Real-time assays with molecular beacons and other fluorescent nucleic acid hybridization probes

Efficient enrichment of free target sequences in an integrated microfluidic device for point-of-care detection systems

Nucleic acid biomarker detection has great importance in the diagnosis of disease, the monitoring of disease progression and the classification of patients according to treatment decision making. Nucleic acid biomarkers found in the blood of patients have generated a lot of interest due to the possibility of being detected non-invasively which makes them ideal for monitoring and screening tests and particularly amenable to point-of-care (POC) or self-testing. A major challenge to POC molecular diagnostics is the need to enrich the target to optimise detection. In this work, we describe a microfabricated device for the enrichment of short dsDNA target sequences, which is especially valuable for potential detection methods, as it improves the probability of effectively detecting the target in downstream analyses. The device integrated a heating element and a temperature sensor with a microfluidic chamber to carry out the denaturation of the dsDNA combined with blocking-probes to enrich the target. This procedure was validated by fluorescence resonance energy transfer (FRET) technique, labelling DNA with a fluorophore and a quencher. As proof of concept, a 23-mer long dsDNA sequence corresponding to the L858R mutation of the EGFR gene was used. The qualitative results obtained determined that the most optimal blocking rate was obtained with the incorporation of 11/12-mer blocking-probes at a total concentration of 6 μM. This device is a powerful DNA preparation tool, which is an indispensable initial step for subsequent detection of sequences via nucleic acid hybridisation methods.

DNAzyme Nanomachine with Fluorogenic Substrate Delivery Function: Advancing Sensitivity in Nucleic Acid Detection

We have developed a hook-equipped DNA nanomachine (HDNM) for the rapid detection of specific nucleic acid sequences without a preamplification step. HDNM efficiently unwinds RNA structures and improves the detection sensitivity. Compared to the hookless system, HDNM offers an 80-fold and 13-fold enhancement in DNA and RNA detection, respectively, reducing incubation time from 3 to 1 h.

DNA-Aptamer-Based qPCR Using Light-Up Dyes for the Detection of Nucleic Acids

Quantitative polymerase chain reaction (qPCR) is widely used in detection of nucleic acids, but existing methods either lack sequence-specific detection or are costly because they use chemically modified DNA probes. In this work, we apply a DNA aptamer and light-up dye-based chemistry for qPCR for nucleic acid quantification. In contrast to the conventional qPCR, in our method, we observe an exponential decrease in fluorescence upon DNA amplification. The qPCR method we developed produced consistent Ct vs log10 (DNA amount) standard curves, which have a linearfit with R2 value > 0.99. This qPCR technique was validated by quantifying gene targets from Streptococcus zooepidemicus (SzhasB) and Mycobacterium tuberculosis (MtrpoB). We show that our strategy is able to successfully detect DNA at as low as 800 copies/μL. To the best of our knowledge, this is the first study demonstrating the application of light-up dyes and DNA aptamers in qPCR.

RT-IVT method allows multiplex real-time quantification of in vitro transcriptional mRNA production

For the past 30 years, in vitro transcription (IVT) technology has been extensively used for RNA production or for basic transcriptional mechanism research. However, methods for mRNA quantification still need to be improved. In this study, we designed a RT-IVT method using binary fluorescence quencher (BFQ) probes and the PBCV-1 DNA ligase to quantify mRNA production in real-time by fluorescence resonance energy transfer (FRET) and RNA-splinted DNA ligation. Compared with existing methods, the RT-IVT method is inexpensive and non-radioactive, and can detect mRNA production in unpurified systems in real-time and shows high sensitivity and selectivity. The activity of T7 RNA polymerase and Escherichia coli RNA polymerase holoenzyme was then characterized with this method. We then multiplexed the real-time mRNA quantification for three T7 promoters on a RT-PCR thermocycler by using BFQ probes with different colored fluorophores that were specific for each target. Ultimately, we created an inexpensive multiplexed method to quantify mRNA production in real-time, and future research could use these methods to measure the affinity of transcriptional repressors to their target DNA sequence.

Graphene Oxide-Functionalized Thread-Based Electrofluidic Approach for DNA Hybridization

A novel, low-cost, and disposable thread-based electrofluidic analytical method employing isotachophoresis (ITP) was developed for demonstrating surface DNA hybridization. This approach was based on graphene oxide (GO) surface-functionalized zones on nylon threads as a binding platform to trap a fluorescently labeled isotachophoretically focused single-stranded DNA (ssDNA) band, resulting in quenching of the fluorescence, which signaled quantitative trapping. In the event of an isotachophoretically focused complementary DNA (cDNA) band passing over the GO-trapped ssDNA zone, surface hybridization of the ssDNA and cDNA to form double-stranded DNA (dsDNA) band occurred, which is released from the GO-coated zones, resulting in restoration of the fluorescent signal as it exits the GO band and migrates further along the thread. This controllable process demonstrates the potential of the GO-functionalized thread-based microfluidic analytical approach for DNA hybridization and its visualization, which could be adapted into point-of-care (POC) diagnostic devices for real-world applications.

