KF polymerase-based fluorescence aptasensor for the label-free adenosine detection

Aptamers used for biosensors and targeted therapy

Aptamers are single-stranded nucleic acid sequences that can bind to target molecules with high selectivity and affinity. Most aptamers are screened in vitro by a combinatorial biology technique called systematic evolution of ligands by exponential enrichment (SELEX). Since aptamers were discovered in the 1990s, they have attracted considerable attention and have been widely used in many fields owing to their unique advantages. In this review, we present an overview of the advancements made in aptamers used for biosensors and targeted therapy. For the former, we will discuss multiple aptamer-based biosensors with different principles detected by various signaling methods. For the latter, we will focus on aptamer-based targeted therapy using aptamers as both biotechnological tools for targeted drug delivery and as targeted therapeutic agents. Finally, challenges and new perspectives associated with these two regions were further discussed. We hope that this review will help researchers interested in aptamer-related biosensing and targeted therapy research.

Fluorometric determination of agrA gene transcription in methicillin-resistant Staphylococcus aureus with a graphene oxide-based assay using strand-displacement polymerization recycling and hybridization chain reaction

A graphene oxide (GO)-based fluorescent bioassay was developed to quantify agrA gene transcription (its mRNA) in methicillin-resistant Staphylococcus aureus (MRSA). This method is based on the use of Klenow fragment (KF)-assisted target recycling amplification and hybridization chain reaction (HCR). A triple complex was designed that contained a capture probe (CP), a trigger probe (TP), and a help probe (HP), which were partially complementary to one another. In the absence of the target, all the oligonucleotides labeled with carboxyfluorescein (FAM) are adsorbed onto the surface of GO by π-stacking interactions. This adsorption quenches the FAM signal. On the contrary, the target RNA causes the triple complex to disintegrate and initiates strand-displacement polymerization reaction (SDPR) and HCR in the presence of the appropriate raw materials, including the primer, KF, dNTPs, hairpin 1 (H1), and hairpin 2 (H2), generating double-stranded DNA (dsDNA) products. These dsDNA products are repelled by GO and produce strong fluorescence, measured at excitation/emission wavelengths of 480/514 nm. The fluorescent signal is greatly amplified by SYBR Green I (SGI) due to the synergistic effect of dsDNA-SGI. The target was assayed with this method at concentrations in the range 10 fM to 100 pM, and the detection limit (LOD) was 10 fM. This method also displayed good applicability in the analysis of real samples. It provides a new way of monitoring biofilm formation and studying the mechanisms of drug actions. Graphical abstract Schematic representation of the graphene oxide-based fluorescent bioassay for agrA gene transcription in methicillin-resistant Staphylococcus aureus by using strand-displacement polymerization recycling and hybridization chain reaction.

Splitting aptamers and nucleic acid enzymes for the development of advanced biosensors

In analogy to split-protein systems, which rely on the appropriate fragmentation of protein domains, split aptamers made of two or more short nucleic acid strands have emerged as novel tools in biosensor set-ups. The concept relies on dissecting an aptamer into a series of two or more independent fragments, able to assemble in the presence of a specific target. The stability of the assembled structure can further be enhanced by functionalities that upon folding would lead to covalent end-joining of the fragments. To date, only a few aptamers have been split successfully, and application of split aptamers in biosensing approaches remains as promising as it is challenging. Further improving the stability of split aptamer target complexes and with that the sensitivity as well as efficient working modes are important tasks. Here we review functional nucleic acid assemblies that are derived from aptamers and ribozymes/DNAzymes. We focus on the thrombin, the adenosine/ATP and the cocaine split aptamers as the three most studied DNA split systems and on split DNAzyme assemblies. Furthermore, we extend the subject into split light up RNA aptamers used as mimics of the green fluorescent protein (GFP), and split ribozymes.

The sensitive detection of ATP and ADA based on turn-on fluorescent copper/silver nanoclusters

A simple turn-on fluorescence strategy is proposed for the detection of ATP based on DNA-stabilized copper/silver nanoclusters (DNA-Cu/Ag NCs). The fluorescence intensity of DNA-Cu/Ag NCs increases significantly in the presence of ATP, because the specific interaction between ATP and its aptamer causes two darkish Cu/Ag NCs to be situated at the 5' and 3' termini close to each other. A limit of detection (LOD) of 7.0 μM is found, in a linear range of 2-18 mM, and the proposed sensor is simple, sensitive, and selective. Additionally, the DNA-Cu/Ag NCs/ATP system is further developed into a sensor for ADA detection and demonstrates a linear response to ADA from 5 to 50 U/L with a LOD of 5 U/L. The proposed method is also shown to be successful in detecting ATP and ADA in a solution of fetal bovine serum.

