G-quadruplex fluorescent probe-mediated real-time rolling circle amplification strategy for highly sensitive microRNA detection

Detection of fucosylated extracellular vesicles miR-4732-5p related to diagnosis of early lung adenocarcinoma by the electrochemical biosensor

Our preliminary investigation has identified the potential of serum fucosylated extracellular vesicles (EVs) miR-4732-5p in the early diagnosis of lung adenocarcinoma (LUAD) by a fucose-captured strategy utilizing lentil lectin (LCA)-magnetic beads and subsequent screening of high throughput sequencing and validation of real-time quantitative polymerase chain reaction (RT-qPCR). Considering the relatively complicated procedure, expensive equipment, and stringent laboratory condition, we have constructed an electrochemical biosensor assay for the detection of miR-4732-5p. miR-4732-5p is extremely low in serum, down to the fM level, so it needs to be detected by highly sensitive electrochemical methods based on the Mg2+-dependent DNAzyme splitting nucleic acid lock (NAL) cycle and hybridization chain reaction (HCR) signal amplification. In this study, signal amplification is achieved through the dual amplification reactions using NAL cycle in combination with HCR. In addition, hybridized DNA strands bind to a large number of methylene blue (MB) molecules to enhance signaling. Based on the above strategy, we further enhance our signal amplification strategies to improve detection sensitivity and accuracy. The implementation of this assay proceeded as follows: initially, miR-4732-5p was combined with NAL, and then Mg2+-dependent DNAzyme splitted NAL to release auxiliary DNA (S1) strands, which were subsequently captured by the immobilized capture probe DNA (C1) strands on the electrode surface. Following this, abundant quantities of DNA1 (H1) and DNA2 (H2) tandems were generated by HCR, and S1 strands then hybridized with the H1 and H2 tandems through base complementary pairing. Finally, MB was bonded to the H1 and H2 tandems through π-π stacking interaction, leading to the generation of a signal current upon the detection of a potential capable of inducing a redox change of MB by the electrode. Furthermore, we evaluated the performance of our developed electrochemical biosensor assay. The results demonstrated that our assay is a reliable approach, characterized by its high sensitivity (with a detection limit of 2.6 × 10-17 M), excellent specificity, good accuracy, reproducibility, and stability. Additionally, it is cost-effective, requires simple operation, and is portable, making it suitable for the detection of serum fucosylated extracellular vesicles miR-4732-5p. Ultimately, this development has the potential to enhance the diagnostic efficiency for patients with early-stage LUAD.

Elongation and Ligation-Mediated Differential Coding for Label-Free and Locus-Specific Analysis of 8-Oxo-7,8-dihydroguanine in DNA

Oxidative DNA damage is closely associated with the occurrence of numerous human diseases and cancers. 8-Oxo-7,8-dihydroguanine (8-oxoG) is the most prevalent form of DNA damage, and it has become not only an oxidative stress biomarker but also a new epigenetic-like biomarker. However, few approaches are available for the locus-specific detection of 8-oxoG because of the low abundance of 8-oxoG damage in DNA and the limited sensitivity of existing assays. Herein, we demonstrate the elongation and ligation-mediated differential coding for label-free and locus-specific analysis of 8-oxoG in DNA. This assay is very simple without the involvement of any specific labeled probes, complicated steps, and large sample consumption. The utilization of Bsu DNA polymerase can specifically initiate a single-base extension reaction to incorporate dATP into the opposite position of 8-oxoG, endowing this assay with excellent selectivity. The introduction of cascade amplification reaction significantly enhances the sensitivity. The proposed method can monitor 8-oxoG with a limit of detection of 8.21 × 10-19 M (0.82 aM), and it can identify as low as 0.001% 8-oxoG damage from a complex mixture with excessive undamaged DNAs. This method can be further applied to measure 8-oxoG levels in the genomic DNA of human cells under diverse oxidative stress, holding prospect potential in the dynamic monitoring of critical 8-oxoG sites, early clinical diagnosis, and gene damage-related biomedical research.

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.

Visual Detection of Stem-Loop Primer Amplification (SPA) Products without Denaturation Using Peroxidase-like DNA Machines (PxDM)

Rapid, inexpensive, and accurate determination of nucleic acids is a decisive factor in evaluating population's health and monitoring treatment at point-of-care (POC) settings. Testing systems with visual outputs can provide instrument-free signal detection. Isothermal amplification technologies can substitute conventional polymerase chain reaction (PCR) testing due to compatibility with the POC diagnostic. Here, we have visually detected DNA fragments obtained by stem-loop-primer-assisted isothermal amplification (SPA), but not those obtained by PCR or LAMP amplification using DNA nanomachines with peroxidase-like activity (PxDM) with sensitivity to a single nucleotide substitution. Compared to the diagnostics with conventional loop-mediated isothermal amplification (LAMP), the PxDM method produces no false positive signals with the non-specific amplification products. The study suggests that PxDM, in conjunction with SPA isothermal amplification, can become a valid platform for POC testing systems.