OWL2: a molecular beacon-based nanostructure for highly selective detection of single-nucleotide variations in folded nucleic acids

Hybridization probes have been used in the detection of specific nucleic acids for the last 50 years. Despite the extensive efforts and the great significance, the challenges of the commonly used probes include (1) low selectivity in detecting single nucleotide variations (SNV) at low (e.g. room or 37 °C) temperatures; (2) low affinity in binding folded nucleic acids, and (3) the cost of fluorescent probes. Here we introduce a multicomponent hybridization probe, called OWL2 sensor, which addresses all three issues. The OWL2 sensor uses two analyte binding arms to tightly bind and unwind folded analytes, and two sequence-specific strands that bind both the analyte and a universal molecular beacon (UMB) probe to form fluorescent 'OWL' structure. The OWL2 sensor was able to differentiate single base mismatches in folded analytes in the temperature range of 5-38 °C. The design is cost-efficient since the same UMB probe can be used for detecting any analyte sequence.

Conformational-switch biosensors as novel tools to support continuous, real-time molecular monitoring in lab-on-a-chip devices

Recent years have seen continued expansion of the functionality of lab on a chip (LOC) devices. Indeed LOCs now provide scientists and developers with useful and versatile platforms across a myriad of chemical and biological applications. The field still fails, however, to integrate an often important element of bench-top analytics: real-time molecular measurements that can be used to "guide" a chemical response. Here we describe the analytical techniques that could provide LOCs with such real-time molecular monitoring capabilities. It appears to us that, among the approaches that are general (i.e., that are independent of the reactive or optical properties of their targets), sensing strategies relying on binding-induced conformational change of bioreceptors are most likely to succeed in such applications.

Oligonucleotide-Chemosensor Conjugate as a Dual Responsive Detection Platform and Its Application for Simultaneous Detection of ATP and Zn2

Simultaneous detection, which helps understand complex physiological processes and accurately diagnose diseases, has been achieved using dual responsive probes. The dual responsive probe can ideally distinguish four cases, which are a combination of the absence and presence of two analytes, with characteristic fluorescence emissions. Owing to the demanding conditions of its development, most previous studies have focused on the simple linkage between small-molecule chemosensors that have an individual target and spectral range. In this study, a new dual responsive detection platform, oligonucleotide-chemosensor conjugate, was developed using a linkage between versatile oligonucleotide probes and small-molecule chemosensors to expand the applicable scaffold and detectable target for simultaneous detection. As a proof of concept, the ATP aptamer probe and Zn²⁺ chemosensor were conjugated as the levels of ATP and Zn²⁺ are intimately correlated in several signaling pathways and diseases. Each probe could detect an analyte independently within a conjugate probe, and simultaneous detection was also demonstrated without spectral crosstalk or interference between the receptors. In addition, the introduced cholesterol modification allowed the developed probe to detect changes in analytes on the plasma membrane of live cells through flow cytometry and confocal microscopy.

Comparison of Quenching Efficiencies in Long Triple-Labeled and Double-Labeled TaqMan Oligodeoxynucleotide Probes

Although triple labeling of molecular beacons has been documented to improve quenching efficiencies and studies generally assume similar benefits at long TaqMan probes, a limited number of works have studied this issue in TaqMan probes. We therefore prepared a series of long triple-labeled oligodeoxynucleotide probes with 6-carboxyfluorescein as a fluorophore at the 5'-end and BlackBerry (BBQ-650) or azaphthalocyanine quenchers at the 3'-end and in the intrastrand position and systematically compared their quenching efficiencies with those of the corresponding double-labeled probes including important control probes. A model polymerase chain reaction (PCR) assay enabled the determination of the quenching efficiencies of static and Förster resonance energy transfer (FRET) quenching in the target probes. The type of probe had no effect on the static quenching ability. Importantly, FRET quenching of double-labeled probes with a quencher at the 3'-end showed a statistically insignificant difference from the control probe without any quencher, indicating the need to shift the quencher closer to the fluorophore in long probes. Shortening the distance between the fluorophore and the quencher played a key role in FRET quenching, whereas the introduction of an additional quencher only slightly improved the quenching efficiency. BBQ-labeled probes had lower quenching efficiencies than azaphthalocyanine probes. The methodologies and relationships described above seem, however, to be universal and applicable to any quencher.

PlexProbes enhance qPCR multiplexing by discriminating multiple targets in each fluorescent channel

