Specific discrimination and universal signal amplification for RNA detection by coupling toehold exchange with RCA through nucleolytic conversion of a structure-switched hairpin probe

Highly sensitive detection of microRNA-21 by nitrogen-doped carbon dots-based ratio fluorescent probe via nuclease-assisted rolling circle amplification strategy

Highly sensitive and selective detection of microRNA-21 (miRNA-21) in biological samples is critical for the disease diagnosis and cancer treatment. In this study, a nitrogen-doped carbon dots (N-CDs)-based ratio fluorescence sensing strategy was constructed for miRNA-21 detection with high sensitivity and excellent specificity. Bright-blue N-CDs (λ_ex/λ_em = 378 nm/460 nm) were synthesized by facile one-step microwave-assisted pyrolysis method by using uric acid as the single precursor, and the absolute fluorescence quantum yield and fluorescence lifetime of N-CDs were 35.8% and 5.54 ns separately. The padlock probe hybridized with miRNA-21 firstly and then was cyclized by T4 RNA ligase 2 to form a circular template. At the present of dNTPs and phi29 DNA polymerase, the oligonucleotide sequence in miRNA-21 was prolonged to hybridize with the surplus oligonucleotide sequences in circular template, generating long and reduplicated oligonucleotide sequences containing abundant guanine nucleotides. Separate G-quadruplex sequences were generated after the addition of Nt.BbvCI nicking endonuclease, and then hemin bound with G-quadruplex sequence to construct the G-quadruplex DNAzyme. Such G-quadruplex DNAzyme catalyzed the redox reaction of o-phenylenediamine (OPD) with H₂O₂, finally producing the yellowish-brown 2,3-diaminophenazine (DAP) (λ_em = 562 nm). Due to the inner filter effect between N-CDs and DAP, the ratio fluorescence signal of DAP with N-CDs was utilized for sensitive detection of miRNA-21 with detection limit of 0.87 pM. Such approach has practical feasibility and excellent specificity for miRNA-21 analysis during highly homological miRNA family in HeLa cell lysates and human serum samples.

A label-free fluorescent aptasensor based on a novel exponential rolling circle amplification for highly sensitive ochratoxin A detection

Rapid and sensitive analysis of ochratoxin A (OTA) plays an important role in food safety. Here, an aptasensor based on novel exponential rolling circle amplification (ERCA) was proposed for ultrasensitive and label-free fluorescence detection of OTA. The attachment of OTA to its aptamer could release H and rapidly hybridize with CT to initiate rolling circle amplification (RCA). The amplicons could further displace H from APH to initiate recycled RCA, achieving exponential growth of amplification products that contained G4 dimers for lighting up ThT. Benefiting from the exponential amplification efficiency of the ERCA strategy and the high fluorescence quantum yield of G4 dimer/ThT, this strategy exhibited a wide linear range from 10 fg/mL to 10 ng/mL with a detection limit of 4.3 fg/mL. In addition, the aptasensor displayed satisfactory recoveries in real sample analysis. We believe that this novel aptasensor possesses promising application prospects in food safety and medicine detection.

The fluorescent aptasensor based on CRISPR-Cas12a combined with TdT for highly sensitive detection of cocaine

Ultrasensitive and specific detection of cocaine is of great significance for monitoring cocaine abuse. Herein, a fluorescent aptasensor via coupling CRISPR-Cas12a, with magnetic nanoparticles (MNPs), split-aptamer, and terminal deoxynucleotidyl transferase (TdT), was developed for the detection of cocaine. In short, the complete cocaine aptamer is split into two parts, one is modified on magnetic nanoparticles (MNPs) and the other is free. The presence of cocaine will mediate the binding of these two segments. Then TdT will mediate the extension to form an ultra-long sequence that can bind with multiple CRISPR-Cas12a resulting in the trans-cleavage activity of CRISPR-Cas12a being triggered. Thence, the DNA reporter which is bi-labeled with fluorophore and quencher is cleaved resulting in the generation of a fluorescence signal. The developed fluorescent aptasensor realizes the detection of cocaine with excellent sensitivity and specificity. The detection limit is low down to 33 pM, and the linear range is from 330 to 1.65 × 10⁵ pM. Most importantly, this fluorescent aptasensor can be successfully applied to the determination of cocaine in human plasma samples.

