Confined DNA tetrahedral molecular sieve for size-selective electrochemiluminescence sensing.

Size selectivity is crucial in highly accurate preparation of biosensors. Herein, we described an innovative electrochemiluminescence (ECL) sensing platform based on the confined DNA tetrahedral molecular sieve (DTMS) for size-selective recognition of nucleic acids and small biological molecule. Firstly, DNA template (T) was encapsulated into the inner cavity of DNA tetrahedral scaffold (DTS) and hybridized with quencher (Fc) labeled probe DNA to prepare DTMS, accordingly inducing Ru(bpy)32+ and Fc closely proximate, resulting the sensor in a "signal-off" state. Afterwards, target molecules entered the cavity of DTMS to realize the size-selective molecular recognition while prohibiting large molecules outside of the DTMS, resulting the sensor in a "signal-on" state due to the release of Fc. The rigid framework structure of DTS and the anchor of DNA probe inside the DTS effectively avoided the nuclease degradation of DNA probe, and nonspecific protein adsorption, making the sensor possess potential application prospect for size-selective molecular recognition in diagnostic analysis with high accuracy and specificity.

DNA tetrahedral scaffold-corbelled 3D DNAzyme walker for electrochemiluminescent aflatoxin B1 detection.

The reaction efficiency of surface-based DNA walker can directly affect the properties of a biosensor. Herein, three-dimensional (3D) DNAzyme walker were first fixed on the top of DNA tetrahedral scaffold to improve the immobilization efficiency. Ferrocene (Fc) that labeled at substrate strand ends effectively quenched the electrochemiluminescence (ECL) signal of Ru(bpy)2(cpaphen)2+, yielding the sensor in a "signal-off" state. Upon the addition of aflatoxin B1 (AFB1), 3D DNAzyme walker was activated and fueled by Na+, accordingly releasing Fc and recovering the ECL signal of Ru(bpy)2(cpaphen)2+. Due to the high movement efficiency of such 3D DNAzyme walker, ultrasensitive detection of AFB1 was achieved in the range of 1.0 fg mL-1-10 ng mL-1, with a detection limit of 0.58 fg mL-1. Moreover, satisfactory results were obtained while detecting AFB1 in corn and peanut samples, suggesting it has a potential application in food safety analysis.

Two-step förster resonance energy transfer amplification for ratiometric detection of pathogenic bacteria in food samples.

We described a two-step förster resonance energy transfer (FRET) system for ratiometric Staphylococcus aureus (S. aureus) detection based on a dual-recognition proximity binding-induced toehold strand displacement reactions (TSDR). Ru(bpy)32+ and platinum nanoparticles (Pt NPs) labeled DNA (Ru-S3 and Pt NPs-S4) hybridized to enable the occurrence of the primary FRET using Ru(bpy)32+ as the energy donor and Pt NPs as the energy acceptor. TSDR happened by integrating vancomycin hydrochloride labeled S1 (Van-S1) and gold nanoclusters labeled S2-aptamer (Au NCs-S2-aptamer) with S. aureus. The single DNA segments of Van-S1 bond to the terminal toehold of Ru-S3, displacing Pt-S4, inducing the secondary FRET using Au NCs as the energy donor and Ru(bpy)32+ as the energy acceptor. This two-step FRET system efficiently improved the reaction efficiency of S. aureus with a detection limit of 1.0 CFU/mL. Furthermore, satisfactory results obtained while detecting S. aureus in food samples, indicating a great potential for food analysis.

CTnI diagnosis in myocardial infarction using G-quadruplex selective Ir(â ¢) complex as effective electrochemiluminescence probe.

In this work, strong electrochemiluminescence (ECL) emission was achieved by using one type of the G-quadruplex selective iridium (III) complex as an efficient ECL signal probe. Based on the typical sandwich immunoreaction between the cardiac troponin-I antigen (cTnI) and its corresponding antibody, iridium (III) complex was introduced according to its specific interaction with G-quadruplex DNA that modified on the surface of negatively charged gold nanoparticles ((-)AuNPs), inducing an increased ECL signal, which was proportional to cTnI concentration. Based on of this, quantitative detection of cTnI could be realized in the range of 5.0 fg/mL-100 ng/mL, with a detection limit of 1.67 fg/mL. Moreover, the proposed immunosensor was successfully applied for the diagnosis of cTnI in human serums from healthy individuals and acute myocardial infarction (AMI) patients, suggesting a great potential application value in the early diagnosis of AMI.

