parameters of binding are presented, and its biochemical implications are discussed. The number of binding sites can also be increased, and up to four sites are introduced in a single DNA sequence. Finally, the different sequences are made into fluorescent biosensors based on the structure-switching signaling aptamer design. The one-site aptamer has 3.8-fold higher sensitivity at lower adenosine concentration with a limit of detection of 9.1 μM adenosine, but weaker fluorescence signal at higher adenosine concentrations, consistent with a moderate cooperativity in the original aptamer. This work has offered insights into a classic aptamer for the relationship between the number of binding sites and sensitivity, and a shorter aptamer for improved biosensor design.

Ultrasensitive and Accurate Assay of Human Methyltransferase Activity at the Single-Cell Level Based on a Single Integrated Magnetic Microprobe

Human DNA methyltransferase (MTase) activity expression patterns and inhibition response are linked to related cancer initiation, progression, and therapeutic responses. Sensitive and accurate human MTase activity assay in cancer cells, especially at the single-cell level, is essential for biological study, clinical diagnosis, and therapy. Here, we developed an ultrasensitive and accurate DNA (cytosine-5)-methyltransferase 1 (Dnmt1) activity assay at the single-cell level based on a single integrated magnetic microprobe of functionalized double-stranded DNA (dsDNA) anchored to a single magnetic microbead surface. Functionalized dsDNA is designed with a hemimethylated DNA site for Dnmt1 recognition and a single-stranded tail to trigger in situ rolling circle amplification (RCA). Under the action of Dnmt1, hemimethylated dsDNA could be recognized and catalyzed to fully methylated dsDNA, which would protect them from the cleavage of BssHII. However, the dsDNA without full methylation would be cut by BssHII, making single-stranded tail separated from the single integrated microprobe. Subsequently, full methylation-protected in situ RCA could be performed, and multiple signal probes were hybridized to the single integrated microprobe for amplified signal accumulation. Finally, Dnmt1 activity could be evaluated by reading the fluorescence of the single integrated microprobe. Meanwhile, to minimize matrix interferences, magnetic separation was performed in the process. In this strategy, the single integrated magnetic microprobe was provided with integrated capacities of target recognition, signal amplification, signal accumulation, and matrix isolation. Therefore, an ultralow detection limit of 0.007 U/mL Dnmt1 was obtained, and accurate Dnmt1 activity assays in multiple cell lysates at the single-cell level were achieved. Furthermore, the inhibition effect of RG108 was evaluated conveniently. These results indicate that the single integrated magnetic microprobe-based strategy is an excellent candidate for sensitive monitoring of Dnmt1 activity and screening of anticancer drugs.

Genetically Encoded Fluorescent RNA Sensor for Ratiometric Imaging of MicroRNA in Living Tumor Cells

Light-up RNA aptamers are valuable tools for fluorescence imaging of RNA in living cells and thus for elucidating RNA functions and dynamics. However, no light-up RNA sensor has been reported for imaging of microRNAs (miRs) in mammalian cells. We report a novel genetically encoded RNA sensor for fluorescent imaging of miRs in living tumor cells using a light-up RNA aptamer that binds to sulforhodamine and separates it from a conjugated contact quencher. On the basis of the structural switching mechanism for molecular beacon, we show that the RNA sensor activates high-contrast fluorescence from the sulforhodamine-quencher conjugate when its stem-loop responsive motif hybridizes with target miR. The RNA sensor can be stably expressed within a designed tRNA scaffold in tumor cells and deliver light-up response to miR target. We also realize the RNA sensor for dual-emission, ratiometric imaging by coexpression of RNA sensor with GFP, enabling quantitative studies of target miR in living cells. Our design may provide a new paradigm for developing robust, sensitive light-up RNA sensors for RNA imaging applications.

Imaging Endogenous Metal Ions in Living Cells Using a DNAzyme-Catalytic Hairpin Assembly Probe

DNAzymes are a promising platform for metal ion detection, and a few DNAzyme-based sensors have been reported to detect metal ions inside cells. However, these methods required an influx of metal ions to increase their concentrations for detection. To address this major issue, the design of a catalytic hairpin assembly (CHA) reaction to amplify the signal from photocaged Na+ -specific DNAzyme to detect endogenous Na+ inside cells is reported. Upon light activation and in the presence of Na+ , the NaA43 DNAzyme cleaves its substrate strand and releases a product strand, which becomes an initiator that trigger the subsequent CHA amplification reaction. This strategy allows detection of endogenous Na+ inside cells, which has been demonstrated by both fluorescent imaging of individual cells and flow cytometry of the whole cell population. This method can be generally applied to detect other endogenous metal ions and thus contribute to deeper understanding of the role of metal ions in biological systems.