Accurate and rapid quantification of PD-L1 positive exosomes by a triple-helix molecular probe

Programmed death ligand-1 (PD-L1) positive exosomes (P-Exo) have been widely used for tumor diagnosis. However, accurate and rapid quantification of P-Exo remains challenging due to the heterogeneity of clinical individuals and isolation techniques. In this study, the triple-helix molecular probe (THMP) coupled with high-affinity silica-based TiO₂ magnetic beads was used to isolate exosomes and to analyze the relative abundance of P-Exo in total exosomes (T-Exo). By employing this strategy, the entire analysis was completed within 70 min and the detection limit for P-Exo was 880 particles μL^-1. Additionally, the relative abundance of P-Exo in T-Exo (RA_P-Exo/T-Exo) was calculated from their fluorescence ratio, which could avoid errors due to differences in samples and separation methods, and identify 1.5 × 10³ P-Exo from 5 × 10⁶ T-Exo per microliter. RA_P-Exo/T-Exo values were not only effective in distinguishing healthy volunteers from breast cancer patients, but also highly positively correlated with the stage of breast carcinoma. Overall, this strategy opens a new avenue for rapid and quantitative analysis of P-Exo, providing an opportunity for precise diagnosis and prediction of treatment efficacy in cancer.

Construction of a self-sufficient DNA circuit for amplified detection of kanamycin

Improper use of kanamycin can lead to trace kanamycin residues in animal-derived foods, which can pose a potential threat to public health. Isothermal enzyme-free DNA circuits have provided a versatile toolbox for detecting kanamycin residues in complicated food samples, yet they are always limited by low amplification efficiency and intricate design. Herein, we present a simple-yet-robust nonenzymatic self-driven hybridization chain reaction (SHCR) amplifier for kanamycin determination with 5800-fold sensitivity over that of the conventional HCR circuit. The analyte kanamycin-activated SHCR circuitry can generate numerous new initiators to promote the reaction and improve the amplification efficiency, thus achieving an exponential signal gain. With precise target recognition and multilayer amplification capability, our self-sustainable SHCR aptasensor facilitated the highly sensitive and reliable analysis of kanamycin in buffer, milk, and honey samples, thus holding great potential for the amplified detection of trace contaminants in liquid food matrices.

Interference reduction isothermal nucleic acid amplification strategy for COVID-19 variant detection

Common reference methods for COVID-19 variant diagnosis include viral sequencing and PCR-based methods. However, sequencing is tedious, expensive, and time-consuming, while PCR-based methods have high risk of insensitive detection in variant-prone regions and are susceptible to potential background signal interference in biological samples. Here, we report a loop-mediated interference reduction isothermal nucleic acid amplification (LM-IR-INA) strategy for highly sensitive single-base mutation detection in viral variants. This strategy exploits the advantages of nicking endonuclease-mediated isothermal amplification, luminescent iridium(III) probes, and time-resolved emission spectroscopy (TRES). Using the LM-IR-INA strategy, we established a luminescence platform for diagnosing COVID-19 D796Y single-base substitution detection with a detection limit of 2.01 × 10⁵ copies/μL in a linear range of 6.01 × 10⁵ to 3.76 × 10⁸ copies/μL and an excellent specificity with a variant/wild-type ratio of significantly less than 0.0625%. The developed TRES-based method was also successfully applied to detect D796Y single-base substitution sequence in complicated biological samples, including throat and blood, and was a superior to steady-state technique. LM-IR-INA was also demonstrated for detecting the single-base substitution D614G as well as the multiple-base mutation H69/V70del without mutual interference, indicating that this approach has the potential to be used as a universal viral variant detection strategy.

Photoacoustic detection of SARS-CoV-2 spike N501Y single-nucleotide polymorphism based on branched rolling circle amplification

Rapid and accurate diagnosis of SARS-CoV-2 single-nucleotide variations is an urgent need for the initial detection of local circulation and monitoring the alternation of dominant variant. In this proof-of-concept study, a homogeneous and isothermal photoacoustic biosensor is demonstrated for rapid molecular amplification and detection of a synthetic DNA corresponding to SARS-CoV-2 spike N501Y. Branched rolling circle amplification produces single-stranded amplicons that can aggregate detection probe-modified AuNPs, which induces a strong photoacoustic signal at 640 nm due to both the surface plasmon resonance shift and the size-dependent effect of laser-induced nanobubbles, achieving a sub-femtomolar detection limit within a total assay time of 80 min. The limit of detection can be kept when measuring 5% serum samples. Moreover, the proposed biosensor is highly specific for single-nucleotide polymorphism discrimination and robust against background DNA.