Programmable design of isothermal nucleic acid diagnostic assays through abstraction-based models

Accelerating the design of nucleic acid amplification methods remains a critical challenge in the development of molecular tools to identify biomarkers to diagnose both infectious and non-communicable diseases. Many of the principles that underpin these mechanisms are often complex and can require iterative optimisation. Here we focus on creating a generalisable isothermal nucleic acid amplification methodology, describing the systematic implementation of abstraction-based models for the algorithmic design and application of assays. We demonstrate the simplicity, ease and flexibility of our approach using a software tool that provides amplification schemes de novo, based upon a user-input target sequence. The abstraction of reaction network predicts multiple reaction pathways across different strategies, facilitating assay optimisation for specific applications, including the ready design of multiplexed tests for short nucleic acid sequence miRNAs or for difficult pathogenic targets, such as highly mutating viruses.

Detecting nucleic acids often requires choosing between different amplification mechanisms. Here the authors present a generalisable and programmable isothermal methodology, demonstrated in clinical applications, including for multiplexed detection of short miRNAs.

Interference Reduction Biosensing Strategy for Highly Sensitive microRNA Detection

MicroRNAs are potential biomarkers for human cancers and other diseases due to their roles as post-transcriptional regulators for gene expression. However, the detection of miRNAs by conventional methods such as RT-qPCR, in situ hybridization, northern blot-based platforms, and next-generation sequencing is complicated by short length, low abundance, high sequence homology, and susceptibility to degradation of miRNAs. In this study, we developed a nicking endonuclease-mediated interference reduction rolling circle amplification (NEM-IR-RCA) strategy for the ultrasensitive and highly specific detection of miRNA-21. This method exploits the advantages of the optical properties of long-lived iridium(III) probes, in conjunction with time-resolved emission spectroscopy (TRES) and exponential rolling circle amplification (E-RCA). Under the NEM-IR-RCA-based signal enhancement processes, the limit of detection of miRNA-21 was down to 0.0095 fM with a linear range from 0.05 to 100 fM, which is comparable with the conventional RT-qPCR. Unlike RT-qPCR, the strategy was performed at a lower and constant temperature without heating/cooling cycles and reverse transcription. The strategy could clearly discriminate between matched and mismatched targets, demonstrating high specificity. Moreover, the potential application of this method was demonstrated in cancer cells and mouse serum samples, showing good agreement with RT-qPCR results. Apart from miRNA-21 detection, this platform could be also adapted for detecting other miRNAs, such as let-7a and miRNA-22, indicating its excellent potential for biomedical research and clinical diagnostics.

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

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

Highly sensitive fluorescence biosensing of BCR-ABL1 fusion gene based on exponential transcription-triggered hemin catalysis

Simple, sensitive and specific detection of the transcription level of BCR-ABL1 mRNA possesses vital clinical significance in diagnosis and treatment of chronic myeloid leukemia (CML). In this study, an innovative fluorescence biosensing methodology has been developed for sensitive and specific detection of BCR-ABL1 mRNA by integrating high-efficiency of exponential transcription and superior catalytic performance of DNA-grafted hemin. Exponential transcription was triggered by BCR-ABL1 mRNA to produce plenty of RNA products. They can specifically hybridize with circular dual-labeled hemin (DLH) probe to dissociate the intramolecular hemin dimmers into highly active hemin monomers for catalyzing fluorescence substrate tyramine. This exponential transcription-triggered hemin catalysis (ET-HC) strategy showed highly sensitive and specific for BCR-ABL1 detection with a limit of detection at 0.5 aM and a good linear range from 2 aM to 200 fM. This method was successfully applied to directly detect as low as 0.001% e13a2 transcript isoforms from complex genomic RNA extraction. Compared with clinical routine, the overall process is a thermostatic reaction and eliminates additional reverse transcription operation. Therefore, the developed ET-HC strategy might provide a promising alternative tool for precise diagnosis and personalized treatment of CML.