A reusable ratiometric electrochemical biosensor on the basis of the binding of methylene blue to DNA with alternating AT base sequence for sensitive detection of adenosine

We develop a reusable ratiometric electrochemical biosensor on the basis of the binding of methylene blue (MB) to DNA with alternating AT base sequence for sensitive detection of adenosine. We design a strand 1 with MB-modified thymine (T) base in the proximal 3' termini as the capture probe for its immobilization on the gold electrode and a 3' termini ferrocene (Fc)-modified aptamer for the recognition of adenosine. The hybridization of strand 1 with the aptamer leads to the formation of a double-stranded DNA (dsDNA) and consequently the away of MB from the electrode surface and the close of Fc to the electrode surface, generating a small value of I_MB/I_Fc (I_MB and I_Fc are the peak currents of MB and Fc, respectively). In the presence of adenosine, its binding with the aptamer induces the release of Fc from the electrode surface and the close of MB to the electrode surface, generating a large value of I_MB/I_Fc. As a result, adenosine may be accurately quantified by the measurement of ratiometric signal (I_MB/I_Fc). This ratiometric electrochemical biosensor can be simply fabricated and exhibits high sensitivity with a limit of detection of as low as 90.8pM and a large dynamic range from 0.1nM to 100μM. Moreover, this biosensor demonstrates good performance with excellent selectivity, regeneration capability, high reliability and good reproducibility, and may become a universal platform for the detection of various biomolecules which can be recognized by aptamers, holding great potential for further applications in biomedical research and clinical diagnosis.

Thioflavin T behaves as an efficient fluorescent ligand for label-free ATP aptasensor

Here, we for the first time demonstrated thioflavin T (ThT) as an efficient fluorescent ligand for 27-mer ATP-binding aptamer (ABA27), providing a novel signal readout mode for label-free selective ATP detection. ABA27 can promote the fluorescence emission of ThT with an unprecedentedly high efficiency, attributed to the specific structure of ABA27 rather than the G-tracts. Polyacrylamide gel electrophoresis, fluorescence spectroscopy, and fluorometric titration reveal that ThT interacts with ABA27 with a lower binding affinity (Kd ~89 μM) than ATP, which allows ATP to easily compete with ThT for the DNA binder. In the presence of ThT, adding ATP induces ABA27 to undergo a structural change, thereby not favoring the binding to ThT, verified by circular dichroism and UV-Vis absorption spectroscopy. As a result, the fluorescence intensity of ThT decreases dramatically, enabling the sensitive detection of ATP with high selectivity over other analogs. Such a sensing strategy may make ThT able to serve as a facile signal reporter for DNA nanomechanical devices fueled with ATP. Graphical Abstract The principle of the displacement of ThT by ATP.

Binding induced colocalization activated hybridization chain reaction on the surface of magnetic nanobead for sensitive detection of adenosine

Herein, a sensitive and enzyme-free assay for adenosine detection has been developed on the basis of binding induced colocalization activated hybridization chain reaction (HCR) strategy on the surface of magnetic nanobead. First, the recognition probe was fabricated and divided into two parts: the Apt-1 that composed a part of adenosine aptamer and toehold domain, and the Apt-2 that consisted of another part of adenosine aptamer and branch migration domain. The Apt-1 was immobilized on a streptavidin-magnetic nanobead (streptavidin-MNBs) that played the roles of enrichment and separation. Then the recognition event of adenosine could bring the two parts of aptamer together and induce the colocalization of toehold domain and branch migration domain, which could serve as an integrated initiator to trigger the HCR, producing a long nicked double-stranded polymer. Finally, the intercalating dye SYBR Green I was inserted into the polymer, generating an enhanced fluorescence signal. In this strategy, the initiator was divided into two parts and could be suppressed effectively in the absence of adenosine. Utilizing the separated function, the spontaneous hybridization of H1 and H2 could be avoided, and a low background could be acquired. Moreover, through the double amplification of HCR and multimolecules binding of SYBR Green I, highly sensitive and enzyme-free detection were achieved. The detection limit for adenosine detection was 2.0×10(-7)mol/L, which was comparable or superior to the previous aptasensors. Importantly, adenosine analysis in human urines has been performed, and this strategy could significantly distinguish the adenosine content in normal human urines and cancer patient urines, suggesting that this proposed assay will become a reliable and sensitive adenosine detection method in early clinical diagnosis and medical research.