Recent Developments in G-Quadruplex Binding Ligands and Specific Beacons on Smart Fluorescent Sensor for Targeting Metal Ions and Biological Analytes

The G-quadruplex structure is crucial in several biological processes, including DNA replication, transcription, and genomic maintenance. G-quadruplex-based fluorescent probes have recently gained popularity because of their ease of use, low cost, excellent selectivity, and sensitivity. This review summarizes the latest applications of G-quadruplex structures as detectors of genome-wide, enantioselective catalysts, disease therapeutics, promising drug targets, and smart fluorescence probes. In every section, sensing of G-quadruplex and employing G4 for the detection of other analytes were introduced, respectively. Since the discovery of the G-quadruplex structure, several studies have been conducted to investigate its conformations, biological potential, stability, reactivity, selectivity for chemical modification, and optical properties. The formation mechanism and advancements for detecting different metal ions (Na⁺, K⁺, Ag⁺, Tl⁺, Cu⁺/Cu²⁺, Hg²⁺, and Pb²⁺) and biomolecules (AMP, ATP, DNA/RNA, microRNA, thrombin, T4 PNK, RNase H, ALP, CEA, lipocalin 1, and UDG) using fluorescent sensors based on G-quadruplex modification, such as dye labels, artificial nucleobase moieties, dye complexes, intercalating dyes, and bioconjugated nanomaterials (AgNCs, GO, QDs, CDs, and MOF) is described herein. To investigate these extremely efficient responsive agents for diagnostic and therapeutic applications in medicine, fluorescence sensors based on G-quadruplexes have also been employed as a quantitative visualization technique.

Unified-amplifier based primer exchange reaction (UniAmPER) enabled detection of SARS-CoV-2 from clinical samples

Primer exchange reaction (PER) is an emergent method for non-templated synthesis of single stranded DNA molecules. PER has been shown to be effective in cell imaging systems and for detection of macromolecules. A particular application of PER is to detect a specific target nucleic acid. To this endeavor, two coupled DNA hairpins, a detector and an amplifier, play in accordance to extend a target nucleic acid with a concatemer DNA sequence. Here we introduced unified-amplifier based primer exchange reaction (UniAmPER) that beneficially extends the target by a unified-amplifier. The unified-amplifier operates as both detector and amplifier hairpins. The extension resulted in synthesis of concatemer G-rich sequences. The G-rich sequences were expected to form G-quadruplex (GQ) structures. Presence of the GQ structures were investigated by peroxidase activity of GQs in presence ^{of hemin, H}2°2 ^{and 3,3',5,5'-Tetramethylbenzidine (TMB) as well as by fluorescence signal} generation upon intercalation of thioflavin T (ThT). The presented unified-amplifier in this study facilitates application of PER systems for development of colorimetric or fluorogenic biosensors. As a proof of principle, the method has been applied for detection of reversely transcribed cDNAs from clinical SARS-CoV-2 samples.

A rapid and label-free DNA-based interference reduction nucleic acid amplification strategy for viral RNA detection

Common reference methods for COVID-19 diagnosis include thermal cycling amplification (e.g. RT-PCR) and isothermal amplification methods (e.g. LAMP and RPA). However, they may not be suitable for direct detection in environmental and biological samples due to background signal interference. Here, we report a rapid and label-free interference reduction nucleic acid amplification strategy (IR-NAAS) that exploits the advantages of luminescent iridium(III) probes, time-resolved emission spectroscopy (TRES) and multi-branch rolling circle amplification (mbRCA). Using IR-NAAS, we established a luminescence approach for diagnosing COVID-19 RNAs sequences RdRp, ORF1ab and N with a linear range of 0.06-6.0 × 10⁵ copies/mL and a detection limit of down to 7.3 × 10⁴ copies/mL. Moreover, the developed method was successfully applied to detect COVID-19 RNA sequences from various environmental and biological samples, such as domestic sewage, and mice urine, blood, feces, lung tissue, throat and nasal secretions. Apart from COVID-19 diagnosis, IR-NAAS was also demonstrated for detecting other RNA viruses, such as H1N1 and CVA10, indicating that this approach has great potential approach for routine preliminary viral detection.