The probe technology described in this paper facilitates detection and discrimination of multiple targets in a single fluorescent channel during PCR. This provides a strategy for doubling the number of targets that can be analysed simultaneously on existing PCR instruments. These probes are referred to as PlexProbes and produce fluorescence that can be switched ‘on’ or ‘off’ in the presence of target by manipulating the temperature. During PCR, fluorescence can be measured at multiple temperatures allowing discrimination of specific targets at defined temperatures. In a single fluorescent channel, a model duplex assay allowed either real-time or endpoint detection of Chlamydia trachomatis (CT) at 52°C and end-point detection of Neisseria gonorrhoeae (GC) at 74°C. Using this model system, as few as 40 copies of each specific target could be detected as single infection or co-infection, regardless of the presence or absence of the other target. A PlexProbe prototype assay for sexually transmitted infections (PP-STI) which simultaneously enables detection and differentiation of six targets using only three fluorescent channels was then constructed and evaluated. The PP-STI assay detects GC (2 gene targets), CT, Mycoplasma genitalium (MG), Trichomonas vaginalis (TV) and an internal control (IC). To evaluate assay performance, a panel of archived clinical samples (n = 337) were analysed using PP-STI and results compared to those obtained with a commercially available diagnostic assay. The overall agreement between results obtained with the PP-STI assay and the reference test was greater than 99.5%. PlexProbes offer a method of detecting more targets from a single diagnostic test, empowering physicians to make evidence-based treatment decisions while conserving time, labour, sample volume and reagent costs.

Fundamental photophysics of isomorphic and expanded fluorescent nucleoside analogues

Fluorescent nucleoside analogues (FNAs) are structurally diverse mimics of the natural essentially non-fluorescent nucleosides which have found numerous applications in probing the structure and dynamics of nucleic acids as well as their interactions with various biomolecules. In order to minimize disturbance in the labelled nucleic acid sequences, the FNA chromophoric groups should resemble the natural nucleobases in size and hydrogen-bonding patterns. Isomorphic and expanded FNAs are the two groups that best meet the criteria of non-perturbing fluorescent labels for DNA and RNA. Significant progress has been made over the past decades in understanding the fundamental photophysics that governs the spectroscopic and environmentally sensitive properties of these FNAs. Herein, we review recent advances in the spectroscopic and computational studies of selected isomorphic and expanded FNAs. We also show how this information can be used as a rational basis to design new FNAs, select appropriate sequences for optimal spectroscopic response and interpret fluorescence data in FNA applications.

Label-Free Homogeneous microRNA Detection in Cell Culture Medium Based on Graphene Oxide and Specific Fluorescence Quenching

Label-free homogeneous optical detection of low concentration of oligonucleotides using graphene oxide in complex solutions containing proteins remains difficult. We used a colloidal graphene oxide (GO) as a fluorescent probe quencher to detect microRNA-21 spiked-in cell culture medium, overcoming previously reported problematic aspects of protein interference with graphene oxide. We used a "turn off" assay for specific quenching-based detection of oligo DNA-microRNA hybridization in solution. A fluorescein conjugated 30-mer single-stranded DNA (ssDNA) probe was combined with a complementary synthetic microRNA (18 nucleotides) target. The probe-target hybridization was detected by specific quenching due to photoinduced electron transfer (PET). On the next step, GO captures and quenches the unhybridized probe by fluorescence resonance energy transfer (FRET) in the presence of cell culture medium supplemented with platelet lysate, 0.1% sodium dodecyl sulfate (SDS), 0.1% Triton X-100 and 50% formamide. This resulted in sensitive measurement of the specific probe-target complexes remaining in solution. The detection is linear in the range of 1 nM and 8 nM in a single 100 μL total volume assay sample containing 25% cell culture medium supplemented with platelet lysate. We highlight a general approach that may be adopted for microRNA target detection within complex physiological media.

Application of Short Pre-enrichment, and Double Chemistry Real-Time PCR, Combining Fluorescent Probes and an Intercalating Dye, for Same-Day Detection and Confirmation of Salmonella spp. and Escherichia coli O157 in Ground Beef and Chicken Samples

Molecular methods, particularly those based on real-time PCR (qPCR), have become a popular approach to detect pathogens in food samples. This technique may take advantage of hydrolysis fluorescent probes for increased specificity. Even though suitable, this approach loses the capacity of performing result confirmation by melt curve analysis. In the current study, we developed an alternative approach, combining fluorescent probes along with an intercalating dye (SYBR Green) in order to simultaneously detect, and confirm the result, of two foodborne pathogens (Salmonella spp. and Escherichia coli O157). This new approach named double chemistry qPCR was combined with a short pre-enrichment in order to obtain a multiplex “same-day” detection method for the selected pathogens. The evaluation of the novel method in spiked food samples (ground beef and chicken breast) obtained values of relative sensitivity, specificity, and accuracy higher than 95%, and Cohen’s kappa of 0.92, with a Limit of Detection₉₅ below 5 cfu/25 g, demonstrating its reliability. In addition to this, the method was challenged by inoculating heat-stressed bacteria as well as dead ones. It was observed that it was also possible to detect stressed bacteria with an initial inoculation level below 10 cfu/25 g. Also, it was noticed that high initial concentration of either pathogen (higher than 10⁴ cfu/25 g) was needed in order to generate false positive results due to the presence of dead bacteria, thus the method presents potential for its application in the specific detection of live microorganisms.