G-quadruplex-selective iridium(III) complex as a novel electrochemiluminescence probe for switch-on assay of double-stranded DNA.

In this work, we synthesized an iridium(III) complex and studied its selective ability to interact with a specific G-quadruplex DNA sequence (GTGGGTAGGGCGGGTTGG). Results showed that the iridium(III) complex exhibits high selectivity for the G-quadruplex DNA and could be used as an efficient electrochemiluminescence (ECL) probe in a switch-on assay format for the detection of double-stranded DNA (dsDNA). To construct the assay, a hairpin-structured capture probe (CP) which was modified by thiol at its 3' end and contained the G-quadruplex sequence at its 5' end was firstly immobilized on a gold electrode. Upon the specific recognition of the dsDNA sequence with the corresponding CP, the hairpin structure of the CP was opened to free G-quadruplex sequence, forming the G-quadruplex structure with the assistance of K+. Then, the iridium(III) complex was able to specifically interact with the G-quadruplex to produce an obvious ECL signal that was proportional to the dsDNA concentration. Notably, this iridium(III) complex/G-quadruplex-based strategy was universal and was not limited to the analysis of DNA using specific sequences, thus opening a new avenue for the application of the G-quadruplex-selective iridium(III) complex in the field of ECL.

Zettomole electrochemical HIV DNA detection using 2D DNA-Au nanowire structure, hemin/G-quadruplex and polymerase chain reaction multi-signal synergistic amplification.

Multi-signal synergistically amplified electrochemical sensing of HIV DNA was proposed based on two-dimensional (2D) DNA-Au nanowire structure coupled with hemin/G-quadruplex and polymerase chain reaction (PCR). In the design, by using target HIV DNA as the template, PCR generated numbers of double-stranded DNA (dsDNA) with free single-stranded DNA (ssDNA) tails on one side and free G-quadruplex sequences on the other side. Then, the ssDNA tails of the PCR products were hybridized with the capture probe (CP) to introduce the hemin/G-quadruplex to the electrode surface as a redox-active reporter and to amplify the electrochemical signal as mimic peroxidase catalysis in the presence of H2O2. Meanwhile, (+)AuNPs were electrostatically adsorbed onto dsDNA surface for the formation of 2D DNA-Au nanowire structure, amplifying the electrochemical signal further as another mimic peroxidase and electric conductor together. By effectively combining these signal amplification processes, ultrasensitive HIV DNA detection was achieved with a detection limit of 1.3 aM, indicating that it has potential application in clinical diagnosis.

Polymerase chain reaction-based ultrasensitive detection of HBV DNA via G-quadruplex selective iridium(III) complex luminescent probe.

Polymerase chain reaction (PCR) is the gold standard for low-abundant DNA detection. Here, to expand the application of PCR with novel detecting methods, we developed a label-free fluorescent sensor for ultrasensitive and one-step detection of hepatitis B virus (HBV) DNA using the G-quadruplex selective iridium(III) complex luminescent probe. By using HBV DNA as the template with two hairpin structure primers that contained oxyethylene glycol tethers, PCR amplification occurred and generated numbers of specific PCR products with free G-quadruplex sequences at both ends. Such free G-quadruplex sequences can change into G-quadruplex structure with the help of K+, resulting in a strong luminescence intensity upon their binding with the G-quadruplex selective iridium(III) complex. The luminescence intensity increase was proportional to the concentration of PCR products, and indirectly related with HBV DNA concentration. Moreover, the utilization of the iridium(III) complex effectively improved the specificity of the sensor, while PCR paved the way for the ultrasensitive detection of DNA in the linear range of 3.0 fM to 800 pM, with a detection limit of 1.6 fM. Notably, this assay was successfully used to detect HBV DNA in normal and patient serum samples, indicating a potential application for biomolecular analysis.

Ultrasensitive and label-free detection of ATP by using gold nanorods coupled with enzyme assisted target recycling amplification.