Functional nucleic acid biosensors utilizing rolling circle amplification

Functional nucleic acids regulate rolling circle amplification to produce multiple detection outputs suitable for the development of point-of-care diagnostic devices.

Recent advances in biological detection with rolling circle amplification: design strategy, biosensing mechanism, and practical applications

Rolling circle amplification (RCA) is a simple and isothermal DNA amplification technique that is used to generate thousands of repeating DNA sequences using circular templates under the catalysis of DNA polymerase.

Recent advances of fluorescent biosensors based on cyclic signal amplification technology in biomedical detection

The cyclic signal amplification technology has been widely applied for the ultrasensitive detection of many important biomolecules, such as nucleic acids, proteins, enzymes, adenosine triphosphate (ATP), metal ions, exosome, etc. Due to their low content in the complex biological samples, traditional detection methods are insufficient to satisfy the requirements for monitoring those biomolecules. Therefore, effective and sensitive biosensors based on cyclic signal amplification technology are of great significance for the quick and simple diagnosis and treatment of diseases. Fluorescent biosensor based on cyclic signal amplification technology has become a research hotspot due to its simple operation, low cost, short time, high sensitivity and high specificity. This paper introduces several cyclic amplification methods, such as rolling circle amplification (RCA), strand displacement reactions (SDR) and enzyme-assisted amplification (EAA), and summarizes the research progress of using this technology in the detection of different biomolecules in recent years, in order to provide help for the research of more efficient and sensitive detection methods.

Application of the Dimeric G-Quadruplex and toehold-mediated strand displacement reaction for fluorescence biosensing of ochratoxin A

Ochratoxin A (OTA) is one of the most toxic mycotoxins that exists in various agro-products and foods. Here, a non-label and enzyme-free fluorescence biosensor for highly specific detection of OTA has been developed by the combination of toehold-mediated strand displacement reaction (TMSD) and G-quadruplex dimer/ThT (G-dimer/ThT). The DNA duplex (aptamer-IP) is composed of the anti-OTA aptamer and a single stranded initiation probe (IP). In the presence of OTA, the attachment of target to aptamer leads to the liberation of the IP, which activates the cycle TMSD amplifications of two hairpin probes (H1 and H2) accompanied by the production of numerous H1-H2 assemblies. This double-stranded H1-H2 structure results in the proximity between the 5'-end overhang tail of H1 and the 3'-end stem of H2 to liberate the pre-blocked G-dimer sequence for lighting up ThT. In addition, the method displayed a stable fluorescence emission in the high-salt media. It was successfully applied to analyze OTA in real food samples. Hence, the constructed fluorescence biosensing platform might provide a new way for OTA and other toxin analysis detection.

When Nanoworlds Collide: Implementing DNA Amplification, Nanoparticles, Molecules, and FRET into a Single MicroRNA Biosensor

Isothermal nucleic acid amplification strategies have been combined with nanotechnology for advanced biosensing, material design, and biomedical applications. However, merging phenomena and materials of different nanoscales with the aim of exploiting all their benefits at once has remained a challenging endeavor. Here, we exemplify the various problems one can encounter when combining the nanodimensions of lanthanide complexes (∼2 nm), Förster resonance energy transfer (FRET, ∼5 nm), quantum dots (QDs, ∼20 nm), and rolling circle amplification (RCA, ∼250 nm) into a single microRNA biosensor and how these challenges can be overcome. Six different approaches, including simple FRET-RCA, enzyme-digesting FRET-RCA, and FRET-hyperbranched-RCA were investigated. We demonstrated specific miR-21 detection with 80 fM limit of detection and multiplexing capability with FRET from a Tb complex to different QDs. The detailed view on the various complex multi-nanodimensional assay systems elucidated the limited clinical translation of such sophisticated multicomponent nanobiosensors.

Graphene oxide assisted light-up aptamer selection against Thioflavin T for label-free detection of microRNA

We selected an aptamer against a fluorogenic dye called Thioflavin T (ThT). Aptamers are single-stranded DNA that can bind a specific target. We selected the ThT aptamer using graphene oxide assisted SELEX and a low-cost Open qPCR instrument. We optimized, minimized, and characterized the best aptamer candidate against ThT. The aptamer, ThT dye, and the enzymatic strand displacement amplification (SDA) were used in a label-free approach to detect the micro RNA miR-215 in saliva and serum. The aptamer confers higher specificity than intercalating dyes but without expensive covalently modified DNA probes. This isothermal, low-cost, simple method can detect both DNA and RNA. The target, miR-215, was detected with a limit of detection of 2.6 nM.

