Biosensing using hairpin DNA probes
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Jiahao Huang
Jiahao Huang received his MS in biomedical engineering (2010) and BS in biotechnology (2007) from Hunan University, P.R. China. Currently, he is a PhD candidate in the Department of Mechanical and Aerospace Engineering at The Hong Kong University of Science and Technology. His research interests are in the field of DNA-based optical biosensors.
Jueqi Wu received her BS in environmental engineering (2012) from Sun Yat-sen University, P.R. China. She is a MPhil. student in the Environmental Engineering Program at The Hong Kong University of Science and Technology. Her research areas include the applications of metal particles and biosensors in environmental science.
Zhigang Li received his PhD in mechanical engineering (2005) from the University of Delaware, USA, and his MEng in thermal engineering (1999) from Tsinghua University, P.R. China. He is now an Associate Professor in the Department of Mechanical and Aerospace Engineering at The Hong Kong University of Science and Technology. His research interests mainly focus on micro/nano-fluidics, microscale/nanoscale transport phenomena, and biodetection.
Abstract
Hairpin DNA probes (HDPs) are specially designed single-stranded DNA and have excellent sensing specificity. The past decade has witnessed the fast development of HDP-based biosensors due to the tremendous applications in biology, medicine, environmental science, and engineering. Their detectable targets include nucleic acids, proteins, small molecules, and metal ions. In this review, we summarize the recent progress in HDP-based biosensors by categorizing them into molecular beacon (MB)-based sensing in homogeneous systems and other HDP-based solid-state sensors. The basic design of MBs with diverse signaling pairs is introduced first. Then, various detectable targets and the detection principles of all HDP-based biosensors are extensively discussed. Furthermore, the methods for amplifying the response signal and improving the detection performance are covered. Finally, the limitations and possible solutions about the sensors are discussed.
Introduction
Nucleic acid-based biosensors have received great attention in the past decade (Willner et al. 2008). Nucleic acids are promising molecular probes due to the ease of chemical synthesis and functional modification, the specificity for base pairing, and the predictability for intermolecular or intramolecular interactions (Wang et al. 2009b). Hairpin DNA probes (HDPs) are perhaps one of the most versatile nucleic acid probes that have been widely employed for biosensor construction in aqueous/homogeneous systems (Ma et al. 2013b) and solid-state-based platforms (Lubin and Plaxco 2010). An HDP is a single-stranded DNA assuming a stem-loop structure due to the complementary sequences at its ends (Figure 1A). For homogeneous sensing systems, HDPs are usually labeled with two photoluminescent species (one donor and one acceptor) at their two ends for signal generation under structural changes. Such labeled HDPs are called molecular beacons (MBs; Figure 1B). MBs were first reported by Tyagi and Kramer (1996). The stem part of an MB brings the donor dye (commonly referred to as fluorophore, F) and the acceptor dye (fluorescence quencher, Q) in close proximity, which, in the absence of the target, ensures a low background signal by efficiently quenching the fluorescence emission of the donor. The loop region is specially designed to be complementary to a target sequence. In the presence of targets, the targets hybridize with the loop of the MB and open the hairpin structure of the MB, leading to the separation of the fluorophore (donor) and quencher (acceptor) and consequently the restoration of the fluorescence signal (Figure 1C). Generally, an MB probe can be regarded as an elegant biosensor that switches between two different signaling conformations (closed and open states) in an analyte-dependent manner. The optical signal generated upon the addition of an analyte is proportional to the amount of the analytes, which forms the basis of MB-based detection methods.
Hairpin probes and basic detection mechanism.
(A) An HDP, (B) an MB, and (C) structural change of an MB in binding with a target DNA. A, acceptor dye; D, donor dye; F, fluorophore; Q, quencher.
Other than the MB-based homogeneous sensing methods, some other HDPs can also perform sensing functions after being immobilized on solid surfaces (Lubin and Plaxco 2010). In solid-state HDP-based sensors, HDPs are usually labeled with optically or electrochemically sensitive signaling tags and tethered to a solid substrate. In the presence of targets, they undergo conformational changes when binding with the targets and translate the recognition events into detectable signals. The solid-state HDP platforms have been widely used for the determination of DNA (Du et al. 2003, Fan et al. 2003), RNA (Pang et al. 2014), thrombin (Radi et al. 2006, Wang et al. 2009c), ATP (Wu et al. 2013a), cocaine (Baker et al. 2006), and Hg2+ (Gao et al. 2013c, Xiong et al. 2015).
Recently, HDPs have been widely used in a variety of areas, including life science, medical diagnosis, pathogen identification, and food safety monitoring. This review covers the advances in the design of HDPs, especially MBs with diverse signaling pairs, and summarizes the detectable targets and the generic sensing principles of HDP-based biosensors, with a focus on the homogenous and solid-state sensing schemes. Typical HDP-based amplification methods, including enzyme-free and enzyme-assisted approaches, are also extensively discussed.
MB-based sensing in homogeneous systems
Signaling labels of MBs
The stems of MBs can improve the selectivity of MB-based biosensors because they set a conformational constraint, which preferentially favors the interaction between MBs and perfectly matched targets. The sensitivity, however, strongly depends on the signaling pair (fluorophore and quencher) labeled on the MBs and may be deteriorated due to the incomplete signal quenching in the closed state and/or the inadequate signal generation from the open state of MBs. To improve the sensitivity, different labels for MBs have been reported to enhance the signal-to-background ratio (SBR; the ratio of the signals with and without the presence of the targets).
Figure 2 lists MBs with diverse signaling labels, varying from inorganic materials (Joshi and Tor 2001) to organic compounds (Wabuyele et al. 2003, Kim et al. 2007a), including nanomaterials (Dubertret et al. 2001, Ebrahimi et al. 2014), metal complexes (Wilson and Johansson 2003), conjugated polymers (Yang et al. 2005c, Lee et al. 2007), superquenchers (SQs; Yang et al. 2005b, Lee et al. 2009, Lovell et al. 2010), and other materials with outstanding photophysical properties.
Various MBs.
(A) Regular MBs: (a) an MB modified with common signaling pairs (one fluorophore and one quencher), (b) an MB with two different fluorophores attached, (c) an MB labeled with two identical units, (d) an MB with guanosine-rich sequences acting as the quencher, (e) an MB with an abasic site trapping a fluorescent dye for signal generation, and (f) an MB with a fluorescent dye tethered in the loop region. (B) Nanomaterial-labeled MBs: (a) an MB with an AuNP acting as the quencher; (b) an MB with an AgNP acting as the quencher; (c) an MB with a QD and a fluorophore acting as the energy donor and acceptor; (d) an MB with a QD and a quencher acting as the energy donor and acceptor; and (e) an MB with a QD and an AuNP acting as the energy donor and acceptor. (C) Multiple-labeled MBs: (a) an MB with an SQ; (b) an MB including a multiple-pyrene moiety; (c) an MB with a dual-fluorophore unit (F1 and F2) and a quencher acting as the energy donor and acceptor; and (d) an MB with a conjugated polymer. (D) Metal complex-based MBs: (a) an MB with transition metal complexes, (b) an MB with copper complexes, and (c) an MB with thymine-Hg2+-thymine complexes. CC, copper complex; E, electrochemical tag; F, fluorophore; FC, fluorophore chain; MF, multiple fluorophore; Ml, mercury ion; Q, quencher; TMC, transition metal complex.
Common signaling pairs
Most MBs have one fluorophore and one quencher at their ends. The first MB (Tyagi and Kramer 1996) consisted of a 5-bp stem and an 18-base loop portion with 5-(2′-aminoethyl) aminonaphthalene-1-sulfonic acid as the fluorophore located at its 5′-end and 4-(4′-dimethylaminophenylazo) benzoic acid (DABCYL) as the quencher at the 3′-end (Figure 2A,a). Other fluorophores commonly used in MB designs include carboxyfluorescein (Kolpashchikov 2006, Sun and Fan 2012), Cy3 (Yu et al. 2014), Cy5 (Watanabe et al. 2014), and Texas red (Tyagi et al. 1998). Besides DABCYL (Huang et al. 2012a, 2013), Black Hole Quencher 1 and 2 (BHQ1 and BHQ2; Kang et al. 2011) are popular quenchers.
MBs labeled with two different fluorophores
MBs labeled with two fluorophores (Figure 2A,b) were constructed to monitor intermolecular interactions among structured nucleic acids, including hairpin structures (Ota et al. 1998). For such dual-fluorophore-labeled MB probes, the fluorescence resonance energy transfer (FRET) between the two fluorophores could be greatly reduced in the occurrence of MB-target hybridization. In addition to in vitro sensing, the measurement of the change in FRET made it possible for in vivo target detection, which could not be performed using the traditional MBs due to the employment of nonfluorescent quenchers.
Dual-fluorophore-labeled MBs have advantages over the conventional ones (Zhang et al. 2001). In the presence of DNA targets, the fluorescence intensity of one fluorophore, say, F1 (coumarin; Figure 2A,b), increased, whereas that of the other fluorophore, F2 (carboxyfluorescein), decreased. The signal ratio of the two fluorophores could greatly improve the detection sensitivity. Moreover, this ratiometric probe exhibited self-referencing feature, which led to exceptional properties, such as excellent quantitative linearity and low interference from photobleaching and other environmental factors that would affect optical measurements.
MBs labeled with two identical units
MBs could also be labeled with two identical entities (Figure 2A,c). A new aptamer probe with two pyrene molecules labeled at its ends was developed for protein detection (Yang et al. 2005a). In the presence of target protein (platelet-derived growth factor), the structure of this aptamer probe was disrupted and adopted a closed hairpin conformation, bringing the two pyrene molecules close to each other. Then, its emission wavelength was shifted from its monomer (≈400 nm for pyrene monomer) to excimer (~485 nm for pyrene excimer) emission wavelength. One fascinating characteristic of the pyrene excimer is that its fluorescence lifetime (100 ns) is much longer than that of most of the biological background species (<10 ns). This new aptamer probe made it possible to directly detect targets in complex biological samples without any separation or purification processes.
Nesterova et al. (2009) labeled MBs with two identical, water-soluble phthalocyanine molecules (Figure 2A,c), a class of near-infrared fluorophores (emission wavelengths are >700 nm) that could avoid significant autofluorescence (emission wavelengths are in the visible spectrum: 400–700 nm). Without the targets, the two phthalocyanine molecules formed a nonfluorescent dimer. In the presence of DNA targets, however, the phthalocyanine dimers were separated due to the conformational changes of the MBs, leading to fluorescence restoration.
Based on the same design concept (Figure 2A,c), another interesting electrochemical MB probe modified with two identical units (carminic acid) was reported for the sensitive determination of HIV-1 gag gene (Li et al. 2014a). Unlike MB-based optical detection methods, this new probe represented one of the rare cases that the MB probe could be smartly designed as an electrochemical carrier to perform sensing function in homogeneous solutions. Furthermore, this immobilization-free electrochemical method differed from other electrochemical DNA sensors, where the immobilization operations were indispensable, as discussed later.
Single-labeled MBs
Single-labeled MBs were also developed (Stohr et al. 2005), where the signal from the fluorophore was quenched by the guanosine residues in the stem portion instead of a quencher (Figure 2A,d). Compared with the dual-labeled MBs, single-labeled MBs were cheaper and could be easily used in sensing systems involving solid surface supports.
Figure 2A,e shows an example, where the stem sequence contains an abasic site (Sato et al. 2011). Some fluorescent ligands could specifically bind to the abasic site and cause fluorescence quenching. Upon hybridization with the target DNA, the MB was opened and the ligand would be released from the abasic site, leading to fluorescence generation.
Another single-labeled MB was synthesized by attaching fluorene to the loop part (Figure 2A,f; Hwang et al. 2004). Fluorene was chosen as the fluorophore due to its better quantum yields and less bulky structure than other commonly used fluorophores. Compared to the background signal observed from these fluorene-labeled MBs, the fluorescence intensity of single-base-mismatched duplexes was weaker, whereas the fluorescence signal of the fully matched duplexes was much stronger. This made it feasible to well distinguish the presence of one-base-mismatched sequences.
Nanomaterial-based signaling moieties
Dubertret et al. (2001) reported the first example to apply gold nanoparticles (AuNPs) as quenchers for MBs (Figure 2B,a), and the quenching efficiency was found to reach up to 99.45%, which was approximately 2 orders of magnitude higher than the first MB in 1996 (Tyagi and Kramer 1996). With such high quenching efficiency, single nucleotide discrimination could be easily achieved.
AuNP-based MBs (Figure 2B,a) were used for real-time monitoring of RNA synthesis (Rosa et al. 2012). The employment of AuNPs could protect MBs from being digested by nucleases without sacrificing the quenching and sensing capabilities of MBs. Therefore, AuNP-based MBs did not suffer from degradation issues, which might be faced by the conventional MBs in certain applications. They could be applied for real-time assessment of RNA transcription and inhibition.
In addition to AuNPs, silver nanoparticles (AgNPs) were also employed as nanoquenchers (Figure 2B,b; Peng et al. 2009). AgNPs could quench nonspecific fluorescence and enhance signal generation when the fluorophore was separated from them. It was demonstrated that AgNPs could enhance the signal by 10-fold compared with AuNPs for the same amount of targets, and a detection limit of 100 pM could be easily achieved using AgNP-based MBs. However, the detection performance strongly depended on the particle size of AgNPs.
Quantum dots (QDs) are another nanomaterial superior to traditional organic dyes due to their exceptional optical properties, such as broad absorption spectra, narrow emission spectra, high quantum yield, and excellent photostability. Attempts were made to explore the possibility to use QD-based MBs for biosensing. Liu et al. (2011) developed a two-photon excited MB with QDs as the energy donor and carboxy-X-rhodamine (ROX) as the energy acceptor (Figure 2B,c). It could reduce the direct excitation of the acceptor to eliminate possible errors and false-positive results. Moreover, it was free of interferences from autofluorescence or scattering light, which could find potential applications in complicated biological matrices.
Figure 2B,d shows another QD-based MB, where tiny yet robust silica-coated QDs were used as fluorophores (Wu et al. 2011). Such QDs were less than 10 nm in diameter and allowed efficient fluorescence energy transfer. QD-based MB sensors were able to selectively detect 0.1 nM DNA very quickly. They were also sufficiently stable in a wide range of pH values (from 1 to 14) and even in environments of high salt concentration (up to 2 M).
The combination of QDs and AuNPs for MBs is worth mentioning. As shown in Figure 2B,e, MBs were labeled with CdSe-ZnS core-shell QDs and AuNPs, which acted as the donor and the acceptor, respectively. These QD/AuNP-based MBs were designed for visualizing virus replications in living cells (Yeh et al. 2010). Tat peptide was modified onto the QD surface to provide approximately 100% noninvasive delivery of the QD/AuNP-based MBs within 2 h. Such MBs addressed the major problems in applying the conventional MBs for in vivo viral detection, as the modest half-life of the conventional MBs was approximately 50 min due to cytoplasmic nuclease degradation and the lack of noninvasive intracellular delivery (Bratu et al. 2003).
It should be noted that graphene, which exhibits excellent optical, electrochemical, and electronic properties (Bonanni and Pumera 2011, Tang et al. 2011a, Chen et al. 2012), is emerging as an appealing nanomaterial for biosensor constructions. Due to its distinct adsorption properties for single- and double-stranded DNA and its remarkable fluorescence quenching ability (Wang et al. 2010a, 2013b, 2014g, Tang et al. 2012a), it has been widely used in MB-based sensing systems (Lu et al. 2010a,b, Wang et al. 2011b).
Multiple-labeled MBs
Yang et al. (2005b) reported a novel molecular assembly of DABCYL molecules to form SQs with quenching efficiency as high as 99.7% (Figure 2C,a), which led to a 320-fold enhancement of fluorescent signal, far better than the 14-fold enhancement of the regular MBs with the same monomer quencher.
Another multiple-labeled MB probe was engineered for real-time detection of DNA sequences (Conlon et al. 2008), which contained one to four pyrene monomers on the 5′-end and DABCYL quencher on its 3′-end (Figure 2C,b). Each additional pyrene monomer resulted in a significant increase in the excimer emission intensity, quantum yield, and lifetime of the target-bound MBs. Benefited from its relatively long fluorescence lifetime (~40 ns) and large Stokes shift (130 nm), this new MB probe could be used under high autofluorescence (e.g., cell growth media) without special sample pretreatment or washing steps.
Figure 2C,c shows a wavelength-shifting MB probe developed with two fluorophores as an FRET pair (F1 and F2) at one end and a quencher (Q) at the other end (Tyagi et al. 2000). One of the fluorophores was called the harvester fluorophore (F1) that absorbed light, and the other was named emitter fluorophore (F2) that emitted light after gaining energy from the harvester fluorophore (F1). Without the DNA targets, the MB probe was dark, because the energy absorbed by the harvester fluorophore (F1) was quickly transferred to the quencher (Q) and produced no fluorescence emission. In the presence of the DNA targets, however, they induced conformational reorganization in the MBs and the energy absorbed by the harvester fluorophore (F1) could be efficiently transferred to the emitter fluorophore (F2), and then the fluorescence emission of the emitter fluorophore (F2) was generated. This could substantially improve the fluorescence response of MBs if they contained a fluorophore that could not efficiently absorb energy from light sources. This wavelength-shifting MB probe offered some insights for reliable multiplex genetic analyses.
Fluorescent amplifying polymers were also employed in MBs (Figure 2C,d; Yang et al. 2005c). Simple, rapid, and well-controlled procedures were developed to conjugate MBs with polymers in a precise 1:1 molecular ratio. Lee et al. (2007) also proposed a polymer-conjugated MB probe, where the polymer was water soluble and could be used as an efficient signal amplifying unit. This is superior to the previous work (Yang et al. 2005c), where surfactants were needed due to the poor water solubility of polymers.
Metal complex-based MBs
Joshi and Tor (2001) developed MBs tagged with metal complexes, which were called “metallobeacons” (Figure 2D,a). Ruthenium complex-containing nucleoside served as a fluorophore, whereas osmium complex-containing nucleoside acted as a quencher. The fluorescence emission of the MBs was sufficiently quenched in the form of stem-loop structure and then was dramatically recovered upon hybridization with complementary DNA sequences.
The Eu3+ complex of chlorosulfonylated tetradentate β-diketone (L) was also applied as a donor dye (Li et al. 2011b) in MBs (Figure 2D,a). The detection limit was approximately 20 times lower than that of the conventional MBs. The long luminescence duration of Eu3+ (~0.8 ms) could be easily distinguished from the short fluorescence lifetime of the background (<10 ns) and the scattering light. As low as 500 pM DNA in cell media could be determined quantitatively without any sample pretreatment.