Abnormal concentration of adenosine triphosphate (ATP) is directly asscociate with several diseases. Thus, sensitive detection of ATP is essential to early diagnosis of disease. Herein, we described an ultrasensitive strategy for ATP detection by using positively charged gold nanorods ((+)AuNRs) as an efficient fluorescence quenching platform, coupled with exonuclease Ⅲ (Exo Ⅲ) assisted target recycling amplification. To construct the sensor, DNA template that contained ATP aptamer was used for the formation of Ag nanoclusters signal probe (DNA/AgNCs), the structure of it could change to duplex after the interaction of it with ATP. Such DNA template or duplex DNA product could electrostatically adsorb onto (+)AuNRs surface, resulting in the quenching of the fluorescence signal due to the vicinity of AgNCs to (+)AuNRs. With the addition of Exo Ⅲ, DNA duplex could be hydrolyzed and released from (+)AuNRs surface, leading to the recovery of a strong fluorescent signal, while ATP could be regenerated for next target recycling. Combing the good fluorescence quenching ability of (+)AuNRs and the Exo Ⅲ assisted signal amplification, a low detection limit of 26 pM was achieved for ATP detection. Notably, the proposed method can be successfully applied for detecting ATP in serum samples, indicating a potential application value in early cancer diagnosis.

One-step and ultrasensitive ATP detection by using positively charged nano-gold@graphene oxide as a versatile nanocomposite.

A versatile nanocomposite was simply prepared based upon the electrostatic adsorption of positively charged gold nanoparticles with negatively charged graphene oxide (nano-gold@GO), and utilized as a novel fluorescence quenching platform for ultrasensitive detection of adenosine triphosphate (ATP). In the designed system, DNA-stabilized Ag nanoclusters (DNA/AgNCs) were used as fluorescent probes, DNA duplex was formed in the presence of ATP, and they can electrostatically adsorb onto the surface of nano-gold@GO to quench the fluorescence signal. Upon the addition of exonuclease III (Exo III), the DNA duplex would be hydrolyzed into DNA fragments and resulted in the recovery of the fluorescence signals due to the diffusion of AgNCs away from nano-gold@GO. Based on these, sensitive detection of ATP was realized with a detection range of 5.0 pM-20 nM. Notably, a good recovery in the range of 94-104% was obtained when detecting ATP in human serum samples, indicating a promising application value in early disease diagnosis. Graphical abstract A functional positively charged nano-gold@graphene oxide was fabricated and utilized as an enhanced fluorescence quenching platform for the detection of ATP, coupled with exonuclease III-assisted signal amplification.

Electrochemical aptamer-based determination of protein tyrosine kinase-7 using toehold-mediated strand displacement amplification on gold nanoparticles and graphene oxide.

An electrochemical method is described for ultrasensitive determination of protein tyrosine kinase-7 (PTK7). It is based on (a) the use of positively charged gold nanoparticles (AuNPs) and negatively charged graphene oxide (GO), and (b) of toehold-mediated strand displacement amplification. A hairpin probe 2 (HP2) containing the sgc8 aptamer was used to modify a glassy carbon electrode (GCE). Its hairpin structure is opened in the presence of PTK7 to form the PTK7-HP2 complex. The exposed part of HP2 partly hybridizes with hairpin probe 1 (HP1) that was immobilizing on the AuNPs and GO modified GCE. On addition of the hairpin probe 3 that was labeled with the redox probe Methylene Blue (MB-HP3), toehold-mediated strand displacement occurs due to complementary hybridization of HP1 with MB-HP3. This causes the release of PTK7-HP2 into the solution and makes it available for the next reaction. Under optimal conditions, PTK7 can be quantified by voltammetry (typically performed at -0.18 V) with a detection limit of 1.8 fM. The assay possesses high selectivity for PTK7 due to the employment of the aptamer. It was successfully applied to the determination of PTK7 in the debris of malignant melanoma A375 cells. Graphical abstract Schematic representation of the enzyme-free electrochemical sensor for ultrasensitive determination of protein tyrosine kinase-7 (PTK7) based on the toehold-mediated strand displacement reaction amplification on gold nanoparticles and graphene oxide.

Fluorometric determination of mercury(II) using positively charged gold nanoparticles, DNA-templated silver nanoclusters, T-Hg(II)-T interaction and exonuclease assisted signal amplification.