Recent advances in rolling circle amplification-based biosensing strategies-A review

Rolling circle amplification (RCA) is an efficient enzymatic isothermal reaction that using circular probe as a template to generate long tandem single-stranded DNA or RNA products under the initiation of short DNA or RNA primers. As a simplified derivative of natural rolling circle replication which synthesizes copies of circular nucleic acids molecules such as plasmids, RCA amplifies the circular template rapidly without thermal cycling and finds various applications in molecular biology. Compared with other amplification strategies, RCA has many obvious advantages. Firstly, because of the strict complementarity required in ligation of a padlock probe, it endows the RCA reaction with high specificity and can even be utilized to distinguish single base mismatches. Secondly, through the introduction of multiple primers, exponential amplification can be achieved easily and leads to a good sensitivity. Thirdly, RCA products can be customized by manipulating circular templates to generate functional nucleic acids such as aptamer, DNAzymes and restriction enzyme sites. Moreover, the RCA has good biocompatibility and is especially suitable for in situ detection. Therefore, RCA has attracted considerable attention as an efficient and potential tool for highly sensitive detection of biomarkers. Herein, we comprehensively introduce the fundamental principles of RCA technology, summarize it from three aspects including initiation mode, amplification mode and signal output mode, and discuss the recent application of RCA-based biosensor in this review.

Dimeric G-Quadruplex: An Effective Nucleic Acid Scaffold for Lighting Up Thioflavin T

As a recently identified higher-order quadruplex (G4) structure, the G4 dimer possesses unique structure and biological functions. In this work, we found accidentally that two tandem PW17 (one known G4-forming DNA) sequences can fold into a stable G4 dimer, and the G4 dimer can enhance dramatically the fluorescence intensity of thioflavin T (ThT). The G4 dimer/ThT fluorescence intensity is about ninefold that of the corresponding G4 monomer/ThT. Meanwhile, compared with the common G4/ThT system, G4 dimer/ThT exhibited more stable fluorescence emission in the media with various concentrations of Na⁺ and K⁺. On the basis of these findings, G4 dimer/ThT was used as a fluorescence indicator to construct one arched DNA probe for label-free detection of DNA. By incorporating a G4 dimer sequence in amplified products, we further designed one rolling circle amplification-based biosensing strategy to show the utility of this G4 dimer/ThT fluorescence indicator. This study demonstrates that dimeric G4 is an effective nucleic acid scaffold for lighting up ThT, showing promising applications in a label-free bioassay.

Detection of microRNAs by the combination of Exonuclease-III assisted target recycling amplification and repeated-fishing strategy

A simple but effective method for the detection of miRNAs was proposed by integrating exonuclease-III assisted target recycling amplification and repeated-fishing strategy. In the proposed method, exonuclease-III assisted target recycling amplification reaction is adopted to produce a large amount of DNA fragments with fluorescence group at its 5' end in the presence of the target miRNA, which are then repeatedly fished out from the reaction mixture by a gold foil modified with a capture probe and transferred into a so-called 'product tube'. The amount of the target miRNA can then be determined from the fluorescence measurement of the solution in the 'product tube'. Application to the detection of miRNA-155 in samples of KH-2 and BRSA-2B cells revealed that the proposed method could achieve sensitive and accurate quantification of the target miRNA with a limit of detection of 36 fM and recovery rates in the range from 96.2% to 105%. Its simplicity, sensitivity and resistance to possible fluorescence interferences in complex biological samples make the proposed method a potentially competitive alternative for miRNAs detection in complex biological samples.

Label-free fluorometric detection of microRNA using isothermal rolling circle amplification generating tandem G-quadruplex

MicroRNAs (miRNAs) play an important role in various biological processes and have been regarded as promising diagnostic biomarkers for solid tumors in the field of clinical diagnostics. In this study, we developed a simple label/quencher-free fluorometric system for sensitive and selective miRNA detection using isothermal gene amplification such as rolling circle amplification generating tandem G-quadruplex DNA structures (GQ-RCA). The closed-circular dumbbell-shaped padlock DNA was designed to be complementary to its corresponding target miRNA. In the presence of the target miRNA, a long stretch of ssDNA with tandem G-quadruplex sequence repeats was readily generated by RCA, initiated by phi29 DNA polymerase through DNA synthesis priming at the 3'-OH of the target miRNA annealed to the padlock DNA. The RCA product harboring tandem G-quadruplex was monitored with fluorophore Thioflavin T (ThT) that emits strong fluorescence only when it intercalates into the G-quadruplex. The GQ-RCA assay enabled us to detect miRNA as low as 4.9 fM with a linear range from 25.6 fM to 80 pM within 0.5 h. In addition, our system was applied to the miRNA samples present in human plasma, showing its potential use in the clinical diagnosis of cancer.