Divide and Control: Comparison of Split and Switch Hybridization Sensors

Hybridization probes have been intensively used for nucleic acid analysis in medicine, forensics and fundamental research. Instantaneous hybridization probes (IHPs) enable signalling immediately after binding to a targeted DNA or RNA sequences without the need to isolate the probe-target complex (e. g. by gel electrophoresis). The two most common strategies for IHP design are conformational switches and split approach. A conformational switch changes its conformation and produces signal upon hybridization to a target. Split approach uses two (or more) strands that independently or semi independently bind the target and produce an output signal only if all components associate. Here, we compared the performance of split vs switch designs for deoxyribozyme (Dz) hybridization probes under optimal conditions for each of them. The split design was represented by binary Dz (BiDz) probes; while catalytic molecular beacon (CMB) probes represented the switch design. It was found that BiDz were significantly more selective than CMBs in recognition of single base substitution. CMBs produced high background signal when operated at 55°C. An important advantage of BiDz over CMB is more straightforward design and simplicity of assay optimization.

A Both-End Blocked Peroxidase-Mimicking DNAzyme for Low-Background Chemiluminescent Sensing of miRNA

G-quadruplex DNAzymes that exhibited peroxidase-like activity have been shown to be appealing reporters for amplified readout of biosensing events simply by their formation or dissociation in the presence of analytes. For low background signaling, the efficient preblock of DNAzymes is critically important. Herein, we report a both-end blocked DNAzyme beacon strategy for chemiluminescent biosensing. The catalytic activity of peroxidase-mimicking DNAzyme can be inactivated fully by fixing both ends of the DNAzyme sequence, and easily recovered via a strand displace reaction between the miRNA and the block DNA. The efficient block and recovery of DNAzymes provide the both-end blocked beacon the highest signal-to-background ratio (over 25) among the reported DNAzymes for amplification-free detection of miRNA. As a result, the beacon allowed detection of subpicomolar miRNA without any labeling and amplification procedures, which is about 40-fold more sensitive than the traditional hairpin fluorescence beacon. Also, it exhibited excellent discrimination ability that can distinguish single-base mismatch miRNA. The simplicity, high sensitivity, and selectivity provided by the beacon make it a promising alternative tool for nucleic acid detection.

DNA Dendrimer-Streptavidin Nanocomplex: an Efficient Signal Amplifier for Construction of Biosensing Platforms

We develop a DNA dendrimer-streptavidin (SA) nanocomplex as a novel signal amplifier to create biosensing platforms for disease-related species. The DNA dendrimer-SA nanocomplex is fabricated by cross-linking the nonlinear hybridization chain reaction based DNA dendrimer with the SA-coupled linker DNA and possesses multiple sticky ends, a high molecular weight, and a hyperbranched nanostructure with large numbers of DNA duplexes. Taking advantage of the DNA dendrimer-SA nanocomplex and a label-free quartz crystal microbalance (QCM) technology, we first construct a mass-sensitive QCM biosensing platform for nucleic acids, which displays high selectivity and sensitivity, with a detection limit of 0.062 nM KRAS gene fragment. Then we present a fluorescent sensing strategy toward HeLa cells by functionalizing the DNA dendrimer-SA nanocomplex using the sgc8 aptamer and the SYBR Green I intercalating dye. The spiked recoveries of targets in physiological media are greater than 90%, demonstrating potential application of created biosensing platforms in clinical diagnosis. This work expands the rule set of designing DNA nanomaterials for development of biosensing strategies, and provides universal platforms for detecting disease-related species through simply altering the related capture and reporter DNA sequences.