Colorimetric determination of DNA using an aptamer and plasmonic nanoplatform

A facile plasmonic nanoplatform was developed for rapid and sensitive determination of nucleic acid. Hg²⁺-regulated molecular beacon (MB, hairpin) containing rich thymine (T) bases at both ends is used as the probe. A hairpin structure can be formed in the MB probe due to the strong binding of Hg²⁺ to T. However, in the presence of target DNA, the hairpin structure is opened owing to target DNA-specific hybridization with the aptamer. Simultaneously, the opened MB interacts with poly(diallyldimethylammonium chloride) (PDDA) and hinders PDDA-induced aggregation of AuNPs, accompanied by a color change from blue to red and a decrease in absorption ratio (A₆₂₀/A₅₂₀). Hence, a good linear relationship was observed between the decreased absorption ratio (A₆₂₀/A₅₂₀) and DNA concentration ranging from 0.02 to 2 nmol/L with a low detection limit of 4.42 pmol/L. Moreover, this nanoplatform has been successfully utilized to discriminate between perfect target and mismatch sequences. More importantly, the bioassay is simple, versatile, rapid, and cost-effective compared with other common methods, which holds great promise for clinical diagnosis and biomedical application. Graphical abstract.

RNA knockdown by synthetic peptidyl-oligonucleotide ribonucleases: behavior of recognition and cleavage elements under physiological conditions

Sequence-specific protein-based ribonucleases are not found in nature. Absolute sequence selectivity in RNA cleavage in vivo normally requires multi-component complexes that recruit a guide RNA or DNA for target recognition and a protein-RNA assembly for catalytic functioning (e.g. RNAi molecular machinery, RNase H). Recently discovered peptidyl-oligonucleotide synthetic ribonucleases selectively knock down pathogenic RNAs by irreversible cleavage to offer unprecedented opportunities for control of disease-relevant RNA. Understanding how to increase their potency, selectivity and catalytic turnover will open the translational pathway to successful therapeutics. Yet, very little is known about how these chemical ribonucleases bind, cleave and leave their target. Rational design awaits this understanding in order to control therapy, particularly how to overcome the trade-off between sequence specificity and potency through catalytic turnover. We illuminate this here by characterizing the interactions of these chemical RNases with both complementary and non-complementary RNAs using T_m profiles, fluorescence, UV-visible and NMR spectroscopies. Crucially, the level of counter cations, which are tightly-controlled within cellular compartments, also controlled these interactions. The oligonucleotide component dominated interaction between conjugates and complementary targets in the presence of physiological levels of counter cations (K⁺), sufficient to prevent repulsion between the complementary nucleic acid strands to allow Watson-Crick hydrogen bonding. In contrast, the positively-charged catalytic peptide interacted poorly with target RNA, when counter cations similarly screened the negatively-charged sugar-phosphate RNA backbones. The peptide only became the key player, when counter cations were insufficient for charge screening; moreover, only under such non-physiological conditions did conjugates form strong complexes with non-complementary RNAs.Communicated by Ramaswamy H. Sarma.

Lack of specificity associated with using molecular beacons in loop mediated amplification assays

Background

Loop mediated isothermal amplification of nucleic acid templates is a rapid, sensitive and specific method suitable for molecular diagnostics. However the complexity of primer design and the number of primers involved can lead to false positives from non-specific primer interactions. Standard methods of LAMP detection utilise the increasing concentrations of DNA or inorganic pyrophosphate and therefore lack specificity for identifying the desired LAMP amplification. Molecular beacons used in PCR reactions are target specific and may enhance specificity with LAMP.

Results

We present a potential molecular beacon approach to LAMP detection targeting the single stranded region between loops, and test this for LAMP molecular beacons targeting the 35S promoter and NOS terminator sequences commonly used in GM crops. From these studies we show that molecular beacons used in LAMP, despite providing a change in fluorescent intensity with amplification, appear not to anneal to specific target sequences and therefore target specificity is not a benefit of this method. However, molecular beacons demonstrate a change in fluorescence which is indicative of LAMP amplification products. We identify the LAMP loop structure as likely to be responsible for this change in signal.

Conclusions

Molecular beacons can be used to detect LAMP amplification but do not provide sequence specificity. The method can be used to determine effectively LAMP amplification from other primer-driven events, but does not discriminate between different LAMP amplicons. It is therefore unsuitable for multiplex LAMP reactions due to non-specific detection of LAMP amplification.

Electronic supplementary material

The online version of this article (10.1186/s12896-019-0549-z) contains supplementary material, which is available to authorized users.

Fluorophore-Quencher Interactions Effect on Hybridization Characteristics of Complementary Oligonucleotides

Nucleic acids are often covalently modified with fluorescent reporter molecules to create a hybridization state-dependent optical signal. Designing such a nucleic acid reporter involves selecting a fluorophore, quencher, and fluorescence quenching design. This report outlines the effect that these choices have on the DNA hybridization characteristics by examining six fluorophores in four quenching schemes: a quencher molecule offset from the fluorophore by 0, 5, or 10 bases, and nucleotide quenching. The similar binding characteristics of left-handed L-DNA were evaluated in comparison with right-handed DNA to quantify the effect of each quenching scheme. These results were applied to the Adaptive PCR method, which monitors fluorescently-labeled L-DNA as a sentinel for analogous unlabeled D-DNA in the reaction. All of the tested fluorophores and quenching schemes increased the annealing temperature of the oligonucleotide pairs by values ranging from 0.5 to 8.5 °C relative to unlabeled oligonucleotides. The design with the smallest increase (0.5 °C) was a sense strand with a FAM fluorophore and an anti-sense strand with Black Hole Quencher 2 offset by 10 bases from the FAM. An identical design that did not offset the quencher molecules resulted in a shift in annealing temperature of 5 °C. PCR was performed using temperature switching based on each of these L-DNA designs, and efficiency was significantly increased for the 10-base offset design, which had the smallest shift in annealing temperature. These results highlight the importance of selecting an appropriate fluorescence quenching scheme for nucleic acid optical signals.