Loop-mediated isothermal amplification (LAMP) reaction as viable PCR substitute for diagnostic applications: a comparative analysis study of LAMP, conventional PCR, nested PCR (nPCR) and real-time PCR (qPCR) based on Entamoeba histolytica DNA derived from faecal sample

Background

This study reports the analytical sensitivity and specificity of a Loop-mediated isothermal amplification (LAMP) and compares its amplification performance with conventional PCR, nested PCR (nPCR) and real-time PCR (qPCR). All the assays demonstrated in this study were developed based on Serine-rich Entamoeba histolytica protein (SREHP) gene as study model.

Results

A set of SREHP gene specific LAMP primers were designed for the specific detection of Entamoeba histolytica. This set of primers recorded 100% specificity when it was evaluated against 3 medically important Entamoeba species and 75 other pathogenic microorganisms. These primers were later modified for conventional PCR, nPCR and qPCR applications. Besides, 3 different post-LAMP analyses including agarose gel electrophoresis, nucleic acid lateral flow immunoassay and calcein-manganese dye techniques were used to compare their limit of detection (LoD). One E. histolytica trophozoite was recorded as the LoD for all the 3 post-LAMP analysis methods when tested with E. histolytica DNA extracted from spiked stool samples. In contrast, none of the PCR method outperformed LAMP as both qPCR and nPCR recorded LoD of 100 trophozoites while the LoD of conventional PCR was 1000 trophozoites.

Conclusions

The analytical sensitivity comparison among the conventional PCR, nPCR, qPCR and LAMP reveals that the LAMP outperformed the others in terms of LoD and amplification time. Hence, LAMP is a relevant alternative DNA-based amplification platform for sensitive and specific detection of pathogens.

Reverse transcription-based loop-mediated isothermal amplification strategy for real-time miRNA detection with phosphorothioated probes

A novel reverse transcription-based loop-mediated isothermal amplification (LAMP) strategy for miRNA detection has been developed. This method consists of two stem-loop probes inspired by the dumbbell-shaped amplicons and inner primers used in conventional LAMP reactions. Termed "terminal hairpin formation and self-priming" (THSP), this reaction incorporates phosphorothioated (PS) modifications to achieve DNA folding and extension without primers. The final signal is monitored by a sequence-specific detection probe, which minimizes the background noise. We suggest that our rapid, facile, and reliable LAMP method will be a promising candidate for detecting miRNA in biomedical applications.

DNAzyme-Based Target-Triggered Rolling-Circle Amplification for High Sensitivity Detection of microRNAs

MicroRNAs regulate and control the growth and development of cells and can play the role of oncogenes and tumor suppressor genes, which are involved in the occurrence and development of cancers. In this study, DNA fragments obtained by target-induced rolling-circle amplification were constructed to complement with self-cleaving deoxyribozyme (DNAzyme) and release fluorescence biomolecules. This sensing approach can affect multiple signal amplification permitting fluorescence detection of microRNAs at the pmol L⁻¹ level hence affording a simple, highly sensitive, and selective low cost detection platform.

Circular Nucleic Acids: Discovery, Functions and Applications

Circular nucleic acids (CNAs) are nucleic acid molecules with a closed-loop structure. This feature comes with a number of advantages including complete resistance to exonuclease degradation, much better thermodynamic stability, and the capability of being replicated by a DNA polymerase in a rolling circle manner. Circular functional nucleic acids, CNAs containing at least a ribozyme/DNAzyme or a DNA/RNA aptamer, not only inherit the advantages of CNAs but also offer some unique application opportunities, such as the design of topology-controlled or enabled molecular devices. This article will begin by summarizing the discovery, biogenesis, and applications of naturally occurring CNAs, followed by discussing the methods for constructing artificial CNAs. The exploitation of circular functional nucleic acids for applications in nanodevice engineering, biosensing, and drug delivery will be reviewed next. Finally, the efforts to couple functional nucleic acids with rolling circle amplification for ultra-sensitive biosensing and for synthesizing multivalent molecular scaffolds for unique applications in biosensing and drug delivery will be recapitulated.

