Biomimetic sensing based on chemically induced assembly of a signaling DNA aptamer on a fluid bilayer membrane

Digital nanoreactors to control absolute stoichiometry and spatiotemporal behavior of DNA receptors within lipid bilayers

Interactions between membrane proteins are essential for cell survival but are often poorly understood. Even the biologically functional ratio of components within a multi-subunit membrane complex-the native stoichiometry-is difficult to establish. Here we demonstrate digital nanoreactors that can control interactions between lipid-bound molecular receptors along three key dimensions: stoichiometric, spatial, and temporal. Each nanoreactor is based on a DNA origami ring, which both templates the synthesis of a liposome and provides tethering sites for DNA-based receptors (modelling membrane proteins). Receptors are released into the liposomal membrane using strand displacement and a DNA logic gate measures receptor heterodimer formation. High-efficiency tethering of receptors enables the kinetics of receptors in 1:1 and 2:2 absolute stoichiometries to be observed by bulk fluorescence, which in principle is generalizable to any ratio. Similar single-molecule-in-bulk experiments using DNA-linked membrane proteins could determine native stoichiometry and the kinetics of membrane protein interactions for applications ranging from signalling research to drug discovery.

Split Aptamers Immobilized on Polymer Brushes Integrated in a Lab-on-Chip System Based on an Array of Amorphous Silicon Photosensors: A Novel Sensor Assay

Innovative materials for the integration of aptamers in Lab-on-Chip systems are important for the development of miniaturized portable devices in the field of health-care and diagnostics. Herein we highlight a general method to tailor an aptamer sequence in two subunits that are randomly immobilized into a layer of polymer brushes grown on the internal surface of microfluidic channels, optically aligned with an array of amorphous silicon photosensors for the detection of fluorescence. Our approach relies on the use of split aptamer sequences maintaining their binding affinity to the target molecule. After binding the target molecule, the fragments, separately immobilized to the brush layer, form an assembled structure that in presence of a "light switching" complex [Ru(phen)2(dppz)]2+, emit a fluorescent signal detected by the photosensors positioned underneath. The fluorescent intensity is proportional to the concentration of the target molecule. As proof of principle, we selected fragments derived from an aptamer sequence with binding affinity towards ATP. Using this assay, a limit of detection down to 0.9 µM ATP has been achieved. The sensitivity is compared with an assay where the original aptamer sequence is used. The possibility to re-use both the aptamer assays for several times is demonstrated.

Targeted liposomal drug delivery: a nanoscience and biophysical perspective

Liposomes are a unique platform for drug delivery, and a number of liposomal formulations have already been commercialized. Doxil is a representative example, which uses PEGylated liposomes to load doxorubicin for cancer therapy. Its delivery relies on the enhanced permeability and retention (EPR) effect or passive targeting. Drug loading can be achieved using both standard liposomes and also those containing a solid core such as mesoporous silica and poly(lactide-co-glycolide) (PLGA). Developments have also been made on active targeted delivery using bioaffinity ligands such as small molecules, antibodies, peptides and aptamers. Compared to other types of nanoparticles, the surface of liposomes is fluid, allowing dynamic organization of targeting ligands to achieve optimal binding to cell surface receptors. This review article summarizes development of liposomal targeted drug delivery systems, with an emphasis on the biophysical properties of lipids. In both passive and active targeting, the effects of liposome size, charge, fluidity, rigidity, head-group chemistry and PEGylation are discussed along with recent examples. Most of the examples are focused on targeting tumors or cancer cells. Finally, a few examples of commercialized formulations are described, and some future research opportunities are discussed.

Bifunctional DNA Duplexes Permit Efficient Incorporation of pH Probes into Liposomes

Enzyme-mediated proton transport across biological membranes is critical for many vital cellular processes. pH-sensitive fluorescent dyes are an indispensable tool for investigating the molecular mechanism of proton-translocating enzymes. Here, we present a novel strategy to entrap pH-sensitive probes in the lumen of liposomes that has several advantages over the use of soluble or lipid-coupled probes. In our approach, the pH sensor is linked to a DNA oligomer with a sequence complementary to a second oligomer modified with a lipophilic moiety that anchors the DNA conjugate to the inner and outer leaflets of the lipid bilayer. The use of DNA as a scaffold allows subsequent selective enzymatic removal of the probe in the outer bilayer leaflet. The method shows a high yield of insertion and is compatible with reconstitution of membrane proteins by different methods. The usefulness of the conjugate for time-resolved proton pumping measurements was demonstrated by using two large membrane protein complexes.

