G-quadruplex − based homogenous fluorescence platform for ultrasensitive DNA detection through isothermal cycling and cascade signal amplification

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.

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.

Parallel [TG(GA)3]n-homoduplexes/thioflavin T: an intense and stable fluorescent indicator for label-free biosensing

Different from the classical antiparallel DNA double-stranded structure, parallel DNA duplexes possess unique structures and potential biological functions. In this work, we found that the parallel DNA homoduplex from the [TG(GA)3]n sequence ([TG(GA)3]n-dsDNA) can dramatically enhance the fluorescence of thioflavin T (ThT), and the fluorescence enhancement is proportional to the number (n) of TG(GA)3 units in [TG(GA)3]n. Compared with the traditional G-quadruplex/ThT system, [TG(GA)3]n/ThT showed more stable and stronger fluorescence emission. In addition, coupled with an isothermal exponential amplification reaction, [TG(GA)3]3/ThT was used as a label-free fluorescent probe to detect microRNA, and the [TG(GA)3]3/ThT probe exhibited higher sensitivity than the G-quadruplex/ThT probe. This work provides a new paradigm to design label-free fluorescent biosensing/imaging systems.

Metal-Free Photoinduced Atom Transfer Radical Polymerization for Highly Sensitive Detection of Lung Cancer DNA

Convenient and sensitive detection of biomolecules is of great significance to disease diagnosis. In this work, a metal-free photoinduced atom transfer radical polymerization (photoATRP) by a reductive quenching pathway as a novel strategy is applied to achieve lung cancer DNA detection. Thiolated PNA is exploited to specifically recognize target DNA, and the initiator of photoATRP is linked to the electrode surface via phosphate-Zr⁴⁺ -carboxylate. Under the excitation of blue light, the reductive quenching pathway is activated with eosin Y (EY) as photoredox catalyst and N,N,N',N'',N'-pentamethyldiethylenetriamine (PMDETA) as electron donor, and numerous polymeric chains are formed. Under optimal conditions, the linear range of this strategy is from 0.1 pm to 10 nm (R² =0.989) with a limit of detection (LOD) of 1.4 fm (14 zmol in 10 μL). The variety of possible light sources for photoATRP and simple operation endow this biosensor with great potential for practical applications.

Symmetric exponential amplification reaction-based DNA nanomachine for the fluorescent detection of nucleic acids

By introducing palindromic sequences into the classical exponential amplification reaction (EXPAR), we constructed a new palindromic fragment-incorporated multifunctional hairpin probe (P-HP)-mediated symmetric exponential amplification reaction (S-EXPAR), to significantly reduce the background signal caused by inherent nonspecific amplification. A G-triplex/ThT complex was used as the signal reporter for the proposed label-free DNA nanomachine. The P-HP consists of five functional regions: a C-rich region (C), a target DNA recognition region (T′), two nicking sites (X′) and a palindromic fragment (P). When target DNA (T) hybridizes with P-HP, the palindromic fragment at the 3′ end of P-HP is fully exposed. Then, the P-HP/T duplexes hybridize with each other through the exposed P, and EXPAR occurs automatically and continuously on both sides of P under the synergistic effect of polymerase and nicking endonuclease. This is called the S-EXPAR assay. In this system, one T converts to a large number of G-triplex fragments, which can combine with ThT within a short time. The G-triplex/ThT complexes formed act as the signal reporter in a label-free and environmentally friendly format. In this way, the limit of detection of this method is as low as 10 pM with a dynamic response range of 10 pM to 300 nM. In addition, this method can detect other nucleic acids by simply changing the T′ region of the P-HP. Thus, the proposed DNA nanomachine is a potential alternative method for nucleic acid detection.

This label-free and ultra-low background signal DNA nanomachine was based on P-HP mediated S-EXPAR and the G-triplex/ThT complex.

Ultrasensitive DNA detection based on target-triggered rolling circle amplification and fluorescent poly(thymine)-templated copper nanoparticles

We describe a novel strategy for the ultrasensitive detection of target DNA based on rolling circle amplification (RCA) coupled with fluorescent poly(thymine)-templated copper nanoparticles (poly T-CuNPs). In the presence of target DNA, a padlock DNA probe that consists of two regions: a target DNA-specific region and a poly(adenine) region, is circularized by the ligation reaction, and the subsequent RCA reaction is promoted to generate long, concatemeric, single-stranded DNA (ssDNA) with a lot of repetitive poly T sequences. As a result, a large number of poly T-CuNPs are formed, exhibiting a highly fluorescent signal. However, in the absence of target DNA or in the presence of non-specific target DNA, the padlock DNA probe is not circularized and the subsequent RCA is not executed, leading to no production of fluorescent poly T-CuNPs. With this simple strategy, we successfully analyzed the target DNA with the ultralow detection limit of 7.79 aM, a value that is 3 or 7 orders of magnitude lower than those of previous RCA-based fluorescent DNA detection strategies. In addition, the developed system was demonstrated to selectively discriminate non-specific target DNAs with one-base mismatch, suggesting potential application in the accurate diagnosis of single nucleotide polymorphisms or mutations.

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

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

Sensitive DNA detection using cascade amplification strategy based on conjugated polyelectrolytes and hybridization chain reaction

A novel cascade amplification strategy that combines the molecular wire effects of CPEs with the signal amplification capability of the HCR has been developed for sensitive DNA detection.

Rapid and sensitive detection of potassium ion based on K(+)-induced G-quadruplex and guanine chemiluminescence

A simple and rapid method for detection of potassium ion (K(+)) based on a guanine chemiluminescence (CL) system is presented. In this system, one guanine-rich DNA molecule is used as the recognition element. K(+) can cause the guanine-rich DNA to form a G-quadruplex conformation, resulting in remarkable quenching of the guanine CL intensity of guanine-rich DNA. The CL intensity of this CL system decreased with increasing K(+) concentration, revealing a linear relationship in K(+) concentration range from 3 × 10(-5) to 1 × 10(-3) M. A complete detection process can be accomplished in about 5 min. Other common cations (such as Na(+), NH4 (+), Mg(2+), Ca(2+), Zn(2+), and Pb(2+)) did not notably interfere with K(+) detection. The mechanism of this strategy is also discussed. The sensing strategy is low cost and simple without the requirement of complex labeling of probe DNA. The scheme is applicable to the detection of other guanine-rich aptamer-binding chemicals or biomolecules.