Detection of Several Homologous MicroRNAs by a Single Smart Probe System Consisting of Linear Nucleic Acid Blockers

We report a universal smart probe (SP) that is capable of detecting several homologous let-7 microRNAs (miRNAs). While the SP is complementary to let-7a, and therefore, strongly binds to this target, due to sequence homology, the SP also has equal propensity to non-specifically hybridize with let-7b and let-7c, which are homologous to let-7a. The fluorescence signal of the SP was switched off in the absence of any homologous member target, but the signal was switched on when any of the three homologous members was present. With the assistance of nucleic acid blockers (NABs), this SP system can discriminate between homologous miRNAs. We show that the SP can discriminate between let-7a and the other two sequences by using linear NABs (LNABs) to block non-specific interactions between the SP and these sequences. We also found that LNABs used do not cross-react with the let-7a target due to the low LNABs:SP molar ratio of 6:1 used. Overall, this SP represents a universal probe for the recognition of a homologous miRNA family. The assay is sensitive, providing a detection limit of 6 fmol. The approach is simple, fast, usable at room temperature, and represents a general platform for the in vitro detection of homologous microRNAs by a single fluorescent hairpin probe.

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

Herein, we combined toehold exchange with ligation-free rolling circle amplification (RCA) by programming nucleolytic conversion of hairpin probe into sensors, allowed for both high specific recognition and universal signal amplification for RNA detection. The rational engineered HP ensured highly specific recognition based on toehold exchange and allowed the pre-formed circular template for RCA to be shared for different RNAs detection. Generally, detecting different RNA requires different circular template for signal amplification. In this paper, the circular template for RCA was independent of the sequences of the target, so the signal amplification system was an universal one for different RNAs detection. Taking miRNA let-7d as a model target, this method showed a wide linear range from 1 fM to 1 nM with a detection limit of 0.46 fM and exhibited a remarkable selectivity even in distinguishing homologous miRNAs with 1-nt or 2-nt difference. To evaluate the potential of the method, it was applied to analysis the let-7d concentration in human serum, total RNA, and cell lysates. In conclusion, we believe this method exhibits potential for both specific discrimination and universal signal amplification for RNA analysis in complex matrices.

Label-Free Telomerase Detection in Single Cell Using a Five-Base Telomerase Product-Triggered Exponential Rolling Circle Amplification Strategy

Telomerase is a universal biomarker of malignant tumors. Sensitive and reliable analysis for telomerase activity is of vital importance for both early diagnosis and therapy of malignant tumors. Herein, a novel fluorescent strategy was proposed for sensitive and label-free detection of telomerase activity. One highlight of this strategy is that an exponential signal amplification can be triggered by a very short telomerase extension product (TEP). Without adding dATP, the designed telomerase primer can be easily controlled to extend five bases (GGGTT) to give short TEP with definite length. The resulting short TEP can then be constructed as a circular rolling circle amplification (RCA) template and thus initiate a nicking enzyme-mediated exponential RCA, producing G-rich amplification products that can be sensitively probed via specific binding between the fluorescent dye Thioflavin T (ThT) and the nucleic acid G-quadruplexes. Elevated telomerase translocation efficiency, combined with exponential signal amplification and specific probing of RCA products by ThT, endow the sensing platform with extraordinarily high detection sensitivity. The requirement for short TEP increases the possibility to analyze telomerase with low activity. The proposed sensing platform can achieve sensitive telomerase activity detection in individual cells, even with the interference of accumulated normal cells. It was also demonstrated to show excellent capability in screening for the inhibitors of telomerase. Therefore, the proposed sensing platform has great potential for not only clinical diagnosis but also anticancer drug development.

Base excision repair mediated cascading triple-signal amplification for the sensitive detection of human alkyladenine DNA glycosylase

DNA glycosylase (DG) plays a significant role in repairing DNA lesions, and the dysregulation of DG activity is associated with a variety of human pathologies. Thus, the detection of DG activity is essential for biomedical research and clinical diagnosis. Herein, we develop a facile fluorometric method based on the base excision repair (BER) mediated cascading triple-signal amplification for the sensitive detection of DG. The presence of human alkyladenine DNA glycosylase (hAAG) can initiate the cleavage of the substrate at the mismatched deoxyinosine site by endonuclease IV (Endo IV), resulting in the breaking of the DNA substrate. The cleaved DNA substrate functions as both a primer and a template to initiate strand displacement amplification (SDA) to release primers. The released primers can further bind to a circular template to induce an exponential primer generation rolling circle amplification (PG-RCA) reaction, producing a large number of primers. The primers that resulted from the SDA and PG-RCA reaction can induce the subsequent recycling cleavage of signal probes, leading to the generation of a fluorescence signal. Taking advantage of the high amplification efficiency of triple-signal amplification and the low background signal resulting from single uracil repair-mediated inhibition of nonspecific amplification, this method exhibits a low detection limit of 0.026 U mL^-1 and a large dynamic range of 4 orders of magnitude for hAAG. Moreover, this method has distinct advantages of simplicity and low cost, and it can further quantify the hAAG activity from HeLa cell extracts, holding great potential in clinical diagnosis and biomedical research.