Cell-Surface-Anchored Ratiometric DNA Nanoswitch for Extracellular ATP Imaging

The precise detection of extracellular ATP, although a challenging task, is of great significance for understanding the related cell processes. Herein, we developed a ratiometric DNA nanoswitch by employing a DNA tweezer and split aptamer. The nanoswitch is composed of three specially designed ssDNA strands, namely, the central strands O1, O2, and O3. This nanoswitch can be anchored on the cell membrane by cholesterol labeled at the 3' end of O3. Initially, the DNA tweezer adopts an open state, separating the dual fluorophores and giving rise to a low FRET (fluorescence resonance energy transfer) ratio. The presence of ATP induces the binding of the two split aptamers to alter the structure of the nanoswitch from the open state to the closed state, bringing the donor and the acceptor closer together and generating high FRET efficiency. The results demonstrated that the ratiometric DNA nanoswitch can be applied for quantitative analysis and real-time monitoring of the changes in extracellular ATP. We believe that the cell surface-anchored DNA nanoswitch has promising prospects for use as a powerful tool for the understanding of different ATP-related physiological activities.

DNA Oligonucleotide-Functionalized Liposomes: Bioconjugate Chemistry, Biointerfaces, and Applications

Interfacing DNA with liposomes has produced a diverse range of programmable soft materials, devices, and drug delivery vehicles. By simply controlling liposomal composition, bilayer fluidity, lipid domain formation, and surface charge can be systematically varied. Recent development in DNA research has produced not only sophisticated nanostructures but also new functions including ligand binding and catalysis. For noncationic liposomes, a DNA is typically covalently linked to a hydrophobic or lipid moiety that can be inserted into lipid membranes. In this article, we discuss fundamental biointerfaces formed between DNA and noncationic liposomes. The methods to prepare such conjugates and the interactions at the membrane interfaces are also discussed. The effect of DNA lateral diffusion on fluid bilayer membranes and the effect of membrane on DNA assembly are emphasized. DNA hybridization can be programmed to promote fusion of lipid membranes. Representative applications of this conjugate for drug delivery, biosensor development, and directed assembly of materials are briefly described toward the end. Some future research directions are also proposed to further understand this biointerface.

Membrane sculpting by curved DNA origami scaffolds

Membrane sculpting and transformation is essential for many cellular functions, thus being largely regulated by self-assembling and self-organizing protein coats. Their functionality is often encoded by particular spatial structures. Prominent examples are BAR domain proteins, the ‘banana-like’ shapes of which are thought to aid scaffolding and membrane tubulation. To elucidate whether 3D structure can be uncoupled from other functional features of complex scaffolding proteins, we hereby develop curved DNA origami in various shapes and stacking features, following the presumable design features of BAR proteins, and characterize their ability for membrane binding and transformation. We show that dependent on curvature, membrane affinity and surface density, DNA origami coats can indeed reproduce the activity of membrane-sculpting proteins such as BAR, suggesting exciting perspectives for using them in bottom-up approaches towards minimal biomimetic cellular machineries.

BAR domain proteins feature a “banana-like” shape which is thought to aid membrane scaffolding and membrane tubulation. Here authors use DNA origami mimicking BAR domains, giant unilamellar vesicles and fluorescence imaging to study how different BAR domain shapes bind and deform membranes.

Zn2+ Induced Irreversible Aggregation, Stacking, and Leakage of Choline Phosphate Liposomes