Brunner and Kraemer (2004) introduced Cu2+ complexes as intramolecular quenchers for MBs (Figure 2D,b), where the quenching process was induced by the intramolecular coordination of a phenolate donor of fluorescein with a free coordination site of a copper(II) 5-(2-pyridinyl)pyrazole (pypz) complex. These MBs held a great potential to eliminate nonspecific interactions between MBs and complicated biosamples to avoid false-positive signals.
Metal complex-based MBs could be employed for DNA detection. Hou et al. (2012) proposed an MB with fluorescein at its 3′-end as the fluorophore, whereas its 5′-end consisted of a thymidine-rich sequence (Figure 2D,c). Due to the special interactions between thymidine and Hg2+, as well as the fluorescence quenching capability of Hg2+, these Hg2+-quenched MBs could be used as a “turn-on” fluorescence sensor for DNA determination.
Section summary
It is seen that there are a great number of ways to label HDPs to form MBs. The sensing performance of MB-based detection methods greatly depends on the SBR. To improve the detection quality, the background signal should be reduced, whereas the response signal needs to be enhanced. MBs are also required to be resistant to nuclease degradation in physiological media. Furthermore, the signals reported from ideal MBs should not be affected by the autofluorescence from complicated biological systems. Most of the newly developed MBs are based on such ideas. This may be achieved using new materials with striking optical properties. The recent advances in MBs with diverse signaling entities discussed previously are summarized in Table 1.
MBs labeled with different signaling pairs.
| Design concept | Donor | Acceptor | Corresponding model in Figure 2 | References |
|---|---|---|---|---|
| Classical type | 5-(2′-Aminoethyl) aminonaphthalene-1-sulfonic acid | DABCYL | A,a | Tyagi and Kramer 1996 |
| Cy3 | BHQ2 | Yu et al. 2014 | ||
| Cy5 | Iowa Black RQ | Watanabe et al. 2014 | ||
| Texas red and carboxyfluorescein | BHQ1 and BHQ2 | Kang et al. 2011 | ||
| Coumarin, EDANS, fluorescein, Lucifer yellow, BODIPY, eosin, and Texas red | DABCYL | Tyagi et al. 1998 | ||
| Carboxyfluorescein | DABCYL | Kolpashchikov 2006, Huang et al. 2012a, 2013, Sun and Fan 2012 | ||
| Two different fluorophores | Coumarin | Carboxyfluorescein | A,b | Zhang et al. 2001 |
| Thiazole orange | Thiazole red | Holzhauser and Wagenknecht 2011 | ||
| Cy3 | Cy5 | Kim et al. 2007a | ||
| Cy5 | Cy5.5 | Wabuyele et al. 2003 | ||
| Fluorescein | Texas red | Barilero et al. 2009 | ||
| Two identical units | Pyrene | Pyrene | A,c | Yang et al. 2005a, Huang et al. 2010, Zheng et al. 2010, 2011, Meng et al. 2012 |
| Phthalocyanine | Phthalocyanine | Nesterova et al. 2009 | ||
| Chlorin e6 | Chlorin e6 | Gao et al. 2011 | ||
| Carminic acid | Carminic acid | Cheng et al. 2010, Li et al. 2014a | ||
| Hemin | Hemin | Wang et al. 2014f | ||
| Quencher free | Carboxyfluorescein, Alexa 488, rhodamine 6G, tetramethylrhodamine, and oxazine derivative MR121, ATTO665 | Guanosine residues in DNA probes | A,d | Piestert et al. 2003, Stohr et al. 2005, Marme et al. 2006, Song and Zhao 2009, Su et al. 2012 |
| ATMND | Abasic sites in DNA probes | A,e | Sato et al. 2011, Song et al. 2012 | |
| Fluorene | – | A,f | Hwang et al. 2004 | |
| 2-Aminopurine | – | Dirks and Pierce 2004, EI-Yazbi and Loppnow 2012 | ||
| Nanomaterial based | Fluorescein, rhodamine 6G, Texas red, Cy5, ROX, and Cy3 | AuNP | B,a | Dubertret et al. 2001, Beni et al. 2010, Jayagopal et al. 2010, Rosa et al. 2012, Ebrahimi et al. 2014, Qian et al. 2014 |
| Carboxyfluorescein, tetramethylrhodamine, carboxyrhodamine 6G | AgNP | B,b | Wabuyele and Vo-Dinh 2005, Peng et al. 2009, 2012, Zhou et al. 2014b | |
| QD | ROX, Cy5, and Cal610 | B,c | Chen et al. 2007, Liu et al. 2011 | |
| QD | Iowa Black FQ, DABCYL, and BHQ2 | B,d | Li et al. 2011c, Wu et al. 2010b, 2011 | |
| QD | AuNP | B,e | Yeh et al. 2010, Deng et al. 2014a, Zhang et al. 2014a | |
| Multiple moieties attached | Carboxyfluorescein | SQ (three DABCYLs) | C,a | Yang et al. 2005b, Lee et al. 2009 |
| Pyropheophorbide | SQ (three DABCYLs) | Lovell et al. 2010 | ||
| Several pyrene molecules | DABCYL | C,b | Conlon et al. 2008 | |
| 6-Carboxyrhodamine 6G, tetramethylrhodamine, Texas red, and fluorescein | DABCYL | C,c | Tyagi et al. 2000 | |
| Poly (phenylene ethynylene) | DABCYL | C,d | Yang et al. 2005c, Lee et al. 2007 | |
| Metal complex included | Europium complex | BHQ2 | D,a | Li et al. 2011b |
| Ruthenium complex | Osmium complex, BHQ2 | Joshi and Tor 2001, Wilson and Johansson 2003, Huang and Marti 2012 | ||
| Lanthanides | BHQ2 | Krasnoperov et al. 2010 | ||
| Fluorescein | Copper complex | D,b | Brunner and Kraemer 2004 | |
| Fluorescein | Thymidine-Hg2+-thymidine | D,c | Hou et al. 2012 |
Detectable targets and sensing mechanisms for MB-based homogeneous sensing systems
General mechanism of MB-based homogeneous sensing systems
The fluorophores attached on MBs generate fluorescence emission, when the MBs hybridize with the targets and experience structural changes (Figure 1C). The signal generation is instantaneous in homogenous solutions and does not require any washing steps to separate the MB-target complexes from the excess amount of unreacted MBs. This is due to the presence of the stem structure, which can lock the MBs from dissociation and ensure substantial background suppression. The MB-based detection methods can be treated as instantaneous schemes with only one “mix-and-measure” step. Depending on how the signals are generated due to the conformational changes of MBs, MB-based sensing methods can be categorized into two groups: “open-to-signal” and “close-to-signal” (Huang et al. 2014a).
MB-based “open-to-signal” biosensors
“Open-to-signal” sensors are those where the signals are generated due to the opening of MBs. Figure 3 illustrates some typical examples of such sensing methods. Other than the most popular applications for nucleic acid determination (Figure 3A), as discussed in the previous section, the principle of “open-to-signal” MB-based sensing strategies has also been extended for detecting a variety of other analytes, including proteins (Hall et al. 2007, Vallee-Belisle et al. 2011), nucleases (He et al. 2014, Lian et al. 2015), small molecules (Sharon et al. 2010, Machado et al. 2014), and metal ions (Ono and Togashi 2004, Stobiecka et al. 2012).
Basic principles of MB-based homogeneous sensing systems (open-to-signal).
Determination of nucleic acids (A), proteins or small molecules (B), nucleases (C), metal ions (D) and (E), ligases (F), polymerases (G), and MTases (H).
Tang et al. (2008) constructed a novel and versatile intramolecular signal transduction aptamer probe to identify aptamer-target binding events (Figure 3B). Upon the addition of the targets (ATP or thrombin), the conformation of the aptamer probe changed and then the fluorophore was forced away from the quencher, allowing fluorescence recovery. This new aptamer probe integrated the aptamer sequence, competitor probe, and signaling base pairs into one molecule and could be used for target recognition based on intramolecular displacement. Compared to intermolecular DNA hybridization, this intramolecular hybridization design exhibited better performance in terms of probe optimization and hybridization rate.
Employing a similar principle to that depicted in Figure 3B, Vallee-Belisle et al. (2011) introduced “transcription factor beacons” (target-activated fluorescent DNA probes) to record the interactions of specific DNA binding events. Such “transcription factor beacons” could be used for rapid and reliable determination of three transcription factors (TATA binding protein, Myc-Max, and nuclear factor-κB). Their binding activities could be directly monitored in crude nuclear extracts.
Wu et al. (2014c) reported a sensitive and simple “turn-on” fluorescence strategy for the detection of 3′-5′-exonuclease activity of exonuclease III (Figure 3C). As exonuclease III was introduced, it digested the MB from its 3′-end and then removed the quencher from the MB, leading to fluorescence enhancement. The results demonstrated that MBs could be potentially applied for the detection of exonuclease III in real biological samples.
Xu et al. (2011) proposed a new fluorescent probe for the detection of Hg2+ in aqueous media. The sensor employed thymine-rich sequences for the selective recognition of Hg2+ with structure-switching fluorophore-quencher pairs in MBs for signaling (Figure 3D). The presence of Hg2+ facilitated the hybridization between MBs and the single-stranded DNA probes, liberating the fluorescence of the fluorophore.
Zhang et al. (2010b) introduced a new concept of “catalytic beacon,” which could be used for Pb2+ detection with the assistance of DNAzyme. As illustrated in Figure 3E, the DNAzyme strand bound with the MB to form a complex structure. After Pb2+ activated the catalytic activity of the DNAzyme, the DNAzyme substrate (shown as the red region) caged in the MB sequences was cut into two pieces, resulting in stem dissociation and fluorescence increase. An amplified fluorescence signal could be easily expected, as one DNAzyme could catalyze the cleavage of multiple DNAzyme substrates in MBs.
Tang et al. (2003) developed an MB-based platform, which, for the first time, made it possible for the real-time monitoring of nucleic acid ligation. As shown in Figure 3F, two half-matching short DNA strands were employed and they bound with the MB to form a nick. In the presence of DNA ligase, it bound to the nick and catalyzed the ligation of the two short fragments to form a longer DNA strand. Then, the ligation product opened the MB and released the quenched fluorescence of the MB. This detection mechanism was revised later for the real-time investigation of nucleic acid phosphorylation processes (Tang et al. 2005).
Ma et al. (2006) reported a novel MB-based system for real-time DNA polymerase detection. As shown in Figure 3G, a primer was designed to hybridize with the MB and then form a primer-MB complex. When DNA polymerase appeared, it bound to the 3′-end of the primer-MB complex and catalyzed the nucleotide incorporation into the primer to generate a longer DNA strand. Then, the long polymerization product became able to open the MB and led to fluorescence restoration.
Li et al. (2007) proposed a novel method to study methyltransferase (MTase) activity using MBs coupled with enzyme-linkage reactions. As depicted in Figure 3H, MTase methylated the MBs at the recognition site, yielding the methylated DNA products, which were then recognized and cleaved by DpnI endonuclease, restoring the fluorescence of the MBs.
Table 2 provides the details about how the sensing mechanisms in Figure 3 are employed for the determination of a wide range of targets.
MB-based “open-to-signal” schemes for sensing various targets.
| Target of interest | Sensing scheme in Figure 3 | References |
|---|---|---|
| Nucleic acids | ||
| DNA | A | Tyagi et al. 1998, 2000, Zhang et al. 2001, Lee et al. 2007 |
| RNA | Harry et al. 2010, Baker et al. 2012, Su et al. 2015 | |
| Proteins | ||
| Transcription factor | B | Vallee-Belisle et al. 2011 |
| Thrombin | Cheng et al. 2010, Tang et al. 2010, Li et al. 2013a | |
| Platelet-derived growth factor | Tan et al. 2014 | |
| Vascular endothelial growth factor | Hall et al. 2007 | |
| Cdc42-binding kinase | Tok et al. 2010 | |
| HIV-1 reverse transcriptase | Liang et al. 2011 | |
| Small molecules | ||
| ATP | Tang et al. 2008, Nielsen et al. 2010, Wang et al. 2010b, Li et al. 2013a | |
| Adenosine monophosphate | Sharon et al. 2010, Machado et al. 2014 | |
| Flavin mononucleotide | Hall et al. 2007 | |
| Cocaine | Nie et al. 2013 | |
| ATP | F | Ma et al. 2012, 2013a |
| Nicotinamide adenine dinucleotide | Tang et al. 2011b | |
| Nucleases | ||
| RNase H | C | Liu et al. 2013a |
| Exonuclease III | Wu et al. 2014c | |
| S1 nuclease | Li et al. 2000 | |
| Glycosylase | Li et al. 2014b | |
| Endonuclease | Jiang et al. 2014a | |
| Polynucleotide kinase | He et al. 2014, Lian et al. 2015 | |
| Ligase | F | Tang et al. 2003, Liu et al. 2005 |
| Polynucleotide kinase | Tang et al. 2005, Chen et al. 2013a | |
| Polynucleotide kinase | G | Ma et al. 2007a |
| Polymerase | Ma et al. 2006 | |
| Telomerase | Ding et al. 2010 | |
| Endonuclease | Ma et al. 2007b | |
| MTase | H | Li et al. 2007, Wei et al. 2014, Zhao et al. 2014 |
| Metal ions | ||
| Hg2+ | D | Yang et al. 2009, Xu et al. 2011, Wu et al. 2013b |
| Ag+ | Xiao et al. 2013 | |
| Pb2+ | E | Wang et al. 2009a, Zhang et al. 2010b |
| Cu2+ | Li et al. 2013b |
MB-based “close-to-signal” biosensors
“Close-to-signal” sensors report the presence of targets due to the formation of MBs. This sensing technique is termed “reverse MB” method and has been used for detecting a variety of targets (Figure 4).
Schematic of the generic principles of MB-based homogeneous sensing systems (close-to-signal).
Detection of nucleic acids (A and B), proteins or small molecules (C), metal ions (D and E), and phosphatase (F).
Larkey et al. (2014) presented an innovative double-stranded DNA probe (Figure 4A). One single-stranded DNA (shown as the black strand) contained complementary sequences at the ends, which were labeled with a fluorophore and a quencher, respectively. The other DNA probe (shown as the blue strand) was label-free and designed to be complementary to a target sequence. In the presence of the target DNA, the target interacted with the DNA probe (the blue strand) and released the labeled strand (the black one), leading to the generation of a hairpin structure. The reduced fluorescence signal due to the formation of MBs was then used to report the hybridization events. With locked nucleic acid bases incorporated into its sequence, this probe could help reduce false-positive signals from nuclease degradation.
Wabuyele et al. (2003) developed a rapid and robust mutation detection approach capable of identifying point mutations directly from unamplified DNA samples with the help of DNA ligases. As depicted in Figure 4B, two helper DNA probes, whose complementary ends were labeled with fluorescent dyes, were smartly designed to recognize the target sequence. In the presence of perfectly matched DNA templates, the DNA ligase covalently joined the two adjacent helper probes to give rise to a longer single-stranded probe, which subsequently formed into an MB due to the complementary sequence at the ends and allowed FRET between the two dyes to occur.
Li and Ho (2008) proposed a generic method to convert aptamer-target recognition events into optical signals using the idea of “reverse MB.” As indicated in Figure 4C, an unmodified aptamer probe (the blue strand) specific to the targets (ATP or thrombin) served as the molecular recognition element and a competitor probe (the black strand) labeled with a FRET pair that acted as the signal transduction unit. Without targets, these two probes hybridized to each other. In the presence of the target, it selectively recognized the aptamer probe and the competitor probe was displaced to adopt a hairpin structure (MB), which enabled FRET and reduced signal generation.
Using a similar idea, Huang et al. (2010) designed a competition-mediated pyrene-switching aptasensor for the detection of lysozyme in human serum. As seen in Figure 4C, pyrene molecules (not shown in the figure) were modified at both ends of a competitor DNA strand (the black strand), which was partially complementary to an aptamer sequence (the blue strand). Upon the addition of lysozyme, it bound with the aptamer probe in a selective manner and removed the competitor DNA strand, which could return to its initial hairpin-shaped structure. Then, the emission wavelength switched from the pyrene monomer peak to its excimer peak.
Ono and Togashi (2004) reported the first example using thymine-rich DNA probes for the selective detection of Hg2+ in aqueous solutions (Figure 4D). The probe was functionalized with a fluorophore and a quencher at its two ends. It consisted of thymine-rich mercury-binding sequences at the ends and linker sequences in the middle region. In the presence of Hg2+, Hg2+ stabilized the thymine-thymine mismatched base pairs in the probe and then induced the formation of hairpin structures, leading to FRET processes.
Meng et al. (2012) proposed a fluorescence sensor for Zr4+ recognition via a target-mediated reverse MB scheme. As sketched in Figure 4E, two helper probes were modified with two phosphate-functionalized molecules, which were typically used to selectively bind with Zr4+. Each helper probe was labeled with a pyrene molecule. Zr4+ brought the two probes into close proximity, which promoted the formation of a hairpin structure and resulted in a pyrene excimer fluorescence signal.
Song et al. (2010) developed a powerful platform for high-throughput analyses of DNA 3′-phosphatases and their inhibitors based on a universal MB. The platform consisted of two hairpin probes, which bound to each other (Figure 4F). 3′-PO4 termini in one of the hairpin probes (the blue strand) could be hydrolyzed by enzymes with 3′-phosphatase activities, and then a 3′-OH termini would be generated. With the assistance of a polymerase, once the 3′-OH termini in the hairpin probe was yielded, it was immediately elongated by the polymerase to replace the single-stranded DNA probe (the black strand). Then, the complementary ends of the single-stranded DNA probe forced itself to assume a hairpin structure and the fluorescence signal was quenched.
Table 3 summarizes the diverse examples about how the detection principles in Figure 4 are employed for detecting different types of targets. The “open-to-signal” MB-based sensors are commonly regarded as “turn-on” biosensors, whereas the “close-to-signal” MB-based sensors are referred to as “turn-off” biosensors. For the “turn-off” biosensors, targets are reported by the significant loss of the initially strong signals. As it is difficult to completely suppress the original signal, the detection sensitivity of “turn-off” methods is usually limited (Lubin and Plaxco 2010). Another drawback associated with “turn-off” biosensors is that false signals could be generated due to the unexpected quenching entities in complicated biological samples. In contrast, “turn-on” sensors can generate considerably strong signals to improve the detection limit, especially for methods employing signal amplification techniques.