Split Dapoxyl Aptamer for Sequence-Selective Analysis of Nucleic Acid Sequence Based Amplification Amplicons

Hybridization probes have been used for the detection of single nucleotide variations (SNV) in DNA and RNA sequences in the mix-and-read formats. Among the most conventional are Taqman probes, which require expensive quantitative polymerase chain reaction (qPCR) instruments with melting capabilities. More affordable isothermal amplification format requires hybridization probes that can selectively detect SNVs isothermally. Here we designed a split DNA aptamer (SDA) hybridization probe based on a recently reported DNA sequence that binds a dapoxyl dye and increases its fluorescence ( Kato, T.; Shimada, I.; Kimura, R.; Hyuga, M., Light-up fluorophore-DNA aptamer pair for label-free turn-on aptamer sensors. Chem. Commun. 2016 , 52 , 4041 - 4044 ). SDA uses two DNA strands that have low affinity to the dapoxyl dye unless hybridized to abutting positions at a specific analyte and form a dye-binding site, which is accompanied by up to a 120-fold increase in fluorescence. SDA differentiates SNV in the  inhA gene of Mycobacterium tuberculosis at ambient temperatures and detects a conserved region of the Zika virus after isothermal nucleic acid sequence based amplification (NASBA) reaction. The approach reported here can be used for detection of isothermal amplification products in the mix-and-read format as an alternative to qPCR.

Speciation of fungi using real time PCR with molecular beacons: Can we solve the enigma of diagnosis of invasive fungal disease?

BACKGROUND

Invasive fungal diseases (IFDs) are difficult to diagnose and associated with high mortality rates, especially in the immunosuppressed. Species of Aspergillus and Candida are the cause of majority of invasive fungal disease however IFDs are also caused by Fusarium, Zygomycetes, Trichosporon, etc. Early detection is crucial for appropriate antifungal therapy. Blood cultures usually fail to isolate filamentous fungi, while detection of circulating beta-d-glucan or galactomannan antigens show variable sensitivity and specificity. There is a need of reliable, sensitive and specific diagnostic tests for IFDs.

METHODS

A real-time Polymerase Chain Reaction (PCR) assay with a universal primer/molecular beacon system was developed for detecting and speciating most of the pathogenic fungi implicated in IFD. A single-reaction assay was designed targeting a carefully selected region of the ITS2 and ITS5 subunits of the fungal rDNA gene along with four molecular beacons capable of differential hybridization to the amplicons of different species. This generated a signature set of melting temperatures using the standard strains. The assay was tested on clinical specimens from patients with suspected invasive fungal disease.

RESULTS

The assay was tested on 72 clinical samples and 72 healthy controls. Of these, 22 clinical samples (6/8 proven; 13/29 probable; 3/35 possible IFD, classified by the EORTC/MSG criteria) were positive by PCR and generated a set of melting temperatures enabling identification of the causative fungus. The assay was negative in all healthy controls.

CONCLUSION

The molecular beacon assay is a promising tool providing a rapid method for detection and monitoring of invasive fungal disease in immunosuppressed patients.

The owl sensor: a 'fragile' DNA nanostructure for the analysis of single nucleotide variations

Analysis of single nucleotide variations (SNVs) in DNA and RNA sequences is instrumental in healthcare for the detection of genetic and infectious diseases and drug-resistant pathogens. Here we took advantage of the developments in DNA nanotechnology to design a hybridization sensor, named the 'owl sensor', which produces a fluorescence signal only when it complexes with fully complementary DNA or RNA analytes. The novelty of the owl sensor operation is that the selectivity of analyte recognition is, at least in part, determined by the structural rigidity and stability of the entire DNA nanostructure rather than exclusively by the stability of the analyte-probe duplex, as is the case for conventional hybridization probes. Using two DNA and two RNA analytes we demonstrated that owl sensors differentiate SNVs in a wide temperature range of 5 °C-32 °C, a performance unachievable by conventional hybridization probes including the molecular beacon probe. The owl sensor reliably detects cognate analytes even in the presence of 100 times excess of single base mismatched sequences. The approach, therefore, promises to add to the toolbox for the diagnosis of SNVs at ambient temperatures.