The authors describe a method for detection of Hg2+ by using positively charged gold nanoparticles ((+)AuNPs) as a quencher of the fluorescence of DNA-capped silver nanoclusters (DNA-AgNCs) which are negatively charged. In the presence of Hg2+, a DNA duplex is formed through T-Hg2+-T coordination chemistry. The duplex can be digested by exonuclease III to form smaller DNA fragments. This leads to the release of the AgNCs and the recovery of fluorescence, best measured at excitation/emission wavelengths of 460/530 nm. The (+)AuNPs and Hg2+ are also released and can be reused for target recycling signal amplification. Based on these findings, a method is worked out for the determination of Hg2+ that works in the 5.0 pM to 10 nM concentration range and has a detection limit as low as 2.3 pM. It is highly selective because of the highly specific formation of T-Hg2+-T bonds. Graphical abstract By using ultrastable and positively charged gold nanoparticles as fluorescence quenchers and exonuclease assisted signal amplification, a method is developed for the sensitive and selective detection of Hg2+ in water samples.

A gold nanoparticle based fluorescent probe for simultaneous recognition of single-stranded DNA and double-stranded DNA.

A fluorescent method is described for simultaneous recognition of single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). It is based on the quenching of the fluorescence of fluorophore labeled DNA probes by gold nanoparticles (AuNPs). To demonstrate feasibility, two DNA probes labeled with spectrally different fluorophores were designed. The first DNA probe (P1) was modified with 6-carboxyfluorescein (FAM; with green fluorescence, peaking at 518 nm), while the second (P2) was modified with carboxy-X-rhodamine (ROX; with yellow fluorescence, 610 nm). The fluorescence signals of the labels are quenched if P1 or P2 are adsorbed on AuNPs. Upon addition of ssDNA and dsDNA, hybridization occurs between P1 and ssDNA to form a dsDNA. In contrast, P2 hybridizes with dsDNA such that a triplex DNA is formed. As a result, the dsDNA and the triplex DNA, respectively, are desorbed from the surface of the AuNPs so that quenching no longer can occur and strong fluorescence can be observed. Under the optimal conditions, ssDNA and dsDNA can be detected simultaneously via the green and yellow fluorescence, respectively. The detection limits can be as low as 330 pM. In particular, the method has excellent selectivity for the target DNAs over control DNAs. Graphical abstract A gold nanoparticle based fluorescent probe for simultaneous recognition of single-stranded DNA and double-stranded DNA is developed based on the fluorescence quenching of gold nanoparticles to different fluorophore labeled DNA probes.

An ultrasensitive and switch-on platform for aflatoxin B1 detection in peanut based on the fluorescence quenching of graphene oxide-gold nanocomposites.

Graphene oxide-gold nanocomposites (GO/AuNCs) were prepared and used as a novel fluorescence quenching platform for ultrasensitive detection of aflatoxin B1 (AFB1) coupled with hybridization chain reaction (HCR) amplification. In the designed system, two fluorophore labeled hairpin probes (HP1/HP2) were introduced, and the fluorescence signals of them were effectively quenched due to the adsorption on GO/AuNCs. Associate probe (AP) was used for the specific recognition of AFB1, and the stem-loop structure of it was opened. Meanwhile, the exposed section of AP was utilized as an initiator for the happen of HCR between HP1 and HP2, a strong fluorescence signal was obtained due to the formation of long nicked dsDNA duplex and desorbed of them from the surface of GO/AuNCs. Under the optimal conditions, GO/AuNCs displayed 94% of the quenching efficiency to the fluorescent probes, and a detection limit down to 0.03pg/mL was obtained for AFB1 detection. In particular, the assay exhibited excellent selectivity for the detection of AFB1 against other interfering agents that normally coexist with AFB1 in mildewed agriculture products. Moreover, the assay could realize the detection of AFB1 effectively in peanut samples.

Label-Free Platform for MicroRNA Detection Based on the Fluorescence Quenching of Positively Charged Gold Nanoparticles to Silver Nanoclusters.