The interaction between lipids and metal ions is important for metal sensing, cellular signal transduction, and oxidative lipid damage. While most previous work overlooked the phosphate group of lipids for metal binding, we herein highlight its importance. Phosphocholine (PC) and its headgroup inversed choline phosphate (CP) lipids were used to prepare liposomes. From dynamic light scattering (DLS), Zn²⁺ causes significant aggregation or fusion of the CP liposomes, but not PC liposomes. The size change induced by Zn²⁺ is not fully reversed by adding EDTA, implying liposome fusion induced by Zn²⁺. Isothermal titration calorimetry (ITC) shows that binding between Zn²⁺ and CP liposomes is endothermic with a K_d of 110 μM Zn²⁺, suggesting an entropy driven reaction likely due to the release of bound water. In comparison, no heat was detected by titrating Zn²⁺ into PC liposomes or Ca²⁺ into CP liposomes. Furthermore, Zn²⁺ causes a transient leakage of the CP liposomes, and further leakage is observed upon removing Zn²⁺ by EDTA. Transmission electron microscopy (TEM) with negative stained samples showed multilamellar CP lipid structures attributable to Zn²⁺ sandwiched between lipid bilayers, leading to a proposed reaction mechanism. This work provides an interesting system for studying metal interacting with terminal phosphate groups in liposomes, affecting the size, charge, and membrane integrity of the liposomes.

Liposomes and lipid bilayers in biosensors

Biosensors for the rapid, specific, and sensitive detection of analytes play a vital role in healthcare, drug discovery, food safety, and environmental monitoring. Although a number of sensing concepts and devices have been developed, many longstanding challenges to obtain inexpensive, easy-to-use, and reliable sensor platforms remain largely unmet. Nanomaterials offer exciting possibilities for enhancing the assay sensitivity and for lowering the detection limits down to single-molecule resolution. In this review, we present an overview of liposomes and lipid bilayers in biosensing applications. Lipid assemblies in the form of spherical liposomes or two-dimensional planar membranes have been widely used in the design of biosensing assays; in particular, we highlight a number of recent promising developments of biosensors based on liposomes in suspension, liposome arrays, and lipid bilayers arrays. Assay sensitivity and specificity are discussed, advantages and drawbacks are reviewed, and possible further developments are outlined.

New insights into a classic aptamer: binding sites, cooperativity and more sensitive adenosine detection

The DNA aptamer for adenosine (also for AMP and ATP) is a highly conserved sequence that has recurred in a few selections. It it a widely used model aptamer for biosensor development, and its nuclear magnetic resonance structure shows that each aptamer binds two AMP molecules. In this work, each binding site was individually removed by rational sequence design, while the remaining site still retained a similar binding affinity and specificity as confirmed by isothermal titration calorimetry. The thermodynamic parameters of binding are presented, and its biochemical implications are discussed. The number of binding sites can also be increased, and up to four sites are introduced in a single DNA sequence. Finally, the different sequences are made into fluorescent biosensors based on the structure-switching signaling aptamer design. The one-site aptamer has 3.8-fold higher sensitivity at lower adenosine concentration with a limit of detection of 9.1 μM adenosine, but weaker fluorescence signal at higher adenosine concentrations, consistent with a moderate cooperativity in the original aptamer. This work has offered insights into a classic aptamer for the relationship between the number of binding sites and sensitivity, and a shorter aptamer for improved biosensor design.

Fluorescence Resonance Energy Transfer-Based DNA Nanoprism with a Split Aptamer for Adenosine Triphosphate Sensing in Living Cells

We have developed a DNA nanoprobe for adenosine triphosphate (ATP) sensing in living cells, based on the split aptamer and the DNA triangular prism (TP). In which nucleic acid aptamer was split into two fragments, the stem of the split aptamer was respectively labeled donor and acceptor fluorophores that underwent a fluorescence resonance energy transfer if two ATP molecules were bound as target molecule to the recognition module. Hence, ATP as a target induced the self-assembly of split aptamer fragments and thereby brought the dual fluorophores into close proximity for high fluorescence resonance energy transfer (FRET) efficiency. In the in vitro assay, an almost 5-fold increase in F_A/F_D signal was observed, the fluorescence emission ratio was found to be linear with the concentration of ATP in the range of 0.03-2 mM, and the nanoprobe was highly selective toward ATP. For the strong protecting capability to nucleic acids from enzymatic cleavage and the excellent biocompatibility of the TP, the DNA TP nanoprobe exhibited high cellular permeability, fast response, and successfully realized "FRET-off" to "FRET-on" sensing of ATP in living cells. Moreover, the intracellular imaging experiments indicated that the DNA TP nanoprobe could effectively detect ATP and distinguish among changes of ATP levels in living cells. More importantly, using of the split aptamer and the FRET-off to FRET-on sensing mechanism could efficiently avoid false-positive signals. This design provided a strategy to develop biosensors based on the DNA nanostructures for intracellular molecules analysis.