MB-based “close-to-signal” schemes for sensing various targets.
| Target of interest | Sensing scheme in Figure 4 | References |
|---|---|---|
| Nucleic acids | ||
| DNA | B | Wabuyele et al. 2003, Peng et al. 2010 |
| RNA | A | Larkey et al. 2014 |
| Proteins | ||
| Lysozyme | C | Huang et al. 2010 |
| Platelet-derived growth factor | C | Yang et al. 2005a |
| Prostate-specific antigen | C | Ren et al. 2014 |
| Thrombin | C | Li and Ho 2008 |
| Thrombin | E | Heyduk and Heyduk 2005 |
| Small molecules | ||
| Adenosine | Wu et al. 2010a | |
| Cocaine | Wu et al. 2010a | |
| Coralyne | Park and Park 2015 | |
| ATP | C | Li and Ho 2008, Kitamura et al. 2013 |
| Adenosine | Liu and Lu 2006 | |
| Cocaine | Liu and Lu 2006 | |
| Coralyne | Kuo and Tseng 2013 | |
| Coralyne | D | Hung and Tseng 2014, Lin and Tseng 2014 |
| Biothiol | Zhang et al. 2013b | |
| Metal ions | ||
| Hg2+ | Ono and Togashi 2004, Chiang et al. 2008, Stobiecka et al. 2012 | |
| Ag+ | Lin and Tseng 2014 | |
| Zn2+ | Rajendran and Ellington 2008 | |
| Zr4+ | E | Meng et al. 2012 |
| Nucleases | ||
| Phosphatase | F | Song et al. 2010 |
| Polymerase | Song et al. 2009 |
HDP-based solid-state sensing methods
MB-based sensing schemes are usually performed in homogeneous solution systems. Some other HDP-based biosensors, however, are constructed on solid surfaces. Figure 5 illustrates some basic formats of HDP-based solid-state biosensing platforms, where HDPs were immobilized on surfaces for reporting hybridization or binding events using diverse signal readouts (optical or electrical).
General principles for the determination of nucleic acids through HDP-based solid-phase biosensing platforms.
(A) A label-free scheme, (B) an intercalating agent-based biosensor, (C and D) sensors with optically (C) or electrochemically (D) sensitive signaling tags, and (E) an enzyme-linked amplified detection strategy. ECL, electrochemiluminescence; eT, electron transfer; Fl, fluorescence intensity; QCM, quartz crystal microbalance.
Kjallman et al. (2008) introduced an electrochemical HDP-based DNA sensor, where HDPs were initially tethered to an electrode (Figure 5A). In the presence of the target DNA, the HDPs were opened by the targets, leading to variations in the electron transfer behavior at the hairpin probe-modified electrode, which could be reported by impedance spectroscopy. This is the simplest design of label-free electrochemical DNA biosensors for the direct determination of analytes.
Based on a similar principle, Zhang et al. (2010a) developed an electrochemical DNA biosensor for the simultaneous detections of multiplexed DNA targets. In this sensor, thiolated HDPs were attached onto an electrode surface with methylene blue as a hybridization redox indicator (Figure 5B). Methylene blue exhibits higher affinity with single-stranded DNA than with double-stranded DNA. The oxidation currents of methylene blue decreased due to the reduced single-stranded DNA sequences after the hybridization events between HDPs and DNA targets, which could be monitored by square wave voltammetry.
Du et al. (2003) reported a prototypical “molecular beacon” biosensor on gold surfaces. It was the first implementation where the substrate was used as a quenching agent. In this sensor, fluorophore-tagged DNA hairpins were attached to a gold substrate, which caused fluorescence quenching. The target DNA opened the HDP on the substrate and separated the fluorophore from the gold substrate, leading to the fluorescence emission (Figure 5C).
Fan et al. (2003) designed an electrochemical DNA biosensor where an HDP possessing a ferrocene group was immobilized on an electrode (Figure 5D). The stem-loop structure of the HDP brought the ferrocene tag into close proximity to the electrode surface. A large redox current was generated due to the efficient redox of the ferrocene and the rapid electron transfer between the ferrocene and the electrode. In the presence of the target DNA, the conformational change of the hairpin probe separated the ferrocene label from the electrode surface and reduced the redox currents.
Liu et al. (2008) described an enzyme-based electrochemical DNA sensor for the specific detection of femtomolar DNA targets. As depicted in Figure 5E, an HDP labeled with digoxigenin was fixed at an electrode surface. In the absence of DNA targets, the probe adopted a stem-loop structure, which made digoxigenin inaccessible by a horseradish peroxidase-linked anti-digoxigenin antibody due to the high steric hindrance. When targets appeared, the HDP changed its conformation, forcing digoxigenin away from the electrode. Then, the horseradish peroxidase-linked anti-digoxigenin antibody was able to bind with digoxigenin, and an enzymatically amplified electrochemical current signal was produced to report the presence of the target DNA.
Wei et al. (2008) developed an HDP-based enzyme-aided electrochemical DNA sensor for the sensitive determination of salivary RNA (Figure 5E). Without binding with the targets, the hairpin probe closed itself and the anti-fluorescein-horseradish peroxidase could not bind with the fluorescein modified at the end of the HDP. Upon the addition of the target RNA, the hairpin was opened and subsequently bound with the anti-fluorescein-horseradish peroxidase, leading to an amplified current signal to indicate the hybridization events.
Figure 5 illustrates the schematic diagrams for nucleic acid determination on solid-phase substrates. Other than nucleic acid targets, these strategies could also be applied to detect other targets through other interaction mechanisms, including DNA/RNA-aptamer/target binding as well as DNA/RNA-substrate/enzyme reactions. The targets can be either aptamer-relevant molecules, such as cocaine (Baker et al. 2006), ATP (Goda and Miyahara 2011, 2012), thrombin (Radi et al. 2006), and enzyme/DNAzyme-involved species, including Pb2+ (Lin et al. 2011) and ligase (Luan et al. 2010). The means for monitoring the specific interaction events upon the addition of targets range from optical [fluorescence, surface-enhanced Raman scattering (SERS), and surface plasmon resonance (SPR)] to electrochemical [electrochemical impedance spectroscopy (EIS), differential pulse voltammetry, and alternating current voltammetry] readouts. The details about the aforementioned HDP-based solid-state sensors can be found in Table 4.
HDP-based solid-phase platforms for detecting a wide range of analytes.
| Sensing scheme in Figure 5 | Target of interest | Detection technique | References |
|---|---|---|---|
| A | DNA | EIS | Bonanni and Pumera 2011, Zhang et al. 2014b |
| SPR | Meneghello et al. 2014 | ||
| QCM | Papadakis et al. 2012 | ||
| Human immunoglobulin E | Electrochemical current | Wu et al. 2009 | |
| Ligase | SPR | Luan et al. 2010 | |
| ATP | EIS | Goda and Miyahara 2011 | |
| Pb2+ | EIS | Lin et al. 2011 | |
| B | DNA | Alternating current voltammetry | Farjami et al. 2010, Zhang et al. 2010a |
| Differential pulse voltammetry | Chen et al. 2008 | ||
| ATP | Field-effect transistor | Goda and Miyahara 2012 | |
| Bleomycin | ECL | Li et al. 2013c | |
| C | DNA | ECL | Wu et al. 2014a |
| Fluorescence | Du et al. 2003, Piestert et al. 2003, Peng et al. 2012 | ||
| SERS | Wang et al. 2014a | ||
| RNA | SERS | Pang et al. 2014 | |
| DNA-binding protein and histone protein | Fluorescence | Wang et al. 2011a | |
| Hg2+ | Fluorescence | Gao et al. 2013c | |
| D | DNA | Alternating current voltammetry | Fan et al. 2003, Farjami et al. 2011 |
| Square wave voltammetry | Du et al. 2014 | ||
| ECL | Yao et al. 2013 | ||
| Thrombin | ECL | Wang et al. 2009c | |
| Differential pulse voltammetry | Radi et al. 2006 | ||
| ATP | Square wave voltammetry | Wu et al. 2013a | |
| Cocaine | Alternating current voltammetry | Baker et al. 2006 | |
| Hg2+ | Square wave voltammetry | Zhuang et al. 2013a | |
| Differential pulse voltammetry | Xiong et al. 2015 | ||
| E | DNA | Differential pulse voltammetry | Wang et al. 2014c |
| Amperometric current | Liu et al. 2008, Wei et al. 2008, Avila et al. 2015 | ||
| Absorbance | Niu et al. 2011 | ||
| QCM | Wang et al. 2012 | ||
| RNA | Photoelectrochemistry | Yin et al. 2014 | |
| Amperometric current | Cai et al. 2013 |
ECL, electrochemiluminescence; QCM, quartz crystal microbalance.
Brief summary of homogeneous and solid-state sensing systems
Both MB-based homogeneous sensing systems and HDP-based solid-state biosensing platforms have their own advantages and limitations. MB-based sensing methods exhibit great performance in terms of assay time and operation procedures. This is because the hybridization kinetics is faster in homogenous solutions than that on solid surfaces, where the high steric hindrance and slow diffusion kinetics are critical issues. In addition, homogenous sensing systems are less labor-intensive compared with solid-state platforms, because they do not require immobilization procedures to attach DNA probes on substrates and/or tedious washing steps to remove unbound DNA probes. Another problem associated with solid-state sensing approaches is the potential distortion of biomolecular interactions. This is because the immobilized biomolecules can lead to undesirable orientation for binding, lower the degrees of freedom available for capturing the targets, and cause nonspecific binding with targets. However, HDP-based solid-state biosensing platforms, especially electrochemical biosensors, have very low background signals, which greatly improve the detection sensitivity. In addition, they do not require sophisticated instrumentation and are usually cost-effective. Sometimes, electrochemical sensing chips can be regenerated and reused many times without significant signal loss, which can lower the detection cost. It is worth mentioning that HDP-based electrochemical biosensors can avoid the photobleaching problem of dye labels, which might be encountered in HDP-based fluorescent methods.
Signal amplification techniques
A major problem of using traditional MBs for biosensing is that each target molecule can hybridize with only one MB for signal generation. This 1:1 hybridization ratio greatly hinders the signal gain and the detection limit. To improve the detection sensitivity, various signal amplification schemes have been developed, which can be divided into two groups: enzyme-free and enzyme-assisted strategies.
Enzyme-free amplified methods
Huang et al. (2012a) proposed an enzyme-free strategy for the amplified detection of DNA using two MBs. The working mechanism is demonstrated in Figure 6A,a. The two MBs, MB1 and MB2, were specially designed such that they were complementary to each other. Furthermore, part of MB1 was complementary to the target. In the presence of the DNA target, it interacted with and opened MB1. This exposed the rest sequences of MB1 to MB2, and the hybridization between MB1 and MB2 was triggered. MB2 acted as a strand competitor and finally replaced the target DNA in hybridization with MB1. The freed target then went through another reaction cycle and led to more hybridizations between MB1 and MB2, which enhanced the fluorescence generation. This method was later modified by replacing the MBs with two label-free HDPs and applied to the determination of BRCA-1 gene (Huang et al. 2014c), where the signal enhancement was due to the continuous generation of DNA duplex from the two HDPs and their interaction with SYBR Green I molecules. Such target recycling methods are usually called catalytic hairpin assembly (CHA; Li et al. 2011a), and many CHA-based sensing systems have been reported for the effective detections of different analytes.
Schematic of strategies used in HDP-based amplification platforms.
(A) Enzyme-free schemes using CHA (a) and HCR (b) and (B) enzyme-assisted methods with the employment of different enzymes: nicking endonuclease (a), exonuclease III (b), and polymerase (c).
For the first time, Dirks and Pierce (2004) demonstrated an innovative technique for DNA detection based on chain reactions between two sets of HDPs (HP1 and HP2; Figure 6A,b). One of the HDPs (HP1) was labeled with a fluorophore, 2-aminopurine, whose fluorescence was highly quenched when embedded into double-stranded DNA structures. HP1 and HP2 were complementary in a staggered configuration and could coexist stably in the absence of targets. When targets appeared, the target DNA acted as an initiator strand and triggered the hybridizations between the target and HP1, which was then followed by the continuous hybridizations between HP1 and HP2, leading to the substantial fluorescence suppression. This strategy is named hybridization chain reaction (HCR; Dirks and Pierce 2004) and has been widely used to detect a wide ranges of targets. For example, Huang et al. (2014b) reported an HCR-based method for the sensitive and selective detection of Hg2+ in homogeneous solutions. The detection selectivity was ensured by the thymine-Hg2+-thymine coordination. The detection sensitivity was greatly improved through the HCRs as well as the suppression of background signal due to the employment of the graphene oxide (Huang et al. 2014b). A similar idea has been used for the detection of biothiols (Ge et al. 2014).
Enzyme-assisted amplified approaches
Signal amplifications could also be realized by employing certain enzymes that can perform special functions when interacting with DNA probes, including nucleotide incorporation, ligation, and cleavage reactions. PCR (Li and Rothberg 2004), ligase chain reaction (Yan et al. 2010), and rolling circle amplification (RCA; Jiang et al. 2013, Zhuang et al. 2014) are popular methods for amplified DNA detection. Recently, some novel approaches have been developed using tailored DNA probes and their corresponding enzymes, including nicking enzyme, exonuclease III, and polymerase.
Li et al. (2008) introduced a nicking enzyme-aided method for the amplified detection of DNA. As illustrated in Figure 6B,a, after the target hybridized with the MB, the nicking enzyme cleaved the MB and the target was released from the target/MB complex. The liberated target then reacted with and opened another MB. Therefore, the target was used as a catalyst to trigger the continuous digestion of MBs and gave rise to an amplified response signal.
Zuo et al. (2010) demonstrated a simple and sensitive method for DNA detection by employing MBs and exonuclease III. The preferred substrates of exonuclease III are blunt or recessed 3′-termini. As seen in Figure 6B,b, the MB contained exonuclease III-resistant protruding 3′-termini. Exonuclease III would catalyze MB cleavage only when the target bound with the MB, where a blunt 3′-terminus was formed in the MB/target complex. The released target remained intact and could join another reaction circle to facilitate the digestion of MBs by exonuclease III, allowing remarkable signal amplification.
Guo et al. (2009) proposed a new MB-involved method based on polymerase-mediated strand extension instead of strand cleavage. As shown in Figure 6B,c, the sensing system contained both MBs and primers. In the presence of the target DNA, it hybridized with the MB, which, once opened, could bind with the primer; then, the strand elongation would be initiated by the polymerase. The newly yielded probe forced the target to detach from the MB and this newly yielded probe/MB complex produced fluorescence signals. The liberated target became available to repeat the reactions (i.e. liberation after hybridization) to magnify the signal.
The above strategies could be combined for the determination of various analytes. For instance, Li et al. (2012) first engineered two enzyme-free signal amplification processes, HCR and CHA, for the fluorescent detection of nucleic acid analytes. In addition to the fluorescent methods, electrochemical approaches have also been developed for the detection of DNA based on the ingenious combination of CHA and HCR (Liu et al. 2013d). There are a number of other methods that couple CHA or HCR with some enzyme-assisted schemes, which are regarded as dual/multiple-amplified strategies. Table 5 shows the amplification methods demonstrated in Figure 6, which have been well incorporated into HDP-based biosensing systems.
Sensing methods using signal amplification techniques.
| Amplification principle | Sensing scheme in Figure 6 | Target of interest | References |
|---|---|---|---|
| Enzyme free | |||
| CHA | A,a | DNA | Allen et al. 2012, Huang et al. 2012a, 2014a, Jiang et al. 2014b, Li et al. 2011a, 2014c,d, Qian et al. 2015 |
| RNA | Bhadra and Ellington 2014, Zhu et al. 2015 | ||
| Thrombin | Xu et al. 2015 | ||
| Polynucleotide kinase | Hou et al. 2014 | ||
| ATP | Li et al. 2011a | ||
| Adenosine | Quan et al. 2015 | ||
| Pb2+ | Chen et al. 2013b | ||
| HCR | A,b | DNA | Dirks and Pierce 2004, Huang et al. 2011, Shimron et al. 2012, Huang et al. 2013, 2015, Liu et al. 2013c |
| RNA | Choi et al. 2014, Sternberg and Pierce 2014 | ||
| Thrombin | Ang and Yung 2014 | ||
| Human IgG | Dai et al. 2014 | ||
| Cytokine and chemokine | Choi et al. 2011 | ||
| Carcinoembryonic antigen | Xu et al. 2013b | ||
| Interferon-γ | Zhao et al. 2012 | ||
| EBNA-1 | Song et al. 2014 | ||
| Adenosine | Yang et al. 2013 | ||
| Biothiol | Ge et al. 2014 | ||
| Hg2+ | Huang et al. 2014b | ||
| Ag+ | Liu et al. 2014 | ||
| Pb2+ | Zhuang et al. 2013b | ||
| Enzyme assisted | |||
| Nicking endonuclease | B,a | DNA | Li et al. 2008, Gao et al. 2013a, Liu et al. 2013f |
| Thrombin | Xue et al. 2012 | ||
| IgE | Feng et al. 2012 | ||
| L-histidine | Kong et al. 2011 | ||
| MTase | Zhao et al. 2013 | ||
| Exonuclease III | B,b | DNA | Zuo et al. 2010, Cai et al. 2014, Gao and Li 2014, Liu et al. 2015a |
| RNA | Cui et al. 2013 | ||
| Thrombin | Huang et al. 2012b | ||
| Platelet-derived growth factor | Bi et al. 2014 | ||
| MTase | Xing et al. 2014 | ||
| ATP | Liu et al. 2013e | ||
| Streptavidin | Zhou et al. 2013 | ||
| Bleomycin | Gao et al. 2013b | ||
| Hg2+ | Xuan et al. 2013 | ||
| Pb2+ | Xu et al. 2013a | ||
| Ag+ | Wang et al. 2014d | ||
| Polymerase | B,c | DNA | Guo et al. 2009, Jiao et al. 2012, Xuan et al. 2012 |
| RNA | Tian et al. 2013, Deng et al. 2014b | ||
| Platelet-derived growth factor | Qiu et al. 2011, Tang et al. 2012b | ||
| Vascular endothelial growth factor | Cheng et al. 2012 | ||
| ATP | Guo et al. 2011 | ||
| Hg2+ | Zhu et al. 2011 | ||
| Dual/multiple amplification schemes | |||
| CHA+CHA | – | DNA | Song et al. 2015 |
| CHA+HCR | – | DNA | Li et al. 2012, Liu et al. 2013d |
| CHA+HCR | – | RNA | Wu et al. 2014b |
| CHA+HCR | – | Thrombin | Niu et al. 2012 |
| CHA+RCA | – | DNA | Jiang et al. 2013 |
| CHA+RCA | – | RNA | Zhuang et al. 2014 |
| CHA+exonuclease III | – | DNA | Tao et al. 2015, Wu et al. 2015 |
| HCR+exonuclease III | – | DNA | Ren et al. 2015 |
| HCR+polymerase | – | DNA | Wang et al. 2013a |
| Nicking endonuclease+polymerase | – | DNA | Connolly and Trau 2010, Liu et al. 2013b, Ma et al. 2014a |
| Nicking endonuclease+polymerase | – | RNA | Ma et al. 2014b |
| Nicking endonuclease+polymerase | – | Cocaine | Shlyahovsky et al. 2007 |
| Nicking endonuclease+polymerase | – | Ag+ | Freage et al. 2014 |
| Exonuclease I+polymerase | – | DNA | Su et al. 2014 |
| Polymerase+λ exonuclease | – | Interferon-γ | Zhou et al. 2014a |
| Nicking endonuclease+polymerase+ligase | – | DNA | Wang et al. 2014e |
| Nicking endonuclease+polymerase+λ exonuclease | – | DNA | Liu et al. 2015a,b |
| Nicking endonuclease+polymerase+λ exonuclease | – | RNA | Duan et al. 2013 |
Section summary
To compare these two groups of amplification methods, enzyme-free schemes are relatively simpler and cheaper because enzymes are usually expensive and sequence specific. However, the assay time of enzyme-free schemes are usually longer than enzyme-assisted methods. For enzyme-assisted methods, enzyme activities can be easily affected by many factors, such as temperature, pH value, and enzyme concentration. Moreover, the detection procedures may involve complicated operations, such as thermal cycling steps. Nevertheless, enzyme-assisted methods always exhibit superior sensitivity to enzyme-free approaches.