A novel strategy was developed for microRNA-155 (miRNA-155) detection based on the fluorescence quenching of positively charged gold nanoparticles [(+)AuNPs] to Ag nanoclusters (AgNCs). In the designed system, DNA-stabilized Ag nanoclusters (DNA/AgNCs) were introduced as fluorescent probes, and DNA-RNA heteroduplexes were formed upon the addition of target miRNA-155. Meanwhile, the (+)AuNPs could be electrostatically adsorbed on the negatively charged single-stranded DNA (ssDNA) or DNA-RNA heteroduplexes to quench the fluorescence signal. In the presence of duplex-specific nuclease (DSN), DNA-RNA heteroduplexes became a substrate for the enzymatic hydrolysis of the DNA strand to yield a fluorescence signal due to the diffusion of AgNCs away from (+)AuNPs. Under the optimal conditions, (+)AuNPs displayed very high quenching efficiency to AgNCs, which paved the way for ultrasensitive detection with a low detection limit of 33.4 fM. In particular, the present strategy demonstrated excellent specificity and selectivity toward the detection of target miRNA against control miRNAs, including mutated miRNA-155, miRNA-21, miRNA-141, let-7a, and miRNA-182. Moreover, the practical application value of the system was confirmed by the evaluation of the expression levels of miRNA-155 in clinical serum samples with satisfactory results, suggesting that the proposed sensing platform is promising for applications in disease diagnosis as well as the fundamental research of biochemistry.

A cyclometalated iridium(III) complex used as a conductor for the electrochemical sensing of IFN-γ.

A novel iridium(III) complex was prepared and used as a conductor for sensitive and enzyme-free electrochemical detection of interferon gamma (IFN-γ). This assay is based on a dual signal amplification mechanism involving positively charged gold nanoparticles ((+)AuNPs) and hybridization chain reaction (HCR). To construct the sensor, nafion (Nf) and (+)AuNPs composite membrane was first immobilized onto the electrode surface. Subsequently, a loop-stem structured capture probe (CP) containing a special IFN-γ interact strand was modified onto the (+)AuNP surface via the formation of Au-S bonds. Upon addition of IFN-γ, the loop-stem structure of CP was opened, and the newly exposed "sticky" region of CP then hybridized with DNA hairpin-1 (H1), which in turn opened its hairpin structure for hybridizing with DNA hairpin-2 (H2). Happen of HCR between H1 and H2 thus generated a polymeric duplex DNA (dsDNA) chain. Meanwhile, the iridium(III) complex could interact with the grooves of the dsDNA polymer, producing a strong current signal that was proportional to IFN-γ concentration. Thus, sensitive detection of IFN-γ could be realized with a detection limit down to 16.3 fM. Moreover, satisfied results were achieved by using this method for the detection of IFN-γ in human serum samples.

Sensitive detection of miRNA by using hybridization chain reaction coupled with positively charged gold nanoparticles.

Positively charged gold nanoparticles (+)AuNPs can adsorb onto the negatively charged surface of single-stranded DNA (ssDNA) or double-stranded DNA (dsDNA). Herein, long-range dsDNA polymers could form based on the hybridization chain reaction (HCR) of two hairpin probes (H1 and H2) by using miRNA-21 as an initiator. (+)AuNPs could adsorb onto the negatively charged surface of such long-range dsDNA polymers based on the electrostatic adsorption, which directly resulted in the precipitation of (+)AuNPs and the decrease of (+)AuNPs absorption spectra. Under optimal conditions, miRNA-21 detection could be realized in the range of 20 pM-10 nM with a detection limit of 6.8 pM. In addition, (+)AuNPs used here are much more stable than commonly used negatively charged gold nanoparticles ((-)AuNPs) in mixed solution that contained salt, protein or other metal ions. Importantly, the assay could realize the detection of miRNA in human serum samples.

Fluorescence recognition of double-stranded DNA based on the quenching of gold nanoparticles to a fluorophore labeled DNA probe.

An ultrasensitive fluorescent platform for sequence-specific recognition of double-stranded DNA (dsDNA) based on the quenching of gold nanoparticles (AuNPs) to a fluorophore labeled DNA probe was developed. The target dsDNA could hybridize with the loop portion of the molecular beacon (MB) to form a triplex DNA structure and opened the "stem-loop" structure of the MB; such triplex DNA was used as an assistant probe (AP). Meanwhile, a fluorophore labeled DNA-AuNP probe that contained a specific enzyme cleavage site was introduced and its fluorescence signal was efficiently quenched due to the vicinity of the fluorophore to the AuNP surface. Such a DNA-AuNP probe could hybridize with the 5' stem portion of the MB in the AP to form duplex DNA strands that contained a specific enzyme cleavage site for the nicking enzyme assisted cleavage reaction, and resulted in the release of the fluorophore from the AuNP surface and the recovery of the fluorescence signal. Because the AP remains intact during such a cleavage process, it could be reused to hybridize with the next DNA-AuNP probe and trigger the nicking nuclease assisted signal amplification. Under optimal conditions, a low detection limit of 3.8 pM was obtained for dsDNA detection, and the assay has high sequence specificity for dsDNA detection.