Optical and Structural Characterization of a Chronic Myeloid Leukemia DNA Biosensor

Selective base pairing is the foundation of DNA recognition. Here, we elucidate the molecular and structural details of a FRET-based two-component molecular beacon relying on steady-state fluorescence spectroscopy, small-angle X-ray scattering (SAXS), microscale thermophoresis (MST), and differential electrophoretic mobility. This molecular beacon was designed to detect the most common fusion sequences causing chronic myeloid leukemia, e14a2 and e13a2. The emission spectra indicate that the self-assembly of the different components of the biosensor occurs sequentially, triggered by the fully complementary target. We further assessed the structural alterations leading to the specific fluorescence FRET signature by SAXS, MST, and the differential electrophoretic mobility, where the size range observed is consistent with hybridization and formation of a 1:1:1 complex for the probe in the presence of the complementary target and revelator. These results highlight the importance of different techniques to explore conformational DNA changes in solution and its potential to design and characterize molecular biosensors for genetic disease diagnosis.

Fluorogenic bidirectional displacement probe-based real-time isothermal DNA amplification and specific visual detection of products

A unique fluorogenic nucleic acid probe has been devised, called the fluorogenic bidirectional displacement probe, which can serve as both the primer and the signal indicator of amplification products for the development of the real-time isothermal DNA amplification and its visual detection of products with high sensitivity and specificity.

Detection of cofilin mRNA by hybridization-sensitive double-stranded fluorescent probes

We have developed hybridization-sensitive fluorescent oligonucleotide probes that, in the presence of quencher strands, undergo efficient fluorescence quenching through the formation of partial DNA/DNA duplexes.

Triplex DNA Nanostructures: From Basic Properties to Applications

Triplex nucleic acids have recently attracted interest as part of the rich "toolbox" of structures used to develop DNA-based nanostructures and materials. This Review addresses the use of DNA triplexes to assemble sensing platforms and molecular switches. Furthermore, the pH-induced, switchable assembly and dissociation of triplex-DNA-bridged nanostructures are presented. Specifically, the aggregation/deaggregation of nanoparticles, the reversible oligomerization of origami tiles and DNA circles, and the use of triplex DNA structures as functional units for the assembly of pH-responsive systems and materials are described. Examples include semiconductor-loaded DNA-stabilized microcapsules, DNA-functionalized dye-loaded metal-organic frameworks (MOFs), and the pH-induced release of the loads. Furthermore, the design of stimuli-responsive DNA-based hydrogels undergoing reversible pH-induced hydrogel-to-solution transitions using triplex nucleic acids is introduced, and the use of triplex DNA to assemble shape-memory hydrogels is discussed. An outlook for possible future applications of triplex nucleic acids is also provided.

Aptasensor based optical detection of glycated albumin for diabetes mellitus diagnosis

Glycated albumin (GA) has been reported as an important biomarker for diabetes mellitus. This study investigates an optical sensor comprised of deoxyribonucleic acid (DNA) aptamer, semiconductor quantum dot and gold (Au) nanoparticle for the detection of GA. The system functions as a 'turn on' sensor because an increase in photoluminescence intensity is observed upon the addition of GA to the sensor. This is possibly because of the structure of the DNA aptamer, which folds to form a large hairpin loop before the addition of the analyte and is assumed to open up after the addition of target to the sensor in order to bind to GA. This pushes the quantum dot and the Au nanoparticle away causing an increase in photoluminescence. A linear increase in photoluminescence intensity and quenching efficiency of the sensor is observed as the GA concentration is varied between 0-14 500 nM. Time based photoluminescence studies with the sensor show the decrease in binding rate of the aptamer to the target within a specific time period. The sensor was found to have a higher selectivity towards GA than other control proteins. Further investigation of this simple sensor with greater number of clinical samples can open up avenues for an efficient diagnosis and monitoring of diabetes mellitus when used in conjunction with the traditional method of glucose level monitoring.

Fluorinated molecular beacons as functional DNA nanomolecules for cellular imaging

Molecular beacons (MBs) are simple, but practical, fluorescent nanoprobes widely used to detect small molecules, nucleic acids and proteins. However, some challenges still remain when MBs are employed in complex biological environments, such as instability and non-target interference. To meet such challenges, we have designed and synthesized fluorinated molecular beacons (FMBs) as functional DNA nanomolecules for cellular imaging, in which the stem sequence is simply composed of artificial nucleotides with 3,5-bis(trifluoromethyl)benzene (F) as the surrogate base of natural A, T, C and G bases. The introduction of F base into MBs significantly increases their hydrophobicity, and the stem is formed by the assembly of self-complementary base F nucleotides through hydrophobic interactions. Fluorescence studies revealed that FMBs confer improved stability over conventional MBs. To demonstrate the application of FMBs for cellular imaging, we constructed an FMB to detect mRNA in MCF-7 cells, and the FMB was proven to be a practical nanoprobe for cellular imaging of mRNA.

DNA nanotechnology for nucleic acid analysis: multifunctional molecular DNA machine for RNA detection

The Nobel prize in chemistry in 2016 was awarded for 'the design and synthesis of molecular machines'. Here we designed and assembled a molecular machine for the detection of specific RNA molecules. An association of several DNA strands, named multifunctional DNA machine for RNA analysis (MDMR1), was designed to (i) unwind RNA with the help of RNA-binding arms, (ii) selectively recognize a targeted RNA fragment, (iii) attract a signal-producing substrate and (iv) amplify the fluorescent signal by catalysis. MDMR1 enabled detection of 16S rRNA at concentrations ∼24 times lower than that by a traditional deoxyribozyme probe.