DNA Nanostructures on Membranes as Tools for Synthetic Biology

Over the last decade, functionally designed DNA nanostructures applied to lipid membranes prompted important achievements in the fields of biophysics and synthetic biology. Taking advantage of the universal rules for self-assembly of complementary oligonucleotides, DNA has proven to be an extremely versatile biocompatible building material on the nanoscale. The possibility to chemically integrate functional groups into oligonucleotides, most notably with lipophilic anchors, enabled a widespread usage of DNA as a viable alternative to proteins with respect to functional activity on membranes. As described throughout this review, hybrid DNA-lipid nanostructures can mediate events such as vesicle docking and fusion, or selective partitioning of molecules into phase-separated membranes. Moreover, the major benefit of DNA structural constructs, such as DNA tiles and DNA origami, is the reproducibility and simplicity of their design. DNA nanotechnology can produce functional structures with subnanometer precision and allow for a tight control over their biochemical functionality, e.g., interaction partners. DNA-based membrane nanopores and origami structures able to assemble into two-dimensional networks on top of lipid bilayers are recent examples of the manifold of complex devices that can be achieved. In this review, we will shortly present some of the potentially most relevant avenues and accomplishments of membrane-anchored DNA nanostructures for investigating, engineering, and mimicking lipid membrane-related biophysical processes.

A target triggered proximity combination-based fluorescence sensing strategy for adenosine detection

Adenosine is a potent physiological and pharmacological regulator, and its abnormal level is closely related to disease development. The sensitive and specific detection of adenosine is crucial for health evaluation and disease diagnosis. In this work, a target triggered proximity combination-based fluorescence sensing strategy is developed for the sensitive and specific detection of adenosine. A difunctional probe showing target recognition and signal amplification is designed, by integration of DNA linker-connected split aptamer fragments with a fragment-elongated polymerase/nicking template. The presence of adenosine would glue the split aptamers, which triggers the two distal aptamer fragments to combine with each other into proximity. The approaching aptamer fragment ends then initiate the strand displacement amplification (SDA) reaction, generating numerous DNA primers. The DNA primers further hybridize with a padlock probe and initiate the rolling circle amplification (RCA) reaction, producing numerous G-quadruplex sequences. The G-quadruplex sequences finally bind with Thioflavin T to obtain enhanced fluorescence signals. The method exhibits a linear correlation within the adenosine concentration range from 5.0 × 10^-7 M to 2.0 × 10^-5 M (R = 0.999) with a detection limit of 8.4 × 10^-8 M, and a good selectivity to distinguish adenosine from its analogues. The recoveries of adenosine in human serum are from 91% to 94%, demonstrating that the system works well in biological fluids. The proposed sensing strategy is anticipated to hold promise in biochemical research, clinical diagnosis and disease treatment.

Using the Rubik's Cube to directly produce paper analytical devices for quantitative point-of-care aptamer-based assays

In this article, we describe a facile method named as Rubik's Cube stamping (RCS) for equipment-free fabrication of microfluidic paper-based analytical devices (μPADs). RCS is inspired by the worldwide ubiquitous RC toy and requires no specialized electric equipment other than a classical six-faced RC that is assembled with home-made small iron components. It can pattern various rosin microstructures in paper simply by either using different functional faces of the modified RC or applying its internal pivot mechanism to adjust the components' patterning forms on one functional face. Such a versatile stamping method is quite simple and inexpensive, and thus holds potential for producing rosin-patterned μPADs by untrained users in resource-limited environments such as small laboratories and private clinics, or even at home and in the field. Moreover, a set of one-channel devices are fabricated to design a point-of-care aptamer-based assay with near sample-in-answer-out capability that integrates enzymatic reactions for robust yet efficient signal amplification and a personal glucometer for portable, user-friendly, rapid and quantitative readout. Its utility is well demonstrated with the sensitive and specific detection of adenosine as a model target in buffer samples and undiluted human urine within several minutes. With the advantages of low cost, simplicity, portability, rapidity, and aptamer variety, this general point-of-care assay system reported here may find broad applications including home healthcare, field-based environmental monitoring or food analysis and emergency situations.

A membrane-anchored fluorescent probe for detecting K(+) in the cell microenvironment

Cell-surface fluorescent probes are effective tools in cell biology and engineering. Here, we for the first time report a diacyllipid-aptamer conjugate-based fluorescent probe which could anchor on cell membrane for real-time tracking of potassium ions in the cell microenvironment.