Development of a luminescent G-quadruplex-selective iridium(III) complex for the label-free detection of adenosine

A panel of six luminescent iridium(III) complexes were synthesized and evaluated for their ability to act as G-quadruplex-selective probes. The novel iridium(III) complex 1 was found to be highly selective for G-quadruplex DNA, and was employed for the construction of a label-free G-quadruplex-based adenosine detection assay in aqueous solution. Two different detection strategies were investigated for adenosine detection, and the results showed that initial addition of adenosine to the adenosine aptamer gave superior results. The assay exhibited a linear response for adenosine in the concentration range of 5 to 120 μM (R(2) = 0.992), and the limit of detection for adenosine was 5 μM. Moreover, this assay was highly selective for adenosine over other nucleosides, and exhibited potential use for biological sample analysis.

A turn-on fluorescent aptasensor for adenosine detection based on split aptamers and graphene oxide

A simple, sensitive and selective turn-on fluorescent aptasensor for adenosine detection was developed based on target-induced split aptamer fragment conjunction and different interactions of graphene oxide and the two states of the designed aptamer sequences.

Hairpin assembly circuit-based fluorescence cooperative amplification strategy for enzyme-free and label-free detection of small molecule

Here, we developed an enzyme-free, label-free, and sensitive fluorescence cooperative amplification strategy based on a hairpin assembly circuit which coupled catalytic hairpin assembly (CHA) with hybridization chain reaction (HCR) for small molecule adenosine. A double-strand DNA probe with aptamer-catalysis strand (Apt-C) and inhibit strand (Inh) was designed for adenosine recognition and signal transduction which was named as Apt-C/Inh. Hairpins H1 and H2 were employed for constructing the CHA, and hairpins H3 and H4 for the HCR. Through the binding of adenosine and the Apt-C, the Inh was released from the Apt-C/Inh. Then the free Apt-C initiated the CHA through successively opening H1 and H2, generating H1/H2 complex and recyclable Apt-C. Next, the released Apt-C entered another CHA cycle, and the H1/H2 complex further initiated the HCR of H3 and H4 which induced the formation of the concatemers of H3/H4 complex. Such a process brought the two ends of hairpins H3 into close proximity, yielding numerous integrated G-quadruplexes which were initially sequestered in the stem and two terminals of H3. Finally, N-methyl mesoporphyrin IX (NMM) was added to generate an enhanced fluorescence signal. In the proposed strategy, driven only by the energy from hybridization, one target could trigger multiple HCR events via CHA-based target-cycle, leading to a remarkable enzyme-free amplification for adenosine. The detection limit could achieve as low as 9.7 × 10(-7) mol L(-1). Furthermore, G-quadruplexes were applied to construct label-free hairpin assembly circuit, which made it more simple and cost-effective. The satisfactory recoveries were obtained when detecting adenosine in spiked human serum and urine samples, demonstrating the feasibility of this detection strategy in biological samples.

Label-free fluorescence dual-amplified detection of adenosine based on exonuclease III-assisted DNA cycling and hybridization chain reaction

In this work, we constructed a label-free and dual-amplified fluorescence aptasensor for sensitive analysis of adenosine based on exonuclease III (Exo III)-assisted DNA cycling and hybridization chain reaction (HCR). Firstly, we fabricated a trifunctional probe that consisting of the catalytic strand, the aptamer sequence and a streptavidin-magnetic nanobead (streptavidin-MNB). The streptavidin-MNB played a role of enrichment and separation to achieve a low background. The aptamer sequence was employed as a recognition element to bind the target adenosine, leading to the releasing of the catalytic stand. Then, the catalytic strand induced the Exo III-assisted DNA cycling reaction and produced a large amount of DNA fragments, which got a primary amplification. Subsequently, the DNA fragments acted as trigger strands to initiate HCR, forming nicked double helices with multiple G-quadruplex structures, which achieved a secondary amplification. Finally, the G-quadruplex structures bonded with the N-nethyl mesopor-phyrin IX (NMM) and yielded an enhanced fluorescence signal, realizing the label-free detection. In the proposed strategy, a small amount of adenosine can be converted to a large amount of DNA triggers, leading to a significant amplification for the target. This method exhibited a high sensitivity toward adenosine with a detection limit of 4.2×10(-7) mol L(-1), which was about 10 times lower than that of the reported label-free strategies. Moreover, this assay can significantly distinguish the content of adenosine in urine samples of cancer patients and normal human, indicating that our method will offer a new strategy for reliable quantification of adenosine in medical research and early clinical diagnosis.