Peptide nucleic acid probe-based fluorescence melting curve analysis for rapid screening of common JAK2, MPL, and CALR mutations

BACKGROUND

We developed and evaluated the feasibility of peptide nucleic acid (PNA)-based fluorescence melting curve analysis (FMCA) to detect common mutations in myeloproliferative neoplasms (MPNs).

METHODS

We have set up two separate reactions of PNA-based FMCA: JAK2 V617F &CALR p.Leu367fs*46 (set A) and MPL W515L/K &CALR p.Lys385fs*47 (set B). Clinical usefulness was validated with allele-specific real-time PCR, fragment analysis, Sanger sequencing in 57 BCR-ABL1-negative MPNs.

RESULTS

The limit of detection (LOD) of PNA-based FMCA was approximately 10% for each mutation and interference reactions using mixtures of different mutations were not observed. Non-specific amplification was not observed in normal control. PNA-based FMCA was able to detect all JAK2 V617F (n=20), CALR p.Leu367fs*46 (n=10) and p.Lys385fs*47 (n=8). Three of six MPL mutations were detected except three samples with low mutant concentration in out of LOD. JAK2 exon 12 mutations (n=7) were negative without influencing V617F results. Among six variant CALR exon 9 mutations, two were detected by this method owing to invading of probe binding site.

CONCLUSIONS

PNA-based FMCA for detecting common JAK2, MPL, and CALR mutations is a rapid, simple, and sensitive technique in BCR-ABL1-negative MPNs with >10% mutant allele at the time of initial diagnosis.

Fluorescence based fiber optic and planar waveguide biosensors. A review

The application of optical biosensors, specifically those that use optical fibers and planar waveguides, has escalated throughout the years in many fields, including environmental analysis, food safety and clinical diagnosis. Fluorescence is, without doubt, the most popular transducer signal used in these devices because of its higher selectivity and sensitivity, but most of all due to its wide versatility. This paper focuses on the working principles and configurations of fluorescence-based fiber optic and planar waveguide biosensors and will review biological recognition elements, sensing schemes, as well as some major and recent applications, published in the last ten years. The main goal is to provide the reader a general overview of a field that requires the joint collaboration of researchers of many different areas, including chemistry, physics, biology, engineering, and material science.

Nonequilibrium Hybridization Enables Discrimination of a Point Mutation within 5-40 °C

Detection of point mutations and single nucleotide polymorphisms in DNA and RNA has a growing importance in biology, biotechnology, and medicine. For the application at hand, hybridization assays are often used. Traditionally, they differentiate point mutations only at elevated temperatures (>40 °C) and in narrow intervals (ΔT = 1-10 °C). The current study demonstrates that a specially designed multistranded DNA probe can differentiate point mutations in the range of 5-40 °C. This unprecedentedly broad ambient-temperature range is enabled by a controlled combination of (i) nonequilibrium hybridization conditions and (ii) a mismatch-induced increase of equilibration time in respect to that of a fully matched complex, which we dub "kinetic inversion".

Split Spinach Aptamer for Highly Selective Recognition of DNA and RNA at Ambient Temperatures

Split spinach aptamer (SSA) probes for fluorescent analysis of nucleic acids were designed and tested. In SSA design, two RNA or RNA/DNA strands hybridized to a specific nucleic acid analyte and formed a binding site for low-fluorescent 3,5-difluoro-4-hydroxybenzylidene imidazolinone (DFHBI) dye, which resulted in up to a 270-fold increase in fluorescence. The major advantage of the SSA over state-of-the art fluorescent probes is high selectivity: it produces only background fluorescence in the presence of a single-base-mismatched analyte, even at room temperature. SSA is therefore a promising tool for label-free analysis of nucleic acids at ambient temperatures.

DNA Strand Replacement Mechanism in Molecular Beacons Encoded for the Detection of Cancer Biomarkers

Signaling properties of a fluorescent hairpin oligonucleotide molecular beacon (MB) encoded to recognize protein survivin (Sur) mRNA have been investigated. The process of complementary target binding to SurMB with 20-mer loop sequence is spontaneous, as expected, and characterized by a high affinity constant (K = 2.51 × 10(16) M(-1)). However, the slow kinetics at room temperature makes it highly irreversible. To understand the intricacies of target binding to MB, a detailed kinetic study has been performed to determine the rate constants and activation energy Ea for the reaction at physiological temperature (37 °C). Special attention has been paid to assess the value of Ea in view of reports of negative activation enthalpy for some nucleic acid reactions that would make the target binding even slower at increasing temperatures in a non-Arrhenius process. The target-binding rate constant determined is k = 3.99 × 10(3) M(-1) s(-1) at 37 °C with Ea = 28.7 ± 2.3 kcal/mol (120.2 ± 9.6 kJ/mol) for the temperature range of 23 to 55 °C. The positive high value of Ea is consistent with a kinetically controlled classical Arrhenius process. We hypothesize that the likely contribution to the activation energy barrier comes from the SurMB stem melting (tm = 53.7 ± 0.2 °C), which is a necessary step in the completion of target strand hybridization with the SurMB loop. A low limit of detection (LOD = 2 nM) for target tDNA has been achieved. Small effects of conformational polymorphs of SurMB have been observed on melting curves. Although these polymorphs could potentially cause a negative Ea, their effect on kinetic transients for target binding is negligible. No toehold preceding steps in the mechanism of target binding were identified.