Conclusions and future perspectives
As discussed in the previous sections, HDPs are powerful for biodetections due to their conformational variations and the accompanying signal changes. The flexible signal transduction mechanisms and the specific and robust target recognition capability make HDPs versatile and desirable tools for a variety of biosensing systems. Different signaling pairs, including nanomaterials, metal complexes, conjugated polymers, and pyrene compounds, have been used to function as efficient signaling moieties. Furthermore, the sensing strategies performed in homogeneous solutions and on solid substrates have been widely applied for the detection of various targets, including nucleic acids, proteins, small molecules, and metal ions. Even temperature changes could be monitored using MB-based fluorescent nanothermometers, which might employ dual-fluorophore-labeled MB probes (Barilero et al. 2009), AuNP-MB probes (Ebrahimi et al. 2014), or enantiomeric L-DNA MB probes (Ke et al. 2012). They were simple, safe, stable, and accurate.
Despite the remarkable progress achieved, there is still room for the further improvement for HDP-based biosensors. First of all, HDPs suffer from intracellular enzymatic degradation, which could cause problems in practical applications. Most of the HDP-based biosensing experiments reported previously were performed in ideal buffer solutions. In serum, plasma, or other physiological media, however, they may not work well. To improve the stability of HDPs, a possible approach could be nucleic acid base modification. For example, locked nucleic acid (Wang et al. 2005, Yang et al. 2007, Østergaard et al. 2010, Dong et al. 2011, Larkey et al. 2014), peptide nucleic acid (Socher et al. 2008, Wu et al. 2012, Fischbach et al. 2014), and L-DNA (Kim et al. 2007b, Ke et al. 2012) have been developed to improve HDP performance in terms of nuclease digestion resistance, hybridization efficiency, and nonspecific binding issues.
In addition, the interference from autofluorescence or scattering light in complicated biological systems may pose a challenge for the use of HDPs in cellular environments. Several effective measures can solve such problems. Two-photon excitation (Liu et al. 2011) and photoluminescence up-conversion (Wang et al. 2014b) are useful techniques for autofluorescence discrimination, although they are barely used in HDP-based systems. The preparation of signaling pairs with longer photoluminescence lifetime (Yang et al. 2005a, Huang et al. 2010, Li et al. 2011b) can also make it easy to be distinguished from the background autofluorescence. Additionally, near-infrared fluorophores (Nesterova et al. 2009) are likely to enhance the light contrast by eliminating the background scattering light and/or autofluorescence.
Another critical issue for solid-state HDP-based sensors is the relatively high cost, which is mainly caused by the modification and postsynthetic purification of dual-labeling probes and other tedious modifications to transplant them onto solid surfaces. The high cost could hamper the clinical applications and their commercialization, especially in developing countries. To lower the detection cost, it might be helpful to employ label-free schemes or alternative transducers for signal readout. Zhang et al. (2013a) proposed a label- and immobilization-free homogeneous electrochemical biosensor for the detection of cocaine based on a specially designed HDP, into which the cocaine aptamer sequences and peroxidase-mimicking DNAzyme sequences were integrated. A disposable micropipette tip coupled with a reproducible carbon fiber ultramicroelectrode was employed to monitor DNAzyme catalytic activities in a homogeneous solution at the microliter level. This method exhibited several promising advantages, such as low cost, easy manipulation, rapid response, high sensitivity, and excellent repeatability.
Given the advantages of HDPs and the tremendous demands for biosensing in medicine and biological science, increasing efforts and innovative ideas are needed to improve the detection performance in the future. These will be facilitated by new materials, innovative transduction methodologies, as well as novel measurement tools.
About the authors
Jiahao Huang received his MS in biomedical engineering (2010) and BS in biotechnology (2007) from Hunan University, P.R. China. Currently, he is a PhD candidate in the Department of Mechanical and Aerospace Engineering at The Hong Kong University of Science and Technology. His research interests are in the field of DNA-based optical biosensors.
Jueqi Wu received her BS in environmental engineering (2012) from Sun Yat-sen University, P.R. China. She is a MPhil. student in the Environmental Engineering Program at The Hong Kong University of Science and Technology. Her research areas include the applications of metal particles and biosensors in environmental science.
Zhigang Li received his PhD in mechanical engineering (2005) from the University of Delaware, USA, and his MEng in thermal engineering (1999) from Tsinghua University, P.R. China. He is now an Associate Professor in the Department of Mechanical and Aerospace Engineering at The Hong Kong University of Science and Technology. His research interests mainly focus on micro/nano-fluidics, microscale/nanoscale transport phenomena, and biodetection.
Acknowledgments
This work was supported by the Research Grants Council of the Hong Kong Special Administrative Region under grant nos. 615312 and 16205714.
References
Allen, P. B.; Arshad, S. A.; Li, B. L.; Chen, X.; Ellington, A. D. DNA circuits as amplifiers for the detection of nucleic acids on a paperfluidic platform. Lab. Chip. 2012, 12, 2951–2958.10.1039/c2lc40373kSearch in Google Scholar
Ang, Y. S.; Yung, L. L. Engineering self-contained DNA circuit for proximity recognition and localized signal amplification of target biomolecules. Nuc. Acids Res. 2014, 42, 9523–9530.10.1093/nar/gku655Search in Google Scholar
Avila, B. E. F.; Aaque, E.; Campuzano, S.; Pedrero, M.; Dalkiran, B.; Barderas, R.; Villalonga, R.; Kilic, E.; Pingarron, J. M. Dual functional graphene derivative-based electrochemical platforms for detection of the TP53 gene with single nucleotide polymorphism selectivity in biological samples. Anal. Chem. 2015, 87, 2290–2298.10.1021/ac504032dSearch in Google Scholar
Baker, B. R.; Lai, R. Y.; Wood, M. S.; Doctor, E. H.; Heeger, A. J.; Plaxco, K. W. An electronic, aptamer-based small-molecule sensor for the rapid, label-free detection of cocaine in adulterated samples and biological fluids. J. Am. Chem. Soc. 2006, 128, 3138–3139.10.1021/ja056957pSearch in Google Scholar
Baker, M. B.; Bao, G.; Searles, C. D. In vitro quantification of specific microRNA using molecular beacons. Nuc. Acids Res. 2012, 40, e13.10.1093/nar/gkr1016Search in Google Scholar
Barilero, T.; Saux, T. L.; Gosse, C.; Jullien, L. Fluorescent thermometers for dual-emission-wavelength measurements: molecular engineering and application to thermal imaging in a microsystem. Anal. Chem. 2009, 81, 7988–8000.10.1021/ac901027fSearch in Google Scholar
Beni, V.; Hayes, K.; Lerga, T. M.; O’Sullivan, C. K. Development of a gold nano-particle-based fluorescent molecular beacon for detection of cystic fibrosis associated mutation. Biosens. Bioelectron. 2010, 26, 307–313.10.1016/j.bios.2010.08.043Search in Google Scholar
Bhadra, S.; Ellington, A. D. Design and application of cotranscriptional non-enzymatic RNA circuits and signal transducers. Nuc. Acids Res. 2014, 42, e58.10.1093/nar/gku074Search in Google Scholar
Bi, S.; Luo, B. Y.; Ye, J. Y.; Wang, Z. H. Label-free chemiluminescent aptasensor for platelet-derived growth factor detection based on exonuclease-assisted cascade autocatalytic recycling amplification. Biosens. Bioelectron. 2014, 62, 208–213.10.1016/j.bios.2014.06.057Search in Google Scholar
Bonanni, A.; Pumera, M. Graphene platform for hairpin-DNA based impedimetric genosensing. ACS Nano2011, 5, 2356–2361.10.1021/nn200091pSearch in Google Scholar PubMed
Bratu, D. P.; Cha, B. J.; Mhlanga, M. M.; Kramer, F. R.; Tyagi, S. Visualizing the distribution and transport of mRNAs in living cells. Proc. Natl. Acad. Sci. USA2003, 100, 13308–13313.10.1073/pnas.2233244100Search in Google Scholar
Brunner, J.; Kraemer, R. Copper(II)-quenched oligonucleotide probes for fluorescent DNA sensing. J. Am. Chem. Soc. 2004, 126, 13626–13627.10.1021/ja047252aSearch in Google Scholar
Cai, Z. M.; Song, Y. L.; Wu, Y. F.; Zhu, Z.; Yang, C. J.; Chen, X. An electrochemical sensor based on label-free functional allosteric molecular beacons for detection target DNA/miRNA. Biosens. Bioelectron. 2013, 41, 783–788.10.1016/j.bios.2012.10.002Search in Google Scholar
Cai, Z. M.; Chen, Y. Y.; Li, C. S.; Wu, Y. F.; Yang, C. J.; Wang, Y. R.; Chen, X. A dual-signal amplification method for the DNA detection based on exonuclease III. Biosens. Bioelectron. 2014, 61, 370–373.10.1016/j.bios.2014.05.046Search in Google Scholar
Chen, A. K.; Behlke, M. A.; Tsourkas, A. Avoiding false-positive signals with nuclease-vulnerable molecular beacons in single living cells. Nuc. Acids Res. 2007, 35, e105.10.1093/nar/gkm593Search in Google Scholar
Chen, D.; Feng, H. B.; Li, J. H. Graphene oxide: preparation, functionalization, and electrochemical applications. Chem. Rev. 2012, 112, 6027–6053.10.1021/cr300115gSearch in Google Scholar
Chen, J. H.; Zhang, J.; Wang, K.; Huang, L. Y.; Lin, X. H.; Chen, G. N. Electrochemical biosensor based on hairpin DNA probe using 2-nitroacridone as electrochemical indicator for detection of DNA species related to chronic myelogenous leukemia. Electrochem. Commun. 2008, 10, 1448–1451.10.1016/j.elecom.2008.07.035Search in Google Scholar
Chen, F.; Zhao, Y. X.; Qi, L.; Fan, C. H. One-step highly sensitive florescence detection of T4 polynucleotide kinase activity and biological small molecules by ligation-nicking coupled reaction-mediated signal amplification. Biosens. Bioelectron. 2013a, 47, 218–224.10.1016/j.bios.2013.03.034Search in Google Scholar
Chen, J. H.; Zhou, X. M.; Zeng, L. W. Enzyme-free strip biosensor for amplified detection of Pb2+ based on a catalytic DNA circuit. Chem. Commun. 2013b, 49, 984–986.10.1039/C2CC37598BSearch in Google Scholar
Cheng, G. F.; Shen, B. J.; Zhang, F.; Wu, J. K.; Xu, Y.; He, P. G.; Fang, Y. Z. A new electrochemically active-inactive switching aptamer molecular beacon to detect thrombin directly in solution. Biosens. Bioelectron. 2010, 25, 2265–2269.10.1016/j.bios.2010.03.008Search in Google Scholar
Cheng, W.; Ding, S. J.; Li, Q.; Yu, T. X.; Yin, Y. B.; Ju, H. X.; Ren, G. S. A simple electrochemical aptasensor for ultrasensitive protein detection using cyclic target-induced primer extension. Biosens. Bioelectron. 2012, 36, 12–17.10.1016/j.bios.2012.03.032Search in Google Scholar
Chiang, C. K.; Huang, C. C.; Liu, C. W.; Chang, H. T. Oligonucleotide-based fluorescence probe for sensitive and selective detection of mercury(II) in aqueous solution. Anal. Chem. 2008, 80, 3716–3721.10.1021/ac800142kSearch in Google Scholar
Choi, J.; Love, K. R.; Gong, Y.; Gierahn, T. M.; Love, J. C. Immuno-hybridization chain reaction for enhancing detection of individual cytokine-secreting human peripheral mononuclear cells. Anal. Chem. 2011, 83, 6890–6895.10.1021/ac2013916Search in Google Scholar
Choi, H. M. T.; Beck, V. A.; Pierce, N. A. Next-generation in situ hybridization chain reaction: higher gain, lower cost, greater durability. ACS Nano2014, 8, 4284–4294.10.1021/nn405717pSearch in Google Scholar
Conlon, P.; Yang, C. J.; Wu, Y. R.; Chen, Y.; Martinez, K.; Kim, Y. M.; Stevens, N.; Marti, A. A.; Jockusch, S.; Turro, N. J. Pyrene excimer signaling molecular beacons for probing nucleic acids. J. Am. Chem. Soc. 2008, 130, 336–342.10.1021/ja076411ySearch in Google Scholar
Connolly, A. R.; Trau, M. Isothermal detection of DNA by beacon-assisted detection amplification. Angew. Chem. Int. Ed. 2010, 49, 2720–2723.10.1002/anie.200906992Search in Google Scholar
Cui, L.; Chen, Z. R.; Zhu, Z.; Lin, X. Y.; Chen, X.; Yang, C. J. Stabilization of ssRNA on graphene oxide surface: an effective way to design highly robust RNA probes. Anal. Chem. 2013, 85, 2269–2275.10.1021/ac303179zSearch in Google Scholar
Dai, S.; Xue, Q. W.; Zhu, J.; Ding, Y. S.; Jiang, W.; Wang, L. An ultrasensitive fluorescence assay for protein detection by hybridization chain reaction-based DNA nanotags. Biosens. Bioelectron. 2014, 51, 421–425.10.1016/j.bios.2013.07.068Search in Google Scholar
Deng, L.; Du, Y.; Xu, J. J.; Chen, H. Y. An off-on-off electrochemiluminescence approach for ultrasensitive detection of thrombin. Biosens. Bioelectron. 2014a, 59, 58–63.10.1016/j.bios.2014.03.012Search in Google Scholar
Deng, R. J.; Tang, L. H.; Tian, Q. Q.; Wang, Y.; Lin, L.; Li, J. H. Toehold-initiated rolling circle amplification for visualizing individual microRNAs in situ in single cells. Angew. Chem. Int. Ed. 2014b, 53, 2389–2393.10.1002/anie.201309388Search in Google Scholar
Ding, C. F.; Li, X. L.; Ge, L.; Zhang, S. S. Fluorescence detection of telomerase activity in cancer cells based on isothermal circular strand-displacement polymerization reaction. Anal. Chem. 2010, 82, 2850–2855.10.1021/ac902818wSearch in Google Scholar
Dirks, R. M.; Pierce, N. A. Triggered amplification by hybridization chain reaction. Proc. Natl. Acad. Sci. USA2004, 101, 15275–15278.10.1073/pnas.0407024101Search in Google Scholar PubMed PubMed Central
Dong, H. F.; Ding, L.; Yan, F.; Ji, H. X.; Ju, H. X. The use of polyethylenimine-grafted graphene nanoribbon for cellular delivery of locked nucleic acid modified molecular beacon for recognition of microRNA. Biomaterials2011, 32, 3875–3882.10.1016/j.biomaterials.2011.02.001Search in Google Scholar
Du, H.; Disney, M. D.; Miller, B. L.; Krauss, T. D. Hybridization-based unquenching of DNA hairpins on Au surfaces: prototypical “molecular beacon” biosensors. J. Am. Chem. Soc. 2003, 125, 4012–4013.10.1021/ja0290781Search in Google Scholar
Du, Y.; Lim, B. J.; Li, B. L.; Jiang, Y. S.; Sessler, J. L.; Ellington, A. D. Reagentless, ratiometric electrochemical DNA sensors with improved robustness and reproducibility. Anal. Chem. 2014, 86, 8010–8016.10.1021/ac5025254Search in Google Scholar
Duan, R. X.; Zuo, X. L.; Wang, S. T.; Quan, X. Y.; Chen, D. L.; Chen, Z. F.; Jiang, L.; Fan, C. H.; Xia, F. Lab in a tube: ultrasensitive detection of microRNAs at the single-cell level and in breast cancer patients using quadratic isothermal amplification. J. Am. Chem. Soc. 2013, 135, 4604–4607.10.1021/ja311313bSearch in Google Scholar
Dubertret, B.; Calame, M.; Libchaber, A. J. Single-mismatch detection using gold-quenched fluorescent oligonucleotides. Nat. Biotechnol. 2001, 19, 365–370.10.1038/86762Search in Google Scholar
Ebrahimi, S.; Akhlaghi, Y.; Kompany-Zareh, M.; Rinnan, A. Nucleic acid based fluorescent nanothermometers. ACS Nano2014, 8, 10372–10382.10.1021/nn5036944Search in Google Scholar