A highly sensitive homogeneous electrochemical assay for alkaline phosphatase activity based on single molecular beacon-initiated T7 exonuclease-mediated signal amplification

A highly sensitive homogeneous electrochemical assay for alkaline phosphatase activity based on single molecular beacon-initiated T7 exonuclease-assisted signal amplification.

Enzyme-free fluorescent-amplified aptasensors based on target-responsive DNA strand displacement via toehold-mediated click chemical ligation

A new target-responsive DNA strand displacement system via toehold-mediated click chemical ligation was designed and prepared for enzyme-free fluorescent-amplified aptasensors. The aptasensors significantly amplified fluorescent signals in response to targets based on target recycling processes.

Riboswitches based on kissing complexes for the detection of small ligands

Biosensors derived from aptamers were designed for which folding into a hairpin shape is triggered by binding of the cognate ligand. These aptamers (termed aptaswitches) thus switch between folded and unfolded states in the presence and absence of the ligand, respectively. The apical loop of the folded aptaswitch is recognized by a second hairpin called the aptakiss through loop-loop or kissing interactions, whereas the aptakiss does not bind the unfolded aptaswitch. Therefore, the formation of a kissing complex signals the presence of the ligand. Aptaswitches were designed that enable the detection of GTP and adenosine in a specific and quantitative manner by surface plasmon resonance when using a grafted aptakiss or in solution by anisotropy measurement with a fluorescently labeled aptakiss. This approach is generic and can potentially be extended to the detection of any molecule for which hairpin aptamers have been identified, as long as the apical loop is not involved in ligand binding.

Capillary gel electrophoresis-coupled aptamer enzymatic cleavage protection strategy for the simultaneous detection of multiple small analytes

This novel, multi small-analyte sensing strategy is the result of combining the target-induced aptamer enzymatic protection approach with the CGE-LIF (capillary gel electrophoresis with laser-induced fluorescence) technique. The implemented assay principle is based on an analysis of the phosphodiesterase I (PDE I)-mediated size variation of a fluorescein-labeled aptamer (FApt), the enzyme catalyzing the removal of nucleotides from DNA in the 3' to 5' direction. In the absence of the target, the unfolded aptamer was enzymatically cleaved into short DNA fragments. Upon target binding, the DNA substrate was partially protected against enzymatic hydrolysis. The amount of bound aptamer remaining after the exonuclease reaction was proportional to the concentration of the target. The CGE technique, which was used to determine the separation of FApt species from DNA digested products, permitted the quantification of adenosine (A), ochratoxin A (O), and tyrosinamide (T) under the same optimized enzymatic conditions. This assay strategy was subsequently applied to the simultaneous detection of A, O, and T in a single capillary under buffered conditions using corresponding FApt probes of different lengths (23, 36, and 49 nucleotides, respectively). Additionally, the detection of these three small molecules was successfully achieved in a complex medium (diluted, heat-treated human serum) showing a good recovery. It is worth noting that the multiplexed analysis was accomplished for targets with different charge states by using aptamers possessing various structural features. This sensing platform constitutes a rationalized and reliable approach with an expanded potential for a high-throughput determination of small analytes in a single capillary.

Fluorescence sensing of adenosine deaminase based on adenosine induced self-assembly of aptamer structures

A new approach is proposed for simple detection of adenosine deaminase (ADA) based on adenosine induced self-assembly of two pieces of single-stranded DNA (ssDNA). These ssDNA are two fragments of the aptamer that has a strong affinity for adenosine and are labeled with carboxyfluorescein and black hole quencher-1, respectively. The complementarities of the bases in the two pieces of ssDNA are insufficient to form a stable structure. In the presence of adenosine, however, the ssDNA can be assembled into the intact aptamer tertiary structure, which results in fluorescence quenching of the carboxyfluorescein-labeled aptamer fragment. As a result, the adenosine-ssDNA complex shows a low background signal, which is rather desired for achieving sensitive detection. Reaction of the complex with ADA causes a great fluorescence enhancement by converting adenosine into inosine that has no affinity for the aptamer. This behaviour leads to the development of a simple and sensitive fluorescent method for assaying ADA activity, with a detection limit of 0.05 U mL(-1), which is more sensitive than most of the existing approaches. Furthermore, the applicability of the method has been demonstrated by detecting ADA in mouse serum samples.