Nucleic Acid Nanostructures for Chemical and Biological Sensing

The nanoscale features of DNA have made it a useful molecule for bottom-up construction of nanomaterials, for example, two- and three-dimensional lattices, nanomachines, and nanodevices. One of the emerging applications of such DNA-based nanostructures is in chemical and biological sensing, where they have proven to be cost-effective, sensitive and have shown promise as point-of-care diagnostic tools. DNA is an ideal molecule for sensing not only because of its specificity but also because it is robust and can function under a broad range of biologically relevant temperatures and conditions. DNA nanostructure-based sensors provide biocompatibility and highly specific detection based on the molecular recognition properties of DNA. They can be used for the detection of single nucleotide polymorphism and to sense pH both in solution and in cells. They have also been used to detect clinically relevant tumor biomarkers. In this review, recent advances in DNA-based biosensors for pH, nucleic acids, tumor biomarkers and cancer cell detection are introduced. Some challenges that lie ahead for such biosensors to effectively compete with established technologies are also discussed.

Electronic control of DNA-based nanoswitches and nanodevices

Here we demonstrate that we can rationally and finely control the functionality of different DNA-based nanodevices and nanoswitches using electronic inputs. To demonstrate the versatility of our approach we have used here three different model DNA-based nanoswitches triggered by heavy metals and specific DNA sequences and a copper-responsive DNAzyme. To achieve electronic-induced control of these DNA-based nanodevices we have applied different voltage potentials at the surface of an electrode chip. The applied potential promotes an electron-transfer reaction that releases from the electrode surface a molecular input that ultimately triggers the DNA-based nanodevice. The use of electronic inputs as a way to finely activate DNA-based nanodevices appears particularly promising to expand the available toolbox in the field of DNA nanotechnology and to achieve a better hierarchical control of these platforms.

A Modular, DNA-Based Beacon for Single-Step Fluorescence Detection of Antibodies and Other Proteins

A versatile platform for the one-step fluorescence detection of both monovalent and multivalent proteins has been developed. This system is based on a conformation-switching stem-loop DNA scaffold that presents a small-molecule, polypeptide, or nucleic-acid recognition element on each of its two stem strands. The steric strain associated with the binding of one (multivalent) or two (monovalent) target molecules to these elements opens the stem, enhancing the emission of an attached fluorophore/quencher pair. The sensors respond rapidly (<10 min) and selectively, enabling the facile detection of specific proteins even in complex samples, such as blood serum. The versatility of the platform was demonstrated by detecting five bivalent proteins (four antibodies and the chemokine platelet-derived growth factor) and two monovalent proteins (a Fab fragment and the transcription factor TBP) with low nanomolar detection limits and no detectable cross-reactivity.

Fluorescence-based bioassays for the detection and evaluation of food materials

We summarize here the recent progress in fluorescence-based bioassays for the detection and evaluation of food materials by focusing on fluorescent dyes used in bioassays and applications of these assays for food safety, quality and efficacy. Fluorescent dyes have been used in various bioassays, such as biosensing, cell assay, energy transfer-based assay, probing, protein/immunological assay and microarray/biochip assay. Among the arrays used in microarray/biochip assay, fluorescence-based microarrays/biochips, such as antibody/protein microarrays, bead/suspension arrays, capillary/sensor arrays, DNA microarrays/polymerase chain reaction (PCR)-based arrays, glycan/lectin arrays, immunoassay/enzyme-linked immunosorbent assay (ELISA)-based arrays, microfluidic chips and tissue arrays, have been developed and used for the assessment of allergy/poisoning/toxicity, contamination and efficacy/mechanism, and quality control/safety. DNA microarray assays have been used widely for food safety and quality as well as searches for active components. DNA microarray-based gene expression profiling may be useful for such purposes due to its advantages in the evaluation of pathway-based intracellular signaling in response to food materials.

Single excitation three color folded DNA probe for SNP typing

A single fluorophore based multi-color emission probing system has been developed in the current work. dA(py) was used as the fluorophore, which was incorporated at the end of folded oligonucleotide with increasing number. Depending on the number of dA(py) three different color emissions were observed, viz. blue, green and red. These interesting color patterns were further used for the SNP typing of DNA. Every folded oligonucleotide with different number of dA(py)s could discriminate the full matched target sequence from one base mismatched and two base mismatched sequences. Furthermore, this probing system has a simple structure and also exhibited large Stokes shifted signal for discriminating the target DNA.