EI-Yazbi, A. F.; Loppnow, G. R. 2-Aminopurine hairpin probes for the detection of ultraviolet-induced DNA damage. Anal. Chim. Acta2012, 726, 44–49.10.1016/j.aca.2012.03.021Search in Google Scholar
Fan, C. H.; Plaxco, K. W.; Heeger, A. J. Electrochemical interrogation of conformational changes as a reagentless method for the sequence-specific detection of DNA. Proc. Natl. Acad. Sci. USA2003, 100, 9134–9137.10.1073/pnas.1633515100Search in Google Scholar
Farjami, E.; Clima, L.; Gothelf, K. V.; Ferapontova, E. E. DNA interactions with a methylene blue redox indicator depend on the DNA length and are sequence specific. Analyst2010, 135, 1443–1448.10.1039/c0an00049cSearch in Google Scholar
Farjami, E.; Clima, L.; Gothelf, K. V.; Ferapontova, E. E. “Off-on” electrochemical hairpin-DNA-based genosensor for cancer diagnostics. Anal. Chem. 2011, 83, 1594–1602.10.1021/ac1032929Search in Google Scholar
Feng, K. J.; Kong, R. M.; Wang, H.; Zhang, S. F.; Qu, F. L. A universal amplified strategy for aptasensors: enhancing sensitivity through allostery-triggered enzymatic recycling amplification. Biosens. Bioelectron. 2012, 38, 121–125.10.1016/j.bios.2012.05.008Search in Google Scholar
Fischbach, M.; Resch-Genger, U. R.; Seitz, O. Protease probes that enable excimer signaling upon scission. Angew. Chem. Int. Ed. 2014, 53, 11955–11959.10.1002/anie.201406909Search in Google Scholar
Freage, L.; Wang, F. A.; Orbach, R.; Willner, I. Multiplexed analysis of genes and of metal ions using enzyme/DNAzyme amplification machineries. Anal. Chem. 2014, 86, 11326–11333.10.1021/ac5030667Search in Google Scholar
Gao, Y.; Li, B. X. Exonuclease III-assisted cascade signal amplification strategy for label-free and ultrasensitive chemiluminescence detection of DNA. Anal. Chem. 2014, 86, 8881–8887.10.1021/ac5024952Search in Google Scholar
Gao, Y.; Qiao, G. M.; Zhuo, L. H.; Li, N.; Liu, Y.; Tang, B. A tumor mRNA-mediated bi-photosensitizer molecular beacon as an efficient imaging and photosensitizing agent. Chem. Commun. 2011, 47, 5316–5318.10.1039/c1cc10557dSearch in Google Scholar
Gao, F. L.; Lei, J. P.; Ju, H. X. Assistant DNA recycling with nicking endonuclease and molecular beacon for signal amplification using a target-complementary arched structure. Chem. Commun. 2013a, 49, 4006–4008.10.1039/c3cc40723cSearch in Google Scholar
Gao, F. L.; Lei, J. P.; Ju, H. X. Ultrasensitive fluorescence detection of bleomycin via exonuclease III-aided DNA recycling amplification. Chem. Commun. 2013b, 49, 7561–7563.10.1039/c3cc44275fSearch in Google Scholar
Gao, W. H.; Zhang, A.; Chen, Y. S.; Chen, Z. X.; Chen, Y. W., Lu, F. S.; Chen, Z. G. A novel probe density controllable electrochemiluminescence biosensor for ultra-sensitive detection of Hg2+ based on DNA hybridization optimization with gold nanoparticles array patterned self-assembly platform. Biosens. Bioelectron. 2013c, 49, 139–145.10.1016/j.bios.2013.05.013Search in Google Scholar
Ge, J.; Huang, Z. M.; Xi, Q.; Yu, R. Q.; Jiang, J. H.; Chu, X. A novel graphene oxide based fluorescent nanosensing strategy with hybridization chain reaction signal amplification for highly sensitive biothiol detection. Chem. Commun. 2014, 50, 11879–11882.10.1039/C4CC05309ESearch in Google Scholar
Goda, T.; Miyahara, Y. Thermo-responsive molecular switches for ATP using hairpin DNA aptamers. Biosens. Bioelectron. 2011, 26, 3949–3952.10.1016/j.bios.2011.02.041Search in Google Scholar
Goda, T.; Miyahara, Y. A hairpin DNA aptamer coupled with groove binders as a smart switch for a field-effect transistor biosensor. Biosens. Bioelectron. 2012, 32, 244–249.10.1016/j.bios.2011.12.022Search in Google Scholar
Guo, Q. P.; Yang, X. H.; Wang, K. M.; Tan, W. H.; Li, W.; Tang, H. X.; Li, H. M. Sensitive fluorescence detection of nucleic acids based on isothermal circular strand-displacement polymerization reaction. Nuc. Acids Res. 2009, 37, e20.10.1093/nar/gkn1024Search in Google Scholar
Guo, S. J.; Du, Y.; Yang, X.; Dong, S. J.; Wang, E. K. Solid-state label-free integrated aptasensor based on graphene-mesoporous silica-gold nanoparticle hybrids and silver microspheres. Anal. Chem. 2011, 83, 8035–8040.10.1021/ac2019552Search in Google Scholar
Hall, B.; Hesselberth, J. R.; Ellington, A. D. Computational selection of nucleic acid biosensors via a slip structure model. Biosens. Bioelectron. 2007, 22, 1939–1947.10.1016/j.bios.2006.08.019Search in Google Scholar
Harry, S. R.; Hicks, D. J.; Amiri, K. I.; Wright, D. W. Hairpin DNA coated gold nanoparticles as intracellular mRNA probes for the detection of tyrosinase gene expression in melanoma cells. Chem. Commun. 2010, 46, 5557–5559.10.1039/c001969kSearch in Google Scholar
He, H. Z.; Leung, K. H.; Wang, W.; Chan, D. S. H.; Leung, C. H.; Ma, D. L. Label-free luminescence switch-on detection of T4 polynucleotide kinase activity using a G-quadruplex-selective probe. Chem. Commun. 2014, 50, 5313–5315.10.1039/C3CC47444ESearch in Google Scholar
Heyduk, E.; Heyduk, T. Nucleic acid-based fluorescence sensors for detecting proteins. Anal. Chem. 2005, 77, 1147–1156.10.1021/ac0487449Search in Google Scholar
Holzhauser, C.; Wagenknecht, H. A. In-stem-labeled molecular beacons for distinct fluorescent color readout. Angew. Chem. Int. Ed. 2011, 50, 7268–7272.10.1002/anie.201101968Search in Google Scholar
Hou, P.; Long, Y. F.; Zhao, J.; Wang, J. X.; Zhou, F. M. A thymidine-terminated molecular beacon for selective Hg2+ or sequence-specific DNA assay. Spectrochim. Acta Pt. A2012, 86, 76–79.10.1016/j.saa.2011.10.005Search in Google Scholar
Hou, T.; Wang, X. Z.; Liu, X. J.; Lu, T. T.; Liu, S. F.; Li, F. Amplified detection of T4 polynucleotide kinase activity by the coupled λ exonuclease cleavage reaction and catalytic assembly of bimolecular beacons. Anal. Chem. 2014, 86, 884–890.10.1021/ac403458bSearch in Google Scholar
Huang, K. W.; Marti, A. A. Optimizing the sensitivity of photoluminescent probes using time-resolved spectroscopy: a molecular beacon case study. Anal. Chem. 2012, 84, 8075–8082.10.1021/ac3019894Search in Google Scholar
Huang, J.; Zhu, Z.; Bamrungsap, S.; Zhu, G. Z.; You, M. X.; He, X. X.; Wang, K. M.; Tan, W. H. Competition-mediated pyrene-switching aptasensor: probing lysozyme in human serum with a monomer-excimer fluorescence switch. Anal. Chem. 2010, 82, 10158–10163.10.1021/ac102277pSearch in Google Scholar
Huang, J.; Wu, Y. R.; Chen, Y.; Zhu, Z.; Yang, X. H.; Yang, C. J.; Wang, K. M.; Tan, W. H. Pyrene-excimer probes cased on the hybridization chain reaction for the detection of nucleic acids in complex biological fluids. Angew. Chem. Int. Ed. 2011, 50, 401–404.10.1002/anie.201005375Search in Google Scholar
Huang, J. H.; Su, X. F.; Li, Z. G. Enzyme-free and amplified fluorescence DNA detection using bimolecular beacons. Anal. Chem. 2012a, 84, 5939–5943.10.1021/ac3004727Search in Google Scholar
Huang, Y.; Zhao, S. L.; Chen, Z. F.; Shi, M.; Chen, J.; Liang, H. An amplified chemiluminescence aptasensor based on bi-resonance energy transfer on gold nanoparticles and exonuclease III-catalyzed target recycling. Chem. Commun. 2012b, 48, 11877–11879.10.1039/c2cc37130hSearch in Google Scholar
Huang, J. H.; Zheng, Q. B.; Kim, J. K.; Li, Z. G. A molecular beacon and graphene oxide-based fluorescent biosensor for Cu2+ detection. Biosens. Bioelectron. 2013, 45, 379–383.10.1016/j.bios.2012.12.056Search in Google Scholar
Huang, J.; Yang, X. H.; He, X. X.; Wang, K. M.; Liu, J. B.; Shi, H.; Wang, Q.; Guo, Q. P.; He, D. G. Design and bioanalytical applications of DNA hairpin-based fluorescent probes. Trends Analyt. Chem. 2014a, 53, 11–20.10.1016/j.trac.2013.08.007Search in Google Scholar
Huang, J. H.; Gao, X.; Jia, J. J.; Kim, J. K.; Li, Z. G. Graphene oxide-based amplified fluorescent biosensor for Hg2+ detection through hybridization chain reactions. Anal. Chem. 2014b, 86, 3209–3215.10.1021/ac500192rSearch in Google Scholar
Huang, J. H.; Su, X. F.; Li, Z. G. Enzyme- and label-free amplified fluorescence DNA detection usinghairpin probes and SYBR Green I. Sens. Actuat. B Chem. 2014c, 200, 117–122.10.1016/j.snb.2014.04.032Search in Google Scholar
Huang, J. H.; Ye, L.; Gao, X.; Li, H.; Xu, J. B.; Li, Z. G. Molybdenum disulfide-based amplified fluorescence DNA detection using hybridization chain reactions. J. Mater. Chem. B2015, 3, 2395–2401.10.1039/C4TB01986ESearch in Google Scholar
Hung, S. Y.; Tseng, W. H. A polyadenosine-coralyne complex as a novel fluorescent probe for the sensitive and selective detection of heparin in plasma. Biosens. Bioelectron. 2014, 57, 186–191.10.1016/j.bios.2014.02.010Search in Google Scholar
Hwang, G. T.; Seo, Y. J.; Kim, B. H. A highly discriminating quencher-free molecular beacon for probing DNA. J. Am. Chem. Soc. 2004, 126, 6528–6529.10.1021/ja049795qSearch in Google Scholar
Jayagopal, A.; Halfpenny, K. C.; Perez, J. W.; Wright, D. W. Hairpin DNA-functionalized gold colloids for the imaging of mRNA in live cells. J. Am. Chem. Soc. 2010, 132, 9789–9796.10.1021/ja102585vSearch in Google Scholar
Jiang, Y. S.; Li, B. L.; Milligan, J. N.; Bhadra, S.; Ellington, A. D. Real-time detection of isothermal amplification reactions with thermostable catalytic hairpin assembly. J. Am. Chem. Soc. 2013, 135, 7430–7433.10.1021/ja4023978Search in Google Scholar
Jiang, X. Q.; Guo, S. M.; Zhang, M.; Zhou, M.; Ye, B. C. DNA-hosted Hoechst dyes: application for label-free fluorescent monitoring of endonuclease activity and inhibition. Analyst2014a, 139, 5682–5685.10.1039/C4AN01373ESearch in Google Scholar
Jiang, Y. S.; Bhadra, S.; Li, B. L.; Ellington, A. D. Mismatches improve the performance of strand-displacement nucleic acid circuits. Angew. Chem. Int. Ed. 2014b, 53, 1845–1848.10.1002/anie.201307418Search in Google Scholar
Jiao, A. L.; Zheng, J.; Hu, Y. P.; Zhu, G. Z.; Li, J. S.; Li, H. M.; Yang, R. H.; Tan, W. H. Hybridization-triggered isothermal signal amplification coupled with MutS for label-free and sensitive fluorescent assay of SNPs. Chem. Commun. 2012, 48, 5659–5661.10.1039/c2cc30882gSearch in Google Scholar
Joshi, H. S.; Tor, Y. Metal-containing DNA hairpins as hybridization probes. Chem. Commun. 2001, 549–550.10.1039/b100036pSearch in Google Scholar
Kang, W. J.; Cho, Y. L.; Chae, J. R.; Lee, J. D.; Choi, K. J.; Kim, S. Molecular beacon-based bioimaging of multiple microRNAs during myogenesis. Biomaterials2011, 32, 1915–1922.10.1016/j.biomaterials.2010.11.007Search in Google Scholar
Ke, G. L.; Wang, C. M.; Ge, Y.; Zheng, N. F.; Zhu, Z.; Tang, C. J. L-DNA molecular beacon: a safe, stable, and accurate intracellular nano-thermometer for temperature sensing in living cells. J. Am. Chem. Soc. 2012, 134, 18908–18911.10.1021/ja3082439Search in Google Scholar
Kim, H.; Kane, M. D.; Kim, S.; Dominguez, W.; Applegate, B. M.; Savikhin, S. A molecular beacon DNA microarray system for rapid detection of E. coli O157:H7 that eliminates the risk of a false negative signal. Biosens. Bioelectron. 2007a, 22, 1041–1047.10.1016/j.bios.2006.04.032Search in Google Scholar
Kim, Y.; Yang, C. J.; Tan, W. H. Superior structure stability and selectivity of hairpin nucleic acid probes with an L-DNA stem. Nuc. Acids Res. 2007b, 35, 7279–7287.10.1093/nar/gkm771Search in Google Scholar
Kitamura, Y.; Yamamoto, S.; Osawa, Y.; Matsuura, H.; Ihara, T. Versatile allosteric molecular devices based on reversible formation of luminous lanthanide complexes. Chem. Commun. 2013, 49, 285–287.10.1039/C2CC36979FSearch in Google Scholar
Kjallman, T. H. M.; Peng, H.; Soeller, C.; Travas-Sejdic J. Effect of probe density and hybridization temperature on the response of an electrochemical hairpin-DNA sensor. Anal. Chem. 2008, 80, 9460–9466.10.1021/ac801567dSearch in Google Scholar
Kolpashchikov, D. M. A binary DNA probe for highly specific nucleic acid recognition. J. Am. Chem. Soc. 2006, 128, 10625–10628.10.1021/ja0628093Search in Google Scholar
Kong, R. M.; Zhang, X. B.; Chan, Z.; Meng, H. M.; Song, Z. L.; Tan, W. H.; Shen, G. L.; Yu, R. Q. Unimolecular catalytic DNA biosensor for amplified detection of L-histidine via an enzymatic recycling cleavage strategy. Anal. Chem. 2011, 83, 7603–7607.10.1021/ac2018926Search in Google Scholar
Krasnoperov, L. N.; Marras, S. A. E.; Kozlov, M.; Wirpsza, L.; Mustaev, A. Luminescent probes for ultrasensitive detection of nucleic acids. Bioconjug. Chem. 2010, 21, 319–327.10.1021/bc900403nSearch in Google Scholar
Kuo, C. Y.; Tseng, W. L. Adenosine-based molecular beacons as light-up probes for sensing heparin in plasma. Chem. Commun. 2013, 49, 4607–4609.10.1039/c3cc40976gSearch in Google Scholar
Larkey, N. E.; Almlie, C. K.; Tran, V.; Egan, M.; Burrows, S. M. Detection of miRNA using a double-strand displacement biosensor with a self-complementary fluorescent reporter. Anal. Chem. 2014, 86, 1853–1863.10.1021/ac403866gSearch in Google Scholar
Lee, K.; Povlich, L. K.; Kim, J. Label-free and self-signal amplifying molecular DNA sensors based on bioconjugated polyelectrolytes. Adv. Funct. Mater. 2007, 17, 2580–2587.10.1002/adfm.200700218Search in Google Scholar
Lee, K.; Rouillard, J. M.; Kim, B. G.; Gulari, E.; Kim, J. Conjugated polymers combined with a molecular beacon for label-free and self-signal-amplifying DNA microarrays. Adv. Funct. Mater. 2009, 19, 3317–3325.10.1002/adfm.200901175Search in Google Scholar
Li, N.; Ho, C. M. Aptamer-based optical probes with separated molecular recognition and signal transduction modules. J. Am. Chem. Soc. 2008, 130, 2380–2381.10.1021/ja076787bSearch in Google Scholar
Li, X. H.; Rothberg, L. J. Label-free colorimetric detection of specific sequences in genomic DNA amplified by the polymerase chain reaction. J. Am. Chem. Soc. 2004, 126, 10958–10961.10.1021/ja048749nSearch in Google Scholar
Li, J. J.; Geyer, R.; Tan, W. H. Using molecular beacons as a sensitive fluorescence assay for enzymatic cleavage of single-stranded DNA. Nuc. Acids Res. 2000, 28, e52.10.1093/nar/28.11.e52Search in Google Scholar
Li, J.; Yan, H. F.; Wang, K. M.; Tan, W. H.; Zhou, X. W. Hairpin fluorescence DNA probe for real-time monitoring of DNA methylation. Anal. Chem. 2007, 79, 1050–1056.10.1021/ac061694iSearch in Google Scholar
Li, J. J.; Chu, Y. Z.; Lee, B. Y. H.; Xie, X. S. Enzymatic signal amplification of molecular beacons for sensitive DNA detection. Nuc. Acids Res. 2008, 36, e36.10.1093/nar/gkn033Search in Google Scholar
Li, B. L.; Ellington, A. D.; Chen, X. Rational, modular adaptation of enzyme-free DNA circuits to multiple detection methods. Nuc. Acids Res. 2011a, 39, e110.10.1093/nar/gkr504Search in Google Scholar
Li, J. S.; Zhou, W. Y.; Ouyang, X. Y.; Yu, H.; Yang, R. H.; Tan, W. H.; Yuan, J. L. Design of a room-temperature phosphorescence-based molecular beacon for highly sensitive detection of nucleic acids in biological fluids. Anal. Chem. 2011b, 83, 1356–1362.10.1021/ac102710wSearch in Google Scholar
Li, Y. Q.; Guan, L. Y.; Wang, J. H.; Zhang, H. L.; Chen, J.; Lin, S.; Chen, W.; Zhao, Y. D. Simultaneous detection of dual single-base mutations by capillary electrophoresis using quantum dot-molecular beacon probe. Biosens. Bioelectron. 2011c, 26, 2317–2322.10.1016/j.bios.2010.10.002Search in Google Scholar