A new method for the detection of adenosine based on time-resolved fluorescence sensor

In this work, we report a thrombin binding aptamer complex based time-resolved fluorescence sensor for small molecule detection. The sensor employs two strands (DNA1 and DNA2) of oligonucleotides. This two strands of oligonucleotides contain two aptamer (α-aptamer and β-aptamer) respectively. DNA1 and DNA2 were labeled with biotin and DIG at the 3'-end, respectively. Binding of the α-aptamer and β-aptamer to the thrombin promotes the hybridization between the complementary stem sequences attached to the two oligonucleotide sequences. The hybridization then brings biotin to be hidden in the shield part on DNA1, shielding biotin from being approached by the streptavidin modified on the microplate due to the steric hindrance effect of the shield part of DNA1. Result in the thrombin-aptamer complex cannot be modified on the surface of microplate which further leads to no signal reported. The strategy integrates the distinguishing features of aptamer and fluorescent techniques. As a proof-of-principle, adenosine in serum was detected with a detection limit of 0.5 nM. A nice detection limit and linear relationship were obtained.

A novel enzyme-free and label-free fluorescence aptasensor for amplified detection of adenosine

A novel enzyme-free and label-free fluorescence aptasensor based on target-catalyzed hairpin self-assembly is developed for amplified detection of adenosine. This aptasensor contains four DNA strands termed as aptamer-catalysis strand, inhibit strand, hairpin structures H1 and H2 which are partially complementary. Meanwhile, a sequence that can form DNA G-quadruplex is partly hidden in the stem of H2. In the absence of adenosine, aptamer-catalysis strand is inhibited, and cannot trigger the self-assembly between H1 and H2. Upon the addition of adenosine, the binding event of aptamer and adenosine triggers the self-assembly between H1 and H2, resulting in the formation of G-quadruplex at the end of H1-H2 complex. The addition of N-methyl mesoporphyrin IX, which has a pronounced structural selectivity for G-quadruplex, generates label-free fluorescence signal. In the optimum conditions, we could detect adenosine as low as 6 μM by monitoring the change in fluorescence intensity. Furthermore, this amplified aptasensor shows high selectivity toward adenosine against its analogs due to the specific recognition ability of the aptamer for the target. Thus, the proposed aptasensor could be used as a simple and selective platform for target detection.

Single-stranded DNA binding protein-assisted fluorescence polarization aptamer assay for detection of small molecules

Here, we describe a new fluorescence polarization aptamer assay (FPAA) strategy which is based on the use of the single-stranded DNA binding (SSB) protein from Escherichia coli as a strong FP signal enhancer tool. This approach relied on the unique ability of the SSB protein to bind the nucleic acid aptamer in its free state but not in its target-bound folded one. Such a feature was exploited by using the antiadenosine (Ade)-DNA aptamer (Apt-A) as a model functional nucleic acid. Two fluorophores (fluorescein and Texas Red) were introduced into different sites of Apt-A to design a dozen fluorescent tracers. In the absence of the Ade target, the binding of the labeled aptamers to SSB governed a very high fluorescence anisotropy increase (in the 0.130-0.200 range) as the consequence of (i) the large global diffusion difference between the free and SSB-bound tracers and (ii) the restricted movement of the dye in the SSB-bound state. When the analyte was introduced into the reaction system, the formation of the folded tertiary structure of the Ade-Apt-A complex triggered the release of the labeled nucleic acids from the protein, leading to a strong decrease in the fluorescence anisotropy. The key factors involved in the fluorescence anisotropy change were considered through the development of a competitive displacement model, and the optimal tracer candidate was selected for the Ade assay under buffer and realistic (diluted human serum) conditions. The SSB-assisted principle was found to operate also with another aptamer system, i.e., the antiargininamide DNA aptamer, and a different biosensing configuration, i.e., the sandwich-like design, suggesting the broad usefulness of the present approach. This sensing platform allowed generation of a fluorescence anisotropy signal for aptamer probes which did not operate under the direct format and greatly improved the assay response relative to that of the most previously reported small target FPAA.