Label-Free Homogeneous Electroanalytical Platform for Pesticide Detection Based on Acetylcholinesterase-Mediated DNA Conformational Switch Integrated with Rolling Circle Amplification

This study addresses the need for sensitive pesticide assay by reporting a new label-free and immobilization-free homogeneous electroanalytical strategy, which combines acetylcholinesterase (AChE)-catalyzed hydrolysis product-mediated DNA conformational switch and rolling circle amplification (RCA) to detect organophosphorous and carbamate pesticides in a "signal-on" mode. When target pesticides were present, AChE activity was inhibited and could not trigger the following DNA conformational change and the RCA reaction, which results in numerous methylene blue (MB) molecules in a free state, generating a strong electrochemical response. This proposed strategy was highly sensitive for omethoate detection with a detection limit as low as 2.1 μg/L and a linear range from 10 to 10 000 μg/L. Furthermore, this strategy was demonstrated to be applicable for pesticide detection in real samples. Thus, this novel label-free homogeneous electroanalytical strategy holds great promise for pesticide detection and can be further exploited for sensing applications in the environment and the food safety field.

Mechanistic Challenges and Advantages of Biosensor Miniaturization into the Nanoscale

Over the past few decades, there has been tremendous interest in developing biosensing systems that combine high sensitivity and specificity with rapid sample-to-answer times, portability, low-cost operation, and ease-of-use. Miniaturizing the biosensor dimensions into the nanoscale has been identified as a strategy for addressing the functional requirements of point-of-care and wearable biosensors. However, it is important to consider that decreasing the critical dimensions of biosensing elements impacts the two most important performance metrics of biosensors: limit-of-detection and response time. Miniaturization into the nanoscale enhances signal-to-noise-ratio by increasing the signal density (signal/geometric surface area) and reducing background signals. However, there is a trade-off between the enhanced signal transduction efficiency and the longer time it takes to collect target analytes on sensor surfaces due to the increase in mass transport times. By carefully considering the signal transduction mechanisms and reaction-transport kinetics governing different classes of biosensors, it is possible to develop structure-level and device-level strategies for leveraging miniaturization toward creating biosensors that combine low limit-of-detection with rapid response times.

Enhancement of RecA-mediated self-assembly in DNA nanostructures through basepair mismatches and single-strand nicks

The use of DNA as a structural material for nanometre-scale construction has grown extensively over the last decades. The development of more advanced DNA-based materials would benefit from a modular approach enabling the direct assembly of additional elements onto nanostructures after fabrication. RecA-based nucleoprotein filaments encapsulating short ssDNA have been demonstrated as a tool for highly efficient and fully programmable post-hoc patterning of duplex DNA scaffold. However, the underlying assembly process is not fully understood, in particular when patterning complex DNA topologies. Here, we report the effect of basepair-mismatched regions and single-strand nicks in the double-stranded DNA scaffold on the yield of RecA-based assembly. Significant increases in assembly yield are observed upon the introduction of unpaired basepairs directly adjacent to the assembly region. However, when the unpaired regions were introduced further from the assembly site the assembly yield initially decreased as the length of the unpaired region was increased. These results suggest that an unpaired region acts as a kinetic trap for RecA-based nucleoprotein filaments, impeding the assembly mechanism. Conversely, when the unpaired region is located directly adjacent to the assembly site, it leads to an increase in efficiency of RecA patterning owing to increased breathing of the assembly site.

Use of β-cyclodextrin-tethered cationic polymer based fluorescence enhancement of pyrene and hybridization chain reaction for the enzyme-free amplified detection of DNA

An enzyme-free amplified DNA detection method based on prominent fluorescence enhancement of a β-cyclodextrin-tethered cationic polymer and a hybridization chain reaction was developed.

Systematic comparison between toehold exchange and toehold displacement: exploration for highly specific and sensitive DNA detection

The detection of nucleic acid variations with high specificity and sensitivity is essential for the good practice of precision medicine.

A C-HCR assembly of branched DNA nanostructures for amplified uracil-DNA glycosylase assays

The amplified and selective detection of uracil-DNA glycosylase was enabled by a two-layered cascaded hybridization chain reaction machinery.

A novel label-free strategy for pathogenic DNA detection based on metal ion binding-induced fluorescence quenching of graphitic carbon nitride nanosheets

A novel label-free fluorescence sensing strategy based on g-C₃N₄ nanosheets and metal ions is designed for pathogenic DNA detection.