Li, B. L.; Jiang, Y.; Chen, X.; Ellington, A. D. Probing spatial organization of DNA strands using enzyme-free hairpin assembly circuits. J. Am. Chem. Soc. 2012, 134, 13918–13921.10.1021/ja300984bSearch in Google Scholar
Li, F.; Du, Z. F.; Yang, L. M.; Tang, B. Selective and sensitive turn-on detection of adenosine triphosphate and thrombin based on bifunctional fluorescent oligonucleotide probe. Biosens. Bioelectron. 2013a, 41, 907–990.10.1016/j.bios.2012.10.007Search in Google Scholar
Li, H.; Huang, X. X.; Kong, D. M.; Shen H. X.; Liu, Y. Ultrasensitive, high temperature and ionic strength variation-tolerant Cu2+ fluorescent sensor based on reconstructed Cu2+-dependent DNAzyme/substrate complex. Biosens. Bioelectron. 2013b, 42, 225–228.10.1016/j.bios.2012.10.070Search in Google Scholar
Li, Y.; Huang, C. C.; Zheng, J. B.; Qi, H. L.; Cao, W.; Wei, Y. M. Label-free electrogenerated chemiluminescence biosensing method for trace bleomycin detection based on a Ru(phen)2+-hairpin DNA composite film electrode. Biosens. Bioelectron. 2013c, 44, 177–182.10.1016/j.bios.2012.12.060Search in Google Scholar
Li, B.; Li, Z. L.; Situ, B.; Dai, Z.; Liu, Q. L.; Wang, Q.; Gu, D. Y.; Zheng, L. Sensitive HIV-1 detection in a homogeneous solution based on an electrochemical molecular beacon coupled with a Nafion-graphene composite film modified screen-printed carbon electrode. Biosens. Bioelectron. 2014a, 52, 330–336.10.1016/j.bios.2013.09.016Search in Google Scholar
Li, C.; Long, Y.; Liu, B.; Xiang, D.; Zhu, H. Z. Real time monitoring guracil excision using uracil-containing molecular beacons. Anal. Chim. Acta2014b, 819, 71–77.10.1016/j.aca.2014.02.002Search in Google Scholar PubMed
Li, C. X.; Li, Y. X.; Xu, X.; Wang, X. Y.; Chen, Y.; Yang, X. D.; Liu, F.; Li, N. Fast and quantitative differentiation of single-base mismatched DNA by initial reaction rate of catalytic hairpin assembly. Biosens. Bioelectron. 2014c, 60, 57–63.10.1016/j.bios.2014.04.007Search in Google Scholar
Li, H. L.; Ren, J. T.; Liu, Y. Q.; Wang, E. K. Application of DNA machine in amplified DNA detection. Chem. Commun. 2014d, 50, 704–706.10.1039/C3CC47147KSearch in Google Scholar
Lian, S.; Liu, C. H.; Zhang, X. B.; Wang, H. H.; Li, Z. P. Detection of T4 polynucleotide kinase activity based on cationic conjugated polymer-mediated fluorescence resonance energy transfer. Biosens. Bioelectron. 2015, 66, 316–320.10.1016/j.bios.2014.11.045Search in Google Scholar
Liang, Y.; Zhang, Z. P.; Wei, H. P.; Hu, Q. X.; Deng, J. Y.; Guo, D. Y.; Cui, Z. Q.; Zhang, X. E. Aptamer beacons for visualization of endogenous protein HIV-1 reverse transcriptase in living cells. Biosens. Bioelectron. 2011, 28, 270–276.10.1016/j.bios.2011.07.031Search in Google Scholar
Lin, J. H.; Tseng, W. L. Design of two and three input molecular logic gates using non-Watson-Crick base pairing-based molecular beacons. Analyst2014, 139, 1436–1441.10.1039/C3AN02298FSearch in Google Scholar
Lin, Z. Z.; Chen, Y.; Li, X. H.; Fang, W. H. Pb2+ induced DNA conformational switch from hairpin to G-quadruplex: electrochemical detection of Pb2+. Analyst2011, 136, 2367–2372.10.1039/c1an15080dSearch in Google Scholar
Liu, J. W.; Lu, Y. Fast colorimetric sensing of adenosine and cocaine based on a general sensor design involving aptamers and nanoparticles. Angew. Chem. Int. Ed. 2006, 45, 90–94.10.1002/anie.200502589Search in Google Scholar
Liu, L. F.; Tang, Z. W.; Wang, K. M.; Tan, W. H.; Li, J.; Guo, Q. P.; Meng, X. X.; Ma, C. B. Using molecular beacon to monitor activity of E. coli DNA ligase. Analyst2005, 130, 350–357.10.1039/b413959cSearch in Google Scholar
Liu, G.; Wan, Y.; Gau, V.; Zhang, J.; Wang, L. H.; Song, S. P.; Fan, C. H. An enzyme-based E-DNA sensor for sequence-specific detection of femtomolar DNA targets. J. Am. Chem. Soc. 2008, 130, 6820–6825.10.1021/ja800554tSearch in Google Scholar
Liu, L. Z.; Li, H.; Qiu, T.; Zhou, G. H.; Wong, K. Y.; He, Z. K.; Liu, Z. H. Construction of a molecular beacon based on two-photon excited fluorescence resonance energy transfer with quantum dot as donor. Chem. Commun. 2011, 47, 2622–2624.10.1039/c0cc04712kSearch in Google Scholar
Liu, B.; Xiang, D.; Long, Y.; Tong, C. Y. Real time monitoring of junction ribonuclease activity of RNase H using chimeric molecular beacons. Analyst2013a, 138, 3238–3245.10.1039/c3an36414cSearch in Google Scholar PubMed
Liu, J.; Chen, L. B.; Lie, P. C.; Dun, B. Y.; Zeng, L. W. A universal biosensor for multiplex DNA detection based on hairpin probe assisted cascade signal amplification. Chem. Commun. 2013b, 49, 5165–5167.10.1039/c3cc41941jSearch in Google Scholar
Liu, P.; Yang, X. H.; Sun, S.; Wang, Q.; Wang, K. M.; Huang, J.; Liu, J. B.; He, L. L. Enzyme-free colorimetric detection of DNA by using gold nanoparticles and hybridization chain reaction amplification. Anal. Chem. 2013c, 85, 7689–7695.10.1021/ac4001157Search in Google Scholar
Liu, S. F.; Wang, Y.; Ming, J. J.; Lin, Y.; Cheng, C. B.; Li, F. Enzyme-free and ultrasensitive electrochemical detection of nucleic acids by target catalyzed hairpin assembly followed with hybridization chain reaction. Biosens. Bioelectron. 2013d, 49, 472–477.10.1016/j.bios.2013.05.037Search in Google Scholar
Liu, S. F.; Wang, Y.; Zhang, C. X.; Lin, Y.; Li, F. Homogeneous electrochemical aptamer-based ATP assay with signal amplification by exonuclease III assisted target recycling. Chem. Commun. 2013e, 49, 2335–2337.10.1039/c3cc39082aSearch in Google Scholar
Liu, S. F.; Zhang, C. X.; Ming, J. J.; Wang, C. F.; Liu, T.; Li, F. Amplified detection of DNA by an analyte-induced Y-shaped junction probe assembly followed with a nicking endonuclease-mediated autocatalytic recycling process. Chem. Commun. 2013f, 49, 7947–7949.10.1039/c3cc45211eSearch in Google Scholar
Liu, G. P.; Yuan, Y. L.; Wei, S. Q.; Zhang, D. J. Impedimetric DNA-based biosensor for silver ions detection with hemin/G-quadruplex nanowire as enhancer. Electroanalysis2014, 26, 27322738.10.1002/elan.201400439Search in Google Scholar
Liu, S. F.; Cheng, C. B.; Liu, T.; Wang, L.; Gong, H. W.; Li, F. Highly sensitive fluorescence detection of target DNA by coupling exonuclease-assisted cascade target recycling and DNAzyme amplification. Biosens. Bioelectron. 2015a, 63, 99–104.10.1016/j.bios.2014.07.023Search in Google Scholar
Liu, S. F.; Liu, T.; Wang, L. Label-free, isothermal and ultrasensitive electrochemical detection of DNA and DNA 3′-phosphatase using a cascade enzymatic cleavage strategy. Chem. Commun. 2015b, 51, 176–179.10.1039/C4CC08140DSearch in Google Scholar
Lovell, J. F.; Chen, J.; Huynh, E.; Jarvi, M. T.; Wilson, B. C.; Zheng, G. Facile synthesis of advanced photodynamic molecular beacon architectures. Bioconjugate Chem. 2010, 21, 1023–1025.10.1021/bc100178zSearch in Google Scholar
Lu, C. H.; Li, J.; Liu, J. J.; Yang, H. H.; Chen, X.; Chen, G. N. Increasing the sensitivity and single-base mismatch selectivity of the molecular beacon using graphene oxide as the “nanoquencher”. Chem. Eur. J. 2010a, 16, 4889–4894.10.1002/chem.200903071Search in Google Scholar
Lu, C. H.; Zhu, C. L.; Li, J.; Liu, J. J.; Chen, X.; Yang, H. H. Using graphene to protect DNA from cleavage during cellular delivery. Chem. Commun. 2010b, 46, 3116–3118.10.1039/b926893fSearch in Google Scholar
Luan, Q. F.; Xue, Y.; Yao, X.; Lu, W. Hairpin DNA probe based surface plasmon resonance biosensor used for the activity assay of E. coli DNA ligase. Analyst2010, 135, 414–418.10.1039/B920228ESearch in Google Scholar
Lubin, A. A.; Plaxco, K. W. Folding-based electrochemical biosensors: the case for responsive nucleic acid architectures. Acc. Chem. Res. 2010, 43, 496–505.10.1021/ar900165xSearch in Google Scholar
Ma, C. B.; Tang, Z. W.; Wang, K. M.; Tan, W. H.; Li, J.; Li, W.; Li, Z. H.; Yang, X. H.; Li, H. M.; Liu, L. F. Real-time monitoring of DNA polymerase activity using molecular beacon. Anal. Biochem. 2006, 353, 141–143.10.1016/j.ab.2006.02.006Search in Google Scholar
Ma, C. B.; Tang, Z. W.; Wang, K. M.; Tan, W. H.; Yang, X. H.; Li, W.; Li, Z. H.; Li, H. M.; Lv, X. Y. Real-time monitoring of nucleic acid dephosphorylation by using molecular beacons. Chem. BioChem. 2007a, 8, 1487–1490.10.1002/cbic.200700171Search in Google Scholar
Ma, C. B.; Tang, Z. W.; Wang, K. M.; Tan, W. H.; Yang, X. H.; Li, W.; Li, Z. H.; Lv, X. Y. Real-time monitoring of restriction endonuclease activity using molecular beacon. Anal. Biochem. 2007b, 363, 294–296.10.1016/j.ab.2007.01.018Search in Google Scholar
Ma, C. B.; Chen, H. C.; Han, R.; He, H. L.; Zeng, W. M. Fluorescence detection of adenosine triphosphate using smart probe. Anal. Biochem. 2012, 429, 8–10.10.1016/j.ab.2012.06.022Search in Google Scholar
Ma, C. B.; Tang, Z. W.; Wang, K. M.; Yang, X. H.; Tan, W. H. A novel sensitive and selective ligation-based ATP assay using a molecular beacon. Analyst2013a, 138, 3013–3017.10.1039/c3an00161jSearch in Google Scholar PubMed
Ma, D. L.; He, H. Z.; Leung, K. H.; Zhong, H. J.; Chan, D. S.; Leung, C. H. Label-free luminescent oligonucleotide-based probes. Chem. Soc. Rev. 2013b, 42, 3427–3440.10.1039/c2cs35472aSearch in Google Scholar
Ma, C. P.; Han, D. A.; Shi, C. A new isothermal nucleic acid detection strategy mediated by a double-nicked beacon. Chem. Commun. 2014a, 50, 3799–3801.10.1039/c3cc49841gSearch in Google Scholar
Ma, C. P.; Liu, S.; Shi, C. Ultrasensitive detection of microRNAs based on hairpin fluorescence probe assisted isothermal amplification. Biosens. Bioelectron. 2014b, 58, 57–60.10.1016/j.bios.2014.02.034Search in Google Scholar
Machado, I.; Ozalp, V. C.; Rezabal, E.; Schafer, T. DNA aptamers are functional molecular recognition sensors in protic ionic liquids. Chem. Eur. J. 2014, 20, 11820–11825.10.1002/chem.201403354Search in Google Scholar
Marme, N.; Friedrich, A.; Muller, M.; Nolte, O.; Wolfrum, J.; Hoheisel, J. D.; Sauer, M.; Knemeyer, J. P. Identification of single-point mutations in mycobacterial 16S rRNA sequences by confocal single-molecule fluorescence spectroscopy. Nuc. Acids Res. 2006, 34, e90.10.1093/nar/gkl495Search in Google Scholar
Meneghello, A.; Antognoli, A.; Sonato, A.; Zacco, G.; Ruffato, G.; Cretaio, E.; Romanato, F. Label-free efficient and accurate detection of cystic fibrosis causing mutations using an azimuthally rotated GC-SPR platform. Anal. Chem. 2014, 86, 11773–11781.10.1021/ac503272ySearch in Google Scholar
Meng, H. M.; Fu, T.; Zhang, X. B.; Wang, N. N.; Tan, W. H.; Shen, G. L.; Yu, R. Q. Efficient fluorescence turn-on probe for zirconium via a target-triggered DNA molecular beacon strategy. Anal. Chem. 2012, 84, 2124–2128.10.1021/ac300005fSearch in Google Scholar
Nesterova, I. V.; Erdem, S. S.; Pakhomov, S.; Hammer, R. P.; Soper, S. A. Phthalocyanine dimerization-based molecular beacons using near-IR fluorescence. J. Am. Chem. Soc. 2009, 131, 2432–2433.10.1021/ja8088247Search in Google Scholar
Nie, J.; Zhang, D. W.; Tie, C.; Zhou, Y. L.; Zhang, X. X. A label-free DNA hairpin biosensor for colorimetric detection of target with suitable functional DNA partners. Biosens. Bioelectron. 2013, 49, 236–242.10.1016/j.bios.2013.05.020Search in Google Scholar
Nielsen, L. J.; Olsen, L. F.; Ozalp, V. C. Aptamers embedded in polyacrylamide nanoparticles: a tool for in vivo metabolite sensing. ACS Nano. 2010, 4, 4361–4370.10.1021/nn100635jSearch in Google Scholar
Niu, Y. J.; Zhao, Y. J.; Fan, A. P. Conformational switching immobilized hairpin DNA probes following subsequent expanding of gold nanoparticles enables visual detecting sequence-specific DNA. Anal. Chem. 2011, 83, 7500–7506.10.1021/ac201755xSearch in Google Scholar
Niu, S. Y.; Qu, L. J.; Zhang, Q.; Lin, J. H. Fluorescence detection of thrombin using autocatalytic strand displacement cycle reaction and a dual-aptamer DNA sandwich assay. Anal. Biochem. 2012, 421, 362–367.10.1016/j.ab.2011.10.001Search in Google Scholar
Ono, A.; Togashi, H. Highly selective oligonucleotide-based sensor for mercury(II) in aqueous solutions. Angew. Chem. Int. Ed. 2004, 43, 4300–4302.10.1002/anie.200454172Search in Google Scholar
Ota, N.; Hirano, K.; Warashina, M.; Andrus, A.; Mullah, B.; Hatanaka, K.; Taira, K. Determination of interactions between structured nucleic acids by fluorescence resonance energy transfer (FRET): selection of target sites for functional nucleic acids. Nuc. Acids Res. 1998, 26, 735–743.10.1093/nar/26.3.735Search in Google Scholar
Pang, Y. F.; Wang, J. F.; Xiao, R.; Wang, S. Q. SERS molecular sentinel for the RNA genetic marker of PB1-F2 protein in highly pathogenic avian influenza (HPAI) virus. Biosens. Bioelectron. 2014, 61, 460–465.10.1016/j.bios.2014.04.018Search in Google Scholar
Papadakis, G.; Tsortos, A.; Bender, F.; Ferapontova, E. E.; Gizeli, E. Direct detection of DNA conformation in hybridization processes. Anal. Chem. 2012, 84, 1854–1861.10.1021/ac202515pSearch in Google Scholar
Park, K. S.; Park, H. G. A DNA-templated silver nanocluster probe for label-free, turn-on fluorescence-based screening of homo-adenine binding molecules. Biosens. Bioelectron. 2015, 64, 618–624.10.1016/j.bios.2014.10.003Search in Google Scholar
Peng, H. I.; Strohsahl, C. M.; Leach, K. E.; Krauss, T. D.; Miller, B. L. Label-free DNA detection on nanostructured Ag surfaces. ACS Nano2009, 3, 2265–2273.10.1021/nn900112eSearch in Google Scholar
Peng, Z. Y.; Soper, S. A.; Pingle, M. R.; Barany, F.; Davis, L. M. Ligase detection reaction generation of reverse molecular beacons for near real-time analysis of bacterial pathogens using single-pair fluorescence resonance energy transfer and a cyclic olefin copolymer microfluidic chip. Anal. Chem. 2010, 82, 9727–9735.10.1021/ac101843nSearch in Google Scholar
Peng, H. I.; Strohsahl, C. M.; Krauss, T. D.; Miller, B. L. Microfluidic nanoplasmonic-enabled device for multiplex DNA detection. Lab Chip2012, 12, 1089–1093.10.1039/c2lc21114aSearch in Google Scholar
Piestert, O.; Barsch, H.; Buschmann, V.; Heinlein, T.; Knemeyer, J. P.; Weston, K. D.; Sauer, M. A single-molecule sensitive DNA hairpin system based on intramolecular electron transfer. Nano Lett. 2003, 3, 979–982.10.1021/nl0341988Search in Google Scholar
Østergaard, M. E.; Cheguru, P.; Papasani, M. R.; Hill, R. A.; Hrdlicka, P. J. Glowing locked nucleic acids: brightly fluorescent probes for detection of nucleic acids in cells. J. Am. Chem. Soc. 2010, 132, 14221–14228.10.1021/ja1057295Search in Google Scholar
Qian, R. C.; Ding, L.; Yan, L. W.; Lin, M. F.; Ju, H. X. Smart vesicle kit for in situ monitoring of intracellular telomerase activity using a telomerase-responsive probe. Anal. Chem. 2014, 86, 8642–8648.10.1021/ac502538wSearch in Google Scholar
Qian, Y.; Tang, D. Q.; Du, L. L.; Zhang, Y. Z.; Zhang, L. X.; Gao, F. L. A novel signal-on electrochemical DNA sensor based on target catalyzed hairpin assembly strategy. Biosens. Bioelectron. 2015, 64, 177–181.10.1016/j.bios.2014.09.001Search in Google Scholar
Qiu, L. P.; Wu, Z. S.; Shen, G. L.; Yu, R. Q. Highly sensitive and selective bifunctional oligonucleotide probe for homogeneous parallel fluorescence detection of protein and nucleotide sequence. Anal. Chem. 2011, 83, 3050–3057.10.1021/ac103274jSearch in Google Scholar