Magnetic-enhanced fluorescence sensing of tumor miRNA by combination of MNPs@PDA with duplex specific nuclease

Highly sensitive and specific detection of miRNA still remains challenging. In this work, a simple and sensitive fluorescence biosensor has been developed for detection of miRNA by combining the magnetic nanoparticles coated with poly-dopamine (MNPs@PDA) with duplex specific nuclease (DSN). The MNPs@PDA could be easily synthesized via autoxidation of dopamine on the surface of magnetic nanoparticles. The MNPs@PDA could specifically bind with the FAM-labeled single-strand DNA (ssDNA) probes via polyvalent metal-mediated coordination, resulting in the quench of fluorescence signal. The MNPs@PDA exhibited good anti-interference performance in complex matrix. The inner filter effect (IFE) of the MNPs@PDA could be eliminated via magnetic separation. In the presence of specific miRNA, DSN digested the DNA in the DNA-miRNA duplexes and small fragments were formed. The force between these small fragments and MNPs@PDA was negligible, resulting in the occurrence of fluorescence signal. Due to the incorporation of DSN, signal amplification was realized via the recycling process. The established method achieved a low detection limit down to 0.42 pM. The linear concentration range was from 5 pM to 5 nM. Moreover, this method also had high specificity. Remarkably, the target miRNAs extracted from human cells were detected by using the sensing platform.

Fluorometric determination of agrA gene transcription in methicillin-resistant Staphylococcus aureus with a graphene oxide-based assay using strand-displacement polymerization recycling and hybridization chain reaction

A graphene oxide (GO)-based fluorescent bioassay was developed to quantify agrA gene transcription (its mRNA) in methicillin-resistant Staphylococcus aureus (MRSA). This method is based on the use of Klenow fragment (KF)-assisted target recycling amplification and hybridization chain reaction (HCR). A triple complex was designed that contained a capture probe (CP), a trigger probe (TP), and a help probe (HP), which were partially complementary to one another. In the absence of the target, all the oligonucleotides labeled with carboxyfluorescein (FAM) are adsorbed onto the surface of GO by π-stacking interactions. This adsorption quenches the FAM signal. On the contrary, the target RNA causes the triple complex to disintegrate and initiates strand-displacement polymerization reaction (SDPR) and HCR in the presence of the appropriate raw materials, including the primer, KF, dNTPs, hairpin 1 (H1), and hairpin 2 (H2), generating double-stranded DNA (dsDNA) products. These dsDNA products are repelled by GO and produce strong fluorescence, measured at excitation/emission wavelengths of 480/514 nm. The fluorescent signal is greatly amplified by SYBR Green I (SGI) due to the synergistic effect of dsDNA-SGI. The target was assayed with this method at concentrations in the range 10 fM to 100 pM, and the detection limit (LOD) was 10 fM. This method also displayed good applicability in the analysis of real samples. It provides a new way of monitoring biofilm formation and studying the mechanisms of drug actions. Graphical abstract Schematic representation of the graphene oxide-based fluorescent bioassay for agrA gene transcription in methicillin-resistant Staphylococcus aureus by using strand-displacement polymerization recycling and hybridization chain reaction.

Triple-helix molecular-switch-actuated exponential rolling circular amplification for ultrasensitive fluorescence detection of miRNAs

We have developed a rapid and high-efficiency fluorescent biosensing platform based on triple-helix molecular-switch (THMS)-actuated exponential rolling circular amplification (RCA) strategy for the ultrasensitive detection of miR-21.