Ultrasensitive electrochemical DNA biosensor based on functionalized gold clusters/graphene nanohybrids coupling with exonuclease III-aided cascade target recycling

In this work, a novel and ultrasensitive electrochemical biosensor was constructed for DNA detection based on functionalized gold clusters/graphene nanohybrids (AuNCs/GR nanobybrids) and exonuclease III (Exo III)-aided cascade target recycling. By utilizing the capacity of GR as universal template, different metal nanoclusters including AuNCs/GR nanobybrids and PtNCs/GR nanohybrids were synthesized through convenient ultrasonic method. Exo III-aided cascade recycling was initiated by target DNA, generating the final cleavage product (S2), which acted as a linkage between capture probe and the functionalized metal nanoclusters/GR conjugates in the construction of the biosensor. The AuNCs/GR-DNA-enzyme conjugates acted as interfaces of enzyme-catalyzed silver deposition reaction, achieving DNA detection ranging from 0.02 fM to 20 pM with a detection limit of 0.057 fM. In addition, PtNCs/GR-DNA conjugates presented peroxidase-like activity and the functionalized PtNCs/GR nanohybrids-based electrochemical biosensor also realized DNA detection by catalyzing the 3,3',5,5'-tetramethylbenzidine-hydrogen peroxide (TMB-H₂O₂) system to produce electrochemical signal. This metal clusters/GR-based multiple-amplified electrochemical biosensor provided an universal method for DNA detection.

Digital DNA detection based on a compact optofluidic laser with ultra-low sample consumption

DNA lasers self-amplify optical signals from a DNA analyte as well as thermodynamic differences between sequences, allowing quasi-digital DNA detection. However, these systems have drawbacks, such as relatively large sample consumption and complicated dye labelling. Moreover, although the lasing signal can detect the target DNA, it is superimposed on an unintended fluorescence background, which persists for non-target DNA samples as well. From an optical point of view, it is thus not truly digital detection and requires spectral analysis to identify the target. In this work, we propose and demonstrate an optofluidic laser that has a single layer of DNA molecules as the gain material. A target DNA produces intensive laser emission comparable to existing DNA lasers, while any unnecessary fluorescence background is successfully suppressed. As a result, the target DNA can be detected with a single laser pulse, in a truly digital manner. Since the DNA molecules cover only a single layer on the surface of the laser microcavity, the DNA sample consumption is a few orders of magnitude lower than that of existing DNA lasers. Furthermore, the DNA molecules are stained by simply immersing the microcavity in the intercalating dye solution, and thus the proposed DNA laser is free of any complex dye-labelling process prior to analysis.

Ultrasensitive non enzymatic multiple immunosensor for tumor markers detection by coupling DNA hybridization chain reaction with intercalated molecules

In this study, we tried coupling the small signal molecules that could intercalate into DNA double helix with hybridization chain reaction (HCR) technique to fabricate a multiple immunosensor. Doxorubicin hydrochloride (DXH) and methylene blue (MB) were used as signal molecules and alpha-fetoprotein (AFP) and carcinoembryonic antigen (CEA) were selected as model biomarkers. The immunosensor mainly consists of three parts as follows: First, two different primary antibodies (Ab₁) immobilized on the surface of gold nanoparticles (Au NPs); Second, secondary antibodies (Ab₂) conjugated with DNA primer; Third, long DNA concatemers from HCR were used as a carrier to intercalate amounts of signal molecules (DXH or MB). A sandwich immunocomplex was formed among primary antibodies, target biomarkers and secondary antibodies conjugated with DNA primer via specific recognition reaction. Afterwards, DNA concatemers intercalating amounts of DXH or MB were linked to DNA primer via DNA hybridization. Square wave voltammetry (SWV) was employed to record the response signals from electroactive molecules DXH and MB, and two distinguishable signals were obtained, which peak potentials were at about -0.30V (corresponding to MB) and -0.70V (corresponding to DXH, both vs SCE), respectively. The signal intensities of MB and DXH were linearly related to the logarithm of biomarkers concentration in the range of 0.05pgmL^-1-25ngmL^-1, and the limit of detection were 0.03pgmL^-1 for CEA and 0.02pgmL^-1 for AFP (at S/N=3), respectively. Furthermore, the immunosensor exhibited a sensitive electrochemical response to biomarkers in human serum samples and the results obtained were in accordance with reference method, indicating the immunosensor can be applied to real sample analysis in clinic diagnosis.