Ultrasensitive electrochemical detection of protein tyrosine kinase-7 by gold nanoparticles and methylene blue assisted signal amplification.

We present here an ultrasensitive and simple strategy for protein tyrosine kinase-7 (PTK7) detection based on the recognition-induced structure change of sgc8 aptamer, and the signal change of methylene blue (MB) that interacted with sandwiched DNA complex. To construct such a sensor, an homogeneous nano-surface was formed firstly on the glass carbon electrode (GCE) by using negatively charged Nafion (Nf) as the inner layer and positively charged gold nanoparticles ((+)AuNPs) as the outer layer, followed by the immobilization of sgc8 aptamer based on Au-S bond. In the presence of helper probe (HP), sandwiched DNA complex was formed between the sgc8 aptamer and the DNA modified gold nanoparticle probe (DNA-AuNPs). Then, a strong current signal was produced due to the capture of abundant MB molecules by both the sandwiched DNA complex and the multiple DNAs that modified on AuNPs surface. However, the specific binding of sgc8 aptamer with PNK7 would trigger a structure transition of it, and directly prevented the following formation of sandwiched structure and the capture of MB. Thus, PTK7 detection could be realized based on monitoring the signal reduction of MB upon incubation of sgc8 aptamer with PTK7. Under optimal conditions, a low detection limit of 372 fM was obtained for PNK7 detection. Due to the employment of sgc8 aptamer, the proposed biosensor exhibited high selectivity to PNK7. Moreover, satisfactory results were obtained when the proposed method was applied for PNK7 detection in cellular debris.

Ultrasensitive electrochemical sensor for Hg(2+) by using hybridization chain reaction coupled with Ag@Au core-shell nanoparticles.

A novel electrochemical biosensor for Hg(2+) detection was reported by using DNA-based hybridization chain reaction (HCR) coupled with positively charged Ag@Au core-shell nanoparticles ((+)Ag@Au CSNPs) amplification. To construct the sensor, capture probe (CP ) was firstly immobilized onto the surface of glass carbon electrode (GCE). In the presence of Hg(2+), the sandwiched complex can be formed between the immobilized CP on the electrode surface and the detection probe (DP) modified on the gold nanoparticles (AuNPs) based on T-Hg(2+)-T coordination chemistry. The carried DP then opened two ferrocene (Fc) modified hairpin DNA (H1 and H2) in sequence and propagated the happen of HCR to form a nicked double-helix. Numerous Fc molecules were formed on the neighboring probe and produced an obvious electrochemical signal. Moreover, (+)Ag@Au CSNPs were assembly onto such dsDNA polymers as electrochemical signal enhancer. Under optimal conditions, such sensor presents good electrochemical responses for Hg(2+) detection with a detection limit of 3.6 pM. Importantly, the methodology has high selectivity for Hg(2+) detection.

Enhanced electrochemical recognition of double-stranded DNA by using hybridization chain reaction and positively charged gold nanoparticles.

Enhanced sequence-specific recognition of double-stranded DNA (dsDNA) was realized by using hybridization chain reaction (HCR) and positively charged gold nanoparticles ((+)AuNPs) dual signal amplification. To construct such a sensor, capture probe was initially assembled onto gold electrode surface. Upon addition of dsDNA, sandwiched DNA complex was formed between the capture probe and the detection probe, then another exposed part of the detection probe opened two alternating DNA hairpins (H1 and H2) in turn and initiated HCR to form a double-helix. Meantime, (+)AuNPs were electrostatically adsorbed onto such double-helix to amplify the electrochemical signal. Upon optimal conditions, the electronic signals of ferrocene (Fc) that modified on H1 and H2 increased linearly with increasing dsDNA concentration over the range from 15 pM to 1.0 nM, with a detection limit of 2.6 pM. Moreover, the proposed method showed good sequence specificity for dsDNA recognition.