Molecularly Imprinted Polymers with DNA Aptamer Fragments as Macromonomers

Molecularly imprinted polymers (MIPs) are produced in the presence of a template molecule. After removing the template, the cavity can selectively rebind the template. MIPs are attractive functional materials with a low cost and high stability, but traditional MIPs often suffer from low binding affinity. This study employs DNA aptamer fragments as macromonomers to improve MIPs. The DNA aptamer for adenosine was first split into two halves, fluorescently labeled, and copolymerized into MIPs. With a fluorescence quenching assay, the importance of imprinting was confirmed. Further studies were carried out using isothermal titration calorimetry (ITC). Compared to the mixture of the free aptamer fragments, their MIPs doubled the binding affinity. Each free aptamer fragment alone cannot bind adenosine, whereas MIPs containing each fragment are effective binders. We further shortened one of the aptamer fragments, and the DNA length was pushed to as short as six nucleotides, yielding MIPs with a dissociation constant of 27 μM adenosine. This study provides a new method for preparing functional MIP materials by combining high-affinity biopolymer fragments with low-cost synthetic monomers, allowing higher binding affinity and providing a method for signaling binding based on DNA chemistry.

Aptamer-based biosensors for biomedical diagnostics

Aptamers are single-stranded nucleic acids that selectively bind to target molecules. Most aptamers are obtained through a combinatorial biology technique called SELEX. Since aptamers can be isolated to bind to almost any molecule of choice, can be readily modified at arbitrary positions and they possess predictable secondary structures, this platform technology shows great promise in biosensor development. Over the past two decades, more than one thousand papers have been published on aptamer-based biosensors. Given this progress, the application of aptamer technology in biomedical diagnosis is still in a quite preliminary stage. Most previous work involves only a few model aptamers to demonstrate the sensing concept with limited biomedical impact. This Critical Review aims to summarize progress that might enable practical applications of aptamers for biological samples. First, general sensing strategies based on the unique properties of aptamers are summarized. Each strategy can be coupled to various signaling methods. Among these, a few detection methods including fluorescence lifetime, flow cytometry, upconverting nanoparticles, nanoflare technology, magnetic resonance imaging, electronic aptamer-based sensors, and lateral flow devices have been discussed in more detail since they are more likely to work in a complex sample matrix. The current limitations of this field include the lack of high quality aptamers for clinically important targets. In addition, the aptamer technology has to be extensively tested in a clinical sample matrix to establish reliability and accuracy. Future directions are also speculated to overcome these challenges.

Colorimetric detection of human chorionic gonadotropin using catalytic gold nanoparticles and a peptide aptamer

We combined catalytic gold nanoparticles (AuNPs) with an hCG-specific peptide aptamer to create a simple, sensitive, label-free colorimetric assay for hCG. The applications of this colorimetric biosensor may be expanded by changing the peptide aptamer.

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.

Dual-aptamer modification generates a unique interface for highly sensitive and specific electrochemical detection of tumor cells

Because circulating tumor cells (CTCs) have been proven to be an important clue of the tumor metastasis, their detection thus plays a pivotal role in the diagnosis and prognosis of cancer. Herein, we fabricate an electrochemical sensor by directly conjugating two cell-specific aptamers, TLS1c and TLS11a, which specifically recognize MEAR cancer cells, to the surface of a glassy carbon electrode (GCE) via the formation of amide bonds. The two aptamers are simultaneously conjugated to the GCE surface via precisely controlled linkers: TLS1c through a flexible linker (a single-stranded DNA T15; ss-TLS1c) and TLS11a through a rigid linker (a double-stranded DNA T15/A15; ds-TLS11a). It is found that such ss-TLS1c/ds-TLS11a dual-modified GCEs show greatly improved sensitivity in comparison with those modified with a single type of aptamer alone or ds-TLS1c/ds-TLS11a with both rigid linkers, suggesting that our optimized, rationally designed electrode-aptamer biosensing interface may enable better recognition and thus more sensitive detection of tumor cells. Through the utilization of this dual-aptamer-modified GCE, as few as a single MEAR cell in 10(9) whole blood cells can be successfully detected with a linear range of 1-14 MEAR cells. Our work demonstrates a rather simple yet well-designed and ultrasensitive tumor cell detection method based on the cell-specific aptamer-modified GCE, showing a promising potential for further CTC-related clinical applications.