Binding induced strand displacement amplification for homogeneous protein assay

An ultrasensitive and homogenous strategy for protein assay was established based upon binding-induced strand displacement amplification (BI-SDA). Binding-Induced DNA strand-displacement occurred between Apt-T•signal DNA and Apt-C, and release of signal DNA upon addition of platelet-derived growth factor (PDGF BB). The released signal DNA further hybridized with multifunctional hairpin DNA probe and induced the strand-displacement amplification in the presence of Klenow Fragment (exo^-) and dNTPs. The BI-SDA product contain G-quaruplex DNA, which could be recognized and reported by the fluorescence of fluorochrome N-methyl porphyrin propionic acid IX (NMM). The fluorescence intensity was proportional to the concentration of PDGF-BB over the range of 1.0×10^-11mol/L -2.0×10^-9mol/L, with a detection limit of 3.6pmol/L. This proposed strategy showed good selectivity and practicality, and might be applied to other proteins in the future.

Strand-Exchange Nucleic Acid Circuitry with Enhanced Thermo-and Structure- Buffering Abilities Turns Gene Diagnostics Ultra-Reliable and Environmental Compatible

Catalytic hairpin assembly (CHA) is one of the most promising nucleic acid amplification circuits based on toehold-mediated strand exchange reactions. But its performance is usually ruined by fluctuated environmental temperatures or unexpected self-structures existing in most real-world targets. Here we present an amide-assistant mechanism that successfully reduces the prevalence of these problems for CHA and maximizes its thermo- and structure- buffering abilities. Such an organic amide-promoted CHA (shortened as OHT-CHA) can unprecedentedly amplify through 4 °C to 60 °C without rebuilding sequences or concerning target complexity. We are then for the first time able to employ it as a direct and universal signal booster for loop mediated isothermal reaction (LAMP). LAMP is one of the most promising point-of-care (POC) gene amplifiers, but has been hard to detect precisely due to structured products and haunted off-target amplicons. OHT-CHA guarantees a significant and reliable signal for LAMP reaction amplified from as little as 10⁻¹⁹ M virus gene. And one single set of OHT-CHA is qualified to any detection requirement, either in real-time at LAMP running temperature (~60 °C), or at end-point on a POC photon counter only holding environmental temperatures fluctuating between 4 °C to 42 °C.

A novel "signal-on/off" sensing platform for selective detection of thrombin based on target-induced ratiometric electrochemical biosensing and bio-bar-coded nanoprobe amplification strategy

A novel dual-signal ratiometric electrochemical aptasensor for highly sensitive and selective detection of thrombin has been designed on the basis of signal-on and signal-off strategy. Ferrocene labeled hairpin probe (Fc-HP), thrombin aptamer and methyl blue labeled bio-bar-coded AuNPs (MB-P3-AuNPs) were rationally introduced for the construction of the assay platform, which combined the advantages of the recognition of aptamer, the amplification of bio-bar-coded nanoprobe, and the ratiometric signaling readout. In the presence of thrombin, the interaction between thrombin and the aptamer leads to the departure of MB-P3-AuNPs from the sensing interface, and the conformation of the single stranded Fc-HP to a hairpin structure to take the Fc confined near the electrode surface. Such conformational changes resulted in the oxidation current of Fc increased and that of MB decreased. Therefore, the recognition event of the target can be dual-signal ratiometric electrochemical readout in both the "signal-off" of MB and the "signal-on" of Fc. The proposed strategy showed a wide linear detection range from 0.003 to 30nM with a detection limit of 1.1 pM. Moreover, it exhibits good performance of excellent selectivity, good stability, and acceptable fabrication reproducibility. By changing the recognition probe, this protocol could be easily expanded into the detection of other targets, showing promising potential applications in disease diagnostics and bioanalysis.

Mapping a multiplexed zoo of mRNA expression

In situ hybridization methods are used across the biological sciences to map mRNA expression within intact specimens. Multiplexed experiments, in which multiple target mRNAs are mapped in a single sample, are essential for studying regulatory interactions, but remain cumbersome in most model organisms. Programmable in situ amplifiers based on the mechanism of hybridization chain reaction (HCR) overcome this longstanding challenge by operating independently within a sample, enabling multiplexed experiments to be performed with an experimental timeline independent of the number of target mRNAs. To assist biologists working across a broad spectrum of organisms, we demonstrate multiplexed in situ HCR in diverse imaging settings: bacteria, whole-mount nematode larvae, whole-mount fruit fly embryos, whole-mount sea urchin embryos, whole-mount zebrafish larvae, whole-mount chicken embryos, whole-mount mouse embryos and formalin-fixed paraffin-embedded human tissue sections. In addition to straightforward multiplexing, in situ HCR enables deep sample penetration, high contrast and subcellular resolution, providing an incisive tool for the study of interlaced and overlapping expression patterns, with implications for research communities across the biological sciences.