Quan, K.; Huang, J.; Yang, X. H.; Yang, Y. J.; Ying, L.; Wang, H.; He, Y.; Wang, K. M. An enzyme-free and amplified colorimetric detection strategy via target-aptamer binding triggered catalyzed hairpin assembly. Chem. Commun. 2015, 51, 937–940.10.1039/C4CC08488HSearch in Google Scholar
Radi, A.; Sanchez, J. L. A.; Baldrich, E.; O’Sullivan, C. K. Reagentless, reusable, ultrasensitive electrochemical molecular beacon aptasensor. J. Am. Chem. Soc. 2006, 128, 117–124.10.1021/ja053121dSearch in Google Scholar
Rajendran, M.; Ellington, A. D. Selection of fluorescent aptamer beacons that light up in the presence of zinc. Anal. Bioanal. Chem. 2008, 390, 1067–1075.10.1007/s00216-007-1735-8Search in Google Scholar
Ren, K. W.; Wu, J.; Yan, F.; Ju, H. X. Ratiometric electrochemical proximity assay for sensitive one-step protein detection. Sci. Rep. 2014, 4, 4360.10.1038/srep04360Search in Google Scholar
Ren, W.; Gao, Z. F.; Li, N. B.; Luo, H. Q. Ultrasensitive and selective signal-on electrochemical DNA detection via exonuclease III catalysis and hybridization chain reaction amplification. Biosens. Bioelectron. 2015, 63, 153–158.10.1016/j.bios.2014.07.028Search in Google Scholar
Rosa, J.; Conde, J.; Fuente, J. M.; Lima, J. C.; Baptista, P. V. Gold-nanobeacons for real-time monitoring of RNA synthesis. Biosens. Bioelectron. 2012, 36, 161–167.10.1016/j.bios.2012.04.006Search in Google Scholar
Sato, Y.; Nishizawa, S.; Teramae, N. Label-free molecular beacon system based on DNAs containing abasic sites and fluorescent ligands that bind abasic sites. Chem. Eur. J. 2011, 17, 11650–11656.10.1002/chem.201100384Search in Google Scholar
Sharon, E.; Freeman, R.; Willner, I. CdSe/ZnS quantum dots-G-quadruplex/hemin hybrids as optical DNA sensors and aptasensors. Anal. Chem. 2010, 82, 7073–7077.10.1021/ac101456xSearch in Google Scholar
Shimron, S.; Wang, F. A.; Orbach, R.; Willner, I. Amplified detection of DNA through the enzyme-free autonomous assembly of hemin/G-quadruplex DNAzyme nanowires. Anal. Chem. 2012, 84, 1042–1048.10.1021/ac202643ySearch in Google Scholar
Shlyahovsky, B.; Li, D.; Weizmann, Y.; Nowarski, R.; Kotler, M.; Willner, I. Spotlighting of cocaine by an autonomous aptamer-based machine. J. Am. Chem. Soc. 2007, 129, 3814–3815.10.1021/ja069291nSearch in Google Scholar
Socher, E.; Bethge, L.; Knoll, A.; Jungnick, N.; Herrmann, A.; Seitz, O. Low-noise stemless PNA beacons for sensitive DNA and RNA detection. Angew. Chem. Int. Ed. 2008, 47, 9555–9559.10.1002/anie.200803549Search in Google Scholar
Song, C.; Zhao, M. P. Real-time monitoring of the activity and kinetics of T4 polynucleotide kinase by a singly labeled DNA-hairpin smart probe coupled with λ exonuclease cleavage. Anal. Chem. 2009, 81, 1383–1388.10.1021/ac802107wSearch in Google Scholar
Song, C.; Zhang, C.; Zhao, M. P. Singly labeled smart probes for real-time monitoring of the kinetics of dNTP misincorporation and single nucleotide extension in DNA intra-molecular polymerization. Biosens. Bioelectron. 2009, 25, 301–305.10.1016/j.bios.2009.07.002Search in Google Scholar
Song, C.; Zhang, C.; Zhao, M. P. Development of a high-throughput screening platform for DNA 3′-phosphatases and their inhibitors based on a universal molecular beacon and quantitative real-time PCR. Chem. Asian J. 2010, 5, 1146–1151.10.1002/asia.200900661Search in Google Scholar
Song, P. S.; Xiang, Y.; Xing, H.; Zhou, Z. J.; Tong, A. J.; Lu, Y. Label-free catalytic and molecular beacon containing an abasic site for sensitive fluorescent detection of small inorganic and organic molecules. Anal. Chem. 2012, 84, 2916–2922.10.1021/ac203488pSearch in Google Scholar
Song, C.; Xie, G. M.; Wang, L.; Liu, L. Z.; Tian, G.; Xiang, H. DNA-based hybridization chain reaction for an ultrasensitive cancer marker EBNA-1 electrochemical immunosensor. Biosens. Bioelectron. 2014, 58, 68–74.10.1016/j.bios.2014.02.031Search in Google Scholar
Song, W. L.; Zhang, Q.; Sun, W. B. Ultrasensitive detection of nucleic acids by template enhanced hybridization followed by rolling circle amplification and catalytic hairpin assembly. Chem. Commun. 2015, 51, 2392–2395.10.1039/C4CC09453KSearch in Google Scholar
Sternberg, J. B.; Pierce, N. A. Exquisite sequence selectivity with small conditional RNAs. Nano Lett. 2014, 14, 4568–4572.10.1021/nl501593rSearch in Google Scholar
Stobiecka, M.; Molinero, A. A.; Chalupa, A.; Hepel, M. Mercury/homocysteine ligation-induced ON/OFF-switching of a T-T mismatch-based oligonucleotide molecular beacon. Anal. Chem. 2012, 84, 4970–4978.10.1021/ac300632uSearch in Google Scholar
Stohr, K.; Hafner, B.; Nolte, O.; Wolfrum, J.; Sauer, M.; Herten, D. P. Species-specific identification of mycobacterial 16S rRNA PCR amplicons using smart probes. Anal. Chem. 2005, 77, 7195–7203.10.1021/ac051447zSearch in Google Scholar
Su, X.; Zhu, X. C.; Zhang, C.; Xiao, X. J.; Zhao, M. P. In situ, real-time monitoring of the 3′ to 5′ exonucleases secreted by living cells. Anal. Chem. 2012, 84, 5059–5065.10.1021/ac300745fSearch in Google Scholar
Su, H. Y.; Meng, X. C.; Guo, Q. P.; Tan, Y. J.; Cai, Q. Y.; Qin, H. T.; Meng, X. X. Label-free DNA sensor with PCR-like sensitivity based on background reduction and target-triggered polymerization amplification. Biosens. Bioelectron. 2014, 52, 417–421.10.1016/j.bios.2013.09.007Search in Google Scholar
Su, Y. C.; Wang, C. H.; Chang, W. H.; Chen, T. Y.; Lee, G. B. Rapid and amplification-free detection of fish pathogens by utilizing a molecular beacon-based microfluidic system. Biosens. Bioelectron. 2015, 63, 196–203.10.1016/j.bios.2014.07.035Search in Google Scholar
Sun, Y. Z.; Fan, X. D. Distinguishing DNA by analog-to-digital-like conversion by using optofluidic lasers. Angew. Chem. Int. Ed. 2012, 51, 1236–1239.10.1002/anie.201107381Search in Google Scholar
Tan, X. H.; Wang, Y.; Armitage, B. A.; Bruchez, M. P. Label-free molecular beacons for biomolecular detection. Anal. Chem. 2014, 86, 10864–10869.10.1021/ac502986gSearch in Google Scholar
Tang, Z. W.; Wang, K. M.; Tan, W. H.; Li, J.; Liu, L. F.; Guo, Q. P.; Meng, X. X.; Ma, C. B.; Huang, S. S. Real-time monitoring of nucleic acid ligation in homogenous solutions using molecular beacons. Nuc. Acids Res. 2003, 31, e148.10.1093/nar/gng146Search in Google Scholar
Tang, Z. W.; Wang, K. M.; Tan, W. H.; Ma, C. B.; Li, J.; Liu, L. F.; Guo, Q. P.; Meng, X. X. Real-time investigation of nucleic acids phosphorylation process using molecular beacons. Nuc. Acids Res. 2005, 33, e97.10.1093/nar/gni096Search in Google Scholar
Tang, Z. W.; Mallikaratchy, P.; Yang, R. H.; Kim, Y.; Zhu, Z.; Wang, H.; Tan, W. H. Aptamer switch probe based on intramolecular displacement. J. Am. Chem. Soc. 2008, 130, 11268–11269.10.1021/ja804119sSearch in Google Scholar
Tang, Z. W.; Zhu, Z.; Mallikaratchy, P.; Yang, R. H.; Sefah, K.; Tan, W. H. Aptamer-target binding triggered molecular mediation of singlet oxygen generation. Chem. Asian J.2010, 5, 783–786.10.1002/asia.200900545Search in Google Scholar
Tang, L.H.; Wang, Y.; Liu, Y.; Li, J.H. DNA-directed self-assembly of graphene oxide with applications to ultrasensitive oligonucleotide assay. ACS Nano2011a, 5, 3817–3822.10.1021/nn200147nSearch in Google Scholar PubMed
Tang, Z. W.; Liu, P.; Ma, C. B.; Yang, X. H.; Wang, K. M.; Tan, W. H.; Lv, X. Y. Molecular beacon based bioassay for highly sensitive and selective detection of nicotinamide adenine dinucleotide and the activity of alanine aminotransferase. Anal. Chem. 2011b, 83, 2505–2510.10.1021/ac102742kSearch in Google Scholar
Tang, L. H.; Chang, H. X.; Liu, Y.; Li, J. H. Duplex DNA/graphene oxide biointerface: from fundamental understanding to specific enzymatic effects. Adv. Funct. Mater. 2012a, 22, 3083–3088.10.1002/adfm.201102892Search in Google Scholar
Tang, L. H.; Liu, Y.; Ali, M. M.; Kang, D. K.; Zhao, W. A.; Li, J. H. Colorimetric and ultrasensitive bioassay based on a dual-amplification system using aptamer and DNAzyme. Anal. Chem. 2012b, 84, 4711–4717.10.1021/ac203274kSearch in Google Scholar
Tao, C. Y.; Yan, Y. R.; Xiang, H.; Zhu, D.; Cheng, W.; Ju, H. X.; Ding, S. J. A new mode for highly sensitive and specific detection of DNA based on exonuclease III-assisted target recycling amplification and mismatched catalytic hairpin assembly. Chem. Commun. 2015, 51, 4220–4222.10.1039/C5CC00385GSearch in Google Scholar
Tian, T.; Xiao, H.; Zhang, X. L.; Peng, S.; Zhang, X. E.; Guo, S.; Wang, S. R.; Liu, S. M.; Zhou, X.; Meyers, C.; Zhou, X. Simultaneously sensitive detection of multiple miRNAs based on a strand displacement amplification. Chem. Commun. 2013, 49, 75–77.10.1039/C2CC36728ASearch in Google Scholar
Tok, J.; Lai, J.; Leung, T.; Li, S. F. Y. Molecular aptamer beacon for myotonic dystrophy kinase-related Cdc42-binding kinase α. Talanta2010, 81, 732–736.10.1016/j.talanta.2010.01.013Search in Google Scholar
Tyagi, S.; Kramer, F. R. Molecular beacons: probes that fluoresce upon hybridization. Nat. Biotechnol. 1996, 14, 303–308.10.1038/nbt0396-303Search in Google Scholar
Tyagi, S.; Bratu, D. P.; Kramer, F. R. Multicolor molecular beacons for allele discrimination. Nat. Biotechnol. 1998, 16, 49–53.10.1038/nbt0198-49Search in Google Scholar
Tyagi, S.; Marras, S. A. E.; Kramer, F. R. Wavelength-shifting molecular beacons. Nat. Biotechnol. 2000, 18, 1191–1196.10.1038/81192Search in Google Scholar
Vallee-Belisle, A.; Bonham, A. J.; Reich, N. O.; Ricci, F.; Plaxco, K.W. Transcription factor beacons for the quantitative detection of DNA binding activity. J. Am. Chem. Soc. 2011, 133, 13836–13839.10.1021/ja204775kSearch in Google Scholar
Wabuyele, M. B.; Vo-Dinh, T. Detection of human immunodeficiency virus type 1 DNA sequence using plasmonics nanoprobes. Anal. Chem. 2005, 77, 7810–7815.10.1021/ac0514671Search in Google Scholar
Wabuyele, M. B.; Farquar, H.; Stryjewski, W.; Hammer, R. P.; Soper, S. A.; Cheng, Y. W.; Barany, F. Approaching real-time molecular diagnostics: single-pair fluorescence resonance energy transfer (spFRET) detection for the analysis of low abundant point mutations in K-ras oncogenes. J. Am. Chem. Soc. 2003, 125, 6937–6945.10.1021/ja034716gSearch in Google Scholar
Wang, L.; Yang, C. J.; Medley, C. D.; Benner, S. A.; Tan, W. H. Locked nucleic acid molecular beacons. J. Am. Chem. Soc. 2005, 127, 15664–15665.10.1021/ja052498gSearch in Google Scholar
Wang, H.; Kim, Y.; Liu, H. P.; Zhu, Z.; Bamrungsap, S.; Tan, W.H. Engineering a unimolecular DNA-catalytic probe for single lead ion monitoring. J. Am. Chem. Soc. 2009a, 131, 8221–8226.10.1021/ja901132ySearch in Google Scholar
Wang, K. M.; Tang, Z. W.; Yang, C. J.; Kim, Y.; Fang, X. H.; Li, W.; Wu, Y. R.; Meddley, C. D.; Cao, Z. H.; Li, J.; Colon, P.; Lin, H.; Tan, W. H. Molecular engineering of DNA: molecular beacons. Angew. Chem. Int. Ed. 2009b, 48, 856–870.10.1002/anie.200800370Search in Google Scholar
Wang, X. Y.; Dong, P.; Yun, W.; Xu, Y.; He, P. G.; Fang, Y. Z. A solid-state electrochemiluminescence biosensing switch for detection of thrombin based on ferrocene-labeled molecular beacon aptamer. Biosens. Bioelectron. 2009c, 24, 3288–3292.10.1016/j.bios.2009.04.019Search in Google Scholar
Wang, Y.; Li, Z. H.; Hu, D. H.; Lin, C. T.; Li, J. H.; Lin, Y. H. Aptamer/graphene oxide nanocomplex for in situ molecular probing in living cells. J. Am. Chem. Soc. 2010a, 132, 9274–9276.10.1021/ja103169vSearch in Google Scholar
Wang, Y. X.; Li, J. S.; Wang, H.; Jin, J. Y.; Liu, J. H.; Wang, K. M.; Tan, W. H.; Yang, R. H. Silver ions-mediated conformational switch: facile design of structure-controllable nucleic acid probes. Anal. Chem. 2010b, 82, 6607–6612.10.1021/ac101114wSearch in Google Scholar
Wang, J.; Onoshima, D.; Aki, M.; Okamoto, Y.; Kaji, N.; Tokeshi, M.; Baba, Y. Label-free detection of DNA-binding proteins based on microfluidic solid-state molecular beacon sensor. Anal. Chem. 2011a, 83, 3528–3532.10.1021/ac200236rSearch in Google Scholar
Wang, Y.; Li, Z. H.; Wang, J.; Li, J. H.; Lin, Y. H. Graphene and graphene oxide: biofunctionalization and applications in biotechnology. Trends Biotechnol. 2011b, 29, 205–212.10.1016/j.tibtech.2011.01.008Search in Google Scholar
Wang, D. Z.; Tang, W.; Wu, X. J.; Wang, X. Y.; Chen, G. J.; Chen, Q.; Li, N.; Liu, F. Highly selective detection of single-nucleotide polymorphisms using a quartz crystal microbalance biosensor based on the toehold-mediated strand displacement reaction. Anal. Chem. 2012, 84, 7008–7014.10.1021/ac301064gSearch in Google Scholar
Wang, C.; Zhou, H.; Zhu, W. P.; Li, H. B.; Jiang, J. H.; Shen, G. L.; Yu, R. Q. Ultrasensitive electrochemical DNA detection based on dual amplification of circular strand-displacement polymerase reaction and hybridization chain reaction. Biosens. Bioelectron. 2013a, 47, 324–328.10.1016/j.bios.2013.03.020Search in Google Scholar
Wang, Y.; Li, Z. H.; Weber, T. J.; Hu, D. H.; Lin, C. T.; Li, J. H.; Lin, Y. H. In situ live cell sensing of multiple nucleotides exploiting DNA/RNA aptamers and graphene oxide nanosheets. Anal. Chem. 2013b, 85, 6775–6782.10.1021/ac400858gSearch in Google Scholar
Wang, H.; Jiang, X. X.; Wang, X.; Wei, X. P.; Zhu, Y.; Sun, B.; Su, Y. Y.; He, S. D.; He, Y. Hairpin DNA-assisted silicon/silver-based surface-enhanced Raman scattering sensing platform for ultrahighly sensitive and specific discrimination of deafness mutations in a real system. Anal. Chem. 2014a, 86, 7368–7376.10.1021/ac501675dSearch in Google Scholar
Wang, J.; Wei, T.; Lin, X. Y.; Zhang, B. H.; Wang, J. X.; Huang, C.; Yuan, Q. Near-infrared-light-mediated imaging of latent fingerprints based on molecular recognition. Angew. Chem. Int. Ed. 2014b, 53, 1616–1620.10.1002/anie.201308843Search in Google Scholar
Wang, L.; Hua, E. Q.; Liang, M.; Ma, C. X.; Liu, Z. L.; Sheng, S. C.; Liu, M.; Xie, G. M.; Feng, W. L. Graphene sheets, polyaniline and AuNPs based DNA sensor for electrochemical determination of BCR/ABL fusion gene with functional hairpin probe. Biosens. Bioelectron. 2014c, 51, 201–207.10.1016/j.bios.2013.07.049Search in Google Scholar
Wang, M. D.; Leung, K. H.; Lin, S.; Chan, D. S. H.; Leung, C. H.; Ma, D. L. A G-quadruplex-based, label-free, switch-on luminescent detection assay for Ag+ ions based on the exonuclease III-mediated digestion of C-Ag+-C DNA. J. Mater. Chem. B2014d, 2, 6467–6471.10.1039/C4TB01140FSearch in Google Scholar
Wang, Q.; Yang, C. Y.; Xiang, Y.; Yuan, R.; Chai, Y. Q. Dual amplified and ultrasensitive electrochemical detection of mutant DNA Biomarkers based on nuclease-assisted target recycling and rolling circle amplifications. Biosens. Bioelectron. 2014e, 55, 266–271.10.1016/j.bios.2013.12.034Search in Google Scholar
Wang, Q. B.; Xu, N.; Gui, Z.; Lei, J. P.; Ju, H. X.; Yan, F. Catalytic activity of a dual-hemin labelled oligonucleotide: conformational dependence and fluorescent DNA sensing. Chem. Commun. 2014f, 50, 15362–15365.10.1039/C4CC07298GSearch in Google Scholar
Wang, Y.; Tang, L. H.; Li, Z. H.; Lin, Y. H.; Li, J. H. In situ simultaneous monitoring of ATP and GTP using a graphene oxide nanosheet-based sensing platform in living cells. Nat. Protoc. 2014g, 9, 1944–1955.10.1038/nprot.2014.126Search in Google Scholar