Application of nucleic acid-lipid conjugates for the programmable organisation of liposomal modules

We present a critical review of recent work related to the assembly of multicompartment liposome clusters using nucleic acids as a specific recognition unit to link liposomal modules. The asymmetry in nucleic acid binding to its non-self complementary strand allows the controlled association of different compartmental modules into composite systems. These biomimetic multicompartment architectures could have future applications in chemical process control, drug delivery and synthetic biology. We assess the different methods of anchoring DNA to lipid membrane surfaces and discuss how lipid and DNA properties can be tuned to control the morphology and properties of liposome superstructures. We consider different methods for chemical communication between the contents of liposomal compartments within these clusters and assess the progress towards making this chemical mixing efficient, switchable and chemically specific. Finally, given the current state of the art, we assess the outlook for future developments towards functional modular networks of liposomes.

Simple and efficient method to purify DNA-protein conjugates and its sensing applications

DNA-protein conjugates are very useful in analytical chemistry for target recognition and signal amplification. While a number of methods for conjugating DNA with proteins are known, methods for purification of DNA-protein conjugates from reaction mixture containing unreacted proteins are much less investigated. In this work, a simple and efficient approach to purify DNA-invertase conjugates from reaction mixture via a biotin displacement strategy to release desthiobiotinylated DNA-invertase conjugates from streptavidin-coated magnetic beads was developed. The conjugates purified by this approach were utilized for quantitative detection of cocaine and DNA using a personal glucose meter through structure-switching DNA aptamer sensors and competitive DNA hybridization assays, respectively. In both cases, the purified DNA-invertase conjugates showed better performance compared to the same assays using unpurified conjugates. The approach demonstrated here can be further expanded to other DNA and proteins to generate purified DNA-protein conjugates for analytical and other applications.

An ultrasensitive colorimetric aptasensor for ATP based on peptide/Au nanocomposites and hemin–G-quadruplex DNAzyme

A self-assembled peptide nanosphere was firstly applied to construct biosensors. A new signal amplification strategy was proposed for colorimetric aptasensor based on PNS/AuNPs composite. The colorimetric aptasensor displayed an ultra-high sensitivity for ATP detection with a LOD of 1.35 pM.

Double-functionalized gold nanoparticles with split aptamer for the detection of adenosine triphosphate

A newly designed functionalization type for gold nanoparticles (AuNP) with split aptamer has been developed for the detection of adenosine triphosphate (ATP). The ATP aptamer was split into two parts with their 5' prime or 3' prime modified with thiol. Both the 5' SH and 3' SH modified strands for each split aptamer fragment were functionalized onto the same AuNP to construct double-functionalized AuNP-DNA conjugates. Thus, the split aptamer can be reassembled into intact folded structure in the presence of ATP molecule with two potential assembly types, which induces the assembly of AuNP-DNA conjugates. In this double-functionalized system, the traditional assembly type might facilitate another assembly type, which was found to give much higher LSPR change in the presence of ATP than the traditional assembly type, and improve the sensitivity for ATP detection. Time courses of the assemble processes with different assembly types, Mg(2+) concentrations, and aptamer fragments densities on AuNP were followed using the absorption ratio at 650 nm and 520 nm. ATP response with this newly designed system was investigated using absorption spectra and dynamic light scattering method.

Direct detection of adenosine in undiluted serum using a luminescent aptamer sensor attached to a terbium complex

Aptamers, single-stranded nucleic acids that can selectively bind to various target molecules, have been widely used for constructing biosensors. A major challenge in this field, however, is direct sensing of analytes in complex biological media such as undiluted serum. While progress has been made in developing an inhomogeneous assay by using a preseparation step to wash away the interferences within serum, a facile strategy for direct detection of targets in homogeneous unprocessed serum is highly desired. We herein report a turn-on luminescent aptamer biosensor for the direct detection of adenosine in undiluted and unprocessed serum, by taking advantage of a terbium chelate complex with long luminescence lifetime to achieve time-resolved detection. The sensor exhibits a detection limit of 60 μM adenosine while marinating excellent selectivity that is comparable to those in buffer. The approach demonstrated here can be applied for direct detection and quantification of a broad range of analytes in biological media by using other aptamers.