Duplex featured polymerase-driven concurrent strategy for detecting of ATP based on endonuclease-fueled feedback amplification

We have developed a novel amplification strategy termed Endo IV-assisted feedback amplification (EFA) taking advantages of rolling circular amplification (RCA) and Endo IV-assisted signal amplification (ESA) biosensing platform for detecting of adenosine triphosphate (ATP). Two kinds of specially programmed DNA complexes were employed into EFA system, one composed of a split aptamer fragment and a circular template, and the other composed of AP probe and the same circular template. Hence, ATP as a target induced the self-assembly of spilt aptamer fragments and initiated RCA reaction generating a linear DNA, which consists of hybridization elements with Complex II and formation elements of G-quadruplex. More importantly, the addition of endonuclease IV can cut the Complex II into two parts, and one of which can be trimming by phi29 DNA polymerase initiating the new round of RCA reaction producing more RCA products. Thus significantly enhanced fluorescent signal can be measured for ATP as expected, and our proposed strategy exhibits improved performances toward ATP ultrasensitive detection with a limit of detection (LOD) as low as 0.09 nM. Additionally, our developed biosensor demonstrates high selectivity and the superiority of simplicity towards ATP. Above these significant aspects, our proximity binding-induced RCA reaction-based fluorescent assay and Endo IV-fueled feedback signal amplification strategy presents an optimal detection performance towards ATP for potential application in related research and clinical diagnosis.

G-quadruplexes as versatile scaffolds for catalysis

This review summarizes the beginning, progress, and prospects of non-canonical DNA-based hybrid catalysts focusing on G-quadruplexes as versatile scaffolds for catalysis.

A DNA encoding loop program: the snowball effect enhanced microRNA visualization in living cells

A DNA encoding loop program (DELP); an Illustration of the DELP clustering process. In the presence of miRNA, multiple seed probes and fuel probes form enlarged GNP clusters, and the fluorescence of the FAM molecules recovers due to the opening of the hairpin DNA.

Advanced microRNA-based cancer diagnostics using amplified time-gated FRET

MicroRNAs (miRNAs) play an important role in cellular functions and in the development and progression of cancer. Precise quantification of endogenous miRNAs from different clinical patient and control samples combined with a one-to-one comparison to standard technologies is a challenging but necessary endeavor that is largely neglected by many emerging fluorescence technologies. Here, we present a simple, precise, sensitive, and specific ratiometric assay for absolute quantification of miRNAs. Isothermally amplified time-gated Förster resonance energy transfer (TG-FRET) between Tb donors and dye acceptors resulted in miRNA assays with single-nucleotide variant specificity and detection limits down to 4.2 ± 0.5 attomoles. Quantification of miR-21 from human tissues and plasma samples revealed the relevance for breast and ovarian cancer diagnostics. Analysis of miR-132 and miR-146a from acute monocytic leukemia cells (THP-1) demonstrated the broad applicability to different miRNAs and other types of clinical samples. Direct comparison to the gold standard RT-qPCR showed advantages of amplified TG-FRET concerning precision and specificity when quantifying low concentrations of miRNAs as required for diagnostic applications. Our results demonstrate that a careful implementation of rolling circle amplification and TG-FRET into one straightforward nucleic acid detection method can significantly advance the possibilities of miRNA-based cancer diagnostics and research.

Functional-DNA-Driven Dynamic Nanoconstructs for Biomolecule Capture and Drug Delivery

The discovery of sequence-specific hybridization has allowed the development of DNA nanotechnology, which is divided into two categories: 1) structural DNA nanotechnology, which utilizes DNA as a biopolymer; and 2) dynamic DNA nanotechnology, which focuses on the catalytic reactions or displacement of DNA structures. Recently, numerous attempts have been made to combine DNA nanotechnologies with functional DNAs such as aptamers, DNAzymes, amplified DNA, polymer-conjugated DNA, and DNA loaded on functional nanoparticles for various applications; thus, the new interdisciplinary research field of "functional DNA nanotechnology" is initiated. In particular, a fine-tuned nanostructure composed of functional DNAs has shown immense potential as a programmable nanomachine by controlling DNA dynamics triggered by specific environments. Moreover, the programmability and predictability of functional DNA have enabled the use of DNA nanostructures as nanomedicines for various biomedical applications, such as cargo delivery and molecular drugs via stimuli-mediated dynamic structural changes of functional DNAs. Here, the concepts and recent case studies of functional DNA nanotechnology and nanostructures in nanomedicine are reviewed, and future prospects of functional DNA for nanomedicine are indicated.