Watanabe, S.; Hagihara, K.; Tsukagoshi, K.; Hashimoto, M. Microbead-based ligase detection reaction assay using a molecular beacon probe for the detection of low-abundance point mutations. Anal. Chem. 2014, 86, 900–906.10.1021/ac403531xSearch in Google Scholar
Wei, F.; Wang, J. H.; Liao, W.; Zimmermann, B. G.; Tong, D. T.; Ho, C. M. Electrochemical detection of low-copy number salivary RNA based on specific signal amplification with a hairpin probe. Nuc. Acids Res. 2008, 36, e65.10.1093/nar/gkn299Search in Google Scholar
Wei, X. F.; Ma, X. M.; Sun, J. J.; Lin, Z. Y.; Guo, L. H.; Qiu, B.; Chen, G. N. DNA methylation detection and inhibitor screening based on the discrimination of the aggregation of long and short DNA on a negatively charged indium tin oxide microelectrode. Anal. Chem. 2014, 86, 3563–3567.10.1021/ac500101tSearch in Google Scholar
Willner, I.; Shlyahovsky, B.; Zayats, M.; Willner, B. DNAzymes for sensing, nanobiotechnology and logic gate applications. Chem. Soc. Rev. 2008, 37, 1153–1165.10.1039/b718428jSearch in Google Scholar
Wilson, R.; Johansson, M. K. Photoluminescence and electrochemiluminescence of a Ru(II)(bpy)3-quencher dual-labeled oligonucleotide probe. Chem. Commun. 2003, 2710–2711.10.1039/b307550hSearch in Google Scholar PubMed
Wu, Z. S.; Zheng, F.; Sheng, G. L.; Yu, R. Q. A hairpin aptamer-based electrochemical biosensing platform for the sensitive detection of proteins. Biomaterials2009, 30, 2950–2955.10.1016/j.biomaterials.2009.02.017Search in Google Scholar
Wu, C. C.; Yan, L.; Wang, C. M.; Lin, H. X.; Wang, C.; Chen, X.; Yang, C. J. A general excimer signaling approach for aptamer sensors. Biosens. Bioelectron. 2010a, 25, 2232–2237.10.1016/j.bios.2010.02.030Search in Google Scholar
Wu, S. M.; Tian, Z. Q.; Zhang, Z. L.; Huang, B. H.; Jiang, P.; Xie, Z. X.; Pang, D. W. Direct fluorescence in situ hybridization (FISH) in Escherichia coli with a target-specific quantum dot-based molecular beacon. Biosens. Bioelectron. 2010b, 26, 491–496.10.1016/j.bios.2010.07.067Search in Google Scholar
Wu, C. S.; Oo, M. K. K.; Cupps, J. M.; Fan, X. D. Robust silica-coated quantum dot-molecular beacon for highly sensitive DNA detection. Biosens. Bioelectron. 2011, 26, 3870–3875.10.1016/j.bios.2011.02.049Search in Google Scholar
Wu, J. C.; Zou, Y.; Li, C. Y.; Sicking, W.; Piantanida, I.; Yi, T.; Schmuck, C. A molecular peptide beacon for the ratiometric sensing of nucleic acids. J. Am. Chem. Soc. 2012, 134, 1958–1961.10.1021/ja2103845Search in Google Scholar
Wu, L.; Zhang, X. H.; Liu, W.; Xiong, E. H.; Chen, J. H. Sensitive electrochemical aptasensor by coupling “signal-on” and “signal-off” strategies. Anal. Chem. 2013a, 85, 8397–8402.10.1021/ac401810tSearch in Google Scholar
Wu, X.; Chen, J.; Zhao, J. X. A reversible fluorescent logic gate for sensing mercury and iodide ions based on a molecular beacon. Analyst2013b, 138, 5281–5287.10.1039/c3an01051aSearch in Google Scholar PubMed
Wu, M. S.; He, L. J.; Xu, J. J.; Chen, H. Y. RuSi@Ru(bpy)32+/Au@Ag2S nanoparticles electrochemiluminescence resonance energy transfer system for sensitive DNA detection. Anal. Chem. 2014a, 86, 4559–4565.10.1021/ac500591nSearch in Google Scholar
Wu, X. Y.; Chai, Y. Q.; Yuan, R.; Zhuo, Y.; Chen, Y. Dual signal amplification strategy for enzyme-free electrochemical detection of microRNAs. Sens. Actuat. B Chem. 2014b, 203, 296–302.10.1016/j.snb.2014.06.131Search in Google Scholar
Wu, X.; Chen, J.; Zhao, J. X. Ultrasensitive detection of 3′-5′ exonuclease enzymatic activity using molecular beacons. Analyst2014c, 139, 1081–1087.10.1039/C3AN02040ASearch in Google Scholar PubMed
Wu, H.; Zhang, K.; Liu, Y. L.; Wang, H. Y.; Wu, J.; Zhu, F. F.; Zou, P. Binding-induced and label-free colorimetric method for protein detection based on autonomous assembly of hemin/G-quadruplex DNAzyme amplification strategy. Biosens. Bioelectron. 2015, 64, 572–578.10.1016/j.bios.2014.09.096Search in Google Scholar
Xiao, Z. Y.; Guo, X. T.; Ling, L. S. Sequence-specific recognition of double-stranded DNA with molecular beacon with the aid of Ag+ under neutral pH environment. Chem. Commun. 2013, 49, 3573–3575.10.1039/c3cc40333eSearch in Google Scholar
Xing, X. W.; Tang, F.; Wu, J.; Chu, J. M.; Feng, Y. Q.; Zhou, X.; Yuan, B. F. Sensitive detection of DNA methyltransferase activity based on exonuclease-mediated target recycling. Anal. Chem. 2014, 86, 11269–11274.10.1021/ac502845bSearch in Google Scholar
Xiong, E. H.; Wu, L.; Zhou, J. W.; Yu, P.; Zhang, X. H.; Chen, J. H. A ratiometric electrochemical biosensor for sensitive detection of Hg2+ based on thymine-Hg2+-thymine structure. Anal. Chim. Acta2015, 853, 242–248.10.1016/j.aca.2014.10.015Search in Google Scholar
Xu, H. F.; Zhu, X.; Ye, H. Z.; Yu, L. S.; Liu, X. X.; Chen, G. N. A simple “molecular beacon”-based fluorescent sensing strategy for sensitive and selective detection of mercury (II). Chem. Commun. 2011, 47, 12158–12160.10.1039/c1cc14265hSearch in Google Scholar
Xu, H.; Xu, P. P.; Gao, S. M.; Zhang, S. X.; Zhao, X. C.; Fan, C. H.; Zuo, X. L. Highly sensitive recognition of Pb2+ using Pb2+ triggered exonuclease aided DNA recycling. Biosens. Bioelectron. 2013a, 47, 520–523.10.1016/j.bios.2013.03.062Search in Google Scholar
Xu, J.; Wu, J.; Zong, C.; Ju, H. X.; Yan, F. Manganese porphyrin-dsDNA complex: a mimicking enzyme for highly efficient bioanalysis. Anal. Chem. 2013b, 85, 3374–3379.10.1021/ac4000688Search in Google Scholar
Xu, Y. Y.; Zhou, W. J.; Zhou, M.; Xiang, X.; Yuan, R.; Chai, Y. Q. Toehold strand displacement-driven assembly of G-quadruplex DNA for enzyme-free and non-label sensitive fluorescent detection of thrombin. Biosens. Bioelectron. 2015, 64, 306–310.10.1016/j.bios.2014.09.018Search in Google Scholar
Xuan, F.; Luo, X. T.; Hsing, I. M. Sensitive immobilization-free electrochemical DNA sensor based on isothermal circular strand displacement polymerization reaction. Biosens. Bioelectron. 2012, 35, 230–234.10.1016/j.bios.2012.02.054Search in Google Scholar
Xuan, F.; Luo, X. T.; Hsing, I. M. Conformation-dependent exonuclease III activity mediated by metal ions reshuffling on thymine-rich DNA duplexes for an ultrasensitive electrochemical method for Hg2+ detection. Anal. Chem. 2013, 85, 4586–4593.10.1021/ac400228qSearch in Google Scholar
Xue, L. Y.; Zhou, X. M.; Xing, D. Sensitive and homogeneous protein detection based on target-triggered aptamer hairpin switch and nicking enzyme assisted fluorescence signal amplification. Anal. Chem. 2012, 84, 3507–3513.10.1021/ac2026783Search in Google Scholar
Yan, J. L.; Li, Z. P.; Liu, C. H.; Cheng, Y. Q. Simple and sensitive detection of microRNAs with ligase chain reaction. Chem. Commun. 2010, 46, 2432–2434.10.1039/b923521cSearch in Google Scholar
Yang, C. J.; Jocksch, S.; Vicens, M.; Turro, N. J.; Tan, W. H. Light-switching excimer probes for rapid protein monitoring in complex biological fluids. Proc. Natl. Acad. Sci. U. S. A. 2005a, 102, 17278–17283.10.1073/pnas.0508821102Search in Google Scholar
Yang, C. J.; Lin, H.; Tan, W. H. Molecular assembly of superquenchers in signaling molecular interactions. J. Am. Chem. Soc. 2005b, 127, 12772–12773.10.1021/ja053482tSearch in Google Scholar
Yang, C. J.; Pinto, M.; Schanze, K.; Tan, W. H. Direct synthesis of an oligonucleotide-poly-(phenylene ethynylene) conjugate with a precise one-to-one molecular ratio. Angew. Chem. Int. Ed. 2005c, 44, 2572–2576.10.1002/anie.200462431Search in Google Scholar
Yang, C. J.; Wang, L.; Wu, Y. R.; Kim, Y.; Medley, C. D.; Lin, H.; Tan, W. H. Synthesis and investigation of deoxyribonucleic acid/locked nucleic acid chimeric molecular beacons. Nuc. Acids Res. 2007, 35, 4030–4041.10.1093/nar/gkm358Search in Google Scholar
Yang, R. H.; Jin, J. Y.; Long, L. P.; Wang, Y. X.; Wang, H.; Tan, W. H. Reversible molecular switching of molecular beacon: controlling DNA hybridization kinetics and thermodynamics using mercury(II) ions. Chem. Commun. 2009, 322–324.10.1039/B816638BSearch in Google Scholar
Yang, B.; Zhang, X. B.; Kang, L. P.; Shen, G. L.; Yu, R. Q.; Tan, W. H. Target-triggered cyclic assembly of DNA-protein hybrid nanowires for dual-amplified fluorescence anisotropy assay of small molecules. Anal. Chem. 2013, 85, 11518–11523.10.1021/ac402781gSearch in Google Scholar
Yao, W.; Wang, L.; Wang, H. Y.; Zhang, X. L.; Li, L.; Zhang, N.; Pan, L.; Xing, N. N. An electrochemiluminescent DNA sensor based on nano-gold enhancement and ferrocene quenching. Biosens. Bioelectron. 2013, 40, 356–361.10.1016/j.bios.2012.08.002Search in Google Scholar
Yeh, H. Y.; Yates, M. V.; Mulchandani, A.; Chen W. Molecular beacon-quantum dot-Au nanoparticle hybrid nanoprobes for visualizing virus replication in living cells. Chem. Commun. 2010, 46, 3914–3916.10.1039/c001553aSearch in Google Scholar
Yin, H. S.; Wang, M.; Zhou, Y. L.; Zhang, X. Y.; Sun, B.; Wang, G. H.; Ai. S. Y. Photoelectrochemical biosensing platform for microRNA detection based on in situ producing electron donor from apoferritin-encapsulated ascorbic acid. Biosens. Bioelectron. 2014, 53, 175–181.10.1016/j.bios.2013.09.053Search in Google Scholar
Yu, H.; Tan, Y. F.; Cunningham, B. T. Smartphone fluorescence spectroscopy. Anal. Chem. 2014, 86, 8805–8813.10.1021/ac502080tSearch in Google Scholar
Zhang, P.; Beck, T.; Tan, W. H. Design of a molecular beacon DNA probe with two fluorophores. Angew. Chem. Int. Ed. 2001, 40, 402–405.10.1002/1521-3773(20010119)40:2<402::AID-ANIE402>3.3.CO;2-9Search in Google Scholar
Zhang, D. D.; Peng, Y. G.; Qi, H. L.; Gao, Q.; Zhang, C. X. Label-free electrochemical DNA biosensor array for simultaneous detection of the HIV-1 and HIV-2 oligonucleotides incorporating different hairpin-DNA probes and redox indicator. Biosens. Bioelectron. 2010a, 25, 1088–1094.10.1016/j.bios.2009.09.032Search in Google Scholar
Zhang, X. B.; Wang, Z. D.; Xing, H.; Xiang, Y.; Lu, Y. Catalytic and molecular beacons for amplified detection of metal ions and organic molecules with high sensitivity. Anal. Chem. 2010b, 82, 5005–5011.10.1021/ac1009047Search in Google Scholar
Zhang, D. W.; Nie, J.; Zhang, F. T.; Xu, L.; Zhou, Y. L.; Zhang, X. X. Novel homogeneous label-free electrochemical aptasensor based on functional DNA hairpin for target detection. Anal. Chem. 2013a, 85, 9378–9382.10.1021/ac402295ySearch in Google Scholar
Zhang, M.; Le, H. N.; Jiang, X. Q.; Ye, B. C. “Molecular beacon”-directed fluorescence of Hoechst dyes for visual detection of Hg(II) and biothiols and its application for a logic gate. Chem. Commun. 2013b, 49, 2133–2135.10.1039/c3cc38427fSearch in Google Scholar
Zhang, H.; Li, M. X.; Fan, M. X.; Gu, J. X.; Wu, P.; Cai, C. X. Electrochemiluminescence signal amplification combined with a conformation-switched hairpin DNA probe for determining the methylation level and position in the Hsp53 tumor suppressor gene. Chem. Commun. 2014a, 50, 2932–2934.10.1039/C3CC49719DSearch in Google Scholar
Zhang, W.; Li, F. H.; Hu, Y. W.; Gan, S. Y.; Han, D. X.; Zhang, Q. X.; Niu, L. Perylene derivative-bridged Au-graphene nanohybrid for label-free HpDNA biosensor. J. Mater. Chem. B2014b, 2, 3142–3148.10.1039/C3TB21817ASearch in Google Scholar PubMed
Zhao, J. J.; Chen, C. F.; Zhang, L. L.; Jiang, J. H.; Yu, R. Q. An electrochemical aptasensor based on hybridization chain reaction with enzyme-signal amplification for interferon-γ detection. Biosens. Bioelectron. 2012, 36, 129–134.10.1016/j.bios.2012.04.013Search in Google Scholar
Zhao, Y. X.; Chen, F.; Wu, Y. Y.; Dong, Y. H.; Fan, C. H. Highly sensitive fluorescence assay of DNA methyltransferase activity via methylation-sensitive cleavage coupled with nicking enzyme-assisted signal amplification. Biosens. Bioelectron. 2013, 42, 56–61.10.1016/j.bios.2012.10.022Search in Google Scholar
Zhao, Y. X.; Chen, F.; Lin, M. L.; Fan, C. H. A methylation-blocked cascade amplification strategy for label-free colorimetric detection of DNA methyltransferase activity. Biosens. Bioelectron. 2014, 54, 565–570.10.1016/j.bios.2013.11.055Search in Google Scholar
Zheng, J.; Li, J. S.; Gao, X. X.; Jin, J. Y.; Wang, K. M.; Tan, W. H.; Yang, R. H. Modulating molecular level space proximity: a simple and efficient strategy to design structured DNA probes. Anal. Chem. 2010, 82, 3914–3921.10.1021/ac1004713Search in Google Scholar
Zheng, J.; Li, J. S.; Jiang, Y.; Jin, J. Y.; Wang, K. M.; Yang, R. H.; Tan, W. H. Design of aptamer-based sensing platform using triple-helix molecular switch. Anal. Chem. 2011, 83, 6586–6592.10.1021/ac201314ySearch in Google Scholar
Zhou, G. H.; Zhang, X.; Ji, X. H.; He, Z. K. Ultrasensitive detection of small molecule-protein interaction via terminal protection of small molecule linked DNA and Exo III-aided DNA recycling amplification. Chem. Commun. 2013, 49, 8854–8856.10.1039/c3cc43621gSearch in Google Scholar
Zhou, W. J.; Gong, X.; Xiang, Y.; Yuan, R.; Chai, Y. Q. Target-triggered quadratic amplification for label-free and sensitive visual detection of cytokines based on hairpin aptamer DNAzyme probes. Anal. Chem. 2014a, 86, 953–958.10.1021/ac403682cSearch in Google Scholar
Zhou, Z. P.; Huang, H. D.; Chen, Y.; Liu, F.; Huang, C. Z.; Li, N. A distance-dependent metal-enhanced fluorescence sensing platform based on molecular beacon design. Biosens. Bioelectron. 2014b, 52, 367–373.10.1016/j.bios.2013.09.013Search in Google Scholar
Zhu, X.; Zhou, X. M.; Xing, D. Ultrasensitive and selective detection of mercury(II) in aqueous solution by polymerase assisted fluorescence amplification. Biosens. Bioelectron. 2011, 26, 2666–2669.10.1016/j.bios.2010.03.013Search in Google Scholar
Zhu, D. B.; Zhang, L.; Ma, W. G.; Lu, S. Q.; Xing, X. B. Detection of microRNA in clinical tumor samples by isothermal enzyme-free amplification and label-free graphene oxide-based SYBR Green I fluorescence platform. Biosens. Bioelectron. 2015, 65, 152–158.10.1016/j.bios.2014.10.019Search in Google Scholar
Zhuang, J. Y.; Fu, L. B.; Tang, D. P.; Xu, M. D.; Chen, G. N.; Yang, H. H. Target-induced structure-switching DNA hairpins for sensitive electrochemical monitoring of mercury (II). Biosens. Bioelectron. 2013a, 39, 315–319.10.1016/j.bios.2012.07.015Search in Google Scholar
Zhuang, J. Y.; Fu, L. B.; Xu, M. D.; Zhou, Q.; Chen, G. N.; Tang, D. P. DNAzyme-based magneto-controlled electronic switch for picomolar detection of lead (II) coupling with DNA-based hybridization chain reaction. Biosens. Bioelectron. 2013b, 45, 52–57.10.1016/j.bios.2013.01.039Search in Google Scholar
Zhuang, J. Y.; Lai, W. Q.; Chen, G. N.; Tang, D. P. A rolling circle amplification-based DNA machine for miRNA screening coupling catalytic hairpin assembly with DNAzyme formation. Chem. Commun. 2014, 50, 2935–2938.10.1039/c3cc49873eSearch in Google Scholar
Zuo, X. L.; Xia, F.; Xiao, Y.; Plaxco, K. W. Sensitive and selective amplified fluorescence DNA detection based on exonuclease III-aided target recycling. J. Am. Chem. Soc. 2010, 132, 1816–1818.10.1021/ja909551bSearch in Google Scholar
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